steel plate

Abstract

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

Description

[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 [Summary of the Invention] [Problems to be Solved by the Invention]

[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 having excellent cold workability. [Means for Solving the Problems]

[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 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%, and the balance consists of Fe and impurities. In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more, the average particle diameter of ferrite grains is 5.0 to 30.0 μm, and the GOS (Grain Orientation Spread) value of ferrite grains is 2.0° or less. [Effects of the Invention]

[0009] In the steel sheet of the present disclosure, excellent cold workability can be obtained. [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-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 strain distribution within the steel sheet. When strain is locally present within the steel sheet, regions with different amounts of plastic deformation occur locally during cold working. In this case, uniform plastic deformation becomes impossible, resulting in non-uniform plastic deformation. As a result, cold workability decreases. Therefore, the inventors focused on ferrite grains, which are the main component of the macrostructure of the steel sheet. If there is variation in the amount of strain within the ferrite grains, there is a high possibility that the amount of plastic deformation will also vary at the ferrite grain level.

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

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

[0016] 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, the average particle size of ferrite grains is 5.0-30.0 μm, and the GOS (Grain Orientation Spread) value of ferrite grains is 2.0° or less.

[0017] The second form of steel sheet is the same as the first form of steel sheet, with a chemical composition in mass% of: 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: 0.0 It contains one or more elements selected from the group consisting of 0.01-0.030%, 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%.

[0018] The steel plate of this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percentage.

[0019] [Features of the steel plate of this embodiment] The steel plate of this embodiment satisfies the following characteristics. (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) The average particle size of the ferrite grains is 5.0 to 30.0 μm. (Feature 4) The GOS value of the ferrite grains is 2.0° or less. The following describes each of its features.

[0020] [(Feature 1) Regarding chemical composition] The chemical composition of the steel sheet in this embodiment contains the following elements:

[0021] 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%.

[0022] 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%.

[0023] 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%.

[0024] P:0.030% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content exceeds 0.030%, the toughness of the steel 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%.

[0025] S: 0.0080% or less Sulfur (S) is an unavoidable impurity. In other words, the S content is greater than 0%. If the S content exceeds 0.0080%, sulfides will be excessively formed. 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%.

[0026] 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%.

[0027] 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 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. Therefore, 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%.

[0028] N: 0.0200% or less Nitrogen (N) is an unavoidable impurity. In other words, the N content is greater than 0%. N combines with Al to form Al nitride. Al nitride 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 become 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%.

[0029] 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.

[0030] [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.

[0031] [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.

[0032] 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%.

[0033] 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%.

[0034] 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%.

[0035] 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%.

[0036] [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%.

[0037] 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%.

[0038] 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%.

[0039] The preferred lower limit for W content is 0.01%. The preferred upper limit for W content is 0.02%.

[0040] The preferred lower limit for the Ta content is 0.01%. The preferred upper limit for the Ta content is 0.02%.

[0041] 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%.

[0042] 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%.

[0043] 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%.

[0044] 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%.

[0045] 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%.

[0046] 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%.

[0047] 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%.

[0048] 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%.

[0049] 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%.

[0050] 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%.

[0051] [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.

[0052] 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.

[0053] 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.

[0054] [(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.

[0055] 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.

[0056] 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.

[0057] 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%.

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

[0059] [Method for measuring 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] The total area ratio (%) of ferrite and cementite particles is calculated based on the total area of ​​ferrite and cementite particles in the five observation fields and the total area of ​​the five observation fields. The total area ratio is rounded to the nearest integer.

[0064] [(Feature 3) Regarding the average particle size of ferrite grains] In the steel sheet of this embodiment, the average particle size of the ferrite grains is further 5.0 to 30.0 μm.

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

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

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

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

[0069] [Method for measuring the average particle size of ferrite grains] The average particle size of ferrite grains can be measured by the following method.

[0070] A test specimen measuring 15 mm in the rolling direction and 10 mm in the width direction, with a thickness of 15 mm, is taken from the center of the steel plate's width. The surface of the test specimen with a thickness of 15 mm in the rolling direction (i.e., the L-section) is defined as the observation surface. The observation surface of the test specimen is mirror-polished. After mirror polishing, etching is performed with a 3% nital etchant. The etching time is 120 seconds. On the etched observation surface, secondary electron images are taken using a scanning electron microscope (SEM) at five arbitrary observation fields at a depth of thickness / 4 from the surface of the steel plate. The grain size number of ferrite is determined by the sectioning method in accordance with JIS G 0551:2020. At this time, the SEM magnification is selected in the range of 500 to 3000x so that the number of ferrite grains cut by one line segment is 10 to 50 in one field of view. The length of the line segment is 1 mm. The line segments are set up so that no cementite particles overlap them; in other words, all overlapping portions are ferrite grains. The section lengths are determined for five observation fields. Here, the section length is the value obtained by dividing the length L of the line segment by the number of times N the line segment intersects the grain boundaries of the ferrite grains. The ferrite grain size number is determined from the arithmetic mean of the section lengths for the five observation fields. The average grain size (μm) of the ferrite grains is determined from the obtained grain size number. The average particle size of the ferrite grains is the value obtained by rounding the second decimal place of the obtained value (i.e., the value to the first decimal place).

[0071] [(Feature 4) Regarding the GOS (Grain Orientation Spread) value of ferrite grains] In this embodiment, the GOS value of the ferrite grains is further 2.0° or less.

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

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

[0074] [Method for measuring GOS] The method for measuring GOS in this embodiment is as follows: A test specimen for measuring the KAM value is taken from the steel plate. The size of the test specimen is not particularly limited, as long as it has a cross-section (hereinafter referred to as the observation surface) that includes the center of the plate thickness and contains three observation fields of 100 μm in the thickness direction and 100 μm in the rolling direction without overlapping with each other.

[0075] The observation surface of the test specimen is polished to a mirror finish. Within the mirror-polished observation surface, three 100 μm × 100 μm measurement fields are set up so that they do not overlap. Electron backscatter diffraction (EBSD) measurements are performed for each observation field. For EBSD measurements, the acceleration voltage is set to 20 kV and the irradiation current to 28 nA. The measurement points in each observation field are set as follows.

[0076] Each observation field is divided into hexagonal pixel units. The distance between the centers of adjacent pixels is set to 0.3 μm. The measurement point is defined as the center position of a pixel whose entire hexagonal shape is contained within the observation field. In other words, pixels whose portion is outside the observation field are excluded from measurement.

[0077] The crystal orientation obtained from EBSD measurements at each measurement point is defined as the crystal orientation of the pixel containing that measurement point. Measurement points with a Confidence Index (CI value) of 0.1 or less are not used in subsequent calculations. Furthermore, pixels containing measurement points with a Confidence Index of 0.1 or less are treated as non-existent.

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

[0079] Among the multiple pixels identified as ferrite, a region of consecutively arranged pixels is identified as a ferrite region. Within the ferrite region, the orientation difference between the target pixel and each of the six adjacent pixels (six pixels that have an edge in contact with each edge of the hexagonal pixel) is calculated. The arithmetic mean of the six obtained orientation differences is defined as the orientation difference of the pixel in question. A group of consecutively arranged pixels whose obtained orientation differences are 5° or less is defined as a group of pixels within the same ferrite grain.

[0080] Based on the group of pixels (multiple pixels) determined to be within the same ferrite grain, the GOS is calculated as follows. Select any pixel (measurement point) within the same ferrite grain. Determine the crystal orientation difference between this pixel and each other pixel (each measurement point) within the same ferrite grain. Perform this operation for all pixels (measurement points) within the ferrite grain. Calculate the average value of the orientation differences between all obtained pixels. Define the calculated value as the GOS value of the crystal grain.

[0081] The GOS value mentioned above is defined by the following formula.

number

[0082] Multiple measurement points within the same ferrite grain will all show the same GOS value. Calculate the arithmetic mean of the GOS values ​​for all ferrite grains in the three measurement regions. This arithmetic mean is defined as the GOS value. The GOS value is rounded to one decimal place by rounding the second decimal place of the obtained value.

[0083] The arithmetic mean of the GOS values ​​of all ferrite grains within the observation field is defined as the GOS value of the ferrite grains in the steel sheet. The GOS value can be determined by analyzing it using OIM Analysis, a well-known analysis software manufactured by TSL Corporation.

[0084] [Effects of the steel plate in this embodiment] The steel sheet of this embodiment satisfies features 1 to 4. Therefore, the steel sheet of this embodiment provides excellent cold workability.

[0085] [Uses of steel plates] The steel sheet of this embodiment is a material for mechanical parts, such as automotive parts. Examples of mechanical parts include automobile springs and washers, and bicycle gears. The steel sheet of this embodiment may also be used for applications other than mechanical parts where excellent cold workability is required.

[0086] [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.

[0087] 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.

[0088] [(Process 1) Hot Rolling Process] In the hot rolling process, hot rolling is performed on a slab that satisfies Feature 1. The slab is manufactured, for example, by the following method: Molten steel that satisfies Feature 1 is produced, in which the content of each element in the chemical composition is within the range of this embodiment. A slab is manufactured using the above molten steel by a casting method. For example, a slab is manufactured using the above molten steel by a well-known continuous casting method. The chemical composition of the manufactured slab satisfies Feature 1.

[0089] 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.

[0090] [(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.

[0091] [(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.

[0092] [(Process 12) Finish Rolling Process] In the finishing rolling process, a tandem rolling mill is used to perform finishing rolling on the intermediate steel sheet (rough bar) to produce a hot-rolled steel sheet. 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 that make up 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.

[0093] [(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.

[0094] [(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.

[0095] [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 is the surface temperature of the steel sheet at the entry side of the downstream rolling stand L, which applies the final reduction in the finish rolling process. 12Set it to 850 °C or higher. (Condition 3) Set the finish rolling temperature FT in the finish rolling process to 850 - 1000 °C. (Condition 4) Among the plurality of rolling stands 1 - L of the tandem rolling mill used in the finish rolling process, at the rolling stand L - 1 immediately preceding the most downstream rolling stand L, σ defined by Equation (1) L-1 is set to 5 - 20. Further, at the most downstream rolling stand L, σ defined by Equation (1) L is set to 30 - 70. σ n = exp(0.753 + 3000 / (T n + 273)) × ε n 0.21 × v n 0.13 (1) Here, n is 1 - 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 side of the rolling stand n. When applying Equation (1) at the rolling stand L - 1, n in Equation (1) is L - 1. When applying Equation (1) at the rolling stand L, n in Equation (1) is L. Note that when applying Equation (1) at the rolling stand L, for T n (°C) in Equation (1), substitute the final finish rolling start temperature T 12 (°C). On the other hand, when applying Equation (1) at the rolling stand L - 1, for Tn (°C) in Equation (1), substitute T 12 + 20 (°C). (Condition 5) Set the cumulative reduction ratio R in the hot rolling process to 60% or more. (Condition 6) Set the coiling temperature CT in the coiling process to 650 - 550 °C. (Condition 7) Set the annealing temperature T3 in the annealing process to 600 - 730 °C, and set the holding time t3 at the annealing temperature T3 to 20 hours or more. The following explains each condition.

[0096] [(Condition 1) Heating temperature T 11 [About] Heating temperature T in the heating process 11 If the temperature is below 1100℃, the ferrite grains in the manufactured steel sheet become excessively small. As a result, the average particle size of the ferrite grains becomes less than 5.0 μm. On the other hand, the heating temperature T 11 If the temperature exceeds 1350℃, the ferrite grains in the manufactured steel sheet become coarse. As a result, the average particle size of the ferrite grains exceeds 30.0 μm. Therefore, the heating temperature T 11 The temperature range is 1100-1350°C.

[0097] [(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 intermediate steel sheet (rough bar) 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. Final finishing rolling start temperature T 12 If the temperature is below 850°C, the ferrite in the manufactured steel sheet will become excessively small. As a result, the average particle size of the ferrite grains will be less than 5.0 μm. Therefore, the final finishing rolling start temperature T 12 It is above 850℃.

[0098] [(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.

[0099] If the finishing rolling temperature (FT) is below 850°C, the ferrite in the manufactured steel sheet becomes excessively small. As a result, the average particle size of the ferrite grains becomes less than 5.0 μm. When the finishing rolling temperature (FT) exceeds 1000°C, the ferrite in the manufactured steel sheet becomes excessively large. As a result, the average particle size of the ferrite grains exceeds 30.0 μm. Therefore, the finishing rolling temperature FT is 850-1000°C.

[0100] [(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 (1) L-1 Let it be 5 to 20. Furthermore, at the downstream rolling stand L of the multiple rolling stands of the tandem rolling mill, σ defined by equation (1) L The pressure is set to 30-70. 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.

[0101] σ defined by equation (1) 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 5, the reduction at rolling stand L-1 is insufficient. In this case, unrecrystallized and recrystallized material will be mixed together. As a result, the GOS value will become excessively large, exceeding 2.0°. On the other hand, σ L-1 If the GOS value exceeds 20, the reduction at rolling stand L-1 is excessive. In this case, recrystallization occurs at the exit side of rolling stand L-1 and the inlet side of rolling stand L. As a result, the GOS value becomes excessively large, exceeding 2.0°.

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

[0103] σ L-1 Let σ be 5-20, and further, L By setting the GOS value to 30-70, recrystallization can be suppressed in the rough bar before passing through rolling stand L-1 and rolling stand L, and then rapidly recrystallized in the hot-rolled steel sheet after passing through rolling stand L. In this case, recrystallization occurs at almost the same timing in the thickness direction and in the direction perpendicular to the thickness of the steel sheet. As a result, excessive strain accumulation in the ferrite grains of the manufactured steel sheet can be suppressed, and the GOS value becomes 2.0° or less.

[0104] [(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).

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

[0106] [(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 grains become coarser. As a result, the average particle size of the ferrite grains exceeds 30.0 μm. On the other hand, if the winding temperature (CT) falls below 550°C, the ferrite grains become excessively fine. As a result, the average particle size of the ferrite grains becomes less than 5.0 μm. Therefore, the winding temperature CT is 650-550°C.

[0107] [(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, or the holding time t3 is less than 20 hours, the annealing is insufficient. In this case, the ferrite grains become excessively fine. As a result, the average grain size of the ferrite grains becomes less than 5.0 μm. On the other hand, if the annealing temperature T3 exceeds 730°C, the ferrite grains become coarser. As a result, the average grain size of the ferrite grains exceeds 30.0 μm.

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

[0109] 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.

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

[0111] [Table 1]

[0112] 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-1 and 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.

[0113] [Table 2]

[0114] 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.

[0115] [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) Measurement test of average ferrite particle size of steel plate (Test 4) Measurement test of GOS value of steel plate (Test 5) Vickers hardness test (Test 6) Ductility Test The following explains Exams 1 through 6.

[0116] [(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.

[0117] [(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 the above-mentioned [Method for measuring the total area ratio of ferrite and cementite particles in the microstructure]. The obtained total area ratios are shown in Table 3. In all test numbers, the total area ratio of ferrite and cementite particles was 95% or higher.

[0118] [Table 3]

[0119] [(Test 3) Measurement test of average ferrite particle size of steel plate] The average ferrite particle size (μm) of the steel plates for each test number was determined in accordance with the method described in [Method for Measuring the Average Particle Size of Ferrite Grains] above. The results are shown in Table 3.

[0120] [(Test 4) Measurement test of GOS value of steel plate] Based on the [Method for Measuring GOS Values] described above, the GOS values ​​of the steel plates for each test number were determined. The results are shown in Table 3.

[0121] [(Test 5) 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 taken as the Vickers hardness of the test specimen. The arithmetic mean of the Vickers hardness of the 10 test specimens obtained was taken as the Vickers hardness (HV) of the test number. The Vickers hardness was rounded to the nearest integer. The obtained Vickers hardness (HV) is shown in the "Cold Workability (Hardness) (HV)" column in Table 3.

[0122] [(Test 6) Ductility Test] 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.

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

[0124] 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.

[0125] 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.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] In test number 36, the Cr content was too high. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, resulting in insufficient cold workability.

[0131] 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.

[0132] In test number 38, the heating temperature T in the heating process was 11 The pressure was too high. As a result, the average particle size of the ferrite grains exceeded 30.0 μm. Consequently, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0133] In test number 39, the heating temperature T in the heating process was 11 The pressure was too low. As a result, the average particle size of the ferrite grains was less than 5.0 μm. Consequently, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, resulting in insufficient cold workability.

[0134] In test number 40, the final finishing rolling start temperature T 12The pressure was too low. As a result, the average particle size of the ferrite grains was less than 5.0 μm. Consequently, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, resulting in insufficient cold workability.

[0135] In test number 41, the finish rolling temperature (FT) was too low. As a result, the average particle size of the ferrite grains was less than 5.0 μm. Consequently, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, resulting in insufficient cold workability.

[0136] In test number 42, the finishing rolling temperature (FT) was too high. As a result, the average particle size of the ferrite grains exceeded 30.0 μm. Consequently, the Vickers hardness was less than 110 HV, and the strength was excessively low.

[0137] 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 5. Therefore, the GOS value exceeded 2.0°. As a result, the elongation at break was less than 40%, and sufficient cold workability could not be obtained.

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

[0139] In test number 45, the σ defined by equation (1) is found at the downstream rolling stand L. L The value exceeded 70. As a result, the average particle size of the ferrite grains was less than 5.0 μm. Furthermore, the GOS value exceeded 2.0°. Consequently, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, resulting in insufficient cold workability.

[0140] In test number 46, the σ defined by equation (1) is found at the downstream rolling stand L. LThe value was less than 30. As a result, the average particle size of the ferrite grains was less than 5.0 μm. Furthermore, the GOS value exceeded 2.0°. Consequently, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, resulting in insufficient cold workability.

[0141] In test number 47, the cumulative reduction ratio R was less than 60%. As a result, the average particle size of the ferrite grains exceeded 30.0 μm. Furthermore, the GOS value exceeded 2.0°. Consequently, the Vickers hardness was less than 110 HV, indicating excessively low strength. In addition, the elongation at break was less than 40%, resulting in insufficient cold workability.

[0142] In test number 48, the winding temperature (CT) was high. As a result, the average particle size of the ferrite grains exceeded 30.0 μm. Consequently, the Vickers hardness was less than 110 HV, resulting in excessively low strength.

[0143] In test number 49, the winding temperature (CT) was low. As a result, the average particle size of the ferrite grains was less than 5.0 μm. Consequently, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, resulting in insufficient cold workability.

[0144] In test number 50, the annealing temperature T3 was too high. As a result, the average particle size of the ferrite grains exceeded 30.0 μm. Consequently, the Vickers hardness was less than 110 HV, resulting in excessively low strength.

[0145] In test number 51, the annealing temperature T3 was low. As a result, the average particle size of the ferrite grains was less than 5.0 μm. Consequently, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, resulting in insufficient cold workability.

[0146] In test number 52, the holding time t3 was short. As a result, the average particle size of the ferrite grains was less than 5.0 μm. Consequently, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, resulting in insufficient cold workability.

[0147] 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. The average particle size of the ferrite grains is 5.0 to 30.0 μm. The GOS (Grain Orientation Spread) value of the ferrite grain is 2.0° or less. steel plate.

2. A steel plate according to claim 1, The aforementioned chemical composition is, in mass%, 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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