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

[Technical Field]

[0001] This disclosure relates to steel sheets, and more particularly to steel sheets that can be used as materials for machine parts, such as automotive parts. [Background technology]

[0002] Steel sheets with a high carbon content (high-carbon steel sheets) are used as steel sheets for machine parts, such as automotive parts. Machine parts can have complex shapes and require high strength. The method for manufacturing machine parts using steel sheets for machine parts is as follows: Cold working is performed on the steel sheet to form the shape of the machine part. After cold working, the steel sheet is hardened and tempered. Through the above manufacturing process, high-strength machine parts are produced. As described above, steel sheets for machine parts are formed into the shape of machine parts 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 has a composition in mass% of C: 0.20-0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.Al: 0.10% or less, N: 0.0050% or less, B: 0.0005-0.0050%, and further contains one or more of Sb, Sn, Bi, Ge, Te, and Se in total of 0.002-0.030%, with the remainder being Fe and unavoidable impurities. In this steel sheet, the proportion of solid-solution B in the B content is 70% or more. Furthermore, the microstructure consists of ferrite and cementite. Furthermore, the cementite density within the ferrite grains is 0.08 grains / μm 2 The following is the case. In the steel sheet described in Patent Document 1, cold workability is improved by adjusting the cementite density within the ferrite grains. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2015 / 146173 [Overview of the project] [Problems that the invention aims to solve]

[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] The purpose of this disclosure is to provide a steel sheet with excellent cold workability. [Means for solving the problem]

[0008] The steel sheet disclosed herein has a chemical composition in mass percent of: C: 0.20-0.70%, Si: 0.07-1.00%, Mn: 0.20-3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010-1.500%, acid-soluble Al: 0.005-0.070%, N: 0.0200% or less, Ti: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, B: 0-0.0035%. It contains %, 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 remainder being Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more, and the average particle size of ferrite is D t The particle size is 5.00 to 30.00 μm. When the thickness of the steel plate is t (mm), the average particle size D of ferrite at a depth of t / 8 mm from the surface of the steel plate in the thickness direction is given by t. t / 8 (μm), and the average particle size D of ferrite at a depth of t / 2 mm from the surface of the steel plate in the thickness direction. t / 2 (μm) and satisfy equation (1). Sample standard deviation S of ferrite particle size at a depth of t / 8 mm. t / 8The sample standard deviation S of the ferrite particle size at a depth of t / 2 mm is 4.50 μm or less. t / 2 The particle size is 4.00 μm or less. 0.90≦D t / 8 / D t / 2 ≤1.10 (1) [Effects of the Invention]

[0009] The steel sheet disclosed herein offers excellent cold workability. [Modes for carrying out the invention]

[0010] The inventors of this invention investigated steel sheets that can be produced with excellent cold workability. As a result, the inventors obtained the following findings.

[0011] The inventors first investigated a suitable chemical composition for steel sheets used in machine parts, such as automotive parts. As a result, the inventors found that the following composition, in mass%, is suitable: C: 0.20-0.70%, Si: 0.07-1.00%, Mn: 0.20-3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010-1.500%, acid-soluble Al: 0.005-0.070%, N: 0.0200% or less, Ti: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, B: 0-0.0035%, Cu: 0-0.20%, W We considered a steel sheet suitable for machine parts applications if its chemical composition contained 0-0.03% of the following elements: 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 remainder being Fe and impurities.

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

[0013] The microstructure of the steel sheet having the above chemical composition is a structure 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 ferrite grains. If the ferrite grains are too small, the cold workability deteriorates. Therefore, as a result of further investigation, the inventors considered that if the average grain size of ferrite is 5.00 μm or more in the above chemical composition, the cold workability is enhanced.

[0014] However, in the steel sheet satisfying the above chemical composition, sufficient cold workability could not be obtained only by setting the average grain size of ferrite to 5.00 μm or more. Therefore, the inventors conducted further investigation.

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

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

[0017] However, even when the variation in ferrite grains at the same depth was suppressed, sufficient cold workability was still sometimes not obtained. Therefore, the inventors considered that not only the variation in ferrite grains in the region at the same depth, but also the variation in ferrite grains in the thickness direction of the plate, affects cold workability. As a result of further investigation, the average particle size D of ferrite at a depth of t / 8 mm from the surface of the steel plate in the thickness direction was found to be... t / 8 (μm), and the average particle size D of ferrite at a depth of t / 2 mm from the surface of the steel plate in the thickness direction. t / 2 We found that excellent cold workability can be obtained if (μm) and satisfy the following equation (1). 0.90≦D t / 8 / D t / 2 ≤1.10 (1)

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

[0019] The steel sheet of the first form has a chemical composition in mass percent of: C: 0.20-0.70%, Si: 0.07-1.00%, Mn: 0.20-3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010-1.500%, acid-soluble Al: 0.005-0.070%, N: 0.0200% or less, Ti: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, B: 0-0.003%. It contains 5% of the following: 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 remainder being Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more, and the average particle size of ferrite is D t The particle size is 5.00 to 30.00 μm. When the thickness of the steel plate is t (mm), the average particle size D of ferrite at a depth of t / 8 mm from the surface of the steel plate in the thickness direction is given by t. t / 8 (μm), and the average particle size D of ferrite at a depth of t / 2 mm from the surface of the steel plate in the thickness direction.t / 2 (μm) and satisfy equation (1). Sample standard deviation S of ferrite particle size at a depth of t / 8 mm. t / 8 The sample standard deviation S of the ferrite particle size at a depth of t / 2 mm is 4.50 μm or less. t / 2 The particle size is 4.00 μm or less. 0.90≦D t / 8 / D t / 2 ≤1.10 (1)

[0020] The second form of steel sheet is the first form of steel sheet, with Ti: 0.001~0.500%, V: 0.001~0.500%, Nb: 0.001~0.500%, B: 0.0001~0.0035%, Cu: 0.01~0.20%, W: 0.01~0.03%, Ta: 0.01~0.03%, Sn: 0.001~0.030%, Sb: 0.001~0.0 It contains one or more elements selected from the group consisting of 30%, Co: 0.001-0.030%, As: 0.001-0.030%, Mg: 0.001-0.030%, Y: 0.001-0.030%, Zr: 0.001-0.030%, La: 0.001-0.030%, Ce: 0.001-0.030%, and Ca: 0.001-0.030%.

[0021] The steel plate of this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percentage.

[0022] [Features of the steel plate of this embodiment] The steel plate of this embodiment satisfies the following features 1 to 5. (Feature 1) The chemical composition, in mass%, is as follows: C: 0.20-0.70%, Si: 0.07-1.00%, Mn: 0.20-3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010-1.500%, Acid-soluble Al: 0.005-0.070%, N: 0.0200% or less, Ti: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, B: 0-0.0035%, Cu It contains 0-0.20% of Calcium, 0-0.03% of W, 0-0.03% of Ta, 0-0.03% of Sn, 0-0.030% of Sb, 0-0.030% of Co, 0-0.030% of As, 0-0.030% of Mg, 0-0.030% of Y, 0-0.030% of Zr, 0-0.030% of La, 0-0.030% of Ce, and 0-0.030% of Ca, with the remainder being Fe and impurities. (Feature 2) In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. (Feature 3) Average particle size D of ferrite t The size is 5.00 to 30.00 μm. (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 thickness direction is given by... t / 8 (μm), and the average particle size D of ferrite at a depth of t / 2 mm from the surface of the steel plate in the thickness direction. t / 2 (μm) and satisfy equation (1). 0.90≦D t / 8 / D t / 2 ≤1.10 (1) (Feature 5) Sampling standard deviation of ferrite particle size at a depth of t / 8 mm from the surface of the steel plate in the thickness direction. t / 8 The sample standard deviation S of ferrite particle size is 4.50 μm or less, and the sample standard deviation S is t / 2 mm deep from the surface of the steel plate in the thickness direction. t / 2 The particle size is 4.00 μm or less. Features 1 through 5 are explained below.

[0023] [(Feature 1) Regarding chemical composition] The chemical composition of the steel sheet in this embodiment contains the following elements:

[0024] C: 0.20~0.70% Carbon (C) enhances the hardenability of steel sheets. As a result, the strength of machine parts is increased by performing quenching during the manufacturing process of machine parts using steel sheets as a material. If the C content is less than 0.20%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the carbon content exceeds 0.70%, the cold workability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.20-0.70%. The preferred lower limit for 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 for the C content is 0.68%, more preferably 0.65%, and even more preferably 0.60%.

[0025] Si: 0.07~1.00% Silicon (Si) deoxidizes steel during the steelmaking stage in the manufacturing process of steel sheets. Furthermore, when tempering is performed in the process of manufacturing machine parts using steel sheets as a material, Si increases the tempering softening resistance of the steel sheet. As a result, the strength of the machine parts is increased. If the Si content is less than 0.07%, the above effects cannot be fully obtained, even if the content of other elements is within the range 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. As a result, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Si content is 0.07 to 1.00%. The preferred lower limit for the Si content is 0.09%, more preferably 0.12%, and even more preferably 0.15%. The preferred upper limit for the Si content is 0.95%, more preferably 0.90%, more preferably 0.80%, more preferably 0.70%, and more preferably 0.60%.

[0026] Mn: 0.20~3.00% Manganese (Mn) enhances the hardenability of steel plates. As a result, the strength of machine parts is increased by performing quenching during the manufacturing process of machine parts using steel plates as the raw material. If the Mn content is less than 0.20%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 3.00%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. As a result, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Mn content is between 0.20% and 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 preferred upper limit for the Mn content is 2.90%, more preferably 2.80%, more preferably 2.50%, more preferably 2.00%, more preferably 1.90%, more preferably 1.70%, and 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 will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the P content is 0.030% or less. A low phosphorus (P) content is preferable. 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%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit for the P content is 0.028%, 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 will form an excessive amount of sulfides. Therefore, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet will decrease. Therefore, the sulfur content is 0.0080% or less. A low sulfur (S) content is preferable. 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%, more preferably 0.0003%, and still more preferably 0.0005%. The preferred upper limit for the S content is 0.0075%, more preferably 0.0070%, more preferably 0.0065%, more preferably 0.0060%, more preferably 0.0055%, and more preferably 0.0050%.

[0029] Cr: 0.010~1.500% Chromium (Cr) enhances the hardenability of steel sheets. As a result, the strength of machine parts is increased by performing quenching during the manufacturing process of machine parts using steel sheets as a material. If the Cr content is less than 0.010%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content exceeds 1,500%, the strength of the steel sheet becomes excessively high. Therefore, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet decreases. Therefore, the Cr content is between 0.010% and 1.500%. The preferred lower limit for the Cr content is 0.015%, more preferably 0.030%, more preferably 0.050%, more preferably 0.080%, and more preferably 0.100%. The preferred upper limit for the Cr content is 1.480%, more preferably 1.450%, more preferably 1.400%, more preferably 1.300%, more preferably 1.100%, more preferably 0.900%, and more preferably 0.700%.

[0030] Acid soluble Al: 0.005~0.070% Aluminum (Al) deoxidizes steel during the steelmaking stage in the manufacturing process of steel sheets. If the acid-soluble Al (sol.Al) content is less than 0.005%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the acid-soluble Al content exceeds 0.070%, excessive Al nitrides are formed, and the austenite grains become finer. In this case, even if the content of other elements is within the range of this embodiment, the hardenability of the steel sheet decreases. Therefore, the acid-soluble Al content is 0.005-0.070%. The preferred lower limit for the acid-soluble Al content is 0.010%, more preferably 0.012%, more preferably 0.015%, and still more preferably 0.020%. The preferred upper limit for the acid-soluble Al content is 0.065%, more preferably 0.060%, 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 the austenite grains during quenching, a process used to manufacture machine parts from steel sheets. This refinement of austenite grains reduces the hardenability of the steel sheet. If the N content exceeds 0.0200%, the austenite grains are excessively refined during quenching. Therefore, even if the content of other elements is within the range 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 for the N content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The preferred upper limit for the N content is 0.0190%, more preferably 0.0180%, more preferably 0.0170%, more preferably 0.0160%, more preferably 0.0150%, and more preferably 0.0140%.

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

[0033] [About Optional Elements] The chemical composition of the steel sheet in this embodiment may further include, in place of some of the 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%. Any of these elements are optional and may not be included. These optional elements will be described below.

[0034] [Regarding Group 1 (Ti, V, Nb, and B)] The chemical composition of the steel sheet according to this embodiment may further include one or more elements selected from the group consisting of Ti, V, Nb, and B in place of a portion of Fe. Any of these elements are optional and may not be included. If included, Ti, V, Nb, and B increase the strength of the steel sheet.

[0035] Ti: 0~0.500% Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. When Ti is present, that is, when the Ti content is greater than 0%, the Ti forms precipitates such as carbides. Therefore, the strength of the steel sheet is increased by precipitation strengthening. Furthermore, Ti combines with N to suppress the formation of nitrides by solid solution B. Even if only a small amount of Ti is present, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.500%, precipitates will form excessively, resulting in an excessively high strength of the steel sheet. In this case, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet will decrease. Therefore, the Ti content is between 0 and 0.500%. The preferred lower limit for the Ti content is 0.001%, more preferably 0.002%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit for the Ti content is 0.400%, more preferably 0.300%, more preferably 0.200%, more preferably 0.100%, and still more preferably 0.080%.

[0036] V: 0~0.500% Vanadium (V) is an optional element and does not need to be present. In other words, the V content may be 0%. When V is present, that is, when the V content is greater than 0%, V forms precipitates such as carbides. Therefore, the strength of the steel sheet is increased by precipitation strengthening. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.500%, precipitates will form excessively, resulting in an excessively high strength of the steel sheet. In this case, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet will decrease. Therefore, the V content is between 0 and 0.500%. The preferred lower limit of the V content is 0.001%, more preferably 0.002%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit for the V content is 0.480%, more preferably 0.450%, more preferably 0.400%, more preferably 0.300%, more preferably 0.200%, more preferably 0.100%, and more preferably 0.080%.

[0037] Nb: 0~0.500% Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. When Nb is present, that is, when the Nb content is greater than 0%, the Nb forms precipitates such as carbides. Therefore, the strength of the steel sheet is increased by precipitation strengthening. In addition, Nb combines with N to suppress the formation of nitrides by solid solution B. Even if only a small amount of Nb is present, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.500%, precipitates will form excessively, resulting in an excessively high strength of the steel sheet. In this case, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet will decrease. Therefore, the Nb content is between 0 and 0.500%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.002%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit for the Nb content is 0.480%, more preferably 0.450%, more preferably 0.400%, more preferably 0.350%, and still more preferably 0.300%.

[0038] B: 0~0.0035% Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. If B is present, that is, if the B content is greater than 0%, B improves the hardenability of the steel plate and increases its strength. Even if only a small amount of B is present, the above effects can be obtained to some extent. However, if the B content exceeds 0.0035%, B compounds will be formed. In this case, the strength of the steel sheet becomes excessively high. Therefore, even if the content of other elements is within the range of this embodiment, the cold workability of the steel sheet will decrease. Therefore, the B content is 0-0.0035%. The preferred lower limit for the B content is 0.0001%, more preferably 0.0002%, more preferably 0.0003%, and still more preferably 0.0005%. The preferred upper limit for the B content is 0.0032%, more preferably 0.0028%, more preferably 0.0025%, more preferably 0.0020%, and more preferably 0.0015%.

[0039] [Regarding Group 2 (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 some of the 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. Any of these elements are optional and may not be included. In other words, the content of these elements may be 0%.

[0040] These elements are all trump elements and are impurities in the steel sheet of this embodiment. Therefore, the Cu content is 0-0.20%, the W content is 0-0.03%, the Ta content is 0-0.03%, the Sn content is 0-0.030%, the Sb content is 0-0.030%, the Co content is 0-0.030%, the As content is 0-0.030%, the Mg content is 0-0.030%, the Y content is 0-0.030%, the Zr content is 0-0.030%, the La content is 0-0.030%, the Ce content is 0-0.030%, and the Ca content is 0-0.030%.

[0041] The preferred lower limit for the Cu content is 0.01%, and more preferably 0.03%. The preferred upper limit for the Cu content is 0.15%, and more preferably 0.10%.

[0042] The preferred lower limit for W content is 0.01%. The preferred upper limit for W content is 0.02%.

[0043] The preferred lower limit for the Ta content is 0.01%. The preferred upper limit for the Ta content is 0.02%.

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

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

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

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

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

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

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

[0051] The preferred lower limit for the La content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the La content is 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0052] The preferred lower limit of the Ce content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Ce content is 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0053] The preferred lower limit for the Ca content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Ca content is 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0054] [Method for measuring the chemical composition of steel sheets] The chemical composition of the steel sheet in this embodiment can be measured using well-known component analysis methods. Specifically, chips are collected from the inside of the steel sheet to a depth of 0.1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. Elemental analysis of the chemical composition is performed on the solution using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). The C and S content is determined by the well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the well-known inert gas melting-thermal conductivity method.

[0055] Furthermore, the content of each element shall be rounded to the minimum digit of the element content specified in this embodiment, based on the significant figures defined in this embodiment. For example, the carbon content of the steel plate in this embodiment is defined to two decimal places. Therefore, the carbon content shall be the value obtained by rounding the third decimal place of the measured value to two decimal places.

[0056] Similarly, for the elemental content of the steel plate in this embodiment, other than the C content, the measured value is rounded to the minimum digit specified in this embodiment, and the value obtained is considered to be the elemental content. Rounding means truncating the value if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0057] [(Feature 2) About Microorganisms] In the microstructure of the steel sheet of this embodiment, the total area ratio of ferrite and cementite particles is 95% or more. In other words, the microstructure of the steel sheet of this embodiment consists substantially of ferrite and cementite particles.

[0058] In the microstructure, the structures other than ferrite and cementite particles are, for example, one or more 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, more preferably 98% or more, and more preferably 99% or more. The microstructure may consist of ferrite and cementite particles.

[0060] The preferred range for the total area ratio of ferrite and cementite particles is 96-100%, more preferably 97-100%, even more preferably 98-100%, and even more preferably 99-100%.

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

[0062] [Method for measuring the total area ratio of ferrite and cementite particles in a microstructure] The total area ratio of ferrite and cementite particles in a microstructure can be measured by the following method. A test specimen is taken from the center of the steel plate's width, having a cross-section (L-section) parallel to the rolling direction and thickness direction of the steel plate. The size of the test specimen is not particularly limited as long as it includes the observation field described later. Of the surface of the test specimen, the L-section of the steel plate is used as the observation surface. The observation surface of the test specimen is polished to a mirror finish. The mirror-polished observation surface is etched using 3% nitric acid alcohol (Nital etching solution). The etching time is 120 seconds. From the etched observation surface, five rectangular observation fields are selected, each 100 μm in the thickness direction of the steel plate and 120 μm in the direction perpendicular to the thickness direction, with the center of the field of view being the center of the thickness direction. The five observation fields are arranged consecutively in a direction perpendicular to the thickness, and the center of the central observation field among the five arranged observation fields is the center of the width of the steel plate.

[0063] For each field of view, a secondary electron image is observed using a 1000x scanning electron microscope (SEM). Within the field of view, ferrite and cementite particles exhibit different contrast and morphology compared to other structures (bainite, martensite, pearlite, etc.). Therefore, ferrite and cementite particles within the field of view are identified based on their contrast and morphology.

[0064] Specifically, ferrite is a white region within the grain that lacks underlying structures such as lath. Bainite and martensite are regions that contain underlying structures. Pearlite is a striped region with a lamellar structure. Cementite grains are darker regions 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 rounded to the nearest integer.

[0066] [(Feature 3) Average particle size D of ferrite t [About] In the steel sheet of this embodiment, the average particle size D of the ferrite is further... t The size is 5.00 to 30.00 μm. The average particle size of ferrite affects cold workability. If the average particle 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 particle size of ferrite is 5.00 μm or more, excellent cold workability can be obtained in steel sheets, provided that features 1, 2, 4, and 5 are satisfied.

[0067] The preferred lower limit for the average particle size of the ferrite is 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] There is no particular upper limit to the average particle size of ferrite. However, if the average particle size of ferrite is excessively large, the strength of the steel sheet will be excessively low. As a result, sufficient strength may not be obtained in machine parts. Therefore, the upper limit for the average particle size of ferrite is 30.00 μm.

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

[0070] [(Feature 4) Surface ferrite particle size D t / 8 and ferrite particle size D at the center of the plate thickness t / 2 [About] In the steel plate of this embodiment, when the thickness of the steel plate is t (mm), the average ferrite particle size at a depth of t / 8 mm from the surface of the steel plate in the thickness direction is defined as "Surface ferrite particle size D t / 8 It is defined as (μm). Furthermore, ferrite grains at a depth of t / 8 mm from the surface of the steel plate in the thickness direction are also called "surface ferrite grains". In addition, the average particle size of ferrite at a depth of t / 2 mm from the surface of the steel plate in the thickness direction is called "center ferrite particle size D t / 2 It is defined as (μm). Furthermore, ferrite grains located at a depth of t / 2 mm in the thickness direction from the surface of the steel plate are also called "ferrite grains at the center of the plate thickness". At this time, the surface ferrite particle size D t / 8 (μm) and ferrite particle size D at the center of the plate thickness t / 2 (μm) satisfies equation (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 particle size D at the center of the plate thickness t / 2 This is an index that shows the particle size ratio. As mentioned above, cold workability is also affected by the variation in ferrite grains in the steel sheet. Specifically, the more uniform the size of the ferrite grains is in the thickness direction of the sheet, the better the cold workability.

[0072] If F1 is less than 0.90, or if F1 is greater than 1.10, the variation between the surface ferrite grain size and the ferrite grain size in the center of the plate thickness is too large. In this case, sufficient cold workability cannot be obtained.

[0073] On the other hand, if F1 is between 0.90 and 1.10, the variation between the size of surface ferrite grains and the size of ferrite grains in the center of the plate thickness is sufficiently small. Therefore, assuming that features 1 to 3 and feature 5 are satisfied, excellent cold workability can be obtained in the steel plate.

[0074] A preferred lower limit for F1 is 0.92, more preferably 0.94, even more preferably 0.96, and even more preferably 0.97. A preferred upper limit for F1 is 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 [About] In this embodiment, the sample standard deviation S of the ferrite particle size at a depth of t / 8 mm from the surface of the steel plate in the thickness direction is further measured. t / 8 The sample standard deviation S of ferrite particle size is 4.50 μm or less, and the sample standard deviation S is t / 2 mm deep from the surface of the steel plate in the thickness direction. t / 2 The particle size is 4.00 μm or less. The following explains Feature 5.

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

[0077] Feature 4 above specifies the variation in ferrite grains in the thickness direction of the steel sheet. On the other hand, Feature 5 specifies the variation in ferrite grains in the rolling direction, which is perpendicular to the thickness direction of the steel sheet. Cold workability is improved when the variation in ferrite grains is suppressed not only in the thickness direction of the steel sheet, but also in the direction perpendicular to the thickness direction.

[0078] Surface ferrite grain size deviation S t / 8 If the thickness exceeds 4.50 μm, or if the ferrite grain deviation S at the center of the plate thickness t / 2 If the thickness exceeds 4.00 μm, the ferrite grains will have too much variation in the direction perpendicular to the plate thickness. In this case, sufficient cold workability cannot be obtained.

[0079] On the other hand, surface ferrite grain deviation S t / 8 The thickness is 4.50 μm or less, and the ferrite grain deviation S at the center of the plate thickness is t / 2 When the thickness is 4.00 μm or less, assuming that features 1 to 4 are satisfied, the variation in ferrite grains is sufficiently small not only in the thickness direction but also in the direction perpendicular to the thickness direction. Therefore, excellent cold workability can be obtained in steel sheets.

[0080] Surface ferrite grain size deviation S t / 8 The preferred upper limit is 4.40 μm, more preferably 4.35 μm, more preferably 4.30 μm, more preferably 4.20 μm, more preferably 4.10 μm, more preferably 4.00 μm, more preferably 3.80 μm, more preferably 3.70 μm, and more preferably 3.60 μm. Surface ferrite grain size deviation S t / 8 The smaller the value, the better. However, the surface ferrite grain deviation S is also important. t / 8Making it excessively small will result in excessively high manufacturing costs. Therefore, the surface ferrite grain deviation S t / 8 The preferred lower limit is 0.10 μm, more preferably 0.30 μm, and even more preferably 0.50 μm.

[0081] Plate thickness, center ferrite grain deviation S t / 2 The preferred upper limit 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. Plate thickness, center ferrite grain deviation S t / 2 The smaller the value, the better. However, the ferrite grain deviation S at the center of the plate thickness is also important. t / 2 Making it excessively small will result in excessively high manufacturing costs. Therefore, the ferrite grain deviation S at the center of the plate thickness is important. t / 2 The preferred lower limit is 0.10 μm, more preferably 0.30 μm, and even more preferably 0.50 μm.

[0082] [Average particle size of ferrite D t , surface ferrite particle size D t / 8 , plate thickness center ferrite particle size D t / 2 Surface ferrite grain deviation S t / 8 , and the ferrite grain deviation S at the center of the plate thickness t / 2 [Regarding measurement methods] Related to Features 3-5, the average particle size D of the ferrite. t , surface ferrite particle size D t / 8 , plate thickness center ferrite particle size D t / 2 Surface ferrite grain deviation S t / 8 , and the ferrite grain deviation S at the center of the plate thickness t / 2 It is measured using the following method:

[0083] A test specimen is taken from the steel plate, having a cross-section that includes the rolling direction and thickness direction of the steel plate. The thickness of the test specimen is the thickness of the steel plate. The cross-section of the test specimen in the rolling direction and thickness direction of the steel plate is designated as the observation surface. The observation surface is mirror-polished. After mirror polishing, etching is performed with a 3% Nital etchant. The etching time is 120 seconds.

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

[0085] Furthermore, from the observation surface, five rectangular observation fields are selected at a depth of t / 8 from the surface of the steel plate in the thickness direction, with dimensions of 100 μm in the thickness direction and 500 μm in the rolling direction. These observation fields are called "surface observation fields." The center of the surface observation fields in the thickness direction corresponds to a depth of t / 8 from the surface of the steel plate in the thickness direction. The five surface observation fields are also arranged continuously in the rolling direction.

[0086] Secondary electron images are taken using a scanning electron microscope (SEM) in each observation field (center observation field and surface observation field). The equivalent circular diameter of the ferrite grains identified in each observation field is then determined. At this time, if at least a portion of the identified ferrite grains is in contact with or intersecting the rectangular edge of the observation field, those ferrite grains are excluded from the measurement. In other words, only ferrite grains whose entirety lies inside the rectangular edge of the observation field and does not come into contact with the edge are included in the measurement. The equivalent circular diameter (μm) of each ferrite grain to be measured is determined. Here, the equivalent circular diameter refers to the diameter of a circle with the same area as the area of ​​the ferrite grain.

[0087] The arithmetic mean of the equivalent circular diameter of all ferrite grains at the center of each of the five thicknesses observed, and the equivalent circular diameter of all surface ferrite grains observed in each of the five surface observation fields, is used to determine the average ferrite particle size D. t Let (μm) be the average particle size D of the ferrite. t This value is the second decimal place obtained by rounding the third decimal place.

[0088] The arithmetic mean of the equivalent circle diameter of all surface ferrite grains measured in the five surface observation fields is defined as the surface ferrite grain size D. t / 8 Let it be (μm). The arithmetic mean of the equivalent circular diameter of all ferrite grains at the center of each of the five plate thicknesses measured in the central observation field is defined as the ferrite grain size D at the center of the plate thickness. t / 2 Let it be (μm). Surface ferrite particle size D t / 8 and ferrite particle size D at the center of the plate thickness t / 2 The value of is the value obtained by rounding the arithmetic mean to two decimal places. Note that F1(=D t / 8 / D t / 2 The value of ) shall be the value obtained by rounding the third decimal place of 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 defined as the surface ferrite grain deviation S. t / 8 Let it be (μm). The sample standard deviation of all ferrite grains at the center of each of the five plate thicknesses observed in the central observation field is defined as the plate thickness central ferrite grain deviation S. t / 2 Let it be (μm). Surface ferrite grain size deviation S t / 8 and the ferrite grain deviation S at the center of the plate thickness t / 2 The value of is the value obtained by rounding the arithmetic mean to two decimal places.

[0090] [Effects of the steel plate in this embodiment] The steel sheet of this embodiment satisfies features 1 to 5. Therefore, the steel sheet of this embodiment provides excellent cold workability.

[0091] [Uses of steel plates] The steel sheet of this embodiment serves as the material for cold-rolled steel sheets used in mechanical parts, such as automotive parts. Examples of mechanical parts include springs and washers for automobiles, and gears for bicycles. The steel sheet of this embodiment may also be used for applications other than mechanical parts where excellent cold workability is required.

[0092] [Method of manufacturing steel plates] An example of a method for manufacturing the steel sheet of this embodiment will be described. The method for manufacturing the steel sheet described below is just one example for manufacturing the steel sheet of this embodiment. Therefore, a steel sheet having the above-described structure may be manufactured by a manufacturing method other than the one described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the steel sheet of this embodiment.

[0093] An example of the steel plate manufacturing method of this embodiment includes the following steps. (Process 1) Hot rolling process (Step 2) Annealing process The following describes each step.

[0094] [(Process 1) Hot Rolling Process] In the hot rolling process, hot rolling is performed on slabs that satisfy Feature 1. The slabs are manufactured, for example, by the following method: Molten steel is produced in which the content of each element in the chemical composition is within the range of this embodiment. A slab is manufactured by a casting method using the above molten steel. For example, a slab having a chemical composition that satisfies Feature 1 is manufactured by a well-known continuous casting method using the above molten steel.

[0095] Hot-rolled steel sheets are produced by hot-rolling the prepared slabs. The hot-rolling process includes the following steps: (Step 10) Heating process (Process 11) Rough rolling process (Process 12) Finishing rolling process (Step 13) Winding process The following describes each step.

[0096] [(Step 10) Heating process] In the heating process, the slab is heated in a heating furnace. The time the slab stays in the heating furnace is, for example, 30 minutes or more, preferably 60 minutes or more. There is no particular upper limit to the time the slab stays in the furnace, but for example, it is 300 minutes.

[0097] [(Process 11) Rough rolling process] In the rough rolling process, a heated slab is roughly 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] [(Process 12) Finish Rolling Process] In the finishing rolling process, a tandem rolling mill is used to perform finish rolling on the rough bar to produce hot-rolled steel sheets. The tandem rolling mill includes multiple 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 for example, it can be 4 to 7 stands. The finishing rolling process is performed without reheating the intermediate steel sheet after rough rolling.

[0099] [(Step 13) Winding process] In the winding process, the hot-rolled steel sheet, which has completed its finish rolling in the hot-rolling process, is cooled and then wound into a coil.

[0100] [(Step 2) Annealing process] In the annealing process, the coiled hot-rolled steel sheet, after the winding process, is subjected to an annealing treatment. The annealing treatment causes the cementite to become spheroidal, forming cementite particles.

[0101] [Regarding conditions in the manufacturing process] The manufacturing process described above also satisfies the following conditions: (Condition 1) Heating temperature T in the heating process 11 The temperature should be between 1100 and 1350°C. (Condition 2) The final finish rolling start temperature T, which is the surface temperature of the steel sheet at the inlet of the most downstream rolling stand L that imparts the final reduction in the finish rolling process 12 shall be 850°C or higher. (Condition 3) The finish rolling temperature FT in the finish rolling process used in the finish rolling process shall be 850 to 1000°C. (Condition 4) Among the plurality of rolling stands of the tandem rolling mill, σ defined by Equation (2) at the rolling stand L-1 in front of the most downstream rolling stand L L-1 shall be 21 to 35. Further, at the most downstream rolling stand L of the plurality of rolling stands of the tandem rolling mill, σ defined by Equation (2) L shall be 71 to 90. σ n = exp(0.753 + 3000 / (T n + 273)) × ε n 0.21 × v n 0.13 (2) Here, n is from 1 to L, ε n is the equivalent plastic strain imparted to the rough bar at the corresponding rolling stand n, and v n is the strain rate (s -1 ) of the rough bar when passing through the corresponding rolling stand n. T n is the surface temperature (°C) of the steel sheet at the inlet of rolling stand n. When applying Equation (2) at rolling stand L-1, n in Equation (2) is L-1. When applying Equation (2) at rolling stand L, n in Equation (2) is L. Note that when applying Equation (2) at rolling stand L, for T n (°C) in Equation (2), the final finish rolling start temperature T 12 (°C) is substituted. On the other hand, when applying Equation (2) at rolling stand L-1, for Tn (°C) in Equation (2), T 12 + 20 (°C) is substituted. (Condition 5) The cumulative reduction ratio R in the hot rolling process shall be 60% or higher. (Condition 6) The coiling temperature CT in the coiling process shall be 650 to 550°C. (Condition 7) The annealing temperature T3 in the annealing process is set to 600-730°C, and the holding time t3 at annealing temperature T3 is set to 20 hours or more. The following explains each condition.

[0102] [(Condition 1) Heating temperature T 11 [About] Heating temperature T in the heating process 11 If the temperature is below 1100℃, the ferrite in the manufactured steel sheet becomes excessively small. As a result, the average particle size D of the ferrite becomes t The particle size becomes less than 5.00 μm. On the other hand, the heating temperature T 11 If the temperature exceeds 1350°C, the ferrite in the manufactured steel sheet becomes coarse. As a result, the average particle size D of the ferrite becomes coarser. t It exceeds 30.00 μm. Heating temperature T 11 If the temperature is between 1100 and 1350°C, assuming other manufacturing conditions are met, the average particle size D of the ferrite in the steel sheet after manufacturing is... t This ranges from 5.00 to 30.00 μm.

[0103] [(Condition 2) Final finish rolling start temperature T 12 [About] In the tandem rolling mill for the finishing rolling process, the surface temperature of the rough bar (intermediate steel sheet) at the entry side of the downstream rolling stand L is defined as the final finishing rolling start temperature T. 12 Let the temperature be (°C). Final finishing rolling start temperature T 12 This is measured by a thermometer positioned on the entry side of the downstream rolling stand L, which applies the final reduction to the intermediate steel plate.

[0104] Final finishing rolling start temperature T 12 If the temperature is below 850°C, the ferrite in the manufactured steel sheet becomes excessively small. As a result, the average particle size D of the ferrite becomes t The thickness becomes less than 5.00 μm. Furthermore, the shear strain in the surface layer of the plate is greater than that in the center of the plate. As a result, the ferrite grains in the surface layer of the plate become excessively small compared to the ferrite grains in the center of the plate. Consequently, F1 becomes less than the lower limit of equation (1). Therefore, the final finishing rolling start temperature T 12Heat it to over 850℃.

[0105] [(Condition 3) Regarding the finishing rolling temperature FT] The surface temperature of the steel sheet (hot-rolled steel sheet) at the exit of the downstream rolling stand L, one of the multiple rolling stands 1 to L that make up the rolling mill in the finishing rolling process, is defined as the "finishing rolling temperature FT (°C)". The finishing rolling temperature FT is measured by a thermometer placed at the exit of the downstream rolling stand L.

[0106] If the finishing rolling temperature FT is less than 850°C, the ferrite in the manufactured steel sheet becomes excessively small. As a result, the average particle size D of the ferrite becomes t This results in a thickness of less than 5.00 μm. Furthermore, the shear strain in the surface layer of the plate becomes larger than that in the center layer. As a result, the ferrite grains in the surface layer become excessively small compared to the ferrite grains in the center layer. Consequently, F1 falls below the lower limit of equation (1). On the other hand, if the finishing rolling temperature FT exceeds 1000°C, the ferrite in the manufactured steel sheet becomes coarse. As a result, the average particle size D of the ferrite becomes coarser. t It exceeds 30.00 μm. Therefore, the finishing rolling temperature FT should be set to 850-1000°C.

[0107] [(Condition 4)σ n (σ L-1 and σ L ) about In the finishing rolling process, the stress applied to rolling stand L-1, which is preceding the downstream rolling stand L, and to rolling stand L itself, affects the size of the ferrite grains on the surface and in the center of the steel sheet. At the rolling stand L-1 preceding the downstream rolling stand L, σ defined by equation (2) L-1 Let this be 21-35. Furthermore, in the tandem rolling mill, the σ defined by equation (2) is used in the downstream rolling stand L of the multiple rolling stands 1-L. LThe pressure is set to 71-90. In this case, light reduction is performed at rolling stand L-1, followed by heavy reduction at the downstream rolling stand L. As a result, recrystallization occurs all at once in the steel sheet after reduction at the downstream rolling stand L, generating ferrite grains of uniform size in both the thickness direction and the rolling direction.

[0108] σ defined by equation (2) n This is an indicator of the stress applied to the rough bar at the rolling stand n. σ L-1 If the value is less than 21, the reduction at rolling stand L-1 is insufficient. In this case, strain does not easily penetrate to the center of the plate thickness. Therefore, the average particle size D of the ferrite grains in the center of the plate thickness is t / 2 However, the average particle size D of the surface ferrite grains t / 8 It becomes excessively large compared to [the other value]. Therefore, F1 falls below the lower limit of equation (1). Furthermore, because the strain does not penetrate sufficiently to the center of the plate thickness, the ferrite grain deviation S at the center of the plate thickness t / 2 It exceeds 4.00 μm.

[0109] On the other hand, σ L-1 If F1 exceeds 35, the reduction at rolling stand L-1 is too large. In this case, excessive recrystallization occurs at the exit side of rolling stand L-1 and the entry side of rolling stand L. Therefore, at the exit side of the final rolling stand L, the driving force for recrystallization in the center of the plate thickness is insufficient. As a result, F1 falls below the lower limit of equation (1). Furthermore, because the strain does not penetrate sufficiently to the center of the plate thickness, the ferrite grain deviation S in the center of the plate thickness is reduced. t / 2 It exceeds 4.00 μm.

[0110] σ L If the value exceeds 90, the reduction at the rolling stand L is excessive. In this case, the ferrite in the manufactured steel sheet becomes excessively small. As a result, the average particle size D of the ferrite becomes excessive. t This becomes less than 5.00 μm. On the other hand, σ L If the ratio is less than 71, the reduction at 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 the ferrite is reduced. tThe size is less than 5.00 μm. Furthermore, the average particle size D of the surface ferrite grains t / 8 The average particle size D of the ferrite grains in the center of the plate thickness t / 2 It becomes excessively small compared to [the other value]. Therefore, F1 falls below the lower limit of equation (1).

[0111] [(Condition 5) Regarding the cumulative reduction ratio R in the hot rolling process] The cumulative reduction ratio R(%) in the hot rolling process is defined as follows: R = (1 - t1 / t0) × 100 Here, t0 is the thickness of the slab (mm), and t1 is the thickness of the hot-rolled steel sheet after finish rolling (mm).

[0112] If the cumulative reduction ratio R is less than 60%, the reduction during the hot rolling process is insufficient. The ferrite in the manufactured steel sheet will be coarse. As a result, the average ferrite particle size Dt will exceed 30.00 μm. Therefore, the cumulative reduction ratio R is 60% or more.

[0113] [(Condition 6) Regarding winding temperature CT] In the winding process, the surface temperature of the steel sheet at the start of winding is defined as the winding temperature CT (°C). If the winding temperature CT exceeds 650°C, the ferrite in the manufactured steel sheet becomes coarse. As a result, the average particle size D of the ferrite becomes coarse. t It exceeds 30.00 μm. On the other hand, if the winding temperature (CT) is less than 550°C, the ferrite in the manufactured steel sheet becomes excessively small. As a result, the average ferrite particle size Dt becomes less than 5.00 μm. Therefore, the winding temperature CT is 650-550°C.

[0114] [(Condition 7) Regarding annealing temperature T3 and holding time t3] The annealing temperature T3 in the annealing process is set to 600-730°C, and the holding time t3 at annealing temperature T3 is set to 20 hours or more. The preferred upper limit for the holding time t3 is 50 hours. If the annealing temperature T3 is less than 600°C, the annealing is insufficient. In this case, the ferrite in the manufactured steel sheet becomes excessively small. As a result, the average ferrite particle size Dt becomes less than 5.00 μm. Furthermore, if the holding time t3 is too short, the annealing is insufficient. In this case as well, the ferrite in the manufactured steel sheet becomes excessively small. As a result, the average particle size D of the ferrite becomes t This becomes less than 5.00 μm. On the other hand, if the annealing temperature T3 exceeds 730°C, the ferrite in the manufactured steel sheet becomes coarse. As a result, the average ferrite particle size Dt exceeds 30.00 μm.

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

[0116] The effects of the steel sheet of this embodiment will be described in more detail below with reference to examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the steel sheet of this embodiment. Therefore, the steel sheet of this embodiment is not limited to this one example of conditions.

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

[0118] [Table 1]

[0119] Specifically, slabs were manufactured by continuous casting of molten steel. A hot rolling process was then carried out on the slabs. Specifically, the heating temperature T shown in Table 2 was used. 11 The slab was heated at (°C) (Condition 1). Then, the slab was roughly rolled in a reverse rolling mill to produce a rough bar (intermediate steel sheet). Finish rolling was performed on the rough bar using a tandem rolling mill consisting of multiple rolling stands to produce a hot-rolled steel sheet. Final finish rolling start temperature T 12 (°C) (Condition 2), finish rolling temperature FT (°C) (Condition 3), σ at rolling stand L-1, which is before the downstream rolling stand L. L-1and σ at the downstream rolling stand L L (Condition 4) The cumulative reduction ratio R (%) of the hot rolling process (Condition 5) was as shown in Table 2.

[0120] [Table 2]

[0121] A coiling process was performed on the hot-rolled steel sheets after the hot-rolling process. The coiling temperature CT (°C) (Condition 6) is shown in Table 2. An annealing process was performed on the steel sheets after the coiling process. The annealing temperature T3 (°C) and the holding time t3 (hours) at annealing temperature T3 (Condition 7) are shown in Table 2. Steel sheets of each test number were manufactured using the above manufacturing process.

[0122] [Evaluation Test] The following evaluation tests were conducted on the steel plates for each test number. (Test 1) Chemical composition measurement test of steel plate (Test 2) Measurement test of the total area ratio of ferrite and cementite particles in steel plates (Test 3) Average ferrite particle size D of steel plate t , surface ferrite particle size D t / 8 , plate thickness center ferrite particle size D t / 2 Surface ferrite grain deviation S t / 8 , and the ferrite grain deviation S at the center of the plate thickness t / 2 Measurement test (Test 4) Vickers hardness test (Test 5) Ductility Test The following explains Exams 1 through 5.

[0123] [(Test 1) Chemical composition measurement test of steel plate] The chemical composition of the steel sheets for each test number was measured in accordance with the method described in the above-mentioned [Method for Measuring the Chemical Composition of Steel Sheets]. The results of the chemical composition of each test number are shown in Table 1.

[0124] [(Test 2) Measurement test of the total area ratio of ferrite and cementite particles in steel plates] The total area ratio (%) of ferrite and cementite particles in the steel sheets for each test number was determined in accordance with the method described in [Method for measuring the total area ratio of ferrite and cementite particles in the microstructure] above. The obtained total area ratios (%) are shown in Table 3.

[0125] [Table 3]

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

[0127] [(Test 4) Vickers hardness test] The Vickers hardness (HV) of the steel plates for each test number was determined using the following method as an indicator of their cold workability. Test specimens were taken from 10 sampling locations located at the center of the steel plate width, spaced at 500 mm intervals in the rolling direction. The size of the test specimens was 15 mm in the rolling direction × 30 mm in the width direction × plate thickness. The surface of the test specimen that included the rolling direction and the plate thickness direction (L section) was used as the measurement surface. On the measurement surface, with a plate thickness of t mm, a Vickers hardness test was performed in accordance with JIS Z 2244-1 (2020) at five measurement points in the thickness direction from the surface of the steel plate: at depths of t / 8, t / 4, t / 2, 3t / 4, and 7t / 8. The test force was set to 98 N. The arithmetic mean of the five hardnesses obtained was defined as the Vickers hardness (HV) of the test specimen. The arithmetic mean of the Vickers hardness of the 10 test specimens obtained was defined as the Vickers hardness of the test number. The Vickers hardness was defined as an integer value obtained by rounding the first decimal place of the obtained result. The obtained Vickers hardness (HV) is shown in the "Cold Workability (Hardness) (HV)" column in Table 3.

[0128] [(Test 5) Ductility Test] Tensile tests were conducted using the following method to indicate the cold workability of the steel plates for each test number. Tensile test specimens were taken from the steel plate, including the center of the plate width. The tensile test specimens were JIS No. 5 specimens. The parallel section was parallel to the rolling direction of the steel plate. Tensile tests were conducted at room temperature in air in accordance with JIS Z 2241 (2011), and the elongation at break (%) was determined. The obtained elongation at break (%) is shown in the "Cold workability (ductility) (%)" column of Table 3.

[0129] [Test Results] Referring to Tables 1 to 3, test numbers 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. As a result, excellent cold workability was obtained.

[0130] On the other hand, in test number 30, the carbon content was too high. As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

[0131] In test number 31, the carbon content was too low. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0132] In test number 32, the Si content was too high. As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

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

[0134] In test number 34, the Mn content was too high. As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

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

[0136] In test number 36, the Cr content was too high. As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

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

[0138] In test number 38, the heating temperature T 11 The average particle size D of the ferrite was too high. t The thickness exceeded 30.00 μm. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0139] In test number 39, the heating temperature T 11 The average particle size D of the ferrite was too low. t The thickness 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 could not be obtained.

[0140] In test number 40, the final finishing rolling start temperature T 12The value was too low. As a result, the average ferrite particle size Dt was less than 5.00 μm. Furthermore, F1 fell below the lower limit of equation (1). Consequently, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

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

[0142] In test number 42, the finishing rolling temperature FT was too high. As a result, the average particle size D of the ferrite was too high. t The thickness exceeded 30.00 μm. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0143] In test number 43, the σ defined by equation (1) is found in the rolling stand L-1, which is the downstream rolling stand L. L-1 The value was less than 21. Therefore, F1 was below the lower limit of equation (1). Furthermore, the ferrite grain deviation S at the center of the plate thickness t / 2 The thickness exceeded 4.00 μm. As a result, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

[0144] In test number 44, σ L-1 The value exceeded 35. Therefore, F1 was below the lower limit of equation (1). Furthermore, the ferrite grain deviation S at the center of the plate thickness t / 2 The thickness exceeded 4.00 μm. As a result, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

[0145] In test number 45, the σ defined by equation (1) is found at the downstream rolling stand L. L The value exceeded 90. As a result, the average particle size Dt of the ferrite was less than 5.00 μm. Consequently, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

[0146] In test number 46, the σ defined by equation (1) is found at the downstream rolling stand L. L The value was less than 71. Therefore, the average particle size Dt of the ferrite was less than 5.00 μm. Furthermore, F1 was below the lower limit of equation (1). As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

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

[0148] In test number 48, the winding temperature CT was high. Therefore, the average particle size D of the ferrite was high. t The thickness exceeded 30.00 μm. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0149] In test number 49, the winding temperature CT was low. Therefore, the average particle size D of the ferrite was low. t The thickness 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 could not be obtained.

[0150] In test number 50, the annealing temperature T3 was high. Therefore, the average grain size D of the ferrite was high. t The thickness exceeded 30.00 μm. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0151] In test number 51, the annealing temperature T3 was low. Therefore, the average grain size D of the ferrite was low. t The thickness 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 could not be obtained.

[0152] In test number 52, the retention time t3 was short. Therefore, the average particle size D of the ferrite was short. tThe thickness 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 could not be obtained.

[0153] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. It is a steel plate, The chemical composition is expressed in mass percent. C: 0.20-0.70%, Si: 0.07-1.00%, Mn: 0.20-3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010-1.500%, 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: 0-0.03%, Ta: 0-0.03%, 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: 0-0.030%, and, Contains Ca: 0-0.030%, The remainder consists of Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more. Average particle size D of ferrite ｔ The size is 5.00 to 30.00 μm. When the thickness of the steel plate is t (mm), the average particle size D of the ferrite at a depth of t / 8 mm from the surface of the steel plate in the thickness direction is given by ｔ／８ (μm) and the average particle size D of the ferrite at a depth of t / 2 mm from the surface of the steel plate in the thickness direction. ｔ／２ (μm) and satisfy equation (1), The sample standard deviation S of the ferrite particle size at the aforementioned depth position t / 8 mm. ｔ／８ It is 4.50 μm or less, The sample standard deviation S of the ferrite particle size at the aforementioned depth position t / 2 mm. ｔ／２ It is 4.00 μm or less. steel plate. 0.90≦D ｔ／８ / D ｔ／２ ≦1.10 (1)

2. A steel plate according to claim 1, Ti: 0.001 to 0.500%, V: 0.001-0.500%, Nb: 0.001-0.500%, B: 0.0001 to 0.0035%, Cu: 0.01-0.20%, W: 0.01-0.03%, Ta: 0.01-0.03%, Sn: 0.001-0.030%, Sb: 0.001 to 0.030%, Co: 0.001 to 0.030%, As: 0.001 to 0.030%, Mg: 0.001-0.030%, Y: 0.001-0.030%, Zr: 0.001 to 0.030%, La: 0.001 to 0.030%, Ce: 0.001–0.030%, and, Contains one or more selected from the group consisting of Ca: 0.001 to 0.030%, steel plate.

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