Steel sheet

Abstract

Provided is a steel sheet that has exceptional quenching properties and exceptional cold-working properties. A steel sheet according to the present disclosure contains, in mass%, 0.15-0.50% of C, 0.01-0.50% of Si, 0.20-1.30% of Mn, 0.100% or less of P, 0.100% or less of S, 0.100% or less of Al, 0.01-1.20% of Cr, and 0.0150% or less of N, the balance being Fe and impurities. In the steel sheet: the total area ratio of ferrite and cementite particles is 95% or higher; the average particle diameter of the ferrite is 5.0-20.0 µm; the Cr concentration [Cr]θ and the Mo concentration [Mo]θ in the cementite particles satisfy expression (1); the average particle diameter of the cementite particles is 1.50 µm or less; the spheroidization ratio of the cementite particles is not less than 70% but less than 85%; and the degree of dispersity DI of the number of cementite particles in minute square segments in which each side measures 10 μm is 45% or less, the degree of dispersity DI being defined by expression (2).　Expression (1): 2[Cr]θ + 3[Mo]θ ≤ 3 / √C Expression (2): DI = σθ / Nθ × 100

Description

steel plate

[0001] The present disclosure relates to a steel sheet, and more particularly to a steel sheet that can be used as a material for machine parts, such as automobile parts.

[0002] Steel sheets with a C content of 0.15% or more are used as materials for mechanical parts, such as automobile parts. Examples of mechanical parts include automobile door parts, seat parts, chains, gears, and clutches. The method for manufacturing these mechanical parts using steel sheets as raw materials is as follows: The steel sheets are cold-worked to form them into the shape of the mechanical parts. The cold-worked steel sheets are then quenched and tempered. High-strength mechanical parts are manufactured through the above manufacturing process. In order to obtain high strength in the mechanical parts after quenching, the steel sheets are required to have excellent hardenability during quenching during the manufacturing process of mechanical parts made from steel sheets. Furthermore, the steel sheets are cold-worked before quenching. Therefore, the steel sheets are required to have not only the above-mentioned excellent hardenability but also excellent cold workability.

[0003] Steel sheets having excellent hardenability and excellent cold workability are proposed in Patent Documents 1 and 2.

[0004] The steel sheet disclosed in Patent Document 1 contains, by mass%, C: 0.20 to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0050% or less, and B: 0.0005 to 0.0050%, and further contains one or more of Sb, Sn, Bi, Ge, Te, and Se in a total amount of 0.002 to 0.030%, with the balance consisting of Fe and unavoidable impurities. In this steel sheet, the proportion of solute B in the B content is 70% or more. Furthermore, the microstructure is composed of ferrite and cementite. Furthermore, the cementite density within the ferrite grains is 0.08 particles / μm ２ The following is the description of Patent Document 1: It is stated that total elongation can be increased by keeping the density of cementite in ferrite grains low.

[0005] The steel sheet disclosed in Patent Document 2 contains, by mass%, C: 0.10% to 0.33%, Si: 0.01% to 0.50%, Mn: 0.40% to 1.25%, P: 0.03% to 0.01%, sol. Al: 0.10% to 0.01%, N: 0.01%, and Cr: 0.50% to 1.50%, with the balance being Fe and unavoidable impurities. The steel sheet has a microstructure containing ferrite and carbides. The volume fraction of the ferrite and carbides in the entire microstructure is 90% or more, and the volume fraction of pro-eutectoid ferrite in the entire microstructure is 20% to 80%. The Mn concentration in the carbides is 0.10 mass% or more and 0.50 mass% or less, and the ratio of the number of carbides having a particle size of 1 μm or more to the total number of carbides is 30% or more and 60% or less. Patent Document 2 describes that by reducing the Mn concentration in the carbides, the carbides become more easily dissolved during quenching, and as a result, hardenability is improved.

[0006] International Publication No. WO 2015 / 146173 International Publication No. WO 2020 / 175665

[0007] However, excellent hardenability and excellent cold workability may be obtained by means other than those disclosed in Patent Documents 1 and 2.

[0008] An object of the present disclosure is to provide a steel sheet having excellent hardenability and excellent cold workability.

[0009] The steel sheet according to the present disclosure has the following configuration.

[0010] A steel plate having a chemical composition, in mass%, of C: 0.15 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities; and in the microstructure, a total area ratio of ferrite and cementite particles is 95% or more, The average particle size of the ferrite is 5.0 to 20.0 μm, and the Cr concentration [Cr] in mass% in the cementite particles is θ and the Mo concentration in mass% in the cementite particles [Mo] θ and the C content in mass % in the steel plate satisfy formula (1), the average particle size of the cementite particles is 1.50 μm or less, and when the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, a spheroidization rate, which is the ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 70 to less than 85%, and the microstructure is divided into minute square sections with a side length of 10 μm, and the average number of the cementite particles in 500 or more minute square sections is N θ and the sample standard deviation of the number of cementite particles in the small square section is defined as σ θ When the dispersion ratio DI defined by the formula (2) is defined as 2 [Cr], the steel sheet has a dispersion ratio DI of 45% or less. θ +3 [Mo] θ ≦3 / √C (1) DI=σ θ / N θ × 100 (2) Here, the C content in mass % in the chemical composition of the steel sheet is substituted for C in the formula (1).

[0011] The steel sheet according to the present disclosure has excellent hardenability and excellent cold workability.

[0012] FIG. 1 is a schematic diagram showing a part of a square region in a method for measuring the dispersion degree DI of cementite particles in a steel sheet.

[0013] The present inventors have conducted research into steel sheets having excellent hardenability and excellent cold workability, and as a result, have made the following findings.

[0014] The present inventors first investigated a steel sheet having excellent hardenability and excellent cold workability from the viewpoint of chemical composition, and as a result, they concluded that a chemical composition consisting of, in mass%, C: 0.15 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities would enable both improved hardenability and improved cold workability. Therefore, the present inventors further investigated means for improving both hardenability and cold workability of a steel sheet having the above-mentioned chemical composition from the viewpoint of microstructure.

[0015] The present inventors first investigated means for improving the hardenability of a steel sheet during quenching in its microstructure. The microstructure of a steel sheet having the above-described chemical composition is substantially composed of ferrite and cementite particles. In order to improve the hardenability of a steel sheet during quenching in the manufacturing process of a mechanical component made from the steel sheet, it is preferable that the cementite particles in the steel sheet are easily dissolved during quenching. In order to improve the solubility of the cementite particles during quenching, it is preferable that the cementite particles have a small particle size. In the case of a steel sheet having the above-described chemical composition, it is effective to make the average particle size of the cementite particles 1.50 μm or less.

[0016] Furthermore, as a result of investigations by the present inventors, it was found that the Mn concentration in cementite particles does not affect the dissolution of cementite particles during quenching. On the other hand, the present inventors found that the Cr and Mo concentrations in cementite particles significantly affect the dissolution of cementite particles during quenching. Specifically, if the Cr and Mo concentrations in cementite particles are high, the cementite particles become less likely to dissolve during quenching.

[0017] As a result of further investigation based on the above findings, the present inventors have found that the Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ It has been found that if the formula (1) is satisfied, cementite particles are easily dissolved during quenching, and the hardenability of the steel sheet is improved. θ +3 [Mo] θ ≦3 / √C (1) Here, the C content in mass % in the chemical composition of the steel sheet is substituted for C in formula (1).

[0018] The present inventors have further investigated means for improving the cold workability of the microstructure of a steel sheet. Increasing the spheroidization rate of cementite particles and increasing the average grain size of ferrite are effective in improving the cold workability of the steel sheet. Therefore, in the steel sheet of this embodiment, the spheroidization rate of cementite particles is set to less than 70% to 85%, and the average grain size of ferrite is set to 5.0 μm or more. Note that a smaller average grain size of ferrite increases the ferrite grain boundary area in the steel sheet. Therefore, due to grain boundary diffusion, the dissolution of cementite particles during quenching is accelerated. Therefore, the average grain size of ferrite is set to 20.0 μm or less.

[0019] When a steel sheet having the above-described chemical composition satisfies the above characteristics, the hardenability and cold workability are improved to a certain extent. However, even when the steel sheet has the above characteristics, there are cases where the cold workability is still low. Therefore, the inventors of the present invention conducted further studies.

[0020] As a result of the investigation, it was found that the distribution of cementite particles in the steel sheet also has a significant effect on the cold workability. Specifically, regions with dense cementite particles have higher strength than regions with coarse cementite particles. In this case, differences in cold workability occur between regions with dense cementite particles and regions with coarse cementite particles. As a result, the cold workability of the steel sheet as a whole deteriorates. Therefore, it was found that distributing cementite particles in the steel sheet as uniformly as possible is extremely effective in improving the cold workability.

[0021] Based on the above findings, the present inventors further investigated. As a result, the microstructure was divided into minute square sections with a side length of 10 μm, and the average number of cementite particles in 500 or more minute square sections was calculated as N θ and the sample standard deviation of the number of cementite particles in a small square section is defined as σ θ When the dispersion DI defined by the formula (2) is 45% or less, it has been found that the hardenability can be improved and the cold workability can also be improved. θ / N θ ×100 (2)

[0022] The above mechanism is a guess. Therefore, it is possible that the steel sheet of this embodiment has excellent hardenability and excellent cold workability due to a mechanism different from the above. However, as will be shown in the examples described later, a steel sheet satisfying the above characteristics can have excellent hardenability and excellent cold workability.

[0023] The steel sheet according to this embodiment, which was completed based on the above findings, has the following configuration.

[0024] [1] A steel plate having a chemical composition, in mass%, of C: 0.15 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities; and in the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. The average particle size of the ferrite is 5.0 to 20.0 μm, and the Cr concentration [Cr] in mass% in the cementite particles is θ and the Mo concentration in mass% in the cementite particles [Mo] θ and the C content in mass % in the steel plate satisfy formula (1), the average particle size of the cementite particles is 1.50 μm or less, and when the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, a spheroidization rate, which is the ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 70 to less than 85%, and the microstructure is divided into minute square sections with a side length of 10 μm, and the average number of the cementite particles in 500 or more minute square sections is N θ and the sample standard deviation of the number of cementite particles in the small square section is defined as σ θ When the dispersion ratio DI defined by the formula (2) is defined as 2 [Cr], the steel sheet has a dispersion ratio DI of 45% or less. θ +3 [Mo] θ ≦3 / √C (1) DI=σ θ / N θ × 100 (2) Here, the C content in mass % in the chemical composition of the steel sheet is substituted for C in the formula (1).

[0025] [2] The steel sheet according to [1], wherein the chemical composition contains one or more elements selected from the group consisting of Mo: 0.001 to 0.500%, Ni: 0.001 to 1.000%, B: 0.0001 to 0.0100%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%.

[0026] The steel sheet of this embodiment will be described in detail below. Note that "%" regarding elements means mass % unless otherwise specified.

[0027] [Features of Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment satisfies the following Features 1 to 7. (Feature 1) The chemical composition, in mass%, is C: 0.15 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities. (Feature 2) In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. (Feature 3) The average grain size of ferrite is 5.0 to 20.0 μm. (Feature 4) The Cr concentration in mass % in cementite particles [Cr] θ and the Mo concentration in mass% in cementite particles [Mo] θ and the C content in mass% in the steel sheet satisfy the formula (1). θ +3 [Mo] θ≦3 / √C (1) Here, C in formula (1) is substituted with the C content in mass % in the chemical composition of the steel sheet. (Feature 5) The average particle size of cementite particles is 1.50 μm or less. (Feature 6) When cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 70 to less than 85%. (Feature 7) The microstructure is divided into minute square sections with a side length of 10 μm, and the average number of cementite particles in 500 or more minute square sections is N θ and the sample standard deviation of the number of cementite particles in a small square section is defined as σ θ When the dispersion ratio DI defined by the formula (2) is 45% or less, DI=σ θ / N θ ×100 (2) Features 1 to 7 will be explained below.

[0028] [(Feature 1) Chemical Composition] The chemical composition of the steel sheet of this embodiment contains the following elements.

[0029] C: 0.15 to 0.50% Carbon (C) improves the hardenability of steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. If the C content is less than 0.15%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.50%, the cold workability of the steel sheet decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.15 to 0.50%. The preferred lower limit of the C content is 0.16%, and more preferably 0.17%. The preferred upper limit of the C content is 0.49%, and more preferably 0.48%.

[0030] Si: 0.01 to 0.50% Silicon (Si) deoxidizes steel during the steelmaking stage of the steel sheet manufacturing process. Furthermore, Si enhances the temper softening resistance of the steel sheet when tempering is performed in a process for manufacturing machine parts using the steel sheet as a raw material. If the Si content is less than 0.01%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.50%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Si content is 0.01 to 0.50%. The preferred lower limit of the Si content is 0.02%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit of the Si content is 0.45%, even more preferably 0.40%, and even more preferably 0.38%.

[0031] Mn: 0.20 to 1.30% Manganese (Mn) improves the hardenability of steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. If the Mn content is less than 0.20%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.30%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Mn content is 0.20 to 1.30%. The preferred lower limit of the Mn content is 0.25%, more preferably 0.30%, and even more preferably 0.35%. The preferred upper limit of the Mn content is 1.25%, more preferably 1.20%, and even more preferably 1.15%.

[0032] P: 0.100% or less Phosphorus (P) is an impurity. If the P content exceeds 0.100%, the toughness of the steel plate decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.100% or less. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is more than 0%, more preferably 0.001%, even more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the P content is 0.090%, even more preferably 0.080%, and even more preferably 0.050%.

[0033] S: 0.100% or less Sulfur (S) is an impurity. If the S content exceeds 0.100%, S forms excessive sulfides. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the S content is 0.100% or less. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is more than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the S content is 0.090%, even more preferably 0.080%, and even more preferably 0.050%.

[0034] Al: 0.100% or less Aluminum (Al) is an impurity. Al combines with N to form AlN. AlN refines austenite grains during heating in the quenching process in the manufacturing of mechanical components using steel sheet as a raw material. Refining austenite grains reduces the hardenability of the steel sheet. If the Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the austenite grains are excessively refined during heating in the quenching process. As a result, the hardenability of the steel sheet is significantly reduced. Therefore, the Al content is 0.100% or less. The Al content is preferably as low as possible. However, an extreme reduction in the Al content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the Al content is more than 0%, more preferably 0.001%, even more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Al content is preferably 0.090%, more preferably 0.080%, still more preferably 0.070%, and still more preferably 0.050%. In the chemical composition of the steel sheet of this embodiment, the Al content means the acid-soluble Al (sol. Al) content.

[0035] Cr: 0.01 to 1.20% Chromium (Cr) improves the hardenability of steel sheets. As a result, by performing hardening in a process for manufacturing mechanical parts using steel sheets as raw materials, the strength of the mechanical parts is increased. If the Cr content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 1.20%, the Cr concentration [Cr] in cementite particles θThe content of Cr in the steel sheet is excessively increased. As a result, cementite particles do not dissolve sufficiently during heating in the quenching process in the process of manufacturing mechanical parts using the steel sheet as a raw material. In this case, the hardenability of the steel sheet is actually reduced. As a result, mechanical parts manufactured using the steel sheet as a raw material do not have sufficient strength. Therefore, the Cr content is 0.01 to 1.20%. A preferred lower limit of the Cr content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. A preferred upper limit of the Cr content is 1.15%, more preferably 1.10%, even more preferably 1.00%, even more preferably 0.70%, even more preferably 0.50%, and even more preferably 0.30%.

[0036] N: 0.0150% or less Nitrogen (N) is an impurity. N combines with Al to form AlN. AlN refines austenite grains during heating in the quenching process in the manufacturing of mechanical components using steel sheet as a raw material. Refinement of austenite grains reduces the hardenability of the steel sheet. If the N content exceeds 0.0150%, even if the contents of other elements are within the ranges of this embodiment, the austenite grains are excessively refined during heating in the quenching process. As a result, the hardenability of the steel sheet is significantly reduced. Therefore, the N content is 0.0150% or less. The N content is preferably as low as possible. However, an extreme reduction in the N content significantly increases manufacturing costs. Therefore, in consideration of industrial production, the lower limit of the N content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0030%, and even more preferably 0.0040%. The upper limit of the N content is preferably 0.0140%, even more preferably 0.0130%.

[0037] The balance of the chemical composition of the steel sheet according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel sheet, and are acceptable within a range that does not adversely affect the steel sheet according to this embodiment.

[0038] The impurities may include elements with the following contents: Cu: 0-0.150%, W: 0-0.150%, Ta: 0-0.150%, Sn: 0-0.050%, Sb: 0-0.050%, Co: 0-0.050%, As: 0-0.050%, Mg: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, Ce: 0-0.050%, and Ca: 0-0.050%.

[0039] [Optional Elements] The chemical composition of the steel sheet of this embodiment may further contain one or more elements selected from the group consisting of: Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%. These optional elements will be described below.

[0040] [First Group: Mo, Ni, and B] The chemical composition of the steel sheet according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo, Ni, and B. All of these elements are optional elements and may not be contained. When contained, Mo, Ni, and B improve the hardenability of the steel sheet.

[0041] Mo: 0 to 0.500% Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When Mo is contained, that is, when the Mo content exceeds 0%, Mo improves the hardenability of the steel sheet. As a result, when quenching is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. Mo also improves the temper softening resistance of the steel sheet when tempering is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material. Even if even a small amount of Mo is contained, the above effects can be achieved to some extent. However, if the Mo content exceeds 0.500%, the strength of the steel sheet becomes excessively high even when the contents of other elements are within the ranges of this embodiment. As a result, the cold workability of the steel sheet is reduced. Therefore, the Mo content is 0 to 0.500%. The preferred lower limit of the Mo content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Mo content is preferably 0.450%, more preferably 0.400%, even more preferably 0.350%, and still more preferably 0.300%.

[0042] Ni: 0 to 1.000% Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content exceeds 0%, Ni enhances the hardenability of the steel sheet. As a result, when quenching is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. Ni also enhances the temper softening resistance of the steel sheet when tempering is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material. Even if even a small amount of Ni is contained, the above effects can be achieved to some extent. However, if the Ni content exceeds 1.000%, the strength of the steel sheet becomes excessively high even when the contents of other elements are within the ranges of this embodiment. As a result, the cold workability of the steel sheet is reduced. Therefore, the Ni content is 0 to 1.000%. The preferred lower limit of the Ni content is 0.001%, more preferably 0.005%, and even more preferably 0.007%. The upper limit of the Ni content is preferably 0.950%, more preferably 0.900%, even more preferably 0.800%, even more preferably 0.700%, and still more preferably 0.600%.

[0043] B: 0 to 0.0100% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When B is contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content exceeds 0.0100%, B compounds will be formed even if the contents of other elements are within the ranges of this embodiment. In this case, the hardenability improvement effect cannot be sufficiently obtained. Furthermore, the cold workability of the steel sheet will be reduced. Therefore, the B content is 0 to 0.0100%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and still more preferably 0.0050%.

[0044] [Regarding Group 2: V, Nb, and Ti] The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of V, Nb, and Ti, instead of a portion of Fe. All of these elements are optional and may not be contained. When contained, V, Nb, and Ti form carbides. These carbides suppress coarsening of austenite grains during heating in the quenching process in the process of manufacturing mechanical components using the steel sheet as a raw material. Therefore, the toughness of the mechanical components is improved.

[0045] V: 0 to 0.500% Vanadium (V) is an optional element and may not be contained. In other words, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms carbides and suppresses coarsening of austenite grains during heating in the quenching process in the process of manufacturing mechanical components using the steel sheet as a raw material. This improves the toughness of the mechanical component. Even if even a small amount of V is contained, the above effect can be achieved to some extent. However, when the V content exceeds 0.500%, V forms excessive carbides and precipitation strengthens the steel sheet. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the V content is 0 to 0.500%. The preferred lower limit of the V content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the V content is preferably 0.480%, more preferably 0.450%, even more preferably 0.400%, even more preferably 0.350%, and still more preferably 0.300%.

[0046] Nb: 0 to 0.500% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms carbides and suppresses coarsening of austenite grains during heating in the quenching process in the process of manufacturing mechanical parts using steel sheet as a raw material. This improves the toughness of the mechanical parts. Furthermore, Nb bonds with N to suppress the formation of nitrides by solute B. This improves the hardenability of the steel sheet due to solute B. Even if even a small amount of Nb is contained, the above effect can be achieved to some extent. However, when the Nb content exceeds 0.500%, Nb forms excessive carbides and precipitation-strengthens the steel sheet. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the Nb content is 0 to 0.500%. The lower limit of the Nb content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Nb content is preferably 0.480%, more preferably 0.450%, even more preferably 0.400%, even more preferably 0.350%, and even more preferably 0.300%.

[0047] Ti: 0 to 0.150% Titanium (Ti) is an optional element and may not be included. In other words, the Ti content may be 0%. When Ti is included, i.e., when the Ti content exceeds 0%, Ti forms carbides and suppresses coarsening of austenite grains during heating in the quenching process in manufacturing mechanical parts using steel sheet as a raw material. This improves the toughness of the mechanical part. Furthermore, Ti bonds with N to suppress the formation of nitrides by solute B. This improves the hardenability of the steel sheet due to solute B. Even if even a small amount of Ti is included, the above effect can be achieved to some extent. However, when the Ti content exceeds 0.150%, Ti forms excessive carbides and precipitation-strengthens the steel sheet. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the Ti content is 0 to 0.150%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Ti content is preferably 0.145%, more preferably 0.130%, more preferably 0.120%, even more preferably 0.100%, and even more preferably 0.080%.

[0048] [(Feature 2) Microstructure] In the microstructure of the steel sheet of this embodiment, the total area ratio of ferrite and cementite particles is 95% or more. In other words, the microstructure of the steel sheet of this embodiment is substantially composed of ferrite and cementite particles.

[0049] In the microstructure, the structure other than ferrite and cementite particles is, for example, one or more types selected from the group consisting of precipitates other than cementite particles, inclusions, bainite, martensite, and pearlite.

[0050] Preferably, the total area ratio of ferrite and cementite particles in the microstructure is 96% or more, more preferably 97% or more, even more preferably 98% or more, and even more preferably 99% or more. The microstructure may be a structure consisting of ferrite and cementite particles.

[0051] If the total area ratio of ferrite and cementite particles is 95% or more, sufficient hardenability and sufficient cold workability can be obtained, provided that Features 1 and 3 to 7 are satisfied.

[0052] [Method for measuring the total area ratio of ferrite and cementite particles in the microstructure] The total area ratio of ferrite and cementite particles in the microstructure can be measured by the following method. A test piece measuring 15 mm in the rolling direction (L direction) of the steel sheet, 10 mm in the width direction (W direction), and the sheet thickness is taken from the center of the sheet width of the steel sheet. Here, the rolling direction can be identified by observing the roll marks formed on the surface of the steel sheet. The roll marks are thin lines formed on the surface of the steel sheet along the direction of the rolling rolls when rolling is performed. In this case, the direction in which the roll marks extend is identified as the rolling direction.

[0053] Of the surfaces of the test piece, a cross section parallel to the rolling direction (a surface of 15 mm × plate thickness in the rolling direction) is defined as the observation surface. The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (nital etchant). Of the etched observation surface, secondary electron images of five arbitrary observation fields are observed using a scanning electron microscope (SEM) at 1000x magnification. Each observation field is a rectangle of 100 μm × 120 μm.

[0054] In the observation field, ferrite and cementite particles exhibit contrast and morphology that differ from other structures (bainite, martensite, pearlite, precipitates other than cementite particles, inclusions, etc.). Specifically, when the observation surface is etched with a nital etchant, regions with high granular brightness can be identified as cementite particles. Regions with lamellar structures can be identified as pearlite. Regions with brightness lower than pearlite and no visible substructure can be identified as ferrite. Regions with brightness higher than ferrite and lower than pearlite and where a visible substructure can be identified can be identified as bainite and martensite. Therefore, ferrite and cementite particles can be identified within the observation field based on contrast and morphology.

[0055] The total area ratio (%) of ferrite and cementite particles is calculated based on the total area of ​​ferrite and cementite particles in the five observation fields and the total area of ​​the five observation fields.

[0056] [(Feature 3) Average grain size of ferrite] In the steel sheet of this embodiment, the average grain size of ferrite is 5.0 to 20.0 μm. If the average grain size of ferrite is less than 5.0 μm, the cold workability of the steel sheet is reduced. On the other hand, if the average grain size of ferrite exceeds 20.0 μm, the grain boundary area is reduced, and the effect of promoting the dissolution of cementite particles by grain boundary diffusion cannot be obtained. In this case, sufficient hardenability cannot be obtained in the hardening process when manufacturing mechanical parts using the steel sheet as a raw material. As a result, the strength of the mechanical parts manufactured using the steel sheet as a raw material is reduced. Therefore, the average grain size of ferrite is 5.0 to 20.0 μm.

[0057] The lower limit of the average particle size of ferrite is preferably 5.2 μm, more preferably 5.5 μm, even more preferably 5.7 μm, even more preferably 6.0 μm, and even more preferably 6.5 μm. The upper limit of the average particle size of ferrite is preferably 19.5 μm, more preferably 19.0 μm, even more preferably 18.5 μm, and even more preferably 18.0 μm.

[0058] [Method for Measuring the Average Grain Size of Ferrite] The average grain size of ferrite can be measured by the following method. A test piece measuring 15 mm in the rolling direction (L direction) of the steel plate, 10 mm in the width direction (W direction), and the plate thickness is taken from the center of the steel plate width. Of the surface of the test piece, a cross section parallel to the rolling direction (the surface 15 mm in the rolling direction × plate thickness) is defined as the observation surface. The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched using a 5% nital etching solution. The average grain size of ferrite is determined on the etched observation surface by the following method. In accordance with JIS G 0551:2020, the ferrite grain size number is determined by the intercept method. At this time, the magnification of the optical microscope is selected so that the number of ferrite grains cut by one line segment is at least 10 in one visual field. After selecting the magnification, the cut lengths are determined for five visual fields. The ferrite grain size number is determined from the arithmetic average of the cut lengths of the five visual fields. The average grain size (μm) of ferrite is determined from the obtained grain size number.

[0059] (Feature 4) Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Regarding the steel sheet of this embodiment, the Cr concentration [Cr] in mass% in cementite particles θ and the Mo concentration in mass% in cementite particles [Mo] θ and the C content in mass% in the steel sheet satisfy the formula (1). θ +3 [Mo] θ ≦3 / √C (1) Here, the C content in mass % in the chemical composition of the steel sheet is substituted for C in formula (1).

[0060] Cr concentration in cementite particles of steel plate [Cr] θ and Mo concentration [Mo] θ If the hardness is too high, cementite particles do not dissolve sufficiently during heating in the quenching step in the process of manufacturing mechanical parts using the steel sheet as a raw material. In this case, the hardenability of the steel sheet is reduced. As a result, mechanical parts manufactured using the steel sheet as a raw material do not have sufficient strength.

[0061] Cr concentration in cementite particles of steel plate [Cr] θand Mo concentration [Mo] θ satisfies the formula (1), the Cr concentration in the cementite particles [Cr] θ and Mo concentration [Mo] θ Therefore, during heating in the above-mentioned quenching step, cementite particles are sufficiently dissolved, and the hardenability of the steel sheet is improved.

[0062] [Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Measurement method] Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ can be measured by the following method: A test piece is taken from the center of the steel plate width, and the size of the test piece is 10 mm x 10 mm x plate thickness.

[0063] The test piece is subjected to constant current electrolysis using a 10% AA-based solution (a solution containing 10% by volume of acetylacetone and a methanol solution containing 1% by mass of tetramethylammonium chloride).

[0064] Specifically, the above-mentioned 10% AA-based solution is prepared. Then, using the 10% AA-based solution, a current density of 20 mA / cm is applied at room temperature. ２ The test piece is subjected to constant current electrolysis while being held at 10°C. After constant current electrolysis, the test piece is removed from the 10% AA solution. The removed test piece is immersed in an alcohol solution. The test piece immersed in the alcohol solution is subjected to ultrasonic cleaning.

[0065] The 10% AA solution used in the constant current electrolysis and the alcohol solution used in the subsequent ultrasonic cleaning are suction filtered through a filter with a mesh size of 0.2 μm to extract the residue.

[0066] The extracted residue is subjected to chemical elemental analysis. Specifically, the residue is dissolved in acid to obtain a solution. The solution is subjected to chemical elemental analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES) to obtain the Cr mass and Mo mass in the residue. Based on the obtained Cr mass, Mo mass, and total mass of the residue, the Cr concentration (mass%) and Mo concentration (mass%) in the residue are determined.

[0067] The obtained residue is substantially composed of cementite particles. In other words, the amount of particles other than cementite particles (inclusions and precipitates other than cementite particles) in the residue is negligibly small. Therefore, the Cr concentration and Mo concentration in the residue are calculated as the Cr concentration in the cementite particles [Cr] θ (mass%) and Mo concentration [Mo] θ (% by mass).

[0068] [(Feature 5) Average particle size of cementite particles] In the steel sheet of this embodiment, the average particle size of the cementite particles is 1.50 μm or less. As described above, if the cementite particles are large, the cementite particles will not dissolve sufficiently during heating in the quenching step in the process of manufacturing a mechanical component using the steel sheet as a raw material. In this case, the hardenability of the steel sheet will be reduced. As a result, the mechanical component manufactured using the steel sheet as a raw material will not have sufficient strength.

[0069] If the average particle size of the cementite particles is 1.50 μm or less, the cementite particles are sufficiently small, and therefore, when heated in the above-mentioned quenching step, the cementite particles are sufficiently dissolved, resulting in improved hardenability of the steel sheet.

[0070] The upper limit of the average particle size of the cementite particles is preferably 1.45 μm, more preferably 1.40 μm, even more preferably 1.35 μm, and even more preferably 1.30 μm. To improve hardenability, a smaller average particle size of the cementite particles is preferable. However, if the average particle size of the cementite particles is too small, the hardness of the steel sheet becomes too high. In this case, the cold workability of the steel sheet deteriorates. Therefore, the lower limit of the average particle size of the cementite particles is preferably 0.05 μm, more preferably 0.10 μm, even more preferably 0.15 μm, and even more preferably 0.20 μm.

[0071] [Method for measuring the average particle size of cementite particles] The average particle size of cementite particles can be determined by the following method. A test piece measuring 15 mm in the rolling direction (L direction) of the steel plate, 10 mm in the plate width direction (W direction), and the plate thickness is taken from the center of the plate width of the steel plate. Of the surface of the test piece, a cross section parallel to the rolling direction (the surface of 15 mm in the rolling direction and the plate thickness) is defined as the observation surface.

[0072] The observation surface is etched using a picral solution. Secondary electron images are taken at five arbitrary observation fields located at a depth of 1 / 4 of the plate thickness from the surface of the etched observation surface. Specifically, the five observation fields are observed at a magnification of 2000 times using a scanning electron microscope (SEM), and the above-mentioned secondary electron images are taken. Each observation field is a rectangle of 50 μm × 60 μm.

[0073] In each secondary electron image, cementite particles are identified based on the contrast. The area of ​​each identified cementite particle is determined, and the circle-equivalent diameter of each cementite particle is determined based on the area. The circle-equivalent diameter thus determined is defined as the particle diameter of the cementite particle. Note that the particle diameter is determined using well-known image processing software. The arithmetic mean value of the particle diameters of the cementite particles obtained in the five observation fields is defined as the average particle diameter (μm) of the cementite particles.

[0074] [(Feature 6) Spheroidization Ratio] In the steel sheet of this embodiment, among the plurality of cementite particles, cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles. The spheroidization ratio, which is the ratio of the total number of spherical cementite particles to the total number of the plurality of cementite particles, is 70 to less than 85%.

[0075] If the spheroidization rate is less than 70%, sufficient cold workability cannot be obtained in the steel sheet even if Features 1 to 5 and 7 are satisfied. Therefore, the spheroidization rate is 70% or more. A higher spheroidization rate is preferable. However, if the spheroidization rate is excessively increased, the solubility of cementite during quenching decreases. The dissolution of cementite proceeds through the solid solution and diffusion of elements from the interface between the cementite and the matrix. Therefore, the more spherical the cementite, the smaller the interface area between the cementite and the matrix. As a result, the solubility of cementite decreases. Therefore, the upper limit of the spheroidization rate is preferably less than 85%, more preferably 84%, and even more preferably 83%.

[0076] [Method for Measuring Spheroidization Ratio] The spheroidization ratio can be measured by the following method. The aspect ratio is determined for each of the cementite particles identified in five observation fields by the above-mentioned [Method for Measuring the Average Particle Diameter of Cementite Particles]. Specifically, the long diameter is defined as the maximum distance obtained when the outline of the cementite particle is sandwiched between two parallel line segments. Furthermore, the short diameter is defined as the distance between the two line segments when the outline of the cementite particle is sandwiched between two line segments parallel to the long diameter (i.e., the width in the direction perpendicular to the long diameter).

[0077] The aspect ratio (= major axis / minor axis) of each cementite particle is calculated based on the obtained major axis and minor axis. Of all cementite particles in the five observation fields, cementite particles with an aspect ratio of 3.0 or less are identified as "spheroidal cementite particles." The ratio of the total number of spherical cementite particles to the total number of multiple cementite particles is defined as the spheroidization rate (%).

[0078] [(Feature 7) Dispersion Degree DI of Cementite Particles] The microstructure of the steel sheet of this embodiment is divided into minute square sections with a side length of 10 μm, and the average number of cementite particles in 500 or more minute square sections is calculated as N θ and the sample standard deviation of the number of cementite particles in a small square section is defined as σ θ When the dispersion ratio DI defined by the formula (2) is 45% or less, DI=σ θ / N θ ×100 (2)

[0079] The dispersion index (DI) of cementite particles is an index of the distribution state of multiple cementite particles in a steel sheet. A lower dispersion index (DI) means that multiple cementite particles are more uniformly dispersed in the steel sheet.

[0080] When the degree of dispersion DI is high, there are regions in the steel sheet where the cementite particles are dense and regions where the cementite particles are coarse. The regions where the cementite particles are dense have higher strength than the regions where the cementite particles are coarse. The cold workability of the regions in the steel sheet where the strength is high is lower than that of the regions where the strength is low. Therefore, there is a difference in cold workability between the regions where the cementite particles are dense and the regions where the cementite particles are coarse, and the cold workability of the steel sheet as a whole is reduced. Therefore, in order to improve the cold workability of the steel sheet, it is preferable that the cementite particles are distributed as uniformly as possible.

[0081] If the dispersity DI is 45% or less, the cementite particles in the steel sheet are distributed sufficiently uniformly, and therefore, the steel sheet can have sufficient cold workability, provided that Features 1 to 6 are satisfied.

[0082] The upper limit of the dispersity DI is preferably 42%, more preferably 40%, more preferably 38%, more preferably 36%, more preferably 34%, more preferably 32%, and even more preferably 30%. The lower limit of the dispersity DI is not particularly limited. However, reducing the lower limit of the dispersity DI infinitely increases the production cost. Therefore, the lower limit of the dispersity DI is preferably 5%, more preferably 10%, and even more preferably 15%.

[0083] [Method for Measuring Dispersity DI] The dispersity DI can be determined by the following method. A test piece measuring 15 mm in the rolling direction (L direction) of the steel plate, 10 mm in the width direction (W direction), and the plate thickness is taken from the center of the plate width of the steel plate. Of the surface of the test piece, a cross section parallel to the rolling direction (i.e., the surface 15 mm in the rolling direction × the plate thickness) is defined as the observation surface.

[0084] The observation surface is etched using a picral solution. Secondary electron images are taken of five or more observation fields of the etched observation surface at a depth of 1 / 4 of the plate thickness from the surface. Specifically, five or more observation fields are observed at a magnification of 1000x using a scanning electron microscope (SEM), and the above-mentioned secondary electron images are taken. Each observation field is a square area of ​​100 μm × 100 μm. Hereinafter, the observation field is referred to as a square area. There is no particular upper limit on the number of square areas (observation fields), but it is, for example, 10 (i.e., 1000 micro-square sections).

[0085] Each square region is divided into 100 microscopic square sections, each 10 μm on a side. The number of cementite particles in each microscopic square section is then counted. Whether or not a cementite particle is contained in a microscopic square section is determined by whether or not the center of gravity of the cementite particle is contained in the microscopic square section. The cementite particle is identified in the secondary electron image based on contrast. The center of gravity of the cementite particle can be identified using well-known image processing software.

[0086] Fig. 1 is a schematic diagram showing a part of a square region. Referring to Fig. 1, the square region 1 is a square of 100 µm x 100 µm. The square region 1 is partitioned (divided) into 100 minute square compartments 10, each having a side length of 10 µm. In each minute square compartment 10, the number of cementite particles CM contained in the minute square compartment 10 is counted.

[0087] Here, attention is focused on the minute square section 10A. In the minute square section 10A, a cementite particle CM1 exists on a boundary BL1 with the minute square section 10C, and a cementite particle CM2 exists on a boundary BL2 with the minute square section 10B. ２ Among these, the center of gravity of the cementite particle CM1 is included in the minute square section 10A. ２ The center of gravity of the cementite particle CM1 is included in the minute square section 10B, not in the minute square section 10A. Therefore, the cementite particle CM1 is counted as a cementite particle included in the minute square section 10A. ２are counted as cementite particles contained in the minute square section 10B, not in the minute square section 10A. The number of cementite particles in the minute square section 10A in FIG.

[0088] By the above method, the number of cementite particles in each minute square section 10 is counted. Then, the arithmetic mean value of the number of cementite particles in each minute square section 10 obtained for the five square regions 1 (i.e., 500 minute square sections 10) is taken as the average number N θ Furthermore, the sample standard deviation of the number of cementite particles in each of the minute square sections 10 obtained from the five square regions 1 (i.e., 500 minute square sections 10) is defined as the sample standard deviation σ θ The average number obtained is defined as N θ and the sample standard deviation σ θ The dispersity DI is calculated based on the formula (2) using the above.

[0089] [Effects of the Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment, which satisfies the above-described Features 1 to 7, exhibits sufficient hardenability during quenching in a process for manufacturing a mechanical component using the steel sheet as a raw material. Furthermore, the steel sheet of the present embodiment exhibits sufficient cold workability.

[0090] [Quenchability] In the steel sheet of this embodiment, the fact that sufficient quenchability is obtained means the following evaluation.

[0091] [Hardenability evaluation method] (A ｃ１ Transformation point and A ｃ３ Measurement of transformation point) A test piece is taken from the center of the sheet width of the steel sheet of this embodiment. The thermal expansion coefficient during heating is measured using a Formaster testing machine. From the obtained thermal expansion coefficient, A ｃ１ Transformation point and A ｃ３ Find the transformation point.

[0092] (Measurement of maximum quenched hardness) A plate-shaped test specimen was taken from the center of the steel plate width. The shape of the plate-shaped test specimen was 15 mm in the rolling direction (L direction) of the steel plate, 30 mm in the width direction (W direction), and plate thickness. The plate-shaped test specimen was heated in a salt bath at 1000°C for 20 minutes. The plate-shaped test specimen was then removed from the salt bath and immersed in water in a water tank for quenching. The quenched plate-shaped test specimen was cut into two equal parts in the W direction. The cut surfaces were mirror-polished. A Vickers hardness test in accordance with JIS Z2244:2009 was performed at three arbitrary points in the center of the thickness direction (T direction) of the polished cut surface. The test force was 98 N. The arithmetic mean value of the obtained Vickers hardness was defined as the maximum quenched hardness HD0 (HV).

[0093] (Evaluation of Hardenability) A plate-shaped test piece is taken from the center of the steel plate width. The shape of the plate-shaped test piece is 15 mm in the rolling direction (L direction) of the steel plate, 30 mm in the plate width direction (W direction), and plate thickness. The plate-shaped test piece is electrically heated at a temperature rising rate of 400°C / sec. ｃ３ The specimen is heated to transformation point + 100°C. After heating, the plate-shaped test piece is immersed in water in a water tank and quenched. The quenched plate-shaped test piece is cut into two equal parts in the W direction. The cut surfaces are mirror-polished. A Vickers hardness test in accordance with JIS Z2244:2009 is performed at three arbitrary points in the center of the plate thickness direction (T direction) of the polished cut surface. The test force is 98 N. The arithmetic mean value of the obtained Vickers hardness is defined as the quenched hardness HD1 (HV). If the obtained quenched hardness HD1 is 95% or more of the maximum quenched hardness HD0, the steel plate is determined to have sufficient hardenability.

[0094] [Cold Workability] In the steel sheet of this embodiment, the cold workability can be evaluated, for example, by the following method.

[0095] [Cold workability evaluation method] A JIS No. 5 plate test piece specified in JIS Z2241:2011 is taken from the center of the plate width of a steel plate. A V-notch is formed in the longitudinal center of the parallel part of the plate test piece so that the depth direction of the V-notch is parallel to the width direction of the parallel part. The V-notch opening angle is 45° and the V-notch depth is 2 mm. The gauge length is 10 mm including the V-notch. The longitudinal direction of the plate test piece is the rolling direction (L direction) of the steel plate.

[0096] A fracture elongation test is carried out using a plate-shaped test piece at room temperature in the atmosphere. The butt elongation after fracture is measured, and the obtained butt elongation (%) is defined as the notch elongation (%). The steel plate of this embodiment has a larger notch elongation than a steel plate that does not satisfy any of Features 1 to 7. For example, when the plate-shaped test piece has a thickness of 2 mm, the steel plate of this embodiment has a notch elongation of 15% or more.

[0097] [Applications of Steel Sheet] The steel sheet of this embodiment is suitable as a material for mechanical parts, such as automobile parts. Examples of mechanical parts include automobile door parts, seat parts, and drivetrain gear parts. The steel sheet of this embodiment may also be used for applications other than mechanical parts that require excellent hardenability and excellent cold workability.

[0098] [Method for manufacturing steel sheet] An example of a method for manufacturing a steel sheet according to this embodiment will be described. The method for manufacturing a steel sheet described below is one example for manufacturing the steel sheet according to this embodiment. Therefore, a steel sheet having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel sheet according to this embodiment.

[0099] An example of the method for manufacturing a steel sheet according to the present embodiment includes the following steps: (Step 1) Material preparation step (Step 2) Hot rolling step (Step 3) Cold rolling step (Step 4) Cold-rolled sheet annealing step In the present embodiment, the annealing step is not performed after the hot rolling step and before the cold rolling step.

[0100] The main manufacturing conditions in steps 1 to 4 are as follows: (Condition 1) Coiling temperature CT in step 2: greater than 550 to 590°C (Condition 2) Cold rolling rate CR in step 3: 15 to 35% (Condition 3) Annealing temperature T1 in step 4: 550 to 700°C (Condition 4) Holding time t1 in step 4: 5 to 20 hours (Condition 5) In steps 2 and 4, FA defined by the following formula (A) is set to 20,520 or less. FA = (700 + 273) × (X + log (td)) (A) Here, an eigenvalue defined by formula (B) described later is substituted for X in formula (A). A total holding time (hours) defined by formula (C) described later is substituted for td.

[0101] Each step will be described below.

[0102] [(Step 1) Material Preparation Step] In the material preparation step, a material satisfying Feature 1 is prepared. The material is produced, for example, by the following method. Molten steel is produced, the content of each element in the chemical composition of which falls within the range of this embodiment. The molten steel is used to produce a material (slab or ingot) by a casting method. For example, a slab is produced by a well-known continuous casting method using the molten steel. Alternatively, an ingot is produced by a well-known ingot-making method using the molten steel.

[0103] [(Step 2) Hot Rolling Step] In the hot rolling step, hot rolling is performed on a prepared material (slab or ingot) to produce a hot-rolled steel sheet. The hot rolling step includes a rough rolling step and a finish rolling step. In the rough rolling step, the material is rough rolled to produce a rough bar (intermediate steel sheet). In the finish rolling step, the rough bar is finish rolled to produce a hot-rolled steel sheet.

[0104] In the rough rolling process, a material (slab or ingot) is heated in a heating furnace. The heated material is rolled using a rough rolling mill to produce a rough bar. The heating temperature of the material in the rough rolling process is, for example, 1050 to 1300°C. The material is stored in the heating furnace for 30 minutes or more, preferably 60 minutes or more. The upper limit of the storage time is not particularly limited, but is, for example, 300 minutes.

[0105] In the finish rolling process, the rough bar is further rolled (finish rolling) using a finish rolling mill to produce a hot-rolled steel sheet. The finish rolling mill includes multiple stands arranged in a row. Each stand is equipped with a pair of work rolls. The surface temperature of the steel sheet at the outlet of the last stand in the finish rolling mill that reduces the steel sheet is defined as the finish rolling temperature (°C). In this embodiment, the finish rolling temperature is 830 to 950°C. Furthermore, the reduction rate of the last stand in the finish rolling mill that reduces the steel sheet is defined as the final pass reduction rate (%). In this embodiment, the final pass reduction rate is 5 to 30%. The hot-rolled steel sheet after finish rolling is wound up into a coil. The coiling temperature CT will be described later. The coiled hot-rolled steel sheet is cooled to room temperature.

[0106] [(Step 3) Cold Rolling Step] In the cold rolling step, cold rolling is performed on the hot-rolled steel sheet after the hot rolling step. Cold rolling is performed using a cold rolling mill. The cold rolling mill may be, for example, a tandem rolling mill having a plurality of cold rolling stands arranged in a row, or a reverse rolling mill having a single rolling stand.

[0107] In the cold rolling process, cold rolling is performed using a tandem rolling mill or a reverse rolling mill to produce a cold-rolled steel sheet. The cold rolling reduction ratio CR in the cold rolling process will be described later.

[0108] In this embodiment, the hot-rolled steel sheet after the hot-rolling process is subjected to the cold-rolling process without being subjected to an annealing treatment. In other words, the hot-rolled steel sheet annealing process is not performed after the hot-rolling process but before the cold-rolling process. In this manufacturing method, strain is accumulated in the steel sheet during the hot-rolling process and the cold-rolling process, and the annealing process is performed after the cold-rolling process. As a result, ferrite of an appropriate size, cementite particles of an appropriate size, and a Cr concentration [Cr] in the cementite particles of an appropriate amount are obtained. θ and Mo concentration [Mo] θ and an appropriate spheroidization rate of the cementite particles.

[0109] [(Step 4) Cold-rolled sheet annealing step] In the cold-rolled sheet annealing step, the cold-rolled steel sheet after the cold rolling step is annealed. In the annealing step, the annealing temperature T1 and the holding time t1 at the annealing temperature T1 are adjusted to adjust the degree of ferrite recrystallization and cementite particle precipitation.

[0110] [Regarding Conditions 1 to 5] In the above-described steps 1 to 4, the following conditions 1 to 5 are satisfied. (Condition 1) Coiling temperature CT in step 2: greater than 550 to 590°C (Condition 2) Cold rolling reduction CR in step 3: 15 to 35% (Condition 3) Annealing temperature T1 in step 4: 550 to 700°C (Condition 4) Holding time t1 in step 4: 5 to 20 hours (Condition 5) In steps 2 and 4, FA defined by the following formula (A) is 20,520 or less. FA = (700 + 273) × (X + log (td)) (A) Here, an eigenvalue defined by formula (B) described later is substituted for X in formula (A). A total holding time (hours) defined by formula (C) described later is substituted for td. Each condition will be described below.

[0111] [(Condition 1) Coiling Temperature CT] In the hot rolling process, the coiling temperature CT is determined based on the spheroidization rate of cementite particles, the dispersity DI of cementite particles, and the Cr concentration [Cr] in the cementite particles. θ and Mo concentration [Mo] θ If the coiling temperature CT exceeds 590°C, even if other conditions are satisfied, the cementite particles generated in the hot-rolled steel sheet are not distributed uniformly, resulting in variations in distribution. Furthermore, if the coiling temperature CT exceeds 590°C, the strain imparted by the hot-rolled steel sheet is excessively reduced. In this case, the spheroidization rate of the cementite particles decreases. Furthermore, Cr and Mo are concentrated in the generated cementite particles, and the Cr concentration [Cr] in the cementite particles decreases. θ and Mo concentration [Mo] θ However, the formula (1) is not satisfied. Therefore, the coiling temperature CT is set to 590°C or less.

[0112] On the other hand, the lower limit of the coiling temperature CT is not particularly limited. However, if the coiling temperature CT is too low, the strain imparted by the hot-rolled steel sheet remains excessively. In this case, the spheroidization of cementite particles is excessively promoted. As a result, the spheroidization rate of cementite particles becomes 85% or more. Therefore, the preferable lower limit of the coiling temperature CT is above 550°C.

[0113] If the coiling temperature CT is greater than 550°C and less than 590°C, a steel sheet satisfying Features 1 to 7 can be produced, provided that other conditions are met.

[0114] [(Condition 2) Cold Rolling Ratio CR] In the cold rolling process, the cold rolling ratio CR is defined by the following formula: Cold Rolling Ratio CR (%) = (1 - (Thickness of cold rolled steel sheet after cold rolling process / Thickness of hot rolled steel sheet before cold rolling process)) x 100

[0115] If the cold rolling rate CR is less than 15%, the strain introduced into the steel sheet is insufficient. In this case, the spheroidization of cementite particles is not promoted in the subsequent annealing process, resulting in a spheroidization rate of less than 70%. If the cold rolling rate CR is less than 15%, the cementite particles generated in the hot-rolled steel sheet are not distributed sufficiently uniformly, resulting in distribution variations. On the other hand, if the cold rolling rate CR is too high, the strain introduced into the steel sheet is excessive. In this case, ferrite is excessively refined, resulting in an average ferrite grain size of less than 5.0 μm. If the strain introduced into the steel sheet is excessive, the spheroidization of cementite particles is excessively promoted in the subsequent annealing process. Therefore, the spheroidization rate of cementite particles is 85% or more.

[0116] If the cold rolling rate CR is 15 to 35%, a steel sheet satisfying Features 1 to 7 can be produced, provided that other conditions are met.

[0117] [(Condition 3) Annealing Temperature T1] In the cold-rolled sheet annealing process, if the annealing temperature T1 is less than 550°C, the recrystallization of ferrite will be insufficient, the steel sheet will not be softened sufficiently, and the cold workability of the steel sheet will be reduced. Furthermore, the spheroidization of cementite particles will be insufficient, and the spheroidization rate of the cementite particles will be less than 70%.

[0118] On the other hand, if the annealing temperature T1 exceeds 700°C, the annealing temperature is too high. In this case, the cementite particles become coarse and the average particle size exceeds 1.50 μm. Furthermore, Cr and Mo are concentrated in the generated cementite particles, and the Cr concentration [Cr] in the cementite particles becomes too high. θ and Mo concentration [Mo] θ However, equation (1) is no longer satisfied.

[0119] If the annealing temperature T1 is 550 to 700°C, a steel sheet satisfying Features 1 to 7 can be produced, provided that other conditions are met.

[0120] [(Condition 4) Holding Time t1 at Annealing Temperature T1] In the cold-rolled sheet annealing process, the holding time t1 at the annealing temperature T1 is determined based on the size of ferrite in the steel sheet, the size of cementite particles, the spheroidization rate of the cementite particles, and the Cr concentration [Cr] in the cementite particles, similarly to the annealing temperature T1. θ and Mo concentration [Mo] θ Specifically, if the holding time t1 is less than 5 hours, the cementite particles are not sufficiently spheroidized.

[0121] On the other hand, if the holding time t1 exceeds 20 hours, the holding time is too long, and the ferrite and cementite particles become coarse. As a result, the average particle size of ferrite exceeds 20.0 μm, and the average particle size of cementite particles exceeds 1.50 μm. Furthermore, Cr and Mo are concentrated in the cementite particles, and the Cr concentration [Cr] in the cementite particles becomes too high. θ and Mo concentration [Mo] θ However, the formula (1) is no longer satisfied. Furthermore, the spheroidization of cementite particles is excessively promoted, and the spheroidization rate of cementite particles becomes 85% or more.

[0122] If the holding time t1 is 5 to 20 hours, a steel sheet satisfying Features 1 to 7 can be manufactured, provided that other conditions are met.

[0123] [(Condition 5) Regarding FA defined by formula (A)] In the coiling step of the hot rolling step and the cold-rolled sheet annealing step, FA defined by the following formula (A) is set to 20,520 or less. FA = (700 + 273) × (X + log (td)) (A) Here, an eigenvalue defined by formula (B) described later is substituted for X in formula (A). A total holding time (hours) defined by formula (C) described later is substituted for td.

[0124] The eigenvalue X is defined by the following formula (B): X=20+(Cr / 52+Mo / 96) ０．２ (B) Here, each element symbol in formula (B) is substituted with the content of the corresponding element in the steel sheet in mass %.

[0125] The total holding time td is defined by the following formula (C): td = ta + tb + tc (C) Here, the holding time (hours) at an assumed coiling temperature of 700°C, converted from the coiling temperature CT, is substituted for ta in formula (C). The holding time (hours) at an assumed annealing temperature of 700°C in hot-rolled sheet annealing, converted from the annealing temperature T0 (°C) and holding time t0 (hours) in the hot-rolled sheet annealing process, is substituted for tb. Note that if the hot-rolled sheet annealing process is not performed, 0 is substituted for tb. The holding time (hours) at an assumed annealing temperature of 700°C in cold-rolled sheet annealing, converted from the annealing temperature T1 (°C) and holding time t1 (hours) in the cold-rolled sheet annealing process, is substituted for tc.

[0126] The above-mentioned ta, tb, and tc are indicators of holding times when the coiling temperature CT (°C) in the hot rolling process, the annealing temperature T0 (°C) in the hot-rolled sheet annealing process, and the annealing temperature T1 (°C) in the cold-rolled sheet annealing process are each converted to 700°C. Specifically, ta, tb, and tc can be determined by the following formulas (D) to (F): ta=10{(CT+273)×(X+log(10)) / (700+273)−X} (D) Here, the coiling temperature (°C) in the hot rolling process is substituted for CT in formula (D). The eigenvalue defined in the above-mentioned formula (B) is substituted for X. tb = 10 {(T0 + 273) × (X + log (t0)) / (700 + 273) - X} (E) Here, the annealing temperature (°C) of the hot-rolled sheet annealing process is substituted for T0 in formula (E). The holding time (hours) of the hot-rolled sheet annealing process is substituted for t0. The eigenvalue defined in the above formula (B) is substituted for X. tc = 10 {(T1 + 273) × (X + log (t1)) / (700 + 273) - X} (F) Here, the annealing temperature (°C) of the cold-rolled sheet annealing process is substituted for T1 in formula (F). The holding time (hours) of the cold-rolled sheet annealing process is substituted for t1. The eigenvalue defined in the above formula (B) is substituted for X.

[0127] FA is an index of suppression of concentration of Cr and Mo in cementite particles. When FA exceeds 20520, the amount of heat applied to the steel sheet in the coiling step in the hot rolling process and the cold-rolled sheet annealing step is too large relative to the Cr content and Mo content in the steel sheet. In this case, Cr and Mo are concentrated in the cementite particles, and the Cr concentration [Cr] in the cementite particles is θ and Mo concentration [Mo] θ When FA is 20520 or less, on the premise that the chemical composition satisfies the characteristic 1, the Cr concentration [Cr] in the cementite particles θ and Mo concentration [Mo] θ is appropriately adjusted. Therefore, formula (1) is satisfied. If the FA is 20520 or less, a steel plate satisfying Features 1 to 7 can be manufactured, provided that other conditions are met.

[0128] Through the above manufacturing process, a steel sheet satisfying Features 1 to 7 can be manufactured.

[0129] The effects of the steel sheet of this embodiment will be described in more detail below using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel sheet of this embodiment. Therefore, the steel sheet of this embodiment is not limited to this one example of conditions.

[0130] Steel sheets having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured.

[0131]

[0132]

[0133] Specifically, molten steel was continuously cast to produce a slab. The slab was then subjected to a hot rolling process. Specifically, the slab was heated at 1150 to 1250°C for 240 minutes. The heated slab was rolled in a roughing mill to produce a rough bar. The rough bar was then rolled using a finishing mill to produce a hot-rolled steel sheet. The finish rolling temperature for each test number was 830 to 950°C. The reduction ratio of the final pass was 5 to 30%. The hot-rolled steel sheet after finish rolling was wound up and formed into a coil. The coiled hot-rolled steel sheet was allowed to cool to room temperature. The coiling temperature CT was 400 to 650°C.

[0134] The hot-rolled steel sheet after the hot rolling process was subjected to a cold rolling process without being subjected to a hot-rolled sheet annealing process. The cold rolling rate CR in the cold rolling process was 5 to 70%. The cold-rolled steel sheet after the cold rolling was subjected to a cold-rolled sheet annealing process. The annealing temperature T1 was 500 to 750°C, and the holding time t1 was 1 to 48 hours. The steel sheet was produced by the above production process.

[0135] [Evaluation Tests] The following tests were carried out on the manufactured steel sheets with each test number: (Test 1) Total area ratio measurement test of ferrite and cementite particles (Test 2) Average ferrite particle size measurement test (Test 3) Cr concentration [Cr] in cementite particles θ and Mo concentration [Mo] θ(Test 4) Measurement test of average particle size of cementite particles (Test 5) Measurement test of spheroidization rate of cementite particles (Test 6) Measurement test of dispersibility of cementite particles DI (Test 7) ​​Hardenability evaluation test (Test 8) Cold workability evaluation test Tests 1 to 8 will be described below.

[0136] [(Test 1) Test for Measuring the Total Area Ratio of Ferrite and Cementite Particles] The total area ratio of ferrite and cementite particles was determined for each test number based on the method described in the above-mentioned [Method for Measuring the Total Area Ratio of Ferrite and Cementite Particles in a Microstructure]. As a result, as shown in Table 2, the total area ratio of ferrite and cementite particles for each test number was 95% or more.

[0137]

[0138] [(Test 2) Average Ferrite Grain Size Measurement Test] The average ferrite grain size of the steel sheet of each test number was determined based on the method described above in [Method for measuring average ferrite grain size]. The results are shown in Table 2.

[0139] [(Test 3) Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Measurement test of the above [Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Based on the method described in [Measurement method for], the Cr concentration [Cr] in the cementite particles of the steel plate of each test number θ and Mo concentration [Mo] θ Furthermore, the obtained Cr concentration [Cr] θ and Mo concentration [Mo] θ Based on this, the following F1 was calculated: F1 = 2 [Cr] θ +3 [Mo] θ Obtained Cr concentration [Cr] θ , Mo concentration [Mo] θ, and F1 are shown in Table 2. When F1 is 3 / √C or less, that is, when formula (1) is satisfied, "E (Excellent)" is shown next to "formula (1)" in Table 2. When F1 exceeds 3 / √C, that is, when formula (1) is not satisfied, "NA (Not Allowed)" is shown next to "formula (1)" in Table 2.

[0140] [(Test 4) Measurement of average particle size of cementite particles] The average particle size of cementite particles in the steel sheet of each test number was determined based on the method described in the above [Method for measuring average particle size of cementite particles]. The obtained average particle sizes of cementite particles are shown in Table 2.

[0141] [(Test 5) Measurement test of spheroidization ratio of cementite particles] The spheroidization ratio of cementite particles of the steel plate of each test number was determined based on the method described in [Method for measuring spheroidization ratio] above. The obtained spheroidization ratios are shown in Table 2.

[0142] [(Test 6) Measurement of Cementite Particle Dispersion DI] Based on the method described in the above [Method for Measuring Dispersion DI], the cementite particle dispersion DI of the steel sheet of each test number was determined. Five square regions were selected (i.e., the number of minute square sections was 500). The obtained cementite particle dispersion DI is shown in Table 2.

[0143] [(Test 7) ​​Hardenability Evaluation Test] Based on the method described in the above [Hardenability Evaluation Method], the hardenability of the steel plate with each test number at the time of quenching was evaluated. In Table 2, "Lower Hardness Limit" indicates the value of the maximum hardness after quenching HD0 x 0.95. If the hardness after quenching HD1 was equal to or greater than the lower limit, it was determined that sufficient hardenability was obtained (indicated by "E (Excellent)" in "Hardenability Evaluation" in Table 2). On the other hand, if the hardness after quenching HD1 was less than the lower limit, it was determined that sufficient hardenability was not obtained (indicated by "NA (Not Allowed)" in "Hardenability Evaluation" in Table 2).

[0144] [(Test 8) Cold Workability Evaluation Test] The cold workability of the steel plate of each test number was evaluated based on the method described in the above-mentioned [Cold Workability Evaluation Method]. The thickness of the plate-shaped test specimen was 2 mm. The notch elongation obtained is shown in Table 2. If the notch elongation was 15% or more, it was determined that sufficient cold workability was obtained.

[0145] [Evaluation Results] With reference to Table 1-1, Table 1-2, and Table 2, the steel plates of Test Nos. 1, 2, 6, 7, 10, 12, 15, 17, 18, 20 to 44, and 46 satisfied Features 1 to 7. Therefore, the obtained quenched hardness HD1 was 95% or more of the maximum quenched hardness HD0, and the steel plates obtained sufficient hardenability. Furthermore, the notch elongation was 15% or more, and sufficient cold workability was obtained.

[0146] On the other hand, in test number 3, the average grain size of ferrite exceeded 20.0 μm. Furthermore, the spheroidization rate was 85% or more. Furthermore, the Cr concentration [Cr] in the cementite grains θ and Mo concentration [Mo] θ Therefore, the obtained hardness HD1 was less than 95% of the maximum hardness HD0, and sufficient hardenability was not obtained.

[0147] In test numbers 4, 8, and 16, the spheroidization rate was 85% or more. Therefore, the obtained quenched hardness HD1 was less than 95% of the maximum quenched hardness HD0, and sufficient hardenability was not obtained.

[0148] In test numbers 5 and 14, the dispersity DI exceeded 45%. Furthermore, the Cr concentration [Cr] in the cementite particles θ and Mo concentration [Mo] θ The formula (1) was not satisfied. Therefore, the obtained hardness HD1 was less than 95% of the maximum hardness HD0, and sufficient hardenability was not obtained. Furthermore, the notch elongation was less than 15%, and sufficient cold workability was not obtained.

[0149] In test number 9, the spheroidization rate was less than 70%. Furthermore, the Cr concentration in the cementite particles [Cr] θ and Mo concentration [Mo] θThe formula (1) was not satisfied. Therefore, the obtained hardness HD1 was less than 95% of the maximum hardness HD0, and sufficient hardenability was not obtained. Furthermore, the notch elongation was less than 15%, and sufficient cold workability was not obtained.

[0150] In test number 11, the spheroidization rate was less than 70%, and therefore the notch elongation was less than 15%, and sufficient cold workability was not obtained.

[0151] In test number 13, the average particle size of the cementite particles exceeded 1.50 μm. Furthermore, the spheroidization rate exceeded 85%. Furthermore, the Cr concentration [Cr] in the cementite particles θ and Mo concentration [Mo] θ Therefore, the obtained hardness HD1 was less than 95% of the maximum hardness HD0, and sufficient hardenability was not obtained.

[0152] In test number 19, the spheroidization rate was 85% or more. Furthermore, the Cr concentration in the cementite particles [Cr] θ and Mo concentration [Mo] θ Therefore, the obtained hardness HD1 was less than 95% of the maximum hardness HD0, and sufficient hardenability was not obtained.

[0153] In test number 45, the average particle size of the cementite particles exceeded 1.50 μm. Furthermore, the Cr concentration [Cr] in the cementite particles θ and Mo concentration [Mo] θ Therefore, the obtained hardness HD1 was less than 95% of the maximum hardness HD0, and sufficient hardenability was not obtained.

[0154] In test number 47, the C content was too high, so the notch elongation was less than 15%, and sufficient cold workability was not obtained.

[0155] In test number 48, the Si content was too high, so the notch elongation was less than 15%, and sufficient cold workability was not obtained.

[0156] In test number 49, the Mn content was too high, so the notch elongation was less than 15%, and sufficient cold workability was not obtained.

[0157] In test number 50, the Mn content was too low, so the obtained hardness HD1 was less than 95% of the maximum hardness HD0, and sufficient hardenability was not obtained.

[0158] In test number 51, the Mn content was too low. Furthermore, the Cr content was too high. Therefore, the Cr concentration in the cementite particles [Cr] θ and Mo concentration [Mo] θ Therefore, the obtained hardness HD1 was less than 95% of the maximum hardness HD0, and sufficient hardenability was not obtained.

[0159] In addition, in Steel No. 36 in Tables 1-1 and 1-2, the C content was too low, and therefore the maximum quenched hardness was too low.

[0160] Steel plates with each test number shown in Table 3 were manufactured. Specifically, slabs were manufactured by continuous casting of molten steel. A hot rolling process was carried out on the slabs. Specifically, the slabs were heated for 240 minutes at the heating temperature shown in Table 3 ("Slab heating temperature" in Table 3). The heated slabs were rolled in a roughing mill to manufacture rough bars. Furthermore, the rough bars were rolled using a finishing mill to manufacture hot-rolled steel plates. The finishing rolling temperature, final pass reduction, and coiling temperature CT for each test number were as shown in Table 3.

[0161]

[0162] For test numbers 1 to 18 and 20, the hot-rolled steel sheets after the hot rolling process were subjected to a cold rolling process without being subjected to a hot-rolled sheet annealing process. The cold rolling reduction ratio CR in the cold rolling process was as shown in Table 3. The cold-rolled steel sheets after the cold rolling were subjected to a cold-rolled sheet annealing process. The annealing temperature T1 and holding time t1 were as shown in Table 3. In the cold-rolled steel sheet annealing process, the cold-rolled steel sheets were furnace-cooled after the holding time t1 had elapsed. For test number 19, annealing (hot-rolled steel sheet annealing) was performed after the hot rolling process and before the cold rolling process. The annealing temperature T0 and the holding time t0 at the annealing temperature were as shown in Table 3. Steel sheets were produced by the above-mentioned production process.

[0163] [Evaluation Tests] The above-described tests 1 to 8 were carried out on the steel sheets with each test number.

[0164] [Evaluation Test] The evaluation results are shown in Table 4.

[0165]

[0166] With reference to Table 1-1, Table 1-2, Table 3, and Table 4, the chemical compositions of test numbers 1 to 10 were appropriate and satisfied conditions 1 to 5 of the manufacturing conditions. Therefore, the steel sheets of these test numbers satisfied features 1 to 7. As a result, sufficient hardenability and sufficient cold workability were obtained.

[0167] On the other hand, in Test No. 11, although the chemical composition was appropriate, the coiling temperature CT was low. Therefore, the spheroidization rate of the cementite particles was too high. As a result, sufficient hardenability was not obtained.

[0168] On the other hand, in Test No. 12, although the chemical composition was appropriate, the coiling temperature CT was high. Therefore, the spheroidization rate of the cementite particles was too low. Furthermore, the dispersity DI of the cementite particles was too high. Furthermore, F1 was too high. As a result, sufficient hardenability was not obtained, and sufficient cold workability was not obtained.

[0169] In test number 13, although the chemical composition was appropriate, the cold rolling rate CR was too low. Therefore, the spheroidization rate of the cementite particles was too low. Furthermore, the dispersity DI of the cementite particles was too high. As a result, sufficient cold workability was not obtained.

[0170] In test number 14, although the chemical composition was appropriate, the cold rolling rate CR was too high. As a result, the average grain size of ferrite was too small. Furthermore, the spheroidization rate of cementite particles was too high. As a result, sufficient hardenability and cold workability were not obtained.

[0171] In test number 15, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too low. As a result, the microstructure was an unrecrystallized structure, and ferrite could not be confirmed. Furthermore, the spheroidization rate of cementite particles was low. As a result, sufficient cold workability was not obtained.

[0172] In Test No. 16, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too high. As a result, the cementite particles were coarse. Furthermore, the spheroidization rate of the cementite particles was high. Furthermore, F1 was too high. As a result, sufficient hardenability was not obtained.

[0173] In test number 17, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing step was too short. Therefore, the spheroidization rate of cementite particles was too low. As a result, sufficient cold workability was not obtained.

[0174] In test number 18, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too long. As a result, the ferrite and cementite particles were coarse. Furthermore, the spheroidization rate of the cementite particles was high. Furthermore, F1 was too high. As a result, sufficient hardenability was not obtained.

[0175] In test number 19, annealing was performed after the hot rolling process but before the cold rolling process. Therefore, FA exceeded 20520. As a result, ferrite and cementite particles were coarse. Furthermore, the spheroidization rate of the cementite particles was high. Furthermore, F1 was too high. As a result, sufficient hardenability was not obtained.

[0176] In test number 20, FA exceeded 20520. Therefore, F1 was too high. As a result, sufficient hardenability was not obtained.

[0177] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A steel plate, the chemical composition of which, in mass %, is as follows: C: 0.15 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. The average grain size of the ferrite is 5.0 to 20.0 μm. The Cr concentration [Cr] in mass % in the cementite particles θ and the Mo concentration [Mo] in mass % in the cementite particles θ and the C content in mass % in the steel plate satisfy formula (1). The average particle size of the cementite particles is 1.50 μm or less. Among the cementite particles, when the cementite particles with an aspect ratio of 3.0 or less are defined as spherical cementite particles, the spheroidization rate, which is the ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is less than 70 to 85%. The microstructure is divided into minute square sections with a side length of 10 μm. The average number of the cementite particles in 500 or more of the minute square sections is defined as N θ and the sample standard deviation of the number of the cementite particles in the minute square section is defined as σ θ When defined in this way, the degree of dispersion DI defined by formula (2) is 45% or less. Steel plate. 2[Cr] θ +3[Mo] θ ≦3 / √C (1) DI = σ θ / N θ ×100 (2) Here, the C in formula (1) is substituted with the C content in mass % in the chemical composition of the steel plate.

2. The steel sheet according to claim 1, wherein the chemical composition contains at least one element selected from the group consisting of Mo: 0.001 to 0.500%, Ni: 0.001 to 1.000%, B: 0.0001 to 0.0100%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%.

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