Patent Publication Number: US-2017356070-A1

Title: Maraging steel

Description:
FIELD OF INVENTION 
     The present invention relates to managing steels. 
     BACKGROUND OF INVENTION 
     Gas turbines and steam turbines for use in thermal power facilities each include a rotor and blades, where the rotor acts as a rotating shaft. The rotor functionally supports the blades and transmits turning force (torque) to a generator. The rotor, as to be exposed to a high-temperature environment at about 500° C., is made of any of heat-resisting materials. Such heat-resisting materials are mainly selected from ferritic heat-resisting steels and Ni-based alloys. 
     Exemplary practically used ferritic heat-resisting steels for rotors are high-chromium ferritic steels such as 12%-Cr steels. The high-chromium ferritic steels, however, are significantly inferior in strength at high temperatures (high-temperature strength) to Ni-based alloys, which are expensive. Austenitic stainless steels, which are widely used as general heat-resisting materials, are not suitable as materials for rotors, which are large-scale members. This is because the austenitic stainless steels have high coefficients of thermal expansion, although they have high-temperature strength lying midway between that of the ferritic heat-resisting steels and that of the Ni-based alloys. In addition to these materials, exemplary known heat-resisting materials for rotors include precipitation-hardened iron-based high heat-resistance alloys (superalloys) such as “A286”, and precipitation-hardened ferritic heat-resisting steels such as maraging steels. 
     Of these heat-resisting materials, the maraging steels are materials strengthened (hardened) by aging precipitation of martensitic phases and intermetallic compounds and are produced via quenching and aging heat treatments. The maraging steels are significantly superior in high-temperature strength to ferritic heat-resisting steels. Disadvantageously, however, the maraging steels have lower toughness when designed to have such a chemical composition and to be subjected to a heat treatment under such conditions as to offer high strength at high temperatures. In particular, steels for rotors of gas turbines and steam turbines for use in thermal power facilities require excellent toughness, because high heat stress is generated when the temperatures of the steels fall down to mom temperature during suspension of operation. 
     There have been proposed various techniques so as to improve both strength and toughness. For example, Japanese Patent No. 5362995 proposes a stainless steel alloy including, by weight: 0.002% to 0.015% carbon (C), 2% to 15% cobalt (Co), 7.0% to 14.0% nickel (Ni), 8.0% to 15.0% chromium (Cr), 0.5% to 2.6% molybdenum (Mo), 0.4% to 0.75% titanium (TO, less than 0.5% tungsten (W), less than 0.7% aluminum (Al), with the balance essentially iron (Fe) and incidental elements and impurities. The alloy avoids copper (Cu) as an alloying constituent, has a lath martensite microstructure having undergone predetermined treatments, and has a volume fraction of retained austenite of less than 15%, essentially without topologically close packed (TCP) intermetallic phases. In the alloy, the carbon (C) is in a dispersion of 0.02% to 0.15% by volume TIC carbide particles. The alloy further includes a dispersion of intermetallic particles primarily of Ni 3 Tiη phase as a strengthening phase. 
     Japanese Unexamined Patent Application Publication (JP-A) No. 2015-61932 proposes a maraging steel excellent in fatigue characteristics. The maraging steel has a chemical composition including, in mass percent: C in a content of 0.015% or less, Ni in a content of 12.0% to 20.0%, Mo in a content of 3.0% to 6.0%, Co in a content of 5.0% to 13.0%, Al in a content of 0.01% to 0.3%, Ti in a content of 0.2% to 2.0%, 0 in a content of 0.0020% or less, N in a content of 0.0020% or less, and Zr in a content of 0.001% to 0.02%, with the balance being Fe and unavoidable impurities. 
     JP-A No. Hei04(1992)-59922 proposes a method for producing a maraging steel. The method includes subjecting a maraging steel to a recrystallization solution treatment, an unrecrystallized solution treatment, and an aging heat treatment. The maraging steel contains, in mass percent, C in a content of 0.05% or less, Si in a content of 0.2% or less, Mn in a content of 0.2% or less, Pin a content of 0.05% or less, S in a content of 0.05% or less, Ni in a content of 10.0% to 21.0%, Co in a content of 9.5% to 15.0%, Mo in a content of 3.0% to 12.0%, Ti in a content of 0.2% to 1.6%, Al in a content of 0.30% or less, and Bin a content of 0.0005% to 0.0020%, and the managing steel has undergone hot forming. In the method, the recrystallization solution treatment is performed as a two-stage treatment including heating in a temperature range of from 1000° C. to 1180° C. for one minute or longer, cooling at a cooling rate of 20° C./min or more, and further heating in a temperature range of from 800° C. to 950° C. for one minute or longer, and then cooling. 
     SUMMARY OF INVENTION 
     The technique described in Japanese Patent No. 5362995 allows a stainless steel alloy to have higher strength and better toughness by adjusting the chemical composition and microstructure of the alloy. The technique evaluates strength and room-temperature toughness, but fails to evaluate strength at high temperatures of about 500° C., to which high temperatures the present invention is to be applied. 
     The technique described in JP-A No. 2015-61932 offers excellent fatigue strength by refinement of TiN inclusions, but fails to evaluate strength at high temperatures of about 500° C., to which high temperatures the present invention is to be applied. Maraging steels offer more excellent high-temperature strength as compared with ferritic heat-resisting steels. The maraging steels, however, do not always maintain such excellent high-temperature strength as intact when controlled to have higher fatigue strength and better toughness. 
     JP-A No. Hei04(1992)-59922 mentions that a maraging steel having strength, toughness, and ductility at better levels is obtained by appropriately controlling the heat treatment conditions. However, the technique described in this literature also fails to evaluate strength at high temperatures to which the present invention is to be applied, as with the technique described in Japanese Patent No. 5362995 and JP-A No. 2015-61932. 
     The present invention has been made under these circumstances and has an object to improve the toughness of a maraging steel which is more inexpensive as compared with Ni-based alloys and has higher strength at high temperatures as compared with ferritic heat-resisting steels and to provide a maraging steel having high-temperature strength and room-temperature toughness both at excellent levels. 
     The present invention has achieved the object and provides, in an embodiment, a maraging steel containing, in combination, in mass percent, C in a content from greater than 0% to 0.02%, Mn in a content from greater than 0% to 0.3%, Si in a content from greater than 0% to 03%, Ni in a content of 10% to 13%, Mo in a content of 0.5% to 3.5%, Co in a content of 9% to 12%, Cr in a content of 1.5% to 4.5%, Ti in a content of 1.5% to 4.5%, and Al in a content of 0.01% to 0.2%, with the remainder consisting of iron and inevitable impurities. In the maraging steel, the total content of the Ti and Mo is 5.0% or less, and the ratio ([Mo]/[Ti]) of the Mo content [Mo] to the Ti content [Ti] is 1.0 or less. 
     The maraging steel according to the present invention preferably has a phosphorus (P) content from greater than 0% to 0.01%, a nitrogen (N) content from greater than 0% to 0.01%, and a sulfur (S) content from greater than 0% to 0.01%, where P, N, and S are present in the inevitable impurities. The maraging steel preferably has a surface hardness in terms of Vickers hardness of 400 Hv or more. 
     The present invention can actually provide a maraging steel which not only has excellent high-temperature strength by aging precipitation of intermetallic compounds, but also offers good room-temperature toughness by controlling the chemical composition and microstructure. The maraging steel as above offers excellent high-temperature strength and good room-temperature toughness and is very useful typically as materials for rotors for use in thermal power facilities. The maraging steel, when applied to materials for rotors for use in thermal power facilities, gives rotors which are inexpensive and still have lighter weights as compared with conventional Ni-based alloy rotors, and can contribute to improved generation efficiency and thereby to CO 2  emission control. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The inventors of the present invention made investigations from various different angles so as to actually provide a maraging steel which features compatibility between high-temperature strength and room-temperature toughness. In particular, to achieve the high-temperature strength, the inventors made intensive investigations on how the chemical composition and the microstructure state determined by aging heat treatment after quenching affect the room-temperature toughness. 
     In regular maraging steels, precipitates to perform precipitation strengthening are generally intermetallic compounds mainly containing Mo. Assume that a maraging steel has such a chemical composition as to tend to form such intermetallic compounds. In this maraging steel, a Laves phase including Fe 2 Mo, which is a binary intermetallic compound, tends to form upon aging heat treatment of the maraging steel. The maraging steel, when containing a larger amount of the Laves phase, tends to readily have lower toughness. In particular, the aging heat treatment of materials to form rotors, which are large-sized members, is performed at a high temperature for a long time, and thereby causes the compound to be readily formed in a large amount, and this lowers the toughness. 
     The inventors then hit on an idea that conversion of the intermetallic compounds mainly containing Mo into such intermetallic compounds as not to adversely affect toughness may actually provide good toughness without occurrence of the above-mentioned problems. After further investigations, the inventors found such a chemical composition as to form intermetallic compounds mainly including Ti, such as Ni 3 Ti intermetallic compound. The present invention has been made on the basis of these findings. 
     A maraging steel having the chemical composition specified in the present invention, when subjected to an aging heat treatment under predetermined conditions, has a microstructure in which finely divided martensite is dispersed in a ferritic phase in which the Ni 3 Ti intermetallic compound is precipitated. This maraging steel offers such properties as to offer a surface hardness in terms of Vickers hardness of 400 Hv or more. 
     As apparent Am the above-mentioned concept, appropriate settings of, among the chemical composition, in particular the Mo and Ti contents and the relationship between them are important in the maraging steel according to the present invention. Conversion into the precipitates of intermetallic compounds as above requires appropriate settings of not only the contents of Mo and Ti and the total contents of them, but also the ratio ([Mo]/[Ti]) of the Mo content [Mo] to the Ti content [Ti]. Reasons for the settings of these factors are as follows. 
     Mo: 0.5% to 3.5%, Ti: 1.5% to 4.5% 
     Molybdenum (Mo) and titanium (Ti) form precipitates of various intermetallic compounds mainly containing these elements and are useful for higher strength and better toughness of the steel. To offer these advantageous effects effectively, the steel is controlled to contain Mo in a content of 0.5% or more and Ti in a content of 1.5% or more, and preferably contains Mo in a content of 1.0% or more and Ti in a content of 2.0% or more. 
     However, the steel, if having an excessively high Mo content, may suffer from the formation of a larger amount of FeMo, which adversely affects toughness. To eliminate or minimize this, the Mo content is controlled to 3.5% or less, preferably 3.0% or less, and more preferably 2.5% or less. The steel, if having an excessively high Ti content, may suffer from insufficient room-temperature durability. To eliminate or minimize this, the Ti content is controlled to 4.5% or less, preferably 4.0% or less, and more preferably 3.5% or less. 
     Total Content of Mo and Ti: 5.0% or less, Ratio ([Mo]/[Ti]): 1.0 or less 
     In addition to the settings of the Mo and Ti contents as above, providing of intermetallic compounds formed in the steel mainly including not Mo, but Ti requires the control of the total content of Mo and Ti to 5.0% or less, and the control of the ratio ([Mo]/[Ti]) to 1.0 or less. 
     Increase or decrease of the total content of Mo and Ti causes toughness and high-temperature strength to vary in a trade-off manner. To keep toughness and high-temperature strength in balance, the total content of Mo and Ti is controlled to 5.0% or less. The steel, if having a total content of Mo and Ti of greater than 5.0%, has satisfactory high-temperature strength, but fails to surely have toughness, because of excessive amounts of precipitated various intermetallic compounds. The total content is preferably 4.0% or less, and more preferably 3.0% or less. The total content in to terms of lower limit is inevitably 2.0% or more on the basis of the contents of the respective elements, but is preferably 2.2% or more. 
     In contrast, the steel, if having a ratio ([Mo]/[Ti]) (namely, mass ratio) of the Mo content [Mo] to the Ti content [Ti] of greater than 1.0, fails to surely have toughness, because of a larger proportion of the Laves phase. The ratio ([Mo]/[Ti]) is preferably 0.8 or less, and more preferably 0.6 or less. The ratio ([Mo]/[Ti]) in terms of lower limit is 0.11 or more on the basis of the respective contents, but is preferably 0.2 or more, and more preferably 0.3 or more. 
     The settings of the total content of Mo and Ti and the ratio ([Mo]/[Ti]) of the Mo content [Mo] to the Ti content [Ti] within the predetermined ranges allows the steel to have toughness and high-temperature strength both at satisfactory levels. However, the aging heat treatment, if performed at an excessively high temperature and/or for an excessively long time, may fail to give sufficient high-temperature strength. To eliminate or minimize this, the temperature and time conditions in the aging are preferably controlled so as to allow the steel to have a surface Vickers hardness of 400 Hv or more, as mentioned below. 
     In the maraging steel according to the present invention, at least Mo and Ti are to be controlled as mentioned above, but, in addition to these elements, elements such as C, Mn, Si, Ni, Co, Cr, and Al are to be controlled within appropriate ranges. Reasons for the settings on these elements are as follows. 
     C: from greater than 0% to 0.02% 
     Carbon (C) forms carbides in a high-temperature environment to allow the steel to have high-temperature strength and high-temperature creep strength at higher levels. However, the carbon content should be minimized so as to maximize the precipitation of intermetallic compounds mainly containing Ti. The steel, if having an excessively high carbon content of greater than 0.02%, may contrarily have lower toughness because of formation of TiC in a larger amount. The carbon content in terms of upper limit is preferably 0.015% or less, and more preferably 0.010% or less. The carbon content in terms of lower limit is preferably 0.001% or more, and more preferably 0.005% or more, so as to allow carbon to offer basic actions. 
     Mn: from greater than 0% to 0.3% 
     Manganese (Mn) has a deoxidation action in molten steel. The element offers the advantageous effect more with an increasing content of the element. To offer the advantageous effect effectively, the Mn content is preferably controlled to 0.005% or more. The Mn content in terms of lower limit is more preferably 0.010% or more, and furthermore preferably 0.015% or more. However, the steel, if having an excessively high Mn content of greater than 0.3%, may fail to include the martensitic phase after quenching, due to increased stability of the austenitic phase. The Mn content in terms of upper limit is preferably 0.2% or less, and more preferably 0.1% or less. 
     Si: from greater than 0% to 0.3% 
     Silicon (Si) has a deoxidation action in molten steel, as with Mn. This element, even when present in a trace amount, effectively allows the steel to have better oxidation resistance. To offer these advantageous effects effectively, the Si content is preferably controlled to 0.005% or more. The Si content in terms of lower limit is preferably 0.010% or more, and furthermore preferably 0.015% or more. However, the steel, if having an excessively high Si content, may suffer from impaired ductility because of excessive work hardening. To eliminate or minimize this, the Si content is controlled to 0.3% or less. The Si content in tams of upper limit is preferably 0.2% or less, and more preferably 0.1% or less. 
     Ni: 10% to 13% 
     Nickel (Ni) is an austenitic phase-stabilizing element which is necessary for austenitization of the microstructure in heating before quenching. This element also allows Ti to be precipitated as the Ni 3 Ti intermetallic compound and thereby allows the steel to have more satisfactory high-temperature strength. To offer these advantageous effects, the Ni content is contained to 10% or more. The Ni content is preferably 10.5% or more, and more preferably 11.0% or more. However, the steel, if having an excessively high Ni content of greater than 13%, may cause higher cost and may cause austenite to remain after quenching. The Ni content in terms of upper limit is preferably 12.5% or less, and more preferably 12.0% or less. 
     Co: 9% to 12% 
     Cobalt (Co) is dissolved as a solute in the steel to offer solid-solution strengthening. To offer the advantageous effect, the Co content is controlled 9% or more. The Co content in terms of lower limit is preferably 9.5% or more, and more preferably 10.0% or more. However, the steel, if having an excessively high Co content, may cause higher cost and may have impaired ductility due to excessively increased strength. To eliminate or minimize these, the Co content in terms of upper limit is controlled to 12% or less, and is preferably 11.5% or less, and more preferably 11.0% or less. 
     Cr: 1.5% to 4.5% 
     Chromium (CO is necessary for better oxidation resistance of the maraging steel. To offer good oxidation resistance, the Cr content is controlled to 1.5% or more. The Cr content in terms of lower limit is preferably 2.0% or more, and more preferably 2.5% or more. However, the steel, if having an excessively high Cr content, may be embrittled due to the formation of o phases in a high-temperature environment in which the steel is used as a product. To eliminate or minimize this, the Cr content in terms of upper limit is controlled to 4.5% or less, and is preferably 4.0% or less, and more preferably 3.5% or less. 
     Al: 0.01% to 0.2% 
     Aluminum (Al) has a deoxidation action in molten steel, as with Mn. To offer the advantageous effect, the Al content is controlled to 0.01% or more. The Al content in terms of lower limit is preferably 0.02% or more, and more preferably 0.03% or more. However, the steel, if having an excessively high Al content, may suffer from formation of coarse inclusions derived from Al. To eliminate or minimize this, the Al content is controlled to 0.2% or less, and is preferably 0.1% or less, and more preferably 0.05% or less. 
     The chemical composition specified in the present invention is as described above, with the remainder being iron and inevitable impurities. Of the inevitable impurities, P, N, and S are preferably decreased to levels as mentioned below. The impurities excluding P, N, and S may include low-melting point impurity metals derived from scrap raw materials, such as Sn, Pb, Sb, As, and Zn. These elements, however, lower grain-boundary strength during hot working and in use in a high-temperature environment and are desirably minimized in content. 
     P: from greater than 0% to 0.01% 
     Phosphorus (P) is an inevitably-contaminated impurity, and causes the steel to have lower weldability with an increasing content thereof. From this viewpoint, phosphorus is preferably minimized, and the phosphorus content is controlled to preferably 0.01% or less, more preferably 0.005% or less, and furthermore preferably 0.001% or less. 
     N: from greater than 0% to 0.01% 
     Nitrogen (N) is also an inevitably-contaminated impurity, fixes Ti as nitrides, and lowers the amounts of formed intermetallic compounds that contribute to higher strength, where Ti is contained as an essential element in the steel according to the present invention. From this viewpoint, nitrogen is preferably minimized, and the nitrogen content is controlled to preferably 0.01% or less, more preferably 0.005% or less, and furthermore preferably 0.001% or less. 
     S: from greater than 0% to 0.01% 
     Sulfur (S) is also an inevitably-contaminated impurity and impairs hot workability necessary typically for forging, with an increasing content thereof. From this viewpoint, sulfur is preferably minimized, and the sulfur content is controlled to preferably 0.01% or less, more preferably 0.005% or less, and furthermore preferably 0.001% or less. 
     The maraging steel according to the present invention has a chemical composition as mentioned above. The steel having the chemical composition can be easily obtained by adjusting proportions of raw materials as appropriate via melting. Ingots obtained by ingot making may be subjected to homogenization or soaking (hereinafter also referred to “soaking treatment”) as needed subjected to hot working to adjust its shape, and then subjected to an appropriate quenching heat treatment and a subsequent aging heat treatment. 
     When the ingots are those obtained by ingot making, the soaking treatment eliminates or minimizes solidifying segregation of the ingots, by holding the ingots in a temperature range of typically from 1250° C. to 1300° C. for about 10 hours. The hot working may be performed while heating the work at a temperature of about 1000° C. or higher. 
     The steel obtained by subjecting an ingot to the soaking treatment and hot working is subjected to quenching so as to form a martensitic phase. The heating temperature in quenching, namely, the heating temperature before cooling is controlled within such a temperature range that the entire steel becomes an austenitic phase and that precipitates undergo solutionization. The steel according to the present invention having the chemical composition as above is preferably subjected to quenching performed at a heating temperature of 900° C. or higher, more preferably 950° C. or higher, and furthermore preferably 1000° C. or higher. However, quenching, if performed at an excessively high heating temperature, may cause the austenitic phase to coarsen, and this may impede the formation of finely divided martensite. From this viewpoint, the heating temperature in quenching is controlled to preferably 1150° C. or lower, more preferably 1100° C. or lower, and furthermore preferably 1050° C. or lower. 
     Cooling in quenching is preferably performed via air cooling or water cooling. Cooling in a temperature range down to 80° C., which is lower than the martensitic transformation start temperature Ms, is preferably performed at a cooling rate of 5° C./hr or more. The cooling rate in this temperature range is more preferably 10° C./hr or more, and furthermore preferably 20° C./hr or more. However, the cooling rate has a ceiling with respect to such large-sized steels and is about 100° C./hr or less. 
     The steel, in which the martensitic phase is formed in the above manner, has very high strength, but has low ductility and toughness, and thus requires an aging heat treatment so as to adjust balance between strength and toughness, where the aging heat treatment corresponds to a tempering heat treatment. 
     The aging heat treatment is performed in such a temperature range as not to increase the austenitic phase, namely, at a temperature lower than the Ac 3  transformation temperature. For the managing steel having the chemical composition as above, the upper limit temperature is 675° C. Accordingly, the temperature and holding time of the aging heat treatment are controlled in a temperature range lower than 675° C. so that the steel has a surface Vickers hardness of 400 Hv or more. 
     The aging heat treatment is not limited in temperature and holding time, except for the temperature upper limit. However, the aging heat treatment, typically when performed at a set temperature of 650° C., can stably give a sufficient hardness when performed for a holding time of 3 hours or shorter. To allow the aging heat treatment to proceed effectively at that temperature, the holding time is preferably at least one hour or longer, and is more preferably 1.5 hours or longer. 
     The present invention will be illustrated in further detail on operation and advantageous effects thereof, with reference to several examples below. It should be noted, however, that the examples are by no means intended to limit the scope of the present invention; and that various modifications and changes in design without deviating from the spirit and scope of the present invention described herein all fall within the technical scope of the present invention. 
     Examples 
     Steels A to I having chemical compositions given in Table 1 were heated and melted using a vacuum induction furnace, cast into 20-kg ingots, subjected to a soaking treatment at 1280° C. for 12 hours, and further subjected to hot forging to be processed into steels having a size of 60 mm in width by 15 mm thickness by L in length. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 Chemical composition* (in mass percent) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Steel 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Ni 
                 Cr 
                 Co 
                 Mo 
                 Ti 
                 Al 
                 N 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 A 
                 0.009 
                 0.018 
                 0.009 
                 0.005 
                 0.001 
                 11.9 
                 3.1 
                 9.8 
                 1.9 
                 2.0 
                 0.07 
                 0.001 
               
               
                 B 
                 0.006 
                 0.008 
                 0.010 
                 0.004 
                 0.001 
                 12.0 
                 3.1 
                 9.8 
                 1.0 
                 2.0 
                 0.09 
                 0.001 
               
               
                 C 
                 0.015 
                 0.056 
                 0.130 
                 0.008 
                 0.001 
                 11.3 
                 2.2 
                 11.3 
                 2.0 
                 2.8 
                 0.05 
                 0.008 
               
               
                 D 
                 0.012 
                 0.182 
                 0.094 
                 0.003 
                 0.001 
                 11.6 
                 2.5 
                 10.3 
                 0.8 
                 2.3 
                 0.06 
                 0.003 
               
               
                 E 
                 0.008 
                 0.087 
                 0.209 
                 0.009 
                 0.001 
                 12.1 
                 2.7 
                 10.8 
                 2.4 
                 2.5 
                 0.04 
                 0.002 
               
               
                 F 
                 0.014 
                 0.116 
                 0.165 
                 0.004 
                 0.002 
                 10.8 
                 2.9 
                 9.9 
                 1.9 
                 2.6 
                 0.05 
                 0.005 
               
               
                 G 
                 0.011 
                 0.143 
                 0.055 
                 0.005 
                 0.001 
                 11.0 
                 2.8 
                 10.1 
                 3.0 
                 1.7 
                 0.04 
                 0.003 
               
               
                 H 
                 0.013 
                 0.221 
                 0.245 
                 0.007 
                 0.001 
                 10.5 
                 3.4 
                 11.0 
                 1.6 
                 4.2 
                 0.02 
                 0.008 
               
               
                 I 
                 0.003 
                 0.014 
                 0.012 
                 0.002 
                 0.001 
                 12.1 
                 3.0 
                 10.3 
                 5.0 
                 2.0 
                 0.10 
                 0.002 
               
               
                   
               
               
                 *Remainder: iron and inevitable impurities excluding P, S. and N 
               
            
           
         
       
     
     The obtained steels were heated at 1000° C. for 15 minutes, subjected to quenching via water-immersion cooling, and each subjected to an aging heat treatment in a temperature range of from 650° C. to 700° C. for a time range of from 2 to 30 hours, under one of four conditions (a), (b), (c), and (d) as follows. 
     Aging Heat Treatment Conditions 
     (a) At a temperature of 650° C. for a holding time of 3 hours 
     (b) At a temperature of 650° C. for a holding time of 30 hours 
     (c) At a temperature of 700° C. for a holding time of 30 hours 
     (d) At a temperature of 650° C. for a holding time of 2 hours 
     Table 2 presents the steel type and the aging heat treatment condition each employed in Tests Nos. 1 to 12, together with the total content of Mo and Ti, and the ratio ([Mo]/[Ti]). 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Total content (in 
                   
                 Aging heat 
               
               
                 Test 
                   
                 mass percent) of 
                 Ratio 
                 treatment 
               
               
                 number 
                 Steel 
                 Mo and Ti 
                 ([Mo]/[Ti]) 
                 condition 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 A 
                 3.9 
                 0.95 
                 (a) 
               
               
                 2 
                 B 
                 3.0 
                 0.50 
                 (a) 
               
               
                 3 
                 B 
                 3.0 
                 0.50 
                 (d) 
               
               
                 4 
                 C 
                 4.8 
                 0.71 
                 (a) 
               
               
                 5 
                 D 
                 3.1 
                 0.35 
                 (a) 
               
               
                 6 
                 E 
                 4.9 
                 0.96 
                 (a) 
               
               
                 7 
                 F 
                 4.5 
                 0.73 
                 (a) 
               
               
                 8 
                 G 
                 4.7 
                 1.76 
                 (a) 
               
               
                 9 
                 H 
                 5.8 
                 0.38 
                 (a) 
               
               
                 10 
                 I 
                 7.0 
                 2.50 
                 (a) 
               
               
                 11 
                 I 
                 7.0 
                 2.50 
                 (b) 
               
               
                 12 
                 I 
                 7.0 
                 2.50 
                 (c) 
               
               
                   
               
            
           
         
       
     
     From the above-prepared steels, flanged round bar test specimens each including a gauge portion of 6 mm in diameter by 30 mm in length were prepared, subjected to high-temperature tensile tests at 500° C. in accordance with the method prescribed in Japanese Industrial Standard (JIS) G 0567:2012, to determine a 0.2% yield strength as a high-temperature strength. A sample, when having a 0.2% yield strength as measured of 750 MPa or more, is judged to surely have excellent high-temperature strength 
     From the above-prepared steels, full-size 2-mmV notch Charpy test specimens in conformity with JIS Z 2242:2005 were prepared, subjected to Charpy impact tests to measure Charpy impact values at 0° C., on the basis of which toughness was evaluated. The present invention is to improve toughness at room temperature of about 25° C. A sample, when having good toughness at 0° C., can be judged to also have good toughness at room temperature. On the basis of these, toughness was evaluated at 0° C. A sample, when having a Charpy impact value as measured of 10.0 J/cm 2  or more, can be judged to offer more excellent toughness as compared with conventional managing steels. The Charpy impact value is preferably 15.0 J/cm 2  or more, and more preferably 17.0 J/cm 2  or more. 
     The above-prepared steels, namely, steels after the aging heat treatment, were subjected to mirror-like finishing via mechanical polishing, followed by measurements of surface Vickers hardness at a load of 500 g. A sample steel, when having a surface Vickers hardness of 400 Hv or more, can be judged to have excellent surface hardness. 
     Evaluation results on the high-temperature strength, Charpy impact value, and Vickers hardness are presented in Table 3. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Test 
                 High-temperature 
                 Charpy impact 
                 Vickers hardness 
               
               
                 number 
                 strength (MPa) 
                 value (J/cm 2 ) 
                 (Hv) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 835 
                 12.3 
                 443 
               
               
                 2 
                 781 
                 17.6 
                 430 
               
               
                 3 
                 847 
                 15.8 
                 458 
               
               
                 4 
                 856 
                 10.2 
                 486 
               
               
                 5 
                 801 
                 21.9 
                 441 
               
               
                 6 
                 859 
                 11.4 
                 479 
               
               
                 7 
                 848 
                 13.6 
                 468 
               
               
                 8 
                 833 
                 6.8 
                 456 
               
               
                 9 
                 892 
                 8.2 
                 497 
               
               
                 10 
                 940 
                 5.9 
                 520 
               
               
                 11 
                 783 
                 5.8 
                 462 
               
               
                 12 
                 736 
                 9.7 
                 386 
               
               
                   
               
            
           
         
       
     
     These results give considerations as follows. Samples of Tests Nos. 1 to 7 are examples which meet all conditions specified in the present invention and are found to offer excellent high-temperature strength and to have better toughness. These samples are also found to have sufficiently high steel surface hardness after the aging heat treatment. 
     In contrast, samples of Tests Nos. 8 to 12 are comparative examples which do not meet one or more of the conditions specified in the present invention and offer at least one of high-temperature strength, toughness, and surface hardness at poor level. 
     Specifically, the sample of Test No. 8 is a sample using Steel G, which has a ratio ([Mo]/[Ti]) of the Mo content to the Ti content of out of the range specified in the present invention. This sample offers lower toughness even it has undergone an aging heat treatment under appropriate conditions. 
     The sample of Test No. 9 is a sample using Steel H, which has a total content of Mo and Ti of out of the range specified in the present invention. This sample offers lower toughness even it has undergone an aging heat treatment under appropriate conditions. 
     The sample of Test No. 10 is a sample using Steel I, which has a total content of Mo and Ti and a ratio ([Mo]/[Ti]) both out of the ranges specified in the present invention. This sample offers lower toughness even it has undergone an aging heat treatment under appropriate conditions. 
     The sample of Test No. 11 is a sample using Steel I, which has a total content of Mo and Ti and a ratio ([Mo]/[Ti]) both out of the ranges specified in the present invention. In addition, this sample has undergone an aging heat treatment for an excessively long holding time. In this sample, the aging heat treatment condition causes the sample to have lower toughness, although it does not so much affect the high-temperature strength and the surface hardness. 
     The sample of Test No. 12 is a sample using Steel I, which has a total content of Mo and Ti and a ratio ([Mo]/[Ti]) both out of the ranges specified in the present invention. In addition, this sample has undergone an aging heat treatment at an excessively high temperature for an excessively long holding time. This sample offers lower toughness, is in the state of over-aging, and has a high-temperature strength and a surface hardness not meeting the predetermined conditions (criteria).