JP4023733B2 - Cold-rolled steel sheet with excellent fatigue characteristics and manufacturing method thereof - Google Patents

Description

【００４６】
【表２】

【００４７】
【表３】

【００４８】
【表４】

【００４９】
【発明の効果】
本発明によれば、従来自動車のＡＴプレート用冷延鋼板の製造に不可欠であった焼鈍処理が不要となり、これを省略した製造工程により、製造コストを大幅に削減しながら、従来材と同等の耐摩耗性及び改良された疲労特性を具備し、ＡＴプレートの要求特性を十分に満たした冷延鋼板が得られる。またこれを打抜き加工して得られるＡＴプレートは、調質のための熱処理を必要とせず、そのまま（硬引き材のまま）実機使用に供することができる。なお熱延鋼板の冷間圧延を酸洗処理の前後に実施する２段階圧延により酸洗処理の負荷が大幅に軽減され、一段と低コスト化効果が得られる。従って本発明は近時の小型大衆車ＮＢＣの開発動向等に関連するオートマチックトランスミッションの低価格化の要請に対処し得るものである。本発明の冷延鋼板は、上記ＡＴプレート用途のみらなず、疲労特性、耐摩耗性及びプレス打抜き性等を要求される各種用途の素材として広く適用され、品質の安定、コスト削減等の効果をもたらすものである。
【図面の簡単な説明】
【図１】実施例欄における発明材（No.1）の冷延鋼板の金属組織を示す図面代用顕微鏡写真（倍率×400）である。
【図２】実施例欄における比較材（No.33）の冷延鋼板の金属組織を示す図面代用顕微鏡写真（倍率×400）である。
【図３】実施例欄における比較材（No.35）の冷延鋼板の金属組織を示す図面代用顕微鏡写真（倍率×400）である。
【図４】実施例欄における比較材（No.11）の冷延鋼板の金属組織を示す図面代用顕微鏡写真（倍率×400）である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a cold-rolled steel sheet excellent in fatigue characteristics and the like, which is suitably used as a separate plate, friction plate, backing plate and the like, which are mainly components of automatic transmissions of automobiles, and a method for manufacturing the same.
[0002]
[Prior art]
Separate plate (also referred to as driven plate or mating plate), friction plate (also referred to as drive plate, core plate or disk), backing plate (retaining plate, etc.) constituting the automatic transmission (AT) of an automobile A member (hereinafter referred to as “AT plate”) such as a reaction plate or an end plate is a molded product obtained by press punching a steel plate into a substantially annular shape. Separate plates and friction plates are alternately arranged via friction materials, and a torque transmission mechanism is configured by assembling a backing plate and the like. The wear resistance and surface roughness of these part materials are functionally important characteristics and are required to satisfy hardness (Hv): 230 or more and surface roughness (Ra): 0.4 μm or less. .
[0003]
Conventionally, machine structural steel specified in JIS G3311 as an AT plate material, mainly S35C cold-rolled steel sheet, is used. This is called “steel making → continuous casting → hot rolling → pickling → annealing → cold rolling → degreasing → Manufactured in the process of “preparation”. In this manufacturing process, the rolling reduction of cold rolling should be 50% or more so as to satisfy the required characteristics (hardness: Hv ≧ 230, surface roughness: Ra ≦ 0.4 μm) of the cold rolled steel sheet for AT plate. Is needed. The “annealing” before cold rolling is hard if it is a hot-rolled steel sheet, which may hinder the stable operation of cold-rolling (rolling rate ≧ 50%). This is because the coarse pearlite structure of the hot-rolled steel sheet is carried over to the cold-rolled steel sheet, and the press punchability of the product steel sheet is deteriorated.
[0004]
In other words, “annealing” before cold rolling is intended to soften the hot-rolled steel sheet and spheroidize the carbide. By performing annealing, stable operation of cold rolling with a reduction rate of 50% or more and product cold rolling are achieved. The press punchability of the steel sheet is maintained. Thus, in the manufacture of the conventional cold rolled steel sheet for AT plate using S35C as a raw material, it is an essential process to anneal the hot rolled steel sheet before cold rolling, and the annealing treatment is usually tight coil annealing (TCA). ).
[0005]
[Problems to be solved by the invention]
Conventional steel plates for AT plates (S35C cold-rolled steel plates) must be annealed before cold rolling, and because the treatment form is TCA annealing, they must be treated for a long time (soaking: about 10 hours). This is a major factor in increasing costs. However, as seen in the recent development trend of the small-sized mass transit car NBC (New Basic Car), the demand for lower prices has become a trend of the times, and the wear resistance of S35C cold-rolled steel sheet is also the same for the automatic transmission. The demand for the development of low-priced materials with higher properties is becoming stronger.
[0006]
Furthermore, as a problem in terms of material characteristics, not only wear resistance but also fatigue characteristics have recently started to be emphasized. The AT plate is repeatedly deformed by the action of repeated stress accompanying the torque transmission drive operation. The amplitude of the bending deformation is very small, but causes fatigue deterioration due to frequent repetition. In particular, the fatigue deterioration of the AT plate due to repeated deflection deformation is a phenomenon that cannot be ignored for high displacement models, and it is wear resistant from the standpoint of stability and durability of the function of the torque transmission mechanism. In addition to the above, fatigue characteristics are also required.
[0007]
The present invention has been made in view of the above, and omits the annealing process before cold rolling, which is essential for the manufacture of conventional AT plates and is a major factor in cost increase, and is equivalent to the conventional material (S35C). As an AT plate material and a material in various fields that require similar characteristics, it has improved fatigue characteristics that can be applied to high displacement models as well as ensuring wear resistance, press punchability, etc. The present invention provides a cold-rolled steel sheet that is suitably used and a method for producing the cold-rolled steel sheet.
[0008]
[Means for Solving the Problems]
The cold-rolled steel sheet of the present invention is, in mass%, C: 0.17 to 0.25%, Si: 0.05% or less, Mn: 0.5 to 0.9%, P: 0.03% or less, S: 0.015% or less, Al: 0.02-0.08%, N: 0.008% or less, Ti: 0.02-0.07%, B: 0.003-0.006%, balance Is cold-rolled with a reduction ratio of 50% or more without annealing treatment of hot-rolled steel sheet consisting of Fe and inevitable impurities , pearlite + cementite fraction: 30% or more, and ferrite grain size: 5-15 μm Is a cold-rolled steel sheet manufactured by
[0009]
The cold-rolled steel sheet according to the present invention is an annealing prior to cold rolling (hot-rolled steel sheet) that has been conventionally required as an effect of the above-mentioned chemical composition, in particular, the regulation of the C content and Si content and the combined addition of specific amounts of Ti and B. Softening and carbide spheroidizing treatment), enabling stable operation of cold rolling (rolling ratio ≧ 50%) and maintaining wear resistance and press punching performance comparable to conventional materials (S35C). And achieving improved fatigue properties. Moreover, this cold-rolled steel sheet does not require heat treatment for tempering after punching, and can be used as it is (as a hard-drawn material) as a material for an AT plate or the like.
[0010]
The cold-rolled steel sheet of the present invention is a pearlite + obtained by hot-rolling a steel slab having the above chemical composition (hot-rolling temperature ≧ Ar ₃ transformation point) and performing hot-rolling at 480-560 ° C. The hot rolled steel sheet having a cementite fraction of 30% or more and a ferrite particle size of 5 to 15 μm is manufactured by a step of cold rolling at a reduction ratio of 50% or more without pickling and annealing. As described later, cold rolling is divided into cold rolling (pre-rolling) before pickling treatment and cold rolling (finish rolling) after pickling treatment, and the total rolling reduction of the preceding and subsequent rolling is 50% or more. It can also be implemented as a two-stage cold rolling so that Therefore, the cold rolling in the present invention includes a normal rolling form performed after the pickling treatment and a two-stage rolling form performed before and after the pickling treatment.
[0011]
The amount of C in the present invention is specified in a range lower than that of the conventional material (S35C). By limiting the amount of C, the amount of pearlite in the hot-rolled steel sheet can be reduced and the hot-rolled steel sheet can be softened. As effects, softening of the hot-rolled steel sheet before cold rolling and carbide (Fe ₃ C) It is possible to omit the annealing treatment for the purpose of spheroidizing.
[0012]
Control of the steel structure based on limiting the amount of Si to a small amount in combination with the above-mentioned definition of C amount is an important requirement of the present invention. Si, as a ferrite former, promotes the ferrite structure of the hot-rolled steel sheet and inhibits the formation of a refined structure. The present invention makes it easy to form a refined structure (ferrite grain size 5-15 μm) advantageous for fatigue characteristics by limiting the amount of Si.
[0013]
Furthermore, the material improvement effect based on the control of the steel structure by adding a certain amount of Ti and B in combination is the most important feature of the present invention. By adding Ti, fine precipitates (size: about 500 to 3000 mm) such as TiC and Ti (C, N) are formed in the steel, and the ferrite structure of the hot-rolled steel sheet is remarkably refined. In the hot-rolled steel sheet, carbides are preferentially precipitated at the ferrite grain boundaries, so that the carbides are uniformly and finely dispersed in the steel as an effect of refinement. This ensures a uniform fine carbide structure (perlite + cementite fraction ≧ 30%) to obtain improved fatigue properties.
[0014]
Further, the solid solution B produced by the addition of B has a grain boundary strengthening action, increases the strength of the matrix, and contributes to the improvement of fatigue strength. The effect of improving the fatigue strength by the addition of B is enhanced by the coexistence of Ti. This is because TiN is easier to produce than BN, so that the production of BN (consumption of B) is suppressed, and the amount of dissolved B increases.
[0015]
The conventional wisdom is that the fatigue strength of steel is governed by the tensile strength and hardness. However, even if the strength (tensile strength and hardness) is the same level, improved fatigue strength in combination with wear resistance, etc. by controlling the form of carbide and ferrite structure of the hot-rolled steel sheet as described above Can be secured.
[0016]
The cold-rolled steel sheet of the present invention is limited to a range in which the amount of C is lower than that of the conventional material (S35C). This reduction in the amount of C makes it possible to omit spheroidizing annealing, but wear resistance and fatigue characteristics. As shown in the examples below, due to the refinement of the ferrite structure, the uniform dispersion of carbides, and the strengthening action of the matrix, based on the definition of the amount of Si and the addition of a certain amount of Ti and B in combination. In addition, the disadvantage caused by the reduction in the amount of C is sufficiently compensated for, and an excessive improvement effect is obtained.
[0017]
The reasons for limiting the steel composition of the present invention are as follows. Element content is mass%.
C: 0.17 to 0.25%
From the viewpoint of increasing the hardness, wear resistance and fatigue properties of the cold-rolled steel sheet, the higher the C content, the more advantageous. However, if it exceeds 0.25%, the carbide of the hot-rolled steel sheet from the viewpoint of stable operation of cold rolling. It becomes impossible to omit annealing for spheroidizing and softening. On the other hand, if the amount of C is too low, it becomes difficult to ensure wear resistance and fatigue characteristics comparable to that of S35C, which is a conventional material. For this reason, the amount of C needs to be 0.17 to 0.25%.
[0018]
Si: 0.05% or less Since Si is an element that promotes the formation of the ferrite structure of the hot-rolled steel sheet as described above, it is difficult to refine the structure of the hot-rolled steel sheet when the content thereof is increased. In order to form a structure advantageous for fatigue characteristics in which carbides are uniformly and finely dispersed, the amount of Si needs to be limited to 0.05% or less. Si is an element necessary as a deoxidizer in the steel melting process, but in the present invention, elements such as Al act as a deoxidizer in addition to Si, so the Si amount is limited to the above range. However, there is no problem in the steel melting process and steel quality.
[0019]
Mn: 0.5 to 0.9%
Mn is added to prevent hot brittleness of the steel and strengthen the matrix. If it is less than 0.5%, the effect is small, the strength of the matrix is insufficient, and the fatigue strength cannot be increased. The effect is increased by increasing the amount, but if it exceeds 0.9%, it becomes excessively hard and the workability is impaired.
[0020]
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 cracking due to slab cracking. To lose. If it is 0.03% or less, there is no substantial adverse effect, so this is the upper limit.
[0021]
S: 0.015% or less S is an impure component, forms a low-melting-point compound at the grain boundary, causes processing cracks, and causes an AT plate to deteriorate surface properties in stamping. This adverse effect can be suppressed and prevented by Mn (formation of MnS). However, when the S content is increased, wear starting from MnS tends to occur due to an increase in the amount of MnS produced, and wear resistance is reduced. If it is 0.015% or less, the actual damage is avoided, so this is the upper limit.
[0022]
Al: 0.02 to 0.08%
Al is an element added as a deoxidizing agent in the melting process of steel, and has an action of fixing N in the steel as AlN. If it is less than 0.02%, the deoxidation action is insufficient. On the other hand, 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. For this reason, it is made into 0.02 to 0.08%.
[0023]
N: 0.008% or less N is an element inevitably mixed. If the content is increased, the amount of nitride (AlN, TiN, etc.) produced increases, leading to excessive hardening, so 0.008% or less is required.
[0024]
Ti: 0.02 to 0.07%
As described above, Ti forms fine precipitates such as TiC and Ti (C, N) in the steel to refine the ferrite structure of the hot-rolled steel sheet, and the carbide is uniformly and finely dispersed. Abrasion and fatigue properties are enhanced. In order to ensure this effect, addition of 0.02% or more is required. On the other hand, if added in a large amount, excessive hardening is caused by excessive formation of fine precipitates, so 0.07% is made the upper limit.
[0025]
B: 0.003 to 0.006%
Most of B forms solid solution B as described above, and strengthens the matrix and enhances the wear resistance and fatigue characteristics by the grain boundary strengthening action and the ferrite grain refining action due to the solid solution B. In order to obtain this effect, addition of at least 0.003% is necessary. However, if it exceeds 0.006%, the ferrite structure becomes hardened due to excessive grain refinement, so this is the upper limit.
[0026]
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 continuous casting, and after surface treatment of the slab is appropriately performed, hot rolling is performed. In the case of continuous casting, the hot slab (slab) may be inserted into a heating furnace as it is and rolled hot.
[0027]
[Hot rolling]
In hot rolling, the hot rolling finishing temperature is adjusted to be just above the Ar ₃ transformation point from the viewpoint of the quality of the hot rolled steel sheet and the hot rolling efficiency. The hot-rolled steel sheet needs to be wound in a temperature range of 480 to 560 ° C. In low temperature winding below 480 ° C., the crystal grain size becomes excessively fine and the hot-rolled steel sheet is hardened. On the other hand, when it exceeds 560 ° C., carbides easily aggregate and are necessary for improving fatigue strength and wear resistance. This is because it is difficult to ensure uniform dispersion of carbide (perlite + cementite fraction ≧ 30%) and the ferrite structure becomes coarse.
[0028]
The hot-rolled steel sheet is required to have a refined ferrite structure having a crystal grain size (JIS G0552 “Appendix 2 (normative) method of determination based on intersecting line segment (grain size)”) of 5 to 15 μm. This is because an extremely fine ferrite structure of less than 5 μm causes excessive hardening of the steel, while a coarse structure exceeding 15 μm makes it difficult to obtain good fatigue strength and wear resistance.
[0029]
The hot-rolled steel sheet needs to have a “pearlite + cementite fraction” of 30% 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. The total number of grid points occupied by carbides is counted and calculated as a ratio to the total number of grid grid points. In order to obtain improved fatigue properties, the pearlite + cementite fraction must be 30% or more. The upper limit is not particularly required from the viewpoint of fatigue characteristics, but if it exceeds 60%, the hardness may be higher than necessary, so that it is 60% or less from the viewpoint of maintaining good workability. desirable.
[0030]
The above-mentioned fine uniform structure of a hot-rolled steel sheet (ferrite particle size: 5-15 μm, pearlite + cementite fraction: 30% or more) is a steel containing a combination of the above-mentioned C content, Si content and a certain amount of Ti and B. The effect of the composition is ensured by the hot rolling conditions. By using such a refined ferrite structure (hot rolled steel sheet), good fatigue characteristics, wear resistance, etc. of the final product (cold rolled steel sheet) can be guaranteed.
[0031]
[Cold rolling]
The hot-rolled steel sheet is subjected to cold rolling after removing the surface scale by pickling treatment. The rolling reduction in cold rolling needs to be 50% or more. This is to obtain the hardness (Hv ≧ 230) and surface roughness (Ra ≦ 0.4 μm) necessary for cold rolling steel sheets for AT plates. The upper limit of the rolling reduction is not particularly limited, but it is not necessary to apply a high rolling reduction exceeding about 60%. The hardness is preferably 240 to 280 Hv. This is because if it is less than 240 Hv, it is close to the lower limit of the specification and is not advantageous in terms of stability of the function of the AT plate. The hardness (Hv) can be adjusted by adjusting the rolling reduction. In cold rolling, the surface roughness of the rolling roll is appropriately controlled so as to ensure the required surface roughness.
[0032]
If desired, the cold rolling is performed in two stages of pre-rolling before pickling and finish rolling after processing. 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. In this case, pre-rolling (before pickling treatment) and finish rolling (after pickling treatment) may not be continuous, but a pre-rolling machine is provided on the entry side of the pickling tank and a finish rolling mill is provided on the exit side. It is advantageous from the standpoint of production efficiency that each is installed and has a pre-rolling-pickling treatment-finish rolling continuous configuration.
[0033]
The pre-rolling (before the pickling treatment) in the two-stage rolling requires that the rolling reduction is 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. The reduction ratio in finish rolling (after pickling treatment) is set so that the total reduction ratio (= pre-rolling reduction ratio + finishing rolling reduction ratio) is 50% or more. The total reduction ratio is adjusted in this way, as in the case described above (cold rolling is carried out in one stage after pickling), and the required characteristics as an AT plate (hardness: Hv ≧ 230, surface roughness) Sa: Ra ≦ 0.4 μm).
[0034]
[Degreasing and refining]
After cold rolling, the steel sheet surface is purified by degreasing (electrolytic cleaning, etc.), and then a specified inspection (special dimensions such as plate thickness and width, surface flaws, etc.) and refining process for shape correction, etc. The product cold-rolled steel sheet is obtained. These treatment steps may be performed according to ordinary methods.
[0035]
When the cold-rolled steel sheet of the present invention thus obtained is applied as an AT plate material for automobiles, etc., it may be formed into a required annular shape by performing press punching, and the obtained molded product is tempered. This heat treatment is not required and can be used as an AT plate as it is (as a hard-drawn material).
[0036]
【Example】
[1] Test steel sheet production Converter and degassed processing of molten steel is subjected to continuous casting to form a slab (200 mm thick), and the following process A (annealing omitted) or process B A cold-rolled steel sheet for AT plates is obtained by (annealing).
A: Hot rolling → Pickling → Cold rolling (1 or 2 rolling) → Degreasing → Refinement (inspection)
B: Hot rolling → Pickling → Annealing → Cold rolling (1 step rolling) → Degreasing → Refinement (inspection)
[0037]
(1) See steel composition table 1 and table 2. Nos. 1 to 10 are examples of the invention, and Comparative Example 1 (No. 11-20) is an example of a conventional material (S35C equivalent material). Comparative Example 2 (No. 21-35) has a steel composition similar to that of the present invention, but the content of any element (indicated by underline) is not within the scope of the present invention, Comparative Example No. 3 (No. 36-40) is an example in which the steel composition defined in the present invention is present, but the hot rolling or cold rolling conditions are not within the scope of the present invention.
[0038]
(2) Hot rolling (1) No.1-10 (Invention example)
Heating temperature: 1230 ° C, hot rolling finishing temperature: 880 ° C, coiling temperature: 540 ° C
Thickness of hot-rolled sheet: 4.0mm
(2) No.11-20 (Comparative Example 1)
Heating temperature: 1230 ° C, hot rolling finishing temperature: 860 ° C, winding temperature: 600 ° C
Thickness of hot-rolled sheet: 4.0mm
(3) No. 21-35 (Comparative Example 2)
Heating temperature: 1230 ° C, hot rolling finishing temperature: 880 ° C, coiling temperature: 540 ° C
Thickness of hot-rolled sheet: 4.0mm
(4) No. 36-40 (Comparative Example 3)
Heating temperature: 1230 ° C, hot rolling finishing temperature: 880 ° C, winding temperature: 470-630 ° C
Thickness of hot-rolled sheet: 3.0-4.0mm
[0039]
(3) Annealing treatment
Conducted for No. 11 to 20 (Comparative Example 2 = S35C material) (No. 1 to 10 and No. 21 to 40 were not annealed).
Annealing method: Tight coil annealing (TCA)
Processing temperature / time: 700 ℃ / 10Hr
[0040]
(3) Cold rolling reduction ratio (total rolling reduction in the case of two-stage rolling): No.1-39 = 55%, No.40 = 40%
Product steel plate thickness: 1.8mm
(4) Degreasing electrolytic cleaning (treatment liquid: sodium orthosilicate)
[0041]
[2] Evaluation of fatigue properties Fatigue strength (σ _W ) is obtained from the SN curve in accordance with the “plane bending fatigue test method for flat metal plates” (JIS Z2275).
(1) Testing machine: Schenck type plane bending fatigue testing machine (manufactured by Tokyo Henki Co., Ltd.)
(2) Capacity: 59 N · m
(3) Test method: double swing plane bending (4) Test piece shape: JIS No. 1 test piece (5) Test speed: 20 Hz
(6) Test waveform: sine wave [0042]
1-4 has shown the steel structure (magnification x400) of a test cold-rolled steel plate.
Fig. 1 shows invention material No. 1 (no annealing), Fig. 2 shows comparative material No. 33 (Si excess, Ti shortage, annealing omitted), and Fig. 3 shows comparative material No. 35 (no addition of Ti and B, annealing omitted). 4 is a comparative material No. 11 (S35C material annealed).
Comparative material No. 33 in FIG. 2 (Si excess, Ti deficiency, annealing omitted) and Comparative material No. 35 in FIG. 3 (without addition of Ti and B, annealing omitted) have a rough pearlite structure. In No. 1 (Fig. 1), carbide (Fe ₃ C) is uniformly and finely dispersed in spite of omission of annealing, and comparative material No. 11 in Fig. 4 (S35C material subjected to annealing). It has the same finely divided fine and homogeneous structure.
[0043]
Tables 3 and 4 show the hardness (Hv) of cold-rolled product steel sheets (measurement load: 10 kg), surface roughness (Ra), ferrite grain size (μm) of hot-rolled steel sheets, and pearlite + cementite fraction (%). In addition, measurement results such as fatigue characteristics are shown together with manufacturing conditions.
Invention examples (No. 1 to 10) sufficiently satisfy the hardness and surface roughness specifications (hardness: Hv ≧ 230, surface roughness: Ra ≦ 0.4 μm) required for the AT plate material, The surface quality is equivalent to or better than the conventional S35C cold-rolled steel sheet (No. 11-20). Moreover, the present invention has improved fatigue properties, and the fatigue strength is about 10% higher than that of conventional materials. It has been confirmed that the wear resistance and press punchability have the same good characteristics as the conventional material (S35C cold-rolled sheet subjected to annealing before cold rolling). These properties are based on the chemical composition of the steel described above and the fine, fine and fine structure.
[0044]
On the other hand, when the comparative materials No. 21 to 40 are seen, No. 21 (C excess) is excessively hardened, and No. 22 (C deficiency) has low hardness and significantly lower fatigue properties than the conventional material (S35C). . Nos. 23 and 24 (Si-excess) are inferior in fatigue characteristics because the ferrite structure of the hot-rolled sheet is coarsened and a fine carbide structure cannot be obtained. In No.25, No.26 (Ti excess) and No.29, No.30 (B excess), the ferrite structure of the hot-rolled sheet becomes too fine and hardened unnecessarily. No.27, No.28 (Ti shortage), No.31, No.32 (B shortage), No.33, No.34 (Si excess, Ti shortage), and No.35 (No addition of Ti, B) ) Are inferior in fatigue properties because the ferrite structure of the hot-rolled sheet is coarsened and a fine carbide structure cannot be obtained.
Further, No. 36, No. 37 (hot rolling coiling temperature is too high), the ferrite structure of the hot rolled sheet is coarsened, and a fine carbide structure cannot be obtained, resulting in poor fatigue characteristics. In No. 38 and No. 39 (hot rolling coiling temperature is too low), the ferrite structure of the hot rolled sheet is excessively refined and hardened unnecessarily. No. 40 (insufficient rolling reduction) has insufficient hardness and fatigue properties remain at a low level.
[0045]
[Table 1]

[0046]
[Table 2]

[0047]
[Table 3]

[0048]
[Table 4]

[0049]
【The invention's effect】
According to the present invention, the annealing process, which has been indispensable for the manufacture of cold rolled steel sheets for AT plates of conventional automobiles, is no longer necessary, and the manufacturing process omitting this eliminates the manufacturing cost and is equivalent to the conventional material. A cold-rolled steel sheet having wear resistance and improved fatigue characteristics and sufficiently satisfying the required characteristics of the AT plate can be obtained. In addition, the AT plate obtained by punching it does not require heat treatment for tempering and can be used as it is (as a hard-drawn material) for actual use. Note that the load of the pickling treatment is greatly reduced by the two-stage rolling in which the cold rolling of the hot-rolled steel sheet is performed before and after the pickling treatment, and a further cost reduction effect is obtained. Therefore, the present invention can cope with a demand for a reduction in the price of an automatic transmission related to the recent development trend of a small passenger car NBC. The cold-rolled steel sheet of the present invention is widely applied as a material for various uses not only for the above AT plate applications but also required for fatigue characteristics, wear resistance, press punchability, etc., and effects such as quality stability and cost reduction. It is what brings.
[Brief description of the drawings]
FIG. 1 is a drawing-substituting micrograph (magnification × 400) showing the metal structure of a cold-rolled steel sheet of the invention material (No. 1) in the Example column.
FIG. 2 is a drawing-substituting micrograph (magnification × 400) showing a metal structure of a cold-rolled steel sheet of a comparative material (No. 33) in an example column.
FIG. 3 is a drawing-substituting micrograph (magnification × 400) showing the metal structure of a cold-rolled steel sheet as a comparative material (No. 35) in the Example column.
FIG. 4 is a drawing-substituting micrograph (magnification × 400) showing the metal structure of the cold-rolled steel sheet of the comparative material (No. 11) in the Example column.

Claims (2)

In mass%, C: 0.17 to 0.25%, Si: 0.05% or less, Mn: 0.5 to 0.9%, P: 0.03% or less, S: 0.015% or less, Al: 0.02-0.08%, N: 0.008% or less, Ti: 0.02-0.07%, B: 0.003-0.006%, the balance consists of Fe and inevitable impurities , Pearlite + cementite fraction: 30% or more, ferrite grain size: 5-15 μm hot rolled steel sheet produced by cold rolling at a reduction ratio of 50% or more without annealing Cold rolled steel sheet. In mass%, C: 0.17 to 0.25%, Si: 0.05% or less, Mn: 0.5 to 0.9%, P: 0.03% or less, S: 0.015% or less, Al: 0.02 to 0.08%, N: 0.008% or less, Ti: 0.02 to 0.07%, B: 0.003 to 0.006%, the balance being Fe and inevitable impurities The slab is hot-rolled at a hot rolling finish temperature: Ar ₃ transformation point or higher, a coiling temperature: 480-560 ° C., and a pearlite + cementite fraction: 30% or higher, and a ferrite particle size: 5-15 μm A method for producing a cold-rolled steel sheet having excellent fatigue properties, comprising obtaining a rolled steel sheet and then cold-rolling the hot-rolled steel sheet at a reduction rate of 50% or more without pickling and annealing.

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