Steel sheet

Abstract

The present invention provides a steel sheet that can provide excellent bendability and excellent quenching properties, and that, when used as a machine component, can provide excellent hydrogen embrittlement resistance. A steel sheet according to the present disclosure comprises, in terms of mass%, 0.50-0.90% C, 0.10-0.50% Si, 0.20-1.30% Mn, not more than 0.100% P, not more than 0.100% S, not more than 0.100% Al, 0.01-1.20% Cr, and not more than 0.0150% N. The total area ratio of ferrite and cementite particles is not less than 95%. The average particle size of the ferrite is not more than 20.0 μm. The average particle diameter of the cementite particles is not more than 1.50 μm. The spheroidization ratio of the cementite is not less than 85%. The C concentration ratio F1 defined by expression (1) below is not more than 0.50, where [C]s is the C content at a position at a depth of 50 μm in the sheet thickness direction from the surface of the steel sheet, and [C]c is the C content at the center position in the sheet thickness direction of the steel sheet.　(1): F1=[C]s / [C]c

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 plates with a high C content (high carbon steel plates) are used as materials for machine parts, such as automobile parts. These machine parts are manufactured using such steel plates as materials as follows: The steel plate is cold-worked to form it into the shape of the machine part; the cold-worked steel plate is then quenched and tempered; high-strength machine parts are manufactured through the above manufacturing process.

[0003] In order to increase the strength of machine parts, it is necessary to improve the hardenability of steel sheets that are used as raw materials for machine parts. Therefore, steel sheets with improved hardenability have been 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 steel sheet of Patent Document 1 improves hardenability by ensuring a sufficient amount of solute B.

[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 composition including ferrite and carbides. The volume ratio of the ferrite and carbides to the entire microstructure is 90% or more, and the volume ratio of pro-eutectoid ferrite to 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 with a particle size of 1 μm or more to the total number of carbides is 30% or more and 60% or less. In the steel plate of Patent Document 2, the Mn concentration in the carbides is reduced. This makes the carbides more likely to dissolve during quenching. As a result, hardenability is improved.

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

[0007] Among high-carbon steel sheets, steel sheets with a C content of 0.50% or more are often used as materials for mechanical parts such as springs and washers. In the manufacturing process of mechanical parts such as springs and washers, the steel sheets used as the raw materials are subjected to bending. Therefore, the steel sheets used as the raw materials are required to have high bendability. Furthermore, mechanical parts after quenching have high strength. Therefore, the mechanical parts are required to have high resistance to hydrogen embrittlement. Patent Documents 1 and 2 do not discuss bendability and resistance to hydrogen embrittlement.

[0008] An object of the present disclosure is to provide a steel sheet that has excellent bendability and excellent hardenability, and that has excellent hydrogen embrittlement resistance when used as a mechanical part.

[0009] The steel plate according to the present disclosure contains, in mass %, C: 0.50 to 0.90%, Si: 0.10 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%, Cu: 0 to 0.15%, W: 0 to 0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, Ca: 0 to 0.050%, and the balance being Fe and impurities, wherein in the microstructure, a total area ratio of ferrite and cementite particles is 95% or more, an average particle size of the ferrite is 20.0 μm or less, and an average particle size of the cementite particles is 1.50 μm or less, 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 85% or more, and when the C content at a depth of 50 μm from the surface of the steel plate in the plate thickness direction is [C]s and the C content at the center position in the plate thickness direction of the steel plate is [C]c, a C concentration ratio F1 defined by formula (1) is 0.50 or less. F1=[C]s / [C]c (1)

[0010] The steel sheet according to the present disclosure has excellent bendability and excellent hardenability, and when used as a mechanical part, has excellent resistance to hydrogen embrittlement.

[0011] The present inventors have conducted research into steel sheets that have excellent hardenability and bendability and that, when used to form mechanical parts, provide excellent hydrogen embrittlement resistance. As a result, the present inventors have made the following findings.

[0012] First, the present inventors investigated, from the viewpoint of chemical composition, improvements in hardenability and bendability in steel sheets having a C content of 0.50% or more, and improvements in hydrogen embrittlement resistance when used as mechanical parts. As a result, the present inventors found that the steel sheets had a composition of, in mass%, C: 0.50 to 0.90%, Si: 0.10 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%, Cu: 0 to 0.15%, W: 0 to 0.15%, and T The inventors believed that a chemical composition consisting of a: 0-0.15%, 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%, Ca: 0-0.050%, and the balance: Fe and impurities could potentially achieve improved hardenability and bendability in steel sheets, and improved hydrogen embrittlement resistance when used in mechanical parts. Therefore, the inventors further investigated means for improving the hardenability, bendability, and hydrogen embrittlement resistance of steel sheets having the above chemical composition from the perspective of microstructure.

[0013] 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 a manufacturing process for producing mechanical components using the steel sheet as a raw material, it is preferable that the cementite particles in the steel sheet are easily dissolved during quenching. In order to improve the solid solubility of the cementite particles during quenching, it is preferable that the particle size of the cementite particles is small. 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.

[0014] The present inventors have further investigated means for improving the bendability of the microstructure of a steel sheet. In order to improve the bendability of a steel sheet, it is effective to increase the spheroidization rate of cementite particles and to adjust the average grain size of ferrite to an appropriate size. Therefore, in the steel sheet of this embodiment, the spheroidization rate of cementite particles is set to 85% or more, and the average grain size of ferrite is set to 20.0 μm or less.

[0015] However, even with a steel sheet having the above-described configuration, there are cases where excellent bendability and excellent hydrogen embrittlement resistance are still not obtained. Therefore, the present inventors conducted further studies. Here, the present inventors focused on the strength of the surface layer of the steel sheet. If the strength of the surface layer of the steel sheet is as high as that of the interior of the steel sheet, sufficient bendability is not obtained, and sufficient hydrogen embrittlement resistance is not obtained when the steel sheet is used as a mechanical part. If the C concentration in the surface layer of the steel sheet is lower than the C concentration in the interior of the steel sheet other than the surface layer, the bendability of the steel sheet is improved, and the strength of a mechanical part manufactured using the steel sheet is ensured, while also improving hydrogen embrittlement resistance.

[0016] Based on the above technical concept, the present inventors conducted further studies. As a result, the present inventors found that, when the C content at a depth of 50 μm from the surface of a steel plate in the plate thickness direction is [C]s and the C content at the center position in the plate thickness direction of the steel plate is [C]c, if the C concentration ratio F1 defined by formula (1) is 0.50 or less, the steel plate can have excellent bendability, and mechanical parts manufactured using the steel plate as a raw material can have excellent hydrogen embrittlement resistance. F1=[C]s / [C]c (1)

[0017] The steel sheet of this embodiment has been completed based on the above-mentioned technical concept, and has the following configuration.

[0018] The steel plate of the first configuration has, in mass %, C: 0.50 to 0.90%, Si: 0.10 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%, Cu: 0 to 0.15%, W: 0 to 0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, Ca: 0 to 0.050%, and the balance being Fe and impurities, wherein in the microstructure, a total area ratio of ferrite and cementite particles is 95% or more, an average particle size of the ferrite is 20.0 μm or less, and an average particle size of the cementite particles is 1.50 μm or less, 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 85% or more, and when the C content at a depth of 50 μm from the surface of the steel plate in the plate thickness direction is [C]s and the C content at the center position in the plate thickness direction of the steel plate is [C]c, a C concentration ratio F1 defined by formula (1) is 0.50 or less. F1=[C]s / [C]c (1)

[0019] The steel plate of the second configuration is the steel plate of the first configuration, and contains one or more elements selected from the group consisting of Mo: 0.001 to 0.500%, Ni: 0.001 to 1.000%, and B: 0.0001 to 0.0100%.

[0020] The steel plate of the third configuration is the steel plate of the first or second configuration, and contains one or more elements selected from the group consisting of V: ​​0.001 to 0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%.

[0021] A steel plate having a fourth configuration is a steel plate having any one of the first to third configurations, and contains one or more elements selected from the group consisting of Cu: 0.01 to 0.15%, W: 0.01 to 0.15%, Ta: 0.01 to 0.15%, Sn: 0.001 to 0.050%, Sb: 0.001 to 0.050%, Co: 0.001 to 0.050%, As: 0.001 to 0.050%, Mg: 0.001 to 0.050%, Y: 0.001 to 0.050%, Zr: 0.001 to 0.050%, La: 0.001 to 0.050%, Ce: 0.001 to 0.050%, and Ca: 0.001 to 0.050%.

[0022] The steel sheet of this embodiment will be described in detail below. Note that "%" regarding elements means mass % unless otherwise specified.

[0023] [Features of Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment satisfies the following features 1 to 6. (Feature 1) The chemical composition, in mass %, is C: 0.50 to 0.90%, Si: 0.10 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%, Cu: 0-0.15%, W: 0-0.15%, Ta: 0-0.15%, 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%, Ca: 0-0.050%, 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 20.0 μm or less. (Feature 4) The average grain size of cementite particles is 1.50 μm or less. (Feature 5) 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 85% or more. (Feature 6) When the C content at a depth of 50 μm from the surface of the steel plate in the plate thickness direction is [C]s and the C content at the center position in the plate thickness direction of the steel plate is [C]c, the C concentration ratio F1 defined by formula (1) is 0.50 or less. F1=[C]s / [C]c (1) Features 1 to 6 will be described below.

[0024] [(Feature 1) Chemical Composition] The chemical composition of the steel sheet of this embodiment contains the following elements.

[0025] C: 0.50 to 0.90% Carbon (C) improves the hardenability of steel sheet. As a result, when hardening is performed in the process of manufacturing a mechanical part using the steel sheet as a raw material, the strength of the mechanical part is increased. If the C content is less than 0.50%, the above effect is not sufficiently obtained. On the other hand, if the C content exceeds 0.90%, the bendability and hydrogen embrittlement resistance of the steel sheet are reduced. Therefore, the C content is 0.50 to 0.90%. A preferred lower limit of the C content is 0.52%, more preferably 0.55%, and even more preferably 0.60%. A preferred upper limit of the C content is 0.88%, more preferably 0.85%, and even more preferably 0.80%. A preferred range of the C content is, for example, 0.52 to 0.88%, more preferably 0.55 to 0.85%, and even more preferably 0.60 to 0.80%.

[0026] Si: 0.10 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 steel sheets when tempering is performed in the process of manufacturing machine parts using the steel sheets as raw materials. Si also promotes decarburization of the steel sheet surface during cold-rolled sheet annealing. If the Si content is less than 0.10%, the above effects cannot be fully achieved. 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. As a result, the bendability of the steel sheet decreases. Therefore, the Si content is 0.10 to 0.50%. The preferred lower limit of the Si content is 0.12%, more preferably 0.15%, and even more preferably 0.20%. The preferred upper limit of the Si content is 0.48%, more preferably 0.44%, and even more preferably 0.40%. The Si content is preferably in the range of 0.12 to 0.48%, more preferably 0.15 to 0.44%, and even more preferably 0.20 to 0.40%.

[0027] Mn: 0.20 to 1.30% Manganese (Mn) improves the hardenability of steel sheet. As a result, when hardening is performed in the process of manufacturing a mechanical part using the steel sheet as a raw material, the strength of the mechanical part is increased. If the Mn content is less than 0.20%, the above effect is not sufficiently achieved. 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. As a result, the bendability of the steel sheet decreases. Therefore, the Mn content is 0.20 to 1.30%. A preferred lower limit of the Mn content is 0.25%, more preferably 0.30%, and even more preferably 0.35%. A preferred upper limit of the Mn content is 1.25%, more preferably 1.20%, and even more preferably 1.15%. A preferred range of the Mn content is, for example, 0.25 to 1.25%, more preferably 0.30 to 1.20%, and even more preferably 0.35 to 1.15%.

[0028] P: 0.100% or less Phosphorus (P) is an impurity. The P content may be 0%. If the P content exceeds 0.100%, the toughness of the steel plate decreases. Therefore, the P content is 0.100% or less. The P content is preferably as low as possible. In other words, a P content of 0% is preferable. However, excessive reduction of the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the P content is 0.090%, more preferably 0.080%, and even more preferably 0.050%. The preferred range of the P content is, for example, 0.001 to 0.090%, more preferably 0.003 to 0.080%, and even more preferably 0.005 to 0.050%.

[0029] S: 0.100% or less Sulfur (S) is an impurity. The S content may be 0%. If the S content exceeds 0.100%, S forms excessive sulfides. This reduces the bendability of the steel sheet. Therefore, the S content is 0.100% or less. The S content is preferably as low as possible. In other words, an S content of 0% is preferable. However, excessive reduction of the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the S content is 0.090%, more preferably 0.080%, and even more preferably 0.050%. The preferred range of the S content is, for example, 0.001 to 0.090%, more preferably 0.003 to 0.080%, and even more preferably 0.005 to 0.050%.

[0030] Al: 0.100% or less Aluminum (Al) is an impurity. The Al content may be 0%. Al bonds with N to form AlN. AlN refines austenite grains during heating for quenching in the process of manufacturing machine parts using steel sheet as a raw material. Refinement of austenite grains reduces the hardenability of the steel sheet. If the Al content exceeds 0.100%, austenite grains are excessively refined during heating for quenching, significantly reducing the hardenability of the steel sheet. Therefore, the Al content is 0.100% or less. A preferred lower limit of the Al content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. A preferred upper limit of the Al content is 0.090%, more preferably 0.080%, even more preferably 0.070%, and even more preferably 0.050%. A preferred range of the Al content is, for example, 0.001 to 0.090%, more preferably 0.005 to 0.070%, and even more preferably 0.010 to 0.050%. In the chemical composition of the steel sheet of this embodiment, the Al content means the acid-soluble Al (sol. Al) content.

[0031] Cr: 0.01 to 1.20% Chromium (Cr) improves the hardenability of steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical part using steel sheet as a raw material, the strength of the mechanical part is increased. If the Cr content is less than 0.01%, the above effect cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 1.20%, the strength of the steel sheet becomes excessively high. As a result, the bendability of the steel sheet decreases. 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%, and even more preferably 0.50%. The Cr content is preferably in the range of 0.02 to 1.15%, more preferably 0.03 to 1.10%, and even more preferably 0.05 to 0.50%.

[0032] N: 0.0150% or less Nitrogen (N) is an unavoidably contained impurity. In other words, the N content is greater than 0%. N combines with Al to form AlN. AlN refines austenite grains during heating in quenching during the process of manufacturing machine parts 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%, austenite grains are excessively refined during heating in quenching, significantly reducing the hardenability of the steel sheet. Therefore, the N content is 0.0150% or less. A preferred lower limit of the N content is 0.0001%, more preferably 0.0005%. A preferred upper limit of the N content is 0.0140%, more preferably 0.0130%. The N content is preferably in the range of 0.0001 to 0.0140%, for example, and more preferably in the range of 0.0005 to 0.0130%.

[0033] 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.

[0034] [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 first to third groups, in place of a portion of Fe. [Group 1] One or more selected from the group consisting of Mo: 0 to 0.500%, Ni: 0 to 1.000%, and B: 0 to 0.0100%. [Group 2] One or more selected from the group consisting of V: ​​0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%. [Group 3] Cu: 0 to 0.15%, W: 0 to 0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, One or more elements selected from the group consisting of: La: 0 to 0.050%, Ce: 0 to 0.050%, and Ca: 0 to 0.050%. These optional elements will be described below.

[0035] [Regarding Group 1 (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.

[0036] 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. Therefore, when quenching is performed in a process for manufacturing a mechanical part using the steel sheet as a raw material, the strength of the mechanical part is increased. Mo also improves the temper softening resistance of the steel sheet when tempering is performed in a process for manufacturing a mechanical part using the steel sheet as a raw material. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. However, when the Mo content exceeds 0.500%, the strength of the steel sheet becomes excessively high. As a result, the bendability of the steel sheet decreases. Therefore, the Mo content is 0 to 0.500%. The preferred lower limit of the Mo content is 0.001%, more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%. 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%. The Mo content is preferably in the range of, for example, 0.001 to 0.450%, even more preferably 0.003 to 0.400%, even more preferably 0.005 to 0.350%, and still more preferably 0.010 to 0.300%.

[0037] 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 improves the hardenability of the steel sheet. Therefore, when quenching is performed in a process for manufacturing a mechanical part using the steel sheet as a raw material, the strength of the mechanical part is increased. Ni also improves the temper softening resistance of the steel sheet when tempering is performed in a process for manufacturing a mechanical part using the steel sheet as a raw material. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, when the Ni content exceeds 1.000%, the strength of the steel sheet becomes excessively high. As a result, the bendability of the steel sheet decreases. Therefore, the Ni content is 0 to 1.000%. The preferred lower limit of the Ni content is 0.001%, more preferably 0.005%, even more preferably 0.007%, and even more preferably 0.010%. 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 even more preferably 0.600%. The Ni content is preferably in the range of, for example, 0.001 to 0.950%, even more preferably 0.005 to 0.900%, even more preferably 0.007 to 0.800%, even more preferably 0.010 to 0.700%, and even more preferably 0.010 to 0.600%.

[0038] 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. Therefore, by performing hardening in a process for manufacturing a mechanical part using the steel sheet as a raw material, the strength of the mechanical part 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 are formed. In this case, the strength of the steel sheet becomes excessively high. As a result, the bendability of the steel sheet decreases. 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 even more preferably 0.0050%. The B content is preferably in the range of, for example, 0.0001 to 0.0090%, even more preferably 0.0003 to 0.0080%, even more preferably 0.0005 to 0.0070%, even more preferably 0.0005 to 0.0060%, and even more preferably 0.0005 to 0.0050%.

[0039] [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 elements and may not be contained. When contained, V, Nb, and Ti form carbides and suppress coarsening of austenite grains during quenching heating in the process of manufacturing mechanical components using the steel sheet as a raw material. Therefore, the toughness of the mechanical components is improved.

[0040] V: 0 to 0.500% Vanadium (V) is an optional element and does not necessarily need to be present. In other words, the V content may be 0%. When V is present, that is, when the V content exceeds 0%, V forms carbides and suppresses coarsening of austenite grains during quenching heating in the process of manufacturing mechanical parts using the steel sheet as a raw material. This improves the toughness of the mechanical parts. Even if even a small amount of V is present, the above effect can be achieved to some extent. However, when the V content exceeds 0.500%, excessive carbides are formed, precipitation strengthening the steel sheet. This reduces the bendability of the steel sheet. 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 preferred upper limit of the V content is 0.480%, more preferably 0.450%, and even more preferably 0.400%. The V content is preferably in the range of, for example, 0.001 to 0.480%, more preferably 0.003 to 0.450%, and even more preferably 0.005 to 0.400%.

[0041] Nb: 0 to 0.500% Niobium (Nb) is an optional element and does not necessarily need to 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 quenching heating in the process of manufacturing mechanical parts using the steel sheet as a raw material. This improves the toughness of the mechanical parts. Nb also 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%, excessive carbides are formed, precipitation strengthening the steel sheet. This reduces the bendability of the steel sheet. Therefore, the Nb content is 0 to 0.500%. The preferred lower limit of the Nb content is 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%. The Nb content is preferably in the range of, for example, 0.001 to 0.480%, even more preferably 0.003 to 0.450%, even more preferably 0.005 to 0.400%, even more preferably 0.005 to 0.350%, and even more preferably 0.005 to 0.300%.

[0042] 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, that is, when the Ti content exceeds 0%, Ti forms carbides and suppresses coarsening of austenite grains during quenching heating in the process of manufacturing mechanical parts using steel sheet as a raw material. This improves the toughness of the mechanical parts. Ti also 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 Ti is included even slightly, the above effect can be achieved to some extent. However, if the Ti content exceeds 0.150%, excessive carbides are formed, precipitation strengthening the steel sheet. This reduces the cold workability of the steel sheet. Therefore, the Ti content is 0 to 0.150%. The preferred lower limit of the Ti content is 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%, even more preferably 0.120%, even more preferably 0.100%, and even more preferably 0.080%. The Ti content is preferably in the range of 0.001 to 0.145%, even more preferably 0.003 to 0.130%, even more preferably 0.005 to 0.120%, even more preferably 0.005 to 0.100%, and even more preferably 0.005 to 0.080%.

[0043] [Regarding the third group (Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca)] 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 Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca. All of these elements are optional elements and may not be contained. In other words, the content of these elements may be 0%.

[0044] These elements are all tramp elements and are considered impurities in the present invention. Therefore, the Cu content is 0 to 0.15%, the W content is 0 to 0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, and Ca: 0 to 0.050%.

[0045] The lower limit of the Cu content is preferably 0.01%, more preferably 0.03%. The upper limit of the Cu content is preferably 0.13%, more preferably 0.10%. The Cu content is preferably in the range of 0.01 to 0.13%, more preferably 0.03 to 0.10%, for example.

[0046] The lower limit of the W content is preferably 0.01%, more preferably 0.03%. The upper limit of the W content is preferably 0.13%, more preferably 0.10%. The W content is preferably in the range of 0.01 to 0.13%, more preferably 0.03 to 0.10%, for example.

[0047] The lower limit of the Ta content is preferably 0.01%, more preferably 0.03%. The upper limit of the Ta content is preferably 0.13%, more preferably 0.10%. The Ta content is preferably in the range of 0.01 to 0.13%, more preferably 0.03 to 0.10%, for example.

[0048] The lower limit of the Sn content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Sn content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The preferred range of the Sn content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0049] The lower limit of the Sb content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Sb content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The preferred range of the Sb content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0050] The lower limit of the Co content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Co content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The preferred range of the Co content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0051] The lower limit of the As content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the As content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The As content is preferably in the range of, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0052] The lower limit of the Mg content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Mg content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The preferred range of the Mg content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0053] The lower limit of the Y content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Y content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The Y content is preferably in the range of 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%, for example.

[0054] The lower limit of the Zr content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Zr content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The Zr content is preferably in the range of 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%, for example.

[0055] The lower limit of the La content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the La content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The La content is preferably in the range of 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%, for example.

[0056] The lower limit of the Ce content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Ce content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The Ce content is preferably in the range of 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%, for example.

[0057] The lower limit of the Ca content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Ca content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%. The Ca content is preferably in the range of 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%, for example.

[0058] [Method for Measuring Chemical Composition of Steel Sheet] The chemical composition of the steel sheet of this embodiment can be measured by a known elemental analysis method. Specifically, chips are collected from the interior of the steel sheet to a depth of 0.1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a known inert gas fusion-thermal conductivity method.

[0059] The content of each element is determined by rounding off the measured value to the nearest significant digit based on the significant digits specified in this embodiment. For example, the C content of the steel sheet in this embodiment is determined to be a value up to two decimal places. Therefore, the C content is determined to be a value up to two decimal places obtained by rounding off the measured value to two decimal places.

[0060] Similarly, the contents of elements other than the C content of the steel sheet of this embodiment are determined by rounding off the measured value to the smallest digit specified in this embodiment, and the value obtained is the content of the element. Note that rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0061] [(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.

[0062] In the microstructure, the structure other than the ferrite and cementite particles is, for example, one or more types selected from the group consisting of bainite, martensite, and pearlite.

[0063] 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.

[0064] The total area ratio of the ferrite and cementite particles is preferably in the range of 96 to 100%, more preferably 97 to 100%, even more preferably 98 to 100%, and still more preferably 99 to 100%.

[0065] If the total area ratio of ferrite and cementite particles is 95% or more, provided that Features 1 and 3 to 6 are satisfied, excellent bendability can be obtained, and high hydrogen embrittlement resistance can be obtained when the steel is used as a mechanical part.

[0066] [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 of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate. Of the surfaces of the test piece, the surface of 15 mm in the rolling direction and 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 3% nitric acid alcohol (nital etchant). Secondary electron images are observed for five arbitrary observation fields of the etched observation surface using a scanning electron microscope (SEM) at 1000x magnification. Each observation field is a rectangle of 100 μm × 120 μm.

[0067] In the observation field, ferrite and cementite particles exhibit different contrast and morphology from other structures (bainite, martensite, pearlite, etc.), and therefore, ferrite and cementite particles are identified in the observation field based on the contrast and morphology.

[0068] 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.

[0069] [(Feature 3) Average grain size of ferrite] In the steel sheet of this embodiment, the average grain size of ferrite is 20.0 μm or less. If the average grain size of ferrite exceeds 20.0 μm, bendability decreases. If the average grain size of ferrite exceeds 20.0 μm, the annealing time becomes longer due to the coarsening of the ferrite grains. In this case, the concentration of alloying elements in the cementite increases, and hardenability decreases. Therefore, the average grain size of ferrite is 20.0 μm or less.

[0070] The upper limit of the average grain size of ferrite is preferably 19.5 μm, more preferably 19.0 μm, even more preferably 18.5 μm, and still more preferably 18.0 μm.

[0071] The lower limit of the average grain size of ferrite is not particularly limited. However, excessive refinement of ferrite excessively increases the yield strength of the steel sheet. In this case, the processing load during bending increases excessively. Therefore, the preferred lower limit of the average grain size of ferrite is 5.0 μm. The more preferred lower limit of the average grain size of ferrite is 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.

[0072] The average particle size of ferrite is preferably in the range of, for example, 5.0 to 20.0 μm, more preferably 5.2 to 19.5 μm, even more preferably 5.5 to 19.0 μm, even more preferably 5.7 to 18.5 μm, even more preferably 6.0 to 18.0 μm, and even more preferably 6.5 to 18.0 μm.

[0073] [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 of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate width. The surface of the test piece measuring 15 mm in the rolling direction and thickness is defined as the observation surface. The observation surface of the test piece is mirror-polished. After mirror-polishing, etching is performed with a 3% nital etching solution. Secondary electron images are taken of the etched observation surface at five arbitrary observation fields at a depth of 1 / 4 of the plate thickness from the surface of the steel plate using a scanning electron microscope (SEM). The ferrite grain size number is determined by the intercept method in accordance with JIS G 0551:2020. The magnification of the SEM is selected in the range of 500 to 3000 times so that the number of ferrite grains cut by one line segment is at least 10 in one field of view. The cut lengths are determined for the five observation fields. The ferrite grain size number is calculated from the arithmetic mean value of the cut lengths of the five observation fields. The average grain size (μm) of ferrite is calculated from the obtained grain size number. The average grain size of ferrite is the value obtained by rounding the obtained value to one decimal place (i.e., the value to one decimal place).

[0074] [(Feature 4) Average Particle Diameter of Cementite Particles] In the steel sheet of this embodiment, the average particle diameter of the cementite particles is 1.50 μm or less. If the average particle diameter of the cementite particles is large, the bendability of the steel sheet decreases. Furthermore, if the average particle diameter of the cementite particles is large, the cementite particles do 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 steel sheet does not have sufficient hardenability. As a result, the mechanical component manufactured using the steel sheet as a raw material does not have sufficient strength.

[0075] When the average particle size of the cementite particles is 1.50 μm or less, the cementite particles are sufficiently small, so that the steel sheet has sufficient bendability. Furthermore, when heated in the quenching process, the cementite particles are sufficiently dissolved, improving the hardenability of the steel sheet.

[0076] 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 still more preferably 1.30 μm.

[0077] To improve hardenability, a smaller average particle size of cementite particles is preferable. However, if the average particle size of cementite particles is too small, the hardness of the steel sheet becomes too high, and the cold workability deteriorates. Therefore, the lower limit of the average particle size of cementite particles is preferably 0.05 μm, more preferably 0.10 μm, even more preferably 0.15 μm, and still more preferably 0.20 μm.

[0078] The average particle size of the cementite particles is preferably in the range of, for example, 0.05 to 1.50 μm, more preferably 0.10 to 1.45 μm, even more preferably 0.15 to 1.40 μm, even more preferably 0.20 to 1.35 μm, and even more preferably 0.20 to 1.30 μm.

[0079] [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 of the steel plate, 10 mm in the width direction, and the thickness is taken from the center of the steel plate. Of the surfaces of the test piece, the surface 15 mm in the rolling direction and the thickness is defined as the observation surface.

[0080] The observation surface of the test piece is mirror-polished. After mirror-polishing, the observation surface is etched using a picral solution. Secondary electron images are taken of five arbitrary observation fields on the etched observation surface, located at a depth of 1 / 4 of the plate thickness from the surface of the steel plate. 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.

[0081] In each secondary electron image, cementite particles are identified based on the contrast. The area of ​​each identified cementite particle is calculated, and the equivalent circle diameter of each cementite particle is calculated based on the area. The calculated equivalent circle diameter is defined as the particle diameter of the cementite particle. Note that the particle diameter is calculated 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. The average particle diameter of the cementite particles is a value obtained by rounding the obtained value to two decimal places (i.e., the value to one decimal place).

[0082] [(Feature 5) 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 85% or more.

[0083] If the spheroidization rate is 85% or more, excellent bendability can be obtained in the steel sheet, provided that Features 1 to 4 and 6 are satisfied. Therefore, in the steel sheet of this embodiment, the spheroidization rate is 85% or more.

[0084] A higher spheroidization rate is preferable. The lower limit of the spheroidization rate is preferably 87%, more preferably 89%, even more preferably 91%, and still more preferably 95%.

[0085] The spheroidization rate is preferably in the range of, for example, 85 to 100%, more preferably 87 to 100%, even more preferably 89 to 100%, even more preferably 91 to 100%, and even more preferably 95 to 100%.

[0086] [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).

[0087] The aspect ratio (= major axis / minor axis) of each cementite particle is determined 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 cementite particles in the five observation fields is defined as the spheroidization rate (%). The spheroidization rate is a value obtained by rounding the obtained value to one decimal place (i.e., an integer).

[0088] [(Feature 6) Regarding the C concentration ratio F1] In the steel plate of this embodiment, when the C content at a depth of 50 μm from the surface of the steel plate in the plate thickness direction is [C]s and the C content at the center position of the steel plate in the plate thickness direction is [C]c, the C concentration ratio F1 defined by formula (1) is 0.50 or less: F1=[C]s / [C]c (1) Note that the center position of the steel plate in the plate thickness direction is located closer to the interior of the steel plate than the depth of 50 μm from the surface of the steel plate in the plate thickness direction.

[0089] When the C concentration ratio F1 is higher than 0.50, the strength of the surface layer portion of the steel plate is high, and the strength of the surface layer portion of a mechanical component manufactured using the steel plate is also high. In this case, the bendability of the steel plate is reduced, and further, the hydrogen embrittlement resistance of the mechanical component manufactured using the steel plate is reduced.

[0090] When the C concentration ratio F1 is 0.50 or less, the strength of the surface layer of the steel sheet can be sufficiently suppressed. Therefore, the steel sheet can have excellent bendability. Furthermore, machine parts manufactured using the steel sheet as a raw material can have excellent hydrogen embrittlement resistance.

[0091] The upper limit of the C concentration ratio F1 is preferably 0.49, more preferably 0.48, even more preferably 0.47, even more preferably 0.46, and still more preferably 0.45.

[0092] The lower limit of the C concentration ratio F1 is not particularly limited. In a steel sheet satisfying Features 1 to 6, the lower limit of the C concentration ratio F1 is, for example, 0.05, more preferably 0.10, and even more preferably 0.15.

[0093] The C concentration ratio F1 is preferably in the range of, for example, 0.05 to 0.50, more preferably 0.10 to 0.49, even more preferably 0.15 to 0.48, even more preferably 0.15 to 0.47, even more preferably 0.15 to 0.46, and even more preferably 0.15 to 0.45.

[0094] [Method for measuring C concentration ratio F1] The C concentration ratio F1 can be measured by the following method. A test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate width. Of the surfaces of the test piece, the surface of 15 mm in the rolling direction and thickness is defined as the observation surface. The observation surface of the test piece is mirror-polished. On the mirror-polished observation surface, the C content (mass%) is measured at 300 measurement positions at a 1-μm pitch in the longitudinal direction of the steel plate, at a depth of 50 μm from the steel plate surface in the thickness direction, using an electron probe microanalyzer (EPMA). The arithmetic mean value of the C content (mass%) at the obtained 300 points is defined as [C]s. Furthermore, the C content (mass%) is measured at 300 measurement positions at a 1-μm pitch in the longitudinal direction of the steel plate at the center position in the thickness direction of the steel plate using an electron probe microanalyzer (EPMA). The arithmetic mean value of the C content at the obtained 300 points is defined as [C]c. Using the obtained [C]s and [C]c, the C concentration ratio F1 is calculated from formula (1). EPMA is measured under the following conditions: Acceleration voltage: 15.0 kV, Exposure current: 0.30 μA, Exposure time: 50 ms, Beam diameter: 1 μm. [C]s (mass%) and [C]c (mass%) are obtained by rounding the obtained values ​​to the nearest tenth (i.e., the value to the second decimal place). The C concentration ratio F1 is obtained by rounding the obtained values ​​to the nearest tenth (i.e., the value to the second decimal place).

[0095] [Effects of the Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment, which satisfies the above-described Features 1 to 6, provides excellent bendability. Furthermore, the steel sheet of the present embodiment provides sufficient hardenability during quenching in the process of manufacturing a mechanical component using the steel sheet as a raw material. Furthermore, the mechanical component manufactured using the steel sheet of the present embodiment as a raw material provides excellent hydrogen embrittlement resistance.

[0096] [Bendability] Bendability is evaluated using the following method. [Bendability Evaluation Method] A plate-shaped test specimen is taken from the center of the width of the steel plate. The shape of the plate-shaped test specimen is 15 mm in the rolling direction of the steel plate, 30 mm in the width direction, and plate thickness. A 90° V-bend test is performed on the plate-shaped test specimen. Specifically, a 90° V-bend is performed at the center of the width (30 mm) of the plate-shaped test specimen using a die and an indentation punch. The bend line formed on the plate-shaped test specimen by the V-bend is parallel to the rolling direction of the steel plate (L-axis bending). The R of the indentation punch used in the 90° V-bend test is 0.1 mm. The presence or absence of cracks on the surface of the plate-shaped test specimen after the 90° V-bend test is visually observed. If no cracks are observed, it is determined that excellent bendability is obtained.

[0097] [Quenchability] The quenchability is evaluated by the following method. [Quenchability evaluation method] (A ｃ１ Measurement of transformation point) A cylindrical test piece with a diameter of 3 mm and a length of 10 mm is taken from the center of the width of the steel plate. The longitudinal direction of the cylindrical test piece is parallel to the rolling direction of the steel plate. The thermal expansion coefficient during heating is measured using a Formaster testing machine. From the obtained thermal expansion coefficient, A ｃ１ Find the transformation point.

[0098] (Measurement of maximum quenched hardness) A plate-shaped test specimen is taken from the center of the steel plate width. The shape of the plate-shaped test specimen is 15 mm in the rolling direction of the steel plate, 30 mm in the width direction, and plate thickness. The plate-shaped test specimen is heated at 1000°C for 20 minutes using a salt bath. The plate-shaped test specimen is then immersed in water in a water tank for quenching. The quenched plate-shaped test specimen is cut into two equal parts in the width direction of the steel plate. The cut surfaces are mirror-polished. A Vickers hardness test in accordance with JIS Z2244:2009 is performed at three arbitrary points at the center position in the thickness direction of the mirror-polished cut surface. The test force is 98 N. The arithmetic mean value of the obtained Vickers hardness is defined as the maximum quenched hardness HD0 (HV).

[0099] (Hardenability Evaluation) 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 of the steel plate, 30 mm in the plate width direction, and plate thickness. ｃ１ A plate-shaped test piece is immersed in a salt bath at 80°C above the transformation point for 10 minutes. The plate-shaped test piece is then removed from the salt bath and immersed in water in a water tank for quenching. The quenched plate-shaped test piece is cut into two equal halves in the plate width 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 on the mirror-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 0.95 times or more the maximum quenched hardness HD0, the steel plate is determined to have sufficient hardenability.

[0100] [Regarding Hydrogen Embrittlement Resistance] Hydrogen embrittlement resistance is evaluated by the following method. [Method for Evaluating Hydrogen Embrittlement Resistance] A plate-shaped test specimen is taken from the center position of the plate width of the steel plate. The shape of the plate-shaped test specimen is 30 mm in the rolling direction of the steel plate × 100 mm in the plate width direction × plate thickness. The plate-shaped test specimen is bent into a U-shape by a press bending method to prepare a U-bend test specimen with a curvature radius R of 2.0 mm. Specifically, U-bending is performed at the center position of the plate width (100 mm) of the plate-shaped test specimen. The bend line formed in the plate-shaped test specimen by U-bending is parallel to the rolling direction of the steel plate (L-axis bending).

[0101] The U-bend test piece is A ｃ１ The U-bend test specimens were immersed in a salt bath at +80°C for 10 minutes. After immersion, the U-bend test specimens were removed from the salt bath and immersed in a water tank for quenching. The quenched U-bend test specimens were tempered at 250°C for 1 hour. After tempering, a pair of ends of the non-bend portions of the U-bend test specimens were tightened with bolts to elastically deform the U-bend test specimens so that the opposing non-bend portions were parallel.

[0102] A delayed fracture accelerated test is conducted on a U-bend test piece with the non-bend portion parallel. Specifically, the U-bend test piece is immersed in hydrochloric acid of pH 1 for 100 hours. After 100 hours, the presence or absence of cracks in the bent portion of the U-bend test piece is visually confirmed. If no cracks are observed, it is determined that excellent hydrogen embrittlement resistance has been obtained.

[0103] [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 springs and washers. The steel sheet of this embodiment may also be used for applications other than mechanical parts that require bendability and hydrogen embrittlement resistance.

[0104] [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.

[0105] 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) Hot-rolled sheet annealing step (Step 4) Cold rolling step (Step 5) Cold-rolled sheet annealing step Each step will be described below.

[0106] [(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.

[0107] [(Step 2) Hot Rolling Step] In the hot rolling step, a prepared material (slab or ingot) is hot rolled to produce a steel plate. The hot rolling step includes a rough rolling step in which the material is rough rolled to produce a rough bar (intermediate steel plate), and a finish rolling step in which the rough bar is finish rolled to produce a steel plate.

[0108] 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, 1100 to 1250°C. The time the material is left in the heating furnace is 30 minutes or more, preferably 60 minutes or more. The upper limit of the time is not particularly limited, but is, for example, 300 minutes.

[0109] In the finish rolling process, the rough bar is further rolled (finish rolling) using a finish rolling mill to produce a steel plate. The finish rolling mill includes a plurality of stands arranged in a row. Each stand is equipped with a pair of work rolls. The surface temperature of the steel plate at the outlet side of the stand that lastly reduces the steel plate among the plurality of stands of the finish rolling mill 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 that reduces the steel plate among the plurality of stands arranged in a row in the finish rolling mill is defined as the final pass reduction rate (%). In this embodiment, the final pass reduction rate is 5 to 25%. The hot-rolled steel plate after finish rolling is wound up into a coil. The coiling temperature CT will be described later. The coiled hot-rolled steel plate is cooled to room temperature.

[0110] [(Step 3) Hot-rolled sheet annealing step] In the hot-rolled sheet annealing step, the hot-rolled steel sheet is annealed under well-known conditions. The annealing temperature in the hot-rolled sheet annealing step is, for example, 500 to 770°C, and the holding time at the annealing temperature is, for example, 5 to 80 hours. In the hot-rolled sheet annealing step, so-called box annealing is performed. Furthermore, annealing is performed in a reducing atmosphere.

[0111] [(Step 4) Cold Rolling Step] In the cold rolling step, the steel sheet after the hot-rolled sheet annealing step is cold-rolled. The cold rolling is performed using a cold rolling mill. The cold reduction ratio CR in the cold rolling step will be described later.

[0112] [(Step 5) Cold-rolled sheet annealing step] In the cold-rolled sheet annealing step, cold-rolled sheet annealing is performed on the cold-rolled steel sheet after the cold rolling step. In the cold-rolled sheet annealing step of this embodiment, open coil annealing (OCA) is applied. Annealing is performed on the cold-rolled steel sheet using an open coil annealing furnace under conditions that satisfy conditions 3 to 6 described below. This adjusts the degree of ferrite recrystallization and cementite particle precipitation. Furthermore, the C concentration ratio F1 is adjusted.

[0113] [Regarding Conditions 1 to 6] In the above-described steps 1 to 5, the following conditions 1 to 6 are satisfied: (Condition 1) Coiling temperature CT in step 2: 550 to 750°C (Condition 2) Cold rolling reduction CR in step 4: more than 35 to 60% (Condition 3) Annealing temperature T1 in step 5: 650 to 750°C (Condition 4) Holding time t1 in step 5: 5 to 40 hours (Condition 5) Annealing furnace atmosphere in step 5: 93 to 97% by volume hydrogen-3 to 7% by volume nitrogen atmosphere (Condition 6) Dew point of annealing furnace in step 5: +25 to +65°C Each condition will be explained below.

[0114] [(Condition 1) Coiling Temperature CT] In the hot rolling process, the coiling temperature CT affects the spheroidization rate of cementite particles. If the coiling temperature CT is 750°C or less, the cementite particles formed in the hot-rolled steel sheet are distributed sufficiently uniformly. In this case, annealing increases the spheroidization rate of cementite particles. Therefore, the coiling temperature CT is preferably 750°C or less. On the other hand, the lower limit of the coiling temperature CT is not particularly limited. However, due to equipment constraints, the preferred lower limit of the coiling temperature CT is 550°C.

[0115] [(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

[0116] If the cold rolling rate CR is more than 35%, sufficient strain is introduced into the steel sheet. In this case, spheroidization of cementite particles is promoted in the subsequent annealing process, and the spheroidization rate becomes 85% or more. On the other hand, if the cold rolling rate CR is 60% or less, the strain introduced into the steel sheet is appropriate. In this case, ferrite is appropriately refined, and the average grain size of ferrite becomes 5.0 μm or more.

[0117] [(Condition 3) Annealing Temperature T1] In the cold-rolled sheet annealing process, the annealing temperature T1 adjusts the spheroidization rate of cementite particles in the steel sheet. If the annealing temperature T1 is less than 650°C, the spheroidization of cementite becomes insufficient, and the spheroidization rate of cementite particles becomes less than 85%. Furthermore, the C concentration ratio F1 exceeds 0.50. On the other hand, if the annealing temperature T1 exceeds 750°C, the annealing temperature is too high. In this case, the spheroidization of cementite becomes insufficient, and the spheroidization rate of cementite particles becomes less than 85%.

[0118] [(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 affects the size of ferrite and the size of cementite particles in the steel sheet. Specifically, if the holding time t1 is less than 5 hours, the cementite particles are not sufficiently spheroidized. On the other hand, if the holding time t1 exceeds 40 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.

[0119] [(Condition 5) Annealing Furnace Atmosphere] The atmosphere of the open coil annealing furnace in the cold-rolled sheet annealing step is a 93 to 97% by volume hydrogen-3 to 7% by volume nitrogen atmosphere. In this case, the C concentration ratio F1 can be set to 0.50 or less.

[0120] [(Condition 6) Dew Point of Annealing Furnace] The dew point of the atmosphere in the open coil annealing furnace in the cold-rolled sheet annealing process is set to +25 to +65°C. Specifically, the dew point is adjusted to the above range using humidified nitrogen. If the dew point is less than +25°C, the C concentration ratio F1 exceeds 0.50. On the other hand, if the dew point exceeds +65°C, the equipment load becomes excessive. Therefore, the dew point in the cold-rolled sheet annealing process is set to +25 to +65°C.

[0121] Through the above manufacturing process, a steel sheet satisfying Features 1 to 6 can be manufactured.

[0122] Note that if annealing is performed in the hot-rolled sheet annealing step so as to satisfy Conditions 5 and 6 instead of a reducing atmosphere, and then annealing is performed in the cold-rolled sheet annealing step in a reducing atmosphere, a steel sheet satisfying Features 1 to 6 cannot be obtained. If annealing satisfying Conditions 5 and 6 is performed in the hot-rolled sheet annealing step, a decarburized layer is formed in the steel sheet after the hot-rolled sheet annealing step. However, the decarburized layer is elongated by the cold-rolling step after the hot-rolled sheet annealing step. As a result, the decarburized layer becomes thinner by the cold-rolling step. Furthermore, if the cold-rolled sheet annealing step is performed, C in the steel sheet diffuses, causing recarburization, and the C concentration is uniform throughout the steel sheet. As a result, a steel sheet satisfying Feature 6 cannot be obtained.

[0123] In this embodiment, annealing is performed in a reducing atmosphere in the hot-rolled sheet annealing step, and then, in the final cold-rolled sheet annealing step, annealing is performed under conditions that satisfy Conditions 3 to 6. This makes it possible to produce a steel sheet having a C concentration ratio of 0.50 or less and satisfying Features 1 to 6.

[0124] 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.

[0125] Steel sheets having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured.

[0126]

[0127]

[0128] Specifically, molten steel was continuously cast to produce a slab with a thickness of 250 mm. The slab was subjected to a hot rolling process. Specifically, the slab was heated at 1100 to 1250°C for 120 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 with a thickness of 3.5 mm. The finish rolling temperature for each test number was 830 to 950°C. The reduction ratio of the final pass was 5 to 25%. 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 in the hot rolling process for each test number was as shown in Table 2.

[0129]

[0130] The hot-rolled steel sheets were subjected to a hot-rolled sheet annealing process. The annealing temperature in the hot-rolled sheet annealing process was 500 to 770°C, and the holding time at the annealing temperature was 5 to 80 hours. The hot-rolled sheet annealing was performed in a reducing atmosphere. The hot-rolled steel sheets after the hot-rolled sheet annealing process were subjected to a cold-rolling process to produce cold-rolled steel sheets. The cold reduction ratios CR in the cold-rolling process were as shown in Table 2. The cold-rolled steel sheets after the cold rolling were subjected to a cold-rolled sheet annealing process. In the cold-rolled sheet annealing process, open coil annealing was performed. The annealing temperature T1, holding time t1, and dew point in the cold-rolled sheet annealing process were as shown in Table 2. The atmosphere in the open coil annealing furnace was 95% by volume hydrogen and 5% by volume nitrogen for all test numbers. Steel sheets were produced by the above-mentioned manufacturing process.

[0131] [Evaluation Tests] The following tests were carried out on the manufactured steel sheets with each test number: (Test 1) Chemical composition measurement test (Test 2) Total area ratio of ferrite and cementite measurement test (Test 3) Average ferrite grain size measurement test (Test 4) Average grain size of cementite grains measurement test (Test 5) Spheroidization rate of cementite grains measurement test (Test 6) C concentration ratio F1 measurement test (Test 7) ​​Bendability evaluation test (Test 8) Hardenability evaluation test (Test 9) Hydrogen embrittlement resistance evaluation test Tests 1 to 9 will be described below.

[0132] [(Test 1) Chemical Composition Measurement Test] The chemical composition of the steel sheets of each steel number was measured based on the method described in the above-mentioned [Method for Measuring Chemical Composition of Steel Sheet]. As a result, the chemical compositions of the steel sheets of each steel number were as shown in Tables 1-1 and 1-2.

[0133] [(Test 2) Test for measuring total area ratio of ferrite and cementite particles] Based on the method described in the above-mentioned [Method for measuring total area ratio of ferrite and cementite particles in microstructure], the total area ratio of ferrite and cementite particles was determined for each test number. As a result, the total area ratio of ferrite and cementite particles was 95% or more for all steel sheets with test numbers.

[0134] [(Test 3) Ferrite average grain size measurement test] The average grain size of ferrite in the steel sheet of each test number was measured based on the method described in [Method for measuring average grain size of ferrite] above. The results are shown in Table 3.

[0135]

[0136] [(Test 4) Measurement of average particle size of cementite particles] The average particle size (μm) of cementite particles in the steel sheet of each test number was determined based on the method described in the above-mentioned [Method for measuring average particle size of cementite particles]. The obtained average particle sizes (μm) of cementite particles are shown in Table 3.

[0137] [(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 3.

[0138] [(Test 6) Measurement Test of C Concentration Ratio F1] Based on the method described in [Method for Measuring C Concentration Ratio F1] above, the C concentration ratio F1 of the steel sheet having each test number was determined. The obtained C concentration ratios F1 are shown in Table 3. In Table 3, in the column "F1≦0.50", "T (True)" indicates that F1 was 0.50 or less. "F (False)" indicates that F1 exceeded 0.50.

[0139] [(Test 7) ​​Bendability Evaluation Test] The bendability of the steel plate of each test number was evaluated based on the method described in the above-mentioned [Bendability Evaluation Method]. The evaluation results are shown in "Bendability" in Table 3. After the 90° V-bend test, the surface of the plate-shaped test specimen was visually inspected for cracks. If no cracks were found, it was determined that excellent bendability was obtained (shown as "E (Excellent)" in the "Bendability" column in Table 3). On the other hand, if cracks were found, it was determined that excellent bendability was not obtained (shown as "B (Bad)" in the "Bendability" column in Table 3).

[0140] [(Test 8) Hardenability Evaluation Test] Based on the method described in the above [Hardenability Evaluation Method], the hardenability of the steel plate of each test number was evaluated during hardening. The "Hardenability Lower Limit (HV)" column in Table 3 indicates the value of the maximum hardenability HD0 x 0.95. If the hardenability HD1 was equal to or greater than the lower limit, it was determined that sufficient hardenability was obtained (shown as "E (Excellent)" in the "Hardenability" column in Table 3). On the other hand, if the hardenability HD1 was less than the lower limit, it was determined that sufficient hardenability was not obtained (shown as "B (Bad)" in the "Hardenability" column in Table 3). Furthermore, if the maximum hardenability HD0 of the steel plate was less than 600 HV, it was determined that sufficient strength was not obtained for use as a mechanical part.

[0141] [(Test 9) Hydrogen Embrittlement Resistance Evaluation Test] Based on the method described in the above [Method for Evaluating Hydrogen Embrittlement Resistance], the hydrogen embrittlement resistance of simulated mechanical parts (U-bend test pieces) simulating mechanical parts manufactured using the steel plates of each test number as materials was evaluated. The evaluation results are shown in the "Hydrogen Embrittlement Resistance" column in Table 3. The presence or absence of cracks in the bent parts of the U-bend test pieces was visually confirmed, and if no cracks were found, it was determined that excellent hydrogen embrittlement resistance was obtained (shown as "E (Excellent)" in the "Hydrogen Embrittlement Resistance" column in Table 3). On the other hand, if cracks were found, it was determined that excellent hydrogen embrittlement resistance was not obtained (shown as "Bad (Bad)" in the "Hydrogen Embrittlement Resistance" column in Table 3).

[0142] [Evaluation Results] With reference to Tables 1-1, 1-2, 2, and 3, the chemical compositions and manufacturing conditions were appropriate for test numbers 1 to 35. Therefore, the steel sheets with these test numbers satisfied Features 1 to 6. As a result, excellent hardenability, excellent bendability, and excellent hydrogen embrittlement resistance were obtained.

[0143] On the other hand, Steel No. 33 had an excessively low C content, and therefore had a low maximum quench hardness HD0 of less than 600 HV, and was unable to obtain sufficient strength required for parts.

[0144] In test number 36, the C content was too high, and therefore sufficient bendability and hydrogen embrittlement resistance were not obtained.

[0145] In test number 37, the Si content was too low. Therefore, decarburization during annealing of the cold-rolled sheet was insufficient, and the C concentration ratio F1 exceeded 0.50. As a result, sufficient bendability and sufficient hydrogen embrittlement resistance were not obtained.

[0146] In test number 38, the Si content was too high, so the strength of the steel sheet was excessively increased due to solid solution strengthening, and sufficient bendability was not obtained.

[0147] In test number 39, the Mn content was too low, so sufficient hardenability was not obtained.

[0148] In test number 40, the Mn content was too high, so the strength of the steel sheet was excessively increased due to solid solution strengthening, and sufficient bendability was not obtained.

[0149] In test number 41, the Cr content was too high, so sufficient hardenability was not obtained, and furthermore, sufficient bendability was not obtained.

[0150] In test number 42, the Mo content was too high, so sufficient hardenability was not obtained. Furthermore, sufficient bendability was not obtained.

[0151] In test numbers 43 and 44, although the chemical composition was appropriate, the holding time t1 at the tempering temperature T1 in the cold-rolled sheet annealing step was too long. As a result, the average grain size of ferrite exceeded 20.0 μm and the average grain size of cementite particles exceeded 1.50 μm. As a result, sufficient hardenability and sufficient bendability were not obtained.

[0152] In test numbers 45 to 47, although the chemical composition was appropriate, the dew point during annealing of the cold-rolled sheet was less than 25°C. Therefore, the C concentration ratio F1 exceeded 0.50. As a result, sufficient bendability and sufficient hydrogen embrittlement resistance were not obtained.

[0153] In test numbers 48 to 50, although the chemical composition was appropriate, the cold rolling rate (CR) was low at 35% or less. Therefore, the spheroidization rate of cementite particles was too low at less than 85%. As a result, sufficient bendability was not obtained.

[0154] In Test No. 51, although the chemical composition was appropriate, the coiling temperature CT was too high. As a result, the spheroidization rate of cementite particles was too low, at less than 85%, and as a result, sufficient hardenability and sufficient bendability were not obtained.

[0155] In test number 52, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too low. As a result, the spheroidization rate of cementite particles was too low, less than 85%. Furthermore, the C concentration ratio F1 exceeded 0.50. As a result, sufficient bendability and hydrogen embrittlement resistance were not obtained.

[0156] 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, In mass%, C: 0.50-0.90%, Si: 0.10 to 0.50%, Mn: 0.20-1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01-1.20%, N: 0.0150% or less, Mo: 0-0.500%, Ni: 0-1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, Cu: 0 to 0.15%, W: 0-0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, Ca: 0 to 0.050%, and The balance is Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more, The average particle size of the ferrite is 20.0 μm or less, The average particle size of the cementite particles is 1.50 μm or less, When the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, a spheroidization rate, which is a ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 85% or more, When the C content at a depth of 50 μm from the surface of the steel plate in the plate thickness direction is [C]s and the C content at the center position in the plate thickness direction of the steel plate is [C]c, the C concentration ratio F1 defined by formula (1) is 0.50 or less. steel plate. F1=[C]s / [C]c (1)

2. The steel sheet according to claim 1, Mo: 0.001-0.500%, Ni: 0.001 to 1.000%, and B: 0.0001 to 0.0100%, steel plate.

3. The steel sheet according to claim 1, V: 0.001-0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%; steel plate.

4. The steel plate according to claim 2, V: 0.001-0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%; steel plate.

5. The steel sheet according to any one of claims 1 to 4, Cu: 0.01 to 0.15%, W: 0.01-0.15%, Ta: 0.01-0.15%, Sn: 0.001 to 0.050%, Sb: 0.001 to 0.050%, Co: 0.001 to 0.050%, As: 0.001 to 0.050%, Mg: 0.001-0.050%, Y: 0.001-0.050%, Zr: 0.001 to 0.050%, La: 0.001 to 0.050%, Ce: 0.001 to 0.050%, and Ca: 0.001 to 0.050%, containing one or more selected from the group consisting of steel plate.