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 0.80%, Mo: 0.70 to less than 1.50%, V: 0.01 to 0.50%, 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 TS of 1300 MPa or more, and in which the number density ND of MC-type carbide in the bolt is 2.0×1022 pieces / m3 or more, and, defining the C content in atom% in MC type carbide as [C], the Mo content in atom% as [Mo], and the V content in atom% as [V], the C ratio Rc defined by formula (1) is 0.30 to 0.42. Rc=[C] / ([C]+[Mo]+[V]) (1).SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to bolts.

Background Art

[0002] Bolts are used in industrial machines, automobiles, buildings typified by bridges, etc. In recent years, with the improvement in performance of industrial machines and automobiles, and the enlargement of buildings and the like, higher strength of bolts has been demanded. Specifically, bolts having a tensile strength of 1300 MPa or more are required.

[0003] In bolts having a tensile strength of 1300 MPa or more, the hydrogen embrittlement susceptibility increases. Therefore, bolts having a tensile strength of 1300 MPa or more are required to have excellent hydrogen embrittlement resistance characteristics.

[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 characteristics.

[0005] The bolts disclosed in Patent Document 1 contain, by mass%, C: 0.22 to 0.40%, Si: 0.10 to 1.50%, Mn: less than 0.20 to 0.40%, Cr: less than 0.70 to 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, Mo: 0.05% or less, V: 0.050% or less, Nb: 0.050% or less, further contain one or more selected from the group consisting of Sb: 0.001 to 0.100%, Sn: 0.001 to 0.100%, and Bi: 0.001 to 0.100%, and further contain O: 0.0020% or less, P: 0.020% or less, S: 0.020% or less, and the balance consists of Fe and impurities. This bolt further satisfies formula (1) (0.50 ≦ C + (1 / 10)×Si + (1 / 5)×Mn + (5 / 22)×Cr ≦ 0.85) and formula (2) (0.003 ≦ Sb + Sn + Bi ≦ 0.100). In this bolt, Patent Document 1 describes that by adjusting the contents of Sb, Sn, and Bi in the chemical composition to satisfy Formula (2), excellent hydrogen embrittlement resistance characteristics can be obtained even when the tensile strength of the shaft portion is 1000 to 1300 MPa.

[0006] The bolt disclosed in Patent Document 2 contains, by mass%, C: 0.22 to 0.40%, Si: 0.10 to 1.50%, Mn: less than 0.20 to 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%, and the balance consists 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 describes that in this bolt, by satisfying Formula (1), the strength of the bolt is increased to 1000 to 1300 MPa, and by satisfying Formula (2), the hydrogen embrittlement resistance characteristics of the bolt are enhanced.

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

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

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

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

Means for Solving the Problems

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

[0012] By 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 0.80%, Mo: less than 0.70 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, and, N: 0.0010 to 0.0300%, and the balance is composed of Fe and impurities, The tensile strength TS is 1300 MPa or more, The number density ND of MC type carbides is 2.0×10 22 pieces / m 3 or more, When the C content in atomic% in the MC type carbide is defined as [C], the Mo content in atomic% is defined as [Mo], and the V content in atomic% is defined as [V], the C ratio Rc defined by the formula (1) is 0.30 to 0.42, bolt. Rc = [C] / ([C] + [Mo] + [V]) (1)

Advantages of the Invention

[0013] The bolt according to the present disclosure has high strength and excellent hydrogen embrittlement resistance characteristics.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0015] The inventors of the present invention examined bolts having a tensile strength TS of 1300 MPa or more and excellent hydrogen embrittlement resistance characteristics. As a result, the inventors obtained the following findings.

[0016] First, the inventors of the present invention examined bolts having high strength and excellent hydrogen embrittlement resistance characteristics from the viewpoint of chemical composition. As a result, the inventors found that, 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 0.80%, Mo: less than 0.70 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0300%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0100%, Zr: 0 to 0.300%, Hf: 0 to 0.100%, Ta: 0 to 0.100%, W: 0 to 0.20%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Ca: 0 to 0.0050%, Bi: 0 to 0.020%, Te: 0 to 0.010%, and the balance being Fe and impurities, it is considered possible to achieve both a tensile strength TS of 1300 MPa or more and excellent hydrogen embrittlement resistance characteristics.

[0017] Therefore, from the perspective of the microstructure of the bolt having the above chemical composition, means for enhancing the strength and hydrogen embrittlement resistance characteristics were investigated. As a result, the inventors have found that even for a bolt having a tensile strength TS of 1300 MPa or more, the hydrogen embrittlement resistance characteristics can be enhanced by dispersing a large number of fine precipitates in the bolt.

[0018] The inventors further considered that the type of precipitate affects the hydrogen embrittlement resistance characteristics of the bolt. Therefore, further investigations were conducted. Here, the inventors focused on MC-type carbides among the alloy carbides. MC-type carbides are less likely to coarsen and are more likely to remain fine compared to other alloy carbides (for example, M2C-type carbides, cementite, etc.). Fine precipitates serve as hydrogen trap sites and absorb hydrogen. Therefore, the amount of hydrogen that can be stored until hydrogen embrittlement cracking occurs (hereinafter referred to as the critical hydrogen amount) can be increased. Therefore, if the number density of MC-type carbides in the bolt is increased, the hydrogen embrittlement resistance characteristics of the bolt may be enhanced.

[0019] Therefore, the inventors investigated the relationship between the number density of MC-type carbides and the critical hydrogen amount in the bolt having the above chemical composition. As a result, when the number density of MC-type carbides is 2.0×10 22 pieces / m 3 or more, it was found that even for a bolt having a tensile strength TS of 1300 MPa or more, a sufficient critical hydrogen amount can be obtained, and as a result, excellent hydrogen embrittlement resistance characteristics may be obtained.

[0020] However, even for a bolt having the above chemical composition and a number density of MC-type carbides of 2.0×10 22 pieces / m 3 or more, there were cases where sufficient hydrogen embrittlement resistance characteristics were still not obtained when the tensile strength TS was 1300 MPa or more.

[0021] Therefore, the inventors further studied. Here, the inventors considered that not only the number density of MC-type carbides but also the composition of MC-type carbides affects the hydrogen storage effect. Therefore, the relationship between the composition of MC-type carbides and the limiting hydrogen amount was studied. As a result, when the C content in atomic % in the MC-type carbide is defined as [C], the Mo content in atomic % is defined as [Mo], and the V content in atomic % is defined as [V], it was found that excellent hydrogen embrittlement resistance characteristics can be obtained even for bolts with a tensile strength TS of 1300 MPa or more if the C ratio Rc defined by formula (1) is 0.30 to 0.42. Rc = [C] / ([C] + [Mo] + [V]) (1)

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

[0023] [1] By 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 0.80%, Mo: less than 0.70 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, and, N: 0.0010 to 0.0300%, and the balance consists of Fe and impurities, the tensile strength TS is 1300 MPa or more, the number density ND of MC-type carbides is 2.0×10 22 pieces / m 3 or more, when the C content in atomic % in the MC-type carbide is defined as [C], the Mo content in atomic % is defined as [Mo], and the V content in atomic % is defined as [V], the C ratio Rc defined by formula (1) is 0.30 to 0.42, bolt. Rc = [C] / ([C] + [Mo] + [V]) (1)

[0024] [2] 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, Zr: 0.300% or less, Hf: 0.100% or less, Ta: 0.100% or less, W: 0.20% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ca: 0.0050% or less, Bi: 0.020% or less, and Te: 0.010% or less, containing one or more elements selected from the group consisting of bolt.

[0025] Hereinafter, the bolt according to the present embodiment will be described in detail. Note that “%” regarding elements means mass % unless otherwise specified.

[0026] [Configuration of Bolt] The shape of the bolt of the present embodiment has a well-known structure. FIG. 1 is a side view showing an example of the bolt of the present embodiment. Referring to FIG. 1, the bolt of the present embodiment includes a head 10, a lower neck portion 11, and a shaft portion 12. The lower neck portion 11 is a portion connecting the head 10 and the shaft portion 12, and its surface is curved. That is, the surface of the lower neck portion 11 has a curvature. The shaft portion 12 extends from the lower neck portion 11 in the central axis direction of the bolt. Threads are formed on at least a part of the circumferential surface of the shaft portion 12.

[0027] [Features of the Bolt of the Present Embodiment] The bolt of the present embodiment has the following features. (Feature A) The chemical composition is as shown below. (Feature B) The tensile strength TS is 1300 MPa or more. (Characteristic C) The number density ND of MC-type carbide is 2.0×10 22 pieces / m 3 or more. (Characteristic D) The C ratio Rc of MC-type carbide is 0.30 to 0.42. The following describes each characteristic.

[0028] [(Characteristic A) Chemical composition] The chemical composition of the bolt according to this embodiment contains the following elements.

[0029] C: 0.30 to 0.50% Carbon (C) increases the hardenability of the steel material and enhances the strength of the bolt. If the C content is less than 0.30%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.50%, even if the contents of other elements are within the range of this embodiment, the hydrogen embrittlement resistance characteristics of the bolt deteriorate. Therefore, the C content is 0.30 to 0.50%. The preferable lower limit of the C content is 0.32%, more preferably 0.35%. The preferable upper limit of the C content is 0.48%, more preferably 0.45%.

[0030] Si: 0.01 to 0.30% Silicon (Si) increases the hardenability of the steel material and enhances the strength of the bolt. If the Si content is less than 0.01%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Si content exceeds 0.30%, even if the contents of other elements are within the range of this embodiment, the hydrogen embrittlement resistance characteristics of the bolt deteriorate. Therefore, the Si content is 0.01 to 0.30%. The preferable lower limit of the Si content is 0.02%, more preferably 0.03%. The preferable upper limit of the Si content is 0.25%, more preferably 0.20%, still more preferably 0.15%.

[0031] Mn: 0.10 - 1.50% Manganese (Mn) enhances the hardenability of steel materials and increases the strength of bolts. If the Mn content is less than 0.10%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 1.50%, even if the contents of other elements are within the range of this embodiment, the hydrogen embrittlement resistance characteristics of the bolt deteriorate. Therefore, the Mn content is 0.10 - 1.50%. The preferable lower limit of the Mn content is 0.15%, more preferably 0.20%. The preferable upper limit of the Mn content is 1.30%, more preferably 1.20%, and even more preferably 1.10%.

[0032] P: 0.030% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is more than 0%. If the P content exceeds 0.030%, even if the contents of other elements are within the range of this embodiment, P segregates at the grain boundaries. As a result, the hydrogen embrittlement resistance characteristics of the bolt deteriorate. Therefore, the P content is 0.030% or less. It is preferable that the P content is as low as possible. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the P content is 0.025%, more preferably 0.020%.

[0033] S: 0.030% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content exceeds 0.030%, even if the contents of other elements are within the range of this embodiment, S segregates at the grain boundaries. As a result, the hydrogen embrittlement resistance characteristics of the bolt 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 the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the S content is 0.025%, more preferably 0.020%.

[0034] Cr: 0.01 - 0.80% Chromium (Cr) increases the hardenability of the steel material and enhances the strength of the bolt. Cr further increases the tempering softening resistance of the steel material and enhances the strength of the bolt. If the Cr content is less than 0.01%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Cr content exceeds 0.80%, even if the contents of other elements are within the scope of this embodiment, the grain boundary segregation of P is promoted. As a result, the hydrogen embrittlement resistance characteristics of the bolt deteriorate. Therefore, the Cr content is 0.01 - 0.80%. The preferable lower limit of the Cr content is 0.03%, more preferably 0.05%. The preferable upper limit of the Cr content is 0.70%, more preferably 0.60%, and even more preferably 0.50%.

[0035] Mo: 0.70 - less than 1.50% Molybdenum (Mo) increases the tempering softening resistance of the steel material and enhances the strength of the bolt. Mo further concentrates in the MC type carbide and enhances the hydrogen trapping function of the MC type carbide. As a result, the hydrogen embrittlement resistance characteristics of the bolt with high strength are enhanced. If the Mo content is less than 0.70%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Mo content is 1.50% or more, even if the contents of other elements are within the scope of this embodiment, the steel material becomes excessively hard. In this case, the cold workability deteriorates. Therefore, the Mo content is 0.70 - less than 1.50%. The preferable lower limit of the Mo content is 0.75%, more preferably 0.80%. The preferable upper limit of the Mo content is 1.40%, more preferably 1.30%.

[0036] V: 0.01 - 0.50% Vanadium (V) forms MC-type carbides together with Mo, enhancing the hydrogen embrittlement resistance of the bolt. If the V content is less than 0.01%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the V content exceeds 0.50%, even if the contents of other elements are within the scope of this embodiment, the steel material becomes overly hard. In this case, the cold workability deteriorates. Therefore, the V content is 0.01 - 0.50%. The preferable lower limit of the V content is 0.03%, more preferably 0.05%. The preferable upper limit of the V content is 0.45%, more preferably 0.40%, even more preferably 0.35%.

[0037] Al: 0.005 - 0.100% Aluminum (Al) deoxidizes the steel. Al further combines with N to form Al nitrides. Al nitrides suppress the coarsening of crystal grains due to the pinning effect. As a result, the hydrogen embrittlement resistance of the bolt is enhanced. If the Al content is less than 0.005%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Al content exceeds 0.100%, even if the contents of other elements are within the scope of this embodiment, coarse Al nitrides are generated. Coarse Al nitrides become the starting points of fracture. Therefore, the workability of the steel material deteriorates. Therefore, the Al content is 0.005 - 0.100%. The preferable lower limit of the Al content is 0.006%, more preferably 0.007%, even more preferably 0.008%. The preferable upper limit of the Al content is 0.090%, more preferably 0.080%, even more preferably 0.070%. In the chemical composition of the steel material of this embodiment, the Al content means the total Al (Total-Al) content.

[0038] N: 0.0010 - 0.0300% Nitrogen (N) combines with Al to form Al nitride. Al nitride suppresses the coarsening of crystal grains due to the pinning effect. As a result, the hydrogen embrittlement resistance characteristics of the bolt are enhanced. If the N content is less than 0.0010%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the N content exceeds 0.0300%, even if the contents of other elements are within the range of this embodiment, coarse nitrides are generated. Coarse nitrides become the starting points of fracture and deteriorate the workability of the steel material. Therefore, the N content is 0.0010 - 0.0300%. The preferable lower limit of the N content is 0.0020%, more preferably 0.0025%, and even more preferably 0.0030%. The preferable upper limit of the N content is 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%.

[0039] The remainder of the chemical composition of the bolt according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition are those mixed in from ores, scraps, or the manufacturing environment as raw materials when the bolt is industrially manufactured, and are those allowed within a range that does not adversely affect the bolt according to this embodiment.

[0040] [Optional Elements] [Regarding Cu, Ni, B, Zr, Hf, Ta, and W] The chemical composition of the bolt according to this embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, B, Zr, Hf, Ta, and W, instead of a part of Fe. These elements are all optional elements and may not be contained. When contained, Cu, Ni, B, Zr, Hf, Ta, and W enhance 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 exceeds 0%, Cu enhances the hardenability of the steel material and increases the strength of the bolt. Even if a little 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 range of this embodiment, the hardenability becomes too high. As a result, the workability of the steel material deteriorates. Therefore, the Cu content is 0 to 0.40%, and when contained, the Cu content is 0.40% or less (exceeding 0 to 0.40%). The preferable lower limit of the Cu content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferable upper limit of the Cu content is 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 exceeds 0%, Ni enhances the hardenability of the steel material and increases the strength of the bolt. Even if a little 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 range of this embodiment, the hardenability becomes too high. As a result, the workability of the steel material deteriorates. Therefore, the Ni content is 0 to 0.40%, and when contained, the Ni content is 0.40% or less (more than 0 to 0.40%). The preferable lower limit of the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable upper limit of the Ni content is 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 increases the hardenability of the steel material and increases the strength of the bolt. B also suppresses the grain boundary segregation of P and enhances the hydrogen embrittlement resistance characteristics of the bolt. Even if a small amount of B is contained, the above effects can be obtained to a certain extent. However, if the B content exceeds 0.0100%, even if the contents of other elements are within the range of this embodiment, coarse B nitrides are generated. Coarse B nitrides become the starting points of fracture. As a result, the workability of the steel material deteriorates. Therefore, the B content is 0 to 0.0100%, and when contained, the B content is 0.0100% or less (more than 0 to 0.0100%). The preferable lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The preferable upper limit of the B content is 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and even more preferably 0.0050%.

[0044] 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 exceeds 0%, Zr enhances the hardenability of the steel material and increases the strength of the bolt. Even if a small amount of Zr is contained, the above effects can be obtained to a certain extent. However, if the Zr content exceeds 0.300%, even if the contents of other elements are within the range of this embodiment, coarse Zr nitrides are formed. The coarse Zr nitrides become the starting points of fracture. As a result, the workability of the steel material deteriorates. Therefore, the Zr content is 0 to 0.300%, and when contained, the Zr content is 0.300% or less (exceeding 0 to 0.300%). The preferable lower limit of the Zr content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferable upper limit of the Zr content is 0.280%, more preferably 0.250%, even more preferably 0.200%, even more preferably 0.150%, and even more preferably 0.100%.

[0045] Hf: 0.100% 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 exceeds 0%, Hf enhances the hardenability of the steel material and increases the strength of the bolt. Even if a small amount of Hf is contained, the above effects can be obtained to a certain extent. However, if the Hf content exceeds 0.100%, even if the contents of other elements are within the range of this embodiment, coarse Hf nitrides are formed. The coarse Hf nitrides become the starting points of fracture. As a result, the workability of the steel material deteriorates. Therefore, the Hf content is 0 to 0.100%, and when contained, the Hf content is 0.100% or less (exceeding 0 to 0.100%). The preferable lower limit of the Hf content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferable upper limit of the Hf content is 0.080%, more preferably 0.070%, still more preferably 0.060%, and even more preferably 0.050%.

[0046] Ta: 0.100% 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 enhances the hardenability of the steel material and increases the strength of the bolt. If even a small amount of Ta is contained, the above effects can be obtained to a certain extent. However, if the Ta content exceeds 0.100%, even if the contents of other elements are within the scope of this embodiment, coarse Ta nitrides are formed. The coarse Ta nitrides become the starting points of fracture. As a result, the workability of the steel material deteriorates. Therefore, the Ta content is 0 to 0.100%, and when contained, the Ta content is 0.100% or less (exceeding 0 to 0.100%). The preferable lower limit of the Ta content is 0.001%, more preferably 0.005%, and still more preferably 0.010%. The preferable upper limit of the Ta content is 0.080%, more preferably 0.070%, still more preferably 0.060%, and even more preferably 0.050%.

[0047] 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 enhances the hardenability of the steel material and increases the strength of the bolt. If even a small amount of W is contained, the above effects can be obtained to a certain extent. However, if the W content exceeds 0.20%, even if the contents of other elements are within the scope of this embodiment, the hardenability becomes too high. As a result, the workability of the steel material deteriorates. Therefore, the W content is 0 to 0.20%, and when contained, the W content is 0.20% or less (exceeding 0 to 0.20%). The preferable lower limit of the W content is 0.01%, more preferably 0.02%. The preferable upper limit of the W content is 0.15%, more preferably 0.12%, still more preferably 0.10%.

[0048] [Regarding 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 part of Fe. These elements are all optional elements and may not be contained. When contained, Ti and Nb form precipitates and refine the crystal grains. As a result, the hydrogen embrittlement resistance characteristics of the bolt are enhanced.

[0049] 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 exceeds 0%, Ti forms fine precipitates such as Ti carbide and refines the crystal grains. As a result, the hydrogen embrittlement resistance characteristics of the bolt are enhanced. Even if a little Ti is contained, the above effects can be obtained to some extent. However, if the Ti content is too high, even if the contents of other elements are within the range of this embodiment, coarse Ti nitride is generated. Coarse Ti nitride becomes a starting point of fracture. As a result, the workability of the steel material deteriorates. Therefore, the Ti content is 0 to 0.100%, and when contained, the Ti content is 0.100% or less (exceeding 0 to 0.100%). The preferable lower limit of the Ti content is 0.001%, more preferably 0.003%, still more preferably 0.005%. The preferable upper limit of the Ti content is 0.090%, more preferably 0.080%, still more preferably 0.075%.

[0050] 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 exceeds 0%, Nb forms fine precipitates such as Nb carbides and refines the crystal grains. As a result, the hydrogen embrittlement resistance property of the bolt is enhanced. Even if 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 range of this embodiment, coarse Nb carbides or the like are generated. Coarse Nb carbides or the like become the starting points of fracture. As a result, the workability of the steel material deteriorates. Therefore, the Nb content is 0 to 0.100%, and when contained, the Nb content is 0.100% or less (more than 0 to 0.100%). The preferable lower limit of the Nb content is 0.001%, more preferably 0.010%, and still more preferably 0.020%. The preferable upper limit of the Nb content is 0.090%, more preferably 0.080%, and still more preferably 0.070%.

[0051] [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 part of Fe. All of these elements are optional elements and may not be contained. When contained, Ca, Bi, and Te enhance the machinability of the steel material.

[0052] 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 Ca exceeds 0%, Ca enhances the machinability of the steel material. Even if a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, even if the contents of other elements are within the range of this embodiment, the workability of the steel material deteriorates. Therefore, the Ca content is 0 to 0.0050%, and when contained, the Ca content is 0.0050% or less (more than 0 to 0.0050%). The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%.

[0053] 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 a little Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.020%, the workability of the steel material deteriorates even if the contents of other elements are within the range of this embodiment. Therefore, the Bi content is 0 to 0.020%, and when contained, the Bi content is 0.020% or less (more than 0 to 0.020%). The preferable lower limit of the Bi content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferable upper limit of the Bi content is 0.018%, more preferably 0.015%.

[0054] 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 Te is more than 0%, Te improves the machinability of the steel material. Even if a little Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.010%, the workability of the steel material deteriorates even if the contents of other elements are within the range of this embodiment. Therefore, the Te content is 0 to 0.010%, and when contained, the Te content is 0.010% or less (more than 0 to 0.010%). The preferable lower limit of the Te content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the Te content is 0.009%, and more preferably 0.008%.

[0055] [Method for Measuring Chemical Composition of Bolt] The chemical composition of the bolt of this embodiment can be measured by a well-known component analysis method. Specifically, using a drill, chips are collected from inside the surface of the shaft part of the bolt at a depth of 1 mm or more. The collected chips are dissolved in an acid to obtain a solution. For the solution, ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed to conduct elemental analysis of the chemical composition. The C content and the S content are determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas fusion-thermal conductivity method.

[0056] Note that each element content is rounded off the fractional digits of the measured value based on the significant figures defined in this embodiment to the value up to the minimum digit of each element content defined in this embodiment. For example, the C content of the steel material of this embodiment is defined as a value up to the second decimal place. Therefore, the C content is the value up to the second decimal place obtained by rounding off the third decimal place of the measured value.

[0057] Similarly, for the other element contents other than the C content of the steel material of this embodiment, the value obtained by rounding off the fractional digits of the measured value to the minimum digit defined in this embodiment is taken as the element content.

[0058] Note that rounding off means discarding if the fractional digit is less than 5 and rounding up if the fractional digit is 5 or more.

[0059] [(Characteristic B) Tensile Strength TS] The bolt according to this embodiment has a tensile strength TS of 1300 MPa or more. The bolt according to this embodiment has the content of each element in the chemical composition within the range of this embodiment (Feature A), and further has Feature C and Feature D. As a result, in the bolt according to this embodiment, excellent hydrogen embrittlement resistance characteristics can be obtained even when the tensile strength TS is 1300 MPa or more.

[0060] The preferable lower limit of the tensile strength TS is 1320 MPa, and more preferably 1350 MPa. The upper limit of the tensile strength TS is not particularly limited. The upper limit of the tensile strength TS of the bolt according to this embodiment is, for example, 1700 MPa, and for example, 1650 MPa.

[0061] [Measurement method of tensile strength TS] In this embodiment, the tensile strength TS can be obtained by the following method. In accordance with JIS Z 2241:2011, a tensile test is carried out in the atmosphere at normal temperature (20 ± 15°C) to obtain the tensile strength TS (MPa). The tensile test piece is taken so as to include the shaft portion of the bolt, and the central axis of the tensile test piece is coaxial with the shaft portion of the bolt.

[0062] [(Feature C) Number density ND of MC type carbide] In the bolt of this embodiment, further, the number density ND of MC type carbide is 2.0×10 22 pieces / m 3 or more.

[0063] MC type carbide is finer compared to other alloy elements such as cementite and MC2 type carbide. Therefore, MC type carbide is more likely to trap hydrogen compared to other alloy carbides. Thus, in the bolt of this embodiment, the number density ND of MC type carbide is increased to increase the critical hydrogen amount of the bolt.

[0064] If the number density ND of MC type carbide is 2.0×10 22 pieces / m 3 or more, excellent hydrogen embrittlement resistance characteristics can be obtained on the premise that other features (A, B, D) are satisfied.

[0065] The preferable lower limit of the number density ND of MC-type carbides is 2.2×10 22 pieces / m 3 and more preferably 2.4×10 22 pieces / m 3 and even more preferably 2.6×10 22 pieces / m 3 . The upper limit of the number density ND of MC-type carbides is not particularly limited. However, when the content of each element in the chemical composition of the bolt is within the range of the present embodiment, the upper limit of the number density ND of MC-type carbides is, for example, 200.0×10 22 pieces / m 3 and more preferably 100.0×10 22 pieces / m 3 .

[0066] [Measurement method of the number density ND in MC-type carbides] The number density ND of MC-type carbides in the bolt can be measured by the following method using a scanning transmission electron microscope (STEM). First, a thin film sample for STEM is prepared by the following method. Cut the shaft part of the bolt perpendicular to the axial direction (longitudinal direction) of the bolt, and collect a disk with an axial thickness of about 2 mm. Use emery paper to polish both sides (front and back) of the disk. At this time, polish so that the front surface of the disk is parallel to the back surface. Define one of the front and back surfaces of the disk as the observation surface. Further mirror-polish the observation surface. Further polish the mirror-polished observation surface using colloidal silica as the abrasive grain.

[0067] MC-type carbides have a specific crystal orientation relationship with the matrix phase (Fe). Specifically, MC-type carbides are plate-like particles extending along the {100} plane of the matrix phase. Therefore, electron backscatter diffraction (EBSD) is performed on the polished observation surface to identify the crystal orientation of the matrix phase. Then, based on the identified crystal orientation of the matrix phase, focused ion beam processing (FIB processing) is performed on the disk so that the vertical direction (observation direction) of the observation surface of the thin film becomes the <001> crystal orientation of the matrix phase, and a thin film sample for STEM is prepared.

[0068] The production of a thin film sample for STEM by FIB processing may be carried out by a well-known method. For example, a thin film sample for STEM is produced by a lift-out method using a gallium (Ga) ion beam with an acceleration voltage of 30 kV.

[0069] In the surface layer of a thin film sample for STEM produced by an ion beam with an acceleration voltage of 30 kV, dislocation loops and amorphous regions exist, which are not suitable for observing MC-type carbides with a size of several nanometers. Therefore, the surface of the thin film sample for STEM is polished using an ion beam with a low acceleration voltage of 1 kV or less. Through the above manufacturing process, a thin film sample for STEM with a thickness of 100 nm or less is produced.

[0070] The produced thin film sample is observed with the optical system of STEM. Specifically, the thin film sample for STEM is tilted so that the incident beam is along the zone axis <001> of the martensite in the matrix phase in the thin film sample for STEM. The observation magnification is set to 320,000 times and the acceleration voltage is set to 300 kV. The detector is set under the conditions of a known high-angle annular dark field (HAADF), a known low-angle annular dark field (LAADF), and a known bright field (BF), and 10 arbitrary observation fields are observed. In each observation field, images are produced under these observation conditions. Among the images generated in each observation field, the image in which the MC-type carbide can be most clearly recognized is adopted.

[0071] The area of each observation field is 280 nm × 280 nm. In all observation fields, the thickness of the STEM thin film is measured by the Log-ratio method of electron energy loss spectroscopy (EELS).

[0072] In the image adopted for each observation field, the precipitates can be identified by contrast. Therefore, among the identified precipitates, those with a maximum length of 2 nm or more are identified. Here, the "maximum length" in the observation by STEM means the maximum line segment length when any two points on the interface between the precipitate and the matrix phase are selected and the entire line segment connecting the two points is included within the precipitate. It is extremely difficult to identify precipitates with a maximum length of less than 2 nm. Therefore, in this embodiment, precipitates with a maximum length of 2 nm or more are identified.

[0073] 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 the identified precipitates.

[0074] Based on the total number of MC-type carbides identified in all observation fields by the above method and the total volume calculated from all observation fields and thickness, the number density ND (pieces / m 3 ) of MC-type carbides is obtained.

[0075] In addition, when the content of each element in the chemical composition is within the range of this embodiment, as a result of obtaining the electron diffraction pattern of the precipitate in STEM observation, the precipitate with a maximum length of 10 nm or less is almost an MC-type carbide, and there are almost no other precipitates other than the MC-type carbide. And the maximum length of other precipitates other than MC-type carbides such as M2C-type carbides and cementite all greatly exceeded 10 nm. Therefore, as the above method for identifying MC-type carbides, instead of the electron diffraction pattern, precipitates with a maximum length of 10 nm or less (that is, a maximum length of 2 to 10 nm) may be identified as MC-type carbides.

[0076] [(Characteristic D) C ratio Rc in MC-type carbide] In the bolt of this embodiment, further, the C ratio Rc in the MC-type carbide is 0.30 to 0.42. Here, the C ratio Rc means the ratio of the number of C atoms among the total number of atoms of C, Mo, and V in the MC-type carbide. The C ratio Rc is defined as follows.

[0077] The C content, Mo content, and V content in atomic % in the MC-type carbide are defined as follows. [C]: C content in atomic % in the MC-type carbide [Mo]: Mo content in atomic % in the MC-type carbide [V]: V content in atomic % in the MC-type carbide In this case, the C ratio Rc in the MC-type carbide is defined by the following formula (1). Rc = [C] / ([C] + [Mo] + [V]) (1) In the bolt of this embodiment, the C ratio Rc in the MC type carbide is 0.30 to 0.42.

[0078] Figure 2 shows that the content of each element in the chemical composition is within the range of this embodiment, the tensile strength TS is 1300 MPa or more, and the number density ND of the MC type carbide is 2.0×10 22 pieces / m 3 This is a graph showing the relationship between the C ratio Rc in the MC type carbide and the limiting hydrogen amount (mass ppm) in bolts where the above is satisfied. Figure 2 was obtained by the test method described in the examples below.

[0079] As described above, the larger the limiting hydrogen amount, the better the hydrogen trapping function and the better the hydrogen embrittlement resistance characteristics. Referring to Figure 2, when the C ratio Rc is 0.42 or less, the limiting hydrogen amount becomes significantly larger compared to the case where the C ratio Rc exceeds 0.42.

[0080] Therefore, in the bolt of this embodiment, the C ratio Rc in the MC type carbide is 0.42 or less.

[0081] Note that the lower limit of the C ratio Rc in the MC type carbide is not particularly limited. However, there is a limit due to the chemical composition for the lower limit of the C ratio Rc in the MC type carbide. In the case of the bolt with the above chemical composition, the lower limit of the C ratio Rc in the MC type carbide is 0.30.

[0082] The preferable upper limit of the C ratio Rc in the MC type carbide is 0.41, more preferably 0.40. The preferable lower limit of the C ratio Rc is 0.31, more preferably 0.32, more preferably 0.33, more preferably 0.34, more preferably 0.35, more preferably 0.36.

[0083] The mechanism by which the limiting hydrogen amount increases when the C ratio Rc in the MC type carbide is 0.30 to 0.42 is not clear. However, the following mechanism is considered.

[0084] The MC-type carbide usually has an atomic ratio of C to M (metal element) of 1:1. In the bolt of the present embodiment having the chemical composition of Feature A, most of the metal elements in the MC-type carbide are V and Mo. Therefore, usually, the ratio of the number of C atoms in the MC-type carbide to the total number of atoms of Mo and V is 1:1. That is, in a normal MC-type carbide, the C ratio Rc is near 0.50.

[0085] In contrast, in the bolt of the present embodiment, the C ratio Rc of the MC-type carbide is set to 0.42 or less. If the C ratio Rc is 0.42 or less, a part of the C sites in the MC-type carbide becomes vacancies. In such an MC-type carbide having lattice defects in the C sites, the hydrogen trapping function is enhanced as compared with the M2C-type carbide and the MC-type carbide having no lattice defects. Therefore, if the C ratio Rc of the MC-type carbide is 0.42 or less, it is considered that the limiting hydrogen amount becomes significantly large and the hydrogen embrittlement resistance characteristics of the bolt are enhanced.

[0086] Since the above mechanism is an estimation, there is a possibility that the limiting hydrogen amount is increased by a different mechanism. However, in the bolt in which the content of each element in the chemical composition is within the range of the present embodiment, and the tensile strength TS is 1300 MPa or more, and the number density ND of the MC-type carbide is 2.0×10 22 pieces / m 3 or more, if the C ratio Rc in the MC-type carbide is 0.42 or less, the increase in the limiting hydrogen amount has been proven in the examples described later.

[0087] [Measurement method of C ratio Rc] The C ratio Rc of the MC-type carbide can be obtained by the following method. A sample is cut out from inside the surface of the shaft part of the bolt at a depth of 1 mm or more. The cut-out sample is subjected to well-known focused ion beam processing or electrolytic polishing to produce a needle-shaped test piece having a tip curvature radius of about 50 nm.

[0088] Perform three-dimensional atom probe analysis on the needle-shaped specimen. Specifically, identify MC-type carbides in the needle-shaped specimen by three-dimensional atom probe analysis. In three-dimensional atom probe analysis, precipitates present in the needle-shaped specimen can be detected three-dimensionally.

[0089] In three-dimensional atom probe analysis, set the laser wavelength (λ) to 355 nm, the laser power to 30 pJ, and the temperature of the needle-shaped specimen to 50 K. The apparatus used for three-dimensional atom probe analysis is not particularly limited. The three-dimensional atom probe analyzer is, for example, the product named LEAP4000XHR manufactured by Ametek, Inc.

[0090] Obtain a three-dimensional atomic map by reconstructing the acquired measurement data. Specifically, use the detection efficiency of the apparatus and adjust the measurement data so that the spacing of the {110} atomic plane is 0.20 nm in the measurement of iron (Fe) to reconstruct the measurement data and obtain a three-dimensional atomic map.

[0091] In the three-dimensional atomic map, the region of the needle-shaped specimen that has been subjected to three-dimensional atom probe analysis is divided into tiny cubes called voxels. Set the side length of a voxel to 1.0 nm. The elemental concentration (atomic %) in a voxel is defined as the value obtained by dividing the number of atoms of the element contained in the voxel by the total number of atoms of all elements in the voxel.

[0092] Define the sum of the C concentration, Mo concentration, and V concentration in each voxel as the specific elemental concentration (atomic %). Create an isoconcentration surface connecting the voxels with a specific elemental concentration of 6%. The region surrounded by the isoconcentration surface has a high specific elemental concentration. Identify the region surrounded by the isoconcentration surface as a precipitate.

[0093] Among the identified precipitates, identify the precipitates with a maximum length of 2 to 10 nm. Here, among the line segments connecting any two points on the surface of the three-dimensionally detected precipitate (that is, the interface between the precipitate and the steel matrix), the line segment that is entirely contained in the precipitate is defined as the "specific line segment". And the maximum length of the specific line segment of the precipitate is defined as the maximum length of the precipitate. The precipitate with a maximum length of 10 nm or less is identified as MC-type carbide.

[0094] As described above, when the content of each element in the chemical composition is within the range of this embodiment, in STEM observation, among the precipitates in the bolt, the precipitates with a maximum length of 2 to 10 nm are almost MC-type carbides, and there are almost no other precipitates other than MC-type carbides. And the maximum lengths of other precipitates other than MC-type carbides such as M2C-type carbides and cementite all greatly exceed 10 nm. Therefore, among the precipitates in the needle-shaped test piece obtained by three-dimensional atom probe analysis, the precipitates with a maximum length of 10 nm or less are identified as MC-type carbides.

[0095] Furthermore, in the three-dimensional atom map, the types of elements contained in the identified MC-type carbides and the number of atoms of each element can also be grasped. Therefore, select any 20 MC-type carbides from the identified multiple MC-type carbides. And for each of the selected MC-type carbides, determine the C content [C] (atomic %), Mo content [Mo] (atomic %), and V content [V] (atomic %) among the elements contained. Based on the obtained [C], [Mo], and [V], the C ratio Rc of each MC-type carbide is determined using Equation (1). The arithmetic mean value of the obtained C ratios Rc of each MC-type carbide is defined as the C ratio Rc in the MC-type carbide.

[0096] [Regarding the effect of the bolt of this embodiment] As described above, the bolt of this embodiment has the following characteristics. (Characteristic A) The chemical composition is within the range of this embodiment. (Characteristic B) The tensile strength TS is 1300 MPa or more. (Characteristic C) The number density ND of MC-type carbides is 2.0×10 22 pieces / m 3The above is the case. (Characteristic D) The C ratio Rc of the MC type carbide is 0.30 to 0.42. In the bolt of the present embodiment having the above-mentioned characteristics A to D, even when the tensile strength TS is as high as 1300 MPa or more, excellent hydrogen embrittlement resistance characteristics can be obtained.

[0097] As an index of the hydrogen embrittlement resistance characteristics, as described above, the limiting hydrogen amount can be used. The higher the limiting hydrogen amount, the more excellent the hydrogen embrittlement resistance characteristics. The limiting hydrogen amount means the upper limit hydrogen amount at which the steel material does not cause delayed fracture.

[0098] In the bolt of the present embodiment, the limiting hydrogen amount is 1.5 mass ppm or more. The preferable lower limit of the limiting hydrogen amount is 2.0 mass ppm, and more preferably 2.5 mass ppm.

[0099] [Method for measuring the limiting hydrogen amount] The limiting hydrogen amount can be determined by the following method. From the inside at a depth of 1 mm or more from the surface of the bolt, a round bar test piece with an annular notch (hereinafter simply referred to as a round bar test piece) is taken. The size of the round bar test piece is not particularly limited. For example, the diameter of the parallel part is 7 mm and the length is 70 mm. An annular notch extending in the circumferential direction is formed at the central position in the longitudinal direction of the round bar test piece. The R (radius of curvature) at the bottom of the notch is 0.175 mm.

[0100] Prepare a plurality of round bar test pieces. By the cathodic hydrogen charging method, hydrogen is charged into the round bar test pieces under various conditions. The cathodic hydrogen charging method is as follows. Prepare an aqueous solution at room temperature (cathodic charge solution) in which 0 to 20 g of ammonium thiocyanate is added to 1 L of a 3 mass% sodium chloride aqueous solution. While immersing the round bar test piece in the cathodic charge solution, a constant current is generated by controlling the cathode current density in the range of 0.03 to 1.00 mA / cm 2 to add hydrogen to the round bar test piece. The hydrogen amount of the round bar test piece is adjusted by adjusting the concentration of ammonium thiocyanate in the cathodic charge solution and the cathode current density.

[0101] After implementing the cathode hydrogen charging method, the round bar test piece is left at room temperature for 96 hours. Then, a zinc plating film is formed on the surface of the hydrogen-charged round bar test piece to prevent the hydrogen inside the round bar test piece from leaking to the outside. A constant load test is carried out at normal temperature and atmospheric pressure to apply a constant load such that a load of 90% of the tensile strength TS is applied to the round bar test piece. The test time is set to a maximum of 100 hours, and if the round bar test piece endures without breaking for more than 100 hours, the test is stopped. The constant load test is carried out using a plurality of round bar test pieces charged with hydrogen under various conditions.

[0102] After 100 hours, the amount of hydrogen in the round bar test piece that did not break is determined. Specifically, first, the zinc plating film on the surface of the round bar test piece is removed. After removing the zinc plating film, the round bar test piece is subjected to temperature-programmed desorption hydrogen analysis using a gas chromatograph. In the temperature-programmed desorption hydrogen analysis, the temperature is raised from room temperature to 600 °C at a rate of 100 °C / hour. The amount of hydrogen released from the round bar test piece during the temperature increase is measured. The measured amount of hydrogen (mass ppm) is taken as the amount of hydrogen (mass ppm) charged in the round bar test piece.

[0103] Of the amount of hydrogen in the round bar test piece that did not break in the 100-hour constant load test, the maximum amount of hydrogen is defined as the "critical hydrogen amount" (mass ppm).

[0104] [Microstructure of the bolt] The microstructure of the bolt of this embodiment contains a hard phase with an area ratio of 90% or more. The microstructure of the bolt mentioned here means the microstructure of the bolt manufactured by quenching and tempering in the manufacturing process described later. Also, the hard phase is composed of martensite and / or bainite. When the bolt microstructure contains other phases in addition to the hard phase, in the bolt microstructure, the remainder 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 with an area ratio of 90% or more, and the remainder is composed of retained austenite. Note that the tensile strength of the bolt correlates with the microstructure.

[0105] [Manufacturing Method] An example of the manufacturing method of the bolt according to this embodiment will be described. The manufacturing method of the bolt described hereinafter is an example for manufacturing the bolt according to this embodiment. Therefore, the bolt having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method described hereinafter. However, the manufacturing method described hereinafter is a preferable example of the manufacturing method of the bolt according to this embodiment.

[0106] An example of the manufacturing method of the bolt according to this embodiment includes the following steps. (Step 1) Steel Material Preparation Step (Step 2) Bolt Manufacturing Step Hereinafter, each step will be described.

[0107] [(Step 1) Steel Material Preparation Step] In the steel material preparation step, a steel material (bolt steel material) that is the material of the bolt is prepared. The bolt steel material may be manufactured. Alternatively, a third party may prepare the bolt steel material. In the steel material preparation step, a steel material whose chemical composition satisfies Feature A is prepared.

[0108] When manufacturing the bolt steel material, for example, the steel material is manufactured by the following method. First, molten steel in which the content of each element in the chemical composition is within the range of this embodiment is manufactured. Using the molten steel, a material is manufactured. For example, using the molten steel, a bloom (cast slab), which is the material, may be manufactured by the continuous casting method, or an ingot, which is the material, may be manufactured by the ingot-making method. Rough rolling (block rolling, or block rolling and hot rolling with a continuous rolling mill) is performed on the manufactured material (bloom or ingot) to manufacture a billet.

[0109] Finish rolling using a continuous rolling mill is performed on the billet to manufacture a steel material (bolt steel material). The steel material is, for example, a round bar or a wire rod.

[0110] [(Step 2) Bolt Manufacturing Step] In the bolt manufacturing step, a bolt is manufactured using the above-described steel material (bolt steel material). The bolt manufacturing step includes the following steps. (21) Wire drawing process (22) Bolt forming process (23) Heat treatment process The following describes each process.

[0111] [(21) Wire drawing process] In the wire drawing process, well-known wire drawing is performed on the above-mentioned steel material to produce steel wire. The wire drawing may be only primary wire drawing, or multiple wire drawings such as secondary wire drawing may be performed.

[0112] [(22) Bolt forming process] In the bolt forming process, the steel wire after the wire drawing process is cut to a predetermined length, and the cut steel wire is cold forged, rolled, or machined by cutting to produce a bolt intermediate product having a head, a lower neck part, and a shaft part.

[0113] The bolt forming process includes the following processes. (221) Head forming process (222) Thread forming process The following describes each process.

[0114] [(221) Head forming process] In the head forming process, cold forging is performed on the steel wire to form an intermediate product (bolt intermediate product) in the shape of a bolt. Specifically, first, the steel wire is cut to a predetermined length. Then, using a punch and a die, the steel wire is forged to form a bolt intermediate product having a head 10, a lower neck part 11, and a shaft part 12.

[0115] In the head forming process, strain is introduced into the bolt intermediate product. The strain introduced into the bolt intermediate product becomes the driving force for generating MC-type carbides in the heat treatment process of the next step. The strain introduced into the intermediate product in the head forming process will be described later.

[0116] [(222) Thread forming process] In the thread forming process, a thread part is formed on at least a part of the shaft portion 12 of the bolt intermediate product. In the thread forming process, a thread part may be formed on at least a part of the shaft portion 12 by well-known upset forging. Also, instead of upset forging, a thread part may be formed on at least a part of the shaft portion 12 by cutting. As will be described later, the thread forming process may be carried out after the head forming process and before the heat treatment process, or may be carried out after the heat treatment process.

[0117] [(23) Heat treatment process] In the heat treatment process, the following steps are carried out on the intermediate product after the bolt forming process. (231) Quenching process (232) Tempering process Hereinafter, each process will be described.

[0118] [(231) Quenching process] In the quenching process, well-known quenching is carried out. The quenching temperature and the holding time at the quenching temperature are not particularly limited. The quenching temperature is, for example, 870 to 970 °C. The holding time at the quenching temperature is, for example, 15 minutes to 240 minutes (4 hours). After the holding time has elapsed, the bolt intermediate product is rapidly cooled. Specifically, water cooling or oil cooling is carried out on the intermediate product.

[0119] [(232) Tempering process] In the tempering process, tempering is carried out on the bolt intermediate product after the quenching process. The conditions in the tempering process are as follows. (Condition 1) Tempering temperature T: 570 to 650 °C (Condition 2) Holding time t at the tempering temperature: 0.5 to 5.0 hours (Condition 3) Tempering parameter Fn: -13500 to 13500 Here, the tempering parameter Fn is defined by Equation (A). Fn = (ε -0.5 ) × ((T + 273) × (20 + Log(t)) × ((Mo / 96) / (Mo / 96 + V / 51)) × (1 + (Mo / 96) / (V / 51 + Mo / 96)) - 25000) (A) In ε in formula (A), the strain applied to the lower part of the bolt neck in the bolt forming process is substituted. In T, the tempering temperature (°C) is substituted. In t, the holding time (hours) at the tempering temperature is substituted. In Mo, the Mo content (% by mass) in the bolt is substituted. In V, the V content (% by mass) in the bolt is substituted.

[0120] The strain ε applied to the lower part of the bolt neck in the bolt forming process is obtained by the following finite element method (FEM analysis). Specifically, for the bolt to be analyzed, an axisymmetric model with a size of 1 / 6 is created based on the shape symmetry. The 1 / 6 axisymmetric model to be analyzed is divided into a plurality of elements with a tetrahedral first-order element so that one side of the tetrahedron is 0.05 to 1.0 mm.

[0121] For the model divided into the above number of elements, a three-dimensional elastoplastic analysis by FEM is performed. For the analysis, a commercially available forging analysis code may be used. A commercially available forging analysis code is, for example, the product name DEFORM-3D.

[0122] It is assumed that the workpiece (bolt steel material) is an elastoplastic body and the die is a rigid body. The flow stress of the workpiece uses the measured value of the spheroidized annealed material of SCM435 specified in JIS G 4053:2016. As the physical property values of the workpiece, a Young's modulus of 210 GPa and a Poisson's ratio of 0.30 are used. The die is set to a size sufficiently large with respect to the workpiece. The Coulomb friction coefficient between the workpiece and the die is set to μ = 0.05. The FEM analysis is performed under the above conditions to obtain the strain ε applied to the lower part of the bolt neck.

[0123] Hereinafter, Conditions 1 to 3 of the tempering process will be described.

[0124] [Regarding the tempering temperature T and the holding time t] The tempering temperature T is 570°C to 650°C. If the tempering temperature T is less than 570°C, the number density ND of MC-type carbides is less than 2.0×10 22 pieces / m 3 If the tempering temperature T exceeds 650°C, the tensile strength TS is less than 1300 MPa. Also, the holding time t at the tempering temperature T is from 0.5 hours to 5.0 hours. If the holding time t is less than 0.5 hours, the number density ND of MC-type carbides becomes less than 2.0×10 22 pieces / m 3 . If the holding time t exceeds 5.0 hours, the tensile strength TS becomes less than 1300 MPa.

[0125] [Regarding the tempering parameter Fn] In the tempering process, furthermore, the tempering parameter Fn defined by the formula (A) is from -13500 to 13500. Fn is a conditional formula regarding the composition and the amount of MC-type carbides generated. If Fn exceeds 13500, the number density ND of MC-type carbides generated becomes less than 2.0×10 22 pieces / m 3 . On the other hand, if Fn is less than -13500, although the number density ND of MC-type carbides becomes 2.0×10 22 pieces / m 3 or more, the C ratio Rc of the MC-type carbides exceeds 0.42.

[0126] If Fn is from -13500 to 13500, in the bolt having a chemical composition satisfying Feature A, the number density ND of MC-type carbides becomes 2.0×10 22 pieces / m 3 or more (Feature C), and furthermore, the C ratio Rc in the MC-type carbides becomes from 0.30 to 0.42 (Feature D). Furthermore, the tensile strength TS can be adjusted to 1300 MPa or more (Feature B).

[0127] By the above manufacturing method, the bolt according to the present embodiment can be manufactured. Note that the above manufacturing method is a preferable example among the manufacturing methods of the bolt according to the present embodiment. Therefore, the bolt having the above configuration may be manufactured by other manufacturing methods other than the above manufacturing method. In short, as long as the bolt of the present embodiment having the above configuration can be manufactured, the manufacturing method is not particularly limited.

[0128] [Regarding other processes] The bolt manufacturing process according to this embodiment may include other processes in addition to the above-described processes. For example, a spheroidizing heat treatment process may be performed after the wire drawing process and before the bolt forming process. Also, the thread forming process may be performed after the heat treatment process instead of after the head forming process and before the heat treatment process. Furthermore, a compressive residual stress applying process may be performed after the heat treatment process.

Example

[0129] The effects of the bolt of this embodiment will be further specifically described by way of examples. The conditions in the following examples are one example of the 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.

[0130] Steel material (bar steel) serving as the material for the bolt having the chemical composition shown in Table 1 was prepared.

[0131]

Table 1

[0132] In Table 1, “-” in the “Chemical Composition” column means that the corresponding element content is 0% in the significant figures (numerical values up to the least significant digit) defined in this embodiment. In other words, it means that when the fractional part in the corresponding element content is rounded off in the significant figures (numerical values up to the least significant digit) defined in the above-described this embodiment, it is 0%.

[0133] For example, in the case of the Cu content, the significant figures defined in this embodiment are numerical values up to the second decimal place. Therefore, the Cu content of Test No. 1 means that as a result of rounding off the numerical value in the third decimal place, it was 0%. Similarly, in the case of the Ni content, the significant figures defined in this embodiment are numerical values up to the second decimal place. Therefore, the Ni content of Test No. 1 means that as a result of rounding off the numerical value in the third decimal place, it was 0%.

[0134] [Steel Material Preparation Process] Steel materials of each steel grade number were manufactured by the following method. For blooms having the chemical compositions described in Table 1, rough rolling (hot rolling with block rolling and continuous rolling mills) was carried out to manufacture billets. The heating temperature of the blooms was 1200 °C. For the manufactured billets, finish rolling was carried out to manufacture round bars with a diameter of 20 mm. The heating temperature of the billets during finish rolling was 1200 °C.

[0135] [Bolting forming process] Using the manufactured steel materials, bolts were manufactured. First, for steel materials with a diameter of 20 mm of each test number shown in Table 2, wire drawing was carried out under the same conditions for each test number to manufacture steel wires with a diameter of 16 mm. For the steel wires of each test number, a head forming process was carried out to manufacture bolt intermediate products in the shape of hexagonal bolts 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 thread portion on the shaft portion of the bolt intermediate product.

[0136]

Table 2

[0137] In addition, the strain ε added to the lower neck part of the bolt intermediate product manufactured by the bolting forming process was obtained by performing the above-mentioned FEM analysis. The obtained strain ε is shown in the "Strain ε" column in the "Bolting forming process" column of Table 2.

[0138] Furthermore, it was visually determined whether or not cracks had occurred in the bolt intermediate product after the bolting forming process. If a crack with a length of 1 mm or more was confirmed on the surface of the bolt intermediate product, it was judged that the cold forging property was low (indicated by "F (Fail)" in the "Cold forging property" column in the "Bolting forming process" column in Table 2). On the other hand, if no crack with a length of 1 mm or more was confirmed on the surface of the bolt intermediate product, it was judged that the cold forging property was good (indicated by "P (Pass)" in the "Cold forging property" column in the "Bolting forming process" column in Table 2). For steel materials with low cold forging properties, the manufacturing was stopped, and the heat treatment process in the next process and the evaluation test described later were not carried out.

[0139] For the bolt intermediate products with test numbers showing good cold forging properties, a heat treatment process was carried out. First, a quenching process was carried out on the bolt intermediate products under the same conditions for each test number. In the quenching process, for each test number, the quenching temperature was 880 - 970 °C and the holding time at the quenching temperature was 60 minutes (1 hour). After the holding time elapsed, the bolts were water-cooled.

[0140] Tempering was carried out on the bolt intermediate products after quenching. The tempering temperature T (°C) and the holding time t (hours) at the tempering temperature T for each test number were as shown in Table 2. Furthermore, Fn defined by formula (A) was as shown in Table 2. Bolts for each test number were manufactured through the above manufacturing process.

[0141] [Evaluation Test] The following evaluation tests were carried out on the bolts for each test number. (Test 1) Chemical Composition Analysis Test (Test 2) Tensile Test (Test 3) Measurement Test of the Number Density ND of MC-Type Carbides (Test 4) Measurement Test of the C Ratio Rc of MC-Type Carbides (Test 5) Measurement Test of the Limiting Hydrogen Content

[0142] [(Test 1) Chemical Composition Analysis Test] Based on the method described in the above [Measurement Method of Bolt Chemical Composition], the chemical composition of the bolts for each test number was analyzed. As a result, the chemical composition of the bolts for each test number was as described in Table 1.

[0143] [(Test 2) Tensile Test] Based on the method described in the above [Measurement Method of Tensile Strength TS], the tensile strength TS (MPa) of the bolts for each test number was determined. The obtained tensile strength TS was shown in the column of "TS (MPa)" in Table 2.

[0144] Note that for the test numbers where the obtained tensile strength TS was less than 1300 MPa, the subsequent evaluation tests (Test 3 - Test 5) were not carried out.

[0145] [(Test 3) Measurement Test of Number Density ND in MC-Type Carbide] Based on the method described in the above [Measurement Method of Number Density ND in MC-Type Carbide], the number density ND (pieces / m 3 ) in the MC-type carbide of the bolts with each test number was determined. The obtained number density ND is shown in the column of "Number Density ND (×10 22 pieces / m 3 )" in Table 2.

[0146] [(Test 4) Measurement Test of C Ratio Rc of MC-Type Carbide] Based on the method described in the above [Measurement Method of C Ratio Rc], the C ratio Rc in the MC-type carbide of the bolts with each test number was determined. The obtained C ratio Rc is shown in the column of "C Ratio Rc" in Table 2.

[0147] [(Test 5) Measurement Test of Limiting Hydrogen Content] Based on the method described in the above [Measurement Method of Limiting Hydrogen Content], the limiting hydrogen content (mass ppm) of the bolts with each test number was measured. The obtained limiting hydrogen content (mass ppm) is shown in the column of "Limiting Hydrogen Content (mass ppm)" in Table 2.

[0148] [Evaluation Test Results] The evaluation results are shown in Table 2. Referring to Table 1 and Table 2, for the bolts with test numbers 1 to 23, the content of each element in the chemical composition was within the range of this embodiment, and the tensile strength TS was 1300 MPa or more. Furthermore, the number density ND of the MC-type carbide was 2.0×10 22 pieces or more, and the C ratio Rc was 0.30 to 0.42. As a result, the limiting hydrogen content of the bolts with these test numbers was 1.5 ppm or more, and they had excellent hydrogen embrittlement resistance characteristics.

[0149] On the other hand, in test number 24, the Mo content in the bolt was too low. Therefore, the tensile strength TS was lower than 1300 MPa.

[0150] In test number 25, the Mo content in the bolt was too high. Therefore, cracks were confirmed in the bolt intermediate product after the bolt forming process, and the cold forging property was low.

[0151] In Test No. 26, the V content in the bolt was too low. Therefore, the number density ND of MC-type carbides in the bolt was too low. As a result, the limiting hydrogen amount was less than 1.5 ppm, and the hydrogen embrittlement resistance property was low.

[0152] In Test No. 27, the V content in the bolt was too high. Therefore, cracks were confirmed in the bolt intermediate product after the bolt forming process, and the cold forging property was low.

[0153] In Test No. 28, the Cr content in the bolt was too high. Therefore, the limiting hydrogen amount was less than 1.5 ppm, and the hydrogen embrittlement resistance property was low.

[0154] In Test No. 29, although the chemical composition of the bolt was appropriate, the tempering temperature T was too low. Therefore, the number density ND of MC-type carbides in the bolt was too low. As a result, the limiting hydrogen amount was less than 1.5 ppm, and the hydrogen embrittlement resistance property was low.

[0155] In Test No. 30, although the chemical composition of the bolt was appropriate, the tempering temperature T was too high. Therefore, the tensile strength TS was lower than 1300 MPa.

[0156] In Test No. 31, although the chemical composition of the bolt was appropriate, the holding time t at the tempering temperature T was too short. Therefore, the number density ND of MC-type carbides in the bolt was too low. As a result, the limiting hydrogen amount was less than 1.5 ppm, and the hydrogen embrittlement resistance property was low.

[0157] In Test No. 32, although the chemical composition of the bolt was appropriate, the holding time t at the tempering temperature T was too long. Therefore, the tensile strength TS was lower than 1300 MPa.

[0158] In Test Nos. 33 to 35, although the chemical composition of the bolt was appropriate, the tempering parameter Fn was too low. Therefore, the C ratio Rc was too high. As a result, the limiting hydrogen amount was less than 1.5 ppm, and the hydrogen embrittlement resistance property was low.

[0159] In Tests Nos. 36 to 38, although the chemical composition of the bolts was appropriate, the tempering parameter Fn was too high. As a result, the number density ND of MC-type carbides in the bolts was too low. Consequently, the limiting hydrogen amount was less than 1.5 ppm, and the hydrogen embrittlement resistance characteristics were low.

[0160] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. by mass%, C: 0.30 - 0.50%, Si: 0.01 - 0.30%, Mn: 0.10 - 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01 - 0.80%, Mo: less than 0.70 - 1.50%, V: 0.01 - 0.50%, Al: 0.005 - 0.100%, and, N: 0.0010 - 0.0300%, and the balance consists of Fe and impurities, the tensile strength TS is 1300 MPa or more, the number density ND of MC type carbides is 2.0×10 ２２ pieces / m ３ or more, when the C content in atomic% in the MC type carbide is defined as [C], the Mo content in atomic% is defined as [Mo], and the V content in atomic% is defined as [V], the C ratio Rc defined by formula (1) is 0.30 - 0.42, Bolt. Rc = [C] / ([C] + [Mo] + [V]) (1)

2. The bolt according to Claim 1, further comprising in place of a part of Fe, Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Zr: 0.300% or less, Hf: 0.100% or less, Ta: 0.100% or less, W: 0.20% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ca: 0.0050% or less, Bi: 0.020% or less, and, Te: 0.010% or less, containing one or more elements selected from the group consisting of bolt.

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