JP4232927B2 - Medium carbon steel cold-rolled steel sheet excellent in press workability and method for producing the same - Google Patents

Description

【００２６】
【表２】

【００２７】
【発明の効果】
本発明の中炭素鋼冷延鋼板は、高硬度を要求されるプレス成形品の素材として有用であり、例えば自動車のオートマチック・トランスミッションに組み込まれるフリクションプレート，セパレートプレート等の構成材料として好適に使用でき、製造歩留まり、品質の安定化、コスト低減等に効果をもたらすものである。
【図面の簡単な説明】
【図１】本発明の冷延鋼板の金属組織（フェライト−パーライト混合組織）を示す図面代用顕微鏡写真である。
【図２】本発明の冷延鋼板の金属組織（フェライト−球状化セメンタイト混合組織）を示す図面代用顕微鏡写真である。
【図３】リング形状を有する打ち抜き成形品の剪断割れの発生状況を模式的に示す図である。
【符号の説明】
１：打ち抜き成形品
２：剪断割れ
ｆ：フェライト
ｐ：パーライト
ｃ：球状化セメンタイト[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an improvement of a medium-carbon steel cold-rolled steel sheet. For example, the medium-carbon steel cold-rolled steel having high hardness and good press workability, which is useful as a constituent material of a friction plate incorporated in an automatic transmission of an automobile, for example. It is related with a steel plate and its manufacturing method.
[0002]
[Prior art]
Friction plates, separate plates, and the like incorporated in automobile automatic transmissions are manufactured from carbon steel cold-rolled steel sheets, processed into a plate shape by pressing, shape correction, surface shot processing, and the like.
These plates need to be hard and wear resistant. Recently, in order to improve the stability and durability of the function, it is desired to increase the strength of the plate. For example, a material having high hardness and wear resistance of about 260 Hv hardness or higher is required. .
[0003]
[Problems to be solved by the invention]
The enhancement of the hardness and wear resistance of the plate material can be dealt with by adjusting the chemical composition of the material steel or the metal structure.
However, in a cold-rolled steel sheet with increased hardness (for example, Hv of about 260 or more), cracks occur on the peripheral side surface (shear surface) of the plate as shown in FIG. There is a problem that it is easy.
The present inventors have made various studies on the cold-rolled steel sheet for press working that requires high hardness, such as a friction plate, for the purpose of improving press workability and preventing shear cracking while ensuring the required hardness. As a result, the amount of non-metallic inclusions in steel, particularly the amount of A-type non-metallic inclusions and the form thereof can be controlled, and the metal structure can be adjusted to a specific structure to meet both requirements of hardness and press workability. As a result, the present invention has been completed.
[0004]
[Means for Solving the Problems]
The cold-rolled steel sheet of the present invention (Claim 1)
C: 0.2 to 0.8% by weight, Si: 0.35% by weight or less, Mn: 0.90% by weight or less, P: 0.03% by weight or less , S: 0.02% by weight or less , Cr: 0.2% by weight or less , Ca: 0.001 to 0.02% by weight , consisting of a medium carbon steel having a chemical composition consisting of the balance Fe and inevitable impurities ,
The cleanliness of steel (JIS G0555) is dA (60 × 400): 0.015% or less, dT (60 × 400): 0.030% or less,
Ferrite - and a woven perlite mix sets.
The method for producing a cold-rolled steel sheet according to the present invention (Claim 2)
Carbon steel in having the aforementioned chemical composition and hot rolling, carried out the winding of the hot rolled steel sheet in a temperature range of A _r3 point -50 ° C. to A _r3 point -200 ° C. (sheet temperature), followed by cold rolling ratio 1 ~ 25% cold rolling .
The cold-rolled steel sheet of the present invention (Claim 3)
The medium carbon steel material having the above chemical composition is hot-rolled, and the hot rolled steel sheet is wound at _{Ar 3} point−50 ° C. to A _r3 point−200 ° C. (sheet temperature). A cold-rolled steel sheet having a predetermined cleanliness (JIS G0555) and a ferrite-spherical cementite mixed structure, which is produced by performing spheroidizing annealing and then performing cold rolling with a cold rolling rate of 30 to 70% .
[0005]
Shear cracks in cold-rolled steel sheets in press work are cracks that develop on the shearing surface as a result of the propagation and connection of microcracks that originate from microcracks that occur in steel due to the action of external forces of shear. . Non-metallic compounds present in steel, especially A-based non-metallic inclusions (in this case, mainly MnS) having a stretched shape due to viscous deformation during rolling, presses to which shear force is applied due to their morphological characteristics. In processing, when it is easy to become a starting point of a fine crack and the metal structure is poor in ductility, the propagation and connection of the generated fine crack is promoted, and it tends to progress to a shear crack.
[0006]
The present invention is based on the above knowledge regarding the mechanism of shear crack generation, and the chemical composition of the steel is controlled to contain S in the steel and contains Ca. As described above, it is possible to suppress and prevent the occurrence of fine cracks as the starting point of shear cracks, and even if fine cracks are generated by making the metal structure a ferrite-pearlite mixed structure or a ferrite-spheroidized cementite mixed structure, the progress It is possible to ensure a high hardness according to the application such as a friction plate while maintaining an appropriate ductility that can suppress and prevent the above.
[0007]
[Chemical composition]
First, the chemical composition of the material steel will be described. All percentages indicating element content are based on weight.
C: 0.2 to 0.8%
C is an element necessary for increasing the hardness. In order to ensure the hardness required for wear-resistant applications such as friction plates, it is necessary to be 0.2% or more. If it exceeds 0.8%, the ductility becomes insufficient, and it becomes difficult to obtain the effect of improving the shear crack resistance.
[0008]
S: 0.02% or less S combines with Mn to form MnS. MnS undergoes viscous deformation during the hot rolling process to become A-based inclusions, and becomes a starting point of shear cracking in the press working of steel sheets. For this reason, it is desirable that the amount of S is as small as possible, and it is necessary to limit it to 0.02% or less.
[0009]
Ca: 0.001 to 0.02%
Ca has a higher reactivity with S than Mn and preferentially reacts with S to produce CaS, thereby suppressing and reducing the production of MnS. CaS is a compound having a remarkably higher melting point than MnS, and unlike MnS, there is no viscous deformation in hot rolling, and it exists in steel while maintaining a spherical shape. Therefore, by fixing S as CaS, it is possible to suppress and reduce the formation of A-based inclusions that are the origin of shear cracking.
In order to obtain this effect, the Ca content needs to be 0.001% or more from the relationship with the upper limit of the S content. However, there is no benefit of adding a large amount exceeding 0.02%, and this is wasteful in cost, so this is the upper limit.
[0010]
The amount of elements such as Si, Mn, and P is defined in the same manner as that in general medium carbon steel. Si is added in the range of 0.35% or less, and Mn is added in the range of 0.90 % or less as a deoxidation / desulfurization element, in order to ensure the deoxidation element in the melting and casting of steel and the flow of molten metal. Ru is. Further, 0.2% or less of Cr is added as a hardenability improving element. P is regulated to 0.03% or less as an impurity harmful to ductility.
[0012]
[dA (60 × 400): 0.015% or less]
[dT (60 × 400): 0.03% or less]
In the press working of the steel sheet to which the shear external force acts as described above, the A-based inclusion becomes a starting point of occurrence of fine cracks (micro voids). In order to obtain improved shear crack resistance, the A-system inclusion cleanliness dA (number of fields of view: 60, magnification: 400 times) [JIS G0555] must be regulated to 0.015% or less. In order to obtain this cleanliness, it is necessary to limit the amount of S and adjust to the chemical composition containing Ca. In addition to the definition of dA, increasing the total cleanliness of the A-type, B-type, and C-type inclusions is effective in suppressing the propagation and connection of microcracks, and for this reason, dT (60 × 400) is preferably 0.03% or less.
[0013]
[Metal structure]
The cold-rolled steel sheet of the present invention has a ferrite-pearlite mixed structure or a ferrite-spheroidized cementite mixed structure.
FIG. 1 shows a ferrite-pearlite mixed structure, and FIG. 2 shows a ferrite-spheroidized cementite mixed structure. In the figure, f is ferrite, p is pearlite, and c is spheroidized cementite (test material in FIG. 1: No. 2 in the example column, sample material in FIG. 2: No. 3 in the example column).
[0014]
The ferrite-pearlite mixed structure (FIG. 1) is a structure in which the ferrite-pearlite mixed structure of the hot-rolled steel sheet is almost carried over to the cold-rolled steel sheet, and the ferrite-spherical cementite mixed structure (FIG. 2) This is a structure formed by annealing (spheroidizing annealing) prior to cold rolling.
By imparting such a metal structure to a cold-rolled steel sheet, even if a microcrack is generated by press working, its propagation and connection (progress to shear cracking) are prevented and prevented, and it is suitable for applications such as friction plates. High hardness is given.
[0015]
The cold-rolled steel sheet of the present invention can be evaluated for shear crack resistance using the total elongation (El) and the hole expansion ratio (λ) as indices. Steel sheets having characteristic values of total elongation (El) of 4% or more and hole expansion ratio (λ) of 15% or more have excellent shear crack resistance. This characteristic value is guaranteed as the effect of controlling the A-based inclusions based on the chemical composition and adjusting the metal structure.
[0016]
The cold-rolled steel sheet of the present invention is manufactured through a step of cold rolling after hot rolling a medium carbon steel slab whose chemical composition is adjusted, and then performing or omitting cementite spheroidizing annealing.
First, a cold rolled steel sheet having a ferrite-pearlite mixed structure will be described.
Hot rolling is performed at A ₃ transformation point or more single-phase austenite temperature region in a conventional manner. The coiling temperature of the hot-rolled steel sheet is important, and it is preferable to adjust it to a slightly low temperature range of _Ar3 point−50 ° C. to _Ar3 point−200 ° C. (plate temperature). If it exceeds _{Ar 3} point −50 ° C., the pearlite becomes coarse and the homogeneity of the structure is impaired. On the other hand, cracking may occur in a low temperature range below _{Ar 3} point −200 ° C.
[0017]
Cold rolling requires that the cold rolling rate be set to a low range of 1 to 25%. This is because if the cold rolling rate exceeds 25%, the ferrite is excessively hardened and the press workability of the steel plate is impaired, and if the cold rolling rate is less than 1%, the hardness of the steel plate is insufficient. The specific cold rolling rate is individually set according to the plate thickness, required hardness, and the like. The resulting cold-rolled steel sheet has a ferrite-pearlite mixed structure in which the metal structure of the hot-rolled steel sheet is substantially maintained as it is.
[0018]
Next, to describe the production of a cold rolled steel sheet having a ferrite-spherical cementite mixed structure, hot rolling is performed in the austenite single-phase temperature range as described above, and winding of the hot rolled steel sheet is also performed. For the same reason as described above, it is necessary to carry out in the temperature range of _{Ar 3} point −50 ° C. to _{Ar 3} point −200 ° C. (plate temperature).
Next, the hot-rolled steel sheet is annealed (cementite spheroidizing process), and after the annealing process, cold rolling is performed. Cold rolling needs to be performed in the range of a cold rolling rate of 30 to 70%. The reason why the cold rolling rate is 30% or more is to increase the hardness by introducing processing strain into the steel sheet softened by the annealing treatment. However, if the cold rolling rate exceeds 70%, the curing proceeds excessively and impairs press workability, so this should not be exceeded. The specific cold rolling rate is individually set according to the plate thickness, required hardness, and the like. The obtained cold-rolled steel sheet has a mixed structure in which spheroidized cementite is dispersed in the ferrite ground.
[0019]
Although the processing method of the spheroidizing annealing performed at the said process is not specifically limited, Preferably a three-stage process is applied. This is a combination of heating in the temperature range in the vicinity of _{A 1} transformation point, specifically, for _example, first holding more 0.5Hr in a temperature range of less than _{A c1} point -50 ° C. _{to A c1} point After performing the stage treatment, the second stage treatment is carried out by heating for 0.5 to 20 hours in the temperature range of A _c1 point to A _c1 point + 100 ° C., and then the A _r1 point −80 ° C. to A _r1 point A treatment of heating and holding for 2 to 60 hours in the temperature range is continuously performed, and a ferrite-spherical cementite mixed structure with good homogeneity can be suitably formed.
[0020]
【Example】
Hot-rolled medium carbon steel slabs with adjusted chemical composition to obtain hot-rolled sheets (thickness 2.0-5.5 mm), spheroidizing annealing or cold rolling with the same treatment omitted Then, a cold-rolled plate (plate thickness of 1.0 to 2.5 mm) having a predetermined plate thickness is obtained. Each test steel sheet was measured for various properties and subjected to a punching test of the friction plate.
[0021]
Inventive Example No. 3 and 5 and Comparative Example No. The annealing conditions for hot-rolled steel sheets 13 and 14 are as follows.
No. 3: 700 ° C. × 20 hr → 745 ° C. × 7 hr → 690 ° C. × 7 hr
No. 5: 710 ° C. × 15 hr
No. 13: 700 ° C. × 20 hr → 745 ° C. × 7 hr → 690 ° C. × 7 hr
No. 14: 720 ° C. × 20 hr
[0022]
Table 1 shows the manufacturing conditions of the test materials, and Table 2 shows the characteristics and the punching test results.
The shear crack occurrence rate in the table is the ratio (%) of the number of shear cracks generated in 800 sheets of plate material collected from the same lot.
Comparative Example No. Looking at 11 and 12 (ferrite-pearlite mixed structure),
No. No. 11 has poor cleanliness of the A-based inclusions, so the hole expansion rate (λ) is low, shear cracking occurs, No. 12 has poor cleanliness of the A-based inclusions, and the structure becomes coarse due to the hot rolling temperature being too high, the hole expansion rate (λ) is low, and shear cracks are generated.
[0023]
Moreover, No. of the comparative example. Nos. 13 and 14 (ferrite-spherical cementite mixed structure) No. 13 has high hardness, but the cleanliness of the A-based inclusions is poor and the hole expansion rate (λ) is low. No. 14, although the cleanliness of the A-based inclusions is good, the total inclusion cleanliness (dT) greatly exceeds 0.03%, so the occurrence of shear cracks is remarkable.
[0024]
On the other hand, the invention examples have good cleanliness of the A-based inclusions, and there is no shear cracking due to appropriate hot and cold rolling, and the use of friction plates and the like. It has the high hardness required.
[0025]
[Table 1]

[0026]
[Table 2]

[0027]
【The invention's effect】
The medium-carbon steel cold-rolled steel sheet of the present invention is useful as a material for press-molded products that require high hardness, and can be suitably used as a constituent material for friction plates, separate plates, etc. incorporated in automatic transmissions of automobiles, for example. This is effective for manufacturing yield, quality stabilization, cost reduction, and the like.
[Brief description of the drawings]
FIG. 1 is a drawing-substituting micrograph showing a metal structure (ferrite-pearlite mixed structure) of a cold-rolled steel sheet of the present invention.
FIG. 2 is a drawing-substituting micrograph showing the metal structure (ferrite-spheroidized cementite mixed structure) of the cold-rolled steel sheet of the present invention.
FIG. 3 is a diagram schematically showing the occurrence of shear cracks in a punched molded product having a ring shape.
[Explanation of symbols]
1: Punched molded product 2: Shear crack f: Ferrite p: Pearlite c: Spheroidized cementite

Claims (3)

C: 0.2 to 0.8% by weight, Si: 0.35% by weight or less, Mn: 0.90% by weight or less, P: 0.03% by weight or less, S: 0.02% by weight or less , Cr: 0.2% by weight or less , Ca: 0.001 to 0.02% by weight , made of medium carbon steel having a chemical composition consisting of the balance Fe and inevitable impurities, and the cleanliness of the steel (JIS G0555) is dA (60 × 400): 0.015% or less, dT (60 × 400) is 0.030% or less, ferritic - carbon steel cold-rolled steel sheet in which excellent press formability having woven perlite mix sets. C: 0.2 to 0.8% by weight, Si: 0.35% by weight or less, Mn: 0.90% by weight or less, P: 0.03% by weight or less, S: 0.02% by weight or less , Cr: 0.2% by weight or less , Ca: 0.001 to 0.02% by weight , a medium carbon steel material having a chemical composition composed of the balance Fe and inevitable impurities is hot-rolled, and winding of the hot-rolled steel sheet is performed at _{Ar 3} carried out in the temperature range of 50 ° C. to a _r3 point -200 ° C. (sheet temperature), followed by a rolling cold rolling ratio 1% to 25% of the cold, ferrite - excellent press formability having pearlite mixed structure Manufacturing method of medium carbon steel cold-rolled steel sheet for press working. C: 0.2 to 0.8% by weight, Si: 0.35% by weight or less, Mn: 0.90% by weight or less, P: 0.03% by weight or less, S: 0.02% by weight or less, Cr: 0.2% by weight or less, Ca: 0.001 to 0.02% by weight , a medium carbon steel material having a chemical composition consisting of the balance Fe and unavoidable impurities is hot-rolled, and winding of the hot-rolled steel sheet is performed at _{Ar 3} points − after at 50 ° C. _{to a r3} point -200 ° C. (sheet temperature) is produced by subjecting a spheroidizing annealing treatment of cementite hot-rolled steel sheet, followed by a rolling cold rolling ratio 30% to 70% of the cold DA (60 × 400): 0.015% or less, dT (60 × 400): 0.030% or less cleanliness (JIS G0555), and excellent workability in press working with a ferrite-spherical cementite mixed structure carbon steel cold-rolled steel sheet in the.

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