steel plate

Abstract

The present invention provides a steel sheet that, even with a C content of more than 0.90%, is able to achieve both excellent quenching properties and excellent cold workability. A steel sheet according to the present disclosure comprises, by mass%, more than 0.90% to 1.30% C, 0.01-0.50% Si, 0.20-1.30% Mn, not more than 0.100% P, not more than 0.100% S, not more than 0.100% Al, 0.01-1.60% Cr, and not more than 0.0150% N, with the remainder being Fe and impurities, wherein: the total area ratio of ferrite and cementite particles is not less than 95%; the average particle size of ferrite is not more than 15.0 μm; in the cementite particles, Cr concentration [Cr]θ is not more than 7.90% and Mo concentration [Mo]θ is not more than 1.50%; the average particle diameter of the cementite particles is not more than 1.50 μm; the maximum particle diameter of the cementite particles is not more than 5.00 μm; and the spheroidization ratio of the cementite particles is not less than 75%.

Description

[Technical Field]

[0001] The present disclosure relates to steel sheets. [Background technology]

[0002] Steel plates with a high C content (high carbon steel plates) are sometimes used as materials for machine parts such as automobile parts and industrial machinery parts. When these machine parts are manufactured using steel plates as materials, the manufacturing method is as follows: The steel plate is cold-worked to form an intermediate product in the shape of the machine part. The intermediate product is then quenched and tempered. High-strength machine parts are manufactured through the above manufacturing process. In order to obtain high strength in the machine parts after quenching, the steel plate is required to have excellent hardenability when quenched during the machine part manufacturing process. Furthermore, the steel plate is cold-worked before quenching. Therefore, the steel plate is required to have not only excellent hardenability but also excellent cold workability.

[0003] Patent Documents 1 and 2 propose steel sheets that have excellent hardenability during quenching and excellent cold workability when used as materials for machine parts.

[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 being 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 in the ferrite grains is 0.08 particles / μm 2 The following is the description: Patent Document 1 describes that total elongation can be increased by keeping the cementite density of ferrite grains low.

[0005] The steel sheet disclosed in Patent Document 2 has a composition containing, 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%, S: 0.01% to 0.10%, N: 0.01% to 0.50%, and Cr: 0.50% to 1.50%, with the balance being Fe and unavoidable impurities, and a microstructure containing 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% by mass or more and 0.50% by 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% to 60%. Patent Document 2 describes that by reducing the Mn concentration in the carbides, the carbides become more easily dissolved during quenching, resulting in improved hardenability. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2015 / 146173 [Patent Document 2] International Publication No. 2020 / 175665 Summary of the Invention [Problem to be solved by the invention]

[0007] Among machine parts, bearings for automobile parts and knitting needles for textile machine parts are required to have high strength and excellent wear resistance. Therefore, the steel sheets used as materials are required to have further improved hardenability. Therefore, steel sheets with a C content of more than 0.90% are sometimes used as steel sheets for high-strength machine parts. Even steel sheets with such high C contents are required to have excellent cold workability.

[0008] An object of the present disclosure is to provide a steel sheet that can achieve both excellent hardenability and excellent cold workability even when the C content exceeds 0.90%. [Means for solving the problem]

[0009] The steel sheet according to the present disclosure has a chemical composition, in mass%, of C: more than 0.90 to 1.30%, 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 1.60%, 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%, and Ti: 0 to 0.150%, with the balance being Fe and impurities. In the microstructure of the steel sheet, the total area ratio of ferrite and cementite particles is 95% or more, and the average grain size of ferrite is 15.0 μm or less. Cr concentration in cementite particles by mass% [Cr] θ is 7.90% or less, and the Mo concentration in mass% in cementite particles [Mo] θ The average particle size of the cementite particles is 1.50 μm or less, and the maximum particle size of the cementite particles is 5.00 μm or less. Among the cementite particles, cementite particles having an aspect ratio of 3.0 or less are considered spherical cementite particles, and the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 75% or more. [Effects of the Invention]

[0010] The steel sheet according to the present disclosure has excellent hardenability and excellent cold workability. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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.

[0012] First, the present inventors investigated, from the viewpoint of chemical composition, improvements in hardenability and cold workability of steel sheets having a C content of more than 0.90%. As a result, the present inventors concluded that a chemical composition consisting of, in mass%, C: more than 0.90 and up to 1.30%, 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 1.60%, 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 being Fe and impurities, would enable both improvements in hardenability and cold workability to be achieved. Therefore, the present inventors further studied the microstructure of the steel sheet having the above-mentioned chemical composition.

[0013] The present inventors first investigated means for improving the hardenability of the microstructure of a steel sheet. The microstructure of a steel sheet having the above-described chemical composition is substantially composed of ferrite and cementite particles. When a mechanical component is manufactured 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 hardenability of the steel sheet during quenching in the manufacturing process of the mechanical component. To improve the solid solubility of the cementite particles during quenching, it is preferable that the particle diameter of the cementite particles is small. In the case of a steel sheet having the above-described chemical composition, the hardenability is improved if the average particle diameter of the cementite particles is 1.50 μm or less.

[0014] Furthermore, as a result of investigations by the present inventors, it was found that the Mn concentration in cementite particles does not affect the solid solution 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 solid solution of cementite particles during quenching. Specifically, if the Cr and Mo concentrations in cementite particles are high, the cementite particles are less likely to be solid-dissolved during quenching.

[0015] Based on the above findings, further investigation was carried out. As a result, the inventors found that the Cr concentration [Cr] in mass% in cementite particles θis 7.90% or less, and the Mo concentration [Mo] θ It has been found that if the Si content is 1.50% or less, cementite particles are easily dissolved during heat treatment, and the hardenability of the steel sheet is improved.

[0016] The present inventors also investigated means for improving the cold workability of the microstructure of a steel sheet. To improve the cold workability of a steel sheet, it is effective to set the average particle size of cementite particles to 1.50 μm or less and further increase the spheroidization rate of the cementite particles. Furthermore, the average particle size of ferrite also affects the cold workability. Therefore, in the steel sheet of this embodiment, the average particle size of cementite particles is set to 1.50 μm or less, the spheroidization rate of cementite particles is set to 75% or more, and the average particle size of ferrite is set to 15.0 μm or less. When the average particle size of ferrite is 15.0 μm or less, the grain boundary area of ​​ferrite increases. In this case, the dissolution of cementite particles is accelerated during quenching due to grain boundary diffusion. Therefore, when the average particle size of ferrite is 15.0 μm or less, not only the cold workability but also the hardenability is improved.

[0017] However, the steel sheet has the above-mentioned chemical composition, the average particle size of the cementite particles is 1.50 μm or less, and the Cr concentration in mass% in the cementite particles [Cr] θ is 7.90% or less, and the Mo concentration [Mo] θ Even when the Cr content is 1.50% or less, the spheroidization rate is 75% or more, and the average grain size of ferrite is 15.0 μm or less, there are still cases where sufficient cold workability cannot be obtained. Therefore, the present inventors conducted further studies. As a result, the present inventors have obtained the following findings.

[0018] If the C content exceeds 0.90%, the size of cementite particles formed during the cooling process of hot working tends to vary. Even if the average particle size of cementite particles is 1.50 μm or less, if the size of cementite particles varies and coarse cementite is present, the coarse cementite can become the starting point for cracks during cold working. As a result, the cold workability of the steel sheet deteriorates.

[0019] Based on the above findings, the present inventors have conducted further studies and have found that, in the above-mentioned steel sheet, if the maximum particle size of the cementite particles is 5.00 μm or less, deterioration of cold workability due to coarse cementite can be suppressed, and both excellent hardenability and excellent cold workability can be achieved.

[0020] The steel sheet according to this embodiment, which was completed based on the above findings, has the following configuration.

[0021] A steel sheet having a first configuration has a chemical composition, in mass%, of C: more than 0.90 to 1.30%, 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 1.60%, 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%, and Ti: 0 to 0.150%, with the balance being Fe and impurities. In the microstructure of the steel sheet, the total area ratio of ferrite and cementite particles is 95% or more, and the average grain size of ferrite is 15.0 μm or less. Cr concentration in cementite particles by mass% [Cr] θ is 7.90% or less, and the Mo concentration in mass% in cementite particles [Mo] θ The average particle size of the cementite particles is 1.50 μm or less, and the maximum particle size of the cementite particles is 5.00 μm or less. Among the cementite particles, those having an aspect ratio of 3.0 or less are considered spherical cementite particles, and the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 75% or more.

[0022] The steel sheet of the second configuration is the steel sheet of the first configuration, and the chemical composition contains, in mass %, one or more elements selected from the group consisting of Mo: 0.001-0.500%, Ni: 0.0001-1.000%, B: 0.0001-0.0100%, V: 0.001-0.500%, Nb: 0.001-0.500%, and Ti: 0.001-0.150%.

[0023] The steel sheet of this embodiment will be described in detail below. Note that "%" relating to elements means mass % unless otherwise specified.

[0024] [Features of the steel sheet according to this embodiment] The steel sheet of this embodiment satisfies the following features 1 to 7. (Feature 1) The chemical composition is, in mass%, C: over 0.90 to 1.30%, 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 1.60%, 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%, and Ti: 0 to 0.150%, with the remainder consisting of 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 15.0 μm or less. (Feature 4) Cr concentration in cementite particles by mass% [Cr] θ is 7.90% or less, and the Mo concentration in mass% in cementite particles [Mo] θ is less than 1.50%. (Feature 5) The average particle size of cementite particles is 1.50 μm or less. (Feature 6) The maximum particle size of cementite particles is 5.00 μm or less. (Feature 7) Among the cementite particles, cementite particles having an aspect ratio of 3.0 or less are considered spherical cementite particles, and the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 75% or more. Features 1 to 7 will be explained below.

[0025] [(Feature 1) Chemical composition] The chemical composition of the steel sheet of this embodiment contains the following elements.

[0026] C: More than 0.90~1.30% Carbon (C) improves the hardenability of steel sheet. As a result, when quenching is performed in a process for manufacturing a mechanical component using steel sheet as a raw material, the strength of the mechanical component is increased. Furthermore, C causes undissolved cementite particles to remain after quenching and tempering in a process for manufacturing a mechanical component using steel sheet as a raw material, thereby improving the wear resistance of the mechanical component. If the C content is 0.90% or less, the above effects 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 1.30%, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet will be reduced, and further, the toughness of mechanical parts manufactured using the steel sheet will be reduced. Therefore, the C content is more than 0.90% to 1.30%. The lower limit of the C content is preferably 0.91%, more preferably 0.93%, and even more preferably 0.95%. The upper limit of the C content is preferably 1.28%, more preferably 1.25%, and even more preferably 1.20%.

[0027] Si: 0.01 to 0.50% Silicon (Si) deoxidizes steel during the steelmaking process of steel sheet production. Si also increases the temper softening resistance of steel sheet when tempered in the process of manufacturing mechanical parts using the steel sheet as a raw material. If the Si content is less than 0.01%, the above effects cannot be sufficiently obtained 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, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.01 to 0.50%. The lower limit of the Si content is preferably 0.02%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Si content is preferably 0.48%, more preferably 0.44%, and even more preferably 0.40%.

[0028] Mn: 0.20 to 1.30% Manganese (Mn) improves the hardenability of steel sheets depending on the amount of manganese dissolved in austenite during heating in the hardening process of manufacturing mechanical parts from steel sheets. As a result, the strength of mechanical products is increased. If the Mn content is less than 0.20%, the above effects 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, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.20 to 1.30%. The lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, and even more preferably 0.35%. The upper limit of the Mn content is preferably 1.25%, more preferably 1.20%, and even more preferably 1.15%.

[0029] P:0.100% or less Phosphorus (P) is an impurity, and the P content exceeds 0%. 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, excessive reduction in the P content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the P content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.050%.

[0030] S: 0.100% or less Sulfur (S) is an impurity, and the S content exceeds 0%. If the S content exceeds 0.100%, S generates 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, excessive reduction of the S content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the S content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the S content is preferably 0.090%, more preferably 0.080%, even more preferably 0.050%, even more preferably 0.030%, and even more preferably 0.025%.

[0031] Al: 0.100% or less Aluminum (Al) is an impurity, and the Al content exceeds 0%. Al bonds with N to form AlN. AlN refines austenite grains during heating in the quenching process in the process of manufacturing mechanical 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%, 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, significantly reducing the hardenability of the steel sheet. Therefore, the Al content is 0.100% or less. The lower limit of the Al content is preferably 0.001%, 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%, even 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.

[0032] Cr: 0.01 to 1.60% Chromium (Cr) improves the hardenability of steel sheet depending on the amount of chromium dissolved in austenite during heating in the hardening process in the manufacturing process of mechanical parts using steel sheet as a raw material. As a result, the strength of the mechanical parts is increased. If the Cr content is less than 0.01%, the above effects 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 1.60%, the Cr concentration in the cementite grains [Cr] θ It becomes difficult to reduce the content of C to 7.90% or less. In this case, the dissolution of cementite particles slows down during heating in the quenching process in the process of manufacturing machine parts using the steel sheet as a raw material. As a result, the hardenability decreases. Furthermore, the strength of the steel sheet becomes excessively high. 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 Cr content is 0.01 to 1.60%. The lower limit of the Cr content is preferably 0.02%, more preferably 0.03%, even more preferably 0.05%, even more preferably 0.10%, even more preferably 0.20%, even more preferably 0.30%, even more preferably 0.40%, even more preferably more than 0.50%, even more preferably 0.52%, and even more preferably 0.55%. The upper limit of the Cr content is preferably 1.57%, more preferably 1.55%, and even more preferably 1.50%.

[0033] N: 0.0150% or less Nitrogen (N) is an impurity, and the N content exceeds 0%. N combines with Al to form AlN. AlN refines austenite grains during heating in the quenching process in the process of manufacturing mechanical 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%, even if the contents of other elements are within the ranges of this embodiment, the austenite grains will be excessively refined during heating in quenching, significantly reducing the hardenability of the steel sheet. Therefore, the N content is 0.0150% or less. The lower limit of the N content is preferably 0.0001%, and more preferably 0.0005%. The upper limit of the N content is preferably 0.0140%, more preferably 0.0130%, and even more preferably 0.0125%.

[0034] The balance of the chemical composition of the steel sheet of this embodiment is 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 of this embodiment.

[0035] The impurities may include the following elements in the amounts listed below. One or more elements selected from the group consisting of 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%, and Ca: 0-0.050% These elements are all tramp elements, and are impurities in the steel sheet of this embodiment.

[0036] [Optional Elements] The chemical composition of the steel sheet of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo: 0-0.500%, Ni: 0-1.000%, B: 0-0.0100%, V: 0-0.500%, Nb: 0-0.500%, and Ti: 0-0.150%. All of these elements are optional elements. These optional elements will be described below.

[0037] [Group 1 (Mo, Ni and B)] The chemical composition of the steel sheet of 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. These elements are optional and may not be contained. When contained, Mo, Ni, and B improve the hardenability of the steel sheet.

[0038] 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 is greater than 0%, Mo dissolves in austenite during heating in the quenching process in the process of manufacturing mechanical parts using steel sheet as a raw material, improving the hardenability of the steel sheet. This increases the strength of the mechanical products. Mo also increases the temper softening resistance of the steel sheet. 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 if the contents of other elements are within the ranges of this embodiment, and therefore the cold workability of the steel sheet deteriorates. Therefore, the Mo content is 0 to 0.500%. The lower limit of the Mo content is preferably more than 0%, more preferably 0.001%, even 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%.

[0039] 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 is greater than 0%, Ni dissolves in austenite during heating in the quenching process in the process of manufacturing mechanical parts using steel sheet as a raw material, improving the hardenability of the steel sheet. This increases the strength of the mechanical parts. Ni also increases the temper softening resistance of the steel sheet. 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 if the contents of other elements are within the ranges of this embodiment, and therefore the cold workability of the steel sheet deteriorates. Therefore, the Ni content is 0 to 1.000%. The lower limit of the Ni content is preferably more than 0%, more preferably 0.001%, even 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%.

[0040] B: 0 to 0.0100% Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When B is contained, that is, when the B content is more than 0%, B dissolved in austenite during heating in the quenching process in the process of manufacturing mechanical parts using the steel sheet as a raw material improves the hardenability of the steel sheet, thereby increasing the strength of the mechanical parts. Even if even a small amount of B is contained, the above effects can be achieved to some extent. However, if the B content exceeds 0.0100%, even if the contents of other elements are within the ranges of this embodiment, B compounds are formed, which results in insufficient hardenability and a deterioration in the cold workability of the steel sheet. Therefore, the B content is 0 to 0.0100%. The lower limit of the B content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and still 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%.

[0041] [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 and suppress coarsening of austenite grains during heating in the quenching step in the process of manufacturing mechanical components using the steel sheet as a raw material. This improves the toughness of the mechanical components.

[0042] V: 0 to 0.500% Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%. When V is contained, that is, when the V content is more than 0%, V forms carbides and suppresses the coarsening of austenite grains during heating in the quenching process of manufacturing machine parts from steel plates. This improves the toughness of the machine parts. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.500%, excessive carbides are formed, which causes precipitation strengthening of the steel sheet, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0 to 0.500%. The lower limit of the V content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, and still more preferably 0.005%. The upper limit of the V content is preferably 0.480%, more preferably 0.450%, and even more preferably 0.400%.

[0043] 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 is more than 0%, Nb forms carbides and suppresses the coarsening of austenite grains during heating in the quenching process of manufacturing mechanical parts using steel plate as a raw material. This improves the toughness of the mechanical parts. Nb also bonds with N and suppresses the formation of nitrides by solute B. This improves the hardenability of the steel plate due to solute B. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.500%, excessive carbides are formed, which causes precipitation strengthening of the steel sheet, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.500%. The lower limit of the Nb content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, and still more preferably 0.005%. The upper limit of the Nb content is preferably 0.480%, more preferably 0.460%, even more preferably 0.430%, even more preferably 0.400%, even more preferably 0.350%, and even more preferably 0.300%.

[0044] Ti: 0 to 0.150% Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content is greater than 0%, Ti forms carbides and suppresses the coarsening of austenite grains during heating in the quenching 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 and suppresses the formation of nitrides by solute B. This improves the hardenability of the steel sheet due to solute B. Even if Ti is contained even a small amount, the above effects can be achieved to some extent. However, if the Ti content exceeds 0.150%, excessive carbides are formed, which causes precipitation strengthening of the steel sheet, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.150%. The lower limit of the Ti content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%. 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 still more preferably 0.080%.

[0045] [Method for measuring the chemical composition of steel sheets] 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. The solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) 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 by a known inert gas fusion-thermal conductivity method.

[0046] [(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, that is, the microstructure of the steel sheet of this embodiment is substantially composed of ferrite and cementite particles.

[0047] In the microstructure, the remaining 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.

[0048] 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.

[0049] If the total area ratio of ferrite and cementite particles is 95% or more, the steel sheet will have sufficient hardenability and sufficient cold workability, provided that Feature 1 and Features 3 to 7 are satisfied.

[0050] [Method for measuring the total area ratio of ferrite and cementite particles in a 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, 10 mm in the width direction, and thickness is taken from the center of the steel plate width. The surface of the test piece parallel to the rolling direction and thickness direction (i.e., the surface 15 mm in the rolling direction and thickness direction) is used 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 using a scanning electron microscope (SEM) at 1000x magnification in five randomly selected observation fields located at a depth of 1 / 4 of the plate thickness from the steel plate surface in the thickness direction. Each observation field is a rectangle measuring 100 μm in the thickness direction and 120 μm in the rolling direction. The observation fields are selected so that the center of the observation field in the thickness direction is located at a depth of 1 / 4 of the plate thickness from the steel plate surface in the thickness direction.

[0051] 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.

[0052] 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 (100 μm×120 μm×5).

[0053] [(Feature 3) Average grain size of ferrite] Furthermore, in the steel sheet of this embodiment, the average grain size of ferrite is 15.0 μm or less. If the average grain size of ferrite exceeds 15.0 μm, the grain boundary area becomes small, 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 decreases.

[0054] When the average grain size of ferrite is 15.0 μm or less, the grain boundary area of ​​ferrite is sufficiently large, and cementite particles dissolve quickly during quenching. Therefore, the hardenability is improved, provided that Features 1, 2, and 4 to 7 are satisfied. Furthermore, when the average grain size of ferrite is 15.0 μm or less, the cold workability of the steel sheet is improved.

[0055] The upper limit of the average grain size of ferrite is preferably 14.5 μm, more preferably 14.0 μm, even more preferably 13.5 μm, and even more preferably 13.0 μm. The lower limit of the average grain size of ferrite is preferably 3.0 μm, more preferably 3.5 μm, and even more preferably 4.0 μm.

[0056] [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, 10 mm in the width direction, and thickness is taken from the center of the steel plate. The cross section of the test piece surface parallel to the rolling direction and thickness direction (i.e., the surface 15 mm in the rolling direction and thickness direction) is used as the observation surface. The observation surface of the test piece is mirror-polished. After mirror-polishing, it is etched with a 3% nital etching solution. The average ferrite grain size of the etched observation surface is determined using the following method. In accordance with JIS G 0551:2020, the ferrite grain size number is determined by the intercept method. The magnification of the optical microscope is selected so that the number of ferrite grains cut by a single line is at least 10 in one field of view. After selecting the magnification, the cut lengths are determined for five fields of view. The ferrite grain size number is determined from the arithmetic mean of the cut lengths of the five fields of view. The average ferrite grain size number (μm) is determined from the obtained grain size number. The average grain size of ferrite is determined by rounding the obtained value to one decimal place.

[0057] (Feature 4) Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ About In the steel sheet of this embodiment, the Cr concentration [Cr] in mass% in the cementite particles θ is 7.90% or less, and the Mo concentration in mass% in cementite particles [Mo] θ is less than 1.50%.

[0058] Cr concentration in cementite particles of steel plate [Cr] θ or Mo concentration [Mo] θ If the hardness is too high, cementite particles will not dissolve sufficiently during heating in the quenching step of manufacturing mechanical parts using the steel sheet as a raw material. In this case, the hardenability of the steel sheet will be reduced. As a result, mechanical parts manufactured using the steel sheet as a raw material will not have sufficient strength.

[0059] Cr concentration in cementite particles of steel plate [Cr] θ is 7.90% or less, and the Mo concentration in the cementite grains [Mo] θ When the Cr concentration in the cementite grains [Cr] is 1.50% or less,θ and Mo concentration [Mo] θ Therefore, on the premise that Features 1 to 3 and Features 5 to 7 are satisfied, cementite particles are sufficiently dissolved during heating in the above-mentioned quenching step, and the hardenability of the steel sheet is improved.

[0060] Cr concentration in cementite particles [Cr] θ The upper limit of is preferably 7.80%, more preferably 7.70%, even more preferably 7.60%, even more preferably 7.50%, even more preferably 7.20%, even more preferably 7.00%, even more preferably 6.50%, even more preferably 5.00%, even more preferably 3.00%, and even more preferably 2.50%. Mo concentration in cementite particles [Mo] θ The upper limit of the content of Cr is preferably 1.40%, more preferably 1.30%, and even more preferably 1.20%. The Cr concentration in cementite particles [Cr] θ is equal to or greater than the Cr content in the chemical composition of the steel sheet. θ is equal to or greater than the Mo content in the chemical composition of the steel sheet.

[0061] [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 in the following way: A test piece is taken from the center of the steel plate width, measuring 10 mm x 10 mm x plate thickness.

[0062] 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).

[0063] 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 After the constant current electrolysis, the test piece is immersed in an alcohol solution and then subjected to ultrasonic cleaning.

[0064] 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.

[0065] 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 in the residue, the Mo mass in the residue, and the total mass of the residue, the Cr concentration in the residue [Cr] θ (mass%) and Mo concentration in residue [Mo] θ Calculate the Cr concentration in the residue (mass%) [Cr] θ (mass%) is the value obtained by rounding off the second decimal place to the nearest tenth. Mo concentration in residue [Mo] θ (mass%) The obtained value is rounded to the second decimal place.

[0066] The obtained residue is substantially composed of cementite particles. In other words, the amount of particles other than cementite particles (inclusions and other carbides other than cementite particles) in the residue is negligibly small. Therefore, the Cr and Mo concentrations in the residue are calculated as the Cr concentration in the cementite particles [Cr] θ (mass%) and Mo concentration [Mo] θ (% by mass).

[0067] [(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. If the average particle size of the cementite particles exceeds 1.50 μm, the cementite particles will 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 will be reduced. As a result, mechanical parts manufactured using the steel sheet as a raw material will not have sufficient strength.

[0068] If the average particle size of the cementite particles is 1.50 μm or less, the cementite particles are sufficiently small. In this case, the cementite particles are sufficiently dissolved during heating in the above-mentioned quenching process. Therefore, provided that Features 1 to 4, Features 6 and 7 are satisfied, the hardenability of the steel sheet is improved. Furthermore, this also contributes to spheroidizing of the cementite, improving the cold workability of the steel sheet.

[0069] 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. 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 due to precipitation strengthening. In this case, the cold workability of the steel sheet deteriorates. Therefore, the preferred lower limit of the average particle size of cementite particles is 0.05 μm, more preferably 0.10 μm, even more preferably 0.15 μm, even more preferably 0.20 μm, even more preferably 0.25 μm, and even more preferably 0.30 μm.

[0070] [Method for measuring the average particle size of cementite particles] The average particle size of the 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 thickness is taken from the center of the steel plate. The surface of the test piece parallel to the rolling direction and thickness direction (i.e., the surface 15 mm in the rolling direction and thickness direction) is defined as the observation surface.

[0071] The observation surface is etched using picral solution. Secondary electron images are taken of five arbitrary observation fields located at a depth of 1 / 4 of the plate thickness from the surface of the etched observation surface. Specifically, a scanning electron microscope (SEM) is used to observe the five observation fields at a magnification of 2000x, and the above-mentioned secondary electron images are taken. Each observation field is a rectangle measuring 50 μm in the plate thickness direction and 60 μm in the rolling direction. The observation field is selected so that the center position of the observation field in the plate thickness direction is at a depth of 1 / 4 of the plate thickness.

[0072] In each secondary electron image, cementite particles are identified based on the contrast. Of the identified cementite particles, those that are entirely contained within the observation field of view are taken as the measurement target. In other words, cementite particles that partially extend beyond the observation field of view are excluded from the measurement target. The area of ​​each cementite particle to be measured is determined, and the circle-equivalent diameter of each cementite particle is calculated based on the area. The calculated circle-equivalent diameter is taken as the particle diameter of the cementite particle. 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.

[0073] [(Feature 6) Maximum particle size of cementite particles] In the steel sheet of this embodiment, the maximum particle size of the cementite particles is 5.00 μm or less. If the maximum particle size of cementite particles exceeds 5.00 μm, the coarse cementite particles in the steel sheet will become the starting point of cracks during cold working of the steel sheet in the manufacturing process of machine parts made from the steel sheet. Therefore, sufficient cold workability cannot be obtained. Furthermore, the Cr concentration [Cr] in the cementite particles θ and Mo concentration [Mo] θ As a result, the hardenability of the steel sheet decreases, and mechanical parts manufactured using the steel sheet as a material do not have sufficient strength.

[0074] When the maximum particle size of the cementite particles is 5.00 μm or less, coarse cementite particles are prevented from remaining after quenching of the steel sheet, and therefore, provided that Features 1 to 5 and 7 are satisfied, the hardenability and cold workability of the steel sheet are improved in mechanical parts manufactured using the steel sheet as a raw material.

[0075] The upper limit of the maximum particle size of the cementite particles is preferably 4.90 μm, more preferably 4.80 μm, even more preferably 4.70 μm, and still more preferably 4.60 μm.

[0076] [Method for measuring the maximum particle size of cementite particles] The maximum particle size of cementite particles can be determined by the above-mentioned [Method for measuring the average particle size of cementite particles]. Specifically, the maximum particle size of cementite particles obtained by the [Method for measuring the average particle size of cementite particles] is defined as the maximum particle size (μm) of cementite particles.

[0077] [(Feature 7) Spheroidization rate] 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 considered to be spherical cementite particles. The spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of the plurality of cementite particles, is 75% or more.

[0078] If the spheroidization rate is 75% or more, the steel sheet will have significantly excellent cold workability, provided that it satisfies Features 1 to 6. Therefore, the steel sheet of this embodiment has a spheroidization rate of 75% or more. A higher spheroidization rate is preferable. The lower limit of the spheroidization rate is preferably 78%, more preferably 80%, even more preferably 83%, and still more preferably 85%. A preferred upper limit of the spheroidization rate is 100%. However, if the spheroidization rate is increased excessively, the production cost increases significantly. Therefore, in consideration of industrial production, the upper limit of the spheroidization rate is, for example, 95%, e.g., 90%.

[0079] [Method for measuring spheroidization rate] The spheroidization rate can be measured by the following method. The aspect ratio is determined for each of the multiple cementite particles measured in five observation fields using the above-mentioned [Method for Measuring the Average Particle Diameter of Cementite Particles]. Specifically, the outline of the cementite particle is sandwiched between two parallel line segments. In this case, the maximum distance between the two parallel line segments sandwiching the cementite particle is defined as the major axis (μm). Furthermore, when the outline of the cementite particle is sandwiched between two line segments parallel to the major axis, the distance between the two line segments (i.e., the width in the direction perpendicular to the major axis) is defined as the minor axis (μm).

[0080] The aspect ratio (= major axis / minor axis) of each cementite particle is calculated based on the obtained major axis (μm) and minor axis (μm). 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 being measured is defined as the spheroidization rate (%). The spheroidization rate is calculated as an integer value obtained by rounding the calculated value to one decimal place.

[0081] [Effects of the steel sheet according to this embodiment] The steel sheet of this embodiment, which satisfies the above-described features 1 to 7, exhibits sufficient hardenability during hardening in the manufacturing process when mechanical components are manufactured using the steel sheet as a raw material. Furthermore, the steel sheet of this embodiment exhibits sufficient cold workability.

[0082] [About hardenability] In the steel sheet of this embodiment, the fact that sufficient hardenability is obtained means the following evaluation.

[0083] [Hardenability evaluation method] (A c1 transformation point measurement) A test piece having a width of 10 mm, a length of 80 mm, and a thickness of the plate thickness is taken from the center of the plate width of the steel plate of this embodiment. The width direction of the test piece is parallel to the longitudinal direction of the steel plate. If the plate thickness exceeds 3 mm, the thickness of the test piece is adjusted to 3 mm by grinding. The thermal expansion coefficient during heating is measured using a plate Formaster testing machine. From the obtained thermal expansion coefficient, A c1 Find the transformation point.

[0084] (Basic hardness measurement) A plate-shaped test piece is taken from the center of the steel plate width, measuring 15 mm in the rolling direction of the steel plate, 30 mm in the plate width direction, and plate thickness. Plate-shaped test specimens were prepared using a salt bath. c1 The specimen is heated at transformation point +100°C for 30 minutes. The plate-shaped test piece is then immersed in 60°C oil in an oil bath for quenching. After quenching, the plate-shaped test piece is cut into two equal parts across the plate width. The cut surfaces are mirror-polished. Vickers hardness tests are performed in accordance with JIS Z2244:2020 at three arbitrary points in the center of the plate thickness on the polished cut surfaces. The test force is 98N. The arithmetic mean of the obtained Vickers hardness values ​​is the basic quenched hardness HD0 (HV). The basic quenched hardness HD0 is an integer value obtained by rounding the obtained value to one decimal place.

[0085] (Hardenability evaluation) A plate-shaped test piece is taken from the center of the steel plate width, measuring 15 mm in the rolling direction of the steel plate, 30 mm in the plate width direction, and plate thickness. A c1 The plate test piece is immersed in a salt bath at 80°C above the transformation point for 10 minutes. The plate test piece is then removed from the salt bath and immersed in oil at 60°C in an oil tank for quenching. After quenching, the plate test piece is cut into two equal halves across the plate width. The cut surfaces are mirror-polished. Vickers hardness tests are performed in accordance with JIS Z2244:2020 at three arbitrary points in the center of the plate thickness on the polished cut surfaces. The test force is 98N. The arithmetic mean of the obtained Vickers hardness values ​​is defined as the quenched hardness HD1 (HV). The quenched hardness HD1 is an integer value obtained by rounding the obtained value to the nearest tenth. When the obtained quench hardness HD1 is 95% or more of the basic quench hardness HD0, the steel plate is determined to have sufficient hardenability.

[0086] [Cold workability] In the steel sheet of this embodiment, the fact that sufficient cold workability is obtained means the following evaluation.

[0087] [Cold workability evaluation method] A JIS No. 5 plate test piece, as specified in JIS Z2241:2022, is taken from the center of the steel plate width. The test piece thickness is 1 mm. The test piece thickness is adjusted by grinding if necessary. A V-notch is formed in the center position of the longitudinal direction of the parallel part of the test piece. The V-notch opening angle is 45° and the V-notch depth is 2 mm. The depth direction of the V-notch corresponds to the width direction of the parallel part of the test piece. The gauge length is 5 mm, including the V-notch part. The longitudinal direction of the test piece is parallel to the rolling direction of the steel plate.

[0088] A test piece is subjected to a breaking elongation test at room temperature in the atmosphere. The butt elongation after breaking is measured, and the obtained butt elongation (breaking elongation) (%) is defined as the notch elongation (%). If the obtained notch elongation is 8.0% or more, it is determined that the steel sheet has sufficient cold workability.

[0089] [Steel plate applications] The steel sheet of this embodiment is suitable as a material for machine parts for automobiles. Examples of machine parts for automobiles include automobile bearings, springs, washers, etc. The steel sheet of this embodiment is suitable as a material for machine parts for textile machinery. Examples of machine parts for textile machinery include knitting needles, etc. The steel sheet of this embodiment can be widely used in applications requiring excellent hardenability and excellent cold workability.

[0090] [Steel plate manufacturing method] An example of a method for manufacturing the steel sheet of this embodiment will be described. The method for manufacturing the steel sheet described below is one example for manufacturing the steel sheet of this embodiment. Therefore, the 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 the method for manufacturing the steel sheet of this embodiment.

[0091] An example of the method for manufacturing the steel sheet according to this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Hot rolling process (Process 3) Cold rolling process (Process 4) Cold-rolled sheet annealing process In this embodiment, the annealing step is not performed after the hot rolling step and before the cold rolling step.

[0092] The main production conditions in the above steps 1 to 4 are as follows. (Condition 1) Average cooling rate CR1 from the finish rolling temperature to the intermediate temperature MT in process 2: 8.5°C / sec or more (Condition 2) Intermediate temperature in process 2 MT:CT + 80°C or less and A c1 Below the transformation point (Condition 3) Coiling temperature CT in process 2: 600 to 700°C (Condition 4) No hot-rolled sheet annealing is performed (Condition 5) Cold rolling rate RR in process 3: 20 to 60% (Condition 6) Annealing temperature T1 in step 4: 550 to 750°C (Condition 7) Retention time t1 in step 4: 10 to 60 hours

[0093] Each step will be described below.

[0094] [(Process 1) Material preparation process] In the material preparation step, a material that satisfies Feature 1 is prepared. The material is produced, for example, by the following method: Molten steel whose chemical composition satisfies Feature 1 is produced. A slab is produced using the molten steel by a well-known continuous casting method.

[0095] [(Process 2) Hot rolling process] In the hot rolling process, hot rolling is performed on a prepared material (slab) to produce a steel plate. The hot rolling process includes a rough rolling process in which the material is roughly rolled to produce a rough bar (intermediate steel plate), and a finish rolling process in which the rough bar is finish rolled to produce a steel plate.

[0096] In the rough rolling process, a material (slab) 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 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.

[0097] 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 multiple 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 last stand of the finish rolling mill that rolls down the steel plate is defined as the finish rolling temperature (°C). In this embodiment, the finish rolling temperature is 800 to 950°C. The hot-rolled steel plate after finish rolling is transported to a coiler on a run-out table and then wound up by the coiler to form a coil. Cooling after finish rolling is performed in two stages. The average cooling rate CR1 in the first stage of cooling, the intermediate temperature MT which is the temperature at which the first stage of cooling is switched to the second stage of cooling, and the coiling temperature CT will be described later. The coiled steel plate is cooled to room temperature.

[0098] [(Process 3) Cold rolling process] In the cold rolling process, cold rolling is performed on the steel sheet after the hot rolling process. Cold rolling is performed using a cold rolling mill. The cold rolling mill is, for example, a reverse rolling mill consisting of one rolling stand, and the rolling stand includes a pair of work rolls.

[0099] In the cold rolling process, cold rolling is performed using the above-mentioned reverse rolling mill to produce a cold-rolled steel sheet. The cold rolling reduction ratio RR in the cold rolling process will be described later.

[0100] In this embodiment, the hot-rolled steel sheet is subjected to the cold rolling process without being annealed after the hot rolling process. In other words, the 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 in the hot rolling process and the cold rolling process, and then the annealing process is performed. As a result, ferrite grains of an appropriate size, cementite grains of an appropriate size, and a Cr concentration [Cr] in the cementite grains are obtained. θ and Mo concentration [Mo] θ The following is obtained.

[0101] [(Process 4) Cold-rolled sheet annealing process] In the cold-rolled sheet annealing process, the cold-rolled steel sheet after the cold rolling process is annealed. In the annealing process, 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.

[0102] [Conditions 1 to 7] In the above-described steps 1 to 4, the following conditions 1 to 7 are satisfied. (Condition 1) Average cooling rate CR1 from the finish rolling temperature to the intermediate temperature MT in process 2: 8.5°C / sec or more (Condition 2) Intermediate temperature in process 2 MT:CT + 80°C or less and A c1 Below the transformation point (Condition 3) Coiling temperature CT in process 2: 600 to 700°C (Condition 4) No hot-rolled sheet annealing is performed (Condition 5) Cold rolling rate RR in process 3: 20 to 60% (Condition 6) Annealing temperature T1 in step 4: 550 to 750°C (Condition 7) Retention time t1 in step 4: 10 to 60 hours Each condition will be explained below.

[0103] [(Condition 1) Average cooling rate CR1] During the cooling stage of the hot rolling process, the average cooling rate (°C / sec) from the finish rolling temperature to the intermediate temperature MT is defined as the average cooling rate CR1 (°C / sec). The steel sheet of this embodiment is a so-called hypereutectoid steel. Therefore, during cooling from the finish rolling temperature, the steel sheet passes through a temperature range where pro-eutectoid cementite is likely to form (pro-eutectoid cementite formation temperature range). If the cooling rate in the pro-eutectoid cementite formation temperature range is too slow, coarse pro-eutectoid cementite will form. The coarse pro-eutectoid cementite that forms tends to remain coarse even after the cold-rolled sheet annealing process. As a result, the maximum particle size of cementite particles in the steel sheet may exceed 5.00 μm.

[0104] To suppress the formation of coarse pro-eutectoid cementite, the cooling rate in the pro-eutectoid cementite formation temperature range must be sufficiently fast. If the average cooling rate CR1 between the finish rolling temperature and the intermediate temperature MT is 8.5°C / sec or higher, the residence time of the steel sheet in the pro-eutectoid cementite formation temperature range is sufficiently short. Therefore, the formation of coarse pro-eutectoid cementite can be suppressed. As a result, the average particle size of the cementite particles becomes 1.50 μm or less, and the maximum particle size of the cementite particles becomes 5.00 μm or less.

[0105] The upper limit of the average cooling rate CR1 is not particularly limited, but due to equipment restrictions, the upper limit of the average cooling rate CR1 is, for example, 50.0°C / sec.

[0106] [(Condition 2) Intermediate temperature MT] In the cooling stage of the hot rolling process, the temperature between the finish rolling temperature and the coiling temperature CT, at which the cooling at the average cooling rate CR1 is stopped, is defined as the intermediate temperature MT (℃). If the intermediate temperature MT is higher than CT+80℃ or A c1 If the intermediate temperature MT is higher than the transformation point, the pro-eutectoid cementite formation temperature range may exist in the hot-rolled steel sheet even in the temperature range below the intermediate temperature MT. Therefore, pro-eutectoid cementite is likely to form below the intermediate temperature MT. In this case, the maximum particle size of the cementite particles in the steel sheet may exceed 5.00 μm. If the intermediate temperature MT is below CT+80°C and A c1If the temperature is below the transformation point, the pro-eutectoid cementite formation temperature range can be cooled at an average cooling rate CR1. Therefore, the formation of pro-eutectoid cementite can be sufficiently suppressed, and the maximum particle size of cementite particles in the steel sheet can be set to 5.00 μm or less. In this embodiment, the intermediate temperature MT is higher than the coiling temperature CT. Furthermore, when the intermediate temperature MT satisfies condition 2, the average cooling rate from the intermediate temperature MT to the coiling temperature CT is slower than the average cooling rate CR1.

[0107] [(Condition 3) Coiling temperature CT] In the hot rolling process, the coiling temperature CT affects the spheroidization rate of cementite particles and the Cr concentration in the cementite particles [Cr]. θ , Mo concentration [Mo] θ If the coiling temperature CT is less than 600°C, the hardness of the hot-rolled steel sheet becomes too high, making it impossible to carry out the subsequent cold rolling process.

[0108] On the other hand, if the coiling temperature CT exceeds 700°C, the Cr concentration in the cementite particles [Cr] θ exceeds 7.90% or Mo concentration [Mo] θ In some cases, the content of Cr exceeds 1.50%. Furthermore, the spheroidization of cementite becomes insufficient, and the spheroidization rate of cementite particles becomes less than 75%. Furthermore, the maximum particle size of cementite particles in the steel sheet may exceed 5.00 μm.

[0109] If the coiling temperature CT is 600 to 700°C, a steel sheet satisfying Features 1 to 7 can be produced, provided that other conditions are met.

[0110] [(Condition 4) Annealing of hot-rolled sheet] In the manufacturing process of this embodiment, hot-rolled sheet annealing is not performed between the hot rolling process and the cold rolling process. As described above, in this manufacturing method, strain is accumulated in the steel sheet in the hot rolling process and the cold rolling process, and then the annealing process is performed. This allows for the formation of ferrite grains of appropriate size, cementite grains of appropriate size, and a Cr concentration [Cr] in the cementite grains. θ and Mo concentration [Mo] θThe strain in hot-rolled steel sheets is smaller than that in cold-rolled steel sheets. Therefore, when hot-rolled steel sheets are annealed to spheroidize the cementite in the hot-rolled steel sheets, it takes time for the spheroidization. In this case, the total annealing heating time, which is the heating time for the hot-rolled steel sheet annealing and the heating time in the subsequent cold-rolled steel sheet annealing process, becomes excessively long. As a result, the ferrite grains and cementite particles become coarse, and the Cr concentration [Cr] in the cementite particles becomes too high. θ and Mo concentration [Mo] θ may become excessive.

[0111] [(Condition 5) Cold rolling rate RR] In the cold rolling process, the cold rolling reduction RR is defined by the following formula: Cold rolling ratio RR (%) = (1 - (thickness of cold-rolled steel sheet after cold rolling process / thickness of hot-rolled steel sheet before cold rolling process)) × 100

[0112] If the cold rolling rate RR is 20% or more, sufficient strain is introduced into the steel sheet. In this case, the spheroidization of cementite is promoted in the subsequent annealing process, and the spheroidization rate becomes 75% or more. On the other hand, if the cold rolling rate RR is less than 20%, the strain introduced into the steel sheet is insufficient. In this case, ferrite recrystallization does not progress, the steel sheet becomes hard, and the cold workability deteriorates. In this case, it becomes difficult to measure the ferrite grain size. If the cold rolling rate RR exceeds 60%, cold rolling becomes difficult, and therefore the cold rolling rate RR is 60% or less.

[0113] If the cold rolling rate RR is 20 to 60%, a steel sheet satisfying Features 1 to 7 can be manufactured, provided that other conditions are met.

[0114] [(Condition 6) Annealing temperature T1] In the cold-rolled sheet annealing process, the annealing temperature T1 controls the spheroidization rate of cementite particles in the steel sheet. If the annealing temperature T1 is less than 550°C, the spheroidization of cementite becomes insufficient, resulting in a spheroidization rate of less than 75%. Furthermore, ferrite recrystallization does not proceed, the steel sheet becomes hard, and the cold workability deteriorates.

[0115] On the other hand, if the annealing temperature T1 exceeds 750°C, the annealing temperature is too high. In this case, the Cr concentration in the cementite grains [Cr] θ exceeds 7.90% or Mo concentration [Mo] θ In addition, the spheroidization of cementite becomes insufficient, and the spheroidization rate of cementite particles becomes less than 75%.

[0116] If the annealing temperature T1 is 550 to 750°C, a steel sheet satisfying Features 1 to 7 can be produced, provided that other conditions are met.

[0117] [(Condition 7) 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 spheroidization rate of cementite particles and the Cr concentration [Cr] in the cementite particles. θ and Mo concentration [Mo] θ Specifically, if the holding time t1 is less than 10 hours, the cementite is not sufficiently spheroidized.

[0118] On the other hand, if the holding time t1 exceeds 60 hours, the holding time is too long. In this case, Cr and Mo are concentrated in the cementite particles, and the Cr concentration [Cr] in the cementite particles increases. θ exceeds 7.90% or Mo concentration [Mo] θ exceeds 1.50%.

[0119] If the holding time t1 is 10 to 60 hours, a steel sheet satisfying Features 1 to 7 can be manufactured, provided that other conditions are met.

[0120] Through the above manufacturing process, a steel sheet satisfying Features 1 to 7 can be manufactured.

[0121] 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. [Example]

[0122] Steel plates having the chemical compositions shown in Tables 1A and 1B were produced.

[0123] [Table 1A]

[0124] [Table 1B]

[0125] 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 1050 to 1300°C for 240 minutes. The heated slab was rolled in a roughing mill to produce a rough bar. The rough bar was then rolled in a finishing mill to produce a steel plate. The finish rolling temperature for each test number was 800 to 950°C. The hot-rolled steel plate after finish rolling was coiled and formed into a coil. The coiled steel plate was allowed to cool to room temperature. The average cooling rate CR1, intermediate temperature MT, and coiling temperature CT in the hot rolling process for each test number are shown in the "CR1 (°C / sec)," "intermediate temperature MT (°C)," and "coiling temperature CT (°C)" columns of Table 2 (Table 2A and Table 2B). Note that the average cooling rate CR2 from the intermediate temperature MT to the coiling temperature CT was less than the average cooling rate CR1 for all test numbers.

[0126] [Table 2A]

[0127] [Table 2B]

[0128] For test numbers 1 to 71, the hot-rolled steel sheets after the hot rolling process were subjected to a cold rolling process without being subjected to hot-rolled sheet annealing. The cold rolling reduction ratio RR in the cold rolling process is shown in the "Cold rolling reduction ratio RR (%)" column in Table 2 (Table 2A and Table 2B). 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 are shown in the "T1 (°C)" and "t1 (hours)" columns in Table 2 (Table 2A and Table 2B). In the cold-rolled sheet annealing process, the steel sheets were furnace-cooled after the holding time t1. For test number 72, hot-rolled sheet annealing was performed after the hot rolling process but before the cold rolling process. The annealing temperature (°C) and holding time (hours) at the annealing temperature in the hot-rolled sheet annealing are shown in the "Annealing temperature (°C)" and "Annealing time (hours)" columns in Table 2 (Table 2A and Table 2B). Steel plates were manufactured through the above manufacturing process.

[0129] [Evaluation test] The following tests were carried out on the manufactured steel plates with each test number. (Test 1) Chemical composition measurement test (Test 2) Total area ratio measurement test for ferrite and cementite particles (Test 3) Ferrite average particle size measurement test (Test 4) Average particle size measurement test for cementite particles (Test 5) Maximum particle size measurement test for cementite particles (Test 6) Measurement test of the spheroidization rate of cementite particles (Test 7) ​​Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Measurement test (Test 8) Hardenability evaluation test (Test 9) Cold workability evaluation test Tests 1 to 9 will be explained below.

[0130] [(Test 1) Chemical composition measurement test] The chemical composition of each steel sheet was measured based on the method described above in [Method for measuring the chemical composition of steel sheets]. As a result, the chemical compositions of each steel sheet were as shown in Tables 1A and 1B.

[0131] [(Test 2) Total Area Ratio Measurement Test 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." The resulting total area ratios of ferrite and cementite particles are shown in the "Total Area Ratio of Ferrite and Cementite Particles (%)" column of Table 3 (Table 3A and Table 3B). In the "Total Area Ratio of Ferrite and Cementite Particles (%)" column of Table 3 (Table 3A and Table 3B), "≧95" indicates that the total area ratio of ferrite and cementite particles was 95% or more. Note that in all test numbers, when a remainder other than ferrite and cementite particles was present in the microstructure, the remainder was one or more types selected from the group consisting of bainite, martensite, and pearlite.

[0132] [Table 3A]

[0133] [Table 3B]

[0134] [(Test 3) Ferrite average grain size measurement test] The average ferrite grain size (μm) of the steel sheet of each test number was determined based on the method described in [Method for measuring the average ferrite grain size] above. The obtained average ferrite grain sizes are shown in the "Ferrite grain size (μm)" column of Table 3 (Table 3A and Table 3B).

[0135] [(Test 4) Average particle size measurement test for cementite particles] The average particle size (μm) of cementite particles in the steel sheets of each test number was determined based on the method described in "Method for measuring the average particle size of cementite particles" above. The obtained average particle sizes of cementite particles are shown in the "Cementite particle size (μm)" column in Table 3 (Table 3A and Table 3B).

[0136] [(Test 5) Maximum particle size measurement test for cementite particles] The maximum particle size (μm) of cementite particles in the steel sheet having each test number was determined based on the method described in the above-mentioned "Method for measuring maximum particle size of cementite particles." If the obtained maximum particle size of cementite particles was 5.00 μm or less, it was marked with "E" (Excellent) in the "Maximum cementite particle size ≦ 5 μm" column in Table 3 (Table 3A and Table 3B). On the other hand, if the maximum particle size of cementite particles exceeded 5.00 μm, it was marked with "NA" (Not Accepted) in the "Maximum cementite particle size ≦ 5 μm" column in Table 3 (Table 3A and Table 3B).

[0137] [(Test 6) Measurement test of the spheroidization rate of cementite particles] The spheroidization ratio (%) of cementite particles of the steel plate having each test number was determined based on the method described in the above-mentioned [Method for measuring spheroidization ratio]. The obtained spheroidization ratios are shown in the "Spheroidization ratio (%)" column of Table 3 (Table 3A and Table 3B).

[0138] [(Test 7) ​​Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Measurement test The above [Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Based on the method described in [Measurement method for Cr concentration in cementite particles of steel sheets of each test number], θ (mass%) and Mo concentration [Mo] θ The Cr concentration (mass%) was calculated. θ and Mo concentration [Mo] θ Table 3 (Table 3A and Table 3B) "[Cr] θ (mass%)" and "[Mo] θ (mass%)" column.

[0139] [(Test 8) Hardenability Evaluation Test] The hardenability of the steel sheets of each test number was evaluated at the time of hardening based on the method described in the above [Hardenability evaluation method]. The obtained hardness HD1 is shown in the "Hardness HD1 (HV)" column of Table 3 (Table 3A and Table 3B). The "Lower limit of hardness (HV)" in Table 3 indicates the value of "Basic hardness HD0 (HV)" in Table 1B x 0.95. c1 The transformation point (℃) is shown in Table 1B. c1 The hardness after quenching HD1 is shown in "Hardness Rating Point (°C)". If the hardness after quenching HD1 was equal to or greater than the lower limit of hardness after quenching, it was determined that sufficient hardenability was obtained (indicated by "E" in the "Hardenability Rating" column in Table 3 (Table 3A and Table 3B)). On the other hand, if the hardness after quenching HD1 was less than the lower limit of hardness after quenching, it was determined that sufficient hardenability was not obtained (indicated by "NA" in the "Hardenability Rating" column in Table 3 (Table 3A and Table 3B)).

[0140] [(Test 9) Cold workability evaluation test] The cold workability of the steel sheets of each test number was evaluated based on the method described in the above-mentioned [Cold workability evaluation method]. The notch elongation obtained is shown in the "Notch elongation (%)" column of Table 3 (Table 3A and Table 3B). If the notch elongation obtained was 8.0% or more, it was determined that sufficient cold workability was obtained. On the other hand, if the notch elongation was less than 8.0%, it was determined that sufficient cold workability was not obtained.

[0141] [Evaluation results] With reference to Table 1A, Table 1B, Table 2 (Table 2A and Table 2B), and Table 3 (Table 3A and Table 3B), test numbers 1 to 46 had appropriate chemical compositions and satisfied conditions 1 to 7 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.

[0142] On the other hand, in test number 47, the C content was too high, and as a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

[0143] In test number 48, the Si content was too high, and as a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

[0144] In test number 49, the Mn content was too low, so sufficient hardenability was not obtained.

[0145] In test number 50, the Mn content was too high, and as a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

[0146] In test numbers 51 and 52, the Cr content was too high. Therefore, the Cr concentration in mass% in the cementite particles [Cr] θ exceeded 7.90%, and therefore sufficient hardenability could not be obtained.

[0147] In test numbers 53 and 54, the Mo content was too high. Therefore, the Mo concentration in mass% in the cementite particles [Mo] θ exceeded 1.50%, and therefore sufficient hardenability was not obtained.

[0148] In Test Nos. 55 and 56, the intermediate temperature MT exceeded the coiling temperature CT + 80°C. As a result, the maximum particle size of the cementite particles exceeded 5.00 µm. In addition, in Test No. 56, the spheroidization rate of the cementite particles was low. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

[0149] In Test Nos. 57 and 58, although the chemical composition was appropriate, the average cooling rate CR1 was too slow. As a result, the maximum particle size of cementite particles exceeded 5.00 μm. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

[0150] In test number 59, the intermediate temperature MT was A c1 The temperature was higher than the transformation point. As a result, the maximum particle size of cementite particles exceeded 5.00 μm. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

[0151] In test numbers 60 and 61, although the chemical composition was appropriate, the coiling temperature CT was too high. As a result, the maximum particle size of cementite particles exceeded 5.00 μm. In addition, the Cr concentration in the cementite particles [Cr] θ The spheroidization rate of the cementite particles was too high, and the spheroidization rate of the cementite particles was low. As a result, sufficient hardenability and cold workability were not obtained.

[0152] In test numbers 62 and 63, although the chemical composition was appropriate, the cold rolling rate RR was too low. As a result, ferrite recrystallization did not progress and was not completed. Therefore, the ferrite grain size could not be measured (shown as "-" in the "Ferrite grain size (μm)" column in Table 3B). As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

[0153] Although the chemical compositions of test numbers 64 and 65 were 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 low, less than 75%. Furthermore, ferrite recrystallization was not complete, and the ferrite grain size could not be measured (shown as "-" in the "Ferrite grain size (μm)" column in Table 3B). As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

[0154] In test numbers 66 and 67, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too high. As a result, the Cr concentration in the cementite particles [Cr] θ The spheroidization rate of the cementite particles was too high, and the spheroidization rate of the cementite particles was low. As a result, sufficient hardenability and cold workability were not obtained.

[0155] In test numbers 68 and 69, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too short. As a result, the spheroidization rate of cementite particles was too low, less than 75%. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

[0156] In test numbers 70 and 71, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too long. As a result, the Cr concentration in the cementite particles [Cr] θ was too high, resulting in insufficient hardenability.

[0157] In test number 72, the hot-rolled sheet was annealed after the hot rolling process and before the cold rolling process. Therefore, the ferrite grain size and the cementite grain size were too large. Furthermore, the Cr concentration [Cr] in the cementite grains was too high. θ was too high, resulting in insufficient hardenability.

[0158] 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 is in mass %: C: more than 0.90 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.20-1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01-1.60%, 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%, and Ti: 0 to 0.150%; 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 15.0 μm or less, Cr concentration in mass% in the cementite particles [Cr] θ is 7.90% or less, and the Mo concentration in mass% in the cementite particles [Mo] θ is 1.50% or less, The average particle size of the cementite particles is 1.50 μm or less, The maximum particle size of the cementite particles is 5.00 μm or less, Among the cementite particles, the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, and 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 75% or more. steel plate.

2. The steel sheet according to claim 1, The chemical composition is in mass %: Mo: 0.001 to 0.500%, Ni: 0.0001 to 1.000%, B: 0.0001 to 0.0100%, V: 0.001-0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%; steel plate.

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