Steel plate

Abstract

The invention relates to a steel plate in which both superior hardenability and superior cold workingability can be achieved, even with a carbon content greater than 0.90%. The steel plate of the invention shall consist, by mass percentage, of C: greater than 0.90 to 1.30%, Si: greater than 0.01 to 0.50%, Mn: greater than 0.20 to 1.30%, P: less than or equal to 0.100%, S: less than or equal to 0.100%, Al: less than or equal to 0.100%, Cr: greater than 0.01 to 1.60%, and N: less than or equal to 0.0150%, with the remainder being Fe and impurities.In this steel plate, the total surface area ratio of ferrite and cementite grains is greater than or equal to 95%, the average grain size of ferrite is less than or equal to 15.0 μm, the concentration of Cr [Cr]θ in cementite grains is less than or equal to 7.90%, and the concentration of Mo [Mo]θ in cementite grains is less than or equal to 1.50%, the average grain size of cementite grains is less than or equal to 1.50 μm, the maximum grain size of cementite grains is less than or equal to 5.00 μm, and the spheroidization rate of cementite grains is greater than or equal to 75%.

Description

steel plate

[0001] The present disclosure relates to steel sheets.

[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 consisting of Fe and unavoidable impurities. In this steel sheet, the proportion of solute B in the B content is 70% or more. Furthermore, the microstructure is composed of ferrite and cementite. Furthermore, the cementite density within the ferrite grains is 0.08 particles / μm ２ The following is the description of Patent Document 1: It is stated that total elongation can be increased by keeping the cementite density of ferrite grains low.

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

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

[0007] 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 such steel sheets with a high C content 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%.

[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 mass% in cementite particles [Cr] θ is 7.90% or less, and the Mo concentration in mass% in the 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 to be 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.

[0010] The steel sheet according to the present disclosure has excellent hardenability and excellent cold workability.

[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 the improvement of hardenability and cold workability in a steel plate having a C content of more than 0.90% from the viewpoint of chemical composition. As a result, the present inventors concluded that a chemical composition consisting of, in mass%, 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%, Ti: 0 to 0.150%, and the balance being Fe and impurities, would enable both improvement of hardenability and improvement of cold workability. 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. 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, the hardenability is improved if the average particle size 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 θ is 7.90% or less, and the Mo concentration [Mo] θIt 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. If 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, if 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 Mo 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 generated during the cooling process of hot working is likely 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 becomes the starting point for cracks during cold working. As a result, the cold workability of the steel sheet is reduced.

[0019] Based on the above findings, the present inventors have conducted further studies and have found that, in the above-described steel sheet, if the maximum particle size of cementite particles is 5.00 μm or less, deterioration in 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 mass% in cementite particles [Cr] θ is 7.90% or less, and the Mo concentration in mass% in the 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 to be 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 plate of the second configuration is the steel plate of the first configuration, and has a chemical composition containing, in mass%, one or more elements selected from the group consisting of Mo: 0.001 to 0.500%, Ni: 0.0001 to 1.000%, B: 0.0001 to 0.0100%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%.

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

[0024] [Features of Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment satisfies the following Features 1 to 7. (Feature 1) The chemical composition contains, in mass%, 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 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) The Cr concentration [Cr] in mass % in cementite particles θ is 7.90% or less, and the Mo concentration in mass% in the cementite particles [Mo] θ is 1.50% or less. (Feature 5) The average particle size of the cementite particles is 1.50 μm or less. (Feature 6) The maximum particle size of the cementite particles is 5.00 μm or less. (Feature 7) Of the cementite particles, cementite particles having an aspect ratio of 3.0 or less are considered to be 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 described 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% to 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 the steel sheet as a raw material, the strength of the mechanical component is increased. C further enhances the wear resistance of the mechanical component by leaving undissolved cementite particles after quenching and tempering in a process for manufacturing a mechanical component using the steel sheet as a raw material. If the C content is 0.90% or less, the above effect cannot be sufficiently achieved 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%, the cold workability of the steel sheet is reduced even if the contents of other elements are within the ranges of this embodiment. Furthermore, the toughness of a mechanical component manufactured using the steel sheet as a raw material is reduced. Therefore, the C content is more than 0.90% to 1.30%. The preferred lower limit of the C content is 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 stage of the steel sheet manufacturing process. Furthermore, Si enhances the temper softening resistance of the steel sheet when tempered in the process of manufacturing machine parts using the steel sheet as a raw material. If the Si content is less than 0.01%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.50%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Si content is 0.01 to 0.50%. The preferred lower limit of the Si content is 0.02%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Si content is 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 quenching process of manufacturing mechanical parts using steel sheets as raw materials. As a result, the strength of mechanical products is increased. If the Mn content is less than 0.20%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.30%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the Mn content is 0.20 to 1.30%. The preferred lower limit of the Mn content is 0.25%, more preferably 0.30%, and even more preferably 0.35%. The preferred upper limit of the Mn content is 1.25%, more preferably 1.20%, and even more preferably 1.15%.

[0029] P: 0.100% or less Phosphorus (P) is an impurity, and the P content is greater than 0%. If the P content exceeds 0.100%, the toughness of the steel plate will decrease 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 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%.

[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 will produce excessive sulfides. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet will deteriorate. 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, 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%, 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 is greater than 0%. Al bonds with N to form AlN. AlN refines austenite grains during heating in the quenching process in the process of manufacturing mechanical components 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. 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%. 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 process of manufacturing 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 effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 1.60%, the Cr concentration [Cr] in cementite particles θ It becomes difficult to reduce the Cr content to 7.90% or less. In this case, the dissolution of cementite particles is delayed 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 is reduced. 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 is reduced. Therefore, the Cr content is 0.01 to 1.60%. A preferred lower limit of the Cr content is 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%. A preferred upper limit of the Cr content is 1.57%, even 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 is greater than 0%. N bonds 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 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%, and even more preferably 0.0125%.

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

[0035] The impurities may include elements with the following contents: one or more selected from the group consisting of 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%, and Ca: 0 to 0.050%. All of these elements are 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 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%. All of these elements are optional elements. These optional elements will be described below.

[0037] [First Group (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. 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.

[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 exceeds 0%, Mo 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. Therefore, the strength of the mechanical product is increased. 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. As a result, the cold workability of the steel sheet is reduced. Therefore, the Mo content is 0 to 0.500%. The preferred lower limit of the Mo content is greater 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 exceeds 0%, Ni dissolved in austenite during heating in the quenching process in the process of manufacturing mechanical components using the steel sheet as a raw material improves the hardenability of the steel sheet. Therefore, the strength of the mechanical components is increased. 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. As a result, the cold workability of the steel sheet is reduced. Therefore, the Ni content is 0 to 1.000%. The preferred lower limit of the Ni content is greater 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 still more preferably 0.600%.

[0040] B: 0 to 0.0100% Boron (B) is an optional element and does not necessarily need to be present. In other words, the B content may be 0%. When B is present, that is, when the B content exceeds 0%, the 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. This increases the strength of the mechanical parts. Even if even a small amount of B is present, the above effect can be achieved to some extent. However, if the B content exceeds 0.0100%, B compounds are formed even if the contents of other elements are within the ranges of this embodiment. In this case, sufficient hardenability cannot be obtained. Furthermore, the cold workability of the steel sheet is reduced. Therefore, the B content is 0 to 0.0100%. The preferred lower limit of the B content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and still more preferably 0.0050%.

[0041] [Regarding Group 2: V, Nb, and Ti] The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of V, Nb, and Ti, instead of a portion of Fe. All of these elements are optional and may not be contained. When contained, V, Nb, and Ti form carbides 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. Therefore, the toughness of the mechanical components is improved.

[0042] V: 0 to 0.500% Vanadium (V) is an optional element and does not necessarily need to be contained. In other words, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms carbides and suppresses coarsening of austenite grains during heating in the quenching process of manufacturing mechanical components using steel sheet as a raw material. This improves the toughness of the mechanical components. Even if even a small amount of V is contained, the above effect can be achieved to some extent. However, when the V content exceeds 0.500%, excessive carbides are formed, resulting in precipitation strengthening of the steel sheet. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the V content is 0 to 0.500%. The preferred lower limit of the V content is greater than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.005%. The upper limit of the V content is preferably 0.480%, more preferably 0.450%, and even more preferably 0.400%.

[0043] Nb: 0 to 0.500% Niobium (Nb) is an optional element and may not be contained. In other words, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms carbides and suppresses coarsening of austenite grains during heating in the quenching process of manufacturing mechanical parts using 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. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the Nb content is 0 to 0.500%. The lower limit of the Nb content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Nb content is preferably 0.480%, even 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 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 heating in the quenching process of manufacturing mechanical parts using steel sheet as a raw material. This improves the toughness of the mechanical parts. Furthermore, Ti bonds with N to suppress the formation of nitrides by solute B. This improves the hardenability of the steel sheet due to solute B. Even if even a small amount of Ti is included, the above effect can be achieved to some extent. However, if the Ti content exceeds 0.150%, excessive carbides are formed, precipitation strengthening the steel sheet. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the Ti content is 0 to 0.150%. The lower limit of the Ti content is preferably 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%, even more preferably 0.130%, even more preferably 0.120%, even more preferably 0.100%, and even more preferably 0.080%.

[0045] [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.

[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. In other words, 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 can 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 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 sheet, 10 mm in the width direction, and thickness is taken from the center of the steel sheet width. Of the surfaces of the test piece, the 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. The mirror-polished observation surface is etched using 3% nitric acid alcohol (nital etchant). Of the etched observation surface, secondary electron images are observed using a scanning electron microscope (SEM) at 1000x magnification at five arbitrary observation fields located at a depth of 1 / 4 thickness from the steel sheet 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 field is selected so that the center position of the observation field in the plate thickness direction is at a depth of 1 / 4 plate thickness from the surface of the steel plate in the plate thickness direction on the observation surface.

[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] 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 the dissolution of cementite particles is rapid 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 particle 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 particle 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 of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate width. A cross section of the surface of the test piece parallel to the rolling direction and thickness direction (i.e., the surface of 15 mm in the rolling direction and thickness) is used 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. The average grain size of ferrite is determined on the etched observation surface by the following method. The ferrite grain size number is determined by the intercept method in accordance with JIS G 0551:2020. In this case, the magnification of the optical microscope is selected so that the number of ferrite grains cut by one line segment is at least 10 in one visual field. After selecting the magnification, the cut lengths are determined for five visual fields. The ferrite grain size number is determined from the arithmetic average of the cut lengths of the five visual fields. The average grain size (μm) of ferrite is calculated from the obtained grain size number, and the average grain size of ferrite is calculated by rounding the obtained value to one decimal place.

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

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

[0059] Cr concentration in cementite particles of steel plate [Cr] θ is 7.90% or less, and the Mo concentration in cementite particles [Mo] θWhen 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 Mo 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 Cr concentration in the cementite grains [Cr] is preferably 1.40%, more preferably 1.30%, and even more preferably 1.20%. 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 by the following method: A test piece is taken from the center of the steel plate width. The size of the test piece is 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. ２After 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] is calculated. θ (mass%) and Mo concentration in residue [Mo] θ Calculate the Cr concentration in the residue [Cr] (mass%) θ The Mo concentration in the residue [Mo] is calculated by rounding the obtained value to the nearest tenths place. θ (mass%) The obtained value is rounded to the second decimal place to obtain the value.

[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 concentration and Mo concentration in the residue are calculated by dividing the Cr concentration in the cementite particles [Cr] by the Mo 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 a quenching step in a process for manufacturing a mechanical component using the steel sheet as a raw material. In this case, the hardenability of the steel sheet will be reduced. As a result, mechanical components 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 preferred upper limit of the average particle size of the cementite particles is 1.45 μm, more preferably 1.40 μm, even more preferably 1.35 μm, and even more preferably 1.30 μm. To improve hardenability, a smaller average particle size of the cementite particles is preferable. However, if the average particle size of the cementite particles is too small, the hardness of the steel sheet becomes too high 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 the 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 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 parallel to the rolling direction and the thickness direction (i.e., the surface 15 mm in the rolling direction and the thickness) is defined as the observation surface.

[0071] The observation surface is etched using a picral solution. Secondary electron images are taken of any five observation fields of view located at a depth of 1 / 4 of the plate thickness from the surface of the etched observation surface. Specifically, the five observation fields are observed at a magnification of 2000 times using a scanning electron microscope (SEM), and the above-mentioned secondary electron images are taken. Each observation field is a rectangle 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 located 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 outside the observation field of view are excluded from the measurement target. The area of ​​each cementite particle to be measured is determined, and the equivalent circle diameter of each cementite particle is calculated based on the area. The calculated equivalent circle diameter is taken as the particle diameter of the cementite particle. Note that the particle diameter is determined using well-known image processing software. The arithmetic mean value of the particle diameters of the cementite particles obtained in the five observation fields of view is taken 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 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 become the starting point of cracks during cold working of the steel sheet in the manufacturing process of mechanical 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 maximum particle size of cementite particles] The maximum particle size of cementite particles can be determined by the above-mentioned [Method for measuring average particle size of cementite particles]. Specifically, the maximum particle size of cementite particles obtained by the [Method for measuring average particle size of cementite particles] is defined as the maximum particle size (μm) of cementite particles.

[0077] [(Feature 7) 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 75% or more.

[0078] If the spheroidization rate is 75% or more, remarkably excellent cold workability can be obtained in the steel sheet, provided that Features 1 to 6 are satisfied. Therefore, in the steel sheet of this embodiment, the spheroidization rate is 75% or more. A higher spheroidization rate is preferable. A preferred lower limit of the spheroidization rate is 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 manufacturing cost will increase significantly. Therefore, considering industrial production, the upper limit of the spheroidization rate is, for example, 95%, for example, 90%.

[0079] [Method for Measuring Spheroidization Ratio] The spheroidization ratio can be measured by the following method. The aspect ratio is determined for each of the multiple cementite particles measured in five observation fields by the above-mentioned [Method for Measuring the Average Particle Diameter of Cementite Particles]. Specifically, the contour line 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 contour line 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 determined 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 to be measured is defined as the spheroidization rate (%). The spheroidization rate is an integer value obtained by rounding the determined value to one decimal place.

[0081] [Effects of the Steel Sheet of the Present Embodiment] When a mechanical part is manufactured using the steel sheet according to the present embodiment, which satisfies the above-described Features 1 to 7, the steel sheet of the present embodiment exhibits sufficient hardenability during quenching during the manufacturing process. Furthermore, the steel sheet of the present embodiment exhibits sufficient cold workability.

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

[0083] [Hardenability evaluation method] (A ｃ１ Measurement of transformation point) 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] (Measurement of basic quenched hardness) 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. The plate-shaped test piece is A using a salt bath. ｃ１The specimen is heated at transformation point + 100°C for 30 minutes. The plate-shaped test piece is then immersed in oil at 60°C in an oil tank and quenched. The quenched plate-shaped test piece is cut into two equal parts in the plate width direction. The cut surfaces are mirror-polished. A Vickers hardness test in accordance with JIS Z2244:2020 is performed at three arbitrary points in the center of the plate thickness on the polished cut surface. The test force is 98 N. The arithmetic mean value of the obtained Vickers hardness is taken as 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. 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 The plate test specimen is immersed in a salt bath at 80°C above the transformation point for 10 minutes. The plate test specimen is then removed from the salt bath and immersed in oil at 60°C in an oil tank for quenching. The quenched plate test specimen is cut into two equal halves in the plate width direction. 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 98 N. 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. If the obtained quenched hardness HD1 is 95% or more of the basic quenched 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] [Method for evaluating cold workability] A JIS No. 5 plate test piece specified in JIS Z2241:2022 is taken from the center of the width of the steel plate. The test piece thickness is 1 mm. The test piece thickness is adjusted by grinding, if necessary. A V-notch is formed at the longitudinal center position 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 portion. 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] [Applications of Steel Sheet] 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 machines. Examples of machine parts for textile machines include knitting needles, etc. The steel sheet of this embodiment can be widely used in applications requiring excellent hardenability and excellent cold workability.

[0090] [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.

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

[0092] The main manufacturing 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 step 2: 8.5°C / sec or more (Condition 2) Intermediate temperature MT in step 2: 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 process 4: 550 to 750°C (Condition 7) Holding time t1 in process 4: 10 to 60 hours

[0093] Each step will be described below.

[0094] [(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 having a chemical composition satisfying Feature 1 is produced. A slab is produced using the molten steel by a well-known continuous casting method.

[0095] [(Step 2) Hot Rolling Step] In the hot rolling step, a prepared material (slab) 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.

[0096] In the rough rolling process, the 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 material is stored in the heating furnace for 30 minutes or more, preferably 60 minutes or more. The upper limit of the storage time is not particularly limited, but is, for example, 300 minutes.

[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 final stand of the finish rolling mill that reduces 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 by the coiler to form a coil. Cooling after finish rolling is carried out in two stages. The average cooling rate CR1 in the first stage of cooling, the intermediate temperature MT which is the switching temperature from the first stage 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] [(Step 3) Cold Rolling Step] In the cold rolling step, the steel sheet after the hot rolling step is cold rolled. Cold rolling is performed using a cold rolling mill. The cold rolling mill is, for example, a reverse rolling mill consisting of a single rolling stand, and the rolling stand includes a pair of work rolls.

[0099] In the cold rolling step, cold rolling is performed using the above-mentioned reverse rolling mill to produce a cold-rolled steel sheet. The cold rolling reduction rate RR in the cold rolling step 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 appropriate size, cementite grains of appropriate size, and a Cr concentration [Cr] in the cementite grains are obtained. θ and Mo concentration [Mo] θ The following is obtained.

[0101] [(Step 4) Cold-rolled sheet annealing step] In the cold-rolled sheet annealing step, the cold-rolled steel sheet after the cold rolling step is annealed. In the annealing step, the annealing temperature T1 and the holding time t1 at the annealing temperature T1 are adjusted to adjust the degree of ferrite recrystallization and cementite particle precipitation.

[0102] [Regarding 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 step 2: 8.5°C / sec or more (Condition 2) Intermediate temperature MT in step 2: CT+80°C or less and A ｃ１ Below the transformation point (Condition 3) Coiling temperature CT in step 2: 600 to 700°C (Condition 4) Hot-rolled sheet annealing is not performed (Condition 5) Cold rolling rate RR in step 3: 20 to 60% (Condition 6) Annealing temperature T1 in step 4: 550 to 750°C (Condition 7) Holding time t1 in step 4: 10 to 60 hours Each condition will be explained below.

[0103] [(Condition 1) Average Cooling Rate CR1] In 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, it 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 is formed. The formed coarse pro-eutectoid cementite is likely to remain coarse even after the cold-rolled sheet annealing process is performed. Therefore, 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, it is necessary to sufficiently increase the cooling rate in the pro-eutectoid cementite formation temperature range. If the average cooling rate CR1 between the finish rolling temperature and the intermediate temperature MT is 8.5°C / sec or more, 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. / second.

[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 (°C). 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 cementite particles in the steel sheet may exceed 5.00 μm. ｃ１ If the temperature is below the transformation point, the pro-eutectoid cementite formation temperature region 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 is determined based on the spheroidization rate of cementite particles and the Cr concentration [Cr] in the cementite particles. θ , Mo concentration [Mo] θ If the coiling temperature CT is less than 600°C, the hardness of the hot-rolled steel sheet becomes excessively 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 [Cr] in the cementite particles θ exceeds 7.90%, or the Mo concentration [Mo] θIn some cases, 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) Regarding annealing of hot-rolled sheet] In the manufacturing process of this embodiment, annealing of hot-rolled sheet 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 an appropriate size, cementite grains of an appropriate size, and a Cr concentration [Cr] in the cementite grains. θ and Mo concentration [Mo] θ The strain in a hot-rolled steel sheet is smaller than that in a cold-rolled steel sheet. Therefore, when spheroidizing the cementite in the hot-rolled steel sheet by annealing the hot-rolled steel sheet, it takes time for the spheroidization. In this case, the total annealing heating time, which is the heating time in 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 large. θ and Mo concentration [Mo] θ may become excessive.

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

[0112] If the cold rolling rate RR is 20% or more, sufficient strain is introduced into the steel sheet. In this case, 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. Note that if the cold rolling rate RR exceeds 60%, cold rolling becomes difficult. 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 produced, provided that other conditions are met.

[0114] [(Condition 6) 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 550°C, the spheroidization of cementite becomes insufficient, and the spheroidization rate of cementite particles becomes 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 [Cr] in the cementite grains θ exceeds 7.90%, or the 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 is determined based on 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 becomes too high. θ exceeds 7.90%, or the 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.

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

[0123]

[0124]

[0125] Specifically, molten steel was continuously cast to produce a slab. The slab was 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 further rolled using 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 wound up 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 columns "CR1 (°C / sec)," "intermediate temperature MT (°C)," and "coiling temperature CT (°C)" in Table 2 (Table 2A and Table 2B). 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]

[0127]

[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 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 and 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 Tests] The following tests were carried out on the steel sheets manufactured with each test number: (Test 1) Chemical composition measurement test (Test 2) Total area ratio measurement test of ferrite and cementite particles (Test 3) Average ferrite particle size measurement test (Test 4) Average particle size measurement test of cementite particles (Test 5) Maximum particle size measurement test of cementite particles (Test 6) Measurement test of spheroidization rate of cementite particles (Test 7) ​​Cr concentration [Cr] in cementite particles θ and Mo concentration [Mo] θ (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 the steel sheets of each Steel No. 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 No. were as shown in Tables 1A and 1B.

[0131] [(Test 2) Test for Measuring Total Area Ratio of Ferrite and Cementite Particles] The total area ratio of ferrite and cementite particles for each test number was determined based on the method described in the above-mentioned [Method for Measuring Total Area Ratio of Ferrite and Cementite Particles in 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. 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]

[0133]

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

[0135] [(Test 4) Measurement of Average Particle Size of Cementite Particles] The average 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 the average particle size of cementite particles]. 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 Cementite Particle Diameter Measurement Test] The maximum cementite particle diameter (μm) of the steel sheet having each test number was determined based on the method described in the above-mentioned [Method for Measuring Maximum Cementite Particle Diameter]. If the obtained maximum cementite particle diameter was 5.00 μm or less, it was marked with "E" (Excellent) in the "Maximum Cementite Particle Diameter≦5 μm" column in Table 3 (Table 3A and Table 3B). On the other hand, if the maximum cementite particle diameter exceeded 5.00 μm, it was marked with "NA" (Not Accepted) in the "Maximum Cementite Particle Diameter≦5 μm" column in Table 3 (Table 3A and Table 3B).

[0137] [(Test 6) Measurement test of spheroidization ratio 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 [Method for measuring spheroidization ratio] above. The obtained spheroidization ratios are shown in the "Spheroidization ratio (%)" column in Table 3 (Table 3A and Table 3B).

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

[0139] [(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 at the time of quenching was evaluated. 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. ｃ１The transformation point (°C) is shown in "A" in Table 1B. ｃ１ If the hardness after quenching HD1 was equal to or greater than the lower limit of the hardness after quenching, it was determined that sufficient hardenability was obtained (indicated by "E" in the "hardenability judgment" 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 the hardness after quenching, it was determined that sufficient hardenability was not obtained (indicated by "NA" in the "hardenability judgment" 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 obtained notch elongation is shown in the "Notch Elongation (%)" column of Table 3 (Table 3A and Table 3B). If the obtained notch elongation 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), in test numbers 1 to 46, the chemical composition was appropriate and conditions 1 to 7 of the manufacturing conditions were satisfied. Therefore, the steel sheets with 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, so 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, so 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, so 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 could not be 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 the 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 is A ｃ１ The temperature was higher than the transformation point. Therefore, the maximum particle size of the 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 the cementite particles exceeded 5.00 μm. In addition, the Cr concentration [Cr] in the cementite particles 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. Therefore, the Cr concentration [Cr] in the cementite particles was 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 Nos. 68 and 69, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing step was too short. Therefore, the spheroidization rate of cementite particles was too low, 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. Therefore, the Cr concentration [Cr] in the cementite particles was θ was too high, resulting in insufficient hardenability.

[0157] In test number 72, hot-rolled sheet annealing was performed after the hot rolling process but before the cold rolling process. Therefore, the ferrite grain size and cementite grain size were too large. Furthermore, the Cr concentration [Cr] in the cementite grains 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 having a chemical composition, in mass%, of 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 balance being Fe and impurities; and in the microstructure, the total area ratio of ferrite and cementite particles is 95% or more, The average grain size of the ferrite is 15.0 μm or less, and the Cr concentration in the cementite grains in mass% [Cr] θ is 7.90% or less, and the Mo concentration in the cementite particles in mass% [Mo] θ a mean particle size of the cementite particles is 1.50% or less, a maximum particle size of the cementite particles is 5.00 μm or less, and among the cementite particles, those cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, and 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 75% or more.

2. The steel plate according to claim 1, the chemical composition of which 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%.