US3539404A

US3539404A - Method of making a low alloy steel

Info

Publication number: US3539404A
Application number: US638436A
Authority: US
Inventors: Anthony F Deretana
Original assignee: Youngstown Sheet and Tube Co
Current assignee: Youngstown Sheet and Tube Co
Priority date: 1967-05-15
Filing date: 1967-05-15
Publication date: 1970-11-10
Anticipated expiration: 1987-11-10

Description

NOV- 10, 1970 A. F. Dx-:lfRETANA METHOD OF MAKING A LOW ALLOY STEEL Filed May l5, 1967 2 Sheets-Sheet 1 IN VENTOR.

ANTHONY F. de RETANA 5, axngoaadwal ATTORNEY.

Nov. 10, 1970 A. F. DE RETANA 3,539,404

METHOD OF MAKING A LOW ALLOY STEEL Filed May l5. 1967 2 Sheets-Sheet 2 INVENTOR.

ANTHONY F. de RETANA ATTR/VEY.

United States Patent O 3,539,404 METHOD OF MAKING A LOW ALLOY STEEL Anthony F. DeRetana, Poland, Ohio, assignor to The Youngstown Sheet and 'IJbe Company, Youngstown, Ohio, a corporation of Ohio Filed May 15, 1967, Ser. No. 638,436 Int. Cl. C21d 1/02 U.S. Cl. 14S-12.4 4 Claims ABSTRACT OF THE DISCLOSURE A low cost, low alloy steel containing not more than about 0.25% molybdenum (Mo) in the presence of carbonitride formers, carbide formers or both in relatively small quantities, as well as a method of treating the steel to attain high yield strength (80,000 p.s.i. or more) and good low temperature impact toughness (transverse charpy V-notch 15 ft. lb. at 50 F. or less).

BACKGROUND OF THE INVENTION The invention is directed to improvements in the production of alloy steels low in Mo by reducing the amount thereof heretofore thought essential and provision of a novel method of treating the steel whereby the properties of strength and toughness heretofore attained by the employment of relatively large amounts of Mo are imparted to it during the cooling stages to which it is subjected following a hot working or other mill process wherein it it heated to an elevated temperature, typically one in the neighborhood of 2100 F.

SUMMARY OF THE INVENTION A principal object of the invention therefore is to provide for subjecting a low Mo steel of predetermined chemical composition containing not over 0.25% Mo to a predetermined cooling operation whereby when the steel finally emerges therefrom its yield strength, ten sile strength and impact transition temperature are within optimum ranges, for example, yield strength of approximately 80,000 p.s.i. or more, tensile strength in the neighborhood of 100,000 p.s.i. and impact transition ternperature 50 F. or less.

It is well known that addition of Mo to steel in amounts approximating 0.5% and upwards with about 0.002% of boron (B) present, results in production of steel of good hardenability, strength and toughness but as far as is known these properties have not been attained to a like degree with less than said quantities of Mo and B, and it is therefore further within contemplation of the invention to provide steel containing not more than about 0.25 Mo and preferably 0.0005 to 0.0015% B with other alloying elements in small amounts and to subject it to a novel processing whereby good yield and tensile strengths and low impact transition temperatures are attained at a cost for the finished steel materially lower because of the lesser amounts of expensive alloying elements used in its production than heretofore believed essential for securement of comparable results.

Other objects, purposes and advantages of the invention will hereinafter more fully appear or be understood from the following more specic description in which reference will be had to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph based on determination of the eiects on transformation temperature of changes in the cooling rate after heat processing of the low alloy steel of my invention;

FIG. 2 is a graphic representation of the effect on 3,539,404 Patented Nov. 10, 1970 DESCRIPTION OF THE PREFERRED EMBODIMENT More particularly the invention will now first be described with more detailed reference to the several alloying elements employed in the steel and the amounts of each contemplated thereby and then to the processing to which the steel containing them is subjected to attain the results set forth and summarized in said graphs, it being understood that steel of any of the several specic analyses given is within the scope of the invention and may be subjected to treatment in accordance with the method thereof utilizing any of several specifically slightly differing processing steps as will hereinafter be more fully explained.

Thus, in general, the invention is directed and is in fact confined to steel containing at least some of the elements within the range of the percentages set forth in the following table in which, it will be understood, the difference between the totals of said elements and is substantially only iron with perhaps very small percentages of sulfur, phosprorus and the like constituting impurities; it is therefore unnecessary to include percentages of iron in each instance:

In addition to these elements, Ni and Cu may be added without detriment to the physical properties and will improve the corrosion resistance as is well known by those versed in the art; it will of course be appreciated that inclusion of an element or elements in the foregoing tabulation is not intended to mean that each or all except C, Mn, Si, Mo, B and Ti are necessary in a given steel.

A number of samples of steels of slightly diiierent analysis yet all within said ranges and after treatment in accordance with my novel method, were subject to tests to determine factors on which the aforesaid graphs are based; two such steels hereinafter designated steel A and steel B respectively, substantially identical in analysis with the exception that steel A contained 0.37% chromium while steel B was free of that element, contained the following in the amounts given:

More specifically, test pieces of these steels simulating plates of gages yl, 1/2 and 5/e" respectively were heated to 2100 F., cooled slightly, and hotworked prior to being subjected to treatment in accordance with the invention with the following results, the temperature of the pieces at the initiation of the treatment being 1700c F. in all cases:

TABLE 2 Spray Spray Air cool cool Air cool e001 time, To rate, time, To rate,

Steel sec. F. Fjsee. min/see. F. F./seo.

e Plate:

A1 and B1 35 1, 050 18. 6 2. 40 750 1. 88

A2 and B2 27 1, 150 20. 4 3.20 750 2.00

A3 and B3. 2l 1, 250 2l. 4 3. 50 750 2. 18 2li" Plate:

A4 and B4 50 1, 050 13. 0 5. 00 750 1.00

A5 and 135---- 40 1, 150 13. 8 6. 00 750 1. 11

B6 25 1, 250 18.0 8. 00 800 0. 94 Plate:

A7 and B7. 100 1, 050 6. 5 6. 20 800 0. 60

A8 and B8... 70 1, 150 7. 9 7. 50 800 0. 75

A9 and B9 55 1, 250 8. 2 9. 00 800 0. 83

When the foregoing pieces, after treatment, were tested to determine yield strength and tensile strength, and some to determine in addition impact transition temperature, the following results were observed:

TABLE 3 Impact Yield Tensile transition strength, strength, temperature p.s.i. p.s.i. FJ l 91, 000 113, 000 91, 000 110, 000 se, 000 107, 000 91, 000 108, 000 87, 00() 107, 500 se, 000 103, 000 97, 000 113, 000 110 95, 000 110, 000 -120 88, 000 107, 000 -90 se, 000 100, 000 -90 82, 000 s0, 00o 03, 000 e5, 400 se, 300 s0, 500 s0, 000 104, 500

1 size eharpy V-noteh, 5 ft. lbs.

Heating to 2100o F.-2200 F., in all instances precedes the predetermined controlled cooling of the steel in accordance with the invention and this may occur as an incident to other mill processes, for example as residual heat imparted in the normal course of production as in a soaking operation. The steel is then mechanically deformed preparatory to the subsequent specied cooling, and it is substantially immaterial what specific apparatus be employed for the spray cooling provided the rates of cooling be not substantially different from those I have found appropriate and so approximately within the limits of the treatments set forth in the foregoing Table 2.

The graph of FIG. l, based on the effect of various cooling rates in 1/2 steel plate of analysis within the ranges of Table 1 and containing, more specifically, 0.21% Mo and 0.0015% B, demonstrates that the cooling rates within the scope of the invention, i.e., about 822 F./sec. during the spray cooling to a temperature above 1050J F. and about 0.7-2.2 F./ sec. during subsequent air cooling confers a structure in the steel which is free of martensite and consists mainly of acicular ferrite and upper bainite.

The graphs of FIGS. 2 and 3 are designed to show the effects, plotted in increments of increasing yield strength in p.s.i., of inclusion in the low Mo steel of my invention of varying amounts of certain of the alloying elements heretofore mentioned; these results were derived from tests of samples simulating 1/2" plate.

Hence within the scope of my invention is the provision of steel of composition within the analysis of Table 1 and treatment thereof by controlled cooling from 1700 F. or higher initially at a relatively rapid rate by spray cooling for a limited period followed by controlled slower cooling in air for a considerably longer one, all to produce low alloy molybdenum steel of characteristics making it eminently suitable for fabrication into articles of commerce.

I claim:

1. A method of treating a steel composition containing U13-0.22% carbon, 0.69-l.28% manganese, 0.l5-0.40'% silicon, (L11-0.25 170 molybdenum, 0.0005-0.0015% boron, 0.0100.050% titanium, about 0.015-.050% of at least one element selected from the group consisting of columbium and vanadium, which method comprises:

(a) heating to about 2200 F.

(b) hot-working to about 1700a F.;

(c) spray-cooling from about 1700" F. to a range about 1250-1000'a F. at a rate of about 6.5-22 F. per second; and

(d) controllably air-cooling to SOO-750 F. at a rate of about 0.66-2.2 F. per second, to thereby produce a steel product having a micro-structure of acicular ferrite and upper bainite and characterized by the absence of martensite and by a yield strength of at least 80,000 psi.

2. A method as dened in claim 1 in which the steel contains columbium selected from said group in the proportion of about 0.0150.030%.

3. A method as defined in claim 1 in which the steel contains vanadium selected from said group in the proportion of about 0.023-0.050%.

4. A method as defined in claim 1 in which the steel contains columbium selected from said group in the proportion of about 0.0150.030% and vanadium selected from said group in the proportion of about 0.023-0.050%.

References Cited UNITED STATES PATENTS 2,857,299 10/1958 Epstein et al 14S-12.4 2,858,206 10/1958 Boyce et al 148-12 X 3,010,822 11/1961 Altenberger et al. 148-36 X 3,102,831 9/1963 Tisdale 148-12 3,163,565 12/1964 Wada 75-123 X 3,251,682 5/1966 Wada 75-123 X 3,290,183 12/1966 Ohtake et al 148-12 3,330,705 7/1967 Madrzyk et al 148-12 FOREIGN PATENTS 28,241 12/ 1965 Japan.

OTHER REFERENCES Irani et al.: Quenched and Tempered Low-Carbon Steels Containing Niobium or Vanadium, Journal of Iron & Steel Institute, July 1966.

HYLAND BIZOT, Primary Examiner