Classifications

• • 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

• the present invention relates to a cold rolled copper plate excellent in strength, punching workability, and heat deformation resistance, which is suitable for use in a clutch plate, a sink port nizer ring, a clutch disk, and the like, which are mechanical parts for automobile transmissions, and a method for producing the same. It is about. Background art
• Automobile transmissions are composed of clutch plates, synchronizer rings, clutch discs, etc., and transmit driving force and absorb heat generated by friction. Such parts are manufactured by punching steel plates into a ring.
• the transmission has a structure that transmits torque by stacking many such ring-shaped plates, and its functions require wear resistance and flatness of the plates. Therefore, the required properties of the steel sheet, which is the raw material, are required to be excellent in hardness, properties during punching (perforated surface properties such as flatness and burrs), and small deformation during heating, that is, excellent heat resistance.
• S 3 5 C cold-rolled steel sheets are mainly used among machine structural steels specified in JISG 3 3 1 1. It was.
• This S 3 5 C cold-rolled steel sheet is manufactured in the process of “slab ⁇ hot rolling ⁇ pickling ⁇ annealing ⁇ cold rolling”.
• S 3 5 C cold-rolled steel sheet contains a lot of C (about 0.35% by mass), so the hot-rolled copper sheet has high hardness, so it aims to spheroidize and soften the carbide before cold rolling. Therefore, annealing for a long time of several hours or more is essential. Therefore, it is very disadvantageous in terms of cost for automobile parts that require low prices.
• Patent Document 1 discloses a technique that omits annealing of a hot-rolled steel sheet before cold rolling.
• a hot-rolled steel sheet with a softness with C of 0.25 mass% or less Cold rolled steel sheets for AT clutch plates have been proposed that have the desired hardness and surface roughness and are excellent in wear resistance and punchability by cold rolling.
• the properties of the punched end face of the copper plate deteriorate significantly during the punching process, and when the temperature rises, thermal distortion occurs and the flatness of the ring-shaped product There was a problem that the remarkably decreased.
• Patent Document 2 as a technique for improving the residual stress after cold rolling described above, a steel sheet after cold rolling is further used with a large roll having a roll diameter of 300 mm or more and a reduction ratio of about 1%.
• a cold-rolled copper sheet for AT clutch plates has been proposed in which residual stress is reduced by rolling down.
• the difference in strain introduced between the front and back surfaces of the steel sheet is reduced, which improves the properties of the punched end face during press punching, but does not release the residual stress inside the copper plate. Deformation due to thermal strain was inevitable when the temperature rose, and the flatness of the ring-shaped product was remarkably lowered.
• Patent Document 3 proposes a steel plate for an AT clutch plate that has been subjected to steel plate surface roughness adjustment processing such as pickling to optimize the surface roughness and is excellent in adhesion to a friction material. After hot rolling and pickling, this sheet is preferably annealed at 500 to 8003 ⁇ 4 for 3 hours or more to spheroidize the carbide, and then after temper rolling or cold rolling at a rolling reduction of 1% or more, the surface roughness of the steel sheet Although the adjustment process is performed, the remarkable decrease in the flatness of the ring-shaped product due to the residual stress after cold rolling has not been solved as in Patent Document 1 and Patent Document 2.
• Patent Document 1 JP 2003-277883 A
• Patent Document 2 JP 2005-200712 A
• Patent Document 3 Japanese Patent Laid-Open No. 2004-107722 Disclosure of Invention
• the present invention advantageously solves the above-mentioned problems, has high hardness suitable for use in clutch plates, rings, clutch disks, etc., and also has end face properties during punching and flatness when temperature rises.
• the object is to provide an excellent cold-rolled steel sheet together with its advantageous manufacturing method.
• annealing is performed after cold rolling to reduce residual strain.
• An open cold-rolled annealed plate may be used as the material.
• the required hardness cannot be obtained.
• the inventors have intensively studied to solve this problem, and as a result, the steel structure does not become a complete recrystallized structure by annealing after cold rolling, but a part of the unrecrystallized structure remains.
• problems caused by residual strain that is, deterioration of end face properties during punching and deterioration of flatness due to thermal strain during temperature rise The knowledge that it is reduced is obtained.
• the present invention was developed after various studies based on the above findings.
• the gist of the present invention is as follows.
• the balance is the composition of Fe and inevitable impurities, the ferrite average grain size is 2-10 / m, and the unrecrystallized rate is 25% or more and 90% or less.
• the slab balance consisting Fe and inevitable impurities, finishing temperature: hot rolling at Ar 3 transformation point or more, ⁇ temperature taken plated at a temperature of at 580-750, followed A method for producing a cold-rolled steel sheet, characterized by cold rolling at a reduction ratio of 65% or more after pickling and then annealing at a temperature of 680 or less by continuous annealing.
• Average ferrite particle diameter 2 to 10 / z m
• the average ferrite grain size is in an appropriate range. If the average ferrite grain size exceeds 10 / z m, the desired hardness cannot be obtained. On the other hand, if it is less than 2 / z m, the hardness is excessively increased and the press punchability is reduced.
• the preferred ferrite average particle size is in the range of 4-8 / zm. ,
• the average ferrite grain size is obtained in accordance with the cutting method described in JIS G 0551 (Appendix) by observing the thickness cross section in the rolling direction of the copper plate.
• Non-recrystallization rate 25% or more and 90% or less
• the greatest feature of the cold-rolled steel sheet of the present invention is that it has a partially recrystallized structure composed of an unrecrystallized structure and a recrystallized structure.
• a partially recrystallized structure composed of an unrecrystallized structure and a recrystallized structure.
• the ratio of the unrecrystallized structure to the recrystallized structure is important in order to combine high hardness, punching workability, and heat distortion resistance.
• the non-recrystallization rate in order to obtain a desired hardness due to the effect of rolling strain, the non-recrystallization rate must be 25% or more. However, when the non-recrystallization rate exceeds 90%, the recrystallized grains are remarkably reduced, punching workability is deteriorated, and the residual stress is excessively increased to deteriorate the flatness. 90% or less. More preferably, it is in the range of 40% to 80%.
• the non-recrystallization rate can be determined by observing the thickness cross section in the rolling direction of the steel sheet, obtaining the ratio (area ratio) of the non-recrystallized structure in the entire structure, and setting this as the non-recrystallized ratio. .
• the amount of C is an important element from the viewpoint of the hardness and wear resistance of cold-rolled steel sheets, and both hardness and wear resistance increase as the C content increases. Therefore, the desired hardness and wear resistance can be obtained. Therefore, the amount of C should be 0.01% or more.
• the C content exceeds 0.15%, the punching workability deteriorates.
• the deformation strain difference between the front and back during punching increases, and the deformation due to thermal strain increases as the temperature rises, so the flatness of the punched material deteriorates. Therefore, the amount of C is limited to the range of 0.01 to 0.15%.
• a preferable amount of C is in the range of 0.05 to 0.15%, and more preferably in the range of 0.10 to 0.15%.
• the Si amount is limited to 0.03% or less. Preferably, it is 0.02% or less, and may be 0%. In the current refinement technology, the lower limit of the Si content is about 0.005% without a significant increase in steelmaking costs.
• Mn is an element that has the effect of fixing S as an impurity in steel as precipitates (MnS) and reducing the adverse effects caused by S.
• the Mn content must be 0.10% or more.
• the amount of Mn exceeds 0.70%, the hardness of the steel sheet excessively increases and the punching processability is lowered. This is because Mn strengthens the steel by solid solution strengthening.
• the Mn content exceeds 0.70%, defects due to scale are likely to occur on the surface of the hot-rolled steel sheet, and even if pickling is performed after hot rolling, the scale can be completely removed. It becomes difficult.
• the amount of Mn was limited to the range of 0.10 to 0.70%.
• the Mn content is preferably 0.50% or less, and a more preferable Mn content is in the range of 0.20 to 0.50%.
• P is an element that strengthens steel by solid solution strengthening. However, if the P content exceeds 0.025%, it may cause slab cracking and surface defects on the steel sheet. In addition, the hardness of the steel is significantly increased, and punching workability deteriorates. Therefore, the P content is limited to 0.025% or less. The preferred amount of P is 0.023% or less. When the amount of P is less than 0.01%, the effect on strengthening is poor, so 0.01% or more is preferable.
• S is an element present as an impurity in steel.
• S when S exceeds 0.025%, coarse inclusions are formed, starting from this, causing work cracks, leading to a significant decrease in punching workability.
• S also affects the scale peelability of hot-rolled steel sheets. If the amount of S exceeds 0.025%, the surface properties after pickling deteriorate, and as a result, the steel sheet after cold rolling is annealed. The surface roughness is also roughened. Therefore, the S content is limited to 0.025% or less. Preferably, it is 0.020% or less.
• A1 is an element to be included for deoxidation of steel. If the amount of A1 is less than 0.01%, a sufficient deoxidation effect cannot be obtained. On the other hand, even if the amount of A1 exceeds 0.05%, the deoxidation effect is saturated. Therefore, the amount of A1 was limited to a range of 0.01 to 0.05%. A preferable amount of A1 is in the range of 0.03 to 0.05%. N: 0.008% or less
• N is an element present in copper as an impurity. If the amount of N exceeds 0.008%, the steel sheet is excessively hardened and the punching workability is lowered. Therefore, the N content is limited to 0.008% or less. Preferably, it is 0.005% or less.
• elements that greatly affect the hardness of the copper plate are C, Mn, and P, and this C * is an index of hardness in the steel plate of the present invention.
• C * is an index of hardness in the steel plate of the present invention.
• HRB hardness
• the condition of the above formula (1) is satisfied for C *.
• the components other than the above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected. Next, the reason why the hardness and surface roughness of the steel sheet are limited as described above in the present invention will be described.
• Transmission has a structure that transmits torque by stacking a number of steel plates punched into a ring. Accordingly, the steel sheet used is required to have wear resistance, and a hardness (HRB) that can ensure the wear resistance: 83 or more is required. When the hardness is less than 83 HRB, a decrease in wear resistance becomes a problem, so it is necessary to set it to 83 HRB or more. If the hardness exceeds 95 HRB, defects in the punching shape and cracks and cracks occur in the steel plate during punching.
• the surface roughness is small.
• the surface roughness is 0.3 / xm or less in terms of arithmetic average roughness Ra.
• the lower limit of the surface roughness that can be achieved without significantly increasing the manufacturing cost is about 0.1 l jt m at the current technical level.
• the slab with the above composition is hot-rolled at a finishing temperature of Ar 3 transformation point or higher, the cutting temperature is 580 to 750 ° C, and the hot-rolled steel sheet is pickled and then reduced.
• Rate After cold rolling at 65% or more, annealing is performed at a temperature of 6803 ⁇ 4 or less in a continuous annealing furnace.
• slab melting and forging are preferably performed using a converter and a continuous forging machine.
• the finishing temperature In hot rolling, the finishing temperature needs to be higher than the Ar 3 transformation point in view of the quality of hot-rolled steel sheet and the efficiency of hot rolling.
• the finishing temperature When the finishing temperature is lower than the Ar 3 transformation point, the ferrite transformation in the hot-rolled steel sheet is promoted, and coarse grains are formed on the surface layer, resulting in a problem that the hardness is lowered.
• Subsequent scraping requires the scraping temperature to be in the range of 580 to 7503 ⁇ 4.
• the temperature is less than 580 ° C, the crystal grains become excessively fine and the hot-rolled steel sheet becomes hard due to cooling strain, resulting in cooling. Impairs hot rolling property.
• a preferred scraping temperature is in the range of 600-720 ° C.
• the Ar 3 transformation point can be obtained by thermal expansion measurement using a differential thermal dilatometer or the like.
• the hot-rolled copper sheet is pickled according to a conventional method, removed from the scale of the steel sheet surface, and then subjected to cold rolling. The rolling reduction of cold rolling needs to be 65% or more.
• the upper limit of the rolling reduction is not particularly limited. However, when cold rolling is performed at a high rolling reduction exceeding 85%, the shape of the steel sheet after rolling is reduced, the thickness accuracy is lowered, and cold rolling is performed. There is a concern that productivity will decrease due to excessive rolling load on the mill. Therefore, the rolling reduction is preferably 85% or less.
• Cold-rolled steel sheets are annealed in a continuous annealing furnace.
• it is particularly important to perform annealing at a temperature lower than the recrystallization completion temperature after cold rolling.
• the annealing temperature exceeds the recrystallization completion temperature, the yarn after annealing! ; Almost 100% of the weave becomes a recrystallized structure, so the rolling strain introduced during cold rolling disappears. Therefore, the desired high hardness cannot be obtained. Therefore, by setting the annealing temperature below the recrystallization completion temperature, a partially recrystallized structure in which unrecrystallized grains and recrystallized grains are mixed can be obtained.
• the ratio of the unrecrystallized ratio is determined by the annealing temperature, and the unrecrystallized ratio can be 25% or more by setting the annealing temperature to 680 or less.
• the lower limit of the annealing temperature is not particularly limited, but is preferably 500 ° C. or more from the viewpoint of the temperature and atmosphere controllability and productivity of the continuous furnace.
• the cooling rate after annealing is not particularly limited, but is preferably about 5 to 25 ° C / s.
• the heat treatment is performed in the temperature range of 320 to 420. This means that the shape stability of the steel sheet and the surface roughness adjustment during temper rolling when temper rolling is applied. Is advantageous.
• the unrecrystallized rate can be adjusted as appropriate according to the annealing temperature.
• the relationship between the unrecrystallized rate and the annealing temperature is, for example, the apparent unrecrystallized rate determined as follows and the unrecrystallized rate.
• a relationship with the annealing temperature for obtaining the above is obtained in advance, and based on this relationship, the annealing temperature for obtaining a desired non-recrystallization rate may be adjusted.
• the hardness of the steel sheet that has been cold-rolled and water-quenched after the cold-rolled steel sheet is measured in order to exclude hardness fluctuations due to the cooling conditions after annealing. Is preferred.
• the relationship between the apparent non-recrystallization rate and the steel plate hardness obtained as described above is obtained in advance, and based on this relationship, the apparent non-recrystallization rate is obtained from the hardness of the steel plate after annealing. It is also possible to estimate the unrecrystallization rate of the copper plate.
• the cold-rolled steel sheet after annealing may be lightly rolled by temper rolling. This is to adjust the surface roughness and further improve the hardness.
• the rolling rate is preferably 2% or more in terms of elongation.
• the upper limit of the rolling rate is not particularly limited, but if the rolling rate is excessively high, the shape of the copper plate varies. Considering the ability of a rolling mill for temper rolling, the rolling rate is preferably 5% or less in terms of elongation.
• the copper plate was cooled at a rate of 10 / s, then kept at 320 to 420 for 2.5 minutes, and then cooled to room temperature. Furthermore, in the temper rolling line, rolling was performed under a light reduction at a rolling rate (elongation rate) of 3.0%.
• Table 1 shows the results of examining the ferrite average grain size, non-recrystallization ratio, surface roughness, hardness, punching workability, and heat deformation resistance of the steel sheet thus obtained.
• Each survey item was measured by the following method.
• the non-recrystallized ratio was obtained by observing the sheet thickness cross section in the rolling direction at a magnification of 800 times to obtain the area ratio of the non-recrystallized structure, which was defined as the non-recrystallized ratio.
• a sample having a size of 20 X 60 imn was cut out from the steel sheet and measured according to the Rockwell hardness test method specified in JIS Z 2245. The measurement was performed at 10 points on the B scale, and the average value was taken as the hardness.
• the same ring-shaped test piece that was evaluated for punching workability was heated at 300 for 30 minutes and then evaluated by the amount of warpage of the test piece when air-cooled to room temperature. A warp amount of 0.1 or less is good.
• the amount of warpage was measured as follows. Heat and air-cooled test material is polished on both sides with # 800 or more emery single abrasive paper, then placed on a surface plate and measured using a contact-type height gauge to measure the height of 10 points in the circumferential direction. The difference from the plate thickness at the same location measured with a meter was determined, and the maximum value was taken as the amount of warpage.
• a slab having the composition shown in Table 2 was heated to 1250 ° C, hot-rolled at the finishing temperature shown in Table 3, cooled on a run-out table, and scraped at 650.
• the thickness of the hot-rolled steel sheet was 3-10 mm.
• the scale was removed by pickling, and then cold-rolled at a reduction ratio in the range of 50 to 80% to obtain a cold rolled steel sheet having a thickness of 1.5 thighs.
• After degreasing the cold-rolled steel sheet it was annealed in a continuous annealing furnace.
• the annealing temperature was set at various temperatures below 680, and the annealing time was 1 minute.
• the copper plate was cooled at a rate of 10 / s, then kept at a temperature range of 320 to 420 ° C for 2.5 minutes, and then cooled to room temperature. Furthermore, in the temper rolling line, the rolling rate (stretching rate) is in the range of 0 to 3.5%.
• the Ar 3 transformation point shown in Table 2 is the differential thermal dilatometer for specimens taken from each copper slab, heated at 1250 for 30 minutes and then cooled at a cooling rate of 1 ° C / second. Measured with
• Table 3 shows the results of examining the average grain size, non-recrystallization ratio, surface roughness, hardness (HRB), punching workability and heat distortion resistance of the steel plates obtained by strengthening.
• the finishing temperature during hot rolling As shown in Table 3, in the copper types A, I and J, which are invention steels, the finishing temperature during hot rolling, the rolling reduction during cold rolling, the annealing temperature, and the rolling rate during temper rolling (elongation rate)
• the rolling rate during temper rolling elongation rate
• (HRB (H)) ' Rockwell hardness (B scale) of a steel plate that does not contain any recrystallized structure.
• (HRB (S))' and (HRB (H)) ' are obtained as follows. It was. The pre-annealed cold-rolled steel sheet was heated at 580 and 780 for 100 seconds or less, then water-quenched, and the hardness (HRB) measurement and microstructure observation were performed on each sample. The sample with a heating temperature of 580 was confirmed to contain no recrystallized structure, and the hardness (HRB) was measured to give (HRB (H)) '.
• the hardness (HR B) was measured after confirming that it had a completely recrystallized structure, and was set as (HRB (S)) '.
• Industrial applicability in accordance with the present invention, by adjusting the copper component, and by making the steel structure a partially recrystallized structure, it is suitable for use in components of automobile transmissions such as clutch plates, with high strength (high hardness). A cold-rolled sheet having excellent punchability and heat distortion resistance can be obtained. According to the present invention, annealing after cold rolling can be performed in a short time of less than 1 hour, and can be performed in a continuous annealing furnace with extremely high production efficiency. It can be manufactured without incurring uplift, and is therefore suitable for use in auto parts with severe price competition.

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