Patent Publication Number: US-5158745-A

Title: High-nitrogen ferritic heat-resisting steel

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a high-nitrogen ferritic heat-resisting steel, more particularly to a high-nitrogen ferritic heat-resisting steel containing chromium, and which is appropriate for use in a high-temperature, high-pressure environment, and to a method of producing the same. 
     2. Description of the Prior Art 
     Recent years have seen a marked increase in the temperatures and pressures under which thermal power plant boilers are required to operate. Some plans already call for operation at 566° C. and 310 ata and it is expected that operation under conditions of 650° C. and 350 ata will be implemented in the future. These are extremely severe conditions from the viewpoint of the boiler materials used. 
     At operating temperatures exceeding 550° C., it has, from the viewpoints of oxidation resistance and high-temperature strength, been necessary to switch from ferritic 2 . 1/4 Cr-1 Mo steel to high-grade austenitic steels such as 18-8 stainless steel. In other words, it has been necessary to adopt expensive materials with properties exceeding what is required. 
     Decades have been spent in search of steels for filling in the gap between 2 . 1/4 Cr-1 Mo steel and austenitic stainless steel. Medium Cr (e.g. 9 Cr and 12 Cr) steel boiler pipes are made of heat-resisting steels that were developed against this background. They achieve high-temperature strength and creep rupture strength on a par with austenitic steels by use of a base metal composition which includes various alloying elements for precipitation hardening and solution hardening. 
     The creep rupture strength of a heat-resisting steel is governed by solution hardening in the case of short-term aging and by precipitation hardening in the case of long-term aging. This is because the solution-hardening elements initially present in solid solution in the steel for the most part precipitate as stable carbides such as M 23  C 6  during aging, and then when the aging is prolonged these precipitates coagulate and enlarge, with a resulting decrease in creep rupture strength. 
     Thus, with the aim of maintaining the creep rupture strength of heat-resisting steels at a high level, a considerable amount of research has been done for discovering ways for avoiding the precipitation of the solution hardening elements and maintaining them in solid solution for as long as possible. 
     For example, Japanese Patent Public Disclosures No. 63(1988)-89644, 61(1986)-231139 and 62(1987)-297435 disclose ferritic steels that achieve dramatically higher creep rupture strength than conventional Mo-containing ferritic heat-resisting steels by the use of W as a solution hardening element. 
     While the solution hardening by W in these steels may be more effective than by Mo, the precipitates are still fundamentally carbides of the M 23  C 6  type, so that it is not possible to avoid reduction of the creep rupture strength following prolonged aging. 
     Moreover, the use of ferritic heat-resisting steels at up to 650° C. has been considered difficult because of their inferior high-temperature oxidation resistance as compared with austenitic heat-resisting steels. A particular problem with these steels is the pronounced degradation of high-temperature oxidation resistance that results from the precipitation of Cr in the form of coarse M 23  C 6  type precipitates at the grain boundaries. 
     The highest temperature for use of ferritic heat-resisting steel has therefore been considered to be 600° C. 
     The need for heat-resisting steels capable of standing up under extremely severe conditions has grown more acute not only because of the increasingly severe operating conditions mentioned earlier but also because of plans to reduce operating costs by extending the period of continuous power plant operation from the current 100 thousand hours up to around 150 thousand hours. 
     Although ferritic heat-resisting steels are somewhat inferior to austenitic steels in high-temperature strength and anticorrosion property, they have a cost advantage. Furthermore, for reasons related to the difference in thermal expansion coefficient, among the various steam oxidation resistance properties they are particularly superior in scale defoliation resistance. For these reasons, they are attracting attention as a boiler material. 
     For the reasons set out above, however, it is clearly not possible with the currently available technology to develop ferritic heat-resisting steels that are capable of standing up for 150 thousand hours under operating conditions of 650° C., 350 ata, that are low in price and that exhibit good steam oxidation resistance. 
     Through their research the inventors developed a high-nitrogen ferritic heat-resisting steel in which W is added in place of Mo as the main solution hardening element, thereby increasing the high-temperature strength, and nitrogen is forcibly added to the ferritic steel to a level of supersaturation, thereby causing dispersed precipitation of fine nitrides and carbo-nitrides which greatly delay the formation of M 23  C 6  precipitates that would otherwise consume large quantities of Cr acting as an oxidation resistance enhancer, and W acting as a solution hardening agent. The inventors found that this steel exhibits stable creep rupture strength, superior high-temperature oxidation resistance and superior low-temperature toughness, and is capable of being applied under conditions of 650° C., 350 ata and 150 thousand hours of continuous operation. 
     There have been few papers published on research into high-nitrogen ferritic heat-resisting steels and the only known published report in this field is Ergebnisse der Werkstoff-Forschung, Band I, Verlag Schweizerische Akademieder Werkstoffwissenschaften &#34;Thubal-Kain&#34;, Zurich, 1987, 161-180. 
     However, the research described in this report is limited to that in connection with ordinary heat-resisting steel and there is no mention of materials which can be used under such severe conditions as 650° C., 350 ata and 150 thousand hours continuous operation. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a high-nitrogen ferritic heat-resisting steel which overcomes the shortcomings of the conventional heat-resisting steels, and particularly to provide such a steel capable of being used under severe operating conditions wherein the decrease in creep rupture strength following prolonged aging and the degradation of high-temperature oxidation resistance caused by precipitation of carbides are mitigated by adding nitrogen to supersaturation so as to precipitate fine nitrides and/or carbo-nitrides which delay the formation of carbides such as the M 23  C 6  precipitates seen in conventional steels 
     Another object of the invention is to provide such a high-nitrogen ferritic heat-resisting steel imparted with superior high-temperature oxidation resistance and creep rupture strength by allowing nitrogen added to beyond the solution limit to precipitate in the form of nitrides and carbo-nitrides. 
     Another object of the invention is to provide a method of producing a high-nitrogen ferritic heat-resisting steel of the aforesaid type. 
     This invention was accomplished in the light of the aforesaid knowledge and, in one aspect, pertains substantially to a high-nitrogen ferritic heat-resisting steel comprising, in weight per cent, 0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.50-3.00% W, 0.005-1.00% Mo; 0.05-0.50% V, 0.02-0.12% Nb and 0.10-0.50% N and being controlled to include not more than 0.050% P, not more than 0.010% S and not more than 0.020% O, and optionally comprising (A) one or both of 0.01-1.00% Ta and 0.01-1.00% Hf and/or (B) one or both of 0.0005-0.10% Zr and 0.01-0.10% Ti, the balance being Fe and unavoidable impurities. 
     Another aspect of the invention pertains to a method of producing such a high-nitrogen ferritic heat-resisting steel wherein the steel components are melted and equilibrated in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas and the resulting melt is thereafter cast or solidified in an atmosphere controlled to have a nitrogen partial pressure of not less than 1.0 ata and a total pressure of not less than 4.0 ata, with the relationship between the partial pressure p and the total pressure P t  being 
     
         10.sup.P &lt;P.sub.t.sup.0.37 +log.sub.10 6 
    
     thereby obtaining sound ingots free of blowholes. 
     The above and other features of the present invention will become apparent from the following description made with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an ingot and the manner in which it is to be cut. 
     FIG. 2 is a graph showing the relationship between the steel nitrogen content and the weight percentage of the total of M 23  C 6  +M 6  C+Cr 2  N+Cr 2  (C,N) among the precipitates in the steel accounted for by M 23  C 6  +M 6  C, and the relationship between the steel nitrogen content and the weight percentage of the total of M 23  C 6  +M 6  C+Cr 2  N+Cr 2  (C,N) among the precipitates in the steel accounted for by Cr 2  N+Cr 2  (C,N). 
     FIG. 3 is a graph showing conditions under which blowholes occur in the ingot in terms of the relationship between the total pressure and nitrogen partial pressure of the atmosphere during casting. 
     FIG. 4 is a perspective view showing the manner in which creep test pieces are taken from a pipe specimen and a rolled plate specimen. 
     FIG. 5 is a graph showing the relationship between steel nitrogen content and extrapolated creep rupture strength at 650° C., 150 thousand hours. 
     FIG. 6 is a graph showing an example of creep test results in terms of stress vs rupture time. 
     FIG. 7 is a graph showing the relationship between steel nitrogen content and Charpy impact absorption energy at 0° C. following aging at 700° C. for 10 thousand hours. 
     FIG. 8 is a graph showing the relationship between steel nitrogen content and the thickness of the oxidation scale formed on the surface of a test piece after oxidation at 650° C. for 10 thousand hours. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The reasons for the limits placed on the components of the high-nitrogen ferritic heat-resisting steel in this invention will now be explained. 
     C is required for achieving strength. Adequate strength cannot be achieved at a C content of less than 0.01%, while at a C content exceeding 0.30% the steel is strongly affected by welding heat and undergoes hardening which becomes a cause for low-temperature cracking. The C content range is therefore set at 0.01-0.30%. 
     Si is important for achieving oxidation resistance and is also required as a deoxidizing agent. It is insufficient for these purposes at a content of less than 0.02%, whereas a content exceeding 0.80% reduces the creep rupture strength. The Si content range is therefore set at 0.02-0.80% 
     Mn is required for deoxidation and also for achieving strength. It has to be added at at least 0.20% for adequately exhibiting its effect. When it exceeds 1.00% it may in some cases reduce creep rupture strength. The Mn content range is therefore set at 0.20-1.00%. 
     Cr is indispensable to oxidation resistance. It also contributes to increasing creep resistance by combining with N and finely precipitating in the base metal matrix in the form of Cr 2  N, Cr 2  (C,N) and the like. Its lower limit is set at 8.0% from the viewpoint of oxidation resistance. Its upper limit is set at 13.0% for maintaining the Cr equivalent value at a low level so as to realize a martensite phase texture. 
     W produces a marked increase in creep rupture strength by solution hardening. Its effect toward increasing creep rupture strength over long periods at high temperatures of 550° C. and higher is particularly pronounced. Its upper limit is set at 3.00% because at contents higher than this level it precipitates in large quantities in the form of carbide and intermetallic compounds which sharply reduce the toughness of the base metal. The lower limit is set at 0.50% because it does not exhibit adequate solution hardening effect at lower levels. 
     Mo increases high-temperature strength through solution hardening. It does not exhibit adequate effect at a content of less than 0.005% and at a content higher than 1.00% it may, when added together with W, cause heavy precipitation of Mo 2  C type oxides which markedly reduce base metal toughness. The Mo content range is therefore set at 0.005-1.00%. 
     V produces a marked increase in the high-temperature strength of the steel regardless of whether it forms precipitates or, like W, enters solid solution in the matrix. When it precipitates, the resulting VN serves as precipitation nuclei for Cr 2  N and Cr 2  (C,N), which has a pronounced effect toward promoting fine dispersion of the precipitates. It has no effect at below 0.05% and reduces toughness at higher than 0.50%. The V content range is therefore set at 0.05-0.50%. 
     Nb increases high-temperature strength by precipitating as NbN and Nb(C,N). Also, similarly to V, it promotes fine precipitate dispersion by forming precipitation nuclei for Cr 2 , Cr 2  (C,N) and the like. The lower limit at which it manifests these effects is 0.02%. Its upper limit is set at 0.12% because when present at higher levels it reduces strength by causing precipitate coagulation and enlargement. 
     N dissolves in the matrix and also forms nitride and carbo-nitride precipitates. As the form of the precipitates is mainly Cr 2  N and Cr 2  (C,N), there is less precipitate-induced consumption of Cr and W than in the case of the M 23  C 6 , M 6  C and other such precipitates observed in conventional steels. N thus increases oxidation resistance and creep rupture strength. At least 0.10% is required for precipitation of nitrides and carbo-nitrides and suppressing precipitation of M 23  C 6 , M 6  C. The upper limit is set at 0.50% for preventing coagulation and enlargement of nitride and carbo-nitride precipitates by the presence of excessive nitrogen. 
     P, S and O are present in the steel according to this invention as impurities. P and S hinder the achievement of the purpose of the invention by lowering strength, while O has the adverse effect of forming oxides which reduce toughness. The upper limits on these elements is therefore set at 0.050%, 0.010% and 0.020%, respectively. 
     The basic components of the steel according to this invention (aside from Fe) are as set out above. Depending on the purpose to which the steel is to be put, however, it may additionally contain (A) one or both of 0.01-1.00% Ta and 0.01-1.00% Hf and/or (B) one or both of 0.0005-0.10% Zr and 0.01-0.10% Ti. 
     At low concentrations Ta and Hf act as deoxidizing agents. At high concentrations they form fine high melting point nitrides and carbo-nitrides and, as such, increase toughness by decreasing the austenite grain size. In addition, they also reduce the degree to which Cr and W dissolve in precipitates and by this effect enhance the effect of supersaturation with nitrogen. Neither element exhibits any effect at less than 0.01%. When either is present at greater than 1.00%, it reduces toughness by causing enlargement of nitride and carbonitride precipitates. The content range of each of these elements is therefore set at 0.01-1.00%. 
     Acting to govern the deoxidation equilibrium in the steel, Zr suppresses the formation of oxides by markedly reducing the amount of oxygen activity. In addition, its strong affinity for N promotes precipitation of fine nitrides and carbo-nitrides which increase creep rupture strength and high-temperature oxidation resistance. When present at less than 0.0005% it does not provide an adequate effect of governing the deoxidation equilibrium, and when present at greater than 0.10% it results in heavy precipitation of coarse ZrN and ZrC which markedly reduce the toughness of the base metal. The Zr content range is therefore set at 0.0005-0.10%. 
     Ti raises the effect of excess nitrogen by precipitating in the form of nitrides and carbo-nitrides. At a content of less than 0.01% it has no effect, while at a content of over 0.10% it results in precipitation of coarse nitrides and carbo-nitrides which reduce toughness. The Ti content range is therefore set at 0.01-0.10%. 
     The aforesaid alloying components can be added individually or in combinations. 
     The object of this invention is to provide a ferritic heat-resisting steel that is superior in creep rupture strength and high-temperature oxidation resistance. Depending on the purpose of use it can be produced by various methods and be subjected to various types of heat treatment. These methods and treatments in no way diminish the effect of the invention. 
     However, in view of the need to supersaturate the steel with nitrogen, it is necessary during casting to raise the pressure of the atmosphere to not less than 4.0 ata and to control the nitrogen partial pressure to not less than 1.0 ata. As an auxiliary gas to be mixed with the nitrogen gas it is appropriate to use an inert gas such as Ar, Ne, Xe or Kr. These casting conditions were determined by the following experiment. 
     Steel of a chemical composition aside from nitrogen as indicated in claims 1-4 was melted in an induction heating furnace installed in a chamber that could be pressurized up to 150 ata. A mixed gas of argon and nitrogen having a prescribed nitrogen partial pressure was introduced into the furnace and maintained at a pressure which was varied from test to test. After the nitrogen and molten metal had reached chemical equilibrium, the molten metal was cast into a mold that had been installed in the chamber beforehand, whereby there was obtained a 5-ton ingot. 
     The ingot was cut vertically as shown in FIG. 1 and the ingot 1 was visually examined for the presence of blowholes. 
     Following this examination, a part of the ingot was placed in a furnace and maintained at 1180° C. for 1 hour and then forged into a plate measuring 50 mm in thickness, 750 mm in width and 4,000 mm in length. 
     This plate was subjected to solution treatment at 1100° C. for 1 hour and to tempering at 760° C. for 3 hours. The steel was then chemically analyzed and the dispersion state and morphology of the nitrides and carbo-nitrides were investigated by observation with an optical microscope, an electron microscope, X-ray diffraction and electron beam diffraction, whereby the chemical structure was determined. 
     Among the precipitates present within the as-heat-treated steel, FIG. 2 shows how the proportion of the precipitates in the steel accounted for by M 23  C 6  type carbides and M 6  C type carbides, and the proportion thereof accounted for by Cr 2  N type nitrides and carbo-nitrides, vary with nitrogen concentration. At a nitrogen concentration of 0.1%, nitrides and carbo-nitrides account for the majority of the precipitates in the steel, while at a nitrogen concentration of 0.15%, substantially 100% of the precipitates are nitrides and carbo-nitrides with virtually no carbides present whatsoever. Thus, for the effect of this invention to be adequately manifested it is necessary for the nitrogen concentration of the steel to be not less than 0.1%. 
     The graph of FIG. 3 shows how the state of blowhole occurrence varies depending on the relationship between the total and nitrogen partial pressure of the atmosphere. For achieving a nitrogen concentration of 0.1% or higher it is necessary to use a total pressure of not less than 4.0 ata. Equilibrium calculation based on Sievert&#39;s law shows that the nitrogen partial pressure in this case is not less than 1.0 ata. 
     Moreover, where for controlling the amount of nitride and carbo-nitride precipitation the nitrogen partial pressure p is maintained at 1.0-6.0 ata (nitrogen concentration within the steel of approximately 0.5 wt. %), it becomes necessary to vary the total pressure P t  between 4.0 and about 100 ata, the actual value selected depending on the nitrogen partial pressure. Namely, it is necessary to use a total pressure falling above the broken line representing the boundary pressure in FIG. 3. 
     When the boundary line of FIG. 3 is determined experimentally it is found to lie at 
     
         10.sup.P =P.sub.t.sup.0.37 +log.sub.10 6 
    
     meaning that the steel according to this invention can be obtained by selecting an atmosphere of a pressure and composition meeting the condition of the inequality 
     
         10.sup.P &lt;P.sub.t.sup.0.337 +log.sub.10 6. 
    
     It is therefore necessary to use furnace equipment enabling pressure and atmosphere control. Without such equipment, it is difficult to produce the steel of the present invention. 
     There are no limitations whatever on the melting method. Based on the chemical composition of the steel and cost considerations, it suffices to select from among processes using a converter, an induction heating furnace, an arc melting furnace or an electric furnace. 
     The situation regarding refining is similar. Insofar as the atmosphere is controlled to a total pressure of not less than 4.0 ata and a nitrogen partial pressure of not less than 1.0 ata, it is both possible and effective to use a ladle furnace, an electro-slag remelting furnace or a zone melting furnace. 
     After casting under a pressurized atmosphere of a total pressure of not less than 4.0 ata and a nitrogen partial pressure of not less than 1.0 ata, it is possible to process the steel into a billet, bloom or plate by casting or hot rolling. Since the steel of this invention includes finely dispersed nitrides and carbo-nitrides, it is superior to conventional ferritic heat-resisting steels in hot-workability. This is also one reason for employing nitrides and carbo-nitrides obtained by adding nitrogen to beyond the solution limit. 
     For processing the steel into products, it is possible to first process it into a round or rectangular billet and then form it into a seamless pipe or tube by hot extrusion or any of various seamless rolling methods. Otherwise it can be formed into a sheet by hot and cold rolling and then made into a welded tube by electric resistance welding. Alternatively, it can be processed into a welded pipe or tube by use of TIG, MIG, SAW, LASER and EB welding, individually or in combination. Moreover, it is possible to expand the size range of products to which the present invention can be applied by following any of the aforesaid processes by hot or warm stretch reduction or sizing. 
     The steel according to the invention can also be provided in the form of a plate or sheet. The plate or sheet can, in its hot-rolled state or after whatever heat treatment is found necessary, be provided as a heat-resisting material in various shapes, without any influence on the effects provided by the invention. 
     The pipe, tube, plate, sheet and variously shaped heat-resisting materials referred to above can, in accordance with their purpose and application, be subjected to various heat treatments, and it is important for them to be so treated for realizing the full effect of the invention. 
     While the production process ordinarily involves normalizing (solution heat treatment) +tempering, it is also possible and useful additionally to carry out one or a combination of two or more of quenching, tempering and normalizing. It is also possible, without influencing the effects of the present invention in any way, to repeatedly carry out one or more of the aforesaid processes to whatever degree is necessary for adequately bringing out the steel properties. 
     The aforesaid processes can be appropriately selected and applied to the manufacture of the steel according to the invention. 
     WORKING EXAMPLES 
     The steels indicated in Tables 1-4, each having a composition according to one of claims 1-4, were separately melted in amounts of 5 tons each in an induction heating furnace provided with pressurizing equipment. The resulting melt was cleaned by ladle furnace processing (under bubbling with a gas of the same composition as the atmosphere) for reducing its impurity content, whereafter the atmosphere was regulated using a mixed gas of nitrogen and argon so as to satisfy the conditions of the inequality shown above. The melt was then cast into a mold and processed into a round billet, part of which was hot extruded to obtain a tube 60 mm in diameter and 10 mm in wall thickness and the remainder of which was subjected to seamless rolling to obtain a pipe 380 mm in diameter and 50 mm in wall thickness. The tube and pipe were subjected to a single normalization at 1100° C. for 1 hour and were then tempered at 760° C. for 3 hours. 
     In addition, a 50 ton ingot was cast and forged into a slab which was hot rolled into 25 mm and 50 mm thick plates. 
     As shown in FIG. 4, creep test pieces 6 measuring 6 mm in diameter were taken along the axial direction 4 of the pipe or tube 3 and along the rolling direction 5 of the plates and subjected to creep test measurement at 650° C. Based on the data obtained, a linear extrapolation was made for estimating the creep rupture strength at 150 thousand hours. A creep rupture strength of 15.0 kg/mm 2  was used as the creep rupture strength evaluation reference value. The creep rupture strength at 650° C., 150 thousand hours is hereinafter defined as the linearly extrapolated value at 150 thousand hours on the creep rupture strength vs rupture time graph. 
     Toughness was evaluated through an accelerated evaluation test in which aging was carried out at 700° C. for 10 thousand hours. JIS No. 4 tension test pieces were cut from the aged steel and evaluated for impact absorption energy. Assuming a water pressure test at 0° C., the toughness evaluation reference value was set at 5.0 kgf.m. 
     High-temperature oxidation resistance was evaluated by suspending a 25 mm×25 mm×5 mm test piece cut from the steel in 650° C. atmospheric air in a furnace for 10 thousand hours and then cutting the test piece parallel to the direction of growth of the scale and measuring the oxidation scale thickness. 
     The 650° C., 150 thousand hour creep rupture strength, the Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours and the oxidation scale thickness after oxidation at 650° C. for 10 thousand hours are shown in Tables 1 to 4, respectively. 
     For comparison, steels of compositions not falling within any of the claims 1 to 4 were melted, processed and tested in the same way as described above. Their chemical compositions and the evaluation results are shown in Table 5. 
     FIG. 5 shows the relationship between the nitrogen content of the steels and the extrapolated creep rupture strength at 650° C., 150 thousand hours. It will be noted that the creep rupture strength attains high values exceeding 15 kg/mm 2  at a steel nitrogen content of 0.1% or higher but falls below 15 kg/mm 2  and fails to satisfy the evaluation reference value that was set at a steel nitrogen content of less than 0.1%. 
     FIG. 6 shows the results of the creep test in terms of stress vs rupture time. A good linear relationship can be noted between stress and rupture time at a steel nitrogen content of not less than 0.1%. On the other hand, when the steel nitrogen content falls below 0.1%, the relationship between stress and rupture time exhibits a pronounced decline in creep rupture strength with increasing time lapse. That is to say, linearity is not maintained. This is because W and the other solution hardening elements precipitate as carbides whose coagulation and enlargement degrades the creep rupture strength property of the base metal. In contrast, at a nitrogen content of 0.1% or higher, fine nitrides and carbo-nitrides are preferentially precipitated so that the formation of carbides is greatly delayed, and since this suppresses the dissolution of the solution hardening elements into carbides, a high creep rupture strength was maintained in the long-term creep test. 
     FIG. 7 shows the relationship between Charpy impact absorption energy at 0° C. following aging at 700° C. for 10 thousand hours and steel nitrogen content. When the steel nitrogen content falls within the range of 0.1-0.5%, the impact absorption energy exceeds 3.0 kgf.m. In contrast, when it falls below 0.1%, there is little or no suppression of grain growth by residual high melting point nitrides during solution treatment and, as a result, the impact absorption energy decreases, and when it exceeds 0.5%, the impact absorption energy is reduced by heavy nitride precipitation. 
     FIG. 8 shows the relationship between the thickness of the oxidation scale formed on the surface of a test piece after oxidation at 650 C for 10 thousand hours and the steel nitrogen content. Although the oxidation scale thickness is between 400 and 800 μm when the steel nitrogen content falls below 0.1%, it decreases to 250 μm or less when the steel nitrogen content is 0.1% or higher. 
     Reference is now made to the comparison steels shown in Table 5. Nos. 161 and 162 are examples in which insufficient steel nitrogen content resulted in a low extrapolated creep rupture strength at 650° C., 150 thousand hours, and also poor high-temperature oxidation resistance. Nos. 163 and 164 are examples in which excessive steel nitrogen content caused heavy precipitation of coarse nitrides and carbo-nitrides, resulting in a Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours of not more than 3.0 kgf.m. No. 165 is an example in which a low W concentration resulted in a low creep rupture strength owing to insufficient solution hardening, notwithstanding that the steel nitrogen content fell within the range of the invention. No. 166 is an example in which heavy precipitation of coarse ZrN caused by a Zr concentration in excess of 0.1% resulted in a Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours of less than 3.0 kgf.m. Nos. 167, 168 and 169 are examples similar to the case of No. 166 except that the elements present in excess were Ta, Hf and Ti, respectively. As a result, heavy precipitation of coarse TaN, HfN and TiN resulted in a Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours of less than 3.0 kgf.m. No. 170 is an example in which the nitrogen partial pressure was 2.2 ata and the total pressure was 2.5 ata, values not satisfying the inequality set forth above, so that many large blowholes formed in the ingot, making it impossible to obtain either a sound ingot or a plate, and leading to a reduction in both the extrapolated creep rupture strength at 650° C., 150 thousand hours and the Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours. 
     
                                           TABLE 1                                 
__________________________________________________________________________
(Wt. %) Invention Steels                                                  
                                                   CS   VE  TO            
No.                                                                       
   C  Si Mn P  S  Nb V  Cr Mo W  Zr  Ta Hf Ti                             
                                             N  O  kg/mm.sup.2            
                                                        kgf ·    
                                                        μm             
__________________________________________________________________________
 1 0.018                                                                  
      0.682                                                               
         0.885                                                            
            0.034                                                         
               0.009                                                      
                  0.058                                                   
                     0.215                                                
                        11.98                                             
                           0.922                                          
                              0.850                                       
                                 --  -- -- --                             
                                             0.255                        
                                                0.019                     
                                                   19.9 4.67              
                                                            176           
 2 0.131                                                                  
      0.796                                                               
         0.532                                                            
            0.029                                                         
               0.002                                                      
                  0.074                                                   
                     0.490                                                
                        8.77                                              
                           0.961                                          
                              1.024                                       
                                 --  -- -- --                             
                                             0.104                        
                                                0.010                     
                                                   16.0 13.9 117          
 3 0.255                                                                  
      0.685                                                               
         0.935                                                            
            0.014                                                         
               0.009                                                      
                  0.046                                                   
                     0.382                                                
                        11.45                                             
                           0.421                                          
                              2.157                                       
                                 --  -- -- --                             
                                             0.143                        
                                                0.004                     
                                                   17.3 12.8 206          
 4 0.011                                                                  
      0.499                                                               
         0.249                                                            
            0.019                                                         
               0.009                                                      
                  0.094                                                   
                     0.432                                                
                        11.00                                             
                           0.985                                          
                              2.256                                       
                                 --  -- -- --                             
                                             0.250                        
                                                0.007                     
                                                   21.0 4.24 175          
 5 0.035                                                                  
      0.387                                                               
         0.626                                                            
            0.029                                                         
               0.007                                                      
                  0.084                                                   
                     0.269                                                
                        8.36                                              
                           0.350                                          
                              1.752                                       
                                 --  -- -- --                             
                                             0.301                        
                                                0.003                     
                                                   23.5 4.40 198          
 6 0.088                                                                  
      0.491                                                               
         0.213                                                            
            0.013                                                         
               0.002                                                      
                  0.037                                                   
                     0.202                                                
                        9.46                                              
                           0.193                                          
                              1.124                                       
                                 --  -- -- --                             
                                             0.170                        
                                                0.014                     
                                                   17.0 16.0 230          
 7 0.226                                                                  
      0.566                                                               
         0.756                                                            
            0.038                                                         
               0.008                                                      
                  0.072                                                   
                     0.091                                                
                        12.71                                             
                           0.698                                          
                              0.829                                       
                                 --  -- -- --                             
                                             0.329                        
                                                0.016                     
                                                   23.4 7.19 226          
 8 0.074                                                                  
      0.763                                                               
         0.813                                                            
            0.012                                                         
               0.009                                                      
                  0.105                                                   
                     0.470                                                
                        9.07                                              
                           0.563                                          
                              2.597                                       
                                 --  -- -- --                             
                                             0.259                        
                                                0.012                     
                                                   24.2 14.8 135          
 9 0.110                                                                  
      0.056                                                               
         0.530                                                            
            0.022                                                         
               0.002                                                      
                  0.057                                                   
                     0.302                                                
                        12.02                                             
                           0.479                                          
                              0.656                                       
                                 --  -- -- --                             
                                             0.430                        
                                                0.015                     
                                                   21.7 6.95 212          
10 0.250                                                                  
      0.151                                                               
         0.857                                                            
            0.016                                                         
               0.003                                                      
                  0.096                                                   
                     0.116                                                
                        12.42                                             
                           0.617                                          
                              2.967                                       
                                 --  -- -- --                             
                                             0.117                        
                                                0.009                     
                                                   22.6 14.2 113          
11 0.260                                                                  
      0.418                                                               
         0.748                                                            
            0.021                                                         
               0.004                                                      
                  0.041                                                   
                     0.452                                                
                        11.29                                             
                           0.686                                          
                              2.505                                       
                                 0.0429                                   
                                     -- -- --                             
                                             0.400                        
                                                0.018                     
                                                   23.7 4.09 236          
12 0.046                                                                  
      0.599                                                               
         0.739                                                            
            0.024                                                         
               0.006                                                      
                  0.061                                                   
                     0.263                                                
                        11.94                                             
                           0.846                                          
                              0.961                                       
                                 0.0253                                   
                                     -- -- --                             
                                             0.146                        
                                                0.015                     
                                                   17.1 14.2 163          
13 0.098                                                                  
      0.465                                                               
         0.593                                                            
            0.033                                                         
               0.008                                                      
                  0.025                                                   
                     0.312                                                
                        12.92                                             
                           0.306                                          
                              2.574                                       
                                 0.0249                                   
                                     -- -- --                             
                                             0.435                        
                                                0.007                     
                                                   24.5 5.48 221          
14 0.183                                                                  
      0.560                                                               
         0.469                                                            
            0.017                                                         
               0.001                                                      
                  0.101                                                   
                     0.238                                                
                        12.70                                             
                           0.257                                          
                              1.592                                       
                                 0.0595                                   
                                     -- -- --                             
                                             0.149                        
                                                0.012                     
                                                   17.7 8.16 138          
15 0.062                                                                  
      0.615                                                               
         0.891                                                            
            0.021                                                         
               0.006                                                      
                  0.057                                                   
                     0.340                                                
                        12.06                                             
                           0.365                                          
                              2.174                                       
                                 0.0488                                   
                                     -- -- --                             
                                             0.349                        
                                                0.018                     
                                                   23.8 11.0 166          
16 0.224                                                                  
      0.791                                                               
         0.425                                                            
            0.034                                                         
               0.006                                                      
                  0.113                                                   
                     0.483                                                
                        12.05                                             
                           0.827                                          
                              2.318                                       
                                 0.0113                                   
                                     -- -- --                             
                                             0.118                        
                                                0.018                     
                                                   24.5 16.6 184          
17 0.146                                                                  
      0.458                                                               
         0.829                                                            
            0.023                                                         
               0.002                                                      
                  0.038                                                   
                     0.080                                                
                        8.63                                              
                           0.346                                          
                              1.264                                       
                                 0.0155                                   
                                     -- -- --                             
                                             0.129                        
                                                0.016                     
                                                   16.7 13.6 221          
18 0.171                                                                  
      0.531                                                               
         0.633                                                            
            0.035                                                         
               0.006                                                      
                  0.050                                                   
                     0.331                                                
                        12.67                                             
                           0.584                                          
                              1.931                                       
                                 0.0299                                   
                                     -- -- --                             
                                             0.256                        
                                                0.015                     
                                                   22.3 4.85 230          
19 0.065                                                                  
      0.385                                                               
         0.401                                                            
            0.019                                                         
               0.001                                                      
                  0.117                                                   
                     0.431                                                
                        12.63                                             
                           0.749                                          
                              2.345                                       
                                 0.0735                                   
                                     -- -- --                             
                                             0.178                        
                                                0.009                     
                                                   18.2 13.5 162          
20 0.169                                                                  
      0.161                                                               
         0.275                                                            
            0.041                                                         
               0.007                                                      
                  0.110                                                   
                     0.233                                                
                        9.43                                              
                           0.515                                          
                              1.688                                       
                                 0.0166                                   
                                     -- -- --                             
                                             0.365                        
                                                0.011                     
                                                   21.3 7.73 122          
21 0.250                                                                  
      0.394                                                               
         0.807                                                            
            0.010                                                         
               0.002                                                      
                  0.096                                                   
                     0.269                                                
                        11.51                                             
                           0.570                                          
                              1.668                                       
                                 --  0.014                                
                                        -- --                             
                                             0.293                        
                                                0.015                     
                                                   18.4 3.71 213          
22 0.072                                                                  
      0.070                                                               
         0.554                                                            
            0.032                                                         
               0.008                                                      
                  0.094                                                   
                     0.405                                                
                        12.37                                             
                           0.116                                          
                              1.542                                       
                                 --  0.346                                
                                        -- --                             
                                             0.442                        
                                                0.016                     
                                                   21.4 11.3 164          
23 0.126                                                                  
      0.110                                                               
         0.459                                                            
            0.035                                                         
               0.002                                                      
                  0.021                                                   
                     0.270                                                
                        10.37                                             
                           0.319                                          
                              1.731                                       
                                 --  0.645                                
                                        -- --                             
                                             0.484                        
                                                0.017                     
                                                   22.0 11.8 224          
24 0.023                                                                  
      0.257                                                               
         0.268                                                            
            0.047                                                         
               0.010                                                      
                  0.100                                                   
                     0.175                                                
                        11.45                                             
                           0.878                                          
                              1.502                                       
                                 --  0.443                                
                                        -- --                             
                                             0.488                        
                                                0.011                     
                                                   22.5 15.3 201          
25 0.118                                                                  
      0.292                                                               
         0.545                                                            
            0.035                                                         
               0.009                                                      
                  0.097                                                   
                     0.432                                                
                        11.62                                             
                           0.986                                          
                              1.305                                       
                                 --  0.146                                
                                        -- --                             
                                             0.138                        
                                                0.004                     
                                                   19.4 3.31 196          
26 0.028                                                                  
      0.343                                                               
         0.627                                                            
            0.046                                                         
               0.002                                                      
                  0.081                                                   
                     0.224                                                
                        9.26                                              
                           0.330                                          
                              2.857                                       
                                 --  0.356                                
                                        -- --                             
                                             0.104                        
                                                0.016                     
                                                   19.8 13.3 164          
27 0.240                                                                  
      0.568                                                               
         0.675                                                            
            0.029                                                         
               0.001                                                      
                  0.072                                                   
                     0.085                                                
                        10.36                                             
                           0.591                                          
                              1.629                                       
                                 --  0.074                                
                                        -- --                             
                                             0.468                        
                                                0.005                     
                                                   15.5 13.0 130          
28 0.229                                                                  
      0.721                                                               
         0.837                                                            
            0.030                                                         
               0.010                                                      
                  0.086                                                   
                     0.327                                                
                        8.33                                              
                           0.465                                          
                              1.848                                       
                                 --  0.372                                
                                        -- --                             
                                             0.297                        
                                                0.011                     
                                                   18.3 12.0 124          
29 0.050                                                                  
      0.546                                                               
         0.848                                                            
            0.038                                                         
               0.004                                                      
                  0.095                                                   
                     0.354                                                
                        11.53                                             
                           0.619                                          
                              2.724                                       
                                 --  0.034                                
                                        -- --                             
                                             0.465                        
                                                0.011                     
                                                   15.2 5.99 250          
30 0.047                                                                  
      0.599                                                               
         0.883                                                            
            0.019                                                         
               0.008                                                      
                  0.116                                                   
                     0.256                                                
                        8.56                                              
                           0.269                                          
                              2.234                                       
                                 --  0.247                                
                                        -- --                             
                                             0.458                        
                                                0.007                     
                                                   17.9 4.49 234          
31 0.274                                                                  
      0.224                                                               
         0.265                                                            
            0.021                                                         
               0.001                                                      
                  0.067                                                   
                     0.335                                                
                        10.15                                             
                           0.076                                          
                              0.809                                       
                                 0.0179                                   
                                     0.129                                
                                        -- --                             
                                             0.138                        
                                                0.011                     
                                                   18.5 3.06 179          
32 0.059                                                                  
      0.358                                                               
         0.757                                                            
            0.032                                                         
               0.003                                                      
                  0.067                                                   
                     0.452                                                
                        12.38                                             
                           0.237                                          
                              2.678                                       
                                 0.0490                                   
                                     0.376                                
                                        -- --                             
                                             0.384                        
                                                0.010                     
                                                   18.6 11.3 114          
33 0.181                                                                  
      0.371                                                               
         0.509                                                            
            0.019                                                         
               0.001                                                      
                  0.086                                                   
                     0.250                                                
                        8.67                                              
                           0.445                                          
                              0.876                                       
                                 0.0223                                   
                                     0.251                                
                                        -- --                             
                                             0.273                        
                                                0.016                     
                                                   20.1 11.1 217          
34 0.262                                                                  
      0.399                                                               
         0.868                                                            
            0.029                                                         
               0.002                                                      
                  0.043                                                   
                     0.253                                                
                        9.91                                              
                           0.378                                          
                              0.786                                       
                                 0.0571                                   
                                     0.459                                
                                        -- --                             
                                             0.477                        
                                                0.005                     
                                                   20.3 12.1 143          
35 0.214                                                                  
      0.216                                                               
         0.397                                                            
            0.011                                                         
               0.003                                                      
                  0.106                                                   
                     0.102                                                
                        12.12                                             
                           0.740                                          
                              1.685                                       
                                 0.0217                                   
                                     0.012                                
                                        -- --                             
                                             0.479                        
                                                0.007                     
                                                   20.8 16.7 144          
36 0.088                                                                  
      0.739                                                               
         0.842                                                            
            0.031                                                         
               0.007                                                      
                  0.044                                                   
                     0.111                                                
                        8.72                                              
                           0.216                                          
                              1.550                                       
                                 0.0349                                   
                                     0.220                                
                                        -- --                             
                                             0.350                        
                                                0.018                     
                                                   23.5 6.79 248          
37 0.086                                                                  
      0.551                                                               
         0.737                                                            
            0.018                                                         
               0.004                                                      
                  0.025                                                   
                     0.126                                                
                        8.08                                              
                           0.972                                          
                              2.042                                       
                                 0.0139                                   
                                     0.135                                
                                        -- --                             
                                             0.162                        
                                                0.003                     
                                                   15.0 12.5 107          
38 0.256                                                                  
      0.183                                                               
         0.243                                                            
            0.038                                                         
               0.002                                                      
                  0.068                                                   
                     0.466                                                
                        8.69                                              
                           0.221                                          
                              2.419                                       
                                 0.0778                                   
                                     0.042                                
                                        -- --                             
                                             0.389                        
                                                0.003                     
                                                   17.5 15.8 198          
39 0.219                                                                  
      0.572                                                               
         0.708                                                            
            0.019                                                         
               0.002                                                      
                  0.042                                                   
                     0.085                                                
                        8.95                                              
                           0.629                                          
                              1.881                                       
                                 0.0474                                   
                                     0.893                                
                                        -- --                             
                                             0.109                        
                                                0.012                     
                                                   17.8 12.4 179          
40 0.179                                                                  
      0.303                                                               
         0.684                                                            
            0.049                                                         
               0.003                                                      
                  0.115                                                   
                     0.420                                                
                        9.82                                              
                           0.894                                          
                              2.155                                       
                                 0.0889                                   
                                     0.370                                
                                        -- --                             
                                             0.199                        
                                                0.006                     
                                                   22.7 4.69 222          
41 0.187                                                                  
      0.297                                                               
         0.323                                                            
            0.035                                                         
               0.009                                                      
                  0.058                                                   
                     0.205                                                
                        9.76                                              
                           0.207                                          
                              1.210                                       
                                 --  -- 0.065                             
                                           --                             
                                             0.329                        
                                                0.006                     
                                                   19.2 9.50 136          
42 0.010                                                                  
      0.703                                                               
         0.969                                                            
            0.025                                                         
               0.009                                                      
                  0.088                                                   
                     0.427                                                
                        12.17                                             
                           0.252                                          
                              0.775                                       
                                 --  -- 0.215                             
                                           --                             
                                             0.377                        
                                                0.009                     
                                                   19.2 8.48 247          
43 0.138                                                                  
      0.506                                                               
         0.381                                                            
            0.046                                                         
               0.004                                                      
                  0.074                                                   
                     0.269                                                
                        10.54                                             
                           0.220                                          
                              1.320                                       
                                 --  -- 0.456                             
                                           --                             
                                             0.422                        
                                                0.006                     
                                                   19.4 4.07 207          
44 0.041                                                                  
      0.617                                                               
         0.703                                                            
            0.026                                                         
               0.006                                                      
                  0.034                                                   
                     0.265                                                
                        8.26                                              
                           0.792                                          
                              2.052                                       
                                 --  -- 0.971                             
                                           --                             
                                             0.332                        
                                                0.002                     
                                                   21.8 12.9 120          
45 0.068                                                                  
      0.413                                                               
         0.885                                                            
            0.012                                                         
               0.007                                                      
                  0.087                                                   
                     0.270                                                
                        8.76                                              
                           0.159                                          
                              2.161                                       
                                 --  -- 0.946                             
                                           --                             
                                             0.187                        
                                                0.019                     
                                                   18.1 16.5 236          
46 0.120                                                                  
      0.416                                                               
         0.665                                                            
            0.010                                                         
               0.004                                                      
                  0.052                                                   
                     0.143                                                
                        10.44                                             
                           0.989                                          
                              0.894                                       
                                 --  -- 0.574                             
                                           --                             
                                             0.254                        
                                                0.014                     
                                                   15.2 3.72 136          
47 0.257                                                                  
      0.393                                                               
         0.211                                                            
            0.010                                                         
               0.008                                                      
                  0.098                                                   
                     0.430                                                
                        9.35                                              
                           0.506                                          
                              1.243                                       
                                 --  -- 0.547                             
                                           --                             
                                             0.211                        
                                                0.018                     
                                                   15.3 5.21 225          
48 0.247                                                                  
      0.102                                                               
         0.467                                                            
            0.039                                                         
               0.002                                                      
                  0.095                                                   
                     0.304                                                
                        11.36                                             
                           0.871                                          
                              1.153                                       
                                 --  -- 0.555                             
                                           --                             
                                             0.140                        
                                                0.007                     
                                                   20.1 16.5 155          
49 0.287                                                                  
      0.086                                                               
         0.393                                                            
            0.045                                                         
               0.002                                                      
                  0.061                                                   
                     0.306                                                
                        12.46                                             
                           0.296                                          
                              1.104                                       
                                 --  -- 0.781                             
                                           --                             
                                             0.115                        
                                                0.012                     
                                                   16.4 5.38 178          
50 0.287                                                                  
      0.092                                                               
         0.929                                                            
            0.014                                                         
               0.005                                                      
                  0.064                                                   
                     0.297                                                
                        11.05                                             
                           0.946                                          
                              2.816                                       
                                 --  -- 0.908                             
                                           --                             
                                             0.208                        
                                                0.009                     
                                                   21.1 9.21 247          
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours         
 VE: Charpy absorption energy at 0° C. after aging at 700° C
 for 10 thousand hours                                                    
 TO: Oxidation scale thickness after oxidation at 650° C. for 10   
 thousand hours in air                                                    
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
(Wt. %) Invention Steels                                                  
                                                    CS                    
                                                    kg/                   
                                                       VE   TO            
No.                                                                       
   C  Si Mn P  S  Nb V  Cr Mo W  Zr  Ta Hf Ti N  O  mm.sup.2              
                                                       kgf ·     
                                                       μm              
__________________________________________________________________________
51 0.297                                                                  
      0.257                                                               
         0.215                                                            
            0.014                                                         
               0.004                                                      
                  0.022                                                   
                     0.350                                                
                        8.01                                              
                           0.509                                          
                              1.727                                       
                                 0.0460                                   
                                     -- 0.130                             
                                           -- 0.289                       
                                                 0.010                    
                                                    21.0                  
                                                       9.55               
                                                           165            
52 0.083                                                                  
      0.346                                                               
         0.812                                                            
            0.032                                                         
               0.009                                                      
                  0.054                                                   
                     0.331                                                
                        11.82                                             
                           0.178                                          
                              2.066                                       
                                 0.0283                                   
                                     -- 0.610                             
                                           -- 0.437                       
                                                 0.011                    
                                                    23.0                  
                                                       16.2 185           
53 0.280                                                                  
      0.120                                                               
         0.471                                                            
            0.016                                                         
               0.003                                                      
                  0.080                                                   
                     0.102                                                
                        10.95                                             
                           0.143                                          
                              1.821                                       
                                 0.0278                                   
                                     -- 0.572                             
                                           -- 0.330                       
                                                 0.004                    
                                                    19.2                  
                                                       10.9 188           
54 0.076                                                                  
      0.503                                                               
         0.944                                                            
            0.015                                                         
               0.008                                                      
                  0.070                                                   
                     0.196                                                
                        11.34                                             
                           0.591                                          
                              1.461                                       
                                 0.0137                                   
                                     -- 0.690                             
                                           -- 0.441                       
                                                 0.012                    
                                                    21.3                  
                                                       3.30 200           
55 0.246                                                                  
      0.076                                                               
         0.774                                                            
            0.014                                                         
               0.006                                                      
                  0.089                                                   
                     0.245                                                
                        8.88                                              
                           0.705                                          
                              1.435                                       
                                 0.0033                                   
                                     -- 0.643                             
                                           -- 0.444                       
                                                 0.011                    
                                                    21.1                  
                                                       9.65 247           
56 0.264                                                                  
      0.550                                                               
         0.513                                                            
            0.022                                                         
               0.003                                                      
                  0.107                                                   
                     0.333                                                
                        9.51                                              
                           0.672                                          
                              2.212                                       
                                 0.0652                                   
                                     -- 0.619                             
                                           -- 0.214                       
                                                 0.005                    
                                                    20.6                  
                                                       4.85 209           
57 0.043                                                                  
      0.115                                                               
         0.310                                                            
            0.046                                                         
               0.002                                                      
                  0.095                                                   
                     0.323                                                
                        11.70                                             
                           0.190                                          
                              0.520                                       
                                 0.0693                                   
                                     -- 0.809                             
                                           -- 0.225                       
                                                 0.014                    
                                                    21.6                  
                                                       5.42 115           
58 0.213                                                                  
      0.087                                                               
         0.311                                                            
            0.014                                                         
               0.004                                                      
                  0.068                                                   
                     0.066                                                
                        8.80                                              
                           0.420                                          
                              1.799                                       
                                 0.0335                                   
                                     -- 0.892                             
                                           -- 0.351                       
                                                 0.006                    
                                                    15.9                  
                                                       6.88 215           
59 0.248                                                                  
      0.224                                                               
         0.668                                                            
            0.012                                                         
               0.001                                                      
                  0.097                                                   
                     0.100                                                
                        9.33                                              
                           0.067                                          
                              2.798                                       
                                 0.0265                                   
                                     -- 0.074                             
                                           -- 0.268                       
                                                 0.002                    
                                                    15.4                  
                                                       5.07 166           
60 0.206                                                                  
      0.297                                                               
         0.748                                                            
            0.049                                                         
               0.004                                                      
                  0.103                                                   
                     0.074                                                
                        11.79                                             
                           0.839                                          
                              2.783                                       
                                 0.0192                                   
                                     -- 0.542                             
                                           -- 0.456                       
                                                 0.006                    
                                                    22.3                  
                                                       9.67 145           
61 0.141                                                                  
      0.436                                                               
         0.682                                                            
            0.013                                                         
               0.007                                                      
                  0.101                                                   
                     0.280                                                
                        12.03                                             
                           0.402                                          
                              2.144                                       
                                 --  0.209                                
                                        0.468                             
                                           -- 0.186                       
                                                 0.004                    
                                                    23.2                  
                                                       9.25 106           
62 0.112                                                                  
      0.410                                                               
         0.635                                                            
            0.010                                                         
               0.006                                                      
                  0.062                                                   
                     0.365                                                
                        8.54                                              
                           0.950                                          
                              2.671                                       
                                 --  0.712                                
                                        0.067                             
                                           -- 0.339                       
                                                 0.007                    
                                                    15.0                  
                                                       13.3 150           
63 0.165                                                                  
      0.744                                                               
         0.345                                                            
            0.029                                                         
               0.002                                                      
                  0.054                                                   
                     0.178                                                
                        9.71                                              
                           0.667                                          
                              1.021                                       
                                 --  0.123                                
                                        0.986                             
                                           -- 0.180                       
                                                 0.010                    
                                                    24.5                  
                                                       10.6 232           
64 0.214                                                                  
      0.031                                                               
         0.765                                                            
            0.036                                                         
               0.003                                                      
                  0.046                                                   
                     0.480                                                
                        8.99                                              
                           0.739                                          
                              2.675                                       
                                 --  0.243                                
                                        0.934                             
                                           -- 0.392                       
                                                 0.017                    
                                                    18.7                  
                                                       10.6 228           
65 0.164                                                                  
      0.349                                                               
         0.836                                                            
            0.039                                                         
               0.004                                                      
                  0.054                                                   
                     0.454                                                
                        12.25                                             
                           0.842                                          
                              1.840                                       
                                 --  0.067                                
                                        0.102                             
                                           -- 0.440                       
                                                 0.011                    
                                                    19.4                  
                                                       9.12 166           
66 0.218                                                                  
      0.308                                                               
         0.724                                                            
            0.045                                                         
               0.005                                                      
                  0.102                                                   
                     0.416                                                
                        8.02                                              
                           0.682                                          
                              2.775                                       
                                 --  0.146                                
                                        0.176                             
                                           -- 0.207                       
                                                 0.017                    
                                                    23.6                  
                                                       15.7 228           
67 0.286                                                                  
      0.441                                                               
         0.577                                                            
            0.043                                                         
               0.001                                                      
                  0.055                                                   
                     0.452                                                
                        12.29                                             
                           0.954                                          
                              2.209                                       
                                 --  0.053                                
                                        0.348                             
                                           -- 0.372                       
                                                 0.008                    
                                                    23.1                  
                                                       11.9 139           
68 0.276                                                                  
      0.099                                                               
         0.928                                                            
            0.020                                                         
               0.008                                                      
                  0.031                                                   
                     0.407                                                
                        8.32                                              
                           0.816                                          
                              1.771                                       
                                 --  0.471                                
                                        0.768                             
                                           -- 0.401                       
                                                 0.008                    
                                                    19.5                  
                                                       14.3 150           
69 0.239                                                                  
      0.215                                                               
         0.342                                                            
            0.022                                                         
               0.009                                                      
                  0.102                                                   
                     0.376                                                
                        9.67                                              
                           0.969                                          
                              0.759                                       
                                 --  0.139                                
                                        0.179                             
                                           -- 0.167                       
                                                 0.009                    
                                                    20.1                  
                                                       4.70 144           
70 0.089                                                                  
      0.165                                                               
         0.970                                                            
            0.038                                                         
               0.001                                                      
                  0.036                                                   
                     0.446                                                
                        9.77                                              
                           0.121                                          
                              0.892                                       
                                 --  0.214                                
                                        0.241                             
                                           -- 0.156                       
                                                 0.016                    
                                                    16.7                  
                                                       13.7 222           
71 0.027                                                                  
      0.081                                                               
         0.551                                                            
            0.015                                                         
               0.006                                                      
                  0.097                                                   
                     0.240                                                
                        11.99                                             
                           0.917                                          
                              2.561                                       
                                 0.0707                                   
                                     0.013                                
                                        0.114                             
                                           -- 0.436                       
                                                 0.014                    
                                                    20.7                  
                                                       15.5 108           
72 0.095                                                                  
      0.495                                                               
         0.430                                                            
            0.020                                                         
               0.005                                                      
                  0.111                                                   
                     0.299                                                
                        9.77                                              
                           0.568                                          
                              2.552                                       
                                 0.0031                                   
                                     0.166                                
                                        0.035                             
                                           -- 0.274                       
                                                 0.015                    
                                                    19.7                  
                                                       13.5 138           
73 0.077                                                                  
      0.022                                                               
         0.788                                                            
            0.042                                                         
               0.005                                                      
                  0.093                                                   
                     0.265                                                
                        9.21                                              
                           0.779                                          
                              1.373                                       
                                 0.0272                                   
                                     0.010                                
                                        0.181                             
                                           -- 0.365                       
                                                 0.014                    
                                                    20.1                  
                                                       9.92 112           
74 0.016                                                                  
      0.748                                                               
         0.562                                                            
            0.048                                                         
               0.004                                                      
                  0.113                                                   
                     0.444                                                
                        8.66                                              
                           0.731                                          
                              0.704                                       
                                 0.0626                                   
                                     0.054                                
                                        0.231                             
                                           -- 0.179                       
                                                 0.014                    
                                                    19.1                  
                                                       14.4 132           
75 0.260                                                                  
      0.082                                                               
         0.296                                                            
            0.045                                                         
               0.008                                                      
                  0.090                                                   
                     0.243                                                
                        11.54                                             
                           0.104                                          
                              2.254                                       
                                 0.0780                                   
                                     0.388                                
                                        0.378                             
                                           -- 0.182                       
                                                 0.019                    
                                                    18.4                  
                                                       8.75 228           
76 0.137                                                                  
      0.511                                                               
         0.940                                                            
            0.041                                                         
               0.004                                                      
                  0.041                                                   
                     0.194                                                
                        8.78                                              
                           0.081                                          
                              1.492                                       
                                 0.0907                                   
                                     0.307                                
                                        0.797                             
                                           -- 0.456                       
                                                 0.005                    
                                                    24.8                  
                                                       9.23 201           
77 0.278                                                                  
      0.620                                                               
         0.241                                                            
            0.023                                                         
               0.009                                                      
                  0.034                                                   
                     0.155                                                
                        8.79                                              
                           0.844                                          
                              2.616                                       
                                 0.0697                                   
                                     0.045                                
                                        0.180                             
                                           -- 0.470                       
                                                 0.010                    
                                                    15.1                  
                                                       4.46 153           
78 0.156                                                                  
      0.541                                                               
         0.342                                                            
            0.017                                                         
               0.005                                                      
                  0.089                                                   
                     0.434                                                
                        9.98                                              
                           0.084                                          
                              2.337                                       
                                 0.0340                                   
                                     0.083                                
                                        0.701                             
                                           -- 0.295                       
                                                 0.003                    
                                                    16.3                  
                                                       4.30 188           
79 0.120                                                                  
      0.051                                                               
         0.607                                                            
            0.014                                                         
               0.008                                                      
                  0.025                                                   
                     0.445                                                
                        8.37                                              
                           0.984                                          
                              2.421                                       
                                 0.0034                                   
                                     0.022                                
                                        0.574                             
                                           -- 0.213                       
                                                 0.014                    
                                                    15.4                  
                                                       4.31 187           
80 0.226                                                                  
      0.467                                                               
         0.788                                                            
            0.019                                                         
               0.006                                                      
                  0.112                                                   
                     0.056                                                
                        9.90                                              
                           0.265                                          
                              0.856                                       
                                 0.0954                                   
                                     0.038                                
                                        0.482                             
                                           -- 0.104                       
                                                 0.002                    
                                                    24.1                  
                                                       14.2 101           
81 0.235                                                                  
      0.360                                                               
         0.620                                                            
            0.040                                                         
               0.009                                                      
                  0.115                                                   
                     0.229                                                
                        10.42                                             
                           0.565                                          
                              0.505                                       
                                 --  -- -- 0.082                          
                                              0.433                       
                                                 0.014                    
                                                    19.2                  
                                                       15.4 106           
82 0.056                                                                  
      0.280                                                               
         0.265                                                            
            0.024                                                         
               0.002                                                      
                  0.057                                                   
                     0.171                                                
                        10.94                                             
                           0.111                                          
                              1.953                                       
                                 --  -- -- 0.017                          
                                              0.482                       
                                                 0.018                    
                                                    17.9                  
                                                       3.78 236           
83 0.037                                                                  
      0.372                                                               
         0.280                                                            
            0.040                                                         
               0.009                                                      
                  0.057                                                   
                     0.408                                                
                        9.95                                              
                           0.580                                          
                              0.629                                       
                                 --  -- -- 0.028                          
                                              0.125                       
                                                 0.009                    
                                                    16.9                  
                                                       16.9 139           
84 0.086                                                                  
      0.379                                                               
         0.392                                                            
            0.015                                                         
               0.009                                                      
                  0.027                                                   
                     0.338                                                
                        8.22                                              
                           0.140                                          
                              1.935                                       
                                 --  -- -- 0.036                          
                                              0.233                       
                                                 0.007                    
                                                    17.8                  
                                                       7.96 143           
85 0.105                                                                  
      0.067                                                               
         0.764                                                            
            0.014                                                         
               0.002                                                      
                  0.044                                                   
                     0.063                                                
                        11.82                                             
                           0.497                                          
                              1.436                                       
                                 --  -- -- 0.065                          
                                              0.280                       
                                                 0.008                    
                                                    18.0                  
                                                       8.27 205           
86 0.017                                                                  
      0.375                                                               
         0.758                                                            
            0.049                                                         
               0.001                                                      
                  0.022                                                   
                     0.113                                                
                        11.72                                             
                           0.350                                          
                              2.072                                       
                                 --  -- -- 0.083                          
                                              0.152                       
                                                 0.014                    
                                                    19.0                  
                                                       13.0 200           
87 0.157                                                                  
      0.256                                                               
         0.504                                                            
            0.024                                                         
               0.008                                                      
                  0.032                                                   
                     0.338                                                
                        11.23                                             
                           0.360                                          
                              0.532                                       
                                 --  -- -- 0.019                          
                                              0.113                       
                                                 0.002                    
                                                    17.8                  
                                                       4.10 233           
88 0.148                                                                  
      0.263                                                               
         0.962                                                            
            0.028                                                         
               0.004                                                      
                  0.041                                                   
                     0.157                                                
                        11.37                                             
                           0.724                                          
                              1.066                                       
                                 --  -- -- 0.067                          
                                              0.243                       
                                                 0.012                    
                                                    21.4                  
                                                       11.9 107           
89 0.043                                                                  
      0.142                                                               
         0.681                                                            
            0.049                                                         
               0.002                                                      
                  0.118                                                   
                     0.103                                                
                        10.56                                             
                           0.641                                          
                              2.890                                       
                                 --  -- -- 0.022                          
                                              0.215                       
                                                 0.019                    
                                                    16.4                  
                                                       11.0 147           
90 0.184                                                                  
      0.044                                                               
         0.213                                                            
            0.012                                                         
               0.003                                                      
                  0.033                                                   
                     0.482                                                
                        9.75                                              
                           0.789                                          
                              2.704                                       
                                 --  -- -- 0.033                          
                                              0.106                       
                                                 0.010                    
                                                    19.7                  
                                                       11.3 235           
91 0.194                                                                  
      0.501                                                               
         0.498                                                            
            0.047                                                         
               0.004                                                      
                  0.103                                                   
                     0.480                                                
                        12.32                                             
                           0.351                                          
                              2.238                                       
                                 0.0009                                   
                                     -- -- 0.074                          
                                              0.185                       
                                                 0.013                    
                                                    23.4                  
                                                       8.33 172           
92 0.271                                                                  
      0.501                                                               
         0.500                                                            
            0.021                                                         
               0.002                                                      
                  0.099                                                   
                     0.408                                                
                        11.80                                             
                           0.027                                          
                              1.969                                       
                                 0.0340                                   
                                     -- -- 0.072                          
                                              0.339                       
                                                 0.016                    
                                                    24.4                  
                                                       4.68 138           
93 0.182                                                                  
      0.572                                                               
         0.368                                                            
            0.021                                                         
               0.004                                                      
                  0.038                                                   
                     0.350                                                
                        9.11                                              
                           0.998                                          
                              2.356                                       
                                 0.0831                                   
                                     -- -- 0.087                          
                                              0.233                       
                                                 0.011                    
                                                    24.2                  
                                                       16.7 232           
94 0.264                                                                  
      0.455                                                               
         0.225                                                            
            0.034                                                         
               0.005                                                      
                  0.088                                                   
                     0.156                                                
                        12.52                                             
                           0.430                                          
                              2.589                                       
                                 0.0177                                   
                                     -- -- 0.091                          
                                              0.339                       
                                                 0.013                    
                                                    20.0                  
                                                       5.44 186           
95 0.287                                                                  
      0.324                                                               
         0.660                                                            
            0.048                                                         
               0.001                                                      
                  0.020                                                   
                     0.375                                                
                        8.21                                              
                           0.054                                          
                              1.936                                       
                                 0.0569                                   
                                     -- -- 0.058                          
                                              0.142                       
                                                 0.005                    
                                                    23.5                  
                                                       8.32 103           
96 0.162                                                                  
      0.308                                                               
         0.599                                                            
            0.031                                                         
               0.006                                                      
                  0.101                                                   
                     0.183                                                
                        9.48                                              
                           0.518                                          
                              2.105                                       
                                 0.0196                                   
                                     -- -- 0.093                          
                                              0.111                       
                                                 0.010                    
                                                    16.7                  
                                                       13.9 233           
97 0.083                                                                  
      0.547                                                               
         0.585                                                            
            0.030                                                         
               0.002                                                      
                  0.050                                                   
                     0.108                                                
                        9.70                                              
                           0.019                                          
                              0.996                                       
                                 0.0722                                   
                                     -- -- 0.077                          
                                              0.119                       
                                                 0.012                    
                                                    16.4                  
                                                       10.9 232           
98 0.252                                                                  
      0.429                                                               
         0.794                                                            
            0.012                                                         
               0.001                                                      
                  0.037                                                   
                     0.479                                                
                        10.02                                             
                           0.267                                          
                              2.957                                       
                                 0.0378                                   
                                     -- -- 0.076                          
                                              0.447                       
                                                 0.015                    
                                                    19.6                  
                                                       9.07 128           
99 0.146                                                                  
      0.093                                                               
         0.967                                                            
            0.026                                                         
               0.001                                                      
                  0.081                                                   
                     0.241                                                
                        11.24                                             
                           0.928                                          
                              0.867                                       
                                 0.0828                                   
                                     -- -- 0.013                          
                                              0.370                       
                                                 0.005                    
                                                    23.0                  
                                                       4.02 250           
100                                                                       
   0.109                                                                  
      0.072                                                               
         0.449                                                            
            0.039                                                         
               0.007                                                      
                  0.099                                                   
                     0.159                                                
                        11.86                                             
                           0.210                                          
                              1.484                                       
                                 0.0758                                   
                                     -- -- 0.042                          
                                              0.363                       
                                                 0.011                    
                                                    23.7                  
                                                       12.2 174           
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours         
 VE: Charpy absorption energy at 0° C. after aging at 700° C
 for 10 thousand hours                                                    
 TO: Oxidation scale thickness after oxidation at 650° C. for 10   
 thousand hours in air                                                    
 
    
     
                                           TABLE 3                                 
__________________________________________________________________________
(Wt. %) Invention Steels                                                  
                                                    CS                    
                                                    kg/                   
                                                       VE   TO            
No.                                                                       
   C  Si Mn P  S  Nb V  Cr Mo W  Zr  Ta Hf Ti N  O  mm.sup.2              
                                                       kgf ·     
                                                       μm              
__________________________________________________________________________
101                                                                       
   0.048                                                                  
      0.124                                                               
         0.995                                                            
            0.018                                                         
               0.007                                                      
                  0.060                                                   
                     0.091                                                
                        10.28                                             
                           0.779                                          
                              1.508                                       
                                 --  0.544                                
                                        -- 0.089                          
                                              0.297                       
                                                 0.011                    
                                                    23.3                  
                                                       8.29               
                                                           153            
102                                                                       
   0.161                                                                  
      0.777                                                               
         0.344                                                            
            0.030                                                         
               0.008                                                      
                  0.084                                                   
                     0.112                                                
                        9.88                                              
                           0.817                                          
                              1.368                                       
                                 --  0.675                                
                                        -- 0.080                          
                                              0.447                       
                                                 0.017                    
                                                    23.9                  
                                                       8.96 142           
103                                                                       
   0.071                                                                  
      0.230                                                               
         0.250                                                            
            0.023                                                         
               0.002                                                      
                  0.088                                                   
                     0.319                                                
                        9.16                                              
                           0.039                                          
                              1.591                                       
                                 --  0.772                                
                                        -- 0.078                          
                                              0.489                       
                                                 0.004                    
                                                    17.2                  
                                                       9.70 241           
104                                                                       
   0.117                                                                  
      0.196                                                               
         0.870                                                            
            0.025                                                         
               0.005                                                      
                  0.042                                                   
                     0.341                                                
                        9.06                                              
                           0.106                                          
                              2.627                                       
                                 --  0.021                                
                                        -- 0.055                          
                                              0.298                       
                                                 0.013                    
                                                    20.2                  
                                                       6.30 182           
105                                                                       
   0.213                                                                  
      0.034                                                               
         0.746                                                            
            0.023                                                         
               0.005                                                      
                  0.064                                                   
                     0.261                                                
                        10.46                                             
                           0.216                                          
                              1.174                                       
                                 --  0.912                                
                                        -- 0.072                          
                                              0.148                       
                                                 0.010                    
                                                    24.7                  
                                                       15.3 215           
106                                                                       
   0.269                                                                  
      0.288                                                               
         0.436                                                            
            0.025                                                         
               0.008                                                      
                  0.074                                                   
                     0.393                                                
                        11.91                                             
                           0.560                                          
                              1.502                                       
                                 --  0.319                                
                                        -- 0.056                          
                                              0.320                       
                                                 0.009                    
                                                    17.5                  
                                                       4.28 144           
107                                                                       
   0.045                                                                  
      0.313                                                               
         0.479                                                            
            0.018                                                         
               0.006                                                      
                  0.089                                                   
                     0.367                                                
                        9.22                                              
                           0.323                                          
                              1.528                                       
                                 --  0.065                                
                                        -- 0.025                          
                                              0.478                       
                                                 0.011                    
                                                    15.8                  
                                                       4.09 225           
108                                                                       
   0.037                                                                  
      0.273                                                               
         0.642                                                            
            0.030                                                         
               0.006                                                      
                  0.030                                                   
                     0.498                                                
                        8.45                                              
                           0.696                                          
                              1.762                                       
                                 --  0.328                                
                                        -- 0.041                          
                                              0.111                       
                                                 0.013                    
                                                    16.2                  
                                                       10.2 108           
109                                                                       
   0.296                                                                  
      0.702                                                               
         0.674                                                            
            0.028                                                         
               0.005                                                      
                  0.064                                                   
                     0.195                                                
                        8.04                                              
                           0.374                                          
                              1.422                                       
                                 --  0.249                                
                                        -- 0.029                          
                                              0.288                       
                                                 0.017                    
                                                    20.5                  
                                                       11.1 120           
110                                                                       
   0.146                                                                  
      0.540                                                               
         0.288                                                            
            0.038                                                         
               0.009                                                      
                  0.109                                                   
                     0.427                                                
                        11.18                                             
                           0.007                                          
                              2.138                                       
                                 --  0.524                                
                                        -- 0.051                          
                                              0.270                       
                                                 0.017                    
                                                    20.8                  
                                                       16.6 142           
111                                                                       
   0.229                                                                  
      0.767                                                               
         0.884                                                            
            0.031                                                         
               0.006                                                      
                  0.037                                                   
                     0.399                                                
                        11.64                                             
                           0.328                                          
                              1.998                                       
                                 0.0334                                   
                                     0.049                                
                                        -- 0.036                          
                                              0.157                       
                                                 0.018                    
                                                    24.0                  
                                                       15.4 127           
112                                                                       
   0.163                                                                  
      0.324                                                               
         0.984                                                            
            0.032                                                         
               0.003                                                      
                  0.064                                                   
                     0.411                                                
                        12.66                                             
                           0.495                                          
                              2.660                                       
                                 0.0659                                   
                                     0.047                                
                                        -- 0.045                          
                                              0.407                       
                                                 0.012                    
                                                    21.7                  
                                                       16.9 101           
113                                                                       
   0.290                                                                  
      0.149                                                               
         0.750                                                            
            0.049                                                         
               0.006                                                      
                  0.059                                                   
                     0.327                                                
                        10.27                                             
                           0.721                                          
                              0.882                                       
                                 0.0903                                   
                                     0.122                                
                                        -- 0.049                          
                                              0.302                       
                                                 0.005                    
                                                    16.0                  
                                                       9.83 168           
114                                                                       
   0.083                                                                  
      0.381                                                               
         0.717                                                            
            0.030                                                         
               0.003                                                      
                  0.041                                                   
                     0.445                                                
                        10.29                                             
                           0.162                                          
                              0.807                                       
                                 0.0752                                   
                                     0.171                                
                                        -- 0.065                          
                                              0.110                       
                                                 0.007                    
                                                    18.6                  
                                                       3.81 132           
115                                                                       
   0.035                                                                  
      0.393                                                               
         0.647                                                            
            0.042                                                         
               0.004                                                      
                  0.029                                                   
                     0.184                                                
                        11.29                                             
                           0.028                                          
                              2.970                                       
                                 0.0407                                   
                                     0.012                                
                                        -- 0.066                          
                                              0.113                       
                                                 0.014                    
                                                    21.6                  
                                                       15.5 171           
116                                                                       
   0.201                                                                  
      0.330                                                               
         0.284                                                            
            0.013                                                         
               0.008                                                      
                  0.042                                                   
                     0.076                                                
                        11.60                                             
                           0.498                                          
                              1.561                                       
                                 0.0525                                   
                                     0.221                                
                                        -- 0.066                          
                                              0.385                       
                                                 0.011                    
                                                    16.7                  
                                                       12.4 162           
117                                                                       
   0.196                                                                  
      0.341                                                               
         0.991                                                            
            0.029                                                         
               0.010                                                      
                  0.049                                                   
                     0.210                                                
                        9.76                                              
                           0.268                                          
                              2.087                                       
                                 0.0831                                   
                                     0.154                                
                                        -- 0.042                          
                                              0.197                       
                                                 0.019                    
                                                    15.9                  
                                                       11.3 134           
118                                                                       
   0.078                                                                  
      0.566                                                               
         0.501                                                            
            0.020                                                         
               0.008                                                      
                  0.067                                                   
                     0.436                                                
                        9.15                                              
                           0.017                                          
                              1.210                                       
                                 0.0477                                   
                                     0.130                                
                                        -- 0.051                          
                                              0.228                       
                                                 0.005                    
                                                    20.9                  
                                                       7.80 115           
119                                                                       
   0.045                                                                  
      0.378                                                               
         0.571                                                            
            0.031                                                         
               0.004                                                      
                  0.116                                                   
                     0.173                                                
                        10.69                                             
                           0.434                                          
                              1.936                                       
                                 0.0174                                   
                                     0.110                                
                                        -- 0.022                          
                                              0.352                       
                                                 0.010                    
                                                    23.7                  
                                                       4.41 208           
120                                                                       
   0.152                                                                  
      0.305                                                               
         0.955                                                            
            0.011                                                         
               0.009                                                      
                  0.116                                                   
                     0.175                                                
                        10.36                                             
                           0.709                                          
                              0.994                                       
                                 0.0109                                   
                                     0.342                                
                                        -- 0.018                          
                                              0.442                       
                                                 0.009                    
                                                    16.3                  
                                                       10.9 217           
121                                                                       
   0.019                                                                  
      0.109                                                               
         0.995                                                            
            0.028                                                         
               0.006                                                      
                  0.084                                                   
                     0.075                                                
                        9.06                                              
                           0.024                                          
                              2.564                                       
                                 --  -- 0.724                             
                                           0.098                          
                                              0.175                       
                                                 0.014                    
                                                    20.3                  
                                                       15.7 168           
122                                                                       
   0.278                                                                  
      0.720                                                               
         0.852                                                            
            0.048                                                         
               0.001                                                      
                  0.088                                                   
                     0.187                                                
                        10.28                                             
                           0.576                                          
                              0.894                                       
                                 --  -- 0.621                             
                                           0.056                          
                                              0.429                       
                                                 0.012                    
                                                    17.6                  
                                                       14.5 239           
123                                                                       
   0.256                                                                  
      0.312                                                               
         0.250                                                            
            0.038                                                         
               0.006                                                      
                  0.099                                                   
                     0.362                                                
                        12.31                                             
                           0.033                                          
                              2.664                                       
                                 --  -- 0.611                             
                                           0.043                          
                                              0.265                       
                                                 0.004                    
                                                    15.4                  
                                                       3.17 238           
124                                                                       
   0.162                                                                  
      0.623                                                               
         0.574                                                            
            0.028                                                         
               0.003                                                      
                  0.084                                                   
                     0.246                                                
                        11.30                                             
                           0.109                                          
                              0.891                                       
                                 --  -- 0.874                             
                                           0.076                          
                                              0.179                       
                                                 0.005                    
                                                    20.3                  
                                                       4.99 112           
125                                                                       
   0.186                                                                  
      0.225                                                               
         0.758                                                            
            0.044                                                         
               0.008                                                      
                  0.114                                                   
                     0.141                                                
                        8.06                                              
                           0.971                                          
                              2.284                                       
                                 --  -- 0.124                             
                                           0.038                          
                                              0.232                       
                                                 0.008                    
                                                    19.2                  
                                                       15.8 119           
126                                                                       
   0.244                                                                  
      0.047                                                               
         0.555                                                            
            0.033                                                         
               0.010                                                      
                  0.106                                                   
                     0.138                                                
                        8.62                                              
                           0.831                                          
                              2.322                                       
                                 --  -- 0.515                             
                                           0.079                          
                                              0.105                       
                                                 0.008                    
                                                    19.1                  
                                                       10.3 210           
127                                                                       
   0.097                                                                  
      0.613                                                               
         0.502                                                            
            0.024                                                         
               0.009                                                      
                  0.028                                                   
                     0.083                                                
                        11.27                                             
                           0.843                                          
                              2.655                                       
                                 --  -- 0.737                             
                                           0.037                          
                                              0.267                       
                                                 0.008                    
                                                    16.7                  
                                                       11.8 111           
128                                                                       
   0.085                                                                  
      0.525                                                               
         0.366                                                            
            0.023                                                         
               0.008                                                      
                  0.050                                                   
                     0.076                                                
                        12.07                                             
                           0.223                                          
                              2.580                                       
                                 --  -- 0.500                             
                                           0.018                          
                                              0.199                       
                                                 0.001                    
                                                    19.0                  
                                                       9.18 229           
129                                                                       
   0.127                                                                  
      0.312                                                               
         0.687                                                            
            0.018                                                         
               0.008                                                      
                  0.040                                                   
                     0.190                                                
                        11.90                                             
                           0.637                                          
                              2.537                                       
                                 --  -- 0.477                             
                                           0.086                          
                                              0.174                       
                                                 0.011                    
                                                    22.8                  
                                                       5.04 215           
130                                                                       
   0.126                                                                  
      0.121                                                               
         0.821                                                            
            0.017                                                         
               0.001                                                      
                  0.068                                                   
                     0.066                                                
                        11.73                                             
                           0.291                                          
                              1.743                                       
                                 --  -- 0.354                             
                                           0.075                          
                                              0.266                       
                                                 0.012                    
                                                    15.0                  
                                                       15.8 171           
131                                                                       
   0.137                                                                  
      0.096                                                               
         0.512                                                            
            0.027                                                         
               0.005                                                      
                  0.051                                                   
                     0.459                                                
                        9.96                                              
                           0.855                                          
                              1.909                                       
                                 0.0215                                   
                                     -- 0.320                             
                                           0.049                          
                                              0.150                       
                                                 0.010                    
                                                    17.5                  
                                                       9.31 203           
132                                                                       
   0.216                                                                  
      0.391                                                               
         0.323                                                            
            0.016                                                         
               0.006                                                      
                  0.060                                                   
                     0.335                                                
                        10.11                                             
                           0.043                                          
                              2.292                                       
                                 0.0553                                   
                                     -- 0.083                             
                                           0.070                          
                                              0.303                       
                                                 0.006                    
                                                    17.2                  
                                                       16.2 113           
133                                                                       
   0.273                                                                  
      0.367                                                               
         0.387                                                            
            0.031                                                         
               0.010                                                      
                  0.062                                                   
                     0.446                                                
                        10.52                                             
                           0.014                                          
                              0.811                                       
                                 0.0056                                   
                                     -- 0.295                             
                                           0.063                          
                                              0.401                       
                                                 0.004                    
                                                    15.9                  
                                                       4.00 228           
134                                                                       
   0.213                                                                  
      0.554                                                               
         0.462                                                            
            0.040                                                         
               0.005                                                      
                  0.076                                                   
                     0.203                                                
                        9.67                                              
                           0.464                                          
                              1.706                                       
                                 0.0411                                   
                                     -- 0.658                             
                                           0.045                          
                                              0.310                       
                                                 0.016                    
                                                    20.5                  
                                                       10.3 202           
135                                                                       
   0.095                                                                  
      0.720                                                               
         0.299                                                            
            0.049                                                         
               0.003                                                      
                  0.021                                                   
                     0.368                                                
                        8.51                                              
                           0.087                                          
                              1.695                                       
                                 0.0810                                   
                                     -- 0.864                             
                                           0.080                          
                                              0.116                       
                                                 0.002                    
                                                    22.9                  
                                                       9.79 214           
136                                                                       
   0.117                                                                  
      0.226                                                               
         0.843                                                            
            0.048                                                         
               0.001                                                      
                  0.117                                                   
                     0.349                                                
                        12.93                                             
                           0.068                                          
                              1.256                                       
                                 0.0452                                   
                                     -- 0.607                             
                                           0.094                          
                                              0.490                       
                                                 0.018                    
                                                    15.0                  
                                                       5.13 215           
137                                                                       
   0.040                                                                  
      0.771                                                               
         0.648                                                            
            0.022                                                         
               0.009                                                      
                  0.029                                                   
                     0.230                                                
                        11.41                                             
                           0.586                                          
                              0.839                                       
                                 0.0996                                   
                                     -- 0.060                             
                                           0.058                          
                                              0.303                       
                                                 0.014                    
                                                    18.6                  
                                                       5.78 234           
138                                                                       
   0.213                                                                  
      0.552                                                               
         0.253                                                            
            0.040                                                         
               0.001                                                      
                  0.029                                                   
                     0.314                                                
                        9.81                                              
                           0.328                                          
                              2.143                                       
                                 0.0647                                   
                                     -- 0.899                             
                                           0.078                          
                                              0.231                       
                                                 0.010                    
                                                    15.5                  
                                                       14.4 150           
139                                                                       
   0.254                                                                  
      0.514                                                               
         0.628                                                            
            0.029                                                         
               0.003                                                      
                  0.084                                                   
                     0.469                                                
                        9.17                                              
                           0.988                                          
                              1.925                                       
                                 0.0089                                   
                                     -- 0.834                             
                                           0.038                          
                                              0.357                       
                                                 0.011                    
                                                    22.6                  
                                                       12.7 213           
140                                                                       
   0.068                                                                  
      0.391                                                               
         0.306                                                            
            0.016                                                         
               0.002                                                      
                  0.064                                                   
                     0.104                                                
                        12.89                                             
                           0.265                                          
                              1.905                                       
                                 0.0516                                   
                                     -- 0.560                             
                                           0.089                          
                                              0.347                       
                                                 0.011                    
                                                    17.0                  
                                                       3.45 156           
141                                                                       
   0.295                                                                  
      0.339                                                               
         0.648                                                            
            0.010                                                         
               0.009                                                      
                  0.039                                                   
                     0.204                                                
                        11.91                                             
                           0.336                                          
                              2.541                                       
                                 --  0.090                                
                                        0.744                             
                                           0.023                          
                                              0.479                       
                                                 0.013                    
                                                    15.5                  
                                                       3.05 155           
142                                                                       
   0.121                                                                  
      0.123                                                               
         0.203                                                            
            0.037                                                         
               0.008                                                      
                  0.025                                                   
                     0.401                                                
                        12.19                                             
                           0.886                                          
                              0.572                                       
                                 --  0.022                                
                                        0.588                             
                                           0.038                          
                                              0.433                       
                                                 0.003                    
                                                    19.8                  
                                                       14.2 237           
143                                                                       
   0.176                                                                  
      0.651                                                               
         0.714                                                            
            0.046                                                         
               0.009                                                      
                  0.042                                                   
                     0.102                                                
                        9.61                                              
                           0.107                                          
                              1.418                                       
                                 --  0.250                                
                                        0.271                             
                                           0.059                          
                                              0.275                       
                                                 0.011                    
                                                    21.9                  
                                                       4.61 132           
144                                                                       
   0.225                                                                  
      0.131                                                               
         0.336                                                            
            0.043                                                         
               0.001                                                      
                  0.059                                                   
                     0.293                                                
                        10.13                                             
                           0.177                                          
                              0.586                                       
                                 --  0.039                                
                                        0.477                             
                                           0.013                          
                                              0.286                       
                                                 0.003                    
                                                    18.7                  
                                                       4.80 242           
145                                                                       
   0.035                                                                  
      0.667                                                               
         0.427                                                            
            0.037                                                         
               0.007                                                      
                  0.096                                                   
                     0.167                                                
                        9.79                                              
                           0.309                                          
                              1.075                                       
                                 --  0.019                                
                                        0.413                             
                                           0.054                          
                                              0.344                       
                                                 0.003                    
                                                    22.1                  
                                                       10.5 222           
146                                                                       
   0.093                                                                  
      0.048                                                               
         0.722                                                            
            0.014                                                         
               0.008                                                      
                  0.068                                                   
                     0.246                                                
                        9.38                                              
                           0.660                                          
                              0.774                                       
                                 --  0.330                                
                                        0.245                             
                                           0.062                          
                                              0.317                       
                                                 0.004                    
                                                    23.8                  
                                                       3.14 246           
147                                                                       
   0.019                                                                  
      0.771                                                               
         0.981                                                            
            0.022                                                         
               0.008                                                      
                  0.048                                                   
                     0.170                                                
                        9.91                                              
                           0.437                                          
                              2.723                                       
                                 --  0.168                                
                                        0.670                             
                                           0.010                          
                                              0.483                       
                                                 0.010                    
                                                    20.9                  
                                                       13.6 122           
148                                                                       
   0.158                                                                  
      0.251                                                               
         0.952                                                            
            0.049                                                         
               0.006                                                      
                  0.102                                                   
                     0.473                                                
                        9.82                                              
                           0.320                                          
                              1.099                                       
                                 --  0.162                                
                                        0.376                             
                                           0.093                          
                                              0.319                       
                                                 0.014                    
                                                    20.1                  
                                                       9.29 174           
149                                                                       
   0.128                                                                  
      0.189                                                               
         0.780                                                            
            0.022                                                         
               0.003                                                      
                  0.099                                                   
                     0.338                                                
                        10.00                                             
                           0.494                                          
                              1.381                                       
                                 --  0.438                                
                                        0.049                             
                                           0.060                          
                                              0.296                       
                                                 0.002                    
                                                    18.3                  
                                                       6.92 133           
150                                                                       
   0.273                                                                  
      0.731                                                               
         0.616                                                            
            0.049                                                         
               0.010                                                      
                  0.059                                                   
                     0.299                                                
                        11.40                                             
                           0.153                                          
                              2.779                                       
                                 --  0.045                                
                                        0.859                             
                                           0.061                          
                                              0.489                       
                                                 0.005                    
                                                    17.5                  
                                                       9.10 249           
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours         
 VE: Charpy absorption energy at 0° C. after aging at 700° C
 for 10 thousand hours                                                    
 TO: Oxidation scale thickness after oxidation at 650° C. for 10   
 thousand hours in air                                                    
 
    
     
                                           TABLE 4                                 
__________________________________________________________________________
(Wt. %) Invention Steels                                                  
                                                    CS                    
                                                    kg/                   
                                                       VE   TO            
No.                                                                       
   C  Si Mn P  S  Nb V  Cr Mo W  Zr  Ta Hf Ti N  O  mm.sup.2              
                                                       kgf ·     
                                                       μm              
__________________________________________________________________________
151                                                                       
   0.213                                                                  
      0.079                                                               
         0.214                                                            
            0.017                                                         
               0.005                                                      
                  0.049                                                   
                     0.220                                                
                        12.50                                             
                           0.975                                          
                              0.789                                       
                                 0.0684                                   
                                     0.078                                
                                        0.529                             
                                           0.025                          
                                              0.376                       
                                                 0.008                    
                                                    24.4                  
                                                       4.45               
                                                           104            
152                                                                       
   0.149                                                                  
      0.326                                                               
         0.523                                                            
            0.013                                                         
               0.004                                                      
                  0.039                                                   
                     0.178                                                
                        11.70                                             
                           0.161                                          
                              0.844                                       
                                 0.0017                                   
                                     0.542                                
                                        0.220                             
                                           0.056                          
                                              0.430                       
                                                 0.014                    
                                                    16.0                  
                                                       16.5 248           
153                                                                       
   0.132                                                                  
      0.057                                                               
         0.488                                                            
            0.026                                                         
               0.009                                                      
                  0.047                                                   
                     0.263                                                
                        9.93                                              
                           0.382                                          
                              2.180                                       
                                 0.0271                                   
                                     0.297                                
                                        0.619                             
                                           0.038                          
                                              0.125                       
                                                 0.018                    
                                                    24.1                  
                                                       6.08 111           
154                                                                       
   0.073                                                                  
      0.589                                                               
         0.264                                                            
            0.042                                                         
               0.008                                                      
                  0.093                                                   
                     0.331                                                
                        10.64                                             
                           0.833                                          
                              1.516                                       
                                 0.0632                                   
                                     0.034                                
                                        0.427                             
                                           0.032                          
                                              0.338                       
                                                 0.013                    
                                                    20.6                  
                                                       3.65 244           
155                                                                       
   0.028                                                                  
      0.512                                                               
         0.402                                                            
            0.029                                                         
               0.003                                                      
                  0.045                                                   
                     0.469                                                
                        9.80                                              
                           0.213                                          
                              1.841                                       
                                 0.0787                                   
                                     0.263                                
                                        0.824                             
                                           0.011                          
                                              0.145                       
                                                 0.005                    
                                                    22.4                  
                                                       5.02 229           
156                                                                       
   0.197                                                                  
      0.361                                                               
         0.251                                                            
            0.015                                                         
               0.004                                                      
                  0.021                                                   
                     0.309                                                
                        10.82                                             
                           0.689                                          
                              1.083                                       
                                 0.0423                                   
                                     0.036                                
                                        0.997                             
                                           0.081                          
                                              0.420                       
                                                 0.004                    
                                                    21.3                  
                                                       12.4 238           
157                                                                       
   0.048                                                                  
      0.663                                                               
         0.354                                                            
            0.027                                                         
               0.008                                                      
                  0.090                                                   
                     0.387                                                
                        12.09                                             
                           0.960                                          
                              0.982                                       
                                 0.0724                                   
                                     0.166                                
                                        0.641                             
                                           0.079                          
                                              0.234                       
                                                 0.005                    
                                                    24.3                  
                                                       8.05 231           
158                                                                       
   0.222                                                                  
      0.409                                                               
         0.472                                                            
            0.013                                                         
               0.004                                                      
                  0.022                                                   
                     0.337                                                
                        12.12                                             
                           0.218                                          
                              2.006                                       
                                 0.0384                                   
                                     0.280                                
                                        0.936                             
                                           0.066                          
                                              0.268                       
                                                 0.003                    
                                                    18.2                  
                                                       15.0 231           
159                                                                       
   0.191                                                                  
      0.515                                                               
         0.348                                                            
            0.019                                                         
               0.007                                                      
                  0.084                                                   
                     0.469                                                
                        11.84                                             
                           0.642                                          
                              2.134                                       
                                 0.0582                                   
                                     0.195                                
                                        0.571                             
                                           0.060                          
                                              0.394                       
                                                 0.019                    
                                                    19.8                  
                                                       8.23 119           
160                                                                       
   0.153                                                                  
      0.738                                                               
         0.528                                                            
            0.034                                                         
               0.005                                                      
                  0.097                                                   
                     0.413                                                
                        12.12                                             
                           0.414                                          
                              1.796                                       
                                 0.0023                                   
                                     0.271                                
                                        0.732                             
                                           0.033                          
                                              0.485                       
                                                 0.011                    
                                                    21.2                  
                                                       4.30 222           
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours         
 VE: Charpy absorption energy at 0° C. after aging at 700° C
 for 10 thousand hours                                                    
 TO: Oxidation scale thickness after oxidation at 650° C. for 10   
 thousand hours in air                                                    
 
    
     
                                           TABLE 5                                 
__________________________________________________________________________
(Wt. %) Comparison Steels                                                 
                                                    CS  VE                
                                                    (kg/                  
                                                        (kgf              
                                                        TOultidot.        
No.                                                                       
   C  Si Mn P  S  Nb V  Cr Mo W  Zr Ta Hf Ti N  O   mm.sup.2)             
                                                        m)   (μm)      
__________________________________________________________________________
161                                                                       
   0.080                                                                  
      0.054                                                               
         0.572                                                            
            0.016                                                         
               0.007                                                      
                  0.075                                                   
                     0.121                                                
                        8.66                                              
                           0.011                                          
                              1.162                                       
                                 0.011                                    
                                    0.255                                 
                                       0.161                              
                                          0.045                           
                                             0.087                        
                                                0.0021                    
                                                    11.4                  
                                                        3.2 420           
162                                                                       
   0.092                                                                  
      0.076                                                               
         0.323                                                            
            0.017                                                         
               0.006                                                      
                  0.045                                                   
                     0.201                                                
                        11.51                                             
                           0.095                                          
                              1.514                                       
                                 0.008                                    
                                    0.344                                 
                                       0.055                              
                                          0.037                           
                                             0.032                        
                                                0.0021                    
                                                    10.3                  
                                                        3.1  430          
163                                                                       
   0.121                                                                  
      0.101                                                               
         0.812                                                            
            0.003                                                         
               0.005                                                      
                  0.042                                                   
                     0.222                                                
                        9.22                                              
                           0.252                                          
                              1.889                                       
                                 0.077                                    
                                    0.337                                 
                                       0.056                              
                                          0.036                           
                                             0.524                        
                                                0.0045                    
                                                    16.1                  
                                                        0.2  120          
164                                                                       
   0.137                                                                  
      0.110                                                               
         0.271                                                            
            0.004                                                         
               0.006                                                      
                  0.043                                                   
                     0.232                                                
                        10.99                                             
                           0.341                                          
                              2.005                                       
                                 0.001                                    
                                    0.188                                 
                                       0.164                              
                                          0.012                           
                                             0.667                        
                                                0.0078                    
                                                    19.9                  
                                                        0.7   90          
165                                                                       
   0.075                                                                  
      0.141                                                               
         0.457                                                            
            0.045                                                         
               0.006                                                      
                  0.037                                                   
                     0.313                                                
                        9.21                                              
                           0.516                                          
                              0.447                                       
                                 0.003                                    
                                    0.345                                 
                                       0.122                              
                                          0.017                           
                                             0.124                        
                                                0.0065                    
                                                    7.5 8.7  140          
166                                                                       
   0.099                                                                  
      0.080                                                               
         0.724                                                            
            0.042                                                         
               0.009                                                      
                  0.048                                                   
                     0.344                                                
                        9.33                                              
                           0.557                                          
                              2.123                                       
                                 0.131                                    
                                    0.895                                 
                                       0.016                              
                                          0.025                           
                                             0.155                        
                                                0.0043                    
                                                    15.5                  
                                                        0.4  130          
167                                                                       
   0.102                                                                  
      0.156                                                               
         0.889                                                            
            0.027                                                         
               0.002                                                      
                  0.062                                                   
                     0.411                                                
                        9.45                                              
                           0.616                                          
                              1.156                                       
                                 0.015                                    
                                    1.212                                 
                                       0.999                              
                                          0.066                           
                                             0.166                        
                                                0.0152                    
                                                    19.4                  
                                                        0.1   80          
168                                                                       
   0.033                                                                  
      0.092                                                               
         0.233                                                            
            0.018                                                         
               0.002                                                      
                  0.063                                                   
                     0.056                                                
                        9.13                                              
                           0.709                                          
                              0.871                                       
                                 0.045                                    
                                    0.962                                 
                                       1.045                              
                                          0.088                           
                                             0.176                        
                                                0.0117                    
                                                    16.7                  
                                                        0.1  120          
169                                                                       
   0.052                                                                  
      0.244                                                               
         0.475                                                            
            0.021                                                         
               0.002                                                      
                  0.051                                                   
                     0.099                                                
                        10.56                                             
                           0.989                                          
                              0.558                                       
                                 0.099                                    
                                    0.037                                 
                                       0.087                              
                                          0.119                           
                                             0.201                        
                                                0.0169                    
                                                    17.1                  
                                                        0.3   80          
170                                                                       
   0.071                                                                  
      0.163                                                               
         0.502                                                            
            0.016                                                         
               0.003                                                      
                  0.049                                                   
                     0.212                                                
                        9.15                                              
                           0.442                                          
                              2.002                                       
                                 0.020                                    
                                    -- -- -- 0.223                        
                                                0.0051                    
                                                    3.1 0.2  110          
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours         
 VE: Charpy absorption energy at 0° C. after aging at 700° C
 for 10 thousand hours                                                    
 TO: Oxidation scale thickness after oxidation at 650° C. for 10   
 thousand hours in air                                                    
 
    
     The present invention provides a high-nitrogen ferritic heat-resisting steel exhibiting a high rupture strength after prolonged creep and superior high-temperature oxidation resistance and, as such, can be expected to make a major contribution to industrial progress.