US3671334A - High-strength steel having aging properties - Google Patents

Abstract

STEELS HAVING THE FOLLOWING COMPOSITION: .08 TO .18% CARBON; .3 TO 1.0% MANAGANESE; .01 TO .05% COLUMBIUM; .008 TO .014% NITROGEN; .10% MAXIMUM SILICON; LESS THAN A TOTAL OF APPROXIMATELY .02% OF THE NITRIDE-FORMING ELEMENTS ALUMINUM, ZIRCONIUM, VANADIUM AND TITANIUM; AND THE BALANCE ESSENTIALLY IRON, AFTER APPROPRIATE PROCESSING, HAVE YIELD STRENGTHS OF 50,000 TO 70,000 P.S.I. IN HOT-ROLLED AND COLD-ROLLED CONDITIONS AND YIELD STRENGTHS OF 70,000 TO 90,000 P.S.I. AFTER STRAINING AND AGING.

Description

June 20, 1972 J BUCHER ETAL 3,671,334

HIGH-STRENGTH STEEL HAVING AGING PROPERTIES Filed Aug. '7, 1970 2 Sheets-Sheet l 4 mvsmons JOHN H. BUCHER BY JOHN F. HELD heir ATTORNEY ITEMPER REDUCTION N w 0 I 4 T C C U w )D m m 13 H E mu 9 9 T M 7 D M z v 8 L 8 L 3 3 o n n N 3 w C 6 6 n a m a t T O O IIC 5 5 U D h I. R N E N g P g m 0 0| d moIl- C n C T F PM W R E w M 13 mm N 7 W 8 W 0 WM 3 3 3 C 3 3 3 2 3 0 3 c 6 6 6 w 6 a 6 a 4 ll 0 6 r o O O 5 o 0 o 5 5 O 9 8 8 7 2 l 0 7 7 6 w 5 w m. l. mm O IFQZMIFW ZOE.. 0ZO m Bt OTIFOZMKkW 2023020 5 June 20, 1972 J. H. BUCHER ETAL 3,671,334

HIGH-STRENGTH STEEL HAVING AGING PROPERTIES Filed Aug. 7. 1970 2 Sheets-Sheet 2 INVENTORS JOHN H. BUCHER JOHN F. HELD theirATTORNEY TEMPER REDUCTION United States Patent 3,671,334 HIGH-STRENGTH STEEL HAVING AGING PROPERTIES John H. Bucher, Bethel Park, and John F. Held, Verona,

Pa., assignors to Jones 8: Laughlin Steel Corporation,

Pittsburgh, Pa.

Filed Aug. 7, 1970, Ser. No. 62,073 Int. Cl. C21d 7/14; C22c 39/54 US. Cl. 148-123: 14 Claims ABSTRACT OF THE DISCLOSURE and cold-rolled conditions and yield strengths of 70,000 to 90,000 p.s.i. after straining and aging.

This invention relates to renitrogenized columbiummodified high-strength steels, strain-aged articles manufactured from such steels and processes for producing the articles.

We have developed steels which have yield strengths of 50,000 to 70,000 p.s.i. in hot-rolled and cold-rolled conditions and yield strengths of 70,000 to 90,000 p.s.i. in a strained and aged condition. The steels have the following composition: .8 to .18% carbon; .3 to 1.0% manganese; .01 to .05% columbium; .008 to .014% nitrogen; .10% maximum silicon; less than a total of approximately .02% of the nitride-forming elements aluminum, zirconium, vanadium, and titanium; and the balance essentially iron.

The steels of our invention have particular utility where there is a requirement for a medium-carbon steel capable of being formed into articles which after forming and aging develop high strengths. Thus, in a hot-rolled condition, the steels of the invention are formed into reinforcing members for commercial and passenger vehicles and in a cold-rolled condition are formed into pillars, crossmembers, seat tie-downs and floor pans for such vehicles. In a particular application, the steels of our invention are formed, painted, and baked and depending on their exact chemistry and processing history have yield strengths of 50,000 to 70,000 p.s.i. before forming and develop a flow stress of 70,000 to 90,000 p.s.i. after forming (straining) and baking (aging).

The amount of reduction the product is subjected to during cold-rolling influences the aged yield strength of the formed article; higher cold reductions are reflected in higher strengths. In addition, temper rolling of both the hot-rolled and cold-rolled product results in higher aged yield strengths. Also the steels of the invention in a coldrolled condition when subjected to a continuous annealing procedure, e.g., normalizing, develop higher aged yield strengths than when batch annealed.

Columbium is, of course, the major contributor to the high strengths of the steels of the invention and the nitrogen is primarily responsible for the increased strengths developed by the steels upon aging. The primary effect of columbium in imparting strength to the steels is to refine the grain size of the steels. Significantly, columbium is the only one of the conventionally employed grain refining elements, i.e., zirconium, vanadium and titanium, which is not also a strong nitride former; and; consequently, is the only one of these elements which can be used in conjunction with nitrogen to produce a strain- 3,671,334 Patented June 20, 1972 aging steel. Correlatively, the zirconium, vanadium, and titanium levels in the steels of the invention are each kept below about .005% so that the formation of nitrides is kept at a minimum. In addition, aluminum, which is also a strong nitride former is maintained at a level below about .005%.

An object of the present invention is to provide' mediumcarbon steels having yield strengths of 50,000 to 70,000 p.s.i. in a hot-rolled or cold-rolled condition and yield strengths of 70,000 to 90,000 p.s.i. after straining and agmg.

Another object of the invention is to provide such steels in the form of columbium-modified renitrogenized steels.

Yet another object of the invention is to provide strain aged articles manufactured from such steels.

Still another object of the invention is to provide proccesses for producing such articles from such steels.

These and other objects and advantages of the present invention will be evident from the following detailed description of the invention with reference to the accompanying drawing in which:

FIG. 1 is a series of curves illustrating the variation in I the aged yield strength and percent total elongation with percent temper reduction of steels of the invention in a hot-rolled condition.

FIG. 2 is a series of curves illustrating the effects that variations in the amount of cold reduction have on the physical properties of a steel of the invention in the strained and aged condition.

FIG. 3 is a series of curves illustrating the effects of a batch annealing operation and a continuous annealing operation on the physical properties of a steel of the invention in the strained and aged condition.

Two steel heats (identified herein as #63387 and #72319) having the compositions set out in the table were produced according to conventional steelmaking practices.

The high nitrogen levels were obtained by making calcium cyanamide ladle additions to the steels. The manned in which nitrogen is added is not critical, however, and other techniques such as the use of high-nitrogen ferromanganese can be employed to renitrogenize the steels. The steels were teemed and processed into hot-rolled strip by conventional processing techniques. Hot-rolled stri-p manufactured from Heat #63387 and Heat #72319 are referred to herein as #63387(H) and #72319(H) respectively. The #63387(H) strip had a yield strength of about 64,000 p.s.i., and the #72319(H) strip had a yield strength of about 60,000 p.s.i.

To determine the strain-aging properties of the hotrolled strip, samples thereof were pulled on a tensiontesting machine so as to develop a strain of 7 /2% in the samples. Thereafter, the samples were heated at 212 F. for 2 hours, cooled to room temperatures and their physical properties determined. This techniques provides an indication of the aged properties which are established in the product upon undergoing a conventional forming operation, such as the forming of reinforcing members for passenger vehicles, and the same or an equivalent heat treatment. Samples were also temper rolled before being strained to ascertain the eflect of temper rolling on the strain-aged properties of the product. The results of the tests are shown in FIG. 1.

Both the #63387(H) product and the #72319(H) product developed an aged yield strength of about 78,000 p.s.i. in the absence of a temper reduction. However, the aged yield strength of the #63387 (H) product increased rapidly with the increased percentage temper reduction, reaching a maximum of about 88,000 p.s.i. at a temper reduction of about 2%, whereas the aged yield strength of the #72319-(H) product increased to a maximum of about 86,000 p.s.i. at a temper reduction of about 2% The percentage total elongation for each product fell as the percent temper reduction increased, indicating the need to keep temper reduction at a minimum where maximum ductility is sought.

To determine the suitability of the steels for cold-rolled applications, samples of the #63387(H) product were cold reduced 50% and 64% by rolling. The cold-reduced material is referred to herein as #63387(C). The #63387(C) samples were batch annealed for 10 hours at 1250 F., the 64% cold-reduced material possessing a yield strength of 53,000 p.s.i. and the 50% cold-reduced material possessing a yield strength of 50,000 p.s.i. upon completion of the annealing cycle. As with the hotrolled strip, the annealed cold-rolled samples were temper rolled various amounts, pre-strained 7 /2%, and aged by heating at 212 F. for 2 hours. The samples were then tested and the results are presented in FIG. 2, the left-hand side of the figure indicating the results for the 64% cold-reduced product and the right-hand side of the figure indicating the results for the 5 0% cold-reduced product.

As with the hot-rolled strip, the aged yield strength of the cold-rolled product increased with increased temper reductions, reaching a value of 70,000 p.s.i. at about a 1% temper reduction for the 64% cold-rolled product and at about a 3% temper reduction for the 50% cold rolled product. The greater strength of the 64% coldrolled product reflects its smaller annealed grain size. The bottom half of FIG. 2, illustrating the percentage total elongation values of the samples, indicates the need, as with the hot-rolled strip, to keep temper reductions at a minimum to obtain optimum ductility.

The steels of the invention demonstrate a more pronounced aging effect when they are continuously annealed rather than batch annealed after undergoing cold reduction. This is shown in FIG. 3 which presents results for samples of #72139(H) product which were cold rolled 55%, annealed for 6 hours at 1210 F. or normalized at 1585 F. for 2 minutes, prestrained 7 /2%, and heated at 212 F. for 2 hours. The samples were also temper rolled various amounts. The samples annealed for 6 hours at 1210 F. possessed a yield strength of about 49,000 p.s.i. Upon straining and aging, the yield strength increased to about 60,000 p.s.i., as shown at the right-hand side of FIG. 3. The samples normalized at 1585 F. for 2 minutes possessed a yield strength of 53,000 p.s.i., and upon straining and aging, the yield strength increased to about 68,000 p.s.i. With both annealing procedures, the aged yield strengths of the samples increased with increased temper reductions, although the yield strengths of the normalized samples were consistently about 8,000 p.s.i. greater than the batch-annealed samples. As with the previous samples, percentage total elongation decreased with increased percentage temper reduction.

In addition to a normalizing treatment, the continuousannealing operation can be elfected by the heat treatment given steel strip on a conventional Sendzimer galvanizing line. A continuous-annealing operation which provides for rapid cooling of the steel increases the amount of carbon in solution in the steel, rendering it susceptible to carbon strain-aging. This effect is additive to the aging produced by the nitrogen in solution and increases the total strain-aging potential of the steel.

Applying the teachings set out herein, steels having yield strengths of between 50,000 to 70,000 p.s.i. in a hot-rolled or cold-rolled and annealed condition and of between 70,000 to 90,000 p.s.i. after strain-aging can be produced from steels of the following composition: carbon, .08 to .18%; manganese, .3 to 1.0%; columbium, .01 to .05%; nitrogen, .008 to .014%; silicon, less than .10%; phosphorus and silicon in normal amounts; and less than a total of about .020% of aluminum, zirconium, vanadium and titanium. However, greater latitude in processing is obtained by the use of steels having the following preferred composition: carbon, .11 to .13%; manganese, .60 to .80%; columbium, .02 to .03%; and nitrogen, .012 to .014%.

The steels of the invention can be strain-aged over a wide range of temperatures and time periods, and the particular application to which the steels are put primarily determines the particular conditions to be used. Generally, the steel after being formed into some commercial article will be heated at a temperature between 200 and 600 F. for a time period of between 5 to minutes to increase its strength level.

We claim:

1. A formed and strain-aged steel article, consisting essentially of iron, 0.8 to .18% carbon, .3 to 1.0% manganese, .01 to .05 columbium, .008 to 0.14% nitrogen, .10% maximum silicon, and containing less than a total of approximately .02% of the nitride-forming elements aluminum, zirconium, vanadium and titanium, there having been sufficient free nitrogen in the steel to impart strain aging properties thereto, said article having a yield strength in excess of about 70,000 p.s.i.

2. The article of claim 1 wherein the carbon content is .11 to .13%, the manganese content is .60 to .80%, the columbium content is .02 to .03% and the nitrogen content is .012 to .0l4%.

3. A process for producing a steel article having a yield strength of above about 70,000 p.s.i. in a formed and strain-aged condition comprising:

(a) providing a steel consisting essentially of iron, 0.8 to .18% carbon, .3 to 1.0% manganese, .01 to 0.5% columbium, .008 to .14% nitrogen, .10% maximum silicon, and containing less than a total of approximately .02% of the nitride-forming elements aluminum, zirconium, vanadium and titanium, there being sufficient free nitrogen in the steel to impart strain aging properties thereto,

(b) hot-rolling the steel,

(c) forming the hot-rolled product into said article,

and

(d) heating the article at a temperature between 200 and 600 F. for 5 to 120 minutes.

4. The process of claim 3 including temper rolling the hot-rolled product before forming the article. I

5. The process of claim 3 wherein the carbon content of the steel is .11 to .13%, the manganese content is .60 to .80%, the columbium content is .02 to .03% and the nitrogen content is .012 to 014%.

6. The process of claim 5 including temper rolling the hot-rolled product before forming the article.

7. A process for producing a steel article having a yield strength of above about 70,000 p.s.i. in a formed and strain-aged condition comprising:

(a) providing a steel consisting essentially of iron, .08 to .18% carbon, .3 to 1.0% manganese, .01 to .05% columbium, .008 to 014% nitrogen, .10% maximum silicon, and containing less than a total of approximately .02% of the nitride-forming elements aluminum, zirconium, vanadium and titanium, there being sufficient free nitrogen in the steel to impart strain aging properties thereto,

(b) hot-rolling the steel,

(c) cold-rolling the hot-rolled product,

((1) annealing the cold-rolled product to restore its ductility,

(e) forming the annealed product into said article, and

(f) heating the article at a temperature between 200 and 600 F. for 5 to 120 minutes.

8. The process of claim 7 wherein step (d) is carried out as a continuous operation.

9. The process of claim 7 including temper rolling the annealed product before forming the article.

10. The process of claim 9 wherein step (d) is carried out as a continuous operation.

11. The process of claim 7 wherein the carbon content of the steel is .11 to .13%, the manganese content is .60 to .80%, the columbium content is .02 to .03% and the nitrogen content is .012 to .014%

12. The process of claim 11 wherein step (d) is carried out as a continuous operation.

13. The process of claim 11 including temper rolling the annealed product before forming the article.

14. The process of claim 13 wherein step (d) is carried out as a continuous operation.

References Cited UNITED STATES PATENTS 2,999,749 9/1961 Saunders et a1. 75-58 3,010,822 1-1/ 1961 Altenburger et a1. 75-123 1 3,102,831 9/1963 Tisdale 148-12 3,247,946 4/ 1966 Klein 148-123 3,254,991 6/1966 Shimmin et al. 75-123 I X 3,402,080 9/1968 Kubota et al. 148-36 3,544,393 12/ 1970 Zanetti 14812 3,558,370 1/1971 Boni 148-36 3,155,549 11/1964 Nakamura 75-124 X 3,180,726 4/ 1965 Nakamura 148-42 X 3,328,211 6/1967 Nakamura 148-12 3,432,368 3/1969 Na'kamura 148-12 FOREIGN PATENTS 685,397 4/1964 Canada 75-123 I 1,101,193 1/ 1968 Great Britain 75-123 J r Small Niobium Additions on the Mechanical Properties of Commercial Mild Steels, Journal of Iron and Steel Institute, January 1963, vol. 201, pp. 43-46.

L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner U.S.. Cl. X.R.

75-123 B, 123 H, 123 J, 123 M, 124; 148-36

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