bolt

Abstract

To provide a bolt that exhibits high strength, superior hydrogen embrittlement resistance, and superior toughness at low temperature.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 0.0200%, with the balance being Fe and impurities. The bolt has a tensile strength TS of 1300 MPa or more, and a number density ND of MC-type carbide of 2.0×1022 particles / m3 or more. Defining the solid-solution Mo level in atomic% in a matrix phase of the bolt as [Mo]SS and the Mo level in atomic% in MC-type carbide as [Mo]MC, formula (1) is satisfied. [Mo]SS / [Mo]MC≥0.50 (1).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 1300 MPa or more.

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

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

[0005] The bolt disclosed in Patent Document 1 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 by adjusting the contents of Sb, Sn, and Bi in the chemical composition of this bolt so as to satisfy formula (2), excellent hydrogen embrittlement resistance can be obtained even if the tensile strength of the shaft is 1000 to 1300 MPa.

[0006] The bolt disclosed in Patent Document 2 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 describes that by satisfying formula (1), the tensile strength of this bolt is increased to 1000 to 1300 MPa, and by satisfying formula (2), the hydrogen embrittlement resistance of the bolt is increased.

[0007] The bolt disclosed in Patent Document 3 contains, by mass%, C: 0.30 to 0.50%, Si: 1.0 to 2.5%, Mn: 0.1 to 1.5%, P: 0.015% or less (excluding 0%), S: 0.015% or less (excluding 0%), Cr: 0.15 to 2.4%, Al: 0.10% or less (excluding 0%), and N: 0.015% or less (excluding 0%), and further contains Cu: 0.10 to 0.50% and Ni: 0.1 to 1.0% so as to satisfy [Ni] / [Cu]≧0.5, and further contains Ti: 0.05 to 0.20% and V: 0.20% or less (including 0%) so as to satisfy [Ti]+[V]: 0.085 to 0.30%, with the remainder consisting of Fe and impurities. Furthermore, this bolt has an austenite grain size number of 9.0 or more in the bolt shank, and the G value (%), which indicates the proportion of carbides precipitated at the austenite grain boundaries in the bolt shank, satisfies formula (1) (G value: (L / L0) × 100≦60, where L is the total length of carbides with a thickness of 50 nm or 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 even with high strength 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] Incidentally, bolts are sometimes used in low-temperature environments such as refrigerated and frozen warehouses and cold regions. In low-temperature environments, the toughness of bolts decreases. Therefore, bolts are also required to have excellent low-temperature toughness. Generally, bolts having high strength as described above have low low-temperature toughness. Patent Documents 1 to 3 discuss increasing the strength of bolts, but do not discuss low-temperature toughness.

[0010] An object of the present disclosure is to provide a bolt that has high strength, excellent hydrogen embrittlement resistance, and excellent low-temperature toughness. [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 0.50%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%, The balance is 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 That's all, The concentration of solid solution Mo in atomic percent in the parent phase of the bolt is [Mo] SS and the Mo concentration in atomic % in the MC type carbide is defined as [Mo] MC When defined as above, equation (1) is satisfied. [Mo] SS / [Mo] MC ≧0.50 (1)

[0012] A bolt according to the present disclosure comprises: The chemical composition is, in mass%, C: 0.30~0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50% P: 0.030% or less, S: 0.030% or less, Cr: 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 0.0200%, Further, it contains one or more selected from the group consisting of Groups 1 to 3, The balance is Fe and impurities. The tensile strength TS is 1300 MPa or more, The number density ND of MC type carbides is 2.0×10 22 pieces / m 3 That's all, The concentration of solid solution Mo in atomic percent in the parent phase of the bolt is [Mo] SS and the Mo concentration in atomic % in the MC type carbide is defined as [Mo] MC When defined as above, equation (1) 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] SS / [Mo] MC ≧0.50 (1) [Effects of the Invention]

[0013] The bolt according to the present disclosure has high strength, excellent hydrogen embrittlement resistance, and excellent low-temperature toughness. [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 from the viewpoint of chemical composition a bolt that has high strength, excellent hydrogen embrittlement resistance, and excellent low-temperature toughness, and as a result, the inventors found a bolt with a composition of, in mass%, C: 0.30 to 0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 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%, N: 0.0010 to 0.0200%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0050%, Zr: 0 to 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, it was thought that a bolt having a chemical composition containing the following would have a tensile strength TS of 1300 MPa or more, and would be able to obtain excellent hydrogen embrittlement resistance and excellent low-temperature toughness.

[0016] Therefore, the present inventors further investigated means for improving hydrogen embrittlement resistance and low-temperature toughness from the viewpoint of the microstructure of the bolt having the above-mentioned chemical composition, and as a result, the following findings were obtained.

[0017] In bolts with the above-mentioned chemical composition, alloy carbides may precipitate. Coarse alloy carbides reduce the low-temperature toughness of the bolt. On the other hand, 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.). Fine precipitates act as hydrogen trapping sites and occlude 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 other words, by increasing the number density of MC carbides in the bolt, it is possible to improve hydrogen embrittlement resistance while maintaining low-temperature toughness.

[0018] Mo dissolved in the matrix of the bolt concentrates at the grain boundaries. Here, the matrix of the bolt refers to the phase in the bolt's microstructure that is primarily iron, excluding precipitates and inclusions. Mo concentrated at the grain boundaries increases the grain boundary strength. As a result, the low-temperature toughness of the bolt is improved.

[0019] Mo is one of the main constituent elements of MC carbides. Therefore, generally, as the amount of MC carbides in a bolt increases, the concentration of Mo dissolved in the matrix of the bolt decreases. In other words, as the concentration of Mo dissolved in the matrix of the bolt increases, the amount of MC carbides in the bolt decreases. In other words, there seems to be a trade-off between the effect of MC carbides in the bolt on improving hydrogen embrittlement resistance and the effect of Mo dissolved in the matrix on improving low-temperature toughness.

[0020] However, the inventors thought that if the Mo concentration in the MC carbides could be reduced and the Mo concentration dissolved in the matrix could be increased, it would be possible to further improve low-temperature toughness without reducing the amount of MC carbides, which improve hydrogen embrittlement resistance.

[0021] Based on the above findings, the present inventors have further investigated. As a result, they have found that the number density ND of MC type carbides is 2.0 × 10 22 pieces / m 3The solid solution Mo concentration in atomic percent in the matrix of the bolt is [Mo] SS and the Mo concentration in atomic percent in the MC type carbide is defined as [Mo] MC The inventors have found that, when formula (1) is satisfied, even a bolt having a tensile strength TS of 1300 MPa or more can have excellent hydrogen embrittlement resistance and excellent low-temperature toughness. [Mo] SS / [Mo] MC ≧0.50 (1)

[0022] The bolt of this embodiment has been completed based on the above technical concept and has the following configuration.

[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 0.50%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%, The balance is 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 That's all, The concentration of solid solution Mo in atomic percent in the parent phase of the bolt is [Mo] SS and the Mo concentration in atomic % in the MC type carbide is defined as [Mo] MC When defined as above, equation (1) is satisfied. [Mo] SS / [Mo] MC ≧0.50 (1)

[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 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. 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 That's all, The concentration of solid solution Mo in atomic percent in the parent phase of the bolt is [Mo] SS and the Mo concentration in atomic % in the MC type carbide is defined as [Mo] MC When defined as above, equation (1) 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] SS / [Mo] MC ≧0.50 (1)

[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 0.50%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0200%, Cu: 0 to 0.40%, Ni : 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 tensile strength TS is 1300 MPa or more. (Feature 3) The number density ND of MC type carbides is 2.0×10 22 pieces / m 3 That's all. (Feature 4) The concentration of solid solution Mo in atomic percent in the matrix of the bolt is [Mo] SS and the Mo concentration in atomic percent in the MC type carbide is defined as [Mo] MC When defined as above, equation (1) is satisfied. [Mo] SS / [Mo] MC ≧0.50 (1)

[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, excellent hydrogen embrittlement resistance, and excellent low-temperature toughness. 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 even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.50%, the low temperature toughness 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 0.42%.

[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 even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.30%, the low-temperature brittleness of the bolt decreases 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 the bolt. If the Mn content is less than 0.10%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.50%, the low temperature toughness 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 and low-temperature toughness of the bolt decrease 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 and low-temperature toughness of the bolt decrease 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 even 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 material, thereby increasing the strength of the bolt. Cr also improves the temper softening resistance of the steel material, thereby increasing the strength of the bolt. If the Cr content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, 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) improves the hardenability of steel materials and increases the strength of bolts. Mo also concentrates at grain boundaries, increasing grain boundary strength. As a result, the low-temperature toughness of the bolt is improved. If the Mo content is less than 0.70%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is 1.50% or 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 the low temperature toughness of the bolt. 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 0.50% Vanadium (V) forms MC carbides together with Mo to enhance the hydrogen embrittlement resistance of the bolt. If the V content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content exceeds 0.50%, the steel material becomes excessively hard even if the contents of other elements are within the ranges of this embodiment, and in this case, the low-temperature toughness of the bolt decreases. Therefore, the V content is 0.01 to 0.50%. The lower limit of the V content is preferably 0.03%, and more preferably 0.05%. The upper limit of the V content is preferably 0.45%, more preferably 0.40%, and even 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 suppress grain coarsening through a pinning effect. As a result, the low-temperature toughness of the bolt is improved. If the Al content is less than 0.005%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.100%, coarse Al nitrides are formed. These coarse Al nitrides become the starting points for fracture. As a result, the low-temperature toughness of the bolt decreases 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 steel material of this embodiment, the Al content means the total Al content.

[0042] N: 0.0010~0.0200% Nitrogen (N) combines with Al to form Al nitrides. Al nitrides suppress grain coarsening through a pinning effect. As a result, the low-temperature toughness of the bolt is improved. If the N content is less than 0.0010%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0200%, coarse nitrides are formed. These coarse nitrides become the starting points for fracture. As a result, the low-temperature toughness of the bolt decreases 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%, and even more preferably 0.0100%.

[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 exceeds 0%, B improves the hardenability of the steel material and increases the strength of the bolt. B also suppresses the grain boundary segregation of P and improves the hydrogen embrittlement resistance and low-temperature toughness 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 low-temperature toughness of the bolt decreases 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%, even more preferably 0.0030%, and even more preferably 0.0025%.

[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 formed. These coarse Zr nitrides become the starting points for fracture. As a result, the low-temperature toughness of the bolt decreases 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%, even more preferably 0.060%, and still more preferably 0.050%.

[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 formed. These coarse Hf nitrides become the starting points for fracture. As a result, the low-temperature toughness of the bolt decreases 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%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Hf content is preferably 0.009%, more preferably 0.008%, even more preferably 0.007%, and still more preferably 0.006%.

[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 formed. These coarse Ta nitrides become the starting points for fracture. As a result, the low-temperature toughness of the bolt decreases 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%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Ta content is preferably 0.009%, more preferably 0.008%, even more preferably 0.007%, and still more preferably 0.006%.

[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 low-temperature toughness 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 low-temperature toughness 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 low-temperature toughness of the bolt decreases 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%, and even more preferably 0.075%.

[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 crystal grains. As a result, the low-temperature toughness 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. These coarse Nb carbides and the like become the starting points for fracture. As a result, the low-temperature toughness of the bolt decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.100%, and when Nb is contained, 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) 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 satisfies Features 1, 3, and 4. As a result, the bolt according to this embodiment has excellent hydrogen embrittlement resistance and low-temperature toughness even when the tensile strength TS is 1300 MPa or more.

[0062] The lower limit of the tensile strength TS is preferably 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, for example, 1650 MPa.

[0063] [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.

[0064] [(Feature 3) Number density of MC type carbides (ND)] In the bolt of this embodiment, the number density ND of the MC type carbide is 2.0 × 10 22 pieces / m 3 That's all.

[0065] MC carbides are finer than other alloy carbides such as cementite and M2C carbides. Therefore, MC carbides are more likely to trap hydrogen than other alloy carbides. In other words, increasing the number density of MC carbides can improve the hydrogen embrittlement resistance of bolts. On the other hand, when the number density ND of MC carbides is 2.0×10 22 pieces / m 3 If the ratio is less than 1 / 2, even if the bolt satisfies Features 1, 2, and 4, excellent hydrogen embrittlement resistance cannot be obtained.

[0066] Therefore, the number density of MC type carbides is 2.0 × 10 22 pieces / m 3 That's all. The preferred lower limit of the number density ND of MC type carbides is 2.5 × 10 22 pieces / m 3 and more preferably 3.0 × 10 22 pieces / m 3 and more preferably 4.0 × 10 22 pieces / m 3 is. The upper limit of the number density ND of the MC carbides is not particularly limited. When the bolt satisfies Features 1, 2, and 4, the upper limit of the number density ND of the MC carbides is, for example, 100.0 × 10 22 pieces / m 3 is 80.0×10 22 pieces / m 3 is 50.0×1022 pieces / m 3 is.

[0067] [Method for measuring the number density ND of MC type carbides] The number density ND of MC type carbides in a bolt can be measured using a scanning transmission electron microscope (STEM) by the following method. First, a thin-film sample for STEM is prepared using the following method. The lower neck of the bolt is cut perpendicular to the axial direction (longitudinal direction) of the bolt, and a disk with an axial thickness of approximately 2 mm (i.e., a disk with a diameter of 3.0 mm and a thickness of approximately 2 mm) is taken, with the center of the circle located 1.5 mm deep from the surface of the lower neck of the bolt. Both sides of the disk (front and back) are polished using emery paper. The front surface of the disk is polished so that it is parallel to the back surface. One of the front or back surfaces of the disk is defined as the observation surface. The observation surface is then mirror-polished. The mirror-polished observation surface is then further polished using colloidal silica as an abrasive.

[0068] MC carbides have a specific crystal orientation relationship with the matrix. Specifically, MC carbides are plate-like particles extending along the {100} plane of the matrix. Therefore, electron backscatter diffraction (EBSD) is performed on the polished observation surface to identify the crystal orientation of the matrix. Based on the identified crystal orientation of the matrix, it is determined that the perpendicular direction (observation direction) of the observation surface of the thin film is the same as that of the matrix. <001> The disk is processed by focused ion beam (FIB) processing to obtain the crystal orientation, and a thin film sample for STEM is prepared.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] The area of ​​each observation field is 280 nm × 280 nm. In all observation fields, the thickness of the thin film sample for STEM is measured using the log-ratio method of electron energy loss spectroscopy (EELS).

[0073] 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.

[0074] 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.

[0075] The number density ND (number / m) of MC carbides was calculated based on the total number of MC carbides identified in all observation fields by the above method and the total volume calculated from the area of ​​all observation fields and the thickness of the STEM thin film sample. 3 ) is found.

[0076] 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.

[0077] [(Feature 4) Regarding Formula (1)] In the bolt of this embodiment, the concentration of solid solution Mo in atomic % in the matrix of the bolt is [Mo] SS and the Mo concentration in atomic percent in the MC type carbide is defined as [Mo] MC When defined as above, equation (1) is satisfied. [Mo] SS / [Mo] MC ≧0.50 (1)

[0078] Define F1 as follows: F1=[Mo] SS / [Mo] MC As mentioned above, Mo dissolved in the matrix concentrates at the grain boundaries, increasing grain boundary strength. Therefore, the higher the Mo concentration dissolved in the matrix, the higher the low-temperature toughness of the bolt. In this case, by reducing the Mo concentration in the MC carbides and increasing the Mo concentration dissolved in the matrix, it is possible to further improve low-temperature toughness while maintaining the amount of MC carbides necessary to obtain excellent hydrogen embrittlement resistance. On the other hand, if F1 is less than 0.50, excellent low-temperature toughness cannot be obtained even if the bolt satisfies Features 1 to 3.

[0079] Therefore, F1 is greater than or equal to 0.50. The lower limit of F1 is preferably 0.60, and more preferably 0.70. There is no particular upper limit for F1. When the bolt satisfies Features 1 to 3, the upper limit for F1 is, for example, 4.00 or 3.50.

[0080] [[Mo] SS and [Mo] MC Measurement method] Solute Mo concentration in atomic % in the parent phase of the bolt [Mo] SS , and Mo concentration in atomic % in MC-type carbides [Mo] MC can be found in the following way: A sample is cut out from the inside of the bolt shaft to a depth of at least 1 mm from the surface. The cut out sample is subjected to well-known focused ion beam processing or electrolytic polishing to produce a needle-shaped test piece with a tip curvature radius of approximately 50 nm.

[0081] Three-dimensional atom probe analysis is performed on the needle-shaped test specimen. Specifically, MC-type carbides in the needle-shaped test specimen are identified by three-dimensional atom probe analysis. Three-dimensional atom probe analysis can detect precipitates present in the needle-shaped test specimen in three dimensions.

[0082] In the three-dimensional atom probe analysis, the laser wavelength (λ) is set to 355 nm, the laser power to 30 pJ, and the temperature of the needle-shaped test piece to 50 K. The observation area is set to 50 nm x 50 nm x 100 nm. The device used for the three-dimensional atom probe analysis is not particularly limited. For example, the three-dimensional atom probe analyzer is a product name LEAP4000XHR manufactured by Ametec Co., Ltd.

[0083] The acquired measurement data is reconstructed to obtain a three-dimensional atomic map. Specifically, the detection efficiency of the instrument is used to adjust the spacing of the {110} atomic planes to 0.20 nm in the measurement of iron (Fe), and the measurement data is reconstructed to obtain a three-dimensional atomic map.

[0084] In a three-dimensional atomic map, the area of ​​the needle-shaped test piece that has been analyzed with a three-dimensional atom probe is divided into tiny cubes called voxels. Each side of a voxel is 1.0 nm. The concentration (atomic %) of an element within a voxel is defined as the number of atoms of that element contained within the voxel divided by the number of atoms of all elements within the voxel.

[0085] The sum of the C concentration, Mo concentration, and V concentration in each voxel is defined as the specific element concentration (atomic %). An isoconcentration surface is created connecting voxels where the specific element concentration is 6%. The specific element concentration is high within the area surrounded by the isoconcentration surface. The area surrounded by the isoconcentration surface is recognized as a precipitate. The area in the observation region other than the area surrounded by the isoconcentration surface is recognized as the parent phase. The Mo concentration in atomic % in the area recognized as the parent phase is defined as [Mo] SS Let's say.

[0086] Among the identified precipitates, precipitates with a maximum length of 2 to 10 nm are identified. Here, among the line segments connecting any two points on the surface of a three-dimensionally detected precipitate (i.e., the interface between the precipitate and the steel matrix), a line segment whose entirety is included in the precipitate is defined as a "specific line segment." The maximum length of the specific line segment of the precipitate is defined as the maximum length of the precipitate. Precipitates with a maximum length of 10 nm or less are identified as MC-type carbides.

[0087] 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, precipitates with a maximum length of 2 to 10 nm are almost all MC carbides, and precipitates other than MC carbides are almost absent. Furthermore, the maximum lengths of precipitates other than MC carbides, such as M2C carbides and cementite, all far exceed 10 nm. Therefore, among the precipitates in the needle-shaped test specimen obtained by three-dimensional atom probe analysis, precipitates with a maximum length of 10 nm or less are identified as MC carbides.

[0088] Among the identified MC carbides, 20 MC carbides are arbitrarily selected. Then, the Mo concentration in atomic % in each selected MC carbide is calculated. The arithmetic mean value of the Mo concentration in atomic % in each MC carbide calculated is defined as the Mo concentration in atomic % in the MC carbide [Mo]. MC It is defined as:

[0089] [Effects of the bolt of this embodiment] The bolt of this embodiment satisfies the above-mentioned features 1 to 4. As a result, the bolt of this embodiment has excellent hydrogen embrittlement resistance and excellent low-temperature toughness even though it has a high tensile strength TS of 1300 MPa or more.

[0090] [Microstructure of the bolt of this embodiment] The microstructure of the bolt of this embodiment contains 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 contains phases other than the hard structure, the remainder of the bolt microstructure is composed of one or more phases selected from the group consisting of pro-eutectoid ferrite and pearlite. Preferably, the microstructure contains a hard structure with an area ratio of 90% or more, and the bolt microstructure correlates with the tensile strength TS. Specifically, if the tensile strength TS of the bolt is 1300 MPa or more, the area ratio of the hard structure in the bolt microstructure is 90% or more.

[0091] [Method for measuring the microstructure of bolts] The microstructure of the bolt of this embodiment can be measured by the following method.

[0092] A sample is taken from the cross section of the bolt's under-neck, perpendicular to the bolt shaft and including a position 1.5 mm deep from the surface of the bolt's under-neck, with the observation surface being a cross section. After polishing the observation surface of the sample, it is etched at room temperature for 10 seconds using 3% nitric acid alcohol (Nital etching solution). Five observation fields are determined on the etched observation surface, with each field centered at a depth of 1.5 mm, so that they do not overlap with each other. The size of each observation field is 90 μm × 120 μm. Each observation field is observed using an FE-SEM at a magnification of 1000x.

[0093] In the observation field, the hard structure can be easily distinguished from the pro-eutectoid ferrite and pearlite by contrast. Pro-eutectoid ferrite is observed as a white region with no visible substructure within the grains. In observation at 1000x magnification, pearlite is observed as a phase with a lamellar structure. The hard structure contains a substructure (lath) and is observed as a region with lower brightness than the pro-eutectoid ferrite. Therefore, pro-eutectoid ferrite and pearlite are identified based on contrast. Specifically, in observation at 1000x magnification, a white structure is recognized as pro-eutectoid ferrite. In observation at 1000x magnification, a structure in which lamellae can be confirmed is recognized as pearlite. In each observation field, structures other than pro-eutectoid ferrite and pearlite are considered to be hard structures.

[0094] 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.

[0095] [Use of the bolt of this embodiment] The bolt of this embodiment can be used as a fastening means for industrial machinery, automobiles, buildings such as bridges, etc. It is particularly suitable as a bolt for use in low-temperature environments such as refrigerated / freezer warehouses and cold regions. The low-temperature environment referred to here specifically means an environment of 0°C or below. The bolt of this embodiment may also be used for applications other than those described above.

[0096] [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.

[0097] 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.

[0098] [(Process 1) Steel material preparation process] In the steel preparation step, steel (steel for bolts) that will be used to make bolts is prepared. The steel for bolts may be manufactured. Alternatively, a third party may prepare the steel for bolts. In the steel preparation step, steel whose chemical composition satisfies Feature 1 is prepared.

[0099] 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.

[0100] [(Step 11) Billet preparation process] First, molten steel is produced whose chemical composition contains elements whose contents satisfy Feature 1. 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.

[0101] [(Process 12) Finishing rolling process] In the finish rolling process, the billet is heated in a heating furnace. The heated billet is then finish rolled. The finish rolled billet is then cooled to produce a steel material (steel material for bolts) that will be used as the raw material for bolts. The steel material is, for example, a steel bar or a wire rod.

[0102] [(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.

[0103] [(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.

[0104] [(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.

[0105] The bolt forming process includes the following steps: (Step 221) Head forming process (Process 222) Thread forming process Each step will be described below.

[0106] [(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.

[0107] 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.

[0108] [(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.

[0109] [(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.

[0110] [(Process 231) Quenching Process] In the quenching step, a well-known quenching method is performed. The quenching temperature and the holding time at the quenching temperature are not particularly limited. The quenching temperature is, for example, 870 to 1050°C. The holding time at the quenching temperature is, for example, 15 to 240 minutes (4 hours). After the holding time has elapsed, the intermediate bolt is quenched. Specifically, the intermediate bolt is water-cooled or oil-cooled.

[0111] [(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.

[0112] [Manufacturing conditions for the manufacturing method of this embodiment] The above-described manufacturing method satisfies the following conditions. (Condition 1) When the total area reduction rate (%) when the billet temperature in the finish rolling process is 900°C or higher is defined as R, and the strain imparted to the intermediate bolt in the head forming process is defined as ε, formula (A) is satisfied. R×ε×V / Mo≧1.0 (A) Here, the V and Mo in formula (A) are substituted with the mass % contents of the corresponding elements in the chemical composition of the bolt. (Condition 2) The tempering temperature T in the tempering step is 570 to 650°C. (Condition 3) The holding time t at the tempering temperature T in the tempering step is 0.50 to 5.00 hours. Conditions 1 to 3 will be explained below.

[0113] [(Condition 1) Formula (A)] FA is defined as R × ε × V / Mo. FA is an index that indicates the likelihood of forming MC carbides with low Mo concentration. During the finish rolling process, when the billet temperature is 900°C or higher, precipitated Mo dissolves into the matrix. When the billet temperature is 900°C or higher and rolling is performed with a large area reduction, Mo diffuses uniformly into the matrix. As mentioned above, the strain imparted to the intermediate bolt acts as the driving force for the formation of MC carbides during the heat treatment process. If the total area reduction R (%) at a billet temperature of 900°C or higher is high, and the strain ε imparted during the head forming process is large, and the V content relative to the Mo content in the bolt's chemical composition is high, MC carbides form before Mo is fully concentrated. As a result, the Mo concentration in the MC carbides decreases, and the Mo concentration in the matrix increases. Therefore, the higher the FA, the higher the F1 of the manufactured bolt.

[0114] If FA is 1.0 or more, F1 is 0.50 or more. There is no particular upper limit for FA, but in consideration of normal industrial production, it is, for example, 45.0.

[0115] The strain ε imparted to the intermediate bolt during the head forming process is found using the following finite element method (FEM analysis). Specifically, a 1 / 6-size axisymmetric model is created for the bolt to be analyzed based on the symmetry of its shape. The 1 / 6-size axisymmetric model to be analyzed is divided into multiple elements using tetrahedral primary elements with each side of the tetrahedron measuring 0.05 to 1.00 mm.

[0116] A three-dimensional elastic-plastic analysis using FEM is performed on the model divided into the above number of elements. A commercially available forging analysis code can be used for the analysis. An example of a commercially available forging analysis code is DEFORM-3D, manufactured by Scientific Forming Technologies Corporation.

[0117] The workpiece (steel for bolts) is assumed to be an elastoplastic body, and the die a rigid body. The flow stress of the workpiece is the measured value of spheroidized annealed SCM435 material specified in JIS G 4053:2018. The physical properties of the workpiece are Young's modulus: 210 GPa, Poisson's ratio: 0.30. The die is set to a size sufficiently large compared to the workpiece. The Coulomb friction coefficient between the workpiece and die is set to μ = 0.05. An FEM analysis is performed under the above conditions to determine the strain imparted to the neck portion 11 of the intermediate bolt. The obtained strain is taken as the strain ε imparted to the intermediate bolt during the head forming process.

[0118] [(Condition 2) Tempering temperature T] The tempering temperature T is 570°C to 650°C. If the tempering temperature T is less than 570°C, the number density ND of the MC type carbides is 2.0 × 10 22 pieces / m 3 If the tempering temperature T exceeds 650°C, the tensile strength TS will be less than 1300 MPa.

[0119] [(Condition 3) Holding time t at tempering temperature T] The holding time t at the tempering temperature T is 0.50 to 5.00 hours. If the holding time t at the tempering temperature T is less than 0.50 hours, the number density ND of the MC type carbides is 2.0 × 10 22 pieces / m 3 If the holding time t at the tempering temperature T exceeds 5.00 hours, the tensile strength TS will be less than 1300 MPa.

[0120] The bolt according to this embodiment can be manufactured by the above manufacturing method. Note that the above manufacturing method is a preferred example of the manufacturing method for the bolt according to this embodiment. Therefore, a bolt having the above configuration may be manufactured by a manufacturing method other than the above manufacturing method. In short, the manufacturing method is not particularly limited as long as it can manufacture the bolt according to this embodiment having the above configuration.

[0121] [Other processes] The bolt manufacturing process according to this embodiment may include other steps in addition to those described above. For example, a spheroidizing heat treatment step may be performed after the wire drawing step and before the bolt forming step. Furthermore, the thread forming step may be performed after the heat treatment step, rather than after the head forming step and before the heat treatment step. Furthermore, a compressive residual stress imparting step may be performed after the heat treatment step. [Example]

[0122] 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.

[0123] Steel materials (steel bars) having the chemical compositions shown in Tables 1A and 1B were prepared as bolt materials.

[0124] [Table 1A]

[0125] [Table 1B]

[0126] [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 heating temperature of the billets in the finish rolling process was 1200°C. The total area reduction rate R (%) when the billet temperature in the finish rolling process was 900°C or higher was as shown in Table 2.

[0127] [Table 2]

[0128] [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.

[0129] The strain ε imparted to the intermediate bolt during the head forming process was determined by the above-mentioned FEM analysis. The determined strain ε is shown in Table 2.

[0130] A heat treatment process was carried out on the intermediate bolts of each test number. First, a quenching process was carried out on the intermediate bolts under the same conditions for each test number. In the quenching process, the quenching temperature was 880 to 1050°C, and the holding time at the quenching temperature was 60 minutes (1 hour). After the holding time had elapsed, the intermediate bolts were water-cooled.

[0131] The intermediate bolts after the quenching process were subjected to a tempering process. The tempering temperature T (°C) and the holding time t (hours) at the tempering temperature T for each test number are shown in Table 2.

[0132] 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 1300 MPa or more in the tensile test described below.

[0133] [Evaluation test] The following evaluation tests were carried out on the bolts with each test number. (Test 1) Tensile test (Test 2) MC type carbide number density ND measurement test (Test 3) [Mo] SS and [Mo] MC Measurement test (Test 4) Hydrogen embrittlement resistance evaluation test (Test 5) Low temperature toughness evaluation test

[0134] [(Test 1) Tensile test] The tensile strength TS (MPa) of the bolts with each test number was determined based on the method described above in [Method for measuring tensile strength TS]. The obtained tensile strengths TS are shown in Table 2.

[0135] For test numbers for which the obtained tensile strength TS was less than 1300 MPa, the subsequent evaluation tests (Tests 2 to 5) were not carried out.

[0136] [(Test 2) MC type carbide number density ND measurement test] Based on the method described in the above [Method for measuring the number density ND of MC type carbides], the number density ND (number / m 3 The number density ND obtained is shown in Table 2.

[0137] [(Test 3) [Mo] SS and [Mo] MC Measurement test The above [[Mo] SS and [Mo] MC Based on the method described in [Measuring Method of Mo], the solid solution Mo concentration in atomic % in the parent phase of the bolt of each test number [Mo] SS , and Mo concentration in atomic % in MC-type carbides [Mo] MC The obtained [Mo] SS and [Mo] MC F1 was calculated based on the above. F1 was calculated by rounding the obtained value to two decimal places. The obtained F1 is shown in Table 2.

[0138] [(Test 4) Hydrogen embrittlement resistance evaluation test] The hydrogen embrittlement resistance of the bolts of each test number was measured by the following method. A round bar test specimen with a circular notch (hereinafter simply referred to as a round bar test specimen) was taken from the inside of the bolt, at least 1 mm deep from the surface. The size of the round bar test specimen was 7 mm in diameter at the parallel part and 70 mm in length. A circular notch extending circumferentially was formed at the center position in the longitudinal direction of the round bar test specimen. The R (radius of curvature) of the bottom of the notch was 0.175 mm.

[0139] 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 by a voltage was generated to add hydrogen to the round bar specimen.

[0140] After the cathodic hydrogen charging method, the round bar test specimens were left at room temperature for 72 hours. A zinc plating coating was then formed on the 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.

[0141] If the round bar test piece lasted for 100 hours or more without fracture, it was determined that excellent hydrogen embrittlement resistance was obtained (indicated as "E" in the "Hydrogen embrittlement resistance" column in Table 2). On the other hand, if the round bar test piece fractured in less than 100 hours, it was determined that excellent hydrogen embrittlement resistance was not obtained (indicated as "B" in the "Hydrogen embrittlement resistance" column in Table 2).

[0142] [(Test 5) Low-temperature toughness evaluation test] The low temperature toughness of the bolts of each test number was evaluated by the following method. First, a V-notch test specimen conforming to JIS Z 2242:2018 was taken from the center position of a cross section perpendicular to the longitudinal direction of each bolt for each test number. The size of the V-notch test specimen was 10 mm x 10 mm x 55 mm. The longitudinal direction of the V-notch test specimen was parallel to the central axis of the bolt. A V-notch was formed at the center of one of the surfaces of the V-notch test specimen parallel to the longitudinal direction, in a direction perpendicular to the longitudinal direction. The V-notch had a depth of 2 mm, a V-notch angle of 45°, and a V-notch tip radius of 0.25 mm. Two V-notch test specimens were prepared for each test number.

[0143] Using a V-notch test piece, a Charpy impact test was carried out in air at −25° C. in accordance with JIS Z 2242:2018 to determine the absorbed energy (J).

[0144] The arithmetic mean value of the absorbed energy per unit area of ​​the two obtained V-notch test specimens was defined as the low-temperature toughness value (J) of the bolt with that test number. If the low-temperature toughness value was 20J or more, it was judged to have excellent low-temperature toughness (indicated as "E" in the "Low-temperature toughness" column in Table 3). If the low-temperature toughness value was less than 20J, it was judged to have not obtained excellent low-temperature toughness (indicated as "B" in the "Low-temperature toughness" column in Table 3).

[0145] [Test Results] Referring to Tables 1A, 1B, and 2, the bolts of test numbers 1 to 17 satisfied features 1 to 4. Therefore, even when the tensile strength TS was 1300 MPa or more, excellent hydrogen embrittlement resistance and excellent low-temperature toughness were obtained.

[0146] On the other hand, in test numbers 18 and 19, FA was too low in the manufacturing process, and therefore F1 did not satisfy formula (1), and as a result, excellent low-temperature toughness was not obtained.

[0147] In test numbers 20 and 21, the tempering temperature T was too low, and therefore the number density ND of the MC type carbides was too low, resulting in failure to obtain excellent hydrogen embrittlement resistance.

[0148] In test numbers 22 and 23, the tempering temperature T was too high, resulting in a low tensile strength TS of less than 1300 MPa.

[0149] In test numbers 24 and 25, the holding time t at the tempering temperature T was too short, and therefore the number density ND of the MC carbides was too low, resulting in failure to obtain excellent hydrogen embrittlement resistance.

[0150] In test numbers 26 and 27, the holding time t at the tempering temperature T was too long. As a result, the tensile strength TS was low, less than 1300 MPa.

[0151] 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: 0.01-0.50%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%; The balance is Fe and impurities. The tensile strength TS is 1300 MPa or more, The number density ND of the MC type carbide is 2.0 × 10 ２２ pieces / m ３ That's all, The solid solution Mo concentration in atomic % in the parent phase of the bolt is [Mo] ＳＳ and the Mo concentration in atomic % in the MC type carbide is defined as [Mo] ＭＣ When defined as above, equation (1) is satisfied. bolt. [Mo] ＳＳ / [Mo] ＭＣ ≧0.50 (1)

2. A bolt, The chemical composition, in mass%, is C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01-0.80%, Mo: 0.70 to less than 1.50% V: 0.01-0.50%, 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. The tensile strength TS is 1300 MPa or more, The number density ND of the MC type carbide is 2.0 × 10 ２２ pieces / m ３ That's all, The solid solution Mo concentration in atomic % in the parent phase of the bolt is [Mo] ＳＳ and the Mo concentration in atomic % in the MC type carbide is defined as [Mo] ＭＣ When defined as above, equation (1) is satisfied. 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 [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] ＳＳ / [Mo] ＭＣ ≧0.50 (1)

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

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

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

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