Steel plate

Abstract

A steel plate having excellent cold workability is provided. This steel plate contains, in terms of mass%, 0.20-0.70% of C, 0.07-1.00% of Si, 0.20-3.00% of Mn, up to 0.030% of P, up to 0.0080% of S, 0.010-1.500% of Cr, 0.005-0.070% of acid-soluble Al, and up to 0.0200% of N and has an average ferrite grain diameter Dt of 5.00-30.00 μm. When the thickness of the steel plate is expressed by t (mm), then the average ferrite grain diameter Dt / 8 (μm) at a depth of t / 8 mm in the thickness direction from a surface of the steel plate and the average ferrite grain diameter Dt / 2 (μm) at a depth of t / 2 mm satisfy formula (1). At a depth of t / 8 mm, the ferrite has a sample standard deviation St / 8 of grain diameter of 4.50 μm or less. At a depth of t / 2 mm, the ferrite has a sample standard deviation St / 2 of grain diameter of 4.00 μm or less.　(1): 0.90≤Dt / 8 / Dt / 2≤1.10

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, and the average grain size D of ferrite is 0.030% or more. ｔ When the thickness of the steel plate is t (mm), the average grain size D of ferrite at a depth of t / 8 mm from the surface of the steel plate in the plate thickness direction is ｔ／８ (μm), and the average grain size D of ferrite at a depth of t / 2 mm from the surface of the steel sheet in the sheet thickness direction ｔ／２ (μm) satisfies the formula (1). The sample standard deviation S of the ferrite grain size at the depth position of t / 8 mm ｔ／８ is 4.50 μm or less, and the sample standard deviation S of the ferrite grain size at the depth position of t / 2 mm ｔ／２ is 4.00 μm or less. ｔ／８ / D ｔ／２ ≦1.10 (1)

[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. In order to improve the cold workability of such a steel sheet, the inventors focused on the size of the ferrite grains. If the ferrite grains are too small, the cold workability deteriorates. As a result of further investigation, the inventors concluded that if the average grain size of ferrite is 5.00 μm or more in the above-described chemical composition, the cold workability will be improved.

[0014] However, in steel sheets satisfying the above-mentioned chemical composition, sufficient cold workability could not be obtained simply by making the average grain size of ferrite 5.00 μm or more. Therefore, the present inventors conducted further studies.

[0015] Here, the inventors focused on the variation in the size of individual ferrite grains in a steel sheet. For example, if there is a large variation in the size of ferrite grains at the same depth from the surface of a steel sheet, regions with different amounts of plastic deformation will occur locally during cold working. In this case, uniform plastic deformation will not occur, and non-uniform plastic deformation will occur. As a result, cold workability will be reduced.

[0016] Therefore, the inventors considered that suppressing the variation in ferrite grains in the same region at the same depth position from the surface of the steel plate in the plate thickness direction as much as possible would improve the cold workability. As a result of further investigation, when the thickness of the steel plate is t mm, the sample standard deviation S of the ferrite grain size at a depth position of t / 8 mm from the surface of the steel plate in the plate thickness direction is ｔ／８ is 4.50 μm or less, and the sample standard deviation S of the ferrite grain size at a depth of t / 2 mm from the surface of the steel plate in the plate thickness direction ｔ／２ It was thought that if the thickness is 4.00 μm or less, further improvement in cold workability can be obtained.

[0017] However, even if the variation in ferrite grains at the same depth position is suppressed, there are cases where sufficient cold workability is still not obtained. Therefore, the inventors thought that not only the variation in ferrite grains in the region at the same depth position but also the variation in ferrite grains in the sheet thickness direction affects the cold workability. Therefore, as a result of further investigation, it was found that the average grain size D of ferrite at a depth of t / 8 mm from the surface of the steel sheet in the sheet thickness direction is ｔ／８ (μm), and the average grain size D of ferrite at a depth of t / 2 mm from the surface of the steel sheet in the sheet thickness direction ｔ／２ It has been found that excellent cold workability can be obtained if the following formula (1) is satisfied: 0.90≦D ｔ／８ / D ｔ／２ ≦1.10 (1)

[0018] The steel sheet of this embodiment has been completed based on the above technical concept and has the following configuration.

[0019] 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 being Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more, and the average grain size D of ferrite is 0.5% or more. ｔ When the thickness of the steel plate is t (mm), the average grain size D of ferrite at a depth of t / 8 mm from the surface of the steel plate in the plate thickness direction is ｔ／８ (μm), and the average grain size D of ferrite at a depth of t / 2 mm from the surface of the steel sheet in the sheet thickness direction ｔ／２ (μm) satisfies the formula (1). The sample standard deviation S of the ferrite grain size at the depth position of t / 8 mm ｔ／８ is 4.50 μm or less, and the sample standard deviation S of the ferrite grain size at the depth position of t / 2 mm ｔ／２ is 4.00 μm or less. ｔ／８ / D ｔ／２ ≦1.10 (1)

[0020] The steel sheet of the second embodiment is the steel sheet of the first embodiment, further comprising 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 0.0 30%, 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%.

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

[0022] [Features of Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment satisfies the following features 1 to 5. (Feature 1) The chemical composition is, 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: 0.010 to 1.500%, and the steel sheet of the present embodiment satisfies the following features 1 to 5. : 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 grain size D of ferrite ｔ (Feature 4) When the thickness of the steel plate is t (mm), the average grain size D of ferrite at a depth of t / 8 mm from the surface of the steel plate in the plate thickness direction is ｔ／８(μm), and the average grain size D of ferrite at a depth of t / 2 mm from the surface of the steel sheet in the sheet thickness direction ｔ／２ (μm) satisfies the formula (1). 0.90≦D ｔ／８ / D ｔ／２ ≦1.10 (1) (Feature 5) The sample standard deviation S of the ferrite grain size at a depth of t / 8 mm from the surface of the steel plate in the plate thickness direction ｔ／８ is 4.50 μm or less, and the sample standard deviation S of the ferrite grain size at a depth of t / 2 mm from the surface of the steel plate in the plate thickness direction ｔ／２ The thickness is 4.00 μm or less. Features 1 to 5 will be explained below.

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

[0024] 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%.

[0025] 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%.

[0026] 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%.

[0027] 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 sheet 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%.

[0028] 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%, S forms excessively large amounts of sulfides. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the S content is 0.0080% or less. The S content is preferably 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 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the S content is preferably 0.0075%, 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%.

[0029] 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%.

[0030] 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 (sol. Al) content is less than 0.005%, the above-mentioned 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 acid-soluble Al content exceeds 0.070%, Al nitrides are excessively formed, resulting in refinement of austenite grains. In this case, 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 upper limit of the acid-soluble Al content is preferably 0.065%, more preferably 0.060%, even more preferably 0.055%, and still more preferably 0.050%.

[0031] 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 AlN. AlN refines austenite grains during heating for quenching in the process of manufacturing machine 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. The 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%.

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

[0033] [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.

[0034] [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.

[0035] 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%.

[0036] 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%.

[0037] 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%.

[0038] 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%.

[0039] [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%.

[0040] 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%.

[0041] 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%.

[0042] The lower limit of the W content is preferably 0.01%, and the upper limit of the W content is preferably 0.02%.

[0043] The lower limit of the Ta content is preferably 0.01%, and the upper limit of the Ta content is preferably 0.02%.

[0044] 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%.

[0045] 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%.

[0046] 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%.

[0047] 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%.

[0048] 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%.

[0049] 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%.

[0050] 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%.

[0051] 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%.

[0052] 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%.

[0053] 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%.

[0054] [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.

[0055] 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.

[0056] 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.

[0057] [(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.

[0058] 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.

[0059] 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.

[0060] 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%.

[0061] If the total area ratio of ferrite and cementite particles is 95% or more, excellent cold workability can be obtained, provided that Feature 1 and Feature 3 to Feature 5 are satisfied.

[0062] [Method for measuring the total area ratio of ferrite and cementite particles in the microstructure] The total area ratio of ferrite and cementite particles in the microstructure can be measured by the following method. A test specimen 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 specimen 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 specimen. The observation surface of the test specimen 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 has its center centered at the center position of the plate thickness. The five observation fields are arranged consecutively in the direction perpendicular to the plate thickness, and the center position of the central observation field of the five arranged observation fields is set to the center position of the plate width of the steel plate.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] [(Feature 3) Average grain size of ferrite D ｔ Regarding the average grain size D of ferrite in the steel sheet of this embodiment, ｔ is 5.00 to 30.00 μm. The average grain size of ferrite affects cold workability. If the average grain size of ferrite is less than 5.00 μm, the ferrite is excessively fine grained. In this case, sufficient cold workability cannot be obtained. If the average grain size of ferrite is 5.00 μm or more, excellent cold workability can be obtained in the steel sheet, provided that Features 1, 2, 4, and 5 are satisfied.

[0067] The lower limit of the average particle size of ferrite is preferably 5.20 μm, more preferably 5.50 μm, even more preferably 5.70 μm, even more preferably 6.00 μm, and even more preferably 6.50 μm.

[0068] The upper limit of the average grain size of ferrite is not particularly limited. However, if the average grain size of ferrite is excessively large, the strength of the steel sheet becomes 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 is 30.00 μm.

[0069] The upper limit of the average particle size of ferrite is preferably 29.00 μm, more preferably 28.00 μm, even more preferably 27.00 μm, even more preferably 26.00 μm, even more preferably 24.00 μm, even more preferably 22.00 μm, even more preferably 20.00 μm, even more preferably 19.00 μm, and even more preferably 18.50 μm.

[0070] [(Feature 4) Surface ferrite grain size D ｔ／８ and ferrite grain size D at the center of the plate thickness ｔ／２ Regarding the steel plate of this embodiment, when the thickness of the steel plate is t (mm), the average grain size of ferrite at a depth of t / 8 mm from the surface of the steel plate in the plate thickness direction is referred to as the "surface ferrite grain size D ｔ／８ The ferrite grains at a depth of t / 8 mm from the surface of the steel sheet in the thickness direction are also referred to as "surface ferrite grains." The average grain size of ferrite at a depth of t / 2 mm from the surface of the steel sheet in the thickness direction is defined as "mid-thickness ferrite grain size D ｔ／２ The ferrite grains at a depth of t / 2 mm from the surface of the steel sheet in the sheet thickness direction are also referred to as "sheet thickness center ferrite grains." In this case, the surface ferrite grain size D ｔ／８ (μm) and thickness center ferrite grain size D ｔ／２ (μm) satisfies the formula (1). 0.90≦D ｔ／８ / D ｔ／２ ≦1.10 (1)

[0071] F1 is defined as follows: F1 = D ｔ／８ / D ｔ／２ F1 is the surface ferrite grain size D ｔ／８ and ferrite grain size D at the center of the plate thickness ｔ／２As mentioned above, the cold workability is also affected by the variation in ferrite grains in the steel sheet. Specifically, the cold workability improves when the size of the ferrite grains in the thickness direction is as uniform as possible.

[0072] If F1 is less than 0.90 or more than 1.10, the variation in size between the surface ferrite grains and the ferrite grains at the center of the sheet thickness is too large, and in this case, sufficient cold workability cannot be obtained.

[0073] On the other hand, if F1 is 0.90 to 1.10, the variation in size between the surface ferrite grains and the central ferrite grains in the sheet thickness direction is sufficiently small, and therefore, provided that Features 1 to 3 and 5 are satisfied, the steel sheet can have excellent cold workability.

[0074] The lower limit of F1 is preferably 0.92, more preferably 0.94, even more preferably 0.96, and even more preferably 0.97. The upper limit of F1 is preferably 1.08, more preferably 1.06, even more preferably 1.04, and even more preferably 1.03.

[0075] (Feature 5) Sample standard deviation S of surface ferrite grains ｔ／８ and the sample standard deviation S of the ferrite grains at the center of the plate thickness ｔ／２ In this embodiment, the sample standard deviation S of the ferrite grain size at a depth of t / 8 mm from the surface of the steel plate in the plate thickness direction is further ｔ／８ is 4.50 μm or less, and the sample standard deviation S of the ferrite grain size at a depth of t / 2 mm from the surface of the steel plate in the plate thickness direction ｔ／２ The characteristic 5 will be described below.

[0076] The sample standard deviation of the ferrite grain size at a depth of t / 8 mm from the surface of the steel plate in the plate thickness direction is referred to as the "surface ferrite grain deviation S ｔ／８ ". The sample standard deviation of the ferrite grain size at a depth of t / 2 mm from the surface of the steel plate in the plate thickness direction is defined as "plate thickness center ferrite grain deviation S ｔ／２ " is defined as

[0077] Feature 4 above specifies the variation of ferrite grains in the thickness direction of the steel plate. On the other hand, Feature 5 specifies the variation of ferrite grains in the rolling direction, which is the direction perpendicular to the thickness direction of the steel plate. Suppressing the variation of ferrite grains not only in the thickness direction of the steel plate but also in the direction perpendicular to the thickness direction improves cold workability.

[0078] Surface ferrite grain deviation S ｔ／８ is more than 4.50 μm, or the thickness center ferrite grain deviation S ｔ／２ If the diameter exceeds 4.00 μm, the ferrite grains will vary too much in the direction perpendicular to the thickness direction, and sufficient cold workability will not be obtained.

[0079] On the other hand, the surface ferrite grain deviation S ｔ／８ is 4.50 μm or less, and the thickness center ferrite grain deviation S ｔ／２ When the average grain size is 4.00 μm or less, the variation in ferrite grains is sufficiently small not only in the sheet thickness direction but also in the direction perpendicular to the sheet thickness direction, provided that Features 1 to 4 are satisfied. Therefore, the steel sheet can have excellent cold workability.

[0080] Surface ferrite grain deviation S ｔ／８ The upper limit of the surface layer ferrite grain deviation S is preferably 4.40 μm, more preferably 4.35 μm, even more preferably 4.30 μm, even more preferably 4.20 μm, even more preferably 4.10 μm, even more preferably 4.00 μm, even more preferably 3.80 μm, even more preferably 3.70 μm, and even more preferably 3.60 μm. ｔ／８ However, the surface ferrite grain deviation S ｔ／８ If the deviation S of the surface ferrite grains is too small, the manufacturing cost will be too high. ｔ／８ The lower limit of the thickness is preferably 0.10 μm, more preferably 0.30 μm, and even more preferably 0.50 μm.

[0081] Ferrite grain deviation S at the center of plate thickness ｔ／２The preferred upper limit of the thickness center ferrite grain deviation S is 3.90 μm, more preferably 3.80 μm, even more preferably 3.75 μm, even more preferably 3.50 μm, even more preferably 3.30 μm, and even more preferably 3.10 μm. ｔ／２ However, the smaller the deviation of ferrite grains at the center of the thickness, S ｔ／２ If the deviation S is made too small, the manufacturing cost will be excessively high. ｔ／２ The lower limit of the thickness is preferably 0.10 μm, more preferably 0.30 μm, and even more preferably 0.50 μm.

[0082] [Average grain size of ferrite D ｔ , surface ferrite grain size D ｔ／８ , ferrite grain size at the center of the plate thickness D ｔ／２ , surface ferrite grain deviation S ｔ／８ , and the thickness center ferrite grain deviation S ｔ／２ Regarding the measurement method of the average particle diameter D of ferrite related to Features 3 to 5 ｔ , surface ferrite grain size D ｔ／８ , ferrite grain size at the center of the plate thickness D ｔ／２ , surface ferrite grain deviation S ｔ／８ , and the thickness center ferrite grain deviation S ｔ／２ is measured in the following way:

[0083] A test piece having a cross section including the rolling direction and thickness direction of the steel plate is taken from the steel plate. The thickness of the test piece is the thickness of the steel plate. The cross section of the surface of the test piece in the rolling direction and thickness direction of the steel plate is used as the observation surface. The observation surface is mirror-polished. After mirror-polishing, etching is performed with a 3% nital etching solution. The etching time is 120 seconds.

[0084] Five rectangular observation fields of 100 μm in the thickness direction and 500 μm in the rolling direction are selected from the observation surface at a depth of t / 2 from the surface of the steel plate in the thickness direction. These observation fields are called "thickness center observation fields." The center position of the thickness center observation field in the thickness direction corresponds to a depth of t / 2 from the surface of the steel plate in the thickness direction. The five thickness center observation fields are arranged continuously in the rolling direction.

[0085] Furthermore, five rectangular observation fields of view, each measuring 100 μm in the thickness direction and 500 μm in the rolling direction, are selected from the observation surface at a depth of t / 8 from the surface of the steel sheet in the thickness direction. These observation fields are referred to as "surface layer observation fields." The center position of the surface layer observation fields in the thickness direction corresponds to a depth of t / 8 from the surface of the steel sheet in the thickness direction. The five surface layer observation fields are arranged continuously in the rolling direction.

[0086] Secondary electron images are taken using a scanning electron microscope (SEM) in each observation field (plate thickness center observation field and surface layer observation field). Then, the circle-equivalent diameter of the ferrite grains identified in each observation field is determined. At this time, if at least a portion of the identified ferrite grain is in contact with or intersects with the rectangular edge (side) of the observation field, the ferrite grain is excluded from the measurement target. In other words, a ferrite grain that is entirely located inside the rectangular edge of the observation field and is not in contact with the edge is the measurement target. The circle-equivalent diameter (μm) of each ferrite grain to be measured is determined. Here, the circle-equivalent diameter means the diameter of a circle with the same area as the ferrite grain.

[0087] The arithmetic mean value of the circle equivalent diameters of all ferrite grains at the center of the plate thickness that were measured in the five central observation fields of the plate thickness and the circle equivalent diameters of all surface layer ferrite grains that were measured in the five surface layer observation fields was calculated as the average ferrite grain diameter D ｔ (μm) Average grain size of ferrite D ｔ is the value obtained by rounding off to the second decimal place.

[0088] The arithmetic mean value of the circle equivalent diameters of all the surface layer ferrite grains measured in the five surface layer observation fields was defined as the surface layer ferrite grain diameter D ｔ／８ The arithmetic mean value of the circle equivalent diameters of all the ferrite grains at the center of the thickness that were measured in the five central observation fields of the thickness was defined as the center ferrite grain diameter D ｔ／２ (μm) Surface ferrite grain size D ｔ／８ and ferrite grain size D at the center of the plate thickness ｔ／２ The value of F1 (= D) is the value obtained by rounding off the third decimal place of the arithmetic mean value to the second decimal place.ｔ／８ / D ｔ／２ ) shall be the value obtained by rounding off the calculated value to two decimal places.

[0089] The sample standard deviation of all surface ferrite grains measured in the five surface observation fields is called the surface ferrite grain deviation S ｔ／８ The sample standard deviation of all the ferrite grains at the center of the thickness that were measured in the five central observation fields of the thickness is called the center ferrite grain deviation S ｔ／２ (μm) Surface ferrite grain deviation S ｔ／８ and thickness center ferrite grain deviation S ｔ／２ The value of shall be the value obtained by rounding off the arithmetic mean to two decimal places.

[0090] [Effects of the Steel Sheet of the Present Embodiment] The steel sheet of the present embodiment satisfies Features 1 to 5. Therefore, the steel sheet of the present embodiment has excellent cold workability.

[0091] [Applications of Steel Sheet] The steel sheet of this embodiment is used as a raw material for cold-rolled steel sheets 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.

[0092] [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.

[0093] 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.

[0094] [(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 is produced, the contents of each element in the chemical composition of which fall within the ranges of this embodiment. The molten steel is used to produce a slab by a casting method. For example, a slab having a chemical composition that satisfies Feature 1 is produced by a well-known continuous casting method using the molten steel.

[0095] 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.

[0096] [(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.

[0097] [(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.

[0098] [(Step 12) Finish Rolling Step] In the finish rolling step, a tandem rolling mill is used to perform finish rolling on the rough bar to produce a hot-rolled steel sheet. 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 sheet after rough rolling.

[0099] [(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.

[0100] [(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.

[0101] [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 used in the finish rolling step is 850 to 1000° C. (Condition 4) Of the multiple rolling stands of the tandem rolling mill, the σ defined by formula (2) in the rolling stand L-1 in the front stage of the most downstream rolling stand L is Ｌ－１ is set to 21 to 35. Furthermore, in the most downstream rolling stand L of the multiple rolling stands of the tandem rolling mill, σ defined by the formula (2) Ｌ is set to 71 to 90. σ ｎ =exp(0.753+3000 / (T ｎ + 273)) × ε ｎ ０．２１ ×v ｎ ０．１３ (2) where n is 1 to L and ε ｎ is 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 (2) is applied to rolling stand L-1, n in formula (2) is L-1. When formula (2) is applied to rolling stand L, n in formula (2) is L. When formula (2) is applied to rolling stand L, T in formula (2) ｎ (°C) indicates the final finish rolling start temperature T １２ On the other hand, when applying the formula (2) to the rolling stand L-1, Tn (°C) in the formula (2) 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.

[0102] [(Condition 1) Heating temperature T １１ Regarding the heating temperature T １１ If the temperature is less than 1100°C, the ferrite in the produced steel sheet will be too small. ｔ On the other hand, the heating temperature T １１ If the temperature exceeds 1350°C, the ferrite in the produced steel sheet will become coarse. As a result, the average grain size D of ferrite ｔ exceeds 30.00 μm. Heating temperature T １１ If the temperature is 1100 to 1350°C, the average grain size D of ferrite in the steel sheet after manufacturing is ｔ is 5.00 to 30.00 μm.

[0103] [(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 １２ is measured by a temperature gauge arranged on the inlet side of the most downstream rolling stand L which applies the final rolling reduction to the intermediate steel plate.

[0104] Final finish rolling start temperature T １２ If the temperature is less than 850°C, the ferrite in the produced steel sheet will be too small. ｔis less than 5.00 μm. Furthermore, the shear strain in the surface layer portion of the sheet thickness becomes larger than that in the central portion of the sheet thickness. Therefore, the ferrite grains in the surface layer portion of the sheet thickness become excessively small compared to the ferrite grains in the central portion of the sheet thickness. As a result, F1 becomes less than the lower limit of formula (1). Therefore, the final finish rolling start temperature T １２ The temperature is set to 850°C or higher.

[0105] [(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 constitute 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.

[0106] If the finish rolling temperature FT is less than 850°C, the ferrite in the produced steel sheet will be too small. As a result, the average grain size D of ferrite ｔ is less than 5.00 μm. Furthermore, the shear strain in the surface layer portion of the plate thickness becomes larger than that in the central portion of the plate thickness. Therefore, the ferrite grains in the surface layer portion of the plate thickness become excessively small compared to the ferrite grains in the central portion of the plate thickness. As a result, F1 becomes less than the lower limit of formula (1). On the other hand, when the finish rolling temperature FT exceeds 1000°C, the ferrite in the manufactured steel plate becomes coarse. As a result, the average grain size D of ferrite ｔ Therefore, the finish rolling temperature FT is set to 850 to 1000°C.

[0107] [(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. In the rolling stand L-1 preceding the most downstream rolling stand L, σ defined by the formula (2) Ｌ－１ is set to 21 to 35. Furthermore, in the most downstream rolling stand L of the plurality of rolling stands 1 to L of the tandem rolling mill, σ defined by the formula (2) is set to 21 to 35. Ｌis set to 71 to 90. 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 reduction in the most downstream rolling stand L, and ferrite grains of uniform size are generated in the sheet thickness direction and rolling direction.

[0108] σ defined by equation (2) ｎ is a measure of the stress imparted to the rough bar in rolling stand n. Ｌ－１ If the rolling reduction ratio is less than 21, the reduction ratio in the rolling stand L-1 is insufficient. In this case, the strain does not easily penetrate to the center of the plate thickness. Therefore, the average diameter D of the ferrite grains at the center of the plate thickness ｔ／２ is the average grain size D of the surface ferrite grains ｔ／８ Therefore, F1 is less than the lower limit of formula (1). Furthermore, since the strain does not sufficiently penetrate to the center of the thickness, the ferrite grain deviation S ｔ／２ exceeds 4.00 μm.

[0109] On the other hand, σ Ｌ－１ If F exceeds 35, the reduction in the rolling stand L-1 is too large. In this case, excessive recrystallization occurs at the exit side of the rolling stand L-1 but at the entry side of the rolling stand L. Therefore, at the exit side of the final rolling stand L, the driving force for recrystallization at the center of the thickness of the plate is insufficient. Therefore, F1 becomes less than the lower limit of formula (1). Furthermore, since the strain does not sufficiently penetrate to the center of the thickness of the plate, the center ferrite grain deviation S ｔ／２ exceeds 4.00 μm.

[0110] σ Ｌ If the rolling reduction in the rolling stand L exceeds 90, the ferrite grain size in the produced steel sheet becomes excessively small. ｔ is less than 5.00 μm. Ｌ If the rolling ratio is less than 71, the rolling reduction in the rolling stand L is insufficient. In this case, fine ferrite grains are generated from the unrecrystallized region. As a result, the average grain size D of ferrite ｔ Furthermore, the average particle size D of the surface layer ferrite grains is less than 5.00 μm. ｔ／８ is the average grain size of ferrite grains at the center of the plate thickness D ｔ／２Therefore, F1 is less than the lower limit of the formula (1).

[0111] [(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.

[0112] If the cumulative reduction rate R is less than 60%, the reduction in the hot rolling process is insufficient. The ferrite in the produced steel sheet becomes coarse. As a result, the average grain size Dt of ferrite exceeds 30.00 μm. Therefore, the cumulative reduction rate R is 60% or more.

[0113] [(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 in the produced steel sheet becomes coarse. As a result, the average grain size D of ferrite ｔ exceeds 30.00 μm. On the other hand, if the coiling temperature CT is less than 550°C, the ferrite in the produced steel sheet will be excessively small. As a result, the average grain size Dt of ferrite will be less than 5.00 μm. Therefore, the coiling temperature CT is 650 to 550°C.

[0114] [(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 If the holding time t is less than 600°C, the annealing is insufficient. In this case, the ferrite in the produced steel sheet becomes excessively small. As a result, the average grain size Dt of the ferrite becomes less than 5.00 μm. ３ If the annealing time is too short, the annealing is insufficient. In this case, the ferrite in the produced steel sheet becomes too small. As a result, the average grain size D of the ferrite ｔOn the other hand, the annealing temperature T ３ If the temperature exceeds 730°C, the ferrite in the produced steel sheet becomes coarse, and as a result, the average grain size Dt of the ferrite exceeds 30.00 µm.

[0115] The steel sheet of this embodiment is manufactured by the above manufacturing method.

[0116] 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.

[0117] Slabs having the chemical compositions shown in Table 1 were prepared.

[0118]

[0119] 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.

[0120]

[0121] 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.

[0122] [Evaluation Tests] The following evaluation tests were carried out on the steel sheets with each test number: (Test 1) Measurement of the chemical composition of the steel sheets (Test 2) Measurement of the total area ratio of ferrite and cementite particles in the steel sheets (Test 3) Measurement of the average ferrite grain size D of the steel sheets ｔ , surface ferrite grain size D ｔ／８ , ferrite grain size at the center of the plate thickness D ｔ／２ , surface ferrite grain deviation S ｔ／８ , and the thickness center ferrite grain deviation S ｔ／２ Measurement tests (Test 4) Vickers hardness test (Test 5) Ductility test Tests 1 to 5 will be explained below.

[0123] [(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.

[0124] [(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.

[0125]

[0126] [(Test 3) Average ferrite grain size D of steel plate ｔ , surface ferrite grain size D ｔ／８ , ferrite grain size at the center of the plate thickness D ｔ／２ , surface ferrite grain deviation S ｔ／８ , and the thickness center ferrite grain deviation S ｔ／２ Measurement test of the above-mentioned [Average particle size D of ferrite ｔ , surface ferrite grain size D ｔ／８ , ferrite grain size at the center of the plate thickness D ｔ／２ , surface ferrite grain deviation S ｔ／８ , and the thickness center ferrite grain deviation S ｔ／２The average ferrite grain size D of the steel sheet of each test number was measured in accordance with the method described in the section on the measurement method for the average ferrite grain size D of the steel sheet of each test number. ｔ (μm), surface ferrite grain size D ｔ／８ (μm), ferrite grain size at the center of the plate thickness D ｔ／２ (μm), surface ferrite grain deviation S ｔ／８ (μm), and the thickness center ferrite grain deviation S ｔ／２ The results are shown in Table 3.

[0127] [(Test 4) 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 (HV) of the test specimen. The arithmetic mean value of the Vickers hardness of the obtained 10 test pieces was taken as the Vickers hardness of the 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.

[0128] [(Test 5) Ductility Test] A tensile test was conducted as follows to obtain an index of the cold workability of the steel plate of each test number. A tensile test specimen was taken from the steel plate including the center position of the plate width. The tensile test specimen was a JIS No. 5 test specimen. The parallel portion was parallel to the rolling direction of the steel plate. 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.

[0129] [Test Results] Referring to Tables 1 to 3, Test Nos. 1 to 29 satisfied Features 1 to 5. 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.

[0130] On the other hand, 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.

[0131] In test number 31, the C content was too low, resulting in a Vickers hardness of less than 110 HV and excessively low strength.

[0132] 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.

[0133] In test number 33, the Si content was too low, resulting in a Vickers hardness of less than 110 HV and excessively low strength.

[0134] 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.

[0135] In test number 35, the Mn content was too low, resulting in a Vickers hardness of less than 110 HV and excessively low strength.

[0136] 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.

[0137] In test number 37, the Cr content was too low, resulting in a Vickers hardness of less than 110 HV and excessively low strength.

[0138] In test number 38, the heating temperature T １１ Therefore, the average grain size of ferrite D ｔ As a result, the Vickers hardness was less than 110 HV, and the strength was too low.

[0139] In test number 39, the heating temperature T １１ Therefore, the average grain size of ferrite D ｔ As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability was not obtained.

[0140] In test number 40, the final finish rolling start temperature T １２ was too low. Therefore, the average grain size Dt of ferrite was less than 5.00 μm. Furthermore, F1 was less than the lower limit of formula (1). As a result, the Vickers hardness exceeded 160 HV, the fracture elongation was less than 40%, and sufficient cold workability was not obtained.

[0141] In test number 41, the finish rolling temperature FT was too low. As a result, the average grain size Dt of ferrite was less than 5.00 μm. Furthermore, F1 was less than the lower limit of formula (1). As a result, the Vickers hardness exceeded 160 HV, the fracture elongation was less than 40%, and sufficient cold workability was not obtained.

[0142] In test number 42, the finish rolling temperature FT was too high. Therefore, the average grain size D of ferrite ｔ As a result, the Vickers hardness was less than 110 HV, and the strength was too low.

[0143] 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 was less than 21. Therefore, F1 was less than the lower limit of the formula (1). Furthermore, the thickness center ferrite grain deviation S ｔ／２ As a result, the elongation at break was less than 40%, and sufficient cold workability was not obtained.

[0144] In test number 44, σ Ｌ－１ exceeded 35. Therefore, F1 was less than the lower limit of the formula (1). Furthermore, the thickness center ferrite grain deviation S ｔ／２ As a result, the elongation at break was less than 40%, and sufficient cold workability was not obtained.

[0145] In test number 45, the rolling speed was determined by the following equation (1) in the most downstream rolling stand L. Ｌ The average grain size Dt of ferrite was less than 5.00 μm. As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability was not obtained.

[0146] In test number 46, the rolling speed was determined by the following equation (1) in the most downstream rolling stand L. Ｌ was less than 71. Therefore, the average grain size Dt of ferrite was less than 5.00μ. Furthermore, F1 was less than the lower limit of formula (1). Therefore, the Vickers hardness exceeded 160 HV, the fracture elongation was less than 40%, and sufficient cold workability was not obtained.

[0147] In test number 47, the cumulative rolling reduction R was less than 60%, and therefore the average grain size Dt of ferrite exceeded 30.00 μm. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0148] In test number 48, the coiling temperature CT was high. Therefore, the average grain size D of ferrite ｔ As a result, the Vickers hardness was less than 110 HV, and the strength was too low.

[0149] In test number 49, the coiling temperature CT was low. Therefore, the average grain size D of ferrite ｔ As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability was not obtained.

[0150] In test number 50, the annealing temperature T ３ Therefore, the average grain size of ferrite D ｔ As a result, the Vickers hardness was less than 110 HV, and the strength was too low.

[0151] In test number 51, the annealing temperature T ３ Therefore, the average grain size of ferrite D ｔAs a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability was not obtained.

[0152] In test number 52, the retention time t ３ Therefore, the average grain size of ferrite D ｔ As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability was not obtained.

[0153] 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%, 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; in the microstructure, the total area ratio of ferrite and cementite particles is 95% or more; and the average grain size D of ferrite is 100% or more. ｔ is 5.00 to 30.00 μm, and when the thickness of the steel plate is t (mm), the average grain size D of ferrite at a depth of t / 8 mm from the surface of the steel plate in the plate thickness direction is ｔ／８ (μm), and the average grain size D of the ferrite at a depth of t / 2 mm from the surface of the steel plate in the plate thickness direction ｔ／２ (μm) satisfies the formula (1), and the sample standard deviation S of the ferrite grain size at the t / 8 mm depth position ｔ／８ is 4.50 μm or less, and the sample standard deviation S of the grain size of the ferrite at the depth position of t / 2 mm ｔ／２ The steel sheet has a grain size of 4.00 μm or less. ｔ／８ / D ｔ／２ ≦1.10 (1) 2. The steel sheet according to claim 1, comprising: 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%, Zr: 0.001 to 0.030%, A steel plate containing one or more elements selected from the group consisting of La: 0.001 to 0.030%, Ce: 0.001 to 0.030%, and Ca: 0.001 to 0.030%.

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