Cold-rolled steel sheet and method for manufacturing same

Abstract

A cold-rolled steel sheet containing a high carbon content of 0.80 mass% to 1.25 mass% that achieves both excellent machinability and blanking workability. The cold-rolled steel sheet has a defined chemical composition, and cementite particles having an area of 0.06 µm2 or more have an average particle size of 0.65 µm or less, an A value is 2.0 mass% or more, a number density of voids having an area of 0.01 µm or more is 50,000 / mm2 or more, and a Vickers hardness is 200 HV or more and 400 HV or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to cold-rolled steel sheets, and more particularly to a cold-rolled steel sheet having excellent machinability and blanking workability, and a method of producing the same.

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

[0003] On the other hand, steel components such as textile machinery components, bearing components, machine blades, and household blades are cut using metal saws, milling cutters, end mills, and the like during a forming process before quenching. When the cold-rolled steel sheet used as a material has poor machinability, cutting tools wear out quickly, which increases the frequency of cutting tool replacement, which is disadvantageous in terms of productivity and costs. Therefore, cold-rolled steel sheets are required to have excellent machinability.

[0004] Further, in the case of steel components such as knitting needles that are components in textile machinery, the outer shape is cut out by blanking prior to cutting work. Therefore, cold-rolled steel sheets are required to have excellent blanking workability, that is, to be less likely to produce burrs or rollover on blanked end surfaces.

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

[0006] For example, in Patent Literature (PTL) 1, a technology is proposed for improving the machinability of steel containing C: 0.70 mass% to 1.10 mass%, by controlling the number and average particle size of carbides in a spheroidized microstructure.

[0007] Further, in PTL 2, a technology is proposed for improving the machinability of steel by controlling an average value of C content to a specific range in a region from a outer periphery to a depth of 200 µm in a steel wire rod containing C: 0.75 mass% to 1.2 mass%.

[0008] In PTL 3 and PTL 4, technologies are proposed for improving blanking workability in steel sheets containing C: 0.30 mass% to 1.30 mass%, by controlling a ratio of a number of carbides on ferrite grain boundaries to a number of carbides inside ferrite grains within a specific range.

[0009] In PTL 5, a technology is proposed for improving blanking workability by introducing 100 or more voids per mm 2< into a steel microstructure of a steel sheet containing C: 0.70 mass% to 0.95 mass%.CITATION LISTPatent Literature

[0010] PTL 1: JP 2005-307320 A PTL 2: JP 2001-049388 A PTL 3: JP 2008-303415 A PTL 4: JP 2009-215612 A PTL 5: JP 2011-012316 A SUMMARY(Technical Problem)

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

[0012] For example, the technology proposed in PTL 1 improves the machinability of steel by limiting the number of carbides. However, the steel is soft, and therefore significant rollover occurs during blanking.

[0013] Further, the technology proposed in PTL 2 improves machinability by decreasing an amount of C in a 200 µm surface layer through decarburization. However, for applications such as knitting needles, thin cold-rolled steel sheets having a thickness of about 0.4 mm are used, and the technology of PTL 2 cannot be applied to such thin cold-rolled steel sheets. This is because the technology of PTL 2 decreases the amount of C over almost the entire thickness, resulting in insufficient strength. Further, the steel of PTL 2 is also soft, and therefore significant rollover occurs when blanking.

[0014] In the technologies proposed in PTL 3 and PTL 4, blanking workability is improved by increasing the number of carbides within ferrite grains compared to the number of carbides at ferrite grain boundaries. However, even when a steel sheet is obtained by this method, when the steel sheet is soft, significant rollover occurs when blanking. On the other hand, when the steel sheet is hard, machinability is insufficient.

[0015] Further, the technology proposed in PTL 5 makes it possible to decrease occurrence of burrs when blanking by introducing voids into the steel microstructure. However, the steel sheet is soft, and therefore significant rollover occurs when blanking. Further, machinability was insufficient.

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

[0017] The present disclosure is made in view of the above circumstances, and it would be helpful to achieve both excellent machinability and blanking workability in a cold-rolled steel sheet containing a high carbon content of 0.80 mass% to 1.25 mass%.(Solution to Problem)

[0018] As a result of studies, the inventors arrived at the following discoveries. (1) By appropriately controlling a particle size of cementite, Mn amount and Cr amount in the cementite, and a number density of voids, the machinability of the cold-rolled steel sheet can be improved. (2) By appropriately controlling Vickers hardness, the blanking workability of the cold-rolled steel sheet can be improved. (3) By appropriately controlling a chemical composition of a steel slab used and production conditions of the cold-rolled steel sheet, it is possible to appropriately control the particle size of cementite, the Mn amount and Cr amount in the cementite, the number density of voids, and the Vickers hardness.

[0019] The present disclosure is based on the discoveries described above, and primary features of the present disclosure are as described below. 1. A cold-rolled steel sheet comprising a chemical composition containing (consisting of), in mass%, C: 0.80 % to 1.25 %, Si: 0.10 % to 1.0 %, Mn: 0.20 % to 3.0 %, P: 0.001 % to 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 %, with the balance being Fe and inevitable impurity, and the total content of Nb, Ti, and V being 0.24 mass% or less, wherein an average particle size of cementite particles having an area of 0.06 µm 2< or more is 0.65 µm or less, and an A value defined by the following expression (1) is 2.0 mass% or more, a number density of voids having an area of 0.01 µm 2< or more is 50,000 voids / mm 2< or more, a Vickers hardness is 200 HV or more and 400 HV or less, A = 3 / 7 C Mn + C Cr where, in expression (1), C Mn is Mn content in mass% in the cementite particles having an area of 0.06 µm 2< or more, and C Cr is Cr content in mass% in the cementite particles having an area of 0.06 µm 2< or more. 2. The cold-rolled steel sheet according to 1, above, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of: 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. 3. A method of producing a cold-rolled steel sheet, the method comprising: heating a steel slab comprising the chemical composition according to 1 or 2, above, under conditions including a slab heating temperature of 1100 °C or higher and a slab heating time of 60 min or longer; hot rolling the heated steel slab under conditions including a rolling finish temperature exceeding Tc, as defined by the following expression (2), to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet under conditions including an average cooling rate of 20 °C / s or more and a cooling stop temperature of Tc or lower; coiling the cooled hot-rolled steel sheet at a coiling temperature of 530 °C or higher and Tc or lower; subjecting the hot-rolled steel sheet after coiling to first annealing once or more, under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer; and subjecting the hot-rolled steel sheet after the first annealing to cold rolling at a rolling ratio of 15 % or more and second annealing at an annealing temperature of 600 °C or higher and Tc or lower for an annealing time of 3 h or longer, once or more, and then final cold rolling at a rolling ratio of 20 % or more and 80 % or less, where the element symbols in expression (2) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0. (Advantageous Effect)

[0020] According to the present disclosure, it is possible to achieve both excellent machinability and blanking workability in a cold-rolled steel sheet containing a high carbon content of 0.80 mass% to 1.25 mass%. Therefore, the cold-rolled steel sheet is very well suited as a material for various steel components, including textile machinery components, bearing components, machine blades, and household blades.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the accompanying drawings: FIG. 1 is a schematic diagram illustrating a cutting test method; FIG. 2 is a schematic cross-section diagram illustrating a state of a test piece cut in a cutting test; FIG. 3 is a schematic diagram of a graph in which cutting resistance F (N) measured in a cutting test is approximated by a cubic spline curve; and FIG. 4 is a cross-section diagram illustrating a state of an end face of a disk sample 10 after blanking. DETAILED DESCRIPTION

[0022] A detailed description is provided below. The present disclosure is not limited to the following embodiments.[Chemical composition]

[0023] The cold-rolled steel sheet according to the present disclosure has the chemical composition described above. The reasons for the above limitations are described below. Hereinafter, "%" as a unit of content indicates "mass%" unless otherwise specified.C: 0.80 % to 1.25 %

[0024] Carbon is an essential element in material for various steel components, including textile machinery components, bearing components, machine blades, and household blades, in order to improve hardness after quenching. Further, C is an element necessary for producing cementite. Further, according to the present disclosure, the number density of voids is controlled as described below, but when C content is less than 0.80 %, the required number density of voids cannot be obtained. This is because cementite serves as the initiation point for void formation. The C content is therefore 0.80 % or more. The C content is preferably 0.85 % or more. The C content is more preferably 0.90 % or more. On the other hand, when the C content exceeds 1.25 %, surface scale becomes firm during slab heating, resulting in degradation of surface characteristics. Further, when the C content exceeds 1.25 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The C content is therefore 1.25 % or less. The C content is preferably 1.20 % or less. The C content is more preferably 1.10 % or less.Si: 0.10 % to 1.0 %

[0025] Si is an element having an effect of increasing strength of the cold-rolled steel sheet by solid solution strengthening. To obtain the above effect, Si content is 0.10 % or more. The Si content is preferably 0.15 % or more. The Si content is more preferably 0.20 % or more. On the other hand, an excess of Si degrades surface characteristics as a result of surface scale becoming firm during heating. Further, when the Si content exceeds 1.0 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Si content is therefore 1.0 % or less. The Si content is preferably 0.80 % or less. The Si content is more preferably 0.55 % or less.Mn: 0.20 % to 3.0 %

[0026] Mn is an element that dissolves in cementite, and improves the hardness of cementite and increases the number density of voids. Further, the growth of cementite during annealing becomes rate-determined by the diffusion rate of Mn, and coarsening of cementite can be suppressed. However, when Mn content is less than 0.20 %, the required number density of voids cannot be obtained. The Mn content is therefore 0.20 % or more. The Mn content is preferably 0.30 % or more. The Mn content is more preferably 0.40 % or more. The Mn content is even more preferably 0.50 % or more. On the other hand, when the Mn content exceeds 3.0 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Mn content is therefore 3.0 % or less. The Mn content is preferably 2.50 % or less. The Mn content is more preferably 2.0 % or less. The Mn content is even more preferably 1.5 % or less.P: 0.001 % to 0.05 %

[0027] The addition of a trace amount of P has a strength improving effect on the cold-rolled steel sheet due to solid solution strengthening. To achieve this effect, P content is 0.001 % or more. The P content is preferably 0.003 % or more. On the other hand, when the P content exceeds 0.05 %, the toughness of the steel slab decreases due to grain boundary embrittlement, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The P content is therefore 0.05 % or less. The P content is preferably 0.04 % or less.S: 0.03 % or less

[0028] S is an element that causes grain boundary embrittlement during slab casting, and when S is present in excess, cracks may occur during casting of the steel slab, resulting in a significant decrease in productivity. S content is therefore 0.03 % or less. The S content is preferably 0.02 % or less. The lower the S content, the better, and therefore there is no particular lower limit for the S content. Accordingly, the lower limit of the S content may be 0 %. However, excessively reducing the S content causes an increase in production costs. Therefore, from a production cost viewpoint, the S content is preferably 0.0001 % or more. The S content is more preferably 0.0005 % or more.Al: 0.001 % to 0.1 %

[0029] Al is an element necessary for deoxidation during steelmaking. Al content is therefore 0.001 % or more. On the other hand, when Al is excessive, nitrides and oxides are formed, which decreases the toughness of the steel slab, and cracks may occur during the slab production or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Al content is therefore 0.1 % or less. The Al content is preferably 0.08 % or less.N: 0.001 % to 0.01 %

[0030] N is an element that refines grain size by forming fine nitrides, thereby improving the strength of the cold-rolled steel sheet. N content is therefore 0.001 % or more. On the other hand, when N is excessive, N combines with Al to form nitrides, which decreases the toughness of the steel slab, and this may cause cracks during the slab production or during slab heating before hot rolling, resulting in a significant decrease in productivity. The N content is therefore 0.01 % or less. The N content is preferably 0.008 % or less.O: 0.0100 % or less

[0031] O is present as an oxide and is an element that causes embrittlement of the slab. When O is excessive, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. The O content is therefore 0.0100 % or less. The O content is preferably 0.0050 % or less. A lower limit of the O content is not particularly limited, and may be 0 %. However, excessively decreasing the O content causes an increase in production costs. Therefore, from the viewpoint of production costs, the O content is preferably 0.0001 % or more.Cr: 0.56 % to 2.0 %

[0032] Cr is an element that dissolves in cementite, and can improve the hardness of cementite and generate more voids. Further, the growth of cementite during annealing becomes rate-determined by the diffusion rate of Cr, and coarsening of cementite can be suppressed. To obtain the above effects, Cr content is 0.56 % or more. The Cr content is preferably 0.60 % or more. The Cr content is more preferably 0.70 % or more. On the other hand, when the Cr content exceeds 2.0 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Cr content is therefore 2.0 % or less. The Cr content is preferably 1.5 % or less. The Cr content is more preferably 1.3 % or less.

[0033] The chemical composition described above contains at least one element selected from the group consisting of Nb, Ti, and V. In order to obtain a desired void number density, it is necessary to contain at least one of Nb, Ti, or V in the following amounts.Nb: 0.029 % to 0.24 %

[0034] Nb is an element that forms carbides and can cause more voids to be generated in the cold-rolled steel sheet. When Nb is contained, in order to obtain these effects, Nb content is 0.029 % or more. On the other hand, when the Nb content is excessive, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Nb content is therefore 0.24 % or less. The Nb content is preferably 0.20 % or less. The Nb content is more preferably 0.10 % or less.Ti: 0.01 % to 0.21 %

[0035] Ti is an element that forms carbides and can cause more voids to be generated in the cold-rolled steel sheet. When Ti is contained, in order to obtain these effects, Ti content is 0.01 % or more. On the other hand, when the Ti content is excessive, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Ti content is therefore 0.21 % or less. The Ti content is preferably 0.15 % or less. The Ti content is more preferably 0.10 % or less.V: 0.01 % to 0.21 %

[0036] V is an element that forms carbides and can cause more voids to be generated in the cold-rolled steel sheet. When V is contained, in order to obtain these effects, V content is 0.01 % or more. On the other hand, when the V content is excessive, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The V content is therefore 0.21 % or less. The V content is preferably 0.15 % or less. The V content is more preferably 0.10 % or less.Nb + Ti + V: 0.24 % or less

[0037] When the total content of Nb, Ti, and V in the chemical composition exceeds 0.24 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The total content of Nb, Ti, and V is therefore 0.24 % or less. The total content is preferably 0.22 % or less. The total content is more preferably 0.20 % or less. On the other hand, a lower limit of the total content is not particularly limited, but 0.01 % is a practical lower limit. The lower limit corresponds to a case where only either Ti or V is contained in an amount of 0.01 %. The total content may be 0.020 % or more. The total content may be 0.025 % or more. The total content may be 0.030 % or more.

[0038] The cold-rolled steel sheet according to an embodiment of the present disclosure has a chemical composition consisting of the above components, with the balance being Fe and inevitable impurity. The inevitable impurity includes H.

[0039] Further, the chemical composition of the cold-rolled steel sheet according to another embodiment of the present disclosure may optionally further contain at least one of the following elements.Ta: 0.10 % or less

[0040] Ta has an effect of further improving the wear resistance of steel components by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when Ta content exceeds 0.10 %, a large amount of coarse precipitates and inclusions are formed, and the toughness of the steel slab decreases. The Ta content is therefore 0.10 % or less. The Ta content is preferably 0.08 % or less. On the other hand, a lower limit of the Ta content is not particularly limited. However, from the viewpoint of enhancing the effect of Ta addition, the Ta content is preferably 0.01 % or more.W: 0.10 % or less

[0041] W has an effect of further improving the wear resistance of steel components by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when W content exceeds 0.10 %, a large amount of coarse precipitates and inclusions are formed, and the toughness of the steel slab decreases. The W content is therefore 0.10 % or less. The W content is preferably 0.08 % or less. On the other hand, a lower limit of the W content is not particularly limited. However, from the viewpoint of enhancing the effect of W addition, the W content is preferably 0.01 % or more.B: 0.0100 % or less

[0042] B segregates at austenite grain boundaries during hot rolling or annealing, and has an effect of further improving hardenability. However, when B content exceeds 0.0100 %, the toughness of the steel slab decreases. The B content is therefore 0.0100 % or less. The B content is more preferably 0.0080 % or less. On the other hand, a lower limit of the B content is not particularly limited. From the viewpoint of enhancing the effect of B addition, the B content is preferably 0.0003 % or more.Mo: 1.00 % or less

[0043] Mo is an element that has an effect of further improving hardenability. However, when Mo content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Mo content is therefore 1.00 % or less. The Mo content is preferably 0.80 % or less. On the other hand, a lower limit of the Mo content is not particularly limited. However, from the viewpoint of enhancing the effect of Mo addition, the Mo content is preferably 0.01 % or more.Co: 1.00 % or less

[0044] Co is an element that has an effect of further improving hardenability. However, when Co content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Co content is therefore 1.00 % or less. The Co content is preferably 0.80 % or less. On the other hand, a lower limit of the Co content is not particularly limited. From the viewpoint of enhancing the effect of Co addition, the Co content is preferably 0.001 % or more.Ni: 1.00 % or less

[0045] Ni is an element that has an effect of further improving hardenability. However, when Ni content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Ni content is therefore 1.00 % or less. The Ni content is preferably 0.80 % or less. On the other hand, a lower limit of the Ni content is not particularly limited. However, from the viewpoint of enhancing the effect of Ni addition, the Ni content is preferably 0.01 % or more.Cu: 1.00 % or less

[0046] Cu is an element that has an effect of further improving hardenability. However, when Cu content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Cu content is therefore 1.00 % or less. The Cu content is preferably 0.80 % or less. On the other hand, a lower limit of the Cu content is not particularly limited. However, from the viewpoint of enhancing the effect of Cu addition, the Cu content is preferably 0.01 % or more.Sn: 0.200 % or less

[0047] Sn is an element that has an effect of further improving hardenability. However, when Sn content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Sn content is therefore 0.200 % or less. The Sn content is preferably 0.100 % or less. On the other hand, a lower limit of the Sn content is not particularly limited. However, from the viewpoint of enhancing the effect of Sn addition, the Sn content is preferably 0.001 % or more.Sb: 0.200 % or less

[0048] Sb is an element that has an effect of suppressing decarburization. By suppressing decarburization, the strength of the steel sheet can be further improved. However, when Sb content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Sb content is therefore 0.200 % or less. The Sb content is preferably 0.100 % or less. On the other hand, a lower limit of the Sb content is not particularly limited. However, from the viewpoint of enhancing the effect of Sb addition, the Sb content is preferably 0.001 % or more.Ca: 0.0100 % or less

[0049] Ca is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Ca content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Ca content is therefore 0.0100 % or less. The Ca content is preferably 0.0050 % or less. On the other hand, a lower limit of the Ca content is not particularly limited. However, from the viewpoint of enhancing the effect of Ca addition, the Ca content is preferably 0.0005 % or more.Mg: 0.0100 % or less

[0050] Mg is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Mg content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Mg content is therefore 0.0100 % or less. The Mg content is preferably 0.0050 % or less. On the other hand, a lower limit of the Mg content is not particularly limited. However, from the viewpoint of enhancing the effect of Mg addition, the Mg content is preferably 0.0005 % or more.REM: 0.0100 % or less

[0051] REM (rare earth metals) are elements that have an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when REM content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The REM content is therefore 0.0100 % or less. The REM content is preferably 0.0050 % or less. On the other hand, a lower limit of the REM content is not particularly limited. However, from the viewpoint of enhancing the effect of REM addition, the REM content is preferably 0.0005 % or more.Zr: 0.100 % or less

[0052] Zr is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Zr content exceeds 0.100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Zr content is therefore 0.100 % or less. The Zr content is preferably 0.080 % or less. On the other hand, a lower limit of the Zr content is not particularly limited. However, from the viewpoint of enhancing the effect of Zr addition, the Zr content is preferably 0.001 % or more.Te: 0.100 % or less

[0053] Te is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Te content exceeds 0.100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Te content is therefore 0.100 % or less. The Te content is preferably 0.080 % or less. On the other hand, a lower limit of the Te content is not particularly limited. However, from the viewpoint of enhancing the effect of Te addition, the Te content is preferably 0.001 % or more.Hf: 0.10 % or less

[0054] Hf is an element that has an effect of spheroidizing the shape of nitrides and sulfides and improving ultimate deformability of the steel sheet. However, when Hf content exceeds 0.10 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. Th Hf content is therefore 0.10 % or less. The Hf content is preferably 0.08 % or less. On the other hand, a lower limit of the Hf content is not particularly limited. However, from the viewpoint of enhancing the effect of Hf addition, the Hf content is preferably 0.01 % or more.Bi: 0.200 % or less

[0055] Bi is an element that mitigates segregation. However, when Bi content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Bi content is therefore 0.200 % or less. The Bi content is preferably 0.100 % or less. On the other hand, a lower limit of the Bi content is not particularly limited. However, from the viewpoint of enhancing the effect of Bi addition, the Bi content is preferably 0.001 % or more.

[0056] The elements from Ta to Bi have effects of further improving the properties of the cold-rolled steel sheet of the present disclosure, and can be added to the steel sheet of the present disclosure. In the above description, the lower limit values of the preferred content of these optionally-added elements are indicated. However, when the content of these elements is lower than the preferred lower limit, the effect of adding the elements is decreased, but the properties of the cold-rolled steel sheet still satisfy requirements. Accordingly, the addition of these elements is not essential, and the lower limit of content may be 0 %.[Cementite]

[0057] The following describes cementite contained in the cold-rolled steel sheet of the present disclosure.Average particle size of cementite: 0.65 µm or less

[0058] According to the present disclosure, machinability and blanking 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. When the average particle size of cementite particles having an area of 0.06 µm 2< or more is larger than 0.65 µm, the number density of voids cannot be set within a desired range. The average particle size of cementite particles having an area of 0.06 µm 2< or more is therefore 0.65 µm or less. The average particle size of cementite is preferably 0.63 µm or less. The average particle size of cementite is more preferably 0.60 µm or less. The average particle size of cementite is even more preferably 0.55 µm or less. The average particle size of cementite is most preferably 0.40 µm or less. Hereinafter, the "average particle size of cementite particles having an area of 0.06 µm 2< or more" may be simply referred to as the "average particle size of cementite". On the other hand, a lower limit of the average particle size of the cementite is not particularly limited, but from the viewpoint of manufacturability, the average particle size of the cementite may be 0.30 µm or more, or 0.35 µm or more.

[0059] The average particle size of cementite is defined as a value measured by electron backscatter diffraction (EBSD) at a cross-section in the rolling direction at a 1 / 2 thickness position of the cold-rolled steel sheet. More specifically, measurements may be made by a method described in the EXAMPLES section of the present disclosure.A value: 2.0 mass% or more

[0060] When Mn and Cr are dissolved in cementite, the hardness of the cementite increases, and the harder the cementite, the more easily voids are generated in the cold-rolled steel sheet. Therefore, according to the present disclosure, the amounts of Mn and Cr present in cementite are controlled in order to increase the number density of voids. Specifically, the A value defined by the following expression (1) is 2.0 mass% or more. When the A value is less than 2.0 mass%, the void generation ability of cementite decreases, and as a result, obtaining a desired number density of voids becomes difficult. The A value is preferably 3.0 mass% or more. The A value is more preferably 4.0 mass% or more. The A value is even more preferably 5.0 mass% or more. A = 3 / 7 C Mn + C Cr

[0061] Here, in expression (1), C Mn is Mn content in mass% in the cementite particles having an area of 0.06 µm 2< or more, and C Cr is Cr content in mass% in the cementite particles having an area of 0.06 µm 2< or more. On the other hand, although there is no particular upper limit to the A value, in order to increase the A value, increasing the amounts of Mn and Cr added as alloying elements is necessary, and when the A value exceeds 20 mass%, manufacturability is adversely affected. Therefore, from the viewpoint of manufacturability, the A value is preferably 20 mass% or less. The A value is more preferably 15 mass% or less. The A value is even more preferably 12 mass% or less.

[0062] C Mn and C Cr are defined as values measured by an electron probe microanalyzer (EPMA) at a cross-section in the rolling direction at a 1 / 2 thickness position of the cold-rolled steel sheet. More specifically, measurements may be made by a method described in the EXAMPLES section of the present disclosure.[Voids]

[0063] The following is a description of the voids in the cold-rolled steel sheet.Number density of voids: 50,000 / mm 2< or more

[0064] The number density of voids having an area of 0.01 µm 2< or more is very important for improving the machinability and blanking workability of the cold-rolled steel sheet. The reasons for this are as follows.

[0065] When the cold-rolled steel sheet is cut, the material first undergoes plastic deformation, and voids are generated at locations of high plastic strain. The generated voids then grow, and the cold-rolled steel sheet is cut by the voids connecting. At this time, a large cutting resistance is exhibited, particularly while the material undergoes plastic deformation and until voids are generated. Accordingly, by dispersing voids in the cold-rolled steel sheet in advance, the cutting resistance can be decreased and the machinability of the cold-rolled steel sheet can be improved.

[0066] Further, when the cold-rolled steel sheet is blanked, the material undergoes plastic deformation and voids are generated in areas of high plastic strain, just as in cutting work. The generated voids then grow and fracture occurs due to voids connecting, completing the blanking. At this time, rollover occurs due to the blanking, while the material undergoes plastic deformation and until voids are generated. Accordingly, by dispersing voids in the cold-rolled steel sheet in advance, the amount of plastic deformation until fracture during blanking can be decreased, and rollover can be decreased. In this way, blanking workability is improved.

[0067] Therefore, in order to obtain excellent machinability and blanking workability, the number density of voids having an area of 0.01 µm 2< or more is 50,000 / mm 2< or more. The number density of voids is preferably 80,000 / mm 2< or more. The number density of voids is more preferably 100,000 / mm 2< or more. On the other hand, an upper limit of the number density of voids is not particularly limited. The number density of voids is preferably 1,000,000 / mm 2< or less.

[0068] The number density of voids is defined as a value measured using a scanning electron microscope (SEM) on a cross-section of the cold-rolled steel sheet in the rolling direction at a 1 / 2 thickness position of the sheet. More specifically, measurements may be made by a method described in the EXAMPLES section of the present disclosure.[Vickers hardness]

[0069] The following describes Vickers hardness of the cold-rolled steel sheet of the present disclosure.Vickers hardness: 200 HV to 400 HV

[0070] The Vickers hardness of the cold-rolled steel sheet has a large effect on blanking workability and machinability. When the Vickers hardness of the cold-rolled steel sheet is less than 200 HV, significant rollover occurs during blanking, resulting in poor blanking workability. In order to obtain the desired blanking workability, the cold-rolled steel sheet has a Vickers hardness of 200 HV or more. The Vickers hardness is preferably 240 HV or more. The Vickers hardness is more preferably 280 HV or more. The Vickers hardness is even more preferably 300 HV or more. On the other hand, when 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 400 HV or less. The Vickers hardness is preferably 360 HV or less. The Vickers hardness is more preferably 340 HV or less.

[0071] The Vickers hardness is defined as a value at a cross-section in the rolling direction at a 1 / 2 thickness position of the cold-rolled steel sheet. More specifically, measurements may be made by a method described in the EXAMPLES section of the present disclosure.[Sheet thickness]

[0072] The sheet thickness of the cold-rolled steel sheet is not particularly limited and may be any thickness. The sheet thickness is preferably 0.1 mm or more. The sheet thickness is more preferably 0.2 mm or more. Further, an upper limit of the sheet thickness is not particularly limited. The sheet thickness is particularly 2.5 mm or less. The sheet thickness is more preferably 1.6 mm or less. The sheet thickness is even more preferably 0.8 mm or less. When the sheet thickness is 0.2 mm or more and 0.8 mm or less, the cold-rolled steel sheet is particularly suitable for use as a material for textile machinery components such as knitting needles and the like.[Method of producing cold-rolled steel sheet]

[0073] The following describes a method of producing the cold-rolled steel sheet according to an embodiment. In the following explanation, temperature indicated in "°C" indicates a surface temperature (the temperature at a surface of the steel slab, the steel sheet, or the like).

[0074] The cold-rolled steel sheet may be produced by performing the following processes in sequence, starting with a steel slab having the chemical composition described above. (1) Heating (2) Hot rolling (3) Cooling (4) Coiling (5) First annealing (6) Cold rolling (7) Second annealing (8) Final cold rolling (1) Heating

[0075] First, a steel slab having the chemical composition described above is heated. The method of 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-mentioned chemical composition can be produced in a converter and then formed into a steel slab by a casting method such as continuous casting. Further, casting methods other than continuous casting, such as ingot casting followed by blooming, may be used. Further, the steel slab may be produced by an electric furnace steelmaking process. In such a case, scrap may be used as the raw material.Slab heating temperature: 1100 °C or higher

[0076] Heating is carried out prior to hot rolling to homogenize the components and dissolve carbides such as cementite and segregations present in the steel slab. However, when the heating temperature of the steel slab (slab heating temperature) during the heating is lower than 1100 °C, cementite cannot be sufficiently dissolved, and as a result, the average particle size of cementite in the finally obtained cold-rolled steel sheet cannot be set within the desired range. The slab heating temperature is therefore 1100 °C or higher. The slab heating temperature is preferably 1150 °C or higher. On the other hand, an upper limit of the slab heating temperature is not particularly limited. The slab heating temperature is preferably 1350 °C or lower.Slab heating time: 60 min or longer

[0077] When the heating time of the steel slab (slab heating time) during the heating is shorter than 60 min, cementite cannot be sufficiently dissolved, and as a result, the average particle size of cementite in the finally obtained cold-rolled steel sheet cannot be set within the desired range. The slab heating time is therefore 60 min or longer. The slab heating time is preferably 90 min or longer. On the other hand, an upper limit of the slab heating time is not particularly limited. The slab heating time is preferably 300 min or shorter. The slab heating time is more preferably 240 min or shorter.

[0078] The heating may be carried out by any method, but use of a heating furnace is preferred.(2) Hot rolling

[0079] The heated steel slab is then hot rolled to obtain a hot-rolled steel sheet. In the hot rolling, rough rolling and finishing rolling may be carried out according to conventional methods.Rolling finish temperature: exceeding Tc

[0080] When the rolling finish temperature in the hot rolling is Tc or lower, coarse cementite is generated in the hot-rolled steel sheet, and this coarse cementite remains in the finally obtained cold-rolled steel sheet. As a result, the desired cementite grain size cannot be obtained, and the desired machinability and blanking workability cannot be obtained. Therefore, the rolling finish temperature in the hot rolling exceeds Tc. On the other hand, an upper limit of the rolling finish temperature is not particularly limited, and may be 980 °C or lower, or 950 °C or lower.

[0081] Here, Tc (°C) is a value that can be used as an index of the temperature at which a transformation involving cementite occurs, and is calculated by the following expression (2).

[0082] Here, the element symbols in expression (2) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0.(3) CoolingAverage cooling rate: 20 °C / s or more

[0083] After the finishing rolling is completed, cooling is started, then stopped at a cooling stop temperature. When the average cooling rate in the cooling is less than 20 °C / s, the average particle size of cementite cannot be set within the desired range. This is because a slow cooling rate results in the formation of coarse cementite, which remains in the finally obtained cold-rolled steel sheet. Therefore, the average cooling rate is 20 °C / s or more. The average cooling rate is preferably 30 °C / s or more. The average cooling rate is more preferably 50 °C / s or more. On the other hand, although there is no particular upper limit to the average cooling rate, when the cooling rate is excessively high, controlling the cooling stop temperature becomes difficult. The average cooling rate is therefore preferably 500 °C / s or less.Cooling stop temperature: Tc or lower

[0084] When the cooling stop temperature in the cooling is higher than Tc, the average particle size of cementite cannot be set within the desired range. This is because when the cooling stop temperature is high, coarse cementite is formed, and this coarse cementite remains in the finally obtained cold-rolled steel sheet. The cooling stop temperature is therefore Tc or lower. The cooling stop temperature is preferably Tc - 20 °C or lower. On the other hand, a lower limit of the cooling stop temperature is not particularly limited. According to an embodiment of the present disclosure, the cooling stop temperature may be 530 °C or higher. Typically, the cooling stop temperature is preferably equal to or higher than a coiling temperature in the subsequent coiling.(4) CoilingCoiling temperature: 530 °C or higher and Tc or lower

[0085] After the cooling is stopped, the cooled hot-rolled steel sheet is coiled. At this time, when the coiling temperature exceeds Tc, coarse cementite is generated, and this coarse cementite remains in the finally obtained cold-rolled steel sheet. The coiling temperature is therefore Tc or lower. The coiling temperature is preferably Tc - 20 °C or lower. The coiling temperature is more preferably Tc - 40 °C or lower. On the other hand, when the coiling temperature is lower than 530 °C, volume expansion due to transformation during coiling results in a poor coiling shape. The coiling temperature is therefore 530 °C or higher. The coiling temperature is preferably 550 °C or higher. The coiling temperature is more preferably 600 °C or higher.(5) First annealing

[0086] Annealing temperature: 600 °C or higher and Tc or lower Annealing time: 3 h or longer

[0087] The hot-rolled steel sheet after the coiling is subjected to the first annealing under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer. The microstructure of the hot-rolled steel sheet after the coiling is a pearlitic microstructure lined with plate-like cementite and ferrite. The pearlitic microstructure is stable, and therefore does not homogenize without prolonged holding at a high temperature. In order to break up the pearlitic microstructure and allow the subsequent cold rolling and annealing process to produce the desired cementite, the annealing temperature needs to be 600 °C or higher and the annealing time needs to be 3 h or longer. The annealing temperature is preferably 620 °C or higher. Further, the annealing time is preferably 4 h or longer. On the other hand, when the annealing temperature is higher than Tc, phase transformation starts preferentially from one portion, and a locally coarse microstructure is formed. As a result, the microstructure becomes non-uniform, and the desired cementite particle size cannot be obtained. The annealing temperature is therefore Tc or lower. The annealing temperature is preferably Tc - 20 °C or lower. The annealing temperature is more preferably Tc - 40 °C or lower. On the other hand, although there is no particular upper limit to the annealing time, when too long, productivity decreases. The annealing time is therefore preferably 50 h or shorter. The annealing time is more preferably 30 h or shorter.

[0088] By carrying out the first annealing under the above conditions, the pearlitic microstructure can be broken up, and the desired cementite can be more easily formed in the subsequent cold rolling and annealing processes.

[0089] The first annealing can be carried out any number of times, but from the viewpoint of enhancing the effect of the annealing, the first annealing is preferably carried out two or more times. On the other hand, there is no particular upper limit to the number of times the first annealing is carried out, but even when the first annealing is carried out more than three times, the effect is saturated. The first annealing is therefore preferably carried out three times or less. When the first annealing is carried out two or more times, each annealing may be carried out under the above conditions. The annealing conditions for each time may be the same or different from each other.

[0090] Prior to the first annealing, the hot-rolled steel sheet is preferably pickled.(6) Cold rolling(7) Second annealing

[0091] Plate-like cementite is formed in the steel sheet after hot rolling. This plate-like cementite is stable and therefore prone to remain for a long time. The plate-like cementite is coarse, and therefore when 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 blanking workability cannot be obtained. Therefore, in order to refine and spheroidize the plate-like cementite by heating during annealing, the hot-rolled steel sheet after the first annealing is subjected to cold rolling and second annealing at least once.Rolling ratio: 15 % or more

[0092] The cold rolling causes the cementite to be deformed, broken up, and fragmented, thereby making it possible to obtain a desired average cementite particle size. When the rolling ratio in the cold rolling is less than 15 %, the effect cannot be obtained. The rolling ratio is therefore 15 % or more. The rolling ratio is preferably 25 % or more. The rolling ratio is more preferably 35 % or more. The rolling ratio is even more preferably 45 % or more. On the other hand, an upper limit of the rolling ratio is not particularly limited. The rolling ratio is preferably 85 % or less. The rolling ratio is more preferably 80 % or less. Annealing temperature: 600 °C or higher and Tc or lower Annealing time: 3 h or longer

[0093] In the second annealing, the cementite that has been deformed, broken up, and fragmented by the cold rolling can be refined and spheroidized. At the same time, Mn and Cr can be concentrated in the cementite. To obtain these effects, the annealing temperature in the second annealing needs to be 600 °C or higher, and the annealing time needs to be 3 h or longer. The annealing temperature is preferably 620 °C or higher. Further, the annealing time is preferably 4 h or longer. On the other hand, when the annealing temperature is higher than Tc, phase transformation starts preferentially from one portion, and a locally coarse microstructure is formed. As a result, the microstructure becomes non-uniform, and the desired cementite particle size cannot be obtained. The annealing temperature is therefore Tc or lower. The annealing temperature is preferably Tc - 20 °C or lower. The annealing temperature is more preferably Tc - 40 °C or lower. On the other hand, although there is no particular upper limit to the annealing time, when too long, productivity decreases. The annealing time is therefore preferably 30 h or shorter. The annealing time is more preferably 20 h or shorter.

[0094] By carrying out the cold rolling and the 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.

[0095] The cold rolling and the second annealing can be carried out any number of times, but from the viewpoint of enhancing the above-described effects, the process is preferably carried out two or more times. When the cold rolling and the second annealing are carried out two or more times, the cold rolling and the second annealing may be repeated alternately. On the other hand, there is no particular upper limit to the number of times that the cold rolling and the second annealing are carried out, but even when the number of times is more than five, the effect is saturated. The cold rolling and the second annealing are therefore preferably carried out five times or less. When the cold rolling and the second annealing are carried out two or more times, each iteration of the cold rolling and the second annealing may be carried out under the above conditions. The conditions for each iteration may be the same or different from each other.

[0096] When the cold rolling and the second annealing are carried out only once, the rolling ratio of the cold rolling is preferably 70 % or more from the viewpoint of reliably refining cementite.(8) Final cold rolling

[0097] After the cold rolling and the second annealing are carried out as described above, the steel sheet is further subjected to final cold rolling. By carrying out the final cold rolling, the number density of voids can be adjusted to a desired range. Further, the desired Vickers hardness can be obtained.Rolling ratio: 20 % to 80 %

[0098] However, when the rolling ratio in the final cold rolling is less than 20 %, the effects described above cannot be obtained. The rolling ratio in the final cold rolling is therefore 20% or more. The rolling ratio is preferably 25 % or more. The rolling ratio is more preferably 30 % or more. On the other hand, when the rolling ratio in the final cold rolling exceeds 80 %, the Vickers hardness of the cold-rolled steel sheet may exceed 400 HV. When the Vickers hardness exceeds 400 HV, the desired machinability cannot be obtained. The rolling ratio of the final cold rolling is therefore 80 % or less. The rolling ratio is preferably 70 % or less. The rolling ratio is more preferably 60 % or less. The rolling ratio is even more preferably 50 % or less.

[0099] By satisfying the above conditions, it is possible to produce the cold-rolled steel sheet that has excellent machinability and blanking workability. The finally obtained cold-rolled steel sheet may be subjected to further optional surface treatment.EXAMPLES

[0100] In order to confirm the effects of the present disclosure, cold-rolled steel sheets were produced according to the following procedure.

[0101] First, steels having the chemical compositions listed in Table 1 were melted in a converter and made into steel slabs by continuous casting. Each steel slab was then subjected to heating, hot rolling, cooling, coiling, first annealing, cold rolling, second annealing, and final cold rolling in sequence to produce a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm. Each process was carried out under the conditions listed in Tables 2 and 3, and the first annealing, cold rolling, and second annealing were carried out the number of times listed in Tables 2 and 3.

[0102] Next, for each of the obtained cold-rolled steel sheets, the average particle size of cementite, the amount of Mn and Cr dissolved in the cementite particles, the number density of voids, and Vickers hardness were measured by the following procedures. The measurement results are listed in Tables 4 and 5. In Table 4, for convenience, the A values are rounded to two decimal places, but the determination of whether the A value satisfies the conditions of the present disclosure is made based on the value calculated by expression (1).(Average particle size of cementite)

[0103] Test pieces for microstructure observation were taken from the obtained cold-rolled steel sheets. Next, rolling direction cross-sections (L-sections) of the test pieces for microstructure observation were each polished to a mirror surface. Thereafter, finishing polishing was carried out using a colloidal silica solution.

[0104] Next, the polished surface of the test piece for microstructure observation was measured using electron backscatter diffraction (EBSD). The measurement was carried out under the conditions of an electron beam accelerating voltage of 20 keV, a measurement interval of 0.14 µm steps, a measurement field of view of 60 µm × 45 µm, and a measurement position at 1 / 2 thickness.

[0105] 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 produced by TSL Solutions was used. Next, from the identified cementite particles, only cementite particles having an area of 0.06 µm 2< or more were selected, and an average value was calculated.

[0106] The above processes were carried out at each of the three measurement positions, and an average of the three average values obtained was taken as the average particle size of cementite having an area of 0.06 µm 2< or more.(Mn amount and Cr amount in cementite)

[0107] Test pieces for microstructure observation were taken from the obtained cold-rolled steel sheets. L-sections of the test pieces for microstructure observation were each 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, using an electron probe micro analyzer (EPMA), the Mn amount and Cr amount were measured at three locations with respect to one cementite particle having an area of 0.06 µm 2< or more under the conditions of an electron beam accelerating voltage of 15 keV, an irradiation current of 1.0 × 10 -8< A, and a measurement time of 1000 ms, and an average value of the three locations was calculated. The Mn amount and Cr amount in ten cementite particles each having an area of 0.06 µm 2< or more were similarly measured, and the measured values of the ten particles were averaged to obtain the Mn amount and Cr amount in cementite.(Number density of voids)

[0108] Test pieces for microstructure observation were taken from the obtained cold-rolled steel sheets. L-sections of the test pieces for microstructure observation were each polished to a mirror surface. Thereafter, the polished surface of the test piece for microstructure observation was imaged at ten locations at a 1 / 2 thickness position using a scanning electron microscope (SEM) at an accelerating voltage of 15 keV and a magnification of 3000 times to obtain microstructure images.

[0109] From the obtained microstructure images, voids having an area of less than 0.01 µm 2< were excluded by image processing, and the number of voids having an area of 0.01 µm 2< or more was counted and divided by the area of the microstructure image to calculate the number density. Similar measurements were carried out in ten fields of view, and an average value of the obtained number densities was calculated to be the number density of voids having an area of 0.01 µm 2< or more.(Vickers hardness)

[0110] Test pieces for measuring Vickers hardness were taken from the obtained cold-rolled steel sheets. L-sections of the test pieces for measuring Vickers hardness were each polished to a mirror surface. Thereafter, measurements were made in accordance with JIS Z 2244:2009 at a 1 / 2 sheet thickness position, a load of 1 kgf, and five measurement points, and the measured values at the five points were averaged to obtain the Vickers hardness of the cold-rolled steel sheet.

[0111] Further, the machinability and the blanking workability of each of the obtained cold-rolled steel sheets were evaluated using the following procedure. The evaluation results are listed in Tables 4 and 5.(Machinability)

[0112] The machinability of the cold-rolled steel sheets was evaluated based on a cutting life distance in a cutting test. The specific procedure is described below with reference to the drawings. Here, FIG. 1 is a schematic diagram illustrating a cutting test method, and FIG. 2 is a schematic cross-section diagram illustrating a state of a cut test piece.• Creation of test piece

[0113] 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 illustrated in FIG. 1, the test piece 1 for machinability evaluation was attached to a fixing jig 2 by an adhesive. The adhesive used was Aron Alpha EXTRA ®< (Aron Alpha EXTRA is a registered trademark in Japan, other countries, or both), a fast-acting, multi-purpose instant adhesive produced by Aron Alpha Co., Ltd. The dimensions of the fixing jig 2 were 50 mm (width) × 150 mm (length) × 30 mm (depth). When the test piece was attached, it was fixed in a vice for 8 h or longer to secure flatness of the test piece.• Cutting test

[0114] Next, a cutting test was carried out using the test piece 1 for machinability evaluation. Specifically, first, the test piece 1 for machinability evaluation 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 test piece 1 for machinability evaluation in the direction indicated by arrow A, and the metal saw was moved in the direction indicated by arrow B to carry out cutting by down-cutting. The metal saw used was made of cemented carbide having a Vickers hardness of about 1680, a Young's modulus of about 570 GPa, and a specific gravity of about 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.

[0115] The metal saw was used to cut the test piece 1 for machinability evaluation over a total length of 150 mm from one end to the other end, and then the metal saw was returned to the one end, the cutting position was shifted downward (in the direction of arrow C) by 2 mm, and cutting was carried out again. By repeating the above procedure, cutting was carried out at intervals of 2 mm as illustrated in FIG. 2, and cutting was terminated when the metal saw broke.

[0116] The cutting conditions were a rotation speed of 3000 / min, a feed rate of 24 mm / min, and a depth of cut of 0.2 mm, and no cutting oil was used. The measurement resolution of the dynamometer 3 was set to 1000 Hz. In FIG. 2, reference sign 4 denotes a metal saw, and arrow ND indicates the thickness direction of the test piece 1 for machinability evaluation, that is, the cold-rolled steel sheet.

[0117] Next, a cutting life was calculated from the cutting resistance measured in the cutting test. FIG. 3 is a schematic diagram of a graph in which the cutting resistance F, in N, measured in the cutting test is approximated by a cubic spline function, and the horizontal axis represents the cutting distance in mm. The cutting resistance F was calculated from the cutting resistance in each direction measured by the dynamometer using the following expression (3). F = √ F x 2 + F y 2 + F z 2

[0118] Here, F is the cutting resistance, F x is the cutting resistance in the x-axis direction, F y is the cutting resistance in the y-axis direction, and F z is the cutting resistance in the z-axis direction.

[0119] The cutting life was determined from the cutting distance at which a large burr began to appear before the metal saw broke. Here, the point where a large burr began to appear (indicated by an arrow in FIG. 3) is the cutting distance where the gradient of the approximation curve is ≥ (maximum gradient) × 1 / 3, and this is defined as the cutting life distance. When the cutting life distance was 1148 mm or more, the machinability was rated as good, and when less than 1148 mm, the machinability was rated as poor.(Blanking workability)

[0120] The blanking workability of the cold-rolled steel sheets was evaluated based on a rollover ratio in a blanking test. The specific procedures were as follows.

[0121] First, test pieces were taken from the obtained cold-rolled steel sheets. The dimensions of the test pieces were 30 mm (width) x 30 mm (length), and the thickness was the same as that of the cold-rolled steel sheet.

[0122] Next, a center of each test piece was blanked out using a cylindrical punch having a diameter of 10.0 mm to obtain a disk sample having a diameter of 10 mm. The clearance in the blanking was 12.5 %. Here, the clearance is a percentage (%) of a gap between the punch and the die, relative to the thickness of the test piece.

[0123] The obtained disk sample was cut, and rollover d RD at an end surface in the rolling direction and rollover d TD at an end surface in the direction perpendicular to the rolling direction were measured using an optical microscope. FIG. 4 is a schematic cross-section diagram illustrating a state of an end face of a disk sample 10. Here, 11 is rollover, 12 is a shear surface, and 13 is a fracture surface, and a length d of the rollover 11 in the sheet thickness direction is used as d RD and d TD . The rollover ratio D (%) was calculated from the obtained rollover value and the sheet thickness t in mm of the disk sample 10 by the following expression (4). D = d RB + d TD / 2 / t × 100 %

[0124] When the calculated rollover ratio was 10 % or less, the blanking workability was rated as good, and when more than 10 %, the blanking workability was rated as poor.[Table 1]

[0125] Table 1Steel sample IDChemical composition (mass%)Nb + Ti + V (mass%)Tc (°C)RemarksCSiMnPSAlNOCrNbTiVOtherA0.910.210.510.0080.00360.0090.00340.00050.700.080---0.080735Disclosed steelB0.850.331.120.0050.00370.0080.00300.00210.560.0390.02--0.059730Disclosed steelC1.100.100.520.0070.00400.0040.00350.00071.800.101-0.08-0.181751Disclosed steelD1.250.320.820.0080.00330.0040.00290.00100.750.0950.060.08-0.235736Disclosed steelE1.050.150.500.0050.00130.0040.00330.00081.08-0.12--0.120740Disclosed steelF1.080.320.280.0090.00390.0920.00420.00061.31--0.19-0.190751Disclosed steelG0.980.230.750.0040.00070.0050.00330.00150.61-0.100.09-0.190732Disclosed steelH1.070.201.130.0110.00280.0030.00320.00051.420.094--Ta 0.05, Mo 0.4, Cu 0.12, Sb 0.010.094741Disclosed steelI0.970.551.010.0090.00190.0070.00450.00100.730.095---0.095741Disclosed steelJ1.130.150.350.0100.00140.0040.00290.00051.110.062--Mo 0.120.062742Disclosed steelK0.860.182.980.0060.00180.0050.00360.00100.930.136---0.136712Disclosed steelL0.920.801.070.0090.00380.0090.00520.00050.820.121--Zr 0.06, Tc 0.040.121749Disclosed steelM1.080.200.700.0450.02810.0420.00340.00111.590.0290.05--0.079748Disclosed steelN0.870.990.900.0080.00340.0090.00500.00120.870.078--Hf 0.05, Bi 0.080.078757Disclosed steelO1.090.242.450.0060.00230.0100.00340.00941.270.046--W 0.04, Mg 0.005, REM 0.0050.046725Disclosed steelP1.050.220.730.0360.01520.0080.00290.00130.750.192---0.192734Disclosed steelQ1.080.121.930.0040.00290.0710.00300.00091.460.097--Cu 0.15, Sb 0.010.097731Disclosed steelR0.850.340.490.0090.00200.0080.00940.00061.100.077--Ni 0.05, Cu 0.120.077745Disclosed steelS0.870.251.200.0050.00040.0030.00740.00090.750.140--Ca 0.0050.140730Disclosed steelT0.870.290.710.0040.00390.0030.00350.00120.990.031--B 0.00150.031741Disclosed steelU1.090.310.620.0090.00190.0090.00100.00060.750.184--Co 0.006, Sn 0.050.184738Disclosed steela0.750.281.020.0050.00280.0070.00350.00050.720.033---0.033732Comparative steelb0.870.240.150.0080.00230.0050.00380.00070.590.053---0.053738Comparative steelc0.890.350.350.0130.00210.0040.00380.00060.420.048---0.048737Comparative steeld0.810.260.450.0050.00210.0080.00350.00100.590.020---0.020736Comparative steele1.060.331.020.0090.00160.0100.00220.00161.090.261---0.261740Comparative steelf0.980.291.710.0090.00340.0080.00140.00121.050.1910.08--0.271731Comparative steelg1.100.330.770.0100.00180.0060.00320.00091.48-0.150.20-0.350749Comparative steelh1.081.341.720.0090.00320.0060.00430.00061.230.152---0.152764Comparative steeli1.020.423.540.0130.00420.0080.00520.00091.050.123---0.123715Comparative steelj1.050.531.940.0110.00180.0070.00350.00072.360.112---0.112758Comparative steel [Table 2]

[0126] Table 2NoSteel sample IDHeatingHot rollingCoolingCoilingFirst annealingCold rollingSecond annealingTimes *Final cold rollingRemarksSlab heating temp. (°C)Slab heating time (min)Rolling finish temp. (°C)Average cooling rate (°C / s)Cooling stop temp. (°C)Coiling temp. (°C)Heating temp. (°C)Annealing time (h)TimesRolling ratio (%)Heating temp. (°C)Holding time (h)Rolling ratio (%)1A1180709003068066071072456807235Example2B1200809108065062067082306307340Example3B12008091080650620670815063012240Example4B12008091080650620670827564010135Example5B1200809108065062067082306308340Example6C12209087540600580690625063010225Example7D117080860206205706702122564014530Example8E115070850506405907002124564011225Example9F112065845306606206901624067014235Example10G117080880357006606802022563012525Example11H12009086575590550710825066014225Example12I12209090050660650680102456409235Example13I12209090050660650680102706408135Example14I10303080060660650710102457008235Comparative Example15I12209070080660650710102457008235Comparative Example16I12209080010660650700102457008235Comparative Example17I122090900808007707001024570014235Comparative Example18I1220909005066065055022456006225Comparative Example19I12209090050660650770102456908235Comparative Example20I1220909005066065062031455502235Comparative Example21I122090900506606506801024577014235Comparative Example22I12209090050660650710102107009235Comparative Example23I12209090050660650680102456409210Comparative Example* Number of cold rolling and second annealing iterations [Table 3]

[0127] Table 3NoSteel sample IDHeatingHot rollingCoolingCoilingFirst anncalingCold rollingSecond annealingTimes *Final cold rollingRemarksSlab heating temp. (°C)Slab heating time (min)Rolling finish temp. (°C)Average cooling rate (°C / s)Cooling stop temp. (°C)Coiling temp. (°C)Heating temp. (°C)Annealing time (h)TimesRolling ratio (%)Heating temp. (°C)Holding time (h)Rolling ratio (%)24I12209090050660650680102456407290Comparative Example25J1230100950406306106701125065016235Example26K114060860306506007001623565013230Example27L11909092035570520680925066014230Example28M1180808752573071070062406303330Example29N1150708704570067067052306404335Example30O122090900506005707102124566010225Example31P129070850456005906801025064016230Example32Q107021084540580540700252356507220Example33R1250180910706205906802423066012240Example34S1150100930100550510680192506309225Example35T117070870355805807001325067012225Example36U116010090030570530650202506607235Example37a11908090545670640710723569014235Example38b11708090050710660700132456809240Example39c1200110910456406206901123068010225Example40d112055730256405906701622564012225Example41eslab crack occurred during continuous castingComparative Example42fslab crack occurred during continuous castingComparative Example43gslab crack occurred during continuous castingComparative Example44hslab crack occurred during continuous castingComparative Example45islab crack occurred during continuous castingComparative Example46jslab crack occurred during continuous castingComparative Example* Number of cold rolling and second annealing iterations [Table 4]

[0128] Table 4NoSteel sample IDCementiteVoidsVickers hardnessPropertiesRemarksAverage particle size *1 (µm)CMn (mass%)CCr (mass%)A value (3 / 7 C Mn + C Cr ) (mass%)Number density *2 (per mm 2< )HVMachinabilityBlanking workability1A0.642.52.03.150000303goodgoodExample2B0.435.21.74.074000310goodgoodExample3B0.435.21.74.074000310goodgoodExample4B0.455.51.84.268000305goodgoodExample5B0.435.21.74.074000310goodgoodExample6C0.373.15.67.077000338goodgoodExample7D0.454.32.54.464000283goodgoodExample8E0.452.53.14.257000317goodgoodExample9F0.512.04.75.669000333goodgoodExample10G0.453.71.93.554000256goodgoodExample11H0.436.75.38.274000347goodgoodExample12I0.445.12.44.671000321goodgoodExample13I0.445.12.44.671000321goodgoodExample14I0.715.62.65.039000321poorgoodComparative Example15I0.725.62.65.038000321poorgoodComparative Example16I0.685.62.65.040000321poorgoodComparative Example17I0.675.62.65.040000321poorgoodComparative Example18I0.664.01.93.727000311poorpoorComparative Example19I0.805.52.65.035000321poorgoodComparative Example20I0.673.31.63.131000321poorpoorComparative Example21I0.786.12.95.638000321poorgoodComparative Example22I0.675.62.65.040000321poorgoodComparative Example23I0.465.12.44.617000192poorpoorComparative Example*1: Average particle size of cementite particles having area of 0.06 µm 2< or more *2: Number density of voids having area of 0.01 µm 2< or more [Table 5]

[0129] Table 5NoSteel sample IDCementiteVoidsVickers hardnessPropertiesRemarksAverage particle size *1 (µm)C Mn (mass%)C Cr (mass%)A value (3 / 7 C Mn + C Cr ) (mass%)Number density *2 (per mm 2< )HVMachinabilityBlanking workability24I0.425.12.44.6113000405poorgoodComparative Example25J0.472.23.84.869000336goodgoodExample26K0.3815.84.010.896000342goodgoodExample27L0.476.03.05.667000315goodgoodExample28M0.383.64.76.380000339goodgoodExample29N0.444.42.74.671000313goodgoodExample30O0.4112.94.910.583000357goodgoodExample31P0.454.02.54.363000319goodgoodExample32Q0.399.85.19.373000346goodgoodExample33R0.483.03.95.274000311goodgoodExample34s0.416.02.55.166000303goodgoodExample35T0.514.13.55.357000299goodgoodExample36U0.523.32.43.959000327goodgoodExample37a0.585.52.54.947000295poorgoodComparative Example38b0.660.71.51.846000293poorgoodComparative Example39c0.681.61.11.835000281poorgoodComparative Example40d0.502.31.82.847000277poorgoodComparative Example41cslab crack occurred during continuous castingComparative Example42fslab crack occurred during continuous castingComparative Example43gslab crack occurred during continuous castingComparative Example44hslab crack occurred during continuous castingComparative Example45islab crack occurred during continuous castingComparative Example46jslab crack occurred during continuous castingComparative Example*1: Average particle size of cementite particles having area of 0.06 µm 2< or more *2: Number density of voids having area of 0.01 µm 2< or more REFERENCE SIGNS LIST

[0130] 1test piece for machinability evaluation 2fixing jig 3dynamometer 10disk sample (cold-rolled steel sheet) 11rollover 12shear surface 13fracture surface drollover tsheet thickness

Examples

examples

[0100]In order to confirm the effects of the present disclosure, cold-rolled steel sheets were produced according to the following procedure.

[0101]First, steels having the chemical compositions listed in Table 1 were melted in a converter and made into steel slabs by continuous casting. Each steel slab was then subjected to heating, hot rolling, cooling, coiling, first annealing, cold rolling, second annealing, and final cold rolling in sequence to produce a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm. Each process was carried out under the conditions listed in Tables 2 and 3, and the first annealing, cold rolling, and second annealing were carried out the number of times listed in Tables 2 and 3.

[0102]Next, for each of the obtained cold-rolled steel sheets, the average particle size of cementite, the amount of Mn and Cr dissolved in the cementite particles, the number density of voids, and Vickers hardness were measured by the following procedures. The me...

TECHNICAL FIELD

[0001] The present disclosure relates to cold-rolled steel sheets, and more particularly to a cold-rolled steel sheet having excellent machinability and blanking workability, and a method of producing the same.

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

[0003] On the other hand, steel components such as textile machinery components, bearing components, machine blades, and household blades are cut using metal saws, milling cutters, end mills, and the like during a forming process before quenching. When the cold-rolled steel sheet used as a material has poor machinability, cutting tools wear out quickly, which increases the frequency of cutting tool replacement, which is disadvantageous in terms of productivity and costs. Therefore, cold-rolled steel sheets are required to have excellent machinability.

[0004] Further, in the case of steel components such as knitting needles that are components in textile machinery, the outer shape is cut out by blanking prior to cutting work. Therefore, cold-rolled steel sheets are required to have excellent blanking workability, that is, to be less likely to produce burrs or rollover on blanked end surfaces.

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

[0006] For example, in Patent Literature (PTL) 1, a technology is proposed for improving the machinability of steel containing C: 0.70 mass% to 1.10 mass%, by controlling the number and average particle size of carbides in a spheroidized microstructure.

[0007] Further, in PTL 2, a technology is proposed for improving the machinability of steel by controlling an average value of C content to a specific range in a region from a outer periphery to a depth of 200 µm in a steel wire rod containing C: 0.75 mass% to 1.2 mass%.

[0008] In PTL 3 and PTL 4, technologies are proposed for improving blanking workability in steel sheets containing C: 0.30 mass% to 1.30 mass%, by controlling a ratio of a number of carbides on ferrite grain boundaries to a number of carbides inside ferrite grains within a specific range.

[0009] In PTL 5, a technology is proposed for improving blanking workability by introducing 100 or more voids per mm 2< into a steel microstructure of a steel sheet containing C: 0.70 mass% to 0.95 mass%.CITATION LISTPatent Literature

[0010] PTL 1: JP 2005-307320 A PTL 2: JP 2001-049388 A PTL 3: JP 2008-303415 A PTL 4: JP 2009-215612 A PTL 5: JP 2011-012316 A SUMMARY(Technical Problem)

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

[0012] For example, the technology proposed in PTL 1 improves the machinability of steel by limiting the number of carbides. However, the steel is soft, and therefore significant rollover occurs during blanking.

[0013] Further, the technology proposed in PTL 2 improves machinability by decreasing an amount of C in a 200 µm surface layer through decarburization. However, for applications such as knitting needles, thin cold-rolled steel sheets having a thickness of about 0.4 mm are used, and the technology of PTL 2 cannot be applied to such thin cold-rolled steel sheets. This is because the technology of PTL 2 decreases the amount of C over almost the entire thickness, resulting in insufficient strength. Further, the steel of PTL 2 is also soft, and therefore significant rollover occurs when blanking.

[0014] In the technologies proposed in PTL 3 and PTL 4, blanking workability is improved by increasing the number of carbides within ferrite grains compared to the number of carbides at ferrite grain boundaries. However, even when a steel sheet is obtained by this method, when the steel sheet is soft, significant rollover occurs when blanking. On the other hand, when the steel sheet is hard, machinability is insufficient.

[0015] Further, the technology proposed in PTL 5 makes it possible to decrease occurrence of burrs when blanking by introducing voids into the steel microstructure. However, the steel sheet is soft, and therefore significant rollover occurs when blanking. Further, machinability was insufficient.

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

[0017] The present disclosure is made in view of the above circumstances, and it would be helpful to achieve both excellent machinability and blanking workability in a cold-rolled steel sheet containing a high carbon content of 0.80 mass% to 1.25 mass%.(Solution to Problem)

[0018] As a result of studies, the inventors arrived at the following discoveries. (1) By appropriately controlling a particle size of cementite, Mn amount and Cr amount in the cementite, and a number density of voids, the machinability of the cold-rolled steel sheet can be improved. (2) By appropriately controlling Vickers hardness, the blanking workability of the cold-rolled steel sheet can be improved. (3) By appropriately controlling a chemical composition of a steel slab used and production conditions of the cold-rolled steel sheet, it is possible to appropriately control the particle size of cementite, the Mn amount and Cr amount in the cementite, the number density of voids, and the Vickers hardness.

[0019] The present disclosure is based on the discoveries described above, and primary features of the present disclosure are as described below. 1. A cold-rolled steel sheet comprising a chemical composition containing (consisting of), in mass%, C: 0.80 % to 1.25 %, Si: 0.10 % to 1.0 %, Mn: 0.20 % to 3.0 %, P: 0.001 % to 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 %, with the balance being Fe and inevitable impurity, and the total content of Nb, Ti, and V being 0.24 mass% or less, wherein an average particle size of cementite particles having an area of 0.06 µm 2< or more is 0.65 µm or less, and an A value defined by the following expression (1) is 2.0 mass% or more, a number density of voids having an area of 0.01 µm 2< or more is 50,000 voids / mm 2< or more, a Vickers hardness is 200 HV or more and 400 HV or less, A = 3 / 7 C Mn + C Cr where, in expression (1), C Mn is Mn content in mass% in the cementite particles having an area of 0.06 µm 2< or more, and C Cr is Cr content in mass% in the cementite particles having an area of 0.06 µm 2< or more. 2. The cold-rolled steel sheet according to 1, above, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of: 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. 3. A method of producing a cold-rolled steel sheet, the method comprising: heating a steel slab comprising the chemical composition according to 1 or 2, above, under conditions including a slab heating temperature of 1100 °C or higher and a slab heating time of 60 min or longer; hot rolling the heated steel slab under conditions including a rolling finish temperature exceeding Tc, as defined by the following expression (2), to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet under conditions including an average cooling rate of 20 °C / s or more and a cooling stop temperature of Tc or lower; coiling the cooled hot-rolled steel sheet at a coiling temperature of 530 °C or higher and Tc or lower; subjecting the hot-rolled steel sheet after coiling to first annealing once or more, under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer; and subjecting the hot-rolled steel sheet after the first annealing to cold rolling at a rolling ratio of 15 % or more and second annealing at an annealing temperature of 600 °C or higher and Tc or lower for an annealing time of 3 h or longer, once or more, and then final cold rolling at a rolling ratio of 20 % or more and 80 % or less, where the element symbols in expression (2) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0. (Advantageous Effect)

[0020] According to the present disclosure, it is possible to achieve both excellent machinability and blanking workability in a cold-rolled steel sheet containing a high carbon content of 0.80 mass% to 1.25 mass%. Therefore, the cold-rolled steel sheet is very well suited as a material for various steel components, including textile machinery components, bearing components, machine blades, and household blades.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the accompanying drawings: FIG. 1 is a schematic diagram illustrating a cutting test method; FIG. 2 is a schematic cross-section diagram illustrating a state of a test piece cut in a cutting test; FIG. 3 is a schematic diagram of a graph in which cutting resistance F (N) measured in a cutting test is approximated by a cubic spline curve; and FIG. 4 is a cross-section diagram illustrating a state of an end face of a disk sample 10 after blanking. DETAILED DESCRIPTION

[0022] A detailed description is provided below. The present disclosure is not limited to the following embodiments.[Chemical composition]

[0023] The cold-rolled steel sheet according to the present disclosure has the chemical composition described above. The reasons for the above limitations are described below. Hereinafter, "%" as a unit of content indicates "mass%" unless otherwise specified.C: 0.80 % to 1.25 %

[0024] Carbon is an essential element in material for various steel components, including textile machinery components, bearing components, machine blades, and household blades, in order to improve hardness after quenching. Further, C is an element necessary for producing cementite. Further, according to the present disclosure, the number density of voids is controlled as described below, but when C content is less than 0.80 %, the required number density of voids cannot be obtained. This is because cementite serves as the initiation point for void formation. The C content is therefore 0.80 % or more. The C content is preferably 0.85 % or more. The C content is more preferably 0.90 % or more. On the other hand, when the C content exceeds 1.25 %, surface scale becomes firm during slab heating, resulting in degradation of surface characteristics. Further, when the C content exceeds 1.25 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The C content is therefore 1.25 % or less. The C content is preferably 1.20 % or less. The C content is more preferably 1.10 % or less.Si: 0.10 % to 1.0 %

[0025] Si is an element having an effect of increasing strength of the cold-rolled steel sheet by solid solution strengthening. To obtain the above effect, Si content is 0.10 % or more. The Si content is preferably 0.15 % or more. The Si content is more preferably 0.20 % or more. On the other hand, an excess of Si degrades surface characteristics as a result of surface scale becoming firm during heating. Further, when the Si content exceeds 1.0 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Si content is therefore 1.0 % or less. The Si content is preferably 0.80 % or less. The Si content is more preferably 0.55 % or less.Mn: 0.20 % to 3.0 %

[0026] Mn is an element that dissolves in cementite, and improves the hardness of cementite and increases the number density of voids. Further, the growth of cementite during annealing becomes rate-determined by the diffusion rate of Mn, and coarsening of cementite can be suppressed. However, when Mn content is less than 0.20 %, the required number density of voids cannot be obtained. The Mn content is therefore 0.20 % or more. The Mn content is preferably 0.30 % or more. The Mn content is more preferably 0.40 % or more. The Mn content is even more preferably 0.50 % or more. On the other hand, when the Mn content exceeds 3.0 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Mn content is therefore 3.0 % or less. The Mn content is preferably 2.50 % or less. The Mn content is more preferably 2.0 % or less. The Mn content is even more preferably 1.5 % or less.P: 0.001 % to 0.05 %

[0027] The addition of a trace amount of P has a strength improving effect on the cold-rolled steel sheet due to solid solution strengthening. To achieve this effect, P content is 0.001 % or more. The P content is preferably 0.003 % or more. On the other hand, when the P content exceeds 0.05 %, the toughness of the steel slab decreases due to grain boundary embrittlement, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The P content is therefore 0.05 % or less. The P content is preferably 0.04 % or less.S: 0.03 % or less

[0028] S is an element that causes grain boundary embrittlement during slab casting, and when S is present in excess, cracks may occur during casting of the steel slab, resulting in a significant decrease in productivity. S content is therefore 0.03 % or less. The S content is preferably 0.02 % or less. The lower the S content, the better, and therefore there is no particular lower limit for the S content. Accordingly, the lower limit of the S content may be 0 %. However, excessively reducing the S content causes an increase in production costs. Therefore, from a production cost viewpoint, the S content is preferably 0.0001 % or more. The S content is more preferably 0.0005 % or more.Al: 0.001 % to 0.1 %

[0029] Al is an element necessary for deoxidation during steelmaking. Al content is therefore 0.001 % or more. On the other hand, when Al is excessive, nitrides and oxides are formed, which decreases the toughness of the steel slab, and cracks may occur during the slab production or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Al content is therefore 0.1 % or less. The Al content is preferably 0.08 % or less.N: 0.001 % to 0.01 %

[0030] N is an element that refines grain size by forming fine nitrides, thereby improving the strength of the cold-rolled steel sheet. N content is therefore 0.001 % or more. On the other hand, when N is excessive, N combines with Al to form nitrides, which decreases the toughness of the steel slab, and this may cause cracks during the slab production or during slab heating before hot rolling, resulting in a significant decrease in productivity. The N content is therefore 0.01 % or less. The N content is preferably 0.008 % or less.O: 0.0100 % or less

[0031] O is present as an oxide and is an element that causes embrittlement of the slab. When O is excessive, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. The O content is therefore 0.0100 % or less. The O content is preferably 0.0050 % or less. A lower limit of the O content is not particularly limited, and may be 0 %. However, excessively decreasing the O content causes an increase in production costs. Therefore, from the viewpoint of production costs, the O content is preferably 0.0001 % or more.Cr: 0.56 % to 2.0 %

[0032] Cr is an element that dissolves in cementite, and can improve the hardness of cementite and generate more voids. Further, the growth of cementite during annealing becomes rate-determined by the diffusion rate of Cr, and coarsening of cementite can be suppressed. To obtain the above effects, Cr content is 0.56 % or more. The Cr content is preferably 0.60 % or more. The Cr content is more preferably 0.70 % or more. On the other hand, when the Cr content exceeds 2.0 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Cr content is therefore 2.0 % or less. The Cr content is preferably 1.5 % or less. The Cr content is more preferably 1.3 % or less.

[0033] The chemical composition described above contains at least one element selected from the group consisting of Nb, Ti, and V. In order to obtain a desired void number density, it is necessary to contain at least one of Nb, Ti, or V in the following amounts.Nb: 0.029 % to 0.24 %

[0034] Nb is an element that forms carbides and can cause more voids to be generated in the cold-rolled steel sheet. When Nb is contained, in order to obtain these effects, Nb content is 0.029 % or more. On the other hand, when the Nb content is excessive, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Nb content is therefore 0.24 % or less. The Nb content is preferably 0.20 % or less. The Nb content is more preferably 0.10 % or less.Ti: 0.01 % to 0.21 %

[0035] Ti is an element that forms carbides and can cause more voids to be generated in the cold-rolled steel sheet. When Ti is contained, in order to obtain these effects, Ti content is 0.01 % or more. On the other hand, when the Ti content is excessive, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The Ti content is therefore 0.21 % or less. The Ti content is preferably 0.15 % or less. The Ti content is more preferably 0.10 % or less.V: 0.01 % to 0.21 %

[0036] V is an element that forms carbides and can cause more voids to be generated in the cold-rolled steel sheet. When V is contained, in order to obtain these effects, V content is 0.01 % or more. On the other hand, when the V content is excessive, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The V content is therefore 0.21 % or less. The V content is preferably 0.15 % or less. The V content is more preferably 0.10 % or less.Nb + Ti + V: 0.24 % or less

[0037] When the total content of Nb, Ti, and V in the chemical composition exceeds 0.24 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. The total content of Nb, Ti, and V is therefore 0.24 % or less. The total content is preferably 0.22 % or less. The total content is more preferably 0.20 % or less. On the other hand, a lower limit of the total content is not particularly limited, but 0.01 % is a practical lower limit. The lower limit corresponds to a case where only either Ti or V is contained in an amount of 0.01 %. The total content may be 0.020 % or more. The total content may be 0.025 % or more. The total content may be 0.030 % or more.

[0038] The cold-rolled steel sheet according to an embodiment of the present disclosure has a chemical composition consisting of the above components, with the balance being Fe and inevitable impurity. The inevitable impurity includes H.

[0039] Further, the chemical composition of the cold-rolled steel sheet according to another embodiment of the present disclosure may optionally further contain at least one of the following elements.Ta: 0.10 % or less

[0040] Ta has an effect of further improving the wear resistance of steel components by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when Ta content exceeds 0.10 %, a large amount of coarse precipitates and inclusions are formed, and the toughness of the steel slab decreases. The Ta content is therefore 0.10 % or less. The Ta content is preferably 0.08 % or less. On the other hand, a lower limit of the Ta content is not particularly limited. However, from the viewpoint of enhancing the effect of Ta addition, the Ta content is preferably 0.01 % or more.W: 0.10 % or less

[0041] W has an effect of further improving the wear resistance of steel components by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when W content exceeds 0.10 %, a large amount of coarse precipitates and inclusions are formed, and the toughness of the steel slab decreases. The W content is therefore 0.10 % or less. The W content is preferably 0.08 % or less. On the other hand, a lower limit of the W content is not particularly limited. However, from the viewpoint of enhancing the effect of W addition, the W content is preferably 0.01 % or more.B: 0.0100 % or less

[0042] B segregates at austenite grain boundaries during hot rolling or annealing, and has an effect of further improving hardenability. However, when B content exceeds 0.0100 %, the toughness of the steel slab decreases. The B content is therefore 0.0100 % or less. The B content is more preferably 0.0080 % or less. On the other hand, a lower limit of the B content is not particularly limited. From the viewpoint of enhancing the effect of B addition, the B content is preferably 0.0003 % or more.Mo: 1.00 % or less

[0043] Mo is an element that has an effect of further improving hardenability. However, when Mo content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Mo content is therefore 1.00 % or less. The Mo content is preferably 0.80 % or less. On the other hand, a lower limit of the Mo content is not particularly limited. However, from the viewpoint of enhancing the effect of Mo addition, the Mo content is preferably 0.01 % or more.Co: 1.00 % or less

[0044] Co is an element that has an effect of further improving hardenability. However, when Co content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Co content is therefore 1.00 % or less. The Co content is preferably 0.80 % or less. On the other hand, a lower limit of the Co content is not particularly limited. From the viewpoint of enhancing the effect of Co addition, the Co content is preferably 0.001 % or more.Ni: 1.00 % or less

[0045] Ni is an element that has an effect of further improving hardenability. However, when Ni content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Ni content is therefore 1.00 % or less. The Ni content is preferably 0.80 % or less. On the other hand, a lower limit of the Ni content is not particularly limited. However, from the viewpoint of enhancing the effect of Ni addition, the Ni content is preferably 0.01 % or more.Cu: 1.00 % or less

[0046] Cu is an element that has an effect of further improving hardenability. However, when Cu content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Cu content is therefore 1.00 % or less. The Cu content is preferably 0.80 % or less. On the other hand, a lower limit of the Cu content is not particularly limited. However, from the viewpoint of enhancing the effect of Cu addition, the Cu content is preferably 0.01 % or more.Sn: 0.200 % or less

[0047] Sn is an element that has an effect of further improving hardenability. However, when Sn content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Sn content is therefore 0.200 % or less. The Sn content is preferably 0.100 % or less. On the other hand, a lower limit of the Sn content is not particularly limited. However, from the viewpoint of enhancing the effect of Sn addition, the Sn content is preferably 0.001 % or more.Sb: 0.200 % or less

[0048] Sb is an element that has an effect of suppressing decarburization. By suppressing decarburization, the strength of the steel sheet can be further improved. However, when Sb content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Sb content is therefore 0.200 % or less. The Sb content is preferably 0.100 % or less. On the other hand, a lower limit of the Sb content is not particularly limited. However, from the viewpoint of enhancing the effect of Sb addition, the Sb content is preferably 0.001 % or more.Ca: 0.0100 % or less

[0049] Ca is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Ca content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Ca content is therefore 0.0100 % or less. The Ca content is preferably 0.0050 % or less. On the other hand, a lower limit of the Ca content is not particularly limited. However, from the viewpoint of enhancing the effect of Ca addition, the Ca content is preferably 0.0005 % or more.Mg: 0.0100 % or less

[0050] Mg is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Mg content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Mg content is therefore 0.0100 % or less. The Mg content is preferably 0.0050 % or less. On the other hand, a lower limit of the Mg content is not particularly limited. However, from the viewpoint of enhancing the effect of Mg addition, the Mg content is preferably 0.0005 % or more.REM: 0.0100 % or less

[0051] REM (rare earth metals) are elements that have an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when REM content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The REM content is therefore 0.0100 % or less. The REM content is preferably 0.0050 % or less. On the other hand, a lower limit of the REM content is not particularly limited. However, from the viewpoint of enhancing the effect of REM addition, the REM content is preferably 0.0005 % or more.Zr: 0.100 % or less

[0052] Zr is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Zr content exceeds 0.100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Zr content is therefore 0.100 % or less. The Zr content is preferably 0.080 % or less. On the other hand, a lower limit of the Zr content is not particularly limited. However, from the viewpoint of enhancing the effect of Zr addition, the Zr content is preferably 0.001 % or more.Te: 0.100 % or less

[0053] Te is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Te content exceeds 0.100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Te content is therefore 0.100 % or less. The Te content is preferably 0.080 % or less. On the other hand, a lower limit of the Te content is not particularly limited. However, from the viewpoint of enhancing the effect of Te addition, the Te content is preferably 0.001 % or more.Hf: 0.10 % or less

[0054] Hf is an element that has an effect of spheroidizing the shape of nitrides and sulfides and improving ultimate deformability of the steel sheet. However, when Hf content exceeds 0.10 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. Th Hf content is therefore 0.10 % or less. The Hf content is preferably 0.08 % or less. On the other hand, a lower limit of the Hf content is not particularly limited. However, from the viewpoint of enhancing the effect of Hf addition, the Hf content is preferably 0.01 % or more.Bi: 0.200 % or less

[0055] Bi is an element that mitigates segregation. However, when Bi content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Bi content is therefore 0.200 % or less. The Bi content is preferably 0.100 % or less. On the other hand, a lower limit of the Bi content is not particularly limited. However, from the viewpoint of enhancing the effect of Bi addition, the Bi content is preferably 0.001 % or more.

[0056] The elements from Ta to Bi have effects of further improving the properties of the cold-rolled steel sheet of the present disclosure, and can be added to the steel sheet of the present disclosure. In the above description, the lower limit values of the preferred content of these optionally-added elements are indicated. However, when the content of these elements is lower than the preferred lower limit, the effect of adding the elements is decreased, but the properties of the cold-rolled steel sheet still satisfy requirements. Accordingly, the addition of these elements is not essential, and the lower limit of content may be 0 %.[Cementite]

[0057] The following describes cementite contained in the cold-rolled steel sheet of the present disclosure.Average particle size of cementite: 0.65 µm or less

[0058] According to the present disclosure, machinability and blanking 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. When the average particle size of cementite particles having an area of 0.06 µm 2< or more is larger than 0.65 µm, the number density of voids cannot be set within a desired range. The average particle size of cementite particles having an area of 0.06 µm 2< or more is therefore 0.65 µm or less. The average particle size of cementite is preferably 0.63 µm or less. The average particle size of cementite is more preferably 0.60 µm or less. The average particle size of cementite is even more preferably 0.55 µm or less. The average particle size of cementite is most preferably 0.40 µm or less. Hereinafter, the "average particle size of cementite particles having an area of 0.06 µm 2< or more" may be simply referred to as the "average particle size of cementite". On the other hand, a lower limit of the average particle size of the cementite is not particularly limited, but from the viewpoint of manufacturability, the average particle size of the cementite may be 0.30 µm or more, or 0.35 µm or more.

[0059] The average particle size of cementite is defined as a value measured by electron backscatter diffraction (EBSD) at a cross-section in the rolling direction at a 1 / 2 thickness position of the cold-rolled steel sheet. More specifically, measurements may be made by a method described in the EXAMPLES section of the present disclosure.A value: 2.0 mass% or more

[0060] When Mn and Cr are dissolved in cementite, the hardness of the cementite increases, and the harder the cementite, the more easily voids are generated in the cold-rolled steel sheet. Therefore, according to the present disclosure, the amounts of Mn and Cr present in cementite are controlled in order to increase the number density of voids. Specifically, the A value defined by the following expression (1) is 2.0 mass% or more. When the A value is less than 2.0 mass%, the void generation ability of cementite decreases, and as a result, obtaining a desired number density of voids becomes difficult. The A value is preferably 3.0 mass% or more. The A value is more preferably 4.0 mass% or more. The A value is even more preferably 5.0 mass% or more. A = 3 / 7 C Mn + C Cr

[0061] Here, in expression (1), C Mn is Mn content in mass% in the cementite particles having an area of 0.06 µm 2< or more, and C Cr is Cr content in mass% in the cementite particles having an area of 0.06 µm 2< or more. On the other hand, although there is no particular upper limit to the A value, in order to increase the A value, increasing the amounts of Mn and Cr added as alloying elements is necessary, and when the A value exceeds 20 mass%, manufacturability is adversely affected. Therefore, from the viewpoint of manufacturability, the A value is preferably 20 mass% or less. The A value is more preferably 15 mass% or less. The A value is even more preferably 12 mass% or less.

[0062] C Mn and C Cr are defined as values measured by an electron probe microanalyzer (EPMA) at a cross-section in the rolling direction at a 1 / 2 thickness position of the cold-rolled steel sheet. More specifically, measurements may be made by a method described in the EXAMPLES section of the present disclosure.[Voids]

[0063] The following is a description of the voids in the cold-rolled steel sheet.Number density of voids: 50,000 / mm 2< or more

[0064] The number density of voids having an area of 0.01 µm 2< or more is very important for improving the machinability and blanking workability of the cold-rolled steel sheet. The reasons for this are as follows.

[0065] When the cold-rolled steel sheet is cut, the material first undergoes plastic deformation, and voids are generated at locations of high plastic strain. The generated voids then grow, and the cold-rolled steel sheet is cut by the voids connecting. At this time, a large cutting resistance is exhibited, particularly while the material undergoes plastic deformation and until voids are generated. Accordingly, by dispersing voids in the cold-rolled steel sheet in advance, the cutting resistance can be decreased and the machinability of the cold-rolled steel sheet can be improved.

[0066] Further, when the cold-rolled steel sheet is blanked, the material undergoes plastic deformation and voids are generated in areas of high plastic strain, just as in cutting work. The generated voids then grow and fracture occurs due to voids connecting, completing the blanking. At this time, rollover occurs due to the blanking, while the material undergoes plastic deformation and until voids are generated. Accordingly, by dispersing voids in the cold-rolled steel sheet in advance, the amount of plastic deformation until fracture during blanking can be decreased, and rollover can be decreased. In this way, blanking workability is improved.

[0067] Therefore, in order to obtain excellent machinability and blanking workability, the number density of voids having an area of 0.01 µm 2< or more is 50,000 / mm 2< or more. The number density of voids is preferably 80,000 / mm 2< or more. The number density of voids is more preferably 100,000 / mm 2< or more. On the other hand, an upper limit of the number density of voids is not particularly limited. The number density of voids is preferably 1,000,000 / mm 2< or less.

[0068] The number density of voids is defined as a value measured using a scanning electron microscope (SEM) on a cross-section of the cold-rolled steel sheet in the rolling direction at a 1 / 2 thickness position of the sheet. More specifically, measurements may be made by a method described in the EXAMPLES section of the present disclosure.[Vickers hardness]

[0069] The following describes Vickers hardness of the cold-rolled steel sheet of the present disclosure.Vickers hardness: 200 HV to 400 HV

[0070] The Vickers hardness of the cold-rolled steel sheet has a large effect on blanking workability and machinability. When the Vickers hardness of the cold-rolled steel sheet is less than 200 HV, significant rollover occurs during blanking, resulting in poor blanking workability. In order to obtain the desired blanking workability, the cold-rolled steel sheet has a Vickers hardness of 200 HV or more. The Vickers hardness is preferably 240 HV or more. The Vickers hardness is more preferably 280 HV or more. The Vickers hardness is even more preferably 300 HV or more. On the other hand, when 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 400 HV or less. The Vickers hardness is preferably 360 HV or less. The Vickers hardness is more preferably 340 HV or less.

[0071] The Vickers hardness is defined as a value at a cross-section in the rolling direction at a 1 / 2 thickness position of the cold-rolled steel sheet. More specifically, measurements may be made by a method described in the EXAMPLES section of the present disclosure.[Sheet thickness]

[0072] The sheet thickness of the cold-rolled steel sheet is not particularly limited and may be any thickness. The sheet thickness is preferably 0.1 mm or more. The sheet thickness is more preferably 0.2 mm or more. Further, an upper limit of the sheet thickness is not particularly limited. The sheet thickness is particularly 2.5 mm or less. The sheet thickness is more preferably 1.6 mm or less. The sheet thickness is even more preferably 0.8 mm or less. When the sheet thickness is 0.2 mm or more and 0.8 mm or less, the cold-rolled steel sheet is particularly suitable for use as a material for textile machinery components such as knitting needles and the like.[Method of producing cold-rolled steel sheet]

[0073] The following describes a method of producing the cold-rolled steel sheet according to an embodiment. In the following explanation, temperature indicated in "°C" indicates a surface temperature (the temperature at a surface of the steel slab, the steel sheet, or the like).

[0074] The cold-rolled steel sheet may be produced by performing the following processes in sequence, starting with a steel slab having the chemical composition described above. (1) Heating (2) Hot rolling (3) Cooling (4) Coiling (5) First annealing (6) Cold rolling (7) Second annealing (8) Final cold rolling (1) Heating

[0075] First, a steel slab having the chemical composition described above is heated. The method of 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-mentioned chemical composition can be produced in a converter and then formed into a steel slab by a casting method such as continuous casting. Further, casting methods other than continuous casting, such as ingot casting followed by blooming, may be used. Further, the steel slab may be produced by an electric furnace steelmaking process. In such a case, scrap may be used as the raw material.Slab heating temperature: 1100 °C or higher

[0076] Heating is carried out prior to hot rolling to homogenize the components and dissolve carbides such as cementite and segregations present in the steel slab. However, when the heating temperature of the steel slab (slab heating temperature) during the heating is lower than 1100 °C, cementite cannot be sufficiently dissolved, and as a result, the average particle size of cementite in the finally obtained cold-rolled steel sheet cannot be set within the desired range. The slab heating temperature is therefore 1100 °C or higher. The slab heating temperature is preferably 1150 °C or higher. On the other hand, an upper limit of the slab heating temperature is not particularly limited. The slab heating temperature is preferably 1350 °C or lower.Slab heating time: 60 min or longer

[0077] When the heating time of the steel slab (slab heating time) during the heating is shorter than 60 min, cementite cannot be sufficiently dissolved, and as a result, the average particle size of cementite in the finally obtained cold-rolled steel sheet cannot be set within the desired range. The slab heating time is therefore 60 min or longer. The slab heating time is preferably 90 min or longer. On the other hand, an upper limit of the slab heating time is not particularly limited. The slab heating time is preferably 300 min or shorter. The slab heating time is more preferably 240 min or shorter.

[0078] The heating may be carried out by any method, but use of a heating furnace is preferred.(2) Hot rolling

[0079] The heated steel slab is then hot rolled to obtain a hot-rolled steel sheet. In the hot rolling, rough rolling and finishing rolling may be carried out according to conventional methods.Rolling finish temperature: exceeding Tc

[0080] When the rolling finish temperature in the hot rolling is Tc or lower, coarse cementite is generated in the hot-rolled steel sheet, and this coarse cementite remains in the finally obtained cold-rolled steel sheet. As a result, the desired cementite grain size cannot be obtained, and the desired machinability and blanking workability cannot be obtained. Therefore, the rolling finish temperature in the hot rolling exceeds Tc. On the other hand, an upper limit of the rolling finish temperature is not particularly limited, and may be 980 °C or lower, or 950 °C or lower.

[0081] Here, Tc (°C) is a value that can be used as an index of the temperature at which a transformation involving cementite occurs, and is calculated by the following expression (2).

[0082] Here, the element symbols in expression (2) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0.(3) CoolingAverage cooling rate: 20 °C / s or more

[0083] After the finishing rolling is completed, cooling is started, then stopped at a cooling stop temperature. When the average cooling rate in the cooling is less than 20 °C / s, the average particle size of cementite cannot be set within the desired range. This is because a slow cooling rate results in the formation of coarse cementite, which remains in the finally obtained cold-rolled steel sheet. Therefore, the average cooling rate is 20 °C / s or more. The average cooling rate is preferably 30 °C / s or more. The average cooling rate is more preferably 50 °C / s or more. On the other hand, although there is no particular upper limit to the average cooling rate, when the cooling rate is excessively high, controlling the cooling stop temperature becomes difficult. The average cooling rate is therefore preferably 500 °C / s or less.Cooling stop temperature: Tc or lower

[0084] When the cooling stop temperature in the cooling is higher than Tc, the average particle size of cementite cannot be set within the desired range. This is because when the cooling stop temperature is high, coarse cementite is formed, and this coarse cementite remains in the finally obtained cold-rolled steel sheet. The cooling stop temperature is therefore Tc or lower. The cooling stop temperature is preferably Tc - 20 °C or lower. On the other hand, a lower limit of the cooling stop temperature is not particularly limited. According to an embodiment of the present disclosure, the cooling stop temperature may be 530 °C or higher. Typically, the cooling stop temperature is preferably equal to or higher than a coiling temperature in the subsequent coiling.(4) CoilingCoiling temperature: 530 °C or higher and Tc or lower

[0085] After the cooling is stopped, the cooled hot-rolled steel sheet is coiled. At this time, when the coiling temperature exceeds Tc, coarse cementite is generated, and this coarse cementite remains in the finally obtained cold-rolled steel sheet. The coiling temperature is therefore Tc or lower. The coiling temperature is preferably Tc - 20 °C or lower. The coiling temperature is more preferably Tc - 40 °C or lower. On the other hand, when the coiling temperature is lower than 530 °C, volume expansion due to transformation during coiling results in a poor coiling shape. The coiling temperature is therefore 530 °C or higher. The coiling temperature is preferably 550 °C or higher. The coiling temperature is more preferably 600 °C or higher.(5) First annealing

[0086] Annealing temperature: 600 °C or higher and Tc or lower Annealing time: 3 h or longer

[0087] The hot-rolled steel sheet after the coiling is subjected to the first annealing under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer. The microstructure of the hot-rolled steel sheet after the coiling is a pearlitic microstructure lined with plate-like cementite and ferrite. The pearlitic microstructure is stable, and therefore does not homogenize without prolonged holding at a high temperature. In order to break up the pearlitic microstructure and allow the subsequent cold rolling and annealing process to produce the desired cementite, the annealing temperature needs to be 600 °C or higher and the annealing time needs to be 3 h or longer. The annealing temperature is preferably 620 °C or higher. Further, the annealing time is preferably 4 h or longer. On the other hand, when the annealing temperature is higher than Tc, phase transformation starts preferentially from one portion, and a locally coarse microstructure is formed. As a result, the microstructure becomes non-uniform, and the desired cementite particle size cannot be obtained. The annealing temperature is therefore Tc or lower. The annealing temperature is preferably Tc - 20 °C or lower. The annealing temperature is more preferably Tc - 40 °C or lower. On the other hand, although there is no particular upper limit to the annealing time, when too long, productivity decreases. The annealing time is therefore preferably 50 h or shorter. The annealing time is more preferably 30 h or shorter.

[0088] By carrying out the first annealing under the above conditions, the pearlitic microstructure can be broken up, and the desired cementite can be more easily formed in the subsequent cold rolling and annealing processes.

[0089] The first annealing can be carried out any number of times, but from the viewpoint of enhancing the effect of the annealing, the first annealing is preferably carried out two or more times. On the other hand, there is no particular upper limit to the number of times the first annealing is carried out, but even when the first annealing is carried out more than three times, the effect is saturated. The first annealing is therefore preferably carried out three times or less. When the first annealing is carried out two or more times, each annealing may be carried out under the above conditions. The annealing conditions for each time may be the same or different from each other.

[0090] Prior to the first annealing, the hot-rolled steel sheet is preferably pickled.(6) Cold rolling(7) Second annealing

[0091] Plate-like cementite is formed in the steel sheet after hot rolling. This plate-like cementite is stable and therefore prone to remain for a long time. The plate-like cementite is coarse, and therefore when 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 blanking workability cannot be obtained. Therefore, in order to refine and spheroidize the plate-like cementite by heating during annealing, the hot-rolled steel sheet after the first annealing is subjected to cold rolling and second annealing at least once.Rolling ratio: 15 % or more

[0092] The cold rolling causes the cementite to be deformed, broken up, and fragmented, thereby making it possible to obtain a desired average cementite particle size. When the rolling ratio in the cold rolling is less than 15 %, the effect cannot be obtained. The rolling ratio is therefore 15 % or more. The rolling ratio is preferably 25 % or more. The rolling ratio is more preferably 35 % or more. The rolling ratio is even more preferably 45 % or more. On the other hand, an upper limit of the rolling ratio is not particularly limited. The rolling ratio is preferably 85 % or less. The rolling ratio is more preferably 80 % or less. Annealing temperature: 600 °C or higher and Tc or lower Annealing time: 3 h or longer

[0093] In the second annealing, the cementite that has been deformed, broken up, and fragmented by the cold rolling can be refined and spheroidized. At the same time, Mn and Cr can be concentrated in the cementite. To obtain these effects, the annealing temperature in the second annealing needs to be 600 °C or higher, and the annealing time needs to be 3 h or longer. The annealing temperature is preferably 620 °C or higher. Further, the annealing time is preferably 4 h or longer. On the other hand, when the annealing temperature is higher than Tc, phase transformation starts preferentially from one portion, and a locally coarse microstructure is formed. As a result, the microstructure becomes non-uniform, and the desired cementite particle size cannot be obtained. The annealing temperature is therefore Tc or lower. The annealing temperature is preferably Tc - 20 °C or lower. The annealing temperature is more preferably Tc - 40 °C or lower. On the other hand, although there is no particular upper limit to the annealing time, when too long, productivity decreases. The annealing time is therefore preferably 30 h or shorter. The annealing time is more preferably 20 h or shorter.

[0094] By carrying out the cold rolling and the 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.

[0095] The cold rolling and the second annealing can be carried out any number of times, but from the viewpoint of enhancing the above-described effects, the process is preferably carried out two or more times. When the cold rolling and the second annealing are carried out two or more times, the cold rolling and the second annealing may be repeated alternately. On the other hand, there is no particular upper limit to the number of times that the cold rolling and the second annealing are carried out, but even when the number of times is more than five, the effect is saturated. The cold rolling and the second annealing are therefore preferably carried out five times or less. When the cold rolling and the second annealing are carried out two or more times, each iteration of the cold rolling and the second annealing may be carried out under the above conditions. The conditions for each iteration may be the same or different from each other.

[0096] When the cold rolling and the second annealing are carried out only once, the rolling ratio of the cold rolling is preferably 70 % or more from the viewpoint of reliably refining cementite.(8) Final cold rolling

[0097] After the cold rolling and the second annealing are carried out as described above, the steel sheet is further subjected to final cold rolling. By carrying out the final cold rolling, the number density of voids can be adjusted to a desired range. Further, the desired Vickers hardness can be obtained.Rolling ratio: 20 % to 80 %

[0098] However, when the rolling ratio in the final cold rolling is less than 20 %, the effects described above cannot be obtained. The rolling ratio in the final cold rolling is therefore 20% or more. The rolling ratio is preferably 25 % or more. The rolling ratio is more preferably 30 % or more. On the other hand, when the rolling ratio in the final cold rolling exceeds 80 %, the Vickers hardness of the cold-rolled steel sheet may exceed 400 HV. When the Vickers hardness exceeds 400 HV, the desired machinability cannot be obtained. The rolling ratio of the final cold rolling is therefore 80 % or less. The rolling ratio is preferably 70 % or less. The rolling ratio is more preferably 60 % or less. The rolling ratio is even more preferably 50 % or less.

[0099] By satisfying the above conditions, it is possible to produce the cold-rolled steel sheet that has excellent machinability and blanking workability. The finally obtained cold-rolled steel sheet may be subjected to further optional surface treatment.EXAMPLES

[0100] In order to confirm the effects of the present disclosure, cold-rolled steel sheets were produced according to the following procedure.

[0101] First, steels having the chemical compositions listed in Table 1 were melted in a converter and made into steel slabs by continuous casting. Each steel slab was then subjected to heating, hot rolling, cooling, coiling, first annealing, cold rolling, second annealing, and final cold rolling in sequence to produce a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm. Each process was carried out under the conditions listed in Tables 2 and 3, and the first annealing, cold rolling, and second annealing were carried out the number of times listed in Tables 2 and 3.

[0102] Next, for each of the obtained cold-rolled steel sheets, the average particle size of cementite, the amount of Mn and Cr dissolved in the cementite particles, the number density of voids, and Vickers hardness were measured by the following procedures. The measurement results are listed in Tables 4 and 5. In Table 4, for convenience, the A values are rounded to two decimal places, but the determination of whether the A value satisfies the conditions of the present disclosure is made based on the value calculated by expression (1).(Average particle size of cementite)

[0103] Test pieces for microstructure observation were taken from the obtained cold-rolled steel sheets. Next, rolling direction cross-sections (L-sections) of the test pieces for microstructure observation were each polished to a mirror surface. Thereafter, finishing polishing was carried out using a colloidal silica solution.

[0104] Next, the polished surface of the test piece for microstructure observation was measured using electron backscatter diffraction (EBSD). The measurement was carried out under the conditions of an electron beam accelerating voltage of 20 keV, a measurement interval of 0.14 µm steps, a measurement field of view of 60 µm × 45 µm, and a measurement position at 1 / 2 thickness.

[0105] 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 produced by TSL Solutions was used. Next, from the identified cementite particles, only cementite particles having an area of 0.06 µm 2< or more were selected, and an average value was calculated.

[0106] The above processes were carried out at each of the three measurement positions, and an average of the three average values obtained was taken as the average particle size of cementite having an area of 0.06 µm 2< or more.(Mn amount and Cr amount in cementite)

[0107] Test pieces for microstructure observation were taken from the obtained cold-rolled steel sheets. L-sections of the test pieces for microstructure observation were each 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, using an electron probe micro analyzer (EPMA), the Mn amount and Cr amount were measured at three locations with respect to one cementite particle having an area of 0.06 µm 2< or more under the conditions of an electron beam accelerating voltage of 15 keV, an irradiation current of 1.0 × 10 -8< A, and a measurement time of 1000 ms, and an average value of the three locations was calculated. The Mn amount and Cr amount in ten cementite particles each having an area of 0.06 µm 2< or more were similarly measured, and the measured values of the ten particles were averaged to obtain the Mn amount and Cr amount in cementite.(Number density of voids)

[0108] Test pieces for microstructure observation were taken from the obtained cold-rolled steel sheets. L-sections of the test pieces for microstructure observation were each polished to a mirror surface. Thereafter, the polished surface of the test piece for microstructure observation was imaged at ten locations at a 1 / 2 thickness position using a scanning electron microscope (SEM) at an accelerating voltage of 15 keV and a magnification of 3000 times to obtain microstructure images.

[0109] From the obtained microstructure images, voids having an area of less than 0.01 µm 2< were excluded by image processing, and the number of voids having an area of 0.01 µm 2< or more was counted and divided by the area of the microstructure image to calculate the number density. Similar measurements were carried out in ten fields of view, and an average value of the obtained number densities was calculated to be the number density of voids having an area of 0.01 µm 2< or more.(Vickers hardness)

[0110] Test pieces for measuring Vickers hardness were taken from the obtained cold-rolled steel sheets. L-sections of the test pieces for measuring Vickers hardness were each polished to a mirror surface. Thereafter, measurements were made in accordance with JIS Z 2244:2009 at a 1 / 2 sheet thickness position, a load of 1 kgf, and five measurement points, and the measured values at the five points were averaged to obtain the Vickers hardness of the cold-rolled steel sheet.

[0111] Further, the machinability and the blanking workability of each of the obtained cold-rolled steel sheets were evaluated using the following procedure. The evaluation results are listed in Tables 4 and 5.(Machinability)

[0112] The machinability of the cold-rolled steel sheets was evaluated based on a cutting life distance in a cutting test. The specific procedure is described below with reference to the drawings. Here, FIG. 1 is a schematic diagram illustrating a cutting test method, and FIG. 2 is a schematic cross-section diagram illustrating a state of a cut test piece.• Creation of test piece

[0113] 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 illustrated in FIG. 1, the test piece 1 for machinability evaluation was attached to a fixing jig 2 by an adhesive. The adhesive used was Aron Alpha EXTRA ®< (Aron Alpha EXTRA is a registered trademark in Japan, other countries, or both), a fast-acting, multi-purpose instant adhesive produced by Aron Alpha Co., Ltd. The dimensions of the fixing jig 2 were 50 mm (width) × 150 mm (length) × 30 mm (depth). When the test piece was attached, it was fixed in a vice for 8 h or longer to secure flatness of the test piece.• Cutting test

[0114] Next, a cutting test was carried out using the test piece 1 for machinability evaluation. Specifically, first, the test piece 1 for machinability evaluation 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 test piece 1 for machinability evaluation in the direction indicated by arrow A, and the metal saw was moved in the direction indicated by arrow B to carry out cutting by down-cutting. The metal saw used was made of cemented carbide having a Vickers hardness of about 1680, a Young's modulus of about 570 GPa, and a specific gravity of about 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.

[0115] The metal saw was used to cut the test piece 1 for machinability evaluation over a total length of 150 mm from one end to the other end, and then the metal saw was returned to the one end, the cutting position was shifted downward (in the direction of arrow C) by 2 mm, and cutting was carried out again. By repeating the above procedure, cutting was carried out at intervals of 2 mm as illustrated in FIG. 2, and cutting was terminated when the metal saw broke.

[0116] The cutting conditions were a rotation speed of 3000 / min, a feed rate of 24 mm / min, and a depth of cut of 0.2 mm, and no cutting oil was used. The measurement resolution of the dynamometer 3 was set to 1000 Hz. In FIG. 2, reference sign 4 denotes a metal saw, and arrow ND indicates the thickness direction of the test piece 1 for machinability evaluation, that is, the cold-rolled steel sheet.

[0117] Next, a cutting life was calculated from the cutting resistance measured in the cutting test. FIG. 3 is a schematic diagram of a graph in which the cutting resistance F, in N, measured in the cutting test is approximated by a cubic spline function, and the horizontal axis represents the cutting distance in mm. The cutting resistance F was calculated from the cutting resistance in each direction measured by the dynamometer using the following expression (3). F = √ F x 2 + F y 2 + F z 2

[0118] Here, F is the cutting resistance, F x is the cutting resistance in the x-axis direction, F y is the cutting resistance in the y-axis direction, and F z is the cutting resistance in the z-axis direction.

[0119] The cutting life was determined from the cutting distance at which a large burr began to appear before the metal saw broke. Here, the point where a large burr began to appear (indicated by an arrow in FIG. 3) is the cutting distance where the gradient of the approximation curve is ≥ (maximum gradient) × 1 / 3, and this is defined as the cutting life distance. When the cutting life distance was 1148 mm or more, the machinability was rated as good, and when less than 1148 mm, the machinability was rated as poor.(Blanking workability)

[0120] The blanking workability of the cold-rolled steel sheets was evaluated based on a rollover ratio in a blanking test. The specific procedures were as follows.

[0121] First, test pieces were taken from the obtained cold-rolled steel sheets. The dimensions of the test pieces were 30 mm (width) x 30 mm (length), and the thickness was the same as that of the cold-rolled steel sheet.

[0122] Next, a center of each test piece was blanked out using a cylindrical punch having a diameter of 10.0 mm to obtain a disk sample having a diameter of 10 mm. The clearance in the blanking was 12.5 %. Here, the clearance is a percentage (%) of a gap between the punch and the die, relative to the thickness of the test piece.

[0123] The obtained disk sample was cut, and rollover d RD at an end surface in the rolling direction and rollover d TD at an end surface in the direction perpendicular to the rolling direction were measured using an optical microscope. FIG. 4 is a schematic cross-section diagram illustrating a state of an end face of a disk sample 10. Here, 11 is rollover, 12 is a shear surface, and 13 is a fracture surface, and a length d of the rollover 11 in the sheet thickness direction is used as d RD and d TD . The rollover ratio D (%) was calculated from the obtained rollover value and the sheet thickness t in mm of the disk sample 10 by the following expression (4). D = d RB + d TD / 2 / t × 100 %

[0124] When the calculated rollover ratio was 10 % or less, the blanking workability was rated as good, and when more than 10 %, the blanking workability was rated as poor.[Table 1]

[0125] Table 1Steel sample IDChemical composition (mass%)Nb + Ti + V (mass%)Tc (°C)RemarksCSiMnPSAlNOCrNbTiVOtherA0.910.210.510.0080.00360.0090.00340.00050.700.080---0.080735Disclosed steelB0.850.331.120.0050.00370.0080.00300.00210.560.0390.02--0.059730Disclosed steelC1.100.100.520.0070.00400.0040.00350.00071.800.101-0.08-0.181751Disclosed steelD1.250.320.820.0080.00330.0040.00290.00100.750.0950.060.08-0.235736Disclosed steelE1.050.150.500.0050.00130.0040.00330.00081.08-0.12--0.120740Disclosed steelF1.080.320.280.0090.00390.0920.00420.00061.31--0.19-0.190751Disclosed steelG0.980.230.750.0040.00070.0050.00330.00150.61-0.100.09-0.190732Disclosed steelH1.070.201.130.0110.00280.0030.00320.00051.420.094--Ta 0.05, Mo 0.4, Cu 0.12, Sb 0.010.094741Disclosed steelI0.970.551.010.0090.00190.0070.00450.00100.730.095---0.095741Disclosed steelJ1.130.150.350.0100.00140.0040.00290.00051.110.062--Mo 0.120.062742Disclosed steelK0.860.182.980.0060.00180.0050.00360.00100.930.136---0.136712Disclosed steelL0.920.801.070.0090.00380.0090.00520.00050.820.121--Zr 0.06, Tc 0.040.121749Disclosed steelM1.080.200.700.0450.02810.0420.00340.00111.590.0290.05--0.079748Disclosed steelN0.870.990.900.0080.00340.0090.00500.00120.870.078--Hf 0.05, Bi 0.080.078757Disclosed steelO1.090.242.450.0060.00230.0100.00340.00941.270.046--W 0.04, Mg 0.005, REM 0.0050.046725Disclosed steelP1.050.220.730.0360.01520.0080.00290.00130.750.192---0.192734Disclosed steelQ1.080.121.930.0040.00290.0710.00300.00091.460.097--Cu 0.15, Sb 0.010.097731Disclosed steelR0.850.340.490.0090.00200.0080.00940.00061.100.077--Ni 0.05, Cu 0.120.077745Disclosed steelS0.870.251.200.0050.00040.0030.00740.00090.750.140--Ca 0.0050.140730Disclosed steelT0.870.290.710.0040.00390.0030.00350.00120.990.031--B 0.00150.031741Disclosed steelU1.090.310.620.0090.00190.0090.00100.00060.750.184--Co 0.006, Sn 0.050.184738Disclosed steela0.750.281.020.0050.00280.0070.00350.00050.720.033---0.033732Comparative steelb0.870.240.150.0080.00230.0050.00380.00070.590.053---0.053738Comparative steelc0.890.350.350.0130.00210.0040.00380.00060.420.048---0.048737Comparative steeld0.810.260.450.0050.00210.0080.00350.00100.590.020---0.020736Comparative steele1.060.331.020.0090.00160.0100.00220.00161.090.261---0.261740Comparative steelf0.980.291.710.0090.00340.0080.00140.00121.050.1910.08--0.271731Comparative steelg1.100.330.770.0100.00180.0060.00320.00091.48-0.150.20-0.350749Comparative steelh1.081.341.720.0090.00320.0060.00430.00061.230.152---0.152764Comparative steeli1.020.423.540.0130.00420.0080.00520.00091.050.123---0.123715Comparative steelj1.050.531.940.0110.00180.0070.00350.00072.360.112---0.112758Comparative steel [Table 2]

[0126] Table 2NoSteel sample IDHeatingHot rollingCoolingCoilingFirst annealingCold rollingSecond annealingTimes *Final cold rollingRemarksSlab heating temp. (°C)Slab heating time (min)Rolling finish temp. (°C)Average cooling rate (°C / s)Cooling stop temp. (°C)Coiling temp. (°C)Heating temp. (°C)Annealing time (h)TimesRolling ratio (%)Heating temp. (°C)Holding time (h)Rolling ratio (%)1A1180709003068066071072456807235Example2B1200809108065062067082306307340Example3B12008091080650620670815063012240Example4B12008091080650620670827564010135Example5B1200809108065062067082306308340Example6C12209087540600580690625063010225Example7D117080860206205706702122564014530Example8E115070850506405907002124564011225Example9F112065845306606206901624067014235Example10G117080880357006606802022563012525Example11H12009086575590550710825066014225Example12I12209090050660650680102456409235Example13I12209090050660650680102706408135Example14I10303080060660650710102457008235Comparative Example15I12209070080660650710102457008235Comparative Example16I12209080010660650700102457008235Comparative Example17I122090900808007707001024570014235Comparative Example18I1220909005066065055022456006225Comparative Example19I12209090050660650770102456908235Comparative Example20I1220909005066065062031455502235Comparative Example21I122090900506606506801024577014235Comparative Example22I12209090050660650710102107009235Comparative Example23I12209090050660650680102456409210Comparative Example* Number of cold rolling and second annealing iterations [Table 3]

[0127] Table 3NoSteel sample IDHeatingHot rollingCoolingCoilingFirst anncalingCold rollingSecond annealingTimes *Final cold rollingRemarksSlab heating temp. (°C)Slab heating time (min)Rolling finish temp. (°C)Average cooling rate (°C / s)Cooling stop temp. (°C)Coiling temp. (°C)Heating temp. (°C)Annealing time (h)TimesRolling ratio (%)Heating temp. (°C)Holding time (h)Rolling ratio (%)24I12209090050660650680102456407290Comparative Example25J1230100950406306106701125065016235Example26K114060860306506007001623565013230Example27L11909092035570520680925066014230Example28M1180808752573071070062406303330Example29N1150708704570067067052306404335Example30O122090900506005707102124566010225Example31P129070850456005906801025064016230Example32Q107021084540580540700252356507220Example33R1250180910706205906802423066012240Example34S1150100930100550510680192506309225Example35T117070870355805807001325067012225Example36U116010090030570530650202506607235Example37a11908090545670640710723569014235Example38b11708090050710660700132456809240Example39c1200110910456406206901123068010225Example40d112055730256405906701622564012225Example41eslab crack occurred during continuous castingComparative Example42fslab crack occurred during continuous castingComparative Example43gslab crack occurred during continuous castingComparative Example44hslab crack occurred during continuous castingComparative Example45islab crack occurred during continuous castingComparative Example46jslab crack occurred during continuous castingComparative Example* Number of cold rolling and second annealing iterations [Table 4]

[0128] Table 4NoSteel sample IDCementiteVoidsVickers hardnessPropertiesRemarksAverage particle size *1 (µm)CMn (mass%)CCr (mass%)A value (3 / 7 C Mn + C Cr ) (mass%)Number density *2 (per mm 2< )HVMachinabilityBlanking workability1A0.642.52.03.150000303goodgoodExample2B0.435.21.74.074000310goodgoodExample3B0.435.21.74.074000310goodgoodExample4B0.455.51.84.268000305goodgoodExample5B0.435.21.74.074000310goodgoodExample6C0.373.15.67.077000338goodgoodExample7D0.454.32.54.464000283goodgoodExample8E0.452.53.14.257000317goodgoodExample9F0.512.04.75.669000333goodgoodExample10G0.453.71.93.554000256goodgoodExample11H0.436.75.38.274000347goodgoodExample12I0.445.12.44.671000321goodgoodExample13I0.445.12.44.671000321goodgoodExample14I0.715.62.65.039000321poorgoodComparative Example15I0.725.62.65.038000321poorgoodComparative Example16I0.685.62.65.040000321poorgoodComparative Example17I0.675.62.65.040000321poorgoodComparative Example18I0.664.01.93.727000311poorpoorComparative Example19I0.805.52.65.035000321poorgoodComparative Example20I0.673.31.63.131000321poorpoorComparative Example21I0.786.12.95.638000321poorgoodComparative Example22I0.675.62.65.040000321poorgoodComparative Example23I0.465.12.44.617000192poorpoorComparative Example*1: Average particle size of cementite particles having area of 0.06 µm 2< or more *2: Number density of voids having area of 0.01 µm 2< or more [Table 5]

[0129] Table 5NoSteel sample IDCementiteVoidsVickers hardnessPropertiesRemarksAverage particle size *1 (µm)C Mn (mass%)C Cr (mass%)A value (3 / 7 C Mn + C Cr ) (mass%)Number density *2 (per mm 2< )HVMachinabilityBlanking workability24I0.425.12.44.6113000405poorgoodComparative Example25J0.472.23.84.869000336goodgoodExample26K0.3815.84.010.896000342goodgoodExample27L0.476.03.05.667000315goodgoodExample28M0.383.64.76.380000339goodgoodExample29N0.444.42.74.671000313goodgoodExample30O0.4112.94.910.583000357goodgoodExample31P0.454.02.54.363000319goodgoodExample32Q0.399.85.19.373000346goodgoodExample33R0.483.03.95.274000311goodgoodExample34s0.416.02.55.166000303goodgoodExample35T0.514.13.55.357000299goodgoodExample36U0.523.32.43.959000327goodgoodExample37a0.585.52.54.947000295poorgoodComparative Example38b0.660.71.51.846000293poorgoodComparative Example39c0.681.61.11.835000281poorgoodComparative Example40d0.502.31.82.847000277poorgoodComparative Example41cslab crack occurred during continuous castingComparative Example42fslab crack occurred during continuous castingComparative Example43gslab crack occurred during continuous castingComparative Example44hslab crack occurred during continuous castingComparative Example45islab crack occurred during continuous castingComparative Example46jslab crack occurred during continuous castingComparative Example*1: Average particle size of cementite particles having area of 0.06 µm 2< or more *2: Number density of voids having area of 0.01 µm 2< or more REFERENCE SIGNS LIST

[0130] 1test piece for machinability evaluation 2fixing jig 3dynamometer 10disk sample (cold-rolled steel sheet) 11rollover 12shear surface 13fracture surface drollover tsheet thickness

Examples

examples

[0100]In order to confirm the effects of the present disclosure, cold-rolled steel sheets were produced according to the following procedure.

[0101]First, steels having the chemical compositions listed in Table 1 were melted in a converter and made into steel slabs by continuous casting. Each steel slab was then subjected to heating, hot rolling, cooling, coiling, first annealing, cold rolling, second annealing, and final cold rolling in sequence to produce a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm. Each process was carried out under the conditions listed in Tables 2 and 3, and the first annealing, cold rolling, and second annealing were carried out the number of times listed in Tables 2 and 3.

[0102]Next, for each of the obtained cold-rolled steel sheets, the average particle size of cementite, the amount of Mn and Cr dissolved in the cementite particles, the number density of voids, and Vickers hardness were measured by the following procedures. The me...

Claims

1. A cold-rolled steel sheet comprising a chemical composition containing, in mass%, C: 0.80 % to 1.25 %, Si: 0.10 % to 1.0 %, Mn: 0.20 % to 3.0 %, P: 0.001 % to 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 %, with the balance being Fe and inevitable impurity, and the total content of Nb, Ti, and V being 0.24 mass% or less, wherein an average particle size of cementite particles having an area of 0.06 µm2 or more is 0.65 µm or less, and an A value defined by the following expression (1) is 2.0 mass% or more, a number density of voids having an area of 0.01 µm2 or more is 50,000 voids / mm2 or more, a Vickers hardness is 200 HV or more and 400 HV or less, A = 3 / 7 C Mn + C Cr where, in expression (1), CMn is Mn content in mass% in the cementite particles having an area of 0.06 µm2 or more, and CCr is Cr content in mass% in the cementite particles having an area of 0.06 µm2 or more.

2. The cold-rolled steel sheet according to claim 1, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of: 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.

3. A method of producing a cold-rolled steel sheet, the method comprising: heating a steel slab comprising the chemical composition according to claim 1 or 2, under conditions including a slab heating temperature of 1100 °C or higher and a slab heating time of 60 min or longer; hot rolling the heated steel slab under conditions including a rolling finish temperature exceeding Tc, as defined by the following expression (2), to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet under conditions including an average cooling rate of 20 °C / s or more and a cooling stop temperature of Tc or lower; coiling the cooled hot-rolled steel sheet at a coiling temperature of 530 °C or higher and Tc or lower; subjecting the hot-rolled steel sheet after coiling to first annealing once or more, under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer; and subjecting the hot-rolled steel sheet after the first annealing to cold rolling at a rolling ratio of 15 % or more and second annealing at an annealing temperature of 600 °C or higher and Tc or lower for an annealing time of 3 h or longer, once or more, and then final cold rolling at a rolling ratio of 20 % or more and 80 % or less, where the element symbols in expression (2) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0.

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