bolt

Abstract

To provide a bolt that has high strength and excellent resistance to hydrogen embrittlement.SOLUTION: A bolt according to the present disclosure contains, in mass%, 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: 0.01 to 0.30%, Mo: 1.50 to 2.50%, V: 0.01 to 0.30%, Al: 0.005 to 0.100% and N: 0.0010 to 0.0300%, and the balance being Fe and impurities, and has a tensile strength of 1400 MPa or more, and in which the MC-type carbide in the bolt has a Mo ratio of 60 atom % or more among metal elements excluding Fe.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a bolt. [Background technology]

[0002] Bolts are used in industrial machinery, automobiles, buildings such as bridges, etc. In recent years, with the increasing performance of industrial machinery and automobiles and the increasing size of buildings, etc., there has been a demand for higher strength bolts. Specifically, there is a demand for bolts with a tensile strength of, for example, 1400 MPa or more.

[0003] Bolts with a tensile strength of 1400 MPa or more may be more susceptible to hydrogen embrittlement. Hydrogen embrittlement can cause delayed fracture of bolts. Therefore, bolts with a tensile strength of 1400 MPa or more are required to have excellent hydrogen embrittlement resistance.

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

[0005] The bolt disclosed in Patent Document 1 contains, in mass %, C: 0.22 to 0.40%, Si: 0.10 to 1.50%, Mn: 0.20 to less than 0.40%, Cr: 0.70 to less than 1.60%, Al: 0.005 to 0.060%, Ti: 0.010 to 0.050%, B: 0.0003 to 0.0040%, N: 0.0015 to 0.0080%, Cu: 0.50% or less, Ni: 0.30% or less, M The bolt contains 0.05% or less O, 0.050% or less V, 0.050% or less Nb, and one or more elements selected from the group consisting of 0.001-0.100% Sb, 0.001-0.100% Sn, and 0.001-0.100% Bi, and also contains 0.0020% or less O, 0.020% or less P, and 0.020% or less S, with the balance being Fe and impurities. The 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). As a result, Patent Document 1 states that this bolt has a tensile strength of 1000 to 1300 MPa in the shank and also has excellent hydrogen embrittlement resistance.

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

[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%), and further contains Cu: 0.10 to 0.50% and Ni: 0.1 to 1.0% so as to satisfy [Ni] / [Cu]≧0.5, and further contains Ti: 0.05 to 0.20% and V: 0.20% or less (including 0%) so as to satisfy [Ti]+[V]: 0.085 to 0.30%, with the remainder consisting of Fe and impurities. Furthermore, this bolt has an austenite grain size number of 9.0 or higher in the bolt shank, and the G value (%), which indicates the proportion of carbides precipitated at the austenite grain boundaries in the bolt shank, satisfies formula (1) (G value: (L / L0) × 100≦60, where L is the total length of carbides with a thickness of 50 nm or higher precipitated at the austenite grain boundaries, and L0 is the length of the austenite grain boundaries). As a result, Patent Document 3 states that this bolt has a tensile strength of 1400 MPa or higher and also exhibits excellent hydrogen embrittlement resistance. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2019-218584 [Patent Document 2] International Publication No. 2017 / 094487 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-163865 Summary of the Invention [Problem 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 other than those disclosed in Patent Documents 1 to 3.

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

[0011] The bolt according to the present disclosure has the following configuration.

[0012] A bolt, In mass%, C: 0.30~0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50% P:0.030% or less, S: 0.030% or less, Cr: 0.01 to 0.30%, Mo: 1.50-2.50%, V: 0.01 to 0.30%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0300%, The balance is Fe and impurities. The tensile strength is 1400 MPa or more, In the MC type carbide in the bolt, the ratio of Mo among metal elements excluding Fe is 60 atomic % or more. bolt. [Effects of the Invention]

[0013] Bolts according to the present disclosure have high strength and excellent resistance to hydrogen embrittlement. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a graph showing the relationship between the Mo ratio (atomic %) in MC carbides and the limiting hydrogen content (ppm by mass) for a bolt whose chemical composition contains elements within the range of this embodiment and whose tensile strength is 1400 MPa or more. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have conducted research into bolts having a tensile strength of 1400 MPa or more and excellent hydrogen embrittlement resistance, and as a result have made the following discoveries.

[0016] First, the inventors investigated bolts with high strength and excellent hydrogen embrittlement resistance from the perspective of chemical composition. As a result, the inventors found that the composition was, 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: 0.01-0.30%, Al: 0.005-0.100%, N: 0.0010-0.0300%, Cu: 0-0.40%, Ni: 0-0.40%, and B: 0-0. It was thought that a bolt with a chemical composition of Cr: 0.0100%, Ti: 0-0.100%, Nb: 0-0.100%, Zr: 0-0.300%, Ca: 0-0.0050%, Bi: 0-0.100%, Te: 0-0.100%, and the remainder being Fe and impurities could potentially achieve both a tensile strength of 1400 MPa or more and excellent hydrogen embrittlement resistance.

[0017] Therefore, from the viewpoint of the microstructure of bolts with the above chemical composition, we investigated ways to improve the strength and hydrogen embrittlement resistance. As a result, we found that by dispersing a large number of fine precipitates in the bolt, it is possible to improve the hydrogen embrittlement resistance even for bolts with a tensile strength of 1400 MPa or more.

[0018] The inventors further considered that the type of precipitates affects the hydrogen embrittlement resistance of the bolt. Therefore, after further investigation, the inventors focused on MC-type carbides among alloy carbides. MC-type carbides are less likely to coarsen and tend to remain fine compared to other alloy carbides (e.g., M2C-type carbides, cementite, etc.). Therefore, if MC-type carbides are formed in the bolt, the hydrogen embrittlement resistance of the bolt may be improved.

[0019] Therefore, the inventors of the present invention thought that if Mo and V were further added to the above-mentioned chemical composition, MC type carbides would be formed in the bolt, improving the hydrogen embrittlement resistance of the bolt. Therefore, the inventors further investigated the chemical composition of the bolt taking into consideration the formation of MC type carbides. As a result, the following was found to be the result: 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: 0.01-0.30%, Mo: 1.50-2.50%, V: 0.01-0.30%, Al: 0.005-0.100%, N: 0.0010-0.0300%, Cu: 0-0.40%, Ni: 0-0.40%, B: 0-0.0100% The inventors considered that if a bolt had a chemical composition consisting of Ti: 0-0.100%, Nb: 0-0.100%, Zr: 0-0.300%, Ca: 0-0.0050%, Bi: 0-0.100%, Te: 0-0.100%, and the balance being Fe and impurities, fine MC type carbides would be formed in the bolt, and excellent hydrogen embrittlement resistance would be possible even if the tensile strength was increased to 1400 MPa or more.

[0020] However, even with bolts having the above chemical composition, there were cases where it was not possible to achieve both a tensile strength of 1400 MPa or more and excellent hydrogen embrittlement resistance. Therefore, the inventors conducted further research. As a result, they found that increasing the Mo concentration in the MC carbide improved the hydrogen trapping function of the MC carbide.

[0021] Therefore, the inventors investigated and studied the relationship between the Mo concentration in the MC carbides and the hydrogen embrittlement resistance of bolts with the above-mentioned chemical composition. As a result, the inventors found that if the Mo ratio among the metal elements in the MC carbides of a bolt with the above-mentioned chemical composition is 60 atomic % or more, it is possible to achieve both a tensile strength of 1400 MPa or more and excellent hydrogen embrittlement resistance.

[0022] The bolt according to this embodiment, which was completed based on the above findings, has the following configuration.

[0023] [1] A bolt, In mass%, C: 0.30~0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50% P:0.030% or less, S: 0.030% or less, Cr: 0.01 to 0.30%, Mo: 1.50-2.50%, V: 0.01 to 0.30%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0300%, The balance is Fe and impurities. The tensile strength is 1400 MPa or more, In the MC type carbide in the bolt, the ratio of Mo among metal elements excluding Fe is 60 atomic % or more. bolt.

[0024] [2] [1] The bolt according to [1], further comprising: Instead of part of Fe, Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.300% or less, Ca: 0.0050% or less, Bi: 0.100% or less, and Te: 0.100% or less, Contains one or more elements selected from the group consisting of bolt.

[0025] The bolt according to this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0026] Bolt Configuration The bolt has a head and a shank. The shank extends from the neck (under-neck) of the head in the direction of the central axis of the bolt. The shank includes a threaded portion having threads formed on its circumferential surface.

[0027] [Chemical composition] The chemical composition of the bolt according to this embodiment contains the following elements:

[0028] C: 0.30 to 0.50% Carbon (C) improves the hardenability of steel and increases the strength of the bolt. If the C content is less than 0.30%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.50%, the hydrogen embrittlement resistance of the bolt will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.30 to 0.50%. The lower limit of the C content is preferably 0.32%, and more preferably 0.35%. The upper limit of the C content is preferably 0.48%, and more preferably 0.45%.

[0029] Si: 0.01 to 0.30% Silicon (Si) improves the hardenability of steel and increases the strength of the bolt. If the Si content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.30%, the hydrogen embrittlement resistance of the bolt will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.01 to 0.30%. The lower limit of the Si content is preferably 0.02%, and more preferably 0.03%. The upper limit of the Si content is preferably 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0030] Mn: 0.10 to 1.50% Manganese (Mn) improves the hardenability of steel and increases the strength of the bolt. If the Mn content is less than 0.10%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.50%, the hydrogen embrittlement resistance of the bolt will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.10 to 1.50%. The lower limit of the Mn content is preferably 0.15%, and more preferably 0.20%. The upper limit of the Mn content is preferably 1.30%, and more preferably 1.10%.

[0031] P:0.030% or less Phosphorus (P) is an impurity. In other words, the lower limit of the P content is greater than 0%. If the P content exceeds 0.030%, P segregates at grain boundaries even if the contents of other elements are within the ranges specified in this embodiment. As a result, the hydrogen embrittlement resistance of the bolt deteriorates. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the P content is preferably 0.025%, and more preferably 0.020%.

[0032] S: 0.030% or less Sulfur (S) is an impurity. In other words, the lower limit of the S content is greater than 0%. If the S content exceeds 0.030%, S will segregate at grain boundaries even if the contents of other elements are within the ranges specified in this embodiment. As a result, the hydrogen embrittlement resistance of the bolt will deteriorate. Therefore, the S content is 0.030% or less. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the S content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the S content is preferably 0.025%, and more preferably 0.020%.

[0033] Cr: 0.01 to 0.30% Chromium (Cr) improves the hardenability of steel material, thereby increasing the strength of the bolt. Cr also improves the temper softening resistance of the steel material, thereby increasing the strength of the bolt. If the Cr content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 0.30%, the hydrogen embrittlement resistance of the bolt will be reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.01 to 0.30%. The lower limit of the Cr content is preferably 0.02%, and more preferably 0.03%. The upper limit of the Cr content is preferably 0.25%, and more preferably 0.20%.

[0034] Mo: 1.50-2.50% Molybdenum (Mo) concentrates in MC carbides and enhances their hydrogen trapping function. As a result, Mo enhances the hydrogen embrittlement resistance of high-strength bolts. If the Mo content is less than 1.50%, the Mo ratio among the metal elements in the MC carbides in the bolt will be 60 atomic % or less, and the above effects will not be fully achieved even if the contents of other elements are within the ranges specified in this embodiment. On the other hand, if the Mo content exceeds 2.50%, the Mo ratio in the MC carbides decreases, which reduces the hydrogen embrittlement resistance of the bolt. Therefore, the Mo content is 1.50 to 2.50%. The lower limit of the Mo content is preferably 1.60%, and more preferably 1.70%. The upper limit of the Mo content is preferably 2.40%, and more preferably 2.30%.

[0035] V: 0.01 to 0.30% Vanadium (V) forms MC carbides together with Mo to improve the hydrogen embrittlement resistance of the bolt. If the V content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content exceeds 0.30%, the Mo ratio to the metal elements in the MC carbides decreases even if the contents of other elements are within the ranges of this embodiment, resulting in a decrease in the hydrogen embrittlement resistance of the bolt. Therefore, the V content is 0.01 to 0.30%. The lower limit of the V content is preferably 0.03%, and more preferably 0.05%. The upper limit of the V content is preferably 0.25%, and more preferably 0.20%.

[0036] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. Furthermore, Al combines with N to form Al nitrides. Al nitrides suppress grain coarsening through a pinning effect. As a result, the hydrogen embrittlement resistance of the bolt is improved. If the Al content is less than 0.005%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, coarse Al nitrides are formed, which become the starting points for fracture, thereby deteriorating the workability of the steel material. Therefore, the Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably 0.006%, more preferably 0.007%, and even more preferably 0.008%. The upper limit of the Al content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.070%. In the chemical composition of the steel material of this embodiment, the Al content means the total Al content.

[0037] N: 0.0010~0.0300% Nitrogen (N) combines with Al to form Al nitrides, which have a pinning effect that suppresses grain coarsening. As a result, the hydrogen embrittlement resistance of the bolt is improved. If the N content is less than 0.0010%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0300%, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed. The coarse nitrides become the starting points for fracture, which reduces the workability of the steel material. Therefore, the N content is 0.0010 to 0.0300%. The lower limit of the N content is preferably 0.0020%, more preferably 0.0025%, and even more preferably 0.0030%. The upper limit of the N content is preferably 0.0290%, more preferably 0.0280%, even more preferably 0.0270%, even more preferably 0.0250%, even more preferably 0.0200%, even more preferably 0.0150%, and even more preferably 0.0100%.

[0038] The remainder of the chemical composition of the bolt according to this embodiment is Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the bolt, and are acceptable as long as they do not adversely affect the bolt according to this embodiment.

[0039] [Optional Elements] The chemical composition of the bolt of this embodiment further includes, in place of a portion of Fe, Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.300% or less, Ca: 0.0050% or less, Bi: 0.100% or less, and Te: 0.100% or less, It may contain one or more elements selected from the group consisting of: These optional elements will be explained below.

[0040] [Group 1: Cu, Ni and B] The chemical composition of the bolt according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu, Ni, and B. These elements are all optional elements and may not be contained. When contained, Cu, Ni, and B improve the hardenability of the steel material and increase the strength of the bolt.

[0041] Cu:0.40% or less Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When Cu is contained, that is, when the Cu content is more than 0%, Cu improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 0.40%, even if the contents of other elements are within the ranges of this embodiment, the hardenability becomes too high, resulting in a decrease in the workability of the steel material. Therefore, the Cu content is 0 to 0.40%, and when Cu is contained, the Cu content is 0.40% or less (more than 0 to 0.40%). The lower limit of the Cu content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Cu content is preferably 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0042] Ni: 0.40% or less Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content is more than 0%, Ni improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content exceeds 0.40%, even if the contents of other elements are within the ranges of this embodiment, the hardenability becomes too high, resulting in a decrease in the workability of the steel material. Therefore, the Ni content is 0 to 0.40%, and when Ni is contained, the Ni content is 0.40% or less (more than 0 to 0.40%). The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Ni content is preferably 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0043] B: 0.0100% or less Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When B is contained, that is, when the B content is more than 0%, B improves the hardenability of the steel material and increases the strength of the bolt. B also suppresses the grain boundary segregation of P and improves the hydrogen embrittlement resistance of the bolt. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0100%, even if the contents of other elements are within the ranges of this embodiment, coarse B nitrides are formed. The coarse B nitrides become the starting points for cracks. As a result, the workability of the steel material is reduced. Therefore, the B content is 0 to 0.0100%, and when B is contained, the B content is 0.0100% or less (more than 0 to 0.0100%). The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and even more preferably 0.0050%.

[0044] [Group 2: Ti, Nb, and Zr] The chemical composition of the bolt according to this embodiment may further contain one or more elements selected from the group consisting of Ti, Nb, and Zr in place of a portion of Fe. These elements are optional and may not be included. When contained, Ti, Nb, and Zr form precipitates and refine the crystal grains. As a result, the hydrogen embrittlement resistance of the bolt is improved.

[0045] Ti:0.100% or less Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content is more than 0%, Ti forms fine precipitates such as Ti carbides and refines the crystal grains. As a result, the hydrogen embrittlement resistance of the bolt is improved. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse Ti nitrides are formed, which become the starting points for cracks, resulting in a decrease in the workability of the steel material. Therefore, the Ti content is 0 to 0.100%, and when Ti is contained, the Ti content is 0.100% or less (more than 0 to 0.100%). The lower limit of the Ti content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Ti content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.075%.

[0046] Nb: 0.100% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content is more than 0%, Nb forms fine precipitates such as Nb carbides and refines the grains. As a result, the hydrogen embrittlement resistance of the bolt is improved. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse Nb carbides and the like are generated. The coarse Nb carbides and the like become the starting points for cracks. As a result, the workability of the steel material is reduced. Therefore, the Nb content is 0 to 0.100%, and when Nb is contained, the Nb content is 0.100% or less (more than 0 to 0.100%). The lower limit of the Nb content is preferably 0.001%, more preferably 0.005%, even more preferably 0.010%, and still more preferably 0.020%. The upper limit of the Nb content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0047] Zr: 0.300% or less Zirconium (Zr) is an optional element and may not be contained, that is, the Zr content may be 0%. When Zr is contained, that is, when the Zr content is more than 0%, Zr forms fine precipitates such as Zr nitrides and refines the crystal grains. As a result, the hydrogen embrittlement resistance of the bolt is improved. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.300%, even if the contents of other elements are within the ranges of this embodiment, coarse Zr nitrides are formed. The coarse Zr nitrides become the starting points for cracks. As a result, the workability of the steel material is reduced. Therefore, the Zr content is 0 to 0.300%, and when Zr is contained, the Zr content is 0.300% or less (more than 0 to 0.300%). The lower limit of the Zr content is preferably 0.001%, more preferably 0.005%, even more preferably 0.010%, and still more preferably 0.020%. The upper limit of the Zr content is preferably 0.280%, more preferably 0.250%, even more preferably 0.200%, even more preferably 0.150%, and still more preferably 0.100%.

[0048] [Group 3: Ca, Bi and Te] The chemical composition of the bolt according to this embodiment may further contain one or more elements selected from the group consisting of Ca, Bi, and Te, in place of a portion of Fe. These elements are optional and may not be included. When contained, Ca, Bi, and Te improve the machinability of the steel material.

[0049] Ca:0.0050% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%. When Ca is contained, that is, when the Ca content is more than 0%, Ca improves the machinability of the steel material. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, the workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0 to 0.0050%, and when Ca is contained, the Ca content is 0.0050% or less (more than 0 to 0.0050%). The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Ca content is preferably 0.0040%, and more preferably 0.0030%.

[0050] Bi:0.100% or less Bismuth (Bi) is an optional element and may not be contained, that is, the Bi content may be 0%. When Bi is contained, that is, when Bi is more than 0%, Bi improves the machinability of the steel material. Even if even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.100%, the workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Bi content is 0 to 0.100%, and when Bi is contained, the Bi content is 0.100% or less (more than 0 to 0.100%). The lower limit of the Bi content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Bi content is preferably 0.080%, and more preferably 0.060%.

[0051] Te: 0.100% or less Tellurium (Te) is an optional element and may not be contained, that is, the Te content may be 0%. When Te is contained, that is, when the Te content is more than 0%, Te improves the machinability of the steel material. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.100%, the workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.100%, and when Te is contained, the Te content is 0.100% or less (more than 0 to 0.100%). The lower limit of the Te content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Te content is preferably 0.080%, and more preferably 0.060%.

[0052] [Method for measuring the chemical composition of bolts] The chemical composition of the bolt of this embodiment can be measured using a known elemental analysis method. Specifically, chips are collected using a drill from the inside of the bolt shaft to a depth of 1 mm or more from the surface. The collected chips are dissolved in acid to obtain a solution. The solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) to perform elemental analysis of the chemical composition. The C content and S content are determined using the known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the known inert gas fusion-thermal conductivity method.

[0053] The content of each element is determined by rounding off the measured value to the nearest significant digit based on the significant digits specified in this embodiment. For example, the C content of the steel material in this embodiment is determined to one decimal place. Therefore, the C content is determined to one decimal place by rounding off the measured value to two decimal places.

[0054] Similarly, the contents of other elements other than the C content of the steel material of this embodiment are also determined by rounding off the measured value to the smallest digit specified in this embodiment, and this is the content of the element.

[0055] Rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0056] [Tensile strength] The bolt according to this embodiment has a tensile strength of 1400 MPa or more. The bolt according to this embodiment has the above-mentioned chemical composition, and in the MC carbides in the bolt, the Mo ratio among the metal elements is 60 atomic % or more. As a result, the bolt according to this embodiment has excellent hydrogen embrittlement resistance even when the tensile strength is 1400 MPa.

[0057] The lower limit of the tensile strength is preferably 1420 MPa, more preferably 1450 MPa, even more preferably 1500 MPa, and even more preferably 1550 MPa. The upper limit of the tensile strength is not particularly limited. The upper limit of the tensile strength of the bolt according to this embodiment is, for example, 1800 MPa, for example, 1750 MPa.

[0058] [Method for measuring tensile strength] In this embodiment, the tensile strength can be determined by the following method. A tensile test is conducted in air at room temperature (20±15°C) in accordance with JIS Z 2241:2011 to obtain the tensile strength (MPa). A tensile test specimen is taken so as to include the bolt shank, and the central axis of the tensile test specimen is coaxial with the bolt shank.

[0059] [Mo ratio in MC type carbide] In the bolt according to this embodiment, the ratio of Mo to the metal elements other than Fe in the MC type carbides is 60 atomic % or more.

[0060] Here, "the ratio of Mo among the metal elements excluding Fe in the MC type carbide is 60 atomic % or more" means the ratio (atomic %) of the Mo content contained in the MC type carbide to the total content of metal elements excluding Fe, when the total content of metal elements excluding Fe contained in the MC type carbide is taken as 100 atomic %.

[0061] Fig. 1 is a graph showing the relationship between the Mo ratio (atomic %) in MC carbides and the limiting hydrogen content (ppm by mass) for a bolt whose chemical composition has a tensile strength of 1400 MPa or more and whose element contents are within the range of this embodiment. Fig. 1 was created based on the results of the Examples described below.

[0062] A higher critical hydrogen content means a better hydrogen trapping function and better hydrogen embrittlement resistance. Referring to Figure 1, in a bolt whose chemical composition contains elements within the range of this embodiment and whose tensile strength is 1400 MPa or more, if the Mo ratio in the MC carbides is 60 atomic % or more, the critical hydrogen content is significantly higher than when the Mo ratio in the MC carbides is less than 60 atomic %.

[0063] Therefore, in the bolt of this embodiment, the content of each element in the chemical composition is within the range of this embodiment, and assuming that the tensile strength is 1400 MPa or more, the Mo ratio among the metal elements excluding Fe in the MC type carbides in the bolt is 60 atomic % or more.

[0064] The mechanism by which the critical hydrogen content increases when the Mo ratio in MC carbides is high is unclear. V forms MC carbides, and Mo typically forms M2C carbides. However, MC carbides with a high Mo content have a higher hydrogen trapping function than MC carbides whose M sites are mainly composed of V or M2C carbides whose M sites are mainly composed of Mo. Therefore, if the Mo ratio in MC carbides is set to 60 atomic % or more, the critical hydrogen content will increase significantly, and it is thought that the hydrogen embrittlement resistance of the bolt will be improved.

[0065] The above-mentioned mechanism is speculative, and it is possible that the critical hydrogen content is increased by a different mechanism. However, it has been proven in the examples described below that in a bolt whose chemical composition contains elements within the range of this embodiment and whose tensile strength is 1400 MPa or greater, if the Mo ratio among the metal elements excluding Fe in the MC carbides is 60 atomic % or greater, the critical hydrogen content is increased.

[0066] The lower limit of the Mo ratio in the MC carbide is preferably 62% in atomic percent, more preferably 64%, and even more preferably 66%.

[0067] [Method for measuring the Mo ratio in MC type carbides] The Mo ratio in MC carbides can be determined using the following method: A sample is cut from the inside of the bolt shaft, at least 1 mm deep from the surface. The cut sample is then subjected to well-known focused ion beam processing to create a needle-shaped test piece with a tip curvature radius of approximately 50 nm.

[0068] Three-dimensional atom probe analysis is performed on the needle-shaped test specimen. MC carbides in the needle-shaped test specimen are identified by the three-dimensional atom probe analysis using the following method. Three-dimensional atom probe analysis can detect precipitates present in the needle-shaped test specimen in three dimensions.

[0069] In the three-dimensional atom probe analysis, the laser wavelength (λ) is set to 355 nm, the laser power is set to 30 pJ, and the temperature of the needle-shaped test piece is set to 50 K. The apparatus used for the three-dimensional atom probe analysis is not particularly limited. For example, the three-dimensional atom probe analyzer is a product name LEAP4000XHR manufactured by Ametec Co., Ltd.

[0070] The acquired measurement data is reconstructed to obtain a three-dimensional atomic map. Specifically, the detection efficiency of the instrument is used to adjust the spacing of the {110} atomic planes to 0.2 nm in the measurement of iron (Fe), and the measurement data is reconstructed to obtain a three-dimensional atomic map.

[0071] In a three-dimensional atomic map, the area of ​​the needle-shaped test piece that has been analyzed with a three-dimensional atom probe is divided into tiny cubes called voxels. Each side of a voxel is 1.0 nm. The concentration (atomic %) of an element within a voxel is defined as the number of atoms of that element contained within the voxel divided by the number of atoms of all elements within the voxel.

[0072] The sum of the C, Mo, and V concentrations in each voxel is defined as the specific element concentration (atomic %). An isoconcentration surface is created connecting voxels where the specific element concentration is 20%. The area surrounded by the isoconcentration surface has a high concentration of the specific element. The area surrounded by the isoconcentration surface is recognized as a precipitate.

[0073] Among the identified precipitates, precipitates with a maximum length of 10 nm or less are identified. Here, among the line segments connecting any two points on the surface of a three-dimensionally detected precipitate (i.e., the interface between the precipitate and the steel matrix), a line segment whose entirety is included in the precipitate is defined as a "specific line segment." The maximum length of the specific line segment of the precipitate is then defined as the maximum length of the precipitate. Precipitates with a maximum length of 10 nm or less are identified as MC-type carbides.

[0074] When the content of each element in the chemical composition is within the range of this embodiment, most of the precipitates in the bolt with a maximum length of 10 nm or less are MC carbides, and precipitates other than MC carbides are almost absent. The maximum lengths of precipitates other than MC carbides, such as M2C carbides and cementite, all far exceed 10 nm. This point will be explained below.

[0075] A sample was taken from the interior of the shank of the bolt of this embodiment, at a depth of 1 mm or more from the surface, to prepare a thin film specimen with a thickness of 50 nm. The prepared thin film specimen was observed with a transmission electron microscope (TEM) to identify precipitates based on contrast. The maximum length of the identified precipitates was determined. Furthermore, crystal structure analysis was performed on the identified precipitates to determine their crystal structure and identify the type of precipitate. As a result, in the bolt having the chemical composition of this embodiment, precipitates with a maximum length of 10 nm or less were almost entirely MC-type carbides, and other precipitates were almost absent. The maximum lengths of precipitates other than MC-type carbides (such as M2C-type carbides and cementite) greatly exceeded 10 nm.

[0076] From the above results, among the precipitates in the three-dimensional atomic map obtained by three-dimensional atom probe analysis, precipitates with a maximum length of 10 nm or less are identified as MC-type carbides.

[0077] Among the identified MC carbides, 20 MC carbides are arbitrarily selected. Then, the total number of atoms of metal elements excluding Fe and the number of Mo atoms in each selected MC carbide are calculated. The ratio of the number of Mo atoms to the total number of atoms of metal elements excluding Fe is defined as the Mo ratio (atomic %). A metal element is an element whose simple substance forms a metal. Therefore, Si, B, P, S, C, N, and O are not included in the metal elements.

[0078] The arithmetic mean value of the Mo ratios determined for each of the selected 20 MC carbides is defined as the Mo ratio (atomic %) among the metal elements excluding Fe in the MC carbides of the bolt.

[0079] [Effects of the bolt of this embodiment] As described above, the bolt of this embodiment has the following configuration. (1) The content of each element in the chemical composition is within the range of this embodiment. (2) The tensile strength is 1400 MPa or more. (3) The ratio of Mo among the metal elements excluding Fe in the MC type carbide is 60 atomic % or more. A bolt having the above-described configuration exhibits excellent hydrogen embrittlement resistance even when the tensile strength is as high as 1400 MPa or more.

[0080] The critical hydrogen content can be used as an index of hydrogen embrittlement resistance. The higher the critical hydrogen content, the better the hydrogen embrittlement resistance. The critical hydrogen content refers to the upper limit of the hydrogen content at which the steel does not suffer from delayed fracture.

[0081] [Method for measuring limit hydrogen amount] The limiting hydrogen content can be determined by the following method. A round bar test piece with an annular notch (hereinafter simply referred to as a round bar test piece) is taken from the inside of the bolt, at a depth of 1 mm or more from the surface. The size of the round bar test piece is not particularly limited, but for example, the diameter of the parallel part is 7 mm and the length is 70 mm. A circular notch extending circumferentially is formed at the center position in the longitudinal direction of the round bar test piece. The radius of the bottom of the notch is 0.175 mm.

[0082] A number of round bar test specimens are prepared. Hydrogen is charged to the round bar test specimens under various conditions using the cathodic hydrogen charging method. The cathodic hydrogen charging method is as follows: A room temperature aqueous solution (cathodic hydrogen charging solution) is prepared by adding 0 to 20 g of ammonium thiocyanate to 1 L of a 3 mass % sodium chloride aqueous solution. The round bar test specimens are immersed in the cathodic hydrogen charging solution for 72 hours, with the cathodic current density set to 0.03 to 1.00 mA / cm. 2 A constant current controlled within the range of 1000 V is generated to add hydrogen to the rod test specimen. The amount of hydrogen in the rod test specimen is adjusted by adjusting the concentration of ammonium thiocyanate in the cathodic hydrogen charging solution and the cathode current density.

[0083] After performing the cathodic hydrogen charging method, the round bar test specimen is left at room temperature for 96 hours. A zinc plating film is then formed on the surface of the hydrogen-charged round bar test specimen to prevent the hydrogen inside the round bar test specimen from leaking out. A constant load test is conducted at room temperature and atmospheric pressure, in which a constant load is applied to the round bar test specimen so that a load of 90% of its tensile strength is applied. The test time is a maximum of 100 hours, and the test is stopped if the round bar test specimen can withstand more than 100 hours without breaking. A constant load test is conducted using multiple round bar test specimens that have been hydrogen-charged under various conditions.

[0084] The amount of hydrogen in the round bar test specimens that did not break after 100 hours was determined. Specifically, the zinc plating coating was first removed from the surface of the round bar test specimen. After the zinc plating coating was removed, the round bar test specimens were subjected to thermal desorption hydrogen analysis using gas chromatography. In the thermal desorption hydrogen analysis, the temperature was raised from room temperature to 600°C at a rate of 100°C / hour. The amount of hydrogen released from the round bar test specimen during the temperature rise was measured. The measured amount of hydrogen (ppm by mass) was taken as the amount of hydrogen (ppm by mass) charged into the round bar test specimen.

[0085] The maximum hydrogen content of the round bar test piece that did not break in the 100-hour constant load test is defined as the "limit hydrogen content" (ppm by mass).

[0086] In the bolt of this embodiment, for example, the limit hydrogen content is 1.0 mass ppm or more, and the lower limit of the limit hydrogen content is preferably 1.2 mass ppm, and more preferably 1.4 mass ppm.

[0087] [Bolt microstructure] The microstructure of the bolt of this embodiment contains a hard phase at an area ratio of 90% or more. The bolt microstructure here refers to the microstructure of a bolt manufactured by quenching and tempering in the manufacturing process described below. The hard phase is composed of martensite and / or bainite. When the bolt microstructure contains phases other than the hard phase, the remainder of the bolt microstructure other than the hard phase is composed of one or more selected from the group consisting of retained austenite, ferrite, and pearlite. Preferably, the microstructure contains a hard phase at an area ratio of 90% or more, and the remainder is composed of retained austenite. The tensile strength of the bolt is correlated with the microstructure.

[0088] [Manufacturing method] An example of a method for manufacturing a bolt according to this embodiment will be described below. The method for manufacturing a bolt according to this embodiment will be described below as an example. Therefore, a bolt having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a bolt according to this embodiment.

[0089] An example of a method for manufacturing a bolt according to this embodiment includes the following steps. (1) Steel material preparation process (2) Bolt manufacturing process Each step will be described below.

[0090] [(1) Steel preparation process] In the steel material preparation step, a steel material (steel material for bolts) that will be used as the material for bolts is prepared. The steel material for bolts may be manufactured. Alternatively, steel material for bolts supplied by a third party may be prepared. In the steel material preparation step, a steel material is prepared in which the content of each element in the chemical composition falls within the range of this embodiment.

[0091] When manufacturing steel for bolts, for example, the steel is manufactured by the following method. First, molten steel is manufactured, the content of each element in the chemical composition of which falls within the range of this embodiment. A raw material is manufactured using the molten steel. For example, a bloom (stripe) as the raw material may be manufactured using the molten steel by a continuous casting method, or an ingot as the raw material may be manufactured using the molten steel by an ingot casting method. The manufactured raw material (bloom or ingot) is subjected to rough rolling (blooming, or blooming and hot rolling in a continuous rolling mill) to manufacture a billet. The billet is subjected to finish rolling using a continuous rolling mill to manufacture a steel material (steel for bolts). The steel material is, for example, a steel bar or a wire rod.

[0092] [(2) Bolt manufacturing process] In the bolt manufacturing process, bolts are manufactured using the above-mentioned bolt steel material. The bolt manufacturing process includes the following steps. (21) Wire drawing process (22) Cold forging process (23) Quenching and tempering process Each step will be described below.

[0093] [(21) Wire drawing process] In the wire drawing process, the bolt steel material is subjected to a well-known wire drawing process to produce a steel wire. The wire drawing process may be a primary wire drawing process only, or multiple wire drawing processes such as a secondary wire drawing process may be performed.

[0094] [(22) Cold forging process] In the cold forging step, the steel wire after the wire drawing step is subjected to well-known cold forging to manufacture an intermediate product in the shape of a bolt.

[0095] [(23) Quenching and tempering process] In the quenching and tempering step, the intermediate product is quenched and tempered.

[0096] [Quenching] Quenching is carried out by a well-known method. The quenching temperature and the holding time at the quenching temperature are not particularly limited. The quenching temperature is, for example, 840 to 970°C. The holding time at the quenching temperature is, for example, 15 to 360 minutes (6 hours). After the holding time has elapsed, the intermediate product is quenched. Specifically, the intermediate product is water-cooled or oil-cooled.

[0097] [Tempering] The intermediate product after quenching is tempered under the following conditions: Tempering temperature T: 570~660℃ Holding time at tempering temperature: 0.5 to 6.0 hours Tempering condition formula: Fn1 defined by formula (A) is 3500 to 155000 Fn1=T×t×[Mo] / [V] (A) Here, the tempering temperature T (°C) is substituted for T in formula (A). The tempering time t (hours) is substituted for t in formula (A). The Mo content (mass%) in the steel is substituted for [Mo] in formula (A). The V content (mass%) in the steel is substituted for [V] in formula (A). These tempering conditions will be explained below.

[0098] [Tempering temperature T] If the tempering temperature T is too high, the tensile strength of the bolt will be less than 1400 MPa. Therefore, the tempering temperature T is 570 to 660° C. The lower limit of the tempering temperature T is a well-known temperature. [Holding time t at tempering temperature T] The holding time t at the tempering temperature T is within a known range. In this embodiment, the holding time t at the tempering temperature T is 0.5 to 6.0 hours.

[0099] [Fn1] Furthermore, in the tempering, Fn1 defined by the formula (A) is set to 3500 to 155000. Fn1=T×t×[Mo] / [V] (A)

[0100] Fn1 is a parameter related to the formation of MC carbides. If Fn1 is less than 3500, the MC carbides do not contain enough Mo. As a result, the Mo ratio of the MC carbides in the bolt is less than 60 atomic %. Therefore, Fn1 is 3500 or more.

[0101] If Fn1 is too high, the strength of the bolt will decrease. Therefore, Fn1 should be 155,000 or less.

[0102] The preferred lower limit of Fn1 is 4000, more preferably 5000, more preferably 6000, more preferably 7000, more preferably 8000, and more preferably 10000. The preferred upper limit of Fn1 is 100000, more preferably 80000, more preferably 60000, and more preferably 50000.

[0103] The bolt according to this embodiment can be manufactured by the above manufacturing method. Note that the above manufacturing method is a preferred example of the manufacturing method for the bolt according to this embodiment. Therefore, a bolt having the above configuration may be manufactured by a manufacturing method other than the above manufacturing method. In short, the manufacturing method is not particularly limited as long as it can manufacture the bolt according to this embodiment having the above configuration.

[0104] [Other processes] The bolt manufacturing process according to this embodiment may include other steps in addition to those described above. For example, a spheroidizing heat treatment step may be performed after the wire drawing step and before the cold forging step. Furthermore, a rolling step may be performed after the cold forging step and before the quenching and tempering step, or after the quenching and tempering step to form threads. Furthermore, a compressive residual stress imparting step may be performed after the quenching and tempering step. All of these steps are optional and may not be performed. [Example]

[0105] The effects of the bolt of this embodiment will be explained more specifically using examples. The conditions in the following examples are 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.

[0106] Steel material (steel bar) having the chemical composition shown in Table 1 was prepared as the material for the bolts.

[0107] [Table 1]

[0108] In Table 1, "-" means that the content of the corresponding element is 0% in significant figures (numerical values ​​down to the least significant digit) as defined in this embodiment. In other words, the content of the corresponding element is 0% when fractions are rounded up or down to the nearest significant digit as defined in this embodiment. For example, the Cu content specified in this embodiment is specified as a numerical value up to two decimal places, so that in test number 1 in Table 1, the measured Cu content was 0% when rounded to two decimal places. In addition, the Ni content specified in this embodiment is specified as a numerical value up to two decimal places. Therefore, in test number 1 in Table 1, the measured Ni content was 0% when rounded to two decimal places. Rounding off means that if the digit (fraction) below the specified minimum digit is less than 5, it is rounded down, and if it is 5 or more, it is rounded up.

[0109] [Steel preparation process] Steel materials of each steel grade number were produced by the following method. Blooms having the chemical compositions shown in Table 1 were subjected to rough rolling (blooming and hot rolling in a continuous rolling mill) to produce billets. The heating temperature of the blooms was 1200°C. The produced billets were subjected to finish rolling to produce steel bars with a diameter of 20 mm. The heating temperature of the billets during finish rolling was 1200°C.

[0110] [Bolt manufacturing process] Bolts were manufactured using the produced steel material. First, the 20 mm diameter steel material of each test number shown in Table 2 was subjected to wire drawing under the same conditions for each test number to produce a steel wire of 16 mm diameter. The steel wire of each test number was subjected to cold forging (bolt forming) under the same conditions for each test number to produce an intermediate product in the shape of a hexagonal bolt with a diagonal distance of 30 mm, a head height of 10 mm, and a nominal length of 100 mm.

[0111] [Table 2]

[0112] The manufactured intermediate products were subjected to quenching and tempering. For quenching, the quenching temperature was 920°C and the holding time at the quenching temperature was 60 minutes (1 hour) for each test number. After the holding time had elapsed, the intermediate products were water-cooled.

[0113] The intermediate products after quenching were tempered. The tempering temperature T (°C), holding time t (hours) at the tempering temperature, and Fn1 for each test number are shown in the "Tempering temperature T (°C)", "Holding time t (hours)", and "Fn1" columns in Table 2. Bolts with each test number were manufactured using the above manufacturing process.

[0114] [Evaluation test] The following evaluation tests were carried out on the bolts with each test number.

[0115] [Chemical composition analysis test] The chemical composition of the bolts with each test number was analyzed using the method described above for measuring the chemical composition of the bolts. As a result, the chemical composition of the bolts with each test number was as shown in Table 1.

[0116] [Tensile test] The tensile strength TS (MPa) of the bolts with each test number was determined based on the above-described [Method for measuring tensile strength]. The determined tensile strength TS is shown in the "TS (MPa)" column in Table 2.

[0117] For test numbers for which the obtained tensile strength was less than 1400 MPa, the subsequent Mo ratio measurement test and limit hydrogen amount measurement test were not carried out.

[0118] [Mo ratio measurement test in MC type carbide] The Mo ratio (atomic %) of the metal elements in the MC carbide of the bolt with each test number was determined based on the above-described [Method for measuring the Mo ratio in MC carbide]. The determined Mo ratios are shown in the "Mo ratio (atomic %)" column in Table 2.

[0119] [Limit hydrogen amount measurement test] The limiting hydrogen amount (ppm by mass) of the bolts with each test number was measured based on the above-described [Method for measuring limiting hydrogen amount]. The obtained limiting hydrogen amount (ppm by mass) is shown in the "Limiting hydrogen amount (ppm by mass)" column in Table 2.

[0120] [Evaluation test results] Referring to Tables 1 and 2, the chemical compositions of the bolts with test numbers 1 to 23 were appropriate. Furthermore, the Mo ratio in the MC carbides was 60 atomic % or more. As a result, the tensile strength was high at 1,400 MPa or more. Furthermore, the limiting hydrogen content was high at 1.0 mass ppm or more, and the hydrogen embrittlement resistance was excellent.

[0121] On the other hand, in test number 24, the Mo content was too low. Therefore, the Mo ratio in the MC type carbides was less than 60 atomic %. As a result, the limiting hydrogen content was less than 1.0 mass ppm, and the hydrogen embrittlement resistance was poor.

[0122] In test number 25, the Mo content was too high, so the limiting hydrogen content was less than 1.0 ppm by mass, and the hydrogen embrittlement resistance was poor.

[0123] In test number 26, the V content was too low, so the limiting hydrogen content was less than 1.0 ppm by mass, and the hydrogen embrittlement resistance was poor.

[0124] In test number 27, the V content was too high. As a result, the Mo ratio in the MC carbides was less than 60 atomic %. As a result, the limiting hydrogen content was less than 1.0 mass ppm, and the hydrogen embrittlement resistance was poor.

[0125] In test number 28, the Cr content was too high, so the limiting hydrogen content was less than 1.0 ppm by mass, and the hydrogen embrittlement resistance was poor.

[0126] In test number 29, the tempering temperature T was too high, and as a result, the tensile strength was less than 1400 MPa.

[0127] In test numbers 30 and 31, the tempering temperature T and holding time t were within appropriate ranges, but Fn1 was too low. As a result, the Mo ratio in the MC carbides was less than 60 atomic %. As a result, the limiting hydrogen content was less than 1.0 ppm by mass, and the hydrogen embrittlement resistance was poor.

[0128] In test Nos. 32 and 33, although the tempering temperature T and holding time t were within appropriate ranges, Fn1 was too high, resulting in a tensile strength TS of less than 1400 MPa.

[0129] 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 can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A bolt, In mass%, 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: 0.01-0.30%, Mo: 1.50-2.50%, V: 0.01-0.30%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0300%; The balance is Fe and impurities. The tensile strength is 1400 MPa or more, In the MC type carbide in the bolt, the ratio of Mo among metal elements excluding Fe is 60 atomic % or more. bolt.

2. The bolt of claim 1 further comprising: Instead of a part of Fe, Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.300% or less, Ca: 0.0050% or less, Bi: 0.100% or less, and Te: 0.100% or less, Contains one or more elements selected from the group consisting of bolt.

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