bolt

Abstract

To provide a bolt having high strength and excellent hydrogen embrittlement resistance.SOLUTION: A bolt according to the present disclosure contains, 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: less than 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0200%, with the balance comprised of Fe and impurities, satisfies the equation (1), and has a tensile strength (TS) of 1200 MPa or more. When the total area ratio (%) of MC-type carbides is defined as SMC and the thickness (nm) of a thin film sample in the observation field is defined as t, SMC' expressed by equation (2) and the total area ratio SC(%) of cementite satisfy equation (3). Mo / V>3.00 (1) SMC'=SMC / t (2) SMC' / SC≥0.010 (3).SELECTED DRAWING: None

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 1200 MPa or more.

[0003] Bolts with such high strength are more susceptible to hydrogen embrittlement. Bolts that experience hydrogen embrittlement are more likely to experience delayed fracture. Therefore, 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 that have high strength and excellent hydrogen embrittlement resistance.

[0005] The bolt disclosed in Patent Document 1 contains, in mass %, C: 0.22 to 0.40%, Si: 0.10 to 1.50%, Mn: 0.20 to less than 0.40%, Cr: 0.70 to less than 1.60%, Al: 0.005 to 0.060%, Ti: 0.010 to 0.050%, B: 0.0003 to 0.0040%, N: 0.0015 to 0.0080%, Cu: 0.50% or less, Ni: 0.30% or less, M The bolt contains 0.05% or less O, 0.050% or less V, 0.050% or less Nb, and one or more elements selected from the group consisting of 0.001-0.100% Sb, 0.001-0.100% Sn, and 0.001-0.100% Bi, and also contains 0.0020% or less O, 0.020% or less P, and 0.020% or less S, with the balance being Fe and impurities. The bolt further satisfies formula (1) (0.50≦C+(1 / 10)×Si+(1 / 5)×Mn+(5 / 22)×Cr≦0.85) and formula (2) (0.003≦Sb+Sn+Bi≦0.100). Patent Document 1 states that this bolt has excellent hydrogen embrittlement resistance even at high strength because the contents of Sb, Sn, and Bi in the chemical composition satisfy formula (2).

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

[0007] The bolt disclosed in Patent Document 3 contains, by mass%, C: 0.30 to 0.50%, Si: 1.0 to 2.5%, Mn: 0.1 to 1.5%, P: 0.015% or less (excluding 0%), S: 0.015% or less (excluding 0%), Cr: 0.15 to 2.4%, Al: 0.10% or less (excluding 0%), and N: 0.015% or less (excluding 0%), and further contains Cu: 0.10 to 0.50% and Ni: 0.1 to 1.0% so as to satisfy [Ni] / [Cu]≧0.5, and further contains Ti: 0.05 to 0.20% and V: 0.20% or less (including 0%) so as to satisfy [Ti]+[V]: 0.085 to 0.30%, with the remainder consisting of Fe and impurities. Furthermore, the austenite grain size number of the bolt shank is 9.0 or more, and the G value (%), which indicates the proportion of carbides precipitated at the austenite grain boundaries of the bolt shank, satisfies formula (1) (G value = (L / L0) × 100 ≦ 60). In formula (1), L is the total length of carbides with a thickness of 50 nm or more precipitated at the austenite grain boundaries, and L0 is the length of the austenite grain boundaries. Patent Document 3 states that this bolt achieves excellent hydrogen embrittlement resistance by suppressing carbide precipitation at grain boundaries. [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 that has 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: 0.01 to 0.80%, Mo: 0.70 to less than 1.50% V: 0.01 to less than 0.50% Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%, The balance is Fe and impurities. Satisfying equation (1), The tensile strength TS is 1200 MPa or more, When a thin film sample for STEM made from the bolt was observed using a scanning transmission electron microscope (STEM), the total area ratio (%) of MC type carbides in an observation field of 280 nm × 280 nm was defined as S MC and When the thickness (nm) of the STEM thin film sample in the observation field is defined as t, S expressed by equation (2) MC and, When a cross section perpendicular to the axial direction of the bolt is observed using a field emission scanning electron microscope (FE-SEM), the total area ratio S of cementite in an observation field of 9 μm × 12 μm is C (%) and Equation (3) is satisfied. Mo / V>3.00 (1) S MC ´=S MC / t (2) S MC ´ / S C ≧0.010 (3) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition of the bolt in mass %.

[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: 0.01 to 0.80%, Mo: 0.70 to less than 1.50% V: 0.01 to less than 0.50% Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%, Further, it contains one or more selected from the group consisting of Groups 1 to 3, The balance is Fe and impurities. Satisfying equation (1), The tensile strength TS is 1200 MPa or more, When a thin film sample for STEM made from the bolt was observed using a scanning transmission electron microscope (STEM), the total area ratio (%) of MC type carbides in an observation field of 280 nm × 280 nm was defined as S MC and When the thickness (nm) of the STEM thin film sample in the observation field is defined as t, S expressed by equation (2) MC and, When a cross section perpendicular to the axial direction of the bolt is observed using a field emission scanning electron microscope (FE-SEM), the total area ratio S of cementite in an observation field of 9 μm × 12 μm is C (%) and Equation (3) is satisfied. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, 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, Te: 0.010% or less, and Sn: 0.100% or less, one or more selected from the group consisting of Mo / V>3.00 (1) S MC ´=S MC / t (2) S MC ´ / S C ≧0.010 (3) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition of the bolt in mass %. [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] First, the inventors investigated a bolt that has high strength and excellent hydrogen embrittlement resistance from the viewpoint of chemical composition, and as a result, the inventors found that the bolt has, 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 less than 0.50%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0200%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0050%, Zr: 0. It was thought that a bolt having a chemical composition containing Fe: 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 Sn: 0-0.100%, with the balance being Fe and impurities, could potentially achieve a tensile strength TS of 1200 MPa or more and excellent hydrogen embrittlement resistance.

[0016] MC carbides precipitate in bolts with the above-mentioned chemical composition. Among alloy carbides, MC carbides in particular are less likely to coarsen and tend to remain fine compared to other alloy carbides (e.g., M2C carbides, cementite, etc.). The fine precipitates act as hydrogen trapping sites and absorb hydrogen. This increases the amount of hydrogen that can be stored before hydrogen embrittlement cracking occurs (hereinafter referred to as the limiting hydrogen amount). In this way, MC carbides in bolts improve the hydrogen embrittlement resistance of the bolt.

[0017] The MC carbides formed in a bolt having the above-mentioned chemical composition contain Mo and V. V is more likely to form MC carbides than Mo. Therefore, in order to improve the hydrogen embrittlement resistance of a bolt, it is thought that increasing the V content more than the Mo content is effective. However, through studies conducted by the present inventors, it has been found that the hydrogen embrittlement resistance of a bolt can be further improved by increasing the Mo content relative to the V content. Specifically, the hydrogen embrittlement resistance of a bolt is improved when the chemical composition of the bolt satisfies the following formula (1). Mo / V>3.00 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition of the bolt in mass %.

[0018] The following mechanism is presumed to be the reason why satisfying formula (1) improves the hydrogen embrittlement resistance of bolts. MC type carbides may have sites (C vacancies) where C should be present. MC type carbides with C vacancies trap hydrogen in the C vacancies, which enhances the hydrogen absorption function. Here, the higher the Mo content relative to the V content, the higher the C vacancy rate of the MC carbides that are formed. If the chemical composition of the bolt satisfies formula (1), the hydrogen absorption function of the MC carbides will be sufficiently enhanced, and the hydrogen embrittlement resistance of the bolt will be improved.

[0019] However, even bolts having the above-mentioned chemical composition and satisfying formula (1) sometimes fail to provide excellent hydrogen embrittlement resistance. The present inventors further investigated the cause of this and found that coarse cementite was formed at the grain boundaries in bolts that did not provide excellent hydrogen embrittlement resistance.

[0020] Cementite is preferentially formed at grain boundaries. It is believed that the coarser the cementite present at the grain boundaries, the lower the grain boundary strength. In other words, the higher the total area ratio of cementite in a bolt, the lower the bolt's resistance to hydrogen embrittlement. On the other hand, the hydrogen absorption ability of individual MC carbides improves as the MC carbides become larger. As the individual MC carbides become larger, the total area ratio of MC carbides in the bolt increases. In other words, the higher the total area ratio of MC carbides in the bolt, the better the bolt's resistance to hydrogen embrittlement. Therefore, in order to obtain a bolt with excellent hydrogen embrittlement resistance, it is necessary to adjust the ratio of the total area ratio of MC carbides to the total area ratio of cementite in the bolt so that it is sufficiently high.

[0021] Based on the above findings, the inventors further investigated the relationship between the total area ratio of MC carbides, the total area ratio of cementite, and the hydrogen embrittlement resistance of bolts. As a result, when a thin film sample for STEM prepared from a bolt having the above-mentioned chemical composition and satisfying formula (1) was observed using a scanning transmission electron microscope (STEM), the total area ratio (%) of MC carbides in an observation field of 280 nm × 280 nm was found to be S MC and the thickness (nm) of the STEM thin film specimen in the observation field is defined as t. S expressed by equation (2) MC When a cross section perpendicular to the axial direction of the bolt was observed using a field emission scanning electron microscope (FE-SEM), the total area ratio S of cementite in an observation field of 9 μm × 12 μm was C The present inventors have found that, if the relationship between the tensile strength (TS) and the hydrogen embrittlement resistance (%) satisfies the formula (3), even a bolt having a tensile strength (TS) of 1200 MPa or more can have excellent hydrogen embrittlement resistance. S MC ´=S MC / t (2) S MC ´ / S C ≧0.010 (3)

[0022] The bolt of this embodiment was completed based on the above findings and has the following configuration. Note that the above mechanism is speculation. Therefore, it is possible that the bolt of this embodiment can achieve excellent hydrogen embrittlement resistance through a mechanism different from that described above. However, it has been proven in the examples described below that a bolt having the following configuration can achieve excellent hydrogen embrittlement resistance.

[0023] 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: 0.01 to 0.80%, Mo: 0.70 to less than 1.50% V: 0.01 to less than 0.50% Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%, The balance is Fe and impurities. Satisfying equation (1), The tensile strength TS is 1200 MPa or more, When a thin film sample for STEM made from the bolt was observed using a scanning transmission electron microscope (STEM), the total area ratio (%) of MC type carbides in an observation field of 280 nm × 280 nm was defined as S MC and When the thickness (nm) of the STEM thin film sample in the observation field is defined as t, S expressed by equation (2) MC and, When a cross section perpendicular to the axial direction of the bolt is observed using a field emission scanning electron microscope (FE-SEM), the total area ratio S of cementite in an observation field of 9 μm × 12 μm is C (%) and Equation (3) is satisfied. Mo / V>3.00 (1) S MC ´=S MC / t (2) S MC ´ / S C ≧0.010 (3) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition of the bolt in mass %.

[0024] 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: 0.01 to 0.80%, Mo: 0.70 to less than 1.50% V: 0.01 to less than 0.50% Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%, Further, it contains one or more selected from the group consisting of Groups 1 to 3, The balance is Fe and impurities. Satisfying equation (1), The tensile strength TS is 1200 MPa or more, When a thin film sample for STEM made from the bolt was observed using a scanning transmission electron microscope (STEM), the total area ratio (%) of MC type carbides in an observation field of 280 nm × 280 nm was defined as S MC and When the thickness (nm) of the STEM thin film sample in the observation field is defined as t, S expressed by equation (2) MC and, When a cross section perpendicular to the axial direction of the bolt is observed using a field emission scanning electron microscope (FE-SEM), the total area ratio S of cementite in an observation field of 9 μm × 12 μm is C (%) and Equation (3) is satisfied. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, 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, Te: 0.010% or less, and Sn: 0.100% or less, one or more selected from the group consisting of Mo / V>3.00 (1) S MC ´=S MC / t (2) S MC ´ / S C ≧0.010 (3) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition of the bolt in mass %.

[0025] The third configuration of bolts is A bolt of a second configuration, The chemical composition includes the first group.

[0026] The fourth configuration bolt is A bolt of the second or third configuration, The chemical composition includes the second group.

[0027] The fifth configuration bolt is A bolt having any one of the second to fourth configurations, The chemical composition includes the third group.

[0028] The bolt according to this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0029] 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.

[0030] [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 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 less than 0.50%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0200%, Cu: 0 to 0.40%, N The alloy contains i: 0-0.40%, 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 Sn: 0-0.100%, with the remainder being Fe and impurities. (Feature 2) The chemical composition further satisfies formula (1). Mo / V>3.00 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition of the bolt in mass %. (Feature 3) The tensile strength TS is 1200 MPa or more. (Feature 4) When a thin film specimen made from a bolt was observed using a scanning transmission electron microscope (STEM), the total area ratio (%) of MC-type carbides in an observation field of 280 nm × 280 nm was defined as S. MC and the thickness (nm) of the STEM thin film specimen in the observation field is defined as t. S expressed by equation (2) MC When a cross section perpendicular to the axial direction of the bolt was observed using a field emission scanning electron microscope (FE-SEM), the total area ratio S of cementite in an observation field of 9 μm × 12 μm was C (%) satisfies equation (3). S MC ´=S MC / t (2) S MC ´ / S C ≧0.010 (3)

[0031] 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.

[0032] [(Feature 1) Chemical composition] The chemical composition of the bolt of this embodiment contains the following elements.

[0033] 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. 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%, more preferably 0.35%, and even more preferably 0.38%. The upper limit of the C content is preferably 0.48%, more preferably 0.45%, and even more preferably 0.43%.

[0034] 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. 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%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Si content is preferably 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0035] Mn: 0.10 to 1.50% Manganese (Mn) improves the hardenability of steel and increases the strength of bolts. If the Mn content is less than 0.10%, the above effects cannot be sufficiently obtained. 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%, more preferably 0.20%, and even more preferably 0.25%. The upper limit of the Mn content is preferably 1.30%, more preferably 1.20%, and even more preferably 1.10%.

[0036] P:0.030% or less Phosphorus (P) is an impurity. If the P content exceeds 0.030%, P segregates at grain boundaries. As a result, the hydrogen embrittlement resistance of the bolt decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible. However, excessive 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 even more preferably 0.003%. The upper limit of the P content is preferably 0.025%, more preferably 0.023%, and even more preferably 0.020%.

[0037] S: 0.030% or less Sulfur (S) is an impurity. If the S content exceeds 0.030%, S segregates at grain boundaries. As a result, the hydrogen embrittlement resistance of the bolt decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.030% or less. The S content is preferably as low as possible. However, excessive 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 still more preferably 0.003%. The upper limit of the S content is preferably 0.025%, more preferably 0.023%, and even more preferably 0.020%.

[0038] Cr: 0.01 to 0.80% Chromium (Cr) improves the hardenability of steel and increases the strength of bolts. Cr also increases the temper softening resistance of steel, thereby increasing the strength of bolts. If the Cr content is less than 0.01%, the above effects cannot be fully achieved. On the other hand, Cr promotes the penetration of hydrogen into the bolt, and if the Cr content exceeds 0.80%, 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 0.01 to 0.80%. The lower limit of the Cr content is preferably 0.03%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Cr content is preferably 0.70%, more preferably 0.60%, and even more preferably 0.50%.

[0039] Mo: 0.70 to less than 1.50% Molybdenum (Mo) increases the temper softening resistance of steel materials and increases the strength of bolts. Mo also concentrates in MC carbides, enhancing the hydrogen absorption ability of the MC carbides. As a result, the hydrogen embrittlement resistance of high-strength bolts is improved. If the Mo content is less than 0.70%, the above effects cannot be fully achieved. On the other hand, if the Mo content is 1.50% or more, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material becomes excessively high, resulting in a decrease in cold workability. Therefore, the Mo content is 0.70 to less than 1.50%. The lower limit of the Mo content is preferably 0.75%, more preferably 0.80%, and even more preferably 0.85%. The upper limit of the Mo content is preferably 1.49%, more preferably 1.40%, and even more preferably 1.30%.

[0040] V: 0.01 to less than 0.50% Vanadium (V) forms MC-type carbides together with Mo to enhance the hydrogen embrittlement resistance of the bolt. If the V content is less than 0.01%, this effect cannot be sufficiently obtained. On the other hand, if the V content is 0.50% or more, even if the contents of other elements are within the ranges of this embodiment, the steel material becomes excessively hard, resulting in a decrease in cold workability. Therefore, the V content is 0.01 to less than 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 0.49%, more preferably 0.45%, even more preferably 0.40%, and still more preferably 0.35%.

[0041] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. Furthermore, Al combines with N to form Al nitrides. Al nitrides have a pinning effect that suppresses grain coarsening. As a result, the hydrogen embrittlement resistance of the bolt is improved. If the Al content is less than 0.005%, the above effect cannot be fully achieved. On the other hand, if the Al content exceeds 0.100%, coarse Al nitrides are generated. These coarse Al nitrides become the starting points for fracture. As a result, the workability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment. 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 bolt of this embodiment, the Al content means the total Al content.

[0042] N: 0.0010~0.0200% Nitrogen (N) bonds with aluminum to form aluminum 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 effect cannot be fully achieved. On the other hand, if the N content exceeds 0.0200%, coarse nitrides are formed. These coarse nitrides become the starting points for fracture. As a result, the workability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment. 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%, even more preferably 0.0130%, even more preferably 0.0110%, and even more preferably 0.0080%.

[0043] 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 and scrap, or from the manufacturing environment, during industrial production of the bolt, and are acceptable as long as they do not adversely affect the bolt according to this embodiment.

[0044] [Optional Elements] The chemical composition of the bolt of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Groups 1 to 3. All of these elements are optional and may not be contained. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, 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, Te: 0.010% or less, and Sn: 0.100% or less, one or more selected from the group consisting of These optional elements will be explained below.

[0045] [Group 1 (Cu, Ni, B, Zr, Hf, Ta and W)] The chemical composition of the bolt according to this embodiment may further contain the above-mentioned elements of Group 1 in place of a portion of Fe. These elements are optional elements, and all of them improve the hardenability of the steel material and increase the strength of the bolt.

[0046] Cu: 0.40% or less Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When Cu is contained, that is, when the Cu content is more than 0%, Cu improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. On the other hand, if the Cu content exceeds 0.40%, the hardenability becomes too high, and as a result, the workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 0.40%, and when Cu is contained, the Cu content is 0.40% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Cu content is preferably 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0047] 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 improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. On the other hand, if the Ni content exceeds 0.40%, the hardenability becomes too high, and as a result, the workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.40%, and when Ni is contained, the Ni content is 0.40% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Ni content is preferably 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0048] 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 fracture. As a result, the 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, the B content 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 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, and still more preferably 0.0030%.

[0049] 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 nitrides are generated. These coarse Zr nitrides become the starting points for fracture. As a result, the 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 when Zr is contained, the Zr content 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%, and still more preferably 0.060%.

[0050] 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 nitrides are generated. These coarse Hf nitrides become the starting points for fracture. As a result, the 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, the Hf content is 0.010% or less. The lower limit of the Hf content is preferably 0.001%, and more preferably 0.002%. The upper limit of the Hf content is preferably 0.008%, more preferably 0.007%, even more preferably 0.006%, and still more preferably 0.005%.

[0051] 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 nitrides are generated. These coarse Ta nitrides become the starting points for fracture. As a result, the 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 when Ta is contained, the Ta content is 0.010% or less. The lower limit of the Ta content is preferably 0.001%, and more preferably 0.002%. The upper limit of the Ta content is preferably 0.008%, more preferably 0.007%, even more preferably 0.006%, and still more preferably 0.005%.

[0052] 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, resulting in a decrease in the workability of the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0 to 0.20%, and when W is contained, the W content is 0.20% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.05%. The upper limit of the W content is preferably 0.15%, more preferably 0.12%, and even more preferably 0.10%.

[0053] [Regarding Group 2 (Ti and Nb)] The chemical composition of the bolt according to this embodiment may further contain the above-mentioned elements of Group 2 in place of part of Fe. These elements are optional elements, and all of them improve the hydrogen embrittlement resistance of the bolt.

[0054] 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 nitrides are formed, which become the starting points for fracture. As a result, the bolt's workability deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.100%, and when Ti is contained, the Ti content 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%, even more preferably 0.075%, even more preferably 0.050%, and still more preferably 0.020%.

[0055] 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 exceeds 0.100%, coarse Nb carbides and the like are generated. The coarse Nb carbides and the like become the starting points of fracture. As a result, even if the contents of other elements are within the ranges of this embodiment, the workability of the steel material is reduced. Therefore, the Nb content is 0 to 0.100%, and when Nb is contained, the Nb content is 0.100% or less. 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%.

[0056] [Group 3 (Ca, Bi, Te and Sn)] The chemical composition of the bolt according to this embodiment may further contain the above-mentioned elements of Group 3 in place of a portion of Fe. These elements are optional elements, and all of them improve the machinability of the steel material.

[0057] 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 workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0 to 0.0050%, and when Ca is contained, the Ca content is 0.0050% or less. 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%, more preferably 0.0035%, and even more preferably 0.0030%.

[0058] 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 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 when Bi is contained, the Bi content 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%, more preferably 0.015%, and even more preferably 0.013%.

[0059] 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 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 when Te is contained, the Te content 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%, more preferably 0.008%, and even more preferably 0.007%.

[0060] Sn: 0.100% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When Sn is contained, that is, when the Sn content is more than 0%, Sn improves the machinability of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. On the other hand, if the Sn content exceeds 0.100%, the workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.100%, and when Sn is contained, the Sn content is 0.100% or less. The lower limit of the Sn content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Sn content is preferably 0.090%, more preferably 0.070%, and even more preferably 0.050%.

[0061] [(Feature 2) Regarding Formula (1)] The chemical composition of the bolt of this embodiment further satisfies formula (1). Mo / V>3.00 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition of the bolt in mass %.

[0062] F1 is defined as F1 = Mo / V. F1 is an index of the C void ratio of the MC carbide. As mentioned above, the higher the Mo content relative to the V content, the higher the C void ratio of the MC carbide formed. As a result, the hydrogen absorption function of the MC carbide is enhanced, and the hydrogen embrittlement resistance of the bolt is improved. On the other hand, if F1 is 3.00 or less, the C void ratio of the MC carbide is not sufficiently increased. In this case, even if the bolt satisfies Features 1, 3, and 4, it will not be able to achieve excellent hydrogen embrittlement resistance.

[0063] Therefore, F1 is greater than 3.00. The lower limit of F1 is preferably 3.01, more preferably 3.10, and even more preferably 3.50. The upper limit of F1 is not particularly limited. However, when the content of each element in the chemical composition of the bolt is within the range of this embodiment, the upper limit of F1 is, for example, 150.00, for example, 100.00, for example, 70.00, or for example, 50.00. The value of F1 shall be rounded off to two decimal places.

[0064] [(Feature 3) Tensile strength TS] The bolt according to this embodiment has a tensile strength TS of 1200 MPa or more. The bolt according to this embodiment satisfies Features 1, 2, and 4. As a result, the bolt according to this embodiment has excellent hydrogen embrittlement resistance even when the tensile strength TS is 1200 MPa or more.

[0065] The lower limit of the tensile strength TS is preferably 1250 MPa, and more preferably 1300 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, for example, 1650 MPa.

[0066] [Method for measuring tensile strength TS] In this embodiment, the tensile strength TS can be determined by the following method. A tensile test is conducted in air at room temperature (20±15°C) in accordance with JIS Z 2241:2022 to obtain the tensile strength TS (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.

[0067] [(Feature 4) Regarding formula (3)] Furthermore, in the bolt of this embodiment, when a thin film sample for STEM made from the bolt is observed using a scanning transmission electron microscope (STEM), the total area ratio (%) of MC type carbides in an observation field of 280 nm × 280 nm is S MC and the thickness (nm) of the STEM thin film specimen in the observation field is defined as t. S expressed by equation (2) MC When a cross section perpendicular to the axial direction of the bolt was observed using a field emission scanning electron microscope (FE-SEM), the total area ratio S of cementite in an observation field of 9 μm × 12 μm was C (%) satisfies equation (3). S MC ´=S MC / t (2) S MC ´ / S C ≧0.010 (3)

[0068] S defined by equation (2) MC ´ is an index to more accurately represent the total area ratio of MC carbides in a bolt. By observing a thin film sample taken from a bolt using a scanning transmission electron microscope (STEM), the total area ratio S of MC carbides in an arbitrary observation field can be determined. MC Here, the size of the MC carbides in the bolt is smaller than the thickness of the thin film sample. Therefore, the thicker the thin film sample, the more the total number of MC carbides contained in the observation field. In other words, the total area ratio S of MC carbides in a certain observation field is MC is affected by the thickness of the thin film specimen in the observation field. Therefore, the total area ratio of MC-type carbides, S MCBy dividing by the thickness t of the thin film sample in that observation field, the influence of the thickness of the thin film sample for each observation field can be alleviated.

[0069] F3=S MC ´ / S C is defined as follows. F3 is an index representing the total area ratio of MC carbides relative to the total area ratio of cementite. As mentioned above, cementite is preferentially formed at grain boundaries and reduces grain boundary strength. Therefore, in order to obtain excellent hydrogen embrittlement resistance, it is preferable that the total area ratio of MC carbides relative to the total area ratio of cementite is high. On the other hand, if F3 is less than 0.010, the total area ratio of MC carbides relative to the total area ratio of cementite is too low. In this case, even if the bolt satisfies Features 1 to 3, excellent hydrogen embrittlement resistance cannot be obtained.

[0070] Therefore, F3 is greater than or equal to 0.010. The lower limit of F3 is preferably 0.012, and more preferably 0.015. The upper limit of F3 is not particularly limited. However, when the bolt satisfies Features 1 to 3, the upper limit of F3 is, for example, 0.200.

[0071] [S MC ' calculation method] S of the bolt in this embodiment MC ´ is the total area ratio of MC type carbides, S, as defined by equation (2). MC The total area ratio of MC-type carbides, S, is calculated based on the thickness t (nm) of the thin film specimen in the observation field. MC The thickness (t) (nm) of the thin film sample in the observation field can be measured using a scanning transmission electron microscope (STEM) as follows.

[0072] 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.

[0073] Here, the MC carbides contained in the bolt are plate-shaped particles extending along the {100} plane of the matrix. In other words, there are three types of MC carbides: those whose plate planes are parallel to the (100) plane of the matrix, those whose plate planes are parallel to the (010) plane of the matrix, and those whose plate planes are parallel to the (001) plane of the matrix. Here, the vertical direction (observation direction) of the observation surface of the STEM thin film specimen is parallel to the <001> By adjusting the thin film so that the crystal orientation is parallel to the observation direction, two of the three types of MC carbides are observed, each of which has a plate surface parallel to the observation direction. Specifically, electron backscatter diffraction (EBSD) is performed on the polished observation surface to identify the crystal orientation of the parent phase. Then, based on the identified crystal orientation of the parent phase, it is determined that the vertical direction (observation direction) of the observation surface of the thin film is parallel to the crystal orientation of the parent phase. <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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] The area of ​​each observation field is 280 nm × 280 nm. For all observation fields, the thickness t (nm) of the STEM thin film sample is measured using the log-ratio method of electron energy loss spectroscopy (EELS).

[0078] 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.

[0079] 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.

[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] The total area of ​​the identified MC carbides is calculated. The total area of ​​the identified MC carbides is the sum of the products of the major and minor axes of each MC carbide. Based on the total area of ​​the MC carbides identified in a certain observation field and the size of the observation field, the total area ratio S of the MC carbides in the observation field is calculated. MC Calculate (%).

[0082] The total area ratio S of MC-type carbides in a certain observation field MC (%) and the thickness t (nm) of the STEM thin film sample in the observation field. MC Furthermore, S MC The arithmetic mean value of ' is the S of the bolt in this embodiment. MC ´.

[0083] [S C Measurement method] Total area ratio of cementite S C (%) can be measured by the following method using a field emission scanning electron microscope (FE-SEM).

[0084] A sample is taken from the cross section of the bolt underhead, perpendicular to the bolt shaft and including a depth of 1.5 mm from the surface of the bolt underhead. After polishing the observation surface of the sample, the observation surface is etched at room temperature for 20 seconds using a 6% picral etching solution (a mixture of 6 g of picric acid and 94 mL of ethanol). 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 9 μm × 12 μm. Each observation field is observed using an FE-SEM at a magnification of 10,000x.

[0085] Oxides and sulfides are identified by a known method using an energy dispersive X-ray spectroscopy (EDS) device attached to the FE-SEM. Of the particles within the observation field, particles excluding oxides and sulfides are identified as cementite. The total area of ​​the identified cementite is calculated. The total area of ​​the identified cementite can be calculated, for example, by known image processing. Here, cementite with a maximum length of less than 100 nm has little effect on the hydrogen embrittlement resistance of the bolt. Therefore, the cementite to be identified is cementite with a maximum length of 100 nm or more.

[0086] The total area ratio S of cementite was calculated based on the total area of ​​cementite identified in all the observation fields and the total area of ​​all the observation fields by the above-mentioned method. C Calculate (%).

[0087] [Bolt effect] 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 when the tensile strength TS is as high as 1200 MPa or more.

[0088] [Bolt microstructure] 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 is composed of, for example, one or more selected from the group consisting of pro-eutectoid ferrite and pearlite. The tensile strength TS of the bolt correlates with the microstructure. Specifically, if the tensile strength TS of the bolt is 1200 MPa or more, the area ratio of the hard structure in the bolt microstructure is 90% or more.

[0089] [Method for measuring the microstructure of bolts] The microstructure of the bolt of this embodiment can be measured by the following method.

[0090] 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 x 120 μm. Each observation field is observed using an FE-SEM at a magnification of 1000x.

[0091] 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. In observation at 1000x magnification, pearlite is observed as a phase with a lamellar structure. The hard structure 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.

[0092] 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.

[0093] [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. The bolt of this embodiment may also be used for purposes other than those mentioned above.

[0094] [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.

[0095] 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.

[0096] [(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 Features 1 and 2 is prepared.

[0097] When manufacturing steel material for bolts, an example of a method for manufacturing steel material for bolts includes the following steps. (Step 11) Billet preparation process (Process 12) Finishing rolling process Each step will be described below.

[0098] [(Step 11) Billet preparation process] First, molten steel is produced whose chemical composition contains elements whose contents satisfy Features 1 and 2. A material is then produced using the molten steel. For example, the molten steel may be used to produce a bloom (stripe) as the material by continuous casting, or the molten steel may be used to produce an ingot as the material by ingot casting. The produced material (bloom or ingot) is subjected to rough rolling (slabbing, or slabbing and hot rolling in a continuous rolling mill) to produce a billet.

[0099] [(Process 12) Finishing rolling process] In the finish rolling process, the billet is heated in a heating furnace. The heated billet is then hot-rolled (finish rolling) using a finish rolling mill with multiple rolling stands arranged in a row. The finish-rolled billet is then cooled to produce steel material (steel material for bolts) that will be used as the raw material for bolts. The steel material is, for example, steel bar or wire rod.

[0100] [(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.

[0101] [(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.

[0102] [(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.

[0103] The bolt forming process includes the following steps: (Step 221) Head forming process (Process 222) Thread forming process Each step will be described below.

[0104] [(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.

[0105] 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 subsequent heat treatment process.

[0106] [(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.

[0107] [(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.

[0108] [(Step 231) Quenching process] In the quenching process, the intermediate bolt after the bolt forming process is heated in a heating furnace. The holding time in the heating furnace is not particularly limited. The holding time 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.

[0109] [(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.

[0110] [Manufacturing conditions for the manufacturing method of this embodiment] The above-described manufacturing method satisfies the following conditions. (Condition 1) When the rolling end temperature (°C) in the finish rolling process is defined as T1, the area reduction rate (%) in the final stand in the finish rolling process is defined as r, and the heating temperature (°C) in the quenching process is defined as T2, formula (A) is satisfied. (T1-800)×(T2-800) / r≧700 (A) (Condition 2) When the tempering temperature (°C) in the tempering step is defined as T3 and the holding time (hours) at the tempering temperature T3 (°C) is defined as t3, formula (B) is satisfied. 17000≦(T3+273)×(20+log(t3))≦18600 (B) Here, log in formula (B) means common logarithm. Conditions 1 and 2 will be explained below.

[0111] [(Condition 1) Formula (A)] FA is defined as (T1-800) × (T2-800) / r. FA is an index that represents the degree of suppression of cementite coarsening. In the finish rolling process, if the billet is rolled at a low temperature with a high area reduction, coarse cementite precipitates during the cooling process after rolling. In the finish rolling process, the billet passes through multiple rolling stands arranged in a row to produce steel with a predetermined diameter. The rolling in the final stand, where the billet temperature is lowest and immediately before cooling, has the greatest impact on the amount of cementite generated in the steel produced. The final stand refers to the rolling stand closest to the exit side among the multiple rolling stands arranged in a row. The rolling end temperature T1 (°C) in the finish rolling process can be assumed to be the billet temperature during rolling in the final stand. In other words, the lower the rolling end temperature T1 (°C) in the finish rolling process or the higher the area reduction rate r (%) in the final stand, the more coarse the cementite in the steel produced in the finish rolling process. Furthermore, during the heating process in the quenching process, some of the cementite contained in the intermediate bolt dissolves. However, if the heating temperature T2 (°C) in the quenching step is low, the cementite contained in the intermediate bolt does not dissolve sufficiently.

[0112] In these cases, that is, when the FA is low, a certain amount of coarse cementite has already been generated in the intermediate bolt before the tempering process. If a tempering process that satisfies the above-mentioned condition 2 is carried out in such a state, this cementite will become even coarser. If the FA is less than 700, the cementite in the bolt will become too coarse. As a result, the cementite area ratio SC becomes too high and the bolt does not meet feature 4. Therefore, the FA is at least 700. There is no particular upper limit to the FA, but in consideration of normal industrial production, it is, for example, 7,000.

[0113] [Regarding (Condition 2) Equation (B)] FB is defined as (T3 + 273) × (20 + log(t3)). FB is an index of the amount of heat given to the bolt during the tempering process. During the tempering process, MC carbides precipitate and grow. During the growth process of the MC carbides, they steal carbon from cementite.

[0114] If FB is less than 17,000, the amount of heat given in the tempering process is too small. In this case, the growth of MC carbides does not occur sufficiently. Therefore, the total area ratio S of MC carbides MC S calculated from MC ´ becomes lower, and the cementite area ratio S C As a result, even if the bolt satisfies condition 1 above, it does not satisfy feature 4. On the other hand, if FB exceeds 18,600, the amount of heat applied in the tempering process is too large, resulting in a bolt with a tensile strength TS of less than 1,200 MPa. Therefore, FB is 17,000 to 18,600.

[0115] The bolt of 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 of 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 of this embodiment having the above configuration.

[0116] [Other processes] The manufacturing process for the bolt of 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. Also, the thread forming step may be performed after the heat treatment step, rather than after the head forming step and before the heat treatment step. Furthermore, a compressive residual stress imparting step may be performed after the heat treatment step. [Example]

[0117] 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.

[0118] Steel materials (steel bars) having the chemical compositions shown in Tables 1A and 1B were prepared as bolt materials.

[0119] [Table 1A]

[0120] [Table 1B]

[0121] [Steel preparation process] The steel materials of each test number were produced by the following method. Blooms having the chemical compositions shown in Tables 1A and 1B were subjected to rough rolling (blooming and hot rolling in a continuous rolling mill) to produce billets. The heating temperature of the blooms was 1200°C. The produced billets were subjected to a finish rolling process to produce steel bars with a diameter of 20 mm. The rolling end temperature T1 (°C) in the finish rolling process and the area reduction rate r (%) in the final stand of the finish rolling process were each as shown in Table 2.

[0122] [Table 2]

[0123] [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.

[0124] 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. The heating temperature T2 (°C) in the quenching process was as shown in Table 2. The holding time at heating temperature T2 (°C) was 60 minutes (1 hour). After the holding time had elapsed, the intermediate bolts were water-cooled.

[0125] The intermediate bolts after the quenching process were tempered. The tempering temperature T3 (°C) and the holding time t3 (hours) at the tempering temperature T3 for each test number are shown in Table 2.

[0126] 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 TS of 1200 MPa or more in the tensile test described below.

[0127] [Evaluation test] The following evaluation tests were carried out on the bolts with each test number. (Test 1) Tensile test (Test 2) F3 calculation test (Test 3) Hydrogen embrittlement resistance evaluation test

[0128] [(Test 1) Tensile test] The tensile strength TS (MPa) of the bolts with each test number was determined based on the method described in the above [Method for measuring tensile strength TS]. The determined tensile strength TS is shown in the "TS (MPa)" column in Table 2.

[0129] For test numbers for which the obtained tensile strength TS was less than 1200 MPa, the subsequent evaluation tests (Tests 2 and 3) were not carried out.

[0130] [(Test 2) F3 calculation test] The above-mentioned [S MC Based on the method described in [Calculation method of MC type carbides], the total area ratio S of MC type carbides in the bolts of each test number is calculated. MC The total area ratio S of the MC-type carbides was measured. MC Based on the thickness t (nm) of the thin film sample in the observation field, the S of the bolt for each test number MC ´ was calculated. The above-mentioned [S C Based on the method described in [Measuring Method of Cementite in Bolts of Each Test Number], the total area ratio S of cementite in the bolts of each test number was C (%) was measured. The obtained S MC ´ and S C The F3 values ​​obtained are shown in Table 2.

[0131] [(Test 3) Hydrogen embrittlement resistance evaluation test] The hydrogen embrittlement resistance of the bolts of each test number was evaluated 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.

[0132] The round bar test specimens were charged with hydrogen by the cathodic hydrogen charging method under the following conditions. A room-temperature solution (cathode charging solution) was prepared by adding 3 g of ammonium thiocyanate to 1 L of a 3 mass % sodium chloride solution. The round bar test piece was immersed in the cathode charging solution for 72 hours at a cathode current density of 0.05 mA / cm. 2 A constant current controlled within the range of 1000 kJ / s was generated to add hydrogen to the round bar specimen.

[0133] After the cathodic hydrogen charging method, the round bar test specimens were left at room temperature for 48 hours. A zinc plating coating was then formed on the hydrogen-charged surface of the round bar test specimens to prevent hydrogen from leaking out. 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 the tensile strength TS was applied. The test time was a maximum of 100 hours, and the test was stopped if the round bar test specimens lasted for more than 100 hours without fracture.

[0134] If the round bar test piece lasted for more than 100 hours without breaking, it was judged to have excellent hydrogen embrittlement resistance (indicated as "E" in the "Hydrogen embrittlement resistance" column in Table 2). On the other hand, if the round bar test piece broke before 100 hours had elapsed, it was judged to have poor hydrogen embrittlement resistance (indicated as "B" in the "Hydrogen embrittlement resistance" column in Table 2).

[0135] [Test Results] Referring to Tables 1A, 1B and 2, the bolts with test numbers 1 to 17 satisfied features 1 to 4. Therefore, even when the tensile strength TS was 1200 MPa or more, excellent hydrogen embrittlement resistance was obtained.

[0136] On the other hand, in test numbers 18 and 19, F1 was too low, and as a result, excellent hydrogen embrittlement resistance was not obtained.

[0137] In test numbers 20 and 21, FA was too low in the manufacturing process, and therefore F3 was too low, resulting in poor hydrogen embrittlement resistance.

[0138] In test numbers 22 and 23, FB was too low in the tempering process, and therefore F3 was too low, resulting in failure to obtain excellent hydrogen embrittlement resistance.

[0139] In test numbers 24 and 25, the FB in the tempering process was too high, resulting in a low tensile strength TS of less than 1200 MPa.

[0140] 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: 0.01-0.80%, Mo: 0.70 to less than 1.50% V: 0.01 to less than 0.50% Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%; The balance is Fe and impurities. Formula (1) is satisfied, The tensile strength TS is 1200 MPa or more, When a thin film sample for STEM made from the bolt was observed using a scanning transmission electron microscope (STEM), the total area ratio (%) of MC type carbides in an observation field of 280 nm x 280 nm was determined as S ＭＣ and When the thickness (nm) of the STEM thin film sample in the observation field is defined as t, S represented by formula (2) ＭＣ and, When a cross section perpendicular to the axial direction of the bolt is observed using a field emission scanning electron microscope (FE-SEM), the total area ratio S of cementite in an observation field of 9 μm × 12 μm is Ｃ (%) and Satisfying formula (3), bolt. Mo / V>3.00 (1) S ＭＣ ´=S ＭＣ / t (2) S ＭＣ ´ / S Ｃ ≧0.010 (3) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition of the bolt in mass %.

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: 0.01-0.80%, Mo: 0.70 to less than 1.50% V: 0.01 to less than 0.50% Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%; Further, it contains one or more selected from the group consisting of Groups 1 to 3, The balance is Fe and impurities. Formula (1) is satisfied, The tensile strength TS is 1200 MPa or more, When a thin film sample for STEM made from the bolt was observed using a scanning transmission electron microscope (STEM), the total area ratio (%) of MC type carbides in an observation field of 280 nm x 280 nm was determined as S ＭＣ and When the thickness (nm) of the STEM thin film sample in the observation field is defined as t, S represented by formula (2) ＭＣ and, When a cross section perpendicular to the axial direction of the bolt is observed using a field emission scanning electron microscope (FE-SEM), the total area ratio S of cementite in an observation field of 9 μm × 12 μm is Ｃ (%) and Satisfying formula (3), bolt. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, 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 [Second group] 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, Te: 0.010% or less, and Sn: 0.100% or less, one or more selected from the group consisting of Mo / V>3.00 (1) S ＭＣ ´=S ＭＣ / t (2) S ＭＣ ´ / S Ｃ ≧0.010 (3) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition of the bolt in mass %.

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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