Steel sheet

Abstract

Provided is a steel sheet that has excellent quenching properties and cold-working properties. This steel sheet comprises, by mass%, 0.50%–0.90% of C, 0.01%–0.50% of Si, 0.20%–1.30% of Mn, no more than 0.100% of P, no more than 0.100% of S, no more than 0.100% of Al, 0.01%–1.20% of Cr, and no more than 0.0150% of N, the remainder being Fe and impurities. The total area ratio of ferrite and cementite particles is at least 95%, the average particle diameter of the ferrite is 5.0–20.0 μm, the Cr concentration [Cr]θ in the cementite particles is less than 2.00%, the Mo concentration [Mo]θ in the cementite particles is no more than 1.00%, the average particle diameter of the cementite particles is no more than 1.50 μm, and the spheroidization rate of the cementite particles is at least 85%.

Description

steel plate

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

[0002] Steel sheets with a C content of 0.15% or more are used as materials for machine parts, such as automobile parts. The method for manufacturing these machine parts using steel sheets as raw materials is as follows: The steel sheets are cold-worked to form them into the shape of the machine parts. The cold-worked steel sheets are 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 sheets are required to have excellent hardenability when quenched during the manufacturing process of machine parts made from steel sheets. Furthermore, the steel sheets are cold-worked before quenching. Therefore, the steel sheets are required to have not only the above-mentioned excellent hardenability but also excellent cold workability.

[0003] 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 density of cementite in ferrite grains low.

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

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

[0007] Among mechanical parts, springs, washers, and the like are required to have high strength. To increase the strength, further improvement in the hardenability of the steel sheet used as the material is required. C in steel sheet is an effective element for improving the hardenability of steel sheet. Therefore, steel sheets with an increased C content of 0.50% or more are used for these mechanical parts. Even in such steel sheets with a high C content, not only hardenability but also cold workability is required.

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

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

[0010] A steel plate having a chemical composition, in mass%, of C: 0.50 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities; and in the microstructure, a total area ratio of ferrite and cementite particles is 95% or more, The average particle size of the ferrite is 5.0 to 20.0 μm, and the Cr concentration [Cr] in mass% in the cementite particles is θ is less than 2.00%, 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 when the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, a spheroidization rate, which is the ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 85% or more.

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

[0012] 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.

[0013] First, the present inventors investigated the improvement of hardenability and cold workability in steel sheets having a C content of 0.50% or more from the viewpoint of chemical composition. As a result, the present inventors concluded that a chemical composition consisting of, in mass%, C: 0.50 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities would enable both improvement of hardenability and improvement of cold workability. Therefore, the present inventors further investigated means for improving both hardenability and cold workability of a steel sheet having the above-mentioned chemical composition from the viewpoint of microstructure.

[0014] 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-mentioned chemical composition is a structure substantially consisting of ferrite and cementite particles. In order to improve the hardenability of a steel sheet during quenching in a process for manufacturing a mechanical part using the steel sheet as a raw material, it is preferable that the cementite particles in the steel sheet are easily dissolved during quenching. In order to improve the solid solubility of the cementite particles during quenching, it is preferable that the particle size of the cementite particles is small. In the case of a steel sheet having the above-mentioned chemical composition, it is effective to make the average particle size of the cementite particles 1.50 μm or less.

[0015] 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.

[0016] Based on the above findings, further investigation was carried out. As a result, the inventors found that the Cr concentration [Cr] in the cementite particles θis less than 2.00%, and the Mo concentration [Mo] θ It has been found that if the content of C is 1.00% or less, cementite particles are easily dissolved during quenching, and the hardenability of the steel sheet is improved.

[0017] The present inventors have further investigated means for improving the cold workability of the microstructure of a steel sheet. In order to improve the cold workability of a steel sheet, it is effective to increase the spheroidization rate of cementite particles and to coarsen the average grain size of ferrite. Therefore, in the steel sheet of this embodiment, the spheroidization rate of cementite particles is set to 85% or more, and the average grain size of ferrite is set to 5.0 μm or more. Note that a smaller average grain size of ferrite increases the ferrite grain boundary area in the steel sheet. Therefore, due to grain boundary diffusion, the dissolution of cementite particles becomes faster during quenching. Therefore, the average grain size of ferrite is set to 20.0 μm or less.

[0018] The steel sheet according to this embodiment, which was completed based on the above findings, has the following configuration.

[0019] [1] A steel plate having a chemical composition, in mass%, of C: 0.50 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities; and in the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. The average particle size of the ferrite is 5.0 to 20.0 μm, and the Cr concentration [Cr] in mass% in the cementite particles is θ is less than 2.00%, and the Mo concentration in mass% in the cementite particles [Mo] θthe average particle size of the cementite particles is 1.50 μm or less, and when the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, a spheroidization rate, which is the ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 85% or more.

[0020] [2] The steel sheet according to [1], wherein the chemical composition contains one or more elements selected from the group consisting of Mo: 0.001 to 0.500%, Ni: 0.001 to 1.000%, B: 0.0001 to 0.0100%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%.

[0021] The steel sheet of this embodiment will be described in detail below. Note that "%" regarding elements means mass % unless otherwise specified.

[0022] [Features of Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment satisfies the following Features 1 to 6. (Feature 1) The chemical composition, in mass%, is C: 0.50 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities. (Feature 2) In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. (Feature 3) The average grain size of ferrite is 5.0 to 20.0 μm. (Feature 4) The Cr concentration in mass % in cementite particles [Cr] θ is less than 2.00%, and the Mo concentration in mass% in the cementite particles [Mo] θis 1.00% or less. (Feature 5) The average particle size of the cementite particles is 1.50 μm or less. (Feature 6) When cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 85% or more. Features 1 to 6 will be described below.

[0023] [(Feature 1) Chemical Composition] The chemical composition of the steel sheet of this embodiment contains the following elements.

[0024] C: 0.50 to 0.90% Carbon (C) improves the hardenability of steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. If the C content is less than 0.50%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.90%, the cold workability of the steel sheet decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.50 to 0.90%. The preferred lower limit of the C content is 0.52%, more preferably 0.55%, and even more preferably 0.60%. The preferred upper limit of the C content is 0.88%, more preferably 0.85%, and even more preferably 0.80%.

[0025] Si: 0.01 to 0.50% Silicon (Si) deoxidizes steel during the steelmaking stage of the steel sheet manufacturing process. Furthermore, Si enhances the temper softening resistance of the steel sheet when tempering is performed in a process for manufacturing machine parts using the steel sheet as a raw material. If the Si content is less than 0.01%, the above effect cannot be sufficiently 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%.

[0026] Mn: 0.20 to 1.30% Manganese (Mn) improves the hardenability of steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. If the Mn content is less than 0.20%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.30%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Mn content is 0.20 to 1.30%. The preferred lower limit of the Mn content is 0.25%, more preferably 0.30%, and even more preferably 0.35%. The preferred upper limit of the Mn content is 1.25%, more preferably 1.20%, and even more preferably 1.15%.

[0027] P: 0.100% or less Phosphorus (P) is an impurity. If the P content exceeds 0.100%, the toughness of the steel plate decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.100% or less. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is more than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the P content is 0.090%, even more preferably 0.080%, and even more preferably 0.050%.

[0028] S: 0.100% or less Sulfur (S) is an impurity. If the S content exceeds 0.100%, S forms excessively large amounts of sulfides. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the S content is 0.100% or less. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is more than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the S content is 0.090%, even more preferably 0.080%, and even more preferably 0.050%.

[0029] Al: 0.100% or less Aluminum (Al) is an impurity. Al combines with N to form AlN. AlN refines austenite grains during heating in the quenching process in the manufacturing of mechanical components using steel sheet as a raw material. Refining austenite grains reduces the hardenability of the steel sheet. Therefore, if the Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the austenite grains are excessively refined during heating in the quenching process. As a result, the hardenability of the steel sheet is significantly reduced. Therefore, the Al content is 0.100% or less. The Al content is preferably as low as possible. However, an extreme reduction in the Al content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the Al content is greater than 0%, more preferably 0.001%, even more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Al content is preferably 0.090%, more preferably 0.080%, still more preferably 0.070%, and still more preferably 0.050%. In the chemical composition of the steel sheet of this embodiment, the Al content means the acid-soluble Al (sol. Al) content.

[0030] Cr: 0.01 to 1.20% Chromium (Cr) improves the hardenability of steel sheets. As a result, by performing hardening in a process for manufacturing mechanical parts using steel sheets as raw materials, the strength of the mechanical parts is increased. If the Cr content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 1.20%, the Cr concentration [Cr] in cementite particles θThe content of Cr in the steel sheet is excessively increased. As a result, cementite particles do not dissolve sufficiently during heating in the quenching process in the process of manufacturing mechanical components using the steel sheet as a raw material. In this case, the hardenability of the steel sheet is actually reduced. As a result, mechanical components manufactured using the steel sheet as a raw material do not have sufficient strength. Therefore, the Cr content is 0.01 to 1.20%. A preferred lower limit of the Cr content is 0.02%, more preferably 0.03%, even more preferably 0.05%, even more preferably 0.08%, even more preferably 0.10%, even more preferably 0.13%, and even more preferably 0.18%. A preferred upper limit of the Cr content is 1.15%, even more preferably 1.10%, even more preferably 1.00%, even more preferably 0.70%, and even more preferably 0.50%.

[0031] N: 0.0150% or less Nitrogen (N) is an impurity. N combines with Al to form AlN. AlN refines austenite grains during heating in the quenching process in the manufacturing of mechanical components using steel sheet as a raw material. Refining austenite grains reduces the hardenability of the steel sheet. Therefore, if the N content exceeds 0.0150%, even if the contents of other elements are within the ranges of this embodiment, the austenite grains are excessively refined during heating in the quenching process. As a result, the hardenability of the steel sheet is significantly reduced. Therefore, the N content is 0.0150% or less. The N content is preferably as low as possible. However, an extreme reduction in the N content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the N content is more than 0%, more preferably 0.0001%, and even more preferably 0.0005%. The upper limit of the N content is preferably 0.0140%, more preferably 0.0130%, and even more preferably 0.0120%.

[0032] The balance of the chemical composition of the steel sheet according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel sheet, and are acceptable within a range that does not adversely affect the steel sheet according to this embodiment.

[0033] The impurities may include elements with the following contents: 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%.

[0034] [Optional Elements] The chemical composition of the steel sheet of this embodiment may further contain one or more elements selected from the group consisting of: Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%. These optional elements will be described below.

[0035] [First Group: Mo, Ni, and B] The chemical composition of the steel sheet according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo, Ni, and B. All of these elements are optional elements and may not be contained. When contained, Mo, Ni, and B improve the hardenability of the steel sheet.

[0036] Mo: 0 to 0.500% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. In other words, the Mo content may be 0%. When Mo is contained, that is, when the Mo content exceeds 0%, Mo improves the hardenability of the steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical part using the steel sheet as a raw material, the strength of the mechanical part is increased. Furthermore, when tempering is performed in a process for manufacturing a mechanical part using the steel sheet as a raw material, Mo improves the temper softening resistance of the steel sheet. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. However, when the Mo content exceeds 0.500%, the Mo concentration [Mo] in the cementite particles θ The Mo content is excessively increased. As a result, cementite particles do not dissolve sufficiently during heating in the quenching process in the process of manufacturing mechanical parts using the steel sheet as a raw material. In this case, the hardenability of the steel sheet is actually reduced. As a result, mechanical parts manufactured using the steel sheet as a raw material do not have sufficient strength. Therefore, the Mo content is 0 to 0.500%. A preferred lower limit of the Mo content is 0.001%, more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%. A preferred upper limit of the Mo content is 0.450%, more preferably 0.400%, even more preferably 0.350%, and even more preferably 0.300%.

[0037] Ni: 0 to 1.000% Nickel (Ni) is an optional element and may not be contained. In other words, the Ni content may be 0%. When Ni is contained, that is, when the Ni content exceeds 0%, Ni enhances the hardenability of the steel sheet. As a result, when quenching is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. Ni also enhances the temper softening resistance of the steel sheet when tempering is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material. Even if even a small amount of Ni is contained, the above effect can be achieved to some extent. However, if the Ni content exceeds 1.000%, the strength of the steel sheet will be 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 will be reduced. Therefore, the Ni content is 0 to 1.000%. The preferred lower limit of the Ni content is 0.001%, more preferably 0.005%, 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%.

[0038] B: 0 to 0.0100% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When B is contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content exceeds 0.0100%, B compounds will be formed even if the contents of other elements are within the ranges of this embodiment. In this case, the hardenability improvement effect cannot be sufficiently obtained. Furthermore, the cold workability of the steel sheet will be reduced. Therefore, the B content is 0 to 0.0100%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and still more preferably 0.0050%.

[0039] [Regarding Group 2: V, Nb, and Ti] The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of V, Nb, and Ti, instead of a portion of Fe. All of these elements are optional and may not be contained. When contained, V, Nb, and Ti form carbides. These carbides suppress coarsening of austenite grains during heating in the quenching process in the process of manufacturing mechanical components using the steel sheet as a raw material. Therefore, the toughness of the mechanical components is improved.

[0040] V: 0 to 0.500% Vanadium (V) is an optional element and may not be contained. In other words, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms carbides and suppresses coarsening of austenite grains during heating in the quenching process in the process of manufacturing mechanical components using the steel sheet as a raw material. This improves the toughness of the mechanical component. Even if even a small amount of V is contained, the above effect can be achieved to some extent. However, when the V content exceeds 0.500%, V forms excessive carbides and precipitation strengthens the steel sheet. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the V content is 0 to 0.500%. The preferred lower limit of the V content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the V content is preferably 0.480%, more preferably 0.450%, and even more preferably 0.400%.

[0041] Nb: 0 to 0.500% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms carbides and suppresses coarsening of austenite grains during heating in the quenching process in the process of manufacturing mechanical parts using steel sheet as a raw material. This improves the toughness of the mechanical parts. Furthermore, Nb bonds with N to suppress the formation of nitrides by solute B. This improves the hardenability of the steel sheet due to solute B. Even if even a small amount of Nb is contained, the above effect can be achieved to some extent. However, when the Nb content exceeds 0.500%, Nb forms excessive carbides and precipitation-strengthens the steel sheet. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the Nb content is 0 to 0.500%. The lower limit of the Nb content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Nb content is preferably 0.480%, more preferably 0.450%, even more preferably 0.400%, even more preferably 0.350%, and even more preferably 0.300%.

[0042] Ti: 0 to 0.150% Titanium (Ti) is an optional element and may not be included. In other words, the Ti content may be 0%. When Ti is included, i.e., when the Ti content exceeds 0%, Ti forms carbides and suppresses coarsening of austenite grains during heating in the quenching process in manufacturing mechanical parts using steel sheet as a raw material. This improves the toughness of the mechanical part. Furthermore, Ti bonds with N to suppress the formation of nitrides by solute B. This improves the hardenability of the steel sheet due to solute B. Even if even a small amount of Ti is included, the above effect can be achieved to some extent. However, when the Ti content exceeds 0.150%, Ti forms excessive carbides and precipitation-strengthens the steel sheet. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the Ti content is 0 to 0.150%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Ti content is preferably 0.145%, more preferably 0.130%, more preferably 0.120%, even more preferably 0.100%, and even more preferably 0.080%.

[0043] [(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.

[0044] In the microstructure, the structure other than ferrite and cementite particles is, for example, one or more types selected from the group consisting of precipitates other than cementite particles, inclusions, bainite, martensite, and pearlite.

[0045] 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.

[0046] If the total area ratio of ferrite and cementite particles is 95% or more, sufficient hardenability and sufficient cold workability can be obtained, provided that Feature 1 and Feature 3 to Feature 6 are satisfied.

[0047] [Method for measuring the total area ratio of ferrite and cementite particles in the microstructure] The total area ratio of ferrite and cementite particles in the microstructure can be measured by the following method. A test piece measuring 15 mm in the rolling direction (L direction) of the steel sheet, 10 mm in the sheet width direction, and sheet thickness is taken from the center of the sheet width of the steel sheet. Here, the rolling direction can be identified by observing the roll marks formed on the surface of the steel sheet. The roll marks are thin lines formed on the surface of the steel sheet along the direction of the rolling rolls when rolling is performed. In this case, the direction in which the roll marks extend is identified as the rolling direction.

[0048] Of the surfaces of the test piece, a cross section parallel to the rolling direction (a surface of 15 mm × plate thickness in the rolling direction) is defined as the observation surface. The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (nital etchant). Of the etched observation surface, secondary electron images of five arbitrary observation fields are observed using a scanning electron microscope (SEM) at 1000x magnification. Each observation field is a rectangle of 100 μm × 120 μm.

[0049] In the observation field, ferrite and cementite particles exhibit contrast and morphology that differ from other structures (bainite, martensite, pearlite, precipitates other than cementite particles, inclusions, etc.). Specifically, when the observation surface is etched with a nital etching solution, areas with high granular brightness can be identified as cementite particles. Areas with lamellar structures can be identified as pearlite. Areas with a brightness lower than pearlite and no visible substructure can be identified as ferrite. Areas with a brightness higher than ferrite and lower than pearlite and where a substructure is observed can be identified as bainite and martensite. Therefore, ferrite and cementite particles can be identified within the observation field based on contrast and morphology.

[0050] The total area ratio (%) of ferrite and cementite particles is calculated based on the total area of ​​ferrite and the total area of ​​cementite particles in the five observation fields and the total area of ​​the five observation fields.

[0051] [(Feature 3) Average grain size of ferrite] In the steel sheet of this embodiment, the average grain size of ferrite is 5.0 to 20.0 μm. If the average grain size of ferrite is less than 5.0 μm, the cold workability of the steel sheet is reduced. On the other hand, if the average grain size of ferrite exceeds 20.0 μm, the grain boundary area is reduced, and the effect of promoting the dissolution of cementite particles by grain boundary diffusion cannot be obtained. In this case, sufficient hardenability cannot be obtained in the hardening process when manufacturing mechanical parts using the steel sheet as a raw material. As a result, the strength of the mechanical parts manufactured using the steel sheet as a raw material is reduced. Therefore, the average grain size of ferrite is 5.0 to 20.0 μm.

[0052] The lower limit of the average particle size of ferrite is preferably 5.2 μm, more preferably 5.5 μm, even more preferably 5.7 μm, even more preferably 6.0 μm, and even more preferably 6.5 μm. The upper limit of the average particle size of ferrite is preferably 19.5 μm, more preferably 19.0 μm, even more preferably 18.5 μm, and even more preferably 18.0 μm.

[0053] [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. Of the surface of the test piece, a cross section parallel to the rolling direction (the surface measuring 15 mm in the rolling direction and thickness) is defined as the observation surface. The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched using a 5% nital etching solution. The average grain size of ferrite is determined on the etched observation surface by the following method. In accordance with JIS G 0551:2020, the ferrite grain size number is determined by the intercept method. In this case, the magnification of the optical microscope is selected so that the number of ferrite grains intercepted by one line segment is at least 10 in one visual field. After selecting the magnification, the intercept lengths are determined for five visual fields. The ferrite grain size number is determined from the arithmetic mean of the intercept lengths of the five visual fields. The average grain size (μm) of ferrite is determined from the obtained grain size number.

[0054] (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 less than 2.00%, and the Mo concentration in mass% in the cementite particles [Mo] θ is 1.00% or less.

[0055] Cr concentration in cementite particles of steel plate [Cr] θ and Mo concentration [Mo] θ If the hardness is too high, cementite particles do not dissolve sufficiently during heating in the quenching step in the process of manufacturing mechanical parts using the steel sheet as a raw material. In this case, the hardenability of the steel sheet is reduced. As a result, mechanical parts manufactured using the steel sheet as a raw material do not have sufficient strength.

[0056] Cr concentration in cementite particles of steel plate [Cr] θ is less than 2.00%, and the Mo concentration in the cementite particles [Mo] θ When the Cr concentration in the cementite grains [Cr] is 1.00% or less, θ and Mo concentration [Mo] θTherefore, during heating in the above-mentioned quenching step, cementite particles are sufficiently dissolved, and the hardenability of the steel sheet is improved.

[0057] Cr concentration in cementite particles [Cr] θ The upper limit of Mo is preferably 1.90%, more preferably 1.80%, and even more preferably 1.70%. Mo concentration in cementite particles [Mo] θ The upper limit of the content of Si is preferably 0.90%, more preferably 0.80%, and even more preferably 0.70%.

[0058] Cr concentration in cementite particles [Cr] θ The lower limit of Cr concentration [Cr] is not particularly limited. θ The most preferable lower limit of Mo is 0%, but it may remain at 0.01% or more, or 0.10% or more. Mo concentration in cementite particles [Mo] θ The lower limit of Mo concentration [Mo] is not particularly limited. θ The most preferable lower limit of is 0%, but there may be cases where 0.01% or more, or 0.03% or more remains.

[0059] [Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Measurement method] Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ can be measured by the following method: A test piece is taken from the center of the steel plate width, and the size of the test piece is 10 mm x 10 mm x plate thickness.

[0060] 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).

[0061] 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. ２The test piece is subjected to constant current electrolysis while being held at 10°C. After constant current electrolysis, the test piece is removed from the 10% AA solution. The removed test piece is immersed in an alcohol solution. The test piece immersed in the alcohol solution is subjected to ultrasonic cleaning.

[0062] 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.

[0063] The extracted residue is subjected to chemical elemental analysis. Specifically, the residue is dissolved in acid to obtain a solution. The solution is subjected to chemical elemental analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES) to obtain the Cr mass and Mo mass in the residue. Based on the obtained Cr mass, Mo mass, and total mass of the residue, the Cr concentration (mass%) and Mo concentration (mass%) in the residue are determined.

[0064] The obtained residue is substantially composed of cementite particles. In other words, the amount of particles other than cementite particles (inclusions and precipitates other than cementite particles) in the residue is negligibly small. Therefore, the Cr concentration and Mo concentration in the residue are calculated as the Cr concentration in the cementite particles [Cr] θ and Mo concentration [Mo] θ It is considered to be.

[0065] [(Feature 5) Average particle size of cementite particles] In the steel sheet of this embodiment, the average particle size of the cementite particles is 1.50 μm or less. As described above, if the cementite particles are large, the cementite particles will not dissolve sufficiently during heating in the quenching step in the process of manufacturing a mechanical component using the steel sheet as a raw material. In this case, the hardenability of the steel sheet will be reduced. As a result, the mechanical component manufactured using the steel sheet as a raw material will not have sufficient strength.

[0066] If the average particle size of the cementite particles is 1.50 μm or less, the cementite particles are sufficiently small, and therefore, when heated in the above-mentioned quenching step, the cementite particles dissolve sufficiently, thereby improving the hardenability of the steel sheet.

[0067] The upper limit of the average particle size of the cementite particles is preferably 1.45 μm, more preferably 1.40 μm, even more preferably 1.35 μm, and even more preferably 1.30 μm. To improve hardenability, a smaller average particle size of the cementite particles is preferable. However, if the average particle size of the cementite particles is too small, the hardness of the steel sheet becomes too high. In this case, the cold workability of the steel sheet deteriorates. Therefore, the lower limit of the average particle size of the cementite particles is preferably 0.05 μm, more preferably 0.10 μm, even more preferably 0.15 μm, and even more preferably 0.20 μm.

[0068] [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 plate thickness is taken from the center of the plate width of the steel plate. Of the surface of the test piece, a cross section parallel to the rolling direction (the surface of 15 mm in the rolling direction and the plate thickness) is defined as the observation surface.

[0069] The observation surface is etched using a picral solution. Secondary electron images are taken at five arbitrary observation fields located at a depth of 1 / 4 of the plate thickness from the surface of the etched observation surface. Specifically, the five observation fields are observed at a magnification of 2000 times using a scanning electron microscope (SEM), and the above-mentioned secondary electron images are taken. Each observation field is a rectangle of 50 μm × 60 μm.

[0070] In each secondary electron image, cementite particles are identified based on the contrast. The area of ​​each identified cementite particle is determined, and the circle-equivalent diameter of each cementite particle is determined based on the area. The circle-equivalent diameter thus determined is defined as the particle diameter of the cementite particle. Note that the particle diameter is determined using well-known image processing software. The arithmetic mean value of the particle diameters of the cementite particles obtained in the five observation fields is defined as the average particle diameter (μm) of the cementite particles.

[0071] [(Feature 6) Spheroidization Ratio] In the steel sheet of this embodiment, among the plurality of cementite particles, cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles. The spheroidization ratio, which is the ratio of the total number of spherical cementite particles to the total number of the plurality of cementite particles, is 85% or more.

[0072] If the spheroidization rate is 85% or more, the steel sheet will have significantly excellent cold workability, provided that Features 1 to 5 are satisfied. Therefore, the steel sheet of this embodiment has a spheroidization rate of 85% or more. A higher spheroidization rate is preferable. The preferred lower limit of the spheroidization rate is 87%, more preferably 89%, even more preferably 91%, and even more preferably 95%.

[0073] [Method for Measuring Spheroidization Ratio] The spheroidization ratio can be measured by the following method. The aspect ratio is determined for each of the cementite particles identified in five observation fields by the above-mentioned [Method for Measuring the Average Particle Diameter of Cementite Particles]. Specifically, the long diameter is defined as the maximum distance obtained when the outline of the cementite particle is sandwiched between two parallel line segments. Furthermore, the short diameter is defined as the distance between the two line segments when the outline of the cementite particle is sandwiched between two line segments parallel to the long diameter (i.e., the width in the direction perpendicular to the long diameter).

[0074] The aspect ratio (= major axis / minor axis) of each cementite particle is calculated based on the obtained major axis and minor axis. Of all cementite particles in the five observation fields, cementite particles with an aspect ratio of 3.0 or less are identified as "spheroidal cementite particles." The ratio of the total number of spherical cementite particles to the total number of multiple cementite particles is defined as the spheroidization rate (%).

[0075] [Effects of the Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment, which satisfies the above-described Features 1 to 6, exhibits sufficient hardenability during quenching in a process for manufacturing a mechanical component using the steel sheet as a raw material. Furthermore, the steel sheet of the present embodiment exhibits sufficient cold workability.

[0076] [Quenchability] In the steel sheet of this embodiment, the fact that sufficient quenchability is obtained means the following evaluation.

[0077] [Hardenability evaluation method] (A ｃ１ Measurement of transformation point) A test piece is taken from the center of the sheet width of the steel sheet of this embodiment. The thermal expansion coefficient during heating is measured using a Formaster testing machine. From the obtained thermal expansion coefficient, A ｃ１ Find the transformation point.

[0078] (Measurement of maximum quenched hardness) A plate-shaped test specimen was taken from the center of the steel plate width. The shape of the plate-shaped test specimen was 15 mm in the rolling direction (L direction) of the steel plate, 30 mm in the width direction (W direction), and plate thickness. The plate-shaped test specimen was heated in a salt bath at 1000°C for 20 minutes. The plate-shaped test specimen was then removed from the salt bath and immersed in water in a water tank for quenching. The quenched plate-shaped test specimen was cut into two equal parts in the W direction. The cut surfaces were mirror-polished. A Vickers hardness test in accordance with JIS Z2244:2009 was performed at three arbitrary points in the center of the thickness direction (T direction) of the polished cut surface. The test force was 98 N. The arithmetic mean value of the obtained Vickers hardness was defined as the maximum quenched hardness HD0 (HV).

[0079] (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 (L direction) of the steel plate, 30 mm in the plate width direction (W direction), and plate thickness. ｃ１ The plate test specimen is immersed in a salt bath at transformation point +80°C for 10 minutes. The plate test specimen is then removed from the salt bath and immersed in water in a water tank for quenching. The quenched plate test specimen is cut into two equal halves in the W direction. The cut surfaces are mirror-polished. Vickers hardness tests are performed in accordance with JIS Z2244:2009 at three arbitrary points in the center of the thickness direction (T direction) of the polished cut surfaces. The test force is 98 N. The arithmetic mean value of the obtained Vickers hardness is defined as the quenched hardness HD1 (HV). If the obtained quenched hardness HD1 is 95% or more of the maximum quenched hardness HD0, the steel plate is determined to have sufficient hardenability.

[0080] [Cold Workability] In the steel sheet of this embodiment, the cold workability can be evaluated, for example, by the following method.

[0081] [Cold workability evaluation method] A JIS No. 5 plate test piece specified in JIS Z2241:2011 is taken from the center of the plate width of a steel plate. A V-notch is formed in the longitudinal center of the parallel part of the plate test piece so that the depth direction of the V-notch is parallel to the width direction of the parallel part. The V-notch opening angle is 45° and the V-notch depth is 2 mm. The gauge length is 10 mm including the V-notch. The longitudinal direction of the plate test piece is the rolling direction (L direction) of the steel plate.

[0082] A fracture elongation test is carried out using a plate-shaped test piece at room temperature in the atmosphere. The butt elongation after fracture is measured, and the obtained butt elongation (%) is defined as the notch elongation (%). The steel plate of this embodiment has a larger notch elongation than a steel plate that does not satisfy any of Features 1 to 6. For example, when the plate-shaped test piece has a thickness of 2 mm, the steel plate of this embodiment has a notch elongation of 5.0% or more and -33 × C content (%) of the steel plate + 32.5% or more.

[0083] [Applications of Steel Sheet] The steel sheet of this embodiment is suitable as a material for mechanical parts, such as automobile parts. Examples of mechanical parts include automobile springs and washers. The steel sheet of this embodiment may also be used for applications other than mechanical parts that require excellent hardenability and excellent cold workability.

[0084] [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.

[0085] 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.

[0086] The main manufacturing conditions in the above steps 1 to 4 are as follows: (Condition 1) Coiling temperature CT in step 2: 400 to 600°C (Condition 2) Cold rolling rate CR in step 3: more than 35 to 60% (Condition 3) Annealing temperature T1 in step 4: 550 to 720°C (Condition 4) Holding time t1 in step 4: more than 25 to 80 hours

[0087] Each step will be described below.

[0088] [(Step 1) Material Preparation Step] In the material preparation step, a material satisfying Feature 1 is prepared. The material is produced, for example, by the following method. Molten steel is produced, the content of each element in the chemical composition of which falls within the range of this embodiment. The molten steel is used to produce a material (slab or ingot) by a casting method. For example, a slab is produced by a well-known continuous casting method using the molten steel. Alternatively, an ingot is produced by a well-known ingot-making method using the molten steel.

[0089] [(Step 2) Hot Rolling Step] In the hot rolling step, hot rolling is performed on a prepared material (slab or ingot) to produce a hot-rolled steel sheet. The hot rolling step includes a rough rolling step in which the material is rough rolled to produce a rough bar (intermediate steel sheet), and a finish rolling step in which the rough bar is finish rolled to produce a hot-rolled steel sheet.

[0090] In the rough rolling process, a material (slab or ingot) is heated in a heating furnace. The heated material is rolled using a rough rolling mill to produce a rough bar. The heating temperature of the material in the rough rolling process is, for example, 1100 to 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.

[0091] In the finish rolling process, the rough bar is further rolled (finish rolling) using a finish rolling mill to produce a hot-rolled steel sheet. The finish rolling mill includes multiple stands arranged in a row. Each stand is equipped with a pair of work rolls. The surface temperature of the steel sheet at the outlet of the last stand in the finish rolling mill that reduces the steel sheet is defined as the finish rolling temperature (°C). In this embodiment, the finish rolling temperature is 830 to 950°C. Furthermore, the reduction rate of the last stand in the finish rolling mill that reduces the steel sheet is defined as the final pass reduction rate (%). In this embodiment, the final pass reduction rate is 5 to 30%. The hot-rolled steel sheet after finish rolling is wound up into a coil. The coiling temperature CT will be described later. The coiled hot-rolled steel sheet is cooled to room temperature.

[0092] [(Step 3) Cold Rolling Step] In the cold rolling step, cold rolling is performed on the hot-rolled steel sheet after the hot rolling step. Cold rolling is performed using a cold rolling mill. The cold rolling mill is, for example, a reverse rolling mill consisting of a single rolling stand, and the rolling stand includes a pair of work rolls.

[0093] 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 ratio CR in the cold rolling step will be described later.

[0094] In this embodiment, the hot-rolled steel sheet after the hot-rolling process is subjected to the cold-rolling process without being subjected to an annealing treatment. In other words, the hot-rolled sheet annealing process is not performed after the hot-rolling process but before the cold-rolling process. In this manufacturing method, strain is accumulated in the steel sheet during the hot-rolling process and the cold-rolling process, and the annealing process is performed after the cold-rolling process. As a result, ferrite 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.

[0095] [(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.

[0096] [Regarding Conditions 1 to 4] In the above-described steps 1 to 4, the following conditions 1 to 4 are satisfied: (Condition 1) Coiling temperature CT in step 2: 400 to 600°C (Condition 2) Cold rolling rate CR in step 3: more than 35 to 60% (Condition 3) Annealing temperature T1 in step 4: 550 to 720°C (Condition 4) Holding time t1 in step 4: more than 25 to 80 hours Each condition will be explained below.

[0097] [(Condition 1) 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 600°C or less, the cementite particles formed in the hot-rolled steel sheet are distributed sufficiently uniformly. Furthermore, the strain imparted by the hot-rolled steel sheet is appropriately maintained. In this case, the spheroidization rate of the cementite particles is increased by performing an annealing process after the cold rolling process. Furthermore, the concentration of Cr and Mo in the formed cementite particles can be suppressed, and the Cr concentration [Cr] in the cementite particles can be reduced. θ is less than 2.00%, and the Mo concentration [Mo] θ Therefore, the coiling temperature CT is preferably set to 600° C. or less.

[0098] On the other hand, the lower limit of the coiling temperature CT is not particularly limited. However, due to equipment restrictions, the lower limit of the coiling temperature CT is preferably 400°C.

[0099] If the coiling temperature CT is 400 to 600°C, a steel sheet satisfying Features 1 to 6 can be produced, provided that other conditions are met.

[0100] [(Condition 2) Cold Rolling Ratio CR] In the cold rolling process, the cold rolling ratio CR is defined by the following formula: Cold Rolling Ratio CR (%) = (1 - (Thickness of cold rolled steel sheet after cold rolling process / Thickness of hot rolled steel sheet before cold rolling process)) x 100

[0101] If the cold rolling rate CR exceeds 35%, sufficient strain is introduced into the steel sheet. In this case, in the subsequent annealing process, spheroidization of cementite particles is promoted, and the spheroidization rate becomes 85% or more. On the other hand, if the cold rolling rate CR exceeds 60%, excessive strain is introduced into the steel sheet. In this case, ferrite is excessively refined, and the average grain size of ferrite becomes less than 5.0 μm. If the cold rolling rate CR exceeds 60%, the lamellar structure of pearlite is further broken down. In this case, spheroidization of cementite particles is promoted. As a result, Cr and Mo are concentrated in the cementite particles, and the Cr concentration [Cr] in the cementite particles increases. θ becomes 2.00% or more, or Mo concentration [Mo] θ exceeds 1.00%.

[0102] If the cold rolling rate CR is more than 35% to 60%, a steel sheet satisfying Features 1 to 6 can be produced, provided that other conditions are met.

[0103] [(Condition 3) Annealing Temperature T1] In the cold-rolled sheet annealing process, the annealing temperature T1 adjusts the spheroidization rate of cementite particles in the steel sheet. If the annealing temperature T1 is less than 550°C, the spheroidization of cementite becomes insufficient, and the spheroidization rate of cementite particles becomes less than 85%.

[0104] On the other hand, if the annealing temperature T1 exceeds 720°C, the annealing temperature is too high. In this case, the spheroidization of cementite particles becomes insufficient, and the spheroidization rate of cementite particles becomes less than 85%. Furthermore, the total area ratio of ferrite and cementite particles in the microstructure becomes less than 95%.

[0105] If the annealing temperature T1 is 550 to 720°C, a steel sheet satisfying Features 1 to 6 can be produced, provided that other conditions are met.

[0106] [(Condition 4) Holding Time t1 at Annealing Temperature T1] In the cold-rolled sheet annealing process, the holding time t1 at the annealing temperature T1 is determined based on the size of ferrite in the steel sheet, the size of cementite particles, and the Cr concentration [Cr] in the cementite particles. θ and Mo concentration [Mo] θ Specifically, if the holding time t1 is 25 hours or less, the cementite particles are not sufficiently spheroidized.

[0107] On the other hand, if the holding time t1 exceeds 80 hours, the holding time is too long, and the ferrite and cementite particles become coarse. As a result, the average particle size of ferrite exceeds 20.0 μm, and the average particle size of cementite particles exceeds 1.50 μm. Furthermore, Cr and Mo are concentrated in the cementite particles, and the Cr concentration [Cr] in the cementite particles becomes too high. θ becomes 2.00% or more, or Mo concentration [Mo] θ exceeds 1.00%.

[0108] If the holding time t1 is more than 25 to 80 hours, a steel sheet satisfying Features 1 to 6 can be manufactured, provided that other conditions are met.

[0109] Through the above manufacturing process, a steel sheet satisfying Features 1 to 6 can be manufactured.

[0110] 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.

[0111] Steel sheets having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured.

[0112]

[0113]

[0114] 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 1100 to 1250°C for 240 minutes. The heated slab was rolled in a roughing mill to produce a rough bar. The rough bar was then rolled using a finishing mill to produce a hot-rolled steel sheet. The finish rolling temperature for each test number was 830 to 950°C. The reduction ratio of the final pass was 5 to 30%. The hot-rolled steel sheet after finish rolling was wound up and formed into a coil. The coiled hot-rolled steel sheet was allowed to cool to room temperature. The coiling temperature CT in the hot rolling process for each test number was as shown in Table 2.

[0115]

[0116] For test numbers 1 to 48, the hot-rolled steel sheets after the hot rolling process were subjected to a cold rolling process without being subjected to a hot-rolled sheet annealing process. The cold rolling reduction ratios CR in the cold rolling process were as shown in Table 2. The cold-rolled steel sheets after the cold rolling were subjected to a cold-rolled sheet annealing process. The annealing temperature T1 and holding time t1 were as shown in Table 2. In the cold-rolled sheet annealing process, the steel sheets were furnace-cooled after the holding time t1 had elapsed. For test number 49, the hot-rolled sheet annealing process was performed after the hot rolling process and before the cold rolling process. The annealing temperature and the holding time at the annealing temperature were as shown in Table 2. Steel sheets were produced by the above-mentioned production process.

[0117] [Evaluation Tests] The following tests were carried out on the manufactured steel sheets with each test number: (Test 1) Total area ratio measurement test of ferrite and cementite particles (Test 2) Average ferrite particle size measurement test (Test 3) Cr concentration [Cr] in cementite particles θ and Mo concentration [Mo] θ (Test 4) Measurement test of average particle size of cementite particles (Test 5) Measurement test of spheroidization rate of cementite particles (Test 6) Hardenability evaluation test (Test 7) ​​Cold workability evaluation test Tests 1 to 7 will be described below.

[0118] [(Test 1) Test for Measuring the Total Area Ratio of Ferrite and Cementite Particles] The total area ratio of ferrite and cementite particles was determined for each test number based on the method described in the above-mentioned [Method for Measuring the Total Area Ratio of Ferrite and Cementite Particles in a Microstructure]. As a result, as shown in Table 3, the total area ratios of ferrite and cementite particles for all test numbers other than test number 46 were 95% or more.

[0119]

[0120] [(Test 2) Ferrite Average Grain Size Measurement Test] The average grain size of ferrite in the steel sheet of each test number was determined based on the method described in [Method for measuring average grain size of ferrite] above. The results are shown in Table 3.

[0121] [(Test 3) Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Measurement test of the above [Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Based on the method described in [Measurement method for], the Cr concentration [Cr] in the cementite particles of the steel plate of each test number θ and Mo concentration [Mo] θ The obtained Cr concentration [Cr] θ and Mo concentration [Mo] θ is shown in Table 3.

[0122] [(Test 4) Measurement of average particle size of cementite particles] The average particle size of cementite particles in the steel sheet of each test number was determined based on the method described in [Method for measuring average particle size of cementite particles] above. The obtained average particle sizes of cementite particles are shown in Table 3.

[0123] [(Test 5) Measurement test of spheroidization ratio of cementite particles] The spheroidization ratio of cementite particles of the steel plate of each test number was determined based on the method described in [Method for measuring spheroidization ratio] above. The obtained spheroidization ratios are shown in Table 3.

[0124] [(Test 6) Hardenability Evaluation Test] The hardenability of the steel plate with each test number was evaluated based on the method described in the above-mentioned [Hardenability Evaluation Method]. The "hardenability lower limit" in Table 3 indicates the value of the maximum hardenability HD0 x 0.95. If the hardenability HD1 was equal to or greater than the hardenability lower limit, it was determined that sufficient hardenability was obtained (shown as "E (Excellent)" in the "hardenability judgment" in Table 3). On the other hand, if the hardenability HD1 was less than the hardenability lower limit, it was determined that sufficient hardenability was not obtained (shown as "NA (Not Allowed)" in the "hardenability judgment" in Table 3). Furthermore, if the hardenability HD1 was less than 600 HV, it was determined that the part strength was not sufficient.

[0125] [(Test 7) ​​Cold Workability Evaluation Test] The cold workability of the steel sheet of each test number was evaluated based on the method described in the above-mentioned [Cold Workability Evaluation Method]. The thickness of the plate-shaped test specimen was 2 mm. The obtained notch elongation is shown in Table 3. If the obtained notch elongation was 5.0% or more and equal to or greater than the notch elongation target (-33 × C content (%) of the steel sheet + 32.5%), the steel sheet was judged to have sufficient cold workability (shown as "E (Excellent)" in the "Cold Workability Evaluation" column in Table 3). On the other hand, if the obtained notch elongation was less than 5.0% or less than the notch elongation target (-33 × C content (%) of the steel sheet + 32.5%), the steel sheet was judged to have insufficient cold workability (shown as "NA (Not Allowed)" in the "Cold Workability Evaluation" column in Table 3).

[0126] [Evaluation Results] With reference to Table 1-1, Table 1-2, Table 2, and Table 3, in test numbers 1 to 35, the chemical composition was appropriate and the manufacturing conditions 1 to 4 were satisfied. Therefore, the steel sheets with these test numbers satisfied features 1 to 6. As a result, sufficient hardenability and sufficient cold workability were obtained.

[0127] On the other hand, in test number 36, the C content was too high, so the notch elongation was less than 5.0%, and sufficient cold workability was not obtained.

[0128] In test number 37, the Si content was too high, so the notch elongation was less than the target notch elongation, and sufficient cold workability was not obtained.

[0129] In test number 38, the Mn content was too low. As a result, the hardness after quenching HD1 was less than 600 HV, and it was determined that the part strength was not sufficient. Furthermore, sufficient hardenability was not obtained.

[0130] In test number 39, the Mn content was too high, so the notch elongation was less than the target notch elongation, and sufficient cold workability was not obtained.

[0131] In test number 40, the Cr content was too high. Therefore, the Cr concentration in mass% in the cementite particles [Cr] θ Therefore, sufficient hardenability was not obtained.

[0132] In test number 41, the Mo content was too high. Therefore, the Mo concentration in mass% in the cementite particles [Mo] θ exceeded 1.00%, and therefore sufficient hardenability could not be obtained.

[0133] In test number 42, although the chemical composition was appropriate, the coiling temperature CT was too high. Therefore, the spheroidization rate of the cementite particles was too low. As a result, sufficient cold workability was not obtained. Furthermore, the Cr concentration [Cr] in the cementite particles θ As a result, sufficient hardenability was not obtained.

[0134] In Test No. 43, although the chemical composition was appropriate, the cold rolling rate CR was too low. Therefore, the spheroidization rate of cementite particles was too low. As a result, the notch elongation was less than 5.0%, and sufficient cold workability was not obtained.

[0135] In test number 44, although the chemical composition was appropriate, the cold rolling rate CR was too high. Therefore, the average grain size of ferrite was less than 5.0 μm. As a result, sufficient cold workability was not obtained. Furthermore, the Cr concentration [Cr] in the cementite particles was θ was too high, resulting in insufficient hardenability.

[0136] In Test No. 45, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing step was too low. Therefore, the spheroidization rate of cementite particles was low at less than 85%. As a result, the notch elongation was less than 5.0%, and sufficient cold workability was not obtained.

[0137] In Test No. 46, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too high. As a result, pearlite precipitated, and the total area ratio of ferrite and cementite particles was less than 95%. Furthermore, the spheroidization rate of the cementite particles was low. As a result, sufficient cold workability was not obtained.

[0138] In Test No. 47, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing step was too short. As a result, the spheroidization rate of cementite particles was low at less than 85%. As a result, the notch elongation was less than 5.0%, and sufficient cold workability was not obtained.

[0139] In test number 48, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too long. As a result, the ferrite and cementite particles were coarse. Furthermore, the Cr concentration [Cr] in the cementite particles θ was too high, resulting in insufficient hardenability.

[0140] In test number 49, annealing was performed after the hot rolling process and before the cold rolling process. Therefore, the Cr concentration in the cementite particles [Cr] θ was too high, resulting in insufficient hardenability.

[0141] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A steel plate, the chemical composition of which, by mass%, is as follows: C: 0.50 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance: Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more, the average particle size of the ferrite is 5.0 to 20.0 μm, and the Cr concentration [Cr] in the cementite particles by mass% θ is less than 2.00%, and the Mo concentration [Mo] in the cementite particles by mass% θ is 1.00% or less. The average particle size of the cementite particles is 1.50 μm or less. When the cementite particles with an aspect ratio of 3.0 or less among the cementite particles are defined as spherical cementite particles, the spheroidization rate, which is the ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 85% or more. Steel plate.

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

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