US3759705A - Chromium containing alloy steel and articles - Google Patents

Abstract

ALLOY STEEL SUITED TO ELEVATE-TEMPERATURE APPLICATIONS AND SHEET, STRIP, AND THE LIKE FASHIONED OF THE SAME OF READY FORMABILITY AND COMPARATIVELY LOW COST CONTAINING ABOUT 16% TO 19% CHROMIUM, SILICON ABOUT .5% TO ABOUT 1.6% TO ABOUT 2.7% WITH THE SUM OF THE SILICON AND ALUMINUM CONTENTS NOT EXCEEDING 3.9%, CARBON NOT OVER .1% AND PREFERABLY LESS THAN .07%, MANGANESE UP TO ABOUT 1% BUT PREFERABLY LESS THAN .50% NITROGEN UP TO ABOUT .05%, ABOUT .15% TO ABOUT .8% TITANIUM AND/OR ABOUT .15% TO ABOUT 1.25% COLUMBIUM, AND REMAINDER SUBSTANTIALLY ALL IRON.

Description

United States Patent 01 hoe 3,759,705 CHROMIUM-CONTAINING ALLOY STEEL AND ARTICLES David L. Chalk, Monroe, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio No Drawing. Continuation-impart of abandoned application Ser. No. 748,971, July 31, 1968. This application June 10, 1971, Ser. No. 151,892

Int. Cl. C22c 37/10, 39/02 US. Cl. 75-424- 9 Claims ABSTRACT OF THE DISCLOSURE Alloy steel suited to elevated-temperature applications and sheet, strip, and the like fashioned of the same of ready formability and comparatively low cost containing about 16% to 19% chromium, silicon about .5 to about 1.6% to about 2.7% with the sum of the silicon and aluminum contents not exceeding 3.9%, carbon not over .1% and preferably less than .07%, manganese up to about 1% but preferably less than .50%, nitrogen up to about .05%, about .15% to about .8% titanium and/or about .15 to about 1.25% columbium, and remainder substantially all iron.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my copending application Ser. No. 748,971 dated July 31, 1968, entitled Chromium-Containing Alloy Steel and Articles and now abandoned.

As a matter of introduction, my invention is concerned with the straight chromium grades of stainless steel of low carbon content.

One of the objects of the invention is the provision of a stainless steel which is comparatively inexpensive, readily lending itself to the production of sheet, strip and the like at minimum mill cost.

Another object is the provision of such a steel and sheet, strip, and other mill products fashioned of the same which readily lend themselves to a variety of forming and fabricating operations such as pressing, bending, drawing, deep-drawing, shaping, trimming, welding and brazing as in the production of a host of articles of ultimate use.

A further object is the provision of an alloy steel which is suited to elevated-temperature duty in the presence of an oxidizing atmosphere or other corrosive atmosphere over prolonged periods of use with minimum attention or replacement as in furnace parts, combustion chambers, combustion nozzles, and the like.

Other objects of my invention in part will become apparent and in part particularly pointed to in the description which follows.

In short, therefore, my invention resides in the combination of ingredients and the relation between the same, as well as in the converted and fabricated products fashioned of the same, all as more particularly described herein, the scope of the application of which is set out in the claims at the end of the specification.

BACKGROUND OF THE INVENTION As an aid to a better understanding of certain features of my invention, it may be noted at this point that there are several grades of stainless steel available to hightemperature applications. Thus, for example, there is the well-known AISI Type 430, nominally analyzing carbon .12% max., manganese 1.00% max., phosphorus, .040% max., sulphur .030% max., silicon 1.00% max., chromium 14.00% to 18.00%, and remainder substantially iron. This is a general-purpose stainless steel which is nonhardenable, that is, it is neither hardened by cold-working nor by heat-treatment. It is particularly suited to use in the form of decoratve trim, automobile trim, and other similar applications where appeal to the eye at reasonable cost is desired. In industry, because of its corrosion-resisting qualities and comparatively low cost, it is employed in nitric acid tanks, in annealing baskets, and other applications where certain corrosive atmospheres are encountered.

Another stainless steel widely accepted in industry is the Type 442. This steel analyzes carbon .20% max., manganese 1.00% max., phosphorus .040% max., sulphur 030% max., silicon 1.00% max., chromium 18.00% to 23.00%, and remainder substantially iron. Although the steel works reasonably well, it unfortunately is characterized by poor weldability. The steel, however, is well suited to furnace parts, combustion chambers, nozzles, and the like, where oxidation under elevated-temperature use is directly encountered. Because of the higher chromium content, the steel is somewhat more costly than the Type 430. But, like the Type 430, the steel is not hardenable by thermal treatment or otherwise.

A further known and used steel is the Type 446, this analyzing carbon ..20% max., manganese 1.50% max., phosphorus .040% max., sulphur. 030% max., silicon 1.00% max., chromium 23.00% to 27.00%, and remainder substantially iron. This steel, like the Type 442, does not readily lend itself to welding. Neither is it hardenab-le by heat-treatment, nor, indeed, by cold-working. The steel, however, is resistant to scaling at elevated temperatures, resistant to sulphur-bearing gases, and resistant to oxidizing atmospheres. Although somewhat more costly than the Type 442 because of the higher chromium content, the steel nevertheless enjoys rather wide usage in industrial applications where resistance to corrosion and resistance to oxidation at elevated temperatures is an essential requirement.

In spite of the many desirable characteristics found in known steels, many are too expensive for wide application throughout the art. Moreover, the poor welding characteristics which are found in several of these steels further limit their field of use.

An object of the present invention, therefore, is to overcome some of the shortcomings of the steels of the prior art and provide an alloy steel, more particularly a stainless steel, Which readily lends itself to a variety of common working and forming operations, such as cutting, blanking, drawing, bending, and the like; which is readily weldable in the worked or formed condition; and which is resistant to scaling and corrosion at elevated temperatures, being well suited to a variety of applications where corrosive and/or oxidizing atmospheres are encountered at elevated temperatures, which steel and various converted products fashioned thereof, particularly the flatrolled products, are less costly than the steels of the prior art.

SUMMARY OF THE INVENTION Referring now to the practice of my invention, I provide a chromium-silicon-aluminum-titanium steel which in broad composition essentially consists of about 16% to about 19% chromium, about .5% to about 1.4% silicon, with aluminum about 1.6% to about 2.7 carbon not exceeding .1% and preferably less than .070% and especially less than .06%, manganese up to about 1% but preferably less than 50% and, better yet, not over .45 with the further ingredient titanium in the amount of about .15% to about .8% and/or columbium in the amount of about .15% to about 1.0% or even about 1.25%, and remainder substantially all iron. In my steel it is the ingredient titanium rather than the ingredient columbium which customarily is employed for reasons more fully given hereafter. The steel of course additionally contains phosphorus, sulphur and nitrogen, the phosphorus being present in amounts up to about .040%, the sulphur in amounts up to about .030%, and the nitrogen in amounts up to about .05 preferably to only .03%. of rather restricted composition balance, the steel is wholly ferritic and in no sense hardenable by heat treatment.

The ingredients essential to my steel are chromium, silicon, aluminum and titanium and/or columbium. And in my view the amounts of these ingredients are in every sense critical, for where they are preserved, the desired combination of properties is had; where they are not, one or more of the desired properties is lost. To a somewhat lesser extent, criticality exists in the permissible amounts of the ingredients carbon, manganese and nitrogen, and, to an even lesser extent, of the ingredients phosphorus and sulphur.

In my steel the chromium content must be at least about 16%, for otherwise the desired resistance to oxidation and corrosion at elevated temperatures is not had. But where the chromium exceeds about 19%, hot-working becomes more difiicult, and the cost of converting the steel from ingot and slab to the common flat-rolled products is substantially increased. Moreover, with the higher chromium content the cost of the metal itself is increased.

The silicon content of my steel is in the amount of about .5% to about 1.4%. At least about .5% silicon is required in order to achieve fluidity of the metal in furnace and ladle, and significant freedom from segregation effects. Moreover, a silicon content of at least .5 and, better yet, at least about .8% or about 1%, is required for significant resistance to oxidation at elevated temperatures. A silicon content not exceeding 1.4% is required in my steel, however, for with a higher silicon content a tendency toward embrittlement, with sacrifice of workability in the mill and formability by the customer fabricator, is noted.

And while an aluminum content of at least about 1.6%, in combination with the silicon present, is desired in order to achieve good resistance to scaling at elevated temperatures, aluminum in an amount exceeding about 2.7% is not acceptable because of difficulties in teeming and in ingot production; actually, for a best combination of properties, there is employed an aluminum content not exceeding about 2.3%. With an excessive aluminum content, objectionable quantities of aluminum oxide are formed in tapping and in teeming. And these oxides are entrapped in the surface of the metal as it rises in the mold, causing folds and subcutaneous defects, the removal of which is both time-consuming and costly. Moreover, with an excessive aluminum content, as with an excessive silicon content, there is a loss of ductility and the metal works with difiiculty and with increased cost.

For a desired combination of strength, weldability, formability and heat resistance, the sum of the silicon and aluminum contents must not exceed 3.9%. Actually, a best combination of properties is had where the sum of these two ingredients does not exceed 3.7% or even 3.5%.

And finally, in my steel, the ingredient titanium or, in some instances, columbium with or without titanium is essential. Titanium in the amount of at least about .15% is necessary, but titanium exceeding about .6% and certainly in amount exceeding about .8% is not desired. For a best combination of heat-resisting characteristics and fabricability, the amount of titanium is at least about four times the sume of the carbon and nitrogen contents, or as a practical matter, since a nitrogen analysis is not easily run in commercial production, the titanium content is at least about six times the carbon content. Titanium, however, should not exceed some fifteen times the carbon content, certainly not over about twenty times the carbon content, for an excess of titanium not only is unnecessarily costly, but also lends an undesired hardness and stiffness to the metal, thereby decreasing formability.

In my steel, the titanium combines with the carbon present in the metal and also the nitrogen, giving titanium carbides or titanium nitrides or titanium cyanonitrides, which efiectively remove the ingredients carbon and nitrogen from the steel. Removal of these ingredients is important, for both carbon and nitrogen are austeniteforming elements and it is only with their removal that the desired ferritic structure is achieved. This result can also be had by the use of columbium rather than titanium. Although columbium is not quite as effective as titanium, it nevertheless may be employed with benefit.

Where by reason of the particular milling process employed as, for example, in vacuum melting or in vacuum treatment, that is, degassing subsequent to melting, most of the carbon and nitrogen are eliminated in the processing of the metal; the requirement for titanium and/or columbium is minimized. The amount of titanium required, however, is at least about .15 with a maximum of about .8% and the amount of columbium, where employed, is at least about .15% but not over about 1.25%. Where both titanium and columbium are used together, the sum total of the two is at least about .15 with the two not exceeding about 1.25 Inasmuch as titanium is considerably less costly than columbium, it is titanium that customarily is employed, and this without the columbium addition.

The ingredient carbon generally is viewed as an impurity and is present in amounts not exceeding .1%, and for best properties, less than .070%. With a higher carbon content, ductility and workability suffer. For a best combination of results the carbon is less than 06%, preferably not exceeding .05%, this to achieve maximum ductility, formability and weldability with a minimum tendency to form austenite. Although the carbon content of my steel may closely approach zero, such as .Ol% or even .005 it customarily amounts to at least .02% or even .03%, this in a measure contributing to fluidity in furnace, ladle and mold.

In my steel nitrogen customarily does not exceed .03%, certainly it is not more than about .05%, even with airmelting. A higher nitrogen content is not acceptable because it increases the hardness of the metal at rolling temperatures, tends to form austenite and requires undue amounts of titanium for eifective elimination, all at increased cost.

Manganese may be present in my steel, this in amounts up to about 1%. For best results, however, the manganese content is maintained below .50% and, even better, not over .45 Similarly, any nickel present, this as an impurity, is in amount less than about Inasmuch as both nickel and manganese are austenite-forming ingredients, I find it important for a best combination of properties to maintain both at a minimum, with nickel not exceeding .35 and manganese less than 50%, preferably not over .45 as noted, especially whenever the carbon, another austenite-former, approaches .l%. The ingredients phosphorus and sulphur commonly present in stainless steel as impurities are in amounts up to about 040% phosphorus and up to about 030% sulphur. While greater amounts of these two ingredients may be present for certain applications, in my best steel the amounts of these ingredients are limited as noted.

As suggested above, my stainless steel conveniently is melted in the electric arc furnace, but where desired it may be melted in the vacuum furnace or, indeed, melted in the former and remelted in the latter in accordance with known and used techniques. The steel handles well in the furnace, it teems well, and the molds readily strip from the ingot. The steel can also be continuously cast with success.

The metal works well in the mill, this at commonly used temperatures, as in the conversion of the ingots into billet, bar and flat-rolled products. Moreover, and of particular consequence, the metal works well in the cold-mill, as in the production of sheet and strip, this with maximum reduction and minimum requirement for intermediate anneal. A short routing through the mill, with minimum consumption of time and use of anneal ing equipment, and consequent minimum handling, results. This I attribute to the wholly ferritic character of the metal promoted in large part by the presence of titanium and/or columbium and the limited chromium content.

The steel in the form of plate, sheet, strip, or the like, or in the form of bar and wire, is well suited to the production of a wide variety of products, apparatus and equipment, or parts therefor, employing common forming and fabricating operations, such as pressing, bending, drawing, cutting, shaping and trimming. Moreover, the metal readily lends itself to welding and brazing without hardening upon cooling, this in substantial measure being due to the titanium and/or columbium taking carbon out of solution and preventing the formation of martensite. The steel and various products fashioned of the same are well adapted to high-temperature applications where oxidizing or corrosive atmospheres are encountered as, for example, in furnace parts, combustion chambers, nozzles, and the like, as well as in more severe applications customarily requiring the higher chromium and substantially more expensive steels of the prior art such as the Types 442 and 446, which steels are more costly in the ingot and more difiicult and expensive in processing through the mill.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Now my steel in broad composition essentially consists of the ingredients chromium, silicon, aluminum and titanium (with columbium substituted for titanium in whole or in part in certain applications) in which some latitude is allowed for each of these ingredients as noted above. A best combination of properties, however, is enfioyed only where these ingredients are present in more limited andrestricted amount. Thus, one preferred steel according to my invention essentially consists of about 17% to about 19% chromium, about 1% to about 1.4% or even about .5% to about 1.4% silicon, about 1.6% up to about 2.3% and in any event not over about 2.7% aluminum with the sum of the silicon and aluminum contents not over about 3.9%, better yet, not over 3.7% and for a best combination of properties not over 3.5%, about .15% or .2% up to about .6% titanium, with carbon less than .07%, especially about .02% to about .06%, and remainder substantially all iron. In this steel, for best scaling resistance, the titanium content amounts to at least about six times the carbon content. Manganese may be present in amounts up to about 1% or, better yet, not over .45%, and nickel up to about 35%, or something less than .50%. Incidental amounts of phosphorus, sulphur and nitrogen are present, as noted above. The steel is wholly ferritic and enjoys a combination of low cost in the ingot, low cost in processing through the mill, and yet good weldability in fabrication and good resistance to oxidation and corrosion in elevated-temperature applications.

A further preferred steel of somewhat more limited composition essentially consists of about 17% to about 19% chromium, about .8% to about 1.2% silicon, about 1.6% or even about 1.7% to about 2.3% aluminum, about .2% to about 5% or .6% or even .8% titanium, carbon about .02% to about 06% but in any event not exceeding .l%, and remainder substantially all iron. The amount of titanium is at least about four times the sum of the carbon and nitrogen contents or at least about six times the carbon content. Titainum, however,

should not exceed twenty times the carbon content for best formability. In this steel, the manganese content amounts to about .1% to less than .50% or,better yet, not over .45%. And nickel, present as an impurity, is in an amount less than .50% and preferably in amount not exceeding .35 Such a steel enjoys a maximum combination of beneficial properties, all at minimum cost.

A still further steel essentially consists of about 16% to about 18% chromium, about .5% to about .8% or even to about 1% silicon, aluminum about 1.7% to about 2.3% or even about 1.6% 'to about 2.7%, with titanium about .15% to about .6%, more particularly about .2% to about .5 or to about .6%, carbon up to about .06% and preferably about .03% to about .06%, and remainder substantially all iron. Here again the amount of titanium should be at least four times the sum of the carbon and nitrogen contents, especially at least about six times but not over twenty times the carbon content for best results. While manganese may be present in amounts up to about 1%, it preferably is present in the amount of about .1% to less than .50% or preferably not over .45%. And, here again, nickel, where present, is in an amount less than .50%, not ex ceeding .35 for best results.

The various preferred steels of my invention readily lend themselves to a combination of hot-working and cold-working, with a minimum number of steps in routing through the mill, and with consequent savings in cost over the processing that must be used on presently available straight chromium heat-resisting stainless steels. Moreover, the steels are wholly ferritic, and readily weldable with an absence of undue hardening as a result of the welding operation as commonly encountered in the known ferritic high-temperature steels, i.e., Types 430, 442 and 446.

As particularly illustrative of the heat-resisting quality .of my steel, this as compared with the well-known and widely used Types 442 and 446, both of substantially greater cost, I give below in Table 1(a) the chemical compositions of fourteen steels, seven according to my invention. In Table I(b) there is presented the oxidation resulting from heating eleven of these steels (four according to the invention) for hours in still air at dilfering temperatures ranging from 1500 F. to 2200 F.

TABLE I(a).-CHEMICAL COMPOSITION OF FOURTEEN STAINLESS STEELS Percent Si Cr .Al Ti N l Steels enjoying a best combination of properties. 2 Steels according to invention.

Samples of eleven of the steels of Table I(a) were maintained for 100 hours in still air at different temperatures ranging from 1500 F. up to 2000 F. for one group of samples and up to 2200 F. for the remaining group. Actually, the temperatures employed for the various groups of samples spanned the range from 1500 F. to 2200 F. by 100-degree increments. The oxidation encountered at the various temperatures of test is expressed as the gain in weight per unit of area and reported in terms of milligrams per square inch in Table -I(b) below.

TABLE I(b).-OXIDATION TEST RESULTS ON ELEVEN OF THE STEELS OF TABLE I(a) IN STILL AIR FOR 100 HOURS (Weight gain/unit area, rugs/sq. in. at temp. shown) Heat 442 F. F. F; F. F. F. F. F.

Type 442 4. 2 6. 3 12. 17. 2 33. 2 Type 446. 8 5. 1 10. 17. 4 18. 2 Code 27 13. 2 23. 5 45. 5 Code 28 6. 1 13. 5 28. 0 Code 29 3. 7 5. 7 11. 9 Code 30 2. 7 7. 5 88. 4 Code 31 1 3. 4 6. 1 ll. 5 Code 33--. 12. 8 26. 0 57. 0 Code 38--. 13. 3 21. 9 38. 7 13. 5 23. 4 46. 0

l Steels enjoying a best combination of properties. 1 Steels according to invention.

While the steel of Type 442 commonly is recommended for service in air up to 1800 F. and the steel of Type 446 for service in air up to 2000 F., it clearly appears from the test data presented in Table I(b) above that the steel of my invention is superior to both at temperatures up to 1800" F. Note that the steels of Types 442 and 446 at 1800 F. suffer a gain in weight of 17.2 and 17.4 rugs/sq. in. respectively after 100-hour treatment at temperature. Contrasted with that gain, the steel of interest is only increased in weight by 5.7 and 6.1 mgs./sq. in. for the three examples given above (Codes 29 and 31). It will be seen that the present steel enjoys at 1800 F. about the same resistance to oxidation as the steel of Type 442 at only 1600" F. (6.3 mgs./sq. in.).

It also will be seen that while the steel of Type 446 at 2000 F. sustains a gain in weight of 53.8 mgs./sq. in. in 100-h0ur treatment, the illustrative steels of interest (Codes 29 and 31) respectively suffer a gain in weight of only 20.6 and 23.5 mgs./sq. in. And even at 2100 F. and 2200 F., the gain in Weight of the illustrative steels (Codes 29, 31 and 767) is about the same as that reached by the steel of Type 446 with treatment at only 2000 F.

dition and in the condition following welding, but without anneal or other treatment subsequent to the welding operation. And while the steels of Codes 28 and 38 evidenced excessive weight gains (3.9 and 4.2 mgs./sq. in in the unwelded and welded conditions respectively for Code 28 and 10.3 and 10.4 mgs./sq. in. for the Code 38), the steels according to my invention, namely Codes 29, 31 and 578, gained only 3.6, 3.4 and 3.9 rngs./sq. in. respectively in the unwelded condition. In the welded condition, only the steels of Codes 31 and 578 were tested, these with respective weight gains of only 2.8 and 3.7 rugs/sq. in.

The mechanical properties of the steels of Table I(a) are given below in Table I(c). These tests were performed on samples of cold-rolled strip of .050" thickness, annealed at 1650 F. for one minute and pickled in a nitrichydrofiuoric acid bath. The properties of yield point in kilopounds per square inch (k.s.i.), percent yield point elongation, ultimate tensile strength in k.s.i., percent elongation in 2", and Rockwell hardness on the B-scale are reported as the average values of longitudinal tests performed on duplicate samples.

TABLE I(c).-MECHANICAL PROPERTIES OF THE STEELS OF TABLE I(a) Si plus Her d- Al, per- Y.P., Y.P.E., U.1.S., Percent ness cent 'Ii/C k.s.i. percent k.s.i. E. in 2 RB 1 Steels enjoying a best combination of properties.

1 Steels according to invention.

The gain in Weight comes to 23.1 and 27.0 rugs/sq. in. for the steels of Codes 29 and 31 at the 2100 F. treatment and only 58.7 and 33.0 mgs./sq. in. at the 2200" F. treatment as compared to the gain of 53.8 mgs./ sq. in. for the Type 446 steel at 2000 F. The steel 767 sulfers a gain of only 21.5 mg s./sq. in. at 2200 F.

While the steel of Code 30 enjoys many of the desired characteristics of the steel according to my invention, the resistance to scaling at 2000 F. and above leaves something to be desired, for at that temperature the steel suffers a weight gain of 168 mgs./sq. in. as against only 20.6 and 23.5 rugs/sq. in. respectively for the steels of Codes 29 and 31. This shortcoming in oxidation resistance I attribute to the comparatively low titanium content of only .25 with ratio of titanium to carbon of only 3.7 to 1, as against the desired 6 to 1 minimum had with the steels of Codes 29 and 31 (8.5 to 1 for Code 29 and 7.8 to l for Code 31). Certain of the steels reported in Table I(a) were subjected to a further test, that is, exposure at some 1750 F. to 2000 F. both in the unwelded con- In comparing the mechanical properties of the steels given above, it will be immediately seen that the steel of Type 442 is singularly deficient in strength; its strength is exceeded by all six of the specific examples according to my invention (some 53.9, 60.4, 53.9, 60.4, 61.0 and 60.4 k.s.i. yield point strength for my steels in the order presented, as against 46.5 k.s.i. for the steel of Type 442). While the steel of Type 446 is possessed of greater strength than the specific examples of my steel, the ductility is lacking (only 22.5% elongation in 2" for Type 446 as compared to 31.5%, 29.5%, 31.5%, 29.5% and 31.2% for the respective examples of my steel). And note that while the several illustrative examples of that steel are no harder than the steels of Types 442 and 446, it is the steels with the ratios of titanium to carbon of some 8 to 1, 10 to 1 or 12 to 1 and the lower sums of the silicon and aluminum contents, 1.80% to 3.17%) which enjoy maximum softness and minimum stiffness with best formability (steels 29 and 31, 4504 and 4505 with respective hardnesses of Rockwell B 85.0, 89.2, 85.0 and 89.2). The

steels 578 and 767 with the highest ratios of titanium to carbon contents (15.5 and 16.0 respectively) and highest sums of the silicon and aluminum contents (3.45% and 3.36% respectively) reveal greatest hardness (Rockwell B 92.1 and 90.1 respectively), thus pointing to the excessive hardness and stiffness, with loss of formability, where there is had a titanium-to-carbon ratio much above 12 or 15 to 1, certainly where it is above to 1, and the sum of the silicon and aluminum contents exceeds 3.5%, more especially where the sum exceeds 3.7% and certainly where it exceeds 3.9%.

In my steels, therefore, it is seen that there is had a good combination of strength and ductility not fully enjoyed by the steels of the prior art.

Olsen cup values taken on samples of the Types 442 and 446, as well as Codes 29, 4504 and 4505, this revealing the comparative deep-drawing properties, further establish the superiority of the steel of my invention. While the steels of Types 442 and 446 have Olsen cup values of .335 and .270 inches, two examples of my steel (Codes 29 and 4504) have values of .355 and one (Code 4505) the value of .340 inches.

All of the above steels were found to be ferritic with an equi-axed grain structure, this being somewhat coarser for my steel than the steel of Type 446.

My steel, moreover, enjoys short-time elevated temperature properties which are superior to the steel of Type 442 and equal to those of the steel of Type 446 at temperature of 1100 F. and more. Actually, the yield strength of my steel is superior to both of the prior steels at temperatures of 1100 F. While the better of the two prior steels (Type 446) is characterized by short-time yield strength of 22,500 p.s.i. at a temperature of 1100" F., my steel has a yield strength of 27,500 p.s.i. at that temperature. While the yield figures are not available for the 1200 F. testing, the tensile strength of my steel is found to be 28,300 p.s.i. while that of the Type 446 is 23,000 p.s.i.

Thus, in conclusion, it will be seen that I provide in my invention a stainless steel in which there are achieved the various objects and advantages hereinbefore set out. The steel is strong, ductile and, because of its superior oxidation resistance at elevated temperatures, is better suited to the wide variety of applications commonly fulfilled by the known AISI Types 442 and 446, applications where there are encountered oxidizing and corrosive media at elevated temperatures. While my steel is well suited to duty up to some 1800 F., the steel in which a best combination of properties is had, as illustrated by the steel of Code 29, is well suited to applications up to 2000 F. or oven 2100 F. or 2200 F. Moreover, the steel of my invention readily lends itself to conversion in the mill by way of a short routing, with maximum reductions and minimum interruptions for intermediate anneal, all at minimum cost.

Moreover, the metal in the form of sheet, strip and other flat-rolled products lends itself to a wide variety of forming and fabricating operations. The metal is wholly ferritic and readily weldable, this without an increase in hardness upon cooling from welding temperatures.

Inasmuch as many embodiments may be made of the steel of my invention, and various changes may be made in the several specific embodiments set out above, it is to be understood that all matter described herein is to be interpreted by way of illustration and not by way of limitation.

I claim:

1. Alloy steel characterized by good cold-formability together with improved resistance to oxidation at temperatures up to some 1800 F. to 2200 F. or more, and consisting essentially of about 16% to about 19% chromium, about .5% to about 1.4% silicon, about 1.6% to about 2.7% aluminum with the sum of the silicon and aluminum contents not over 3.9%, carbon not exceeding .1%, manganese up to about 1%, nitrogen up to about .05

10 ingredient of the group consisting of about .15 to about .8% titanium and about .15 to about 1.25% columbium, and being at least about four times the sum of the carbon and nitrogen contents, and remainder iron.

2; Alloy steel characterized by good cold-formability together with improved resistance to oxidation at temperatures up to some 1800 F. to 2200" F. or more, and consisting essentially of about 17% to about 19% chromium, about .5% to about 1.4% silicon, about 1.6% to about 2.7 aluminum with the sum of the silicon and aluminum contents not over 3.7%, carbon less than .070%, manganese up to about 1%, nitrogen up to about .05 about .15 to about .6% titanium and at least about six times the carbon content, and remainder iron.

3. Alloy steel characterized by good cold-formability together with improved resistance to oxidation at temperatures up to some 1800 F. to 2200 F. or more, and consisting essentially of about 17% to about 19% chromium, about .8% to about 1.2% silicon, about 1.7% to about 2.3% aluminum, nickel not exceeding .35 about .02% to about .06% carbon, about .1% to less than 50% manganese, nitrogen up to about .05%, about .2% to about .6% titanium and at least about six but not over about twenty times the carbon content, and remainder iron.

4. Alloy steel characterized by good cold-formability together with improved resistance to oxidation at temperatures up to some 1800 F. to 2200 F. or more, and consisting essentially of about 16% to about 18% chromium, about .5 to about 1% silicon, about 1.6% to about 2.7% aluminum, carbon up to about .06%, manganese up to about 1%, nitrogen up to about .05 about .15% to about .6% titanium and being at. least about six but not over about twenty times the carbon content, and remainder iron.

5. Alloy steel characterized by good cold-formability together with improved resistance to oxidation at temperatures up to some 1800" F. to 2200' F. or more, and consisting essentially of about 16% to about 18% chromium, about .5 to about 1% silicon, about 1.7% to about 2.3% aluminum, nickel not exceeding .35 carbon about .03 to about .06%, manganese about .1% to less than .50%, nitrogen up to about .05 about ..2% to about .5 titanium and being at least about four times the sum of the carbon and nitrogen contents, and remainder iron.

6. Flat-rolled or drawn products characterized by good cold-formability together with improved resistance to oxidation at temperatures up to some 1800" F. to 2200 F. or more, and consisting essentially of about 17% to about 19% chromium, about .8% to about 1.2% silicon, over 1.5% to about 2.3% aluminum, carbon not exceeding .1%, manganese less than .50%, nitrogen not exceeding about .03%, ingredient of the group consisting of about .2% to about .8% titanium and about .15% to about 1.00% columbium and being at least about six times the carbon content, and remainder iron.

7. Flat-rolled products characterized by good cold-formability together with improved resistance to oxidation at temperatures up to some 1800 F. to 2200 F. or more, and consisting essentially of about 16% to about 18% chromium, about .5 to about 1% silicon, about 1.7% to about 2.3% aluminum, about 03% to about .06% carbon, nitrogen up to about .05 about .2% to about .6% titanium and being at least about six times but not over about twenty times the carbon content, and remainder Iron.

8. Bar, rod and wire characterized by good cold-formability together with improved resistance to oxidation at temperatures up to some 1800 F. to 2200 F. or more, and consisting essentially of about 16% to about 19% chromium, about .5 to about 1.4% silicon, about 1.6% to about 2.7% aluminum with the sum of the silicon and aluminum content not exceeding 3.9%, carbon less than .07%, nitrogen not exceeding .03%, ingredient of the 11 group consisting of about .15% to about .8% titanium and about .15 to about 1.00% columbium and being at least about six but not over about twenty times the carbon content, and remainder iron.

9. Bar, rod and wire characterized by good cold-formability together with improved resistance to oxidation at temperatures up to some 1800 F. to 2200 F. or more, and consisting essentially of about 17% to about 19% chromium, about .5 to about 1.4% silicon, about 1.6% to about 2.3% aluminum, carbon less than .07%, nitrogen not exceeding about .03%, about .2% to about .6% titanium and being at least about six times the carbon content, and remainder iron.

References Cited UNITED STATES PATENTS 1,467,562 9/1923 Armstrong 75l24 5 1,508,032 9/1924 Smith 75l24 2,288,660 7/ 1942 Kantzow 75l26 F 2,848,323 4/1958 Harris 75l26 D 2,905,577 9/1959 Harris 75l26 D 3,615,367 10/1971 Tanczyn 75-126 D 75l26 D, 126 F US. Cl. X.R.

UNITED STATES PATENT OFFICE GERTIFICATE 0F QQRRECTWN Patent 759,105 Dated jeptemher l 8 "l9? 2 n I Inventor(s) David L. Chalk It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, The Abstract of the Disclosure should read as follows:

Alloy steel suited to elevated-temperature applications and sheet, strip, and the like fashioned of the same of ready formability and comparatively low cost containing about 16% *to 19% chromium, silicon about 5% to about 11%, aluminum flabout 1.6% to about 2.7% with the sum of the silicon and fialuminum contents not exceeding 3.9%, carbon not over 1% [and preferably less than 07%, manganese up to about 1% but preferably less than 50%, nitrogen up to about .O5%, about .1575 to about .8% titanium and/or about .15% to about 1.25% columbium, and remainder substantially all iron.

Signed and sealed this 26th day of March 197b,.

(SEAL) Attest:

EDWARD MFLETCHER,JR. c. MARSHALL DANN Atte sting Officer Commissioner of Patents FORM PO-WSO (10-69) USCOMMDC wand,

A U.S GOVERNMENT PRINTING OFFICE: I969 0-3iS-334.