US3423252A - Thermomechanical treatment of steel - Google Patents

Thermomechanical treatment of steel Download PDF

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US3423252A
US3423252A US444676A US3423252DA US3423252A US 3423252 A US3423252 A US 3423252A US 444676 A US444676 A US 444676A US 3423252D A US3423252D A US 3423252DA US 3423252 A US3423252 A US 3423252A
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steel
austenite
ferrite
fibers
grains
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Raymond A Grange
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United States Steel Corp
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United States Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment

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  • the common constructional or engineering steels 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.
  • 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.
  • 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.
  • 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:
  • 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:
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • Example 1 relates to the treatment of an AISI 4320 steel containing:
  • 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.
  • 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.
  • 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.
  • the impact values of the products tempered to various levels of tensile strength are plotted in FIGURE 1.
  • 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:
  • 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.
  • 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.
  • Example 3 Samples from a hot rolled steel plate of the following composition:
  • 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.
  • 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:
  • Example 5 The steel was a plain carbon 1020 of the following composition:
  • thermomechanical treatment imparts exceptional low temperature toughness as well as raising the yield and tensile strengths above the values obtainable by conventional normalizing.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.

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US444676A 1965-04-01 1965-04-01 Thermomechanical treatment of steel Expired - Lifetime US3423252A (en)

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3502514A (en) * 1968-01-30 1970-03-24 United States Steel Corp Method of processing steel
US3645801A (en) * 1968-12-20 1972-02-29 Bethlehem Steel Corp Method of producing rolled steel having high-strength and low-impact transition temperature
US4047979A (en) * 1976-10-08 1977-09-13 United States Steel Corporation Heat treatment for improving the toughness of high manganese steels
US4067756A (en) * 1976-11-02 1978-01-10 The United States Of America As Represented By The United States Department Of Energy High strength, high ductility low carbon steel
US4088511A (en) * 1976-07-29 1978-05-09 Lasalle Steel Company Steels combining toughness and machinability
US4186037A (en) * 1975-09-12 1980-01-29 Italsider S.P.A. Thermal treatment of intermediate quenching and quick tempering through eddy currents and a device for applying said treatment to high productivity rolling plants for flat products
US4426235A (en) 1981-01-26 1984-01-17 Kabushiki Kaisha Kobe Seiko Sho Cold-rolled high strength steel plate with composite steel structure of high r-value and method for producing same
WO1984002354A1 (en) * 1982-12-09 1984-06-21 Univ California High strength, low carbon, dual phase steel rods and wires and process for making same
US4613385A (en) * 1984-08-06 1986-09-23 Regents Of The University Of California High strength, low carbon, dual phase steel rods and wires and process for making same
US4753691A (en) * 1986-02-25 1988-06-28 Nippon Steel Corporation Method of directly softening rolled machine structural steels

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS552743A (en) 1978-06-22 1980-01-10 Nippon Kokan Kk <Nkk> Steel excellent in damping performance and manufacture thereof
DE3326642C2 (de) * 1983-07-23 1986-07-24 Berchem & Schaberg Gmbh, 4650 Gelsenkirchen Verfahren zur Herstellung eines Schmiedestückes, insbesondere eines Gesenkschmiedestückes, aus einer niedriglegierten Stahllegierung

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
None *

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3502514A (en) * 1968-01-30 1970-03-24 United States Steel Corp Method of processing steel
US3645801A (en) * 1968-12-20 1972-02-29 Bethlehem Steel Corp Method of producing rolled steel having high-strength and low-impact transition temperature
US4186037A (en) * 1975-09-12 1980-01-29 Italsider S.P.A. Thermal treatment of intermediate quenching and quick tempering through eddy currents and a device for applying said treatment to high productivity rolling plants for flat products
US4088511A (en) * 1976-07-29 1978-05-09 Lasalle Steel Company Steels combining toughness and machinability
US4047979A (en) * 1976-10-08 1977-09-13 United States Steel Corporation Heat treatment for improving the toughness of high manganese steels
US4067756A (en) * 1976-11-02 1978-01-10 The United States Of America As Represented By The United States Department Of Energy High strength, high ductility low carbon steel
US4426235A (en) 1981-01-26 1984-01-17 Kabushiki Kaisha Kobe Seiko Sho Cold-rolled high strength steel plate with composite steel structure of high r-value and method for producing same
WO1984002354A1 (en) * 1982-12-09 1984-06-21 Univ California High strength, low carbon, dual phase steel rods and wires and process for making same
US4613385A (en) * 1984-08-06 1986-09-23 Regents Of The University Of California High strength, low carbon, dual phase steel rods and wires and process for making same
US4753691A (en) * 1986-02-25 1988-06-28 Nippon Steel Corporation Method of directly softening rolled machine structural steels

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NL6604399A (de) 1966-10-03
ES324784A1 (es) 1967-04-16
BE678804A (de) 1966-09-30
GB1137952A (en) 1968-12-27
DE1508450A1 (de) 1969-10-23

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