阅读说明：本技术 高碳冷轧钢板及其制造方法 (High carbon cold rolled steel sheet and method for manufacturing same ) 是由 宫本友佳 松井洋一郎 佐藤省吾 横田毅 于 2019-02-20 设计创作，主要内容包括：本发明提供一种精密冲裁加工性优异的高碳冷轧钢板及其制造方法。将具有预定组成的铸片直接或者临时冷却进行再加热后,进行粗轧制,粗轧制结束后,进行在Ar<Sub>3</Sub>相变点以上的温度区域结束精轧的精轧,将从精轧结束温度到660℃的温度区域以平均冷却速度30℃/s～70℃/s进行冷却,将在500℃～660℃卷取的热轧钢板直接或者酸洗后,进行在650～720℃的温度区域的退火温度保持的一次箱式退火,之后在20～50％的压下率进行冷轧后,进行在650～720℃的温度区域的退火温度保持的二次箱式退火,从而制造上述高碳冷轧钢板。(The invention provides a high-carbon cold-rolled steel sheet with excellent fine blanking processability and a manufacturing method thereof. Reheating a cast slab having a predetermined composition by direct or temporary cooling, rough rolling, and Ar after finishing the rough rolling 3 Finishing the finish rolling of the finish rolling in a temperature range of not less than the transformation point, cooling the temperature range from the finish rolling temperature to 660 ℃ at an average cooling rate of 30 ℃/s to 70 ℃/s, subjecting the hot-rolled steel sheet coiled at 500 ℃ to 660 ℃ to primary box annealing at an annealing temperature of 650 ℃ to 720 ℃ directly or after pickling, and then subjecting the steel sheet to 20 DEG CThe high-carbon cold-rolled steel sheet is produced by cold rolling at a reduction ratio of 50% and then performing secondary box annealing at an annealing temperature of 650 to 720 ℃.)

1. A high-carbon cold-rolled steel sheet having the following composition and structure,

the composition contains, in mass%, C: 0.45-0.75%, Si: 0.10 to 0.50%, Mn: 0.50-1.00%, P: 0.03% or less, S: 0.01% or less, sol.al: 0.10% or less, N: 0.0150% or less, Cr: 0.10 to less than 0.40%, the balance being Fe and unavoidable impurities,

the structure is such that the average grain size of cementite is 0.40 to 0.75 μm, the average spacing between cementites is 1.5 to 8.0 μm, the spheroidization ratio of cementite is 75% or more, and the average grain size of ferrite is 4.0 to 10.0. mu.m,

the shear surface ratio of the punched end surface after fine blanking by a die in which the clearance between the punch and the die is 25 μm or less is 90% or more, and the arithmetic mean roughness Ra of the shear surface of the punched end surface is less than 1.0 μm.

2. The high-carbon cold-rolled steel sheet according to claim 1, having a cross-sectional hardness of HV160 or less.

3. A method for producing a high-carbon cold-rolled steel sheet according to claim 1 or 2,

the cast slab having the composition is directly or temporarily cooled and reheated, then rough-rolled,

after the rough rolling is finished, Ar is carried out₃Finishing finish rolling of finish rolling is finished in a temperature region above the transformation point,

a hot-rolled steel sheet wound at 500 to 660 ℃ is subjected to primary box annealing at an annealing temperature of 650 to 720 ℃ directly or after pickling while cooling at an average cooling rate of 30 to 70 ℃/s in a temperature range from the finish rolling temperature to 660 ℃, then is subjected to cold rolling at a reduction ratio of 20 to 50%, and then is subjected to secondary box annealing at an annealing temperature of 650 to 720 ℃.

Technical Field

The present invention relates to a high-carbon cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to a high-carbon cold-rolled steel sheet which is excellent in precision punching workability, in which an end face having a reduced fracture surface causing fatigue life is obtained in precision punching suitable for slab processing of automobile parts, chain parts, and the like, and a die is less likely to be worn, and a method for manufacturing the same.

Background

High carbon cold-rolled steel sheets are sometimes used as slabs for automobile drive train parts and chain parts. In automobile drive train parts and chain parts, in order to obtain a punched end face having a smooth shape, the parts are often manufactured by fine blanking, while fine blanking is a processing method with a small clearance, and therefore, a high load is applied to a die, particularly, a high load is applied to a punching punch, and a problem arises in die life due to punch wear and the like. These high-carbon cold-rolled steel sheets used as slabs contain a certain amount or more of carbon in order to obtain a predetermined hardness after heat treatment. Such a high-carbon cold-rolled steel sheet having a high C content is subjected to heat treatment such as quenching and tempering, thereby increasing the strength and improving the fatigue life.

Since the high-carbon cold-rolled steel sheet has a high C content, carbon in the steel precipitates as hard cementite, and the high-carbon cold-rolled steel sheet has a large amount of carbon, which makes it difficult to process the steel in the as-hot-rolled state. Therefore, it is generally used to improve workability by spheroidizing cementite by annealing after hot rolling and dispersing the cementite appropriately.

The fine blanking process to be performed in the present invention will be described with reference to fig. 1. The fine blanking process to which the present invention is directed is a fine blanking process in which a high carbon steel sheet is used as a slab and a gap of 25 μm or less is processed using a die (die) and a punch (punch). Fig. 1 is a conceptual diagram illustrating a punched end face after fine blanking. Hereinafter, in the present specification, the punched end surface is also referred to as an "end surface". The end face after the fine blanking process is generally composed of a shear surface (a in fig. 1) generated by contacting with a cutter and plastically deforming and smoothly cutting and a fracture surface (b in fig. 1) generated when a crack occurs and the material is separated. After the heat treatment, in order to ensure a predetermined fatigue life, it is preferable to suppress the fracture surface having a large end surface roughness as much as possible and to reduce the surface roughness of the shear surface. Further, since the fine blanking is a method of processing with a small clearance, a high load is applied to the die, particularly to the blanking punch, and the die life is shortened as compared with the usual blanking. In order to prolong the life of the die, the surface roughness of the shearing surface is preferably small.

When the ductility of the steel sheet is too large or too small, the die life is also shortened. For example, if the cementite is too soft during spheroidizing annealing, the fluidity of the steel sheet tends to be improved during blanking (punching), but since the fluidity is too good, the steel sheet contacts the punch too much, wear of the punch increases, and the life of the punch decreases. On the other hand, when the spheroidization of cementite is insufficient during annealing and the steel sheet is too hard, punch wear loss or the like occurs, that is, the punch life is reduced. Therefore, in general, a high-carbon cold-rolled steel sheet used for blanking is hot-rolled and then annealed to spheroidize cementite so as to include a region having an appropriate hardness in the total length of the entire width in the longitudinal direction and the width direction, and then cold-rolled to adjust the hardness.

For example, patent document 1 proposes a method for producing a high-carbon steel strip containing, in mass%, C: 0.20 to 0.80%, Si: 0.3% or less, Mn: 0.60-1.60%, sol.Al: 0.010 to 0.100%, Ca: the steel is hot-rolled at a temperature of 0.0100% or less, coiled at 550 to 680 ℃, acid-washed, cold-rolled 1 st time at a reduction ratio of 10 to 80%, intermediate-annealed at 650 to 725 ℃, cold-rolled 2 nd time at a reduction ratio of 5 to 25%, and then heat-treated to form the product.

Patent document 2 proposes a medium-high carbon hot-rolled steel sheet having the following structure: contains, in mass%, C: 0.10 to 0.70%, Si: 0.01-1.0%, Mn: 0.1-3.0%, P: 0.001-0.025%, S: 0.0001 to 0.010% of Al: 0.001-0.10%, N: 0.001 to 0.01%, ferrite grain size of 10 to 50 μm, cementite grain size of 0.1 to 2.0 μm, cementite spheroidization ratio of 85% or more, hardness HV100 to 160, and excellent punchability.

Patent document 3 proposes a method for producing a high-carbon steel strip excellent in cold rolling workability and fatigue life after heat treatment, containing, in wt%, C: 0.20 to 1.20%, Si: 0.05-0.30%, P: less than 0.020%, which is produced by repeating cold rolling of 20 to 80% and annealing at 650 to 720 ℃ 1 or more times after hot rolling.

Patent document 4 proposes a steel sheet containing, in mass%, C: 0.25 to 0.6%, Si: 2% or less, Mn: 2% or less, P: 0.02% or less, S: 0.02% or less, Cr: 2% or less, V: 0.05 to 0.5%, hardness HV180 to 350, and excellent bending workability and punching workability.

Patent document 5 proposes a high carbon steel sheet containing, in mass%, C: 0.45-0.90%, Si: 0.001 to 0.5% or less, Mn: 0.2-2.0%, P: 0.03% or less, S: 0.005% or less, Al: 0.001-0.10%, N: 0.01% or less, further containing Cr: 0.005 to 1.0%, Mo: 0.005-1.0%, Cu: 0.005-1.0%, Ni: 0.005-1.0%, Ti: 0.005-0.3%, Nb: 0.005-0.3%, V: 0.005-0.3%, B: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and a hardness difference DeltaHVt of 10 or less between t/2 and t/4 (t: plate thickness) at a depth of HV150 or less, and is excellent in workability.

Patent document 6 proposes a steel sheet containing, in mass%, C: 0.1 to 0.5%, Si: 0.5% or less, Mn: 0.2-1.5%, P: 0.03% or less, S: 0.02% or less, and if necessary, Al: 0.1% or less, further Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01-0.1% and B: 0.0005 to 0.005%, and the area ratio of ferrite having an average grain size of 1 to 20 [ mu ] m and an aspect ratio of 2 or less to the total ferrite amount is 70% or more, the spheroidization ratio of carbide is 90% or more, and the amount of ferrite grain boundary carbide is 40% or more.

Patent document 7 proposes a steel sheet containing, in mass%, C: 0.1 to 0.5%, Si: 0.5% or less, Mn: 0.2-1.5%, P: 0.03% or less, S: 0.02% or less, and further optionally, Al: 0.1% or less, and Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01-0.1% and B: 0.0005-0.005%, 1 or 2 or more kinds of ferrite having an average grain diameter of 1-10 μm, a spheroidization ratio of carbide of 80% or more, and a ferrite grain boundary carbide amount of 40% or more, and is excellent in fine blanking workability.

Patent document 8 proposes a high carbon steel sheet containing, in mass%, C: 0.65-0.90%, Si: 0.01 to 0.50% or less, Mn: 0.1-2.00%, P: 0.0200% or less, S: 0.0200% or less and Cr: 0.20 to 2.00%, and further optionally 1 or 2 or more of Al, Mo, Ni, Cu, B, Nb, V, Ti, W, Ta, Mg, Ca, Y, Zr, La, Ce, N, O, Sn, Sb, and As, wherein the spheroidization ratio defined by the number of carbides having an aspect ratio of less than 3 is 80 to 99%, the average grain diameter converted to a circle equivalent diameter is 0.2 to 1.5 μm, and the standard deviation σ of the carbide diameter is 0.10 to 0.45.

Patent document 1: japanese laid-open patent publication No. 11-264049

Patent document 2: japanese patent laid-open publication No. 2015-117406

Patent document 3: japanese patent laid-open No. 2000-34542

Patent document 4: japanese patent laid-open publication No. 2010-235965

Patent document 5: japanese patent laid-open publication No. 2017-179596

Patent document 6: japanese laid-open patent publication No. 2007-270331

Patent document 7: japanese patent laid-open No. 2007-231416

Patent document 8: japanese patent laid-open publication No. 2016 and 222990.

Disclosure of Invention

Patent document 1 proposes a high-carbon steel strip in which the spheroidization ratio of cementite in the steel is 80% or more, the average grain size is 0.8 μm or less, and the tensile strength of the steel is 600 to 700N/mm²The high carbon steel strip is hot-rolled and pickled, and then subjected to 1 cold rolling, annealing, and 2 cold rolling to obtain an end face having a fracture surface reduced as much as possible. However, there is no description about a production method in which a hot-rolled steel sheet having a curl after hot rolling is subjected to primary box annealing, cold rolling, and secondary box annealing as it is or after acid washing, and the tensile strength is less than 600N/mm²The steel having the hardness of (3) is not discussed, and sufficient cold workability cannot be obtained in the high carbon steel strip disclosed in patent document 1.

The medium-high carbon hot-rolled steel sheet described in patent document 2 is a technique relating to a hot-rolled steel sheet having a steel hardness of HV100 to 160 and excellent cold workability, but a sheet thickness of 3.5mm or more, and the present invention is technically different from the target cold-rolled steel sheet and does not describe cold rolling and annealing before and after the cold rolling.

Patent document 3 proposes a method for producing a high carbon steel strip having excellent cold rolling workability and fatigue life after heat treatment, which can achieve predetermined workability by adjusting steel components and conditions of cold rolling and annealing after hot rolling, and does not describe hot rolling and grain sizes of cementite and ferrite.

Patent document 4 proposes a steel sheet excellent in bending workability and punching workability, but does not describe a steel containing Cr in an amount of 0.61% or more, and less than 0.61% of Cr.

In patent document 5, since the chain is also used for the intended purpose, it is estimated that fine blanking workability is also considered as the required workability. However, patent document 5 does not describe a cold rolling process, since the structure and hardness are adjusted only in the annealing process after hot rolling.

Patent document 6 proposes a cold-rolled steel sheet excellent in fine blanking workability, in which the ferrite grain size, the spheroidization ratio of carbide, the carbide amount in ferrite grain boundaries, and the like are restricted even with respect to the structure of the base material, and describes that the influence thereof on Rz of the punched end face, which is an index of fine blanking workability, is exerted, but there is no description that the average grain spacing of carbide is involved and the influence thereof is not exerted. Further, the amount of Cr for obtaining a predetermined fine blanking workability is not described.

Patent document 7 proposes a hot-rolled steel sheet having excellent fine blanking workability, which is technically different from the cold-rolled steel sheet to be the subject of the present invention, and does not describe cold rolling and annealing before and after the cold rolling.

Patent document 8 proposes a high carbon steel sheet having preferable stretch formability, describes a method of performing 2 times of annealing after 1 time of cold rolling in a continuous annealing furnace for 1800 seconds or less, and does not describe a method of performing 2 times of annealing in a box annealing. Further, no indication of precision blanking workability is described.

The invention aims to provide a high-carbon cold-rolled steel sheet with excellent fine blanking processability and a manufacturing method thereof.

More specifically, the present invention provides a high-carbon cold-rolled steel sheet and a method for manufacturing the same, having the following structure: a cold-rolled steel sheet is produced by performing primary box annealing, cold rolling and secondary box annealing while setting a predetermined finish rolling temperature, an average cooling rate until coiling, and a coiling temperature for a steel slab containing 0.10 to less than 0.40% of Cr, wherein the average grain size of cementite is 0.40 to 0.75 [ mu ] m, the average interval between cementites is 1.5 to 8.0 [ mu ] m, the spheroidization ratio of cementite is 75% or more, the average grain size of ferrite is 4.0 to 10.0 [ mu ] m, the shear surface ratio of a punched end surface after blanking by a die having a clearance between a punching punch and a die of 25 [ mu ] m or less is 90% or more, and the arithmetic average roughness Ra of the shear surface of the punched end surface is less than 1.0 [ mu ] m, and the fine blanking workability is excellent.

In the present specification, the high-carbon cold-rolled steel sheet means a cold-rolled steel sheet having a C content of 0.45 mass% or more.

In the present specification, a cold-rolled steel sheet having excellent fine blanking properties means a cold-rolled steel sheet in which the shear surface percentage of the punched end surface after fine blanking by a die having a clearance between a punch and a die of 25 μm or less is 90% or more and the arithmetic mean roughness Ra of the shear surface of the punched end surface is less than 1.0 μm.

The present inventors have conducted extensive studies on the relationship between the finish rolling temperature of steel containing 0.10% or more and less than 0.40% of Cr, the cooling rate until coiling, the coiling temperature, the primary annealing temperature, the reduction of cold rolling, the secondary annealing temperature, and the fine blanking workability.

As a result, it was found that, in the fine blanking workability of the high-carbon cold-rolled steel sheet, the average grain size of cementite, the spheroidization ratio of cementite, and further the average grain size of ferrite in the steel structure were significantly affected, and that the shear surface ratio of the end surface after the fine blanking was 90% or more and the arithmetic mean roughness Ra of the shear surface was less than 1.0 μm, when the average grain size of cementite was 0.40 to 0.75 μm, the average interval between cementites was 1.5 to 8.0 μm, the spheroidization ratio of cementite was 75% or more, and the average grain size of ferrite was 4.0 to 10.0 μm.

The present invention has been completed based on this situation, and the following is a gist.

[1] A high-carbon cold-rolled steel sheet having the following composition and structure,

the composition contains, in mass%, C: 0.45-0.75%, Si: 0.10 to 0.50%, Mn: 0.50-1.00%, P: 0.03% or less, S: 0.01% or less, sol.al: 0.10% or less, N: 0.0150% or less, supplemented Cr: 0.10% or more and less than 0.40%, the balance being Fe and unavoidable impurities,

the structure is such that the average grain size of cementite is 0.40 to 0.75 μm, the average spacing between cementites is 1.5 to 8.0 μm, the spheroidization ratio of cementite is 75% or more, and the average grain size of ferrite is 4.0 to 10.0. mu.m,

the shear surface ratio of the punched end surface after fine blanking by a die in which the clearance between the punch and the die is 25 μm or less is 90% or more, and the arithmetic mean roughness Ra of the shear surface of the punched end surface is less than 1.0 μm.

[2] The high-carbon cold-rolled steel sheet according to [1], wherein the cross-sectional hardness is HV160 or less.

[3] A method for producing a high-carbon cold-rolled steel sheet according to [1] or [2],

the cast slab having the above composition is directly or temporarily cooled and reheated, then rough rolled,

after the rough rolling is finished, Ar is carried out₃Finishing the finish rolling in a temperature region above the transformation point,

a hot-rolled steel sheet wound at 500 to 660 ℃ is subjected to primary box annealing at an annealing temperature of 650 to 720 ℃ directly or after pickling while cooling at an average cooling rate of 30 to 70 ℃/s in a temperature range from the finish rolling temperature to 660 ℃, then is subjected to cold rolling at a reduction ratio of 20 to 50%, and then is subjected to secondary box annealing at an annealing temperature of 650 to 720 ℃.

According to the present invention, a high-carbon cold-rolled steel sheet having excellent fine blanking workability can be provided.

The high-carbon cold-rolled steel sheet of the present invention is preferably used as a slab for automobile parts and chain parts, which require fine blanking workability, and particularly preferably used as a slab for automobile drive system parts such as timing chains.

Drawings

Fig. 1 is a conceptual diagram illustrating a punched end face after fine blanking.

Detailed Description

The high-carbon cold-rolled steel sheet and the method for producing the same according to the present invention will be described in detail below. The unit of the component content, that is, "%" means "% by mass" unless otherwise specified.

1) Composition of

C：0.45～0.75％

C is an element important for obtaining the strength after quenching. When the C content is less than 0.45%, the C content needs to be 0.45% or more because a desired hardness cannot be obtained by heat treatment such as quenching and tempering after the steel sheet is processed into a part. However, if the C content exceeds 0.75%, the steel becomes hard, and cold workability such as toughness and fine blanking workability deteriorates. Therefore, the C content is 0.45 to 0.75%. After quenching, the C content is preferably 0.50% or more, more preferably 0.51% or more, and further preferably 0.53% or more, in order to obtain more excellent hardness. In the case of processing a strictly workable member, that is, a member having a high degree of workability and being difficult to mold, the C content is preferably 0.70% or less, more preferably 0.67% or less, and still more preferably 0.65% or less.

Si：0.10～0.50％

Si is added as a deoxidizer together with Al in refining steel. However, if Si is excessively contained, Si oxide is generated in grain boundaries during heat treatment, and there is an increased risk of lowering the fatigue strength. Therefore, the Si content is 0.50% or less. The Si content is preferably 0.45% or less, more preferably 0.40% or less, and further preferably 0.35% or less. On the other hand, Si is an element that increases the temper softening resistance after heat treatment. The Si content is 0.10% or more in order to obtain a desired hardness even in tempering in a wide temperature range after quenching. The Si content is preferably 0.15% or more, and more preferably 0.16% or more.

Mn：0.50～1.00％

Mn is an element that improves hardenability and enhances strength by solid solution strengthening. When the Mn content exceeds 1.00%, the ribbon structure is developed due to Mn segregation, the structure becomes nonuniform, and the steel is hardened by solid solution strengthening, and cold workability is lowered. Therefore, the Mn content is 1.00% or less. The Mn content is preferably 0.95% or less, more preferably 0.90% or less, and further preferably 0.85% or less. On the other hand, if the Mn content is less than 0.50%, the total hardenability begins to decrease, and therefore the Mn content is 0.50% or more. The Mn content is preferably 0.52% or more, more preferably 0.55% or more.

P: less than 0.03%

P is an element that increases strength by solid-solution strengthening. If the P content exceeds 0.03%, grain boundary embrittlement occurs, and the toughness after quenching deteriorates. However, the P content is 0.03% or less. In order to obtain more excellent toughness after quenching, the P content is preferably 0.02% or less. P reduces cold workability and toughness after quenching, and therefore, the smaller the P content, the more preferable the refining cost increases when P is excessively reduced, and therefore, the P content is preferably 0.005% or more.

S: less than 0.01%

S forms sulfides, and reduces cold workability of the high-carbon cold-rolled steel sheet and toughness after quenching, and therefore is an element that needs to be reduced. If the S content exceeds 0.01%, cold workability of the high-carbon cold-rolled steel sheet and toughness after quenching are significantly deteriorated. Therefore, the S content is set to 0.01% or less. In order to obtain more excellent cold rolling workability and toughness after quenching, the S content is preferably 0.004% or less, and more preferably 0.0040% or less. Since S decreases cold workability and toughness after quenching, the smaller the S content, the better the refining cost increases when S is excessively decreased, and therefore the S content is preferably 0.0005% or more.

Al: less than 0.10%

If the al content exceeds 0.10%, AlN is produced during heating in the quenching treatment and the austenite grains are too fine, and the formation of the ferrite phase is promoted during cooling, so that the microstructure becomes ferrite and martensite, and the hardness after quenching is lowered. Therefore, the sol.al content is 0.10% or less. The al content is preferably 0.06% or less. Note that sol.al has an effect of deacidification, and in order to sufficiently deacidify the sol.al content is preferably 0.005% or more, more preferably 0.010% or more, and further preferably 0.015% or more.

N: 0.0150% or less

If the N content exceeds 0.0150%, the formation of AlN causes the austenite grains to be excessively fine during heating in the quenching treatment, promotes the formation of a ferrite phase during cooling, and lowers the hardness after quenching. Therefore, the N content is 0.0150% or less. The lower limit is not particularly limited, and N is an element that forms AlN or Cr-based nitride, thereby appropriately suppressing the growth of austenite grains during heating in the quenching treatment and improving the toughness after quenching, and therefore the N content is preferably 0.0005% or more.

Cr: more than 0.10 percent and less than 0.40 percent

Cr is an element that delays spheroidization of cementite in steel, and is an important element for improving hardenability in heat treatment. If the amount is less than 0.10%, the spheroidization of cementite excessively progresses, a predetermined average cementite particle diameter cannot be obtained, ferrite is likely to occur during quenching due to hardenability, and a sufficient effect cannot be obtained, so that the Cr content is 0.10% or more. On the other hand, when the Cr content is 0.40% or more, spheroidization of cementite is not easily performed, and a predetermined spheroidization ratio of cementite cannot be obtained. As a result, the steel sheet before quenching is hardened, a predetermined average distance between cementites cannot be obtained, and a fracture surface is likely to occur in the end face or the surface roughness Ra of the sheared surface of the end face becomes large in, for example, fine blanking. Therefore, the Cr content is less than 0.40%. In particular, when a member having a shear surface of an end surface and a surface roughness Ra and a fracture surface of the end surface, which are likely to occur, is machined, further excellent workability is required, and therefore the Cr content is preferably 0.35% or less.

The other components are Fe and inevitable impurities. In addition, when scrap is used As a raw material of the high-carbon cold-rolled steel sheet of the present invention, there is a case where 0.003% or more of 1 or 2 or more of Sn, Sb and As is inevitably mixed, and when 0.02% or less of these elements are all contained, the hardenability of the high-carbon cold-rolled steel sheet of the present invention is not hindered, and therefore, the high-carbon cold-rolled steel sheet of the present invention may contain Sn As an unavoidable impurity: 0.003 to 0.02%, Sb: 0.003-0.02% and As: 0.003-0.02% of 1 or more than 2.

2) Tissue of

The high-carbon cold-rolled steel sheet of the present invention has a structure containing ferrite and cementite. In the structure of the high carbon cold rolled steel sheet of the present invention, the total of ferrite and cementite is 95% or more in terms of area ratio. The total of ferrite and cementite is preferably 97% or more in area percentage, and may be 100%. The remaining part when the area ratio of ferrite and cementite is less than 100% is 1 or 2 selected from pearlite and bainite.

2-1) average particle diameter of cementite: 0.40-0.75 μm

When cementite having a large particle size is present, the cementite is broken in fine blanking and a fracture surface is generated at the end face using the broken cementite as a starting point, and therefore the average particle size of the cementite is 0.75 μm or less. The average particle diameter of the cementite is preferably 0.73 μm or less, more preferably 0.71 μm or less. On the other hand, when the cementite is too fine, the number of cementite particles of 0.1 μm or less is increased, the hardness of the steel is increased, and the fracture surface is increased at the end face in the fine blanking process, so that the average particle diameter of the cementite particles is 0.40 μm or more. The average particle diameter of the cementite is preferably 0.42 μm or more, more preferably 0.44 μm or more. The average grain size is an average value obtained by grinding a cross section parallel to the rolling direction of a test piece taken from the center of the width of the steel sheet, etching the cross section, and then calculating the circle equivalent diameter of all cementites detected at a position of 1/4 mm in thickness by a scanning electron microscope at a magnification of 2000 times.

2-2) average spacing between cementites: 1.5-8.0 mu m

At the position where large deformation is applied in fine blanking, voids occur between cementite grains at ferrite grain boundaries, and cracks are likely to grow. These cracks develop during the forming process after the fine blanking process, and a fracture surface occurs. When the average interval between cementite bodies is less than 1.5 μm, the starting points of voids excessively increase, so that cracks easily occur, and the length of the fracture surface at the end surface increases, thereby deteriorating fine blanking workability. Therefore, the average interval between cementites is 1.5 μm or more. The average interval between cementites is preferably 1.7 μm or more, and more preferably 2.0 μm or more. When the average interval between cementites exceeds 8.0 μm, the cementite becomes too coarse per 1 cementite, cracks are likely to occur, and a portion where the length of the fracture surface of the end face increases is likely to occur. Therefore, the average interval between cementites is 8.0 μm or less. The average interval between cementites is preferably 7.7 μm or less, more preferably 7.5 μm or less. The average interval between cementites is determined as follows: a cross section (1/4 position of the sheet thickness) parallel to the rolling direction of a test piece taken from the center of the sheet width of the steel sheet was observed with a scanning electron microscope at a magnification of 2000 times, the cementite and the structure other than the cementite were binarized using Image analysis software GIMP, the intervals of the cementite were determined using analysis software Image-J, and the total was divided by the number of intervals.

2-3) spheroidization ratio of cementite: over 75 percent

The spheroidized carburized body is preferable because the ductility of the steel is improved and the workability is improved. Since the occurrence of a fracture surface at the end face during punching is greatly suppressed when the spheroidization ratio of cementite is 75% or more, and a predetermined shear surface ratio is easily obtained, the spheroidization ratio of cementite in the structure of the high carbon cold rolled steel sheet of the present invention is 75% or more. The spheroidization ratio of the cementite is preferably 77% or more, more preferably 80% or more. The method for determining the spheroidization ratio of the cementite of the present invention is as follows. The cross section (1/4 position of the sheet thickness) parallel to the rolling direction of the test piece taken from the center of the sheet width of the steel sheet was observed with a scanning electron microscope at a magnification of 2000 times, the cementite and the structure other than the cementite were binarized using Image analysis software GIMP, the area and the peripheral length of each cementite were determined using analysis software Image-J, the circularity coefficient of each cementite was calculated using the following equation, and the average value thereof was determined to obtain the spheroidization ratio of the cementite.

Circularity coefficient 4 pi area/(circumference length)²

2-4) average grain size of ferrite: 4.0-10.0 mu m

The average grain size of ferrite is a factor that strongly restricts workability including hardness and fine blanking workability of the steel sheet. When the ferrite grain size is small, the steel sheet becomes high in hardness due to the fine and strengthened steel, and the workability is lowered. The ferrite has an average grain size of 4.0 μm or more in order to obtain predetermined hardness and workability. Preferably 5.0 μm or more. On the other hand, in fine blanking, when the average grain size of ferrite exceeds 10.0 μm, tear drops are likely to occur at the end face, and the fine blanking workability is lowered. Thus, the average grain size of ferrite is 10.0 μm or less. Preferably 8.0 μm or less. The average ferrite grain size was determined by the method described in examples using the cutting method (defined in JIS G0551).

3) Fine blanking workability

3-1) shear surface rate of end face is more than 90%

After the heat treatment, in order to ensure a predetermined fatigue life, it is desirable to suppress the fracture surface having a large surface roughness of the end face as much as possible and to reduce the surface roughness of the end face, and therefore the shear surface ratio of the end face is 90% or more. Preferably 95% or more. The shear surface ratio of the end face is obtained by the following formula.

Shear plane ratio of end face (length of shear plane/length of entire end face) × 100

The length of the sheared surface and the length of the entire end surface in the above formula are the length of the sheared surface in the plate thickness direction at the center of the plate width of the punched plate and the length of the entire end surface (the total length of the sheared surface fracture surfaces) when the steel plate is fine-blanked by the die in which the clearance between the punching punch and the die is set to 25 μm or less and the plate having 4 angles of 10mmR, 40mm in length × 60mm in width, is punched out. The shear surface ratio of the end face was an average value of values calculated at the center of the width of the punched plate in which 2 portions were present. In the case of performing fine blanking with a die in which the clearance between the blanking punch and the die is set to 25 μm or less, the die is worn out more at the portion where the steel sheet contacts the die. Since a die having insufficient strength has insufficient wear resistance and is worn early, it is preferable to use a die made of SKD steel that can secure a predetermined strength as a die. The clearance between the punch and the die of the die is preferably 2 μm or more.

3-2) arithmetic average roughness Ra of shear surface of end face: less than 1.0 μm

Since fine blanking is a method in which the clearance between the blanking punch and the die is small, a high load is applied to the die, particularly to the blanking punch, and the die life is shortened as compared with ordinary blanking. In order to prolong the life of the die, it is desirable that the shear surface of the end face has a smaller surface roughness, and therefore the arithmetic mean roughness Ra of the shear surface of the end face is less than 1.0 μm. The arithmetic average roughness Ra of the shear plane of the end face is preferably 0.8 μm or less, and more preferably 0.5 μm or less.

The arithmetic average roughness Ra of the shear surface of the end face is as follows: the steel sheet is fine-blanked with a die having a clearance between a blanking punch and a die of 25 μm or less, and the length of 5.0mm is measured in the plate width direction at the plate thickness center of the plate width center of the blanked plate at the time of blanking the plate having 4 corners of 10mmR of 40mm length × 60mm width. The arithmetic mean roughness Ra of the shear surface of the end face was an average value of values obtained at the center of the plate thickness at the center of the plate width where the punched plate had 2 portions.

4) Mechanical Properties

In order to improve the dimensional accuracy of products such as chains and the life (wear resistance) of punching dies, it is important to control the mechanical properties in addition to the shape control of cementite for suppressing the formation of fracture surfaces on the end surfaces during the fine punching process as described in the above 2). When the hardness of the high-carbon cold-rolled steel sheet is high, the fracture surface tends to increase at the end surface, and the die wear becomes severe, and therefore the hardness (cross-sectional hardness) of the high-carbon cold-rolled steel sheet is preferably HV160 or less. The sectional hardness was determined by the method described in examples. In the present specification, the heat treatment conditions to be performed after working and the hardness of the steel sheet after heat treatment are not described, but the high carbon cold rolled steel sheet of the present invention is used after being subjected to heat treatment (quenching and tempering) after working.

5) Manufacturing method

The preferred method for producing the high-carbon cold-rolled steel sheet of the present invention will be described below. In the present invention, unless otherwise specified, the finishing temperature, coiling temperature, and other temperatures may be surface temperatures of hot-rolled steel sheet or the like, and measured by a radiation thermometer or the like. The average cooling rate is (cooling start temperature-cooling stop temperature)/(cooling time from cooling start temperature to cooling stop temperature), unless otherwise specified.

The steel having the composition loaded in the above 1) is cast by a known method such as converter or electric furnace by a known method such as continuous casting to form a cast slab, and then hot rolled including rough rolling and finish rolling is performed directly or after reheating by cooling. First, cast pieces (steel slabs) are roughly rolled into slabs. The conditions for rough rolling need not be particularly specified, and can be performed by a usual method.

5-1) finish rolling finishing temperature: ar (Ar)₃Above the phase transition point

After the rough rolling is finished, Ar is carried out₃Finishing finish rolling of finish rolling is finished in a temperature region above the transformation point. Finish rolling finishing temperature is less than Ar₃At the transformation point, coarse ferrite grains are formed after hot rolling and after annealing (primary box annealing, secondary box annealing), and the fine blanking workability is significantly reduced. Therefore, the finish rolling finishing temperature is Ar₃Above the transformation point. The upper limit of the finish rolling finish temperature is not particularly limited, but is preferably set to 1000 ℃ or lower in order to smoothly perform cooling after finish rolling. Also, in the present invention, Ar₃The phase change point can be determined from a phase change meter. Specifically, when a cylindrical test piece of 3 mm. phi. was temporarily heated to 900 ℃ from room temperature and cooled, Ar₃The transformation point is a temperature corresponding to the initial transformation point of the thermal expansion curve during cooling.

5-2) temperature region from finish rolling finish temperature to 660 ℃: the average cooling speed is 30-70 ℃/s

The pearlite formation pattern after hot rolling differs depending on the average cooling rate in the temperature range from the finish rolling temperature to 660 ℃. The average cooling rate in the temperature range is 30 ℃/s or more because the pearlite having large lamellar intervals is obtained when the average cooling rate in the temperature range is small, and a predetermined cementite cannot be obtained after the primary box annealing, the cold rolling, and the secondary box annealing. On the other hand, if the average cooling rate is too high, bainitic ferrite is obtained, and the hot-rolled steel sheet itself becomes hard. Even after the subsequent steps, the steel sheet becomes hard and the desired hardness cannot be obtained, and therefore the average cooling rate in the temperature range is 70 ℃/s or less. The average cooling rate in the temperature range is preferably 65 ℃/s or less, and more preferably 60 ℃/s or less.

5-3) coiling temperature: 500-660 DEG C

The hot rolled steel sheet after finish rolling is wound into a coil shape. If the coiling temperature is too high, the strength of the hot-rolled steel sheet becomes too low, and when the hot-rolled steel sheet is coiled into a coil shape, the coil may be deformed by its own weight, which is not preferable from the viewpoint of handling. Therefore, the upper limit of the winding temperature is set to 660 ℃. On the other hand, if the coiling temperature is too low, the hot-rolled steel sheet is not preferably hardened. Therefore, the lower limit of the winding temperature is set to 500 ℃. The coiling temperature is preferably 550 ℃ or higher.

5-4) primary box annealing temperature: annealing temperature in the temperature range of 650-720 DEG C

In order to achieve a desired thickness, cold rolling is required, the load on the rolling mill is reduced, the cold rolling property is improved, and the steel which is the final product has a desired hardness, so that primary annealing is required. When the annealing temperature is less than 650 ℃, the cold rolling property is poor, and the acceleration of spheroidizing of cementite is reduced, so that the steel of the final product is hardened, and therefore, the annealing temperature of the primary box annealing is 650 ℃ or more. The annealing temperature in the first box annealing is preferably 660 ℃ or higher, and more preferably 670 ℃ or higher. On the other hand, since spheroidizing excessively proceeds and cementite coarsens when the annealing temperature of the primary box annealing exceeds 720 ℃, the annealing temperature of the primary box annealing is 720 ℃ or lower. In addition, the holding time at the annealing temperature is preferably 20 hours or more from the viewpoint of spheroidizing the cementite. From the viewpoint of workability, the holding time of the annealing temperature is preferably 40h or less.

5-5) reduction of cold rolling: 20 to 50 percent

In order to achieve a desired sheet thickness and a predetermined ferrite grain size, cold rolling is required. When the reduction ratio of cold rolling is less than 20%, the thickness of the hot-rolled steel sheet must be reduced to achieve a desired thickness, and it is difficult to control the reduction ratio. Further, recrystallization is difficult, and it is difficult to obtain a desired hardness without recrystallization. Therefore, the reduction ratio in cold rolling needs to be 20% or more. On the other hand, if the reduction ratio of the cold rolling exceeds 50%, the thickness of the hot-rolled steel sheet needs to be increased, and it is difficult to obtain a uniform structure in the total thickness direction at the above-mentioned average cooling rate. Further, since the grain size is small and smaller than the ferrite grain size determined after recrystallization, the reduction ratio in cold rolling needs to be 50% or less.

5-6) secondary box annealing temperature: annealing temperature in the temperature range of 650-720 DEG C

After the cold rolling, secondary annealing is required to obtain a desired hardness. When the secondary box annealing temperature is less than 650 ℃, recrystallization is not easily performed and desired hardness cannot be obtained, so the secondary box annealing temperature is 650 ℃ or higher. The secondary box annealing temperature is preferably 660 ℃ or higher, and more preferably 670 ℃ or higher. On the other hand, the secondary box annealing temperature is 720 ℃ or lower because the predetermined average grain size of cementite cannot be obtained when the secondary box annealing temperature exceeds 720 ℃. From the viewpoint of obtaining a desired hardness, the holding time of the annealing temperature is preferably 20 hours or more. From the viewpoint of workability, the holding time of the annealing temperature is preferably 40h or less.

The high-carbon cold-rolled steel sheet of the present invention is subjected to temper rolling as necessary after the secondary box annealing, and is subjected to a treatment such as degreasing according to a usual method, and in this case, can be subjected to fine blanking and the like. The fine blanking is preferably performed according to a usual method and is usually performed under conditions such as a suitable selection of a clearance between a die and a punch in order to obtain a good end face. After the completion of the working, heat treatment such as quenching, tempering, and austenitizing can be performed according to a conventional method, whereby desired hardness and fatigue strength can be obtained.

The high-carbon cold-rolled steel sheet of the present invention is not particularly limited, and the thickness is preferably 3.0mm or less, and more preferably 2.5mm or less. The thickness is preferably 0.8mm or more, more preferably 1.2mm or more, though not particularly limited.