US3423252A

US3423252A - Thermomechanical treatment of steel

Info

Publication number: US3423252A
Application number: US444676A
Authority: US
Inventors: Raymond A Grange
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1965-04-01
Filing date: 1965-04-01
Publication date: 1969-01-21
Anticipated expiration: 1986-01-21
Also published as: BE678804A; GB1137952A; DE1508450A1; NL6604399A; ES324784A1

Description

Jan. 21, 1969 A, GRANGE 3,423,252

THERMOMECHANICAL TREATMENT OF STEEL" Filed April 1, 1965 Sheet of 2 IMP/167' VALUE A7 75%. I4 t E MM 2 5 l2 a K) b E E 0; /0 k a Q 8 ll 6 5 IMPACT VALUE AT-3/5 F. g? 4 LEGEND THERMOMECHAN/CALLY t I; TREATED SAJE 4320 sr EL 2 3f g CONVENTION/ILL) OUENCHED AND a rEMPEnEo ME 4320 TEEL 0 I40 I I I I I YIELD STRENGTH -/000 p. S. l'.

Attorney Jan. 21, 1969 Filed April 1, 1965 R. A. GRANGE Sheet 2 of 2v CONVENTION/ILL Y OUENCHED AND TEMPERED SAE 4320 STEEL o o 4 2 Q m g 3 1 Q 400 1 L g o 3 THERMOMECHAN/CALLY Q TREATED SAE 4320 STEEL I40 I60 I80 200 220 TENS/LE STRENGTH I000 p. S. I'.

INVENTOR.

RA YMOND A. GRANGE A I f arney United States Patent Claims ABSTRACT OF THE DISCLOSURE A method of producing a highly fibrous structure in hardenalble hypoeutectoid steel wherein the steel is heated to produce a structure composed of a mixture of austenite and ferrite grains, drastically reducing the steel to elongate such grains while maintaining such structure and thereafter cooling the steel to transform the austenite to the desired rnicrostructure.

The common constructional or engineering steels, whether plain carbon or low alloy variety, are hypoeutectoid steels. Accordingly, such steels constitutionally are mixtures of two phases, ferrite and carbide, in which the dispersion of the carbide phase largely determines the mechanical properties. It is conventional therefore, to heat treat these steels subsequent to hot working to develop particular combinations of properties. In the latter regard normalizing, which refines the grain and results in a microstructure consisting of pearlite interspersed between equi-axed grains of ferrite, has been used to improve formability and machinability and moderately increase strength and toughness; and, where maximum strength and toughness have been desired, it has been the practice to quench and temper to produce single constituent-microstructures of tempered martensite or bainite. While the latter treatment develops the highest level of strength combined with the best low temperature toughness available heretofore, formability of the materials is decreased.

I have discovered a new thermomechanical method, for the treatment of such treatment of such steels which produces a fine fibrous microstructure characterized by fibers of ferrite interspersed with fibers of a harder rnicroconstituent and imparts improved combinations of formability and machina-bility, strength and toughness, particularly low temperature toughness. This is evident from the drawing wherein:

FIGURE 1 is a graph showing a comparison of impact values of specimens of SAE 4320 steel having a conventional martensitic microstructure and specimens having a microstructnre of fibrous ferrite plus fibrous martensite resulting from the thermomechanical treatment of this invention, both tempered to the various tensile strengths indicated thereon; and

FIGURE 2 is a graph showing acomparison of diamond pyramid hardness values of specimens of SAE 4320 steel having a conventional martensitic microstructure and specimens having a microstructure of fibrous ferrite plus fibrous martensite resulting from the thermomechanical treatment, both tempered to the various tensile strengths indicated thereon.

As will be subsequently shown, this new method may be operated (1) to produce a fibrous structure of ferrite and pearlite to impart formability the equivalent of that achieved by normalizing together with considerably higher strength and better low temperature toughness than is produced by such conventional treatment, or (2) to produce a fibrous structure of ferrite and martensite;

bainite or mixtures of the latter to impart the same strength level but coupled with better formability and low temice perature toughness than is achieved by conventional quench and temper heat treatments.

The method is applicable to hardenable hypoeutectoid steel containing carbon between 0.05 and 016% and which may contain up to a total of 5% of the common alloying elements. Hardenable steel as used herein is defined as a steel which is predominantly ferritic at room temperature, becomes substantially austenitic on heating to suitable elevated temperature and thereafter becomes ferritic on cooling to room temperature. Regardless of alloy content of the steel, the best response to the treatment is obtained over the range 0.1 to .4% carbon.

The method of the present invention comprises the three steps:

(1) Heating-The steel must be brought to a predetermined temperature, T, within its critical range and at which it 'will transform to a mixture of approximately 50-50 austenite and ferrite. Temperature T varies with the carbon, maganese, silicon, chromium and nickel contents of the steel. It, of course, can be determined experimentally but is satisfactorily calculated from the following equation:

where Other elements which may be present, e.g. sulphur, phosphorus, aluminum, molybdenum, copper, etc., at least in the amounts present in conventional constructional steels, have little effect on the temperature, T, and consequently may be ignored. Heating may be carried out by raising the steel to treating temperature or alternatively by heating it above its critical range to effect full austenization and then cooling to the treating temperature to allow partial transformation of the austenite to ferrite. The steel must be soaked at temperature for suflicient time to uniformly heat throughout its mass and insure a reproducible mixture of austenite and ferrite. The actual temperature of the steel must be within 35 of calculated temperature T since achievement of the results of the invention require the presence of at least but not more than austenite in the subsequent steps.

(2) Mechanical working.Upon completion of heating, the steel consists of a mixture of equi-axed ferrite and austenite grains. The second step in the process comprises subjecting the steel in this condition to a drastic plastic deformation to substantiall elongate these grains. At least 25% deformation must be effected in this step and larger deformations of 50% or more are preferable; in general, the greater the deformation, the greater the ultimate benefit. It is equally important that the temperature of the steel during working is not allowed to vary appreciably from its temperature at the start of deformation, since the mixture of austenite and ferrite 'rnust persist into the final step of the process. The transformation of austenite to ferrite and vice versa, however, is not immediate upon change of temperature. Thus in practicing my invention, I have found that variations within the range +25 F. and -50 F. of the initial temperature have no substantial adverse affect when the working is completed within normal times, Conditions of working which result in a rise of more than 25 F. however must be avoided, and undue delays in completion of the working will require return of the steel to the heating step. Except as qualified above the time spent in mechanical working is not critical and the necessary deformation may be done in a single stage as by extrusion or impact-forging or in multiple stages as by several successive rolling passes or many forging blows.

(3) Transformation-Soaking at temperature T concentrates substantially all of the carbon of the steel in solid solution in the austenite grains formed at this temperature and constituting approximately 50% of its volume; the working step elongates these grains and the interspersed grains of ferrite into a fiber-like structure oriented in the principal direction of their elongation. The final step of the process contemplates the controlled transformation of the austenite fibers into fibers of a stronger, harder microconstituent. Inasmuch as several transformation products and the conditions which govern the production of each are known, several alternatives are available in the practice of the final step. Thus immediately following working, the austenite fibers may be transformed to fibers of martensite by rapidly quenching the steel from working temperature T to below its M temperature, or to fibers of bainite or mixtures of martensite and bainite by suitable and known modifications of the quenching procedure; or by use of relatively slow cooling, e.g., quenching in still air, the austenite fibers may be transformed to fibers consisting substantially of pearlite. Regardless of the rate of quench used, the steel will possess exceptionally good low temperature toughness; thus the choice of quench will depend primarily upon the levels of strength and formability desired. In the latter regard, the highest strength will be achieved by quenching to matensite while the best formability will be afforded by cooling to produce pearlite fibers.

The following specific examples will serve to illustrate application of the method in the treatment of particular steel compositions and the results and advantages thereof over conventional treatments.

Example 1 relates to the treatment of an AISI 4320 steel containing:

C 0.21 Mn 0.73 P .002 S .010 Si .25 Ni 1.80 Cr 0.80 Mo 0.22

For treatment by the method of the present invention, treating temperature was determined in accordance with the formula, page 2, and found to be about 1375 F. A half-inch thick plate of this steel was heated to this temperature, soaked for one hour, then rolled at this temperature to a thickness of 0.17 inch (approximately 65% reduction), following which it was immediately quenched in brine to below its M temperature. Metall-ographic examination of specimens of the so-treated material showed a fibrous mixed microstructure comprised of elongated ferrite interspersed in elongated volumes of martensite, the latter constituting about 55 to 60% of the mixture. For comparison, a similar plate of the same steel was not rolled at 1700 F. to the same thickness, following which it was given a conventional hardening treatment by soaking at 1700" F. and then brine quenching to below its M point. Metallographic examination showed the conventional treated product had a fully martensitic structure throughout the thickness thereof.

Both of the above products were subsequently tempered and their impact properties investigated over the range 75 F. to -315 F. using quarter-width Charpy V-notch (CVN) specimens. At room temperature (75 F.), both had substantially the same impact value, between 12 to 16 ft./lbs. depending upon the tempering temperature, and both exhibited a ductile fracture; however, in the conventional quenched and tempered specimens, transition to brittle fracture occurred at about 200 F.,

whereas the thermomechanical treated specimens broke with a ductible fracture even at -3l5 F. This superiority in low temperature toughness was found to exist at all tempering temperatures over the range 400 to 1000 F. Over this range, the highest impact value, i.e., energy absorption, was obtained in material tempered at 400 F.; the lowest, in material tempered in the neighborhood of 800 F. For comparison, the impact values of the products tempered to various levels of tensile strength are plotted in FIGURE 1. I

The mechanical properties of both products, tempered at 400 F. for two hours, are tabulated below in Table I:

1 0.2% ofiset yield strength. 2 At 315 F. using quarter-width OVN specimens.

As shown in the above tabulation, the tensile and yield strengths of A151 4320 steel are only about 8,000 p.s.i. less when treated by the method of the present invention and tempered at 400 F. This difference decreased as the tempering temperature was increased, the materials showing substantially the same tensile and yield values when tempered at 1000 F. The small amount of these differences in strength are remarkable considering that the thermomechanically treated material was comprised of almost /2 proeutectoid ferrite. The expected effect of such a volume of ferrite would be to soften and weaken. Hardness measurements on the two products tempered to the same level of tensile strength confirm the softening but deny the weakening, e.g., if both are tempered to 200,000 p.s.i. tensile strength, the conventionally treated fully martensitic material will have a diamond pyramid hardness of about 430 while that of the thermomechanically treated specimen will be about 400. Based on conventional interpretation of hardness, a drop of 30 points in this range should be accompanied by a drop of about 15,000 p.s.i. in tensile strength. The hardness vs. tensile strength curves for the two materials over the range of 140,000 to 220,000 p.s.i. tensile are shown in FIGURE 2. It is apparent from these curves that the present invention affords softer and therefore more formable and machinable steel without the sacrifice of strength heretofore thought necessary.

I attribute the unexpectedly high levels of strength achieved by my treatment to the higher carbon of the martensite in combination with the fibrous nature of the microstructure produced; the marked improvement in low temperature toughness, formability and machinability, to the large portion of ferrite fibers interleaving the fibers of the harder stronger microconstituent of this structure.

Example 2.--The effect of the fibrous nature, compared to the non-fibrous condition developed by thermal treatment, of the structure produced by the method of the present invention is shown in the following example wherein steel of the following composition:

Mn .73 P .008

S .005 Si .29 Ni 1.76 Cr .46

was subjected to:

Treatment A (thermomechanical).Bars of the steel, %-inch thick were heated to 1700 F. for twenty minutes, then quenched into a lead pot at 1350 F. and held for one hour, after which they were rolled in five passes to 0.165-inch thickness (percent reduction=78% To insure that the rolling was conducted at 1350 F., the samples were returned to the lead pot after each pass. After the last pass, the steel was quenched in brine and subsequently tempered at 400 F. for two hours. Metallographic examination showed the treatment produced the desired fibrous microstructure of ferrite and martensite in proportions of about 5050.

Treatment B (thermal).Bars of the steel previously reduced to 0.165-inch thick by conventional rolling practices were given treatment identical with the above except that the mechanical working step was omitted. Metallographic examination of this product showed a substantially uniform mixture of equi-axed grains of ferrite and martensite; in addition to the lack of a fibrous character, the structure was considerably coarser than that produced in Treatment A.

The mechanical properties of the steel as modified by Treatments A and B are tabulated below in Table II:

TABLE II Treat- Treatment merit B A" (Thermo- (Thermal) mechanical) Tensile strength, p.s.i 143, 800 165,200 Yield strength, p.s.i- 97, 200 122, 200 Elongation, percent in 1 9. 5 11. 5 CVN Impact-Fracture Tran perature, F 30 275 It is evident that all properties are enhanced by the thermomechanical treatment and specifically that the fibrous nature of the structure produced by .this treatment is a major factor in the improvement, particularly as regards the improvement in low temperature toughness.

The effect of temperature in the mechanical working step or, more precisely, the effect of the volume of austenite fraction present, are illustrated in Example 3 below. Samples from a hot rolled steel plate of the following composition:

Other elements: Normal residual amounts C .18 Mn .67 Si .17 Ni 1.07

TABLE III Treating Temperature, F 1,300

Volume Fraction of Austenite prior to Quench 0. 1 0.25 0.6 0.8 0.2% Ofiset Yield Strength (p.s.i.) 66, 9090 77, 6;)0 77,403 111,302

Elongation (percent in 1) 6 Impact Value at 80 F. (FL/Lb.) 12. 5 10 13. 5 10 Impact Value at 200" F. (Ft./Lb.) 1 6 11 7.5

1 Quarter-width CVN specimens.

It is evident that optimum effects, particularly as regards ductility and low temperature toughness, are associated with an austenite volume fraction in the neighborhood of 0.5 in the working step. Accordingly, it is preferable to conduct the present treatment at a temperature as near as practical to the mid-temperature of the critical range of the steel. As previously explained, this temperature varies with the chemical composition of the steel but may be determined or calculated using the formula above described. The above data show benefits are largely lost if the austenite fraction is as low as 0.25 or as high as 0.8%. However, I have found the benefits of the invention are largely obtained over the range of /3 to austenite. Accordingly, the treatment may be conducted at any temperature which insures the presence in and maintenance of austenite within this range during the mechanical working step.

As mentioned earlier, quenching the steel following the thermomechanical treatment so as to transform the austenite fibers to fibers of martensite imparts the highest strength level. Where ease of forming is a primary consideration, cooling so as to transform to pearlite fibers should be practiced. The following examples illustrate the application of the present method to the latter end and the results thereof.

Example 4.--A plate of the 0.2% carbon-1.0% nickel steel used in Example 3 was heated to 1700 F. and quenched to below the M point after which it was soaked at 1350" F. for one hour, rolled at this temperature to effect 65% reduction in thickness and allowed to cool in still air to room temperature. The preliminary heating to 1700 F. and quenching refines the grain of the steel and, while not essential to the results of the present invention, is nevertheless beneficial and represents a preferred practice. The microstructure of the product consisted of fine fibers of ferrite and pearlite oriented in the direction of rolling. For comparison, a second plate of the same steel was given a conventional normalizing treatment consisting of soaking at 1700- F. for 20 minutes and air cooling. The microstructure of this product was fine pearlite and equi-axed grains of ferrite. The mechanical properties of the two products are compared below in Table IV:

TABLE IV Treatment Thermo- Normalmechanical ized Tensile strength, p.s.i- 87, 500 78, 400 Yield strength, p.s i 74, 800 50,900 Elongation in l"... 24. 5 28 CVN Impact-Fracture Transition Temperature, F 280 The superiority for the plate processed by the thermomechanical method of the present invention is self-evident. Yield strength is almost 50% higher and, the low temperature toughness is markedly improved. In fact, the low temperature toughness is better than that achievable in his grade of steel when conventionally quenched and tempered.

Example 5.The steel was a plain carbon 1020 of the following composition:

C .20 Mn 0.53 P .016 S .030 Si 0.24 A1 0.031

TABLE V Treatment Therrno- Normalmechanical ized Tensile strength, p.s.i 73,000 67, 500 Yield strength, p.s.i 57, 45, 000 Elongation, percent in 1 33. 5 38 CVN Impact-Fracture Transition Temperature, F 40 As in the previous example, it is evident that the thermomechanical treatment imparts exceptional low temperature toughness as well as raising the yield and tensile strengths above the values obtainable by conventional normalizing.

While I have described certain specific embodiments of my invention, it is obvious that modifications can be made without departing from the scope of the appended claims.

I claim:

1. A method of treating hardenable hypoeutectoid steel to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within its critical range to produce a microstructure that is at least /a but not more than austeuite with the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing said steel to substantially elongate said grains, and cooling said steel to transform said austenite.

2. A method of treating hardenable hypoeutectoid steel to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within its critical range to produce a microstructure that is at least /3 but more than /3 austenite with the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing said steel to effect a reduction of at least 25% to substantially elongate said grains while maintaining the temperature of the steel in the range of +25 to 50 F. of said heating temperature, and cooling said steel to transform said austenite.

3. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within its critical range to produce a microstructure comprising at least austenite but not more than /3 austenite with the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing said steel to effect a reduction of at least 25% to substantially elongate said grains into fibers, and cooling said drastically reduced steel at a rate to transform the austenite to pearlitic, martensitic or bainitic microconstituents and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite.

4. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within its critical range to produce a microstructure comprising at least /3 but not more than /a austenite with the balance ferrite and wherein grains of austenite are inter-spersed with grains of ferrite, drastically reducing said steel to effect a reduction of at least 25 to substantially elongate said grains into fibers, and cooling said drastically reduced steel at a rate to substantially transform the austenite to pearlite to produce steel characterized by fibers of pearlite interspersed with fibers of ferrite.

5. A method of treatin ghardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within its critical ange to produce a microstructure comprising at least /3 but not more than austenite with the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing said steel to effect a reduction of at least 25% to substantially elongate said grains into fibers, cooling said drastically reduced steel at a rate to transform the austenite to martensitic or bainitic microconstituents and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite and tempering the steel to the desired tensile strength.

6. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within its critical range to produce a microstructure comprising at least /3 but not more than /3 austenite with the balance ferrite and Wherein grains of austenite are interspersed with grains of ferrite, drastically reducing said steel to effect a reduction of at least 25% to substantially elongate said grains into fibers while maintaining the temperature of the steel in the range of +25 to 50 F. of said heating temperature, and cooling said drastically reduced steel at a rate to transform the austenite to pearlitic, martensitic or bainitic and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite.

7. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within 35 of the middle of its critical range to produce a microstructure comprising at least but not more than austenite and the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing the steel to effect a reduction of at least 25% to substantially elongate said grains into fibers, and cooling said drastically reduced steel at a rate to transform the austenite to pearlitic, martensitic or bainitic microconstituents and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite.

8. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within 35 of the middle of its critical range to produce a microstructure that is at least 6 but not more than /3 austenite and the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing the steel to effect a reduction of at least 25% to substantially elongate said grains into fibers, and cooling said drastically reduced steel at a rate to substantially transform the austenite to pearlite to produce steel characterized by fibers of pearlite interspersed with fibers of ferrite.

9. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within 35 of the middle of its critical range to produce a microstructure comprising at least /3 but not more than /2, austenite and the balanced ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing the steel to effect a reduction of at least 25 to substantially elongate said grains into fibers, cooling said drastically reduced steel at a rate to transform the austenite to martensitic or bainitic microconstituents and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite and tempering the steel to the desired tensile strength.

10. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within 35 of the middle of its critical range to produce a microstructure comprising at least /3 "but not more than /a austenite with the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing the steel to effect a reduction of at least 25% to substantially elongate said grains into fibers while maintaining the temperature of the steel within the range of +25 to -50 F. of said heating temperature, and cooling said drastically reduced steel at a rate to transform the austenite to martensitic or bainitic microconstituents and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite.

11. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than alloying elements to provide 'high strength combined with good formability, machinability and toughness comprising heating the steel to within 35 of the middle of its critical range as determined by the formula where T=the treating temperature in F.

1333- (25Xpercent Mn) +(40Xpercent Si) (26 Xpercent Ni) (42 X percent Cr), and A,=1570(323 X percent C) (25 Xpercent Mn) (80 X percent Si) (32 X percent Ni) (3 X percent Cr) to produce a microstructure comprising at least /3 and not more than austenite with the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing the steel to effect a reduction of at least 25% to substantially elongate said grains, and cooling said drastically reduced steel at a rate to transform the austenite to pearlitic, martensitic or bainitic microconstituents and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite.

12. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within 35 of the middle of its critical range as determined by the formula n+ f T- 2 where T=the treating temperature in F. A 133 3- (25 Xpercent Mn) (40 X percent Si) (26 X percent Ni) (42 X percent Cr), and A l570(323X percent C) (25 X percent Mn) 80 X percent Si) (32 X percent Ni) (3 X percent Cr) to produce a microstructure comprising at least /3 and not more than austenite with the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing the steel to effect a reduction of at least 25% to substantially elongate said grains into fibers, and cooling said drastically reduced steel at a rate to substantially transform the austenite to pearlite to produce steel characterized by fibers of pearlite interspersed with fibers of ferrite.

13. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within 35 of the middle of its critical range as determined by the formula where T=the treating temperature in F. A 1333 (25 X percent Mn) (40X percent Si) (26X percent Ni) (42 X percent Cr), and A =1570(323 X percent C) (25 Xpercent Mn) (80 X percent Si) (32 X percent Ni) (3 Xpercent Cr) to produce a microstructure comprising at least /3 but not more than /3 austenite with the balance ferrite and wherein grains of austenite are interspersed with grams of ferrite, drastically reducing the steel to effect a reduction of at least 25% to substantially elongate said grains into fibers, cooling said drastically reduced steel at a rate to transform the austenite fibers to martensitic or bainitic microconstituents and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite and tempering the steel to the desired tensile strength.

14. A method of treating hardenable hypoeutectoid steel containing between 0.05 and 0.6% carbon and less than 5% alloying elements to provide high strength combined with good formability, machinability and toughness comprising heating the steel to within 35 of the middle of its critical range as determined by the formula where T=the treating temperature in F. A 1333 (25 X percent Mn) (40 X percent Si) (26X percent Ni) 42 X percent Cr), and A;=1570(323 Xpercent C) (25 Xpercent Mn) ('80 X percent Si) (32 X percent Ni) 3 X percent Cr) to produce a microstructure comprising at least /3 but not more than /a austenite with the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing the steel to effect a reduction of at least 25 to substantially elongate said grains into fibers while maintaining the temperature of the steel within the range of +25 to 50" F of said heating temperature, and cooling said drastically reduced steel at a rate to transform the austenite to pearlitic, martensitic or bainitic microconstituents and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite.

15. A method of treating hardenable hypoeutectoid steel containing between 0.10 and 0.4% carbon and less than 5% alloying elements to provide high strength and good formability, machinability and toughness comprising heating the steel to within 35 of the middle of its critical range as determined by the formula where T=the treating temperature in F.

A =1333 (25 X percent Mn) (40 X percent Si) (26 X percent Ni) 42 X percent Cr), and

A l570( 323 X percent C) (25 Xpercent Mn) (X percent Si) (32 X percent Ni) (3 X percent Cr) to produce a microstructure comprising at least /3 but not more than /3 austenite with the balance ferrite and wherein grains of austenite are interspersed with grains of ferrite, drastically reducing the steel to effect a reduction of at least 25% to substantially elongate said grains into fibers while maintaining the temperature of the steel within the range of +25 to -50 F. of said heating temperature, cooling said drastically reduced steel at a rate to transform the austenite to martensitic or bainitic microconstituents and mixtures thereof to produce steel characterized by fibers of said microconstituents interspersed with fibers of ferrite and tempering said steel to the desired tensile strength.

16. A new article of manufacture comprising hardenable hypoeutectoid steel containing between .05 and 0.6% carbon and less than 5% alloying elements having a fibrous microstructure produced by the method of claim 1.

17. A new article of manufacture comprising hardenable hypoeutectoid steel containing between .05 and 0.6% carbon and less than 5% alloying elements having a fibrous microstructure produced by the method of claim 15.

References Cited Hot-Cold Working of Steel to Improve Strength, 5

DMIC Report 192, October 1963, pp. 9-18 relied on.

Metals Eng. Quarterly, vol. 1, No. 1, February 1961, pp. 41 and 47 relied on.

L. DEWAYNE RUTLEDGE, P imary Examiner.

WAYLAND W. STALLARD, Assistant Examiner.

US. Cl. X.R. 148l2.4

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,423,252 January 21, 1969 Raymond A. Grange It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below: Column 1, line 42, after "treatment", first occurrence, cancel "of such treatment". Column 3, line 60, "not" should read hot Column 4, line 2, ductible-" should read ductile line 8, after "the" second occurrence, insert two Column 6, line 45, "his" should read this Column 7, line 1, "treatin ghardenable" should read treating hardenable Column 8, line 14, after itic" insert microconstituents line 53, "balanced" should read balance Signed and sealed this 24th day of March 1970.

( E Attest:

WILLIAM E. SCHUYLER, IR.

Commissioner of Patents