US20110048588A1 - Cold-rolled steel sheet and method for manufacturing the same - Google Patents

Cold-rolled steel sheet and method for manufacturing the same Download PDF

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Publication number
US20110048588A1
US20110048588A1 US12/919,780 US91978009A US2011048588A1 US 20110048588 A1 US20110048588 A1 US 20110048588A1 US 91978009 A US91978009 A US 91978009A US 2011048588 A1 US2011048588 A1 US 2011048588A1
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steel sheet
cold
less
rolling
rolled steel
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Inventor
Tetsuya Mega
Nobuyuki Nakamura
Takashi Kobayashi
Yasunobu Nagataki
Takayoshi Kabeya
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KABEYA, TAKAYOSHI, MEGA, TETSUYA, NAGATAKI, YASUNOBU, KOBAYASHI, TAKASHI, NAKAMURA, NOBUYUKI
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/40—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rings; for bearing races

Definitions

  • This disclosure relates to a cold-rolled steel sheet which is suitable for use in clutch plates, synchronizer rings, and clutch discs that are mechanical parts of automotive transmissions and which has excellent strength, punching workability, and heat deformation resistance and also relates to a method for manufacturing the cold-rolled steel sheet.
  • Automotive transmissions are composed of clutch plates, synchronizer rings, clutch discs, and the like and have a function of transmitting a driving force and a function of absorbing heat generated by friction. Such parts are manufactured by punching steel sheets into ring-shaped plates.
  • the transmissions have a structure, formed by piling up the ring-shaped plates, transmitting torque and need to have wear resistance and plate flatness. Therefore, properties required for steel sheets that are materials for the transmissions are excellent hardness, punching properties (punched surface properties such as flatness and burring), and deformation resistance during heating, that is, heat deformation resistance.
  • S35C cold-rolled steel sheets which have been conventionally used principally as materials for clutch plates for automotive transmissions (hereinafter referred to as ATs).
  • the S35C cold-rolled steel sheets are manufactured in such a manner that slabs are subjected to the following steps: hot rolling, pickling, annealing, and then cold rolling. Since hot-rolled steel sheets for the S35C cold-rolled steel sheets contain a large amount of C (about 0.35 mass percent) and therefore have high hardness, the hot-rolled steel sheets need to be annealed for a long time not less than several hours for the purpose of spheroidizing and softening a carbide before being cold-rolled. This is very disadvantageous in cost for automotive parts that need to be inexpensive.
  • JP 2003-277883 A discloses a technique in which the annealing of a hot-rolled steel sheet that is unsubjected to cold rolling is omitted. That is, the document proposes a cold-rolled steel sheet for AT clutch plates.
  • the cold-rolled steel sheet has desired hardness, desired surface roughness, excellent wear resistance, and excellent punching workability and is manufactured in such a manner that a soft hot-rolled steel sheet containing 0.25 mass percent C is cold-rolled at a rolling reduction of 50% or more.
  • the steel sheet has a problem that properties of a punched edge thereof are extremely deteriorated during punching because of residual stress caused by cold rolling and also has a problem that ring-shaped products have extremely low flatness due to thermal strain caused by an increase in temperature.
  • JP 2005-200712 A discloses a technique for improving residual stress caused by cold rolling. That is, this document proposes a cold-rolled steel sheet, having reduced residual stress, for AT clutch plates.
  • This cold-rolled steel sheet is manufactured in such a manner that a steel sheet is cold-rolled and then further rolled at a rolling reduction of about 1% under light load using large rolls with a diameter of 300 mm or more.
  • the difference in strain between the front and back of the steel sheet is reduced and therefore properties of a punched edge thereof are improved during press punching; however, ring-shaped products have extremely low flatness because the residual stress in the steel sheet is not relieved and therefore deformation due to thermal strain caused by an increase in temperature cannot be avoided.
  • JP 2004-107722 A proposes a steel sheet, having excellent adhesion to a friction material, for AT clutch plates.
  • the surface roughness thereof is optimized by a surface roughness-adjusting treatment such as pickling.
  • the steel sheet is hot-rolled, pickled, annealed at 500-800° C. for three hours or more for the purpose of spheroidizing a carbide, temper- or cold-rolled at a rolling reduction of 1% or more, and then subjected to a surface roughness-adjusting treatment.
  • Ring-shaped products have extremely low flatness due to residual stress caused by cold rolling. This has not been solved as similar to those described in JP 2003-277883 A and JP 2005-200712 A.
  • a cold-rolled steel sheet in which residual strain is relieved by annealing subsequent to cold rolling may be used as a material instead of directly using a cold-rolled steel sheet as a material in a conventional way.
  • a required hardness cannot be obtained.
  • C, Mn, and P which are components of steel, significantly contribute to the hardness of a steel sheet and, therefore, the use of an appropriate amount of these components is effective in stably obtaining a required hardness.
  • the mean diameter of ferrite needs to be within an appropriate range to achieve a sufficient hardness.
  • the mean diameter of ferrite is greater than 10 ⁇ m, a desired hardness cannot be achieved.
  • the mean diameter of ferrite is less than 2 ⁇ m, the hardness is excessively high and therefore the press punching properties are low.
  • the mean diameter of ferrite is preferably within a range from 4 to 8 ⁇ m.
  • the mean diameter of ferrite is determined by observing a cross section of a steel sheet that is perpendicular to the rolling direction thereof in accordance with the cutting method specified in JIS G 0551 (appendix).
  • the cold-rolled steel sheet has a partially recrystallized grain structure consisting of an unrecrystallized grain structure and a recrystallized grain structure.
  • the steel sheet needs to contain a certain amount of unrecrystallized grains in which rolling strain applied to the steel sheet during cold rolling remains to secure the hardness of a steel sheet. That is, to balance high hardness, punching workability, and heat deformation resistance, the ratio of an unrecrystallized grain structure to a recrystallized grain structure is important.
  • the degree of unrecrystallization needs to be 25% or more to obtain a desired hardness by the effect of cold rolling.
  • the degree of unrecrystallization is greater than 90%, the number of recrystallized grains is extremely small. This causes a deterioration in punching workability and a deterioration in flatness due to an extreme increase in residual strain. Therefore, the degree of unrecrystallization is preferably 90% or less and more preferably within a range from 40% to 80%.
  • the degree of unrecrystallization may be determined in such a manner that the percentage (area percentage) of an unrecrystallized grain structure in a microstructure is determined by observing a cross section of a steel sheet that is perpendicular to the rolling direction thereof.
  • C is an element that is important in view of the hardness and wear resistance of the cold-rolled steel sheet.
  • An increase in content of C increases the hardness and wear resistance thereof. Therefore, to obtain a desired hardness and wear resistance, the content of C is 0.01% or more.
  • the content of C is greater than 0.15%.
  • punching workability is deteriorated.
  • the difference in deformation strain between the front and back is increased during punching and deformation due to thermal strain is increased during heating. This causes a deterioration in flatness of a punching material. Therefore, the content of C is limited to a range from 0.01% to 0.15%.
  • the content of C is preferably within a range from 0.05% to 0.15% and more preferably 0.10% to 0.15%.
  • the content of S is limited to 0.03% or less.
  • the content of S is preferably 0.02% or less and may be 0%. In current refining technology, the lower limit of the content of S is about 0.005% without significantly increasing the steel production cost.
  • Mn is an element that fixes S, which is present in steel as an impurity, in the form of a precipitate (MnS) to reduce negative effects caused by S.
  • the content of Mn needs to be 0.10% or more to obtain this advantage.
  • Mn hardens steel by solid solution hardening.
  • the content of Mn is greater than 0.70%, defects resulting from scales are likely to be caused on surfaces of a hot-rolled steel sheet and it is difficult to completely remove the scales by performing pickling after hot rolling.
  • the content of Mn is limited to a range from 0.10% to 0.70%.
  • the content of Mn is preferably 0.50% or less and more preferably within a range from 0.20% to 0.50%.
  • P is an element that hardens steel by solid solution hardening.
  • the content of P is greater than 0.025%, slab cracking and/or surface defects of steel sheets are caused. Furthermore, a significant increase in hardness of steel is caused and therefore punching workability is deteriorated. Therefore, the content of P is limited to 0.025% or less.
  • the content of P is preferably 0.023% or less.
  • the content of P is less than 0.01%, the hardening effect thereof is low. Therefore, the content of P is preferably 0.01% or more.
  • S is an element that is present in steel as an impurity. Coarse inclusions are formed when the content of S is greater than 0.025%. The inclusions cause work cracking, leading to a serious deterioration in punching workability. S affects the scale removal of hot-rolled steel sheets. When the content of S is greater than 0.025%, surface properties of a pickled steel sheet are deteriorated. This increases the surface roughness of the steel sheet that has been cold-rolled and then annealed. Therefore, the content of S is limited to 0.025% or less and is preferably 0.020% or less.
  • Al is an element used for the deoxidization of steel.
  • the content of Al is less than 0.01%, a sufficient deoxidization effect cannot be obtained.
  • the content of Al is greater than 0.05%, such a deoxidization effect is saturated. Therefore, the content of Al is limited to a range from 0.01% to 0.05%.
  • the content of Al is preferably within a range from 0.03% to 0.05%.
  • N is an element that is present in steel as an impurity.
  • the content of N is greater than 0.008%, a steel sheet is excessively hardened and, therefore, punching workability thereof is deteriorated. Therefore, the content of N is limited to 0.008% or less and is preferably 0.005% or less.
  • C* is an index representing the hardness of the steel sheet.
  • a reason for specifying C* is described below with reference to FIG. 1 prepared on the basis of Example 1 below.
  • HRB hardness
  • C* satisfies conditions defined by Formula (1).
  • Components other than those described above are Fe and unavoidable impurities. It is possible that the steel sheet contains other components than those described above unless the advantages obtained are reduced.
  • a transmission has a structure in which several ring-shaped plates prepared by punching a steel sheet are stacked to transmit a torque. Therefore, the steel sheet used needs to have wear resistance and therefore needs to have a hardness (HRB) of 83 or more, which is sufficient to secure the wear resistance thereof.
  • HRB hardness
  • the hardness thereof is less than 83 in HRB, a reduction in wear resistance is problematic.
  • the hardness thereof needs to be 83 or more in HRB.
  • the hardness thereof is greater than 95 in HRB, the shape of a punched piece is faulty or cracks or fractures are caused in the steel sheet during punching. Therefore, the hardness thereof is preferably 95 or less in HRB.
  • the steel sheet preferably has a surface roughness Ra of 0.3 ⁇ m or less on an arithmetic average basis.
  • the lower limit of the surface roughness thereof is about 0.1 ⁇ m at the current technology level unless manufacturing cost is not significantly increased.
  • a slab having the above composition is hot-rolled at a finishing temperature not lower than the Ar 3 transformation point thereof.
  • the hot-rolled steel sheet is coiled at a coiling temperature of 580° C. to 750° C., pickled, and then cold-rolled at a rolling reduction of 65% or more.
  • the steel sheet is annealed at a temperature of 680° C. or lower in a continuous annealing furnace.
  • a method for producing the slab is not particularly limited and may be a usual one.
  • a steel converter and a continuous caster are preferably used for such production and casting, respectively, of the slab.
  • the finishing temperature In hot rolling, the finishing temperature needs to be not lower than the Ar 3 transformation point in view of the quality and rolling efficiency of the hot-rolled steel sheet.
  • the finishing temperature When the finishing temperature is lower than the Ar 3 transformation point, the ferritic transformation of the hot-rolled steel sheet is accelerated and therefore the following problem is caused: a problem that the hot-rolled steel sheet has a reduced hardness because coarse grains are formed in surface layers.
  • the coiling temperature In subsequent coiling, the coiling temperature needs to be within a range from 580° C. to 750° C. When the coiling temperature is lower than 580° C., crystal grains are extremely fine and the hot-rolled steel sheet is hardened by cooling strain, which inhibits cold rollability.
  • the coiling temperature is higher than 750° C.
  • the mean diameter of ferrite is increased after coiling, surface properties of the steel sheet are deteriorated because the formation of scales on surfaces of the steel sheet is excessive accelerated, and the surface roughness thereof is significantly deteriorated.
  • the coiling temperature is preferably within a range from 600° C. to 720° C.
  • the Ac 3 transformation point can be determined by the measurement of thermal expansion using a differential thermal dilatometer or the like.
  • the hot-rolled steel sheet is pickled by a usual process and scales are removed from surfaces of the hot-rolled steel sheet, the hot-rolled steel sheet is cold-rolled.
  • the rolling reduction of cold rolling needs to be 65% or more. This is because the hardness is increased by causing ferrite grains to be fine and the surface roughness is reduced during annealing performed subsequently to cold rolling.
  • the lower limit of the rolling reduction is not particularly limited. In the case of cold-rolling the hot-rolled steel sheet at a rolling reduction exceeding 85%, the following problems are concerned: the faulty shape of the hot-rolled steel sheet, a reduction in thickness accuracy of the hot-rolled steel sheet, and an extreme increase in load of a cold rolling mill. Therefore, the rolling reduction is preferably 85% or less.
  • the cold-rolled steel sheet is annealed in the continuous annealing furnace. It is particularly important that annealing is performed at a temperature not higher than the recrystallization finishing temperature after cold rolling. When the annealing temperature exceeds the recrystallization finishing temperature, substantially 100% of the microstructure is transformed into a recrystallized grain structure by annealing and therefore rolling strain induced during cold rolling is lost. Thus, a desired high hardness cannot be obtained.
  • a partially recrystallized grain structure in which unrecrystallized grains and recrystallized grains coexist can be obtained by setting the annealing temperature to the recrystallization finishing temperature or lower.
  • the degree of unrecrystallization is determined by the annealing temperature. Hence, the degree of unrecrystallization can be adjusted to 25% or more by setting the annealing temperature to 680° C. or lower.
  • the lower limit of the annealing temperature is not particularly limited and is preferably 500° C. or higher in view of the control of the temperature and atmosphere of the continuous annealing furnace and the production efficiency thereof.
  • the cooling rate of the annealed steel sheet is not particularly limited and is preferably about 5 to 25° C./s. In view of the shape stability of the steel sheet and the adjustment of the surface roughness of the steel sheet during temper rolling in the case of performing temper rolling, it is advantageous that the steel sheet is heat-retained within a temperature range from 320° C. to 420° C. in the course of cooling.
  • the degree of unrecrystallization can be appropriately adjusted by controlling the annealing temperature.
  • the relationship between the degree of unrecrystallization and the annealing temperature for example, the relationship between an apparent degree of unrecrystallization determined as described below and an annealing temperature for achieving the apparent degree of unrecrystallization is determined in advance and an annealing temperature for achieving a desired degree of unrecrystallization may be determined on the basis of this relationship:
  • the apparent degree of unrecrystallization is preferably determined in such a manner that the cold-rolled steel sheet is annealed, water-quenched, and then measured for hardness.
  • the degree of unrecrystallization of a steel sheet can be estimated in such a manner that the relationship between the apparent degree of unrecrystallization determined as described above and the hardness of the steel sheet is determined in advance and the apparent degree of unrecrystallization is determined from the hardness of the annealed steel sheet on the basis of this relationship.
  • the annealed cold-rolled steel sheet may be temper-rolled under light load. This is because the surface roughness thereof is adjusted and the hardness thereof is further improved.
  • the rolling reduction is preferably 2% or more in terms of elongation.
  • the upper limit of the rolling reduction is not particularly limited. When the rolling reduction is excessively high, the steel sheet is uneven in shape. In consideration of the capacity of a rolling mill used for temper rolling, the rolling reduction is preferably 5% or less in terms of elongation.
  • the steel sheets After being annealed, the steel sheets were cooled at a rate of 10° C./s, heat-retained at a temperature of 320° C. to 420° C. for 2.5 minutes, and then cooled to room temperature. The steel sheets were further rolled at a rolling reduction (elongation) of 3.0% under light load on a temper rolling line.
  • the degree of unrecrystallization was determined in the same manner as that used to determine the mean diameter of ferrite. That is, a cross section of each steel sheet that was perpendicular to the rolling direction thereof was observed at 800 times magnification and the area percentage of an unrecrystallized grain structure was thereby determined. The area percentage thereof was determined to be the degree of unrecrystallization.
  • the arithmetic average surface roughness Ra was determined in accordance with the measurement method specified in JIS B 0601.
  • a sample with a size of 20 mm ⁇ 60 mm was cut out of each steel sheet and then measured in accordance with the Rockwell hardness test method specified in JIS Z 2245. Ten portions of the sample were measured on the B-scale and the measurements thereof were averaged, whereby the hardness (HRB) was determined.
  • a ring-shaped specimen having an inner diameter of 140 mm and an outer diameter of 160 mm was prepared from each steel sheet in such a manner that the steel sheet was punched with a press-type punching machine at a clearance equal to 10% of the thickness of the steel sheet (a thickness of 1.5 mm).
  • the punched edge of the steel sheet that was perpendicular to the rolling direction thereof was observed with an optical microscope with a magnification of ten to 20 times.
  • the steel sheet was evaluated in accordance with standards below:
  • the ring-shaped specimen used to evaluate punching workability was heated at 300° C. for 30 minutes and then air-cooled to room temperature.
  • the camber of the resulting specimen was evaluated.
  • a specimen with a camber of 0.1 mm or less is evaluated to be good.
  • the camber thereof was measured as described below. After both surfaces of the specimen heated and then air-cooled were polished with a piece of 800 or finer emery paper, the specimen was put on a platen. Circumferentially arranged ten portions of the specimen were measured for height with a contact-type height gauge and were also measured for thickness with a micrometer. The difference between the height and thickness of each portion was determined. The maximum difference was determined to be the camber.
  • every inventive steel has a desired mean diameter of ferrite, degree of unrecrystallization, and surface roughness and is excellent in hardness (HRB), punching workability, and heat deformation resistance.
  • the steel sheets After being annealed, the steel sheets were cooled at a rate of 10° C./s, heat-retained at a temperature of 320° C. to 420° C. for 2.5 minutes, and then cooled to room temperature. The steel sheets were further rolled at a rolling reduction (elongation) of 0% to 3.5% under light load on a temper rolling line.
  • Temper Mean unre- temper- temper- rolling temper- rolling diameter crystal- Surface Hard- ature ature reduction ature reduction of ferrite lization roughness ness Punching Camber Steels No.
  • Steels A, I, and J which are inventive steels, are useful in obtaining a desired mean diameter of ferrite, a desired degree of unrecrystallization, a desired surface roughness, excellent hardness (HRB), excellent punching workability, and excellent heat deformation resistance when finishing temperatures during hot rolling, rolling reductions during cold rolling, annealing temperatures, and rolling reductions (elongations) during temper rolling are within appropriate ranges.
  • Steels B to H which are comparative steels, have a mean diameter of ferrite, a degree of unrecrystallization, and a surface roughness of which one is outside an appropriate range although finishing temperatures during hot rolling, rolling reductions during cold rolling, and annealing temperatures are within appropriate ranges. Therefore, obtained hardness (HRB), punching workability, or heat deformation resistance is not as good as expected herein.
  • (HRB(S))′ and (HRB(H))′ were determined as described below.
  • the unannealed cold-rolled steel sheets were heated at 580° C. or 780° C. for a time of 100 seconds or shorter and then water-quenched. Samples taken therefrom were measured for hardness (HRB) and observed for microstructure. After it was confirmed that the samples heated at 580° C. contained no recrystallized grain structure, these samples were measured for hardness (HRB), whereby (HRB(H))′ was determined. After it was confirmed that the samples heated at 780° C. contained a completely recrystallized grain structure, these samples were measured for hardness (HRB), whereby (HRB(S))′ was determined.
  • the following sheet can be obtained in such a manner that steel components are adjusted and the microstructure of steel is transformed into a partially recrystallized grain structure: a cold-rolled steel sheet which is suitable for use in automotive transmission parts such as clutch plates and which has high strength (high hardness), excellent punching workability, and excellent heat deformation resistance.
  • Annealing subsequent to cold rolling may be performed for a short time shorter than one hour and can be performed using a continuous annealing furnace with extremely high production efficiency.
  • the cold-rolled steel sheet can be manufactured without a significant increase in cost and therefore is suitable for use in automotive parts experiencing severe price competition.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
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US12/919,780 2008-02-29 2009-02-26 Cold-rolled steel sheet and method for manufacturing the same Abandoned US20110048588A1 (en)

Applications Claiming Priority (5)

Application Number Priority Date Filing Date Title
JP2008-050916 2008-02-29
JP2008050916 2008-02-29
JP2008-287692 2008-11-10
JP2008287692A JP5320990B2 (ja) 2008-02-29 2008-11-10 冷延鋼板およびその製造方法
PCT/JP2009/054102 WO2009107856A1 (ja) 2008-02-29 2009-02-26 冷延鋼板およびその製造方法

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US (1) US20110048588A1 (ko)
JP (1) JP5320990B2 (ko)
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US20170067132A1 (en) * 2014-03-07 2017-03-09 Nippon Steel & Sumitomo Metal Corporation Middle/high carbon steel sheet and method for manufacturing same
US10161462B2 (en) 2013-01-31 2018-12-25 Nisshin Steel Co., Ltd. Clutch plate for multiplate wet clutch and method of manufacturing the same

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US9234268B2 (en) 2011-07-29 2016-01-12 Nippon Steel & Sumitomo Metal Corporation High-strength galvanized steel sheet excellent in bendability and manufacturing method thereof
US10161462B2 (en) 2013-01-31 2018-12-25 Nisshin Steel Co., Ltd. Clutch plate for multiplate wet clutch and method of manufacturing the same
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WO2009107856A1 (ja) 2009-09-03
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TWI395822B (zh) 2013-05-11
JP2009228128A (ja) 2009-10-08

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