JP5046400B2 - Method for producing cold-rolled steel sheet with excellent recrystallization softening resistance and cold-rolled steel sheet for automatic transmission - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet excellent in recrystallization softening resistance (characteristic showing recrystallization softening resistance), and in particular, a method for producing a cold-rolled steel sheet suitably used as a plate material for an automatic transmission of an automobile, and cold rolling It relates to steel plates.

  Separate plates, friction plates, backing plates, etc. (hereinafter abbreviated as “AT plates”) that make up automatic transmissions for automobiles are formed by stamping steel plates into a generally annular shape, and these plate members are combined. A torque transmission mechanism is configured.

Steel plates for AT plates require a certain level of wear resistance and a smooth surface for press molding, and are required to satisfy hardness (Hv): 230 or more and surface roughness Ra: 0.4 μm or less. Conventionally, JIS G3311 steel for machine structural use (typically S35C) has been used as a material for AT plates. The manufacturing process is as follows.
`` Steel making → Continuous casting → Hot rolling → Steel pickling → Annealing → Cold rolling → Temper rolling → Refinement ”
The cold rolling in the above process is high pressure rolling with a reduction rate of 50% or more. The reason for the high pressure reduction is to satisfy the hardness and surface roughness (Hv ≧ 230, Ra ≦ 0.4μm) required for AT plates, and “annealing” should be performed before cold rolling. What is done is that the hot-rolled steel sheet is hard, so that stable operation of high-pressure reduction rolling is difficult, and a coarse pearlite structure is carried over to the cold-rolled steel sheet, resulting in poor press-punching of the cold-rolled steel sheet. This is because musiness or roughness tends to occur on the punched surface of the punched molded product (AT plate). The annealing is performed as a tight coil batch process (soaking: about 10 hours).

The annealing process before the cold rolling in the above manufacturing process (which is a long-time heat treatment) is a major factor in increasing the cost, while maintaining the stable operation of the cold rolling while omitting the annealing process and the required material characteristics ( A method of manufacturing a cold-rolled steel sheet having hardness and surface properties has been proposed (Patent Document 1), and further improvements in the steel composition (C amount limitation, combined addition of Ti and B, etc.) and cleanliness of hot-rolled steel sheets Improved material properties (press punchability, etc.) over conventional materials as a control effect on the degree (form and amount of non-metallic inclusions) and the structure of hot-rolled steel sheet (ferrite grain size, ferrite + cementite fraction, etc.) ) Has also been proposed (Patent Documents 2 to 4).
JP 2003-277883 A JP 2004-162153 A JP 2004-292939 A JP 2007-2111260 A

The AT plate obtained by press punching the cold-rolled steel sheet is required to have a flatness of 0.15 mm or less as an important requirement for the torque transmission function. However, the plate surface distortion (deformation) occurs in press punching due to the larger diameter of the AT plate, the narrower size of the punching bar, and the more complicated shape due to the higher displacement and weight reduction of automobiles. A tendency becomes remarkable, and it is difficult to guarantee required flatness.
For this reason, a press temper process is performed as a shape correction process for giving the punched AT plate the required flatness. This process is performed by pressing the AT plate with a jig and heating and holding it in a predetermined temperature range (about 400 to 550 ° C.) for a certain time.

The shape correction (improvement of flatness) effect by the press temper treatment is promoted while increasing the treatment temperature, but the treatment temperature (about 400 to 550 ° C.) overlaps with the recrystallization temperature range, so that the softness accompanying recrystallization. It is difficult to guarantee the required hardness of the product AT plate as a result. Although the patent documents 1 to 4 disclose the omission of the annealing process in the manufacturing process of the cold rolled steel sheet for AT plate and the improvement of the press punching processability, the recrystallization softening in the heat treatment after the press punching process is disclosed. There is no description including the phenomenon and suggestion of the countermeasure, and the problem of the decrease in the hardness of the plate cannot be solved.
In view of the above, the present invention is an improvement of the manufacturing method of the cold-rolled steel sheet that eliminates the annealing treatment before cold rolling. (Characteristics showing resistance to crystal softening), effectively suppressing and mitigating softening (decrease in hardness) in press tempering after press punching, material properties such as hardness required for AT plates and good shape quality ( The present invention provides a method for producing a cold-rolled steel sheet and a cold-rolled steel sheet that can be provided with flatness.

The method for producing a cold-rolled steel sheet according to the present invention (claim 1)
In mass%, C: 0.15-0.25%, Si: 0.25% or less, Mn: 0.3-0.9%, P: 0.03% or less, S: 0.015% or less, Al: 0.01-0.08%, N: 0.008% or less, Cr: 0.05-0.5%, Ti: 0.01-0.05%, B: 0.002-0.005 %, The balance is extracted from a slab having a composition composed of Fe and inevitable impurities by heating to a temperature exceeding 1230 ° C. in a heating furnace, hot rolling finishing temperature: Ar ₃ transformation point or higher, and winding temperature: 500 to After hot rolling at 600 ° C., a hot rolled steel sheet having a ferrite-pearlite mixed structure with a ferrite crystal grain size of 5 to 15 μm and a pearlite + cementite fraction of 40% or more is obtained, and the hot rolled steel sheet is pickled. Cold rolling at a reduction rate of 30% or more without annealing It is characterized in.

  The cold-rolled steel sheet for AT plates of the present invention (Claim 6) is made of a cold-rolled steel sheet produced by the above production method.

The greatest feature of the present invention is that high-temperature heating, which greatly exceeds the conventional processing temperature, is applied to the heat treatment of the slab, together with the regulation of the steel composition (combined addition of C amount, Ti, B, Cr, etc.). It is said.
The extraction temperature in the heating furnace of the slab having the low / medium carbon steel composition, which is the subject of the present invention, is generally carried out in the temperature range of 1180 to 1230 ° C. The reason why high-temperature heating exceeding 1230 ° C. is not performed is because there is no benefit of such high-temperature heating, and it is recognized that it is a wasteful consumption of heat energy. In addition, it is thought that the austenite grains of the slab become excessively coarse as the temperature rises, and the structure of the subsequent hot-rolled steel sheet cannot be refined, resulting in a decrease in material properties of the product cold-rolled steel sheet. is there.

On the other hand, the present invention to which high-temperature heating exceeding 1230 ° C. is applied is resistant to recrystallization softening of cold-rolled steel sheets without causing the above-mentioned quality defects (deterioration of the steel sheet surface quality) that are concerned about high-temperature heating. Realized to dramatically improve the characteristics. The product cold-rolled steel sheet has favorable characteristics such as hardness, wear resistance, and press punchability required for applications such as AT plates.
As an effect of improving the recrystallization softening resistance of the cold-rolled steel sheet according to the present invention, in the AT plate application, softening in press temper treatment after press punching is effectively mitigated and reduced, and the large diameter size and the narrow width size of the crosspiece Even in the case of an AT plate having a shape that tends to cause distortion as described above, it becomes easy to ensure good flatness (≦ 0.15 mm) while maintaining the required mechanical strength. Such an effect of improving the material properties of the present invention sufficiently compensates for an increase in the heat energy cost required for increasing the slab extraction temperature.

Although details of the mechanism by which the recrystallization softening property of cold-rolled steel sheets is enhanced by increasing the slab extraction temperature are not clear, sufficient solidification of the TiC and Ti (C, N) in the slab steel to the austenite phase is not clear. It is considered that the effect is due to the formation of a uniformly dispersed structure as ultrafine precipitates when melting is achieved and re-deposited in the subsequent hot rolling step. Moreover, it is thought that it is due to the formation of a structure in which ultrafine precipitates are uniformly dispersed because of having favorable characteristics such as hardness, wear resistance and press punching workability.
The formation of such an ultrafine uniform structure and improved material properties of the cold-rolled steel sheet are based on the steel composition of the present invention.

  Regarding the steel composition of the present invention, the hot rolled steel sheet is softened by reducing the pearlite content of the hot rolled steel sheet by limiting the C content (lower C composition than the conventional S35C carbon steel), and before hot rolling. No need for annealing. In addition to this C content regulation, the control of the steel structure based on the combined addition of Ti and B and the material improvement effect are important. As a production effect of fine precipitates (size: about 500 to 3000 mm) such as TiC and Ti (C, N) by addition of Ti, the ferrite structure of the hot-rolled steel sheet is remarkably refined. In hot-rolled steel sheets, carbide preferentially precipitates at the ferrite grain boundaries, and as a result of grain refinement, the carbide is uniformly and finely dispersed in the steel, and there is a structure that is advantageous in terms of wear resistance, recrystallization softening resistance, etc. It is formed. The refinement effect is enhanced by the solid solution B in the steel coexisting with Ti. This is because the TiN production tendency is larger than that of BN, and the BN production reaction is suppressed. Further, the solid solution B contributes to strengthening of the grain boundary of the hot-rolled steel sheet and refinement of the ferrite structure, and is effective in improving the strength and wear resistance of the matrix. As these effects, the material properties of the cold-rolled steel sheet are enhanced, and even after the press temper treatment in combination with the improvement of the recrystallization softening resistance, it surpasses the S35C cold-rolled steel sheet, which is a typical conventional material for AT plate applications. Hardness and other mechanical properties are guaranteed.

The reasons for limiting the steel composition of the present invention are as follows. All element contents are mass%.
C: 0.15-0.25%
From the point of increasing the hardness and wear resistance of the cold-rolled steel sheet, the higher the amount of C, the more advantageous. However, if it exceeds 0.25%, the annealing for softening and softening the carbide of the hot-rolled steel sheet is omitted. I can't. On the other hand, if the amount of C is less than 0.15%, it is difficult to ensure the same wear resistance as S35C, which is a conventional material, so this is the lower limit.

Si: 0.25% or less Si is a deoxidizing element in the melting process of steel. The addition amount is sufficient up to 0.25%. Addition exceeding this amount causes reduction in pickling property of the hot-rolled steel sheet and surface defects due to residual scale after pickling, and lowers the surface quality of the AT plate, so this is the upper limit.

Mn: 0.3 to 0.9%
Mn is effective in preventing hot brittleness of steel and strengthening the matrix. If it is less than 0.3%, the effect is small, the strength of the matrix is insufficient, and the wear resistance is also lowered. The effect is increased by increasing the amount, but if it exceeds 0.9%, it becomes excessively hard and the workability is impaired.

P: 0.03% or less P is an impure component, and if present in a large amount in steel, it causes a decrease in grain boundary strength and causes embrittlement of cold-rolled steel sheets at room temperature, and stability of press-formed products such as AT plates. Sexuality is impaired. If it is 0.03% or less, there is no substantial adverse effect, so this is the upper limit.

S: 0.015% or less S has the effect of suppressing hot brittleness by forming MnS, but if it is too much, it may cause work cracks starting from MnS, and AT plates may cause deterioration of surface properties in stamping. Invite. In addition, wear starting from MnS tends to occur and wear resistance is reduced. If it is 0.015% or less, the actual damage is avoided, so this is the upper limit.

Al: 0.01 to 0.08%
Al is added as a deoxidizer in the steel melting process. It also has the effect of fixing N in steel as AlN. If it is less than 0.01%, the deoxidation action is insufficient, while if it exceeds 0.08%, the cleanliness of the steel is impaired, surface flaws occur, and the surface quality of the steel sheet is deteriorated.

N: 0.008% or less N is an inevitable impurity element. When the content is increased, the amount of nitride (AlN, TiN, etc.) generated increases, resulting in excessive hardening, and therefore it is necessary to be 0.008% or less.

Cr: 0.05-0.5%
Cr dissolves in the steel and partly forms carbides (precipitated particles) to act as solid solution strengthening and precipitation strengthening. With this matrix strengthening function, it is possible to set a reduction ratio (cold rolling ratio) in cold rolling necessary for securing the required hardness (Hv ≧ 230) in AT plate applications to be low. Lowering the cold rolling rate is effective in reducing the compressive residual stress in the center of the thickness of the cold rolled steel sheet and improving the flatness of the AT plate after press punching. To obtain these effects, 0.05% or more must be added. If it exceeds 0.5%, the effect is almost saturated.

Ti: 0.01 to 0.05%
Ti generally has the effect of forming TiS to avoid hot brittleness. Furthermore, as described above, Ti forms fine precipitates such as TiC and Ti (C, N) in steel to refine the ferrite structure of the hot-rolled steel sheet, and as a result, carbide is uniformly and finely dispersed and wear resistant. Increases nature. In order to secure this effect, addition of 0.01% or more is required. If added in a large amount, excessive hardening is caused by excessive formation of fine precipitates, so 0.05% is made the upper limit.

B: 0.002 to 0.005%
Most of B forms solid solution B as described above, and strengthens the matrix and enhances wear resistance by strengthening grain boundaries and refining the ferrite structure. To obtain this effect, 0.002% or more must be added. However, if it exceeds 0.005%, the ferrite structure becomes hardened due to excessive grain refinement, so this is the upper limit.

  Ni, Cu and the like are unavoidable elements in the normal melting process, but if they are in the range of 0.03% or less, mixing is allowed without substantial detriment.

Next, the manufacturing process of the cold rolled steel sheet of the present invention will be described.
[Smelting and casting of steel]
First, steel melted to a predetermined chemical composition in a steelmaking furnace is made into a slab by ingot-making, ingot rolling, or by continuous casting, and after surface treatment of the slab is appropriately performed, it is hot-rolled. In the case of continuous casting, the hot cast slab (slab) may be directly charged into a heating furnace and hot rolled.

[Slab heating and extraction in heating furnace]
The point that the slab was subjected to high temperature extraction (extraction temperature: more than 1230 ° C.) is the greatest feature of the present invention. The “extraction temperature” in the present invention refers to the highest temperature reached in the furnace (slab temperature).
The extraction temperature by the heat treatment of the slab is said to require about 1180 ° C. or more in order to satisfy the homogenization of the slab steel structure and other requirements, but as described above, an example of high temperature extraction exceeding 1230 ° C. Is not found in the past. In contrast to this, the present invention to which high temperature extraction exceeding 1230 ° C. is applied makes it possible to greatly improve the recrystallization softening resistance without causing material deterioration of the product steel plate, which has been a concern in the past. This material improvement effect is due to the fact that the high temperature heat treatment of the slab promotes the disappearance of solid solution and segregation of carbides in the slab steel, and achieves high homogenization of the austenite single phase structure. In addition, it is considered that the required material properties are ensured based on the chemical composition of the steel material of the present invention without the material deterioration of the product steel plate, which has been a concern in the past, regardless of the increase in the extraction temperature. It is done.

In the slab heat treatment in the present invention, there is no benefit of applying a high temperature exceeding 1280 ° C., and the heat energy cost is wasted, and quality defects of the product steel plate (grain boundary oxidation and thereby deterioration of the surface quality of the product steel plate) There is also a risk of causing. Accordingly, the temperature is preferably up to 1280 ° C. More preferably, it is 1250-1270 degreeC.
The heating and holding time at the slab extraction temperature in the furnace may be about 30 minutes or more.

[Hot rolling]
Hot rolling is performed according to a conventional method. From the viewpoint of the quality of the hot-rolled steel sheet and the hot-rolling efficiency, the hot-rolling finishing temperature is adjusted just above the Ar ₃ transformation point. The steel sheet is preferably wound in a temperature range of 500 to 600 ° C. Low temperature winding below 500 ° C causes the crystal grain size to become excessively fine and hardens the hot-rolled steel sheet. On the other hand, high temperature winding above 600 ° C tends to agglomerate carbides, making it difficult to ensure uniform dispersion. At the same time, the ferrite structure becomes coarse, and as a result, the effect of improving the wear resistance and recrystallization softening property of the cold-rolled steel sheet is impaired. Preferably it is 500-550 degreeC.

  The hot-rolled steel sheet is required to have a refined ferrite structure with a crystal grain size (JIS G0552 “Appendix 2 (normative) method of determination based on intersecting line segment (grain size)”) of 5 to 15 μm. A fine ferrite structure of less than 5 μm causes excessive hardening, while a coarse structure exceeding 15 μm lacks the effect of improving wear resistance and recrystallization softening resistance. Preferably it is 5-10 micrometers. The fine crystal grain size is ensured by the hot rolling conditions as an effect of the steel composition containing both the regulation of the C amount and a fixed amount of Ti, B, and Cr.

  The hot-rolled steel sheet needs to have a “pearlite + cementite fraction” of 40% or more as measured by the point calculation method as an index of the area ratio of carbide in addition to the definition of the ferrite grain size. Here, the point calculation method refers to the quantitative analysis method (page 264) of the optical microscope structure described in the Metal Handbook (edited by the Japan Institute of Metals, Maruzen Co., Ltd.), revised 6th edition, and grids within the microscope field of view. And counting the total number of grid points occupied by carbides, and calculating the ratio to the total number of grid grid points. By this pearlite + cementite fraction of 40% or more, the effect of improving the recrystallization softening resistance of the cold-rolled steel sheet is more clearly expressed.

The fine uniform structure of the hot-rolled steel sheet (ferrite particle size: 5-15 μm, pearlite + cementite fraction: 40% or more) is a steel composition containing both the C content, the Cr content and a fixed amount of Ti and B. As an effect, it is ensured by the slab extraction conditions and hot rolling conditions of the heating furnace.
The finely divided ferrite structure and carbide dispersion structure of the hot-rolled steel sheet make the dispersion of carbide in steel in the final product (cold-rolled steel sheet) more uniform and finer, resulting in wear resistance and resistance to press tempering. The improvement effect of the recrystallization softening property is guaranteed.

[Cold rolling]
The hot-rolled steel sheet is subjected to cold rolling after the surface scale is removed by pickling treatment. The rolling reduction in cold rolling is 30% or more. This is to obtain the hardness (Hv ≧ 230) and surface roughness (Ra ≦ 0.4 μm) necessary for a cold rolled steel sheet for AT plates. The effect increases as the rolling reduction increases, but the flatness after press punching decreases because the compressive stress at the center of the plate thickness increases as the rolling reduction increases. In particular, when the stamping of an AT plate having a large diameter and a narrow width of a crosspiece is performed, the influence becomes large. For this reason, the rolling reduction should be limited to 60% or less. More preferably, it is 40 to 50%.
In addition, as for the rolling roll (work roll) in cold rolling, management of roll surface roughness is suitably implemented so that the surface roughness (Ra <= 0.4micrometer) of the cold-rolled steel plate obtained may be ensured.

  If desired, the cold rolling can be carried out in two stages: pre-rolling before pickling and finish rolling after treatment. As a scale crushing effect by pre-rolling (before pickling treatment), the descaling property is greatly improved, and the pickling treatment time can be greatly shortened and the cost can be reduced. Pre-rolling (before pickling treatment) and finish rolling (after pickling treatment) do not need to be continued, but a pre-rolling machine is installed on the entrance side of the pickling tank, and a finishing rolling mill is installed on the exit side. It is advantageous from the viewpoint of production efficiency to adopt a continuous configuration of pre-rolling-pickling treatment-finish rolling.

  In addition, it is desirable to regulate the rolling reduction of pre-rolling (before pickling treatment) in the two-stage rolling to 25% or less. This is because if pre-rolling is performed at a high rolling reduction exceeding this range, wrinkles due to the pressing of the scale onto the steel sheet surface may occur, and the surface quality may be impaired. Preferably it is 10 to 20%. The rolling reduction of finish rolling (after pickling treatment) is set so that the total rolling reduction (= pre-rolling rolling reduction + finish rolling rolling reduction) is 30% or more. The adjustment of the total reduction ratio of the two-stage rolling is the same as the above-mentioned characteristics (implemented in one stage after pickling) (required hardness: Hv ≧ 230, surface roughness: Ra ≦ 0.4 μm).

[Temper rolling]
After cold rolling, the steel sheet surface is purified by degreasing treatment (electrolytic cleaning, etc.) according to a conventional method, and temper rolling is performed for the purpose of correcting the shape of the steel sheet and alleviating residual stress. The elongation in temper rolling is preferably adjusted to 1% or less. The shape correction effect by temper rolling is almost saturated at an elongation of 1%, and an elongation exceeding that only hardens the steel sheet unnecessarily. In addition, from the viewpoint of the stress relaxation effect, it is preferable to use a large diameter roll of 300 mm or more as a work roll for temper rolling.

[Preparation]
After cold rolling, the product cold-rolled steel sheet is subjected to a refining process for shape correction by levelers, etc., predetermined inspection (specific dimensions such as plate thickness and width, surface flaws, etc.) and trimming of the steel plate edge, etc. obtain.

[Press punching and press tempering]
When the cold-rolled steel sheet is used for AT plate applications, it is subjected to press punching to obtain a press-formed product having a predetermined shape. A press temper process is performed as a process for correcting the distortion (deformation) associated with the press punching of the molded product and obtaining the required flatness. In this process, a press punched molded product is sandwiched from both sides with a pressing jig and heated for a predetermined time under a predetermined pressure, and the processing conditions are appropriately set according to the material of the press punched molded product. The In the S35C carbon steel plate molding (conventional material of AT plate), a temperature of about 400 to 500 ° C. is applied. This is because a high temperature treatment (for example, 550 ° C.) exceeding that causes a significant decrease in hardness (recrystallization softening) and cannot satisfy the required hardness (Hv: 230 or more).
Unlike this, the cold-rolled steel sheet press-formed product of the present invention has a reduced hardness (recrystallization) even at a high temperature exceeding 500 ° C. (for example, 550 ° C., 600 ° C.) as shown in Examples below. There is little softening), press tempering can be achieved efficiently, and the required hardness (Hv: 230 or more) and flatness (flatness: 0.15 mm or less) can be sufficiently secured.

The molten steel subjected to melting and component adjustment by a converter and degassing apparatus is subjected to continuous casting to form a slab (200 mm thickness), and a cold-rolled steel sheet is obtained by the following process (process i or process b).
Process a (omission of annealing before cold rolling):
“Hot rolling → pickling → cold rolling (1 or 2 rolling) → temper rolling → refining (inspection)”
Process b (annealing before cold rolling):
“Hot rolling → Pickling → Annealing → Cold rolling (1 step rolling) → Temper rolling → Refining (inspection)”

[1] Test steel composition (Table 1)
No. 1-6 are invention examples,
Nos. 11 to 17 have a steel composition similar to that of the inventive example, but the element content of any of C, Cr, Ti, and B (underlined) is a comparative example that is out of the definition of the present invention,
Nos. 21 to 24 are examples of conventional materials (JIS G3311 S35C equivalent material).

[2] Manufacturing conditions (see Table 2)
The invention example, comparative example A, comparative example B (B1, B2), and comparative example C in Table 2 are classified as follows according to steel composition and slab extraction temperature.

Classification Steel composition (Table 1) Slab extraction temperature (Table 2)
Invention Example Steel A (No.1 ~ 6) 1260 ℃ (High temperature extraction)
Comparative Example A Steel A (No. 1-6) 1230 ° C
Comparative Example B (B1) Steel B (No. 11-17) 1260 ° C (high temperature extraction)
(B2) Steel B (No.11 ~ 17) 1230 ℃
Comparative Example C Steel C (No. 21-24) 1230 ° C

(2.1) Hot rolling:
Extraction temperature Finishing temperature Winding temperature
<Invention example> 1260 ° C 860 ° C 540 ° C
<Comparative Example A> 1230 ° C 860 ° C 540 ° C
<Comparative Example B1> 1260 ° C 860 ° C 540 ° C
<Comparative Example B2> 1230 ° C 860 ° C 540 ° C
<Comparative Example C> 1230 ° C 850 ° C 600 ° C

(2.2) Annealing treatment:
Invention Example and Comparative Example A: No annealing Comparative Example B (B1, B2): No annealing Comparative Example C: With annealing (tight coil annealing: 700 ° C. × 10 Hr)

(2.3) Cold rolling:
Total reduction ratio: In the case of two-stage rolling, this is the total reduction ratio of the two-stage rolling before and after.
<Invention example, comparative example A> Total rolling reduction: 35 to 55% Finished plate thickness: 1.8 mm
<Comparative example B1, Comparative example B2> Total rolling reduction: 45 to 65% Finished plate thickness: 1.8mm
<Comparative example C> Total rolling reduction: 55% Finished plate thickness: 1.8mm

(2.4) Temper rolling:
Temper rolling ratio (elongation): 0.8%
Work roll diameter: 350mm

[3] Material property test and evaluation method for cold-rolled steel sheet
(1) Wear resistance test Using Ogoshi type rapid wear tester (see Fig. 1).
Press the rotating disk (mating material) (2) against the test material ( ₁ ), measure the width (b ₀ ) of the wear mark on the surface of the test material after the test, and calculate the wear amount A (mm ³ ) from the volume of the wear part. calculate. The wear resistance is evaluated by the specific wear amount [= A / (P × L) (mm ³ / kg · m)].
Test environment: Room temperature (14 ℃), in the air Rotating disc: SK5 / 400Hv (tempered by quenching and tempering)
Disc radius (r) 30mm, Disc thickness (B) 3.0mm
Wear distance (L): 200m
Load (P): 61.7N
Wear speed (V): 4m / sec

(2) Press punchability test An annular AT plate (separate plate) is press punched.
(2.1) Punching conditions Press machine: 200-ton mechanical press Stroke length: 250mm
Number of strokes: 25spm
Clearance: 10% (plate thickness 1.8mm)
Punching dimensions: Inner diameter 105 mm x Outer diameter 127 mm

(2.2) Punchability evaluation method Judgment of the punching end face is made as follows.
Observation method: Observation cross section by scanning electron microscope (magnification: 20 times): Longitudinal cross section of steel sheet Evaluation criteria: ○ ... Beautiful punched end face (no burrs or cracks)
× ... Muscle / crack (1 or more) is found on the punched end face

(3) Evaluation test of anti-recrystallization softening property A press-punched AT plate is subjected to heat treatment, and the improvement effect of the anti-recrystallization softening property is evaluated based on the hardness after the heat treatment.
Treatment temperature: 400 ° C, 500 ° C, 600 ° C
Processing time: 60 minutes

[4] Evaluation of various properties of the test steel sheet Table 2 shows the properties of the cold-rolled steel sheet together with the properties (crystal grain size, P + C fraction) of the hot-rolled steel sheet.
Judgment of crystal grain size of hot-rolled steel sheet based on crossing line defined in JIS G0552 Annex 2 (paragraph 0028)
P + C fraction is measured by point calculation (see paragraph 0029)
The hardness (Hv) of the cold-rolled steel sheet is a value measured at a load of 98N.
Table 3 shows the test results of the recrystallization softening resistance after the cold-rolled steel sheet press punching, together with the slab extraction temperature and the initial hardness of the cold-rolled steel sheet (hardness before the test) (slab extraction temperature in the table) The initial hardness is transferred from Table 2).

(4.1) Hardness, wear resistance, surface roughness and press punchability of cold-rolled steel sheet (Table 2):
Invention examples (No. 1 to 6) have higher wear resistance than No. 21 to 24 (corresponding to conventional AT plate materials) of Comparative Example C, and the hardness required for AT plates ( Hv) and surface roughness (Ra) specifications (Hv ≧ 230, Ra ≦ 0.4 μm) are sufficiently satisfied. Moreover, the press punching property is also good, and it has a beautiful press punching end surface property equivalent to that of the conventional material (S35C carbon steel AT plate).
Comparative Example A (Nos. 1 to 6) has the same wear resistance and press punchability as Comparative Example C, but the invention example is inferior in recrystallization softening resistance as described later. Absent.

Furthermore, looking at Comparative Example B (B1, B2),
No. 11 of Comparative Example B1 and No. 11 of Comparative Example B2 (insufficient amount of C in steel) have insufficient hardness and wear resistance is significantly lower than that of Comparative Example C (conventional AT plate S35C steel). .
No. 12 of Comparative Example B1 and No. 12 of Comparative Example B2 (excess amount of C in steel) have good wear resistance but are excessively hardened. Inferior to sex.
No. 13 of Comparative Example B1, No. 13 of Comparative Example B2 (insufficient Ti amount of steel),
No. 14 of Comparative Example B1, No. 14 of Comparative Example B2 (insufficient B amount of steel)
No. 17 of Comparative Example B1 and No. 17 of Comparative Example B2 (insufficient amount of Ti, B, Cr in steel)
Both are inferior in wear resistance as compared with Invention Example and Comparative Example C (conventional material of AT plate),
No. 16 of Comparative Example B1 and No. 16 of Comparative Example B2 (excess amount of Cr in steel)
It is excessively hardened and has poor press punchability.

(4.2) Recrystallization softening resistance (Table 3):
In the table, the value after the heat treatment is less than Hv230 (required hardness lower limit value of AT plate) is underlined.
The inventive example maintains a sufficient hardness exceeding Hv230 at the processing temperatures of 500 ° C., 550 ° C., and 600.
In Comparative Example A (steel composition is the same as that of the invention example), a decrease in hardness in the treatment at 600 ° C. is remarkable. The substantial difference in manufacturing conditions between the comparative example A and the inventive example is only the slab extraction temperature (comparative example A is low temperature extraction). This indicates that the high temperature extraction of the slab according to the present invention is an important factor for improving the recrystallization softening resistance.
In Comparative Example C (a typical conventional material of an AT plate, an S35C carbon steel equivalent material), the hardness of Hv230 or higher was maintained at 500 ° C, but Hv230 could not be satisfied at 550 ° C, and the hardness decreased at 600 ° C. Becomes even larger, and the difference from the invention example is obvious.

Further, when looking at Comparative Example B1, there is an example (No. 17) in which the hardness is already insufficient by the treatment at 550 ° C., and at 600 ° C., all the examples (No. 11 to 17) are reduced to a level lower than Hv230. Yes. This comparative example B1 is the same high-temperature slab extraction as the inventive example, and the only substantial difference from the inventive example is that the steel composition deviates from the provisions of the present invention. From this, it can be seen that the provision of the steel composition of the present invention is an essential requirement in combination with the high temperature extraction of the slab in order to obtain a remarkable improvement effect of the anti-recrystallization softening property.
Comparative Example B2 is an example in which both the steel composition and the slab extraction temperature are out of the definition of the present invention, and the recrystallization softening resistance is far from that of the inventive example.

  Since the cold-rolled steel sheet of the present invention has a high resistance to recrystallization softening, it is possible to effectively apply a press temper treatment for improving the flatness of a press punched molded product. In the field of AT plates, along with complication of shape such as increasing the diameter of the plate and narrowing the width of the crosspiece, the tendency of distortion of the plate plate surface tends to become prominent. Since the recrystallization softening phenomenon (decrease in hardness) can be effectively avoided, a high temperature is applied to the press temper treatment, and the shape correction effect is achieved more effectively while suppressing the decrease in hardness, and the required flatness is achieved. It becomes easy to ensure the property.

According to the present invention, a cold-rolled steel sheet for various uses that requires a certain hardness, surface roughness, etc., including an automotive AT plate, is manufactured in a process in which the annealing treatment before cold rolling is omitted. Can do. Moreover, the cold-rolled steel sheet has a high resistance to recrystallization softening.
Compared with the conventional manufacturing of cold-rolled steel sheets for AT plates (which requires an annealing process before cold rolling), the cost saving effect of the present invention in which the annealing process is omitted is large.
When the cold-rolled steel sheet according to the present invention is used for an AT plate, it is possible to sufficiently guarantee the required properties of hardness of Hv230 or higher and surface roughness of Ra 0.4 μm or lower.

Since the cold-rolled steel sheet according to the present invention has a high degree of recrystallization softening property, the press tempering process is performed as a shape correction process (such as a process for restoring the flatness of the plate plate surface) of a molded product obtained by press punching. In the case of carrying out the process, the softening (decrease in hardness) accompanying the recrystallization phenomenon can be effectively suppressed and mitigated, and the required hardness can be maintained. In particular, with AT plates for automobiles, it becomes difficult to ensure flatness as the shape of the AT plate increases due to the increased displacement and weight reduction, and the shape of the punching bar portion becomes narrower. However, according to the present invention, it is possible to stably apply a press tempering process and stably assure good flatness while maintaining a required hardness.
The cold-rolled steel sheet according to the present invention can be suitably used for various applications that require a certain hardness, surface roughness, anti-recrystallization softening property, etc., in addition to the use of AT plates.

It is explanatory drawing which shows the test point of abrasion resistance evaluation in an Example.

  1: Test material 2: Rotating disk (mating material)

Claims (6)

In mass%, C: 0.15-0.25%, Si: 0.25% or less, Mn: 0.3-0.9%, P: 0.03% or less, S: 0.015% or less, Al: 0.01-0.08%, N: 0.008% or less, Cr: 0.05-0.5%, Ti: 0.01-0.05%, B: 0.002-0.005 %, The balance is extracted from a slab having a composition composed of Fe and inevitable impurities by heating to a temperature exceeding 1230 ° C. in a heating furnace, hot rolling finishing temperature: Ar ₃ transformation point or higher, and winding temperature: 500 to After hot rolling at 600 ° C., a hot rolled steel sheet having a ferrite-pearlite mixed structure with a ferrite crystal grain size of 5 to 15 μm and a pearlite + cementite fraction of 40% or more is obtained, and the hot rolled steel sheet is pickled. Cold rolling at a reduction rate of 30% or more without annealing Excellent production method for cold rolled steel sheet 耐再 crystal softening properties characterized by.   The method for producing a cold-rolled steel sheet having excellent recrystallization softening resistance according to claim 1, wherein the slab extraction temperature from the heating furnace is 1250 to 1280 ° C.   The method for producing a cold-rolled steel sheet having excellent recrystallization softening resistance according to claim 1 or 2, wherein the rolling reduction in cold rolling is 35 to 55%.   The cold rolling is composed of a first stage pre-rolling before the pickling treatment of the hot-rolled steel strip and a second stage rolling which is finish cold rolling after the pickling treatment. The rolling reduction of the pre-rolling is 25. The method for producing a cold-rolled steel sheet having excellent recrystallization softening resistance according to any one of claims 1 to 3. An excellent recrystallization softening property having a hardness of Hv: 230 or more and a surface roughness of Ra: 0.4 μm or less, produced by the production method according to any one of claims 1 to 4. Cold rolled steel sheet.   A cold-rolled steel sheet for an automatic transmission member comprising the cold-rolled steel sheet according to claim 5 and excellent in recrystallization softening resistance.

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