Steel plate

Abstract

Provided is a steel sheet which is capable of obtaining excellent bendability and excellent hardenability, and which is capable of obtaining excellent hydrogen embrittlement resistance when made into a machine component. This steel sheet contains, in mass%, 0.50-0.90% of C, 0.10-0.50% of Si, 0.20-1.30% of Mn, 0.100% or less of P, 0.100% or less of S, 0.100% or less of Al, 0.01-1.20% of Cr, and 0.0150% or less of N, the total area ratio of ferrite and cementite particles being 95% or more, the average particle diameter of ferrite being 20.0 [mu] m or less, the average particle diameter of cementite particles being 1.50 [mu] m or less, the spheroidization rate of cementite being 85% or more, and the spheroidization rate of cementite being 1.50% or less. When [C] s is the C content at a depth of 50 [mu] m from the surface of the steel sheet in the sheet thickness direction, and [C] c is the C content at the center of the steel sheet in the sheet thickness direction, the C concentration ratio F1 defined by formula (1) is 0.50 or less. F1 = [C] s / [C] c (1)

Description

Technical Field

[0001] The present disclosure relates to a steel sheet, and more particularly, to a steel sheet that can be used as a blank for mechanical parts such as automobile parts. Background Art

[0002] Steel sheets with high carbon content (high-carbon steel sheets) are used as blanks for mechanical parts, particularly automotive components. The method for manufacturing these mechanical parts using these steel sheets is as follows. The steel sheets are cold-formed to form the shape of the mechanical part. The cold-formed steel sheets are then quenched and tempered. Through these manufacturing processes, high-strength mechanical parts are produced.

[0003] In order to increase the strength of mechanical parts, it is required to improve the hardenability of steel sheets used as blanks for mechanical parts. Therefore, Patent Documents 1 and 2 propose steel sheets with improved hardenability.

[0004] The steel plate disclosed in Patent Document 1 has the following composition: in mass %, it contains C: 0.20-0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.Al: 0.10% or less, N: 0.0050% or less, B: 0.0005-0.0050%, and further contains one or more of Sb, Sn, Bi, Ge, Te, and Se in a total amount of 0.002-0.030%, with the remainder being Fe and unavoidable impurities. In this steel plate, the proportion of the amount of solid-solution B to the B content is 70% or more. Furthermore, the microstructure is composed of ferrite and cementite. Furthermore, the density of cementite in the ferrite grains is 0.08 pieces / μm 2 In the steel sheet of Patent Document 1, the hardenability is improved by ensuring the amount of solid-solution B.

[0005] The steel sheet disclosed in Patent Document 2 has a composition comprising, by mass%, C: 0.10% to 0.33%, Si: 0.01% to 0.50%, Mn: 0.40% to 1.25%, P: 0.03% to 0.01%, S: 0.01% to 0.10%, sol. Al: 0.01% to 0.01%, N: 0.50% to 1.50%, and Cr: 0.50% to 1.50%, with the balance being Fe and unavoidable impurities. The steel sheet disclosed in Patent Document 2 has a microstructure comprising ferrite and carbides. The volume ratio of ferrite and carbides to the total microstructure is 90% or more, and the volume ratio of proeutectoid ferrite to the total microstructure is 20% to 80%. The Mn concentration in the carbides is 0.10% to 0.50% by mass, and the proportion of carbides with a particle size of 1 μm or greater to the total number of carbides is 30% to 60%. In the steel plate disclosed in Patent Document 2, the Mn concentration in the carbides is reduced. This facilitates the dissolution of the carbides during quenching, resulting in improved hardenability.

[0006] Prior art literature

[0007] Patent Literature

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

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

[0010] Problems to be solved by the invention

[0011] However, high-carbon steel sheets with a carbon content of 0.50% or more are often used as raw materials for mechanical parts such as springs and washers. During the manufacturing process for these parts, the raw steel sheets are bent. Therefore, high bendability is required.

[0012] Furthermore, mechanical parts have high strength after quenching. Therefore, mechanical parts are required to have high hydrogen embrittlement resistance. Patent Documents 1 and 2 do not investigate bendability and hydrogen embrittlement resistance.

[0013] An object of the present disclosure is to provide a steel sheet that can obtain excellent bendability and excellent hardenability, and can obtain excellent hydrogen embrittlement resistance when manufactured into a mechanical part.

[0014] Solutions for solving problems

[0015] The steel plate of the present disclosure is calculated as mass %.

[0016] C: 0.50~0.90%,

[0017] Si: 0.10-0.50%,

[0018] Mn: 0.20-1.30%,

[0019] P: 0.100% or less,

[0020] S: 0.100% or less,

[0021] Al: 0.100% or less,

[0022] Cr: 0.01~1.20%,

[0023] N: 0.0150% or less,

[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%,

[0030] Cu: 0-0.15%,

[0031] W: 0~0.15%

[0032] Ta: 0~0.15%,

[0033] Sn: 0~0.050%,

[0034] Sb: 0~0.050%,

[0035] Co: 0-0.050%,

[0036] As: 0~0.050%,

[0037] Mg: 0~0.050%,

[0038] Y: 0~0.050%,

[0039] Zr: 0~0.050%,

[0040] La: 0~0.050%,

[0041] Ce: 0~0.050%,

[0042] Ca: 0 to 0.050%, and

[0043] Balance: Fe and impurities,

[0044] In the microstructure, the total area ratio of ferrite and cementite particles is more than 95%,

[0045] The average particle size of the ferrite is 20.0 μm or less.

[0046] The average particle size of the cementite particles is 1.50 μm or less.

[0047] When the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, the ratio of the total number of the spherical cementite particles to the total number of the cementite particles, i.e., the spheroidization rate, is 85% or more;

[0048] When the C content at a position 50 μm deep from the surface of the steel plate in the plate thickness direction is defined as [C]s and the C content at the center position in the plate thickness direction of the steel plate is defined as [C]c, the C concentration ratio F1 defined by formula (1) is 0.50 or less.

[0049] F1=[C]s / [C]c (1)

[0050] Effects of the Invention

[0051] The steel sheet disclosed herein can achieve excellent bendability and hardenability, and can achieve excellent hydrogen embrittlement resistance when manufactured into mechanical parts. DETAILED DESCRIPTION

[0052] The present inventors have studied steel sheets having excellent hardenability and bendability and exhibiting excellent hydrogen embrittlement resistance when used as mechanical parts.

[0053] First, the present inventors studied the improvement of the hardenability and bendability of steel sheets having a C content of 0.50% or more, and the improvement of hydrogen embrittlement resistance when used as mechanical parts, from the perspective of chemical composition. The present inventors found that, if the steel sheet contains, in terms of mass%, C: 0.50-0.90%, Si: 0.10-0.50%, Mn: 0.20-1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01-1.20%, N: 0.0150% or less, Mo: 0-0.500%, Ni: 0-1.000%, B: 0-0.0100%, V: 0-0.500%, Nb: 0-0.500%, Ti: 0-0.150%, Cu: 0-0. A chemical composition comprising 15% Ni: 0-0.15%, W: 0-0.15%, Ta: 0-0.15%, Sn: 0-0.050%, Sb: 0-0.050%, Co: 0-0.050%, As: 0-0.050%, Mg: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, Ce: 0-0.050%, Ca: 0-0.050%, and the balance being Fe and impurities, is likely to improve the hardenability and bendability of the steel sheet, as well as the hydrogen embrittlement resistance when manufactured into mechanical parts. Therefore, the present inventors have further investigated means for improving the hardenability, bendability, and hydrogen embrittlement resistance of steel sheets having the above chemical composition from the perspective of their microstructure.

[0054] The present inventors first studied means for improving the hardenability of steel plates during quenching in the microstructure. The microstructure of the steel plate having the above-mentioned chemical composition is essentially composed of ferrite and cementite particles. During quenching in the manufacturing process when a mechanical part is manufactured using the steel plate as a blank, in order to improve the hardenability of the steel plate, it is preferred that the cementite particles in the steel plate are easily dissolved during quenching. In order to improve the solid solubility of the cementite particles during quenching, it is preferred that the particle size of the cementite particles is small. In the case of the steel plate having the above-mentioned chemical composition, it is effective to set the average particle size of the cementite particles to be less than 1.50 μm.

[0055] The present inventors have also studied means for improving the bendability of steel sheets through microstructure. To improve the bendability of steel sheets, increasing the spheroidization rate of cementite particles and maintaining an appropriate average ferrite particle size are effective. Therefore, in the steel sheet of this embodiment, the spheroidization rate of cementite particles is set to 85% or higher, and the average ferrite particle size is set to 20.0 μm or less.

[0056] However, even with the above-mentioned steel plate, excellent bendability and excellent hydrogen embrittlement resistance are sometimes not obtained. Therefore, the present inventors have conducted further research. Here, the present inventors have focused on the strength of the surface layer of the steel plate. If the strength of the surface layer of the steel plate is as high as that of the interior of the steel plate, sufficient bendability cannot be obtained, and sufficient hydrogen embrittlement resistance cannot be obtained when the steel plate is made into a mechanical component. If the carbon concentration in the surface layer of the steel plate is lower than the carbon concentration in the interior of the steel plate outside the surface layer, the bendability of the steel plate is improved, and the strength of the mechanical component manufactured using the steel plate as a blank is ensured, while the hydrogen embrittlement resistance is also improved.

[0057] Based on the above technical concept, the present inventors conducted further research and found that, when the C content at a depth of 50 μm from the surface of the steel plate in the plate thickness direction is defined as [C]s and the C content at the center position in the plate thickness direction of the steel plate is defined as [C]c, if the C concentration ratio F1 defined by formula (1) is 0.50 or less, the steel plate can have excellent bendability and a mechanical component manufactured using the steel plate as a blank can have excellent hydrogen embrittlement resistance.

[0058] F1=[C]s / [C]c (1)

[0059] The steel plate of this embodiment is completed based on the above-mentioned technical concept, and has the following structure.

[0060] The steel plate of the first constitution is calculated as mass %.

[0061] C: 0.50~0.90%,

[0062] Si: 0.10-0.50%,

[0063] Mn: 0.20-1.30%,

[0064] P: 0.100% or less,

[0065] S: 0.100% or less,

[0066] Al: 0.100% or less,

[0067] Cr: 0.01~1.20%,

[0068] N: 0.0150% or less,

[0069] Mo: 0~0.500%,

[0070] Ni: 0-1.000%,

[0071] B: 0~0.0100%,

[0072] V: 0~0.500%,

[0073] Nb: 0~0.500%,

[0074] Ti: 0~0.150%,

[0075] Cu: 0-0.15%,

[0076] W: 0~0.15%

[0077] Ta: 0~0.15%,

[0078] Sn: 0~0.050%,

[0079] Sb: 0~0.050%,

[0080] Co: 0-0.050%,

[0081] As: 0~0.050%,

[0082] Mg: 0~0.050%,

[0083] Y: 0~0.050%,

[0084] Zr: 0~0.050%,

[0085] La: 0~0.050%,

[0086] Ce: 0~0.050%,

[0087] Ca: 0 to 0.050%, and

[0088] Balance: Fe and impurities,

[0089] In the microstructure, the total area ratio of ferrite and cementite particles is more than 95%,

[0090] The average particle size of the ferrite is 20.0 μm or less.

[0091] The average particle size of the cementite particles is 1.50 μm or less.

[0092] When the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, the ratio of the total number of the spherical cementite particles to the total number of the cementite particles, i.e., the spheroidization rate, is 85% or more;

[0093] When the C content at a position 50 μm deep from the surface of the steel plate in the plate thickness direction is defined as [C]s and the C content at the center position in the plate thickness direction of the steel plate is defined as [C]c, the C concentration ratio F1 defined by formula (1) is 0.50 or less.

[0094] F1=[C]s / [C]c (1)

[0095] The second steel plate is the first steel plate, which contains a material selected from

[0096] Mo: 0.001~0.500%,

[0097] Ni: 0.001~1.000%, and

[0098] B: 0.0001~0.0100%

[0099] One or more of the group consisting of.

[0100] The third steel plate is a steel plate of the first or second structure, which contains a material selected from

[0101] V: 0.001~0.500%,

[0102] Nb: 0.001~0.500%, and

[0103] Ti: 0.001~0.150%

[0104] One or more of the group consisting of.

[0105] The steel plate of the fourth configuration is a steel plate of any one of the first to third configurations, and contains a steel plate selected from

[0106] Cu: 0.01-0.15%,

[0107] W: 0.01~0.15%,

[0108] Ta: 0.01~0.15%,

[0109] Sn: 0.001~0.050%,

[0110] Sb: 0.001~0.050%,

[0111] Co: 0.001~0.050%,

[0112] As: 0.001~0.050%,

[0113] Mg: 0.001~0.050%,

[0114] Y: 0.001~0.050%,

[0115] Zr: 0.001~0.050%,

[0116] La: 0.001~0.050%,

[0117] Ce: 0.001-0.050%, and

[0118] Ca: 0.001~0.050%

[0119] One or more of the group consisting of.

[0120] Hereinafter, the steel sheet of the present embodiment will be described in detail. Note that "%" of an element means mass % unless otherwise specified.

[0121] [Features of the Steel Sheet of the Present Embodiment]

[0122] The steel plate of the present embodiment satisfies the following features 1 to 6.

[0123] (Feature 1)

[0124] The chemical composition is as follows in mass %: C: 0.50-0.90%, Si: 0.10-0.50%, Mn: 0.20-1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01-1.20%, N: 0.0150% or less, Mo: 0-0.500%, Ni: 0-1.000%, B: 0-0.0100%, V: 0-0.500%, Nb: 0-0.500%, Ti :0~0.150%, Cu: 0~0.15%, W: 0~0.15%, Ta: 0~0.15%, Sn: 0~0.050%, Sb: 0~0.050%, Co: 0~0.050%, As: 0~0.050%, Mg: 0~0.050%, Y: 0~0.050%, Zr: 0~0.050%, La: 0~0.050%, Ce: 0~0.050%, Ca: 0~0.050%, and the balance: Fe and impurities.

[0125] (Feature 2)

[0126] In the microstructure, the total area ratio of ferrite and cementite particles is more than 95%.

[0127] (Feature 3)

[0128] The average grain size of ferrite is 20.0 μm or less.

[0129] (Feature 4)

[0130] The average particle size of cementite particles is 1.50 μm or less.

[0131] (Feature 5)

[0132] When cementite particles having an aspect ratio of 3.0 or less among cementite particles are defined as spherical cementite particles, the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 85% or more.

[0133] (Feature 6)

[0134] When the C content at a position 50 μm deep from the surface of the steel plate in the plate thickness direction is defined as [C]s and the C content at the center position in the plate thickness direction of the steel plate is defined as [C]c, the C concentration ratio F1 defined by formula (1) is 0.50 or less.

[0135] F1=[C]s / [C]c (1)

[0136] Features 1 to 6 will be described below.

[0137] [(Feature 1) About chemical composition]

[0138] The chemical composition of the steel plate of this embodiment contains the following elements.

[0139] C: 0.50~0.90%

[0140] Carbon (C) improves the hardenability of steel sheets. As a result, quenching during the manufacturing process of mechanical parts using steel sheets as blanks increases the strength of the parts. If the C content is less than 0.50%, these effects are not fully achieved. On the other hand, if the C content exceeds 0.90%, the bendability and hydrogen embrittlement resistance of the steel sheet decrease. Therefore, the C content is between 0.50% and 0.90%.

[0141] The lower limit of the C content is preferably 0.52%, more preferably 0.55%, and even more preferably 0.60%.

[0142] The upper limit of the C content is preferably 0.88%, more preferably 0.85%, and even more preferably 0.80%.

[0143] The preferred range of the C content is, for example, 0.52 to 0.88%, more preferably 0.55 to 0.85%, and even more preferably 0.60 to 0.80%.

[0144] Si: 0.10-0.50%

[0145] Silicon (Si) deoxidizes steel during the steelmaking stage in the steel plate manufacturing process. Si further improves the steel plate's resistance to temper softening when tempering is performed during the process of manufacturing mechanical parts using the steel plate as a blank. Si further promotes decarburization of the steel plate surface during cold-rolled plate annealing. If the Si content is less than 0.10%, the above-mentioned effects cannot be fully achieved. On the other hand, if the Si content exceeds 0.50%, the strength of the steel plate becomes too high due to solid solution strengthening. Therefore, the bendability of the steel plate decreases. Therefore, the Si content is 0.10 to 0.50%.

[0146] The lower limit of the Si content is preferably 0.12%, more preferably 0.15%, and even more preferably 0.20%.

[0147] The upper limit of the Si content is preferably 0.48%, more preferably 0.44%, and even more preferably 0.40%.

[0148] The preferred range of the Si content is, for example, 0.12 to 0.48%, more preferably 0.15 to 0.44%, and even more preferably 0.20 to 0.40%.

[0149] Mn: 0.20~1.30%

[0150] Manganese (Mn) improves the hardenability of steel sheets. As a result, by quenching the steel sheets during the manufacturing process of mechanical parts, the strength of the parts is improved. If the Mn content is less than 0.20%, the above effects cannot be fully achieved. On the other hand, if the Mn content exceeds 1.30%, the strength of the steel sheet becomes too high due to solid solution strengthening. As a result, the bendability of the steel sheet is reduced. Therefore, the Mn content is 0.20-1.30%.

[0151] The lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, and even more preferably 0.35%.

[0152] The upper limit of the Mn content is preferably 1.25%, more preferably 1.20%, and even more preferably 1.15%.

[0153] The preferred range of the Mn content is, for example, 0.25 to 1.25%, more preferably 0.30 to 1.20%, and even more preferably 0.35 to 1.15%.

[0154] P: 0.100% or less

[0155] Phosphorus (P) is an impurity. The P content can be 0%. If the P content exceeds 0.100%, the toughness of the steel sheet decreases. Therefore, the P content is 0.100% or less.

[0156] The P content is preferably as low as possible. Specifically, the P content is preferably 0%. However, excessively reducing the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0157] The upper limit of the P content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.050%.

[0158] The preferred range of the P content is, for example, 0.001 to 0.090%, more preferably 0.003 to 0.080%, and even more preferably 0.005 to 0.050%.

[0159] S: 0.100% or less

[0160] Sulfur (S) is an impurity. The S content can be 0%. If the S content exceeds 0.100%, S forms excessive sulfides, which reduces the bendability of the steel sheet. Therefore, the S content is 0.100% or less.

[0161] The S content is preferably as low as possible. Specifically, the S content is preferably 0%. However, excessively reducing the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0162] The upper limit of the S content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.050%.

[0163] The preferred range of the S content is, for example, 0.001 to 0.090%, more preferably 0.003 to 0.080%, and even more preferably 0.005 to 0.050%.

[0164] Al: 0.100% or less

[0165] Aluminum (Al) is an impurity. The Al content can be 0%. Al combines with N to form AlN. AlN refines the austenite grains during the quenching process in the process of manufacturing mechanical parts using steel plates as blanks. The refinement of the austenite grains reduces the hardenability of the steel plate. If the Al content exceeds 0.100%, the austenite grains will be excessively refined during the quenching process, and the hardenability of the steel plate will be significantly reduced. Therefore, the Al content is 0.100% or less.

[0166] The lower limit of the Al content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0167] The upper limit of the Al content is preferably 0.090%, more preferably 0.080%, further preferably 0.070%, and further preferably 0.050%.

[0168] The preferred range of the Al content is, for example, 0.001 to 0.090%, more preferably 0.005 to 0.070%, and even more preferably 0.010 to 0.050%.

[0169] In the chemical composition of the steel sheet according to the present embodiment, the Al content refers to the acid-soluble Al (sol. Al) content.

[0170] Cr: 0.01~1.20%

[0171] Chromium (Cr) improves the hardenability of steel sheets. As a result, quenching during the manufacturing process of mechanical parts using steel sheets as blanks increases the strength of the parts. If the Cr content is less than 0.01%, this effect cannot be fully achieved. On the other hand, if the Cr content exceeds 1.20%, the strength of the steel sheet becomes too high, thus reducing the bendability of the steel sheet. Therefore, the Cr content is 0.01-1.20%.

[0172] The lower limit of the Cr content is preferably 0.02%, more preferably 0.03%, and even more preferably 0.05%.

[0173] The upper limit of the Cr content is preferably 1.15%, more preferably 1.10%, more preferably 1.00%, more preferably 0.70%, and still more preferably 0.50%.

[0174] The preferred range of the Cr content is, for example, 0.02 to 1.15%, more preferably 0.03 to 1.10%, and even more preferably 0.05 to 0.50%.

[0175] N: 0.0150% or less

[0176] Nitrogen (N) is an unavoidable impurity. That is, the N content exceeds 0%. N combines with Al to form AlN. AlN refines the austenite grains during the quenching process in the process of manufacturing mechanical parts using steel plates as blanks. The refinement of the austenite grains reduces the hardenability of the steel plate. If the N content exceeds 0.0150%, the austenite grains will be excessively refined during the quenching process, significantly reducing the hardenability of the steel plate. Therefore, the N content is 0.0150% or less.

[0177] The lower limit of the N content is preferably 0.0001%, more preferably 0.0005%.

[0178] The upper limit of the N content is preferably 0.0140%, more preferably 0.0130%.

[0179] The preferred range of the N content is, for example, 0.0001 to 0.0140%, more preferably 0.0005 to 0.0130%.

[0180] The balance of the chemical composition of the steel sheet of this embodiment is Fe and impurities. Impurities in the chemical composition refer to substances that are introduced from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the steel sheet, and are permitted to the extent that they do not adversely affect the steel sheet of this embodiment.

[0181] About Optional Elements

[0182] The chemical composition of the steel sheet according to the present embodiment may further contain one or more elements selected from the group consisting of the first group to the third group in place of a portion of Fe.

[0183] [Group 1]

[0184] Choose Free

[0185] Mo: 0~0.500%,

[0186] Ni: 0~1.000%, and

[0187] B: 0~0.0100%

[0188] One or more of the group consisting of

[0189] [Group 2]

[0190] Choose Free

[0191] V: 0~0.500%,

[0192] Nb: 0-0.500%, and

[0193] Ti: 0~0.150%

[0194] One or more of the group consisting of

[0195] [Group 3]

[0196] Choose Free

[0197] Cu: 0-0.15%,

[0198] W: 0~0.15%

[0199] Ta: 0~0.15%,

[0200] Sn: 0~0.050%,

[0201] Sb: 0~0.050%,

[0202] Co: 0-0.050%,

[0203] As: 0~0.050%,

[0204] Mg: 0~0.050%,

[0205] Y: 0~0.050%,

[0206] Zr: 0~0.050%,

[0207] La: 0~0.050%,

[0208] Ce: 0-0.050%, and

[0209] Ca: 0~0.050%

[0210] One or more of the group consisting of

[0211] These optional elements are described below.

[0212] [About the first group (Mo, Ni and B)]

[0213] The chemical composition of the steel sheet of this embodiment may further contain one or more elements selected from the group consisting of Mo, Ni, and B in place of a portion of Fe. These elements are arbitrary and may not be present. When present, Mo, Ni, and B improve the hardenability of the steel sheet.

[0214] Mo: 0~0.500%

[0215] Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%.

[0216] When Mo is contained, that is, when the Mo content exceeds 0%, Mo improves the hardenability of the steel sheet. Therefore, by quenching the steel sheet during the process of manufacturing mechanical parts, the strength of the mechanical parts is improved. Mo further improves the steel sheet's resistance to temper softening when tempering the steel sheet during the process of manufacturing mechanical parts. Even a small amount of Mo can achieve the above effects to some extent. However, if the Mo content exceeds 0.500%, the strength of the steel sheet becomes too high. As a result, the bendability of the steel sheet decreases. Therefore, the Mo content is 0 to 0.500%.

[0217] The lower limit of the Mo content is preferably 0.001%, more preferably 0.003%, further preferably 0.005%, and further preferably 0.010%.

[0218] The upper limit of the Mo content is preferably 0.450%, more preferably 0.400%, further preferably 0.350%, and further preferably 0.300%.

[0219] The preferred range of the Mo content is, for example, 0.001 to 0.450%, more preferably 0.003 to 0.400%, further preferably 0.005 to 0.350%, and further preferably 0.010 to 0.300%.

[0220] Ni: 0~1.000%

[0221] Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%.

[0222] When Ni is contained, that is, when the Ni content exceeds 0%, Ni improves the hardenability of the steel sheet. Therefore, by quenching the steel sheet during the process of manufacturing mechanical parts, the strength of the mechanical parts is improved. Ni further improves the steel sheet's resistance to temper softening when tempering is performed during the process of manufacturing mechanical parts using the steel sheet as a blank. The above effects can be achieved to some extent even with a small amount of Ni. However, if the Ni content exceeds 1.000%, the strength of the steel sheet becomes too high. As a result, the bendability of the steel sheet decreases. Therefore, the Ni content is 0 to 1.000%.

[0223] The lower limit of the Ni content is preferably 0.001%, more preferably 0.005%, further preferably 0.007%, and further preferably 0.010%.

[0224] The upper limit of the Ni content is preferably 0.950%, more preferably 0.900%, more preferably 0.800%, more preferably 0.700%, and even more preferably 0.600%.

[0225] The preferred range of the Ni content is, for example, 0.001 to 0.950%, more preferably 0.005 to 0.900%, more preferably 0.007 to 0.800%, more preferably 0.010 to 0.700%, and even more preferably 0.010 to 0.600%.

[0226] B: 0~0.0100%

[0227] Boron (B) is an optional element and may not be contained. That is, the B content may be 0%.

[0228] When B is present, that is, when the B content exceeds 0%, B improves the hardenability of the steel sheet. Therefore, quenching during the process of manufacturing mechanical parts from the steel sheet as a blank increases the strength of the mechanical parts. Even a small amount of B can achieve this effect to some extent. However, if the B content exceeds 0.0100%, B compounds are formed. In this case, the strength of the steel sheet becomes excessively high, resulting in a decrease in the bendability of the steel sheet. Therefore, the B content is preferably between 0 and 0.0100%.

[0229] The lower limit of the B content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0230] The upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, and still more preferably 0.0050%.

[0231] The preferred range of the B content is, for example, 0.0001 to 0.0090%, more preferably 0.0003 to 0.0080%, more preferably 0.0005 to 0.0070%, more preferably 0.0005 to 0.0060%, and even more preferably 0.0005 to 0.0050%.

[0232] [About the second group (V, Nb and Ti)]

[0233] The chemical composition of the steel plate of this embodiment may further 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, which suppress the coarsening of austenite grains during quenching and heating in the process of manufacturing mechanical parts using the steel plate as a blank. This improves the toughness of the mechanical parts.

[0234] V: 0~0.500%

[0235] Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%.

[0236] When V is present, that is, when the V content exceeds 0%, V forms carbides, which inhibit the coarsening of austenite grains during quenching and heating in the process of manufacturing mechanical parts using the steel plate as a blank. This improves the toughness of the mechanical parts. Even a small amount of V can achieve this effect to some extent. However, if the V content exceeds 0.500%, excessive carbides are formed, causing precipitation strengthening of the steel plate. Consequently, the bendability of the steel plate is reduced. Therefore, the V content is preferably between 0 and 0.500%.

[0237] The lower limit of the V content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0238] The upper limit of the V content is preferably 0.480%, more preferably 0.450%, and further preferably 0.400%.

[0239] The preferred range of the V content is, for example, 0.001 to 0.480%, more preferably 0.003 to 0.450%, and even more preferably 0.005 to 0.400%.

[0240] Nb: 0~0.500%

[0241] Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%.

[0242] When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms carbides, which inhibit the coarsening of austenite grains during quenching heating in the process of manufacturing mechanical parts using steel plates as blanks. Therefore, the toughness of the mechanical parts is improved. In addition, Nb combines with N to inhibit the formation of nitrides by solid solution B. As a result, the hardenability of the steel plate achieved by solid solution B is improved. As long as a small amount of Nb is contained, the above effect can be obtained to a certain extent. However, if the Nb content exceeds 0.500%, carbides are formed excessively, causing precipitation strengthening of the steel plate. Therefore, the bendability of the steel plate is reduced. Therefore, the Nb content is 0 to 0.500%.

[0243] The lower limit of the Nb content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0244] The upper limit of the Nb content is preferably 0.480%, more preferably 0.450%, further preferably 0.400%, further preferably 0.350%, further preferably 0.300%.

[0245] The preferred range of the Nb content is, for example, 0.001 to 0.480%, more preferably 0.003 to 0.450%, more preferably 0.005 to 0.400%, more preferably 0.005 to 0.350%, and even more preferably 0.005 to 0.300%.

[0246] Ti: 0~0.150%

[0247] Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%.

[0248] When Ti is contained, that is, when the Ti content exceeds 0%, Ti forms carbides, which inhibit the coarsening of austenite grains during quenching heating in the process of manufacturing mechanical parts using steel plates as blanks. Therefore, the toughness of mechanical parts is improved. In addition, Ti combines with N to inhibit the formation of nitrides by solid solution B. As a result, the hardenability of the steel plate achieved by solid solution B is improved. As long as a small amount of Ti is contained, the above-mentioned effect can be obtained to a certain extent. However, if the Ti content exceeds 0.150%, carbides are formed excessively, causing precipitation strengthening of the steel plate. Therefore, the cold workability of the steel plate is reduced. Therefore, the Ti content is 0 to 0.150%.

[0249] The lower limit of the Ti content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0250] The upper limit of the Ti content is preferably 0.145%, more preferably 0.130%, more preferably 0.120%, more preferably 0.100%, and still more preferably 0.080%.

[0251] The Ti content preferably ranges from 0.001 to 0.145%, more preferably from 0.003 to 0.130%, more preferably from 0.005 to 0.120%, more preferably from 0.005 to 0.100%, and even more preferably from 0.005 to 0.080%.

[0252] [Regarding the third group (Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca)]

[0253] The chemical composition of the steel sheet of this embodiment may further contain one or more elements selected from the group consisting of Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca in place of a portion of Fe. These elements are optional and may not be present. In other words, the content of these elements may be 0%.

[0254] These elements are all impurity elements and are considered impurities in the present invention. Therefore, the Cu content is 0-0.15%, the W content is 0-0.15%, Ta: 0-0.15%, Sn: 0-0.050%, Sb: 0-0.050%, Co: 0-0.050%, As: 0-0.050%, Mg: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, Ce: 0-0.050%, and Ca: 0-0.050%.

[0255] The lower limit of the Cu content is preferably 0.01%, more preferably 0.03%.

[0256] The upper limit of the Cu content is preferably 0.13%, more preferably 0.10%.

[0257] The preferred range of the Cu content is, for example, 0.01 to 0.13%, more preferably 0.03 to 0.10%.

[0258] The lower limit of the W content is preferably 0.01%, more preferably 0.03%.

[0259] The upper limit of the W content is preferably 0.13%, more preferably 0.10%.

[0260] The preferred range of the W content is, for example, 0.01 to 0.13%, more preferably 0.03 to 0.10%.

[0261] The lower limit of the Ta content is preferably 0.01%, more preferably 0.03%.

[0262] The upper limit of the Ta content is preferably 0.13%, more preferably 0.10%.

[0263] The preferred range of the Ta content is, for example, 0.01 to 0.13%, more preferably 0.03 to 0.10%.

[0264] The lower limit of the Sn content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0265] The upper limit of the Sn content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0266] The preferred range of the Sn content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0267] The lower limit of the Sb content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0268] The upper limit of the Sb content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0269] The preferred range of the Sb content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0270] The lower limit of the Co content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0271] The upper limit of the Co content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.

[0272] The preferred range of the Co content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0273] The lower limit of the As content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0274] The upper limit of the As content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0275] The preferred range of the As content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0276] The lower limit of the Mg content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0277] The upper limit of the Mg content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0278] The preferred range of the Mg content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0279] The lower limit of the Y content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0280] The upper limit of the Y content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0281] The preferred range of the Y content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0282] The lower limit of the Zr content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0283] The upper limit of the Zr content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0284] The preferred range of the Zr content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0285] The lower limit of the La content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0286] The upper limit of the La content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0287] The preferred range of the La content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0288] The lower limit of the Ce content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0289] The upper limit of the Ce content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0290] The preferred range of the Ce content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0291] The lower limit of the Ca content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0292] The upper limit of the Ca content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0293] The preferred range of the Ca content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.

[0294] [Method for measuring the chemical composition of steel plates]

[0295] The chemical composition of the steel plate of this embodiment can be measured by a known component analysis method. Specifically, a drill is used to collect chips from the inside of the steel plate at a depth of more than 0.1 mm from the surface of the steel plate. The collected chips are dissolved in acid to obtain a solution. The solution is subjected to inductively coupled plasma atomic emission spectrometry (ICP-AES, Inductively Coupled Plasma Atomic Emission Spectrometry) to perform elemental analysis of the chemical composition. The C content and the S content are determined by the known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the known inert gas melting-thermal conductivity method.

[0296] It should be noted that the content of each element is rounded off based on the significant figures specified in this embodiment, and the decimal point of the measured value is used to obtain the value of the content of each element specified in this embodiment. For example, the C content of the steel plate in this embodiment is specified by the value to the second decimal place. Therefore, the C content is the value obtained by rounding off the third decimal place of the measured value to the second decimal place.

[0297] The same applies to the contents of other elements in the steel sheet of this embodiment, except for the C content. The measured value is the value obtained by rounding off the decimal point up to the minimum digit specified in this embodiment as the element content. It should be noted that rounding off means discarding the decimal point if it is less than 5 and rounding up the decimal point if it is 5 or greater.

[0298] [(Feature 2) Microstructure]

[0299] In the microstructure of the steel sheet of the present embodiment, the total area ratio of ferrite and cementite particles is 95% or more. In other words, the microstructure of the steel sheet of the present embodiment is substantially composed of ferrite and cementite particles.

[0300] In the microstructure, the structure other than ferrite and cementite particles is, for example, one or more selected from the group consisting of bainite, martensite, and pearlite.

[0301] The total area ratio of ferrite and cementite particles in the microstructure is preferably 96% or more, more preferably 97% or more, more preferably 98% or more, and even more preferably 99% or more. The microstructure may also be composed of ferrite and cementite particles.

[0302] The total area ratio of ferrite and cementite particles is preferably in the range of 96 to 100%, more preferably 97 to 100%, further preferably 98 to 100%, and further preferably 99 to 100%.

[0303] If the total area ratio of ferrite and cementite particles is 95% or more, excellent bendability can be obtained on the premise that characteristics 1 and characteristics 3 to 6 are satisfied, and high hydrogen embrittlement resistance can be obtained when the material is made into a mechanical part.

[0304] [Method for measuring the total area ratio of ferrite and cementite particles in the microstructure]

[0305] The total area ratio of ferrite and cementite particles in the microstructure can be measured by the following method.

[0306] A test piece of 15 mm in the rolling direction of the steel plate × 10 mm in the width direction × the thickness of the plate is collected from the center of the width of the steel plate. Among the surfaces of the test piece, the surface of 15 mm in the rolling direction × the thickness of the plate is defined as the observation surface. The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched with 3% nital (nitric acid ethanol etching solution). For any 5 observation fields in the etched observation surface, observe the secondary electron image with a 1000x scanning electron microscope (SEM: Scanning Electron Microscope). Each observation field is a rectangle of 100 μm × 120 μm.

[0307] In the observation field, ferrite and cementite particles show different contrast and different morphology from other structures (bainite, martensite, pearlite, etc.). Therefore, based on the contrast and morphology, the ferrite and cementite particles in the observation field are identified.

[0308] The total area ratio (%) of ferrite and cementite particles was determined based on the total area of ferrite and cementite particles in the five observation fields and the total area of the five observation fields.

[0309] [(Feature 3) About the average grain size of ferrite]

[0310] In the steel sheet of the present embodiment, the average grain size of ferrite is 20.0 μm or less.

[0311] If the average ferrite grain size exceeds 20.0 μm, bendability decreases. If the average ferrite grain size exceeds 20.0 μm, the annealing time becomes even longer due to the coarsening of the ferrite grains. In this case, alloying elements are concentrated in the cementite, reducing hardenability. Therefore, the average ferrite grain size is kept below 20.0 μm.

[0312] The upper limit of the average grain size of ferrite is preferably 19.5 μm, more preferably 19.0 μm, further preferably 18.5 μm, further preferably 18.0 μm.

[0313] There is no particular lower limit for the average ferrite particle size. However, excessive ferrite refinement can excessively increase the yield strength of the steel sheet. In this case, the working load during bending increases excessively. Therefore, the preferred lower limit for the average ferrite particle size is 5.0 μm. A more preferred lower limit for the average ferrite particle size is 5.2 μm, more preferably 5.5 μm, more preferably 5.7 μm, more preferably 6.0 μm, and even more preferably 6.5 μm.

[0314] The preferred range of the average grain size of ferrite is, for example, 5.0 to 20.0 μm, more preferably 5.2 to 19.5 μm, more preferably 5.5 to 19.0 μm, more preferably 5.7 to 18.5 μm, more preferably 6.0 to 18.0 μm, and even more preferably 6.5 to 18.0 μm.

[0315] [Method for measuring the average particle size of ferrite]

[0316] The average particle size of ferrite can be measured by the following method.

[0317] A test piece measuring 15 mm in the rolling direction x 10 mm in the width direction x thickness was collected from the center of the steel plate. The surface of the test piece measuring 15 mm in the rolling direction x thickness was defined as the observation surface. The observation surface of the test piece was mirror-polished. After mirror polishing, the specimen was etched with 3% Nital. Secondary electron images were captured using a scanning electron microscope (SEM) at five observation fields at a depth of 4 / thickness from the steel plate surface on the etched observation surface. The ferrite grain-level index was determined using the intercept method in accordance with JIS G0551:2020. The SEM magnification was selected within the range of 500x to 3000x so that the number of ferrite grains intersected by a line segment in each field of view was at least 10. The intercept length was calculated for each of the five observation fields. The ferrite grain-level index was calculated as the arithmetic mean of the intercept lengths for the five observation fields. The average grain size (μm) of ferrite was determined from the obtained grain size index.

[0318] The average grain size of ferrite is a value obtained by rounding off the obtained value to the second decimal place (that is, a value to the first decimal place).

[0319] [(Feature 4) Average particle size of cementite particles]

[0320] In the steel sheet of the present embodiment, the average particle size of cementite particles is 1.50 μm or less.

[0321] If the average particle size of cementite particles is large, the bendability of the steel sheet decreases. Furthermore, if the average particle size of cementite particles is large, the cementite particles will not fully melt during the quenching process in the process of manufacturing mechanical parts using the steel sheet as a blank. In this case, the steel sheet will not have sufficient hardenability. As a result, the mechanical parts manufactured using the steel sheet as a blank will not have sufficient strength.

[0322] When the average particle size of cementite particles is 1.50 μm or less, the cementite particles are sufficiently small. Therefore, the steel sheet can achieve sufficient bendability. Furthermore, during heating in the quenching process, the cementite particles fully dissolve, improving the hardenability of the steel sheet.

[0323] The upper limit of the average particle size of cementite particles is preferably 1.45 μm, more preferably 1.40 μm, further preferably 1.35 μm, further preferably 1.30 μm.

[0324] To improve hardenability, the average particle size of cementite particles is preferably small. However, if the average particle size of cementite particles is too small, the hardness of the steel sheet becomes too high, and the cold workability decreases. Therefore, the preferred lower limit of 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.

[0325] The average particle size of the cementite particles preferably ranges from 0.05 to 1.50 μm, more preferably from 0.10 to 1.45 μm, more preferably from 0.15 to 1.40 μm, more preferably from 0.20 to 1.35 μm, and even more preferably from 0.20 to 1.30 μm.

[0326] [Method for measuring the average particle size of cementite particles]

[0327] The average particle size of cementite particles can be determined by the following method.

[0328] A test piece measuring 15 mm in the rolling direction x 10 mm in the width direction x thickness was collected from the center of the steel plate. The surface of the test piece measuring 15 mm in the rolling direction x thickness was defined as the observation surface.

[0329] The observation surface of the test piece is mirror-polished. After mirror-polishing, the observation surface is etched with a bitter alcohol solution. Secondary electron images are taken in any five observation fields at a distance of plate thickness / 4 depth from the surface of the steel plate on the etched observation surface. Specifically, a scanning electron microscope (SEM) is used to observe the five observation fields at a magnification of 2000 times, and the secondary electron images are taken. Each observation field is a rectangle of 50 μm × 60 μm.

[0330] In each secondary electron image, based on the contrast, specific cementite particles are identified. The area of each specific cementite particle is obtained, and based on the area, the equivalent circle diameter of each cementite particle is obtained. The obtained equivalent circle diameter is used as the particle size of the cementite particle. It should be noted that the particle size is implemented using known image processing software.

[0331] The arithmetic mean value of the particle sizes of cementite particles obtained in five observation fields was defined as the average particle size (μm) of cementite particles.

[0332] The average particle size of cementite particles is a value obtained by rounding off the obtained value to the third decimal place (ie, a value to the second decimal place).

[0333] [(Feature 5) About spheroidization rate]

[0334] In the steel sheet of this embodiment, cementite particles having an aspect ratio of 3.0 or less among the plurality of cementite particles are defined as spherical cementite particles. The spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 85% or more.

[0335] If the spheroidization rate is 85% or more, the steel sheet can obtain excellent bendability on the premise of satisfying Characteristics 1 to 4 and Characteristics 6. Therefore, in the steel sheet of this embodiment, the spheroidization rate is 85% or more.

[0336] The spheroidization rate is preferably high. The lower limit of the spheroidization rate is preferably 87%, more preferably 89%, more preferably 91%, and even more preferably 95%.

[0337] The preferred range of the spheroidization rate is, for example, 85 to 100%, more preferably 87 to 100%, more preferably 89 to 100%, more preferably 91 to 100%, and even more preferably 95 to 100%.

[0338] [Measurement method of spheroidization rate]

[0339] The spheroidization rate can be measured by the following method.

[0340] The aspect ratio was determined for each of the multiple cementite particles specified in the five observation fields using the above-mentioned [Method for Determining the Average Particle Size of Cementite Particles]. Specifically, the longest diameter was defined as the maximum distance between two parallel line segments sandwiching the contour of the cementite particle. Furthermore, the short diameter was defined as the distance between two line segments parallel to the long diameter sandwiching the contour of the cementite particle (i.e., the width in a direction perpendicular to the long diameter).

[0341] Based on the obtained major and minor diameters, the aspect ratio (= major diameter / minor diameter) of each cementite particle was determined. Among all cementite particles in the five observation fields, cementite particles with an aspect ratio of 3.0 or less were identified as "spherical cementite particles." The ratio of the total number of spherical cementite particles to the total number of cementite particles in the five observation fields was defined as the spheroidization rate (%).

[0342] The sphericity ratio is a value obtained by rounding off the first decimal place of the obtained value (ie, an integer).

[0343] [(Feature 6) Regarding C concentration ratio F1]

[0344] In the steel plate of this embodiment, when the C content at a position with a depth of 50 μm from the surface of the steel plate in the plate thickness direction is defined as [C]s and the C content at the center position in the plate thickness direction of the steel plate is defined as [C]c, the C concentration ratio F1 defined by formula (1) is 0.50 or less.

[0345] F1=[C]s / [C]c (1)

[0346] The center position of the steel plate in the plate thickness direction is located closer to the inside of the steel plate than a position at a depth of 50 μm from the surface of the steel plate in the plate thickness direction.

[0347] When the C concentration ratio F1 exceeds 0.50, the surface strength of the steel sheet is high, and the surface strength of the mechanical parts manufactured using the steel sheet as a blank is also high. In this case, the bendability of the steel sheet is reduced, and the hydrogen embrittlement resistance of the mechanical parts manufactured using the steel sheet as a blank is reduced.

[0348] When the C concentration ratio F1 is 0.50 or less, the strength of the surface layer of the steel sheet can be sufficiently suppressed. Consequently, the steel sheet can achieve excellent bendability. Furthermore, mechanical parts manufactured using the steel sheet as a blank can achieve excellent hydrogen embrittlement resistance.

[0349] The upper limit of the C concentration ratio F1 is preferably 0.49, more preferably 0.48, more preferably 0.47, more preferably 0.46, and even more preferably 0.45.

[0350] The lower limit of the C concentration ratio F1 is not particularly limited. In the steel sheet satisfying characteristics 1 to 6, the lower limit of the C concentration ratio F1 is, for example, 0.05, more preferably 0.10, and even more preferably 0.15.

[0351] The preferred range of the C concentration ratio F1 is, for example, 0.05 to 0.50, more preferably 0.10 to 0.49, more preferably 0.15 to 0.48, more preferably 0.15 to 0.47, more preferably 0.15 to 0.46, and even more preferably 0.15 to 0.45.

[0352] [Method for measuring C concentration ratio F1]

[0353] The C concentration ratio F1 can be measured by the following method.

[0354] A test piece measuring 15 mm in the rolling direction of the steel plate and 10 mm in the width direction and thickness of the steel plate was collected from the center of the steel plate width. Of the surfaces of the test piece, the surface measuring 15 mm in the rolling direction and thickness of the steel plate was defined as the observation surface. The observation surface of the test piece was mirror-polished. In the mirror-polished observation surface, the C content (mass %) was measured using an electron probe microanalyzer (EPMA) at a depth of 50 μm from the steel plate surface in the thickness direction, for 300 measurement positions with a spacing of 1 μm in the longitudinal direction of the steel plate. The arithmetic mean of the C content (mass %) of the obtained 300 points was set as [C]s. Furthermore, at the center position of the steel plate thickness direction, the C content (mass %) was measured using an electron probe microanalyzer (EPMA) at a measurement position with a spacing of 1 μm in the longitudinal direction of the steel plate. The arithmetic mean of the C content (mass %) of the obtained 300 points was set as [C]c. The obtained [C]s and [C]c were used to determine the C concentration ratio F1 from the formula (1). EPMA was measured under the following conditions.

[0355] Accelerating voltage: 15.0 kV

[0356] Irradiation current: 0.30μA

[0357] Irradiation time: 50ms

[0358] Beam diameter: 1 μm

[0359] [C]s (mass %) and [C]c (mass %) are values obtained by rounding off the obtained values to the third decimal place (i.e., values obtained by rounding off the obtained values to the second decimal place). The C concentration ratio F1 is a value obtained by rounding off the obtained values to the third decimal place (i.e., values obtained by rounding off the obtained values to the second decimal place).

[0360] [Effects of the Steel Sheet of the Present Embodiment]

[0361] The steel sheet of this embodiment, which satisfies the above-mentioned characteristics 1 to 6, can achieve excellent bendability. Furthermore, the steel sheet of this embodiment can achieve sufficient hardenability during quenching during the manufacturing process of mechanical parts made from the steel sheet. Furthermore, mechanical parts manufactured from the steel sheet of this embodiment can achieve excellent hydrogen embrittlement resistance.

[0362] [About bendability]

[0363] The bendability was evaluated by the following method.

[0364] [Bendability evaluation method]

[0365] A plate-shaped test piece is collected from the center of the plate width of the steel plate. The shape of the plate-shaped test piece is 15 mm along the rolling direction of the steel plate × 30 mm along the plate width direction × plate thickness. A 90° V bending test is performed on the plate-shaped test piece. Specifically, a die and a press punch are used to perform a 90° V bending process at the center of the plate width (30 mm) of the plate-shaped test piece. The bending line formed on the plate-shaped test piece by the V-bending process is parallel to the rolling direction of the steel plate (L-axis bending). The R of the press punch used in the 90°V bending test is set to 0.1 mm. Visually observe whether there are cracks on the surface of the plate-shaped test piece after the 90°V bending test. In the case where no cracks are confirmed, it is judged that excellent bendability can be obtained.

[0366] [About hardenability]

[0367] The hardenability was evaluated by the following method.

[0368] [Quenchability Evaluation Method]

[0369] (A c1 Phase transition point determination)

[0370] A cylindrical test piece with a diameter of 3 mm and a length of 10 mm was collected from the center of the steel plate. The length direction of the cylindrical test piece was parallel to the rolling direction of the steel plate. The thermal expansion coefficient during heating was measured using a Formaster testing machine. A was calculated from the obtained thermal expansion coefficient. c1 Phase transition point.

[0371] (Maximum quenching hardness test)

[0372] A plate-shaped test piece was collected from the center of the steel plate width. The shape of the plate-shaped test piece was 15 mm in the rolling direction of the steel plate × 30 mm in the plate width direction × plate thickness.

[0373] Heat the plate test piece at 1000°C for 20 minutes using a salt bath. Then, quench the plate test piece by immersing it in water in a water tank. Cut the quenched plate test piece into two equal parts along the width direction of the steel plate. Mirror-polish the cut surface. Perform a Vickers hardness test in accordance with JIS Z 2244: 2009 at any three locations in the center of the plate thickness direction of the mirror-polished cut surface. At this time, set the test force to 98 N. The arithmetic mean of the obtained Vickers hardness is defined as the maximum quenching hardness HD0 (HV).

[0374] (Quenchability Evaluation)

[0375] A plate-shaped test piece was collected from the center of the steel plate width. The shape of the plate-shaped test piece was 15 mm in the rolling direction of the steel plate × 30 mm in the plate width direction × plate thickness.

[0376] In A c1Immerse the plate-shaped test piece in a salt bath at the phase transformation point + 80°C for 10 minutes. Then, immerse the plate-shaped test piece taken out of the salt bath in water in a water tank for quenching. Cut the quenched plate-shaped test piece into two equal parts along the plate width direction. Mirror-polish the cut surface. Perform a Vickers hardness test according to JIS Z2244:2009 at any three locations in the center of the plate thickness direction of the mirror-polished cut surface. At this time, set the test force to 98N. The arithmetic mean of the obtained Vickers hardness is defined as the quenching hardness HD1 (HV). When the obtained quenching hardness HD1 is more than 0.95 times the maximum quenching hardness HD0, it is judged that the steel plate can obtain sufficient hardenability.

[0377] [Regarding hydrogen embrittlement resistance]

[0378] The hydrogen embrittlement resistance was evaluated by the following method.

[0379] [Evaluation method for hydrogen embrittlement resistance]

[0380] A plate-shaped test piece is collected from the center of the width of the steel plate. The shape of the plate-shaped test piece is 30 mm along the rolling direction of the steel plate × 100 mm along the width direction × plate thickness. The plate-shaped test piece is bent into a U-shape using a press bending method to produce a U-shaped bent test piece with a curvature radius R of 2.0 mm. Specifically, a U-shaped bending process is performed at the center of the width (100 mm) of the plate-shaped test piece. The bending line formed on the plate-shaped test piece by the U-shaped bending process is parallel to the rolling direction of the steel plate (L-axis bending).

[0381] Place the U-shaped bending test piece at A c1 Immerse the U-bend test piece in a salt bath at +80°C for 10 minutes. Remove the immersed U-bend test piece from the salt bath and quench it in a water tank. Temper the quenched U-bend test piece at 250°C for 1 hour. Elastically deform the U-bend test piece by fastening the opposing non-bend portions of the tempered U-bend test piece with bolts so that the two ends are parallel.

[0382] A delayed fracture acceleration test was conducted on U-bend test pieces with the non-bend portions parallel. Specifically, the U-bend test pieces were immersed in hydrochloric acid at pH 1 for 100 hours. After 100 hours, the bent portion of the U-bend test pieces was visually inspected for cracks. If no cracks were observed, it was determined that excellent hydrogen embrittlement resistance was achieved.

[0383] [Application of steel plates]

[0384] The steel sheet of this embodiment is suitable for use as blanks for mechanical parts, typified by automotive parts. Mechanical parts include, for example, automotive springs and washers. It should be noted that the steel sheet of this embodiment can also be used for applications other than mechanical parts requiring bendability and hydrogen embrittlement resistance.

[0385] [Method for manufacturing steel sheet]

[0386] An example of a method for manufacturing a steel plate according to this embodiment will be described. The method described below is an example for manufacturing the steel plate according to this embodiment. Therefore, a steel plate having the above-described structure can also be manufactured using a method other than the method described below. However, the method described below is a preferred example of a method for manufacturing a steel plate according to this embodiment.

[0387] An example of the method for manufacturing a steel plate according to the present embodiment includes the following steps.

[0388] (Process 1) Blank preparation process

[0389] (Process 2) Hot rolling process

[0390] (Process 3) Hot-rolled plate annealing process

[0391] (Process 4) Cold Rolling Process

[0392] (Process 5) Cold rolled sheet annealing process

[0393] Hereinafter, each step will be described.

[0394] [(Process 1) Blank preparation process]

[0395] In the billet preparation step, a billet meeting feature 1 is prepared. The billet is produced, for example, by the following method. Molten steel having a chemical composition within the ranges of the present embodiment is produced. The molten steel is used to produce billets (slabs or ingots) by casting. For example, the molten steel is used to produce slabs by a known continuous casting method. Alternatively, the molten steel is used to produce ingots by a known ingot casting method.

[0396] [(Process 2) Hot rolling process]

[0397] In the hot rolling process, the prepared billet (slab or ingot) is hot rolled to produce steel plate. The hot rolling process includes a rough rolling process in which the billet is roughly rolled to produce a rough steel bar (intermediate steel plate) and a finish rolling process in which the rough steel bar is finish rolled to produce steel plate.

[0398] In the rough rolling process, a billet (slab or ingot) is heated in a heating furnace. The heated billet is rolled using a roughing mill to produce rough steel bars. The billet is heated to a temperature of, for example, 1100-1250°C in the rough rolling process. The billet is kept in the heating furnace for at least 30 minutes, preferably at least 60 minutes. The upper limit of the time in the furnace is not particularly limited, but is, for example, 300 minutes.

[0399] In the finishing process, the rough steel bar is further rolled (finishing rolled) using a finishing mill to produce a steel plate. The finishing mill includes a plurality of rolling mills arranged in a row. Each rolling mill is equipped with a pair of working rolls. The surface temperature of the steel plate at the outlet side of the last rolling mill that presses down the steel plate among the plurality of rolling mills in the finishing mill is defined as the finishing temperature (°C). In the present embodiment, the finishing temperature is 830 to 950°C. In addition, the reduction rate in the last rolling mill that applies pressure to the steel plate among the plurality of rolling mills arranged in a row in the finishing mill is defined as the reduction rate of the final pass (%). In the present embodiment, the reduction rate of the final pass is 5 to 25%. The hot-rolled steel plate after finish rolling is coiled to form a coil. The coiling temperature CT will be described later. The hot-rolled steel plate formed into a coil is cooled to room temperature.

[0400] [(Process 3) Hot-rolled sheet annealing process]

[0401] In the hot-rolled steel sheet annealing process, the hot-rolled steel sheet is annealed under known conditions. The annealing temperature in the hot-rolled steel sheet annealing process is, for example, 500 to 770°C, and the holding time at the annealing temperature is, for example, 5 to 80 hours. This hot-rolled steel sheet annealing process employs so-called box annealing. Annealing is performed in a reducing atmosphere.

[0402] [(Process 4) Cold Rolling Process]

[0403] In the cold rolling process, the steel sheet after the hot-rolled sheet annealing process is cold rolled. The cold rolling is performed using a cold rolling mill. The cold rolling rate CR in the cold rolling process will be described later.

[0404] [(Process 5) Cold-rolled sheet annealing process]

[0405] In the cold-rolled sheet annealing process, the cold-rolled steel sheet undergoes cold annealing after the cold rolling process. In this embodiment, the cold-rolled sheet annealing process employs open coil annealing (OCA). Using an open coil annealing furnace, the cold-rolled steel sheet is annealed under conditions 3 to 6, described below. This annealing adjusts the degree of ferrite recrystallization and cementite particle precipitation. Furthermore, the carbon concentration ratio F1 is adjusted.

[0406] [Regarding conditions 1 to 6]

[0407] In the above-mentioned steps 1 to 5, the following conditions 1 to 6 are satisfied.

[0408] (Condition 1) Coiling temperature CT in step 2: 550-750°C

[0409] (Condition 2) Cold rolling ratio CR in step 4: more than 35% and 60% or less

[0410] (Condition 3) Annealing temperature T1 in step 5: 650-750°C

[0411] (Condition 4) Holding time t1 in step 5: 5 to 40 hours

[0412] (Condition 5) Atmosphere of the annealing furnace in step 5: 93-97 volume % hydrogen-3-7 volume % nitrogen atmosphere

[0413] (Condition 6) Dew point of the annealing furnace in step 5: +25 to +65°C

[0414] Each condition is described below.

[0415] [(Condition 1) Regarding coiling temperature CT]

[0416] During the hot rolling process, the coiling temperature (CT) affects the spheroidization rate of cementite particles. If the coiling temperature (CT) is below 750°C, the cementite particles generated in the hot-rolled steel sheet are sufficiently evenly distributed. Annealing at this point improves the spheroidization rate of cementite particles. Therefore, the coiling temperature (CT) is preferably below 750°C. While there is no specific lower limit for the coiling temperature (CT), due to equipment limitations, the preferred lower limit is 550°C.

[0417] [(Condition 2) Regarding the cold rolling ratio CR]

[0418] In the cold rolling process, the cold rolling ratio CR is defined by the following formula.

[0419] Cold rolling ratio CR (%) = (1-(thickness of cold-rolled steel sheet after cold rolling process / thickness of hot-rolled steel sheet before cold rolling process)) × 100

[0420] If the cold rolling ratio (CR) exceeds 35%, sufficient strain is introduced into the steel sheet. In this case, spheroidization of cementite particles is promoted in the subsequent annealing step, achieving a spheroidization rate of 85% or higher. On the other hand, if the cold rolling ratio (CR) is 60% or lower, the strain introduced into the steel sheet is appropriate. At this point, ferrite is appropriately refined, with an average ferrite grain size of 5.0 μm or higher.

[0421] [(Condition 3) Regarding Annealing Temperature T1]

[0422] During the cold-rolled sheet annealing process, the annealing temperature T1 adjusts the spheroidization rate of the steel sheet's cementite particles. If the annealing temperature T1 is below 650°C, cementite spheroidization is insufficient, resulting in a spheroidization rate of less than 85%. Furthermore, the carbon concentration ratio F1 exceeds 0.50. On the other hand, if the annealing temperature T1 exceeds 750°C, the annealing temperature is too high. In this case, cementite spheroidization is insufficient, resulting in a spheroidization rate of less than 85%.

[0423] [(Condition 4) Regarding the holding time t1 at the annealing temperature T1]

[0424] During the cold-rolled sheet annealing process, the hold time t1 at the annealing temperature T1 affects the size of the ferrite and cementite particles in the steel sheet. Specifically, if the hold time t1 is less than 5 hours, the cementite particles will not fully spheroidize. On the other hand, if the hold time t1 exceeds 40 hours, the hold time is too long, causing the ferrite and cementite particles to coarsen. As a result, the average ferrite particle size exceeds 20.0 μm, or the average cementite particle size exceeds 1.50 μm.

[0425] [(Condition 5) Regarding the atmosphere of the annealing furnace]

[0426] The atmosphere of the coil annealing furnace in the cold-rolled sheet annealing step is set to 93-97 volume % hydrogen-3-7 volume % nitrogen. In this case, the C concentration ratio F1 can be set to 0.50 or less.

[0427] [(Condition 6) Regarding the dew point of the annealing furnace]

[0428] The dew point of the atmosphere in the coil annealing furnace during the cold-rolled sheet annealing process is set to +25°C to +65°C. Specifically, humidified nitrogen is used to adjust the dew point to this range. If the dew point is below +25°C, the carbon concentration ratio F1 exceeds 0.50. On the other hand, if the dew point exceeds +65°C, the equipment load becomes excessive. Therefore, the dew point during the cold-rolled sheet annealing process is set to +25°C to +65°C.

[0429] Through the above-described manufacturing steps, a steel plate satisfying characteristics 1 to 6 can be manufactured.

[0430] It should be noted that if annealing is performed in a manner that satisfies conditions 5 and 6 in the hot-rolled sheet annealing process instead of in a reducing atmosphere, and if annealing is performed in a reducing atmosphere in the cold-rolled sheet annealing process, a steel sheet that satisfies characteristics 1 to 6 cannot be obtained. When annealing that satisfies conditions 5 and 6 is performed in the hot-rolled sheet annealing process, a decarburized layer is formed on the steel sheet after the hot-rolled sheet annealing process. However, the decarburized layer is stretched by the cold rolling process after the hot-rolled sheet annealing process. As a result, the decarburized layer becomes thinner due to the cold rolling process. Furthermore, if the cold-rolled sheet annealing process is performed, the C in the steel sheet diffuses and forms complex carbon, making the C concentration uniform throughout the steel sheet. As a result, a steel sheet that satisfies characteristic 6 cannot be obtained.

[0431] In this embodiment, the hot-rolled sheet annealing step is performed in a reducing atmosphere, and further, the final cold-rolled sheet annealing step is performed under conditions satisfying conditions 3 to 6. This allows the carbon concentration ratio of the steel sheet to be set to 0.50 or less, and enables the production of a steel sheet satisfying characteristics 1 to 6.

[0432] The effects of the steel sheet of this embodiment will be described in more detail below using examples. The conditions in the following examples are examples of conditions used to confirm the feasibility and effects of the steel sheet of this embodiment. Therefore, the steel sheet of this embodiment is not limited to this example of conditions.

[0433] Example

[0434] Steel plates having the chemical compositions shown in Table 1-1 and Table 1-2 were produced.

[0435] [Table 1-1]

[0436] Table 1-1

[0437]

[0438] [Table 1-2]

[0439] Table 1-2

[0440]

[0441] Specifically, molten steel is continuously cast to produce slabs with a thickness of 250 mm. The slabs are subjected to a hot rolling process. Specifically, the slabs are heated at 1100-1250°C for 120 minutes. The heated slabs are rolled using a roughing mill to produce crude steel bars. Furthermore, the crude steel bars are rolled using a finishing mill to produce hot-rolled steel plates with a thickness of 3.5 mm. The finishing rolling temperature of each test number is 830-950°C. The reduction ratio of the final pass is 5-25%. The hot-rolled steel plates after finish rolling are coiled to form coils. The hot-rolled steel plates formed into coils are naturally cooled to room temperature. The coiling temperature CT in the hot rolling process of each test number is shown in Table 2.

[0442] [Table 2]

[0443] Table 2

[0444]

[0445] The hot-rolled steel sheet is subjected to a hot-rolled sheet annealing process. The annealing temperature in the hot-rolled sheet annealing process is 500 to 770°C, and the holding time at the annealing temperature is 5 to 80 hours. The hot-rolled sheet annealing is carried out in a reducing atmosphere. The hot-rolled steel sheet after the hot-rolled sheet annealing process is subjected to a cold rolling process to manufacture the cold-rolled steel sheet. The cold rolling rate CR in the cold rolling process is shown in Table 2. The cold-rolled steel sheet after cold rolling is subjected to a cold-rolled sheet annealing process. In the cold-rolled sheet annealing process, unwinding annealing is carried out. The annealing temperature T1, holding time t1 and dew point in the cold-rolled sheet annealing process are shown in Table 2. It should be noted that the atmosphere of the unwinding annealing furnace is set to an atmosphere of 95% hydrogen by volume and 5% nitrogen by volume in any test number. Steel sheets are manufactured through the above manufacturing process.

[0446] [Evaluation test]

[0447] The following tests were performed on the manufactured steel plates of each test number.

[0448] (Test 1) Chemical composition measurement test

[0449] (Test 2) Test for measuring the total area ratio of ferrite and cementite

[0450] (Test 3) Ferrite average grain size measurement test

[0451] (Test 4) Average particle size measurement test of cementite particles

[0452] (Test 5) Test for measuring the spheroidization rate of cementite particles

[0453] (Test 6) Test for measuring C concentration ratio F1

[0454] (Test 7) Bendability evaluation test

[0455] (Test 8) Hardenability Evaluation Test

[0456] (Test 9) Hydrogen embrittlement resistance evaluation test

[0457] Hereinafter, Tests 1 to 9 will be described.

[0458] [(Test 1) Chemical composition measurement test]

[0459] The chemical composition of the steel plate of each steel number was measured according to the method described in the above-mentioned [Method for Measuring Chemical Composition of Steel Plate]. The chemical composition of the steel plate of each steel number is shown in Table 1-1 and Table 1-2.

[0460] [(Test 2) Test for measuring the total area ratio of ferrite and cementite particles]

[0461] The total area ratio of ferrite and cementite particles was determined for each test number using the method described in the [Method for Determining the Total Area Ratio of Ferrite and Cementite Particles in Microstructure] above. The total area ratio of ferrite and cementite particles was 95% or greater for all steel plates with the test numbers.

[0462] [(Test 3) Ferrite average grain size measurement test]

[0463] The average ferrite grain size of the steel plate of each test number was determined based on the method described in the above-mentioned [Method for Measuring Average Ferrite Grain Size]. The results are shown in Table 3.

[0464] [Table 3]

[0465] Table 3

[0466]

[0467] [(Test 4) Test for measuring average particle size of cementite particles]

[0468] The average particle size (μm) of cementite particles of the steel plate of each test number was determined according to the method described in [Method for measuring the average particle size of cementite particles]. The obtained average particle size (μm) of cementite particles is shown in Table 3.

[0469] [(Test 5) Test for measuring the spheroidization rate of cementite particles]

[0470] The spheroidization rate of cementite particles in the steel plate of each test number was determined based on the method described in the above-mentioned [Measurement Method of Spheroidization Rate]. The obtained spheroidization rates are shown in Table 3.

[0471] [(Test 6) Test for measuring C concentration ratio F1]

[0472] The carbon concentration ratio F1 of the steel plate for each test number was determined using the method described in the [Method for Determining Carbon Concentration Ratio F1] above. The obtained carbon concentration ratio F1 is shown in Table 3. In Table 3, in the "F1 ≤ 0.50" column, "T (True)" indicates that F1 is 0.50 or less. "F (False)" indicates that F1 exceeds 0.50.

[0473] [(Test 7) Bendability Evaluation Test]

[0474] The bendability of the steel plates of each test number was evaluated based on the method described in the above-mentioned [Bendability Evaluation Method]. The evaluation results are shown in the "Bendability" column of Table 3. The surface of the plate-shaped test piece after the 90° V-bend test was visually observed for the presence or absence of cracks. If no cracks were confirmed, it was judged that excellent bendability could be obtained (indicated by "E (Excellent)" in the "Bendability" column of Table 3). On the other hand, if cracks were confirmed, it was judged that excellent bendability could not be obtained (indicated by "B (Bad)" in the "Bendability" column of Table 3).

[0475] [(Test 8) Hardenability Evaluation Test]

[0476] Based on the method described in the above-mentioned [Quenchability Evaluation Method], the hardenability of the steel plate of each test number during quenching was evaluated. The "Quenching Hardness Lower Limit (HV)" column in Table 3 shows the value of the maximum quenching hardness HD0×0.95. If the quenching hardness HD1 is above the quenching hardness lower limit, it is judged that sufficient hardenability is obtained (indicated by "E (Excellent)" in the "Quenching Hardness" column in Table 3). On the other hand, if the quenching hardness HD1 is less than the quenching hardness lower limit, it is judged that sufficient hardenability is not obtained (indicated by "B (Bad)" in the "Quenching Hardness" column in Table 3). Furthermore, when the maximum quenching hardness HD0 in the steel plate is less than 600HV, it is judged that sufficient strength is not obtained as a mechanical component.

[0477] [(Test 9) Hydrogen embrittlement resistance evaluation test]

[0478] Based on the method described in the above-mentioned [Hydrogen embrittlement resistance evaluation method], a mechanical component manufactured using the steel plate of each test number as a blank was simulated, and the hydrogen embrittlement resistance of the simulated mechanical component (U-shaped bending test piece) was evaluated. The evaluation results are shown in the "Hydrogen embrittlement resistance" column in Table 3. Visually confirm whether there are cracks in the bent portion of the U-shaped bending test piece. If no cracks are confirmed, it is judged that excellent hydrogen embrittlement resistance can be obtained (indicated by "E (Excellent)" in the "Hydrogen embrittlement resistance" column in Table 3). On the other hand, if cracks are confirmed, it is judged that excellent hydrogen embrittlement resistance cannot be obtained (indicated by "B (Bad)" in the "Hydrogen embrittlement resistance" column in Table 3).

[0479] [Evaluation results]

[0480] Referring to Tables 1-1, 1-2, 2, and 3, Test Nos. 1 to 35 had appropriate chemical compositions and manufacturing conditions. Therefore, the steel plates of these test Nos. satisfied Characteristics 1 to 6. As a result, excellent hardenability, excellent bendability, and excellent hydrogen embrittlement resistance were achieved.

[0481] On the other hand, Steel No. 33 has an excessively low C content. Therefore, the maximum quenching hardness HD0 is as low as less than 600 HV, and sufficient strength required as a component cannot be obtained.

[0482] In test number 36, the C content was too high, so sufficient bendability and sufficient hydrogen embrittlement resistance could not be obtained.

[0483] In test number 37, the Si content was too low. Therefore, decarburization during cold-rolled sheet annealing was insufficient, and the C concentration ratio F1 exceeded 0.50. As a result, sufficient bendability and hydrogen embrittlement resistance could not be achieved.

[0484] In test number 38, the Si content was too high. Therefore, the strength of the steel sheet became too high due to solid solution strengthening, and sufficient bendability could not be obtained.

[0485] In test number 39, the Mn content was too low, so sufficient hardenability could not be obtained.

[0486] In test number 40, the Mn content was too high. Therefore, the strength of the steel sheet became too high due to solid solution strengthening, and sufficient bendability could not be obtained.

[0487] In test number 41, the Cr content was too high, so sufficient hardenability could not be obtained. Furthermore, sufficient bendability could not be obtained.

[0488] In test number 42, the Mo content was too high, so sufficient hardenability could not be obtained. Furthermore, sufficient bendability could not be obtained.

[0489] In Test Nos. 43 and 44, the chemical composition was appropriate, but the holding time t1 at the tempering temperature T1 during the cold-rolled sheet annealing step was too long. Consequently, the average ferrite grain size exceeded 20.0 μm, and the average cementite grain size exceeded 1.50 μm. Consequently, sufficient hardenability and bendability were not achieved.

[0490] In test numbers 45 to 47, the chemical compositions were appropriate, but the dew point during cold-rolled sheet annealing was lower than 25° C. Therefore, the C concentration ratio F1 exceeded 0.50. As a result, sufficient bendability and hydrogen embrittlement resistance could not be achieved.

[0491] In test numbers 48 to 50, the chemical composition was appropriate, but the cold rolling ratio CR was as low as 35% or less. Therefore, the spheroidization rate of cementite particles was too low, less than 85%, and as a result, sufficient bendability could not be obtained.

[0492] In Test No. 51, the chemical composition was appropriate, but the coiling temperature CT was too high. Therefore, the spheroidization rate of cementite particles was too low, less than 85%. As a result, sufficient hardenability and bendability could not be obtained.

[0493] In Test No. 52, the chemical composition was appropriate, but the annealing temperature T1 during the cold-rolled sheet annealing step was too low. Consequently, the spheroidization rate of the cementite particles was too low, less than 85%. Furthermore, the carbon concentration ratio F1 exceeded 0.50. Consequently, sufficient bendability and hydrogen embrittlement resistance could not be achieved.

[0494] The embodiments of the present disclosure have been described above. However, the above embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above embodiments and can be implemented by appropriately modifying the above embodiments without departing from the scope of the present disclosure.

Claims

1. A steel plate comprising, by mass%, C: 0.50 to 0.90%, Si: 0.10-0.50%, Mn: 0.20-1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr：0.01～1.20％、 N: 0.0150% or less, Mo: 0~0.500%, Ni: 0-1.000%, B：0～0.0100％、 V：0～0.500％、 Nb: 0~0.500%, Ti: 0~0.150%, Cu: 0-0.15%, W：0～0.15％、 Ta: 0~0.15%, Sn: 0~0.050%, Sb: 0~0.050%, Co: 0-0.050%, As: 0~0.050%, Mg: 0~0.050%, Y：0～0.050％、 Zr：0～0.050％、 La: 0~0.050%, Ce: 0~0.050%, Ca: 0 to 0.050%, and Balance: Fe and impurities, In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more, the average particle size of the ferrite is 20.0 μm or less, The average particle size of the cementite particles is less than 1.50 μm, When the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, a spheroidization rate, which is a ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 85% or more; When the C content at a position 50 μm deep from the surface of the steel plate in the plate thickness direction is defined as [C]s and the C content at the center position in the plate thickness direction of the steel plate is defined as [C]c, the C concentration ratio F1 defined by formula (1) is 0.50 or less, F1=[C]s / [C]c (1).

2. The steel plate according to claim 1, comprising Mo: 0.001~0.500%, Ni: 0.001~1.000%, and B：0.0001～0.0100％ One or more of the group consisting of.

3. The steel plate according to claim 1 or claim 2, comprising V：0.001～0.500％、 Nb: 0.001~0.500%, and Ti: 0.001~0.150% One or more of the group consisting of.

4. The steel plate according to any one of claims 1 to 3, comprising a Cu: 0.01-0.15%, W：0.01～0.15％、 Ta: 0.01~0.15%, Sn: 0.001~0.050%, Sb: 0.001~0.050%, Co: 0.001~0.050%, As: 0.001~0.050%, Mg: 0.001~0.050%, Y: 0.001 ~0.050%, Zr: 0.001~0.050%, La: 0.001~0.050%, Ce: 0.001~0.050%, and Ca: 0.001~0.050% One or more of the group consisting of.

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