Steel materials and mechanical structural parts

Abstract

The present invention provides a steel material having excellent cold forgeability. The steel material of the present invention has a chemical composition described in the specification, defines the number density (pieces / mm 2 ) of Mn sulfides as ND0, defines the number density (pieces / mm 2 ) of Cu-Ni-containing Mn sulfides as ND1, and defines the number density (pieces / mm 2 ) of Ti-containing Mn sulfides as ND2, and when ND1 (pieces / mm 2 ) is 1.00 or more, satisfies formula (1) and formula (2), ND1+ND2≥2.00 (1), (ND1+ND2) / ND0<0.25 (2).

Description

Technical Field

[0001] This invention relates to steel and mechanical structural components made from the same steel. Background Technology

[0002] For mechanical structural components used in industrial machinery, automobiles, and other applications, high strength is required. Therefore, steel is generally used as the raw material for these components. In the manufacturing process of mechanical structural components, especially bolts, cold forging is mostly used instead of hot forging in the steel-based manufacturing process. This is to improve yield and reduce manufacturing costs.

[0003] The manufacturing process of cold-forged bolts is as follows: Steel as raw material is drawn. The drawn steel is then subjected to heat treatment (e.g., annealing) to soften the steel. The heat-treated steel is then cold-forged to produce an intermediate part in the shape of a bolt, having a head and a shaft. The intermediate part is then quenched and tempered to manufacture the bolt.

[0004] In recent years, from the perspectives of energy conservation and further cost reduction, research has been conducted on omitting the heat treatment process before cold forging. To omit the heat treatment before cold forging, the steel used as the raw material for bolts requires excellent cold forging properties. Furthermore, even in cases where the final product has a complex shape, being able to perform cold forging instead of hot forging can reduce manufacturing costs. In this case, excellent cold forging properties are also required for the steel used as the raw material for bolts.

[0005] Japanese Patent Application Publication No. 2006-274373 (Patent Document 1) and International Publication No. 2020 / 090149 (Patent Document 2) disclose techniques for improving the cold forging properties of steel used as raw material for bolts.

[0006] The bolt steel disclosed in Patent Document 1 has the following composition by mass percentage: C: 0.07~0.15%, Si: 0.2% or less, Mn: 0.5~2%, P: 0.015% or less, S: 0.015% or less, Cr: 2% or less, Al: 0.005~0.08%, N: 0.01% or less, and the carbon equivalent (Ceq = C + Si / 7 + Mn / 6 + Cr / 9) is 0.50% or less, with the remainder consisting of iron and unavoidable impurities. In this steel, by limiting the carbon equivalent, the precipitation of cementite at grain boundaries can be suppressed, thus mitigating the embrittlement of the steel. Furthermore, Patent Document 1 describes how excellent cold forging properties can be obtained by including the above alloying elements within appropriate ranges.

[0007] The bolt steel disclosed in Patent Document 2 contains, by mass%, 0.18-0.24% C, 0.10-0.22% Si, 0.60-1.00% Mn, 0.010-0.050% Al, 0.65-0.95% Cr, 0.010-0.050% Ti, 0.0015-0.0050% B, 0.0050-0.0100% N, and less than 0.025% P. The composition of the bolt steel disclosed in Patent Document 2 is as follows: S: 0.025% or less (including 0), Cu: 0.20% or less (including 0), and Ni: 0.30% or less (including 0), satisfying the range of 0.45≤C+Si / 24+Mn / 6+Ni / 40+Cr / 5≤0.60 and N≤0.519Al+0.292Ti, with the remainder being Fe and unavoidable impurities, and a microstructure of bainite comprising 95% or more in area ratio. The old austenite grains in this microstructure have a grain size number of 6 or more, and a strength deviation of less than 100 MPa. In the aforementioned steel, the area ratio of the bainite structure is increased, and the old austenite grains are made finer. Therefore, Patent Document 2 describes the ability to obtain a greater Bauschinger effect, which reduces the deformation resistance during cold forging of the bolt head.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2006-274373

[0011] Patent Document 2: International Publication No. 2020 / 090149 Summary of the Invention

[0012] The technical problem that the invention aims to solve

[0013] However, cold forging properties can also be improved by means different from those disclosed in Patent Documents 1 and 2.

[0014] The purpose of this disclosure is to provide steel with excellent cold forging properties, and mechanical structural components that achieve excellent cold forging properties during the manufacturing process.

[0015] Technical solutions for solving technical problems

[0016] The steel of this invention contains, by mass%,

[0017] C: 0.04% to less than 0.20%

[0018] Si: 0.01~0.35%

[0019] Mn: 0.20~1.00%

[0020] Al: 0.001~0.100%

[0021] Ti: 0.001~0.100%

[0022] Cu: 0.01~0.40%,

[0023] Ni: 0.01~0.30%

[0024] Cr: 0.01~0.30%

[0025] Mo: 0.001~0.200%

[0026] Sn: 0.001~0.100%

[0027] P: below 0.040%

[0028] S: Below 0.040%

[0029] N: below 0.0150%

[0030] O: Below 0.0030%

[0031] B: 0~0.0010%

[0032] Nb: 0~0.050%,

[0033] V: 0~0.15%,

[0034] Sb: 0~0.050%,

[0035] As: 0~0.050%,

[0036] Pb: 0~0.090%,

[0037] Ca: 0~0.0050%, and

[0038] Mg: 0~0.0050%,

[0039] The remaining portion consists of Fe and impurities.

[0040] The number density (numbers / mm) of Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass, and a Mn content of 10% or more. 2 Defined as ND0,

[0041] The number density (numbers / mm²) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of 5% or more, and a Mn content of 10% or more.2 Defined as ND1,

[0042] The number density (numbers / mm²) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more. 2 When defined as ND2,

[0043] ND1 (pieces / mm) 2 The value is above 1.00.

[0044] Satisfying equations (1) and (2).

[0045] ND1+ND2≥2.00 (1)

[0046] (ND1+ND2) / ND0<0.25 (2).

[0047] The mechanical structure component disclosed herein is characterized in that...

[0048] By mass%, it contains

[0049] C: 0.04% to less than 0.20%

[0050] Si: 0.01~0.35%

[0051] Mn: 0.20~1.00%

[0052] Al: 0.001~0.100%

[0053] Ti: 0.001~0.100%

[0054] Cu: 0.01~0.40%,

[0055] Ni: 0.01~0.30%

[0056] Cr: 0.01~0.30%

[0057] Mo: 0.001~0.200%

[0058] Sn: 0.001~0.100%

[0059] P: below 0.040%

[0060] S: Below 0.040%

[0061] N: below 0.0150%

[0062] O: Below 0.0030%

[0063] B: 0~0.0010%

[0064] Nb: 0~0.050%,

[0065] V: 0~0.15%,

[0066] Sb: 0~0.050%,

[0067] As: 0~0.050%,

[0068] Pb: 0~0.090%,

[0069] Ca: 0~0.0050%, and

[0070] Mg: 0~0.0050%,

[0071] The remaining portion consists of Fe and impurities.

[0072] The number density (numbers / mm) of Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass, and a Mn content of 10% or more. 2 Defined as ND0,

[0073] The number density (numbers / mm²) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of 5% or more, and a Mn content of 10% or more. 2 Defined as ND1,

[0074] The number density (numbers / mm²) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more. 2 When defined as ND2,

[0075] ND1 (pieces / mm) 2 The value is above 1.00.

[0076] Satisfying equations (1) and (2).

[0077] ND1+ND2≥2.00 (1)

[0078] (ND1+ND2) / ND0<0.25 (2).

[0079] Invention Effects

[0080] Excellent cold forging properties can be obtained in the steel of this invention. The mechanical structural components disclosed herein exhibit excellent cold forging properties during the manufacturing process. Detailed Implementation

[0081] The inventors first conducted research on steels capable of achieving excellent cold forging properties from the viewpoint of chemical composition. As a result, the inventors concluded that, by mass%, the steel should contain: C: 0.04% to less than 0.20%, Si: 0.01% to 0.35%, Mn: 0.20% to 1.00%, Al: 0.001% to 0.100%, Ti: 0.001% to 0.100%, Cu: 0.01% to 0.40%, Ni: 0.01% to 0.30%, Cr: 0.01% to 0.30%, Mo: 0.001% to 0.200%, Sn: 0.001% to 0.100%. A chemical composition consisting of P: less than 0.040%, S: less than 0.040%, N: less than 0.0150%, O: less than 0.0030%, B: 0~0.0010%, Nb: 0~0.050%, V: 0~0.15%, Sb: 0~0.050%, As: 0~0.050%, Pb: 0~0.090%, Ca: 0~0.0050%, Mg: 0~0.0050%, and the remainder being Fe and impurities, can yield excellent cold forging properties.

[0082] However, even steels meeting the above chemical composition sometimes exhibit low cold forgeability. Therefore, the inventors, from a microstructural perspective, investigated methods to improve cold forgeability. As a result, the inventors obtained the following insights.

[0083] (A) In the manufacturing process of bolts made from steel, a quenching process is performed to improve the strength of the bolts. Cu and Ni are dispersed in the matrix phase of the steel, improving the hardenability of the steel. Therefore, the strength of bolts manufactured from steel is further improved. However, even with small amounts of Cu and Ni, the strength of the steel increases after the hot working process. As a result, the cold forging properties of the steel decrease.

[0084] On the other hand, when Mn sulfides are present near Cu and Ni, Cu and Ni are enriched in the Mn sulfides. Therefore, during the hot working process of steel, the localized Cu and Ni concentrations around the Mn sulfides decrease, reducing hardenability. As a result, the ferrite volume fraction of the steel increases, improving its cold forging properties.

[0085] Here, Mn sulfides are defined as inclusions with a S content of 10% or more and a Mn content of 10% or more in elemental concentration analysis using EDX as described later.

[0086] (B) Cu and Ni in steel that are not enriched in Mn sulfides during the hot working process diffuse again during heating in the quenching process when manufacturing bolts. If the number density of Cu and Ni enriched Mn sulfides (hereinafter referred to as "Cu-Ni-containing Mn sulfides") is already high before the quenching process, it is difficult for Cu and Ni to enrich into Mn sulfides during the quenching process. Therefore, the local reduction in hardenability around Mn sulfides can be suppressed. As a result, bolts made from steel can ensure sufficient strength. That is, in order to maintain the strength of bolts made from steel while improving the cold forgeability of steel, it is effective to increase the number density of Cu-Ni-containing Mn sulfides in the steel.

[0087] Here, Cu-Ni-containing Mn sulfides are defined as inclusions that, in elemental concentration analysis using EDX (mass %) as described later, have an S content of 10% or more, a combined Cu and Ni content of 5% or more, and a Mn content of 10% or more.

[0088] (C) Mn sulfides in steel are generally coarse and elongate in the rolling direction. Such Mn sulfides become the initiation point of cracks during cold forging. Among Mn sulfides, the Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides mentioned above are finer and less prone to elongation in the rolling direction of the steel compared to other Mn sulfides (hereinafter referred to as "ordinary Mn sulfides"). Therefore, Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides are less likely to become the initiation point of cracks during cold forging. That is, by increasing the number density of Cu-Ni-containing Mn sulfides and the number density of Ti-containing Mn sulfides, the cold forgeability of the steel is improved.

[0089] Here, Ti-containing Mn sulfides are defined as inclusions that, in elemental concentration analysis using EDX (mass %) as described later, have an S content of 10% or more, a combined Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more.

[0090] (D) In ​​the steel with the above chemical composition, Mn sulfides can mainly consist of Cu-Ni-containing Mn sulfides, Ti-containing Mn sulfides, and ordinary Mn sulfides. During the Mn sulfide formation process, if the number density of Cu-Ni-containing Mn sulfides and the number density of Ti-containing Mn sulfides are high, the amount of sulfur typically used for Mn sulfide growth is reduced. In this case, the higher the number density of ordinary Mn sulfides, the more effectively the coarsening of individual ordinary Mn sulfides is suppressed. Therefore, the ordinary Mn sulfides also become finer. As a result, ordinary Mn sulfides are less likely to become the initiation point for cracks during cold forging, further improving the cold forgeability of the steel.

[0091] Based on the above insights, the inventors investigated and studied the relationship between the number density of Mn sulfides, the number density of Cu-Ni-containing Mn sulfides, the number density of Ti-containing Mn sulfides, and cold forging properties. The results showed that the number density (numbers / mm²) of Mn sulfides with an equivalent circle diameter of 1.0 μm or more is defined as ND0, and the number density (numbers / mm²) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more is defined as... 2 ) is defined as ND1, which is the number density (numbers / mm) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more. 2 When ND1 is defined as ND2, if ND1 (pieces / mm) 2 If the value is 1.00 or higher, satisfying equations (1) and (2), then steel with excellent cold forging properties can be obtained.

[0092] ND1+ND2≥2.00 (1)

[0093] (ND1+ND2) / ND0<0.25 (2).

[0094] The steel and mechanical structure components of this embodiment are based on the above technical concept and have the following structure.

[0095] The steel used in the first structure, by mass percentage, contains [amount missing].

[0096] C: 0.04% to less than 0.20%

[0097] Si: 0.01~0.35%

[0098] Mn: 0.20~1.00%

[0099] Al: 0.001~0.100%

[0100] Ti: 0.001~0.100%

[0101] Cu: 0.01~0.40%,

[0102] Ni: 0.01~0.30%

[0103] Cr: 0.01~0.30%

[0104] Mo: 0.001~0.200%

[0105] Sn: 0.001~0.100%

[0106] P: below 0.040%

[0107] S: Below 0.040%

[0108] N: below 0.0150%

[0109] O: Below 0.0030%

[0110] B: 0~0.0010%

[0111] Nb: 0~0.050%,

[0112] V: 0~0.15%,

[0113] Sb: 0~0.050%,

[0114] As: 0~0.050%,

[0115] Pb: 0~0.090%,

[0116] Ca: 0~0.0050%, and

[0117] Mg: 0~0.0050%,

[0118] The remaining portion consists of Fe and impurities.

[0119] The number density (numbers / mm) of Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass, and a Mn content of 10% or more. 2 ND0 is defined as the number density (numbers / mm²) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more by mass%, a combined Cu and Ni content of 5% or more, and a Mn content of 10% or more. 2 ND1 is defined as the number density (numbers / mm²) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more. 2 When ND1 is defined as ND2, ND1 (pieces / mm) 2 If the value is 1.00 or higher, then equations (1) and (2) are satisfied.

[0120] ND1+ND2≥2.00 (1)

[0121] (ND1+ND2) / ND0<0.25 (2).

[0122] The steel for the second structure, based on the steel for the first structure, contains, by mass percentage, selected free...

[0123] B: 0.0001~0.0010%

[0124] Nb: 0.001~0.050%,

[0125] V: 0.01~0.15%,

[0126] Sb: 0.001~0.050%

[0127] As: 0.001~0.050%

[0128] Pb: 0.001~0.090%,

[0129] Ca: 0.0001~0.0050%, and

[0130] Mg: 0.0001~0.0050% of one or more of the group.

[0131] The mechanical structural components of the first structure, by mass%, contain,

[0132] C: 0.04% to less than 0.20%

[0133] Si: 0.01~0.35%

[0134] Mn: 0.20~1.00%

[0135] Al: 0.001~0.100%

[0136] Ti: 0.001~0.100%

[0137] Cu: 0.01~0.40%,

[0138] Ni: 0.01~0.30%

[0139] Cr: 0.01~0.30%

[0140] Mo: 0.001~0.200%

[0141] Sn: 0.001~0.100%

[0142] P: below 0.040%

[0143] S: Below 0.040%

[0144] N: below 0.0150%

[0145] O: Below 0.0030%

[0146] B: 0~0.0010%

[0147] Nb: 0~0.050%,

[0148] V: 0~0.15%,

[0149] Sb: 0~0.050%,

[0150] As: 0~0.050%,

[0151] Pb: 0~0.090%,

[0152] Ca: 0~0.0050%, and

[0153] Mg: 0~0.0050%,

[0154] The remaining portion consists of Fe and impurities. The number density (numbers / mm) of Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass, and a Mn content of 10% or more is given. 2 ND0 is defined as the number density (numbers / mm²) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more by mass%, a combined Cu and Ni content of 5% or more, and a Mn content of 10% or more. 2 ND1 is defined as the number density (numbers / mm²) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more. 2 When ND1 is defined as ND2, ND1 (pieces / mm) 2 If the value is 1.00 or higher, then equations (1) and (2) are satisfied.

[0155] ND1+ND2≥2.00 (1)

[0156] (ND1+ND2) / ND0<0.25 (2).

[0157] The mechanical structural components of the second structure, based on the mechanical structural components of the first structure, contain, by mass%, selected free...

[0158] B: 0.0001~0.0010%

[0159] Nb: 0.001~0.050%,

[0160] V: 0.01~0.15%,

[0161] Sb: 0.001~0.050%

[0162] As: 0.001~0.050%

[0163] Pb: 0.001~0.090%,

[0164] Ca: 0.0001~0.0050%, and

[0165] Mg: 0.0001~0.0050% of one or more of the group.

[0166] The mechanical structural component of the third structure is a bolt, which is based on the mechanical structural component of the first or second structure.

[0167] The steel and mechanical structure components of this embodiment will be described below. In the following description, unless otherwise specified, the percentage of element content refers to mass%.

[0168] [Characteristics of the steel used in this embodiment]

[0169] The steel used in this embodiment meets the following characteristics.

[0170] (Feature 1)

[0171] The chemical composition, by mass%, contains: C: 0.04% to less than 0.20%, Si: 0.01% to 0.35%, Mn: 0.20% to 1.00%, Al: 0.001% to 0.100%, Ti: 0.001% to 0.100%, Cu: 0.01% to 0.40%, Ni: 0.01% to 0.30%, Cr: 0.01% to 0.30%, Mo: 0.001% to 0.200%, Sn: 0.001% to 0.100%. The percentages are as follows: P: less than 0.040%, S: less than 0.040%, N: less than 0.0150%, O: less than 0.0030%, B: 0~0.0010%, Nb: 0~0.050%, V: 0~0.15%, Sb: 0~0.050%, As: 0~0.050%, Pb: 0~0.090%, Ca: 0~0.0050%, and Mg: 0~0.0050%, with the remainder consisting of Fe and impurities.

[0172] (Feature 2)

[0173] The number density ND1 of Cu-Ni-containing Mn sulfides is 1.00 per mm. 2 above.

[0174] (Feature 3)

[0175] Mn sulfide number density ND0 (numbers / mm) 2 The number density of Cu-Ni-containing Mn sulfides, ND1 (numbers / mm²), 2 ) and the number density of Ti-containing Mn sulfides ND2 (numbers / mm 2 ) satisfies equations (1) and (2).

[0176] ND1+ND2≥2.00 (1)

[0177] (ND1+ND2) / ND0<0.25 (2)

[0178] The following describes features 1 through 3.

[0179] [(Feature 1) Chemical Composition]

[0180] The steel in this embodiment contains the following elements in its chemical composition.

[0181] C: 0.04% to less than 0.20%

[0182] Carbon (C) improves the hardenability of steel and increases the strength of bolts made from steel. If the C content is less than 0.04%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0183] On the other hand, if the carbon content is 0.20% or higher, the strength of the steel is excessively increased even if the contents of other elements are within the range of this embodiment. As a result, the cold forging properties of the steel are reduced.

[0184] Therefore, the C content is 0.04% to less than 0.20%.

[0185] The preferred lower limit for C content is 0.06%, and more preferably 0.08%.

[0186] The preferred upper limit for the C content is 0.19%, further preferably 0.17%, and even more preferably 0.15%.

[0187] Si: 0.01~0.35%

[0188] Silicon (Si) strengthens steel through solid solution treatment. Si further improves the hardenability of the steel. As a result, the strength of bolts manufactured from steel is increased. If the Si content is less than 0.01%, the above-mentioned effects cannot be fully achieved even if the contents of other elements are within the range of this embodiment.

[0189] On the other hand, if the Si content exceeds 0.35%, the strength of the steel will be excessively increased even if the contents of other elements are within the range of this embodiment. Therefore, the cold forging properties of the steel are reduced.

[0190] Therefore, the Si content is 0.01~0.35%.

[0191] The preferred lower limit for Si content is 0.02%, and more preferably 0.03%.

[0192] The preferred upper limit for the Si content is 0.32%, more preferably 0.30%, and even more preferably 0.25%.

[0193] Mn: 0.20~1.00%

[0194] Manganese (Mn) strengthens steel through solid solution treatment. Mn further improves the hardenability of steel. As a result, the strength of bolts manufactured from steel is increased. 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.

[0195] On the other hand, if the Mn content exceeds 1.00%, coarse Mn sulfides are excessively formed. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0196] Therefore, the Mn content is 0.20~1.00%.

[0197] The preferred lower limit for Mn content is 0.22%, further preferably 0.25%, and even more preferably 0.30%.

[0198] The preferred upper limit for Mn content is 0.95%, and more preferably 0.90%.

[0199] Al: 0.001~0.100%

[0200] Aluminum (Al) is used to deoxidize steel. If the Al content is less than 0.001%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0201] On the other hand, if the Al content exceeds 0.100%, coarse Al-based inclusions will form even if the contents of other elements are within the range of this embodiment. These coarse Al-based inclusions become the initiation point for cracks during cold forging. Therefore, the cold forgeability of the steel is reduced.

[0202] Therefore, the Al content is 0.001~0.100%.

[0203] The preferred lower limit for Al content is 0.005%, and more preferably 0.010%.

[0204] The preferred upper limit for Al content is 0.080%, and more preferably 0.070%.

[0205] In this embodiment, Al content refers to the total Al content.

[0206] Ti: 0.001~0.100%

[0207] Titanium (Ti) forms Ti-containing Mn sulfides. As described above, by forming Ti-containing Mn sulfides, the coarsening of Mn sulfides can be suppressed. Furthermore, Ti-containing Mn sulfides are less prone to elongation during processing compared to ordinary Mn sulfides. Therefore, in cold-forged steel, the occurrence of cracks originating from elongated Mn sulfides can be suppressed. As a result, the cold forgeability of the steel is improved. If the Ti content is less than 0.001%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0208] On the other hand, if the Ti content exceeds 0.100%, coarse Ti carbides, Ti nitrides, and Ti carbonitrides will form even if the contents of other elements are within the range of this embodiment. These Ti inclusions or Ti precipitates become the initiation point for cracks during cold forging. As a result, the cold forgeability of the steel is reduced.

[0209] Therefore, the Ti content is 0.001~0.100%.

[0210] The preferred lower limit for the Ti content is 0.003%, more preferably 0.010%, and even more preferably 0.015%.

[0211] The preferred upper limit for Ti content is 0.080%, and more preferably 0.070%.

[0212] Cu: 0.01~0.40%

[0213] Copper (Cu) strengthens steel through solid solution treatment. Cu further improves the hardenability of steel. As a result, the strength of bolts manufactured from steel is increased. If the Cu 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.

[0214] On the other hand, if the Cu content exceeds 0.40%, the strength of the steel will become excessively high even if the contents of other elements are within the range of this embodiment. Therefore, the cold forging properties of the steel are reduced.

[0215] Therefore, the Cu content is 0.01~0.40%.

[0216] The preferred lower limit for Cu content is 0.02%, and more preferably 0.04%.

[0217] The preferred upper limit for the Cu content is 0.38%, more preferably 0.34%, and even more preferably 0.30%.

[0218] Ni: 0.01~0.30%

[0219] Nickel (Ni) strengthens steel through solid solution treatment. Ni further improves the hardenability of steel. As a result, the strength of bolts manufactured from steel is increased. If the Ni 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.

[0220] On the other hand, if the Ni content exceeds 0.30%, the strength of the steel will become excessively high even if the contents of other elements are within the range of this embodiment. Therefore, the cold forging properties of the steel are reduced.

[0221] Therefore, the Ni content is 0.01~0.30%.

[0222] The preferred lower limit for Ni content is 0.02%, and more preferably 0.04%.

[0223] The preferred upper limit for Ni content is 0.29%, further preferably 0.27%, even more preferably 0.24%, and even more preferably 0.20%.

[0224] Cr: 0.01~0.30%

[0225] Chromium (Cr) strengthens steel through solid solution treatment. Cr further improves the hardenability of steel. As a result, the strength of bolts manufactured from steel is increased. 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.

[0226] On the other hand, if the Cr content exceeds 0.30%, the strength of the steel will become excessively high even if the contents of other elements are within the range of this embodiment. Therefore, the cold forging properties of the steel are reduced.

[0227] Therefore, the Cr content is 0.01~0.30%.

[0228] The preferred lower limit for Cr content is 0.02%, and more preferably 0.04%.

[0229] The preferred upper limit for the Cr content is 0.27%, more preferably 0.25%, even more preferably 0.20%, and even more preferably 0.15%.

[0230] Mo: 0.001~0.200%

[0231] Molybdenum (Mo) improves the hardenability of steel and increases the strength of bolts made from steel. If the Mo content is less than 0.001%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0232] On the other hand, if the Mo content exceeds 0.200%, the strength of the steel will become excessively high even if the contents of other elements are within the range of this embodiment. Therefore, the cold forging properties of the steel are reduced.

[0233] Therefore, the Mo content is 0.001~0.200%.

[0234] The preferred lower limit for the Mo content is 0.005%, more preferably 0.008%, and even more preferably 0.010%.

[0235] The preferred upper limit for the Mo content is 0.180%, more preferably 0.150%, and even more preferably 0.100%.

[0236] Sn: 0.001~0.100%

[0237] Tin (Sn) segregates at the interface between the parent phase and Mn sulfides, causing the steel to become embrittled. This, in turn, improves the machinability of the steel. However, if the Sn content is less than 0.001%, even with the contents of other elements within the range specified in this embodiment, the aforementioned effects cannot be fully achieved.

[0238] However, if the Sn content exceeds 0.100%, Sn will excessively segregate. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0239] Therefore, the Sn content is 0.001~0.100%.

[0240] The preferred lower limit for Sn content is 0.002%, and more preferably 0.004%.

[0241] The preferred upper limit for the Sn content is 0.092%, further preferably 0.090%, even more preferably 0.080%, and even more preferably 0.070%.

[0242] P: below 0.040%

[0243] Phosphorus (P) is an impurity. If the P content exceeds 0.040%, P excessively segregates at the grain boundaries, reducing grain boundary strength. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0244] Therefore, the P content is below 0.040%.

[0245] The phosphorus (P) content is preferably as low as possible. However, if the P content is reduced excessively, the manufacturing cost increases. Therefore, considering typical industrial production, the preferred lower limit for the P content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%.

[0246] The preferred upper limit for the P content is 0.035%, further preferably 0.030%, and even more preferably 0.025%.

[0247] S: below 0.040%

[0248] Sulfur (S) is an impurity. S combines with Mn to form Mn sulfides. If the S content exceeds 0.040%, coarse Mn sulfides are formed in excess. These coarse Mn sulfides become the initiation point for cracks during cold forging. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0249] Therefore, the sulfur content is below 0.040%.

[0250] The sulfur (S) content is preferably as low as possible. However, if the S content is reduced excessively, the manufacturing cost increases. Therefore, considering typical industrial production, the preferred lower limit for the S content is more than 0%, more preferably 0.001%, and even more preferably 0.002%.

[0251] The preferred upper limit for the sulfur content is 0.035%, more preferably 0.030%, and even more preferably 0.025%.

[0252] N: below 0.0150%

[0253] Nitrogen (N) is an impurity. N combines with elements such as Al, Ti, and B to form nitrides. If the N content exceeds 0.0150%, coarse nitrides are formed in excess. These coarse nitrides become the initiation point for cracks during cold forging. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0254] Therefore, the N content is below 0.0150%.

[0255] The nitrogen (N) content is preferably as low as possible. However, if the N content is reduced excessively, the manufacturing cost increases. Therefore, considering typical industrial production, the preferred lower limit for the N content is more than 0%, more preferably 0.0001%, even more preferably 0.0010%, and even more preferably 0.0020%.

[0256] The preferred upper limit for the nitrogen content is 0.0130%, further preferably 0.0100%, and even more preferably 0.0080%.

[0257] O: Below 0.0030%

[0258] Oxygen (O) is an impurity. O combines with other elements in the steel to form oxides. If the O content exceeds 0.0030%, coarse oxides are excessively formed. These coarse oxides become the initiation point for cracks during cold forging. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0259] Therefore, the O content is below 0.0030%.

[0260] The oxygen content is preferably as low as possible. However, if the oxygen content is reduced excessively, the manufacturing cost will increase. Therefore, considering typical industrial production, the preferred lower limit for the oxygen content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%.

[0261] The preferred upper limit for the O content is 0.0027%, further preferably 0.0024%, and even more preferably 0.0020%.

[0262] The remaining portion of the chemical composition of the steel in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to impurities that are introduced during the industrial manufacturing of steel from raw materials such as ore, waste, or the manufacturing environment, and are permitted within a range that does not adversely affect the steel of this embodiment.

[0263] [Optional Elements]

[0264] In the chemical composition of the steel of this embodiment, a portion of the Fe is further replaced by a selected...

[0265] B: 0~0.0010%

[0266] Nb: 0~0.050%,

[0267] V: 0~0.15%,

[0268] Sb: 0~0.050%,

[0269] As: 0~0.050%,

[0270] Pb: 0~0.090%,

[0271] Ca: 0~0.0050%, and

[0272] Mg: 0 to 0.0050% of one or more of the group.

[0273] These elements are all arbitrary. The following is a description of these arbitrary elements.

[0274] [Group 1: B, Nb, and V]

[0275] The chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of B, Nb, and V to replace a portion of the Fe. These elements all improve the strength of bolts manufactured from steel.

[0276] B: 0~0.0010%

[0277] Boron (B) can be any element, or it can be absent. That is, the B content can be 0%.

[0278] When present, i.e., when the boron content is above 0%, boron (B) improves the hardenability of steel. As a result, the strength of bolts made from steel is increased. This effect can be achieved to some extent even with a small amount of boron.

[0279] However, if the boron content exceeds 0.0010%, coarse boron nitrides are formed. These coarse boron nitrides become the initiation point for cracks during cold forging. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0280] Therefore, the B content is 0~0.0010%, and when it is present, the B content is below 0.0010%.

[0281] The preferred lower limit for the B content is 0.0001%, further preferably 0.0002%, and even more preferably 0.0003%.

[0282] The preferred upper limit for the B content is 0.0009%, further preferably 0.0008%, and even more preferably 0.0006%.

[0283] Nb: 0~0.050%

[0284] Niobium (Nb) can be any element, or it can be absent. That is, the Nb content can be 0%.

[0285] When Nb is present, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as carbides, nitrides, and carbonitrides. These Nb precipitates strengthen the bolts made from steel through precipitation. Even a slight presence of Nb can achieve these effects to some extent.

[0286] However, if the Nb content exceeds 0.050%, coarse Nb precipitates are formed. These coarse Nb precipitates become the initiation point for cracks during cold forging. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0287] Therefore, the Nb content is 0~0.050%, and in the case where it is present, the Nb content is below 0.050%.

[0288] The preferred lower limit for Nb content is 0.001%, more preferably 0.002%, and even more preferably 0.005%.

[0289] The preferred upper limit for Nb content is 0.045%, further preferably 0.040%, and even more preferably 0.035%.

[0290] V: 0~0.15%

[0291] Vanadium (V) can be any element, or it can be absent. That is, the V content can be 0%.

[0292] When V is present, meaning the V content exceeds 0%, V precipitates such as carbides and carbonitrides. These V precipitates strengthen bolts made from steel through precipitation reinforcement. Even a small amount of V can achieve these effects to some extent.

[0293] However, if the V content exceeds 0.15%, coarse V precipitates are formed. These coarse V precipitates become the initiation point for cracks during cold forging. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0294] Therefore, the V content is 0~0.15%, and in the case of presence, the V content is below 0.15%.

[0295] The preferred lower limit for the V content is 0.01%, more preferably 0.02%, and even more preferably 0.04%.

[0296] The preferred upper limit for the V content is 0.13%, more preferably 0.10%, and even more preferably 0.08%.

[0297] [Group 2: Sb, As, and Pb]

[0298] The chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of Sb, As, and Pb to replace a portion of the Fe. These elements all improve the machinability of the steel.

[0299] Sb: 0~0.050%

[0300] Antimony (Sb) can be any element, or it can be absent. That is, the Sb content can be 0%.

[0301] When Sb is present, i.e., the Sb content is higher than 0%, Sb segregates at the interface between the parent phase and Mn sulfides, causing the steel to become embrittled. Therefore, the machinability of the steel is improved. Even a slight presence of Sb can achieve the above effects to some extent.

[0302] However, if the Sb content exceeds 0.050%, excessive Sb segregation occurs. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0303] Therefore, the Sb content is 0~0.050%, and in the case where it is present, the Sb content is below 0.050%.

[0304] The preferred lower limit for the Sb content is 0.001%, more preferably 0.002%, and even more preferably 0.004%.

[0305] The preferred upper limit for the Sb content is 0.045%, further preferably 0.040%, and even more preferably 0.035%.

[0306] As: 0~0.050%

[0307] Arsenic (As) can be any element, or it can be absent. That is, the As content can be 0%.

[0308] When As is present, i.e., the As content is higher than 0%, As segregates at the interface between the parent phase and Mn sulfides, causing the steel to become embrittled. Therefore, the machinability of the steel is improved. Even a slight presence of As can achieve the above effects to some extent.

[0309] However, if the As content exceeds 0.050%, As will excessively segregate. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0310] Therefore, the As content is 0~0.050%, and in the case where it is present, the As content is below 0.050%.

[0311] The preferred lower limit for the As content is 0.001%, further preferably 0.002%, and even more preferably 0.004%.

[0312] The preferred upper limit for the As content is 0.045%, further preferably 0.040%, and even more preferably 0.035%.

[0313] Pb: 0~0.090%

[0314] Lead (Pb) can be any element, or it can be absent. That is, the Pb content can be 0%.

[0315] When Pb is present, i.e., the Pb content is higher than 0%, Pb segregates at the interface between the parent phase and Mn sulfides, causing the steel to become embrittled. Therefore, the machinability of the steel is improved. Even a slight presence of Pb can achieve the above effects to some extent.

[0316] However, if the Pb content exceeds 0.090%, Pb will excessively segregate. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0317] Therefore, the Pb content is 0~0.090%, and when it is present, the Pb content is below 0.090%.

[0318] The preferred lower limit for Pb content is 0.001%, more preferably 0.002%, and even more preferably 0.004%.

[0319] The preferred upper limit for Pb content is 0.080%, further preferably 0.070%, and even more preferably 0.060%.

[0320] [Group 3: Ca and Mg]

[0321] The chemical composition of the steel in this embodiment may also replace a portion of the Fe and contain one or more elements selected from the group consisting of Ca and Mg. These elements all refine the Mn sulfides in the steel, improving the cold forging properties of the steel.

[0322] Ca: 0~0.0050%

[0323] Calcium (Ca) is an arbitrary element and can be absent altogether. That is, the Ca content can be 0%. When Ca is present, i.e., when Ca exceeds 0%, Ca refines Mn sulfides. Therefore, the cold forging properties of steel are improved. Even a slight presence of Ca can achieve the above effects to some extent.

[0324] However, if the Ca content exceeds 0.0050%, coarse Ca oxides are formed. These coarse Ca oxides become the initiation point for cracks during cold forging. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0325] Therefore, the Ca content is 0~0.0050%, and when it is present, the Ca content is below 0.0050%.

[0326] The preferred lower limit for Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%.

[0327] The preferred upper limit for Ca content is 0.0040%, and more preferably 0.0030%.

[0328] Mg: 0~0.0050%

[0329] Magnesium (Mg) is an optional element, and it can be absent altogether. That is, the Mg content can be 0%. When Mg is present, i.e., when Mg exceeds 0%, it refines Mn sulfides. Therefore, the cold forging properties of the steel are improved. Even a slight presence of Mg can achieve the above effects to some extent.

[0330] However, if the Mg content exceeds 0.0050%, coarse Mg oxides are formed. These coarse Mg oxides become the initiation point for cracks during cold forging. Therefore, even if the contents of other elements are within the range of this embodiment, the cold forgeability of the steel is reduced.

[0331] Therefore, the Mg content is 0~0.0050%, and when it is present, the Mg content is less than 0.0050%.

[0332] The preferred lower limit for Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%.

[0333] The preferred upper limit for Mg content is 0.0040%, and more preferably 0.0030%.

[0334] [(Feature 2) Number density of Cu-Ni-containing Mn sulfides ND1]

[0335] As described above, in the elemental concentration analysis using EDX described later, inclusions (particles) with an S content of 10% or more, a combined Cu and Ni content of 5% or more, and a Mn content of 10% or more are defined as Cu-Ni-containing Mn sulfides.

[0336] In the steel of this embodiment, the number density ND1 (numbers / mm) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more is... 2 The value is above 1.00.

[0337] When Mn sulfides are typically present near Cu and Ni, Cu and Ni usually accumulate in the Mn sulfides. As a result, Cu-Ni-containing Mn sulfides are formed. As described above, in order to improve the cold forgeability of steel while maintaining the strength of bolts made from steel, a number density ND1 (numbers / mm²) of Cu-Ni-containing Mn sulfides is preferred. 2 High. If the number density of Cu-Ni-containing Mn sulfides ND1 (numbers / mm²) is high. 2 If the value is 1.00 or higher, then the steel can achieve excellent cold forging properties, provided that it meets characteristics 1 and 3.

[0338] The preferred lower limit for the number density ND1 is 1.10, more preferably 1.30, and even more preferably 1.50.

[0339] There is no specific upper limit for the number density ND1. When steel satisfies features 1 and 3, the upper limit for the number density ND1 is, for example, 5.00, 4.00, or 3.00.

[0340] [(Feature 3) Equation (1) and Equation (2)]

[0341] As described above, in the elemental concentration analysis using EDX described later, inclusions (particles) with an S content of 10% or more and an Mn content of 10% or more are defined as Mn sulfides.

[0342] In the elemental concentration analysis using EDX described later, which is expressed as a percentage by mass, inclusions (particles) with an S content of 10% or more, a combined Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more are defined as Ti-containing Mn sulfides.

[0343] In the steel of this embodiment, the number density ND0 (numbers / mm) of Mn sulfides with an equivalent circle diameter of 1.0 μm or more 2 The number density ND1 (numbers / mm) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more. 2 The number density ND2 (numbers / mm) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more. 2 It also satisfies equations (1) and (2).

[0344] ND1+ND2≥2.00 (1)

[0345] (ND1+ND2) / ND0<0.25 (2)

[0346] Equations (1) and (2) will be explained below.

[0347] [Equation (1)]

[0348] F1 = ND1 + ND2. As mentioned above, Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides are generally finer than Mn sulfides and are less likely to elongate axially in the steel. Therefore, Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides are less likely to become the initiation point of cracks during cold forging. In addition, if the number density of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides is high, the amount of S used for the growth of Mn sulfides is usually reduced. Therefore, under the premise of satisfying the above equation (2), the coarsening of Mn sulfides is usually suppressed. As a result, the cold forgeability of the steel is improved.

[0349] If the combined number density ND1 of Cu-Ni Mn sulfides and the number density ND2 of Ti Mn sulfides is greater than 2.00, then the number densities of Cu-Ni Mn sulfides and Ti Mn sulfides are sufficiently high. Therefore, excellent cold forging properties can be obtained in the steel.

[0350] The preferred lower limit for F1 is 2.10, further preferably 2.20, even more preferably 2.40, and even more preferably 2.60.

[0351] There is no specific upper limit for F1. When the steel satisfies both feature 1 and feature 2, the upper limit of F1 is, for example, 10.00, 8.00, 7.00, or 6.50.

[0352] Additionally, F1 is the value obtained by rounding the third decimal place of the obtained number (i.e., the value of the second decimal place).

[0353] [Equation (2)]

[0354] F2 = (ND1 + ND2) / ND0. F2 represents the ratio of the number density of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides to the number density of Mn sulfides. When the total number density F1 of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides satisfies equation (1), if the number density ND0 of Mn sulfides is high, then the number density of Mn sulfides is generally high. As mentioned above, when the total number density F1 of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides satisfies equation (1), the amount of S available for the growth of Mn sulfides is generally reduced.

[0355] If F2 is less than 0.25, the number density of Mn sulfides is usually high enough under the condition that the amount of S available in the growth of Mn sulfides is limited. Therefore, the Mn sulfides usually become sufficiently fine. Thus, excellent cold forging properties can be obtained in steel under the premise of satisfying equation (1).

[0356] The lower limit of F2 is not particularly limited. The preferred lower limit of F2 is 0.01, further preferably 0.03, even more preferably 0.04, and even more preferably 0.05.

[0357] The preferred upper limit for F2 is 0.24, further preferably 0.23, even more preferably 0.22, and even more preferably 0.20.

[0358] Additionally, F2 is the value obtained by rounding the third decimal place of the obtained number (i.e., the value of the second decimal place).

[0359] [Methods for determining number densities ND0, ND1, and ND2]

[0360] The number density of Mn sulfides ND0, the number density of Cu-Ni-containing Mn sulfides ND1, and the number density of Ti-containing Mn sulfides ND2 are determined by the following method.

[0361] Five test pieces were selected, each having a cross-section encompassing the axial and radial directions of the steel, and a surface extending to a depth of R / 2 from the steel's surface. The R / 2 depth refers to the central portion of a line segment (i.e., radius R) connecting the steel's surface to its center. The dimensions of each test piece were not particularly limited. Among the surfaces of each test piece, the surface encompassing the axial and radial directions of the steel, i.e., the surface containing the aforementioned R / 2 depth, was designated as the target surface. The R / 2 depth corresponds to the center position of the target surface.

[0362] The collected test specimens were embedded in resin. The surface of the resin-embedded test specimens was then ground. The observation area within the ground surface was observed using a scanning electron microscope (SEM) with compositional analysis capabilities. The observation area was a rectangle of 1200 μm × 960 μm centered at a depth of R / 2. The longer side of the observation area corresponded to the axial direction of the steel. During observation, the observation area was divided into 36 non-repeating fields of view of 200 μm × 160 μm, and each field of view was observed at 500x magnification.

[0363] Within the observation area, particles (precipitates or inclusions) with an equivalent circle diameter greater than 1.0 μm were identified based on the Z-contrast of the reflected electron image. It should be noted that in the reflected electron image, particles are displayed with a contrast darker than the parent phase. Furthermore, the equivalent circle diameter refers to the diameter of a circle whose area is converted to the area of ​​the particle. Elemental concentration analysis using energy-dispersive X-ray spectrophotometry (EDX) was performed on each identified particle to determine Mn sulfides. The EDX analysis (elemental concentration analysis) used the EDX-standard-free method. The accelerating voltage was set to 20 kV, and the elements to be quantified were C, Si, Mn, P, S, Cr, Ti, Cu, Ni, Ca, N, O, and Al.

[0364] In the EDX analysis results of each particle, when the total content of the above quantitative elements in mass% is set to 100%, if the S content in mass% is 10% or more and the Mn content is 10% or more, the particle is identified as a Mn sulfide.

[0365] When the total content of the above-mentioned quantitative elements in mass% is set to 100%, and the content of S in mass% is 10% or more, the total content of Cu and Ni is 5% or more, and the content of Mn is 10% or more, the particle is specifically identified as a Cu-Ni-containing Mn sulfide.

[0366] When the total content of the above quantitative elements in mass% is set to 100%, and the content of S in mass% is 10% or more, the total content of Cu and Ni is less than 5%, the content of Ti is 10% or more, and the content of Mn is 10% or more, the particle is identified as a Ti-containing Mn sulfide.

[0367] Calculate the total number of Mn sulfides with an equivalent circle diameter greater than 1.0 μm in each observation region of the five test pieces. Based on the total number of Mn sulfides and the total area of ​​each observation region in the five test pieces, calculate the number density ND0 (numbers / mm) of Mn sulfides with an equivalent circle diameter greater than 1.0 μm. 2 ).

[0368] Similarly, the total number of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or greater, determined in each observation region of the five test pieces, was calculated. Based on the total number of Cu-Ni-containing Mn sulfides and the total area of ​​each observation region in the five test pieces, the number density ND1 (numbers / mm²) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or greater was calculated. 2 ).

[0369] Similarly, the total number of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or greater, determined in each observation region of the five test pieces, was calculated. Based on the total number of Ti-containing Mn sulfides and the total area of ​​each observation region in the five test pieces, the number density ND2 (numbers / mm²) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or greater was calculated. 2 ).

[0370] In addition, the number densities ND0, ND1 and ND2 are values ​​obtained by rounding the third decimal place of the obtained numerical value (i.e., the value of the second decimal place).

[0371] [The effect of the steel in this embodiment]

[0372] As described above, the steel of this embodiment satisfies features 1 to 3. Therefore, the steel of this embodiment exhibits excellent cold forging properties.

[0373] [The shape of the steel in this embodiment]

[0374] The steel used in this embodiment is either a bar or wire. A bar or wire is a steel material with a circular cross-section perpendicular to the axial direction and extending in a rod shape. The steel can be wound into a coil or cut into a specified length. The diameter of the steel cross-section is, for example, 4 to 20 mm.

[0375] [Applications of the steel used in this embodiment]

[0376] The steel of this embodiment can be used as a raw material for mechanical structural components such as bolts. The steel of this embodiment is particularly suitable as a raw material for bolts, fastening devices used in industrial machinery, automobiles, bridges, and buildings. Furthermore, the steel of this embodiment can also be used for applications other than those described above.

[0377] [Steel Manufacturing Methods]

[0378] An example of a method for manufacturing the steel according to this embodiment will be described. The method for manufacturing the steel described below is an example for manufacturing the steel according to this embodiment. Therefore, the steel having the above 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 according to this embodiment.

[0379] An example of the steel manufacturing method of this embodiment includes the following steps.

[0380] (Process 1) Refining Process

[0381] (Process 2) Casting process

[0382] (Process 3) Hot working process

[0383] The following is a description of each process.

[0384] (Process 1) Refining Process

[0385] In the refining process, molten steel with a chemical composition satisfying characteristic 1 above is produced. The refining method is not particularly limited; any known method may be used. For example, molten iron produced using a known method is refined in a converter (primary refining). The molten steel tapped from the converter is then subjected to a known secondary refining. Through these processes, molten steel with a chemical composition satisfying characteristic 1 is produced.

[0386] [(Process 2) Casting Process]

[0387] In the casting process, molten steel produced through a refining process is used to manufacture steel ingots (slabs) through continuous casting.

[0388] [(Process 3) Hot working process]

[0389] In the hot working process, steel ingots are hot-worked to produce steel products. The hot working process includes the following steps.

[0390] (Process 31) Billet rolling process

[0391] (Process 32) Finishing rolling process

[0392] The following is a description of each process.

[0393] (Process 31) Billet rolling process

[0394] In the billet rolling process, steel ingots are hot-rolled (bill rolling) using a billet rolling mill to produce steel billets. When a continuous rolling mill with multiple stands arranged in a row is located downstream of the billet rolling mill, the billets can also be hot-rolled using a continuous rolling mill to further reduce the billet size. The heating temperature in the billet rolling process can be within a known temperature range, for example, 1100~1300°C. The steel billets produced by the billet rolling process are then naturally cooled (air-cooled) to room temperature before the finishing rolling process.

[0395] [(Process 32) Finishing rolling process]

[0396] In the finishing rolling process, the steel billet is hot-rolled (finished rolling) to produce the steel product of this embodiment. Specifically, the steel billet after the initial rolling process is heated in a heating furnace. The heating temperature is, for example, 1000~1250°C. The heated steel billet is then hot-rolled (finished rolling) using a finishing mill to produce a circular steel product with a desired cross-section. The finishing mill is, for example, a continuous rolling mill in which multiple rolling stands are arranged in a row perpendicular to the reduction direction of adjacent rolling stands.

[0397] [Manufacturing conditions in the manufacturing method of this embodiment]

[0398] In the above manufacturing method, the following conditions must be met.

[0399] (Condition 1)

[0400] The total residence time t1 for raw materials heated to a temperature above 1200℃ in subsequent processes is set to less than 120 minutes.

[0401] (Condition 2)

[0402] In the total reduction of area (TRR) of the finishing rolling process, the total reduction of area (RR) in the finishing rolling process where the steel temperature is below 980℃ is included. 980 The proportion X (%) is defined by the following formula.

[0403] X=RR 980 / TRR×100

[0404] At this point, equation (A) is satisfied.

[0405] X-√(Ti+Cu+Ni+1) / (2×S))≥0 (A)

[0406] Here, the content of the corresponding element in terms of mass% is substituted into the element symbols in Equation (A).

[0407] (Condition 3)

[0408] In the finishing rolling process, the maximum reduction of area in a single pass where the steel temperature exceeds 1000℃ is defined as Y (%). At this time, the following formula (B) is satisfied.

[0409] Y-15 / √(Cu+0.5×Ni+0.2)>0 (B)

[0410] Here, the content of the corresponding element in terms of mass% is substituted into the element symbols in Equation (B).

[0411] The following explains conditions 1 through 3.

[0412] [Condition 1: The total residence time t1 is when the raw material is heated to a temperature above 1200℃]

[0413] In processes following the hot working process (i.e., from the start of steel heating in the billet rolling process to the completion of finishing rolling in the finishing rolling process and the cooling of the finished steel to room temperature), if the total residence time t1 at a raw material heating temperature of 1200°C or higher exceeds 120 minutes, Cu, Ni, and Ti precipitates coarsen. This coarsening of precipitates is caused by Ostwald curing. During Ostwald curing, fine Cu, Ni, and Ti precipitates disappear as they enter the coarsened precipitates. Therefore, the amounts of Cu, Ni, and Ti available for forming Cu-Ni Mn sulfides and Ti-containing Mn sulfides decrease. Consequently, the number density of Cu-Ni Mn sulfides and Ti-containing Mn sulfides decreases. At this point, Equation (1) is not satisfied in the manufactured steel. Therefore, the total residence time t1 at 1200°C or higher is less than 120 minutes.

[0414] The preferred upper limit for the total residence time t1 above 1200°C is 110 minutes, further preferably 100 minutes, and even more preferably 90 minutes.

[0415] There is no particular limitation on the lower limit of the total residence time t1 above 1200°C. A preferred lower limit is 0 minutes, but for industrial production considerations, 10 minutes is further preferred, 20 minutes is even more preferred, and 30 minutes is still even more preferred. It should be noted that if the heating temperature of the raw material in subsequent processes does not reach 1200°C or above, the total residence time t1 above 1200°C is set to 0 minutes.

[0416] [Condition 2: Formula (A)]

[0417] Defined as FA = X - √(Ti + Cu + Ni + 1) / (2 × S). FA is an index representing the ease of segmentation of Mn sulfides during the finishing rolling process. In the finishing rolling process, Mn sulfides tend to elongate in the rolling direction of the steel. Especially in the finishing rolling process, Mn sulfides elongate easily when the steel temperature is below 980°C. Furthermore, compared to Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides, Mn sulfides are generally significantly more prone to elongation.

[0418] If FA satisfies equation (A), then a sufficient amount of reduction is applied to the steel at a temperature below 980°C. In this case, especially after the Mn sulfides elongate in the rolling direction, they are divided into multiple and refined, reducing the number of coarse Mn sulfides that would become the initiation point of cracks during cold forging. As a result, the number density of Mn sulfides increases compared to the number density of Cu-Ni and Ti-containing Mn sulfides, and the manufactured steel satisfies equation (2).

[0419] The preferred lower limit for FA is 0.5, further preferably 0.8, and even more preferably 1.0.

[0420] There is no particular upper limit for FA, but it is preferably 10.0, more preferably 8.0, even more preferably 6.0, and even more preferably 5.0.

[0421] In addition, if we consider typical industrial production, the lower limit of X is, for example, 3.0%, and the upper limit of X is, for example, 15.0%.

[0422] [Condition 3: Equation (B)]

[0423] FB is defined as FB = Y - 15 / √(Cu + 0.5 × Ni + 0.2). FB is an index representing the ease with which Cu and Ni are enriched in Mn sulfides during the finishing rolling process. In hot rolling using a finishing mill with multiple rolling stands, the reduction of area of ​​the billet passing through each rolling stand is defined as "reduction of area per pass".

[0424] In the finishing rolling process, if the steel temperature exceeds 1000℃, Cu and Ni in the steel easily diffuse. Therefore, rolling with a large reduction of area at a steel temperature exceeding 1000℃ further promotes the diffusion of Cu and Ni, and facilitates their enrichment in Mn sulfides. If FB is below 0, the enrichment of Cu and Ni in Mn sulfides is insufficient. In this case, the steel does not satisfy characteristic 2. Therefore, FB exceeds 0.

[0425] The preferred lower limit for FB is 0.1, further preferably 0.2, even more preferably 0.3, and even more preferably 0.4.

[0426] There is no particular limit to the upper limit of FB, but it is preferably 15.0, further preferably 12.0, and even more preferably 8.0.

[0427] In addition, if we consider typical industrial production, the lower limit of Y is, for example, 15.0%, and the upper limit of Y is, for example, 35.0%.

[0428] The steel of this embodiment is manufactured through the above manufacturing process.

[0429] [Regarding the mechanical structure components of this embodiment]

[0430] The mechanical structure components of this embodiment are made from the steel used in this embodiment. For example, the mechanical structure components of this embodiment are bolts. For example, the mechanical structure components of this embodiment can also be nuts. For example, the mechanical structure components of this embodiment can also be hollow components or cup-shaped components with an open end.

[0431] [Features of the mechanical structure components in this embodiment]

[0432] The mechanical structure components of this embodiment satisfy the following characteristics.

[0433] (Feature 4)

[0434] The chemical composition, by mass%, contains: C: 0.04% to less than 0.20%, Si: 0.01% to 0.35%, Mn: 0.20% to 1.00%, Al: 0.001% to 0.100%, Ti: 0.001% to 0.100%, Cu: 0.01% to 0.40%, Ni: 0.01% to 0.30%, Cr: 0.01% to 0.30%, Mo: 0.001% to 0.200%, Sn: 0.001% to 0.10%. The composition is as follows: 0%, P: less than 0.040%, S: less than 0.040%, N: less than 0.0150%, O: less than 0.0030%, B: 0~0.0010%, Nb: 0~0.050%, V: 0~0.15%, Sb: 0~0.050%, As: 0~0.050%, Pb: 0~0.090%, Ca: 0~0.0050%, and Mg: 0~0.0050%, with the remainder consisting of Fe and impurities.

[0435] (Feature 5)

[0436] The number density ND1 of Cu-Ni-containing Mn sulfides is 1.00 per mm. 2 above.

[0437] (Feature 6)

[0438] Mn sulfide number density ND0 (numbers / mm) 2The number density of Cu-Ni-containing Mn sulfides, ND1 (numbers / mm²), 2 ) and the number density of Ti-containing Mn sulfides ND2 (numbers / mm 2 ) satisfies equations (1) and (2).

[0439] ND1+ND2≥2.00 (1)

[0440] (ND1+ND2) / ND0<0.25 (2)

[0441] The number density ND0 of Mn sulfides in mechanical structural components, the number density ND1 of Cu-Ni-containing Mn sulfides, and the number density ND2 of Ti-containing Mn sulfides were determined based on the methods described in [Methods for Determining Number Densities ND0, ND1, and ND2]. However, the test piece used for the measurements was collected with a cross-section encompassing the depth direction from the surface of the mechanical structural component as the object surface. The center position of the object surface corresponds to a depth of 1 mm from the surface of the mechanical structural component. The observation area on the object surface was set as a rectangle of 1200 μm × 960 μm centered at a depth of 1 mm from the surface of the mechanical structural component. The long side of the observation area was perpendicular to the depth direction from the surface of the mechanical structural component.

[0442] The function of each element in feature 4 is the same as that of the corresponding element in feature 1 of the steel in this embodiment. Furthermore, the technical significance of feature 5 is the same as that of feature 2 of the steel in this embodiment. The technical significance of feature 6 is the same as that of feature 3 of the steel in this embodiment. Therefore, in the mechanical structure component of this embodiment that satisfies features 4 to 6, excellent cold forging properties can be obtained during the manufacturing process.

[0443] [Manufacturing methods for mechanical structural components]

[0444] The mechanical structure component of this embodiment is manufactured using the steel of this embodiment as raw material through a known method. The chemical composition of the mechanical structure component manufactured using the known method is the same as that of the steel used as raw material. Furthermore, the number density of Mn sulfides ND0, the number density of Cu-Ni-containing Mn sulfides ND1, and the number density of Ti-containing Mn sulfides ND2 in the mechanical structure component manufactured using the known method are almost unchanged compared to the steel used as raw material. Therefore, as long as the mechanical structure component of this embodiment is manufactured using the steel of this embodiment as raw material and a known method that satisfies features 1 to 3, features 4 to 6 can be satisfied.

[0445] As an example of a mechanical structure component in this embodiment, the manufacturing process of a bolt will be described. The manufacturing method of the bolt, as a mechanical structure component in this embodiment, includes, for example, a wire drawing process, a cold forging process, and a quenching and tempering process.

[0446] In the wire drawing process, the steel of this embodiment is subjected to a known wire drawing process to manufacture steel wire. The wire drawing process can be a single wire drawing or multiple wire drawing processes, such as a double wire drawing. In the cold forging process, the steel wire after the wire drawing process is subjected to a known cold forging (upsetting) process to manufacture a bolt-shaped semi-finished product.

[0447] In the quenching and tempering process, the intermediate part is quenched and tempered. Quenching is performed by known methods. The quenching temperature is, for example, 840~970°C. The holding time at the quenching temperature is, for example, 15 minutes to 360 minutes (6 hours). After the holding time, the intermediate part is quenched rapidly. Specifically, the intermediate part is water-cooled or oil-cooled. The quenched intermediate part is then tempered. The tempering temperature is, for example, 400~600°C. The holding time at the tempering temperature is, for example, 0.5~6.0 hours.

[0448] The above manufacturing method enables the production of bolts that serve as mechanical structure components according to this embodiment. The mechanical structure components of this embodiment exhibit excellent cold forging properties during the manufacturing process. Therefore, even without performing a softening heat treatment before cold forging, cracking of the raw material and deterioration of the die during cold forging can be suppressed in the manufacturing process of the mechanical structure components of this embodiment.

[0449] Example

[0450] The effects of the steel in this embodiment are further illustrated through examples. The conditions in the following examples are examples used to confirm the feasibility and effects of the steel in this embodiment. Therefore, the steel in this embodiment is not limited to this single example.

[0451] By performing well-known refining processes, steel with the chemical composition shown in Table 1 (Table 1A and Table 1B) is produced.

[0452] [Table 1A]

[0453]

[0454] [Table 1B]

[0455]

[0456] Specifically, molten steel is used to manufacture steel ingots via continuous casting. The manufactured steel ingots are then subjected to a roughing rolling process to produce steel billets. In the roughing rolling process, the steel ingots are heated to 1100~1300℃ and hot-rolled using a roughing mill. The produced steel billets are then cooled to room temperature. The produced steel billets are then subjected to a finishing rolling process. In the finishing rolling process, the steel billets are heated to 1000~1250℃. The heated steel billets are then hot-rolled using a finishing mill to produce wire rod (steel) with a diameter of 10.0 mm. Through the above manufacturing processes, steel products of various test numbers are produced.

[0457] The total residence time t1 (minutes) of steel (ingot, billet, and finished steel) at a temperature above 1200°C, from the start of ingot heating in the roughing rolling process to the cooling of the steel to room temperature after the finishing rolling process, is shown in the "t1 (minutes)" column of Table 2. In the finishing rolling process, the proportions X (%) and FA of the total reduction of area (RR) 980 in the finishing rolling process where the steel temperature is below 980°C are shown in the "X (%)" and "FA" columns of Table 2. In the finishing rolling process, the maximum reduction of area (Y (%)) and FB in a single pass where the billet temperature exceeds 1000°C are shown in the "Y (%)" and "FB" columns of Table 2.

[0458] [Table 2]

[0459]

[0460] [Evaluation Test]

[0461] The following evaluation tests were performed on the steel for each test number manufactured.

[0462] (Experiment 1) Determination of number density ND0, ND1 and ND2

[0463] (Experiment 2) Ultimate Compression Test

[0464] The following is a description of each experiment.

[0465] [(Experiment 1) Determination of Number Density ND0, ND1 and ND2]

[0466] Based on the methods described above for determining the number densities ND0, ND1, and ND2, the number density ND0 (numbers / mm) of Mn sulfides with an equivalent circle diameter of 1.0 μm or more is calculated. 2 The number density ND1 (numbers / mm) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more. 2 The number density ND2 (numbers / mm) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more. 2 The results are shown in Table 2 under “ND0 (pieces / mm)”.2 "ND1 (pieces / mm)" column, "ND1 (pieces / mm)" column 2 The columns “ND2 (pieces / mm)” and “ND2 (pieces / mm)” are also relevant. 2 )"column.

[0467] [(Experiment 2) Ultimate Compression Test]

[0468] To evaluate cold forging properties, a limit compression test was conducted to determine the ultimate compressibility of the steel. Specifically, multiple limit compression ratio test pieces were collected from steel (wire) of each test number. The limit compression ratio test pieces were cylindrical, with a diameter of 8 mm and a length of 12 mm. The length direction of the limit compression ratio test pieces was parallel to the axial direction of the steel of each test number. Furthermore, the central axis of the limit compression ratio test pieces corresponded to the central axis of the steel of each test number. A longitudinal slit was formed on the side (circumferential surface) of the test piece. The slit angle was 30 degrees, the slit depth was 0.46 mm, and the radius of curvature at the slit tip was 0.15 mm. The length of the slit was the same as the length of the test piece, 12 mm. Furthermore, recesses were formed on a pair of end faces of the test piece to secure it to the end face constraint mold. The recesses were formed at the center of each end face and were conical in shape. The diameter of the opening of the recess on the end face (equivalent to the bottom of the cone) is 2mm, and the angle of the vertex of the section containing the central axis of the conical recess is 120°.

[0469] In the ultimate compression test, a 500-ton hydraulic press was used. The ultimate compression test was conducted on the prepared test pieces using the following method: For each test piece, a die with an end face constraint having a protrusion corresponding to the pit of the test piece was used, and cold compression was performed at a speed of 15 mm / s. Compression was stopped when a microcrack of 0.5 mm or more appeared at the bottom of the notch, and the compression ratio (%) at this point was calculated. This test was performed a total of 5 times, and the compression ratio (%) with a cumulative failure probability of 50% was calculated. A compression ratio of 55% or higher was rated as "E (Excellent)," indicating excellent cold forging properties. Conversely, a compression ratio less than 55% was rated as "NA (Not Accepted)," indicating poor cold forging properties. The evaluation results are shown in the "Ultimate Compression Ratio" column of Table 2.

[0470] [Experimental Results]

[0471] Referring to Tables 1A, 1B, and 2, the steels tested from 1 to 15 meet characteristics 1 to 3. Therefore, these steels exhibit excellent cold forging properties.

[0472] On the other hand, in experiment number 16, the carbon content was too high. As a result, excellent cold forging properties were not obtained.

[0473] In experiment number 17, the Si content was too high. As a result, excellent cold forging properties were not obtained.

[0474] In experiment number 18, the Mn content was too high. As a result, excellent cold forging properties were not obtained.

[0475] In experiment number 19, the Al content was too high. As a result, excellent cold forging properties were not obtained.

[0476] In experiment number 20, the Ti content was too high. As a result, excellent cold forging properties were not obtained.

[0477] In experiment number 21, the Cu content was too high. Furthermore, the FA content was too low. Therefore, F2 did not satisfy equation (2). As a result, excellent cold forging properties were not obtained.

[0478] In experiment number 22, the Ni content was too high. Furthermore, the FA content was too low. Therefore, F2 did not satisfy equation (2). As a result, excellent cold forging properties were not obtained.

[0479] In experiment number 23, the Cr content was too high. As a result, excellent cold forging properties were not obtained.

[0480] In experiment number 24, the Mo content was too high. As a result, excellent cold forging properties were not obtained.

[0481] In experiment number 25, the Sn content was too high. As a result, excellent cold forging properties were not obtained.

[0482] In experiment number 26, the phosphorus content was too high. As a result, excellent cold forging properties were not obtained.

[0483] In experiment number 27, the sulfur content was too high. As a result, excellent cold forging properties were not obtained.

[0484] In experiment number 28, the nitrogen content was too high. As a result, excellent cold forging properties were not obtained.

[0485] In experiment number 29, the oxygen content was too high. As a result, excellent cold forging properties were not obtained.

[0486] In tests 30 and 31, the total residence time t1 above 1200℃ was too long. Therefore, F1 does not satisfy equation (1). As a result, excellent cold forging properties cannot be obtained.

[0487] In tests 32 and 33, FA was too low. Therefore, F2 did not satisfy equation (2). As a result, excellent cold forging properties were not obtained.

[0488] In tests 34 and 35, FB was too low. Therefore, ND1 was too low. As a result, excellent cold forging properties were not obtained.

[0489] The embodiments of the present invention have been described above. However, the above embodiments are merely illustrative examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and can be implemented by appropriately modifying the above embodiments without departing from its spirit.

Claims

1. A type of steel, characterized in that, By mass%, it contains C: 0.04% to less than 0.20% Si: 0.01~0.35% Mn: 0.20~1.00% Al：0.001~0.100%、 Ti: 0.001~0.100% Cu: 0.01~0.40%, Ni: 0.01~0.30% Cr：0.01~0.30%、 Mo: 0.001~0.200% Sn: 0.001~0.100% P: below 0.040% S: Below 0.040% N: below 0.0150% O: Below 0.0030% B：0~0.0010%、 Nb: 0~0.050%, V：0~0.15%、 Sb: 0~0.050%, As: 0~0.050%, Pb: 0~0.090%, Ca: 0~0.0050%, and Mg: 0~0.0050%, The remaining portion consists of Fe and impurities. The number density (numbers / mm) of Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass, and a Mn content of 10% or more. 2 Defined as ND0, The number density (numbers / mm²) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of 5% or more, and a Mn content of 10% or more. 2 Defined as ND1, The number density (numbers / mm²) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more. 2 When defined as ND2, ND1 (pieces / mm) 2 The value is above 1.

00. Satisfying equations (1) and (2). ND1+ND2≥2.00 (1) (ND1+ND2) / ND0<0.25 (2).

2. The steel according to claim 1, characterized in that, By mass%, it contains selected ingredients. B：0.0001~0.0010%、 Nb: 0.001~0.050%, V：0.01~0.15%、 Sb: 0.001~0.050% As: 0.001~0.050% Pb: 0.001~0.090%, Ca: 0.0001~0.0050%, and Mg: 0.0001~0.0050% of one or more of the group.

3. A component for a mechanical structure, characterized in that, By mass%, it contains C: 0.04% to less than 0.20% Si: 0.01~0.35% Mn: 0.20~1.00% Al：0.001~0.100%、 Ti: 0.001~0.100% Cu: 0.01~0.40%, Ni: 0.01~0.30% Cr：0.01~0.30%、 Mo: 0.001~0.200% Sn: 0.001~0.100% P: below 0.040% S: Below 0.040% N: below 0.0150% O: Below 0.0030% B：0~0.0010%、 Nb: 0~0.050%, V：0~0.15%、 Sb: 0~0.050%, As: 0~0.050% Pb: 0~0.090%, Ca: 0~0.0050%, and Mg: 0~0.0050%, The remaining portion consists of Fe and impurities. The number density (numbers / mm) of Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass, and a Mn content of 10% or more. 2 Defined as ND0, The number density (numbers / mm²) of Cu-Ni-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of 5% or more, and a Mn content of 10% or more. 2 Defined as ND1, The number density (numbers / mm²) of Ti-containing Mn sulfides with an equivalent circle diameter of 1.0 μm or more, a S content of 10% or more by mass%, a combined Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more. 2 When defined as ND2, ND1 (pieces / mm) 2 The value is above 1.

00. Satisfying equations (1) and (2). ND1+ND2≥2.00 (1) (ND1+ND2) / ND0<0.25 (2).

4. The mechanical structure component according to claim 3, characterized in that, By mass%, it contains selected ingredients. B：0.0001~0.0010%、 Nb: 0.001~0.050%, V：0.01~0.15%、 Sb: 0.001~0.050% As: 0.001~0.050% Pb: 0.001~0.090%, Ca: 0.0001~0.0050%, and Mg: 0.0001~0.0050% of one or more of the group.

5. The mechanical structure component according to claim 3 or 4, characterized in that, The mechanical structure component is a bolt.

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