steel sheet

Abstract

This disclosure provides a steel sheet with excellent hardenability and excellent cold workability. The steel sheet disclosed herein has the following composition by mass %: C: 0.15~0.50%, 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 and Mo concentration [Mo] θ To satisfy equation (1), the average particle size of the cementite particles is less than 1.50 μm, the spheroidization rate of the cementite particles is greater than 70% and less than 85%, and the dispersion DI of the number of cementite particles in a small square partition with a side length of 10 μm, as defined by equation (2), is less than 45%. 2[Cr] θ +3[Mo] θ ≤3 / √C (1) DI=σ θ / N θ ×100 (2)

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 blanks for mechanical parts, such as automotive components. Examples of these mechanical parts include door components, seat components, chains, gears, and clutches. The method for manufacturing these mechanical parts using steel sheets as blanks 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 produced. To achieve 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 blanks. 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 documents 1 and 2 propose steel plates with excellent hardenability and excellent cold workability.

[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. This 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. The volume percentage of ferrite and carbides relative to the overall microstructure is 90% or more, and the volume percentage of proeutectoid ferrite relative to the overall microstructure is 20% to 80% or more. 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] However, excellent hardenability and excellent cold workability can also be obtained by means other than those disclosed in Patent Documents 1 and 2.

[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: 0.15~0.50%

[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 The Mo concentration [Mo] in the above-mentioned cementite particles, expressed as a percentage by mass. θ The C content (by mass%) in the above steel plates satisfies equation (1).

[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 70% or more and less than 85%.

[0036] The microstructure was divided into tiny square partitions with a side length of 10 μm. The average number of cementite particles in more than 500 of these tiny square partitions was defined as N. θ The sample standard deviation of the number of cementite particles in the aforementioned tiny square partition is defined as σ. θ When the dispersion DI defined by equation (2) is below 45%.

[0037] 2[Cr]θ +3[Mo] θ ≤3 / √C (1)

[0038] DI=σ θ / N θ ×100 (2)

[0039] Here, the C content in the chemical composition of the steel plate mentioned above is substituted into C in Equation (1).

[0040] The effects of the invention

[0041] The steel plate disclosed herein has excellent hardenability and excellent cold workability. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of a portion of a square region in a method for determining the dispersion (DI) of cementite particles within a steel plate. Detailed Implementation

[0043] 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.

[0044] First, the inventors conducted research on steel sheets exhibiting excellent hardenability and cold workability from the perspective of chemical composition. As a result, the inventors concluded that a chemical composition, by mass percent, of C: 0.15~0.50%, Si: 0.01~0.50%, Mn: 0.20~1.30%, P: ≤0.100%, S: ≤0.100%, Al: ≤0.100%, Cr: 0.01~1.20%, N: ≤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, can achieve a balance between improved hardenability and improved cold workability. 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.

[0045] The inventors first investigated methods to improve hardenability during quenching 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 solubility of the cementite particles during quenching, it is preferable that the cementite particles have a small particle size. In the case of steel plates having the above-described chemical composition, it is effective to make the average particle size of the cementite particles 1.50 μm or less.

[0046] Furthermore, the inventors' research results indicate that the Mn concentration in cementite particles has no effect on the dissolution 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 dissolution of cementite particles during quenching. Specifically, if the Cr and Mo concentrations in the cementite particles are high, the cementite particles become difficult to dissolve during quenching.

[0047] Based on the above insights, further research was conducted, and the inventors discovered that if the Cr concentration [Cr] in the cementite particles is... θ and Mo concentration [Mo] θ If equation (1) is satisfied, the cementite particles become easier to dissolve during quenching, and the hardenability of the steel plate is improved.

[0048] 2[Cr] θ +3[Mo] θ ≤3 / √C (1)

[0049] Here, C in equation (1) is replaced with the C content in the chemical composition of the steel plate, expressed as a percentage by mass.

[0050] 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 70% or more and less than 85%, 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.

[0051] By making the steel sheet with the above chemical composition meet the above characteristics, hardenability and cold workability are improved to some extent. However, even steel sheets with the above characteristics still have low cold workability. Therefore, the inventors conducted further research.

[0052] The study results indicate that the distribution of cementite particles in steel plates also significantly affects cold workability. Specifically, the strength is higher in areas with dense cementite particles compared to areas with sparse cementite particles. In this case, cold workability differs between dense and sparse cementite particle areas. Consequently, the overall cold workability of the steel plate decreases. Therefore, it is determined that a highly effective way to improve cold workability is to distribute cementite particles as uniformly as possible within the steel plate.

[0053] Based on the above insights, the inventors conducted further research. The results showed that when the microstructure was divided into tiny square partitions with a side length of 10 μm, and the average number of cementite particles in more than 500 tiny square partitions was defined as N... θ The sample standard deviation of the number of cementite particles in a tiny square partition is defined as σ. θ When the dispersion DI defined by equation (2) is below 45%, it can improve hardenability and cold workability.

[0054] DI=σ θ / N θ ×100 (2)

[0055] The above-described mechanism is hypothetical. Therefore, the steel sheet of this embodiment may also have excellent hardenability and excellent cold workability based on a mechanism different from that described above. However, as shown in the embodiments described later, a steel sheet that satisfies the above characteristics can also have excellent hardenability and excellent cold workability.

[0056] The steel plate of this embodiment, based on the above insights, has the following structure. [1]

[0058] A steel plate, the chemical composition of which is (in mass%) is

[0059] C: 0.15~0.50%

[0060] Si: 0.01~0.50%

[0061] Mn: 0.20~1.30%,

[0062] P: Below 0.100%

[0063] S: Below 0.100%

[0064] Al: Below 0.100%

[0065] Cr: 0.01~1.20%

[0066] N: below 0.0150%

[0067] Mo: 0~0.500%,

[0068] Ni: 0~1.000%

[0069] B: 0~0.0100%

[0070] V: 0~0.500%

[0071] Nb: 0~0.500%,

[0072] Ti: 0~0.150%, and

[0073] Balance: Fe and impurities,

[0074] In the microstructure, the total area fraction of ferrite and cementite particles is over 95%.

[0075] The average grain size of the ferrite mentioned above is 5.0~20.0 μm.

[0076] The Cr concentration [Cr] in the above-mentioned cementite particles, expressed as a percentage by mass. θ The Mo concentration [Mo] in the above-mentioned cementite particles, expressed as a percentage by mass. θ The C content (by mass%) in the above steel plates satisfies equation (1).

[0077] The average particle size of the aforementioned cementite particles is less than 1.50 μm.

[0078] 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 70% or more and less than 85%.

[0079] The microstructure was divided into tiny square partitions with a side length of 10 μm. The average number of cementite particles in more than 500 of these tiny square partitions was defined as N. θ The sample standard deviation of the number of cementite particles in the aforementioned tiny square partition is defined as σ. θ When the dispersion DI defined by equation (2) is below 45%.

[0080] 2[Cr] θ +3[Mo] θ ≤3 / √C (1)

[0081] DI=σ θ / N θ ×100 (2)

[0082] Here, the C content in the chemical composition of the steel plate mentioned above is substituted into C in Equation (1). [2]

[0084] According to the steel plate described in [1], wherein,

[0085] The above chemical composition contains selected free radicals

[0086] Mo: 0.001~0.500%,

[0087] Ni: 0.001~1.000%

[0088] B: 0.0001~0.0100%

[0089] V: 0.001~0.500%

[0090] Nb: 0.001~0.500%, and

[0091] Ti: One or more elements in a group consisting of 0.001 to 0.150%.

[0092] 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%.

[0093] [Features of the steel plate in this embodiment]

[0094] The steel plate of this embodiment satisfies the following features 1 to 7.

[0095] (Feature 1)

[0096] The chemical composition, by mass%, is: C: 0.15-0.50%, 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.

[0097] (Feature 2)

[0098] In the microstructure, the total area ratio of ferrite and cementite particles is over 95%.

[0099] (Feature 3)

[0100] The average grain size of ferrite is 5.0~20.0 μm.

[0101] (Feature 4)

[0102] Cr concentration [Cr] in cementite particles (mass%) θ Mo concentration [Mo] in cementite particles, expressed as a percentage by mass. θ The C content in the steel plate, expressed as a percentage by mass, satisfies equation (1).

[0103] 2[Cr] θ +3[Mo] θ ≤3 / √C (1)

[0104] Here, C in equation (1) is replaced with the C content in the chemical composition of the steel plate, expressed as a percentage by mass.

[0105] (Feature 5)

[0106] The average particle size of the cementite particles is less than 1.50 μm.

[0107] (Feature 6)

[0108] 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 70% or more and less than 85%.

[0109] (Feature 7)

[0110] In a microstructure, the average number of cementite particles in more than 500 microsquare partitions with a side length of 10 μm is defined as N. θ The sample standard deviation of the number of cementite particles in a tiny square partition is defined as σ. θ When the dispersion DI defined by equation (2) is below 45%.

[0111] DI=σ θ / N θ ×100 (2)

[0112] The following is an explanation of each feature 1 to feature 7.

[0113] [Regarding (Characteristic 1) Chemical Composition]

[0114] The steel plate of this embodiment contains the following elements in its chemical composition.

[0115] C: 0.15~0.50%

[0116] 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 0.15%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0117] On the other hand, if the C content exceeds 0.50%, the cold workability of the steel plate will decrease even if the contents of other elements are within the range of this embodiment.

[0118] Therefore, the C content is 0.15~0.50%.

[0119] The preferred lower limit for C content is 0.16%, and more preferably 0.17%.

[0120] The preferred upper limit for C content is 0.49%, and more preferably 0.48%.

[0121] Si: 0.01~0.50%

[0122] 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.

[0123] 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.

[0124] Therefore, the Si content is 0.01~0.50%.

[0125] The preferred lower limit for Si content is 0.02%, more preferably 0.05%, more preferably 0.10%, and more preferably 0.15%.

[0126] The preferred upper limit for Si content is 0.45%, more preferably 0.40%, and even more preferably 0.38%.

[0127] Mn: 0.20~1.30%

[0128] 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.

[0129] 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.

[0130] Therefore, the Mn content is 0.20~1.30%.

[0131] The preferred lower limit for Mn content is 0.25%, more preferably 0.30%, and even more preferably 0.35%.

[0132] The preferred upper limit for Mn content is 1.25%, more preferably 1.20%, and even more preferably 1.15%.

[0133] P: below 0.100%

[0134] 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.

[0135] Therefore, the P content is below 0.100%.

[0136] 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 more than 0%, more preferably 0.001%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%.

[0137] The preferred upper limit for the P content is 0.090%, more preferably 0.080%, and even more preferably 0.050%.

[0138] S: Below 0.100%

[0139] 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.

[0140] Therefore, the sulfur content is below 0.100%.

[0141] 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%.

[0142] The preferred upper limit for the sulfur content is 0.090%, more preferably 0.080%, and even more preferably 0.050%.

[0143] Al: below 0.100%

[0144] Aluminum (Al) is an impurity. Al combines with nitrogen to form AlN. During the quenching process in the manufacture of mechanical parts using steel sheets as raw materials, AlN causes the austenite grains to become finer. This austenite grain refinement reduces the hardenability of the steel sheet. 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 fine during the quenching process. As a result, the hardenability of the steel sheet is significantly reduced.

[0145] Therefore, the Al content is below 0.100%.

[0146] 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%.

[0147] The preferred upper limit for Al content is 0.090%, more preferably 0.080%, more preferably 0.070%, and more preferably 0.050%.

[0148] In the chemical composition of the steel plate in this embodiment, the Al content refers to the content of acid-soluble Al (sol.Al).

[0149] Cr: 0.01~1.20%

[0150] 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.

[0151] 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.

[0152] Therefore, the Cr content is 0.01~1.20%.

[0153] The preferred lower limit for Cr content is 0.02%, more preferably 0.03%, and even more preferably 0.05%.

[0154] The preferred upper limit for Cr content is 1.15%, more preferably 1.10%, more preferably 1.00%, more preferably 0.70%, more preferably 0.50%, and more preferably 0.30%.

[0155] N: below 0.0150%

[0156] 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. 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.

[0157] Therefore, the N content is below 0.0150%.

[0158] 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%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0030%, and more preferably 0.0040%.

[0159] The preferred upper limit for N content is 0.0140%, and more preferably 0.0130%.

[0160] 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.

[0161] As impurities mentioned above, they sometimes contain the following elements in varying amounts.

[0162] Cu: 0~0.150%, W: 0~0.150%, Ta: 0~0.150%, 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%.

[0163] [Optional Elements]

[0164] The chemical composition of the steel plate in this embodiment may also contain selected...

[0165] Mo: 0~0.500%,

[0166] Ni: 0~1.000%

[0167] B: 0~0.0100%

[0168] V: 0~0.500%

[0169] Nb: 0~0.500%, and

[0170] Ti: One or more elements in a group consisting of 0 to 0.150%.

[0171] The following is an explanation of these arbitrary elements.

[0172] [Regarding Group 1: Mo, Ni, and B]

[0173] 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.

[0174] Mo: 0~0.500%

[0175] Molybdenum (Mo) can be any element, or it can be absent. That is, the Mo content can be 0%.

[0176] 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. Furthermore, Mo 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.

[0177] However, if the Mo content exceeds 0.500%, 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.

[0178] Therefore, the Mo content is 0~0.500%.

[0179] The preferred lower limit for the Mo content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0180] The preferred upper limit for the Mo content is 0.450%, more preferably 0.400%, more preferably 0.350%, and more preferably 0.300%.

[0181] Ni: 0~1.000%

[0182] Nickel (Ni) can be any element, or it can be absent. That is, the Ni content can be 0%.

[0183] 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-mentioned effects are achieved to some extent even with a small amount of Ni.

[0184] 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.

[0185] Therefore, the Ni content is 0~1.000%.

[0186] The preferred lower limit for Ni content is 0.001%, more preferably 0.005%, and even more preferably 0.007%.

[0187] 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%.

[0188] B: 0~0.0100%

[0189] Boron (B) can be any element, or it can be absent. That is, the B content can be 0%.

[0190] 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 the above-mentioned effect to some extent.

[0191] 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.

[0192] Therefore, the B content is 0~0.0100%.

[0193] The preferred lower limit for the content of B is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0194] 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%.

[0195] [Regarding Group 2: V, Nb, and Ti]

[0196] 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.

[0197] V: 0~0.500%

[0198] Vanadium (V) can be any element, or it can be absent. That is, the V content can be 0%.

[0199] In the presence of V, i.e., when the V content exceeds 0%, V forms carbides, which suppress the coarsening of austenite grains during the quenching process in the manufacture of mechanical parts from steel plates. Therefore, the toughness of the mechanical parts is improved. Even a small amount of V is sufficient to achieve this effect to some extent.

[0200] 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.

[0201] Therefore, the V content is 0~0.500%.

[0202] The preferred lower limit for the V content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0203] The preferred upper limit for the V content is 0.480%, more preferably 0.450%, more preferably 0.400%, more preferably 0.350%, and more preferably 0.300%.

[0204] Nb: 0~0.500%

[0205] Niobium (Nb) can be any element, or it can be absent. That is, the Nb content can be 0%.

[0206] 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 B. This, in turn, improves the hardenability of the steel sheet caused by dissolved B. Even a small amount of Nb can achieve these effects to some extent.

[0207] 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.

[0208] Therefore, the Nb content is 0~0.500%.

[0209] The preferred lower limit for Nb content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0210] 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%.

[0211] Ti: 0~0.150%

[0212] Titanium (Ti) can be any element, or it can be absent. That is, the Ti content can be 0%.

[0213] 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, inhibiting the formation of nitrides from dissolved B. This, in turn, improves the hardenability of the steel sheet caused by dissolved B. Even a small amount of Ti can achieve these effects to some extent.

[0214] 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.

[0215] Therefore, the Ti content is 0~0.150%.

[0216] The preferred lower limit for Ti content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0217] 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%.

[0218] [Regarding (Characteristic 2) Microstructure]

[0219] 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.

[0220] 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.

[0221] 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.

[0222] 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 7, sufficient hardenability and sufficient cold workability can be obtained.

[0223] [Method for determining the total area fraction of ferrite and cementite particles in microstructure]

[0224] The total area ratio of ferrite and cementite particles in a microstructure can be determined by the following methods.

[0225] A test piece measuring 15 mm in the rolling direction (L direction) and 10 mm in the width direction (W direction) and 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.

[0226] 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.

[0227] 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 substructure is not identified can be identified as ferrite. Areas with higher brightness than ferrite and lower brightness than pearlite, where substructure is identified, can be identified as bainite and martensite. Therefore, based on contrast and morphology, ferrite and cementite particles within the field of view are identified.

[0228] 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.

[0229] [Regarding (characteristic 3) the average grain size of ferrite]

[0230] In the steel plate of this embodiment, the average grain size of ferrite is 5.0~20.0 μm.

[0231] 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.

[0232] Therefore, the average grain size of ferrite is 5.0~20.0 μm.

[0233] 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.

[0234] 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.

[0235] [Method for determining the average particle size of ferrite]

[0236] The average particle size of ferrite can be determined by the following methods.

[0237] A test piece measuring 15 mm in the rolling direction (L direction) and 10 mm in the width direction (W direction) and thickness of the steel plate was collected from the center of the plate width. The cross-section of the test piece parallel to the rolling direction (the surface measuring 15 mm × thickness in the rolling direction) 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 truncated section method according to JIS G 0551:2020. The magnification of the optical microscope was selected such that the number of ferrite grains truncated by a single line segment was at least 10 in one field of view. After selecting the magnification, the truncated section lengths were determined for five fields of view. The grain size number of the ferrite was determined by the arithmetic mean of the truncated section lengths of the five fields of view. The average grain size (μm) of ferrite is determined from the obtained grain size number.

[0238] Regarding (characteristic 4) the Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ ]

[0239] In the steel sheet of this embodiment, the Cr concentration [Cr] in the cementite particles, expressed as a percentage by mass, is... θ Mo concentration [Mo] in cementite particles, expressed as a percentage by mass. θ The C content in the steel plate, expressed as a percentage by mass, satisfies equation (1).

[0240] 2[Cr] θ +3[Mo] θ ≤3 / √C (1)

[0241] Here, C in equation (1) is replaced with the C content in the chemical composition of the steel plate, expressed as a percentage by mass.

[0242] 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.

[0243] Cr concentration [Cr] in cementite particles of steel plate θ and Mo concentration [Mo] θ Under the condition of satisfying equation (1), the Cr concentration [Cr] in the cementite particles θ and Mo concentration [Mo] θThe 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.

[0244] [Cr concentration in cementite particles] θ and Mo concentration [Mo] θ [Determination method]

[0245] Cr concentration [Cr] in cementite particles θ and Mo concentration [Mo] θ It can be determined by the following methods.

[0246] 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.

[0247] 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).

[0248] 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.

[0249] 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.

[0250] 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.

[0251] 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 (mass%) and Mo concentration [Mo] θ (quality%).

[0252] [(Feature 5) Average particle size of cementite particles]

[0253] In the steel plate of this embodiment, the average particle size of the cementite particles is less than 1.50 μm.

[0254] 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.

[0255] 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. As a result, the hardenability of the steel plate is improved.

[0256] 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.

[0257] 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.

[0258] [Method for determining the average particle size of cementite particles]

[0259] The average particle size of cementite can be determined by the following method.

[0260] A test piece measuring 15 mm × 10 mm × plate thickness in the rolling direction (L direction) and the width direction (W direction) of the steel plate was collected from the center of the plate width. The section of the test piece parallel to the rolling direction (the surface measuring 15 mm × plate thickness in the rolling direction) was defined as the observation surface.

[0261] The observation surface was etched using a bitter alcohol solution. Secondary electron images were captured at five arbitrary observation fields located at a depth of 4 / 4 of the plate thickness on the etched surface. Specifically, the five observation fields 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.

[0262] 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 cementite particles obtained from five observation fields was taken as the average particle size (μm) of the cementite particles.

[0263] [Regarding (Feature 6) Sphericity]

[0264] 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 70% or more and less than 85%.

[0265] If the spheroidization rate is less than 70%, then even if characteristics 1 to 5 and characteristic 7 are satisfied, sufficient cold workability cannot be obtained in the steel plate. Therefore, the spheroidization rate should be above 70%.

[0266] A high spheroidization rate is preferred. However, if the spheroidization rate is excessively increased, the solubility of cementite during quenching decreases. The dissolution of cementite occurs through solid solution and diffusion of elements from the interface between cementite and the parent phase. Therefore, the closer the cementite is to a spherical shape, the smaller the interfacial area between cementite and the parent phase. As a result, the solubility of cementite decreases. Therefore, the preferred upper limit for the spheroidization rate is less than 85%, more preferably 84%, and even more preferably 83%.

[0267] [Methods for determining sphericity]

[0268] The sphericity can be determined by the following methods.

[0269] According to the above-described method for determining the average particle size of cementite, the aspect ratio of multiple cementite particles identified in five observation fields was determined. 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 (i.e., the width in the direction perpendicular to the major axis) is defined as the minor axis.

[0270] 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 (%).

[0271] [Regarding (Feature 7) Cementite Particle Dispersion DI]

[0272] The microstructure of the steel plate in this embodiment is divided into tiny square partitions with a side length of 10 μm, and the average number of cementite particles in more than 500 tiny square partitions is defined as N. θ The sample standard deviation of the number of cementite particles in a tiny square partition is defined as σ. θ When the dispersion DI defined by equation (2) is below 45%.

[0273] DI=σ θ / N θ ×100 (2)

[0274] The dispersion degree (DI) of cementite particles is an indicator of the distribution of multiple cementite particles in a steel plate. The lower the dispersion degree (DI), the more uniformly the multiple cementite particles are dispersed in the steel plate.

[0275] When the dispersion DI is high, there are regions with dense cementite particles and regions with sparse cementite particles within the steel sheet. The strength of the regions with dense cementite particles is higher than that of the regions with sparse cementite particles. The cold workability of the high-strength regions in the steel sheet is lower than that of the low-strength regions. Therefore, a difference in cold workability arises between the regions with dense and sparse cementite particles, resulting in a decrease in the overall cold workability of the steel sheet. Therefore, to improve the cold workability of the steel sheet, it is preferable that the cementite particles are distributed as uniformly as possible.

[0276] If the dispersion DI is below 45%, the multiple cementite particles in the steel plate are sufficiently and uniformly distributed. Therefore, sufficient cold workability can be obtained in the steel plate, provided that characteristics 1 to 6 are satisfied.

[0277] The preferred upper limit for the dispersion DI is 42%, more preferably 40%, more preferably 38%, more preferably 36%, more preferably 34%, more preferably 32%, and more preferably 30%.

[0278] There is no particular limitation on the lower limit of the dispersion DI. However, if the lower limit of the dispersion DI is reduced indefinitely, the manufacturing cost will increase. Therefore, the preferred lower limit of the dispersion DI is 5%, more preferably 10%, and even more preferably 15%.

[0279] [Method for determining the dispersibility DI]

[0280] The dispersion DI can be determined using the following method.

[0281] A test piece measuring 15 mm × 10 mm × plate thickness in the rolling direction (L direction) and the width direction (W direction) of the steel plate was collected from the center of the plate width. The section of the test piece surface parallel to the rolling direction (i.e., the surface measuring 15 mm × plate thickness in the rolling direction) was defined as the observation surface.

[0282] The observation surface was etched using an picolinate solution. Secondary electron images were captured in any five or more observation fields at a depth of 4 / 4 of the plate thickness on the etched observation surface. Specifically, at least five observation fields were observed using a scanning electron microscope (SEM) at 1000x magnification, and the aforementioned secondary electron images were captured. Each observation field was defined as a 100μm × 100μm square region. Hereinafter, the observation field is referred to as a square region. There is no particular upper limit to the number of square regions (observation fields), for example, 10 (i.e., 1000 tiny square partitions).

[0283] Each square region was divided into 100 tiny square partitions with sides of 10 μm. The number of cementite particles within each tiny square partition was then counted. Whether a cementite particle was contained within a tiny square partition was determined by whether its centroid was located within that partition. It should be noted that in the secondary electron image, cementite particle identification was based on contrast. The centroid location of the cementite particles can be determined using known image processing software.

[0284] Figure 1 This is a schematic diagram representing a portion of a square region. (See reference...) Figure 1 Square region 1 is a 100μm × 100μm square. Square region 1 is divided into 100 tiny square partitions 10 with a side length of 10μm. In each tiny square partition 10, the number of cementite particles CM contained in the tiny square partition 10 is counted.

[0285] Here, we focus on the minute square partition 10A. Within minute square partition 10A, cementite particles CM1 exist on its boundary BL1 with minute square partition 10C, and cementite particles CM2 exist on its boundary BL2 with minute square partition 10B. The centroid of cementite particle CM1 is contained within minute square partition 10A. Conversely, the centroid of cementite particle CM2 is contained within minute square partition 10B but not within minute square partition 10A. Therefore, cementite particle CM1 is counted as cementite particles contained in minute square partition 10A. Conversely, cementite particle CM2 is not counted as cementite particles contained in minute square partition 10A, but rather as cementite particles contained in minute square partition 10B. It should be noted that... Figure 1The number of cementite particles in the tiny square partition 10A is 7.

[0286] Using the method described above, the number of cementite particles in each of the five tiny square partitions 10 is counted. Then, the arithmetic mean of the number of cementite particles in each of the five square regions 1 (i.e., 500 tiny square partitions 10) is defined as the average number N. θ Furthermore, the sample standard deviation of the number of cementite particles in each of the five square regions 1 (i.e., 500 tiny square partitions 10) is defined as the sample standard deviation σ. θ Using the obtained average number N θ and sample standard deviation σ θ The dispersion DI is calculated based on equation (2).

[0287] [The effect of the steel plate in this embodiment]

[0288] The steel sheet of this embodiment, which satisfies features 1 to 7 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.

[0289] [Regarding hardenability]

[0290] In the steel plate of this embodiment, sufficient hardenability is achieved as evaluated below.

[0291] [Methods for evaluating hardenability]

[0292] (A c1 Phase transition point and A c3 Phase transition point determination)

[0293] 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 and A c3 Phase transition point.

[0294] (Determination of maximum quenching hardness)

[0295] 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.

[0296] 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).

[0297] (Hardenability Evaluation)

[0298] 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.

[0299] The plate-shaped test piece was heated by applying an electric current, and heated to A at a heating rate of 400℃ / second. c3 Phase transition point +100℃. The heated plate-shaped test piece is quenched by immersing it in water in a water bath. The quenched plate-shaped test piece is then cut in half along the W direction. The cut surfaces are mirror-polished. Vickers hardness tests according to JIS Z2244:2009 are performed at any three points in the center of the plate thickness direction (T direction) of the polished cut surface. The test force is set to 98 N. The arithmetic mean of the obtained Vickers hardness is defined as the quenching hardness HD1 (HV).

[0300] 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.

[0301] [Regarding cold workability]

[0302] In the steel sheet of this embodiment, cold workability can be evaluated, for example, by the following methods.

[0303] [Methods for evaluating cold workability]

[0304] 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.

[0305] Using a plate-shaped test piece, an elongation at break test was conducted at room temperature and in atmospheric conditions. The elongation at break was measured, and the obtained elongation at break (%) was defined as the notch elongation (%). Compared with steel plates that do not satisfy any of features 1 to 7, 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 15% or more.

[0306] [Applications of steel plates]

[0307] The steel sheet of this embodiment is suitable as a blank for mechanical parts, such as automotive components. Examples of mechanical parts include automotive door components, seat components, and drive system gear components. 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 excellent cold workability.

[0308] [Steel plate manufacturing methods]

[0309] 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.

[0310] An example of the steel plate manufacturing method of this embodiment includes the following steps.

[0311] (Process 1) Blank preparation process

[0312] (Process 2) Hot rolling process

[0313] (Process 3) Cold rolling process

[0314] (Process 4) Cold-rolled sheet annealing process

[0315] 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.

[0316] The main manufacturing conditions for processes 1 to 4 are as follows.

[0317] (Condition 1) The winding temperature CT in process 2: above 550℃ but below 590℃

[0318] (Condition 2) Cold rolling ratio CR in process 3: 15~35%

[0319] (Condition 3) Annealing temperature T1 in process 4: 550~700℃

[0320] (Condition 4) Holding time t1 in process 4: 5~20 hours

[0321] (Condition 5) In steps 2 and 4, the FA defined by the following formula (A) is set to 20520 or less.

[0322] FA = (700 + 273) × (X + log(td)) (A)

[0323] Here, we substitute the characteristic value defined by equation (B) as described later into X in equation (A). We substitute the total holding time (in hours) defined by equation (C) as described later into td.

[0324] The following is a description of each process.

[0325] [(Process 1) Blank Preparation Process]

[0326] 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.

[0327] [(Process 2) Hot rolling process]

[0328] In the hot rolling process, prepared billets (slabs or ingots) are hot-rolled to produce hot-rolled steel sheets. The hot rolling process includes roughing and finishing rolling. In the roughing process, the billets are roughly rolled to produce rough strips (intermediate steel plates). In the finishing process, the rough strips are finished to produce hot-rolled steel sheets.

[0329] 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, 1050~1300°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.

[0330] 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.

[0331] [(Process 3) Cold rolling process]

[0332] In the cold rolling process, hot-rolled steel sheets are cold-rolled after the hot rolling process. Cold rolling is carried out using a cold rolling mill. A cold rolling mill can be, for example, a tandem rolling mill with multiple cold rolling stands arranged in a row, or a reversible rolling mill consisting of a single rolling mill.

[0333] In the cold rolling process, cold rolling is carried out using tandem rolling mills or reversible rolling mills to produce cold-rolled steel sheets. The cold rolling ratio CR in the cold rolling process is described later.

[0334] 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 of appropriate size, cementite particles of appropriate size, and an appropriate Cr concentration [Cr] in the cementite particles are obtained. θ and Mo concentration [Mo] θ And the appropriate spheroidization rate of cementite particles.

[0335] [(Process 4) Cold-rolled sheet annealing process]

[0336] 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.

[0337] [Regarding conditions 1 through 5]

[0338] In the above-mentioned processes 1 to 4, the following conditions 1 to 5 are satisfied.

[0339] (Condition 1) The winding temperature CT in process 2: above 550℃ but below 590℃

[0340] (Condition 2) Cold rolling ratio CR in process 3: 15~35%

[0341] (Condition 3) Annealing temperature T1 in process 4: 550~700℃

[0342] (Condition 4) Holding time t1 in process 4: 5~20 hours

[0343] (Condition 5) In steps 2 and 4, the FA defined by the following formula (A) is set to 20520 or less.

[0344] FA = (700 + 273) × (X + log(td)) (A)

[0345] Here, we substitute the characteristic value defined by equation (B) as described later into X in equation (A). We substitute the total holding time (in hours) defined by equation (C) as described later into td.

[0346] The following is an explanation of each condition.

[0347] [Regarding (Condition 1) winding temperature CT]

[0348] In the hot rolling process, the coiling temperature (CT) affects the spheroidization rate of cementite particles, the dispersion (DI) of cementite particles, and the Cr concentration [Cr] in the cementite particles. θ and Mo concentration [Mo] θ If the coiling temperature (CT) exceeds 590°C, even if other conditions are met, the cementite particles generated in the hot-rolled steel sheet will not be distributed sufficiently uniformly, resulting in deviations in the distribution. Furthermore, if the coiling temperature (CT) exceeds 590°C, the strain applied to the hot-rolled steel sheet is excessively reduced. In this case, the spheroidization rate of the cementite particles decreases. Consequently, Cr and Mo will accumulate in the generated cementite particles, increasing the Cr concentration [Cr] in the cementite particles. θ and Mo concentration [Mo] θ Equation (1) is not satisfied. Therefore, the winding temperature CT is set to below 590℃.

[0349] On the other hand, there is no particular limitation on the lower limit of the coiling temperature (CT). However, if the coiling temperature (CT) is too low, excessive strain is left on the hot-rolled steel sheet. In this case, the spheroidization of cementite particles is excessively promoted. As a result, the spheroidization rate of cementite particles becomes 85% or more. Therefore, the preferred lower limit of the coiling temperature (CT) exceeds 550°C.

[0350] If the coiling temperature CT exceeds 550℃ but is below 590℃, it is possible to manufacture steel plates that meet features 1 to 7, provided that other conditions are met.

[0351] [Regarding (Condition 2) Cold Rolling Ratio CR]

[0352] In the cold rolling process, the cold rolling ratio CR is defined by the following formula.

[0353] Cold rolling yield CR (%) = (1 - (thickness of cold-rolled steel sheet after cold rolling / thickness of hot-rolled steel sheet before cold rolling)) × 100

[0354] If the cold rolling ratio (CR) is less than 15%, the strain introduced into the steel sheet is insufficient. In this case, the spheroidization of cementite particles cannot be promoted in the next annealing process, and the spheroidization rate becomes less than 70%. If the cold rolling ratio (CR) is less than 15%, the cementite particles generated in the hot-rolled steel sheet cannot be distributed sufficiently uniformly, resulting in distribution deviations. On the other hand, if the cold rolling ratio (CR) is too high, the strain introduced into the steel sheet becomes excessive. In this case, the ferrite is excessively refined, and the average ferrite grain size becomes less than 5.0 μm. If the strain introduced into the steel sheet becomes excessive, the spheroidization of cementite particles is further excessively promoted in the next annealing process. Therefore, the spheroidization rate of cementite particles becomes 85% or more.

[0355] If the cold rolling ratio CR is 15~35%, it is possible to manufacture steel plates that meet characteristics 1 to 7, provided that other conditions are met.

[0356] [Regarding (Condition 3) Annealing temperature T1]

[0357] In the annealing process of cold-rolled steel sheets, if the annealing temperature T1 is less than 550℃, the recrystallization of ferrite becomes insufficient, the steel sheet cannot be sufficiently softened, and the cold workability of the steel sheet decreases. Consequently, the spheroidization of cementite particles becomes insufficient, and the spheroidization rate of cementite particles becomes less than 70%.

[0358] On the other hand, if the annealing temperature T1 exceeds 700°C, the annealing temperature is too high. In this case, the cementite particles become coarse, with an average particle size exceeding 1.50 μm. Consequently, Cr and Mo will accumulate in the resulting cementite particles, increasing the Cr concentration [Cr] in the cementite particles. θ and Mo concentration [Mo] θ Equation (1) is not satisfied.

[0359] If the annealing temperature T1 is 550~700℃, it is possible to manufacture steel plates that meet characteristics 1 to 7, provided that other conditions are met.

[0360] [Regarding the holding time t1 at annealing temperature T1 (condition 4)]

[0361] In the annealing process of cold-rolled steel sheets, the holding time t1 at annealing temperature T1 is the same as the annealing temperature T1, affecting the size of ferrite, cementite particles, spheroidization rate of cementite particles, and Cr concentration [Cr] in the cementite particles. θ and Mo concentration [Mo] θ Specifically, if the holding time t1 is less than 5 hours, the cementite particles will not be fully geomorphized.

[0362] On the other hand, if the holding time t1 exceeds 20 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 and Mo concentration [Mo] θ Equation (1) is not satisfied. Consequently, the spheroidization of cementite particles is excessively promoted, and the spheroidization rate of cementite particles becomes more than 85%.

[0363] If the holding time t1 is 5 to 20 hours, it is possible to manufacture steel plates that meet characteristics 1 to 7, provided that other conditions are met.

[0364] [Regarding (Condition 5), FA is defined by equation (A)]

[0365] In the coiling process of hot rolling and the annealing process of cold rolled sheet, FA, as defined by the following formula (A), is set to 20520 or less.

[0366] FA = (700 + 273) × (X + log(td)) (A)

[0367] Here, we substitute the characteristic value defined by equation (B) as described later into X in equation (A). We substitute the total holding time (in hours) defined by equation (C) as described later into td.

[0368] The eigenvalue X is defined by the following equation (B).

[0369] X = 20 + (Cr / 52 + Mo / 96) 0.2 (B)

[0370] Here, the element symbols in Equation (B) are substituted with the content of the corresponding element in the steel plate in terms of mass%.

[0371] The total hold time td is defined by the following formula (C).

[0372] td = ta + tb + tc (C)

[0373] Here, the holding time (in hours) calculated from the winding temperature CT and assumed to be 700°C is substituted into ta in equation (C).

[0374] Substitute into tb the annealing temperature T0 (°C) and holding time t0 (hours) in the hot-rolled plate annealing process, assuming the annealing temperature in the hot-rolled plate annealing process is 700°C and the holding time is t0 (hours). It should be noted that if the hot-rolled plate annealing process is not performed, substitute 0 into tb.

[0375] Substitute into tc the annealing temperature T1 (°C) and holding time t1 (hours) in the cold-rolled sheet annealing process, assuming the annealing temperature in the cold-rolled sheet annealing process is 700°C and the holding time (hours) is calculated.

[0376] The terms ta, tb, and tc mentioned above are indicators of the holding time at 700°C when the coiling temperature CT (°C) of the hot rolling process, the annealing temperature T0 (°C) of the hot-rolled sheet annealing process, and the annealing temperature T1 (°C) of the cold-rolled sheet annealing process are converted respectively. Specifically, ta, tb, and tc can be calculated according to the following equations (D) to (F).

[0377] ta=10 {(CT+273)×(X+log(10)) / (700+273)-X} (D)

[0378] Here, the coiling temperature (°C) of the hot rolling process is substituted into CT in Equation (D). The characteristic value defined by Equation (B) above is substituted into X.

[0379] tb=10 {(T0+273)×(X+log(t0)) / (700+273)-X} (E)

[0380] Here, the annealing temperature (°C) of the hot-rolled sheet annealing process is substituted into T0 in Equation (E). The holding time (hours) of the hot-rolled sheet annealing process is substituted into t0. The characteristic value defined by Equation (B) above is substituted into X.

[0381] tc=10 {(T1+273)×(X+log(t1)) / (700+273)-X} (F)

[0382] Here, the annealing temperature (°C) of the cold-rolled sheet annealing process is substituted into T1 in Equation (F). The holding time (hours) of the cold-rolled sheet annealing process is substituted into t1. The characteristic value defined by Equation (B) above is substituted into X.

[0383] FA is an indicator of the inhibition of Cr and Mo enrichment in cementite particles. When the FA exceeds 20520, excessive heat is applied to the steel sheet during the coiling process in hot rolling and the annealing process in cold rolling, relative to the Cr and Mo content in the steel sheet. In this case, Cr and Mo will accumulate in the cementite particles, and the Cr concentration in the cementite particles [Cr] will increase. θ and Mo concentration [Mo]θ Equation (1) is not satisfied.

[0384] When the FA is below 20520, and assuming the chemical composition meets characteristic 1, the Cr concentration [Cr] in the cementite particles is appropriately adjusted. θ and Mo concentration [Mo] θ Therefore, equation (1) is satisfied. If FA is 20520 or less, it is possible to manufacture steel plates that satisfy features 1 to 7, provided that other conditions are met.

[0385] Through the above manufacturing processes, steel plates that meet the requirements of features 1 to 7 can be manufactured.

[0386] 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.

[0387] Example 1

[0388] Manufacture steel plates having the chemical compositions shown in Tables 1-1 and 1-2.

[0389] [Table 1-1]

[0390]

[0391] [Table 1-2]

[0392]

[0393] 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 1150–1250°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 for 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) is 400–650°C.

[0394] For hot-rolled steel sheets after the hot rolling process, a cold rolling process is performed instead of a hot-rolled sheet annealing process. The cold rolling ratio (CR) in the cold rolling process is 5~70%. The cold-rolled steel sheets are then subjected to a cold-rolled sheet annealing process. The annealing temperature (T1) is 500~750℃, and the holding time (t1) is 1~48 hours. Through the above manufacturing processes, steel sheets are manufactured.

[0395] [Evaluation Test]

[0396] The following tests were performed on the steel plates manufactured for each test number.

[0397] (Experiment 1) Determination of the total area ratio of ferrite and cementite particles

[0398] (Experiment 2) Ferrite Average Grain Size Determination Test

[0399] (Experiment 3) Cr concentration in cementite particles [Cr] θ and Mo concentration [Mo] θ Determination test

[0400] (Experiment 4) Test on the determination of the average particle size of cementite particles

[0401] (Experiment 5) Determination of the spheroidization rate of cementite particles

[0402] (Experiment 6) Determination of the Dispersion DI of Cementite Particles

[0403] (Experiment 7) Hardenability Evaluation Test

[0404] (Experiment 8) Cold workability evaluation test

[0405] The following describes Experiments 1 through 8.

[0406] [(Experiment 1) Determination of the total area ratio of ferrite and cementite particles]

[0407] 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 2, and the total area ratio of ferrite and cementite particles for each test number is above 95%.

[0408] [Table 2]

[0409]

[0410] [(Experiment 2) Ferrite Average Particle Size Determination Test]

[0411] Based on the method described in the above-mentioned [Method for Determining the Average Grain Size of Ferrite], the average grain size of ferrite in the steel plates of each test number was determined. The results are shown in Table 2.

[0412] [(Experiment 3) Cr concentration in cementite particles [Cr]] θ and Mo concentration [Mo] θ [Determination Test]

[0413] Based on the above [Cr concentration in cementite particles] θ and Mo concentration [Mo] θThe 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 Furthermore, based on the obtained Cr concentration [Cr] θ and Mo concentration [Mo] θ Find the following F1.

[0414] F1=2[Cr] θ +3[Mo] θ

[0415] The obtained Cr concentration [Cr] θ , Mo concentration [Mo] θ And F1 is shown in Table 2. When F1 is less than or equal to 3 / √C, that is, when Equation (1) is satisfied, it is indicated by "E (Excellent)" in "Equation (1)" in Table 2. When F1 exceeds 3 / √C, that is, when Equation (1) is not satisfied, it is indicated by "NA (Not Allowed)" in "Equation (1)" in Table 2.

[0416] [(Experiment 4) Determination of the average particle size of cementite particles]

[0417] Based on the method described above in the [Method for Determining the Average Particle Size of Cementite Particles], the average particle size of cementite particles in the steel plates for each test number was determined. The obtained average particle sizes of cementite particles are shown in Table 2.

[0418] [(Experiment 5) Determination of the spheroidization rate of cementite particles]

[0419] 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 2.

[0420] [(Experiment 6) Determination of the Dispersion DI of Cementite Particles]

[0421] Based on the method described in the above-mentioned [Method for Determination of Dispersion DI], the dispersion DI of cementite particles in the steel plates of each test number was determined. The number of selected square regions was set to 5 (i.e., the number of micro square partitions was set to 500). The obtained dispersion DI of cementite particles is shown in Table 2.

[0422] [(Experiment 7) Hardenability Evaluation Test]

[0423] Based on the method described in the [Hardenability Evaluation Method] above, the hardenability of the steel plates during quenching was evaluated for each test number. The "Lower Limit of Quenching Hardness" in Table 2 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 (represented by "E (Excellent)" in the "Hardenability Judgment" in Table 2). On the other hand, if the quenching hardness HD1 is less than the lower limit of quenching hardness, it is judged that insufficient hardenability has been achieved (represented by "NA (Not Allowed)" in the "Hardenability Judgment" in Table 2).

[0424] [(Experiment 8) Cold workability evaluation test]

[0425] 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 2. If the notch elongation is 15% or more, it is judged that sufficient cold workability has been obtained.

[0426] [Evaluation Results]

[0427] Referring to Tables 1-1, 1-2, and 2, the steel plates with test numbers 1, 2, 6, 7, 10, 12, 15, 17, 18, 20-44, and 46 satisfy characteristics 1 to 7. Therefore, the obtained quenching hardness HD1 is more than 95% of the highest quenching hardness HD0, achieving sufficient hardenability in the steel plate. Furthermore, the notch elongation is more than 15%, resulting in sufficient cold workability.

[0428] On the other hand, in experiment number 3, the average particle size of ferrite exceeded 20.0 μm. Furthermore, the spheroidization rate was over 85%. Furthermore, the Cr concentration [Cr] in the cementite particles was... θ and Mo concentration [Mo] θ Equation (1) is not satisfied. Therefore, the obtained quenching hardness HD1 is less than 95% of the highest quenching hardness HD0, and sufficient hardenability is not obtained.

[0429] In tests 4, 8, and 16, the spheroidization rate was above 85%. Therefore, the obtained quenching hardness HD1 was less than 95% of the highest quenching hardness HD0, indicating insufficient hardenability.

[0430] In tests 5 and 14, the dispersion DI exceeded 45%. Furthermore, the Cr concentration [Cr] in the cementite particles... θ and Mo concentration [Mo] θEquation (1) is not satisfied. Therefore, the obtained quenching hardness HD1 is less than 95% of the highest quenching hardness HD0, and sufficient hardenability is not obtained. Furthermore, the notch elongation is less than 15%, and sufficient cold workability is not obtained.

[0431] In experiment number 9, the spheroidization rate was less than 70%. Furthermore, the Cr concentration [Cr] in the cementite particles... θ and Mo concentration [Mo] θ Equation (1) is not satisfied. Therefore, the obtained quenching hardness HD1 is less than 95% of the highest quenching hardness HD0, and sufficient hardenability is not obtained. Furthermore, the notch elongation is less than 15%, and sufficient cold workability is not obtained.

[0432] In test number 11, the spheroidization rate was less than 70%. Therefore, the notch elongation was less than 15%, and sufficient cold workability was not achieved.

[0433] In experiment number 13, the average particle size of the cementite exceeded 1.50 μm. Furthermore, the spheroidization rate exceeded 85%. Furthermore, the Cr concentration [Cr] in the cementite particles was... θ and Mo concentration [Mo] θ Equation (1) is not satisfied. Therefore, the obtained quenching hardness HD1 is less than 95% of the highest quenching hardness HD0, and sufficient hardenability is not obtained.

[0434] In experiment number 19, the spheroidization rate was above 85%. Furthermore, the Cr concentration [Cr] in the cementite particles... θ and Mo concentration [Mo] θ Equation (1) is not satisfied. Therefore, the obtained quenching hardness HD1 is less than 95% of the highest quenching hardness HD0, and sufficient hardenability is not obtained.

[0435] In experiment number 45, the average particle size of the cementite exceeded 1.50 μm. Furthermore, the Cr concentration [Cr] in the cementite particles... θ and Mo concentration [Mo] θ Equation (1) is not satisfied. Therefore, the obtained quenching hardness HD1 is less than 95% of the highest quenching hardness HD0, and sufficient hardenability is not obtained.

[0436] In test number 47, the carbon content was too high. Therefore, the notched elongation was less than 15%, and sufficient cold workability was not achieved.

[0437] In experiment number 48, the Si content was too high. Therefore, the notched elongation was less than 15%, and sufficient cold workability was not achieved.

[0438] In test number 49, the Mn content was too high. Therefore, the notched elongation was less than 15%, and sufficient cold workability was not achieved.

[0439] In test number 50, the Mn content was too low. Therefore, the obtained quenching hardness HD1 was less than 95% of the highest quenching hardness HD0, and sufficient hardenability was not achieved.

[0440] In experiment number 51, the Mn content was too low. Consequently, the Cr content was too high. Therefore, the Cr concentration [Cr] in the cementite particles was... θ and Mo concentration [Mo] θ Equation (1) is not satisfied. Therefore, the obtained quenching hardness HD1 is less than 95% of the highest quenching hardness HD0, and sufficient hardenability is not obtained.

[0441] It should be noted that the carbon content in steel number 36 in Tables 1-1 and 1-2 is too low. Therefore, the maximum quenching hardness is too low.

[0442] Example 2

[0443] Manufacture steel plates for each of the test numbers shown in Table 3.

[0444] Specifically, molten steel is continuously cast to produce slabs. The slabs are then subjected to a hot rolling process. Specifically, the slabs are heated for 240 minutes at the heating temperatures shown in Table 3 (“Slab Heating Temperatures” in Table 3). The heated slabs are rolled using a roughing mill to produce rough strip. Furthermore, the rough strip is rolled using a finishing mill to produce hot-rolled steel sheets. The finishing rolling temperature, final pass reduction, and coiling temperature CT for each test number are shown in Table 3.

[0445] [Table 3]

[0446]

[0447] For tests 1-18 and 20, a cold rolling process was performed on the hot-rolled steel sheets after the hot rolling process, without a hot-rolled annealing process. The cold rolling ratio CR in the cold rolling process is shown in Table 3. A cold-rolled annealing process was then performed on the cold-rolled steel sheets. The annealing temperature T1 and holding time t1 are shown in Table 3. During the cold-rolled annealing process, the cold-rolled steel sheets were furnace-cooled after a holding time t1. For test 19, annealing (hot-rolled annealing) was performed after the hot rolling process and before the cold rolling process. The annealing temperature T0 and the holding time t0 at the annealing temperature are shown in Table 3. Steel sheets were manufactured through the above manufacturing processes.

[0448] [Evaluation Test]

[0449] Tests 1 through 8 were performed on the steel plates for each test number.

[0450] [Evaluation Test]

[0451] The evaluation results are shown in Table 4.

[0452] [Table 4]

[0453]

[0454] Referring to Tables 1-1, 1-2, 3, and 4, in tests 1-10, the chemical composition was appropriate and met conditions 1-5 of the manufacturing conditions. Therefore, the steel plates from these test numbers met characteristics 1-7. As a result, sufficient hardenability and sufficient cold workability were obtained.

[0455] On the other hand, in experiment 11, although the chemical composition was appropriate, the winding temperature (CT) was low. Therefore, the spheroidization rate of the cementite particles was too high. As a result, sufficient hardenability was not achieved.

[0456] On the other hand, in experiment number 12, although the chemical composition was appropriate, the winding temperature CT was high. Therefore, the spheroidization rate of the cementite particles was too low. Consequently, the dispersion DI of the cementite particles was too high. Furthermore, F1 was too high. As a result, sufficient hardenability and cold workability were not achieved.

[0457] In experiment number 13, 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. Consequently, the dispersion (DI) of the cementite particles was too high. As a result, sufficient cold workability was not achieved.

[0458] In experiment number 14, although the chemical composition was appropriate, the cold rolling ratio (CR) was too high. Therefore, the average grain size of ferrite was too small. Consequently, the spheroidization rate of cementite particles was too high. As a result, insufficient hardenability and insufficient cold workability were not achieved.

[0459] In experiment number 15, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too low. Therefore, the microstructure became a non-recrystallized structure, and ferrite could not be identified. Consequently, the spheroidization rate of the cementite particles was low. As a result, sufficient cold workability was not achieved.

[0460] In experiment number 16, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too high. Therefore, the cementite particles were coarse. Consequently, the spheroidization rate of the cementite particles was high. Furthermore, F1 was too high. As a result, sufficient hardenability was not achieved.

[0461] In experiment number 17, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too short. Therefore, the spheroidization rate of the cementite particles was too low. As a result, sufficient cold workability was not achieved.

[0462] In experiment number 18, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too long. Therefore, the ferrite and cementite particles were coarse. Consequently, the spheroidization rate of the cementite particles was high. Furthermore, F1 was too high. As a result, sufficient hardenability was not achieved.

[0463] In experiment number 19, annealing was performed after the hot rolling process and before the cold rolling process. Therefore, FA exceeded 20520. Consequently, the ferrite and cementite particles were coarse. Furthermore, the spheroidization rate of the cementite particles was high. Consequently, F1 was too high. As a result, sufficient hardenability was not achieved.

[0464] In test number 20, FA exceeded 20520. Therefore, F1 was too high. As a result, insufficient hardenability was not achieved.

[0465] The embodiments of this disclosure have been described above. However, the above embodiments are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the above embodiments, and appropriate modifications can be made to the above embodiments without departing from its spirit.

Claims

1. A steel plate having a chemical composition, in mass percent, of the following: C：0.15~0.50%、 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. θ The C content (in mass%) in the steel plate satisfies equation (1). 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 70% or more and less than 85%. The microstructure is divided into tiny square partitions with a side length of 10 μm. The average number of cementite particles in more than 500 of these tiny square partitions is defined as N. θ The sample standard deviation of the number of cementite particles in the tiny square partition is defined as σ. θ When the dispersion DI defined by equation (2) is below 45%, 2[Cr] θ +3[Mon] θ ≤3 / √C (1) DI=σ θ / N θ ×100 (2) Here, C in formula (1) is replaced by the C content in the chemical composition of the steel plate, expressed as a percentage by mass.

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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