bolt

Abstract

To provide a bolt having high strength and excellent low-temperature toughness. [Solution] A bolt according to the present disclosure comprises, 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: over 2.50 to 3.50%, 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, and has a tensile strength TS of 1600 MPa or more, and a P concentration in atomic % at prior austenite grain boundaries of [P] GB and the Mo concentration at the prior austenite grain boundary is [Mo] GB When this is the case, equation (1) is satisfied. [P] GB / [Mo] GB <2.0 (1)

Description

[Technical Field]

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

[0002] Bolts are used in industrial machinery, automobiles, and buildings such as bridges. In recent years, with the increasing performance of industrial machinery and automobiles and the increasing size of buildings, there has been a demand for higher strength bolts. Specifically, there is a demand for bolts with a tensile strength of 1600 MPa or more.

[0003] A high-strength bolt having a tensile strength of 1600 MPa or more is proposed, for example, in Japanese Patent Laid-Open Publication No. 2022-095157 (Patent Document 1).

[0004] The bolt disclosed in Patent Document 1 contains, by mass%, 0.35-0.50% C, 1.52-2.50% Si, 0.10-0.40% Mn, greater than 0-0.015% P, greater than 0-0.010% S, 0.11-0.50% Cu, 0.11-1.0% Ni, 0.1-2.4% Cr, 0.05-0.19% Ti, greater than 0-0.10% Al, 0.0003-0.01% B, and greater than 0-0.015% N, with the balance being Fe and unavoidable impurities. This bolt contains a low Mn content and also contains Cu, Ni, Ti, and B as essential components, with their contents appropriately adjusted. Patent Document 1 states that this allows the bolt to achieve a tensile strength of 1600 MPa or more after tempering and exhibit sufficient delayed fracture resistance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-095157 Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, bolts may be 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 with high strength as described above have low low-temperature toughness. Patent Document 1 considers increasing the strength of bolts, but does not consider low-temperature toughness.

[0007] An object of the present disclosure is to provide a bolt that has high strength and excellent low-temperature toughness. [Means for solving the problem]

[0008] A bolt according to the present disclosure comprises: 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: over 2.50 to 3.50% 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 1600 MPa or more, The P concentration in atomic percent at the prior austenite grain boundary is [P] GB The Mo concentration at the prior austenite grain boundary is [Mo] GB When this is the case, equation (1) is satisfied. [P] GB / [Mo] GB <2.0 (1)

[0009] A bolt according to the present disclosure comprises: 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: over 2.50 to 3.50% 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 elements selected from the group consisting of Groups 1 to 3, with the remainder being Fe and impurities, The tensile strength TS is 1600 MPa or more, The P concentration in atomic percent at the prior austenite grain boundary is [P] GB The Mo concentration at the prior austenite grain boundary is [Mo] GB When this is the case, 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 [P] GB / [Mo] GB <2.0 (1) [Effects of the Invention]

[0010] The bolts according to the present disclosure have high strength and excellent low temperature toughness. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a side view showing an example of a bolt according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors have conducted various studies on bolts that have a tensile strength TS of 1600 MPa or more and excellent low-temperature toughness, and as a result have obtained the following findings.

[0013] As mentioned above, at low temperatures, the toughness of a bolt generally decreases, making it more susceptible to brittle fracture. The fracture surfaces of embrittlement fractures that occur in bolts with reduced toughness contain more intergranular fractures than fracture surfaces with high toughness. In other words, by increasing the strength of the prior austenite grain boundaries in the bolt, it is possible to suppress the initiation and propagation of cracks along the prior austenite grain boundaries, thereby suppressing brittle fracture even at low temperatures. As a result, low-temperature toughness can be improved.

[0014] Here, Mo segregates at prior austenite grain boundaries and can increase the strength of the prior austenite grain boundaries. Therefore, the inventors believed that by increasing the Mo content in the chemical composition of the bolt to more than 2.50%, the strength of the prior austenite grain boundaries would be sufficiently increased and excellent low-temperature toughness would be obtained. Based on the above findings, the inventors investigated the chemical composition of the bolt. The results were as follows: C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10-1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01-0.80%, Mo: over 2.50-3.50%, V: 0.01-0.50%, Al: 0.005-0.100%, N: 0.0010-0.0200%, Cu: 0-0.40%, Ni: 0-0.40%, B: 0-0.0050%, Zr: 0-0.1 It was thought that a bolt having a chemical composition containing the following alloys, with the balance consisting of Fe and impurities, could potentially achieve a tensile strength TS of 1600 MPa or more and excellent low-temperature toughness.

[0015] However, even with bolts having the above-mentioned chemical composition, there were cases in which excellent low-temperature toughness could not be obtained. Therefore, the inventors conducted further investigation, focusing on the concentration of each element at the prior austenite grain boundary.

[0016] As a result, it was found that there is a large variation in the Mo concentration at the prior austenite grain boundaries even among bolts with the same chemical composition. Furthermore, it was found that bolts with low low-temperature toughness had low Mo concentrations at the prior austenite grain boundaries but high P concentrations. P segregates at the prior austenite grain boundaries and embrittles the grain boundaries. This is thought to be the reason for the reduced low-temperature toughness. Based on these results, the inventors conducted further studies focusing on the Mo and P concentrations at the prior austenite grain boundaries. As a result, it was found that the sum of the Mo and P concentrations in atomic % at the prior austenite grain boundaries is almost constant for bolts with the above-mentioned chemical composition. In other words, it is presumed that Mo and P compete for a certain number of segregation sites at the prior austenite grain boundaries (site competition).

[0017] Based on the above findings, in order to improve the low temperature toughness of the bolt, it is preferable to lower the P concentration by increasing the Mo concentration in atomic % at the prior austenite grain boundary as much as possible. Therefore, the inventors have determined that the P concentration in atomic % at the prior austenite grain boundary [P] GB and Mo concentration [Mo] GB Further research was conducted into the relationship between the low temperature toughness of the bolt and the above chemical composition. As a result, it was found that if the following formula (1) is satisfied, a bolt with the above chemical composition can have a tensile strength TS of 1600 MPa or more while also achieving excellent low temperature toughness. [P] GB / [Mo] GB <2.0 (1)

[0018] The bolt of this embodiment has been completed based on the above technical concept and has the following configuration.

[0019] The bolts in the first configuration are: 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: over 2.50 to 3.50% 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 1600 MPa or more, The P concentration in atomic percent at the prior austenite grain boundary is [P] GB The Mo concentration at the prior austenite grain boundary is [Mo] GB When this is the case, equation (1) is satisfied. [P] GB / [Mo] GB <2.0 (1)

[0020] The second configuration of bolts 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: over 2.50 to 3.50% 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 elements selected from the group consisting of Groups 1 to 3, with the remainder being Fe and impurities, The tensile strength TS is 1600 MPa or more, The P concentration in atomic percent at the prior austenite grain boundary is [P] GB The Mo concentration at the prior austenite grain boundary is [Mo] GB When this is the case, 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 [P] GB / [Mo] GB <2.0 (1)

[0021] The third configuration of bolts is A bolt of a second configuration, Contains the first group.

[0022] The fourth configuration bolt is A bolt of the second or third configuration, Contains the second group.

[0023] The fifth configuration bolt is A bolt having any one of the second to fourth configurations, Contains the third group.

[0024] The bolt according to this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0025] 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.

[0026] [Features of the bolt of this embodiment] The bolt of this embodiment satisfies the following features 1 to 3. (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: over 2.50 to 3.50%, 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 balance being Fe and impurities. (Feature 2) The tensile strength TS is 1600 MPa or more. (Feature 3) The P concentration in atomic percent at the prior austenite grain boundary is [P] GB and the Mo concentration at the prior austenite grain boundary is [Mo] GB When this is the case, equation (1) is satisfied. [P] GB / [Mo] GB <2.0 (1) Features 1 to 3 will be explained below.

[0027] [(Feature 1) Chemical composition] The chemical composition of the bolt according to this embodiment contains the following elements:

[0028] C: 0.30 to 0.50% Carbon (C) improves the hardenability of steel and increases the strength of the bolt. If the C content is less than 0.30%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.50%, the 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%.

[0029] Si: 0.01 to 0.30% Silicon (Si) improves the hardenability of steel and increases the strength of the bolt. If the Si content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.30%, the 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%.

[0030] Mn: 0.10 to 1.50% Manganese (Mn) improves the hardenability of steel and increases the strength of the bolt. If the Mn content is less than 0.10%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.50%, the 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%.

[0031] P:0.030% or less Phosphorus (P) is an impurity. If the P content exceeds 0.030%, P segregates excessively at grain boundaries. As a result, even if the contents of other elements are within the ranges of this embodiment, the above-mentioned feature 3 cannot be satisfied, and the low-temperature toughness of the bolt decreases. 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%.

[0032] 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 low-temperature toughness 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 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%.

[0033] 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 forms coarse Cr-based carbides at prior austenite grain boundaries. If the Cr content exceeds 0.80%, the low-temperature toughness of the bolt decreases 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%.

[0034] Mo: Over 2.50 to 3.50% Molybdenum (Mo) improves the hardenability of steel 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 bolts is improved. If the Mo content exceeds 2.50%, the above effects can be sufficiently obtained. On the other hand, if the Mo content is 3.50% or less, the hardness of the steel material can be prevented from becoming excessively high, and as a result, the low-temperature toughness of the bolt is improved. Therefore, the Mo content is more than 2.50% to 3.50%. The lower limit of the Mo content is preferably 2.51%, more preferably 2.60%, and even more preferably 2.70%. The upper limit of the Mo content is preferably 3.40%, and more preferably 3.30%.

[0035] V: 0.01 to 0.50% Vanadium (V) forms MC-type carbides to increase the strength 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%.

[0036] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. Furthermore, Al combines with N to form Al nitrides. Al nitrides suppress grain coarsening through a pinning effect. As a result, the 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.

[0037] 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%.

[0038] 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.

[0039] [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.

[0040] [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.

[0041] Cu: 0.40% or less Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When Cu is contained, that is, when the Cu content is more than 0%, Cu improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. 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%.

[0042] Ni: 0.40% or less Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content exceeds 0%, Ni 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%.

[0043] 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. 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%.

[0044] 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%.

[0045] 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%.

[0046] 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%.

[0047] W: 0.20% or less Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W 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%.

[0048] [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.

[0049] Ti:0.100% or less Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content 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%.

[0050] Nb: 0.100% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content 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%.

[0051] [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.

[0052] Ca:0.0050% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%. When Ca is contained, that is, when 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%.

[0053] Bi:0.020% or less Bismuth (Bi) is an optional element and may not be contained, that is, the Bi content may be 0%. When Bi is contained, that is, when Bi is more than 0%, Bi improves the machinability of the steel material. Even if 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%.

[0054] Te: 0.010% or less Tellurium (Te) is an optional element and may not be contained, that is, the Te content may be 0%. When Te is contained, that is, when 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%.

[0055] 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%.

[0056] [(Feature 2) Tensile strength TS] The bolt according to this embodiment has a tensile strength TS of 1600 MPa or more. The chemical composition of the bolt according to this embodiment satisfies Feature 1 and also Feature 3. As a result, the bolt according to this embodiment has excellent low-temperature toughness even when the tensile strength TS is 1600 MPa or more.

[0057] The lower limit of the tensile strength TS is preferably 1650 MPa, and more preferably 1700 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, 2000 MPa, for example, 1900 MPa.

[0058] [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.

[0059] [(Feature 3) Equation (1)] In the bolt of this embodiment, the P concentration in atomic % at the prior austenite grain boundary is further defined as [P] GB and the Mo concentration at the prior austenite grain boundary is [Mo] GB When this is the case, equation (1) is satisfied. [P] GB / [Mo] GB <2.0 (1)

[0060] As mentioned above, both Mo and P tend to segregate at prior austenite grain boundaries and are in a site competition relationship. Mo segregates at prior austenite grain boundaries to increase grain boundary strength, while P segregates at prior austenite grain boundaries to decrease grain boundary strength and cause embrittlement. Therefore, in order to improve the low-temperature toughness of bolts, it is effective to promote the segregation of Mo at prior austenite grain boundaries and suppress the segregation of P.

[0061] Here, F1 is defined as follows: F1=[P] GB / [Mo] GB If F1 is less than 2.0, the P concentration relative to the Mo concentration at the prior austenite grain boundaries is sufficiently low. In this case, the proportion of Mo occupies a given number of segregation sites is sufficiently high, and the proportion of P is sufficiently low. In other words, the segregation of Mo to the prior austenite grain boundaries sufficiently strengthens the grain boundaries, and grain boundary embrittlement due to the segregation of P is sufficiently suppressed. As a result, a bolt that satisfies Features 1 and 2 can achieve excellent low-temperature toughness. Therefore, F1 should be less than 2.0.

[0062] The lower limit of F1 is not particularly limited. When the bolt satisfies characteristics 1 and 2, the lower limit of F1 is, for example, 0.1. The upper limit of F1 is preferably 1.8, and more preferably 1.6. F1 is the value obtained by rounding off the first decimal place to the nearest tenth.

[0063] [[P] GB and [Mo] GB Measurement method] P concentration in atomic % at prior austenite grain boundaries [P] GB and Mo concentration [Mo] GB can be measured by Auger Electron Spectroscopy (AES) in the following manner.

[0064] First, a round bar test specimen with an annular notch (hereinafter simply referred to as a round bar test specimen) is prepared from the inside of the bolt, at least 1 mm deep from the surface. The size of the round bar test specimen is not particularly limited, but for example, it may be 3 mm in diameter and 17 mm in axial length. Furthermore, the notch is formed in the center of the axial length of the round bar test specimen. The notch is, for example, a V-notch with a depth of 0.7 mm, and is formed around the entire circumference of the round bar test specimen.

[0065] The prepared round bar test piece was placed in a vacuum chamber attached to the AES device, and the degree of vacuum in the chamber was set to 10 -7 The pressure in the chamber is set to 10 Pa or less. The AES device used is, for example, PHI680 manufactured by ULVAC-PHI, Inc. A round bar test piece cooled to -120°C or less in the chamber is subjected to impact fracture. The vacuum in the chamber is set to 10 -7While maintaining the pressure below 10 Pa, the fracture surface of the round bar test piece is observed with a scanning electron microscope (SEM) of the AES device to identify the grain boundary fracture surface. Note that a person skilled in the art can easily distinguish between a smooth grain boundary fracture surface and a rough intragranular fracture surface by observing the fracture surface of the round bar test piece with an SEM. Furthermore, a total of 10 measurement points are identified from three or more grain boundary fracture surfaces. Note that the measurement points identify areas of the grain boundary fracture surface other than precipitates. A person skilled in the art can easily determine whether or not there are precipitates from the contrast.

[0066] The identified measurement points are analyzed for elemental concentration using an AES instrument. In the AES analysis, the acceleration voltage is 10 kV, the sample current is 10 nA, and the target elements are quantified as P, C, Fe, and Mo. In the differential Auger spectrum, the Auger peaks of each element used for quantitative calculation are P: 102-130 eV, C: 234-292 eV, Fe: 685-715 eV, and Mo: 169-199 eV. The peak intensity and relative sensitivity coefficient of the resulting Auger peak of each element are used to obtain a quantitative value. The peak intensity is calculated as the difference between the maximum and minimum values ​​of the peak (the so-called peak-to-peak intensity). The relative sensitivity coefficient can be the value provided by the instrument manufacturer.

[0067] For each measurement point, the ratio of the P content in atomic % and the Mo content to the total content of P, C, Fe, and Mo are calculated. The average value of the P content ratios at a total of 10 measurement points is taken as the P concentration at the prior austenite grain boundary [P]. GB Similarly, the average value of the Mo content ratio at a total of 10 measurement points is defined as the Mo concentration at the prior austenite grain boundary [Mo]. GB (atomic %).

[0068] [Effects of the bolt of this embodiment] The bolt of this embodiment satisfies Features 1 to 3. Therefore, the bolt of this embodiment can obtain excellent low-temperature toughness even though it has a high tensile strength TS of 1600 MPa or more.

[0069] [Microstructure of the bolt of this embodiment] The microstructure of the bolt of this embodiment includes a hard structure with an area ratio of 90% or more. The bolt microstructure here refers to the microstructure of a bolt manufactured by quenching and tempering in the manufacturing process described below. The hard structure is composed of martensite and / or bainite. When the bolt microstructure includes phases other than the hard structure, the remainder of the bolt microstructure is composed of one or more phases selected from the group consisting of pro-eutectoid ferrite and pearlite. The bolt microstructure correlates with the tensile strength TS. Specifically, if the tensile strength TS of the bolt is 1600 MPa or more, the area ratio of the hard structure in the bolt microstructure is 90% or more.

[0070] [Method for measuring the microstructure of bolts] The microstructure of the bolt of this embodiment can be measured by the following method.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] [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.

[0075] [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.

[0076] 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.

[0077] [(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.

[0078] When manufacturing steel for bolts, the steel material preparation process includes the following steps. (Process 11) Casting process (Process 12) Direct hot processing process (Process 13) Finishing rolling process

[0079] [(Process 11) Casting Process] In the casting process, a cast material having a chemical composition that satisfies Feature 1 is produced. Specifically, molten steel is prepared, the content of each element in the chemical composition of which satisfies Feature 1. The prepared molten steel is used to produce a material by a well-known casting method. For example, an ingot is produced by an ingot casting method. Alternatively, a bloom is produced by a continuous casting method. Through the above steps, a cast material (ingot or bloom) is produced.

[0080] [(Process 12) Direct hot processing process] In the direct hot working process, billets are produced by hot working the cast material during cooling (solidifying) after the casting process. Specifically, when the surface temperature of the cast material during cooling (solidifying) falls within the range of 1100 to 900°C, hot working is started to produce billets.

[0081] In this specification, the hot working of a produced cast material when the surface temperature of the cast material during cooling reaches 1100 to 900°C without cooling the cast material to room temperature is referred to as "direct hot working." Specifically, the hot working method is hot rolling (rough rolling).

[0082] As mentioned above, Mo and P compete for segregation sites. In reality, it is assumed that P segregates in the remaining segregation sites occupied by Mo. In other words, the P concentration [P] at the prior austenite grain boundaries in the manufactured bolt is GB Lower the Mo concentration [Mo] GB To improve this property, it is effective to maintain the segregated state of Mo at the grain boundaries during casting, preventing it from diffusing into the grains as much as possible. Mo diffusion is significantly promoted at temperatures above 900°C. Therefore, in processes after the casting process where the steel may be heated to above 900°C, it is necessary to adjust the manufacturing conditions so that the uniform diffusion of segregated Mo can be suppressed.

[0083] Generally, cast materials produced by a casting method are first cooled to room temperature. The cooled cast material is then reheated to a high temperature range of 1100°C or higher in a heating furnace, and then subjected to blooming (rough rolling). This conventional manufacturing process is referred to herein as "hot processing after reheating." In the case of hot processing after reheating, heating is performed from room temperature before blooming, so a large amount of heat is applied to the cast material in the temperature range of 900°C or higher. This promotes the diffusion of Mo segregated at the grain boundaries of the cast material. In this case, the bolt produced will not satisfy Feature 3.

[0084] On the other hand, direct hot working omits the reheating process of the cast material before hot working, which is required in conventional hot working after reheating. This suppresses the diffusion of Mo segregated at the grain boundaries of the cast material. As a result, the bolts manufactured can satisfy Feature 3, provided that other manufacturing conditions are met.

[0085] [(Process 13) Finishing rolling process] In the finish rolling process, the billet is heated in a heating furnace. The heating temperature is, for example, 1000 to 1200°C. The heated billet is finish rolled. The finish rolled billet is cooled to produce a steel material (steel material for bolts) that will be used as a bolt material. The steel material is, for example, a steel bar or a wire rod.

[0086] [(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

[0087] [(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.

[0088] [(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.

[0089] The bolt forming process includes the following steps: (Step 221) Head forming process (Process 222) Thread forming process Each step will be described below.

[0090] [(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.

[0091] [(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.

[0092] [(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

[0093] [(Step 231) Quenching process] In the quenching process, the intermediate bolts after the bolt forming process are quenched. c3 The bolt is heated to above its transformation point and held for a predetermined time. After the holding time has elapsed, the intermediate bolt is rapidly cooled. Specifically, the intermediate bolt is cooled by water or oil.

[0094] In the quenching process, when the quenching temperature is T (°C) and the holding time at the quenching temperature T (°C) is t (minutes), the following formula (A) is satisfied. (T-865)×log(t)≧30 (A) In addition, log in the formula (A) means common logarithm.

[0095] FA is defined as follows: FA = (T-865) × log(t) FA is an index of the amount of heat given to steel during heating in the hardening process. If FA is less than 30, the austenite grains before quenching will be fine. This results in insufficient hardenability of the intermediate bolt, and the core of the intermediate bolt may not be sufficiently hardened. As a result, the manufactured bolt will not meet Feature 2.

[0096] On the other hand, if the FA is 30 or higher, the austenite grains before quenching are of an appropriate size. This ensures sufficient hardenability all the way to the center of the bolt. As a result, the bolts manufactured will meet Feature 2, provided that other manufacturing conditions are met.

[0097] The upper limit of FA is not particularly limited, but is set to 300, for example, in consideration of normal industrial production.

[0098] [(Step 232) Tempering process] In the tempering process, the intermediate bolt product after the quenching process is tempered. In the tempering process, the intermediate bolt product after the quenching process is heated and held for a predetermined time. The tempering temperature is, for example, 570 to 660°C. The holding time at the tempering temperature is, for example, 0.5 to 6.0 hours.

[0099] [Other manufacturing conditions] Furthermore, in the manufacturing process for the bolt of this embodiment, when the maximum rolling reduction in one pass in the rough rolling of the direct hot working process is R1 (%) and the maximum rolling reduction in one pass in the finish rolling process is R2 (%), the following formula (B) is satisfied. R1×R2×T×log(t)≦1900000 (B) In the formula (B), T is the quenching temperature (° C.) in the quenching step, and t is the holding time (minutes) at the quenching temperature T. Furthermore, log means common logarithm.

[0100] FB is defined as follows: FB = R1 × R2 × T × log(t) FB is an index that indicates the ease of diffusion of Mo segregated at grain boundaries. Here, "pass" means the operation of the rolled material passing through one rolling stand. In rough rolling and finish rolling, hot rolling consisting of multiple passes is carried out. Also, "maximum reduction in one pass" means the reduction in the pass with the largest reduction.

[0101] In rough rolling and finish rolling, rolling is performed in a temperature range of 900°C or higher, where Mo is likely to diffuse. If a large reduction rate per pass is applied in such a temperature range, the diffusion of segregated Mo is significantly promoted. Therefore, in order to maintain the segregated state of Mo, it is preferable that the maximum reduction rate R1 (%) per pass in rough rolling and the maximum reduction rate R2 (%) per pass in finish rolling are both small.

[0102] Furthermore, in the quenching process, the intermediate bolt is heated to approximately 900°C. If the quenching temperature T is too high or the holding time t at the quenching temperature T is too long, the amount of heat applied to the intermediate bolt increases. As a result, the diffusion of Mo segregated at the prior austenite grain boundaries is promoted. Therefore, in order to maintain the segregated state of Mo, it is preferable that both the quenching temperature T and the holding time t at the quenching temperature T be short.

[0103] In other words, the smaller the F, the more the uniform diffusion of Mo is suppressed during the bolt manufacturing process, and the more the segregated state can be maintained. If F is 1,900,000 or less, the uniform diffusion of Mo segregated at the grain boundaries is sufficiently suppressed during the bolt manufacturing process. As a result, the manufactured bolt can satisfy Feature 3, provided that other manufacturing conditions are met.

[0104] The lower limit of FB is not particularly limited, but is, for example, 100,000 in consideration of normal industrial production.

[0105] [Other manufacturing 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]

[0106] 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.

[0107] Steel materials (steel bars) having the chemical compositions shown in Tables 1A and 1B were prepared as bolt materials.

[0108] [Table 1A]

[0109] [Table 1B]

[0110] [Steel preparation process] Specifically, the steel materials of each test number were produced by the following method: Ingots having the chemical compositions shown in Tables 1A and 1B were produced by a casting method (casting process).

[0111] In Test Nos. 1 to 17 and 20 to 23, when the surface temperature of the ingot during cooling reached 1100 to 900°C, hot rolling (rough rolling) was performed on the ingot to produce a billet. That is, in these Test Nos., direct hot working was performed (indicated as "direct" in the "hot working" column in Table 2). On the other hand, in Test Nos. 18 and 19, the ingot was cooled to room temperature, reheated to 1200°C in a heating furnace, and then bloomed (rough rolling) to produce a billet (indicated as "reheating" in the "hot working" column in Table 2). The maximum rolling reduction R1 in one pass of rough rolling for each Test No. was as shown in Table 2. In all hot working methods, the cumulative rolling reduction in the direct hot working process was within the range of 35 to 60%.

[0112] [Table 2]

[0113] The produced billets were subjected to a finish rolling process to produce steel materials (round bars) with a diameter of 20 mm. The heating temperature of the billets in the finish rolling process was 1200°C. The maximum reduction ratio R2 in one pass of finish rolling for each test number was as shown in Table 2.

[0114] [Bolt manufacturing process] Bolts were manufactured using the produced steel material. 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 with a diameter of 16 mm. 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.

[0115] 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 quenching temperature T (°C) and the holding time t (minutes) at the quenching temperature T for each test number were as shown in Table 2. After the holding time had elapsed, the intermediate bolts were water-cooled.

[0116] The intermediate bolts after the quenching process were subjected to a tempering process. The tempering temperature was 620°C, and the holding time at the tempering temperature was 2.0 hours.

[0117] 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 1600 MPa or more in the tensile test described below.

[0118] [Evaluation test] The following evaluation tests were carried out on the bolts with each test number. (Test 1) Tensile test (Test 2) [P] GB and [Mo]GB Measurement test (Test 3) Low temperature toughness evaluation test

[0119] [(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 3.

[0120] [Table 3]

[0121] For test numbers for which the obtained tensile strength TS was less than 1600 MPa, the subsequent evaluation tests (Tests 2 and 3) were not carried out.

[0122] [(Test 2) [P] GB and [Mo] GB Measurement test The above [[P] GB and [Mo] GB Based on the method described in [Measurement method for the P concentration in atomic % at the prior austenite grain boundary of the bolts of each test number], GB and Mo concentration [Mo] GB Furthermore, the obtained [P] GB and [Mo] GB Based on this, F1 was calculated. GB , [Mo] GB and F1 are shown in Table 3, respectively.

[0123] [(Test 3) 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.

[0124] 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).

[0125] The arithmetic mean value of the absorbed energy per unit area of ​​the two V-notch test pieces obtained 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 by "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 by "B" in the "Low-temperature toughness" column in Table 3).

[0126] [Evaluation test results] Referring to Tables 1A, 1B, 2 and 3, the bolts of test numbers 1 to 17 satisfied features 1 to 3. Therefore, even when the tensile strength TS was 1600 MPa or more, excellent low-temperature toughness was obtained.

[0127] On the other hand, in test numbers 18 and 19, hot working was performed after reheating instead of direct hot working, so F1 was too high, resulting in low low temperature toughness.

[0128] In test numbers 20 and 21, the FA in the finish rolling process was too low, resulting in a low tensile strength TS of less than 1600 MPa.

[0129] On the other hand, in test numbers 22 and 23, FB was too high in the manufacturing process, and therefore F1 was too high, resulting in low low temperature toughness.

[0130] 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. In mass%, C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01-0.80%, Mo: more than 2.50 to 3.50%, 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 1600 MPa or more, The P concentration in atomic % at the prior austenite grain boundary is [P] ＧＢ The Mo concentration at the prior austenite grain boundary is [Mo] ＧＢ When this is done, equation (1) is satisfied. bolt. [[]] ＧＢ / [Mo] ＧＢ <200 (1)

2. In mass%, C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01-0.80%, Mo: more than 2.50 to 3.50%, V: 0.01-0.50%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0200%; Further, it contains one or more elements selected from the group consisting of Groups 1 to 3, with the remainder consisting of Fe and impurities, The tensile strength TS is 1600 MPa or more, The P concentration in atomic % at the prior austenite grain boundary is [P] ＧＢ The Mo concentration at the prior austenite grain boundary is [Mo] ＧＢ When this is done, 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 [[]] ＧＢ / [Mo] ＧＢ <200 (1)

3. 3. The bolt according to claim 2, containing the first group, bolt.

4. 3. The bolt according to claim 2, containing the second group, bolt.

5. 3. The bolt according to claim 2, containing the third group, bolt.

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