US3507710A - Process for treating steel - Google Patents

Description

United States Patent 3,507,710 PROCESS FOR TREATING STEEL Raymond A. Grange, Washington Township, Westmoreland County, and Clayton E. Groti, Penn Hills Township, Allegheny County, Pa., assignors to United States Steel Corporation, a corporation of Delaware N0 Drawing. Filed Feb. 23, 1967, Ser. No. 617,829 Int. Cl. C21d 1/26, 7/14 U.S. Cl. 148-12 13 Claims ABSTRACT OF THE DISCLOSURE A high-temperature heat treatment applicable to steel to eliminate or minimize the effect of dendritic segregation and, in certain steels, to refine the inherent grain size. The treatment involves exposing such steels to temperature above about 2400 F. for a minimum of minutes. Data are presented to show the improved microstructure grain size and reduced directionality of steel pretreated in accordance with the invention.

This invention relates to a process for improving steel by a high-temperature heat treatment. Still more particularly, the invention concerns a heat treatment which comprises heating steel to a minimum temperature of 2400 F. for at least 10 minutes. Still more particularly, the inven tion concerns heating a steel within the range of about 2400 to about 2500 F. for 10 to 20 minutes.

When steel solidifies from the liquid state, dendritic crystals are usually formed and as a result, dendrites are an unavoidable characteristic of cast steel. The core of dendrite crystals is of higher purity than the last metal solidified. When hot worked into products, steels often exhibit a condition known as banding due to the dendritic segregation. Banding is in most respects undesirable since it enhances directionality in mechanical properties and decreases uniformity of response to heat treating. The elimination or reduction in the severity of banding is, therefore, highly desirable.

One conventional practice to eliminate banding has been a so-called homogenization treatment. However, to be effective, homogenization as practiced conventionally requires heating for many hours at temperature well above 2200 F. Such treatment is of doubtful economic practicality because elaborate and costly means for protectting the steel from decarburization and excessive scaling are necessary due to the long exposure at high temperature. The present invention avoids this problem by providing a high temperature treatment of such short duration that no protection against oxidation is required. The novel heat treatment of the invention markedly eliminates or reduces banding in a Wide variety of steel compositions.

In the practice of the invention, any method of heating and any rate of heating can be used. It is not necessary to start at room temperature, but higher temperature can follow hot-working so as to utilize heat already present in the steel. It is necessary, however, that the steel be exposed to a temperature of at least about 2400 F. for a minimum of 10 minutes. Following such exposure to temperature, the steel is cooled, normally by air cooling. However, quenching in water or water spray cooling is also satisfactory.

Sometimes, even though removed by the high-temperature treatment, banding will return if the steel is mechanically worked and then heat treated again. However, hot or cold-working subsequent to high-temperature treatment results in gradual return of the banding condition as the amount of working is progressively greater. Hotor cold-working to the extent of about 25 reduction will not restore the banded condition. Final hotor cold- Patented Apr. 21, 1970 working to this limited extent may be advisable to produce a steel product with good surface and dimensions.

In a common application of the process of the invention as applied to produce strip or plate, the product would be initially reduced by conventional hot-working to a thickness about 25% greater than the final product. As the strip or plate emerges from the final hot-rolling pass, it is passed through a furnace to bring its temperature throughout to at least 2400 F. and the steel is maintained at substantially this temperature for at least about 10 minutes. The strip or plate would then be passed through rolls to flatten it and reduce it to final dimension. Thereafter, the steel can be handled in any conventional manner. As an alternative, the strip or plate could be hotworked oversize and subsequently in an independent operation reheated to at least 2400 F. for at least 10 minutes and then given a finishing pass to produce a product with good surface and close dimensional tolerance.

Steel treated as described above can be subsequently heated in any conventional manner. For example, it can be annealed, normalized or hardened without return of the banded condition. In addition, treatments involving reaustenitizing can be repeated indefinitely without destroying the beneficial effect of the high-temperature treatment. As a practical matter, the process is applicable to any steel which can withstand a 10-minute heating at 2400 F. without melting, burning or suffering damage of overheating. This includes all constructional carbon and alloy steels. Cast steel with dendritic segregation, as well as banded wrought steel, will benefit from the high-temperature treatment of the invention.

The treatment described herein has other benefits in addition to reducing banding. One such additional advantage is refinement of the steels inherent grain size, i.e. the development of relatively fine grain size upon subsequent conventional miscellaneous heat treatments. Thus, the term inherent grain size refers to the grain coarsening behavior of the steel as it occurs over a range of temperature and time. inherently fine grain steels are those whose austenite grains do not coarsen when heat treated conventionally. The treatment of the invention may be used to reduce banding where banding is a problem or to achieve finer grain heat treated microstructures or both. The advantages of finer grain sizes are well known and include increased yield strength and enhanced toughness. A significant grain refining efiect has consistently been obtained in silicon-killed steels which, as usually processed, are coarse grained. The grain refining of silicon-killed steels by the present method can often achieve as fine a grain structure as aluminum-killed steel. Thus, steels substantially free of aluminum and other grain refining elements classified as inherently coarse grained steels, i.e. having grains coarser than ASTM #5 upon austenitizing at temperatures of 1700 F. or lower, when treated in accordance with the invention become inherently fine grained, i.e. capable of developing a finegrained structure of ASTM #6 and finer when austenitized at 1700 F. or below. Furthermore, a grain refining effect may also be found in aluminum-killed steels in which more or less than the optimum amount of aluminum is present.

A still additional beneficial effect obtained in connection with the high-temperature treatment involves the conversion of long thin inclusions of sulphide or silicate types into less harmful disconnected inclusion particles. In this manner, cracking often associated with such inclusions is minimized.

The following example illustrates a preferred embodiment of the invention.

An 0.5-inch plate of commercially-manufactured steel having the composition described below was heated at 2400 F. for 10 minutes and air cooled.

3 C, 0.25; Mn, 1.40; P, 0.021; S, 0.034; Si, 0.032; Cu, 0.22.

This sample and a like one in the as-rolled condition were then both reaustenitized at 1600 F. and furnace cooled. Tensile and V-notch Charpy impact specimens were prepared from each of the two plates. One half the specimens were taken in the longitudinal direction and the other half in the transverse direction. The results obtained are described in Table I.

TABLE 1 Longitudinal Transverse N High No High pretemp. pi'etemp. treattrcattreattreat- Mechanical property merit merit ment ment Tensile strength, p.s.i 77, 700 77,800 80,000 78,800 Yield strength, p.s.i 48, 700 50, 300 48, 200 49, 900 Elong. in 1, percent. 31. 5 32. 5 26 33 Red. of area, percent 64 66 57 62 Impact energy at +200 F. ft. lb. 77 88. 5 40. 5 60. 0 Impact energy at +75 F. it. ll). 38 43.5 25. 5 31. 5 Impact energy at 0 F. ft. lb 5 19. 5 9 17. 5

1 Average of two or more tests.

As can be seen from the data in Table I, the high temperature treatment in accordance with the invention results in more uniform ductility in the longitudinal and transverse directions. Moreover, strength and toughness are improved as well. In addition, the tendency toward lower ductility and notch toughness in the transverse direction is reduced by the higher temperature treatment.

Examples of the results obtained by practicing the method disclosed herein on other compositions are shown in Table II. All the samples were normalized subsequent to the high temperature treatment or after hot rolling where no pretreatment was given.

The standard AISI steels of Table II above have the following respective compositions:

TABLE III economical and feasible manner while minimizing the undesirable effects of dendritic segregation.

We claim:

1. A grain-refining, high temperature treatment for reducing banding and capable of promoting coalescence of inclusion stringers in a wrought article comprising a steel subject to dendritic segregation on solidification, comprising subjecting the article to be treated, prior to subsequent heat treatment of the article, to a temperature of at least about 2400 F. and less than a temperature siifiicient to cause incipient melting or burning of the article, and maintaining the article within said temperature range for about 10 to 20 minutes.

2. A method in accordance with claim 1 wherein said high temperature heat treatment consists of heating said steel article to a temperature in the range of 2400 to about 2500 F. for 10 to 20 minutes.

3. A method in accordance with claim 1 wherein said steel article is worked after said high temperature heat treatment up to about 25% reduction without restoring a banded condition.

4. A method according to claim 1 wherein said steel article is one which is substantially free of aluminum and other grain refining elements.

5. A method according to claim 1 wherein said high temperature treatment is applied to a steel casting.

6. An article produced in accordance with the method of claim 1.

7. An article produced in accordance with the method of claim 4., wherein the article is characterized by a finer grain size, as compared to the untreated article, after subsequent heat treatment.

8. A method in accordance with claim 2 wherein said high temperature heat treatment is applied to a steel casting.

9. A method in accordance with claim 1 wherein the article comprises a steel selected from the group consisting of carbon and alloy constructional steels.

10. A method in accordance with claim 9, wherein the article is composed of a steel comprising, by weight percent,

dental impurities.

11. A method in accordance with claim 10 wherein the steel contains carbon up to 1.10%, silicon up to 2.20%,

[C0mp0siti0n, weight percent 1 Type of steel 1 S AISI 1020 AISI 1030 A181 1335 AISI 131321-- AISI 2340 AISI 4140- A181 4320.. A181 8620 l AISI standard since these steels are no longer listed as standard grades.

2 Balance iron, except for incidental steelmaking impurities.

3 Optionally, 0.15 to 0.35% Pb.

4 Open hearth or basic oxygen.

5 Acid open hearth.

Investigations have demonstrated that the recommended treatments disclosed herein either markedly reduce or eliminate banding on a wide variety of steels. By employing the conditions described, overheating is avoided. Moreover, the limited treating time decreases the extent of decarburization and scaling obtained in practicing the invention. Thus, the relatively short high-temperature treatment of the invention considerably improves the microstructure and mechanical properties of steel in an grade specification in 1966, except grades 1321 and 2340, for which the 1949 standard specifications are used,

sulfur up to 0.35%, phosphorus up to 0.12%, and lead up to 0.35

12. A method in accordance with claim 11, wherein the steel additionally contains up to about 4% in total of Percent Chromium Up to 1.10 Nickel Up to 3.75 Molybdenum Up to 0.25 Boron Optionally, over 0.0005.

13. An article produced 111 accordance With the process L. DEWAYNE RUTLEDG E, Primary Examiner of claim W. W. STALLARD, Assistant Examiner References Cited UNITED STATES PATENTS US. Cl. X.R. 3,384,476 5/1968 Egnell 14s 12 5 134 3,323,953 6/1967 Lesney 148-12