bolt

Abstract

To provide a bolt having high strength and excellent hydrogen embrittlement resistance.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: more than 0.8% to 1.50%, Mo: 0.01 to 1.50%, V: 0.01 to 0.5%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0200%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.40%, and Sn: 0.001 to 0.100%, with the balance being Fe and impurities, and has a tensile strength of 1100 MPa or more, the number density ND of MC-type carbides is 2.0 * 10 21 pieces / m3 or more, and in a rectangular observation region of 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the shank of the bolt in the cross section including the longitudinal direction and the radial direction of the shank of the bolt, the [TC] segregation degree σ is 0.400 or less.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, and buildings such as bridges. In recent years, with the increasing performance of industrial machinery and automobiles and the increasing size of buildings, there has been a demand for higher strength bolts. Specifically, there is a demand for bolts with a tensile strength of 1100 MPa or more.

[0003] Bolts with such high strength are susceptible to hydrogen embrittlement, so high strength bolts are required to have excellent resistance to hydrogen embrittlement.

[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 resistance to hydrogen embrittlement.

[0005] The bolt disclosed in Patent Document 1 has a chemical composition, in mass %, of 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, and Ni: 0.30% or more. The bolt further contains 0.05% or less Mo, 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 further contains 0.0020% or less O, 0.020% or less P, and 0.020% or less S, with the balance consisting of 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). Patent Document 1 states that by adjusting the contents of Sb, Sn, and Bi in the chemical composition of this bolt so as to satisfy formula (2), excellent hydrogen embrittlement resistance can be obtained even if the tensile strength of the shaft is 1000 to 1300 MPa.

[0006] The bolt disclosed in Patent Document 2 has a chemical composition, in mass%, of 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). Patent Document 2 states that by satisfying formula (1), the tensile strength of the bolt is increased to 1000 to 1300 MPa, and by satisfying formula (2), the hydrogen embrittlement resistance of the bolt is increased.

[0007] The bolt disclosed in Patent Document 3 has a chemical composition, in mass%, of C: 0.30 to 0.50%, Si: 1.0 to 2.5%, Mn: 0.1 to 1.5%, P: 0.015% or less (including 0%), S: 0.015% or less (including 0%), Cr: 0.15 to 2.4%, Al: 0.10% or less (including 0%), and N: 0.015% or less (including 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, 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 balance being Fe and impurities. Furthermore, this bolt has an austenite grain size number of 9.0 or greater 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 greater precipitated at the austenite grain boundaries, and L0 is the length of the austenite grain boundaries). Patent Document 3 states that by suppressing carbide precipitation at the grain boundaries, this bolt achieves excellent hydrogen embrittlement resistance even at high strength. [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] High strength and excellent hydrogen embrittlement resistance are obtained with the bolts disclosed in Patent Documents 1 to 3. However, high strength and excellent hydrogen embrittlement resistance may be obtained by means different from 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] A bolt according to the present disclosure comprises: The chemical composition is, 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: over 0.80~1.50%, Mo: 0.01 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0010~0.0200%, Cu: 0.01 to 0.40% Ni: 0.01 to 0.40%, and Sn: 0.001 to 0.100%; The balance is Fe and impurities. The tensile strength is 1100 MPa or more, The number density ND of MC type carbides is 2.0×10 21 pieces / m 3 That's all, In a cross section including the longitudinal and radial directions of the shank of the bolt, in a rectangular observation area of ​​1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the shank of the bolt, The 62,500 measurement areas, which were divided into 250 in the radial direction and 250 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% [Cu] in each of the obtained measurement areas was calculated. MA , Ni content [Ni] MA, Sn content [Sn] MA and [TC] using equation (1). MA Seeking The [TC] of all the measurement areas MA The arithmetic mean value of [TC] AVE year, [TC] for each measurement area MA The aforementioned [TC] AVE The ratio to [TC] S year, The [TC] of all the measurement areas S When the sample standard deviation of [TC] is the segregation degree σ, The [TC] segregation degree σ is 0.400 or less. [TC] MA =[Cu] MA +[Ni] MA +20[Sn] MA (1)

[0012] A bolt according to the present disclosure comprises: The chemical composition is, 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: over 0.80~1.50%, Mo: 0.01 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0010~0.0200%, Cu: 0.01 to 0.40% Ni: 0.01 to 0.40%, and Sn: 0.001 to 0.100%; Further, it contains one or more selected from the group consisting of Groups 1 to 3, The balance is Fe and impurities. The tensile strength is 1100 MPa or more, The number density ND of MC type carbides is 2.0×10 21 pieces / m 3 That's all, In a cross section including the longitudinal and radial directions of the shank of the bolt, in a rectangular observation area of ​​1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the shank of the bolt, The 62,500 measurement areas, which were divided into 250 in the radial direction and 250 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% [Cu] in each of the obtained measurement areas was calculated. MA , Ni content [Ni] MA , Sn content [Sn] MA and [TC] using equation (1). MA Seeking The [TC] of all the measurement areas MA The arithmetic mean value of [TC] AVE year, [TC] for each measurement area MA The aforementioned [TC] AVE The ratio to [TC] S year, The [TC] of all the measurement areas S When the sample standard deviation of [TC] is the segregation degree σ, The [TC] segregation degree σ is 0.400 or less. [Group 1] B: 0.0050% or less, Zr: 0.100% or less, Hf: 0.010% or less, Ta: 0.010% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ti: 0.100% or less, and Nb: 0.100% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, Bi: 0.020% or less, and Te: 0.010% or less, one or more selected from the group consisting of [TC] MA =[Cu] MA +[Ni] MA +20[Sn] MA (1) [Effects of the Invention]

[0013] The 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 side view showing an example of a bolt according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors first investigated a bolt having high strength and excellent hydrogen embrittlement resistance from the viewpoint of chemical composition, and as a result, the present inventors found that the bolt had a composition of, 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: over 0.80 to 1.50%, Mo: 0.01 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0200%, B: 0 to 0.0050%, Zr: 0 to 0.0050%, It was thought that a bolt with a chemical composition of 0.100%, Hf: 0-0.010%, Ta: 0-0.010%, W: 0-0.20%, Ti: 0-0.100%, Nb: 0-0.100%, Ca: 0-0.0050%, Bi: 0-0.020%, Te: 0-0.010%, and the remainder being Fe and impurities could potentially achieve a tensile strength of 1100 MPa or more and excellent hydrogen embrittlement resistance.

[0016] Therefore, the inventors further investigated means for improving the strength and hydrogen embrittlement resistance of bolts having the above-mentioned chemical composition from the viewpoint of their microstructure. As a result, the inventors discovered that by dispersing a large number of fine alloy carbides in the bolt, it is possible to improve the hydrogen embrittlement resistance of even bolts with a tensile strength of 1100 MPa or more.

[0017] The inventors further considered that the type of alloy carbide affects the hydrogen embrittlement resistance of the bolt. Therefore, further investigations were conducted. Among alloy carbides, the inventors focused on MC-type 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.). Fine alloy carbides act as hydrogen trapping sites and occlude hydrogen. Therefore, the amount of hydrogen that can be stored before hydrogen embrittlement cracking occurs (hereinafter referred to as the limiting hydrogen amount) can be increased. Therefore, increasing the number density of MC-type carbides in a bolt may improve the hydrogen embrittlement resistance of the bolt.

[0018] Therefore, the inventors investigated the relationship between the number density of MC carbides and the limiting hydrogen content in bolts having the above-mentioned chemical composition. As a result, it was found that the number density of MC carbides was 2.0 × 10 21 pieces / m 3 It was found that if this is the case, even bolts with a tensile strength of 1100 MPa or more can obtain a sufficient limiting hydrogen amount, and as a result, excellent hydrogen embrittlement resistance can be obtained.

[0019] However, the number density of MC type carbides is 2.0 × 10 21 pieces / m 3 Even with bolts that meet these specifications, if the tensile strength is 1100 MPa or more, sufficient hydrogen embrittlement resistance may still not be obtained. Therefore, the present inventors conducted further investigations.

[0020] Adjusting the number density of MC carbides as described above is a means of increasing the limit of hydrogen that can be stored inside the bolt, thereby improving hydrogen embrittlement resistance. Hydrogen embrittlement occurs when the amount of hydrogen that penetrates into the bolt from the bolt surface exceeds the limit. Therefore, it is thought that hydrogen embrittlement resistance can be further improved by adopting not only measures to increase the limit of hydrogen, but also measures to suppress hydrogen penetration from the bolt surface.

[0021] Based on the above considerations, the inventors further investigated the chemical composition of bolts as a means for suppressing hydrogen penetration. Cu, Ni, and Sn suppress corrosion of steel. Corrosion suppression suppresses hydrogen generation on the steel surface, which in turn suppresses hydrogen penetration into the steel. Therefore, if a bolt contains Cu, Ni, and Sn, hydrogen penetration into the bolt can be suppressed. Based on the above considerations, the present inventors have determined that the composition of the alloy is, 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: over 0.80 to 1.50%, Mo: 0.01 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0200%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.50%, 0.40%, Sn: 0.001-0.100%, B: 0-0.0050%, Zr: 0-0.100%, Hf: 0-0.010%, Ta: 0-0.010%, W: 0-0.20%, Ti: 0-0.100%, Nb: 0-0.100%, Ca: 0-0.0050%, Bi: 0-0.020%, Te: 0-0.010%, and the balance being Fe and impurities, and the number density ND of the MC type carbides is 2.0 × 10 21 pieces / m 3 It was thought that a bolt with these specifications could potentially achieve a tensile strength of 1100 MPa or more and excellent hydrogen embrittlement resistance.

[0022] However, simply containing Cu, Ni, and Sn may not always provide excellent hydrogen embrittlement resistance. The inventors focused on the distribution of Cu, Ni, and Sn in the surface layer of the bolt. Among Cu, Ni, and Sn, the hydrogen penetration suppression effect of Sn is significantly higher than that of Cu and Ni. Meanwhile, the hydrogen penetration suppression effect of Cu is comparable to that of Ni. Therefore, if the surface layer is divided into multiple microzones, and the concentrations of Cu, Ni, and Sn are distributed in each microzone so that the hydrogen penetration suppression effect in each microzone is comparable, the hydrogen penetration suppression effect in the surface layer will be uniform without variation, and hydrogen penetration can be effectively suppressed. Based on the above findings, the Cu concentration [Cu] in the microzone MA, Ni concentration [Ni] MA , and Sn concentration [Sn] MA Based on this, the hydrogen penetration suppression effect index [TC] MA is defined as follows: [TC] MA =[Cu] MA +[Ni] MA +20[Sn] MA (1)

[0023] As mentioned above, the hydrogen penetration suppression effect of Sn is significantly higher than that of Cu and Ni, so [Sn] in formula (1) MA In each micro area of ​​the surface layer, [TC] MA It is thought that if the values ​​are similar, hydrogen penetration into the surface layer can be suppressed.

[0024] Based on the above findings, the present inventors conducted further studies. As a result, [TC] of all micro-areas MA The arithmetic mean value of [TC] AVE and [TC] for each micro-area MA [TC] AVE The ratio to [TC] S and [TC] for all micro-areas S The sample standard deviation of [TC] is defined as the [TC] segregation degree σ. The present inventors believe that if the [TC] segregation degree σ can be reduced, the [TC] in Equation (1) can be reduced. MA It was thought that this would be uniformly distributed on the surface of the bolt and would be able to sufficiently suppress the penetration of hydrogen into the bolt.

[0025] As a result, the number density ND of the MC type carbide was 2.0 × 10 21 pieces / m 3 Furthermore, the inventors have found that if the [TC] segregation degree σ of the bolt described above is 0.400 or less, the hydrogen penetration suppression effect in the surface layer of the bolt will be uniform without variation, hydrogen penetration will be sufficiently suppressed, and hydrogen embrittlement resistance can be improved even for bolts having a tensile strength of 1100 MPa or more.

[0026] The bolt of this embodiment has been completed based on the above technical concept and has the following configuration.

[0027] The bolts in the first configuration are: The chemical composition is, 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: over 0.80~1.50%, Mo: 0.01 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0010~0.0200%, Cu: 0.01 to 0.40% Ni: 0.01 to 0.40%, and Sn: 0.001 to 0.100%; The balance is Fe and impurities. The tensile strength is 1100 MPa or more, The number density ND of MC type carbides is 2.0×10 21 pieces / m 3 That's all, In a cross section including the longitudinal and radial directions of the shank of the bolt, in a rectangular observation area of ​​1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the shank of the bolt, The 62,500 measurement areas, which were divided into 250 in the radial direction and 250 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% [Cu] in each of the obtained measurement areas was calculated. MA , Ni content [Ni] MA , Sn content [Sn] MA and [TC] using equation (1). MA Seeking The [TC] of all the measurement areas MA The arithmetic mean value of [TC] AVE year, [TC] for each measurement area MA The aforementioned [TC]AVE The ratio to [TC] S year, The [TC] of all the measurement areas S When the sample standard deviation of [TC] is the segregation degree σ, The [TC] segregation degree σ is 0.400 or less. [TC] MA =[Cu] MA +[Ni] MA +20[Sn] MA (1)

[0028] The second configuration of bolts is The chemical composition is, 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: over 0.80~1.50%, Mo: 0.01 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0010~0.0200%, Cu: 0.01 to 0.40% Ni: 0.01 to 0.40%, and Sn: 0.001 to 0.100%; Further, it contains one or more selected from the group consisting of Groups 1 to 3, The balance is Fe and impurities. The tensile strength is 1100 MPa or more, The number density ND of MC type carbides is 2.0×10 21 pieces / m 3 That's all, In a cross section including the longitudinal and radial directions of the shank of the bolt, in a rectangular observation area of ​​1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the shank of the bolt, The 62,500 measurement areas, which were divided into 250 in the radial direction and 250 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% [Cu] in each of the obtained measurement areas was calculated. MA , Ni content [Ni] MA , Sn content [Sn] MA and [TC] using equation (1). MA Seeking The [TC] of all the measurement areas MA The arithmetic mean value of [TC] AVE year, [TC] for each measurement area MA The aforementioned [TC] AVE The ratio to [TC] S year, The [TC] of all the measurement areas S When the sample standard deviation of [TC] is the segregation degree σ, The [TC] segregation degree σ is 0.400 or less. [Group 1] B: 0.0050% or less, Zr: 0.100% or less, Hf: 0.010% or less, Ta: 0.010% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ti: 0.100% or less, and Nb: 0.100% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, Bi: 0.020% or less, and Te: 0.010% or less, one or more selected from the group consisting of [TC] MA =[Cu] MA +[Ni] MA +20[Sn] MA (1)

[0029] The third configuration of bolts is A bolt of a second configuration, The chemical composition includes the first group.

[0030] The fourth configuration bolt is A bolt of the second or third configuration, The chemical composition includes the second group.

[0031] The fifth configuration bolt is A bolt having any one of the second to fourth configurations, The chemical composition includes the third group.

[0032] The bolt according to this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0033] Bolt Configuration The shape of the bolt of this embodiment has a well-known structure. FIG. 1 is a side view showing an example of a bolt of this embodiment. Referring to FIG. 1, the bolt of this embodiment includes a head 10, a neck portion 11, and a shank 12. The neck portion 11 is a portion that connects the head 10 and the shank 12, and has a curved surface. In other words, the surface of the neck portion 11 has a curvature. The shank 12 extends from the neck portion 11 in the direction of the central axis of the bolt. The shank 12 has a thread formed on at least a portion of its circumferential surface.

[0034] [Features of the bolt of this embodiment] The bolt of this embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition is, 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: over 0.80-1.50%, Mo: 0.01-1.50%, V: 0.01-0.50%, Al: 0.005-0.100%, N: 0.0010-0.0200%, Cu: 0.01-0.40%, Ni: 0 0.01-0.40%, Sn: 0.001-0.100%, B: 0-0.0050%, Zr: 0-0.100%, Hf: 0-0.010%, Ta: 0-0.010%, W: 0-0.20%, Ti: 0-0.100%, Nb: 0-0.100%, Ca: 0-0.0050%, Bi: 0-0.020%, and Te: 0-0.010%, with the remainder consisting of Fe and impurities. (Feature 2) The tensile strength is 1100 MPa or more. (Feature 3) The number density ND of MC type carbides is 2.0×10 21 pieces / m 3 That's all. (Feature 4) In a cross section of the bolt shank, including the longitudinal and radial directions, a rectangular observation area measuring 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the bolt shank was divided into 250 parts in the radial direction and 250 parts in the longitudinal direction, and area analysis was carried out using an electron beam microanalyzer on 62,500 measurement areas. The Cu content in mass% in each measurement area was calculated as follows: MA , Ni content [Ni] MA , Sn content [Sn] MA and [TC] using equation (1). MA Calculate the [TC] of all measurement areas. MA The arithmetic mean value of [TC] AVE and [TC] for each measurement area. MA [TC] AVE The ratio to [TC] S and [TC] in all measurement areas. S When the sample standard deviation is taken as the [TC] segregation degree σ, the [TC] segregation degree σ is 0.400 or less. [TC] MA =[Cu] MA +[Ni] MA+20[Sn] MA (1)

[0035] The bolt according to this embodiment satisfies the above-mentioned Features 1 to 4. Therefore, the bolt according to this embodiment has high strength and excellent resistance to hydrogen embrittlement. Features 1 to 4 will be explained below.

[0036] [(Feature 1) Chemical composition] The chemical composition of the bolt of this embodiment contains the following elements.

[0037] 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%.

[0038] 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 0.28%, more preferably 0.26%, and even more preferably 0.15%.

[0039] 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 1.20%.

[0040] P:0.030% or less Phosphorus (P) is an unavoidable impurity. If the P content exceeds 0.030%, P segregates at grain boundaries, resulting in a decrease in the hydrogen embrittlement resistance of the bolt. 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 more than 0%, more preferably 0.001%, even more preferably 0.002%, and still more preferably 0.003%. The upper limit of the P content is preferably 0.025%, and more preferably 0.020%.

[0041] S: 0.030% or less Sulfur (S) is an unavoidable impurity. If the S content exceeds 0.030%, S segregates at grain boundaries, resulting in a decrease in the hydrogen embrittlement resistance of the bolt. 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 more than 0%, more preferably 0.001%, even 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%.

[0042] Cr: More than 0.80~1.50% Chromium (Cr) improves the hardenability of steel material and increases the strength of the bolt. Cr also improves the temper softening resistance of the steel material and increases the strength of the bolt. If the Cr content exceeds 0.80%, the above effects can be effectively obtained, provided that the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr 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 Cr content is more than 0.80% and up to 1.50%. The lower limit of the Cr content is preferably 0.82%, and more preferably 0.84%. The upper limit of the Cr content is preferably 1.45%, and more preferably 1.40%.

[0043] Mo: 0.01 to 1.50% Molybdenum (Mo) increases the temper softening resistance of steel material and increases the strength of bolts. If the Mo 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 Mo content is 1.50% or less, the above effect can be effectively obtained, provided that the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0.01 to 1.50%. The lower limit of the Mo content is preferably 0.05%, and more preferably 0.10%. The upper limit of the Mo content is preferably 1.40%, and more preferably 1.30%.

[0044] V: 0.01 to 0.50% Vanadium (V) forms MC-type carbides to enhance 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 0.50%, the hardness of the steel material becomes excessively high, and in this case, even if the contents of other elements are within the ranges of this embodiment, the cold workability deteriorates. Therefore, the V content is 0.01 to 0.50%. 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 0.45%, and more preferably 0.40%.

[0045] 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%, coarse Al nitrides are formed. These coarse Al nitrides become the starting points for fracture. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel material is reduced. 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.

[0046] N: 0.0010~0.0200% 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 0.0200%, coarse nitrides are formed. These coarse nitrides become the starting points for fracture. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel material is reduced. Therefore, the N content is 0.0010 to 0.0200%. 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 0.0190%, more preferably 0.0180%, even more preferably 0.0170%, even more preferably 0.0150%, and even more preferably 0.0100%.

[0047] Cu: 0.01 to 0.40% Copper (Cu) inhibits hydrogen penetration into the bolt, thereby improving the hydrogen embrittlement resistance of the bolt. If the Cu 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 Cu content exceeds 0.40%, the above effects become saturated even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.01 to 0.40%. The lower limit of the Cu content is preferably 0.02%, 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%.

[0048] Ni: 0.01 to 0.40% Nickel (Ni) inhibits hydrogen penetration into the bolt, thereby improving the hydrogen embrittlement resistance of the bolt. If the Ni 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 Ni content exceeds 0.40%, the above effect saturates. Therefore, the Ni content is 0.01 to 0.40%. The lower limit of the Ni content is preferably 0.02%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Ni content is preferably 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0049] Sn: 0.001 to 0.100% Tin (Sn) inhibits hydrogen penetration into the bolt, thereby improving the hydrogen embrittlement resistance of the bolt. If the Sn content is less than 0.001%, 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 Sn content exceeds 0.100%, the above effects become saturated even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0.001 to 0.100%. The lower limit of the Sn content is preferably 0.002%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Sn content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0050] The remainder of the chemical composition of the bolt according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during industrial production of the steel material that is the material for the bolt, but are not intentionally added and are allowed to the extent that they do not adversely affect the bolt according to this embodiment.

[0051] [Optional Elements] The chemical composition of the bolt of this embodiment may further contain one or more elements selected from the group consisting of first to third groups in place of a portion of Fe. [Group 1] B: 0.0050% or less, Zr: 0.100% or less, Hf: 0.010% or less, Ta: 0.010% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ti: 0.100% or less, and Nb: 0.100% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, Bi: 0.020% or less, and Te: 0.010% or less, one or more selected from the group consisting of These optional elements will be explained below.

[0052] [Group 1 (B, Zr, Hf, Ta and W)] 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 B, Zr, Hf, Ta, and W. All of these elements are optional elements, and they improve the hardenability of the steel material and increase the strength of the bolt.

[0053] B: 0.0050% 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 increases 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. On the other hand, if the B content exceeds 0.0050%, coarse B nitrides are formed. These coarse B nitrides become the starting points for cracks. As a result, the cold workability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0050%, and if B is contained, it is 0.0050% or less. 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 0.0040%, more preferably 0.0030%, even more preferably 0.0020%, and still more preferably 0.0010%.

[0054] Zr: 0.100% 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 improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of Zr is contained, the above effects can be obtained to some extent. On the other hand, if the Zr content exceeds 0.100%, coarse Zr precipitates are formed. These coarse Zr precipitates become the starting points for fracture. As a result, the cold workability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Zr content is 0 to 0.100%, and if contained, it is 0.100% or less. The lower limit of the Zr content is preferably 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Zr content is preferably 0.090%, more preferably 0.080%, even more preferably 0.070%, even more preferably 0.060%, and still more preferably 0.050%.

[0055] Hf:0.010% or less Hafnium (Hf) is an optional element and may not be contained, that is, the Hf content may be 0%. When Hf is contained, that is, when the Hf content is more than 0%, Hf improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of Hf is contained, the above effects can be obtained to some extent. On the other hand, if the Hf content exceeds 0.010%, coarse Hf precipitates are formed. The coarse Hf precipitates become the starting points for fracture. As a result, the cold workability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Hf content is 0 to 0.010%, and if contained, it is 0.010% or less. The lower limit of the Hf content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Hf content is preferably 0.009%, more preferably 0.008%, even more preferably 0.007%, and still more preferably 0.006%.

[0056] Ta: 0.010% or less Tantalum (Ta) is an optional element and may not be contained, that is, the Ta content may be 0%. When Ta is contained, that is, when the Ta content exceeds 0%, Ta improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of Ta is contained, the above effects can be obtained to some extent. On the other hand, if the Ta content exceeds 0.010%, coarse Ta precipitates are formed. The coarse Ta precipitates become the starting points for fracture. As a result, the cold workability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ta content is 0 to 0.010%, and if contained, it is 0.010% or less. The lower limit of the Ta content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Ta content is preferably 0.009%, more preferably 0.008%, even more preferably 0.007%, and still more preferably 0.006%.

[0057] W: 0.20% or less Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of W is contained, the above effects can be obtained to some extent. On the other hand, if the W content exceeds 0.20%, the hardenability becomes too high, and as a result, the cold workability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0 to 0.20%, and if W is contained, it is 0.20% or less. The lower limit of the W content is preferably 0.01%, and more preferably 0.02%. The upper limit of the W content is preferably 0.15%, more preferably 0.12%, and even more preferably 0.10%.

[0058] [Regarding Group 2 (Ti and Nb)] The chemical composition of the bolt according to this embodiment may further contain one or more elements selected from the group consisting of Ti and Nb in place of a portion of Fe. These elements are optional and form precipitates to refine the crystal grains. As a result, the hydrogen embrittlement resistance of the bolt is improved.

[0059] 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. On the other hand, if the Ti content is too high, coarse Ti precipitates are formed, which become the starting points for cracks. As a result, the cold workability of the steel material is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.100%, and if contained, it is 0.100% or less. 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%.

[0060] 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. On the other hand, if the Nb content is too high, coarse Nb precipitates are formed. The coarse Nb precipitates become the starting points for cracks. As a result, the cold workability of the steel material is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.100%, and when Nb is contained, it is 0.100% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.010%, and even 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%.

[0061] [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. All of these elements are optional elements and improve the machinability of the steel material.

[0062] 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. On the other hand, if the Ca content exceeds 0.0050%, the cold 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 if contained, it is 0.0050% or less. 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%.

[0063] Bi:0.020% 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. On the other hand, if the Bi content exceeds 0.020%, the cold 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 0.020%, and if Bi is contained, it is 0.020% or less. 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 0.018%, and more preferably 0.015%.

[0064] Te: 0.010% 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. On the other hand, if the Te content exceeds 0.010%, the cold 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 0.010%, and if contained, it is 0.010% or less. 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 0.009%, and more preferably 0.007%.

[0065] [(Feature 2) Tensile strength] The bolt according to this embodiment has a tensile strength of 1100 MPa or more. The chemical composition of the bolt according to this embodiment satisfies Feature 1, Feature 3, and Feature 4. As a result, the bolt according to this embodiment has excellent hydrogen embrittlement resistance even when the tensile strength is 1100 MPa or more.

[0066] The lower limit of the tensile strength is preferably 1150 MPa, and more preferably 1200 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, 1600 MPa, for example, 1550 MPa.

[0067] [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 2241:2022 to obtain the tensile strength (MPa). The 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.

[0068] [(Feature 3) Number density of MC type carbides (ND)] In the bolt of this embodiment, the number density ND of the MC type carbide is 2.0 × 10 21 pieces / m 3 That's all.

[0069] MC carbides are finer than other alloy carbides such as cementite and M2C carbides. Therefore, MC carbides are more likely to trap hydrogen than other alloy carbides. In other words, increasing the number density of MC carbides can improve the hydrogen embrittlement resistance of bolts. On the other hand, when the number density ND of MC carbides is 2.0×10 21 pieces / m 3 If the ratio is less than 1 / 2, even if the bolt satisfies Features 1, 2, and 4, excellent hydrogen embrittlement resistance cannot be obtained.

[0070] Therefore, the number density of MC type carbides is 2.0 × 10 21 pieces / m 3 That's all. The preferred lower limit of the number density ND of MC type carbides is 2.5 × 10 21 pieces / m 3 and more preferably 3.0 × 10 21 pieces / m 3 and more preferably 4.0 × 10 21 pieces / m 3 is. The upper limit of the number density ND of the MC carbides is not particularly limited. When the bolt satisfies Features 1, 2, and 4, the upper limit of the number density ND of the MC carbides is, for example, 200.0 × 10 21 pieces / m 3 and preferably 100.0×10 21 pieces / m 3 and more preferably 80.0 × 10 21 pieces / m 3 is.

[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) by the following method. First, a thin-film sample for STEM is prepared using the following method. The lower neck of the bolt is cut perpendicular to the axial direction (longitudinal direction) of the bolt, and a disk with an axial thickness of approximately 2 mm (i.e., a disk with a diameter of 3.0 mm and a thickness of approximately 2 mm) is taken, with the center of the circle located 1.5 mm deep from the surface of the lower neck of the bolt. Both sides of the disk (front and back) are polished using emery paper. The front surface of the disk is polished so that it is parallel to the back surface. One of the front or back surfaces of the disk is defined as the observation surface. The observation surface is then mirror-polished. The mirror-polished observation surface is then further polished using colloidal silica as an abrasive.

[0072] MC carbides have a specific crystal orientation relationship with the matrix. Specifically, MC carbides are plate-like particles extending along the {100} plane of the matrix. Therefore, electron backscatter diffraction (EBSD) is performed on the polished observation surface to identify the crystal orientation of the matrix. Based on the identified crystal orientation of the matrix, it is determined that the perpendicular direction (observation direction) of the observation surface of the thin film is the same as that of the matrix. <001> The disk is processed by focused ion beam (FIB) processing to obtain the crystal orientation, and a thin film sample for STEM is prepared.

[0073] The preparation of a thin film sample for STEM by FIB processing may be carried out by a well-known method, for example, by the lift-out method using a gallium (Ga) ion beam at an acceleration voltage of 30 kV.

[0074] The surface of a STEM thin film specimen fabricated using an ion beam with an accelerating voltage of 30 kV contains dislocation loops and amorphous structures, making it unsuitable for observing MC carbides of a few nanometers in size. Therefore, the surface of the STEM thin film specimen is polished using an ion beam with a low accelerating voltage of 1 kV or less. This manufacturing process produces a STEM thin film specimen with a thickness of 100 nm or less.

[0075] The prepared thin film specimen is observed using the STEM optical system. Specifically, the crystal orientation of the martensite parent phase in the STEM thin film specimen is <001> The thin film specimen for STEM is tilted so that the incident light is on the zone axis. The observation magnification is set to 320,000x, and the accelerating voltage is set to 300 kV. The detector is set under known high-angle annular dark-field (HAADF) conditions, known low-angle annular dark-field (LAADF) conditions, and known bright-field (BF) conditions, and 10 observation fields are observed at random. Images are created for each observation field under these observation conditions. Of the images generated for each observation field, the image in which the MC-type carbides can be most clearly recognized is used.

[0076] The area of ​​each observation field is 280 nm × 280 nm. In all observation fields, the thickness of the thin film sample for STEM is measured 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 their contrast. Among the identified precipitates, precipitates with a maximum length of 2 nm or more are identified. Here, the "maximum length" in STEM observation refers to the maximum line length when any two points on the interface between a precipitate and the matrix are selected and the entire line connecting those two points is contained 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 identified.

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

[0079] The number density ND (number / m) of MC carbides was calculated based on the total number of MC carbides identified in all observation fields by the above method and the total volume calculated from the area of ​​all observation fields and the thickness of the STEM thin film sample. 3 ) is found.

[0080] When the content of each element in the chemical composition is within the range of this embodiment, electron diffraction patterns of precipitates obtained by STEM observation showed that precipitates with a maximum length of 10 nm or less were almost all MC carbides, and precipitates other than MC carbides were hardly present. Furthermore, the maximum lengths of precipitates other than MC carbides, such as M2C carbides and cementite, all far exceeded 10 nm. Therefore, instead of using electron diffraction patterns, 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 carbides as described above.

[0081] [(Feature 4) [TC] Segregation σ] In the bolt of this embodiment, in a cross section including the longitudinal and radial directions of the bolt shank, a rectangular observation region of 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the bolt shank was divided into 250 in the radial direction and 250 in the longitudinal direction, and 62,500 measurement areas were subjected to area analysis using an electron beam microanalyzer. The Cu content [Cu] in mass% in each of the obtained measurement areas was MA , Ni content [Ni] MA , Sn content [Sn] MA and [TC] using equation (1). MA Calculate the [TC] of all measurement areas. MA The arithmetic mean value of [TC] AVE and [TC] for each measurement area. MA [TC] AVE The ratio to [TC] S and [TC] in all measurement areas. S When the sample standard deviation is taken as the [TC] segregation degree σ, the [TC] segregation degree σ is 0.400 or less. [TC] MA =[Cu] MA +[Ni] MA +20[Sn] MA (1)

[0082] Cu, Ni, and Sn (hereinafter referred to as hydrogen penetration inhibitor elements) present in the surface layer of a bolt inhibit hydrogen penetration into the bolt. Among Cu, Ni, and Sn, the hydrogen penetration inhibitory effect of Sn is significantly higher than that of Cu and Ni. On the other hand, the hydrogen penetration inhibitory effect of Cu is similar to that of Ni. Therefore, if the surface layer is divided into multiple micro-regions and the concentrations of Cu, Ni, and Sn are distributed in each micro-region so that the hydrogen penetration inhibitory effect in each micro-region is the same, the hydrogen penetration inhibitory effect in the surface layer will be uniform without variation, and hydrogen penetration can be effectively inhibited.

[0083] Specifically, in a cross section including the longitudinal and radial directions of the bolt shank, a rectangular observation area 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the bolt shank is divided into 250 measurement areas in the radial direction and 250 in the longitudinal direction (also referred to as micro-areas in this specification). The measurement areas are subjected to area analysis using an electron beam microanalyzer, and the Cu content [Cu] in mass% in each measurement area is calculated. MA , Ni content [Ni] MA , Sn content [Sn] MA and using equation (1), the hydrogen penetration suppression effect index [TC] MA is defined by equation (1). [TC] MA If is uniformly distributed on the surface layer of the bolt, it is possible to sufficiently suppress the penetration of hydrogen into the bolt.

[0084] Here, [TC] in all measurement areas MA The arithmetic mean value of [TC] AVE and [TC] for each measurement area. MA [TC] AVE The ratio to [TC] S and [TC] in all measurement areas. S The sample standard deviation of [TC] is defined as [TC] segregation σ. By defining [TC] segregation σ, [TC] MA The uniformity of the surface can be evaluated.

[0085] If the [TC] segregation rate σ exceeds 0.400, the [TC] MA The uniformity of [TC] is not sufficient. MA In this case, even if the bolt satisfies Features 1 to 3, it is not possible to sufficiently suppress the penetration of hydrogen. As a result, the bolt cannot achieve sufficient hydrogen embrittlement resistance.

[0086] [TC] If the segregation degree σ is 0.400 or less, [TC] MA As a result, on the premise that the bolt satisfies Features 1 to 3, the bolt can have excellent hydrogen embrittlement resistance.

[0087] The upper limit of the [TC] segregation degree σ is preferably 0.375, more preferably 0.350, and even more preferably 0.325. [TC] The lower limit of the segregation degree σ is not particularly limited, but is, for example, 0.080, and more preferably 0.100.

[0088] [[TC]Method for measuring segregation degree σ] The [TC] segregation degree σ of bolts can be measured by the following method using an electron probe microanalyzer (EPMA). First, a test specimen is taken from a cross section of the bolt shank, including the longitudinal and radial directions, with a rectangular observation area of ​​1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the bolt shank. This rectangular observation area is divided into 250 radial and 250 longitudinal sections, resulting in 62,500 measurement areas.

[0089] Elemental analysis is performed on each measurement area. In elemental analysis, the acceleration voltage is 15 kV, the probe current is 400 nA, the beam diameter is 2 μm, and the integration time is 0.1 seconds. The elements to be measured are Cu, Ni, and Sn, and the contents of Cu, Ni, and Sn in mass% in each measurement area are calculated, and the Cu content [Cu] MA , Ni content [Ni] MA and Sn content [Sn] MA Furthermore, [Cu]MA , [Ni] MA and [Sn] MA Using the following formula (1), [TC] MA Define [TC] MA =[Cu] MA +[Ni] MA +20[Sn] MA (1)

[0090] [TC] obtained in each measurement area MA Using this, the [TC] segregation degree σ in the rectangular observation area is calculated using the following method.

[0091] [TC] in all measurement areas MA The arithmetic mean value of [TC] AVE Furthermore, [TC] in each measurement area is defined as MA [TC] AVE The ratio to [TC] S In other words, [TC] S means the total amount of Cu, Ni, and Sn in each measurement area, excluding the influence of the Cu, Ni, and Sn contents in mass %.

[0092] [TC] in all measurement areas S The sample standard deviation of is calculated. The obtained value is defined as "[TC] segregation degree σ." The significant figure of [TC] segregation degree σ is three decimal places. In other words, [TC] segregation degree σ is the value obtained by rounding off the number to four decimal places.

[0093] [Effects of the bolt of this embodiment] The bolt of this embodiment satisfies the above-mentioned features 1 to 4. As a result, the bolt of this embodiment has excellent hydrogen embrittlement resistance even though it has a high tensile strength of 1100 MPa or more.

[0094] [Microstructure of the bolt of this embodiment] The microstructure of the bolt of this embodiment includes a hard structure with 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 structure is composed of martensite and / or bainite. When the bolt microstructure includes phases other than the hard structure, the remainder of the bolt microstructure other than the hard structure is composed of one or more phases selected from the group consisting of pro-eutectoid ferrite and pearlite. The bolt microstructure correlates with tensile strength. Specifically, if the tensile strength of the bolt is 1100 MPa or more, the area ratio of the hard structure in the bolt microstructure is 90% or more.

[0095] [Method for measuring the microstructure of bolts] The microstructure of the bolt of this embodiment can be measured by the following method.

[0096] A sample is taken from the cross section of the bolt's under-neck, perpendicular to the bolt shaft and including a position 1.5 mm deep from the surface of the bolt's under-neck, with the observation surface being a cross section. After polishing the observation surface of the sample, it is etched at room temperature for 10 seconds using 3% nitric acid alcohol (Nital etching solution). Five observation fields are determined on the etched observation surface, with each field centered at a depth of 1.5 mm, so that they do not overlap with each other. The size of each observation field is 90 μm × 120 μm. Each observation field is observed using an FE-SEM at a magnification of 1000x.

[0097] In the observation field, the hard structure can be easily distinguished from the pro-eutectoid ferrite and pearlite by contrast. Pro-eutectoid ferrite is observed as a white region with no visible substructure within the grains. In observation at 1000x magnification, pearlite is observed as a phase with a lamellar structure. The hard structure contains a substructure (lath) and is observed as a region with lower brightness than the pro-eutectoid ferrite. Therefore, pro-eutectoid ferrite and pearlite are identified based on contrast. Specifically, in observation at 1000x magnification, a white structure is recognized as pro-eutectoid ferrite. In observation at 1000x magnification, a structure in which lamellae can be confirmed is recognized as pearlite. In each observation field, structures other than pro-eutectoid ferrite and pearlite are considered to be hard structures.

[0098] Based on the identified pro-eutectoid ferrite and pearlite, the total area of ​​the pro-eutectoid ferrite and pearlite in the five observation fields is calculated. The total area of ​​the hard structure in the five observation fields is calculated by subtracting the total area of ​​the pro-eutectoid ferrite and pearlite in the five observation fields from the total area of ​​the five observation fields. The area ratio (%) of the hard structure is calculated based on the total area of ​​the hard structure in the five observation fields and the total area of ​​the five observation fields.

[0099] [Use of the bolt of this embodiment] The bolt of this embodiment can be used as a fastening means for industrial machinery, automobiles, buildings such as bridges, etc. It is particularly suitable as a bolt for use in low-temperature environments such as refrigerated / freezer warehouses and cold regions. The low-temperature environment referred to here specifically means an environment of 0°C or below. The bolt of this embodiment may also be used for applications other than those described above.

[0100] [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.

[0101] An example of a method for manufacturing a bolt according to this embodiment includes the following steps. (Process 1) Steel material preparation process (Process 2) Bolt manufacturing process Each step will be described below.

[0102] [(Process 1) Steel material preparation process] In the steel preparation step, steel (steel for bolts) that will be used to make bolts is prepared. The steel for bolts may be manufactured. Alternatively, a third party may prepare the steel for bolts. In the steel preparation step, steel whose chemical composition satisfies Feature 1 is prepared.

[0103] When manufacturing steel material for bolts, an example of a method for manufacturing steel material for bolts includes the following steps. (Process 11) Casting process (Process 12) Blooming rolling process (Process 13) Finishing rolling process Each step will be described below.

[0104] [(Process 11) Casting Process] In the casting process, a cast material having a chemical composition that satisfies Feature 1 is produced. Specifically, molten steel is prepared, the content of each element in the chemical composition of which satisfies Feature 1. The molten steel may be produced using a converter, an electric furnace, or other methods. The prepared molten steel is used to produce a material by a well-known casting method. For example, an ingot is produced by an ingot casting method. Alternatively, a bloom is produced by a continuous casting method. A cast material (ingot or bloom) is produced by the above steps.

[0105] [(Step 12) Blooming rolling process] In the blooming process, the bloom is hot-rolled (blooming) using a blooming mill to produce a billet. If a continuous rolling mill with a plurality of rolling stands arranged in a line is disposed downstream of the blooming mill, the billet may be hot-rolled using the continuous rolling mill to further reduce the size of the billet. In the blooming process, the bloom is heated in a heating furnace. The heating temperature is, for example, 1150 to 1300°C. The billet produced in the blooming process is allowed to cool to room temperature before the finish rolling process.

[0106] [(Process 13) Finishing rolling process] In the finish rolling process, the billet is heated in a heating furnace. The heating temperature is, for example, 1050 to 1200°C. The heated billet is finish rolled. The finish rolled billet is cooled to produce a steel material (steel material for bolts) that will be used as a material for bolts. The steel material is, for example, a steel bar or a wire rod.

[0107] [(Process 2) Bolt manufacturing process] In the bolt manufacturing process, bolts are manufactured using the above-mentioned steel material (steel material for bolts). The bolt manufacturing process includes the following steps. (Step 21) Wire drawing process (Process 22) Bolt forming process (Step 23) Heat treatment step Each step will be described below.

[0108] [(Step 21) Wire drawing process] In the wire drawing process, the steel material is subjected to a well-known wire drawing process to manufacture 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.

[0109] [(Process 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 cut to produce a bolt intermediate product equipped with a head, a neck portion, and a shaft portion.

[0110] The bolt forming process includes the following steps: (Step 221) Head forming process (Process 222) Thread forming process Each step will be described below.

[0111] [(Step 221) Head forming process] In the head forming process, cold forging is performed on the steel wire to form a bolt-shaped intermediate product (a bolt intermediate product). Specifically, the steel wire is first cut to a predetermined length. Then, the steel wire is forged using a punch and a die to form a bolt intermediate product having a head 10, a neck portion 11, and a shank 12.

[0112] In the head forming process, strain is imparted to the intermediate bolt. The strain imparted to the intermediate bolt becomes the driving force for the formation of MC carbides in the next heat treatment process.

[0113] [(Process 222) Thread forming process] In the thread forming process, a thread is formed on at least a portion of the shank 12 of the intermediate bolt product. In the thread forming process, the thread may be formed on at least a portion of the shank 12 by well-known rolling. Alternatively, the thread may be formed on at least a portion of the shank 12 by cutting instead of rolling. As will be described later, the thread forming process may be performed after the heat treatment process, rather than after the head forming process but before the heat treatment process.

[0114] [(Step 23) Heat treatment step] In the heat treatment process, the intermediate bolt after the bolt forming process is subjected to the following process. (Step 231) Quenching process (Step 232) Tempering process Each step will be described below.

[0115] [(Step 231) Quenching process] In the quenching step, a well-known quenching method is performed. The quenching temperature and the holding time at the quenching temperature are not particularly limited. The quenching temperature is, for example, 870 to 1050°C. The holding time at the quenching temperature is, for example, 15 to 240 minutes (4 hours). After the holding time has elapsed, the intermediate bolt is quenched. Specifically, the intermediate bolt is water-cooled or oil-cooled.

[0116] [(Step 232) Tempering process] In the tempering process, the intermediate bolt after the quenching process is tempered by heating the intermediate bolt after the quenching process and holding it for a predetermined period of time.

[0117] [Manufacturing conditions for the manufacturing method of this embodiment] The above-described manufacturing method satisfies the following conditions. (Condition 1) When the number of passes in which the steel material temperature in the blooming process is reduced by an area reduction rate of 10% or more within a range of 1100°C or more is defined as PN1, and the number of passes in which the steel material temperature in the finish rolling process is reduced by an area reduction rate of 10% or more within a range of more than 1000°C is defined as PN2, the following formula (A) is satisfied. 5×PN1+PN2≧25 (A) (Condition 2) The tempering temperature T in the tempering step is 570 to 650°C. (Condition 3) The holding time t at the tempering temperature T in the tempering step is 0.50 to 5.00 hours. Conditions 1 to 3 will be explained below.

[0118] [(Condition 1) Formula (A)] In this embodiment, when the number of passes for rolling at an area reduction rate of 10% or more within a range in which the steel material temperature in the blooming process is 1100°C or more is PN1, and the number of passes for rolling at an area reduction rate of 10% or more within a range in which the steel material temperature in the finish rolling process is above 1000°C is PN2, the following formula (A) is satisfied. 5×PN1+PN2≧25 (A) Note that integers of 1 or greater are substituted for the number of passes PN1 and PN2. Furthermore, the steel material temperature means the surface temperature of the steel material at the delivery side of the rolling stand. The area reduction rate for each pass is calculated using the following formula. Area reduction rate = (1 - area of ​​cross section perpendicular to the longitudinal direction of the steel material after reduction / area of ​​cross section perpendicular to the longitudinal direction of the steel material before reduction) x 100

[0119] FA is defined as follows: FA=5×PN1+PN2 FA is an index for increasing the diffusibility of Cu, Ni, and Sn in a bolt. Here, a "pass" means that the rolled material is subjected to a single reduction (external force) from a pair of work rolls in one rolling stand. In the blooming and finish rolling processes, hot rolling consisting of multiple passes is carried out.

[0120] When the steel temperature in the blooming process is in the range of 1100°C or higher, Cu, Ni, and Sn easily diffuse in the steel. By applying a reduction and imparting strain in this temperature range, the strain becomes the driving force, promoting the diffusion of Cu, Ni, and Sn. Furthermore, by applying a reduction in the finish rolling process when the steel temperature is in the range of over 1000°C, the diffusion of Cu, Ni, and Sn is further promoted. Here, Sn is an element that is particularly prone to segregation compared to Cu and Ni. The number of passes (PN1) in the blooming process has a significant impact on the diffusivity of Sn. Therefore, the FA is defined to increase the contribution of the number of passes (PN1), and the FA is increased. This further increases the diffusivity of Cu, Ni, and Sn in the steel.

[0121] If FA is less than 25, the number of passes PN1 and / or PN2 is too small. Therefore, Cu, Ni, and Sn do not diffuse sufficiently. As a result, even if conditions 2 and 3 are satisfied, the [TC] segregation ratio σ of the bolt exceeds 0.400.

[0122] If the FA is 25 or more, sufficient strain is imparted during the blooming and finish rolling processes, allowing Cu, Ni, and Sn to diffuse sufficiently. As a result, the bolt's [TC] segregation σ becomes 0.400 or less.

[0123] The number of passes PN1 and the number of passes PN2 are not particularly limited, but the number of passes PN1 is preferably 3 or more, and the number of passes PN2 is preferably 10 or more.

[0124] [(Condition 2) Tempering temperature 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 the MC type carbides is 2.0 × 10 21 pieces / m 3 If the tempering temperature T exceeds 650°C, the tensile strength will be less than 1100 MPa.

[0125] [(Condition 3) Holding time t at tempering temperature T] The holding time t at the tempering temperature T is 0.50 to 5.00 hours. If the holding time t at the tempering temperature T is less than 0.50 hours, the number density ND of the MC type carbides is 2.0 × 10 21 pieces / m 3 If the holding time t at the tempering temperature T exceeds 5.00 hours, the tensile strength will be less than 1100 MPa.

[0126] 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.

[0127] [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 bolt forming step. Furthermore, 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 imparting step may be performed after the heat treatment step. [Example]

[0128] 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.

[0129] Steel materials (steel bars) having the chemical compositions shown in Tables 1A and 1B were prepared as bolt materials.

[0130] [Table 1A]

[0131] [Table 1B]

[0132] [Steel preparation process] The steel material of each test number was produced by the following method. Blooms having the chemical compositions shown in Tables 1A and 1B were subjected to rough rolling (hot rolling in a blooming mill and a continuous rolling mill) to produce billets. The heating temperature of the blooms was 1150 to 1300°C. The produced billets were subjected to a finish rolling process to produce steel bars with a diameter of 20 mm. The heating temperature of the billets in the finish rolling process was 1050 to 1200°C. The number of passes PN1 in the blooming process, the number of passes PN2 in the finish rolling process, and FA for each test number were as shown in Table 2.

[0133] [Table 2]

[0134] [Bolt manufacturing process] Bolts were manufactured using the produced steel material (steel bars). First, the 20 mm diameter steel material of each test number was subjected to wire drawing under the same conditions for each test number to produce a steel wire of 16 mm diameter. A head forming process was carried out on the steel wire of each test number to produce a hexagonal bolt-shaped intermediate product with 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 carried out under the same conditions for each test number to form a threaded portion on the shank of the intermediate bolt product.

[0135] A heat treatment process was carried out on the intermediate bolts of each test number. First, a quenching process was carried out on the intermediate bolts under the same conditions for each test number. In the quenching process, the quenching temperature was 880 to 1050°C, and the holding time at the quenching temperature was 60 minutes (1 hour). After the holding time had elapsed, the intermediate bolts were water-cooled.

[0136] The intermediate bolts after the quenching process were subjected to a tempering process. The tempering temperature T (°C) and the holding time t (hours) at the tempering temperature T for each test number are shown in Table 2.

[0137] Bolts with each test number were manufactured using the above manufacturing process. The microstructure of each bolt with each test number was measured based on the method described above in [Method for measuring the microstructure of a bolt]. As a result, the area ratio of the hard structure was 90% or more for all bolts with test numbers that achieved a tensile strength of 1100 MPa or more in the tensile test described below.

[0138] [Evaluation test] The following evaluation tests were carried out on the bolts with each test number. (Test 1) Tensile test (Test 2) MC type carbide number density ND measurement test (Test 3) [TC] Measurement test of segregation degree σ (Test 4) Hydrogen embrittlement resistance evaluation test Each test will be explained below.

[0139] [(Test 1) Tensile test] The tensile strength (MPa) of the bolts with each test number was determined based on the method described above in [Method for measuring tensile strength]. The obtained tensile strengths are shown in Table 2.

[0140] For test numbers for which the obtained tensile strength was less than 1100 MPa, the subsequent evaluation tests (Tests 2 to 4) were not carried out.

[0141] [(Test 2) MC type carbide number density ND measurement test] Based on the method described in the above [Method for measuring the number density ND of MC type carbides], the number density ND (number / m 3 The number density ND obtained is shown in Table 2.

[0142] [(Test 3) [TC] Measurement test of segregation degree σ] The [TC] segregation degree σ of the bolts with each test number was determined based on the method described above in [Method for measuring [TC] segregation degree σ]. The obtained [TC] segregation degrees σ are shown in Table 2.

[0143] [(Test 4) Hydrogen embrittlement resistance evaluation test] The hydrogen embrittlement resistance of the bolts of each test number was measured by the following method. A round bar test specimen with a circular notch (hereinafter simply referred to as a round bar test specimen) was taken from the inside of the bolt, at least 1 mm deep from the surface. The size of the round bar test specimen was 7 mm in diameter at the parallel part and 70 mm in length. A circular notch extending circumferentially was formed at the center position in the longitudinal direction of the round bar test specimen. The R (radius of curvature) of the bottom of the notch was 0.175 mm.

[0144] The round specimens were charged with hydrogen using the cathodic hydrogen charging method under the following conditions: 1 L of a 3 mass % sodium chloride aqueous solution was mixed with 3 g of ammonium thiocyanate to prepare a room temperature aqueous solution (cathode charging solution). The round specimens were immersed in the cathode charging solution for 72 hours at a cathodic current density of 0.05 mA / cm. 2A constant current controlled by a voltage was generated to add hydrogen to the round bar specimen.

[0145] After the cathodic hydrogen charging method, the round bar test specimens were left at room temperature for 72 hours. A zinc plating coating was then formed on the surface of the hydrogen-charged round bar test specimens to prevent hydrogen from leaking out of the round bar test specimens. A constant load test was conducted at room temperature and atmospheric pressure, applying a constant load to the round bar test specimens so that a load of 90% of their tensile strength was applied. The test time was a maximum of 100 hours, and the test was stopped if the round bar test specimens lasted more than 100 hours without fracture.

[0146] If the round bar test piece lasted for 100 hours or more without fracture, it was determined that excellent hydrogen embrittlement resistance was obtained (indicated as "E" in the "Hydrogen embrittlement resistance" column in Table 2). On the other hand, if the round bar test piece fractured in less than 100 hours, it was determined that excellent hydrogen embrittlement resistance was not obtained (indicated as "B" in the "Hydrogen embrittlement resistance" column in Table 2).

[0147] [Test Results] Referring to Tables 1A, 1B and 2, the bolts of test numbers 1 to 17 satisfied features 1 to 4. Therefore, even when the tensile strength was 1100 MPa or more, excellent hydrogen embrittlement resistance was obtained.

[0148] On the other hand, in test numbers 18 and 19, the FA was too low, and therefore the [TC] segregation ratio σ exceeded 0.400, and as a result, excellent hydrogen embrittlement resistance was not obtained.

[0149] In test numbers 20 and 21, the tempering temperature T was too low, and therefore the number density ND of the MC type carbides was too low, resulting in failure to obtain excellent hydrogen embrittlement resistance.

[0150] In test numbers 22 and 23, the tempering temperature T was too high, resulting in a low tensile strength of less than 1100 MPa.

[0151] In test numbers 24 and 25, the holding time t at the tempering temperature T was too short, and therefore the number density ND of the MC carbides was too low, resulting in failure to obtain excellent hydrogen embrittlement resistance.

[0152] In test numbers 26 and 27, the holding time t at the tempering temperature T was too long. As a result, the tensile strength was low at less than 1100 MPa.

[0153] 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, The chemical composition, in mass%, is 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: 0.01-1.50%, V: 0.01-0.50%, Al: 0.005-0.100%, N: 0.0010-0.0200%, Cu: 0.01-0.40%, Ni: 0.01 to 0.40%, and Sn: 0.001 to 0.100%; The balance is Fe and impurities. The tensile strength is 1100 MPa or more, The number density ND of the MC type carbide is 2.0 × 10 ２１ pieces / m ３ That's all, In a cross section including the longitudinal direction and radial direction of the shank of the bolt, in a rectangular observation region extending from the surface of the shank to 1000 μm in the radial direction and 1000 μm in the longitudinal direction, The 62,500 measurement areas, which were divided into 250 in the radial direction and 250 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content [Cu] in mass% in each of the obtained measurement areas was calculated. ＭＡ , Ni content [Ni] ＭＡ , Sn content [Sn] ＭＡ and [TC] using equation (1). ＭＡ Seeking The [TC] of all the measurement areas ＭＡ The arithmetic mean value of [TC] ＡＶＥ year, The [TC] of each measurement area ＭＡ The above [TC] ＡＶＥ The ratio to [TC] Ｓ year, The [TC] of all the measurement areas Ｓ When the sample standard deviation is [TC] segregation degree σ, The [TC] segregation degree σ is 0.400 or less, bolt. [TC] ＭＡ =[Cu] ＭＡ +[N] ＭＡ +20 [Sn] ＭＡ (1)

2. A bolt, The chemical composition, in mass%, is 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: 0.01-1.50%, V: 0.01-0.50%, Al: 0.005-0.100%, N: 0.0010-0.0200%, Cu: 0.01-0.40%, Ni: 0.01 to 0.40%, and Sn: 0.001 to 0.100%; Further, it contains one or more selected from the group consisting of Groups 1 to 3, The balance is Fe and impurities. The tensile strength is 1100 MPa or more, The number density ND of the MC type carbide is 2.0 × 10 ２１ pieces / m ３ That's all, In a cross section including the longitudinal direction and radial direction of the shank of the bolt, in a rectangular observation region extending from the surface of the shank to 1000 μm in the radial direction and 1000 μm in the longitudinal direction, The 62,500 measurement areas, which were divided into 250 in the radial direction and 250 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content [Cu] in mass% in each of the obtained measurement areas was calculated. ＭＡ , Ni content [Ni] ＭＡ , Sn content [Sn] ＭＡ and [TC] using equation (1). ＭＡ Seeking The [TC] of all the measurement areas ＭＡ The arithmetic mean value of [TC] ＡＶＥ year, The [TC] of each measurement area ＭＡ The above [TC] ＡＶＥ The ratio to [TC] Ｓ year, The [TC] of all the measurement areas Ｓ When the sample standard deviation is [TC] segregation degree σ, The [TC] segregation degree σ is 0.400 or less, bolt. [Group 1] B: 0.0050% or less, Zr: 0.100% or less, Hf: 0.010% or less, Ta: 0.010% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ti: 0.100% or less, and Nb: 0.100% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, Bi: 0.020% or less, and Te: 0.010% or less, one or more selected from the group consisting of [TC] ＭＡ =[Cu] ＭＡ +[N] ＭＡ +20 [Sn] ＭＡ (1)

3. 3. The bolt according to claim 2, the chemical composition contains the first group; bolt.

4. 3. The bolt according to claim 2, The chemical composition contains the second group. bolt.

5. 3. The bolt according to claim 2, The chemical composition contains the third group. bolt.

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