steel

Abstract

To provide a steel that can reduce the sensitivity to hydrogen embrittlement in bolts.SOLUTION: A steel disclosed herein includes, in mass%, C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10-1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01-0.80%, Mo: 1.50-2.50%, V: 0.01-0.30%, Al: 0.005-0.100%, and N: 0.0010-0.0300%, with the balance being Fe and impurities. The area ratio of the hard phase is 90% or more. The Vickers hardness is 220-400 HV. The number density of cementite particles with an area of 0.0005 μm2 or more in the hard phase is 4.0 / μm2 or more. Among the cementite particles in the hard phase, the proportion of cementite particles with an area of 0.0005-0.0100 μm2 is 50.0% or more. The sample standard deviation of the cementite area is 0.070 μm2 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to steel materials, and more particularly to steel materials that can be used as bolt materials. [Background technology]

[0002] Bolts are used in industrial machinery, automobiles, buildings such as bridges, etc. 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.

[0003] High strength bolts may be susceptible to hydrogen embrittlement. Hydrogen embrittlement can cause delayed fracture of bolts. Therefore, high strength bolts are required to have low susceptibility to hydrogen embrittlement.

[0004] International Publication No. 2020 / 162616 (Patent Document 1) proposes a steel material that can be used to make bolts, which has high strength and low susceptibility to hydrogen embrittlement.

[0005] The steel material disclosed in Patent Document 1 contains, in mass%, C: 0.35 to 0.45%, Si: 0.02 to 0.10%, Mn: 0.20 to 0.84%, Cr: 0.60 to 1.15%, V: 0.30 to 0.50%, Mo: 0.25 to 0.99%, Al: 0.010 to 0.100%, N: 0.0010 to 0.0150%, P: 0.015% or less, S: 0.015% or less, the balance: Fe and impurities, and satisfies the following formulas (1) and (2): 0.48≦Mo / 1.4+V<1.10 (1) 0.80 <Mo / V<3.00 (2) In this steel, the Mo content and V content in the chemical composition are adjusted to satisfy formulas (1) and (2).This makes it easier for MC carbides to disperse in bolts manufactured using this steel, and as a result, Patent Document 1 describes that the hydrogen embrittlement susceptibility of the bolts is reduced. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2020 / 162616 Summary of the Invention [Problem to be solved by the invention]

[0007] Bolts manufactured using the steel material disclosed in Patent Document 1 have low susceptibility to hydrogen embrittlement. However, the hydrogen embrittlement susceptibility of bolts manufactured using the steel material may be reduced by a means different from that disclosed in Patent Document 1.

[0008] An object of the present disclosure is to provide a steel material that, when used as a material for a bolt, can reduce the hydrogen embrittlement susceptibility of the bolt. [Means for solving the problem]

[0009] The steel material according to the present disclosure has the following configuration.

[0010] A steel material, 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: 1.50-2.50%, V: 0.01 to 0.30%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0300%, The balance is Fe and impurities. The area ratio of the hard phase consisting of bainite and / or martensite is 90% or more, and the Vickers hardness is 220 to 400 HV, 0.0005 μm in the hard phase 2The number density of cementite having an area of 4.0 particles / μm or more 2 or more, and among the plurality of cementite particles in the hard phase, 0.0005 to 0.0100 μm 2 The number ratio of the cementite having an area of 50.0% or more, and the sample standard deviation of the area of the plurality of cementite in the hard phase is 0.070 μm 2 Below is the Steel material. [Effects of the Invention]

[0011] When the steel material according to the present disclosure is used as a material for a bolt, it can reduce the hydrogen embrittlement susceptibility of the bolt. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors have investigated steel materials that, when used as bolt materials, can reduce the hydrogen embrittlement susceptibility of bolts, and as a result, have obtained the following findings.

[0013] First, the inventors investigated, from the viewpoint of chemical composition, steel materials that can be used as bolt materials and that can reduce the hydrogen embrittlement susceptibility of bolts, and as a result, the inventors found that the steel materials contained, 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%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0300%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0100% It was thought that if a steel material had a chemical composition containing Zr: 0-0.300%, Ti: 0-0.100%, Nb: 0-0.100%, Ca: 0-0.0050%, Mg: 0-0.0050%, Bi: 0-0.100%, and Te: 0-0.100%, it might be possible to reduce the hydrogen embrittlement susceptibility of the bolt when used as the material for the bolt.

[0014] The present inventors further investigated means for reducing the hydrogen embrittlement susceptibility of bolts having the above-mentioned chemical composition from the viewpoint of their microstructure, and as a result, they discovered that the hydrogen embrittlement susceptibility of bolts can be reduced by dispersing a large number of fine MC type carbides in the bolts.

[0015] Therefore, the inventors thought that if Mo and V were further added to the above-mentioned chemical composition, fine MC carbides would be formed in the bolt, reducing the hydrogen embrittlement susceptibility of the bolt. Therefore, the inventors further investigated the chemical composition of the steel, taking into account the formation of MC carbides during the bolt manufacturing process. As a result, they found that if the steel of this embodiment satisfies the chemical composition of Feature 1 below, when a bolt is manufactured using this steel, the hydrogen embrittlement susceptibility of the bolt will be reduced to a certain extent.

[0016] (Feature 1) The chemical composition, in mass%, is 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: 1.50-2.50%, V: 0.01-0.30%, Al: 0.005-0.100%, N: 0.0010-0.0 300%, Cu: 0-0.40%, Ni: 0-0.40%, B: 0-0.0100%, Zr: 0-0.300%, Ti: 0-0.100%, Nb: 0-0.100%, Ca: 0-0.0050%, Mg: 0-0.0050%, Bi: 0-0.100%, Te: 0-0.100%, and the balance being Fe and impurities.

[0017] However, even with steel materials having the chemical composition of Feature 1, there have been cases where the hydrogen embrittlement susceptibility of bolts cannot be sufficiently reduced. Therefore, the present inventors have investigated means for further reducing the hydrogen embrittlement susceptibility of bolts. As a result, they have obtained the following findings.

[0018] In the case of a steel material having a chemical composition that satisfies Feature 1, cementite is generated in the steel material. Furthermore, in the manufacturing process of a bolt using a steel material having the above-mentioned chemical composition as a raw material, the steel material is subjected to spheroidizing annealing, quenching, and tempering, thereby generating the above-mentioned fine MC carbides containing Mo and / or V in the bolt.

[0019] To generate fine MC carbides in the bolt, it is preferable that the cementite in the steel be sufficiently dissolved during quenching. The cementite in the steel before quenching contains Mo and V. If the cementite in the steel is layered (i.e., if the cementite is part of pearlite), the cementite will not be sufficiently dissolved during quenching in the bolt manufacturing process. In this case, even if tempering is performed, a sufficient amount of fine MC carbides will not be generated.

[0020] Therefore, in the steel material of this embodiment, the microstructure is made to satisfy the following characteristic 2. (Feature 2) The area ratio of the hard phase consisting of bainite and / or martensite in the microstructure is 90% or more, and the Vickers hardness is 220 to 400 HV.

[0021] In the microstructure of a steel material having a chemical composition that satisfies Feature 1, if the area ratio of the hard phase is 90% or more and the Vickers hardness is 220 to 400 HV, the microstructure of the steel material is substantially bainite. The cementite in the bainite is not layered but granular. Therefore, compared with layered cementite in pearlite, cementite is more likely to be fully dissolved during quenching. Therefore, the microstructure of the steel material is substantially bainite.

[0022] In the microstructure observation method described below, it is easy to distinguish bainite and martensite from other phases, but it is extremely difficult to distinguish bainite from martensite. On the other hand, when a phase consisting of bainite and / or martensite is defined as a "hard phase," if the area ratio of the hard phase in the microstructure is 90% or more and the Vickers hardness is 220 to 400 HV, the microstructure of the steel material is substantially bainite. Therefore, the steel material of this embodiment is specified as Feature 2.

[0023] As described above, the present inventors considered that if a steel material satisfies Features 1 and 2, when a bolt is manufactured using that steel material as a raw material, the hydrogen embrittlement susceptibility of the bolt can be sufficiently reduced. However, even if the steel material satisfies Features 1 and 2, there are still cases where the hydrogen embrittlement susceptibility of the bolt manufactured using the steel material cannot be sufficiently reduced. Therefore, as a result of further investigation, the present inventors have obtained the following findings.

[0024] Even if the microstructure of a steel material is substantially bainite and the cementite is granular, the amount of fine MC carbides formed after tempering is affected by the number density and particle size distribution of cementite. If the number density of cementite is low, sufficient formation sites for MC carbides are not obtained. Furthermore, even if the average particle size of multiple cementite particles in a steel material is small, if the particle size distribution of cementite is broad, the size of the cementite in the steel material varies. In this case, large-sized cementite has higher V and Mo contents than small-sized cementite. Therefore, even if the cementite in a steel material is sufficiently dissolved by quenching, V and Mo are concentrated in the microregions where large-sized cementite is dissolved compared to other microregions (regions where small-sized cementite is dissolved). Therefore, even if the average particle size of cementite is small, if the particle size distribution of cementite is broad, V- and Mo-enriched regions will occur due to the large-sized cementite during quenching. In this case, it is difficult to generate a sufficient amount of fine MC carbides during tempering, which means that hydrogen trapping sites cannot be secured sufficiently, and as a result, the bolt's susceptibility to hydrogen embrittlement is not sufficiently reduced.

[0025] Therefore, in the steel material of this embodiment, in order to suppress the generation of V and Mo segregation regions during quenching, the number density of cementite in the steel material is increased as much as possible, the proportion of fine cementite among multiple cementite particles is increased as much as possible, and the grain size distribution of cementite is made sharp in order to suppress variation in cementite size. Specifically, the steel material satisfying Features 1 and 2 is made to also satisfy the following Feature 3.

[0026] (Feature 3) 0.0005 μm in the hard phase 2 The number density ND of cementite having an area of 4.0 pieces / μm 2 or more, and among the multiple cementites in the hard phase, 0.0005 to 0.0100 μm 2 The number ratio NR of cementite having an area of 0.0005 μm is 50.0% or more, and 2The sample standard deviation σ of the area of multiple cementites in the hard phase having an area of 0.070 μm or more 2 The following is the result.

[0027] In this case, there is a sufficient amount of cementite in the steel material. Furthermore, the cementite is sufficiently fine, and the cementite particle size distribution is sufficiently sharp. In other words, the variation in cementite size is sufficiently suppressed. Therefore, during quenching in the manufacturing process of producing bolts using the steel material, the cementite is sufficiently dissolved, and regions where V and Mo are concentrated are unlikely to occur. As a result, the bolt's susceptibility to hydrogen embrittlement is sufficiently low.

[0028] The above mechanism is a guess. Therefore, it is possible that a bolt manufactured using the steel material of this embodiment as a raw material will have a sufficiently low hydrogen embrittlement susceptibility due to a mechanism different from the above. However, as will be shown in the examples described later, when a bolt is manufactured using a steel material that satisfies Features 1 to 3, the hydrogen embrittlement susceptibility of the bolt will be sufficiently low.

[0029] The bolt according to this embodiment, which was completed based on the above findings, has the following configuration.

[0030] [1] A steel material, 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: 1.50-2.50%, V: 0.01 to 0.30%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0300%, The balance is Fe and impurities. The area ratio of the hard phase consisting of bainite and / or martensite is 90% or more, and the Vickers hardness is 220 to 400 HV, 0.0005 μm in the hard phase 2 The number density of cementite having an area of 4.0 particles / μm or more 2 or more, and among the plurality of cementite particles in the hard phase, 0.0005 to 0.0100 μm 2 The number ratio of the cementite having an area of 50.0% or more, and the sample standard deviation of the area of the plurality of cementite in the hard phase is 0.070 μm 2 Below is the Steel material.

[0031] [2] [1] The steel material according to the present invention further comprises: Instead of part of Fe, Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Zr: 0.300% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Bi: 0.100% or less, and Te: 0.100% or less, Contains one or more elements selected from the group consisting of Steel material.

[0032] The steel material of this embodiment will be described in detail below. Note that "%" relating to elements means mass % unless otherwise specified.

[0033] [Features of the steel material of this embodiment] The steel material of this embodiment has the following features. (Feature 1) The content of each element in the chemical composition is within the range shown below. (Feature 2) The area ratio of the hard phase consisting of bainite and / or martensite is 90% or more, and the Vickers hardness is 220 to 400 HV. (Feature 3) 0.0005 μm in the hard phase 2 The number density ND of cementite having an area of 4.0 pieces / μm 2 or more, and among the multiple cementites in the hard phase, 0.0005 to 0.0100 μm 2 The number ratio NR of cementite having an area of 50.0% or more, and the sample standard deviation σ of the area of multiple cementite in the hard phase is 0.070 μm 2 The following is the result. Below, features 1 to 3 will be explained.

[0034] [(Feature 1) Chemical composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0035] C: 0.30 to 0.50% Carbon (C) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the C content is less than 0.30%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.50%, the hydrogen embrittlement susceptibility of the bolt increases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.30 to 0.50%. The lower limit of the C content is preferably 0.32%, and more preferably 0.35%. The upper limit of the C content is preferably 0.48%, and more preferably 0.45%.

[0036] 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 cold forgeability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.01 to 0.30%. The lower limit of the Si content is preferably 0.02%, and more preferably 0.03%. The upper limit of the Si content is preferably 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0037] Mn: 0.10 to 1.50% Manganese (Mn) improves the hardenability of steel and increases the strength of the bolt. If the Mn content is less than 0.10%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.50%, the hydrogen embrittlement susceptibility of the bolt increases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.10 to 1.50%. The lower limit of the Mn content is preferably 0.15%, and more preferably 0.20%. The upper limit of the Mn content is preferably 1.30%, and more preferably 1.10%.

[0038] P:0.030% or less Phosphorus (P) is an impurity. In other words, the lower limit of the P content is greater than 0%. If the P content exceeds 0.030%, P segregates at grain boundaries even if the contents of other elements are within the ranges specified in this embodiment. As a result, the bolt becomes more susceptible to hydrogen embrittlement. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the P content is preferably 0.025%, and more preferably 0.020%.

[0039] S: 0.030% or less Sulfur (S) is an impurity. In other words, the lower limit of the S content is greater than 0%. If the S content exceeds 0.030%, S will segregate at grain boundaries even if the contents of other elements are within the ranges specified in this embodiment. As a result, the bolt becomes more susceptible to hydrogen embrittlement. Therefore, the S content is 0.030% or less. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the S content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the S content is preferably 0.025%, and more preferably 0.020%.

[0040] 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, if the Cr content exceeds 0.80%, hydrogen penetration into the steel material is promoted even if the contents of other elements are within the ranges of this embodiment, resulting in an increase in the susceptibility of the bolt to hydrogen embrittlement. Therefore, the Cr content is 0.01 to 0.80%. The lower limit of the Cr content is preferably 0.02%, and more preferably 0.03%. The upper limit of the Cr content is preferably 0.70%, and more preferably 0.60%.

[0041] Mo: 1.50-2.50% Molybdenum (Mo) increases the temper softening resistance of steel materials, thereby increasing the strength of bolts. Mo also concentrates in MC carbides, enhancing the hydrogen trapping function of the MC carbides. As a result, Mo reduces the hydrogen embrittlement susceptibility of high-strength bolts. When the Mo content is 1.50% or more and the contents of other elements are within the ranges of this embodiment, the above effects can be sufficiently obtained. On the other hand, if the Mo content exceeds 2.50%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 1.50 to 2.50%. The lower limit of the Mo content is preferably 1.60%, and more preferably 1.70%. The upper limit of the Mo content is preferably 2.40%, and more preferably 2.30%.

[0042] V: 0.01 to 0.30% Vanadium (V) forms MC carbides together with Mo, reducing the hydrogen embrittlement susceptibility 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 0.30%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0.01 to 0.30%. 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 0.25%, and more preferably 0.20%.

[0043] Al: 0.005 to 0.100% Aluminum (Al) is a deoxidizing agent during steel refining. Furthermore, Al combines with N to form Al nitrides. These Al nitrides suppress grain coarsening through a pinning effect during the quenching process in bolt manufacturing. As a result, the bolt is less susceptible to hydrogen embrittlement. If the Al content is less than 0.005%, the above effects cannot be fully achieved, 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%, even if the contents of other elements are within the ranges of this embodiment, coarse Al nitrides are formed, which become the starting points for fracture, thereby deteriorating the cold forgeability of the steel material. 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.

[0044] N: 0.0010~0.0300% Nitrogen (N) combines with Al to form Al nitrides. The Al nitrides suppress grain coarsening through a pinning effect during the quenching process in bolt manufacturing. As a result, the bolt is less susceptible to hydrogen embrittlement. 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 0.0300%, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed. The coarse nitrides become the starting points of fracture. Therefore, the cold forgeability of the steel material is reduced. Therefore, the N content is 0.0010 to 0.0300%. 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 0.0290%, more preferably 0.0280%, even more preferably 0.0270%, even more preferably 0.0250%, even more preferably 0.0200%, even more preferably 0.0150%, and even more preferably 0.0100%.

[0045] The balance of the chemical composition of the steel material according to the present 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 the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material according to the present embodiment.

[0046] [Optional Elements] The chemical composition of the steel material of this embodiment further includes: Instead of part of Fe, Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Zr: 0.300% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Bi: 0.100% or less, and Te: 0.100% or less, It may contain one or more elements selected from the group consisting of: These optional elements will be explained below.

[0047] [Group 1: Cu, Ni, B and Zr] The chemical composition of the steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu, Ni, B, and Zr. These elements are all optional elements and may not be contained. When contained, Cu, Ni, B, and Zr improve the hardenability of the steel material and increase the strength of the bolt.

[0048] 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. However, if the Cu content exceeds 0.40%, the steel material becomes embrittled even if the contents of other elements are within the ranges of this embodiment, resulting in a decrease in the hot workability of the steel material. Therefore, the Cu content is 0 to 0.40%, and when Cu is contained, the Cu content is 0.40% or less (more than 0 to 0.40%). 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%.

[0049] 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 is more than 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. However, if the Ni content exceeds 0.40%, even if the contents of other elements are within the ranges of this embodiment, the hardenability becomes too high, resulting in a decrease in the hot workability of the steel material. Therefore, the Ni content is 0 to 0.40%, and when Ni is contained, the Ni content is 0.40% or less (more than 0 to 0.40%). 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%.

[0050] B: 0.0100% or less Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When B is contained, that is, when the B content is more than 0%, B improves the hardenability of the steel material and increases the strength of the bolt. B also suppresses the grain boundary segregation of P and reduces the hydrogen embrittlement susceptibility of the bolt. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0100%, even if the contents of other elements are within the ranges of this embodiment, coarse B nitrides are formed. The coarse B nitrides become the starting points for cracks. As a result, the cold forgeability of the steel material is reduced. Therefore, the B content is 0 to 0.0100%, and when B is contained, the B content is 0.0100% or less (more than 0 to 0.0100%). 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 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and even more preferably 0.0050%.

[0051] Zr: 0.300% or less Zirconium (Zr) is an optional element and may not be contained, that is, the Zr content may be 0%. When Zr is contained, that is, when the Zr content 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. However, if the Zr content exceeds 0.300%, even if the contents of other elements are within the ranges of this embodiment, coarse Zr nitrides are formed. The coarse Zr nitrides become the starting points for cracks. As a result, the cold forgeability of the steel material is reduced. Therefore, the Zr content is 0 to 0.300%, and when Zr is contained, the Zr content is 0.300% or less (more than 0 to 0.300%). The lower limit of the Zr content is preferably 0.001%, more preferably 0.005%, even more preferably 0.010%, and still more preferably 0.020%. The upper limit of the Zr content is preferably 0.280%, more preferably 0.250%, even more preferably 0.200%, even more preferably 0.150%, and still more preferably 0.100%.

[0052] [Group 2: Ti and Nb] The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of Ti and Nb in place of a portion of Fe. These elements are optional and may not be included. When contained, Ti and Nb form precipitates and refine the crystal grains. As a result, the hydrogen embrittlement susceptibility of the bolt is reduced.

[0053] 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 bolt's susceptibility to hydrogen embrittlement is reduced. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse Ti nitrides are formed, which become the starting points for cracks, resulting in a decrease in the cold forgeability of the steel material. Therefore, the Ti content is 0 to 0.100%, and when Ti is contained, the Ti content is 0.100% or less (more than 0 to 0.100%). 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%.

[0054] Nb: 0.100% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content is more than 0%, Nb forms fine precipitates such as Nb carbides and refines the grains. As a result, the bolt's susceptibility to hydrogen embrittlement is reduced. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse Nb carbides and the like are generated. The coarse Nb carbides become the starting points for cracks. As a result, the cold forgeability of the steel material is reduced. Therefore, the Nb content is 0 to 0.100%, and when Nb is contained, the Nb content is 0.100% or less (more than 0 to 0.100%). The lower limit of the Nb content is preferably 0.001%, more preferably 0.005%, even more preferably 0.010%, and still 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%.

[0055] [Group 3: Ca and Mg] The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of Ca and Mg in place of a portion of Fe. These elements are optional and may not be contained. When contained, Ca and Mg refine MnS in the steel material, thereby reducing the hydrogen embrittlement susceptibility of the bolt.

[0056] 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 exceeds 0%, Ca refines the MnS in the steel material and reduces the hydrogen embrittlement susceptibility of the bolt. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, the cold forgeability 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 (more than 0 to 0.0050%). The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Ca content is preferably 0.0040%, and more preferably 0.0030%.

[0057] Mg: 0.0050% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When Mg is contained, that is, when the Mg content is more than 0%, Mg refines the MnS in the steel material and reduces the hydrogen embrittlement susceptibility of the bolt. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mg content is 0 to 0.0050%, and when Mg is contained, the Mg content is 0.0050% or less (more than 0 to 0.0050%). The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Mg content is preferably 0.0040%, and more preferably 0.0030%.

[0058] [Group 4: Bi and Te] The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of Bi and Te, instead of a portion of Fe. These elements are optional and may not be contained. When contained, Bi and Te improve the machinability of the steel material.

[0059] Bi:0.100% 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. However, if the Bi content exceeds 0.100%, the hot 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 0.100%, and when Bi is contained, the Bi content is 0.100% or less (more than 0 to 0.100%). 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 0.080%, and more preferably 0.060%.

[0060] Te: 0.100% 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. However, if the Te content exceeds 0.100%, the hot 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 0.100%, and when Te is contained, the Te content is 0.100% or less (more than 0 to 0.100%). 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 0.080%, and more preferably 0.060%.

[0061] [Method for measuring the chemical composition of steel] The chemical composition of the steel material of this embodiment can be measured by a known elemental analysis method. Specifically, chips are collected from the interior of the steel material to a depth of 1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. The solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by a known inert gas fusion-thermal conductivity method.

[0062] The content of each element is determined by rounding off the measured value to the nearest significant digit based on the significant digits specified in this embodiment. For example, the C content of the steel material in this embodiment is determined to one decimal place. Therefore, the C content is determined to one decimal place by rounding off the measured value to two decimal places.

[0063] Similarly, the contents of other elements other than the C content of the steel material of this embodiment are also determined by rounding off the measured value to the smallest digit specified in this embodiment, and this is the content of the element.

[0064] Rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0065] [(Feature 2) Microstructure of steel] The microstructure of the steel material of this embodiment has the following characteristics. (Feature 2) The area ratio of the hard phase is 90% or more, and the Vickers hardness of the steel material is 220 to 400 HV. Here, the hard phase consists of bainite and / or martensite.

[0066] The microstructure of the steel material of this embodiment is substantially a bainite structure. However, when microstructure observation is performed using an optical microscope as described below, bainite and martensite cannot be clearly distinguished. On the other hand, the hard phase (bainite and / or martensite) can be easily distinguished from phases other than the hard phase (ferrite, pearlite, etc.) by the above-mentioned microstructure observation. Furthermore, bainite and martensite have different Vickers hardnesses. Specifically, the Vickers hardness of bainite is lower than that of martensite.

[0067] When the content of each element in the chemical composition is within the range of this embodiment, if the area ratio of the hard phase in the microstructure is 90% or more and the Vickers hardness is 220 to 400 HV, the microstructure is substantially bainite.

[0068] The steel material of this embodiment is subjected to a spheroidizing treatment during the bolt manufacturing process. As described above, the microstructure of the steel material of this embodiment is substantially a bainite structure. Therefore, in the steel material after the spheroidizing treatment, the spheroidizing rate of cementite is increased. In this case, the cold forgeability of the steel material can be improved.

[0069] The lower limit of the area ratio of the hard phase in the microstructure of the steel material is preferably 92%, more preferably 94%, and even more preferably 96%.

[0070] The lower limit of the Vickers hardness is preferably 230 HV, more preferably 240 HV, and even more preferably 250 HV. The upper limit of the Vickers hardness of the hard phase is preferably 395 HV, more preferably 390 HV, even more preferably 380 HV, and even more preferably 370 HV.

[0071] [Method for measuring the microstructure of steel] The microstructure of this embodiment can be measured by the following method. In a cross section (transverse section) perpendicular to the longitudinal direction of a steel material, the center position of the line segment (i.e., the radius) connecting the central axis and the steel surface is defined as the "D / 4" position (D means the diameter of the steel cross section).

[0072] A sample is taken from the D / 4 position of the steel material. The surface of the sample that corresponds to the above cross section is defined as the observation surface. The observation surface of the sample is mirror-polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (Nital etching solution). Five randomly selected observation fields (90 μm × 120 μm) on the etched observation surface are observed using an optical microscope at 1000x magnification. In the observation field, the hard phases (bainite and / or martensite) and other phases (proeutectoid ferrite, pearlite, etc.) can be easily distinguished by contrast. Ferrite is observed as a white area. Pearlite is observed as a phase with a lamellar structure. Hard phases are observed as areas that are less bright than ferrite. The area ratio (%) of the hard phase is calculated based on the total area of the hard phase in the five observation fields and the total area of the five observation fields.

[0073] [Vickers hardness measurement method] The Vickers hardness of steel is determined using the following method. A sample is taken with the cross section perpendicular to the longitudinal direction of the steel as the observation surface. The observation surface of the sample is the entire cross section perpendicular to the longitudinal direction of the steel. In other words, the diameter of the observation surface of the sample is D. The observation surface is mirror-polished. On the mirror-polished observation surface, an arbitrary measurement position at a depth of D / 4 in the radial direction from the surface of the steel is designated as measurement position P1. From measurement position P1, measurement positions P2 to P12 are determined at 30° intervals around the center of the observation surface of the sample (i.e., corresponding to the center of the cross section of the steel). Vickers hardness is measured at these 12 measurement positions P1 to P12, which are at a depth of D / 4.

[0074] At each of the measurement positions P1 to P12, a Vickers hardness test is carried out in accordance with JIS Z 2244:2009. The test force is 0.98 N. The arithmetic mean value of the hardness obtained at the 12 measurement points is defined as the Vickers hardness (HV) of the steel material.

[0075] [(Feature 3) Cementite in steel] In the steel material of this embodiment, 0.0005 μm in the hard phase 2 The number density ND of cementite having an area of 4.0 pieces / μm 2 or more, and 0.0005 μm in the hard phase 2 Among the multiple cementite particles having an area of 0.0005 to 0.0100 μm or more, 2 The number ratio NR of cementite having an area of 0.0005 μm in the hard phase is 50.0% or more. 2The sample standard deviation σ of the area of multiple cementites having an area of 0.070 μm or more 2 The following is the result.

[0076] The steel material of this embodiment has a high V content and Mo content. Therefore, when this steel material is used to manufacture a bolt, many fine MC carbides containing Mo are formed by tempering after quenching during the manufacturing process. These fine MC carbides are likely to trap hydrogen. Therefore, the manufactured bolt has a high limit of hydrogen before it breaks. As a result, the bolt's susceptibility to hydrogen embrittlement is reduced.

[0077] In order to achieve the above effects, in the steel material from which the bolt is made, (1) the number density ND of cementite in the hard phase is increased, (2) the proportion of fine cementite among the multiple cementite particles in the hard phase is increased, and (3) the grain size distribution of the cementite is made sharp in order to suppress variation in the size of the cementite. These points will be explained below.

[0078] [Regarding the number density of cementite (above (1))] During the manufacturing process of a bolt made from steel, the higher the number density of cementite in the hard phase of the steel, the higher the number density of MC carbides in the bolt, and the more uniformly the MC carbides are dispersed in the bolt. As a result, the hydrogen embrittlement susceptibility of the bolt is sufficiently low. Specifically, in a steel that satisfies the above-mentioned characteristics 1 and 2, the number density ND of cementite in the hard phase is 4.0 particles / μm 2 If the above conditions are met, the MC type carbides in the bolt are sufficiently finely dispersed, and the susceptibility of the bolt to hydrogen embrittlement is sufficiently reduced.

[0079] The preferred lower limit of the number density ND of cementite in the hard phase is 4.2 particles / μm 2 and more preferably 4.5 particles / μm 2 and more preferably 4.7 particles / μm 2 is. The upper limit of the number density ND of cementite in the hard phase is not particularly limited. The preferred upper limit of the number density ND of cementite in the hard phase is 25.0 particles / μm 2 and more preferably 20.0 particles / μm 2 and more preferably 15.0 particles / μm 2 and more preferably 10.0 particles / μm 2 is.

[0080] [Regarding the particle size distribution of cementite (above (2) and (3))] Even if the microstructure of a steel material satisfying Feature 1 is substantially bainite and the cementite is granular, the amount of fine MC carbides formed in the bolt after tempering is affected by the cementite particle size distribution. Specifically, even if the average particle size of multiple cementite particles in the steel material is small, if the cementite particle size distribution is broad, the size of the cementite particles in the steel material varies. In this case, the hydrogen embrittlement susceptibility of the bolt manufactured using the steel material cannot be sufficiently reduced.

[0081] 0.0005 μm in the hard phase 2 Among the multiple cementite particles having an area of 0.0005 to 0.0100 μm or more, 2 The number ratio NR of cementite having an area of 50.0% or more, and the sample standard deviation σ of the area of multiple cementite in the hard phase is 0.070 μm 2 If the content is less than 100%, the cementite in the steel is sufficiently fine and the cementite particle size distribution is sufficiently sharp. In this case, during quenching in the manufacturing process of producing bolts using the steel, the cementite is sufficiently dissolved and segregated regions of V and Mo are unlikely to occur. As a result, the hydrogen embrittlement susceptibility of the bolt is sufficiently low.

[0082] The lower limit of the number ratio NR is preferably 52.5%, more preferably 55.0%, and even more preferably 57.5%. The upper limit of the number ratio NR is not particularly limited, and may be, for example, 100.0%, for example, 90.0%, for example, 85.0%, or for example, 80.0%.

[0083] The preferred upper limit of the sample standard deviation σ is 0.065 μm 2 and more preferably 0.063 μm 2 and more preferably 0.061 μm 2 is.

[0084] [Method for measuring cementite number density ND, cementite number ratio NR, and sample standard deviation σ of cementite area] 0.0005 μm in the hard phase 2 The number density ND of cementite having an area of 0.0005 μm or more in the hard phase 2 Among the multiple cementite particles having an area of 0.0005 to 0.0100 μm or more, 2 The number ratio of cementite having an area of 0.0005 μm and 2 The sample standard deviation σ of the area of cementite having the above area can be measured by the following method.

[0085] A sample is taken from the D / 4 position on a cross section (transverse section) perpendicular to the longitudinal direction of the steel material. The surface of the sample corresponding to the cross section is defined as the observation surface. After polishing the observation surface of the sample, it is etched for 20 seconds using a 6% picral etching solution (a mixture of 6 g of picric acid and 94 mL of ethanol). Using a scanning electron microscope (SEM), five observation areas (9 μm × 12 μm) at random on the etched observation surface are observed at a magnification of 10,000 times, and photographs of particles (precipitates and inclusions) within the observation areas are taken to generate photographic images. The observation areas are defined as areas within the hard phase.

[0086] Furthermore, in the above-mentioned observation region, oxides and sulfides are identified by a well-known method using EDS (Energy Dispersive X-ray Spectroscopy) attached to the SEM. Of the particles observed by the SEM, particles excluding oxides and sulfides are identified as cementite. The area of the identified cementite is calculated. The area of the identified cementite can be calculated, for example, by well-known image processing.

[0087] 0.0005 μm identified in five observation fields 2 Based on the total number of cementite particles having an area of 100 or more and the total area of the five observation fields, the number density ND (particles / μm 2 ) is found.

[0088] In addition, among the identified cementite, the area of 0.0005 to 0.0100 μm 2 The ratio (%) of the total number of cementite particles to the total number of identified cementite particles is defined as the number ratio NR (%). Furthermore, the sample standard deviation σ is calculated based on the area of each identified cementite particle. Here, the sample standard deviation σ is calculated using the following formula.

[0089]

number

[0090] [Effects of the steel material of this embodiment] As described above, the steel material of this embodiment has the following configuration. (Feature 1) The content of each element in the chemical composition is within the range described in this embodiment. (Feature 2) The area ratio of the hard phase is 90% or more, and the Vickers hardness of the steel is 220 to 400 HV. (Feature 3) In the steel material of this embodiment, 0.0005 μm in the hard phase 2 The number density ND of cementite having an area of 4.0 pieces / μm 2 or more, and 0.0005 μm in the hard phase 2 Among the multiple cementite particles having an area of 0.0005 to 0.0100 μm or more, 2 The number ratio NR of cementite having an area of 0.0005 μm in the hard phase is 50.0% or more. 2 The sample standard deviation σ of the area of multiple cementites having an area of 0.070 μm or more 2 The following is the result. When the steel material having the above-described configuration is used as a material for a bolt, it is possible to reduce the hydrogen embrittlement susceptibility of the bolt.

[0091] Here, in this specification, hydrogen embrittlement susceptibility is evaluated by the following method. The steel material of this embodiment is subjected to wire drawing. The drawn steel material is subjected to well-known spheroidizing annealing. In the spheroidizing annealing, the steel material is heated to 760°C and held at 760°C for 5 hours. After the holding time has elapsed, the steel material is slowly cooled at 5°C / hour. The steel material is then air-cooled until the temperature reaches 650°C to room temperature.

[0092] The spheroidized annealed steel is cold forged (bolt forming) to produce an intermediate product in the shape of a bolt. The intermediate product is then quenched and tempered under well-known conditions. The quenching temperature is 920°C, and the holding time at the quenching temperature is 1 hour. After the holding time has elapsed, the intermediate product is water-cooled. A bolt is produced through these steps.

[0093] A round bar test piece with a circular notch, measuring 7 mm in diameter and 70 mm in length, is taken from the inside of the manufactured bolt, at least 1 mm deep from the surface. A circular notch is formed at the longitudinal center of the test piece. The notch shape is 1.4 mm deep, the notch angle is 60°, and the radius of curvature at the notch base is 0.175 mm.

[0094] Hydrogen is charged to the circularly notched round bar test specimen using the cathodic hydrogen charging method. Specifically, a room temperature cathodic hydrogen charging solution is prepared. The cathodic hydrogen charging solution is an aqueous solution in which 3 g of ammonium thiocyanate is added to 1 L of a 3 mass % sodium chloride aqueous solution.

[0095] A round bar test piece with an annular notch was immersed in the cathodic hydrogen charging solution for 18 hours at a cathodic current density of 0.05 mA / cm. 2 A constant current controlled at 1000 kJ / s is generated to add hydrogen to the annular notched round bar test specimen. After performing the cathodic hydrogen charging method, the annular notched round bar test specimen is left at room temperature for 96 hours. A zinc plating film is then formed on the surface of the hydrogen-charged annular notched round bar test specimen under the same conditions to prevent the hydrogen inside the annular notched round bar test specimen from leaking to the outside.

[0096] A constant load test is carried out at room temperature and atmospheric pressure, in which a constant load is applied to a round bar test piece with an annular notch on which a zinc plating coating has been formed, so that a load equal to the cross-sectional area of the cross section (transverse section) perpendicular to the longitudinal direction of the test piece at the bottom of the notch multiplied by 0.90 times the tensile strength obtained in the tensile test described below using a smooth round bar tensile test piece is applied.

[0097] The tensile test using a smooth round bar tensile test specimen is carried out in the following manner. A smooth round bar tensile test specimen is taken from the inside of the bolt, at least 1 mm deep from the surface. The diameter of the parallel part of the smooth round bar tensile test specimen is 6 mm, and the length of the parallel part is 70 mm. The central axis of the parallel part of the smooth round bar tensile test specimen is coaxial with the central axis of the bolt. A tensile test in accordance with JIS Z 2241:2011 is carried out using the smooth round bar tensile test specimen in air at room temperature (20±15°C), and the tensile strength (MPa) is determined.

[0098] In the above-mentioned constant load test, if the annular notched round bar test piece can withstand 100 hours or more without fracture, it is determined that the hydrogen embrittlement susceptibility is sufficiently low. In the above-mentioned constant load test, the annular notched round bar test piece of the steel material of this embodiment can withstand 100 hours or more without fracture. Therefore, the hydrogen embrittlement susceptibility of a bolt manufactured using the steel material of this embodiment as a raw material is sufficiently low.

[0099] [Steel shape] The shape of the steel material of this embodiment is not particularly limited. The steel material of this embodiment is, for example, a steel bar or a wire rod. That is, the steel material of this embodiment is, for example, a steel bar.

[0100] [Steel use] As described above, the steel material of this embodiment is suitable as a material for bolts. The steel material of this embodiment is particularly suitable as a material for bolts that require a tensile strength of 1300 MPa or more and low hydrogen embrittlement susceptibility. However, the steel material of this embodiment may also be used for applications other than the above-mentioned bolts.

[0101] [Steel manufacturing method] An example of a method for manufacturing a steel material according to this embodiment will be described. The method for manufacturing a steel material described below is one example for manufacturing the steel material according to this embodiment. Therefore, a steel material 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 steel material according to this embodiment.

[0102] An example of the method for manufacturing the steel material according to this embodiment includes the following steps. (Process 1) Direct hot working process after casting (Process 2) Finishing rolling process

[0103] The main production conditions in the above steps 1 and 2 are as follows. (Process 1) When the surface temperature of the casting material during cooling (solidification) reaches the range of 1100-900°C, hot working is performed on the casting material (direct hot working). (Process 2) ·Heating temperature HT: 1000~1200℃ Finishing temperature FT: 900℃ or higher Finishing rolling temperature FT to 820°C, average cooling rate CR1: 1.0 to 2.0°C / sec or less Average cooling rate CR2 from 820 to 400°C: 2.0 to 4.0°C / sec

[0104] In the above-described manufacturing process, the crystal grains in the steel are maintained as coarse as possible until the completion of finish rolling. As a result, during cooling after the completion of finish rolling, the bainite nose in the continuous cooling transformation diagram (CCT diagram) shifts to the long-time side. This lowers the bainite transformation start temperature. If the bainite transformation start temperature is lowered, the formation temperature of cementite, which precipitates with bainite transformation, also becomes lower. If the formation temperature of cementite is lowered, the cementite remains fine. Therefore, a steel satisfying Features 1 to 3 is manufactured. Each step will be described below.

[0105] [(Process 1) Direct hot working process after casting] In the hot working process directly after casting, a cast material is produced, and when the surface temperature of the cast material during cooling (solidification) reaches a range of 1100 to 900°C, hot working is started to produce a billet, which allows the production of a billet with coarse crystal grains.

[0106] The post-casting direct hot working process includes the following steps: (Process 11) Casting process (Process 12) Direct hot processing process Each step will be described below.

[0107] [(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.

[0108] [(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.

[0109] In this specification, the term "direct hot working" refers to performing hot working when the surface temperature of a produced cast material reaches 1100 to 900°C without cooling the cast material to room temperature. The hot working method may be hot forging or hot rolling. The cumulative reduction rate in the reduction step is not particularly limited, but is, for example, 35% or more.

[0110] Generally, the casting material produced by the casting method is first cooled to room temperature. Then, the casting material cooled to room temperature is heated in a heating furnace. c3 The cast material is heated to above the transformation point, and then blooming (rough rolling) is performed. This conventional manufacturing process is referred to as "hot working after reheating" in this specification. In the case of hot working after reheating, a reverse transformation (transformation from α phase to γ phase) occurs in the cast material during heating before blooming. This reverse transformation refines the crystal grains.

[0111] On the other hand, in this embodiment, the grain refinement due to the reverse transformation is avoided. Specifically, when the surface temperature of the cast material during cooling in the casting process falls within the range of 1100 to 900°C, that is, when the surface temperature of the cast material falls within the range of A r3 The cast material is hot forged or hot rolled at or above the transformation point to produce a billet. In this case, the cast material is hot worked without undergoing reverse transformation, which prevents the crystal grains of the produced billet from becoming finer. The upper limit of the cumulative rolling reduction in the working step is not particularly limited, but is, for example, 80%.

[0112] [(Process 2) Finishing rolling process] In the finish rolling process, the billet produced in the hot working process directly after casting is heated. The heated billet is finish rolled. The finish rolled billet is cooled to produce steel. The finish rolling process meets the following manufacturing conditions: ·Heating temperature HT: 1000~1200℃ Finishing temperature FT: 900℃ or higher Finishing rolling temperature FT to 820°C, average cooling rate CR1: 1.0 to 2.0°C / sec or less Average cooling rate CR2 from 820 to 400°C: 2.0 to 4.0°C / sec Each manufacturing condition will be explained below.

[0113] [About heating temperature HT] The heating temperature HT in the heating furnace in the finish rolling process is 1000 to 1200°C. If the heating temperature HT of the heating furnace in the finish rolling process is less than 1000°C, the precipitates in the billet do not dissolve sufficiently. In this case, the undissolved precipitates have a pinning effect, which suppresses the coarsening of crystal grains in the billet. In this case, the cementite in the steel after the finish rolling process becomes coarse, and Feature 3 is not satisfied. On the other hand, if the heating temperature HT exceeds 1200°C, the manufacturing cost increases. Furthermore, cracks are more likely to occur in the billet during finish rolling. Therefore, the heating temperature HT is 1000 to 1200°C.

[0114] The lower limit of the heating temperature HT is preferably 1020°C, more preferably 1040°C, and even more preferably 1060°C. The upper limit of the heating temperature HT is preferably 1180°C, more preferably 1160°C, and even more preferably 1140°C. The holding time at the heating temperature HT is not particularly limited and is, for example, 0.5 to 4.0 hours.

[0115] [Finishing rolling temperature FT] In the finish rolling process, hot rolling (finish rolling) is carried out using a continuous rolling mill equipped with multiple rolling stands arranged in a row. In hot rolling using a continuous rolling mill, the temperature of the steel material at the outlet side of the stand that lastly rolled down the steel material is defined as the finish rolling temperature FT (°C). Note that the steel material temperature means the surface temperature of the steel material.

[0116] The finish rolling temperature FT is 900°C or higher. If the finish rolling temperature FT is less than 900°C, the pinning effect of precipitates will suppress the coarsening of crystal grains in the steel material. In this case, the cementite in the steel material after the finish rolling process will become coarse, and Feature 3 will not be satisfied.

[0117] If the finish rolling temperature is 900°C or higher, the crystal grains after finish rolling can be maintained in a coarse state. As a result, fine cementite can be produced in the subsequent cooling process, and Feature 3 is satisfied.

[0118] [Cooling after finish rolling] After the finish rolling is completed, the finish-rolled steel is cooled. By meeting the manufacturing conditions in the above process, the crystal grains in the finish-rolled steel remain coarse. Therefore, in steel with the chemical composition of Feature 1, the bainite nose in the CCT diagram is shifted to the long-time side. In other words, the transformation start temperature of bainite is lower.

[0119] Therefore, in the cooling after finish rolling, when the steel material temperature is in the finish rolling temperature range of FT to 820°C, the cooling rate (average cooling rate CR1) is made as slow as possible to maintain the crystal grains as coarse as possible. Then, when the steel material temperature is in the range of 820 to 400°C, which corresponds to the bainite transformation temperature range, the cooling rate (average cooling rate CR2) is made faster to suppress the growth of cementite that is formed with the bainite transformation. The average cooling rates CR1 and CR2 will be explained below.

[0120] [About the average cooling rate CR1] The average cooling rate CR1 means the arithmetic mean value of the cooling rate in the range in which the steel temperature is from the finish rolling temperature FT to 820°C.

[0121] If the average cooling rate CR1 exceeds 2.0°C / s, the cooling rate in this temperature range is too fast. In this case, the grains in the steel become finer. As a result, the bainite nose shifts to the shorter time side in the CCT diagram for steel with the chemical composition of Feature 1. In this case, the bainite transformation temperature increases. As a result, cementite formed during bainite transformation grows during cooling. As a result, the steel after the finish rolling process no longer satisfies Feature 3.

[0122] The lower limit of the average cooling rate CR1 is not particularly limited. Taking into consideration the facility capacity, the lower limit of the average cooling rate CR1 is 1.0°C / sec.

[0123] The average cooling rate CR1 has a lower limit of preferably 1.1° C. / sec, more preferably 1.2° C. / sec, and an upper limit of preferably 1.9° C. / sec, more preferably 1.8° C. / sec.

[0124] [About the average cooling rate CR2] The average cooling rate CR2 means the arithmetic mean value of the cooling rate in the steel temperature range from 820 to 400°C.

[0125] The steel temperature range of 820 to 400°C includes the temperature range in which bainite transformation occurs in steel produced under the above-mentioned production conditions using a cast material with a chemical composition that satisfies Feature 1. Therefore, in the temperature range of 820 to 400°C, the cooling rate is increased to a degree that does not result in excessive formation of martensite. This suppresses the growth of cementite that forms during bainite transformation, and maintains the cementite in a fine state.

[0126] If the average cooling rate CR2 is less than 2.0°C / sec, the cooling rate in the temperature range of 820 to 400°C is too slow. In this case, the cementite that is formed becomes coarse. As a result, the steel material after the finish rolling process no longer satisfies Feature 3.

[0127] On the other hand, if the average cooling rate CR2 exceeds 4.0°C / s, the cooling rate in the temperature range of 820 to 400°C is too fast. In this case, martensite is excessively formed. As a result, in the steel material after the finish rolling process, although the area ratio of the hard phase is 90% or more, the Vickers hardness exceeds 400HV.

[0128] If the average cooling rate CR2 is 2.0 to 4.0°C / sec, the steel material after the finish rolling process satisfies Features 1 to 3, provided that other manufacturing conditions are met.

[0129] The preferred lower limit of the average cooling rate CR2 is 2.2° C. / sec, more preferably 2.4° C. / sec, and even more preferably 2.6° C. / sec. The preferred upper limit of the average cooling rate CR2 is 3.8° C. / sec, more preferably 3.6° C. / sec, and even more preferably 3.4° C. / sec.

[0130] The average cooling rates CR1 and CR2 can be calculated using the following method. Cooling of the steel material is carried out in a cooling equipment line located downstream of the continuous rolling mill that performs finish rolling. Thermometers are installed at multiple locations on the cooling equipment line from upstream to downstream. The steel material temperature as it passes through each thermometer and the passage time are detected. The average cooling rates CR1 and CR2 are calculated based on the obtained steel material temperature and passage time.

[0131] The steel material of this embodiment is manufactured by the above manufacturing process.

[0132] [Bolt manufacturing process] For reference, a manufacturing process for a bolt made of the steel material of this embodiment will be described. An example of the manufacturing process for a bolt made of the steel material of this embodiment includes the following steps. ·Wire drawing process Spheroidizing annealing process Cold forging process ·Quenching and tempering process Each step will be described below.

[0133] [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.

[0134] [Spheroidizing annealing process] In the spheroidizing annealing step, the steel wire after the wire drawing step is subjected to spheroidizing annealing. Spheroidizing annealing may be performed under well-known conditions. For example, the steel wire is heated to 720 to 800°C. Then, it is held at 720 to 800°C for 1.0 to 6.0 hours. Then, it is slowly cooled to 650°C at a cooling rate of 3 to 10°C / hour. Then, it is cooled to room temperature.

[0135] [Cold forging process] In the cold forging step, the steel wire after the wire drawing step is subjected to well-known cold forging to manufacture an intermediate product in the shape of a bolt.

[0136] [Quenching and tempering process] In the quenching and tempering step, the intermediate product is quenched and tempered.

[0137] [Quenching] The intermediate product after cold forging is quenched by a known method. The quenching temperature and the holding time at the quenching temperature are not particularly limited. The quenching temperature is, for example, 840 to 970°C. The holding time at the quenching temperature is, for example, 15 to 360 minutes (6 hours). After the holding time has elapsed, the intermediate product is rapidly cooled. Specifically, the intermediate product is water-cooled or oil-cooled.

[0138] [Tempering] The intermediate product after quenching is subjected to well-known tempering. The tempering conditions are, for example, as follows: 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.

[0139] By the above manufacturing method, the bolt made of the steel material of this embodiment can be manufactured.

[0140] [Other processes] The bolt manufacturing process may include other steps in addition to those described above. For example, a thread rolling step may be performed after the cold forging step and before the quenching and tempering step to form a thread. Furthermore, a compressive residual stress imparting step may be performed after the quenching and tempering step. All of these steps are optional and may not be performed. [Example]

[0141] The effects of the steel material 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 steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

[0142] Steel materials (round bars) having the chemical compositions shown in Tables 1-1 and 1-2 were prepared as bolt materials.

[0143] [Table 1-1]

[0144] [Table 1-2]

[0145] The steel materials of each test number were produced by the following method: Ingots having the chemical compositions shown in Tables 1-1 and 1-2 were produced by a casting method (casting process).

[0146] In test numbers 1 to 25 and 28 to 35, hot forging was performed to produce billets when the surface temperature of the cast material during cooling after the casting process reached 1100 to 900°C. That is, in these test numbers, direct hot working was performed (indicated as "direct" in the "hot working" column in Table 2). On the other hand, in test numbers 26 and 27, the cast material was cooled to room temperature, reheated to 1200°C in a heating furnace, and then hot forging was performed to produce billets (indicated as "after reheating" in the "hot working" column in Table 2). In all hot working methods, the area reduction rate was within the range of 35 to 60%.

[0147] [Table 2]

[0148] The produced billets were subjected to a finish rolling process to produce steel materials (round bars) with a diameter of 20 mm.

[0149] The heating temperature HT (°C) in the finish rolling process, the finish rolling temperature FT (°C), the average cooling rate CR1 (°C / sec) at which the steel temperature was changed from the finish rolling temperature FT to 820°C, and the average cooling rate CR2 (°C / sec) at which the steel temperature was changed from 820°C to 400°C were as shown in Table 2. Steel materials (round bars) with each test number were produced by the above production process.

[0150] [Evaluation test] The manufactured steel materials were subjected to the following evaluation tests. (Test 1) Chemical composition measurement test (Test 2) Microstructure observation test (Test 3) Vickers hardness test (Test 4) Measurement test of cementite number density ND, cementite number ratio NR, and sample standard deviation σ (Test 5) Steel wiredrawability evaluation test (Test 6) Cold forgeability evaluation test of steel (Test 7) Hydrogen embrittlement susceptibility evaluation test for steel bolts Each evaluation test will be described below.

[0151] [(Test 1) Chemical composition measurement test] The chemical composition of the steel material of each test number was measured based on the above-mentioned [Method for measuring chemical composition of steel material]. As a result, the chemical composition of the steel material of each test number was as shown in Tables 1-1 and 1-2.

[0152] [(Test 2) Microstructure Observation Test] The area ratio (%) of the hard phase in the microstructure of the steel material of each test number was determined based on the above-mentioned [Method for measuring the microstructure of steel material]. The results obtained are shown in the "Hard phase area ratio (%)" column under "Microstructure" in Table 2.

[0153] [(Test 3) Vickers hardness test] The Vickers hardness (HV) of the steel material of each test number was determined according to the above-mentioned [Vickers hardness measurement method]. The obtained Vickers hardness (HV) is shown in the "Vickers hardness (HV)" column in the "Microstructure" section of Table 2.

[0154] [(Test 4) Measurement test of cementite number density ND, cementite number ratio NR, and sample standard deviation σ] For each test number, 0.0005 μm in the hard phase 2 The number density ND (pieces / μm 2 ), among the multiple cementites in the hard phase, 0.0005 to 0.0100 μm 2 The number ratio NR (%) of cementite having an area of 0.0005 μm 2 The sample standard deviation σ (μm 2 ) was determined based on the above-mentioned [Method for measuring the cementite number density ND, cementite number ratio NR, and sample standard deviation σ of cementite area]. 2 ), number ratio NR (%) and sample standard deviation σ (μm 2 ) in the "Cementite" column in Table 2. 2 )," "Number ratio NR (%)" and "Sample standard deviation σ (μm 2 )" column.

[0155] [(Test 5) Steel wiredrawability evaluation test] The following wiredrawing process was carried out on the steel material with each test number. 250 kg of steel material with each test number was subjected to lubrication treatment under the same conditions, and then wiredrawing was carried out. The cumulative reduction in area of the die during wiredrawing was set to 25%. If no breakage occurred during wiredrawing, the wiredrawability was evaluated as excellent (shown as "P (Pass)" in the "Wiredrawability" column in Table 2). On the other hand, if breakage occurred during wiredrawing, the wiredrawability was evaluated as poor (shown as "F (Fail)" in the "Wiredrawability" column in Table 2). Note that tests 6 to 7 were not carried out on steel materials with test numbers that had poor wiredrawability.

[0156] [(Test 6) Cold forgeability evaluation test of steel material] The cold forgeability of the steel material of each test number was evaluated by the following method. First, the steel material was subjected to spheroidizing annealing. In the spheroidizing annealing, the steel material was heated to 760°C. Then, it was held at 760°C for 5 hours. After the holding time had elapsed, the steel material was slowly cooled at 5°C / hour. Then, the steel material was air-cooled from 650°C to room temperature.

[0157] A smooth test piece was taken from the D / 4 depth position in the cross section perpendicular to the longitudinal direction of the steel after spheroidizing annealing. The smooth test piece was cylindrical, with a diameter of 8 mm and a length of 12 mm. The longitudinal direction of the smooth test piece was parallel to the longitudinal direction of the steel.

[0158] A limit compression test was conducted on the smooth test specimens in accordance with the test method described in the Cold Forging Subcommittee Materials Research Group's Journal of Plasticity and Processing, Vol. 22, No. 241 (1981-2), pp. 139-144. Specifically, the smooth test specimens were cold compressed at room temperature in air at a rate of 10 mm / min using end-face restraint dies. Compression was stopped when microcracks of 0.5 mm or more appeared in the smooth test specimens, and the compression ratio (%) at that point was calculated. This measurement was performed a total of 10 times to determine the compression ratio (%) at which the cumulative failure probability was 50%, and this compression ratio was designated the limit compression ratio (%).

[0159] When the limit compression ratio was 70% or more, the cold forgeability was judged to be excellent (indicated as "70 or more" in the "Limit compression ratio (%)" column in Table 2). On the other hand, when the limit compression ratio was less than 70%, the cold forgeability was judged to be poor (the limit compression ratio (%) is shown in the "Limit compression ratio (%)" column in Table 2). Note that for test numbers with poor cold forgeability, Test 7 was not performed.

[0160] [(Test 7) Hydrogen embrittlement susceptibility evaluation test for steel bolts] The hydrogen embrittlement susceptibility evaluation test for the bolts made of the steel material with each test number was carried out by the following method.

[0161] First, the steel material of each test number was subjected to wire drawing under the same conditions to produce a steel wire with a diameter of 16 mm. The steel wire of each test number was subjected to spheroidizing annealing under the same conditions as those performed in [(Test 6) Steel material cold forgeability evaluation test]. The steel wires after spheroidizing annealing were subjected to cold forging (bolt forming) under the same conditions to produce intermediate bolt-shaped products with the same thread root diameter of 14 mm for each test number.

[0162] The manufactured intermediate products were subjected to quenching and tempering. For quenching, the quenching temperature was 920°C and the holding time at the quenching temperature was 1 hour for each test number. After the holding time had elapsed, the intermediate products were water-cooled.

[0163] The intermediate product after quenching was tempered at a tempering temperature of 620°C for 2.0 hours. Bolts with each test number were manufactured using the above manufacturing process.

[0164] The following hydrogen embrittlement susceptibility evaluation test was carried out on the bolts with each test number. A round bar specimen with a circular notch, measuring 7 mm in diameter and 70 mm in length, was taken from the inside of the bolt, at least 1 mm deep from the surface. A circular notch was formed at the longitudinal center of the specimen. The notch shape was 1.4 mm deep, the notch angle was 60°, and the radius of curvature at the notch base was 0.175 mm.

[0165] Hydrogen was charged to the circularly notched round bar specimens using the cathodic hydrogen charging method. Specifically, a room temperature cathodic hydrogen charging solution was prepared. The cathodic hydrogen charging solution was an aqueous solution containing 3 g of ammonium thiocyanate added to 1 L of a 3 mass % sodium chloride aqueous solution.

[0166] A round bar test piece with an annular notch was immersed in the cathodic hydrogen charging solution for 18 hours at a cathodic current density of 0.05 mA / cm. 2 A constant current controlled at 1000 kJ / s was generated to load hydrogen into the circularly notched round bar specimens.

[0167] After the cathodic hydrogen charging method, the circularly notched round bar test specimens were left at room temperature for 96 hours. After that, a zinc plating film was formed on the surface of the hydrogen-charged circularly notched round bar test specimens under the same conditions to prevent hydrogen from leaking out of the specimens.

[0168] A constant load test was conducted at room temperature and atmospheric pressure on a zinc-plated round bar specimen, in which a constant load was applied to the bar specimen at room temperature so that the load was 0.90 times the cross-sectional area of the cross section (transverse section) perpendicular to the longitudinal direction of the specimen at the notch root multiplied by the tensile strength obtained in the tensile test using a smooth round bar specimen (described below). The maximum test time was 100 hours. If the bar specimen survived for 100 hours or more without fracture in the constant load test, it was evaluated as having sufficiently low hydrogen embrittlement susceptibility (indicated by "100 or more" in the "Endurance time (hours)" column under "Hydrogen embrittlement susceptibility" in Table 2). On the other hand, if the bar specimen fractured in less than 100 hours in the constant load test, it was evaluated as having high hydrogen embrittlement susceptibility (indicated by the "Endurance time (hours)" column under "Hydrogen embrittlement susceptibility" in Table 2).

[0169] The tensile tests using smooth round bar tensile test specimens were carried out in the following manner. A smooth round bar tensile test specimen was taken from the inside of the bolt with each test number, at least 1 mm deep from the surface. The diameter of the parallel part of the smooth round bar tensile test specimen was 6 mm, and the length of the parallel part was 70 mm. The central axis of the parallel part of the smooth round bar tensile test specimen was coaxial with the central axis of the bolt.

[0170] Using smooth round bar tensile test specimens, tensile tests in accordance with JIS Z 2241:2011 were carried out in air at room temperature (20±15°C) to determine the tensile strength (MPa). The obtained tensile strengths are shown in the "Tensile strength (MPa)" column under "Hydrogen embrittlement susceptibility" in Table 2.

[0171] [Evaluation results] With reference to Tables 1-1, 1-2, and 2, the steels of test numbers 1 to 22 had appropriate chemical compositions and manufacturing conditions. As a result, the area ratio of the hard phase was 90% or more, and the Vickers hardness was 220 to 400 HV. Furthermore, the area ratio of the hard phase of the steels was 0.0005 μm 2 The number density ND of cementite having an area of 4.0 pieces / μm 2 or more, and among the multiple cementites in the hard phase, 0.0005 to 0.010 μm 2 The number ratio NR of cementite having an area of 0.0005 μm is 50.0% or more, and 2 The sample standard deviation σ of the area of multiple cementites with an area of 0.070 μm 2 Therefore, the wire drawing processability was excellent. Furthermore, the cold forging processability after spheroidizing treatment was excellent. Furthermore, the hydrogen embrittlement susceptibility of the bolts manufactured using the steel material was sufficiently low.

[0172] In test number 23, the Mo content was too high, which resulted in poor cold forgeability after spheroidizing treatment.

[0173] In test number 24, the V content was too low, which resulted in the bolts made from the steel being highly susceptible to hydrogen embrittlement.

[0174] In test number 25, the V content was too high, and therefore the cold forgeability after spheroidizing treatment was poor.

[0175] In test numbers 26 and 27, although the chemical composition was appropriate, the cast materials were reheated and then hot worked. As a result, the cementite number density ND was 4.0 particles / μm. 2 The cementite number ratio NR is less than 50.0%, and the sample standard deviation σ of the cementite area is 0.070 μm 2 As a result, bolts made from steel were highly susceptible to hydrogen embrittlement.

[0176] In test number 28, although the chemical composition was appropriate, the heating temperature HT in the finish rolling process was too low, and the finish rolling temperature FT was too low. As a result, the cementite number density ND was 4.0 particles / μm. 2 The cementite number ratio NR is less than 50.0%, and the sample standard deviation σ of the cementite area is 0.070 μm 2 As a result, bolts made from steel were highly susceptible to hydrogen embrittlement.

[0177] In test numbers 29 and 30, although the chemical composition was appropriate, the average cooling rate CR1 was too fast. As a result, the cementite number density ND was 4.0 particles / μm 2 The cementite number ratio NR is less than 50.0%, and the sample standard deviation σ of the cementite area is 0.070 μm 2 As a result, bolts made from steel were highly susceptible to hydrogen embrittlement.

[0178] In test numbers 31 and 32, although the chemical composition was appropriate, the average cooling rate CR2 was too slow. As a result, the cementite number density ND was 4.0 particles / μm 2 The cementite number ratio NR is less than 50.0%, and the sample standard deviation σ of the cementite area is 0.070 μm 2 As a result, bolts made from steel were highly susceptible to hydrogen embrittlement.

[0179] In test numbers 33 and 34, although the chemical composition was appropriate, the average cooling rate CR2 was too fast. As a result, although the area ratio of the hard phase was 90% or more, the Vickers hardness exceeded 400 HV. As a result, the wiredrawability was poor.

[0180] In test number 35, the C content was too low, so the cementite number density ND was 4.0 particles / μm 2 As a result, bolts made from steel were highly susceptible to hydrogen embrittlement.

[0181] 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 steel material, 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: 1.50-2.50%, V: 0.01-0.30%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0300%; The balance is Fe and impurities. The area ratio of the hard phase consisting of bainite and / or martensite is 90% or more, and the Vickers hardness is 220 to 400 HV, 0.0005 μm in the hard phase ２ The number density of cementite having an area of 4.0 pieces / μm ２ or more, and among the plurality of cementites in the hard phase, 0.0005 to 0.0100 μm ２ The number ratio of the cementite having an area of 50.0% or more, and the sample standard deviation of the area of the plurality of cementite in the hard phase is 0.070 μm ２ Below is the Steel material.

2. The steel material according to claim 1, further comprising: Instead of a part of Fe, Cu: 0.40% or less, Ni: 0.40% or less, B: 0.0100% or less, Zr: 0.300% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Bi: 0.100% or less, and Te: 0.100% or less, Contains one or more elements selected from the group consisting of Steel material.

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