HIGH TEMPERATURE FERRITIC STEEL

Abstract: An alloy steel having good formability, cyciic oxidation re­ sistance and creep strength at elevated temperatures above 1000°F and particularly above about 1500°F (816°C) after a fi­ nal anneal at 1850° to 2050°F (1010° to 1120°C), comprising 0.05% maximum carbon, about 2% maximum manganese, 1.0% to 2.0% silicon, 6% to 25% chromium, up to about 5% molybde­ num, with the sum of chromium and molybdenum being at least 8.0%, 0.05% maximum nitrogen, at least one of titanium, zirconium, tantalum and columbium in an amount equal to the stoichiometric equivalent of the carbon plus nitrogen contents, and containing at least 0.1% uncombined columbium, less than 0.5% aluminum and balance essentially iron.

Description: HIGH TEMPERATURE FERRITIC STEEL This invention relates to a ferritic steel having improved cyclic oxidation resistance and creep strength at elevated temperature. More particularly, in the form of cold rolled strip1 sheet, bar, rod and wire which has been subjected to a final anneal at 1850 to 2050"F (1010 to 11200C), a preferred steel of the invention having Åa ferritic microstructure exhibits the above properties by reason of purposeful addition of silicon, a carbide and nitride former, and columbium within critical limits. Control of aluminum to a low value confers excellent weldability and formability without sacrifice of other properties. A synergistic improvement in creep strength and improved cyclic oxidation resistance at elevated temperature results from the combination of a silicon addition within the broad range of 0.8% to 2.25%, addition of sufficient carbide and nitride former to combine with substantially all the carbon and the nitrogen, addition of a small amount of columbium substantially all of which will be uncombined as a result of the carbide and nitride former addition, and a final high temperature anneal. The combination of properties is achieved throughout a wide range of chromium levels, viz. from about 1% to about 25%, but a fully ferritic microstructure may not be obtained at chromium plus mclybdenum levels less than about 8%. The automotive industry is a large user of flat rolled ferritic stainless steels for engine exhaust components. A standard stainless steel for this purpose has a nominal composition of about 0.03% maximum carbon, about 0.25% manganese, residual phosphorus and sulfur, about 0.5% silicon, about 128 chromium, about 0.2% nickel, about 0.48 titanium, about 0.1% maximum aluminum, about 0.02% maximum nitrogen, and balance essentially iron. The present invention provides a substitute for the above stainless steel, having improved properties, not only for automotive exhaust components, but also for powder metal articles and welded articles. A steel having substantially improved elevated temperature strength and oxidation resistance, in comparison to the above standard steel, is disclosed in United States Patent 4,261,739. In broad ranges the steel of this patent consists essentially of, in weight percent, from about 0.018 to 0.068 carbon, about 1% maximum manganese, about 2% maximum silicon, about 1% to about 20% chromium, about 0.5 maximum nickel, about 0.5% to about 2% aluminum, about 0.01% to 0.05% nitrogen, 1.0% maximum titanium, with a minimum titanium content of 4 times the percent carbon plus 3.5 times the percent nitrogen, about 0.1% to 1.0% columbium, with the sum total of titanium plus columbium not exceeding about 1.2%, and remainder essentially iron. A preferred steel in accordance with this patent has a nominal composition of about 0.028 carbon, about 0.258 manganese, about 0.02% phosphorus, about 0.005% sulfur, about 0.5% silicon, about 12.0% chromium, about 0.208 nickel, about 0.02% nitrogen, about 0.38 titanium, about 0.68 columbium, about 1.2 aluminum, and balance essentially iron. Such a preferred steel exhibits optimum elevated temperature strength and oxidation resistance in the cold rolled form when it is subjected to a final anneal at 18500 to 2050QF. While this patent recognizes that aluminum in excess of about 1% can affect weldability adversely, relatively high aluminum levels, with a minimum of about 0.75%, must nevertheless be present in order to obtain the excellent elevated temperature oxidation resistance of this steel. Accordingly, the likelihood of poor weldability under some types of welding operations is present in the steel of this patent. The present invention constitutes a discovery that silicon can be substituted at least partially for aluminum and also partially for chromium, with a consequent improvement in weldability while at the same time retaining excellent oxidation resistance and creep strength at elevated temperature. An article entitled "Influence of Columbium on the 870"C Creep Properties of 188 Chromium Ferritic Stainless Steels" by J. N. Johnson, SAE Technical Paper Series, 810035, February 1981, reports tests on an 18% chromium steel containing molybdenum, titanium and columbium. In the test samples, silicon ranged from 0.08% to 0.74%, and uncombined columbium ranged from 0.118 to 0.58%. It was concluded on the basis of reported tests that a significant improvement in 8700C creep strength of 182 chromium steels was obtained with the combination of about 0.5% free (uncombined) columbium and a high final annealing temperature at 925 to 11500C (about 1700 to about 21000F). In these test samples, aluminum was absent except in one sample which contained 1.89% aluminum, 0.71% silicon, 0.35% titanium and no columbirn. This article contains no discussion regarding the effect of silicon or aluminum, other than reference to a Laves phase which, although primarily intermetallic compounds of iron-molybdenum or iron-columbium, may contain substitutional elements such as chromium, manganese and silicon. "Effect of Molybdenum on Creep Properties of a Ferritic 18Cr-Nb-Ti Steel for Catalytic Converters", J. D. Redmond et al, Journal of Metals, February 19, 1981, pages 19 - 25 reports the effect of molybdenum and columbium on creep-rupture properties of an 18% chromium steel. It is concluded that an additional strengthening mechanism in molybdenum-containing steels may result from the change in composition of the Laves phase where columbium decreases with increasing molybdenum contents. The displaced columbium is then available for further dispersion strengthening by carbide precipitation. Ferritic, chromium-containing steels containing one or more of aluminum, titanium, columbium, silicon or zirconium are disclosed in United States Patents 3,909,250; 3,782,925 and 3,759,705, and British Patent 1,262,588. These alloys, while exhibiting improved oxidation resistance at elevated temperature, nevertheless have poor creep strength at elevated temperature and possible weldability problems. NASA TN-D 7966 published in 1975, discloses modifications in 15% and 18% chromium ferritic steels wherein it was concluded that addition of 0.45% to 1.25% tantalum to a nominal 18% chromium, 28 aluminum, 1% silicon and 0.5% titanium steel provided the greatest improvement in fabricability, tensile strength and stress-to-rupture strength at 1800 F, along with oxidation resistance and corrosion resistance at elevated temperature. After cold rolling to final thickness, a final anneal at 1000C was conducted in the processing of these test alloys. An article by H. E. Evans et al, in Oxidation of Metals, Vol. 19, Nos. 1/2, 1983, pages 1 - 18, describes the influence of silicon on the oxidation resistance of nitrided austenitic stainless steels of nominal 2% chromium - 258 nickel composition. A series of such steels, also containing froim 0.005% to 0.050% carbon, 0.42% to 0.74% manganese, 1.44% to 1.56% titanium, and 0.05% to 0.21% columbium, was prepared with silicon levels ranging from 0.05t to 2.35%. cold rolled strips were nitrided at 14230K (2102"F) and tested for oxidation resistance at 11230K (15620F). It was found that chromium-rich oxide surface films developed in all cases, and the film thickness increased parabolically with time. The parabolic rate constant was at a minimum at 0.92% silicon. The reason for failure of higher silicon levels (about 1.5% to 2.35%) to improve oxidation resistance was postulated as being perhaps due to removal of silicon from solution by precipitation. The present invention constitutes a discovery that improvement in weldability can be combined with excellent cyclic oxidation resistance and creep strength at elevated temperature above 1000 F (538"C) and particularly above 1500"F (816"C) in a ferritic steel. This is achieved in a preferred ferritic steel by substitution of silicon format least part of the aluminum required in prior art steels having high oxidation resistance, by providing a relatively small content of uncombined columbium with reliance on titanium, zirconium, tantalum and/or columbium to combine with carbon and nitrogen, and by subjecting the ferritic steel to a final anneal at 18500 to 20500F (10100C to 11200C). While the cyclic oxidation resistance at elevated temperature of the steel of the present invention is slightly inferior to that of the previously mentioned United States Patent 4,261,739, creep strength of the present steel is slightly superior to that of said patent, and cyclic oxidation resistance and creep strength are substantially superior to that of the above-mentioned standard steel used for engine exhaust components which is commonly designated as Type 409. It is an object of the present invention to provide a substantially ferritic steel at all temperatures having a wide chromium range which exhibits the combination of excellent oxidation resistance and strength at elevated temperature together with excellent weldability and which at the same time contains a minimum of expensive alloying ingredients. According to the broadest aspect of the invention there is provided an alloy steel exhibiting good formability and improved cyclic oxidation resistance and creep strength at elevated temperatures above 1000cm (538"C) after a final anneal at 18500 to 20500F (1010 to 1120"C), consisting essentially of, in weight percent, 0.05% maximum carbon, about 2% maximum manganese, 1.0% to 2.0% silicon, 6% to 25% chromium, up to about 5% molybdenum, with the sum of chromium and molybdenum being at least 8%, 0.05% maximum nitrogen, at least one of titanium, zirconium, tantalum, and columbium, said titanium, zirconium, tantalum and columbium being present in an amount at least equal to the stoichiometric equivalent of the percent carbon plus the percent nitrogen, and containing at least 0.18 uncombined columbium, less than 0.5% aluminum, and balance essentially iron. A preferred ferritic steel within the above broad ranges which combines the further desirable properties of weldability and formability, consists essentially of, in weight percent, about 0.03% maximum carbon, about 1% maximum manganese, 1.0% to 2.0% silicon, 8% to 20% chromium, about 0.5% maximum molybdenum, about 0.03% maximum nitrogen, about 0.5% maximum titanium with a minimum titanium content of 4 times the percent carbon plus 3.5 times the percent nitrogen, about 0.3% maximum columbium with at least 0.10% uncombined columbium, less than 0.5% aluminum, and balance essentially iron. Uncombined columbium will be understood to mean that which is not combined with carbon and/or nitrogen. Reference is made to the accompanying drawing which is a graphic comparison of sag resistance as a function of silicon content at two different final annealing temperatures. As disclosed in the above-mentioned United States Patent 4,261,739, conventional final annealing temperatures for ferritic steels range from about 1400 to about 1700"F (760 to 925"C). As was the case in that patent, it has been found that a higher final anneal within the range of 1850 to 2050'F (1010 to 11200C) contributes significantly to improved elevated temperature creep strength in the steel of the present invention. Improvement in high temperature creep strength may be attributed to an increase in the final grain sizes, solid solution strengthening of the ferritic matrix, and the presence of carbide and nitride precipitates of titanium, zirconium, tantalum, and/or columbium which pin the grain boundaries, thus retarding the creep mechanism. A columbium-silicon rich Laves phase which improves creep strength, apparently develops at a lower columbium level than obtained in United States Patent 4,261,739 due to synergism with silicon. Surprisingly, cyclic oxidation resistance is also dramatically improved due to the higher silicon level either with or without a higher final anneal and either with or without a columbium addition. A broad maximum of 0.05% carbon and 0.05% nitrogen must be observed in order to maintain a fully ferritic structure and to minimize the amounts of the carbide and nitride forming element or elements needed to stabilize the steel. Preferably carbon and nitrogen are each restricted to a maximum of about 0.038. Manganese could be present for its strengthening effect, but a broad maximum of about 2%, and a preferred maximum of 1%, should be observed, since it does not form ferrite and may adversely affect the oxidation resistance of ferritic steels. Phosphorus and sulfur may be present in the usual residual amounts without adverse effect. Chromium may range between about 1% and 25% in order to obtain a desired level of corrosion and oxidation resistance at minimum cost, for a particular application. A preferred range of about 8% to about 20% chromium confers the properties usually associated with a ferritic stainless steel. It is a feature of the present invention that up to about 2% chromium is replaced by the purposeful silicon addition without loss of oxidation, especially cyclic, resistance. Molybdenum additions are permitted up to about 5% to promote a ferritic structure at all temperatures. It also improves corrosion resistance and high temperature creep strength. Silicon is essential within the broad range of 0.8% to about 2.25%, with a preferred range of about 1.0% to about 2.0%. This silicon addition at least partially replaces aluminum or higher chromium levels used in prior art ferritic steels to provide high temperature (above 1500 F) oxidation resistance, and the replacement of aluminum by silicon minimizes the detrimental effect of aluminum on weldability. Silicon is of course a ferrite former. Aluminum is restricted to a broad maximum of about 2%, and for improved weldability is preferably restricted to less than 0.5%. With titanium present, the nitrogen in the steel preferentially combines with titanium rather than aluminum, thereby avoiding the adverse effect of aluminum nitrides in causing porosity in weld areas. A carbide and nitride forming element is added in an amount at least equal to the stoichiometric equivalent of the carbon plus nitrogen contents. Titanium is preferred and, if used, is present in a minimum amount of 4 times the percent carbon plus 3.5 times the percent nitrogen. A broad maximum of 1.0% should be observed, and a preferred maximum of 0.58 should be observed with carbon and nitrogen each at a preferred maximum of 0.03%. When titanium, aluminum and columbium are present, titanium preferentially combines with nitrogen, and probably with carbon, although it is possible that some of the carbon may combine with columbium. The objective is to tie up as much as possible of the carbon and nitrogen with titanium or other carbide and nitride formers, leaving columbium present in uncombined form. Uncombined columbium is preferably used when chromium plus molybdenum is at least 88 and is limited to a broad maximum of 0.5%, and preferably to a maximum of 0.3%. At least 0.1% free or uncombined columbium is the minimum effective amount. For reasons explained above, the titanium addition permits the amount of total columbium addition to be minimized, which is advantageous from the standpoint of cost. The amount of uncombined columbium needed for increased creep strength at elevated temperature has been found to be relatively low, and as little as 0.10% and preferably about 0.20% uncombined columbium has been found to be effective for these purposes, due to the synergistic effect of the silicon addition. Nickel may be added in amounts up to about 5% where additional toughness is needed, if the level of ferrite formers is high enough to avoid excessive austenite formation, i.e., less than 10% austenite, and preferably less than 5%. Any one or more of the preferred ranges indicated above can be used with any one or more of the broad ranges for the remaining elements set forth above. A series of experimental heats of steels of the invention has been prepared and tested, along with comparative steels in which silicon or columbium are outside the ranges of the present invention. Comparative tests have also been run on Type 409 and on the steel of United States Patent 4,261,739. The compositions of these steels are set forth in Table I. Creep strength, as measured by sag resistance tests, is reported in Table II for 0.060 inch sheet at 1600 F, and in Table III for 0.045 inch sheet at 1500 F. It will be noted that several different final anneal temperatures were used, and the results show that a high temperature final anneal at 1850 to 20500F significantly improves the sag resistance and hence creep strength of the cold rolled sheet. Heats 6 and 7 in Table II exhibited improved creep strength after anneals at 19500F and 2050 F, respectively, in comparison to an anneal at 18500F. In contrast to this, Heat 8, containing 0.44% silicon but otherwise within the composition limits of the steel of the invention, exhibited inferior sag resistance after an anneal at 19500F, in comparison to an anneal at 18500F. A representative steel of U.S. Patent 4,261,739, was inferior to Heat 7 after a final anneal at 19500F. Referring to Table III, Heats 9 and 10, which contained 1.94% and 2.428 silicon respectively, but no columbium, were inferior to Heats 4 and 5 (containing columbium) in sag resistance at the annealing temperature of 19500F. Referring to the drawing it is noted that a series of non columbium bearing steels exhibited a substantial increase in sag resistance as silicon was gradually increased, when the steels were subjected to a final anneal at 1950"F. On the other hand when the same steels were subjected to a final anneal at 1650 , the sag resistance decreased with increasing silicon contents. In both cases the effect is substantially linear. Table IV summarizes mechanical properties of Heats 4 and 5 under different final annealing conditions. It will be noted that the yield strength and tensile strength of samples subjected to annealing at 19500F are slightly lower than those annealed at 16500F but the elongation values are somewhat higher. Table V summarizes Olson cup values of Gas Tungsten Arc autogenous weldments of a steel of the invention and three comparative steels. It will be noted that the formability and ductility of the weld areas in the steel of the invention were relatively high. Heat 10, containing 2.42% silicon, exhibited low values, thus establishing criticality of the maximum of 2.25% silicon. Heat 11, a steel of U.S.P. 4,261,739, was inferior to the steels of the invention in weldability due to its aluminum content of 0.91%. Table VI contains cyclic oxidation resistance test results conducted at 1700"F while Table VII contains similar test results conducted at 17500F. The use of cyclic oxidation resistance tests rather than static tests is believed to simulate more closely the particular application of the steel of the present invention for engine exhaust components. Accordingly, improved cyclic oxidation resistance is of greater significance than static oxidation resistance. It is evident from Tables VI and VII that Heats 4 and 5, these being steels of the invention, have cyclic oxidation resistance substantially superior to that of Heat 12 which is the conventional Type 409 alloy currently used for engine exhaust components. On the other hand, Heat 11 which is a steel of United States Patent 4,261,739, is definitely superior to all the steels which were tested. From the above description it is evident that the invention includes within its scope alloy steel strip, sheet, plate, bar, rod and wire annealed at 18500 to 2050"F having the above broad composition which exhibits improved cyclic oxidation resistance and creep strength at temperatures above 1000"F. Good results are obtained at temperatures of at least 1500F and up to about 16000F or higher in the higher chromium alloys, i.e. where chromium is from about 6% to 25%, chromium plus molybdenum total at least 8%, and at least 0.1% uncombined columbium is present. In one embodiment of the steel of the invention chromium ranges from about 1% to about 8%, and no uncombined columbium is present. In another embodiment, which is ferritic at all temperatures, chromium ranges from about 6% to about 25%, with the sum of chromium plus molybdenum being at least 8%, and at least 0.18 uncombined columbium is present. An embodiment exhibiting an optimum combination of properties consists essentially of 0.03% maximum carbon, about 1% maximum manganese, about 1.4% silicon, about 11% chromium, 0.03% maximum nitrogen, about 0.5% maximum titanium with a minimum titanium content of 4 times the percent carbon plus 3.5 times the percent nitrogen, about 0.2% uncombined columbium, less than 0.5% aluminum, and balance essentially iron. The invention further provides a welded article for high temperature service fabricated from alloy steel strip, sheet, plate, bar, rod and wire, which has been subjected to a final anneal at 1850 to 20500F and exhibiting improved formability, cyclic oxidation resistance and creep strength at temperatures above 1000"F, the steel having the broad composition set forth above. Improved cyclic oxidation resistance and creep strength at temperatures of at least 15000F are obtained in the ferritic steel alloy where chromium ranges from about 68 to 25%, chromium plus molybdenum total at least 8%, and at least 0.18 uncombined columbium is present. Automotive exhaust components for high temperature service are provided by the invention fabricated from alloy steel having the broad composition set forth above and exhibiting improved cyclic oxidation resistance and creep strength at temperatures above 1000"F. Improved cyclic oxidation resistance and creep strength at temperatures of at least 1500F are obtained in ferritic automotive exhaustscomponents of the invention where chromium ranges from about 6% to 25%, chromium plus molybdenum total at least 8%, and at least 0.1% uncombined columbium is present. The invention also provides forged, cast and powder metal articles having the broad composition set forth above. Improved cyclic oxidation resistance and creep strength at temperatures of at least 15000F are obtained in ferritic articles of the above type where chromium ranges from about 6% to 25%, chromium plus molybdenum total at least 8%, and at least 0.1% uncombined columbium is present. The steel of the present invention achieves the objective of providing improved cyclic oxidation resistance and creep strength at elevated temperature, in comparison to the conventional Type 409, together with improved weldability and creep strength as compared to the steel of United States Patent 4,261,739 with a reduction in expensive columbium as allowed by the discovery of the unique synergistic effect introduced by silicon when present in the alloys of this invention. TABLE I Compositions- Weight Percent Heat No. C Mn P S Si Cr Ni Al Ti N Cb 1 .023 .27 .023 .016 1.18 6.49 .19 .026 .35 .014 2 .022 .28 .021 .016 1.18 8.21 .19 .027 .35 .012 3 .025 .26 .022 .016 1.13 9.88 .18 < .020 .21 .012 4* .019 .28 .023 .010 1.09 10.27 .18 .028 .31 .016 .15 5* .020 .28 .022 .010 1.10 10.19 .19 .030 .33 .018 .29 6* .020 .40 .020 .005 1.03 11.27 .43 .024 .22 .015 .19 7* .019 .40 .020 .005 1.53 11.27 .43 < .020 .18 .015 .19 8 .019 .40 .020 .005 .44 11.27 .43 .024 .24 .015 .19 9 .015 .27 .021 .011 1.94 11.04 .20 .052 .41 .016 10 .018 .27 .021 .010 2.42 11.06 .20 .049 .43 .014 11** .030 .33 .016 .011 .70 11.66 .22 .91 .44 .016 .52 12*** .014 .28 .019 .002 .58 11.15 .17 .060 .41 .012 13 .015 .26 .022 .011 1.45 11.08 .20 .047 .35 .015 * Steels of the invention ** Steel of U.S.P. 4,261,739 *** Type 409 TABLE II Sag Resistance - 1600 'F 0.060" Sheet Sag Deflection - Inch Heat No. %Si 20 hrs. 100 hrs. 1850 OF Final Anneal 6* 1.03 0.065 0.160 7* 1.53 0.058 0.135 8 0.44 0.283 0.887 1950 F Final Anneal 6* 1.03 0.048 0.112 7* 1.53 0.029 0.069 8 0.44 0.591 > 1.350 Steel of USP 4,261,739 0.05 0.10 2050 F Final Anneal 6* 1.03 0.027 0.061 7* 1.53 0.028 0.058 8 0.44 0.258 0.742 * Steels of the invention T A B L E III Sag Resistance - 15000F Heat No. % Si Sag Deflection - Indi 20 hrs. 100 hrs. 1650 F Final Anneal 3* 1.13 .136 .262 9* 1.94 .225 .474 10 2.42 .328 .561 13* 1.45 .193 .420 19500F Final Anneal 4** 1.09 .031 .052 5** 1.10 .045 .067 9 1.94 .072 .128 10 2.42 .051 .107 13 1.45 .083 .143 Average of duplicate samples - samples .045" sheet except 9 and 10 which were 0.040" sheet. * annealing treatment outside of invention ** Steels of the invention TABLE IV Mechanical Properties Final Anneal 0.2% Y.S. U.T.S. % Elong. Hardness Heat No. F ksi ksi in 2" HRB 4 1650 39.7 67.2 32.5 75.5 4* 1950 36.1 61.6 34.5 74 5 1650 48.6 75.6 24 81.5 5* 1950 37.3 82.3 25.5 79.5 * Steels of the invention TABLE V Olsen Values - Welds Heat No. Orientation Cup Height - In. 3 Root .368 Face .358 5* Root .335 Face .353 10 Root .215 Face .318 11 Root .203 Face .181 * Steels of the invention TABLE VI Cyclic Oxidation Resistance 1700'F Weight Gain in mg/cmê Cycles Heat No. 142 274 373 613 948 1 6.89 10.51 12.54 20.94 41.59 2 .45 .69 .82 .98 1.18 3 .26 .35 .38 .44 .50 5* .38 .52 .65 .73 .85 9 .42 .60 .76 .88 1.01 10 .46 .66 .70 .96 1.06 11 .16 .15 .17 .19 .23 12 .83 - 1.21 2.22(after 752 cycles) Average of duplicate samples * Steels of the invention TABLE VII Cyclic Oxidation Resistance - 17500F Weight Gain in Cycles Heat No. 59 240 1 30.70 80.33 2 3.05 8.06 3 3.02 4.10 4* .34 .69 5* .38 .69 9 .39 .70 10 .38 .70 11 .30 .31 12 8.59 28.50 Average of duplicate samples except Ht. 11 * Steels of the invention

Claims: Claims: 1. Annealed ferritic steel exhibiting improved cyclic oxidation resistance and creep strength at temperatures of at least 1500F (8160C), after a final anneal at 1850 to 2050"F (1010 to 1120"C), consisting essentially of, in weight percent, 0.05% maximum carbon, about 2% maximum manganese, 1.0% to 2.0% silicon, 6% to 25% chromium, up to about 5% molybdenum, with the sum of chromium and molybdenum being at least 8%, 0.05% maximum nitrogen, at least one of titanium, zirconium, tantalum and columbium, with said titanium, zirconium, tantalum and columbium being present in an amount at least equal to the stoichiometric equivalent of the percent carbon plus the percent nitrogen, and containing at least 0.1% uncombined columbium, less than 0.5% aluminum, and balance essentially iron. 2. The steel claimed in claim 1, consisting essentially of about 0.03% maximum carbon, about 1% maximum manganese, about 1.0% to about 2.0% silicon, about 88 to about 20% chromium, about 0.58 maximum molybdenum, about 0.03% maximum nitrogen, about 0.5% maximum titanium with a minimum titanium content of 4 times the percent carbon plus 3.5 times the percent nitrogen, about 0.3% maximum columbium with at least 0.10% uncombined columbium, less than 0.5% aluminum, and balance essentially iron. 3. The steel claimed in claim 1 or 2, including up to about 5% nickel. 4. The steel claimed in claim 1 or 2, wherein said uncombined columbium is at least about 0.2%. 5. A ferritic steel as claimed in claim 1, exhibiting good formability and improved cyclic oxidation resistance and creep strength at temperatures of at least 1500"F (816rC) after a final anneal at 1850 to 20500F (1010 to 11200C), consisting essentially of, in weight percent, 0.03% maximum carbon, about 1% maximum manganese, about 1.4% silicon, about 118 chromium, 0.03% maximum nitrogen, about 0.5% maximum titanium with a minimum titanium content of 4 times the percent carbon plus 3.5 times the percent nitrogen, about 0.28 uncombined columbium, less than 0.52 aluminum, and balance essentially iron. 6. Alloy steel strip, sheet, plate, bar, rod and wire annealed at 18500 to 20500F which exhibits improved oxidation resistance and creep strength at elevated temperatures of at least 1500 (816"C), said steel consisting essentially of, in weight percent, 0.05% maximum carbon, about 2% maximum manganese, 1.0% to 2.0% silicon, 6% to 25% chromium, up to about 5% molybdenum, with the sum of chromium and molybdenum being at least 8%, 0.05% maximum nitrogen, at least one of titanium, zirconium, tantalum and columbium, with said titanium, zirconium, tantalum and columbium being present in an amount at least equal to the stoichiometric equivalent of the percent carbon plus the percent nitrogen, and containing at least 0.1% uncombined columbium, less than 0.5% aluminum, and balance essentially iron. 7. Welded article for high temperature service fabricated from alloy steel strip, sheet, plate, bar, rod and wire which has been subjected to a final anneal at 18500 to 20500F (10100 to 11200C) and exhibiting improved formability, cyclic oxidation resistance and creep strength at temperatures of at least 1500 F (8160C), said steel consisting essentially of, in weight percent, 0.05% maximum carbon, about 2% maximum manganese, 1.0% to 2.0% silicon, 6% to 25% chromium, up to about 5% maximum molybdenum, with the sum of chromium and molybdenum being at least 8%, about 0.058 maximum nitrogen, a carbide and nitride forming element chosen from the group consisting of titanium, zirconium, tantalum, columbium, and mixtures thereof, said element being present in an amount at least equal to the stoichiometric equivalent of the percent carbon plus the percent nitrogen, and containing at least 0.1% uncombined columbium, less than 0.5% aluminum, and balance essentially iron. 8. Automotive exhaust components for high temperature service fabricated from an alloy steel which has been subjected to a final anneal at 18500 to 2050"F (1010 to 1120"C) and exhibiting improved cyclic oxidation resistance and creep strength at temperatures of at least 1500"F (816"C) said steel consisting essentially of in weight percent, about 0.05% maximum carbon, about 28 maximum manganese, 1.0% to 2.0% silicon, 6% to 25% chromium, up to about 5% maximum molybdenum, with the sum of chromium and molybdenum being at least 8.0% about 0.05% maximum nitrogen, a carbide and nitride forming element chosen from the group consisting of titanium, zirconium, tantalum, columbium and mixtures thereof, said element being present in an amount at least equal to the stoichiometric equivalent of the percent carbon plus the percent nitrogen, and containing at least 0.1% uncombined columbium, less than 0.5% aluminum, and balance essentially iron. 9. Forged, cast and powder metal articles consisting essentially of in weight percent, about 0.05% maximum carbon, about 2% maximum manganese, 1.0% to 2.0% silicon, 6% to 25% chromium, up to about 5% maximum molybdenum, with the sum of chromium and molybdenum being at least 8.0%, about 0.05% maximum nitrogen, a carbide and nitride forming element chosen from the group consisting of titanium, zirconium, tantalum, columbium, and mixtures thereof, said element being present in an amount at least equal to the stoichiometric equivalent of the percent carbon plus the percent nitrogen, and containing at least 0.18 uncombined columbium, less than 0.5% aluminum, and balance essentially iron.

Inventors: GORMAN, MARK D.
Applicants: ARMCO ADVANCED MATERIALS CORPORATION
Application year: 1984
Main classification: C22C38
IPC codes: C22C38