steel sheet

Abstract

This invention provides a steel sheet with excellent hardenability and excellent cold workability. The steel sheet disclosed herein has the following composition by mass %: C: 0.50~0.90%, Si: 0.01~0.50%, Mn: 0.20~1.30%, P: less than 0.100%, S: less than 0.100%, Al: less than 0.100%, Cr: 0.01~1.20%, N: less than 0.0150%, and the balance being Fe and impurities. The total surface area of ​​ferrite and cementite particles is 95% or more, the average grain size of ferrite is 5.0~20.0 μm, and the Cr concentration in the cementite particles is [Cr]. θ Less than 2.00%, Mo concentration [Mo] θ The content is below 1.00%, the average particle size of cementite particles is below 1.50 μm, and the spheroidization rate of cementite particles is above 85%.

Description

Technical Field

[0001] This disclosure relates to steel sheets, and more specifically, to steel sheets that can be used as blanks for mechanical parts, such as automotive parts. Background Technology

[0002] Steel sheets with a carbon content of 0.15% or higher are used as raw materials for mechanical parts, such as automotive components. The method for manufacturing these mechanical parts using steel sheets as raw materials is as follows: The steel sheet is cold-worked to form the shape of the mechanical part. The cold-worked steel sheet is then quenched and tempered. Through these manufacturing processes, high-strength mechanical parts are manufactured. To obtain high strength in the quenched mechanical parts, excellent hardenability is required for the steel sheet during the quenching process in the manufacturing of mechanical parts using steel sheets as raw materials. Furthermore, the steel sheet is cold-worked before quenching. Therefore, for steel sheets, not only excellent hardenability but also excellent cold workability is required.

[0003] Patent Document 1 and Patent Document 2 propose steel plates that have excellent hardenability and excellent cold workability when quenched as blanks for use as mechanical parts.

[0004] The steel plate disclosed in Patent Document 1 has the following composition by mass: 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 also contains 0.002-0.030% of one or more of Sb, Sn, Bi, Ge, Te, and Se, with the balance being Fe and unavoidable impurities. In this steel plate, the amount of dissolved B accounts for more than 70% of the total B content. 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 described in Patent Document 1: by suppressing the cementite density within the ferrite grains to a lower level, the total elongation can be improved.

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

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: International Publication No. 2015 / 146173

[0009] Patent Document 2: International Publication No. 2020 / 175665 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] Furthermore, springs, washers, and other components in mechanical parts require high strength. To improve strength, the hardenability of the steel sheet used as raw material needs to be further improved. Carbon (C) in steel sheet is an effective element for improving hardenability. Therefore, steel sheets used for these mechanical parts have a C content of 0.50% or higher. In such high-C-content steel sheets, not only hardenability but also cold workability is required.

[0012] The purpose of this disclosure is to provide a steel sheet with excellent hardenability and excellent cold workability.

[0013] Solution for solving the problem

[0014] The steel plate disclosed herein has the following composition.

[0015] A steel plate, the chemical composition of which is (in mass%) is

[0016] C: 0.50~0.90%

[0017] Si: 0.01~0.50%

[0018] Mn: 0.20~1.30%,

[0019] P: Below 0.100%

[0020] S: Below 0.100%

[0021] Al: Below 0.100%

[0022] Cr: 0.01~1.20%

[0023] N: below 0.0150%

[0024] Mo: 0~0.500%,

[0025] Ni: 0~1.000%

[0026] B: 0~0.0100%

[0027] V: 0~0.500%

[0028] Nb: 0~0.500%,

[0029] Ti: 0~0.150%, and

[0030] Balance: Fe and impurities,

[0031] In the microstructure, the total area fraction of ferrite and cementite particles is over 95%.

[0032] The average grain size of the ferrite mentioned above is 5.0~20.0 μm.

[0033] The Cr concentration [Cr] in the above-mentioned cementite particles, expressed as a percentage by mass. θ Less than 2.00%, the Mo concentration [Mo] in the above cementite particles, expressed as a percentage by mass. θ Below 1.00%,

[0034] The average particle size of the aforementioned cementite particles is less than 1.50 μm.

[0035] When cementite particles with an aspect ratio of 3.0 or less are defined as spherical cementite particles, the ratio of the total number of spherical cementite particles to the total number of cementite particles, i.e., the spheroidization rate, is 85% or more.

[0036] The effects of the invention

[0037] The steel plate disclosed herein has excellent hardenability and excellent cold workability. Detailed Implementation

[0038] The inventors have conducted research on steel sheets exhibiting excellent hardenability and excellent cold workability. As a result, the inventors have obtained the following insights.

[0039] First, the inventors conducted research on improving the hardenability and cold workability of steel plates with a C content of 0.50% or more from the perspective of chemical composition. As a result, the inventors concluded that if the chemical composition (by mass%) is C: 0.50~0.90%, Si: 0.01~0.50%, Mn: 0.20~1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01~1.20%, N: 0.0150% or less, Mo: 0~0.500%, Ni: 0~1.000%, B: 0~0.0100%, V: 0~0.500%, Nb: 0~0.500%, Ti: 0~0.150%, with the balance being Fe and impurities, then both improved hardenability and improved cold workability can be achieved. Therefore, the inventors further investigated, from the viewpoint of microstructure, methods that can simultaneously improve hardenability and cold workability of steel plates having the above-mentioned chemical composition.

[0040] The inventors first investigated methods that improve hardenability in the microstructure of steel plates. The microstructure of the steel plate having the above-described chemical composition is essentially composed of ferrite and cementite particles. During quenching in the manufacturing process of mechanical parts using steel plates as raw materials, it is preferable that the cementite particles in the steel plate readily dissolve during quenching to improve the hardenability of the steel plate. To improve the solid solubility of the cementite particles during quenching, it is preferable that the cementite particles have a small particle size. In the case of the steel plate having the above-described chemical composition, it is effective to make the average particle size of the cementite particles 1.50 μm or less.

[0041] Furthermore, the inventors' research results indicate that the Mn concentration in cementite particles has no effect on the solid solution of cementite particles during quenching. On the other hand, the inventors found that the Cr and Mo concentrations in cementite particles have a significant impact on the solid solution of cementite particles during quenching. Specifically, if the Cr and Mo concentrations in cementite particles are high, the cementite particles become difficult to dissolve during quenching.

[0042] Based on the above insights, further research was conducted. As a result, the inventors discovered that if the Cr concentration [Cr] in the cementite particles... θ Less than 2.00%, Mo concentration [Mo] θ If the content is below 1.00%, the cementite particles become easier to dissolve during quenching, thus improving the hardenability of the steel plate.

[0043] The inventors further investigated methods to improve the cold workability of steel sheets in their microstructure. To improve the cold workability of steel sheets, it is effective to increase the spheroidization rate of cementite particles and to make the average grain size of ferrite coarser. Therefore, in the steel sheet of this embodiment, the spheroidization rate of cementite particles is 85% or more, and the average grain size of ferrite is 5.0 μm or more. It should be noted that when the average grain size of ferrite is small, the ferrite grain boundary area in the steel sheet increases. Therefore, through grain boundary diffusion, the dissolution of cementite particles during quenching is accelerated. Therefore, the average grain size of ferrite is 20.0 μm or less.

[0044] The steel plate of this embodiment, based on the above insights, has the following structure. [1]

[0046] A steel plate, the chemical composition of which is (in mass%) is

[0047] C: 0.50~0.90%

[0048] Si: 0.01~0.50%

[0049] Mn: 0.20~1.30%,

[0050] P: Below 0.100%

[0051] S: Below 0.100%

[0052] Al: Below 0.100%

[0053] Cr: 0.01~1.20%

[0054] N: below 0.0150%

[0055] Mo: 0~0.500%,

[0056] Ni: 0~1.000%

[0057] B: 0~0.0100%

[0058] V: 0~0.500%

[0059] Nb: 0~0.500%,

[0060] Ti: 0~0.150%, and

[0061] Balance: Fe and impurities,

[0062] In the microstructure, the total area fraction of ferrite and cementite particles is over 95%.

[0063] The average grain size of the ferrite mentioned above is 5.0~20.0 μm.

[0064] The Cr concentration [Cr] in the above-mentioned cementite particles, expressed as a percentage by mass. θ Less than 2.00%, the Mo concentration [Mo] in the above cementite particles, expressed as a percentage by mass. θ Below 1.00%,

[0065] The average particle size of the aforementioned cementite particles is less than 1.50 μm.

[0066] When cementite particles with an aspect ratio of 3.0 or less are defined as spherical cementite particles, the ratio of the total number of spherical cementite particles to the total number of cementite particles, i.e., the spheroidization rate, is 85% or more. [2]

[0068] According to the steel plate described in [1], wherein,

[0069] The above chemical composition contains selected free radicals

[0070] Mo: 0.001~0.500%,

[0071] Ni: 0.001~1.000%

[0072] B: 0.0001~0.0100%

[0073] V: 0.001~0.500%

[0074] Nb: 0.001~0.500%, and

[0075] Ti: One or more elements in a group consisting of 0.001 to 0.150%.

[0076] The steel plate of this embodiment will now be described in detail. It should be noted that, unless otherwise specified, "%" related to elements refers to mass%.

[0077] [Features of the steel plate in this embodiment]

[0078] The steel plate of this embodiment satisfies the following features 1 to 6.

[0079] (Feature 1)

[0080] The chemical composition, by mass%, is: C: 0.50~0.90%, Si: 0.01~0.50%, Mn: 0.20~1.30%, P: less than 0.100%, S: less than 0.100%, Al: less than 0.100%, Cr: 0.01~1.20%, N: less than 0.0150%, Mo: 0~0.500%, Ni: 0~1.000%, B: 0~0.0100%, V: 0~0.500%, Nb: 0~0.500%, Ti: 0~0.150%, with the balance being Fe and impurities.

[0081] (Feature 2)

[0082] In the microstructure, the total area ratio of ferrite and cementite particles is over 95%.

[0083] (Feature 3)

[0084] The average grain size of ferrite is 5.0~20.0 μm.

[0085] (Feature 4)

[0086] Cr concentration [Cr] in cementite particles (mass%) θ Less than 2.00%, Mo concentration [Mo] in cementite particles (mass %) θ It is below 1.00%.

[0087] (Feature 5)

[0088] The average particle size of the cementite particles is less than 1.50 μm.

[0089] (Feature 6)

[0090] When cementite particles with an aspect ratio of 3.0 or less are defined as spherical cementite particles, the ratio of the total number of spherical cementite particles to the total number of cementite particles, i.e., the spheroidization rate, is 85% or more.

[0091] The following describes features 1 through 6.

[0092] [Regarding (Characteristic 1) Chemical Composition]

[0093] The steel plate of this embodiment contains the following elements in its chemical composition.

[0094] C: 0.50~0.90%

[0095] Carbon (C) improves the hardenability of steel sheets. As a result, the strength of mechanical parts is increased by performing quenching in the process of manufacturing mechanical parts using steel sheets as raw materials. If the C content is less than 0.50%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0096] On the other hand, if the C content exceeds 0.90%, the cold workability of the steel plate will decrease even if the contents of other elements are within the range of this embodiment.

[0097] Therefore, the C content is 0.50~0.90%.

[0098] The preferred lower limit for C content is 0.52%, more preferably 0.55%, and even more preferably 0.60%.

[0099] The preferred upper limit for C content is 0.88%, more preferably 0.85%, and even more preferably 0.80%.

[0100] Si: 0.01~0.50%

[0101] Silicon (Si) deoxidizes steel during the steelmaking stage of the steel sheet manufacturing process. Furthermore, Si increases the tempering softening resistance of the steel sheet during tempering in the process of manufacturing mechanical parts from steel sheet as raw material. If the Si content is less than 0.01%, the aforementioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0102] On the other hand, if the Si content exceeds 0.50%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, even if the contents of other elements are within the range of this embodiment, the cold workability of the steel sheet will decrease.

[0103] Therefore, the Si content is 0.01~0.50%.

[0104] The preferred lower limit for Si content is 0.02%, more preferably 0.05%, and even more preferably 0.10%.

[0105] The preferred upper limit for the Si content is 0.48%, more preferably 0.44%, and even more preferably 0.40%.

[0106] Mn: 0.20~1.30%

[0107] Manganese (Mn) improves the hardenability of steel sheets. As a result, the strength of mechanical parts is increased by performing quenching in the process of manufacturing mechanical parts using steel sheets as raw materials. If the Mn content is less than 0.20%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0108] On the other hand, if the Mn content exceeds 1.30%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, even if the contents of other elements are within the range of this embodiment, the cold workability of the steel sheet will decrease.

[0109] Therefore, the Mn content is 0.20~1.30%.

[0110] The preferred lower limit for Mn content is 0.25%, more preferably 0.30%, and even more preferably 0.35%.

[0111] The preferred upper limit for Mn content is 1.25%, more preferably 1.20%, and even more preferably 1.15%.

[0112] P: below 0.100%

[0113] Phosphorus (P) is an impurity. If the P content exceeds 0.100%, the toughness of the steel plate will decrease even if the contents of other elements are within the range of this embodiment.

[0114] Therefore, the P content is below 0.100%.

[0115] The phosphorus (P) content is preferably as low as possible. However, extreme reductions in P content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit for P content is over 0%, more preferably 0.001%, more preferably 0.003%, and more preferably 0.005%.

[0116] The preferred upper limit for the P content is 0.090%, more preferably 0.080%, and even more preferably 0.050%.

[0117] S: Below 0.100%

[0118] Sulfur (S) is an impurity. If the S content exceeds 0.100%, excessive S will form sulfides. Therefore, even if the contents of other elements are within the range of this embodiment, the cold workability of the steel sheet will be reduced.

[0119] Therefore, the sulfur content is below 0.100%.

[0120] The sulfur content is preferably as low as possible. However, extreme reductions in sulfur content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit for sulfur content is more than 0%, more preferably 0.001%, more preferably 0.003%, and more preferably 0.005%.

[0121] The preferred upper limit for the sulfur content is 0.090%, more preferably 0.080%, and even more preferably 0.050%.

[0122] Al: below 0.100%

[0123] Aluminum (Al) is an impurity. Al combines with nitrogen to form AlN. During the quenching process in the manufacture of mechanical parts from steel sheets, AlN causes austenite grain refinement. This refinement of the austenite grains reduces the hardenability of the steel sheet. Therefore, if the Al content exceeds 0.100%, even if the contents of other elements are within the range of this embodiment, the austenite grains will become excessively refined during the quenching process. As a result, the hardenability of the steel sheet is significantly reduced.

[0124] Therefore, the Al content is below 0.100%.

[0125] The Al content is preferably as low as possible. However, extreme reductions in Al content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit for Al content is more than 0%, more preferably 0.001%, more preferably 0.005%, and more preferably 0.010%.

[0126] The preferred upper limit for Al content is 0.090%, more preferably 0.080%, more preferably 0.070%, and more preferably 0.050%.

[0127] In the chemical composition of the steel plate in this embodiment, the Al content refers to the content of acid-soluble Al (sol.Al).

[0128] Cr: 0.01~1.20%

[0129] Chromium (Cr) improves the hardenability of steel sheets. As a result, the strength of mechanical parts is increased by performing quenching in the process of manufacturing mechanical parts from steel sheets. If the Cr content is less than 0.01%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0130] On the other hand, if the Cr content exceeds 1.20%, then the Cr concentration [Cr] in the cementite particles... θ The hardening rate is excessive. Therefore, during the quenching process in the manufacture of machine parts from steel plates, the cementite particles do not dissolve sufficiently. In this case, the hardenability of the steel plate actually decreases. As a result, sufficient strength cannot be obtained in machine parts manufactured from steel plates.

[0131] Therefore, the Cr content is 0.01~1.20%.

[0132] The preferred lower limit for Cr content is 0.02%, more preferably 0.03%, more preferably 0.05%, more preferably 0.08%, more preferably 0.10%, more preferably 0.13%, and more preferably 0.18%.

[0133] The preferred upper limit for Cr content is 1.15%, more preferably 1.10%, more preferably 1.00%, more preferably 0.70%, and more preferably 0.50%.

[0134] N: below 0.0150%

[0135] Nitrogen (N) is an impurity. N combines with Al to form AlN. During the quenching process in the manufacture of mechanical parts from steel sheets, AlN causes austenite grain refinement. This refinement of the austenite grains reduces the hardenability of the steel sheet. Therefore, if the N content exceeds 0.0150%, even if the contents of other elements are within the range of this embodiment, the austenite grains will become excessively refined during the quenching process. As a result, the hardenability of the steel sheet is significantly reduced.

[0136] Therefore, the N content is below 0.0150%.

[0137] The nitrogen (N) content is preferably as low as possible. However, extreme reductions in N content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit for N content is more than 0%, more preferably 0.0001%, and even more preferably 0.0005%.

[0138] The preferred upper limit for the nitrogen content is 0.0140%, more preferably 0.0130%, and even more preferably 0.0120%.

[0139] The steel plate of this embodiment has a chemical composition balance of Fe and impurities. Here, impurities in the chemical composition refer to substances that are permissible during the industrial manufacturing of steel plates, which may be introduced from the raw materials such as ore, waste, or the manufacturing environment, and are within the range that do not adversely affect the steel plate of this embodiment.

[0140] The aforementioned impurities may contain the following elements in varying amounts.

[0141] Cu: 0~0.15%, W: 0~0.15%, Ta: 0~0.15%, Sn: 0~0.050%, Sb: 0~0.050%, Co: 0~0.050%, As: 0~0. 050%, Mg: 0~0.050%, Y: 0~0.050%, Zr: 0~0.050%, La: 0~0.050%, Ce: 0~0.050%, Ca: 0~0.050%.

[0142] [Optional Elements]

[0143] The chemical composition of the steel plate in this embodiment may also contain selected...

[0144] Mo: 0~0.500%,

[0145] Ni: 0~1.000%

[0146] B: 0~0.0100%

[0147] V: 0~0.500%

[0148] Nb: 0~0.500%, and

[0149] Ti: One or more elements in a group consisting of 0 to 0.150%.

[0150] The following is an explanation of these arbitrary elements.

[0151] [Regarding Group 1: Mo, Ni, and B]

[0152] The chemical composition of the steel plate in this embodiment may also contain one or more elements selected from the group consisting of Mo, Ni, and B to replace a portion of the Fe. These elements are arbitrary and may not be present. When present, Mo, Ni, and B improve the hardenability of the steel plate.

[0153] Mo: 0~0.500%

[0154] Molybdenum (Mo) can be any element, or it can be absent. That is, the Mo content can be 0%.

[0155] In the presence of Mo, i.e., when the Mo content exceeds 0%, Mo improves the hardenability of the steel sheet. As a result, the strength of the mechanical parts is increased when quenching is performed in the process of manufacturing mechanical parts from steel sheet blanks. Mo further increases the tempering softening resistance of the steel sheet when tempering is performed in the process of manufacturing mechanical parts from steel sheet blanks. The above effects are achieved to some extent even with a small amount of Mo.

[0156] However, if the Mo content exceeds 0.500%, the Mo concentration [Mo] in the cementite particles will increase. θ The hardening rate is excessive. Therefore, during the quenching process in the manufacture of machine parts from steel plates, the cementite particles do not dissolve sufficiently. In this case, the hardenability of the steel plate actually decreases. As a result, sufficient strength cannot be obtained in machine parts manufactured from steel plates.

[0157] Therefore, the Mo content is 0~0.500%.

[0158] The preferred lower limit for the Mo content is 0.001%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%.

[0159] The preferred upper limit for the Mo content is 0.450%, more preferably 0.400%, more preferably 0.350%, and more preferably 0.300%.

[0160] Ni: 0~1.000%

[0161] Nickel (Ni) can be any element, or it can be absent. That is, the Ni content can be 0%.

[0162] In the presence of Ni, i.e., when the Ni content exceeds 0%, Ni improves the hardenability of the steel sheet. As a result, the strength of the mechanical parts is increased by performing quenching in the process of manufacturing mechanical parts from steel sheet blanks. Ni further increases the tempering softening resistance of the steel sheet when tempering is performed in the process of manufacturing mechanical parts from steel sheet blanks. The above effects are achieved to some extent even with a small amount of Ni.

[0163] However, if the Ni content exceeds 1.000%, the strength of the steel sheet will become excessively high, even if the contents of other elements are within the range of this embodiment. As a result, the cold workability of the steel sheet is reduced.

[0164] Therefore, the Ni content is 0~1.000%.

[0165] The preferred lower limit for Ni content is 0.001%, more preferably 0.005%, more preferably 0.007%, and more preferably 0.010%.

[0166] The preferred upper limit for Ni content is 0.950%, more preferably 0.900%, more preferably 0.800%, more preferably 0.700%, and more preferably 0.600%.

[0167] B: 0~0.0100%

[0168] Boron (B) can be any element, or it can be absent. That is, the B content can be 0%.

[0169] In the presence of boron (B), i.e., when the B content exceeds 0%, B improves the hardenability of the steel sheet. As a result, the strength of the mechanical parts is increased by performing quenching in the process of manufacturing mechanical parts from steel sheet as raw material. Even a small amount of B can achieve this effect to some extent.

[0170] However, if the B content exceeds 0.0100%, B compounds will form even if the contents of other elements are within the range of this embodiment. In this case, the effect of improving hardenability cannot be fully obtained. Consequently, the cold workability of the steel sheet decreases.

[0171] Therefore, the B content is 0~0.0100%.

[0172] The preferred lower limit for the content of B is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0173] The preferred upper limit for the content of B is 0.0090%, more preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, and more preferably 0.0050%.

[0174] [Regarding Group 2: V, Nb, and Ti]

[0175] The chemical composition of the steel sheet in this embodiment may also contain one or more elements selected from the group consisting of V, Nb, and Ti to replace a portion of the Fe. These elements are arbitrary and may not be present. When present, V, Nb, and Ti form carbides. These carbides suppress the coarsening of austenite grains during the quenching process in the manufacturing of mechanical parts from steel sheet. Therefore, the toughness of the mechanical parts is improved.

[0176] V: 0~0.500%

[0177] Vanadium (V) can be any element, or it can be absent. That is, the V content can be 0%.

[0178] In the presence of vanadium (V), i.e., when the V content exceeds 0%, V forms carbides. During the quenching process in the manufacture of mechanical parts from steel plates, this carbides suppress the coarsening of austenite grains. Consequently, the toughness of the mechanical parts is improved. Even a small amount of V is sufficient to achieve this effect to some extent.

[0179] However, if the V content exceeds 0.500%, excessive V forms carbides, causing precipitation strengthening of the steel sheet. Therefore, even if the contents of other elements are within the range of this embodiment, the cold workability of the steel sheet will decrease.

[0180] Therefore, the V content is 0~0.500%.

[0181] The preferred lower limit for the V content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0182] The preferred upper limit for the V content is 0.480%, more preferably 0.450%, and even more preferably 0.400%.

[0183] Nb: 0~0.500%

[0184] Niobium (Nb) can be any element, or it can be absent. That is, the Nb content can be 0%.

[0185] In the presence of Nb, i.e., when the Nb content exceeds 0%, Nb forms carbides, which suppress the coarsening of austenite grains during the quenching process in the manufacture of mechanical parts from steel sheets. Therefore, the toughness of the mechanical parts is improved. Furthermore, Nb combines with N, suppressing the formation of nitrides from dissolved boron. This, in turn, improves the hardenability of the steel sheet caused by dissolved boron. Even a small amount of Nb can achieve these effects to some extent.

[0186] However, if the Nb content exceeds 0.500%, excessive Nb forms carbides, causing precipitation strengthening of the steel sheet. Therefore, even if the contents of other elements are within the range of this embodiment, the cold workability of the steel sheet will decrease.

[0187] Therefore, the Nb content is 0~0.500%.

[0188] The preferred lower limit for Nb content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0189] The preferred upper limit for Nb content is 0.480%, more preferably 0.450%, more preferably 0.400%, more preferably 0.350%, and more preferably 0.300%.

[0190] Ti: 0~0.150%

[0191] Titanium (Ti) can be any element, or it can be absent. That is, the Ti content can be 0%.

[0192] In the presence of Ti (i.e., when the Ti content exceeds 0%), Ti forms carbides, which suppress the coarsening of austenite grains during the quenching process in the manufacture of mechanical parts from steel sheets. Therefore, the toughness of the mechanical parts is improved. Furthermore, Ti combines with N (nitrogen), inhibiting the formation of nitrides from dissolved boron (B). This, in turn, improves the hardenability of the steel sheet caused by B solution. Even a small amount of Ti can achieve these effects to some extent.

[0193] However, if the Ti content exceeds 0.150%, excessive Ti forms carbides, causing precipitation strengthening of the steel sheet. Therefore, even if the contents of other elements are within the range of this embodiment, the cold workability of the steel sheet will decrease.

[0194] Therefore, the Ti content is 0~0.150%.

[0195] The preferred lower limit for Ti content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0196] The preferred upper limit for Ti content is 0.145%, more preferably 0.130%, more preferably 0.120%, more preferably 0.100%, and more preferably 0.080%.

[0197] [Regarding (Characteristic 2) Microstructure]

[0198] In the microstructure of the steel plate of this embodiment, the total area fraction of ferrite and cementite particles is 95% or more. That is, the microstructure of the steel plate of this embodiment is substantially composed of ferrite and cementite particles.

[0199] In the microstructure, the structure other than ferrite and cementite particles is, for example, one or more of the group consisting of other precipitates, inclusions, bainite, martensite and pearlite other than cementite particles.

[0200] The preferred microstructure has a total area fraction of ferrite and cementite particles of 96% or more, more preferably 97% or more, more preferably 98% or more, and more preferably 99% or more. The microstructure may also consist of ferrite and cementite particles.

[0201] If the total area ratio of ferrite and cementite particles is above 95%, then, on the premise of satisfying characteristics 1 and characteristics 3 to 6, sufficient hardenability and sufficient cold workability can be obtained.

[0202] [Method for determining the total area fraction of ferrite and cementite particles in microstructure]

[0203] The total area ratio of ferrite and cementite particles in a microstructure can be determined by the following methods.

[0204] A test piece measuring 15 mm in the rolling direction (L direction) and 10 mm in the width direction (thickness) of the steel plate was collected from the center of the plate width. Here, the rolling direction can be determined by observing the roll marks formed on the surface of the steel plate. Roll marks refer to fine striations formed on the surface of the steel plate along the direction of the rolls during rolling. In this case, the direction in which the roll marks extend is determined as the rolling direction.

[0205] The cross-section parallel to the rolling direction (15 mm × plate thickness in the rolling direction) on the surface of the test piece was defined as the observation surface. The observation surface of the test piece was mirror-polished. For the mirror-polished observation surface, 3% nitric acid ethanol (nitric acid ethanol etching solution) was used for etching. Secondary electron images were observed using a 1000x scanning electron microscope (SEM) at any five observation points on the etched observation surface. Each observation field was set as a rectangle of 100 μm × 120 μm.

[0206] Within the field of view, ferrite and cementite particles exhibit distinct contrast and morphology compared to other microstructures (bainite, martensite, pearlite, other precipitates and inclusions besides cementite particles, etc.). Specifically, when the observation surface is etched with nitric acid-ethanol etching solution, areas with high brightness and granular structure can be identified as cementite particles. Areas with layered structure can be identified as pearlite. Areas with lower brightness than pearlite and where no substructure is identified can be identified as ferrite. Areas with higher brightness than ferrite but lower brightness than pearlite, where substructure is observed, can be identified as bainite and martensite. Therefore, based on contrast and morphology, ferrite and cementite particles within the field of view are identified.

[0207] Based on the total area of ​​ferrite and cementite particles in 5 observation fields, and the total area of ​​5 observation fields, calculate the total area ratio (%) of ferrite and cementite particles.

[0208] [Regarding (characteristic 3) the average grain size of ferrite]

[0209] In the steel plate of this embodiment, the average grain size of ferrite is 5.0~20.0 μm.

[0210] If the average grain size of ferrite is less than 5.0 μm, the cold workability of the steel sheet decreases. On the other hand, if the average grain size of ferrite exceeds 20.0 μm, the grain boundary area decreases, and the dissolution-promoting effect of cementite particles due to grain boundary diffusion cannot be obtained. In this case, sufficient hardenability cannot be achieved during the quenching process when manufacturing mechanical parts from steel sheet. Therefore, the strength of mechanical parts manufactured from steel sheet is reduced.

[0211] Therefore, the average grain size of ferrite is 5.0~20.0 μm.

[0212] The preferred lower limit for the average particle size of ferrite is 5.2 μm, more preferably 5.5 μm, more preferably 5.7 μm, more preferably 6.0 μm, and more preferably 6.5 μm.

[0213] The preferred upper limit for the average grain size of ferrite is 19.5 μm, more preferably 19.0 μm, more preferably 18.5 μm, and more preferably 18.0 μm.

[0214] [Method for determining the average particle size of ferrite]

[0215] The average particle size of ferrite can be determined by the following methods.

[0216] A test piece measuring 15 mm in the rolling direction and 10 mm in the width direction and thickness in the plate thickness direction was collected from the center of the steel plate. The cross-section parallel to the rolling direction (the surface measuring 15 mm × plate thickness in the rolling direction) of the test piece was defined as the observation surface. The observation surface of the test piece was mirror-polished. The mirror-polished observation surface was then etched using a 5% nitric acid ethanol etching solution. The average grain size of the ferrite in the etched observation surface was determined using the following method: The grain size number of the ferrite was determined by the truncation method according to JIS G 0551:2020. The magnification of the optical microscope was selected such that the number of ferrite grains intercepted by a single line segment was at least 10 in one field of view. After selecting the magnification, the truncation lengths were determined for five fields of view. The grain size number of the ferrite was determined from the arithmetic mean of the truncation lengths of the five fields of view. The average grain size (μm) of the ferrite was determined from the obtained grain size number.

[0217] Regarding (characteristic 4) the Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ ]

[0218] In the steel sheet of this embodiment, the Cr concentration [Cr] in the cementite particles, expressed as a percentage by mass, is... θ Less than 2.00%, Mo concentration [Mo] in cementite particles (mass %) θ It is below 1.00%.

[0219] Cr concentration [Cr] in cementite particles of steel plate θ and Mo concentration [Mo] θ In high-temperature conditions, during the quenching process in the manufacture of mechanical parts from steel plates, the cementite particles do not dissolve sufficiently. This reduces the hardenability of the steel plate. Consequently, the mechanical parts manufactured from steel plates do not achieve adequate strength.

[0220] Cr concentration [Cr] in cementite particles of steel plate θ Mo concentration [Mo] in cementite particles less than 2.00% θ When the Cr concentration in cementite particles is below 1.00%, [Cr] θ and Mo concentration [Mo] θ The hardenability is sufficiently low. Therefore, during the heating process described above in the quenching step, the cementite particles dissolve completely, improving the hardenability of the steel plate.

[0221] Cr concentration [Cr] in cementite particles θ The preferred upper limit is 1.90%, further preferably 1.80%, and even more preferably 1.70%.

[0222] Mo concentration [Mo] in cementite particles θ The preferred upper limit is 0.90%, further preferably 0.80%, and even more preferably 0.70%.

[0223] Cr concentration [Cr] in cementite particles θ There is no specific lower limit. Cr concentration [Cr] θ The optimal lower limit is 0%, but there may be cases where the residue is above 0.01% or above 0.10%.

[0224] Mo concentration [Mo] in cementite particles θ There is no specific lower limit. Mo concentration [Mo] θ The optimal lower limit is 0%, but there may be cases where the residue is above 0.01% or above 0.03%.

[0225] [Cr concentration in cementite particles] θ and Mo concentration [Mo] θ [Determination method]

[0226] Cr concentration [Cr] in cementite particles θ and Mo concentration [Mo] θ It can be determined by the following methods.

[0227] Test specimens were collected from the center of the steel plate. The dimensions of the test specimens were set to 10 mm × 10 mm × plate thickness.

[0228] The test piece was subjected to constant current electrolysis using a 10% AA-based solution (a solution containing 10% by volume of acetylacetone and 1% by mass of a methanol solution containing tetramethylammonium chloride).

[0229] Specifically, prepare the 10% AA-based solution described above. Then, using the 10% AA-based solution, maintain the current density at 20 mA / cm² at room temperature. 2 The test pieces were then subjected to constant current electrolysis. After constant current electrolysis, the test pieces were removed from the 10% AA solution. The removed test pieces were then immersed in an alcohol solution. The test pieces immersed in the alcohol solution were then ultrasonically cleaned.

[0230] The residue was extracted by filtration using a 0.2 μm mesh filter for the 10% AA solution used in constant current electrolysis and the alcohol solution used in the subsequent ultrasonic cleaning.

[0231] Chemical elemental analysis was performed on the extracted residue. Specifically, the residue was dissolved in acid to obtain a solution. Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to analyze the solution to obtain the mass of Cr and Mo in the residue. Based on the obtained mass of Cr, Mo, and the total mass of the residue, the Cr concentration (mass%) and Mo concentration (mass%) in the residue were determined.

[0232] The resulting residue is essentially composed of cementite particles. That is, the amount of particles other than cementite (inclusions and other precipitates) in the residue is negligible. Therefore, the Cr and Mo concentrations in the residue can be considered as the Cr concentration [Cr] within the cementite particles. θ and Mo concentration [Mo] θ .

[0233] [(Feature 5) Average particle size of cementite particles]

[0234] In the steel plate of this embodiment, the average particle size of the cementite particles is less than 1.50 μm.

[0235] As mentioned above, if the cementite particles are large, they will not dissolve sufficiently during the quenching process in the manufacturing of mechanical parts from steel plates. In this case, the hardenability of the steel plate decreases. Consequently, sufficient strength cannot be obtained in the mechanical parts manufactured from steel plates.

[0236] If the average particle size of the cementite is less than 1.50 μm, the cementite particles are sufficiently small. Therefore, during the heating process in the above-mentioned quenching step, the cementite particles dissolve completely, and the hardenability of the steel plate is improved.

[0237] The preferred upper limit for the average particle size of cementite particles is 1.45 μm, more preferably 1.40 μm, more preferably 1.35 μm, and even more preferably 1.30 μm.

[0238] To improve hardenability, a small average particle size of cementite particles is preferred. However, if the average particle size of cementite particles is too small, the hardness of the steel sheet becomes excessively high. In this case, the cold workability of the steel sheet decreases. Therefore, the preferred lower limit for the average particle size of cementite particles is 0.05 μm, more preferably 0.10 μm, more preferably 0.15 μm, and even more preferably 0.20 μm.

[0239] [Method for determining the average particle size of cementite particles]

[0240] The average particle size of cementite can be determined by the following method.

[0241] A test piece measuring 15 mm × 10 mm × plate thickness in the rolling direction and taken from the center of the steel plate width is collected. The cross section of the test piece surface parallel to the rolling direction (the surface measuring 15 mm × plate thickness in the rolling direction) is defined as the observation surface.

[0242] The observation surface was etched using a bitter alcohol solution. Secondary electron images were captured at five arbitrary observation points on the etched surface at a depth of 4 / 4 of the plate thickness. Specifically, the five observation points were observed at 2000x magnification using a scanning electron microscope (SEM), and the aforementioned secondary electron images were captured. Each observation field was set as a rectangle of 50 μm × 60 μm.

[0243] In each secondary electron image, cementite particles were determined based on contrast. The area of ​​each determined cementite particle was calculated, and based on the area, the equivalent circle diameter of each cementite particle was determined. The calculated equivalent circle diameter was taken as the particle size of that cementite particle. It should be noted that the particle size was determined using well-known image processing software. The arithmetic mean of the particle sizes of the cementite particles obtained from five observation fields was taken as the average particle size (μm) of the cementite particles.

[0244] [Regarding (Feature 6) Sphericity]

[0245] In the steel plate of this embodiment, cementite particles with an aspect ratio of 3.0 or less among a plurality of cementite particles are defined as spherical cementite particles. The ratio of the total number of spherical cementite particles to the total number of a plurality of cementite particles, i.e., the spheroidization rate, is 85% or more.

[0246] If the spheroidization rate is 85% or higher, then, provided that features 1 to 5 are satisfied, significantly superior cold workability can be obtained in the steel sheet. Therefore, in the steel sheet of this embodiment, the spheroidization rate is 85% or higher.

[0247] High sphericity is preferred. The preferred lower limit for sphericity is 87%, further preferably 89%, further preferably 91%, and further preferably 95%.

[0248] [Methods for determining sphericity]

[0249] The sphericity can be determined by the following methods.

[0250] According to the above-described method for determining the average particle size of cementite particles, the aspect ratio of multiple cementite particles determined at five observation fields was calculated. Specifically, the maximum interval obtained when the outline of a cementite particle is clamped by two parallel line segments is defined as the major axis. Furthermore, the interval between the two line segments clamping the outline of the cementite particle with two line segments parallel to the major axis (i.e., the width in the direction perpendicular to the major axis) is defined as the minor axis.

[0251] Based on the obtained major and minor diameters, the aspect ratio (=major diameter / minor diameter) of each cementite particle is calculated. Cementite particles with an aspect ratio of 3.0 or less among all cementite particles in the five observation fields are defined as "spherical cementite particles". The ratio of the total number of spherical cementite particles to the total number of multiple cementite particles is defined as the spheroidization rate (%).

[0252] [The effect of the steel plate in this embodiment]

[0253] The steel sheet of this embodiment, which satisfies features 1 to 6 above, can achieve sufficient hardenability during quenching in the process of manufacturing mechanical parts using this steel sheet as a blank. Furthermore, the steel sheet of this embodiment can achieve sufficient cold workability.

[0254] [Regarding hardenability]

[0255] In the steel plate of this embodiment, sufficient hardenability is achieved as evaluated below.

[0256] [Methods for evaluating hardenability]

[0257] (A c1 Phase transition point determination)

[0258] Test specimens were collected from the center of the steel plate width according to this embodiment. The coefficient of thermal expansion during heating was measured using a Formastor testing machine. A was then calculated from the obtained coefficient of thermal expansion. c1 Phase transition point.

[0259] (Determination of maximum quenching hardness)

[0260] Plate-shaped test pieces were collected from the center of the steel plate width. The shape of the plate-shaped test piece was set to be 15 mm in the rolling direction (L direction) × 30 mm in the width direction (W direction) × plate thickness.

[0261] The plate-shaped test piece was heated at 1000°C for 20 minutes using a salt bath. Then, the plate-shaped test piece removed from the salt bath was quenched by immersing it in water in a water bath. The quenched plate-shaped test piece was cut in half along the W direction. The cut surfaces were mirror-polished. A Vickers hardness test according to JIS Z2244:2009 was performed at any three points along the center of the plate thickness direction (T direction) of the polished cut surface. The test force was set to 98 N. The arithmetic mean of the obtained Vickers hardness was defined as the highest quenched hardness HD0 (HV).

[0262] (Hardenability Evaluation)

[0263] Plate-shaped test pieces were collected from the center of the steel plate width. The shape of the plate-shaped test piece was set to be 15 mm in the rolling direction (L direction) × 30 mm in the width direction (W direction) × plate thickness.

[0264] Immerse the plate-shaped test piece in A c1 The plate was subjected to a salt bath at +80°C (phase transition point) for 10 minutes. Then, the plate-shaped test piece removed from the salt bath was quenched by immersing it in water in a water bath. The quenched plate-shaped test piece was cut in half along the W direction. The cut surfaces were mirror-polished. Vickers hardness tests according to JIS Z2244:2009 were performed at three random points along the center of the plate thickness direction (T direction) of the polished cut surfaces. The test force was set to 98 N. The arithmetic mean of the obtained Vickers hardness was defined as the quenching hardness HD1 (HV).

[0265] If the obtained quenching hardness HD1 is more than 95% of the highest quenching hardness HD0, it is judged that the steel plate has achieved sufficient hardenability.

[0266] [Regarding cold workability]

[0267] In the steel sheet of this embodiment, cold workability can be evaluated, for example, by the following methods.

[0268] [Methods for evaluating cold workability]

[0269] A JIS 5 plate test piece, as specified in JIS Z2241:2011, is collected from the center of the width of the steel plate. A V-notch is formed at the center of the parallel portion along its length, with the depth direction of the V-notch parallel to the width direction of the parallel portion. The opening angle of the V-notch is set to 45°, and the depth of the V-notch is set to 2 mm. The gauge length is set to 10 mm, including the V-notch. The length direction of the plate test piece is defined as the rolling direction (L direction) of the steel plate.

[0270] Using a plate-shaped test piece, an elongation at break test was conducted at room temperature and in atmospheric conditions. The elongation at the butt joint after fracture was measured, and the obtained elongation at the butt joint (%) was defined as the notch elongation (%). Compared with steel plates that do not satisfy any of features 1 to 6, the steel plate of this embodiment has a larger notch elongation. For example, when the thickness of the plate-shaped test piece is 2 mm, the notch elongation in the steel plate of this embodiment is 5.0% or more, and is -33 × C content (%) of the steel plate + 32.5% or more.

[0271] [Applications of steel plates]

[0272] The steel sheet of this embodiment is suitable as a blank for mechanical parts, such as automotive components. Examples of mechanical parts include automotive springs and washers. It should be noted that the steel sheet of this embodiment can also be used for applications other than mechanical parts requiring excellent hardenability and cold workability.

[0273] [Steel plate manufacturing method]

[0274] An example of a method for manufacturing the steel plate according to this embodiment will be described. The method for manufacturing the steel plate described below is an example for manufacturing the steel plate of this embodiment. Therefore, the steel plate having the above-described structure can also be manufactured by other manufacturing methods besides the method described below. However, the manufacturing method described below is a preferred example of the method for manufacturing the steel plate of this embodiment.

[0275] An example of the steel plate manufacturing method of this embodiment includes the following steps.

[0276] (Process 1) Blank preparation process

[0277] (Process 2) Hot rolling process

[0278] (Process 3) Cold rolling process

[0279] (Process 4) Cold-rolled sheet annealing process

[0280] It should be noted that in this embodiment, the annealing process is not performed after the hot rolling process and before the cold rolling process.

[0281] The main manufacturing conditions for processes 1 to 4 are as follows.

[0282] (Condition 1) Winding temperature CT in process 2: 400~600℃

[0283] (Condition 2) Cold rolling ratio CR in process 3: exceeding 35% and below 60%

[0284] (Condition 3) Annealing temperature T1 in process 4: 550~720℃

[0285] (Condition 4) Holding time t1 in process 4: More than 25 hours but less than 80 hours

[0286] The following is a description of each process.

[0287] [(Process 1) Blank Preparation Process]

[0288] In the billet preparation process, a billet satisfying characteristic 1 is prepared. The billet is manufactured, for example, by the following method: Molten steel with the content of each element in its chemical composition within the range of this embodiment is produced. Using the above-mentioned molten steel, a billet (slab or ingot) is manufactured by casting. For example, a slab is manufactured using the above-mentioned molten steel by a known continuous casting method. Alternatively, an ingot is manufactured using the above-mentioned molten steel by a known ingot casting method.

[0289] [(Process 2) Hot rolling process]

[0290] In the hot rolling process, prepared billets (slabs or ingots) are hot rolled to produce hot-rolled steel sheets. The hot rolling process includes: a rough rolling process, which rough rolls the billets to produce rough strips (intermediate steel sheets); and a finish rolling process, which finish rolls the rough strips to produce hot-rolled steel sheets.

[0291] In the roughing rolling process, the billet (slab or ingot) is heated in a furnace. The heated billet is then rolled using a roughing mill to produce rough strip. The heating temperature of the billet in the roughing rolling process is, for example, 1100~1300°C. The time the billet spends in the furnace is 30 minutes or more, preferably 60 minutes or more. There is no particular upper limit to the furnace time, but it can be, for example, 300 minutes.

[0292] In the finishing rolling process, a finishing mill is used to further roll the rough strip (finish rolling) to produce hot-rolled steel sheet. The finishing mill includes multiple stands arranged in a row. Each stand has a pair of work rolls. The surface temperature of the steel sheet exiting from the last stand that presses down the steel sheet is defined as the finishing rolling temperature (°C). In this embodiment, the finishing rolling temperature is 830~950°C. Furthermore, the reduction rate of the last stand that applies pressure to the steel sheet in the multiple stands arranged in a row within the finishing mill is defined as the reduction rate (%) of the final pass. In this embodiment, the reduction rate of the final pass is 5~30%. The finished hot-rolled steel sheet is coiled into a roll. The coiling temperature CT is described later. The coiled hot-rolled steel sheet is cooled to room temperature.

[0293] [(Process 3) Cold rolling process]

[0294] In the cold rolling process, hot-rolled steel sheets are subjected to cold rolling after the hot rolling process. Cold rolling is carried out using a cold rolling mill. A cold rolling mill is, for example, a reversible rolling mill consisting of a single rolling mill frame, which includes a pair of work rolls.

[0295] In the cold rolling process, the aforementioned reversible rolling mill is used to perform cold rolling to produce cold-rolled steel sheets. The cold rolling ratio CR in the cold rolling process is described later.

[0296] In this embodiment, a cold rolling process is performed on the hot-rolled steel sheet after the hot rolling process, but no annealing treatment is performed. That is, the hot-rolled sheet is not annealed after the hot rolling process and before the cold rolling process. In this manufacturing method, strain is accumulated in the steel sheet during the hot rolling and cold rolling processes, and an annealing process is performed after the cold rolling process. As a result, ferrite grain size of appropriate size, cementite grain size of appropriate size, and Cr concentration [Cr] of appropriate concentration in the cementite grain are obtained. θ and Mo concentration [Mo] θ .

[0297] [(Process 4) Cold-rolled sheet annealing process]

[0298] In the annealing process of cold-rolled steel sheets, annealing treatment is performed on the cold-rolled steel sheets after the cold rolling process. In the annealing process, the annealing temperature T1 and the holding time t1 at the annealing temperature T1 are adjusted to adjust the degree of recrystallization of ferrite and precipitation of cementite particles.

[0299] [Regarding conditions 1 through 4]

[0300] In the above-mentioned processes 1 to 4, the following conditions 1 to 4 must be met.

[0301] (Condition 1) Winding temperature CT in process 2: 400~600℃

[0302] (Condition 2) Cold rolling ratio CR in process 3: exceeding 35% and below 60%

[0303] (Condition 3) Annealing temperature T1 in process 4: 550~720℃

[0304] (Condition 4) Holding time t1 in process 4: More than 25 hours but less than 80 hours

[0305] The following is an explanation of each condition.

[0306] [Regarding (Condition 1) winding temperature CT]

[0307] In the hot rolling process, the coiling temperature (CT) affects the spheroidization rate of cementite particles and the Cr concentration [Cr] within the cementite particles. θ , Mo concentration [Mo] θ If the coiling temperature (CT) is below 600°C, the cementite particles generated in the hot-rolled steel sheet are sufficiently and uniformly distributed. Furthermore, the strain imparted to the hot-rolled steel sheet can be appropriately maintained. In this case, by performing an annealing process after the cold rolling process, the spheroidization rate of the cementite particles is increased. Furthermore, the enrichment of Cr and Mo in the generated cementite particles can be suppressed, and the Cr concentration [Cr] in the cementite particles is reduced. θ The Mo concentration [Mo] is less than 2.00%. θThe percentage should be below 1.00%. Therefore, the winding temperature (CT) is preferably set to below 600°C.

[0308] On the other hand, there is no particular limitation on the lower limit of the take-up temperature CT. However, due to equipment limitations, the preferred lower limit of the take-up temperature CT is 400°C.

[0309] If the coiling temperature CT is 400~600℃, it is possible to manufacture steel plates that meet features 1 to 6, provided that other conditions are met.

[0310] [Regarding (Condition 2) Cold Rolling Ratio CR]

[0311] In the cold rolling process, the cold rolling ratio CR is defined by the following formula.

[0312] Cold rolling yield CR (%) = (1 - (thickness of cold-rolled steel sheet after cold rolling / thickness of hot-rolled steel sheet before cold rolling)) × 100

[0313] If the cold rolling ratio (CR) exceeds 35%, sufficient strain is introduced into the steel sheet. In this case, spheroidization of cementite particles is promoted in the subsequent annealing process, with a spheroidization rate exceeding 85%. On the other hand, if the cold rolling ratio (CR) exceeds 60%, the strain introduced into the steel sheet becomes excessive. In this case, ferrite is excessively refined, and the average ferrite grain size becomes less than 5.0 μm. If the cold rolling ratio (CR) exceeds 60%, the layered structure of pearlite is further segmented. In this case, spheroidization of cementite particles is promoted. As a result, Cr and Mo are enriched in the cementite particles, and the Cr concentration [Cr] in the cementite particles increases. θ To reach 2.00% or higher, or a Mo concentration [Mo]. θ More than 1.00%.

[0314] If the cold rolling ratio CR exceeds 35% but is below 60%, it is possible to manufacture steel plates that meet characteristics 1 to 6, provided that other conditions are met.

[0315] [Regarding (Condition 3) Annealing temperature T1]

[0316] In the annealing process of cold-rolled steel sheets, the annealing temperature T1 adjusts the spheroidization rate of cementite particles. If the annealing temperature T1 is less than 550℃, the spheroidization of cementite becomes insufficient, and the spheroidization rate of cementite particles becomes less than 85%.

[0317] On the other hand, if the annealing temperature T1 exceeds 720°C, the annealing temperature is too high. In this case, the spheroidization of cementite particles becomes insufficient, and the spheroidization rate of cementite particles becomes less than 85%. Consequently, the total area ratio of ferrite and cementite particles in the microstructure is less than 95%.

[0318] If the annealing temperature T1 is 550~720℃, it is possible to manufacture steel plates that meet characteristics 1 to 6, provided that other conditions are met.

[0319] [Regarding the holding time t1 at annealing temperature T1 (condition 4)]

[0320] In the annealing process of cold-rolled steel sheets, the holding time t1 at annealing temperature T1 affects the size of ferrite, the size of cementite particles, and the Cr concentration [Cr] in the cementite particles. θ and Mo concentration [Mo] θ Specifically, if the holding time t1 is less than 25 hours, the cementite particles will not be fully geomorphized.

[0321] On the other hand, if the holding time t1 exceeds 80 hours, the holding time is too long, resulting in coarsening of the ferrite and cementite particles. Consequently, the average particle size of the ferrite exceeds 20.0 μm, and the average particle size of the cementite exceeds 1.50 μm. Furthermore, Cr and Mo accumulate in the cementite particles, leading to a high Cr concentration [Cr] in the cementite particles. θ To reach 2.00% or higher, or a Mo concentration [Mo]. θ More than 1.00%.

[0322] If the holding time t1 exceeds 25 hours but is less than 80 hours, it is possible to manufacture steel plates that meet features 1 to 6, provided that other conditions are met.

[0323] Through the above manufacturing processes, steel plates that meet the requirements of features 1 to 6 can be manufactured.

[0324] The effects of the steel plate of this embodiment will be further explained in detail below through examples. The conditions in the following examples are examples of conditions adopted to confirm the feasibility and effects of the steel plate of this embodiment. Therefore, the steel plate of this embodiment is not limited to these examples of conditions.

[0325] Example 1

[0326] Manufacture steel plates having the chemical compositions shown in Tables 1-1 and 1-2.

[0327] [Table 1-1]

[0328]

[0329] [Table 1-2]

[0330]

[0331] Specifically, molten steel is continuously cast to produce slabs. The slabs are then subjected to a hot rolling process. Specifically, the slabs are heated at 1100–1250°C for 240 minutes. The heated slabs are rolled using a roughing mill to produce rough strip. Then, the rough strip is rolled using a finishing mill to produce hot-rolled steel sheets. The finishing rolling temperature for each test number is 830–950°C. The reduction rate in the final pass is 5–30%. The finished hot-rolled steel sheets are coiled into coils. The coiled hot-rolled steel sheets are then allowed to cool naturally to room temperature. The coiling temperature CT in the hot rolling process for each test number is shown in Table 2.

[0332] [Table 2]

[0333]

[0334] For experiments 1-48, the hot-rolled steel sheets were subjected to a cold-rolling process after the hot-rolling process, without undergoing a hot-rolled annealing process. The cold-rolling ratio CR in the cold-rolling process is shown in Table 2. The cold-rolled steel sheets were then subjected to a cold-rolled annealing process. The annealing temperature T1 and holding time t1 are shown in Table 2. During the cold-rolled annealing process, the steel sheets were furnace-cooled after a holding time t1. For experiment 49, the hot-rolled annealing process was performed after the hot-rolling process and before the cold-rolling process. The annealing temperature and holding time at the annealing temperature are shown in Table 2. Steel sheets were manufactured through the above manufacturing processes.

[0335] [Evaluation Test]

[0336] The following tests were performed on the steel plates manufactured for each test number.

[0337] (Experiment 1) Determination of the total area ratio of ferrite and cementite particles

[0338] (Experiment 2) Ferrite Average Grain Size Determination Test

[0339] (Experiment 3) Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Determination test

[0340] (Experiment 4) Test on the determination of the average particle size of cementite particles

[0341] (Experiment 5) Determination of the spheroidization rate of cementite particles

[0342] (Experiment 6) Hardenability Evaluation Test

[0343] (Experiment 7) Cold workability evaluation test

[0344] The following describes Experiments 1 through 7.

[0345] [(Experiment 1) Determination of the total area ratio of ferrite and cementite particles]

[0346] Based on the method described above in "[Method for Determining the Total Area Ratio of Ferrite and Cementite Particles in Microstructure]", the total area ratio of ferrite and cementite particles for each test number was calculated. The results are shown in Table 3. Except for test number 46, the total area ratio of ferrite and cementite particles for all other test numbers was above 95%.

[0347] [Table 3]

[0348]

[0349] [(Experiment 2) Ferrite Average Particle Size Determination Test]

[0350] Based on the method described above in [Method for Determining the Average Grain Size of Ferrite], the average grain size of ferrite in the steel plates for each test number was determined. The results are shown in Table 3.

[0351] [(Experiment 3) Cr concentration in cementite particles [Cr]] θ and Mo concentration [Mo] θ [Determination Test]

[0352] Based on the above [Cr concentration in cementite particles] θ and Mo concentration [Mo] θ The Cr concentration [Cr] in the cementite particles of the steel plate for each test number was determined using the method described in the determination method. θ and Mo concentration [Mo] θ The obtained Cr concentration [Cr] θ and Mo concentration [Mo] θ As shown in Table 3.

[0353] [(Experiment 4) Determination of the average particle size of cementite particles]

[0354] Based on the method described in the above-mentioned [Method for Determining the Average Particle Size of Cementite Particles], the average particle size of cementite particles in the steel plates of each test number was determined. The obtained average particle sizes of cementite particles are shown in Table 3.

[0355] [(Experiment 5) Determination of the spheroidization rate of cementite particles]

[0356] Based on the method described in the above-mentioned [Method for Determining Nodularity], the nodularity of cementite particles in the steel plates of each test number was determined. The obtained nodularity is shown in Table 3.

[0357] [(Experiment 6) Hardenability Evaluation Test]

[0358] Based on the method described in the [Hardenability Evaluation Method] above, the hardenability of the steel plates during quenching for each test number was evaluated. The "Lower Limit of Quenching Hardness" in Table 3 represents the value of the highest quenching hardness HD0 × 0.95. If the quenching hardness HD1 is above the lower limit of quenching hardness, it is judged that sufficient hardenability has been achieved (indicated by "E (Excellent)" in the "Hardenability Judgment" in Table 3). On the other hand, if the quenching hardness HD1 is less than the lower limit of quenching hardness, it is judged that sufficient hardenability has not been achieved (indicated by "NA (Not Allowed)" in the "Hardenability Judgment" in Table 3). Furthermore, if the quenching hardness HD1 is less than 600 HV, it is judged that sufficient component strength is not met.

[0359] [(Experiment 7) Cold workability evaluation test]

[0360] Based on the method described in the above-mentioned [Cold Workability Evaluation Method], the cold workability of the steel plates for each test number was evaluated. The thickness of the plate test pieces was 2 mm. The obtained notch elongation is shown in Table 3. If the obtained notch elongation is 5.0% or more and the notch elongation target (-33 × C content (%) of the steel plate + 32.5%) or more, it is judged that sufficient cold workability has been obtained in the steel plate (indicated by "E (Excellent)" in "Cold Workability Judgment" in Table 3). On the other hand, if the obtained notch elongation is less than 5.0% or less than the notch elongation target (-33 × C content (%) of the steel plate + 32.5%), it is judged that sufficient cold workability has not been obtained in the steel plate (indicated by "NA (Not Allowed)" in "Cold Workability Judgment" in Table 3).

[0361] [Evaluation Results]

[0362] Referring to Tables 1-1, 1-2, 2, and 3, in tests 1-35, the chemical composition was appropriate and conditions 1-4 of the manufacturing conditions were met. Therefore, the steel plates in these tests met characteristics 1-6. As a result, sufficient hardenability and sufficient cold workability were obtained.

[0363] On the other hand, in test number 36, the carbon content was too high. Therefore, the notched elongation was less than 5.0%, and sufficient cold workability was not achieved.

[0364] In experiment number 37, the Si content was too high. Therefore, the notched elongation was less than the target notched elongation, and sufficient cold workability was not achieved.

[0365] In test number 38, the Mn content was too low. Therefore, the quenching hardness HD1 was less than 600 HV, which was deemed insufficient to meet the requirements for part strength. Consequently, insufficient hardenability was not achieved.

[0366] In test number 39, the Mn content was too high. Therefore, the notched elongation was less than the target notched elongation, and sufficient cold workability was not achieved.

[0367] In test number 40, the Cr content was too high. Therefore, the Cr concentration [Cr] in the cementite particles, expressed as a percentage by mass, is... θ It is above 2.00%. Therefore, sufficient hardenability was not achieved.

[0368] In test number 41, the Mo content was too high. Therefore, the Mo concentration [Mo] in the cementite particles, expressed as a percentage by mass, is [%. θ The hardenability is more than 1.00%. Therefore, sufficient hardenability was not achieved.

[0369] In experiment 42, although the chemical composition was appropriate, the winding temperature (CT) was too high. Therefore, the spheroidization rate of the cementite particles was too low. As a result, sufficient cold workability was not achieved. Consequently, the Cr concentration [Cr] in the cementite particles was also low. θ The content is above 2.00%. As a result, sufficient hardenability was not achieved.

[0370] In experiment number 43, although the chemical composition was appropriate, the cold rolling ratio (CR) was too low. Therefore, the spheroidization rate of the cementite particles was too low. As a result, the notched elongation was less than 5.0%, and sufficient cold workability was not achieved.

[0371] In experiment 44, although the chemical composition was appropriate, the cold rolling ratio (Cr) was too high. Therefore, the average grain size of ferrite was less than 5.0 μm. As a result, sufficient cold workability was not achieved. Furthermore, the Cr concentration [Cr] in the cementite particles was also high. θ Too high. As a result, insufficient hardenability was not achieved.

[0372] In experiment number 45, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too low. Consequently, the spheroidization rate of the cementite particles was less than 85%. As a result, the notched elongation was less than 5.0%, and sufficient cold workability was not achieved.

[0373] In experiment 46, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too high. As a result, pearlite precipitated, and the total area ratio of ferrite and cementite particles was less than 95%. Consequently, the spheroidization rate of the cementite particles was low. Consequently, sufficient cold workability was not achieved.

[0374] In experiment 47, although the chemical composition was appropriate, the holding time t1 during the annealing process of the cold-rolled sheet was too short. As a result, the spheroidization rate of the cementite particles was less than 85%. Consequently, the notched elongation was less than 5.0%, and sufficient cold workability was not achieved.

[0375] In experiment 48, although the chemical composition was appropriate, the holding time t1 during the cold-rolled sheet annealing process was too long. Therefore, the ferrite and cementite particles were coarse. Consequently, the Cr concentration [Cr] in the cementite particles was high. θ Too high. As a result, insufficient hardenability was not achieved.

[0376] In experiment number 49, annealing was performed after the hot rolling process and before the cold rolling process. Therefore, the Cr concentration [Cr] in the cementite particles... θ Too high. As a result, insufficient hardenability was not achieved.

[0377] The embodiments of this disclosure have been described above. However, the above embodiments are merely illustrative examples for implementing this disclosure. Therefore, this disclosure is not limited to the above embodiments, and can be implemented with appropriate modifications without departing from its spirit.

Claims

1. A steel plate having a chemical composition, in mass percent, of the following: C：0.50~0.90%、 Si: 0.01~0.50% Mn: 0.20~1.30%, P: Below 0.100% S: Below 0.100% Al: Below 0.100% Cr：0.01~1.20%、 N: below 0.0150% Mo: 0~0.500%, Ni: 0~1.000% B：0~0.0100%、 V：0~0.500%、 Nb: 0~0.500%, Ti: 0~0.150%, and Balance: Fe and impurities, In the microstructure, the total area fraction of ferrite and cementite particles is over 95%. The average grain size of the ferrite is 5.0~20.0 μm. The Cr concentration [Cr] in the cementite particles, expressed as a percentage by mass. θ The Mo concentration [Mo] in the cementite particles, expressed as a percentage by mass, is less than 2.00%. θ Below 1.00%, The average particle size of the cementite particles is less than 1.50 μm. When cementite particles with an aspect ratio of 3.0 or less are defined as spherical cementite particles, the ratio of the total number of spherical cementite particles to the total number of cementite particles, i.e., the spheroidization rate, is 85% or more.

2. The steel plate according to claim 1, wherein, The chemical composition contains selected... Mo: 0.001~0.500%, Ni: 0.001~1.000% B：0.0001~0.0100%、 V：0.001~0.500%、 Nb: 0.001~0.500%, and Ti: One or more elements in a group consisting of 0.001 to 0.150%.

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