Steel sheet

Abstract

A steel sheet according to the present disclosure includes, by mass%, 0.50-0.80% C, greater than 0.50% and less than or equal to 2.10% Si, 0.20-1.20% Mn, less than or equal to 0.100% Al, 0.01-0.60% Cr, and less than or equal to 0.0150% N, the remainder being Fe and unavoidable impurities. The total area ratio of ferrite and cementite grains is greater than or equal to 95%. The average grain size of the ferrite is less than or equal to 20.0 μm, and the average grain size of the cementite is less than or equal to 1.50 μm. The spheroidization ratio of the cementite grains is great than or equal to 85%. The Cr concentration [Cr]θ (mass%) in a residue, the Mo concentration [Mo]θ (mass%) in the residue, and the C content [C] by mass% in the chemical composition of the steel sheet satisfy expression (3). The graphitization ratio GR, which is defined by expression (4) using [C] and the C concentration [C]CB (mass%) in carbides in the residue, is less than or equal to 5.0%.　(3): [Cr]θ+[Mo]θ–2.5 / (√[C])≤0　(4): GR=(1–[C]CB / [C])×100

Description

steel plate

[0001] This disclosure relates to steel plates.

[0002] Steel sheets with a high carbon content (high-carbon steel sheets) are sometimes used as materials for mechanical parts such as automobile parts and industrial machinery parts. When manufacturing these mechanical parts using steel sheets as a material, the manufacturing method is as follows: Cold working is performed on the steel sheet to process it into an intermediate product that has the same shape as the final mechanical part. Hardening and tempering are performed on the intermediate product. Through the above manufacturing process, high-strength mechanical parts are manufactured. In order to obtain high strength in the mechanical parts after hardening, the steel sheet is required to have excellent hardenability during the hardening process in the manufacturing of the mechanical parts. Furthermore, the steel sheet is cold-worked before hardening. Therefore, the steel sheet is required to have not only excellent hardenability but also excellent cold workability.

[0003] Steel sheets having excellent hardenability and excellent cold workability when used as materials for machine parts have been proposed in International Publication No. 2015 / 146173 (Patent Document 1) and International Publication No. 2020 / 175665 (Patent Document 2).

[0004] The steel sheet disclosed in Patent Document 1 has a composition in mass% of C: 0.20-0.40%, 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, B: 0.0005-0.0050%, and further contains one or more of Sb, Sn, Bi, Ge, Te, and Se in total amount of 0.002-0.030%, with the remainder being Fe and unavoidable impurities. In this steel sheet, the proportion of solid-solution B in the B content is 70% or more. Furthermore, the microstructure consists of ferrite and cementite. Furthermore, the cementite density within the ferrite grains is 0.08 grains / μm ２ The following is stated: Patent Document 1 states that the total elongation can be increased by keeping the cementite density of ferrite grains low.

[0005] The steel sheet disclosed in Patent Document 2 has a composition in mass%, containing C: 0.10% to 0.33%, Si: 0.01% to 0.50%, Mn: 0.40% to 1.25%, P: 0.03% or less, S: 0.01% or less, sol. Al: 0.10% or less, N: 0.01% or less, and Cr: 0.50% to 1.50%, with the remainder being Fe and unavoidable impurities, and has a microstructure containing ferrite and carbides. The volume ratio of ferrite and carbides to the total microstructure is 90% or more, and the volume ratio of protereminate ferrite to the total microstructure is 20% to 80%. The Mn concentration in the carbide is 0.10% by mass or more and 0.50% by mass or less, and the proportion of carbides with a particle size of 1 μm or more to the total number of carbides is 30% or more and 60% or less. Patent Document 2 states that by reducing the Mn concentration in the carbide, the carbide becomes easier to dissolve during quenching, and as a result, the hardenability is improved.

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

[0007] Incidentally, among machine parts, springs and washers require high strength. To increase strength, further improvement in the hardenability of the steel sheet material is required. Carbon (C) in steel sheets is an effective element for improving the hardenability of steel sheets. Therefore, steel sheets with a C content of 0.50% or more are sometimes used for these machine parts. Even with such high-C-content steel sheets, not only hardenability but also cold workability is required.

[0008] The purpose of this disclosure is to provide a steel sheet that exhibits excellent hardenability and excellent cold workability.

[0009] The steel sheet of the present disclosure has a chemical composition, in mass %, of C: 0.50 to 0.80%, Si: more than 0.50 to 2.10%, Mn: 0.20 to 1.20%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 0.60%, N: 0.0150% or less, Mo: 0 to 0.350%, 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%, and the balance consists of Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. The average grain size of ferrite is 20.0 μm or less. The average particle size of cementite particles is 1.50 μm or less. Among the cementite particles, the cementite particles with an aspect ratio of 3.0 or less are defined as 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 85% or more. The total mass <M> of Fe, Mn, Cr, and Mo in the residue obtained by the extraction residue analysis method θ is used as the total mass of metal elements in the cementite particles, and the C mass <C> in the cementite particles θ is determined, and the Cr concentration [Cr] in the residue defined by formula (1) θ (mass %), the Mo concentration [Mo] in the residue defined by formula (2) θ (mass %), and the C content [C] in mass % in the chemical composition of the steel sheet satisfy formula (3). Further, [C] and the C concentration [C] of carbides in the residue based on the mass of the steel sheet ＣＢ (mass %) are used, and the graphitization rate GR defined by formula (4) is 5.0% or less. [Cr] θ = mass of Cr in the residue / (<M> θ + <C> θ ) × 100 (1) [Mo] θ = mass of Mo in the residue / (<M> θ + <C> θ ) × 100 (2) [Cr] θ + [Mo] θ - 2.5 / (√[C]) ≤ 0 (3) GR = (1 - [C] ＣＢ / [C]) × 100 (4)

[0010] The steel sheet according to this disclosure provides excellent hardenability and excellent cold workability.

[0011] The inventors of the present invention conducted research on steel sheets having excellent hardenability and excellent cold workability. As a result, the inventors obtained the following findings.

[0012] First, the inventors investigated the chemical composition of steel sheets that can be used as materials for mechanical parts such as springs and washers. Mechanical parts such as springs and washers require not only high strength but also a high elastic limit. Increasing the Si content is effective in obtaining such high strength and a high elastic limit. Si increases the resistance to tempering and softening, and as a result, increases the strength and elastic limit of the steel sheet. Therefore, the inventors considered that a steel sheet having a chemical composition consisting of, by mass%, C: 0.50 to 0.80%, Si: greater than 0.50 to 2.10%, Mn: 0.20 to 1.20%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 0.60%, N: 0.0150% or less, Mo: 0 to 0.350%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%, with the remainder being Fe and impurities, would be suitable as a material for machine parts. Therefore, the inventors further investigated the steel sheet having the above chemical composition from the perspective of microstructure in order to obtain excellent hardenability and excellent cold workability.

[0013] The inventors first investigated means of improving the hardenability of steel sheets in their microstructure. The microstructure of a steel sheet having the above-mentioned chemical composition is substantially composed of ferrite and cementite particles. When manufacturing machine parts using steel sheets as a material, in order to improve the hardenability of the steel sheet during quenching in the manufacturing process of the machine parts, it is preferable that the cementite particles in the steel sheet dissolve easily during quenching. To improve the solid solubility of cementite particles during quenching, it is preferable that the particle size of the cementite particles be small. In the case of a steel sheet having the above-mentioned chemical composition, hardenability is improved if the average particle size of the cementite particles is 1.50 μm or less. Also, if the average particle size of ferrite is 20.0 μm or less, the grain boundary area of ​​the ferrite increases. In this case, grain boundary diffusion accelerates the dissolution of cementite during quenching. Therefore, if the average particle size of ferrite is 20.0 μm or less, hardenability is also improved.

[0014] Furthermore, the inventors' investigations revealed that the Mn concentration in cementite particles does not affect the solid solution of cementite particles during quenching. On the other hand, the inventors found that the Cr and Mo concentrations in cementite particles greatly influence the solid solution of cementite particles during quenching. Specifically, if the Cr and Mo concentrations in cementite particles are high, the cementite particles become less likely to solidify during quenching.

[0015] Based on the above findings, further investigations were conducted. As a result, the inventors determined that the total mass of Fe, Mn, Cr, and Mo in the residue obtained by the extraction residue analysis method <M> θ Let C be the total mass of metallic elements in the cementite particles, and the C mass in the cementite particles < C θ When this is determined, the Cr concentration in the residue [Cr] is defined by formula (1). θ (mass%), Mo concentration in the residue as defined by formula (2) [Mo] θ We found that if the C content [C] in mass percent of the chemical composition of the steel sheet satisfies formula (3), then excellent hardenability can be obtained. [Cr] θ = Mass of Cr in the residue / (<M> θ + <C> θ )×100 (1) [Mo]θ = Mo mass in the residue / (<M> θ + <C> θ )×100 (2) [Cr] θ + [Mo] θ -2.5 / (√[C])≦0 (3)

[0016] The inventors further investigated means of improving the cold workability of the steel sheet in its microstructure. To improve the cold workability of the steel sheet, it is effective to increase the spheroidization rate of cementite particles. In the steel sheet of this embodiment, it was found that excellent cold workability can be obtained if the spheroidization rate of cementite particles is 85% or higher.

[0017] However, even when the steel sheet had the above-mentioned chemical composition, its microstructure consisted substantially of ferrite and cementite particles, the average particle size of the ferrite was 20.0 μm or less, and the average particle diameter of the cementite particles was 1.50 μm or less, satisfying formula (3), sufficient hardenability was still sometimes not obtained. Therefore, the inventors conducted further investigations. As a result, the following findings were obtained.

[0018] In steel sheets with the chemical composition described above, the Si content is high, exceeding 0.50%. Normally, carbon (C) in steel sheets combines with Fe (Fe) to form cementite. Si reduces the stability of cementite. Therefore, the higher the Si content, the easier it is for cementite to decompose and graphite to form. This ease of graphite formation is influenced not only by the Si content but also by the C content. When the C content is 0.50% or higher, and the Si content also exceeds 0.50%, graphite tends to form more easily during the steel sheet manufacturing process.

[0019] Graphite has greater thermal stability compared to cementite. Therefore, during the quenching process in the manufacturing of machine parts, graphite is less likely to melt than cementite. As a result, the hardenability of steel sheets is reduced.

[0020] Based on the above findings, the inventors hypothesized that if graphite formation could be suppressed in a steel sheet having the above-mentioned chemical composition, excellent hardenability could be obtained. Therefore, the inventors conducted further investigations. As a result, the carbon content [C] (mass%) in the chemical composition of the steel sheet and the carbon concentration [C] of carbides in the residue based on the mass of the steel sheet were found to be related. ＣＢ The inventors have found that if the graphitization rate GR, defined by formula (4) using (mass%), is 5.0% or less, excellent hardenability can be obtained even with the aforementioned steel sheet with a high Si content. GR = (1 - [C] ＣＢ / [C])×100 (4)

[0021] Based on the above findings, the steel plate according to this embodiment is as follows:

[0022] The steel sheet of the first form has a chemical composition in mass percent of C: 0.50 to 0.80%, Si: greater than 0.50 to 2.10%, Mn: 0.20 to 1.20%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 0.60%, N: 0.0150% or less, Mo: 0 to 0.350%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%, with the remainder being Fe and impurities. In the microstructure, the total area ratio of ferrite to cementite particles is 95% or more, and the average particle size of ferrite is 20.0 μm or less. The average particle diameter of the cementite particles is 1.50 μm or less, and among the cementite particles, those with an aspect ratio of 3.0 or less are defined as 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 85% or more. The total mass of Fe, Mn, Cr, and Mo in the residue obtained by the extraction residue analysis method is <M> θ Let C be the total mass of metallic elements in the cementite particles, and the C mass in the cementite particles < C θ The Cr concentration in the residue [Cr] defined by equation (1) is calculated as follows: θ (mass%), Mo concentration in the residue as defined by formula (2) [Mo] θ(mass%) and the C content [C] in mass% of the chemical composition of the steel sheet satisfy formula (3). Furthermore, [C] and the C concentration of carbides in the residue based on the mass of the steel sheet [C] ＣＢ The graphitization rate GR, defined by equation (4) using (mass%), is 5.0% or less. [Cr] θ = Mass of Cr in the residue / (<M> θ + <C> θ )×100 (1) [Mo] θ = Mo mass in the residue / (<M> θ + <C> θ )×100 (2) [Cr] θ + [Mo] θ -2.5 / (√[C])≦0 (3) GR=(1-[C] ＣＢ / [C])×100 (4)

[0023] The second form of steel sheet is the first form of steel sheet, and its chemical composition contains one or more elements selected from the group consisting of Mo: 0.001 to 0.350%, 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%, in mass%.

[0024] The steel plate of this embodiment will be described in detail below. Unless otherwise specified, "%" in relation to elements refers to mass percentage.

[0025] [Features of the steel sheet of this embodiment] The steel sheet of this embodiment satisfies the following features 1 to 7. (Feature 1) The chemical composition, in mass%, contains C: 0.50 to 0.80%, Si: greater than 0.50 to 2.10%, Mn: 0.20 to 1.20%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 0.60%, N: 0.0150% or less, Mo: 0 to 0.350%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%, with the remainder being Fe and impurities. (Feature 2) In the microstructure, the total area ratio of ferrite to cementite particles is 95% or more. (Feature 3) The average particle size of ferrite is 20.0 μm or less. (Feature 4) The average particle diameter of cementite particles is 1.50 μm or less. (Feature 5) When cementite particles with an aspect ratio of 3.0 or less are considered spherical cementite particles, the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 85% or more. (Feature 6) The total mass of Fe, Mn, Cr, and Mo in the residue obtained by the extraction residue analysis method <M> θ Let C be the total mass of metallic elements in the cementite particles, and the C mass in the cementite particles < C θ When this is determined, the Cr concentration in the residue [Cr] defined by equation (1) θ (mass%), Mo concentration in the residue as defined by formula (2) [Mo] θ The mass percentage of carbon (C) and the mass percentage of carbon content [C] in the chemical composition of the steel sheet satisfy formula (3). [Cr] θ = Mass of Cr in the residue / (<M> θ + <C> θ )×100 (1) [Mo] θ = Mo mass in the residue / (<M> θ + <C> θ )×100 (2) [Cr] θ + [Mo] θ -2.5 / (√[C]) ≤ 0 (3) (Feature 7) C content [C] (mass%) and C concentration of carbides in the residue based on the mass of the steel plate [C] ＣＢThe graphitization rate GR, defined by equation (4) using (mass%), is 5.0% or less. GR = (1 - [C] ＣＢ / [C]) × 100 (4) Features 1 to 7 will be explained below.

[0026] [(Feature 1) Chemical Composition] The chemical composition of the steel sheet of this embodiment contains the following elements.

[0027] C: 0.50-0.80% Carbon (C) enhances the hardenability of steel sheets. As a result, the strength of machine parts is increased by performing quenching in the manufacturing process of machine parts made from steel sheets. If the C content is less than 0.50%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content exceeds 0.80%, the cold workability of the steel sheet decreases, even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.50-0.80%. 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 0.78%, more preferably 0.75%, and even more preferably 0.70%.

[0028] Si: Greater than 0.50% to 2.10% When tempering is performed during the manufacturing process of machine parts made from steel sheets, silicon (Si) increases the tempering softening resistance of the steel sheet, thereby increasing the strength and elastic limit of the steel sheet after tempering. If the Si content is 0.50% or less, the above effect cannot be sufficiently obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 2.10%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. In this case, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet decreases. If the Si content exceeds 2.10%, the decomposition of cementite particles is further promoted, and the formation of graphite is promoted. Therefore, the Si content is greater than 0.50% to 2.10%. The preferred lower limit of the Si content is 0.51%, more preferably 0.53%, more preferably 0.55%, more preferably 0.60%, more preferably 0.61%, more preferably 0.65%, and more preferably 0.71%. The preferred upper limit of the Si content is 2.05%, more preferably 2.00%, and more preferably 1.90%.

[0029] Mn: 0.20-1.20% Manganese (Mn) enhances the hardenability of steel plates during the quenching process in the manufacturing of machine parts made from steel plates, depending on the amount of solid solution dissolved in austenite. As a result, the strength of the machine parts is increased. If the Mn content is less than 0.20%, the above effect cannot be sufficiently obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 1.20%, the strength of the steel plate becomes excessively high due to solid solution strengthening. Therefore, even if the content of other elements is within the range of this embodiment, the cold workability of the steel plate decreases. Accordingly, the Mn content is 0.20-1.20%. The preferred lower limit of the Mn content is 0.25%, more preferably 0.30%, and still more preferably 0.35%. The preferred upper limit of the Mn content is 1.15%, more preferably 1.10%, and still more preferably 1.05%.

[0030] P: 0.100% or less. Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content exceeds 0.100%, the toughness of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the P content is 0.100% or less. It is preferable that the P content be 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%.

[0031] S: 0.100% or less. Sulfur (S) is an unavoidable impurity. In other words, the S content is greater than 0%. If the S content exceeds 0.100%, S will excessively produce sulfides. Therefore, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet will decrease. Accordingly, the S content is 0.100% or less. It is preferable that the S content be as low as possible. However, excessive reduction of the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the S content is 0.090%, more preferably 0.080%, even more preferably 0.050%, even more preferably 0.030%, and even more preferably 0.025%.

[0032] Al: 0.100% or less. Aluminum (Al) is inevitably present. In other words, the Al content is greater than 0%. Al deoxidizes steel. Even if only a small amount of Al is present, the above effect can be obtained to some extent. On the other hand, if the Al content exceeds 0.100%, coarse inclusions will be excessively generated. In this case, even if the content of other elements is within the range of this embodiment, the workability of the steel sheet will decrease. Therefore, the Al content is 0.100% or less. The preferred lower limit of the Al content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The 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%.

[0033] Cr: 0.01-0.60% Chromium (Cr) enhances the hardenability of steel plates during the quenching process in the manufacturing of machine parts made from steel plates, depending on the amount of solid solution into austenite. As a result, the strength of the machine parts is increased. If the Cr content is less than 0.01%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content exceeds 0.60%, the Cr concentration in the residue [Cr] θ The (mass%) becomes excessively high. In this case, equation (3) is no longer satisfied. Therefore, during the heating process of the quenching step in the manufacturing process of machine parts made from steel plates, the melting of cementite slows down. As a result, even if the content of other elements is within the range of this embodiment, the hardenability decreases. Therefore, the Cr content is 0.01 to 0.60%. The preferred lower limit of the Cr content is 0.02%, more preferably 0.03%, more preferably 0.05%, more preferably 0.10%, more preferably 0.20%, and more preferably 0.30%. The preferred upper limit of the Cr content is 0.57%, more preferably 0.55%, and more preferably 0.50%.

[0034] N: 0.0150% or less. Nitrogen (N) is an unavoidable impurity. In other words, the N content is greater than 0%. If the N content exceeds 0.0150%, coarse nitrides will be excessively formed. In this case, even if the content of other elements is within the range of this embodiment, the workability of the steel sheet will decrease. Furthermore, the toughness and fatigue strength of the steel sheet after heat treatment will decrease. Therefore, the N content is 0.0150% or less. The preferred lower limit of the N content is 0.0001%, and more preferably 0.0005%. The preferred upper limit of the N content is 0.0140%, more preferably 0.0130%, and still more preferably 0.0125%.

[0035] The remainder of the chemical composition of the steel sheet in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the steel sheet, and are permissible within a range that does not adversely affect the steel sheet according to this embodiment.

[0036] The above impurities may contain the following elements in the following amounts: one or more elements selected from the group consisting of Cu: 0-0.30%, W: 0-0.20%, Ta: 0-0.15%, Sn: 0-0.10%, Sb: 0-0.050%, Co: 0-0.10%, As: 0-0.050%, Mg: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, Ce: 0-0.050%, and Ca: 0-0.050%. All of these elements are trump elements and are impurities in the steel sheet of this embodiment. The above impurities may also contain O: 0.0100% or less.

[0037] [Optional Elements] The chemical composition of the steel sheet in this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo: 0-0.350%, Ni: 0-1.000%, B: 0-0.0100%, V: 0-0.500%, Nb: 0-0.500%, and Ti: 0-0.150%. These elements are optional and may not be included. Each optional element will be described below.

[0038] [Group 1 (Mo, Ni, and B)] The chemical composition of the steel sheet in this embodiment may further include one or more elements selected from the group consisting of Mo, Ni, and B in place of a portion of Fe. Any of these elements are optional and may not be included. If included, Mo, Ni, and B enhance the hardenability of the steel sheet.

[0039] Mo: 0-0.350% Molybdenum (Mo) is an optional element and may not be present. In other words, the Mo content may be 0%. When Mo is present, that is, when the Mo content is greater than 0%, the Mo dissolved in the austenite during the quenching process in the manufacturing process of machine parts made from steel sheets enhances the hardenability of the steel sheet. Therefore, the strength of the machine parts is increased. Mo also increases the resistance of the steel sheet to tempering and softening. Even if only a small amount of Mo is present, the above effects can be obtained to some extent. However, if the Mo content exceeds 0.350%, the Mo concentration in the residue [Mo] θ The (mass%) becomes excessively high. In this case, equation (3) is no longer satisfied. Therefore, during the heating process of the quenching step in the manufacturing process of machine parts made from steel plates, the melting of cementite slows down. As a result, even if the content of other elements is within the range of this embodiment, the hardenability decreases. Therefore, the Mo content is 0 to 0.350%. The preferred lower limit of the Mo content is greater than 0%, more preferably 0.001%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Mo content is 0.345%, more preferably 0.320%, more preferably 0.300%, more preferably 0.250%, and more preferably 0.210%.

[0040] Ni: 0-1.000% Nickel (Ni) is an optional element and may not be included. In other words, the Ni content may be 0%. When Ni is included, that is, when the Ni content is greater than 0%, the Ni dissolved in the austenite during the quenching process in the manufacturing process of machine parts made from steel sheets enhances the hardenability of the steel sheet. As a result, the strength of the machine parts increases. Ni also increases the tempering softening resistance of the steel sheet. Even if only a small amount of Ni is included, the above effects can be obtained to some extent. However, if the Ni content exceeds 1.000%, the strength of the steel sheet becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the cold workability of the steel sheet decreases. Therefore, the Ni content is 0-1.000%. The preferred lower limit of the Ni content is greater than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.007%, more preferably 0.010%, and more preferably 0.040%. The preferred upper limit of the Ni content is 0.980%, more preferably 0.900%, more preferably 0.800%, more preferably 0.700%, and more preferably 0.600%.

[0041] B: 0 to 0.0100% Boron (B) is an optional element and may not be present. In other words, the B content may be 0%. When B is present, that is, when the B content is greater than 0%, the B dissolved in the austenite during the quenching process in the manufacturing process of machine parts using steel sheets as material enhances the hardenability of the steel sheet. As a result, the strength of the machine parts is increased. Even if only a small amount of B is present, 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 content of other elements is within the range of this embodiment. In this case, sufficient hardenability cannot be obtained. Furthermore, the cold workability of the steel sheet decreases. 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%, more preferably 0.0003%, and still more preferably 0.0005%. The preferred upper limit for the B content is 0.0090%, more preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, and still more preferably 0.0050%.

[0042] [Second Group (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 in place of a portion of Fe. Any of these elements are optional and may not be included. When included, V, Nb, and Ti form precipitates, which suppress the coarsening of austenite grains during the quenching process in the manufacturing process of machine parts made from steel sheets. As a result, the toughness of the machine parts is improved.

[0043] V: 0-0.500% Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. When V is present, that is, when the V content is greater than 0%, V forms precipitates such as carbides, which suppress the coarsening of austenite grains during the quenching process in the manufacturing process of machine parts made from steel sheets. As a result, the toughness of the machine parts is improved. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.500%, precipitates are excessively generated, and the strength of the steel sheet is excessively increased due to precipitation strengthening. In this case, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet will decrease. Therefore, the V content is 0-0.500%. The preferred lower limit of the V content is greater than 0%, more preferably 0.001%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit for the V content is 0.480%, more preferably 0.450%, and even more preferably 0.400%.

[0044] Nb: 0-0.500% Niobium (Nb) is an optional element and may not be present. In other words, the Nb content may be 0%. When Nb is present, that is, when the Nb content is greater than 0%, Nb forms precipitates such as carbides, which suppress the coarsening of austenite grains during the quenching process in the manufacturing process of machine parts made from steel sheets. As a result, the toughness of the machine parts is improved. In addition, Nb combines with N to suppress the formation of nitrides by solid solution B. This improves the hardenability of the steel sheet due to solid solution B. Even if only a small amount of Nb is present, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.500%, precipitates are excessively generated, and the strength of the steel sheet is excessively increased due to precipitation strengthening. In this case, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet will decrease. Therefore, the Nb content is 0-0.500%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.001%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Nb content is 0.480%, more preferably 0.450%, more preferably 0.400%, and more preferably 0.350%.

[0045] Ti: 0-0.150% 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 is greater than 0%, Ti forms precipitates such as carbides, which suppress the coarsening of austenite grains during the quenching process in the manufacturing process of machine parts made from steel sheets. As a result, the toughness of the machine parts is improved. In addition, Ti combines with N to suppress the formation of nitrides by solid solution B. This improves the hardenability of the steel sheet due to solid solution B. Even if only a small amount of Ti is included, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.150%, precipitates are excessively generated, precipitation-strengthening the steel sheet. As a result, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Ti content is 0-0.150%. The preferred lower limit of the Ti content is greater than 0%, more preferably 0.001%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Ti content is 0.145%, more preferably 0.130%, more preferably 0.120%, more preferably 0.100%, and more preferably 0.080%.

[0046] [Method for Measuring the Chemical Composition of Steel Sheets] The chemical composition of the steel sheet in this embodiment can be measured by a well-known component analysis method. Specifically, chips are collected from the inside 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. Elemental analysis of the chemical composition is performed on the solution using ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry). The C and S content is determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas melting-thermal conductivity method. The O content is determined using a well-known inert gas melting-non-dispersive infrared absorption method.

[0047] [(Feature 2) Regarding the 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 consists substantially of ferrite and cementite particles.

[0048] In the microstructure, the remaining structure other than ferrite and cementite particles consists of one or more elements selected from the group consisting of, for example, bainite and martensite. If pearlite is observed in the microstructure, it is considered a mixed structure of ferrite and cementite particles.

[0049] Preferably, the total area ratio of ferrite and cementite particles in the microstructure is 96% or more, more preferably 97% or more, more preferably 98% or more, and more preferably 99% or more. The microstructure may consist of ferrite and cementite particles.

[0050] Assuming that the total area ratio of ferrite and cementite particles is 95% or more, sufficient hardenability and sufficient cold workability of the steel sheet can be obtained, provided that features 1 and 3 to 7 are satisfied.

[0051] [Method for measuring the total area ratio of ferrite and cementite particles in a microstructure] The total area ratio of ferrite and cementite particles in a microstructure can be measured by the following method.

[0052] A test specimen measuring 15 mm in the rolling direction and 10 mm in the width direction, with a thickness of 15 mm, is taken from the center of the width of the steel plate. The surface of the test specimen parallel to the rolling direction and the thickness direction (i.e., the surface measuring 15 mm in the rolling direction and with a thickness of 15 mm) is designated as the observation surface. The observation surface of the test specimen is mirror-polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (Nital etching solution). Secondary electron images are observed using a 1000x scanning electron microscope (SEM) at five arbitrary observation fields at the center position in the thickness direction of the etched observation surface. Each observation field is a rectangle measuring 100 μm in the thickness direction and 120 μm in the rolling direction. The observation fields are selected so that the center position in the thickness direction of the observation field corresponds to the center position in the thickness direction of the observation surface.

[0053] In the observation field, ferrite and cementite particles exhibit different contrasts and morphologies compared to other structures (bainite, martensite). Therefore, ferrite and cementite particles are identified within the observation field based on their contrast and morphology.

[0054] The total area ratio (%) of ferrite and cementite particles is calculated based on the total area of ​​ferrite and cementite particles in five observation fields and the total area of ​​the five observation fields (100 μm × 120 μm × 5). The total area ratio is rounded to the nearest integer.

[0055] [(Feature 3) Regarding the average particle size of ferrite] In the steel sheet of this embodiment, the average particle size of ferrite is 20.0 μm or less. If the average particle size of ferrite exceeds 20.0 μm, the grain boundary area becomes small, and the effect of promoting the dissolution of cementite by grain boundary diffusion cannot be obtained. In this case, sufficient hardenability cannot be obtained in the quenching process when manufacturing machine parts using the steel sheet as a material.

[0056] When the average grain size of ferrite is 20.0 μm or less, the grain boundary area of ​​the ferrite is sufficiently large, and the dissolution of cementite during quenching becomes faster. Therefore, assuming other characteristics are met, the hardenability is improved.

[0057] The preferred upper limit for the average particle size of ferrite is 19.0 μm, more preferably 18.0 μm, more preferably 15.0 μm, more preferably 14.0 μm, more preferably 13.0 μm, and more preferably 12.0 μm. The preferred lower limit for the average particle size of ferrite is not particularly limited. However, excessive reduction of the average particle size of ferrite increases manufacturing costs. Therefore, the preferred lower limit for the average particle size of ferrite is 3.0 μm, more preferably 3.5 μm, more preferably 4.0 μm, more preferably 4.5 μm, and more preferably 5.0 μm.

[0058] [Method for Measuring the Average Grain Size of Ferrite] The average grain size of ferrite can be measured by the following method. A test piece measuring 15 mm in the rolling direction and 10 mm in the width direction and thickness is taken from the center of the width of the steel plate. The cross section of the surface of the test piece parallel to the rolling direction and thickness direction (i.e., the surface measuring 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 3% nital. The average grain size of ferrite on the etched observation surface is determined by the following method. The crystal grain size number of ferrite is determined by the cutting method in accordance with JIS G 0551:2020. At this time, the magnification of the optical microscope is selected so that the number of ferrite crystal grains cut by one line segment is at least 10 or more in one field of view. After selecting the magnification, the cutting length is determined for five fields of view. The crystal grain size number of ferrite is determined from the arithmetic mean of the cutting lengths of the five fields of view. The average grain size (μm) of the ferrite is determined from the obtained grain size number. The average grain size of the ferrite is rounded to the first decimal place by rounding the second decimal place of the obtained value.

[0059] [(Feature 4) Regarding the 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, they will not melt sufficiently during the quenching process in the manufacturing process of machine parts using the steel sheet as a material. In this case, the hardenability of the steel sheet will decrease. As a result, sufficient strength cannot be obtained in machine parts manufactured using the steel sheet as a material.

[0060] If the average particle size of the cementite particles is 1.50 μm or less, the cementite particles are sufficiently small. Therefore, during heating in the aforementioned quenching process, the cementite particles melt sufficiently, improving the hardenability of the steel sheet.

[0061] The preferred upper limit for the average particle size of cementite particles is 1.45 μm, more preferably 1.40 μm, even more preferably 1.35 μm, and even more preferably 1.30 μm. For improved hardenability, a smaller average particle size of cementite particles is preferable. However, if the average particle size of cementite particles is too small, the hardness of the steel sheet becomes too high. In this case, the cold workability of the steel sheet decreases. Therefore, the preferred lower limit for the average particle size of cementite particles is 0.05 μm, more preferably 0.10 μm, even more preferably 0.15 μm, and even more preferably 0.20 μm.

[0062] [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 and 10 mm in the width direction and thickness is taken from the center of the width of the steel plate. The cross section of the surface of the test piece parallel to the rolling direction (the surface measuring 15 mm in the rolling direction and thickness) is defined as the observation surface.

[0063] Etching is performed on the observation surface using Picral solution. Secondary electron images are taken in five arbitrary observation fields located at a depth of 4 / thickness from the surface of the etched observation surface. Specifically, the five observation fields are observed at a magnification of 2000x using a scanning electron microscope (SEM), and the aforementioned secondary electron images are taken. Each observation field is a rectangle measuring 50 μm in the thickness direction and 60 μm in the rolling direction. The observation fields are selected so that the center position in the thickness direction of the observation field is at a depth of 1 / 4th of the thickness.

[0064] In each secondary electron image, cementite particles are identified based on contrast. The area of ​​each identified cementite particle is determined, and the equivalent circular diameter of each cementite particle is calculated based on the area. The calculated equivalent circular diameter is taken as the particle diameter of that cementite particle. The particle diameter is determined using well-known image processing software. The arithmetic mean of the particle diameters of cementite particles obtained in five observation fields is taken as the average particle diameter of the cementite particles (μm). The average particle diameter of the cementite particles is taken as the value obtained by rounding the third decimal place of the obtained arithmetic mean to two decimal places.

[0065] [(Feature 5) Regarding the spheroidization rate] In the steel sheet of this embodiment, when cementite particles with an aspect ratio of 3.0 or less are defined as spherical cementite particles, the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 85% or more.

[0066] If the spheroidization rate is 85% or higher, then, assuming other characteristics are met, remarkably excellent cold workability can be obtained in the steel sheet. Therefore, in the steel sheet of this embodiment, the spheroidization rate is 85% or higher.

[0067] A higher spheroidization rate is preferable. The preferred lower limit of the spheroidization rate is 88%, more preferably 90%, more preferably 93%, and still more preferably 95%. The preferred upper limit of the spheroidization rate is 100%. However, if the spheroidization rate is increased excessively, the manufacturing cost increases significantly. Therefore, considering industrial production, the upper limit of the spheroidization rate is, for example, 99%, for example 98%, and more preferably 97%.

[0068] [Method for Measuring Spheroidization Rate] The spheroidization rate can be measured by the following method. For each of the multiple cementite particles identified in the five observation fields using the method described above for measuring the average particle diameter of cementite particles, the aspect ratio is determined. Specifically, the contour line of the cementite particle is enclosed by two parallel line segments. At this time, the maximum distance between the two parallel line segments enclosing the cementite particle is defined as the major axis (μm). Furthermore, the distance between the two line segments when the contour line of the cementite particle is enclosed by two line segments parallel to the major axis (i.e., the width in the direction perpendicular to the major axis) is defined as the minor axis (μm).

[0069] Based on the obtained major axis (μm) and minor axis (μm), the aspect ratio (= major axis / minor axis) of each cementite particle is determined. The aspect ratio is rounded to one decimal place by rounding the second decimal place of the obtained value. Of all the cementite particles in the five observation fields, those with an aspect ratio of 3.0 or less are identified as "spherical cementite particles". The ratio of the total number of spherical cementite particles to the total number of multiple cementite particles being measured is defined as the spheroidization rate (%). The spheroidization rate is rounded to one integer by rounding the first decimal place of the obtained value.

[0070] [Regarding (Feature 6) Equation (3)] In the steel sheet of this embodiment, the total mass of Fe, Mn, Cr, and Mo in the residue obtained by the extraction residue analysis method <M> θ Let C be the total mass of metallic elements in the cementite particles, and the C mass in the cementite particles < C θ When this is determined, the Cr concentration in the residue [Cr] defined by equation (1) θ (mass%), Mo concentration in the residue as defined by formula (2) [Mo] θ The mass percentage of carbon (C) and the mass percentage of carbon content [C] in the chemical composition of the steel sheet satisfy formula (3). [Cr] θ = Mass of Cr in the residue / (<M> θ + <C> θ )×100 (1) [Mo] θ = Mo mass in the residue / (<M> θ + <C> θ )×100 (2) [Cr] θ + [Mo]θ -2.5 / (√[C])≦0 (3)

[0071] In the steel sheet of this embodiment, Fe, Mn, Cr, and Mo obtained by the extraction residue analysis method are M ３ It forms cementite particles, which are C-type carbides. Therefore, the Cr concentration in the residue defined by formula (1) [Cr] θ (mass%), and Mo concentration in the residue as defined by formula (2) [Mo] θ (Mass%) is considered to be the Cr concentration (Mass%) and the Mo concentration (Mass%) in the cementite particles.

[0072] F3 is defined as follows: F3 = [Cr] θ + [Mo] θ -2.5 / (√[C]) F3 is an index of the ratio of Cr concentration to Mo concentration in cementite particles in steel sheets. If F3 is greater than 0, the Cr concentration in cementite particles of steel sheets [Cr] θ and / or Mo concentration [Mo] θ The temperature is excessively high. In this case, during the quenching process in the manufacturing of machine parts using steel plates as the raw material, the cementite particles do not melt sufficiently.

[0073] On the other hand, if F3 is 0 or less, the Cr concentration in the cementite particles [Cr] θ and Mo concentration [Mo] θ The coefficient of hardening is sufficiently low. Therefore, during heating in the aforementioned quenching process, the cementite particles melt sufficiently, improving the hardenability of the steel sheet. Consequently, F3 is 0 or less (0.00 or less).

[0074] A preferred upper limit for F3 is -0.10, more preferably -0.20, and even more preferably -0.30. A preferred lower limit for F3 is -3.00, more preferably -2.70, and even more preferably -2.50.

[0075] [Method for measuring F3] F3 can be measured by the following method. A test piece is taken from the center of the width of the steel plate. The size of the test piece is 10 mm x 10 mm x plate thickness.

[0076] For the test piece, constant current electrolysis is carried out 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).

[0077] Specifically, prepare the above-mentioned 10% AA-based solution. Then, using the 10% AA-based solution, at room temperature, maintain the current density at 20 mA / cm ２ and perform constant current electrolysis on the test piece. After the constant current electrolysis, take out the test piece from the 10% AA-based solution. Immerse the taken-out test piece in an alcohol solution. Perform ultrasonic cleaning on the test piece immersed in the alcohol solution.

[0078] The 10% AA-based solution used for the constant current electrolysis and the alcohol solution used for the subsequent ultrasonic cleaning are suction filtered through a filter with a mesh size of 0.2 μm to extract the residue.

[0079] Perform chemical element analysis on the extracted residue. Specifically, dissolve the residue in an acid to obtain a solution. Perform chemical element analysis on the solution using inductively coupled plasma optical emission spectrometry (ICP-OES) to obtain the Fe mass, Mn mass, Cr mass, Mo mass, V mass, Nb mass, and Ti mass in the residue. Among the obtained multiple element masses, Fe, Mn, Cr, and Mo are considered to form M ３ C-type carbides (i.e., cementite). The sum of the Fe mass, Mn mass, Cr mass, and Mo mass is defined as the total mass <M> of the metal elements of cementite in the residue θ .

[0080] Here, taking <M> θ in the residue = the total mass of the metal elements in the cementite particles, from the measured value of <M> θ obtain the C mass <C> θ in the cementite particles. Specifically, based on the Fe mass <Fe> θ , Mn mass θ , Cr mass <Cr> θ , and Mo mass θ and the atomic weights of each element, obtain the total number of moles X Ｍ of the metal elements in the residue using the following formula. X Ｍ = <Fe> θ / A Ｆｅ + θ / A Ｍｎ +<Cr> θ / A Ｃｒ + θ / A Ｍｏ Here, A Ｆｅ is the atomic weight of Fe (=55.85). A Ｍｎ is the atomic weight of Mn (=54.94). A Ｃｒ is the atomic weight of Cr (=52.00). A Ｍｏ is the atomic weight of Mo (=95.95).

[0081] M ３ The number of moles of C forming M Ｍ C-type carbide (cementite) is 1 / 3 of the total number of moles X of metal elements. Therefore, M ３ The number of moles of C in the M Ｃ C-type carbide M Ｃ is obtained by the following equation. M Ｍ = X

[0082] Based on the number of moles of C in the M θ C-type carbide M θ = M<OOO0115> × A Ｃ Here, A Ｃ is the atomic weight of C (=12.01).

[0083] The Cr mass in the residue, the Mo mass in the residue, the total mass <M> of metal elements in the residue θ , and the C mass <C> in the cementite particles θ are used to obtain the Cr concentration [Cr] in the residue by Equation (1) θ (mass%). Further, the Mo concentration [Mo] in the residue is obtained by Equation (2) θ (mass%). The Cr concentration [Cr] θ is the value rounded to the second decimal place after rounding the third decimal place of the obtained value. The Mo concentration [Mo] θ is the value rounded to the second decimal place after rounding the third decimal place of the obtained value. [Cr] θ = Cr mass in the residue / (<M>​​​​θ + <C> θ )×100 (1) [Mo] θ = Mo mass in the residue / (<M> θ + <C> θ ) × 100 (2)

[0084] Furthermore, Cr concentration [Cr] θ (Mass %) and Mo concentration [Mo] θ Based on the mass %) and the C content [C] (mass%) in the chemical composition of the steel sheet obtained by the method described in [Method for measuring the chemical composition of steel sheet], F3 is determined. F3 is the value obtained by rounding the third decimal place of the obtained value to two decimal places.

[0085] [(Feature 7) Regarding the graphitization rate GR] In the steel sheet of this embodiment, the carbon content [C] (mass%) in the chemical composition of the steel sheet and the carbon concentration [C] of carbides in the residue based on the mass of the steel sheet are further specified. ＣＢ The graphitization rate GR, defined by equation (4) using (mass%), is 5.0% or less. GR = (1 - [C] ＣＢ / [C])×100 (4)

[0086] As described above, in the case of a steel sheet having a chemical composition that satisfies Feature 1, the Si content is high, exceeding 0.50%. In the case of a steel sheet where the C content is 0.50% or more but the Si content is 0.50% or less, the C in the steel sheet combines with Fe to form cementite. On the other hand, in the steel sheet of this embodiment, the C content is 0.50% or more, and furthermore, the Si content exceeds 0.50%. When both the C content and Si content are high in this way, Si reduces the stability of cementite and promotes its decomposition. When cementite decomposes, graphite is more easily formed in the steel sheet.

[0087] Graphite has greater thermal stability compared to cementite. Therefore, during the quenching process in the manufacturing of machine parts, graphite is less likely to melt than cementite. As a result, the hardenability of the steel sheet decreases. Furthermore, in the presence of graphite, the steel sheet softens, so although the elongation at break increases, the ultimate deformability decreases. As a result, notch elongation decreases.

[0088] The graphitization rate GR is an indicator of the degree of graphite formation in steel. More specifically, the graphitization rate GR is the concentration of carbon other than carbon contained in the extraction residue in the steel sheet. Therefore, the graphitization rate can be considered as essentially the carbon concentration of graphite. Note that, in terms of measurement, the graphitization rate GR may contain solid-solution carbon other than graphite.

[0089] If the graphitization rate (GR) is 5.0% or less, then although dissolved carbon (C) is present in the steel sheet, graphite is virtually absent. Therefore, excellent hardenability can be obtained during the quenching process in the manufacturing of machine parts.

[0090] The preferred upper limit for the graphitization rate GR is 4.5%, more preferably 4.0%, more preferably 3.5%, more preferably 3.2%, and still more preferably 3.0%.

[0091] The lower limit of the graphitization rate GR is not particularly limited. However, in steel sheets that satisfy characteristic 1, some solid solution carbon may be present. The lower limit of the graphitization rate GR is, for example, 0.3%, 0.4%, or 0.5%.

[0092] [Method for measuring graphitization rate GR] The graphitization rate GR can be measured by the following method. Using the V mass, Nb mass, and Ti mass in the residue obtained by the above-mentioned [Method for measuring F3], the C mass in the carbides other than cementite particles in the residue is determined by the following method.

[0093] Of the multiple elemental masses obtained, V forms VC and Nb forms NbC. Therefore, assuming that all the V in the residue forms VC, the mass of C in VC < C ＶＣ This calculates the mass of C in VC < C ＶＣ This can be calculated using the following formula: <C> ＶＣ =V mass x A Ｃ / A Ｖ Here, A Ｃ This is the atomic weight of C (= 12.01). Ｖ This is the atomic weight of V (= 50.94).

[0094] Similarly, assuming that all the Nb in the residue forms NbC, the mass of C in NbC < C ＮｂＣThis calculates the mass of carbon in NbC < C ＮｂＣ This can be calculated using the following formula: <C> ＮｂＣ =Nb mass x A Ｃ / A Ｎｂ Here, A Ｎｂ This is the atomic weight of Nb (= 92.91).

[0095] Furthermore, of the obtained elemental masses, some of the Ti forms TiN, and the remaining Ti that could not bond with N in the steel forms TiC. Therefore, the mass of N in the dissolved test piece is determined from the mass of the test piece dissolved by the constant current electrolysis described above and the N content (mass%) in the chemical composition of the steel sheet obtained by the method described in [Method for measuring the chemical composition of steel sheet]. Assuming that all of the determined N mass was used to form TiN, the mass of Ti that formed TiN < Ti> ＴｉＮ We will find the following: Specifically, Ti mass < Ti ＴｉＮ It can be calculated using the following formula: <Ti> ＴｉＮ =N mass x A Ｔｉ / A Ｎ Here, A Ｎ A is the atomic weight of N (= 14.01). Ｔｉ This is the atomic weight of Ti (= 47.87).

[0096] <Ti> from the Ti mass in the residue ＴｉＮ The value obtained by subtracting the Ti mass < Ti that forms TiC is ＴｉＣ Assuming that all the remaining Ti that could not bond with N forms TiC, the mass of C in TiC < C ＴｉＣ We will determine the mass of C in TiC < C ＴｉＣ This can be calculated using the following formula: <C> ＴｉＣ = <Ti> ＴｉＣ ×A Ｃ / A Ｔｉ

[0097] The C mass in cementite particles obtained by the [F3 measurement method] < C θ And, the mass of C in VC < C ＶＣ And, the mass of C in NbC < C ＮｂＣ And, the mass of C in TiC < C ＴｉＣ Based on this, the total carbon mass of the carbide < CＣＢ We can find it using the following formula. <C> ＣＢ = <C> θ + <C> ＶＣ + <C> ＮｂＣ + <C> Ｔｉ

[0098] The mass of the test specimen dissolved by the constant current electrolysis described above, and the total C mass < C ＣＢ Based on this, the total carbon concentration of carbides in the residue [C] is calculated based on the mass of the steel plate. ＣＢ Determine the (mass %) carbon content [C] (mass %) in the chemical composition of the steel plate and the obtained carbon concentration [C]. ＣＢ Using (mass%), the graphitization rate GR (%) is calculated using equation (4). GR = (1 - [C] ＣＢ / [C]) × 100 (4) The graphitization rate GR shall be the value obtained by rounding the second decimal place of the obtained value to the first decimal place.

[0099] [Effects of the steel sheet of this embodiment] As described above, the steel sheet of this embodiment satisfies features 1 to 7. Therefore, when manufacturing machine parts using the steel sheet of this embodiment, sufficient hardenability can be obtained during the quenching process in the manufacturing process. Furthermore, sufficient cold workability can be obtained with the steel sheet of this embodiment.

[0100] The thickness of the steel plate in this embodiment is not particularly limited. For example, the lower limit of the steel plate thickness is 0.2 mm. For example, the upper limit of the steel plate thickness is 5.0 mm.

[0101] [Regarding hardenability] In the steel plate of this embodiment, obtaining sufficient hardenability means the following evaluation.

[0102] [Method for evaluating hardenability] (A ｃ１ Transformation point and A ｃ３(Measuring the transformation point) A test piece measuring 10 mm in width and 80 mm in length is taken from the center of the width of the steel plate of this embodiment. The longitudinal direction of the test piece is parallel to the longitudinal direction (rolling direction) of the steel plate. The thickness direction of the test piece is parallel to the thickness direction of the steel plate. If the plate thickness exceeds 3 mm, the thickness of the test piece is adjusted to 3 mm. If the plate thickness is less than 3 mm, the thickness of the test piece is the same as the thickness of the steel plate. The coefficient of thermal expansion during heating is measured using a Formaster testing machine. From the obtained coefficient of thermal expansion, A ｃ１ Transformation point and A ｃ３ Find the point of transformation.

[0103] (Maximum hardness measurement) A plate-shaped test specimen is taken from the center of the width of the steel plate. The shape of the plate-shaped test specimen is 15 mm in the rolling direction of the steel plate x 30 mm in the width direction x plate thickness. The plate-shaped test specimen is subjected to a salt bath A ｃ３ Heat at transformation point + 150°C for 30 minutes. If the test specimen is eutectoid steel or hypereutectoid steel, A ｃ１ The specimen is heated at the transformation point + 150°C for 30 minutes. After that, the plate-shaped test specimen is quenched by immersion in 60°C oil in an oil bath. The quenched plate-shaped test specimen is cut in half along the width direction. The cut surfaces are mirror-polished. A Vickers hardness test is performed at three arbitrary locations at the center of the plate thickness on the polished cut surfaces, in accordance with JIS Z2244:2020. The test force is set to 98 N. The arithmetic mean of the obtained Vickers hardness is defined as the maximum quenched hardness HD0 (HV). The maximum quenched hardness HD0 is defined as an integer value obtained by rounding the first decimal place of the obtained value.

[0104] (Hardenability Evaluation) A plate-shaped test specimen is taken from the center of the width of the steel plate. The shape of the plate-shaped test specimen is 15 mm in the rolling direction of the steel plate x 30 mm in the width direction x plate thickness. A ｃ３ Immerse the plate-shaped test specimen in a salt bath at the transformation point + 80°C for 10 minutes. If the test specimen is eutectoid steel or hypereutectoid steel, A ｃ１A plate-shaped test specimen is immersed in a salt bath at the transformation point + 80°C for 10 minutes. After that, the plate-shaped test specimen is removed from the salt bath and quenched by immersion in 60°C oil in an oil bath. The quenched plate-shaped test specimen is cut in half in the width direction. The cut surfaces are mirror-polished. A Vickers hardness test is performed at three arbitrary locations at the center of the plate thickness on the polished cut surfaces, in accordance with JIS Z2244:2020. The test force is set to 98 N. The arithmetic mean of the obtained Vickers hardness is defined as the quenched hardness HD1 (HV). The quenched hardness HD1 is an integer value obtained by rounding the first decimal place of the obtained value. If the obtained quenched hardness HD1 is 95% or more of the maximum quenched hardness HD0, that is, HD1 ≥ HD0 × 0.95, it is determined that sufficient hardenability is obtained in the steel plate.

[0105] [Regarding cold workability] In the steel sheet of this embodiment, obtaining sufficient cold workability means the following evaluation:

[0106] [Method for evaluating cold workability] A JIS No. 5 plate-shaped test specimen, as specified in JIS Z2241:2022, is taken from the center of the width of the steel plate. The thickness of the test specimen shall be 1.0 mm. If the thickness of the steel plate is greater than 1.0 mm, the test specimen shall be machined to adjust its thickness to 1.0 mm. However, if the thickness of the steel plate is less than 1.0 mm, the thickness of the test specimen shall remain the same as the thickness of the steel plate. V-notches shall be formed on both sides in the width direction of the test specimen at the longitudinal center of the parallel section of the test specimen. The opening angle of each V-notch shall be 45°, and the depth of each V-notch shall be 2.0 mm. The depth direction of each V-notch shall correspond to the width direction of the parallel section of the test specimen. The gauge length shall be 10.0 mm, including the V-notch section. The longitudinal direction of the test specimen shall be parallel to the rolling direction of the steel plate.

[0107] A fracture elongation test is performed on a test specimen at room temperature in air. The butt elongation after fracture is measured, and the obtained butt elongation (fracture elongation) (%) is defined as the notch elongation (%). If the obtained notch elongation is 5% or more and is equal to or greater than the notch elongation target value (%) specified by the following formula, it is determined that sufficient cold workability has been obtained in the steel sheet. Notch elongation target value (%) = -18 × [C] - 4 × [Si] + 35 Here, [C] in the formula is substituted with the C content (mass%) in the chemical composition of the steel sheet, and [Si] is substituted with the Si content (mass%) in the chemical composition of the steel sheet. The notch elongation (%) and the notch elongation target value (%) are integer values ​​obtained by rounding the obtained values ​​to the first decimal place.

[0108] [Applications of the steel plate] The steel plate of this embodiment is suitable as a material for mechanical parts in automotive applications. Examples of mechanical parts in automotive applications include car bearings, springs, washers, etc. The steel plate of this embodiment is suitable as a material for mechanical parts in textile machinery applications.

[0109] [Method for Manufacturing Steel Sheets] An example of a method for manufacturing steel sheets according to this embodiment will be described. The method for manufacturing steel sheets described below is just one example for manufacturing steel sheets according to this embodiment. Therefore, steel sheets having the above-described configuration may be manufactured by other manufacturing methods other than the method described below. However, the method described below is a preferred example of a method for manufacturing steel sheets according to this embodiment.

[0110] An example of the steel sheet manufacturing method of this embodiment includes the following steps: (Step 1) Material preparation step (Step 2) Hot rolling step (Step 3) Hot-rolled sheet annealing step (Step 4) Cold rolling step (Step 5) Cold-rolled sheet annealing step Each step will be described below.

[0111] [(Process 1) Material Preparation Process] In the material preparation process, a material that satisfies characteristic 1 is prepared. The material is manufactured, for example, by the following method: Molten steel whose chemical composition satisfies characteristic 1 is manufactured. A slab is manufactured using the molten steel by a well-known continuous casting method.

[0112] [(Process 2) Hot Rolling Process] In the hot rolling process, the prepared material (slab) is subjected to hot rolling to produce steel plates. The hot rolling process includes a rough rolling process in which the material is roughly rolled to produce intermediate steel plates (rough bars), and a finish rolling process in which the intermediate steel plates are finish rolled to produce steel plates.

[0113] In the rough rolling process, the raw material (slab) is heated in a heating furnace. The heated material is then rolled using a rough rolling mill to produce an intermediate steel plate (rough bar). The heating temperature of the material in the rough rolling process is, for example, 1050 to 1300°C. The time the material stays in the heating furnace is 30 minutes or more, preferably 60 minutes or more. There is no particular upper limit to the time the material stays in the furnace, but for example, it is 300 minutes.

[0114] In the finish rolling process, a finish rolling mill is used to further roll (finish roll) the rough bar to produce a steel sheet. The finish rolling mill includes a plurality of rolling stands arranged in a row. Each rolling stand is equipped with a pair of work rolls. The surface temperature of the steel sheet at the exit of the rolling stand that last reduces the steel sheet among the plurality of rolling stands of the finish rolling mill is defined as the finish rolling temperature (°C). In this embodiment, the finish rolling temperature is 830 to 950°C. The reduction ratio at the rolling stand that last reduces the steel sheet in the finish rolling process (hereinafter referred to as the final reduction stand) is 5 to 30%. Here, the reduction ratio is defined by the following formula: Reduction ratio = (1 - (Steel sheet thickness at the exit of the final reduction stand / Steel sheet thickness at the inlet of the final reduction stand)) × 100

[0115] If the reduction ratio at the final reduction stand is less than 5%, the average particle size of ferrite grains in the manufactured steel sheet may be coarse. If the reduction ratio at the final reduction stand is between 5% and 30%, the average particle size of ferrite grains in the manufactured steel sheet will be within an appropriate range.

[0116] After the finish rolling, the hot-rolled steel sheet is transported to the coiler via a run-out table, where it is wound into a coil. The coiled steel sheet is then cooled to room temperature.

[0117] [(Step 3) Hot Rolled Sheet Annealing Process] In the hot rolled sheet annealing process, the hot rolled steel sheet produced in the hot rolling process is subjected to an annealing treatment for the purpose of softening the steel sheet and spheroidizing the cementite. The annealing temperature T1 (°C) and the holding time t1 (hours) at the annealing temperature T1 in the hot rolled sheet annealing process will be described later.

[0118] [(Process 4) Cold Rolling Process] In the cold rolling process, cold rolling is performed on the hot-rolled steel sheet after the hot-rolled sheet annealing process. Cold rolling is performed using a cold rolling mill. The cold rolling mill is, for example, a reverse-type rolling mill consisting of one rolling stand. The rolling stand includes a pair of work rolls. The cold rolling mill may also be a tandem-type rolling mill having multiple rolling stands arranged in a row. Each rolling stand of the tandem-type rolling mill includes a pair of work rolls.

[0119] In the cold rolling process, cold rolling is carried out using the cold rolling mill described above to produce cold-rolled steel sheets. The cold rolling ratio RR (%) in the cold rolling process will be described later.

[0120] [(Step 5) Cold Rolled Sheet Annealing Process] In the cold rolled sheet annealing process, the cold rolled steel sheet after the cold rolling process is subjected to annealing. In this embodiment, the cold rolled sheet annealing process is performed using box annealing, not continuous annealing. The degree of ferrite recrystallization and cementite particle precipitation is adjusted by adjusting the annealing temperature T2 (°C) and the holding time t2 (hours) at the annealing temperature T2. The annealing temperature T2 (°C) and the holding time t2 (hours) will be described later.

[0121] [Conditions during the manufacturing process] The following conditions 1 to 5 are satisfied in the above-described processes 1 to 5. (Condition 1) The coiling temperature CT in the finish rolling process during the hot rolling process is 550 to 700°C. (Condition 2) The annealing temperature T1 in the hot-rolled sheet annealing process is 675 to 750°C, and the holding time t1 at the annealing temperature is 15 to 50 hours. (Condition 3) The cold rolling ratio RR in the cold rolling process is 20 to 80%. (Condition 4) The annealing temperature T2 in the cold-rolled sheet annealing process is greater than 675°C to 750°C, and the holding time t2 is 5 to 30 hours. (Condition 5) The annealing index AI, defined by equation (A) using the cold rolling ratio RR (%) and the residence time TH (hours) in the temperature range of 625 to 750°C during cold-rolled sheet annealing, and the graphite index GI, defined by equation (B), satisfy equation (C). AI = 3.5 × TH １／２ × (RR / 100) １／３ (A) GI = 5.5 - (GISi × [Si] + GINi × [Ni] + GIAl × [Al] + GIMn × [Mn] + GICr × [Cr] + GIMo × [Mo] + GIV × [V]) (B) GI - AI > 0 (C) Here, RR in equation (A) is the cold rolling rate (%). TH in equation (A) is the residence time (hours) in the temperature range of 625 to 675°C during cold-rolled sheet annealing, and is defined by equation (D). TH = tA + tB (D) Here, tA is substituted with the heating time (hours) in the temperature range of 625 to 675°C during cold-rolled sheet annealing, and tB is substituted with the holding time (hours) in the temperature range of 625 to 675°C during cold-rolled sheet annealing. Furthermore, the [element symbol] in equation (B) is replaced with the mass percentage content of the corresponding element in the chemical composition of the steel sheet. In addition, in equation (B), GISi = 1 GINi = 0.33 GIAl = 1 GIMn = 2.2 × [C] - 3.6 GICr = 5 × [C] - 6.8 GIMo = 0.7 × [C] - 1.4 GIV = 5.7 × [C] - 7.4 Here, [C] is the mass percentage content of C in the chemical composition of the steel sheet. The following explains each condition.

[0122] [Regarding (Condition 1) Winding temperature CT (°C)] In the hot rolling process, the winding temperature CT affects the spheroidization rate of cementite particles and / or the Cr concentration and Mo concentration in the cementite particles.

[0123] If the winding temperature CT exceeds 700°C, the Cr concentration in the residue [Cr] θ and Mo concentration in the residue [Mo] θ The coefficient of spheroidization may increase, and F3 may not satisfy equation (3). In addition, the spheroidization of cementite may be insufficient, and the spheroidization rate of cementite particles may be less than 85%.

[0124] If the winding temperature CT is less than 550°C, the hardness of the hot-rolled steel sheet becomes excessively high, making it impossible to carry out the subsequent cold-rolling process. If the winding temperature CT is between 550°C and 700°C, a steel sheet satisfying features 1 to 7 can be manufactured, provided that other conditions are met.

[0125] [Condition 2: Annealing temperature T1 (°C) and holding time t1 (hours) in the hot-rolled sheet annealing process] If the annealing temperature T1 in the hot-rolled sheet annealing process exceeds 750°C, excessive cementite will dissolve. In this case, the amount of undissolved cementite may become excessively small, or there may be no undissolved cementite at all. In this case, graphite may be generated by the subsequent cold-rolling and cold-rolled sheet annealing processes, and the graphitization rate GR may exceed 5.0%. Also, F3 may not satisfy equation (3). On the other hand, if the annealing temperature T1 is less than 675°C, there is an excess of undissolved cementite. In this case, even if the subsequent cold-rolling and cold-rolled sheet annealing processes are carried out, the cementite particles will not become sufficiently spheroidal. Therefore, in the manufactured steel sheet, the spheroidization rate of cementite particles may be less than 85%. Also, the graphitization rate GR may exceed 5.0%.

[0126] If the holding time t1 at annealing temperature T1 exceeds 50 hours, excess Cr and / or Mo will be dissolved in the cementite particles. In this case, F3 will not satisfy equation (3). Furthermore, the cementite particles will become coarser. On the other hand, if the holding time t1 is less than 15 hours, the spheroidization of the cementite will not be sufficiently formed. As a result, the spheroidization rate of cementite particles in the manufactured steel sheet may be less than 85%. Also, the graphitization rate GR may exceed 5.0%.

[0127] If the annealing temperature T1 is set to 675-750°C and the holding time t1 is set to 15-50 hours, then, assuming other conditions are met, a steel sheet satisfying features 1 to 7 can be manufactured.

[0128] [Condition 3: Regarding the cold rolling ratio RR (%) in the cold rolling process] In the cold rolling process, the cold rolling ratio RR is defined by the following formula: Cold rolling ratio RR (%) = (1 - (thickness of cold-rolled steel sheet after the cold rolling process / thickness of hot-rolled steel sheet before the cold rolling process)) × 100

[0129] If the cold rolling ratio (RR) is less than 20%, insufficient strain is introduced into the steel sheet. In this case, ferrite recrystallization does not proceed, and the ferrite grain size cannot be measured. In this case, the steel sheet hardens, and its cold workability decreases. Therefore, the cold rolling ratio (RR) should be 20% or more. Furthermore, if the cold rolling ratio (RR) exceeds 80%, cold rolling becomes difficult. Therefore, the cold rolling ratio (RR) should be 80% or less.

[0130] If the cold rolling ratio RR is between 20% and 80%, then, assuming other conditions are met, steel sheets satisfying features 1 to 7 can be manufactured.

[0131] [Condition 4: Annealing temperature T2 (°C) and holding time t2 (hours) in the cold-rolled sheet annealing process] If the annealing temperature T2 in the cold-rolled sheet annealing process exceeds 750°C, excessive cementite will dissolve. In this case, pearlite with large lamellar spacing will be formed during cooling after annealing. Since pearlite forms plate-like cementite (i.e., extended cementite particles), the spheroidization rate of cementite may be less than 85%. Also, F3 may not satisfy equation (3). On the other hand, if the annealing temperature T2 is 675°C or less (625 to 675°C), the annealing temperature is in the temperature range where graphite is produced. Therefore, the graphitization rate GR may exceed 5.0%.

[0132] If the holding time t2 at annealing temperature T2 exceeds 30 hours, the cementite and ferrite particles will coarseen. As a result, the average particle size of the cementite particles in the manufactured steel sheet will exceed 1.50 μm, and the average particle size of the ferrite will exceed 20.0 μm. In addition, F3 may not satisfy equation (3).

[0133] If the annealing temperature T2 is between 675°C and 750°C, and the holding time t2 is between 5 and 30 hours, then, assuming other conditions are met, a steel sheet satisfying features 1 to 7 can be manufactured.

[0134] [Condition 5: Regarding AI and GI values] FC is defined as follows: FC = GI - AI FC is an index that indicates the degree to which graphite formation is suppressed in the cold-rolled sheet annealing process. Graphitization in steel sheets is influenced by the ratio of graphitization-promoting elements and graphitization-inhibiting elements in the chemical composition of the steel sheet, the cold rolling rate in the cold-rolling process before the cold-rolled sheet annealing process, and the residence time TH in the temperature range of 625 to 675°C (650°C ± 25°C) in the cold-rolled sheet annealing process, which is the temperature range in which graphite is formed.

[0135] The GI value, defined by formula (B), is an index that indicates the degree to which graphite formation is suppressed from the perspective of the chemical composition of the steel sheet. In other words, a higher GI value tends to result in less graphite formation, while a lower GI value tends to result in more graphite formation.

[0136] Furthermore, the AI ​​value, defined by formula (A), is an indicator of the degree to which graphite is likely to form, from the perspective of manufacturing conditions. A higher AI value, that is, the higher the cold rolling rate RR (%) and / or the longer the residence time TH (hours) in the temperature range of 625 to 675°C, the higher the AI ​​value, and the more likely it is that graphite will be formed as a manufacturing condition.

[0137] Therefore, assuming other conditions are met, if the GI value, which is an indicator that suppresses graphite formation from the standpoint of chemical composition, is higher than the AI ​​value, which is an indicator that promotes graphite formation from the standpoint of manufacturing conditions, then the graphitization rate GR of the manufactured steel sheet will be 5.0% or less.

[0138] Through the above manufacturing process, steel plates that satisfy features 1 to 7 can be produced.

[0139] The effects of the steel sheet of this embodiment will be described in more detail below with reference to examples. The conditions in the following examples are just 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.

[0140] Steel sheets having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured.

[0141]

[0142]

[0143] Specifically, slabs were manufactured by continuous casting of molten steel. A hot rolling process was then carried out on the slabs. Specifically, the slabs were heated at 1100-1300°C for 240 minutes. After heating, the slabs were rolled in a roughing mill to produce rough bars (intermediate steel sheets). Furthermore, the rough bars were rolled using a finishing mill to produce hot-rolled steel sheets. The finishing rolling temperature for each test number was 830-950°C. The reduction ratio at the final reduction stand was 5-30%. The hot-rolled steel sheets after finishing rolling were wound at a winding temperature CT (°C) to form coils. The coiled hot-rolled steel sheets were allowed to cool to room temperature.

[0144] The manufactured hot-rolled steel sheets were subjected to hot-rolled sheet annealing at an annealing temperature T1 (°C) and a holding time t1 (hours). Note that the hot-rolled sheet annealing process was not performed for test numbers 70 and 71.

[0145] For hot-rolled steel sheets after the hot-rolled sheet annealing process (test numbers 1 to 69), or hot-rolled steel sheets after the hot-rolling process (test numbers 70 and 71), a cold-rolling process was performed with a cold-rolling ratio of RR (%). For the cold-rolled steel sheets after cold-rolling, a cold-rolled sheet annealing process was performed with an annealing temperature T2 (°C) and a holding time t2 (hours). Steel sheets were manufactured using the above manufacturing process.

[0146] Table 2 (Tables 2A and 2B) shows the winding temperature CT (°C), annealing temperature T1 (°C) in the hot-rolled sheet annealing process, holding time t1 (hours), cold rolling rate RR (%) in the cold-rolling process, residence time TH (hours) in the temperature range of 625-675°C in the cold-rolled sheet annealing process, annealing temperature T2 (°C) in the cold-rolled sheet annealing process, holding time t2 (hours), AI value, GI value, and FC.

[0147]

[0148]

[0149] [Evaluation Tests] The following tests were performed on each steel plate with the specified test number that was manufactured: (Test 1) Chemical composition measurement test (Test 2) Total area ratio measurement test of ferrite and cementite particles (Test 3) Average particle size measurement test of ferrite (Test 4) Average particle diameter measurement test of cementite particles (Test 5) Cementite spheroidization rate measurement test (Test 6) F3 measurement test (Test 7) ​​Graphitization rate GR measurement test (Test 8) Hardenability evaluation test (Test 9) Cold workability evaluation test Tests 1 to 9 are described below.

[0150] [(Test 1) Chemical Composition Measurement Test] Based on the method described in [Method for Measuring the Chemical Composition of Steel Sheets] above, the chemical composition of each steel sheet was measured. As a result, the chemical composition of each steel sheet was as shown in Table 1. The oxygen content was 0.0100% or less in all test numbers.

[0151] [(Test 2) Measurement Test of Total Area Ratio of Ferrite and Cementite Particles] Based on the method described in [Method for Measuring the Total Area Ratio of Ferrite and Cementite Particles in Microstructure] above, the total area ratio of ferrite and cementite particles was determined for each test number. The obtained total area ratios of ferrite and cementite particles are shown in the "Total Area Ratio of Ferrite and Cementite Particles (%)" column of Table 3 (Tables 3A to 3D). In the "Total Area Ratio of Ferrite and Cementite Particles (%)" column of Table 3, "≧95" indicates that the total area ratio of ferrite and cementite particles was 95% or more. In all test numbers, if there was a remainder other than ferrite and cementite particles in the microstructure, that remainder consisted of one or more types selected from the group consisting of bainite and martensite.

[0152]

[0153]

[0154]

[0155]

[0156] [(Test 3) Ferrite Average Particle Size Measurement Test] Based on the method described in [Method for Measuring the Average Particle Size of Ferrite] above, the average particle size (μm) of ferrite in the steel plates of each test number was determined. The obtained average particle sizes of ferrite are shown in the "Ferrite Particle Size (μm)" column of Table 3.

[0157] [(Test 4) Measurement Test of Average Particle Size of Cementite Particles] Based on the method described in [Method for Measuring Average Particle Size of Cementite Particles] above, the average particle size (μm) of cementite particles in the steel plates for each test number was determined. The obtained average particle size of cementite particles is shown in the "Cementite Particle Size (μm)" column of Table 3.

[0158] [(Test 5) Cementite Spheroidization Rate Measurement Test] Based on the method described in [Method for Measuring Spheroidization Rate] above, the spheroidization rate (%) of cementite particles in the steel plates of each test number was determined. The obtained spheroidization rates are shown in the "Cementite Spheroidization Rate (%)" column of Table 3.

[0159] [(Test 6) F3 Measurement Test] Based on the method described in [F3 Measurement Method] above, the F3 value of the steel plate for each test number was determined. In Table 3, the "X value" is [Cr] θ + [Mo] θ The value is shown. The "2.5 / (√[C])" column shows the value of 2.5 / (√[C]). The "F3" column shows the F3 value.

[0160] [(Test 7) ​​Graphitization Rate GR Measurement Test] Based on the method described in [Method for Measuring Graphitization Rate GR] above, the graphitization rate (%) of the steel plate for each test number was determined. The obtained graphitization rate GR is shown in the "Graphitization Rate GR (%)" column of Table 3.

[0161] [(Test 8) Hardenability Evaluation Test] Based on the method described in [Hardenability Evaluation Method] above, the hardenability of the steel plates of each test number during quenching was evaluated. The obtained A ｃ１ Transformation point (°C) and A ｃ３ The transformation point (°C) is shown in "A" in Table 1B. ｃ１ "Point (°C)" and "A ｃ３The hardness is indicated in "°C (points)". The obtained maximum hardness HD0 is shown in the "HD0 (HV)" column in Table 1B. The value of maximum hardness HD0 × 0.95 (i.e., 95% of maximum hardness HD0) is shown in the "HD0 × 0.95 (HV)" column in Table 3. The hardness HD1 (HV) obtained by the hardenability evaluation test is shown in the "HD1 (HV)" column in Table 3. If the hardness HD1 is 95% or more of the maximum hardness HD0, it is judged that sufficient hardenability has been obtained (indicated as "OK" in the "Hardenability Judgment" column in Table 3). On the other hand, if the hardness HD1 is less than the maximum hardness HD0 × 0.95, it is judged that sufficient hardenability has not been obtained (indicated as "NG" in the "Hardenability Judgment" column in Table 3).

[0162] [(Test 9) Cold Workability Evaluation Test] Based on the method described in [Cold Workability Evaluation Method] above, the cold workability of the steel sheet for each test number was evaluated. The obtained notch elongation is shown in the "Notch Elongation (%)" column of Table 3. The target notch elongation value (%) is also shown in the "Target Notch Elongation Value (%)" column of Table 3. If the obtained notch elongation was 5% or more and also equal to or greater than the target notch elongation value, it was determined that sufficient cold workability was obtained (indicated as "OK" in the "Notch Elongation Judgment" column of Table 3). On the other hand, if the notch elongation was less than 5% or less than the target notch elongation value, it was determined that sufficient cold workability was not obtained (indicated as "NG" in the "Notch Elongation Judgment" column of Table 3).

[0163] [Evaluation Results] Referring to Tables 1 to 3, the chemical composition was appropriate for test numbers 1 to 43, and the manufacturing conditions 1 to 5 were met. Therefore, the steel plates for these test numbers met characteristics 1 to 7. As a result, sufficient hardenability and sufficient cold workability were obtained.

[0164] On the other hand, in test number 44, the carbon content was too high. As a result, the notch elongation was below the target value, and sufficient cold workability could not be obtained.

[0165] In test number 45, the Si content was too high. As a result, the graphitization rate (GR) was too high. Consequently, sufficient hardenability could not be obtained. Furthermore, the notch elongation was below the target value, and sufficient cold workability could not be obtained.

[0166] In test number 46, the Mn content was too low. As a result, sufficient hardenability could not be obtained.

[0167] In test number 47, the Mn content was too high. As a result, the notch elongation was below the target value, and sufficient cold workability could not be obtained.

[0168] In test number 48, the Cr content was too high. Also, F3 did not satisfy formula (3). Therefore, sufficient hardenability could not be obtained.

[0169] In test number 49, the Mo content was too high. Also, F3 did not satisfy formula (3). Therefore, sufficient hardenability could not be obtained.

[0170] In tests 50 and 51, the winding temperature CT (°C) was too high. As a result, in test 50, the spheroidization rate of the cementite particles was low, and F3 did not satisfy equation (3). Similarly, in test 51, the spheroidization rate of the cementite particles was low. Consequently, in test 50, sufficient hardenability was not obtained. Furthermore, in tests 50 and 51, the notch elongation was below the target value, and sufficient cold workability was not obtained.

[0171] In tests 52 and 53, the annealing temperature T1 (°C) in the hot-rolled sheet annealing process was too high. As a result, the graphitization rate GR was high. Furthermore, in test 52, F3 did not satisfy equation (3). Therefore, sufficient hardenability was not obtained in these tests. In addition, the notch elongation was below the target value, and sufficient cold workability was not obtained.

[0172] In tests 54 and 55, the annealing temperature T1 (°C) was too low. As a result, the spheroidization rate of the cementite particles was low. Consequently, the notch elongation was below the target value, and sufficient cold workability could not be obtained.

[0173] In tests 56 and 57, the holding time t1 was too long. As a result, the average particle size of the cementite particles was large. Furthermore, F3 did not satisfy equation (3). Consequently, sufficient hardenability was not obtained.

[0174] In tests 58 and 59, the holding time t1 was too short. As a result, the spheroidization rate of the cementite particles was too low. Consequently, the notch elongation was below the target value, and sufficient cold workability could not be obtained.

[0175] In tests 60 and 61, the cold rolling rate RR (%) was too low. As a result, ferrite recrystallization did not proceed and was not completed. Consequently, the ferrite grain size could not be measured (indicated as "-" in the "Ferrite grain size (μm)" column in Table 3D). As a result, the notch elongation was below the target value, and sufficient cold workability could not be obtained.

[0176] In tests 62 and 63, FC did not satisfy formula (C). Therefore, the graphitization rate GR was too high. As a result, sufficient hardenability was not obtained. Furthermore, notch elongation was below the target value, and sufficient cold workability was not obtained.

[0177] In tests 64 and 65, the annealing temperature T2 was too high. As a result, the spheroidization rate of the cementite particles was too low. Also, in test 64, F3 did not satisfy equation (3). As a result, sufficient hardenability was not obtained in test 64. Furthermore, in tests 64 and 65, the notch elongation was below the target notch elongation value, and sufficient cold workability was not obtained.

[0178] In tests 66 and 67, the annealing temperature T2 was too low. As a result, the graphitization rate GR was too high. Consequently, sufficient hardenability was not achieved. Furthermore, the notch elongation was below the target value, resulting in insufficient cold workability.

[0179] In tests 68 and 69, the holding time t2 was too long. As a result, the average particle size of the cementite was large, the ferrite particle size was also large, and F3 did not satisfy equation (3). Consequently, sufficient hardenability could not be obtained.

[0180] In tests 70 and 71, the hot-rolled sheet annealing process was not performed. As a result, the graphitization ratio (GR) was too high. Consequently, sufficient hardenability was not achieved. Furthermore, the notch elongation was below the target value, resulting in insufficient cold workability.

[0181] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

It is a steel plate, The chemical composition is expressed in mass percent. C: 0.50-0.80%, Si: more than 0.50 to 2.10%, Mn: 0.20-1.20%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01-0.60%, N: 0.0150% or less, Mo: 0 to 0.350%, Ni: 0-1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0-0.500%, and, It contains Ti: 0-0.150%, with the remainder consisting of Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. The average particle size of the ferrite is 20.0 μm or less. The average particle diameter of the cementite particles is 1.50 μm or less. Of the cementite particles, those with an aspect ratio of 3.0 or less are defined as 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 85% or more. Total mass of Fe, Mn, Cr, and Mo in the residue obtained by the extraction residue analysis method <M> θ Let the total mass of the metallic elements in the cementite particles be C mass < C θ Seeking, The Cr concentration in the residue as defined by formula (1) [Cr] θ (mass%), Mo concentration in the residue as defined by formula (2) [Mo] θ (mass%) and the C content [C] in mass% of the chemical composition satisfy formula (3), The above [C] and the C concentration of carbides in the residue [C] based on the mass of the steel plate. ＣＢ The graphitization rate GR, defined by equation (4) using (mass%), is 5.0% or less. steel plate. [Cr] θ = Mass of Cr in the residue / (<M> θ + <C> θ ) × 100 (1) [Mo] θ = Mass of Mo in residue / (<M> θ + <C> θ ) × 100 (2) [Cr] θ +[Mo] θ -2.5 / (√[C])≦0 (3) GR=(1-[C] ＣＢ / [C])×100 (4) 　 A steel plate according to claim 1, The aforementioned chemical composition, in mass%, Mo: 0.001 to 0.350%, Ni: 0.001 to 1.000%, B: 0.0001 to 0.0100%, V: 0.001-0.500%, Nb: 0.001–0.500%, and, Contains one or more selected from the group consisting of Ti: 0.001 to 0.150%, steel plate.

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