bolt

Abstract

To provide a bolt that offers high strength as well as enhanced resistance to hydrogen embrittlement.SOLUTION: A bolt disclosed herein comprises, in mass%, C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10-1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.80-1.50% (excluding 0.80%), Mo: 0.01-0.50% (excluding 0.50%), V: 0.01-0.50%, Al: 0.005-0.100%, N: 0.0010-0.0300%, with the balance being Fe and impurities. The tensile strength TS is 1100 MPa or more. In the bolt, the number density ND of MC carbides is 2.0×1021 / m3 or more. Defining the C content in MC carbides as [C] in atom%, the Mo content as [Mo] in atom%, and the vanadium content as [V] in atom%, the carbon ratio Rc, defined by formula (1), is between 0.25 and 0.42. Formula (1): Rc=[C] / ([C]+[Mo]+[V])SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This disclosure relates to bolts. [Background technology]

[0002] Bolts are used in industrial machinery, automobiles, and buildings such as bridges. In recent years, with the increasing performance of industrial machinery and automobiles, and the growing size of buildings, there has been a demand for higher-strength bolts. Specifically, bolts with a tensile strength of 1100 MPa or more are required.

[0003] Bolts with such high strength are more susceptible to hydrogen embrittlement. Therefore, high-strength bolts require excellent resistance to hydrogen embrittlement.

[0004] Japanese Patent Publication No. 2019-218584 (Patent Document 1), International Publication No. 2017 / 094487 (Patent Document 2), and Japanese Patent Publication No. 2013-163865 (Patent Document 3) propose bolts having high strength and excellent resistance to hydrogen embrittlement.

[0005] The bolt disclosed in Patent Document 1 has the following composition in mass%, C: 0.22-0.40%, Si: 0.10-1.50%, Mn: less than 0.20-0.40%, Cr: less than 0.70-1.60%, Al: 0.005-0.060%, Ti: 0.010-0.050%, B: 0.0003-0.0040%, N: 0.0015-0.0080%, Cu: 0.50% or less, Ni: 0.30% or less, M The material contains o: 0.05% or less, V: 0.050% or less, Nb: 0.050% or less, and further contains one or more selected from the group consisting of Sb: 0.001~0.100%, Sn: 0.001~0.100%, and Bi: 0.001~0.100%, and further contains O: 0.0020% or less, P: 0.020% or less, S: 0.020% or less, with the remainder being Fe and impurities. This bolt further satisfies formula (1) (0.50 ≤ C + (1 / 10) × Si + (1 / 5) × Mn + (5 / 22) × Cr ≤ 0.85) and formula (2) (0.003 ≤ Sb + Sn + Bi ≤ 0.100). Patent Document 1 states that, in this bolt, by adjusting the content of Sb, Sn, and Bi in the chemical composition to satisfy formula (2), excellent hydrogen embrittlement resistance can be obtained even when the tensile strength of the shaft is 1000 to 1300 MPa.

[0006] The bolt disclosed in Patent Document 2 contains, by mass%, C: 0.22-0.40%, Si: 0.10-1.50%, Mn: less than 0.20-0.40%, P: 0.020% or less, S: 0.020% or less, Cr: 0.70-1.45%, Al: 0.005-0.060%, Ti: 0.010-0.045%, B: 0.0003-0.0040%, N: 0.0015-0.0080%, O: 0.0020% or less, Cu: 0-0.50%, Ni: 0-0.30%, Mo: 0-0.04%, V: 0-0.05%, and Nb: 0-0.050%, with the remainder being Fe and impurities. This bolt also satisfies equation (1) (0.50 ≤ C + Si / 10 + Mn / 5 + 5Cr / 22 ≤ 0.85) and equation (2) (Si / Mn > 1.0). Patent Document 2 states that this bolt increases its strength to 1000-1300 MPa by satisfying equation (1), and enhances its resistance to hydrogen embrittlement by satisfying equation (2).

[0007] The bolt disclosed in Patent Document 3 contains, by mass%, C: 0.30 to 0.50%, Si: 1.0 to 2.5%, Mn: 0.1 to 1.5%, P: 0.015% or less (excluding 0%), S: 0.015% or less (excluding 0%), Cr: 0.15 to 2.4%, Al: 0.10% or less (excluding 0%), and N: 0.015% or less (excluding 0%). Further, it contains Cu: 0.10 to 0.50% and Ni: 0.1 to 1.0% such that [Ni] / [Cu] ≥ 0.5. Further, it contains Ti: 0.05 to 0.20% and V: 0.20% or less (including 0%) such that [Ti] + [V]: 0.085 to 0.30%. The balance consists of Fe and impurities. This bolt further has an austenite crystal grain size number of 9.0 or more in the bolt shaft portion, and a G value (%) indicating the ratio of carbides precipitated at the austenite grain boundaries in the bolt shaft portion satisfies the formula (1) (G value: (L / L0) × 100 ≤ 60, where L: the total length of carbides with a thickness of 50 nm or more precipitated at the austenite grain boundaries, and L0: the length of the austenite grain boundaries). It is described in Patent Document 3 that in this bolt, excellent hydrogen embrittlement resistance can be obtained even with high strength by suppressing the precipitation of carbides at grain boundaries.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] The bolts disclosed in Patent Documents 1 to 3 have high strength and excellent hydrogen embrittlement resistance. However, high strength and excellent hydrogen embrittlement resistance may be obtained by means different from those of Patent Documents 1 to 3.

[0010] The purpose of this disclosure is to provide a bolt having high strength and excellent resistance to hydrogen embrittlement. [Means for solving the problem]

[0011] The bolts of this disclosure are, by mass%, C: 0.30~0.50%, Si: 0.01~0.30%, Mn: 0.10~1.50%, P:0.030% or less, S: 0.030% or less, Cr: over 0.80~1.50%, Mo: 0.01 to less than 0.50% V: 0.01~0.50%, Al: 0.005~0.100%, and, It contains N: 0.0010 to 0.0300%, The remainder consists of Fe and impurities. The tensile strength TS is 1100 MPa or more. The number density ND of the MC-type carbide is 2.0 × 10⁻⁶. 21 pieces / m 3 That's all. When the atomic percentage of C in the aforementioned MC-type carbide is defined as [C], the atomic percentage of Mo is defined as [Mo], and the atomic percentage of V is defined as [V], the C ratio Rc defined by equation (1) is between 0.25 and 0.42. Rc = [C] / ([C] + [Mo] + [V]) (1)

[0012] The bolts of this disclosure are, by mass%, C: 0.30~0.50%, Si: 0.01~0.30%, Mn: 0.10~1.50%, P:0.030% or less, S: 0.030% or less, Cr: over 0.80~1.50%, Mo: 0.01 to less than 0.50% V: 0.01~0.50%, Al: 0.005~0.100%, and, It contains N: 0.0010 to 0.0300%, Furthermore, it contains one or more elements selected from the groups consisting of Group 1 to Group 3, with the remainder being Fe and impurities. The tensile strength TS is 1100 MPa or more. The number density ND of the MC-type carbide is 2.0 × 10⁻⁶. 21 pieces / m 3 That's all. When the atomic percentage of C in the aforementioned MC-type carbide is defined as [C], the atomic percentage of Mo is defined as [Mo], and the atomic percentage of V is defined as [V], the C ratio Rc defined by equation (1) is between 0.25 and 0.42. Rc = [C] / ([C] + [Mo] + [V]) (1) [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Zr: 0.300% or less, Hf: 0.100% or less, Ta: 0.100% or less, and, One or more selected from the group consisting of W: 0.20% or less. [Group 2] Ti: 0.100% or less, and, One or more selected from the group consisting of Nb: 0.100% or less. [Group 3] Ca: 0.0050% or less, Bi: 0.020% or less, and, One or more selected from the group consisting of Te: 0.010% or less. [Effects of the Invention]

[0013] The bolts according to this disclosure have high strength and excellent resistance to hydrogen embrittlement. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a side view showing an example of a bolt according to this embodiment. [Figure 2] Figure 2 shows the relationship between the carbon ratio (Rc) in the MC-type carbide and the critical hydrogen content in a bolt where the content of each element in the chemical composition is within the range of this embodiment, the tensile strength (TS) is 1100 MPa or higher, and the number density (ND) of the MC-type carbide is 2.0 × 10²¹ particles / m³ or higher. [Modes for carrying out the invention]

[0015] The inventors investigated bolts having a tensile strength (TS) of 1100 MPa or higher and excellent hydrogen embrittlement resistance. As a result, the inventors obtained the following findings.

[0016] First, the inventors investigated bolts with high strength and excellent hydrogen embrittlement resistance from the viewpoint of chemical composition. As a result, the inventors found that the following composition in mass% is desirable: C: 0.30~0.50%, Si: 0.01~0.30%, Mn: 0.10~1.50%, P: 0.030% or less, S: 0.030% or less, Cr: greater than 0.80~1.50%, Mo: less than 0.01~0.50%, V: 0.01~0.50%, Al: 0.005~0.100%, N: 0.0010~0.0300%, Cu: 0~0.40%, Ni: 0~0.40%, B: 0~0.01 We believe that a bolt having a chemical composition consisting of 00%, Zr: 0-0.300%, Hf: 0-0.100%, Ta: 0-0.100%, W: 0-0.20%, Ti: 0-0.100%, Nb: 0-0.100%, Ca: 0-0.0050%, Bi: 0-0.020%, Te: 0-0.010%, with the remainder being Fe and impurities, could potentially achieve both a tensile strength (TS) of 1100 MPa or more and excellent resistance to hydrogen embrittlement.

[0017] Therefore, we investigated means to improve the strength and hydrogen embrittlement resistance of bolts having the above-mentioned chemical composition from the perspective of their microstructure. As a result, we found that by dispersing a large number of fine precipitates in the bolt, hydrogen embrittlement resistance can be improved even in bolts with a tensile strength TS of 1100 MPa or more.

[0018] The inventors further considered that the type of precipitate affects the hydrogen embrittlement resistance of the bolt. Therefore, they conducted further investigations. Here, the inventors focused on MC-type carbides among alloy carbides. Compared to other alloy carbides (e.g., M2C-type carbides, cementite, etc.), MC-type carbides are less prone to coarsening and tend to remain fine. Fine precipitates act as hydrogen trapping sites and absorb hydrogen. Therefore, it is possible to increase the amount of hydrogen that can be stored before hydrogen embrittlement cracking occurs (hereinafter referred to as the critical hydrogen amount). Consequently, increasing the number density of MC-type carbides in the bolt may improve the hydrogen embrittlement resistance of the bolt.

[0019] Therefore, the inventors investigated the relationship between the number density of MC-type carbides and the critical hydrogen content in bolts having the above-mentioned chemical composition. As a result, the number density of MC-type carbides was 2.0 × 10⁻⁶ 21 pieces / m 3 It was found that, under these conditions, even bolts with a tensile strength (TS) of 1100 MPa or higher can obtain a sufficient limiting hydrogen content, potentially resulting in excellent hydrogen embrittlement resistance.

[0020] However, having the above chemical composition, the number density of MC-type carbides is 2.0 × 10⁻⁶ 21 pieces / m 3 Even with bolts meeting the above criteria, sufficient hydrogen embrittlement resistance could still not be obtained when the tensile strength TS was 1100 MPa or higher.

[0021] Therefore, the inventors conducted further investigations. Here, the inventors considered that not only the number density of MC-type carbides but also the composition of MC-type carbides affects the hydrogen storage effect. Therefore, they investigated the relationship between the composition of MC-type carbides and the critical hydrogen content. As a result, when the atomic percent C content in MC-type carbides is defined as [C], the atomic percent Mo content is defined as [Mo], and the atomic percent V content is defined as [V], it was found that if the C ratio Rc defined by equation (1) is between 0.25 and 0.42, excellent hydrogen embrittlement resistance can be obtained even for bolts with a tensile strength TS of 1100 MPa or more. Rc = [C] / ([C] + [Mo] + [V]) (1)

[0022] Based on the above findings, the bolt according to the present embodiment, which is completed, has the following configuration.

[0023] [1] By mass percentage, C: 0.30 to 0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: More than 0.80 to 1.50%, Mo: 0.01 to less than 0.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0300%, and contains the balance consists of Fe and impurities, the tensile strength TS is 1100 MPa or more, the number density ND of MC-type carbides is 2.0×10 21 pieces / m 3 or more, when the C content in atomic percentage in the MC-type carbide is defined as [C], the Mo content in atomic percentage is defined as [Mo], and the V content in atomic percentage is defined as [V], the C ratio Rc defined by the formula (1) is 0.25 to 0.42, bolt. Rc = [C] / ([C] + [Mo] + [V]) (1)

[0024] [2] By mass percentage, C: 0.30 to 0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: More than 0.80 to 1.50%, Mo: 0.01 to less than 0.50%, V: 0.01~0.50%, Al: 0.005~0.100%, and, It contains N: 0.0010 to 0.0300%, Furthermore, it contains one or more elements selected from the groups consisting of Group 1 to Group 3, with the remainder being Fe and impurities. The tensile strength TS is 1100 MPa or more. The number density ND of the MC-type carbide is 2.0 × 10⁻⁶. 21 pieces / m 3 That's all. When the atomic percentage of C in the aforementioned MC-type carbide is defined as [C], the atomic percentage of Mo is defined as [Mo], and the atomic percentage of V is defined as [V], the C ratio Rc defined by equation (1) is 0.25 to 0.42. bolt. Rc = [C] / ([C] + [Mo] + [V]) (1) [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Zr: 0.300% or less, Hf: 0.100% or less, Ta: 0.100% or less, and, One or more selected from the group consisting of W: 0.20% or less. [Group 2] Ti: 0.100% or less, and, One or more selected from the group consisting of Nb: 0.100% or less. [Group 3] Ca: 0.0050% or less, Bi: 0.020% or less, and, One or more selected from the group consisting of Te: 0.010% or less.

[0025] [3] [2] The bolt described above, The above group 1 contains, bolt.

[0026] [4] A bolt as described in [2] or [3], The following include the second group: bolt.

[0027] [5] A bolt as described in any one of items [2] to [4], The third group contains, bolt.

[0028] The bolt according to this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass %.

[0029] [Bolt configuration] The bolt of this embodiment has a well-known structure. Figure 1 is a side view showing an example of the bolt of this embodiment. Referring to Figure 1, the bolt of this embodiment comprises a head 10, a shank portion 11, and a shaft portion 12. The shank portion 11 is the part that connects the head 10 and the shaft portion 12, and its surface is curved. That is, the surface of the shank portion 11 has curvature. The shaft portion 12 extends from the shank portion 11 in the direction of the central axis of the bolt. The shaft portion 12 has threads formed on at least a part of its circumferential surface.

[0030] [Features of the bolt in this embodiment] The bolt of this embodiment has the following features: (Feature 1) The chemical composition is as shown below. (Feature 2) The tensile strength TS is 1100 MPa or higher. (Feature 3) The number density ND of the MC-type carbide is 2.0 × 10⁻⁶ 21 pieces / m 3 That's all. (Feature 4) The carbon ratio (Rc) in the MC-type carbide is 0.25 to 0.42. The following describes each of its features.

[0031] [(Feature 1) Chemical Composition] The chemical composition of the bolt according to this embodiment contains the following elements:

[0032] C: 0.30~0.50% Carbon (C) enhances the hardenability of steel and increases the strength of bolts. If the C content is less than 0.30%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the carbon content exceeds 0.50%, the hydrogen embrittlement resistance of the bolt will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.30-0.50%. The preferred lower limit for the C content is 0.32%, and more preferably 0.35%. The preferred upper limit for the C content is 0.48%, and more preferably 0.45%.

[0033] Si: 0.01~0.30% Silicon (Si) enhances the hardenability of steel materials, thereby increasing the strength of bolts. If the Si content is less than 0.01%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 0.30%, the hydrogen embrittlement resistance of the bolt decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.01 to 0.30%. The preferred lower limit for the Si content is 0.02%, and more preferably 0.05%. The preferred upper limit for the Si content is 0.28%, and more preferably 0.26%.

[0034] Mn: 0.10~1.50% Manganese (Mn) enhances the hardenability of steel and increases the strength of bolts. If the Mn content is less than 0.10%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 1.50%, the hydrogen embrittlement resistance of the bolt decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Mn content is 0.10-1.50%. The preferred lower limit for the Mn content is 0.15%, and more preferably 0.20%. The preferred upper limit for the Mn content is 1.30%, more preferably 1.20%, and even more preferably 1.10%.

[0035] P:0.030% or less Phosphorus (P) is an impurity. Therefore, the lower limit for P content is greater than 0%. If the P content exceeds 0.030%, P will segregate at the grain boundaries, even if the content of other elements is within the range of this embodiment. As a result, the hydrogen embrittlement resistance of the bolt decreases. Therefore, the P content is 0.030% or less. A low phosphorus (P) content is preferable. However, an extreme reduction in P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the P content is 0.025%, and more preferably 0.020%.

[0036] S: 0.030% or less Sulfur (S) is an impurity. Therefore, the lower limit for S content is greater than 0%. If the S content exceeds 0.030%, S will segregate at the grain boundaries, even if the content of other elements is within the range of this embodiment. As a result, the hydrogen embrittlement resistance of the bolt decreases. Therefore, the sulfur content is 0.030% or less. A low sulfur (S) content is preferable. However, an extreme reduction in S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for S content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the S content is 0.025%, and more preferably 0.020%.

[0037] Cr: More than 0.80~1.50% Chromium (Cr) enhances the hardenability of steel, thereby increasing the strength of bolts. Furthermore, Cr increases the resistance of steel to tempering and softening, further enhancing the strength of bolts. Assuming that the Cr content is greater than 0.80% and the content of other elements is within the range of this embodiment, the above effects can be effectively obtained. On the other hand, if the Cr content exceeds 1.50%, even if the content of other elements is within the range of this embodiment, grain boundary segregation of P is promoted. As a result, the hydrogen embrittlement resistance of the bolt decreases. Therefore, the Cr content is between 0.80% and 1.50%. The preferred lower limit for the Cr content is 0.82%, and more preferably 0.84%. The preferred upper limit for the Cr content is 1.45%, and more preferably 1.40%.

[0038] Mo: 0.01 to less than 0.50% Molybdenum (Mo) increases the tempering softening resistance of steel, thereby increasing the strength of bolts. Furthermore, Mo concentrates in MC-type carbides, enhancing the hydrogen trapping function of MC-type carbides. As a result, the hydrogen embrittlement resistance of high-strength bolts is improved. If the Mo content is less than 0.01%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content is less than 0.50%, sufficient cold workability can be obtained, provided that the content of other elements is within the range of this embodiment. Therefore, the Mo content is less than 0.01-0.50%. The preferred lower limit for the Mo content is 0.02%, and more preferably 0.05%. The preferred upper limit for the Mo content is 0.45%, and more preferably 0.40%.

[0039] V: 0.01~0.50% Vanadium (V), together with Mo, forms MC-type carbides, thereby enhancing the hydrogen embrittlement resistance of bolts. If the V content is less than 0.01%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the V content exceeds 0.50%, the steel becomes excessively hard, even if the content of other elements is within the range of this embodiment. In this case, the cold workability decreases. Therefore, the V content is 0.01-0.50%. The preferred lower limit for the V content is 0.03%, and more preferably 0.05%. The preferred upper limit for the V content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0040] Al: 0.005~0.100% Aluminum (Al) deoxidizes steel. Al further combines with N to form Al nitride. Al nitride suppresses grain coarsening through a pinning effect. As a result, the hydrogen embrittlement resistance of the bolt is increased. If the Al content is less than 0.005%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.100%, coarse Al nitrides will be formed even if the content of other elements is within the range of this embodiment. Coarse Al nitrides become the starting point for fracture. As a result, the workability of the steel material decreases. Therefore, the Al content is between 0.005% and 0.100%. The preferred lower limit for the Al content is 0.006%, more preferably 0.007%, and even more preferably 0.008%. The preferred upper limit for the Al content is 0.090%, more preferably 0.080%, and even more preferably 0.070%. In the chemical composition of the bolt in this embodiment, the Al content refers to the total Al (Total-Al) content.

[0041] N: 0.0010~0.0300% Nitrogen (N) combines with Al to form Al nitride. Al nitride suppresses grain coarsening through a pinning effect. As a result, the hydrogen embrittlement resistance of the bolt is increased. If the N content is less than 0.0010%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the N content exceeds 0.0300%, coarse nitrides will be formed even if the content of other elements is within the range of this embodiment. Coarse nitrides become the starting point for fracture and reduce the workability of the steel material. Therefore, the N content is between 0.0010% and 0.0300%. The preferred lower limit for the N content is 0.0020%, more preferably 0.0025%, and even more preferably 0.0030%. The preferred upper limit for the N content is 0.0290%, more preferably 0.0280%, more preferably 0.0270%, more preferably 0.0250%, more preferably 0.0200%, more preferably 0.0150%, and more preferably 0.0100%.

[0042] The remainder of the chemical composition of the bolt according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are introduced during the industrial manufacture of the bolt from raw materials such as ore, scrap, or the manufacturing environment, and are acceptable within a range that does not adversely affect the bolt according to this embodiment.

[0043] [Optional Elements] The chemical composition of the bolt in this embodiment may further contain, in place of a portion of Fe, one or more substances selected from the following groups 1 to 3. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Zr: 0.300% or less, Hf: 0.100% or less, Ta: 0.100% or less, and, One or more selected from the group consisting of W: 0.20% or less. [Group 2] Ti: 0.100% or less, and, One or more selected from the group consisting of Nb: 0.100% or less. [Group 3] Ca: 0.0050% or less, Bi: 0.020% or less, and, One or more selected from the group consisting of Te: 0.010% or less. The elements in groups 1 through 3 are all arbitrary elements. The following describes groups 1 through 3.

[0044] [Regarding Group 1 (Cu, Ni, B, Zr, Hf, Ta, and W)] The chemical composition of the bolt according to this embodiment may further include one or more elements selected from the group consisting of Cu, Ni, B, Zr, Hf, Ta, and W in place of some of the Fe. Any of these elements are optional and may not be included. If included, Cu, Ni, B, Zr, Hf, Ta, and W enhance the hardenability of the steel and increase the strength of the bolt.

[0045] Cu: 0.40% or less Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. When copper (Cu) is present, that is, when the Cu content is greater than 0%, the Cu enhances the hardenability of the steel, thereby increasing the strength of the bolt. Even a small amount of Cu will provide some degree of the above effect. However, if the Cu content exceeds 0.40%, the hardenability becomes too high, even if the content of other elements is within the range of this embodiment. As a result, the workability of the steel decreases. Therefore, the Cu content is 0-0.40%, and if present, the Cu content is 0.40% or less. The preferred lower limit for the Cu content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Cu content is 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0046] Ni: 0.40% or less Nickel (Ni) is an optional element and does not need to be included. In other words, the Ni content may be 0%. When nickel is present, that is, when the nickel content is greater than 0%, the nickel improves the hardenability of the steel and increases the strength of the bolt. Even if only a small amount of nickel is present, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.40%, the hardenability becomes too high, even if the content of other elements is within the range of this embodiment. As a result, the workability of the steel decreases. Therefore, the Ni content is 0-0.40%, and if present, the Ni content is 0.40% or less. The preferred lower limit for the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Ni content is 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0047] B: 0.0100% or less Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When B is present, that is, when the B content is greater than 0%, B increases the hardenability of the steel and thus the strength of the bolt. Furthermore, B suppresses grain boundary segregation of P, thereby improving the hydrogen embrittlement resistance of the bolt. Even if only a small amount of B is present, the above effects can be obtained to some extent. However, if the B content exceeds 0.0100%, coarse B nitrides will be formed, even if the content of other elements is within the range of this embodiment. These coarse B nitrides become the starting point for fracture. As a result, the workability of the steel material decreases. Therefore, the B content is between 0 and 0.0100%, and if present, the B content is 0.0100% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The preferred upper limit for the B content is 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and even more preferably 0.0050%.

[0048] Zr: 0.300% or less Zirconium (Zr) is an optional element and does not need to be included. In other words, the Zr content may be 0%. If zinc (Zr) is present, that is, if the Zr content is greater than 0%, Zr enhances the hardenability of the steel and increases the strength of the bolt. Even if only a small amount of Zr is present, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.300%, coarse Zr nitrides will be formed, even if the content of other elements is within the range of this embodiment. These coarse Zr nitrides become the starting point for fracture. As a result, the workability of the steel material decreases. Therefore, the Zr content is between 0 and 0.300%, and if present, the Zr content is 0.300% or less. The preferred lower limit of the Zr content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit for the Zr content is 0.280%, more preferably 0.250%, more preferably 0.200%, more preferably 0.150%, and more preferably 0.100%.

[0049] Hf:0.100% or less Hafnium (Hf) is an optional element and does not need to be included. In other words, the Hf content may be 0%. When Hf is present, that is, when the Hf content is greater than 0%, Hf increases the hardenability of the steel and thus increases the strength of the bolt. Even if only a small amount of Hf is present, the above effect can be obtained to some extent. However, if the Hf content exceeds 0.100%, coarse Hf nitrides will be formed, even if the content of other elements is within the range of this embodiment. These coarse Hf nitrides become the starting point for fracture. As a result, the workability of the steel material decreases. Therefore, the Hf content is between 0 and 0.100%, and if present, the Hf content is 0.100% or less. The preferred lower limit of the Hf content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Hf content is 0.080%, more preferably 0.070%, more preferably 0.060%, and still more preferably 0.050%.

[0050] Ta:0.100% or less Tantalum (Ta) is an optional element and does not need to be included. In other words, the Ta content may be 0%. When Ta is present, that is, when the Ta content is greater than 0%, Ta increases the hardenability of the steel and thus increases the strength of the bolt. Even if only a small amount of Ta is present, the above effect can be obtained to some extent. However, if the Ta content exceeds 0.100%, coarse Ta nitrides will be formed, even if the content of other elements is within the range of this embodiment. These coarse Ta nitrides become the starting point for fracture. As a result, the workability of the steel material decreases. Therefore, the Ta content is between 0 and 0.100%, and if present, the Ta content is 0.100% or less. The preferred lower limit for the Ta content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Ta content is 0.080%, more preferably 0.070%, even more preferably 0.060%, and even more preferably 0.050%.

[0051] W: 0.20% or less Tungsten (W) is an optional element and does not need to be included. In other words, the W content may be 0%. When W is present, that is, when the W content is greater than 0%, W increases the hardenability of the steel and thus increases the strength of the bolt. Even if only a small amount of W is present, the above effect can be obtained to some extent. However, if the W content exceeds 0.20%, the hardenability becomes too high, even if the content of other elements is within the range of this embodiment. As a result, the workability of the steel decreases. Therefore, the W content is 0-0.20%, and if present, the W content is 0.20% or less. The preferred lower limit for the W content is 0.01%, and more preferably 0.02%. The preferred upper limit for the W content is 0.15%, more preferably 0.12%, and even more preferably 0.10%.

[0052] [Regarding Group 2 (Ti and Nb)] The chemical composition of the bolt according to this embodiment may further include one or more elements selected from the group consisting of Ti and Nb in place of a portion of Fe. Any of these elements are optional and may not be included. If included, Ti and Nb form precipitates and refine the crystal grains. As a result, the hydrogen embrittlement resistance of the bolt is increased.

[0053] Ti:0.100% or less Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. When Ti is present, that is, when the Ti content is greater than 0%, the Ti forms fine precipitates such as Ti carbides, which refine the crystal grains. As a result, the hydrogen embrittlement resistance of the bolt increases. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. However, if the Ti content is too high, coarse Ti nitrides will form even if the content of other elements is within the range of this embodiment. These coarse Ti nitrides become the starting point for fracture. As a result, the workability of the steel material decreases. Therefore, the Ti content is between 0 and 0.100%, and if present, the Ti content is 0.100% or less. The preferred lower limit for the Ti content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Ti content is 0.090%, more preferably 0.080%, and even more preferably 0.075%.

[0054] Nb: 0.100% or less Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. When Nb is present, that is, when the Nb content is greater than 0%, the Nb forms fine precipitates such as Nb carbides, which refine the crystal grains. As a result, the hydrogen embrittlement resistance of the bolt increases. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. However, if the Nb content is too high, coarse Nb carbides and the like will be formed, even if the content of other elements is within the range of this embodiment. These coarse Nb carbides and the like become the starting point for fracture. As a result, the workability of the steel material decreases. Therefore, the Nb content is between 0 and 0.100%, and if present, the Nb content is 0.100% or less. The preferred lower limit of the Nb content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit for the Nb content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0055] [Regarding Group 3 (Ca, Bi, and Te)] The chemical composition of the bolt according to this embodiment may further include one or more elements selected from the group consisting of Ca, Bi, and Te in place of a portion of Fe. Any of these elements are optional and may not be included. If included, Ca, Bi, and Te improve the machinability of the steel material.

[0056] Ca: 0.0050% or less Calcium (Ca) is an optional element and does not need to be included. In other words, the Ca content may be 0%. When calcium (Ca) is present, that is, when the Ca content is greater than 0%, the Ca improves the machinability of the steel. Even a small amount of Ca will provide some degree of this effect. However, if the Ca content exceeds 0.0050%, the workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Ca content is between 0 and 0.0050%, and if present, the Ca content is 0.0050% or less. The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit for the Ca content is 0.0040%, and more preferably 0.0030%.

[0057] Bi:0.020% or less Bismuth (Bi) is an optional element and does not need to be included. In other words, the Bi content may be 0%. When Bi is present, that is, when Bi is greater than 0%, Bi improves the machinability of the steel. Even if only a small amount of Bi is present, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.020%, the workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Bi content is 0-0.020%, and if present, the Bi content is 0.020% or less. The preferred lower limit of the Bi content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Bi content is 0.018%, and more preferably 0.015%.

[0058] Te: 0.010% or less Tellurium (Te) is an optional element and does not need to be included. In other words, the Te content may be 0%. When Te is present, that is, when Te is greater than 0%, Te improves the machinability of the steel. Even if only a small amount of Te is present, the above effect can be obtained to some extent. However, if the Te content exceeds 0.010%, the workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Te content is 0-0.010%, and if present, the Te content is 0.010% or less. The preferred lower limit for the Te content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Te content is 0.009%, and more preferably 0.008%.

[0059] If the chemical composition of the bolt in this embodiment includes not only essential elements but also optional elements, the chemical composition of the bolt will contain the essential elements and one or more elements selected from the groups consisting of the first to third groups, with the remainder being Fe and impurities. For example, the chemical composition of the bolt in this embodiment may contain the essential elements and one or more elements selected from the first group (Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Zr: 0.300% or less, Hf: 0.100% or less, Ta: 0.100% or less, and W: 0.20% or less), with the remainder being Fe and impurities. The chemical composition of the bolt in this embodiment may also contain the essential elements and one or more elements selected from the second group (Ti: 0.100% or less, and Nb: 0.100% or less), with the remainder being Fe and impurities. The chemical composition of the bolt in this embodiment may include essential elements and one or more elements selected from the third group (Ca: 0.0050% or less, Bi: 0.020% or less, and Te: 0.010% or less), with the remainder being Fe and impurities. The chemical composition of the bolt in this embodiment may include essential elements and elements from the first and second groups, with the remainder being Fe and impurities. The chemical composition of the bolt in this embodiment may include essential elements and elements from the first and third groups, with the remainder being Fe and impurities. The chemical composition of the bolt in this embodiment may include essential elements and elements from the second and third groups, with the remainder being Fe and impurities. The chemical composition of the bolt in this embodiment may include essential elements and elements from the first, second, and third groups, with the remainder being Fe and impurities.

[0060] [Method for measuring the chemical composition of bolts] The chemical composition of the bolt in this embodiment can be measured using a well-known component analysis method in accordance with JIS G0321:2017. Specifically, chips are collected from the inside of the bolt shaft at a depth of 1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. Elemental analysis of the chemical composition is performed on the solution using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). The C and S content is determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas melting-thermal conductivity method.

[0061] Furthermore, the content of each element shall be rounded to the minimum digit of the element content specified in this embodiment, based on the significant figures defined in this embodiment. For example, the carbon content of the bolt in this embodiment is defined to two decimal places. Therefore, the carbon content shall be the value obtained by rounding the third decimal place of the measured value to two decimal places.

[0062] Similarly, for the elemental content of bolts other than the C content in this embodiment, the value obtained by rounding the measured value to the minimum digit specified in this embodiment is defined as the elemental content.

[0063] Rounding means that if the fractional part is less than 5, it is rounded down, and if the fractional part is 5 or greater, it is rounded up.

[0064] [(Feature 2) Tensile strength TS] The bolt according to this embodiment has a tensile strength TS of 1100 MPa or higher. The bolt according to this embodiment has elemental content within the range of this embodiment (feature 1) in its chemical composition, and further possesses features 3 and 4. As a result, the bolt according to this embodiment provides excellent hydrogen embrittlement resistance even with a tensile strength TS of 1100 MPa or higher.

[0065] The preferred lower limit of the tensile strength TS is 1150 MPa, and more preferably 1200 MPa. The upper limit of the tensile strength TS is not particularly limited. For example, the upper limit of the tensile strength TS of the bolt according to this embodiment is 1600 MPa, and for example, 1550 MPa.

[0066] [Method for measuring tensile strength (TS)] In this embodiment, the tensile strength TS can be determined by the following method. A tensile test is performed in air at room temperature (20±15℃) in accordance with JIS Z 2241:2011 to obtain the tensile strength TS (MPa). The tensile test specimen is taken so as to include the shaft portion of the bolt, and the central axis of the tensile test specimen is coaxial with the shaft portion of the bolt.

[0067] [(Feature 3) Number density of MC-type carbides: ND] In the bolt of this embodiment, the number density ND of MC-type carbides is further 2.0 × 10 21 pieces / m 3 That's all.

[0068] MC-type carbides are finer in size compared to other alloying elements such as cementite and MC2-type carbides. Therefore, MC-type carbides trap hydrogen more easily than other alloying carbides. Accordingly, in the bolt of this embodiment, the number density ND of MC-type carbides is increased to raise the limiting hydrogen content of the bolt.

[0069] The number density ND of the MC-type carbide is 2.0 × 10⁻⁶. 21 pieces / m 3 Assuming the above conditions are met, and assuming the other features 1, 2, and 4 are also satisfied, excellent hydrogen embrittlement resistance can be obtained.

[0070] The preferred lower limit of the number density ND of MC-type carbides is 2.2 × 10⁻⁶. 21 pieces / m 3 And more preferably 2.4 × 10 21 pieces / m 3 And more preferably 2.6 × 10 21 pieces / m 3Therefore, there is no particular upper limit to the number density ND of MC-type carbides. However, if the content of each element in the chemical composition of the bolt is within the range of this embodiment, the upper limit to the number density ND of MC-type carbides is, for example, 200.0 × 10 21 pieces / m 3 And more preferably 100.0 × 10 21 pieces / m 3 That is the case.

[0071] [Method for measuring the number density (ND) of MC-type carbides] The number density ND of MC-type carbides in a bolt can be measured using a scanning transmission electron microscope (STEM) in the following way. First, a thin film sample for STEM is prepared using the following method: The lower part of the bolt neck is cut perpendicular to the axial direction (longitudinal direction) of the bolt, and a disc is taken from the lower part of the bolt neck, with the center of the circle at a depth of 1.5 mm from the surface of the lower part of the bolt neck and an axial thickness of approximately 2 mm (i.e., a disc with a diameter of 3.0 mm and a thickness of approximately 2 mm). Both sides (front and back) of the disc are polished using emery paper. At this time, the front surface of the disc is polished so that it is parallel to the back surface. One of the front and back surfaces of the disc is defined as the observation surface. The observation surface is further polished to a mirror finish. The mirror-polished observation surface is further polished using colloidal silica as an abrasive.

[0072] MC-type carbides have a specific crystal orientation relationship with respect to the matrix (Fe). Specifically, MC-type carbides are plate-like particles that extend along the {100} plane of the matrix. Therefore, electron beam backscatter diffraction (EBSD) is performed on the observation surface after polishing to determine the crystal orientation of the matrix. Then, based on the determined crystal orientation of the matrix, the direction perpendicular to the observation surface of the thin film (observation direction) is determined to be the same as the matrix. <001> Thin film samples for STEM are prepared by performing focused ion beam (FIB) processing on a disk to achieve the desired crystal orientation.

[0073] The preparation of thin film samples for STEM by FIB processing can be carried out using well-known methods. For example, thin film samples for STEM can be prepared using the lift-out method with a gallium (Ga) ion beam at an accelerating voltage of 30 kV.

[0074] Thin film samples for STEM fabricated using an ion beam with an acceleration voltage of 30 kV have dislocation loops and amorphous regions on their surface, making them unsuitable for observing MC-type carbides of several nanometers. Therefore, the surface of the thin film sample for STEM is polished using an ion beam with a low acceleration voltage of 1 kV or less. Through this manufacturing process, thin film samples for STEM with a thickness of 100 nm or less are prepared.

[0075] The fabricated thin film sample is observed using a STEM optical system. Specifically, the crystal orientation of the martensite matrix in the thin film sample for STEM is observed. <001> The thin film sample for STEM is tilted so that the crystal zone axis is incident. The observation magnification is set to 320,000x and the acceleration voltage to 300kV. The detector is set to known high-angle annular dark-field (HAADF), known low-angle annular dark-field (LAADF), and known bright-field (BF) conditions, and 10 arbitrary observation fields are observed. Images are produced in each observation field under these observation conditions. Of the images generated in each observation field, the image in which the MC-type carbide is most clearly recognized is selected.

[0076] Each observation field will have an area of ​​280 nm × 280 nm. The thickness of the STEM thin film sample will be measured in all observation fields using the Log-ratio method of electron energy loss spectroscopy (EELS).

[0077] In the images taken in each observation field, precipitates can be identified by contrast. Therefore, among the identified precipitates, those with a maximum length of 2 nm or more are selected. Here, "maximum length" in STEM observation refers to the maximum line segment length when any two points at the interface between the precipitate and the matrix are selected, and the entire line segment connecting those two points is included within the precipitate. Precipitates with a maximum length of less than 2 nm are extremely difficult to identify. Therefore, in this embodiment, precipitates with a maximum length of 2 nm or more are selected.

[0078] Among precipitates with a maximum length of 2 nm or more, MC-type carbides are identified by the following method: Precipitates with a maximum length of 2 nm or more are irradiated with an electron beam to obtain electron diffraction patterns. MC-type carbides and other precipitates have different electron diffraction patterns. Therefore, based on the obtained electron diffraction patterns, MC-type carbides are identified from among the identified precipitates.

[0079] Based on the total number of MC-type carbides identified in all observation fields using the method described above, and the total volume calculated from all observation fields and thicknesses, the number density of MC-type carbides ND (pieces / m³) is determined. 3 )

[0080] Furthermore, when the content of each element in the chemical composition was within the range of this embodiment, STEM observation yielded electron diffraction patterns of precipitates, and precipitates with a maximum length of 10 nm or less were almost entirely MC-type carbides, with very few other precipitates present. The maximum lengths of other precipitates, such as M2C-type carbides and cementite, all significantly exceeded 10 nm. Therefore, instead of the electron diffraction pattern, precipitates with a maximum length of 10 nm or less (i.e., a maximum length of 2 to 10 nm) may be identified as MC-type carbides.

[0081] [(Feature 4) Ratio of C in MC-type carbides (Rc)] In the bolts of this embodiment, the carbon ratio Rc in the MC-type carbide is further 0.25 to 0.42. Here, the carbon ratio Rc refers to the proportion of carbon atoms to the total number of carbon, molybdenum, and volatile atoms in the MC-type carbide. The carbon ratio Rc is defined as follows:

[0082] The atomic percentages of C, Mo, and V content in MC-type carbides are defined as follows: [C]: C content in MC-type carbide in atomic percent [Mo]: Mo content in atomic percent of MC-type carbide [V]: V content in atomic percent of MC-type carbide In this case, the carbon ratio Rc in the MC-type carbide is defined by the following equation (1). Rc = [C] / ([C] + [Mo] + [V]) (1) In the bolt of this embodiment, the carbon ratio Rc in the MC-type carbide is 0.25 to 0.42.

[0083] Figure 2 shows that the content of each element in the chemical composition is within the range of this embodiment, the tensile strength TS is 1100 MPa or higher, and the number density ND of the MC-type carbide is 2.0 × 10⁻⁶. 21 pieces / m 3 The graph above shows the relationship between the carbon ratio Rc in the MC-type carbide and the critical hydrogen content (mass ppm) in the bolt. Figure 2 was obtained by the test method described in the examples below.

[0084] As mentioned above, a higher critical hydrogen content indicates superior hydrogen trapping function and greater resistance to hydrogen embrittlement. Referring to Figure 2, when the C ratio Rc is 0.42 or less, the critical hydrogen content is significantly higher compared to when the C ratio Rc is greater than 0.42.

[0085] Therefore, in the bolt of this embodiment, the carbon ratio Rc in the MC-type carbide is 0.42 or less.

[0086] Furthermore, there is no particular lower limit to the carbon ratio Rc in MC-type carbides. However, there is a limit to the carbon ratio Rc in MC-type carbides due to the chemical composition. In the case of bolt with the above chemical composition, the lower limit to the carbon ratio Rc in MC-type carbides is 0.25.

[0087] The preferred upper limit for the C ratio Rc in the MC-type carbide is 0.41, and more preferably 0.40. The preferred lower limit for the C ratio Rc is 0.26, more preferably 0.27, more preferably 0.28, and still more preferably 0.29.

[0088] The mechanism by which the critical hydrogen content increases when the carbon ratio (Rc) in MC-type carbides is between 0.25 and 0.42 is unclear. However, the following mechanism is possible.

[0089] MC-type carbides typically have a 1:1 atomic ratio of carbon (C) to metal (M). In the bolt of this embodiment having the chemical composition of Feature 1, most of the metal elements in the MC-type carbide are V and Mo. Therefore, the ratio of the number of C atoms to the total number of Mo and V atoms in the MC-type carbide is usually 1:1. In other words, in typical MC-type carbides, the C ratio Rc is around 0.50.

[0090] In contrast, in the bolt of this embodiment, the carbon ratio Rc in the MC-type carbide is set to 0.42 or less. When the carbon ratio Rc is 0.42 or less, some of the carbon sites in the MC-type carbide become voids. In MC-type carbides with lattice defects in such carbon sites, the hydrogen trapping function is enhanced compared to M2C-type carbides or MC-type carbides without lattice defects. Therefore, it is thought that if the carbon ratio Rc in the MC-type carbide is 0.42 or less, the critical hydrogen amount increases significantly, and the hydrogen embrittlement resistance of the bolt is enhanced.

[0091] Since the mechanism described above is a hypothesis, it is possible that the critical hydrogen content is increased by a different mechanism. However, the content of each element in the chemical composition is within the range of this embodiment, the tensile strength TS is 1100 MPa or higher, and the number density ND of the MC-type carbide is 2.0 × 10⁻⁶. 21 pieces / m 3 As demonstrated in the examples described later, if the carbon ratio Rc in the MC-type carbide of the bolt described above is 0.42 or less, the critical hydrogen content increases.

[0092] [Method for measuring the C ratio (Rc)] The carbon ratio (Rc) of MC-type carbides can be determined by the following method: A sample is cut from the inside of the bolt, at a depth of 1 mm or more from the surface of the lower part of the neck. A needle-shaped test piece with a tip radius of curvature of about 50 nm is prepared by performing a well-known focused ion beam processing or electropolishing on the cut sample.

[0093] Three-dimensional atom probe analysis will be performed on needle-shaped test specimens. Specifically, MC-type carbides in the needle-shaped test specimens will be identified using three-dimensional atom probe analysis. Three-dimensional atom probe analysis allows for the three-dimensional detection of precipitates present in the needle-shaped test specimens.

[0094] In three-dimensional atom probe analysis, the laser wavelength (λ) is set to 355 nm, the laser power to 30 pJ, and the temperature of the needle-shaped test specimen to 50 K. The apparatus used for three-dimensional atom probe analysis is not particularly limited. For example, a three-dimensional atom probe analyzer is the LEAP4000XHR manufactured by AMETEK Corporation.

[0095] A three-dimensional atomic map is obtained by reconstructing the acquired measurement data. Specifically, by using the detection efficiency of the instrument and adjusting the spacing of the {110} atomic planes to 0.20 nm in the measurement of iron (Fe), the measurement data is reconstructed to obtain a three-dimensional atomic map.

[0096] In a three-dimensional atomic map, the region of a needle-shaped specimen analyzed using three-dimensional atom probes is divided into tiny cubes called voxels. Each side of a voxel is 1.0 nm long. The elemental concentration (atomic %) within a voxel is defined as the number of atoms of that element contained within the voxel divided by the total number of atoms of all elements contained within the voxel.

[0097] The sum of the C, Mo, and V concentrations in each voxel is defined as the concentration of the specific element (atomic %). An isoconcentration surface is created by connecting voxels where the concentration of the specific element is 6%. The region enclosed by the isoconcentration surface has a high concentration of the specific element. The region enclosed by the isoconcentration surface is identified as a precipitate.

[0098] Among the identified precipitates, those with a maximum length of 2 to 10 nm are identified. Here, a line segment connecting any two points on the surface of the precipitate detected in three dimensions (i.e., the interface between the precipitate and the steel matrix) that is entirely contained within the precipitate is defined as a "specific line segment." The maximum length of this specific line segment of the precipitate is defined as the maximum length of the precipitate. Precipitates with a maximum length of 10 nm or less are identified as MC-type carbides.

[0099] As described above, when the content of each element in the chemical composition is within the range of this embodiment, STEM observation reveals that precipitates in the bolt with a maximum length of 2 to 10 nm are almost entirely MC-type carbides, and there are almost no other precipitates besides MC-type carbides. Furthermore, the maximum lengths of other precipitates besides MC-type carbides, such as M2C-type carbides and cementite, all significantly exceed 10 nm. Therefore, precipitates in needle-shaped test pieces obtained by three-dimensional atom probe analysis with a maximum length of 10 nm or less are identified as MC-type carbides.

[0100] The three-dimensional atomic map also allows us to determine the types of elements contained in the identified MC-type carbides, as well as the number of atoms of each element. Therefore, we select 20 arbitrary MC-type carbides from among the identified multiple MC-type carbides. Then, we determine the C content [C] (atomic %), Mo content [Mo] (atomic %), and V content [V] (atomic %) of the elements contained in each selected MC-type carbide. Based on the obtained [C], [Mo], and [V], we calculate the C ratio Rc of each MC-type carbide using equation (1). The arithmetic mean of the calculated C ratio Rc in each MC-type carbide is defined as the C ratio Rc in the MC-type carbide.

[0101] [Regarding the effects of the bolts in this embodiment] As described above, the bolt of this embodiment has the following features. (Feature 1) The chemical composition is within the range of this embodiment. (Feature 2) The tensile strength TS is 1100 MPa or higher. (Feature 3) The number density ND of the MC-type carbide is 2.0 × 10⁻⁶ 21 pieces / m 3That's all. (Feature 4) The carbon ratio (Rc) in the MC-type carbide is 0.25 to 0.42. In the bolt of this embodiment having the above-described features 1 to 4, excellent hydrogen embrittlement resistance can be obtained even when the tensile strength TS is high, at 1100 MPa or more.

[0102] As mentioned above, the critical hydrogen content can be used as an indicator of hydrogen embrittlement resistance. A higher critical hydrogen content indicates superior hydrogen embrittlement resistance. The critical hydrogen content refers to the upper limit of hydrogen content at which the steel material does not undergo delayed fracture.

[0103] In the bolt of this embodiment, the critical hydrogen content is 1.5 ppm by mass or more. The preferred lower limit of the critical hydrogen content is 2.0 ppm by mass, and more preferably 2.5 ppm by mass.

[0104] [Method for measuring critical hydrogen content] The critical hydrogen content can be determined by the following method: A round bar specimen with an annular notch (hereinafter simply referred to as a round bar specimen) is taken from a depth of 1 mm or more inside the bolt surface. The size of the round bar specimen is not particularly limited, but for example, the diameter of the parallel section is 7 mm and the length is 70 mm. An annular notch extending in the circumferential direction is formed at the center of the longitudinal direction of the round bar specimen. The radius of curvature (R) at the bottom of the notch is 0.175 mm.

[0105] Multiple round bar test specimens are prepared. Hydrogen is charged to the round bar test specimens under various conditions using the cathode hydrogen charging method. The cathode hydrogen charging method is as follows: Prepare a room temperature aqueous solution (cathode charging solution) by adding 0 to 20 g of ammonium thiocyanate to 1 L of 3 mass% sodium chloride aqueous solution. Immerse the round bar test specimens in the cathode charging solution and maintain a cathode current density of 0.03 to 1.00 mA / cm² for 72 hours. 2 A controlled constant current is generated within a specified range to add hydrogen to the round bar specimen. The amount of hydrogen in the round bar specimen is adjusted by controlling the concentration of ammonium thiocyanate in the cathode charging solution and the cathode current density.

[0106] After performing the cathode hydrogen charging method, the round bar specimens are left at room temperature for 96 hours. Then, a zinc plating film is formed on the surface of the hydrogen-charged round bar specimens to prevent hydrogen from leaking out. A constant load test is performed on the round bar specimens at room temperature and atmospheric pressure, applying a constant load equivalent to 90% of the tensile strength TS. The test duration is a maximum of 100 hours; if the round bar specimen withstands for 100 hours or more without fracture, the test is stopped. The constant load test is performed using multiple round bar specimens hydrogen-charged under various conditions.

[0107] After 100 hours, the amount of hydrogen in the unbroken round bar specimen is determined. Specifically, first, the zinc plating film on the surface of the round bar specimen is removed. After removing the zinc plating film, a temperature-induced desorption hydrogen analysis is performed on the round bar specimen using gas chromatography. In the temperature-induced desorption hydrogen analysis, the temperature is increased from room temperature to 600°C at a rate of 100°C / hour. The amount of hydrogen released from the round bar specimen during the temperature increase is measured. The measured amount of hydrogen (mass ppm) is taken as the amount of hydrogen (mass ppm) that was charged into the round bar specimen.

[0108] The maximum amount of hydrogen in a round bar specimen that did not fracture during a 100-hour constant load test is defined as the "critical hydrogen amount" (mass ppm).

[0109] [Microstructure of a bolt] The microstructure of the bolt in this embodiment includes hard tissue accounting for 90% or more of the area. The microstructure of the bolt referred to here means the microstructure of a bolt manufactured by quenching and tempering in the manufacturing process described later. The hard tissue consists of martensite and / or bainite. If the bolt's microstructure includes other phases besides the hard tissue, the remainder of the bolt's microstructure consists of one or more selected from the group consisting of retained austenite, ferrite, and pearlite. Preferably, the microstructure contains 90% or more of hard tissue by area, with the remainder consisting of retained austenite. The tensile strength of the bolt correlates with the microstructure. When the tensile strength of the bolt is 1100 MPa or higher, the area ratio of hard tissue in the bolt's microstructure is 90% or more.

[0110] [Manufacturing method] An example of a bolt manufacturing method according to this embodiment will be described. The bolt manufacturing method described below is just one example for manufacturing a bolt according to this embodiment. Therefore, a bolt having the above-described configuration may be manufactured by a manufacturing method other than the one described below. However, the manufacturing method described below is a preferred example of a bolt manufacturing method according to this embodiment.

[0111] An example of a bolt manufacturing method according to this embodiment includes the following steps. (Process 1) Steel material preparation process (Process 2) Bolt manufacturing process The following describes each step.

[0112] [(Process 1) Steel material preparation process] In the steel material preparation process, steel materials to be used as the material for bolts (steel for bolts) are prepared. The steel for bolts may be manufactured. Alternatively, a third party may prepare the steel for bolts. In the steel material preparation process, steel materials whose chemical composition satisfies characteristic 1 are prepared.

[0113] When manufacturing steel materials for bolts, for example, the steel materials may be manufactured by the following method: First, molten steel is produced in which the content of each element in the chemical composition is within the range of this embodiment. The raw material is then manufactured using the molten steel. For example, a bloom (cast slab), which is the raw material, may be manufactured using the molten steel by a continuous casting method, or an ingot, which is the raw material, may be manufactured using the ingot-making method. The manufactured raw material (bloom or ingot) is subjected to rough rolling (blob rolling, or blob rolling and hot rolling in a continuous rolling mill) to produce a billet.

[0114] Steel materials (steel for bolts) are manufactured by performing finish rolling on billets using a continuous rolling mill. The steel materials are, for example, steel bars or wire rods.

[0115] [(Process 2) Bolt Manufacturing Process] In the bolt manufacturing process, bolts are manufactured using the aforementioned steel materials (steel materials for bolts). The bolt manufacturing process includes the following steps: (21) Wire drawing process (22) Bolt forming process (23) Heat treatment process The following describes each step.

[0116] [(21) Wire drawing process] In the wire drawing process, steel wire is manufactured by performing the well-known wire drawing process on the aforementioned steel material. The wire drawing process may consist of primary drawing only, or it may involve multiple wire drawing processes such as secondary drawing.

[0117] [(22) Bolt forming process] In the bolt forming process, the steel wire after the wire drawing process is cut to a predetermined length, and the cut steel wire is cold forged, rolled, or machined to produce an intermediate bolt with a head, neck, and shaft.

[0118] The bolt forming process includes the following steps: (221) Head forming process (222) Thread forming process The following describes each step.

[0119] [(221) Head forming process] In the head forming process, cold forging is performed on the steel wire to form an intermediate bolt (intermediate bolt). Specifically, first, the steel wire is cut to a predetermined length. Then, using a punch and die, the steel wire is press-formed to form an intermediate bolt having a head 10, a neck portion 11, and a shaft portion 12.

[0120] During the head forming process, strain is introduced into the bolt intermediate product. This strain becomes the driving force for generating MC-type carbides in the subsequent heat treatment process. The strain introduced into the intermediate product during the head forming process will be described later.

[0121] [(222) Thread forming process] In the thread forming process, a threaded portion is formed on at least a part of the shaft portion 12 of the bolt intermediate product. In the thread forming process, the threaded portion may be formed on at least a part of the shaft portion 12 by the well-known rolling method. Alternatively, instead of rolling, the threaded portion may be formed on at least a part of the shaft portion 12 by cutting. As will be described later, the thread forming process may be performed after the heat treatment process, rather than after the head forming process and before the heat treatment process.

[0122] [(23) Heat treatment process] In the heat treatment process, the following steps are performed on the intermediate product after the bolt forming process. (231) Quenching process (232) Tempering process The following describes each step.

[0123] [(231) Quenching process] In the quenching process, a well-known quenching method is performed. The quenching temperature and the holding time at the quenching temperature are not particularly limited. For example, the quenching temperature is 870 to 970°C. The holding time at the quenching temperature is, for example, 15 minutes to 240 minutes (4 hours). After the holding time, the intermediate bolts are rapidly cooled. Specifically, the intermediate bolts are water-cooled or oil-cooled.

[0124] [(232) Tempering process] In the tempering process, the intermediate bolts after the quenching process are tempered. The conditions for the tempering process are as follows: (Condition 1) Tempering temperature T: 570~650℃ (Condition 2) Holding time at tempering temperature t: 0.5~5.0 hours (Condition 3) Tempering parameter Fn: -22000~3500 Here, the tempering parameter Fn is defined by equation (A). Fn=(ε -0.5 )×((T+273)×(20+Log(t))×((Mo / 96) / (Mo / 96+V / 51))×(1+(Mo / 96) / (V / 51+Mo / 96))-25000) (A) In equation (A), ε is substituted for the strain applied to the underside of the bolt neck during the bolt forming process. T is substituted for the tempering temperature (°C). t is substituted for the holding time at the tempering temperature (hours). Mo is substituted for the Mo content (mass%) in the bolt. V is substituted for the V content (mass%) in the bolt.

[0125] The strain ε applied to the underside of the bolt neck during the bolt forming process is determined by the following finite element method (FEM analysis). Specifically, a 1 / 6 scale axisymmetric model is created for the bolt under analysis based on its shape symmetry. This 1 / 6 axisymmetric model is then divided into multiple elements using tetrahedral linear elements, with each side of the tetrahedron ranging from 0.05 to 1.00 mm.

[0126] Three-dimensional elastoplastic analysis using FEM is performed on the model divided into the aforementioned number of elements. A commercially available forging analysis code can be used for the analysis. For example, a commercially available forging analysis code is DEFORM-3D.

[0127] The workpiece (steel for bolts) is assumed to be elastoplastic, and the die is assumed to be rigid. The flow stress of the workpiece is taken from the measured value of spheroidized annealed SCM435 as specified in JIS G 4053:2016. The material properties of the workpiece are set to Young's modulus: 210 GPa and Poisson's ratio: 0.30. The die is set to a sufficiently large size relative to the workpiece. The Coulomb friction coefficient between the workpiece and the die is set to μ = 0.05. An FEM analysis is performed under the above conditions to determine the strain ε applied to the lower part of the bolt neck.

[0128] The following describes conditions 1 to 3 for the tempering process.

[0129] [Regarding tempering temperature T and holding time t] The tempering temperature T is 570°C to 650°C. If the tempering temperature T is less than 570°C, the number density ND of MC-type carbides is 2.0 × 10⁻⁶. 21 pieces / m 3 It will be less than 1100 MPa. If the tempering temperature T is greater than 650°C, the tensile strength TS will be less than 1100 MPa. Furthermore, the holding time t at the tempering temperature T is between 0.5 hours and 5.0 hours. If the holding time t is less than 0.5 hours, the number density ND of MC-type carbides is 2.0 × 10⁻⁶. 21 pieces / m 3 It will be less than 1100 MPa. If the holding time t exceeds 5.0 hours, the tensile strength TS will be less than 1100 MPa.

[0130] [Regarding the tempering parameter Fn] In the tempering process, the tempering parameter Fn, defined by equation (A), is between -22000 and 3500. Fn is a conditional expression relating to the composition and amount of MC-type carbides produced. If Fn exceeds 3500, the number density ND of the produced MC-type carbides is 2.0 × 10⁻⁶. 21 pieces / m 3 It will be less than -22000. On the other hand, if Fn is less than -22000, the number density ND of MC-type carbides is 2.0 × 10⁻⁶. 21 pieces / m 3 Although the above is true, the carbon ratio (Rc) in the MC-type carbide exceeds 0.42.

[0131] If Fn is between -22000 and 3500, then in a bolt having a chemical composition that satisfies characteristic 1, the number density ND of MC-type carbides is 2.0 × 10⁻⁶. 21 pieces / m 3 As described above (Feature 3), the carbon ratio Rc in the MC-type carbide is 0.25 to 0.42 (Feature 4). Furthermore, the tensile strength TS can be adjusted to 1100 MPa or higher (Feature 2).

[0132] The bolt according to this embodiment can be manufactured by the manufacturing method described above. The manufacturing method described above is merely one preferred example of a manufacturing method for the bolt according to this embodiment. Therefore, a bolt having the above-described configuration may also be manufactured by a manufacturing method other than the one described above. In short, the manufacturing method is not particularly limited as long as it can produce a bolt according to this embodiment having the above-described configuration.

[0133] [Regarding other processes] The bolt manufacturing process according to this embodiment may include other steps besides those described above. For example, a spheroidizing heat treatment step may be performed after the wire drawing step and before the bolt forming step. Also, the thread forming step may be performed after the heat treatment step, rather than after the head forming step and before the heat treatment step. Furthermore, a compressive residual stress application step may be performed after the heat treatment step. [Examples]

[0134] The effects of the bolt of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the bolt of this embodiment. Therefore, the bolt of this embodiment is not limited to this one example of conditions.

[0135] Steel materials (steel bars) to be used as bolts were prepared, having the chemical compositions shown in Tables 1-1 and 1-2.

[0136] [Table 1-1]

[0137] [Table 1-2]

[0138] [Steel preparation process] The steel materials for each test number were manufactured using the following method: Rough rolling (blobing rolling and hot rolling in a continuous rolling mill) was performed on blooms having the chemical compositions listed in Tables 1-1 and 1-2 to produce billets. The bloom heating temperature was 1200°C. Finish rolling was then performed on the manufactured billets to produce 20mm diameter steel bars. The billet heating temperature during finish rolling was 1200°C.

[0139] [Bolt manufacturing process] Bolts were manufactured using the manufactured steel materials. First, 20 mm diameter steel materials for each test number shown in Table 2 were drawn under the same conditions for each test number to produce 16 mm diameter steel wires. A head forming process was then performed on the steel wires for each test number to produce intermediate bolts with a hexagonal bolt shape, a diagonal distance of 30 mm, a head height of 10 mm, and a nominal length of 100 mm. After the head forming process, a thread forming process was performed under the same conditions for each test number to form a threaded portion on the shaft of the intermediate bolts.

[0140] [Table 2]

[0141] Furthermore, the strain ε added to the lower part of the bolt neck of the bolt intermediate product manufactured by the bolt forming process was determined by performing the FEM analysis described above. The determined strain ε is shown in the "Strain ε" column of the "Bolt Forming Process" column in Table 2.

[0142] Furthermore, the presence or absence of cracks in the intermediate bolts after the bolt forming process was visually determined. If cracks of 1 mm or longer were found on the surface of the intermediate bolts, the cold forgeability was judged to be poor (indicated as "F (Fail)" in the "Cold Forgeability" column under the "Bolt Forming Process" column in Table 2). On the other hand, if no cracks of 1 mm or longer were found on the surface of the intermediate bolts, the cold forgeability was judged to be good (indicated as "P (Pass)" in the "Cold Forgeability" column under the "Bolt Forming Process" column in Table 2). For steel materials with poor cold forgeability, production was stopped, and the subsequent heat treatment process and the evaluation tests described later were not carried out.

[0143] A heat treatment process was performed on intermediate bolt samples from test numbers that showed good cold forgeability. First, a quenching process was performed on the intermediate bolt samples from each test number under the same conditions. In the quenching process, the quenching temperature was set to 880-970°C for each test number, and the holding time at the quenching temperature was 60 minutes (1 hour). After the holding time, the bolts were water-cooled.

[0144] A tempering process was performed on the intermediate bolts after quenching. The tempering temperature T (°C) and holding time t (hours) at tempering temperature T for each test number are shown in Table 2. Furthermore, Fn, as defined by formula (A), is also shown in Table 2. The bolts for each test number were manufactured using the above manufacturing process.

[0145] [Evaluation Test] The following evaluation tests were performed on each bolt with the specified test number. (Test 1) Chemical composition analysis test (Test 2) Tensile test (Test 3) Number density ND measurement test of MC type carbide (Test 4) Measurement test of the carbon ratio (Rc) in MC-type carbides (Test 5) Measurement test of critical hydrogen content

[0146] [(Test 1) Chemical Composition Analysis Test] Based on the method described in [Method for Measuring the Chemical Composition of Bolts] above, the chemical composition of the bolts for each test number was analyzed. As a result, the chemical composition of the bolts for each test number was as shown in Tables 1-1 and 1-2.

[0147] [(Test 2) Tensile Test] Based on the method described in the above-mentioned [Method for Measuring Tensile Strength TS], the tensile strength TS (MPa) of each test number bolt was determined. The determined tensile strengths TS are shown in the "TS (MPa)" column of Table 2.

[0148] Furthermore, for test numbers where the obtained tensile strength (TS) was less than 1100 MPa, subsequent evaluation tests (Tests 3 to 5) were not performed.

[0149] [(Test 3) Number Density ND Measurement Test of MC-type Carbide] Based on the method described in the above-mentioned [Method for measuring the number density ND of MC-type carbides], the number density ND (pieces / m³) of the MC-type carbides of the bolts for each test number was determined. 3 The number density ND was calculated. The calculated number density ND is shown in Table 2 as "Number density ND (×10 21 pieces / m 3 This is shown in the ")" column.

[0150] [(Test 4) Measurement test of the carbon ratio (Rc) in MC-type carbides] Based on the method described in [Method for Measuring C Ratio Rc] above, the C ratio Rc in the MC-type carbide of each test number bolt was determined. The determined C ratio Rc is shown in the "C Ratio Rc" column of Table 2.

[0151] [(Test 5) Limit Hydrogen Amount Measurement Test] Based on the method described in [Method for Measuring Critical Hydrogen Content] above, the critical hydrogen content (mass ppm) of each test number's bolt was measured. The obtained critical hydrogen content (mass ppm) is shown in the "Critical Hydrogen Content (mass ppm)" column of Table 2.

[0152] [Evaluation test results] The evaluation test results are shown in Table 2. Referring to Table 1-1, Table 1-2, and Table 2, for the bolts with test numbers 1 to 27, the content of each element in the chemical composition was within the range of this embodiment, and the tensile strength TS was 1100 MPa or more. Furthermore, the number density ND of MC-type carbides was 2.0×10 21 or more, and the C ratio Rc was 0.25 to 0.42. As a result, the limiting hydrogen amount of the bolts with these test numbers was 1.5 mass ppm or more, and they had excellent hydrogen embrittlement resistance.

[0153] On the other hand, in test number 28, the Mo content in the bolt was too low. Therefore, the tensile strength TS was lower than 1100 MPa.

[0154] In test number 29, the V content in the bolt was too high. Therefore, cracks were confirmed in the bolt intermediate product after the bolt forming process, and the cold forging property was low.

[0155] In test number 30, the V content in the bolt was too low. Therefore, MC-type carbides were not confirmed in the bolt. As a result, the limiting hydrogen amount was less than 1.5 mass ppm, and the hydrogen embrittlement resistance was low.

[0156] In test number 31, although the chemical composition of the bolt was appropriate, the tempering temperature T was too low. Therefore, the number density ND of MC-type carbides in the bolt was too low. As a result, the limiting hydrogen amount was less than 1.5 mass ppm, and the hydrogen embrittlement resistance was low.

[0157] In test number 32, although the chemical composition of the bolt was appropriate, the tempering temperature T was too high. Therefore, the tensile strength TS was lower than 1100 MPa.

[0158] In test number 33, although the chemical composition of the bolt was appropriate, the holding time t at the tempering temperature T was too short. Therefore, the number density ND of MC-type carbides in the bolt was too low. As a result, the limiting hydrogen amount was less than 1.5 mass ppm, and the hydrogen embrittlement resistance was low.

[0159] In Test No. 34, although the chemical composition of the bolt was appropriate, the holding time t at the tempering temperature T was too long. Therefore, the tensile strength TS was lower than 1100 MPa.

[0160] In Test Nos. 35 to 37, although the chemical composition of the bolt was appropriate, the tempering parameter Fn was too low. Therefore, the C ratio Rc was too high. As a result, the limiting hydrogen amount was less than 1.5 mass ppm, and the hydrogen embrittlement resistance was low.

[0161] In Test Nos. 38 to 40, although the chemical composition of the bolt was appropriate, the tempering parameter Fn was too high. Therefore, the number density ND of MC-type carbides in the bolt was too low. As a result, the limiting hydrogen amount was less than 1.5 mass ppm, and the hydrogen embrittlement resistance was low.

[0162] In Test No. 41, the bolt had a chemical composition corresponding to SCM440 specified in JIS G 4053:2016. Therefore, no MC-type carbides were confirmed in the bolt. As a result, the limiting hydrogen amount was less than 1.5 mass ppm, and the hydrogen embrittlement resistance was low.

[0163] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. In mass percent, C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: more than 0.80 to 1.50%, Mo: 0.01% to less than 0.50% V: 0.01-0.50%, Al: 0.005–0.100%, and, N: Contains 0.0010 to 0.0300%, The remainder consists of Fe and impurities. The tensile strength TS is 1100 MPa or more. The number density ND of the MC-type carbide is 2.0 × 10⁻⁶. ２１ pieces / m ３ That's all. When the atomic percentage of C in the MC-type carbide is defined as [C], the atomic percentage of Mo is defined as [Mo], and the atomic percentage of V is defined as [V], the C ratio Rc defined by formula (1) is 0.25 to 0.

42. bolt. Rc=[C] / ([C]+[Mo]+[V]) (1)

2. In mass percent, C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: more than 0.80 to 1.50%, Mo: 0.01% to less than 0.50% V: 0.01-0.50%, Al: 0.005–0.100%, and, N: Contains 0.0010 to 0.0300%, Furthermore, it contains one or more elements selected from the groups consisting of Group 1 to Group 3, with the remainder being Fe and impurities. The tensile strength TS is 1100 MPa or more. The number density ND of the MC-type carbide is 2.0 × 10⁻⁶. ２１ pieces / m ３ That's all. When the atomic percentage of C in the MC-type carbide is defined as [C], the atomic percentage of Mo is defined as [Mo], and the atomic percentage of V is defined as [V], the C ratio Rc defined by formula (1) is 0.25 to 0.

42. bolt. Rc=[C] / ([C]+[Mo]+[V]) (1) [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Zr: 0.300% or less, Hf: 0.100% or less, Ta: 0.100% or less, One or more selected from the group consisting of W: 0.20% or less. [Group 2] Ti: 0.100% or less, One or more selected from the group consisting of Nb: 0.100% or less. [Group 3] Ca: 0.0050% or less, Bi: 0.020% or less, One or more selected from the group consisting of Te: 0.010% or less.

3. A bolt according to claim 2, The first group contains, bolt.

4. A bolt according to claim 2, The following include the second group: bolt.

5. A bolt according to claim 2, The following contain the third group: bolt.

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