Steel sheet

Abstract

Provided is a steel sheet that exhibits an excellent cold workability. The steel sheet according to the present disclosure has a chemical composition that contains, in mass%, C: 0.20 to 0.70%, Si: 0.07 to 1.00%, Mn: 0.20 to 3.00%, P: not more than 0.030%, S: not more than 0.0080%, Cr: 0.010 to 1.500%, acid-soluble Al: 0.005 to 0.070%, and N: not more than 0.0200%, with the balance being Fe and impurities. In the microstructure of the steel sheet according to the present disclosure, the total area ratio of ferrite and cementite grains is at least 95%; the average grain diameter of the ferrite grains is 5.0 to 30.0 μm; and the grain orientation spread (GOS) value of the ferrite grains is not more than 2.0°.

Description

steel plate

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

[0002] Steel sheets with a high C content (high carbon steel sheets) are used as steel sheets for machine parts, such as automobile parts. Machine parts may have complex shapes and are required to have high strength. The method for manufacturing machine parts using steel sheets for machine parts is as follows: The steel sheet is cold worked to form it into the shape of the machine part. The cold worked steel sheet is then quenched and tempered. High strength machine parts are manufactured through the above manufacturing process. As described above, steel sheets for machine parts are formed into the shape of the machine part by cold working. Therefore, excellent cold workability is required.

[0003] A technique for improving the cold workability of steel sheets is proposed, for example, in International Publication No. 2015 / 146173 (Patent Document 1).

[0004] The steel sheet disclosed in Patent Document 1 contains, by mass%, C: 0.20 to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0050% or less, and B: 0.0005 to 0.0050%, and further contains one or more of Sb, Sn, Bi, Ge, Te, and Se in a total amount of 0.002 to 0.030%, with the balance consisting of Fe and unavoidable impurities. In this steel sheet, the proportion of solute B in the B content is 70% or more. Furthermore, the microstructure is composed of ferrite and cementite. Furthermore, the cementite density within the ferrite grains is 0.08 particles / μm ２ The steel sheet of Patent Document 1 improves cold workability by adjusting the density of cementite within ferrite grains.

[0005] International Publication No. 2015 / 146173

[0006] The steel sheet disclosed in Patent Document 1 has sufficient cold workability. However, the cold workability of the steel sheet may be improved by other means.

[0007] An object of the present disclosure is to provide a steel sheet with excellent cold workability.

[0008] The steel sheet of the present disclosure has a chemical composition, in mass%, of C: 0.20 to 0.70%, Si: 0.07 to 1.00%, Mn: 0.20 to 3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010 to 1.500%, acid-soluble Al: 0.005 to 0.070%, N: 0.0200% or less, Ti: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0035% %, Cu: 0-0.20%, W: 0-0.03%, Ta: 0-0.03%, Sn: 0-0.030%, Sb: 0-0.030%, Co: 0-0.030%, As: 0-0.030%, Mg: 0-0.030%, Y: 0-0.030%, Zr: 0-0.030%, La: 0-0.030%, Ce: 0-0.030%, and Ca: 0-0.030%, with the balance consisting 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 the ferrite grains is 5.0-30.0 μm, and the GOS (Grain Orientation Spread) value of the ferrite grains is 2.0° or less.

[0009] The steel sheet of the present disclosure provides excellent cold workability.

[0010] The present inventors have conducted research into steel sheets that can provide excellent cold workability, and as a result, have made the following findings.

[0011] The present inventors first investigated a chemical composition suitable for steel sheets for use in machine parts, such as automobile parts, and as a result, the present inventors found a steel sheet containing, in mass %, C: 0.20 to 0.70%, Si: 0.07 to 1.00%, Mn: 0.20 to 3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010 to 1.500%, acid-soluble Al: 0.005 to 0.070%, N: 0.0200% or less, Ti: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0035%, Cu: 0 to 0.20%, W: 0.010 to 1.500%, The inventors considered that a steel sheet suitable for use in machine parts would have a chemical composition containing the following elements: 0 to 0.03%, Ta: 0 to 0.03%, Sn: 0 to 0.030%, Sb: 0 to 0.030%, Co: 0 to 0.030%, As: 0 to 0.030%, Mg: 0 to 0.030%, Y: 0 to 0.030%, Zr: 0 to 0.030%, La: 0 to 0.030%, Ce: 0 to 0.030%, and Ca: 0 to 0.030%, with the balance being Fe and impurities.

[0012] Therefore, the present inventors have investigated means for improving the cold workability of steel sheets that satisfy the above chemical composition.

[0013] The microstructure of a steel sheet having the above-described chemical composition is substantially composed of ferrite and cementite particles. To improve the cold workability of such a steel sheet, the inventors focused on the strain distribution within the steel sheet. Localized strain within the steel sheet results in localized regions with different amounts of plastic deformation during cold working. In this case, uniform plastic deformation is not possible, resulting in non-uniform plastic deformation. As a result, cold workability is reduced. Therefore, the inventors focused on ferrite grains, which are the main component of the macrostructure of the steel sheet. If the amount of strain within the ferrite grains varies, the amount of plastic deformation is likely to vary from ferrite grain to ferrite grain.

[0014] Based on the above findings, the present inventors investigated the relationship between the GOS (Grain Orientation Spread) of a steel sheet as an index of the amount of strain per ferrite grain and cold workability. GOS indicates the average orientation difference per crystal grain and can be used as an index of the amount of strain per crystal grain. As a result of further investigation, the present inventors found for the first time that, in a steel sheet having the above-mentioned chemical composition, when the total area ratio of ferrite and cementite particles is 95% or more, excellent cold workability can be obtained if the GOS value of the ferrite grains is 2.0° or less.

[0015] The steel sheet of this embodiment has been completed based on the above technical concept and has the following configuration.

[0016] The steel sheet of the first embodiment has a chemical composition, in mass %, of C: 0.20 to 0.70%, Si: 0.07 to 1.00%, Mn: 0.20 to 3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010 to 1.500%, acid-soluble Al: 0.005 to 0.070%, N: 0.0200% or less, Ti: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.003%. 5%, Cu: 0-0.20%, W: 0-0.03%, Ta: 0-0.03%, Sn: 0-0.030%, Sb: 0-0.030%, Co: 0-0.030%, As: 0-0.030%, Mg: 0-0.030%, Y: 0-0.030%, Zr: 0-0.030%, La: 0-0.030%, Ce: 0-0.030%, and Ca: 0-0.030%, with the balance consisting 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 the ferrite grains is 5.0-30.0 μm, and the GOS (Grain Orientation Spread) value of the ferrite grains is 2.0° or less.

[0017] The steel sheet of the second embodiment is the steel sheet of the first embodiment, and has a chemical composition, in mass %, of Ti: 0.001 to 0.500%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, B: 0.0001 to 0.0035%, Cu: 0.01 to 0.20%, W: 0.01 to 0.03%, Ta: 0.01 to 0.03%, Sn: 0.001 to 0.030%, Sb: 0.0 The steel sheet contains one or more elements selected from the group consisting of: 0.01 to 0.030%, Co: 0.001 to 0.030%, As: 0.001 to 0.030%, Mg: 0.001 to 0.030%, Y: 0.001 to 0.030%, Zr: 0.001 to 0.030%, La: 0.001 to 0.030%, Ce: 0.001 to 0.030%, and Ca: 0.001 to 0.030%.

[0018] The steel sheet of this embodiment will be described in detail below. Note that "%" regarding elements means mass % unless otherwise specified.

[0019] [Features of Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment satisfies the following features: (Feature 1) The chemical composition, in mass %, is: C: 0.20 to 0.70%, Si: 0.07 to 1.00%, Mn: 0.20 to 3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010 to 1.500%, acid-soluble Al: 0.005 to 0.070%, N: 0.0200% or less, Ti: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0035%, Cu: 0.010 to 1.500%, : 0-0.20%, W: 0-0.03%, Ta: 0-0.03%, Sn: 0-0.030%, Sb: 0-0.030%, Co: 0-0.030%, As: 0-0.030%, Mg: 0-0.030%, Y: 0-0.030%, Zr: 0-0.030%, La: 0-0.030%, Ce: 0-0.030%, and Ca: 0-0.030%, with the balance consisting of Fe and impurities. (Feature 2) In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. (Feature 3) The average particle size of the ferrite grains is 5.0-30.0 μm. (Feature 4) The GOS value of the ferrite grains is 2.0° or less. Each feature is described below.

[0020] [(Feature 1) Chemical Composition] The chemical composition of the steel sheet of this embodiment contains the following elements.

[0021] C: 0.20 to 0.70% Carbon (C) improves the hardenability of steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. If the C content is less than 0.20%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.70%, the cold workability of the steel sheet decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.20 to 0.70%. The preferred lower limit of the C content is 0.22%, more preferably 0.25%, even more preferably 0.28%, and even more preferably 0.30%. The preferred upper limit of the C content is 0.68%, even more preferably 0.65%, and even more preferably 0.60%.

[0022] Si: 0.07 to 1.00% Silicon (Si) deoxidizes steel during the steelmaking stage of the steel sheet manufacturing process. Furthermore, when tempering is performed in a process for manufacturing mechanical parts using the steel sheet as a raw material, Si increases the temper softening resistance of the steel sheet. As a result, the strength of the mechanical parts is increased. If the Si content is less than 0.07%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 1.00%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the Si content is 0.07 to 1.00%. The preferred lower limit of the Si content is 0.09%, more preferably 0.12%, and even more preferably 0.15%. The upper limit of the Si content is preferably 0.95%, more preferably 0.90%, even more preferably 0.80%, even more preferably 0.70%, and still more preferably 0.60%.

[0023] Mn: 0.20 to 3.00% Manganese (Mn) improves the hardenability of steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical component using the steel sheet as a raw material, the strength of the mechanical component is increased. If the Mn content is less than 0.20%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 3.00%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Mn content is 0.20 to 3.00%. The preferred lower limit of the Mn content is 0.25%, more preferably 0.30%, even more preferably 0.35%, and even more preferably 0.40%. The upper limit of the Mn content is preferably 2.90%, more preferably 2.80%, even more preferably 2.50%, even more preferably 2.00%, even more preferably 1.90%, even more preferably 1.70%, and even more preferably 1.50%.

[0024] P: 0.030% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content exceeds 0.030%, the toughness of the steel plate decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible. However, excessive reduction of the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the P content is 0.028%, even more preferably 0.025%, even more preferably 0.020%, and even more preferably 0.015%.

[0025] S: 0.0080% or less Sulfur (S) is an unavoidable impurity. In other words, the S content is greater than 0%. If the S content exceeds 0.0080%, excessive sulfides are formed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the S content is 0.0080% or less. The lower the S content, the more preferable it is. However, excessive reduction of the S content significantly increases production costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the S content is 0.0075%, even more preferably 0.0070%, even more preferably 0.0065%, even more preferably 0.0060%, even more preferably 0.0055%, and even more preferably 0.0050%.

[0026] Cr: 0.010 to 1.500% Chromium (Cr) improves the hardenability of steel sheet. As a result, when hardening is performed in a process for manufacturing a mechanical part using the steel sheet as a raw material, the strength of the mechanical part is increased. If the Cr content is less than 0.010%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 1.500%, the strength of the steel sheet becomes excessively high. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Cr content is 0.010 to 1.500%. The preferred lower limit of the Cr content is 0.015%, more preferably 0.030%, even more preferably 0.050%, even more preferably 0.080%, and even more preferably 0.100%. The upper limit of the Cr content is preferably 1.480%, more preferably 1.450%, even more preferably 1.400%, even more preferably 1.300%, even more preferably 1.100%, even more preferably 0.900%, and even more preferably 0.700%.

[0027] Acid-soluble Al: 0.005 to 0.070% Aluminum (Al) deoxidizes steel during the steelmaking process in the steel sheet manufacturing process. If the acid-soluble Al content is less than 0.005%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the acid-soluble Al content exceeds 0.070%, excessive Al nitrides are formed, resulting in refinement of austenite grains. As a result, the hardenability of the steel sheet decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the acid-soluble Al content is 0.005 to 0.070%. The preferred lower limit of the acid-soluble Al content is 0.010%, more preferably 0.012%, even more preferably 0.015%, and even more preferably 0.020%. The preferred upper limit of the acid-soluble Al content is 0.065%, even more preferably 0.060%, even more preferably 0.055%, and even more preferably 0.050%.

[0028] N: 0.0200% or less Nitrogen (N) is an unavoidable impurity. In other words, the N content is greater than 0%. N combines with Al to form Al nitrides. Al nitrides refine austenite grains during heating for quenching in the process of manufacturing mechanical parts using steel sheet as a raw material. Refinement of austenite grains reduces the hardenability of the steel sheet. If the N content exceeds 0.0200%, austenite grains are excessively refined during heating for quenching. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel sheet is significantly reduced. Therefore, the N content is 0.0200% or less. A preferred lower limit of the N content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The upper limit of the N content is preferably 0.0190%, more preferably 0.0180%, even more preferably 0.0170%, even more preferably 0.0160%, even more preferably 0.0150%, and even more preferably 0.0140%.

[0029] The balance of the chemical composition of the steel sheet according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel sheet, and are acceptable within a range that does not adversely affect the steel sheet according to this embodiment.

[0030] [Optional Elements] The chemical composition of the steel sheet of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, B: 0-0.0035%, Cu: 0-0.20%, W: 0-0.03%, Ta: 0-0.03%, Sn: 0-0.030%, Sb: 0-0.030%, Co: 0-0.030%, As: 0-0.030%, Mg: 0-0.030%, Y: 0-0.030%, Zr: 0-0.030%, La: 0-0.030%, Ce: 0-0.030%, and Ca: 0-0.030%. All of these elements are optional and may not be included. These optional elements will be explained below.

[0031] [Regarding Group 1 (Ti, V, Nb, and B)] The chemical composition of the steel sheet according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti, V, Nb, and B. All of these elements are optional elements and may not be contained. When contained, Ti, V, Nb, and B increase the strength of the steel sheet.

[0032] Ti: 0 to 0.500% Titanium (Ti) is an optional element and may not be included. In other words, the Ti content may be 0%. When Ti is included, that is, when the Ti content exceeds 0%, Ti forms precipitates such as carbides. Therefore, the strength of the steel sheet is increased by precipitation strengthening. Ti also combines with N to suppress the formation of nitrides by solute B. Even if even a small amount of Ti is included, the above effect can be achieved to some extent. However, if the Ti content exceeds 0.500%, excessive precipitates are formed, resulting in excessively high strength of the steel sheet. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet will be reduced. Therefore, the Ti content is 0 to 0.500%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.002%, even more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Ti content is preferably 0.400%, more preferably 0.300%, even more preferably 0.200%, even more preferably 0.100%, and still more preferably 0.080%.

[0033] V: 0 to 0.500% Vanadium (V) is an optional element and does not necessarily need to be contained. In other words, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms precipitates such as carbides. Therefore, the strength of the steel sheet is increased by precipitation strengthening. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.500%, excessive precipitates are formed, resulting in an excessively high strength of the steel sheet. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the V content is 0 to 0.500%. The preferred lower limit of the V content is 0.001%, more preferably 0.002%, even more preferably 0.003%, and even more preferably 0.005%. The upper limit of the V content is preferably 0.480%, more preferably 0.450%, even more preferably 0.400%, even more preferably 0.300%, even more preferably 0.200%, even more preferably 0.100%, and even more preferably 0.080%.

[0034] Nb: 0 to 0.500% Niobium (Nb) is an optional element and does not necessarily need to be contained. In other words, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms precipitates such as carbides. Therefore, the strength of the steel sheet is increased by precipitation strengthening. Nb also combines with N to prevent solute B from forming nitrides. Even if even a small amount of Nb is contained, the above effect can be achieved to some extent. However, if the Nb content exceeds 0.500%, excessive precipitates are formed, resulting in an excessively high strength of the steel sheet. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet will be reduced. Therefore, the Nb content is 0 to 0.500%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.002%, even more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Nb content is preferably 0.480%, more preferably 0.450%, even more preferably 0.400%, even more preferably 0.350%, and still more preferably 0.300%.

[0035] B: 0 to 0.0035% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When B is contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel sheet and increases its strength. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0035%, B compounds are formed. In this case, the strength of the steel sheet becomes excessively high. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet decreases. Therefore, the B content is 0 to 0.0035%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the B content is preferably 0.0032%, more preferably 0.0028%, even more preferably 0.0025%, even more preferably 0.0020%, and still more preferably 0.0015%.

[0036] [Regarding the second group (Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca)] The chemical composition of the steel sheet according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca. All of these elements are optional elements and may not be contained. In other words, the content of these elements may be 0%.

[0037] These elements are all tramp elements and are impurities in the steel sheet of this embodiment. Therefore, the Cu content is 0 to 0.20%, the W content is 0 to 0.03%, the Ta content is 0 to 0.03%, the Sn content is 0 to 0.030%, the Sb content is 0 to 0.030%, the Co content is 0 to 0.030%, the As content is 0 to 0.030%, the Mg content is 0 to 0.030%, the Y content is 0 to 0.030%, the Zr content is 0 to 0.030%, the La content is 0 to 0.030%, the Ce content is 0 to 0.030%, and the Ca content is 0 to 0.030%.

[0038] The lower limit of the Cu content is preferably 0.01%, more preferably 0.03%, and the upper limit of the Cu content is preferably 0.15%, more preferably 0.10%.

[0039] The lower limit of the W content is preferably 0.01%, and the upper limit of the W content is preferably 0.02%.

[0040] The lower limit of the Ta content is preferably 0.01%, and the upper limit of the Ta content is preferably 0.02%.

[0041] The lower limit of the Sn content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Sn content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0042] The lower limit of the Sb content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Sb content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0043] The lower limit of the Co content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Co content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0044] The lower limit of the As content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the As content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0045] The lower limit of the Mg content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Mg content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0046] The lower limit of the Y content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Y content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0047] The lower limit of the Zr content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Zr content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0048] The lower limit of the La content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the La content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0049] The lower limit of the Ce content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Ce content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0050] The lower limit of the Ca content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Ca content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0051] [Method for Measuring Chemical Composition of Steel Sheet] The chemical composition of the steel sheet of this embodiment can be measured by a known elemental analysis method. Specifically, chips are collected from the interior of the steel sheet to a depth of 0.1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a known inert gas fusion-thermal conductivity method.

[0052] The content of each element is determined by rounding off the measured value to the nearest significant digit based on the significant digits specified in this embodiment. For example, the C content of the steel sheet in this embodiment is determined to be a value up to two decimal places. Therefore, the C content is determined to be a value up to two decimal places obtained by rounding off the measured value to two decimal places.

[0053] Similarly, the contents of elements other than the C content of the steel sheet of this embodiment are determined by rounding off the measured value to the smallest digit specified in this embodiment, and the value obtained is the content of the element. Note that rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0054] [(Feature 2) Microstructure] In the microstructure of the steel sheet of this embodiment, the total area ratio of ferrite and cementite particles is 95% or more. In other words, the microstructure of the steel sheet of this embodiment is substantially composed of ferrite and cementite particles.

[0055] In the microstructure, the structure other than the ferrite and cementite particles is, for example, one or more types selected from the group consisting of bainite, martensite, and pearlite.

[0056] Preferably, the total area ratio of ferrite and cementite particles in the microstructure is 96% or more, more preferably 97% or more, even more preferably 98% or more, and even more preferably 99% or more. The microstructure may be a structure consisting of ferrite and cementite particles.

[0057] The total area ratio of the ferrite and cementite particles is preferably in the range of 96 to 100%, more preferably 97 to 100%, even more preferably 98 to 100%, and still more preferably 99 to 100%.

[0058] If the total area ratio of ferrite and cementite particles is 95% or more, excellent cold workability can be obtained, provided that Features 1, 3, and 4 are satisfied.

[0059] [Method for Measuring Total Area Ratio of Ferrite and Cementite Particles in Microstructure] The total area ratio of ferrite and cementite particles in the microstructure can be measured by the following method.

[0060] A test piece having a cross section (L cross section) parallel to the rolling direction and thickness direction of the steel plate is taken from the center position of the steel plate width. The size of the test piece is not particularly limited as long as it includes the observation field described below. The L cross section of the steel plate is used as the observation surface of the surface of the test piece. The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (Nital etchant). The etching time is 120 seconds. Five rectangular observation fields are selected from the etched observation surface, each of which is 100 μm in the thickness direction of the steel plate and 120 μm in the direction perpendicular to the thickness direction and includes the center position of the thickness of the steel plate. The five observation fields are arranged consecutively in the direction perpendicular to the thickness, and the center position of the central observation field of the five arranged observation fields is the center position of the width of the steel plate.

[0061] For each observation field, a secondary electron image is observed using a scanning electron microscope (SEM) at 1000x magnification. In the observation field, ferrite and cementite particles exhibit different contrast and morphology from other structures (bainite, martensite, pearlite, etc.). Therefore, ferrite and cementite particles within the observation field are identified based on their contrast and morphology.

[0062] Specifically, ferrite is the white region without any substructure such as lath within the grain. Bainite and martensite are regions that contain substructure. Pearlite is the striped region with a lamellar structure. Cementite particles are the black region that is less bright than ferrite. In other words, cementite appears darker than ferrite.

[0063] The total area ratio (%) of ferrite and cementite particles is calculated based on the total area of ​​ferrite and cementite particles in the five observation fields and the total area of ​​the five observation fields. The total area ratio is calculated as an integer value obtained by rounding the obtained value to one decimal place.

[0064] [(Feature 3) Average Diameter of Ferrite Grains] Furthermore, in the steel sheet of this embodiment, the average diameter of ferrite grains is 5.0 to 30.0 μm.

[0065] The average grain size of ferrite grains affects cold workability. If the average grain size of ferrite grains is less than 5.0 μm, the ferrite grains are excessively fine. In this case, sufficient cold workability cannot be obtained. If the average grain size of ferrite grains is 5.0 μm or more, excellent cold workability can be obtained in the steel sheet, provided that Features 1, 2, and 4 are satisfied.

[0066] The lower limit of the average particle size of the ferrite grains is preferably 5.2 μm, more preferably 5.5 μm, even more preferably 5.7 μm, even more preferably 6.0 μm, even more preferably 6.5 μm, and even more preferably 7.0 μm.

[0067] There is no particular upper limit to the average grain size of ferrite grains. However, if the average grain size of ferrite grains is excessively large, the strength of the steel sheet will be excessively low. As a result, sufficient strength may not be obtained in mechanical parts. Therefore, the upper limit of the average grain size of ferrite grains is 30.0 μm.

[0068] The upper limit of the average particle size of the ferrite grains is preferably 29.0 μm, more preferably 28.0 μm, even more preferably 27.0 μm, even more preferably 26.0 μm, even more preferably 24.0 μm, even more preferably 22.0 μm, and even more preferably 20.0 μm.

[0069] [Method for Measuring Average Particle Size of Ferrite Grains] The average particle size of ferrite grains can be measured by the following method.

[0070] A test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate width. The surface of the test piece measuring 15 mm in the rolling direction and thickness (i.e., the L-section) is defined as the observation surface. The observation surface of the test piece is mirror-polished. After mirror-polishing, etching is performed with a 3% nital etchant. The etching time is 120 seconds. Secondary electron images are taken of the etched observation surface at five arbitrary observation fields at a depth of 1 / 4 of the plate thickness from the surface of the steel plate using a scanning electron microscope (SEM). The ferrite grain size number is determined by the intercept method in accordance with JIS G 0551:2020. The magnification of the SEM is selected in the range of 500 to 3000 times so that the number of ferrite grains cut by one line segment is 10 to 50 in one field of view. The length of the line segment is 1 mm. The line segments are set so that cementite particles do not overlap the line segments, that is, so that all overlapping portions of the line segments are ferrite grains. The cut lengths are determined for the five observation fields. Here, the cut length is the value obtained by dividing the length L of the line segment by the number N of times the line segment intersects with the grain boundaries of ferrite grains. The ferrite grain size number is determined from the arithmetic mean value of the cut lengths of the five observation fields. The average grain size (μm) of the ferrite grains is determined from the obtained grain size number. The average grain size of the ferrite grains is the value obtained by rounding the obtained value to one decimal place (i.e., the value to one decimal place).

[0071] [(Feature 4) GOS (Grain Orientation Spread) Value of Ferrite Grains] Furthermore, in this embodiment, the GOS value of the ferrite grains is 2.0° or less.

[0072] The GOS value indicates the magnitude of the crystal orientation difference (variation in crystal orientation) occurring within each crystal grain, and can be used as an index of the amount of strain for each crystal grain. The larger the GOS value, the more ferrite grains there are with large misorientation within the crystal grains. In this case, the amount of strain accumulated within the ferrite grains becomes excessively large, and as a result, the steel sheet does not have sufficient cold workability. If the GOS value is 2.0° or less, the variation in crystal orientation within the ferrite grains is sufficiently suppressed, and there are a sufficient number of ferrite grains with a small amount of accumulated strain within the grains. Therefore, provided that Features 1 to 3 are satisfied, the steel sheet can have excellent cold workability.

[0073] The upper limit of the GOS value is preferably 1.9°, more preferably 1.8°, and even more preferably 1.7°. The lower limit of the GOS value is not particularly limited. A lower GOS value is preferable. However, excessive reduction of the GOS value increases production costs. Therefore, the lower limit of the GOS value is preferably 0.1°, more preferably 0.2°, and even more preferably 0.3°.

[0074] [Method for Measuring GOS] The method for measuring GOS in this embodiment is as follows. A test piece for measuring the KAM value is taken from a steel sheet. The size of the test piece is not particularly limited as long as it has a cross section (hereinafter referred to as the observation surface) that includes the center position of the sheet thickness and includes three observation fields of 100 μm in the sheet thickness direction and 100 μm in the rolling direction, with no overlapping between them.

[0075] The observation surface of the test piece is mirror-polished. Within the mirror-polished observation surface, three measurement fields of 100 μm × 100 μm are set so that they do not overlap with each other. Electron backscatter diffraction (EBSD) measurement is performed for each observation field. In the EBSD measurement, the acceleration voltage is 20 kV and the probe current is 28 nA. The measurement points in each observation field are set as follows:

[0076] Each observation field is divided into regular hexagonal pixel units. The distance between the centers of adjacent pixels is 0.3 μm. The center position of a regular hexagonal pixel that is entirely contained within the observation field is used as the measurement point. In other words, pixels that are partly outside the observation field are excluded from the measurement target.

[0077] The crystal orientation obtained by EBSD measurement at each measurement point is taken as the crystal orientation of the pixel containing that measurement point. Note that measurement points for which the confidence index (CI value), which indicates the likelihood of the obtained crystal orientation, is 0.1 or less are not used in subsequent calculations. Furthermore, pixels containing measurement points for which the confidence index is 0.1 or less are treated as non-existent.

[0078] The EBSD analysis determines the crystal structure of the pixel containing each measurement point from the diffraction pattern (Kikuchi line pattern) obtained at each measurement point. If the crystal structure at the measurement point is a body-centered cubic (bcc) structure, the pixel containing the measurement point is determined to be ferrite.

[0079] Among the pixels determined to be ferrite, a region of a plurality of consecutively arranged pixels is determined to be a ferrite region. Within the ferrite region, the orientation difference between the target pixel and each of six adjacent pixels (six pixels having sides that contact each side of the hexagonal pixel) is calculated. The arithmetic mean value of the six obtained orientation differences is defined as the orientation difference of the pixel. A group of consecutively arranged pixels whose obtained orientation difference is 5° or less is defined as a group of pixels within the same ferrite grain.

[0080] Based on a group of pixels (multiple pixels) determined to be within the same ferrite grain, the GOS is calculated as follows: An arbitrary pixel (measurement point) within the same ferrite grain is selected. The crystal orientation difference between that pixel and each of the other pixels (measurement points) within the same ferrite grain is calculated. This operation is performed for all pixels (measurement points) within the ferrite grain. The average value of the orientation differences between all the obtained pixels is calculated. The calculated value is defined as the GOS value of the crystal grain.

[0081] The above-mentioned GOS value is defined by the following formula: Here, n is substituted with the number of pixels in the ferrite grain. ｉ The crystal orientation of the i-th pixel in the ferrite grain is substituted for θ. ａｖｅ is assigned the arithmetic mean value of the crystal orientations of all pixels in the ferrite grain.

[0082] All measurement points within the same ferrite grain show the same GOS value. The arithmetic mean of the GOS values ​​of all ferrite grains in the three measurement regions is calculated. The obtained arithmetic mean value is used as the GOS value. The GOS value is the value obtained by rounding the obtained value to one decimal place.

[0083] The arithmetic mean value of the GOS values ​​of all ferrite grains within the observation field is defined as the GOS value of the ferrite grains of the steel sheet. The GOS value can be determined by analysis using known analysis software, OIM Analysis (trade name) manufactured by TSL.

[0084] [Effects of the Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment satisfies Features 1 to 4. Therefore, the steel sheet of the present embodiment has excellent cold workability.

[0085] [Applications of Steel Sheet] The steel sheet of this embodiment is used as a material for mechanical parts, such as automobile parts. Examples of mechanical parts include automobile springs, washers, and bicycle gears. The steel sheet of this embodiment may also be used for applications other than mechanical parts that require excellent cold workability.

[0086] [Method for manufacturing steel sheet] An example of a method for manufacturing a steel sheet according to this embodiment will be described. The method for manufacturing a steel sheet described below is one example for manufacturing the steel sheet according to this embodiment. Therefore, a steel sheet having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel sheet according to this embodiment.

[0087] An example of the method for manufacturing a steel sheet according to this embodiment includes the following steps: (Step 1) Hot rolling step (Step 2) Annealing step Each step will be described below.

[0088] [(Step 1) Hot Rolling Step] In the hot rolling step, hot rolling is performed on a slab that satisfies Feature 1. The slab is produced, for example, by the following method. Molten steel that satisfies Feature 1 and whose chemical composition has an element content within the range of this embodiment is produced. The molten steel is used to produce a slab by a casting method. For example, the molten steel is used to produce a slab by a well-known continuous casting method. The chemical composition of the produced slab satisfies Feature 1.

[0089] The prepared slab is hot-rolled to produce a hot-rolled steel sheet. The hot-rolling process includes the following steps: (Step 10) Heating step (Step 11) Rough rolling step (Step 12) Finish rolling step (Step 13) Coiling step Each step will be described below.

[0090] [(Step 10) Heating Step] In the heating step, the slab is heated in a heating furnace. The time for which the slab is stored in the heating furnace is, for example, 30 minutes or more, preferably 60 minutes or more. The upper limit of the time for storing the slab in the heating furnace is not particularly limited, but is, for example, 300 minutes.

[0091] [(Step 11) Rough Rolling Step] In the rough rolling step, the heated slab is rough rolled using a rough rolling mill to produce an intermediate steel plate (rough bar). The rough rolling mill is, for example, a reverse rolling mill.

[0092] [(Step 12) Finish Rolling Step] In the finish rolling step, a tandem rolling mill is used to perform finish rolling on an intermediate steel plate (rough bar) to produce a hot-rolled steel plate. The tandem rolling mill includes a plurality of rolling stands 1 to L (L is an integer) arranged in a row. Each rolling stand n (n = 1 to L) is equipped with a pair of work rolls. The number of rolling stands n constituting the tandem rolling mill is not particularly limited, but may be, for example, 4 to 7 stands. The finish rolling step is performed without reheating the intermediate steel plate after rough rolling.

[0093] [(Step 13) Winding Step] In the winding step, the hot-rolled steel sheet that has been finish-rolled in the finish rolling step of the hot rolling step is cooled and wound up.

[0094] [(Step 2) Annealing Step] In the annealing step, the coiled hot-rolled steel sheet after the coiling step is annealed to spheroidize the cementite into cementite particles.

[0095] [Conditions in the manufacturing process] The manufacturing process described above further satisfies the following condition: (Condition 1) The heating temperature T １１ (Condition 2) The final finish rolling start temperature T １２ is 850° C. or higher. (Condition 3) The finish rolling temperature FT in the finish rolling step is 850 to 1000° C. (Condition 4) Of the multiple rolling stands 1 to L of the tandem rolling mill used in the finish rolling step, in the rolling stand L-1 in the stage preceding the most downstream rolling stand L, σ defined by formula (1) is Ｌ－１ is set to 5 to 20. Furthermore, in the most downstream rolling stand L, σ defined by equation (1) Ｌ is set to 30 to 70. σ ｎ =exp(0.753+3000 / (T ｎ + 273)) × ε ｎ ０．２１ ×v ｎ ０．１３ (1) where n is 1 to L and ε ｎis the equivalent plastic strain imparted to the rough bar in the corresponding rolling stand n, and v ｎ is the strain rate (s －１ ) T ｎ is the surface temperature (°C) of the steel plate at the entry side of rolling stand n. When formula (1) is applied to rolling stand L-1, n in formula (1) is L-1. When formula (1) is applied to rolling stand L, n in formula (1) is L. When formula (1) is applied to rolling stand L, T ｎ (°C) indicates the final finish rolling start temperature T １２ On the other hand, when formula (1) is applied to the rolling stand L-1, Tn (°C) in formula (1) is substituted with T １２ +20 (°C) is substituted. (Condition 5) The cumulative reduction rate R in the hot rolling process is 60% or more. (Condition 6) The coiling temperature CT in the coiling process is 650 to 550°C. (Condition 7) The annealing temperature T ３ is set to 600 to 730 ° C., and the annealing temperature T ３ Holding time t ３ The time is set to 20 hours or more. Each condition will be explained below.

[0096] [(Condition 1) Heating temperature T １１ Regarding the heating temperature T １１ If the heating temperature T is less than 1100°C, the ferrite grains in the produced steel sheet will be excessively small. As a result, the average grain size of the ferrite grains will be less than 5.0 μm. １１ If the heating temperature T exceeds 1350°C, the ferrite grains in the produced steel sheet will become coarse. As a result, the average grain size of the ferrite grains will exceed 30.0 μm. １１ is 1100 to 1350°C.

[0097] [(Condition 2) Final finish rolling start temperature T １２ Regarding the final finish rolling start temperature T １２ (°C). Final finish rolling start temperature T １２is measured by a thermometer arranged on the inlet side of the most downstream rolling stand L that applies the final reduction to the intermediate steel plate. １２ If the final finish rolling start temperature T is less than 850°C, the ferrite in the produced steel sheet will be excessively small. As a result, the average grain size of the ferrite grains will be less than 5.0 μm. １２ is 850°C or higher.

[0098] [(Condition 3) Regarding the Finish Rolling Temperature FT] The surface temperature of the steel sheet (hot-rolled steel sheet) at the outlet side of the most downstream rolling stand L among the multiple rolling stands 1 to L that make up the rolling mill in the finish rolling process is defined as the "finish rolling temperature" FT (°C). The finish rolling temperature FT is measured by a thermometer arranged at the outlet side of the most downstream rolling stand L.

[0099] If the finish rolling temperature FT is less than 850°C, the ferrite in the produced steel sheet will be excessively small. As a result, the average grain size of the ferrite grains will be less than 5.0 μm. If the finish rolling temperature FT exceeds 1000°C, the ferrite in the produced steel sheet will be excessively large. As a result, the average grain size of the ferrite grains will exceed 30.0 μm. Therefore, the finish rolling temperature FT is 850 to 1000°C.

[0100] [(Condition 4) σ ｎ (σ Ｌ－１ and σ Ｌ In the finish rolling process, the stress applied in the rolling stand L-1 preceding the most downstream rolling stand L and in the rolling stand L affects the size of the surface ferrite grains and the ferrite grains at the center of the sheet thickness. Ｌ－１ is set to 5 to 20. Furthermore, in the most downstream rolling stand L of the multiple rolling stands of the tandem rolling mill, σ defined by the formula (1) Ｌ is set to 30 to 70. In this case, after light reduction in rolling stand L-1, heavy reduction is performed in the most downstream rolling stand L. As a result, recrystallization occurs all at once in the steel sheet after being reduced in the most downstream rolling stand L, and ferrite grains of uniform size are generated in the sheet thickness direction and rolling direction.

[0101] σ defined by equation (1) ｎ is a measure of the stress imparted to the rough bar in rolling stand n. Ｌ－１ If σ is less than 5, the reduction in the rolling stand L-1 is insufficient. In this case, unrecrystallized and recrystallized grains are mixed. As a result, the GOS value becomes excessively large and exceeds 2.0°. On the other hand, σ Ｌ－１ If the rolling reduction in the rolling stand L-1 exceeds 20, the rolling reduction in the rolling stand L-1 is excessive. In this case, recrystallization occurs on the exit side of the rolling stand L-1 but on the entry side of the rolling stand L. As a result, the GOS value becomes excessively large and exceeds 2.0°.

[0102] σ Ｌ If σ is less than 30, the amount of strain imparted to the steel sheet is insufficient. In this case, unrecrystallized grains remain. As a result, the average grain size of ferrite grains becomes less than 5.0 μm. Furthermore, the GOS value becomes excessively large and exceeds 2.0°. On the other hand, σ Ｌ If the strain exceeds 70, the amount of strain applied to the steel sheet is excessive. In this case, the average grain size of ferrite grains becomes less than 5.0 μm. Furthermore, the GOS value becomes excessively large and exceeds 2.0°.

[0103] σ Ｌ－１ is set to 5 to 20, and further, σ Ｌ By setting the value of 30 to 70, it is possible to suppress the occurrence of recrystallization in the rough bar that passes through rolling stand L-1 and before passing through rolling stand L, and to cause recrystallization to occur all at once in the hot-rolled steel sheet that has passed through rolling stand L. In this case, recrystallization occurs at approximately the same time in the thickness direction of the steel sheet and in the direction perpendicular to the thickness direction. As a result, it is possible to suppress the excessive accumulation of strain in the ferrite grains of the produced steel sheet, and the GOS value becomes 2.0° or less.

[0104] [(Condition 5) Cumulative Reduction R in Hot Rolling Process] The cumulative reduction R (%) in the hot rolling process is defined as follows: R = (1 - t1 / t0) x 100, where t0 is the plate thickness (mm) of the slab, and t1 is the plate thickness (mm) of the hot-rolled steel plate after finish rolling.

[0105] If the cumulative reduction rate R is less than 60%, the reduction in the hot rolling process is insufficient. In this case, recrystallization does not occur sufficiently. As a result, the average grain size of ferrite grains exceeds 30.0 μm. Furthermore, the strain within the grains is not sufficiently relieved, and the GOS value exceeds 2.0°. Therefore, the cumulative reduction rate R is 60% or more.

[0106] [(Condition 6) Coiling Temperature CT] In the coiling process, the surface temperature of the steel sheet at the start of coiling is defined as the coiling temperature CT (°C). If the coiling temperature CT exceeds 650°C, the ferrite grains become coarse. As a result, the average grain size of the ferrite grains exceeds 30.0 μm. On the other hand, if the coiling temperature CT is less than 550°C, the ferrite grains become excessively fine. As a result, the average grain size of the ferrite grains becomes less than 5.0 μm. Therefore, the coiling temperature CT is 650 to 550°C.

[0107] [(Condition 7) Annealing temperature T ３ and retention time t ３ Regarding the annealing temperature T ３ is set to 600 to 730 ° C., and the annealing temperature T ３ Holding time t ３ The holding time t is set to 20 hours or more. ３ The preferred upper limit of the annealing temperature T is 50 hours. ３ is less than 600°C, or the holding time t ３ If the annealing time is less than 20 hours, the annealing is insufficient. In this case, the ferrite grains become excessively fine. As a result, the average grain size of the ferrite grains becomes less than 5.0 μm. On the other hand, if the annealing time is less than 20 hours, the annealing time is insufficient. In this case, the ferrite grains become excessively fine. As a result, the average grain size of the ferrite grains becomes less than 5.0 μm. ３ If the temperature exceeds 730° C., the ferrite grains become coarse, and as a result, the average grain size of the ferrite grains exceeds 30.0 μm.

[0108] The steel sheet of this embodiment is manufactured by the above manufacturing method.

[0109] The effects of the steel sheet of this embodiment will be described in more detail below using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel sheet of this embodiment. Therefore, the steel sheet of this embodiment is not limited to this one example of conditions.

[0110] Slabs having the chemical compositions shown in Table 1 were prepared.

[0111]

[0112] Specifically, molten steel was continuously cast into a slab. The slab was then subjected to a hot rolling process. Specifically, the slab was heated at a heating temperature T １１ The slab was heated at a temperature of 1000 K (°C) (Condition 1). The slab was then rough rolled using a reverse rolling mill to produce a rough bar (intermediate steel plate). The rough bar was then finish rolled using a tandem rolling mill consisting of multiple rolling stands to produce a hot-rolled steel plate. １２ (°C) (Condition 2), finish rolling temperature FT (°C) (Condition 3), σ at the rolling stand L-1 in the stage preceding the most downstream rolling stand L Ｌ－１ and σ at the most downstream rolling stand L Ｌ The cumulative reduction rate R (%) in the hot rolling process (Condition 4) and the cumulative reduction rate R (%) in the hot rolling process (Condition 5) were as shown in Table 2.

[0113]

[0114] The hot-rolled steel sheet after the hot rolling process was subjected to a coiling process. The coiling temperature CT (°C) (Condition 6) was as shown in Table 2. The steel sheet after the coiling process was subjected to an annealing process. The annealing temperature T ３ (°C) and annealing temperature T ３ Holding time t ３ (Time) (Condition 7) were as shown in Table 2. Steel plates with each test number were manufactured by the above manufacturing process.

[0115] [Evaluation Tests] The following evaluation tests were carried out on the steel sheets with each test number: (Test 1) Test for measuring the chemical composition of the steel sheets (Test 2) Test for measuring the total area ratio of ferrite and cementite particles in the steel sheets (Test 3) Test for measuring the average grain size of ferrite in the steel sheets (Test 4) Test for measuring the GOS value of the steel sheets (Test 5) Vickers hardness test (Test 6) Ductility test Tests 1 to 6 will be described below.

[0116] [(Test 1) Test for measuring the chemical composition of steel sheets] The chemical composition of the steel sheets of each test number was measured according to the method described in the above [Method for measuring the chemical composition of steel sheets]. As a result, the chemical compositions of each test number were as shown in Table 1.

[0117] [(Test 2) Measurement test of total area ratio of ferrite and cementite particles in steel sheet] The total area ratio (%) of ferrite and cementite particles in the steel sheet of each test number was determined according to the method described in the above-mentioned [Method for measuring total area ratio of ferrite and cementite particles in microstructure]. The obtained total area ratios are shown in Table 3. In all test numbers, the total area ratio of ferrite and cementite particles was 95% or more.

[0118]

[0119] [(Test 3) Measurement of average ferrite grain size in steel sheet] The average ferrite grain size (μm) of the steel sheet of each test number was determined according to the method described in the above [Method for measuring average ferrite grain size]. The obtained results are shown in Table 3.

[0120] [(Test 4) Measurement of GOS Value of Steel Sheet] The GOS value of the steel sheet having each test number was determined based on the above-mentioned [Method for Measuring GOS Value]. The obtained results are shown in Table 3.

[0121] [(Test 5) Vickers Hardness Test] As an index of the cold workability of the steel plate of each test number, the Vickers hardness (HV) was determined by the following method. Test specimens were taken from 10 sampling positions arranged at a 500 mm pitch in the rolling direction at the center of the steel plate width. The size of the test specimen was 15 mm in the rolling direction × 30 mm in the width direction × plate thickness. Of the surfaces of the test specimens, the surface including the rolling direction and plate thickness direction (L cross section) was used as the measurement surface. On the measurement surface, when the plate thickness is t mm, a Vickers hardness test in accordance with JIS Z 2244-1 (2020) was performed at five measurement points in the plate thickness direction from the surface of the steel plate: t / 8 depth position, t / 4 depth position, t / 2 depth position, 3t / 4 depth position, and 7t / 8 depth position. The test force was 98 N. The arithmetic average of the five hardness values ​​obtained was taken as the Vickers hardness of the test specimen. The arithmetic mean value of the Vickers hardness of the obtained 10 test pieces was taken as the Vickers hardness (HV) for that test number. The Vickers hardness was calculated by rounding the obtained result to the nearest integer. The obtained Vickers hardness (HV) is shown in the "Cold workability (Hardness) (HV)" column in Table 3.

[0122] [(Test 6) Ductility Test] A tensile test was conducted as follows to obtain an index of the cold workability of the steel sheet of each test number. A tensile test specimen was taken from the steel sheet including the center position of the sheet width. The tensile test specimen was a JIS No. 5 test specimen. The parallel portion was parallel to the rolling direction of the steel sheet. In accordance with JIS Z 2241 (2011), a tensile test was conducted at room temperature in air to determine the fracture elongation (%). The obtained fracture elongation (%) is shown in the "Cold workability (ductility) (%)" column in Table 3.

[0123] [Test Results] Referring to Tables 1 to 3, Test Nos. 1 to 29 satisfied Features 1 to 4. Therefore, the Vickers hardness was in the range of 110 to 160 HV and the elongation at break was 40% or more. Therefore, excellent cold workability was obtained.

[0124] In test number 30, the C content was too high, resulting in a Vickers hardness exceeding 160 HV and a breaking elongation of less than 40%, meaning that sufficient cold workability was not obtained.

[0125] In test number 31, the C content was too low, resulting in a Vickers hardness of less than 110 HV and excessively low strength.

[0126] In test number 32, the Si content was too high, resulting in a Vickers hardness exceeding 160 HV and a fracture elongation of less than 40%, meaning that sufficient cold workability was not obtained.

[0127] In test number 33, the Si content was too low, resulting in a Vickers hardness of less than 110 HV and excessively low strength.

[0128] In test number 34, the Mn content was too high, resulting in a Vickers hardness exceeding 160 HV and a fracture elongation of less than 40%, meaning that sufficient cold workability was not obtained.

[0129] In test number 35, the Mn content was too low, resulting in a Vickers hardness of less than 110 HV and excessively low strength.

[0130] In test number 36, the Cr content was too high, resulting in a Vickers hardness exceeding 160 HV and a fracture elongation of less than 40%, meaning that sufficient cold workability was not obtained.

[0131] In test number 37, the Cr content was too low, resulting in a Vickers hardness of less than 110 HV and excessively low strength.

[0132] In test number 38, the heating temperature T １１ As a result, the average grain size of the ferrite grains exceeded 30.0 μm, and as a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0133] In test number 39, the heating temperature T １１ The average grain size of the ferrite grains was less than 5.0 μm. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so that sufficient cold workability was not obtained.

[0134] In test number 40, the final finish rolling start temperature T １２The average grain size of the ferrite grains was less than 5.0 μm. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so that sufficient cold workability was not obtained.

[0135] In Test No. 41, the finish rolling temperature FT was too low. Therefore, the average grain size of the ferrite grains was less than 5.0 μm. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so sufficient cold workability was not obtained.

[0136] In test number 42, the finish rolling temperature FT was too high, which caused the average grain size of ferrite grains to exceed 30.0 μm, resulting in a Vickers hardness of less than 110 HV and excessively low strength.

[0137] In test number 43, the rolling stand L-1 before the most downstream rolling stand L was subjected to σ defined by the formula (1). Ｌ－１ The GOS value was less than 5. Therefore, the GOS value exceeded 2.0°. As a result, the elongation at break was less than 40%, and sufficient cold workability was not obtained.

[0138] In test number 44, σ Ｌ－１ exceeded 20. Therefore, the GOS value exceeded 2.0°. As a result, the elongation at break was less than 40%, and sufficient cold workability was not obtained.

[0139] In test number 45, the rolling speed was determined by the following equation (1) in the most downstream rolling stand L. Ｌ The value of GOS exceeded 70. Therefore, the average grain size of ferrite grains was less than 5.0 μm. Furthermore, the GOS value exceeded 2.0°. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so that sufficient cold workability was not obtained.

[0140] In test number 46, the rolling speed was determined by the following equation (1) in the most downstream rolling stand L. Ｌ The average grain size of the ferrite grains was less than 5.0 μm. Furthermore, the GOS value exceeded 2.0°. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so that sufficient cold workability was not obtained.

[0141] In test number 47, the cumulative rolling reduction R was less than 60%. Therefore, the average grain size of ferrite grains exceeded 30.0 μm. Furthermore, the GOS value exceeded 2.0°. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low. Furthermore, the fracture elongation was less than 40%, and sufficient cold workability was not obtained.

[0142] In Test No. 48, the coiling temperature CT was high. Therefore, the average grain size of ferrite grains exceeded 30.0 μm. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0143] In Test No. 49, the coiling temperature CT was low. Therefore, the average grain size of ferrite grains was less than 5.0 μm. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so sufficient cold workability was not obtained.

[0144] In test number 50, the annealing temperature T ３ Therefore, the average grain size of the ferrite grains exceeded 30.0 μm. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0145] In test number 51, the annealing temperature T ３ Therefore, the average grain size of the ferrite grains was less than 5.0 μm. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so that sufficient cold workability was not obtained.

[0146] In test number 52, the retention time t ３ Therefore, the average grain size of the ferrite grains was less than 5.0 μm. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so that sufficient cold workability was not obtained.

[0147] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A steel plate having a chemical composition, in mass%, of C: 0.20 to 0.70%, Si: 0.07 to 1.00%, Mn: 0.20 to 3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010 to 1.500%, acid-soluble Al: 0.005 to 0.070%, N: 0.0200% or less, Ti: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0035%, Cu: 0 to 0.20%, W: 0 to 0.03%, Ta: 0 to 0.03%, Sn: 0 to 0.030%, Sb: 0 to 0.030%, A steel sheet comprising: Co: 0-0.030%, As: 0-0.030%, Mg: 0-0.030%, Y: 0-0.030%, Zr: 0-0.030%, La: 0-0.030%, Ce: 0-0.030%, and Ca: 0-0.030%, with the balance being Fe and impurities; in a microstructure, a total area ratio of ferrite and cementite particles is 95% or more; the average grain size of the ferrite grains is 5.0-30.0 μm; and the ferrite grains have a GOS (Grain Orientation Spread) value of 2.0° or less.

2. A steel sheet according to claim 1, wherein the chemical composition is, in mass%, Ti: 0.001 to 0.500%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, B: 0.0001 to 0.0035%, Cu: 0.01 to 0.20%, W: 0.01 to 0.03%, Ta: 0.01 to 0.03%, Sn: 0.001 to 0.030%, Sb: 0.001 to 0.030%, Co: 0.001 to 0.030%, As: 0.001 to 0.030%, Mg: 0.001 to 0.030%, Y: 0.001 to 0.030%, A steel plate containing one or more elements selected from the group consisting of Zr: 0.001 to 0.030%, La: 0.001 to 0.030%, Ce: 0.001 to 0.030%, and Ca: 0.001 to 0.030%.

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