Cold-rolled steel sheet and its manufacturing method

Abstract

This cold-rolled steel sheet contains a high carbon content of 0.80 to 1.25 mass%, and has both excellent machinability and punching workability. 2 The average grain size of cementite grains having an area of ​​0.65 μm or less, and the A value is 2.0 mass% or more, and the average grain size of cementite grains having an area of ​​0.01 μm or less 2 The density of voids with an area of ​​50,000 / mm 2 and a cold-rolled steel sheet having a Vickers hardness of 200HV or more and 400HV or less.

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled steel sheet, and more particularly to a cold-rolled steel sheet having excellent machinability and punching workability, and a method for producing the same.

[0002] Cold-rolled steel sheets are widely used as a material for manufacturing various steel parts. Among them, cold-rolled steel sheets made from high-carbon steel have high hardness due to quenching after forming into steel parts, and are therefore used in applications requiring wear resistance, such as textile machinery parts, bearing parts, and machine and household cutlery.

[0003] On the other hand, steel parts such as textile machinery parts, bearing parts, and machine and household cutlery are cut using metal saws, milling cutters, and end mills during the forming process before quenching. If the raw material, cold-rolled steel sheet, has poor machinability, cutting tools will wear out quickly, requiring more frequent replacement, which is disadvantageous in terms of productivity and cost. For this reason, cold-rolled steel sheet is required to have excellent machinability.

[0004] Furthermore, for steel parts such as knitting needles used in textile machinery, the outer shape is punched out prior to cutting, so cold-rolled steel sheets are required to have excellent punching workability, i.e., to be less likely to produce burrs or sagging on the punched end surface.

[0005] Therefore, various techniques have been proposed to improve machinability and punching workability.

[0006] For example, Patent Document 1 proposes a technique for improving the machinability of steel containing 0.70 to 1.10 mass % of C by controlling the number and average grain size of carbides in a spheroidized structure.

[0007] Furthermore, Patent Document 2 proposes a technology for improving the machinability of steel by controlling the average C content in a region from the outer periphery to a depth of 200 μm within a specific range in a steel wire material containing 0.75 to 1.2 mass% C.

[0008] Patent Documents 3 and 4 propose a technique for improving punching workability in a steel sheet containing 0.30 to 1.30 mass% C by controlling the ratio of the number of carbides on the ferrite grain boundaries to the number of carbides inside the ferrite grains within a specific range.

[0009] In Patent Document 5, in a steel sheet containing C: 0.70 to 0.95 mass%, 2 A technology has been proposed to improve punching workability by introducing more than 100 voids per square inch into the steel structure. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-307320 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-049388 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-303415 [Patent Document 4] Japanese Patent Application Laid-Open No. 2009-215612 [Patent Document 5] Japanese Patent Application Laid-Open No. 2011-012316 Summary of the Invention [Problem to be solved by the invention]

[0011] However, these conventional techniques have the following problems.

[0012] For example, the technology proposed in Patent Document 1 improves the machinability of steel by limiting the number of carbides. However, because the steel is soft, significant sagging occurs during punching.

[0013] Furthermore, the technology proposed in Patent Document 2 improves machinability by reducing the amount of C in the surface 200 μm layer through decarburization. However, cold-rolled steel sheets with a thickness of approximately 0.4 mm are used for applications such as knitting needles, and the technology of Patent Document 2 cannot be applied to such thin cold-rolled steel sheets. This is because the technology of Patent Document 2 reduces the amount of C across almost the entire thickness, resulting in insufficient strength. Furthermore, the steel of Patent Document 2 is also soft, so significant sagging occurs during punching.

[0014] In the techniques proposed in Patent Documents 3 and 4, punching workability is improved by increasing the number of carbides within ferrite grains compared to the number of carbides at the ferrite grain boundaries. However, even with steel sheets obtained by this method, soft steel sheets suffer from significant sagging during punching. On the other hand, hard steel sheets have insufficient machinability.

[0015] Furthermore, the technology proposed in Patent Document 5 can reduce burrs that occur during punching by introducing voids into the steel structure. However, because the steel sheet is soft, significant sagging occurs during punching. Also, the machinability is insufficient.

[0016] As described above, it has not yet been possible to achieve both high levels of machinability and punching workability.

[0017] The present invention has been made in view of the above circumstances, and aims to achieve both excellent machinability and punching workability in a cold-rolled steel sheet containing a high carbon content of 0.80 to 1.25 mass %. [Means for solving the problem]

[0018] The present inventors have conducted research into methods for solving the above problems and have come to the following findings.

[0019] (1) By appropriately controlling the grain size of cementite, the Mn content and Cr content in the cementite, and the number density of voids, the machinability of the cold-rolled steel sheet can be improved.

[0020] (2) By appropriately controlling the Vickers hardness, the punching workability of the cold-rolled steel sheet can be improved.

[0021] (3) By appropriately controlling the chemical composition of the steel slab used and the manufacturing conditions of the cold-rolled steel sheet, it is possible to appropriately control the grain size of cementite in the cold-rolled steel sheet, the Mn content, the Cr content in the cementite, the void number density, and the Vickers hardness.

[0022] The present invention has been completed based on the above findings, and the gist of the present invention is as follows.

[0023] 1. By mass%, C: 0.80~1.25%, Si: 0.10 to 1.0% Mn: 0.20 to 3.0% P: 0.001~0.05%, S: 0.03% or less, Al: 0.001 to 0.1%, N: 0.001 to 0.01%, O: 0.0100% or less, Cr: 0.56 to 2.0%, and At least one selected from the group consisting of Nb: 0.029 to 0.24%, Ti: 0.01 to 0.21%, and V: 0.01 to 0.21%, The balance consists of Fe and unavoidable impurities, and The total content of Nb, Ti, and V is 0.24% by mass or less. 0.06 μm 2 The average grain size of cementite grains having an area of ​​0.65 μm or less, and the A value defined by the following formula (1) is 2.0 mass% or more, 0.01μm 2The density of voids with an area of ​​50,000 / mm 2 That's all, Cold-rolled steel sheet with a Vickers hardness of 200HV or more and 400HV or less. A=(3 / 7)C Mn +C Cr …(1) Here, C in the above formula (1) Mn is the aforementioned 0.06 μm 2 The Mn content (mass%) in cementite particles having an area of ​​at least Cr is the aforementioned 0.06 μm 2 The Cr content (mass%) in cementite particles having an area of ​​0.1 or more is shown.

[0024] 2. The component composition is, in mass%, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Mo: 1.00% or less Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, 2. The cold-rolled steel sheet according to 1 above, further comprising at least one selected from the group consisting of:

[0025] 3. A steel slab having the chemical composition described in 1 or 2 above is heated at a slab heating temperature of 1100°C or higher for a slab heating time of 60 minutes or longer. The heated steel slab is hot-rolled under the condition of a finish rolling end temperature exceeding Tc defined by the following formula (2) to obtain a hot-rolled steel sheet; The hot-rolled steel sheet is cooled under the conditions of an average cooling rate of 20°C / s or more and a cooling stop temperature of Tc or less, The cooled hot-rolled steel sheet is coiled at a coiling temperature of 530°C or higher and Tc or lower, The hot-rolled steel sheet after coiling is subjected to first annealing at least once under the conditions of an annealing temperature of 600°C or higher and Tc or lower, and an annealing time of 3 hours or longer, A method for producing a cold-rolled steel sheet, comprising: subjecting the hot-rolled steel sheet after the first annealing to cold rolling at a rolling reduction of 15% or more and second annealing at an annealing temperature of 600°C or more and Tc or less for an annealing time of 3 hours or more, at least once; and then further subjecting the hot-rolled steel sheet to final cold rolling at a rolling reduction of 20% or more and 80% or less. Tc(℃)=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr+6.38×W…(2) Here, the element symbols in the above formula (2) represent the content (mass %) of each element, and are set to zero when the element is not contained. [Effects of the Invention]

[0026] According to the present invention, a cold-rolled steel sheet containing a high carbon content of 0.80 to 1.25 mass% can achieve both excellent machinability and punching workability, and therefore the cold-rolled steel sheet of the present invention can be suitably used as a material for various steel parts, including textile machine parts, bearing parts, and machine and household cutlery. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a schematic diagram showing the cutting test method. [Figure 2] FIG. 2 is a cross-sectional view showing the state of the test piece cut in the cutting test. [Figure 3] FIG. 3 is a schematic diagram of a graph in which the cutting resistance F(N) measured in the cutting test is approximated by a cubic spline curve. [Figure 4] FIG. 4 is a schematic cross-sectional view showing the state of the end face of the disk sample 10 after punching. DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention will be described in detail below, but the present invention is not limited to the embodiments.

[0029] [Component composition] The cold-rolled steel sheet of the present invention has the above-mentioned chemical composition. The reasons for the limitations will be explained below. In the following explanation, "%" as a unit of content refers to "mass%" unless otherwise specified.

[0030] C: 0.80 to 1.25% Carbon (C) improves hardness after quenching and is therefore an essential element for various steel parts, including textile machine parts, bearing parts, and machine and household cutlery. Carbon is also an element necessary for cementite formation. In the present invention, the void density is controlled as described below. However, if the C content is less than 0.80%, the required void density cannot be achieved. This is because cementite serves as the starting point for void formation. Therefore, the C content is set to 0.80% or more, preferably 0.85% or more, and more preferably 0.90% or more. On the other hand, if the C content exceeds 1.25%, the surface scale becomes strong during slab heating, resulting in deterioration of surface properties. Furthermore, if the C content exceeds 1.25%, the toughness of the steel slab decreases, which can lead to cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the C content is set to 1.25% or less, preferably 1.20% or less, and more preferably 1.10% or less.

[0031] Si: 0.10 to 1.0% Si is an element that has the effect of increasing the strength of cold-rolled steel sheets through solid solution strengthening. To achieve this effect, the Si content is set to 0.10% or more, preferably 0.15% or more, and more preferably 0.20% or more. On the other hand, excessive Si strengthens the surface scale during heating, resulting in deterioration of surface properties. Furthermore, if Si exceeds 1.0%, the toughness of the steel slab decreases, which can cause cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the Si content is set to 1.0% or less, preferably 0.80% or less, and more preferably 0.55% or less.

[0032] Mn: 0.20 to 3.0% Mn is an element that dissolves in cementite, improving the hardness of the cementite and increasing the void density. Furthermore, the growth of cementite during annealing is controlled by the diffusion rate of Mn, thereby suppressing coarsening of the cementite. However, if the Mn content is less than 0.20%, the required void density cannot be obtained. Therefore, the Mn content is set to 0.20% or more, preferably 0.30% or more, more preferably 0.40% or more, and even more preferably 0.50% or more. On the other hand, if the Mn content exceeds 3.0%, the toughness of the steel slab decreases, which can cause cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the Mn content is set to 3.0% or less, preferably 2.50% or less, more preferably 2.0% or less, and even more preferably 1.5% or less.

[0033] P: 0.001 to 0.05% Adding a small amount of P improves the strength of cold-rolled steel sheets through solid solution strengthening. To achieve this effect, the P content is set to 0.001% or more, preferably 0.003% or more. On the other hand, if the P content exceeds 0.05%, the toughness of the steel slab decreases due to grain boundary embrittlement, which can cause cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the P content is set to 0.05% or less, preferably 0.04% or less.

[0034] S: 0.03% or less S is an element that causes grain boundary embrittlement during slab casting, and excessive S can cause cracks during casting of steel slabs, significantly reducing productivity. Therefore, the S content is set to 0.03% or less, preferably 0.02% or less. On the other hand, the lower the S content, the better, so there is no particular limit to the S content. Therefore, the lower limit of the S content may be 0%. However, excessive reduction of the S content leads to an increase in manufacturing costs. Therefore, from the viewpoint of manufacturing costs, the S content is preferably set to 0.0001% or more, and more preferably 0.0005% or more.

[0035] Al: 0.001 to 0.1% Al is an element necessary for deoxidation during steelmaking. Therefore, the Al content is set to 0.001% or more. On the other hand, excessive Al forms nitrides and oxides, which reduces the toughness of the steel slab. This can cause cracks during the slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the Al content is set to 0.1% or less, preferably 0.08% or less.

[0036] N: 0.001 to 0.01% N is an element that refines grain size by forming fine nitrides, thereby improving the strength of cold-rolled steel sheets. Therefore, the N content is set to 0.001% or more. On the other hand, excessive N combines with Al to form nitrides, which reduces the toughness of the steel slab. This can cause cracks during the slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the N content is set to 0.01% or less, preferably 0.008% or less.

[0037] O: 0.0100% or less O exists as an oxide and is an element that embrittles slabs. Excessive O reduces the toughness of the slab, which can lead to cracks during casting and cooling or heating of the slab, significantly reducing productivity. Therefore, the O content is set to 0.0100% or less, preferably 0.0050% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0%. However, excessive reduction of the O content leads to an increase in manufacturing costs. Therefore, from the viewpoint of manufacturing costs, it is preferable that the O content be 0.0001% or more.

[0038] Cr: 0.56~2.0% Cr is an element that dissolves in cementite, improving the hardness of the cementite and generating more voids. Furthermore, the growth of cementite during annealing is controlled by the diffusion rate of Cr, suppressing coarsening of the cementite. To achieve this effect, the Cr content is set to 0.56% or more, preferably 0.60% or more, and more preferably 0.70% or more. On the other hand, if the Cr content exceeds 2.0%, the toughness of the steel slab decreases, which can cause cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the Cr content is set to 2.0% or less, preferably 1.5% or less, and more preferably 1.3% or less.

[0039] The above-mentioned component composition contains at least one selected from the group consisting of Nb, Ti, and V. In order to obtain a desired void density, it is necessary to contain at least one of Nb, Ti, and V in the following amount.

[0040] Nb: 0.029 to 0.24% Nb is an element that forms carbides and can increase the number of voids in cold-rolled steel sheets. When Nb is contained, the Nb content is set to 0.029% or more to achieve this effect. On the other hand, if Nb is contained in excess, the toughness of the steel slab decreases, which can cause cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the Nb content is set to 0.24% or less, preferably 0.20% or less, and more preferably 0.10% or less.

[0041] Ti: 0.01 to 0.21% Ti is an element that forms carbides and can increase the number of voids in cold-rolled steel sheets. When Ti is contained, the Ti content is set to 0.01% or more to achieve this effect. On the other hand, if Ti is contained in excess, the toughness of the steel slab decreases, which can cause cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the Ti content is set to 0.21% or less, preferably 0.15% or less, and more preferably 0.10% or less.

[0042] V: 0.01 to 0.21% V is an element that forms carbides and can generate more voids in cold-rolled steel sheets. When V is contained, the V content is set to 0.01% or more to achieve this effect. On the other hand, if V is contained in excess, the toughness of the steel slab decreases, which can cause cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the V content is set to 0.21% or less, preferably 0.15% or less, and more preferably 0.10% or less.

[0043] Nb+Ti+V: 0.24% or less If the total content of Nb, Ti, and V in the above-mentioned chemical composition exceeds 0.24%, the toughness of the steel slab decreases, which may cause cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Therefore, the total content of Nb, Ti, and V is set to 0.24% or less, preferably 0.22% or less, and more preferably 0.20% or less. On the other hand, the lower limit of the total content is not particularly limited, but 0.01% is the practical lower limit. The lower limit corresponds to the case where only either Ti or V is contained in an amount of 0.01%. The total content may be 0.020% or more, 0.025% or more, or 0.030% or more.

[0044] The cold-rolled steel sheet according to one embodiment of the present invention has a composition comprising the above-mentioned components with the balance being Fe and unavoidable impurities.

[0045] Furthermore, the chemical composition of the cold-rolled steel sheet in another embodiment of the present invention may optionally further contain at least one of the following elements.

[0046] Ta: 0.10% or less Ta has the effect of further improving the wear resistance of steel parts by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, if the Ta content exceeds 0.10%, a large amount of coarse precipitates and inclusions are formed, reducing the toughness of the steel slab. Therefore, the Ta content is set to 0.10% or less, preferably 0.08% or less. On the other hand, there is no particular lower limit for the Ta content. However, from the viewpoint of enhancing the effect of adding Ta, it is preferable that the Ta content be 0.01% or more.

[0047] W: 0.10% or less W has the effect of further improving the wear resistance of steel parts by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, if the W content exceeds 0.10%, a large amount of coarse precipitates and inclusions are formed, reducing the toughness of the steel slab. Therefore, the W content is set to 0.10% or less, preferably 0.08% or less. On the other hand, there is no particular lower limit for the W content. However, from the viewpoint of enhancing the effect of adding W, it is preferable that the W content be 0.01% or more.

[0048] B: 0.0100% or less B segregates at austenite grain boundaries during hot rolling or annealing, and has the effect of further improving hardenability. However, if the B content exceeds 0.0100%, the toughness of the steel slab decreases. Therefore, the B content is set to 0.0100% or less, more preferably 0.0080% or less. On the other hand, there is no particular lower limit for the B content. However, from the viewpoint of enhancing the effect of adding B, the B content is preferably set to 0.0003% or more.

[0049] Mo: 1.00% or less Mo is an element that has the effect of further improving hardenability. However, if the Mo content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Mo content is set to 1.00% or less, preferably 0.80% or less. On the other hand, there is no particular lower limit for the Mo content. However, from the viewpoint of enhancing the effect of adding Mo, it is preferable that the Mo content be 0.01% or more.

[0050] Co: 1.00% or less Co is an element that has the effect of further improving hardenability. However, if the Co content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Co content is set to 1.00% or less, preferably 0.80% or less. On the other hand, there is no particular lower limit for the Co content. However, from the viewpoint of enhancing the effect of adding Co, it is preferable that the Co content be 0.001% or more.

[0051] Ni: 1.00% or less Ni is an element that has the effect of further improving hardenability. However, if the Ni content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Ni content is set to 1.00% or less, preferably 0.80% or less. On the other hand, there is no particular lower limit for the Ni content. However, from the viewpoint of enhancing the effect of adding Ni, it is preferable that the Ni content be 0.01% or more.

[0052] Cu:1.00% or less Cu is an element that has the effect of further improving hardenability. However, if the Cu content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Cu content is set to 1.00% or less, preferably 0.80% or less. On the other hand, there is no particular lower limit for the Cu content. However, from the viewpoint of enhancing the effect of adding Cu, it is preferable that the Cu content be 0.01% or more.

[0053] Sn: 0.200% or less Sn is an element that has the effect of further improving hardenability. However, if the Sn content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Sn content is set to 0.200% or less, preferably 0.100% or less. On the other hand, there is no particular lower limit for the Sn content. However, from the viewpoint of enhancing the effect of adding Sn, it is preferable that the Sn content be 0.001% or more.

[0054] Sb: 0.200% or less Sb is an element that has the effect of suppressing decarburization. Suppressing decarburization can further improve the strength of steel sheet. However, if the Sb content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Sb content is set to 0.200% or less, preferably 0.100% or less. On the other hand, there is no particular lower limit for the Sb content. However, from the viewpoint of enhancing the effect of adding Sb, it is preferable that the Sb content be 0.001% or more.

[0055] Ca:0.0100% or less Ca is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the Ca content exceeds 0.0100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the Ca content is set to 0.0100% or less, preferably 0.0050% or less. On the other hand, there is no particular lower limit for the Ca content. However, from the viewpoint of enhancing the effect of adding Ca, it is preferable that the Ca content be 0.0005% or more.

[0056] Mg: 0.0100% or less Mg is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the Mg content exceeds 0.0100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0050% or less. On the other hand, there is no particular lower limit for the Mg content. However, from the viewpoint of enhancing the effect of adding Mg, it is preferable that the Mg content be 0.0005% or more.

[0057] REM: 0.0100% or less REM (rare earth metal) is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the REM content exceeds 0.0100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the REM content is set to 0.0100% or less, preferably 0.0050% or less. On the other hand, there is no particular lower limit for the REM content. However, from the viewpoint of enhancing the effect of adding REM, it is preferable that the REM content be 0.0005% or more.

[0058] Zr: 0.100% or less Zr is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the Zr content exceeds 0.100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the Zr content is set to 0.100% or less, preferably 0.080% or less. On the other hand, there is no particular lower limit for the Zr content. However, from the viewpoint of enhancing the effect of adding Zr, it is preferable that the Zr content be 0.001% or more.

[0059] Te: 0.100% or less Te is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the Te content exceeds 0.100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the Te content is set to 0.100% or less, preferably 0.080% or less. On the other hand, there is no particular lower limit for the Te content. However, from the viewpoint of enhancing the effect of adding Te, it is preferable that the Te content be 0.001% or more.

[0060] Hf: 0.10% or less Hf is an element that has the effect of spheroidizing the shape of nitrides and sulfides and improving the ultimate deformability of steel sheets. However, if the Hf content exceeds 0.10%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Hf content is set to 0.10% or less, preferably 0.08% or less. On the other hand, there is no particular lower limit for the Hf content. However, from the viewpoint of enhancing the effect of adding Hf, it is preferable that the Hf content be 0.01% or more.

[0061] Bi:0.200% or less Bi is an element that reduces segregation. However, if the Bi content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Bi content is set to 0.200% or less, preferably 0.100% or less. On the other hand, there is no particular lower limit for the Bi content. However, from the viewpoint of enhancing the effect of adding Bi, it is preferable that the Bi content be 0.001% or more.

[0062] The elements Ta to Bi have the effect of further improving the properties of the cold-rolled steel sheet of the present invention and can be added optionally to the steel sheet of the present invention. The above description indicates the preferred lower limits of the contents of these optionally added elements. However, if the contents of these elements are lower than the preferred lower limits, the effects of adding these elements will be reduced, but the properties of the cold-rolled steel sheet will still meet the required level. Therefore, the addition of these elements is not essential, and the lower limit of the content may be 0%.

[0063] [Cementite] Next, cementite contained in the cold-rolled steel sheet of the present invention will be described.

[0064] Average grain size of cementite: 0.65 μm or less In the present invention, the machinability and punching workability can be improved by controlling the number density of voids. In order to increase the number density of voids, it is effective to finely disperse cementite. 2 If the average grain size of cementite grains having an area of ​​0.06 μm or more is larger than 0.65 μm, the number density of voids cannot be set within the desired range. 2 The average particle size of cementite particles having an area of ​​0.65 μm or less, preferably 0.63 μm or less, more preferably 0.60 μm or less, even more preferably 0.55 μm or less, and most preferably 0.40 μm or less. 2 The "average particle size of cementite particles having an area of ​​0.30 μm or more" may be simply referred to as the "average particle size of cementite." On the other hand, the lower limit of the average particle size of cementite is not particularly limited, but from the viewpoint of manufacturability, the average particle size of cementite may be 0.30 μm or more, or 0.35 μm or more.

[0065] The average grain size of cementite is defined as a value measured by electron backscatter diffraction (EBSD) at a cross section in the rolling direction at a position halfway through the thickness of the cold-rolled steel sheet. More specifically, it can be measured by the method described in the examples.

[0066] A value: 2.0% by mass or more When Mn and Cr are dissolved in cementite, the hardness of the cementite increases, and the harder the cementite, the more likely voids are to be generated in the cold-rolled steel sheet. Therefore, in the present invention, the amounts of Mn and Cr present in the cementite are controlled to increase the void density. Specifically, the A value defined by the following formula (1) is set to 2.0 mass% or more. If the A value is less than 2.0 mass%, the void generation ability of the cementite decreases, and as a result, it becomes difficult to obtain the desired void density. The A value is preferably 3.0 mass% or more, more preferably 4.0 mass% or more, and even more preferably 5.0 mass% or more.

[0067] A=(3 / 7)C Mn +C Cr …(1) Here, C in the above formula (1) Mn is the aforementioned 0.06 μm 2 The Mn content (mass%) in cementite particles having an area of ​​at least Cr is the aforementioned 0.06 μm 2 The Cr content (mass %) in cementite particles having an area of ​​20% or more is 0.015%. On the other hand, although there are no particular limitations on the upper limit of the A value, in order to increase the A value, it is necessary to increase the amounts of alloying elements Mn and Cr added, and if the A value exceeds 20% by mass, manufacturability will be adversely affected. Therefore, from the viewpoint of manufacturability, the A value is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 12% by mass or less.

[0068] In addition, the above C Mn and C Cris defined as a value measured by an electron probe microanalyzer (EPMA) at a cross section in the rolling direction at a position halfway through the thickness of a cold-rolled steel sheet. More specifically, it can be measured by the method described in the Examples.

[0069] [Void] Next, the voids contained in the cold-rolled steel sheet of the present invention will be described.

[0070] Void density: 50,000 / mm 2 End 0.01μm 2 The number density of voids having an area of ​​the above is very important for improving the machinability and punching workability of the cold-rolled steel sheet, for the following reasons.

[0071] When cold-rolled steel sheets are cut, they first undergo plastic deformation, generating voids in areas of high plastic strain. These voids then grow and connect, allowing the cold-rolled steel sheets to be cut. During this process, the cold-rolled steel sheets exhibit high cutting resistance, particularly during the time between the time the material undergoes plastic deformation and the time voids are generated. Therefore, dispersing voids in the cold-rolled steel sheets in advance can reduce cutting resistance and improve the machinability of the cold-rolled steel sheets.

[0072] Furthermore, when cold-rolled steel sheets are punched, similar to cutting, the material undergoes plastic deformation, generating voids in areas of high plastic strain. These voids then grow and connect, resulting in fracture, completing the punching process. During this process, sagging occurs during the punching process while the material is plastically deformed and voids are generated. Therefore, by dispersing voids in the cold-rolled steel sheet in advance, the amount of plastic deformation until fracture during punching can be reduced, thereby reducing sagging. This improves punching workability.

[0073] Therefore, in order to obtain excellent machinability and punching workability, the thickness is 0.01 μm 2 The density of voids with an area of ​​50,000 / mm 2 More than 80,000 pieces / mm2 More preferably, 100,000 particles / mm 2 On the other hand, the upper limit of the number density is not particularly limited, but is preferably 1,000,000 pieces / mm 2 The following is the result.

[0074] The void density is defined as a value measured using a scanning electron microscope (SEM) at a cross section of the cold-rolled steel sheet in the rolling direction at a position halfway through the thickness of the sheet. More specifically, it can be measured by the method described in the examples.

[0075] [Vickers hardness] Next, the Vickers hardness of the cold-rolled steel sheet of the present invention will be described.

[0076] Vickers hardness: 200~400HV The Vickers hardness of a cold-rolled steel sheet has a significant effect on punching workability and machinability. If the Vickers hardness of the cold-rolled steel sheet is less than 200 HV, significant sagging occurs during punching, resulting in poor punching workability. To obtain the desired punching workability, the Vickers hardness of the cold-rolled steel sheet is set to 200 HV or more, preferably 240 HV or more, more preferably 280 HV or more, and even more preferably 300 HV or more. On the other hand, if the Vickers hardness of the cold-rolled steel sheet exceeds 400 HV, the machinability decreases. Therefore, the Vickers hardness of the cold-rolled steel sheet is set to 400 HV or less, preferably 360 HV or less, and even more preferably 340 HV or less.

[0077] The Vickers hardness is defined as a value at a cross section of the cold-rolled steel sheet in the rolling direction at a position halfway through the thickness of the sheet. More specifically, it can be measured by the method described in the examples.

[0078] Plate Thickness The thickness of the cold-rolled steel sheet is not particularly limited and can be any thickness, but is preferably 0.1 mm or more, more preferably 0.2 mm or more. The upper limit of the thickness is also not particularly limited, but is preferably 2.5 mm or less, more preferably 1.6 mm or less, and even more preferably 0.8 mm or less. When the thickness is 0.2 mm or more and 0.8 mm or less, the steel sheet can be particularly suitably used as a material for textile machine parts such as knitting needles.

[0079] [Method of manufacturing cold-rolled steel sheets] Next, a method for producing a cold-rolled steel sheet according to an embodiment of the present invention will be described. In the description, the unit "°C" used for temperature indicates the surface temperature (the temperature on the surface of a steel slab, steel sheet, etc.).

[0080] The cold-rolled steel sheet can be produced by sequentially carrying out the following steps on a steel slab having the above-mentioned chemical composition. (1)Heating (2) Hot rolling (3) Cooling (4) Winding (5) First annealing (6) Cold rolling (7) Second annealing (8) Final cold rolling

[0081] (1)Heating First, a steel slab having the above-described composition is heated. The method for producing the steel slab is not particularly limited, and a steel slab produced by any method can be used. For example, molten steel having the above-described composition can be produced in a converter, and then the slab can be produced by a casting method such as continuous casting. Alternatively, a casting method other than continuous casting, such as an ingot-blooming rolling method, can also be used. Alternatively, the steel slab can be produced by an electric furnace method. In this case, scrap can be used as the raw material.

[0082] Slab heating temperature: 1100℃ or higher Heating prior to hot rolling homogenizes the components and dissolves carbides such as cementite and segregations present in the steel slab. However, if the heating temperature of the steel slab (slab heating temperature) during this heating is less than 1100°C, the cementite cannot be sufficiently dissolved, and as a result, the average grain size of the cementite in the final cold-rolled steel sheet cannot be within the desired range. Therefore, the slab heating temperature is set to 1100°C or higher, preferably 1150°C or higher. On the other hand, the upper limit of the slab heating temperature is not particularly limited, but it is preferably set to 1350°C or lower.

[0083] Slab heating time: 60 minutes or more If the heating time of the steel slab during the heating (slab heating time) is less than 60 minutes, the cementite cannot be sufficiently dissolved, and as a result, the average particle size of the cementite in the finally obtained cold-rolled steel sheet cannot be set within the desired range. Therefore, the slab heating time is set to 60 minutes or more, preferably 90 minutes or more. On the other hand, the upper limit of the slab heating time is not particularly limited, but is preferably set to 300 minutes or less, and more preferably 240 minutes or less.

[0084] The heating can be carried out by any method, but is preferably carried out using a heating furnace.

[0085] (2) Hot rolling The heated steel slab is then hot-rolled to form a hot-rolled steel sheet. In the hot-rolling, rough rolling and finish rolling can be performed according to a conventional method.

[0086] Finishing rolling temperature: above Tc If the finish rolling end temperature in the hot rolling is equal to or lower than Tc, coarse cementite is generated in the hot-rolled steel sheet, and this coarse cementite remains in the final cold-rolled steel sheet. As a result, the desired cementite grain size cannot be obtained, and the desired machinability and punchability cannot be obtained. Therefore, the finish rolling end temperature in the hot rolling is set to be higher than Tc. On the other hand, the upper limit of the finish rolling end temperature is not particularly limited, but may be 980°C or lower, or 950°C or lower.

[0087] Here, Tc (° C.) is a value that can be used as an index of the temperature at which transformation involving cementite occurs, and is calculated by the following formula (2). Tc(℃)=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr+6.38×W ···(2) Here, the element symbols in the above formula (2) indicate the content (mass%) of each element, and are set to zero when the element is not contained.

[0088] (3) Cooling Average cooling rate: 20℃ / s or more After the finish rolling is completed, cooling is started and stopped at the cooling stop temperature. If the average cooling rate during the cooling is less than 20°C / s, the average grain size of cementite cannot be set within the desired range. This is because a slow cooling rate generates coarse cementite, which remains in the final cold-rolled steel sheet. Therefore, the average cooling rate is set to 20°C / s or more, preferably 30°C / s or more, and more preferably 50°C / s or more. On the other hand, although there are no particular limitations on the upper limit of the average cooling rate, an excessively high cooling rate makes it difficult to control the cooling stop temperature. Therefore, it is preferable to set the average cooling rate to 500°C / s or less.

[0089] Cooling stop temperature: Tc or less If the cooling stop temperature in the cooling is higher than Tc, the average grain size of cementite cannot be set within the desired range. This is because a high cooling stop temperature generates coarse cementite, which remains in the finally obtained cold-rolled steel sheet. Therefore, the cooling stop temperature is set to Tc or lower, preferably Tc - 20°C or lower. On the other hand, there is no particular restriction on the lower limit of the cooling stop temperature. In one embodiment of the present invention, the cooling stop temperature may be set to 530°C or higher. Typically, it is preferable that the cooling stop temperature be set to a temperature equal to or higher than the coiling temperature in the next coiling.

[0090] (4) Winding Winding temperature: 530°C or higher and Tc or lower After the cooling is stopped, the cooled hot-rolled steel sheet is wound into a coil. If the coiling temperature exceeds Tc, coarse cementite is generated, and this coarse cementite remains in the finally obtained cold-rolled steel sheet. Therefore, the coiling temperature is set to Tc or lower, preferably Tc-20°C or lower, and more preferably Tc-40°C or lower. On the other hand, if the coiling temperature is lower than 530°C, volume expansion due to transformation during coiling will result in a poor coiled shape. Therefore, the coiling temperature is set to 530°C or higher, preferably 550°C or higher, and more preferably 600°C or higher.

[0091] (5) First annealing Annealing temperature: 600°C or higher, Tc or lower Annealing time: 3 hours or more The hot-rolled steel sheet after coiling is subjected to first annealing under the conditions of an annealing temperature of 600°C or higher and Tc or lower, and an annealing time of 3 hours or longer. The structure of the hot-rolled steel sheet after coiling is a pearlite structure in which cementite and ferrite formed in a plate shape are aligned. Because the pearlite structure is stable, it does not become homogenized unless it is held at a high temperature for a long time. Furthermore, in order to break down the pearlite structure and generate the desired cementite in the subsequent cold rolling and annealing processes, the annealing temperature must be 600°C or higher and the annealing time must be 3 hours or longer. The annealing temperature is preferably 620°C or higher. Furthermore, the annealing time is preferably 4 hours or longer. On the other hand, if the annealing temperature is higher than Tc, phase transformation will preferentially begin in one part, resulting in the formation of a locally coarse structure. As a result, the structure will become non-uniform, and the desired cementite grain size will not be obtained. Therefore, the annealing temperature is set to Tc or lower, preferably Tc-20°C or lower, and more preferably Tc-40°C or lower. On the other hand, although there is no particular upper limit to the annealing time, if it is too long, productivity decreases, so the annealing time is preferably 50 hours or less, and more preferably 30 hours or less.

[0092] By carrying out the first annealing under the above conditions, the pearlite structure can be broken down, and the desired cementite can be more easily formed in the subsequent cold rolling and annealing processes.

[0093] The first annealing can be performed one or more times, but from the viewpoint of enhancing the effect of the annealing, it is preferable to repeat it two or more times. On the other hand, there is no particular upper limit on the number of times the first annealing is performed, but the effect saturates even if it is performed more than three times. Therefore, it is preferable to perform the first annealing three or less times. When the first annealing is repeated two or more times, it is sufficient that each annealing is performed under the above-mentioned conditions. The annealing conditions for each time may be the same or different.

[0094] It is also preferable to subject the hot-rolled steel sheet to pickling prior to the first annealing.

[0095] (6) Cold rolling (7) Second annealing Plate-like cementite is formed in the steel sheet after hot rolling. This plate-like cementite is stable and therefore likely to remain for a long time. Because the plate-like cementite is coarse, if the cementite remains, the average particle size of the cementite in the final cold-rolled steel sheet cannot be set within the desired range. As a result, the desired machinability and punching workability cannot be obtained. Therefore, in order to refine and spheroidize the plate-like cementite by heating in annealing, the hot-rolled steel sheet after the first annealing is subjected to cold rolling and second annealing one or more times.

[0096] Rolling ratio: 15% or more The cold rolling deforms, fragments, and decomposes cementite, thereby obtaining a desired average cementite grain size. If the rolling reduction ratio in the cold rolling is less than 15%, the effect cannot be obtained. Therefore, the rolling reduction ratio is set to 15% or more, preferably 25% or more, more preferably 35% or more, and even more preferably 45% or more. On the other hand, the upper limit of the rolling reduction ratio is not particularly limited, but is preferably 85% or less, and more preferably 80% or less.

[0097] Annealing temperature: 600°C or higher, Tc or lower Annealing time: 3 hours or more The second annealing can refine and spheroidize the cementite deformed, fragmented, and decomposed by the cold rolling. At the same time, Mn and Cr can be concentrated in the cementite. To achieve these effects, the annealing temperature in the second annealing must be 600°C or higher and the annealing time must be 3 hours or longer. The annealing temperature is preferably 620°C or higher. The annealing time is preferably 4 hours or longer. On the other hand, if the annealing temperature is higher than Tc, phase transformation begins preferentially in a certain region, resulting in the formation of a locally coarse structure. As a result, the structure becomes nonuniform, and the desired cementite grain size cannot be obtained. Therefore, the annealing temperature is set to Tc or lower, preferably Tc - 20°C or lower, and more preferably Tc - 40°C or lower. On the other hand, although there is no particular upper limit for the annealing time, an excessively long annealing time reduces productivity. Therefore, the annealing time is preferably set to 30 hours or lower, and more preferably 20 hours or lower.

[0098] By carrying out the cold rolling and second annealing under the above conditions, it is possible to promote the refinement of cementite and the concentration of Mn and Cr in the cementite.

[0099] The cold rolling and second annealing can be performed one or more times, but from the viewpoint of enhancing the above-mentioned effects, it is preferable to repeat them two or more times. When the cold rolling and second annealing are repeated two or more times, the cold rolling and the second annealing can be alternately repeated. On the other hand, there is no particular upper limit on the number of times that the cold rolling and the second annealing are performed, but the effect saturates even if the cold rolling and the second annealing are repeated more than five times. Therefore, it is preferable to perform the cold rolling and the second annealing five or less times. When the cold rolling and the second annealing are repeated two or more times, it is preferable that the cold rolling and the second annealing are performed under the above-mentioned conditions for each time. The conditions for each time may be the same or different.

[0100] When the cold rolling and second annealing are carried out only once, the rolling reduction ratio of the cold rolling is preferably 70% or more, from the viewpoint of reliably refining cementite.

[0101] (8) Final cold rolling After the cold rolling and second annealing are performed as described above, final cold rolling is further performed. By performing final cold rolling, the void density can be adjusted to a desired range. In addition, the desired Vickers hardness can be obtained.

[0102] Rolling ratio: 20~80% However, if the rolling ratio in the final cold rolling is less than 20%, the above-mentioned effects cannot be obtained. Therefore, the rolling ratio in the final cold rolling is set to 20% or more, preferably 25% or more, and more preferably 30% or more. On the other hand, if the rolling ratio in the final cold rolling exceeds 80%, the Vickers hardness of the cold-rolled steel sheet may exceed 400 HV. If the Vickers hardness exceeds 400 HV, the desired machinability cannot be obtained. Therefore, the rolling ratio in the final cold rolling is set to 80% or less, preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less.

[0103] By satisfying the above conditions, a cold-rolled steel sheet having excellent machinability and punching workability can be produced. The finally obtained cold-rolled steel sheet may further be subjected to an optional surface treatment. [Example]

[0104] In order to confirm the effects of the present invention, cold-rolled steel sheets were produced according to the following procedure.

[0105] First, steel having the chemical composition shown in Table 1 was melted in a converter and formed into a steel slab by continuous casting. Next, the steel slab was successively subjected to heating, hot rolling, cooling, coiling, first annealing, cold rolling, second annealing, and final cold rolling to produce a cold-rolled steel sheet with a final thickness of approximately 0.4 mm. Each step was carried out under the conditions shown in Tables 2 and 3, and the first annealing, cold rolling, and second annealing were carried out the number of times shown in Tables 2 and 3.

[0106] Next, for each of the obtained cold-rolled steel sheets, the average cementite grain size, the amount of Mn and Cr dissolved in the cementite grains, the number density of voids, and Vickers hardness were measured by the following procedures. The measurement results are shown in Tables 4 and 5. Note that, for convenience, Table 4 shows values ​​in which the A values ​​are rounded to two decimal places, but whether or not the A values ​​satisfy the conditions of the present invention is determined based on the value calculated by formula (1).

[0107] (average grain size of cementite) First, a test piece for microstructure observation was taken from the obtained cold-rolled steel sheet. Next, the cross section (L cross section) of the test piece for microstructure observation was polished to a mirror surface. Thereafter, a final polishing was performed using a colloidal silica solution.

[0108] Next, the polished surface of the specimen for microstructure observation was measured using electron backscatter diffraction (EBSD) under the following conditions: electron beam acceleration voltage: 20 keV, measurement interval: 0.14 μm steps, measurement field: 60 μm × 45 μm, and measurement position: 1 / 2 the plate thickness position.

[0109] The above measurement was carried out at three locations, and the obtained data (EBSD pattern) was analyzed to identify cementite. For the analysis, analysis software OIM Analysis manufactured by TSL was used. Next, from among the identified cementite particles, 0.06 μm 2 Only cementite particles having an area equal to or greater than this were selected, and their average value was calculated.

[0110] The above process was carried out at each of the three measurement positions, and the average of the three obtained average values ​​was calculated as 0.06 μm 2 The average grain size of cementite having an area of ​​1000 or more was defined as the average grain size of cementite having an area of ​​1000 or more.

[0111] (Mn and Cr content in cementite) A test piece for microstructure observation was taken from the obtained cold-rolled steel sheet. The L-section of the test piece for microstructure observation was polished to a mirror surface. Thereafter, the polished surface of the test piece for microstructure observation was observed by SEM to identify cementite particles. Next, an electron probe microanalyzer (EPMA) was used to identify the cementite particles. 2 For one cementite particle with an area of ​​1.0×10, the electron beam acceleration voltage was 15 keV and the probe current was 1.0×10. -8 A, Measurement time: 1000 ms. The Mn and Cr amounts were measured at three locations, and the average value of the three locations was calculated. 0.06 μm 2 The Mn and Cr contents were similarly measured for 10 cementite particles having the above area, and the measured values ​​for the 10 particles were averaged to obtain the Mn and Cr contents in the cementite.

[0112] (Void density) A test piece for microstructure observation was taken from the obtained cold-rolled steel sheet. The L-section of the test piece for microstructure observation was polished to a mirror surface. Then, the polished surface of the test piece for microstructure observation was photographed at 10 points at 1 / 2 the sheet thickness using a scanning electron microscope (SEM) at an acceleration voltage of 15 keV and a magnification of 3000 times to obtain microstructure images.

[0113] From the obtained tissue image, 0.01 μm 2 Voids with an area of ​​less than 0.01 μm were excluded by image processing. 2 The number of voids with an area of ​​0.01 μm or more was counted and divided by the area of ​​the structure image to calculate the number density. Similar measurements were performed in 10 fields of view, and the average value of the obtained number density was calculated. 2 The number density of voids having an area of ​​1000 or more was determined.

[0114] (Vickers hardness) A test piece for Vickers hardness measurement was taken from the obtained cold-rolled steel sheet. The L-section of the test piece for Vickers hardness measurement was polished to a mirror finish. Thereafter, in accordance with JIS Z 2244:2009, measurements were made at the 1 / 2 position in the sheet thickness direction, a load of 1 kgf, and five measurement points. The values ​​measured at the five points were averaged to obtain the Vickers hardness of the cold-rolled steel sheet.

[0115] Furthermore, the machinability and punching workability of each of the obtained cold-rolled steel sheets were evaluated using the following procedure. The evaluation results are shown in Tables 4 and 5.

[0116] (cutting properties) The machinability of the cold-rolled steel sheets was evaluated based on the cutting life distance in a cutting test. Specific procedures are described below with reference to the drawings. Fig. 1 is a schematic diagram showing the cutting test method, and Fig. 2 is a cross-sectional schematic diagram showing the state of the cut test piece.

[0117] -Creating test specimens First, a test piece 1 for machinability evaluation was taken from the obtained cold-rolled steel sheet. The dimensions of the test piece 1 for machinability evaluation were 35 mm (width) × 150 mm (length), and the thickness was the same as that of the cold-rolled steel sheet. Next, as shown in FIG. 1, the test piece 1 for machinability evaluation was attached to a fixing jig 2 with an adhesive. The adhesive used was Aron Alpha EXTRA (registered trademark), a fast-acting multi-purpose instant adhesive manufactured by Aron Alpha Co., Ltd. The dimensions of the fixing jig 2 were 50 mm (width) × 150 mm (length) × 30 mm (depth). When attaching, the test piece was fixed in a vice for more than 8 hours to ensure flatness.

[0118] Cutting test Next, a cutting test was performed using the machinability evaluation test piece 1. Specifically, first, the machinability evaluation test piece 1 attached to the fixing jig 2 was placed on a dynamometer 3. Next, a metal saw was brought into contact with the surface of the machinability evaluation test piece 1 in the direction indicated by arrow A, and cutting was performed by down-cutting while moving the metal saw in the direction indicated by arrow B. The metal saw used was made of cemented carbide with a Vickers hardness of approximately 1680, a Young's modulus of approximately 570 GPa, and a specific gravity of approximately 14.2, and had 22 teeth, an outer diameter of 11 mm, an inner diameter of 4 mm, and a thickness of 0.13 mm.

[0119] After cutting the machinability evaluation test piece 1 over a total length of 150 mm from one end to the other end with the metal saw, the metal saw was returned to the one end, the cutting position was shifted downward by 2 mm (in the direction of arrow C), and cutting was performed again. By repeating the above procedure, cutting was performed at 2 mm intervals as shown in Figure 2, and cutting was stopped when the metal saw broke.

[0120] The cutting conditions were rotation speed: 3000 / min, feed rate: 24 mm / min, depth of cut: 0.2 mm, and no cutting oil was used. The measurement resolution of the dynamometer 3 was 1000 Hz. In Figure 2, 4 indicates the metal saw, and the arrow ND indicates the machinability evaluation test piece 1, i.e., the thickness direction of the cold-rolled steel sheet.

[0121] Next, the cutting life was calculated from the cutting resistance measured in the cutting test. Figure 3 is a schematic diagram of a graph in which the cutting resistance F (N) measured in the cutting test was approximated by a cubic spline function, with the horizontal axis representing the cutting distance (mm). The cutting resistance F was calculated using the following equation (3) from the cutting resistance in each direction measured by a dynamometer. F=√(F x 2 +F y 2 +F z 2 ) …(3) where F is the cutting force, F x is the cutting force in the x-axis direction, F y is the cutting force in the y-axis direction, F z is the cutting resistance in the z-axis direction.

[0122] The cutting life was calculated from the cutting distance at which burrs began to appear before the metal saw broke. The point at which burrs began to appear (indicated by the arrow in Figure 3) was taken as the cutting distance at which the slope of the approximation curve was greater than or equal to (maximum slope) x 1 / 3, and this was taken as the cutting life distance. If the cutting life distance was 1148 mm or more, the cutting ability was considered good, and if it was less than 1148 mm, the cutting ability was considered poor.

[0123] (Punching processability) The punching workability of the cold-rolled steel sheets was evaluated based on the sagging rate in a punching test. The specific procedure was as follows.

[0124] First, a test piece was taken from the obtained cold-rolled steel sheet, with dimensions of 30 mm (width) x 30 mm (length), and the thickness of the test piece was the same as that of the cold-rolled steel sheet.

[0125] Next, the center of the test piece was punched out using a cylindrical punch with a diameter of 10.0 mm to obtain a disk sample with a diameter of 10 mm. The punching clearance was set to 12.5%, where the clearance is the percentage (%) of the gap between the punch and the die relative to the thickness of the test piece.

[0126] The obtained disk sample was cut, and the sagging d RD and sagging d at the end face in the direction perpendicular to the rolling direction TD was measured using an optical microscope. Figure 4 is a cross-sectional schematic diagram showing the state of the end face of the disk sample 10. Here, 11 is a sag, 12 is a shear surface, 13 is a fracture surface, and d RD and d TD The length d in the thickness direction of the sag 11 was used as the sag ratio. From the obtained sag value and the thickness t (mm) of the disk sample 10, the sag ratio D (%) was calculated by the following formula (4). D=[{(d RD +d TD ) / 2} / t]×100(%)…(4)

[0127] If the calculated sagging rate was 10% or less, the punching workability was judged to be good, and if it was more than 10%, the punching workability was judged to be poor.

[0128] [Table 1]

[0129] [Table 2]

[0130] [Table 3]

[0131] [Table 4]

[0132] [Table 5] [Explanation of symbols]

[0133] 1. Test piece for machinability evaluation 2 Fixing jig 3 Dynamometer 10 Disc sample (cold-rolled steel sheet) 11 Sauce 12 Shear plane 13 Fracture surface d Dare t Plate thickness

Claims

1. In mass%, C: 0.80-1.25%, Si: 0.10-1.0%, Mn: 0.20-3.0%, P: 0.001-0.05%, S: 0.03% or less, Al: 0.001-0.1%, N: 0.001 to 0.01%, O: 0.0100% or less, Cr: 0.56 to 2.0%, and Nb: 0.029 to 0.24%, Ti: 0.01 to 0.21%, and V: 0.01 to 0.21%; The balance consists of Fe and unavoidable impurities, and The total content of Nb, Ti, and V is 0.24% by mass or less. 0.06 μm ２ The average particle size of cementite particles having an area of ​​0.65 μm or less is 0.65 μm or less, and the A value defined by the following formula (1) is 2.0 mass% or more, 0.01 μm ２ The number density of voids having an area of ​​50,000 / mm ２ That's all, A cold-rolled steel sheet having a Vickers hardness of 200 HV or more and 400 HV or less. A=(3 / 7)C Ｍｎ +C Ｃｒ …(1) Here, C in the above formula (1) Ｍｎ is the above 0.06 μm ２ The content (mass%) of Mn in cementite particles having an area of ​​at least Ｃｒ is the above 0.06 μm ２ The Cr content (mass%) in cementite particles having an area of ​​0.1 or more is shown.

2. The component composition is, in mass%, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, The cold-rolled steel sheet according to claim 1, further comprising at least one selected from the group consisting of:

3. The method for producing a cold-rolled steel sheet according to claim 1 or 2, A steel slab having the above-mentioned composition is heated under the conditions of a slab heating temperature of 1100°C or more and a slab heating time of 60 minutes or more, The heated steel slab is hot-rolled under the condition of a finish rolling end temperature exceeding Tc defined by the following formula (2) to obtain a hot-rolled steel sheet; The hot-rolled steel sheet is cooled under the conditions of an average cooling rate of 20°C / s or more and a cooling stop temperature of Tc or less, The cooled hot-rolled steel sheet is coiled at a coiling temperature of 530°C or higher and Tc or lower, The hot-rolled steel sheet after coiling is subjected to first annealing at least once under the conditions of an annealing temperature of 600°C or higher and Tc or lower and an annealing time of 3 hours or longer, The hot-rolled steel sheet after the first annealing is subjected to cold rolling at a rolling ratio of 15% or more and second annealing at an annealing temperature of 600°C or more and Tc or less for an annealing time of 3 hours or more, once or more, and then further subjected to final cold rolling at a rolling ratio of 20% or more and 80% or less. A method for producing a cold-rolled steel sheet. Tc (°C) = 723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr+6.38×W…(2) Here, the element symbols in the above formula (2) represent the content (mass %) of each element, and are set to zero when the element is not contained.

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