US3928087A - Method of strengthening low carbon steel and product thereof - Google Patents

Abstract

Deep drawing steel is strengthened by alloy-nitrogen precipitation strengthening to a minimum average yield strength of 50 ksi. A deoxidized, low carbon steel sheet stock or article formed therefrom, containing from about 0.02 to 0.2% titanium in solution, from about 0.025 to 0.3% columbium in solution, from about 0.025 to about 0.3% zirconium in solution, alone or in admixture, is heat treated at 1,100*-1,350*F in an atmosphere containing ammonia in an amount insufficient, at the temperature and time involved, to permit formation of iron nitride.

Description

United States Patent [1 1 Hook [ METHOD OF STRENGTHENING LOW CARBON STEEL AND PRODUCT THEREOF [75] Inventor: Rollin E. Hook, Dayton, Ohio [73] Assignee: Arrnco Steel Corporation,

Middletown, Ohio 221 Filed: May 30, 1974 211 Appl. No.: 474,514

Related US. Application Data [62] Division of Ser. No. 306,390, Nov. 14, 1972, Pat. No.

[52] US. Cl. 148/36 [51] Int. Cl. C21D 9/48 [58] Field of Search 148/36, 12 C; 75/123 .1, 75/124 [56] References Cited UNITED STATES PATENTS 3,721,587 3/1973 Allten et al. 148/36 51 Dec. 23, 1975 Elias et a1. Elias et a1.

Primary Examiner-W. Stallard Attorney, Agent, or Firm-Melville, Strasser, Foster &

Hoffman ABSTRACT I about 0.025 to 0.3% columbium in solution, from about 0.025 to about 0.3% zirconium in solution, alone or in admixture, is heat treated at l,100l ,350F in an atmosphere containing ammonia in an amount insufficient, at the temperature and time involved, to permit formation of iron nitride.

2 Claims, No Drawings METHOD OF STRENGTHENING LOW CARBON STEEL AND PRODUCT THEREOF This is a division of application Ser. No. 306,390 filed Nov. 14, 1972, now U.S. Pat. No. 3,847,682.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of strengthening stamped or deep drawn articles after forming. Currently available high strength sheet stock cannot be extensively formed directly by stamping or deep drawing because of its limited ductility and drawability. The method of this invention involves the novel concept of producing stamped or deep drawn parts from a low strength, deep drawing quality steel, and subsequently strengthening the parts by alloy-nitrogen precipitation strengthening. Cold rolled and annealed sheet stock can also be strengthened in the same manner before forming to attain higher yield strength than has hitherto been possible.

2. Description of the Prior Art The hardening of steel surfaces by heat treating in an ammonia-containing atmosphere to form an iron-nitrogen austenitic structure which is transformed by quenching to a martensitic structure having high surface hardness, has been practiced for many years. Prior art nitriding practices are described in ASM Metals Handbook, 1948 edition, pages 697-702, and the references cited therein. Under present practice, nitriding is performed on particular types of steels (such as Nitralloy type, austenitic stainless steels, SAE and similar steels) in the machined and heat treated condition to provide great wear resistance, retention of surface hardness at elevated temperature and resistance to certain types ofcorrosion. Reference may also be made to U.S. Pat. No. 3,399,085 issued Aug. 27, 1968, to H. E. Knechtel and H. H. Podgurski, relating to nitriding of a Nitralloy type steel.

The nitriding of steels containing nitride-forming alloying elements is discussed in Transactions AIME, volume 150 (1942), pages 157-171, by L. S. Darken. The nitriding of iron-aluminum alloys in an ammoniahydrogen atmosphere is described in Transactions Met. Soc. AIME, volume 245 (1969), pages l595l602 and in Transactions Mel. Soc. AIME, volume 245 (1969), pages l6031608, by H. H. Podgurski et al.

A comparison of nitrided iron-aluminum alloys and iron-titanium alloys is given in Transactions Met. Soc. AIME, volume 242 1968), pages 2415-2422, by V. A. Phillips and A. V. Seybolt. It was concluded in this article that an alloy containing 1% titanium developed substantially higher surface hardening than a 1% aluminum alloy due to the very small particle size of the titanium nitride which was formed, less than about Angstroms. It was suggested that the nitride particles must be within a range of about 10 to 40 Angstroms or smaller in diameter, in order to produce maximum hardening. The particle size of aluminum nitrides in the aluminum-bearing alloy was substantially coarser.

Boron, Calcium, Columbium and Zirconium in Iron and Steel, by R. A. Grange et 211., John Wiley and Sons, lnc., publishers, pages 173-179, discusses columbium as an alloying element in nitriding steels. It was concluded therein that columbium readily combines with nitrogen at temperatures above 750F if present in excess of the amount required to combine with all the carbon to increase the surface hardness of the steel. U.S. Pat. No. 3,671,334, issued June 20, 1972 to J. H. Bucher et a1, discloses a medium-carbon columbiummodified renitrogenized steel containing less than about 0.02% total of aluminum, zirconium, vanadium and titanium. Sufficient free nitrogen is added to the molten steel before teeming to impart strain aging properties thereto. In the hot rolled or cold rolled condition the steels have a yield strength of 50 to ksi, which is increased to a range of 70 to ksi after straining and aging.

U.S. Pat. No. 3,673,008, issued June 27, 1972, to M. E. Wood, discloses carbonitriding of a columbium-containing steel by heating an article formed from the cold worked steel to a temperature above the strain recrystallization temperature but below the A, critical temperature of the steel in a carbonitriding atmosphere containing hydrocarbons and ammonia.

The purpose of the Wood patent is to prevent strain induced grain coarsening by addition of from 0.006 to 0.018% columbium to a carbon steel containing from 0.05 to 0.15% carbon. The carbonitriding is stated to produce a hard, wearresistant case on the formed article. A person skilled in the art would conclude that the undesirable ferritic grain coarsening is inhibited by columbium-carbide precipitates. Such carbide precipitates must exist in a fine dispersion in order to provide resistance to grain coarsening. A fine dispersion is obtained in steels of the type disclosed in Wood because of the high carbon content which significantly lowers the A critical temperature (to about 1,333F). The behavior of low carbon steels to which the present invention relates would be considered non-analogous to that of medium or high carbon steels since low carbon steels have a substantially higher A, critical temperature, and columbium-carbide particles formed by following the process of the Wood patent would be expected by a person skilled in the art to be too coarse to inhibit ferritic grain growth.

The case hardening of relatively massive parts by nitriding, as practiced conventionally, is distinguishable from the concept of strengthening hot rolled or cold rolled low carbon sheet stock. The prior art suggestions of addition of alloying elements such as columbium for the purpose of case hardening or prevention of grain coarsening, would not provide a person skilled in the art with a teaching which would lead to the solution of the problem of increasing the strength of stamped or deep drawn parts formed from deep drawing quality steel sheet stock.

Despite the above background, no successful approach has as yet been made to the problem of increasing the strength of deep drawn parts or stampings formed from sheet stock without loss of the necessary ductility and drawability of the steel required to make the part. Present practice is still governed by the fundamental precept that enhancement of strength is accomplished only by a sacrifice in ductility, drawabilty, and- /or stretch-ability. To the best of applicant's knowledge the prior art has never previously suggested the application of alloy-nitrogen precipitation strengthening to a deep drawing quality, low carbon steel. As is well known, when such steel in sheet form is subjected to drawing or stamping, the finished article will have areas of low yield strength where the part has not been work hardened by straining or deformation, and will have other areas of high yield strength hardened by straining or deformation in forming the article. Typically the yield strength of unstrained areas is the same as or slightly higher than the yield strength of the steel sheet from which the part was formed, i.e. about 2030 ksi. The areas which have been work hardened may have yield strengths ranging upwardly from about 30 ksi to about 80 or 100 ksi, depending upon the severity of straining or deformation. When such article is subjected to heat treatment. the strained areas exhibit recrystallization and excessive grain growth, with consequent undesirable softening.

The prior art approach, illustrated by the above-mentioned Bucher patent, which utilizes strain-aging by carbon or nitrogen to strengthen a formed article, cannot be applied where deep drawing properties are required. Steels which can be strengthened more than a negligible amount by strain-age hardening inherently possess relatively high strength and low ductility in the hot rolled or cold rolled condition and hence cannot be subjected to deep drawing. Moreover, the gain in strength resulting from strain-age hardening is relatively small, on the order of about l ksi, and virtually no strengthening in unstrained areas of parts formed from such steels can be achievedv SUMMARY It is a principal object of the present invention to provide a process for producing articles by drawing or stamping from a deep drawing quality steel ofa specific composition and subsequently to treat the articles after forming by a nitriding treatment which enhances the strength thereof.

It is a further object of the invention to provide a cold rolled sheet stock, and a method for production thereof, in the thickness range of 0.02 to 0.09 inch having a yield strength of at least about 70 ksi.

The present invention provides a method of increasing the yield strength of a low carbon steel sheet stock of deep drawing quality, and articles formed therefrom, by adding to a killed, drawing quality steel 21 nitrideforming alloying element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution at room temperature is from about 0.02 to 0.2%, columbium in solution is from about 0.025 to 0.3%, and zirconium in solution is from about 0.025 to 0.3% by weight, reducing the steel to final thickness, annealing if an article is to be formed therefrom, and heating the steel, or articles formed therefrom, in an atmosphere comprising ammonia and hydrogen at a temperature between l,l00 and 1,350F for a period of time sufficient to cause reaction of the nitride-forming element with the nitrogen of the ammonia to form small, uniformly dispersed nitrides. The concentration of ammonia in the annealing atmosphere ranges between about 2 and I0 percent by volume and must be insufficient, at the temperature and time involved, to permit formation of iron nitride or an iron nitrogen austenite.

Unlike prior art nitriding practice, the alloy-nitrogen precipitation strengthening process of the present invention avoids the formation of an iron-nitrogen austenitic structure by heating at a higher temperature, for a shorter time, and with a lower ammonia concentration in the atmosphere than a typical nitriding operatlon.

Moreover, no quench is applied after the heat treatment. contrary to conventional practice in nitriding. The present invention does not obtain or seek the properties desired in nitriding other types of steels, viz. high Carbon About 0,002 to 0.0l57( Nitrogen Up to about 0.012

Aluminum Up to about 0.08

Manganese About 0.05 to 0.6

Sulfur Up to about 0035 Oxygen Up to about 0.01

Phosphorus Up to about 0.0]

Silicon Up to about 0.015

Titanium About 0.02 to 0.2 in solution Columhium About 0.025 to 0.3 in solution Zirconium About 0.025 to 0.3 in solution lron Remainder, except for incidental impurities In the above composition all percentages are by weight, and the titanium, columbium and zirconium may be present singly or in admixture, the sum total not exceeding about 0.3 percent.

DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred composition is as follows:

Carbon Less than about 0.010%

Nitrogen About 0.004

Aluminum About 0.02 to 0.04 (total) Manganese About 0.05 to 0.6

Sulfur Up to about 0.035

Oxygen Up to about 0.0l

Phosphorus Residual Silicon Residual Titanium About 008 to (H0 ltotal) and either Columbium About 0.03 to 0.06 (total) Zirconium About 0.03 to 0.06 (total) Iron Remainder, except for incidental impurities As indicated previously, the ammonia concentration in the annealing atmosphere is maintained at a concentration sufficiently low, at the temperature and time involved, to avoid the formation of iron nitride or an austenitic structure, thereby avoiding high surface hardness, low toughness and embrittlement. Preferably, the atmosphere in which the heat treatment is conducted contains from 3 to 6% ammonia by volume, with the balance hydrogen. An inert gas, such as nitrogen or argon may be substituted for part of the hydrogen, provided proper adjustments are made in the ratio of ammonia to hydrogen contents so that the formation of iron nitride or an iron-nitrogen austenite does not occur.

It has been found that a heat treatment conducted in this atmosphere within the temperature range of l ,l00 to l,350F, preferably l,l00 to 1,300F, results in relatively rapid diffusion of nitrogen into the steel and reaction of the nitrogen with the nitride-forming alloying element to form small, uniformly dispersed nitride particles, probably ranging in size between about 20 and about 30 Angstroms. A time of one to three hours at temperature is ordin When heat treating a drawn or stamped article hav ing strained and work hardened areas, it is preferred to provide both titanium and columbium. or both titanium and zirconium, as the nitride-forming elements. The presence of at least about 0.025% columbium or zirconium (as determined by analysis at room temperature) prevents the recrystallization and consequent softening of the strained areas of the formed article when subjected to heat. Thus. in the preferred practice of the invention as applied to deep drawn or stamped articles, the yield strength in the unstrained areas is increased to a minimum of 50 ksi and the yield strength of the strained areas is maintained or even increased.

Where the method of the invention is applied to the strengthening of a cold rolled sheet stock in the unformed condition having a thickness ranging between about 0.02 inch and about 0.09 inch, preferably 0.02 to 0.06 inch, the yield strength will be increased to at least about 70 ksi, a value never previously attainable in a low carbon steel. Such a product has sufficient formability to permit fabrication into articles of various types (other than deep drawn) wherein bends are mainly involved.

According to studies reported by L. S. Darken and R. W. Gurry in Physical Chemistry of Metals, McGraw- Hill Book Company, Inc. (1953), pages 372-395, the maximum ammonia concentrations which can be used within the temperature ranges of l ,l to 1,350F and still avoid the formation of iron nitride, are as follows:

arily sufficient.

comes insufficient. in addition. the nitrides formed at l.350F and above are coarser in size and hence contribute less strengtheriliigeffect. Finally. when heat treating deep drawn or stamped articles haxing cold worked areas. a temperature above l,350F should be avoided because of excessive grain growth and consequent softening.

Within a preferred temperature range of l.l00 to 1.300F and a preferred ammonia concentration of 3 to 6 percent by volume, the heat treatment time can range between about 1 hour and about 2 hours. Under such conditions nitrogen diffuses to a depth sufficient to increase the average yield strength of material having an as-received yield strength of ksi to a minimum of ksi.

The thickness of steel sheet treated in accordance with the process of the invention does not constitute a limitation, although its greatest utility resides in the treatment of hot rolled thin bar ranging in thickness from about 0.06 inch to 0.25 inch and cold rolled strip material ranging in thickness from about 0.02 inch to about 0.09 inch. Thin cold rolled material (i.e. up to about 0.06 inch) heat treated at about l,300F for l to 2 hours will be strengthened by alloy-nitrogen precipitation in finely dispersed form substantially all the way through the thickness and will achieve a yield strength of about ksi. Thicker hot rolled material can be heat treated at somewhat lower temperatures, in which case it will be nitrided only part way through, but to a depth 30 sufficient to obtain an average yield strength in excess of 50 ksi and up to about ksi.

Experimental data are presented in the tables below for a series of heats of steels containing titanium, co-

ll00F about 10 4' ammonia agoui flmmontfl lumbium, zirconium or mixtures thereof. for purposes 21 OUl. I ammonia I I 50F ham 2% ammonia 35 of comparison a typical drawmgquahty aluminumkilled steel sheet containing no nitride-forming alloy other than aluminum has been included.

TABLE I Composition 3 Percent by Weight Example Heat C N 0 Mn Ti Cb Zr 1 B10900? 3 .040 .015 .0029 .30 06st m0) .035tin sol) 2 V8452 .0042 .0036 .004i .019 .3l .004 .10) 3 W453 .0043 .0045 .0022 .019 .31 .031 .1 i0 4 800162V .0044 .0057 .0012 .01 l .4 .030 .l2 5 22503 50v .002 .0036 .019 .32 .047 .005 .066 e 22s077s-v .0042 .003 l .0l 1 .33 .029 .049 .039 7 V7963 .005 5 .0050 001s .017 .30 i2 .l'-J s nausea-v .004 .0030 .015 .33 .040 0x4 .063 9 22609l4-V .003 .0045 .0l4 .32 .044 .078 05s The above values represent equilibrium between ammonia-hydrogen mixtures and solid phases of the iron nitride system at one atmosphere pressure.

It will be apparent from the above information that the temperature and ammonia concentration are inter dependent and should be varied inversely with respect to one another in the practice of the present process. Similarly, time is a further interdependent variable also inversely proportional to the temperature and ammonia concentration. It has been found that the rate of diffusion of the nitrogen into the steel is the controlling factor since the reaction rate of nitrogen with the alloying elements is relatively rapid. Below a temperature of l l00F the rate of diffusion is so slow that the time required at temperature is commercially uneconomical. Above 1,350F. the ammonia concentration must be kept so low that the driving force for diffusion be- Example I was a mill-produced, aluminum-killed drawing-quality heat which was not subjected to vacuum degassing, but was mill hot-rolled and laboratory cold-rolled. Examples 2, 3 and 7 were laboratory-produced vacuum melted heats, subjected to laboratory hot-rolling and cold rolling. Examples 4, S, s. 8 and 9 were mill-produced, vacuum degassed heats. aluminum-killed, mill hot-rolled and laboratory cold-rolled.

In Table II below, properties and nitrogen contents at various stages of processing are reported for representative beats. in the As-Received condition all samples were cold-rolled to 0.040 inch thickness and fully annealed. Samples were also subjected to 20 percent cold reduction to 0.032 inch thickness after annealing in order to simulate strained and/0r deformed areas of drawn articles.

Heat Treated in 39: NH 977r H. By Volume Example 1 Y5. T.S. Condition ksi ksi 7(Elong. Y.P.E. liN

Y.S. T.S. Condition ksi ksi %Elong. /tY.P.E. %N

As-Received 20.7 44.8 40.5 0 .0031 l100F1 hr. 43.8 55.7 29.5 2.5 .0093

-2 hrs. 68.0 74.1 19.0 3.6 1200F-1 hr. 748 82.1 15.0 2.7 .032 -2 hrs. 89.6 98.7 13.0 2.0 .067 l300F-l hr. 73.8 81.9 17.0 2.5 .080

2 hrs. 76.5 86.8 16.0 2.1 Cold-Rolled 63.4 68.2 5.0 0.0 .0031 1100F-1 hr 642 69.3 14.0 3.7 -2 hrs. 84.0 88.3 11.0 3.7 1200F-1 hr. 82.8 86.6 11.0 3.0 -2 hrs. 94.3 98.7 14.0 2.8 1300F 1 hr. 82.4 88.6 16.0 2.8

-2 hrs. 81.4 88.6 16.0 2.8 Example 7 Y5. T.S. Condition ksi ksi %Elong. %Y.P.E.

As-Received 23.1 48.5 37.5 0.0 l100F-1 hr. 32.7 49.8 37.5 1.5 -2 hrs. 41.5 54.2 32.0 4.1 |200"F-1 hr. 54.1 62.7 24.0 5.1 -2 hrs. 66.6 71.5 19.0 3.8 1300F-1 hr. 62.9 73.6 17.5 3.3 -2 hrs. 68.3 79.8 18.0 3.7 ColdRolled 20% 68.7 73.2 3.5 0.0 ll00F-l hr. 577 63.3 13.0 0.0 -2 hrs. 65.2 69.8 14.5 2.2 |200F-1 hr. 64.6 70.5 12.0 2.0 -2 hrs. 86.1 90.2 9.5 2.5 |300F-1 hr. 80.5 87.0 12.0 3.9 -2 hrs. 83.9 92.8 13.0 2.8

Broke near or outside gage mark.

Table 11 indicates that the aluminum-killed drawing quality steel of sample 1 showed very little strengthening when nitrided under the same conditions as the remaining steels of examples 2-7. The moderate increase in yield strength is due primarily to the return of the yield point elongation. In addition, some strengthening occurs as a result of nitrogen in solid solution in the steel.

A more direct comparison of the strengthening effect of titanium to that of aluminum is obtained from examples 2 and 3, example 2 containing only titanium as a nitride former. with example 3 containing the same amount of titanium plus 0.031% aluminum. It is apparent that no beneficial effect with respect to strengthening is obtained by addition of aluminum. The only difference is that the steel of example 2 containing only titanium developed yield point elongation while that of example 3 did not do so at the same total nitrogen concentrations. This is of course due to the fact that aluminum was available to scavenge nitrogen, thus resulting in less nitrogen in solid solution.

It is further apparent that the increase in yield strength produced in the columbium-bearing and zirconium-bearing steels of examples 4 and 7, respectively, is not as great as that for the titanium-bearing steels. However, yield strength in excess of 50 ksi were obtained in both cases by heat treatment at 1,2001,300F for one hour.

Example 5 was an embodiment ofa relatively highly alloyed titanium and columbium-bearing steel which exhibited an increase in yield strength substantially the same as that of examples 2 and 3.

Example 6, illustrative of lower alloying additions of titanium and columbium than example 5, exhibited significant increase in yield strength, although not as high as that of the more highly alloyed example 5.

The yield strengths reported for samples subjected to 20 percent cold reduction (simulating the strain or deformation resulting from deep drawing) show that the strengthening which accompanies the precipitation of alloy nitrides is additive to the cold work strengthening so that a net gain in strength is obtained even though there is a small loss in strength due to partial recovery. The strength advantage in nitrided cold worked material over nitrided as-annealed material is attributable to alloy nitride nucleation and precipitation on dislocations and to enhanced solubility of nitrogen in a strained lattice.

The elongation values after nitriding are relatively high in view of the yield strength levels attained. These elongation values indicate that some limited forming could be performed after nitriding strip material such as bending over a restrike operation. The samples subjected to 20 percent cold reduction increased in elongation values along with an increase in yield strength, because of recovery.

As indicated previously the heat treatment step of the present process results in an increase in nitrogen in solid solution in the steel as well as nitrogen combined as nitrides with titanium, columbium, and/or zirconium. It has been found that the total amount of nitrogen taken up by the steel can exceed that required to satisfy normal equilibrium solution requirements plus that needed to convert the alloys t0 nitrides. This excess nitrogen can be attributed to nitrogen trapped on dislocations. adsorbed at the nitride-ferrite interface, and as enhanced lattice solubility in strained ferrite.

Of further significance in Table 11 are the values reported for nitriding at 1,300F in 3% ammonia for 2 hours. In most instances a decrease in yield strength from the maximum values obtained at 1,200F for two hours occurred. due to the formation of coarser alloy nitride particles. A thin austenite rim formed on the surfaces of samples nitrided at 1,300F; therefore, to avoid the formation of an iron nitrogen austenite rim, the ammonia concentration should be slightly less than 3% at 1,300F for a two-hour heat treatment.

A comparison of the strengthening achieved by heat treating in a 3% ammonia 97% hydrogen mixture with that achieved in a 6% 94% hydrogen mixture is given in Table III below 12 and ammonia concentration in the atmosphere) increases as the square of the thickness. For example, for nitrogen diffusion in pure iron at 1.200F, to reach an average fractional saturation (Le. N Avg/N Equil.) of 0.7 it has been found that one hour is required for sheet stock of 0.040 inch thickness while 5.6 hours is required for sheet stock of 0.090 inch thickness. However, an important feature of the present invention is the discovery that marked increases in average yield 10 strength can be realized within relatively short times.

(i.e. not more than two hours), by partial alloy-nitrogen precipitation strengthening. Table IV below indicates the substantial increase in yield strength achieved by nitriding the titanium-columbium bearing steel of ex- TABLE 111 Effect of Ammonia Concentration Cold-Rolled and Annealed 0.040" Sheets Example & Y.S. T.S. Y.S. T.S Aust. Condition ksi ksi %Elong. %Y.P.E. %N ksi ksi %Elong. %Y.P.E. Rim

Steel 2 1100F1 hr. 37.6 54.2 23.5 0.7 .012 57.3 71.4 18.0 0.8 No

1200F-l hr. 76.4 92.6 14.0 1.4 .047 109.7 122.3 12.5 0.5 No

1300F-1 hr. 88.3 98.3 14.0 1.5 .094 93.0 104.4 3.0 0.7 Yes Steel 3 1100F-1 hr. 33.5 51.2 30.0 0.7 .011 52.8 67.5 20.0 0.5 No

1200F-1 hr. 79.9 89.5 11.0 0.0 .047 121.3 128.6 8.0 0.0 No

1300F-1 hr. 88.1 98.6 12.5 0.0 .11 98.3 104.7 1.5 0.0 Yes Steel 4 1100F-1 hr. 39.8 50.8 35.5 5.0 .012 44.8 55.7 28.0 5.0 No

1200F 1 hr. 51.2 59.3 18.0 2.7 .029 73.6 83.4 9.5 2.5 No

1300F-1 hr. 56.6 70.9 20.0 3.2 .074 64.9 79.7 15.0 2.9 Yes Steel 5 1100"F1 hr. 374 55.6 28.0 0.3 .010 54.8 68.8 18.0 0.7 No

1200F-1 hr. 70.0 81.1 17.0 0.9 .029 107.0 114.5 5.5 0.0 No

1300F 1 hr. 87.0 95.5 15.5 1.9 .098 97.9 110.0 4.0 1.2 Yes Steel 6 l100"F-1 hr. 43.8 55.7 29.5 2.5 .0093 56.4 66.1 18.5 3.8 No

1200F-1 hr. 74.8 82.1 15.0 2.7 .032 98.2 106.7 .0 3.5 No

1300F-1 hr. 73.6 81.9 17.0 2.5 .080 82.6 96.7 11.0 1.7 Yes Steel 7 1100F-1 hr. 32.7 49.8 37.5 1.5 40.6 54.7 30.0 3.9 No

1200F-1 hr. 54.1 62.7 24.0 5.1 66.7 76.1 19.0 4.0 No

1300F-1 hr. 62.9 73.6 17.5 3.3 72.9 84.4 14.0 2.5 Yes It is apparent from Table 111 that any given steel achieves a higher yield strength when nitrided in 6% ammonia under the same time and temperature condi tions than is attained by nitriding in 3% ammonia.

ample 8 in a 3% ammonia 97% hydrogen atmo- Higher strength is obtained by nitriding at 1,200F for 45 sphere within the temperature range of l,100-1 ,300F

one hour in 6% ammonia than by nitriding at 1,200F for two hours in 3% ammonia. However, due to diffusion phenomena, the surface to mid-thickness strength gradient would be greater in a 6% ammonia atmosphere than in a 3% ammonia atmosphere. For some applications it may be desirable to obtain a lower average strength with a lesser gradient to mid-thickness. The present invention makes it possible to select readily temperature, time and ammonia concentrations which will result in a wide range of average yield strengths and surface to mid-thickness strength gradients.

Table 111 again indicates that nitriding in 6% ammonia at 1,300F results in formation of an iron-nitrogen austenite rim which will transform either to martensite or an eutectoid structure, depending upon the cooling rate. In example 5, an austenite rim about 1 mil thick resulted from annealing in 6% ammonia at 1,300F for one hour.

As the thickness of the steel stock subjected to the heat treatment of the present invention increases, the time required to reach saturation at the equilibrium nitrogen content in solution (for a given temperature for l-2 hours. It will be noted that nitriding at 1,200F for only one hour resulted in an average yield strength of 66.5 ksi. Even greater strengthening could be attained by heat treating for longer periods of time or by 50 increasing the ammonia concentration to 6 percent. In

Table [V 1,300F again proved to be an unacceptable temperature when using a 3% ammonia atmosphere because of formation of an austenite rim.

55 TABLE IV Properties of 0.090 lnch Hot Rolled Steel of Exam le 8 Heat Treated in 3% NH 97% The criticality of providing at least about 0.02% titanium in solution is illustrated in Table V below.

TABLE v Effect of Amount of Available Nitride-Forming Element In Table V a steel of the invention containing 0.077% total titanium, 0.037% total columbium, 0.031% aluminum, 0.0035% nitrogen, and remainder substantially iron, was carburized from an original carbon level of 0.0044% to a carbon level of 0.010% and to saturation with carbon in order to vary the amount of titanium in hydrogen gas, the excess nitrogen is removed with only a 10 to percent reduction in yield strength. Removal of the excess nitrogen eliminates welding porosity and significantly reduces the ductile to brittle transition temperature while improving the impact energy values. The present invention thus provides low carbon, high strength steel stock suitable for welding applications.

Table VI demonstrates the above observations regarding the effect of denitriding. A steel of the invention initially containing 0.006% carbon, 0.077% titanium, 0.037% columbium, 0.03 l% aluminum, 0.0035% nitrogen, and balance substantially iron, cold rolled to 0.058 inch thickness and annealed, was nitrided as indicated in Table VI. Sample A was not denitrided, Sample B was partially denitrided, and Sample C was still further denitrided. Both the yield strength and the ductile to brittle transition temperature decreased gradually with the decrease in nitrogen in solution.

TABLE VI Effect of Denitriding Ductile to Y.S. T.S. Measured Culc. Brittle Transi' Sample Treatment ksi ksi %Elong. %N,,,,,,, %N(soln.) tion Temp.F A Nitrided in 6%NH,,-94%H 88.3 [00.7 18 0.10 0.067 0 I200F 3 hrs. B Nitrided as in A, then 80.8 92.0 l7 0.053 0.020 60 denitrided in H l200F 2 hrs. C Nitrided as in A, then 77.] 87.l 16 0.043 0.0) fi0 denitrided l200F 4 hrs.

solution available to react with ammonia in the nitriding operation. It is apparent from Table V that a substantial decrease in yield strength occurs progressively with decrease of available titanium in solution from 0.047 to 0.025 percent and to 0 percent successively.

In order to ascertain the degree of strengthening contributed by nitrogen taken into solid solution in the steel, the samples of Table V were denitrided by heating in a hydrogen atmosphere at about l,200F for 2 hours. Table V reports the yield strength in the denitrided condition and further sets forth the differential at each of the different carbon contents, from which it is apparent that nitrogen taken into solid solution contributes about 20 ksi to the yield strength. It is further apparent that denitrided material at the 0.0l0% carbon level (resulting in 0.025 percent available titanium in solution) retains a substantially increased yield strength of 65.9 ksi in the denitrided condition.

lt has been discovered that the excess nitrogen content of the steels following alloy-nitrogen precipitation strengthening at higher allowable ammonia contents can present weldability problems and result in high ductile to brittle notched sheet Charpy impact transition temperature. The welding problems involve porosity resulting from the liberation of this excess nitrogen as nitrogen gas. These problems can be overcome by special welding techniques. The use of high ammonia concentrations, just less than that which results in the formation of iron nitrides or an iron-nitrogen austenite, is desirable from the standpoint of producing maximum stengthening in the shortest possible time. However, it has been discovered that if nitriding conducted for the purpose of strengthening (and which results in an undesirably high excess nitrogen content in solid solution) is followed by a denitriding step, such as annealing in Steel having the composition specified herein can be melted by any conventional operation such as open hearth, basic oxygen furnace or electric furnace. The molten steel is then vacuum degassed in order to achieve the desired carbon and nitrogen levels, killed preferably with Al, and the nitride-forming alloying element or elements are added to the ladle after degassing with suitable mixing. The melt is then teemed into ingots, or cast into slabs. The solidified ingots or slabs are then subjected to conventional hot rolling and to conventional subsequent processing steps to obtain sheet stock of the desired final thickness. The steel is then subjected to the process of the present invention either in the form of sheet or strip, or after forming into articles by drawing or stamping.

The embodiments of the invention in which an exclusive property or privelege is claimed are defined as follows:

1. Cold reduced and annealed steel sheet stock having a thickness between about 0.02 inch and 0.09 inch, an average yield strength of at least ksi, and sufficient formability to permit fabrication into articles other than deep drawn, consisting essentially of less than about 0.010% carbon, from about 0.05 to about 0.6% manganese, from about 0.02 to about 0.04% total aluminum, up to about 0.035% sulfur, up to about 0.0l% oxygen, residual silicon and phosphorus, at least one nitride-forming element chosen from the group consisting of titanium, titanium and columbium, tita nium and zirconium, and titanium and mixtures of columbium and zirconium, with total titanium ranging between about 0.08 and 0.10 percent, total columbium ranging between about 0.03 and 0.06 percent, total zirconium ranging between about 0.03 and 0.00 percent, sufficient nitrogen to combine substantially Ebm- 16 bium, titanium and zirconium, and titanium and mixtures of columbium and zirconium, with total titanium ranging between about 0.08 and 0.!0 percent, total columbium ranging between about 0.03 and 0.06 percent, total zirconium ranging between about 0.03 and 0.06 percent, sufficient nitrogen to combine substantially completely with said aluminum and said nitrideforming elements, and remainder essentially iron, all percentages being by weight, said article having an average yield strength of at least about 50 ksi.

Claims (2)

1. COLD REDUCED AND ANNEALED STEEL SHEET STOCK HAVING A THICKNESS BETWEEN ABOUT 0.02 INCH AND 0.09 INCH, AN AVERAGE YIELD STRENGTH OF AT LEAST 70 KSI, AND SUFFICIENT FORMABILITY TO PERMIT FABRICATION INTO ARTICLES OTHER THAN DEEP DRAWN, CONSISTING ESSENTIALLY OF LESS THAN ABOUT 0.010% CARBON, FROM ABOUT 0.05 TO ABOUT 0.6% MANGANESE, FROM ABOUT 0.02 TO ABOUT 0.04% TOTAL ALUMINUM, UP TO ABOUT 0.035% SULFUR, UP TO ABOUT 0.01% OXYGEN, RESIDUAL SILICON AND PHOSPHORUS, AT LEAST ONE NITRIDE-FORMING ELEMENT CHOSEN FROM THE GROUP CONSISTING OF TITANIUM, TITANIUM AND COLUMBIUM, TITANIUM AND ZIRCONIUM, AND TITANIUM AND MIXTURES OF COLUMBIUM AND ZIRCONIUM, WITH TOTAL TITANIUM RANGING BETWEEN ABOUT 0.08 AND 0.10 PERCENT, TOTAL COLUMBIUM RANGING BETWEEN ABOUT 0.03 AND 0.06 PERCENT, TOTAL ZIRCONIUM RANGING BETWEEN ABOUT 0.03 AND 0.06 PERCENT, SUFFICIENT NITROGEN TO COMBINE SUBSTANTIALLY COMPLETELY WITH SAID ALUMINUM AND SAID NITRIDE-FORMING ELEMENTS, AND REMAINDER ESSENTIALLY IRON, ALL PERCENTAGES BEING BY WEIGHT.

2. A deep drawn article formed from a cold-reduced and annealed steel sheet stock consisting essentially of less than about 0.010% carbon, from about 0.05 to about 0.6% manganese, from about 0.02 to about 0.04% total aluminum, up to about 0.035% sulfur, up to about 0.01% oxygen, residual silicon and phosphorus, at least one nitride forming element chosen from the group consisting of titanium, titanium and columbium, titanium and zirconium, and titanium and mixtures of columbium and zirconium, with total titanium ranging between about 0.08 and 0.10 percent, total columbium ranging between about 0.03 and 0.06 percent, total zirconium ranging between about 0.03 and 0.06 percent, sufficient nitrogen to combine substantially completely with said aluminum and said nitride-forming elements, and remainder essentially iron, all percentages being by weight, said article having an average yield strength of at least about 50 ksi.