Steel material used as the material of the bolt

Abstract

To provide a steel material capable of preventing invasion of hydrogen.SOLUTION: There is provided a steel material which comprises, by mass%, 0.18 to 0.28% of C, 0.02 to 0.50% of Si, 0.30 to 1.20% of Mn, 0.020% or less of P, 0.020% or less of S, 0.35 to 0.60% of Cu, 0.30 to 0.60% of Ni, 0.0002 to 0.0050% of B, 0.005 to 0.100% of Ti, 0.20 to 0.50% of Mo, 0.005 to 0.060% of Al, 0.010 to 0.500% or less of Sn and 0.0100% or less of N, and the balance Fe with impurities, wherein a Cu segregation ratio σ is 0.050 or less in a rectangular observation area of 1,000 μm in a radial direction and 1,000 μm in a longitudinal direction from a surface of the steel material of cross sections including the longitudinal direction and the radial direction of the steel material.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to steel materials, and more particularly to steel materials used as materials for bolts.

Background Art

[0002] Bolts are used as fastening means for industrial machines, automobiles, bridges, buildings, etc. Among these applications, bridges, buildings, etc. may be built in coastal areas or cold regions. Coastal areas have a corrosive environment with a lot of salt. Also, in cold regions, snow melting salts and antifreeze agents may be used. Snow melting salts and antifreeze agents corrode the steel materials that make up the bolts. That is, cold regions are often corrosive environments.

[0003] In such a corrosive environment, hydrogen embrittlement is likely to occur. Therefore, bolts used in a corrosive environment are required to have excellent hydrogen embrittlement resistance characteristics.

[0004] Techniques related to improving corrosion resistance and hydrogen embrittlement resistance characteristics have been proposed in Japanese Patent Application Laid-Open No. 2008-274367 (Patent Document 1).

[0005] The steel material disclosed in Patent Document 1 contains, by mass%, C: 0.15 to 0.6%, Si: 0.05 to 0.5%, Mn and Cr: a total of 0.5 to 3.5%, P: 0.05% or less, S: 0.03% or less, Cu: less than 0.3%, Ni: less than 1%, O: 0.01% or less, and Sn: 0.05 to 0.50%, with the balance being composed of Fe and impurities, and has a composition with a Cu / Sn ratio of 1 or less. In this document, by containing Sn, the intrusion of hydrogen into the steel material is suppressed, and as a result, the hydrogen embrittlement resistance characteristics are enhanced.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] As disclosed in Patent Document 1, it is known that if the intrusion of hydrogen into a steel material can be suppressed, the hydrogen embrittlement resistance characteristics of the steel material will be enhanced. In order to enhance the hydrogen embrittlement resistance characteristics of the steel material, the intrusion of hydrogen into the steel material may be suppressed by means different from those of Patent Document 1.

[0008] An object of the present disclosure is to provide a steel material capable of suppressing the intrusion of hydrogen.

Means for Solving the Problems

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

[0010] A steel material, by mass%, C: 0.18 to 0.28%, Si: 0.02 to 0.50%, Mn: 0.30 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cu: 0.35 to 0.60%, Ni: 0.30 to 0.60%, B: 0.0002 to 0.0050%, Ti: 0.005 to 0.100%, Mo: 0.20 to 0.50%, Al: 0.005 to 0.060%, Sn: 0.010 to 0.500% or less, and, N: 0.0100% or less, containing, the balance being composed of Fe and impurities, Among the cross-sections including the longitudinal direction and the radial direction of the steel material, in a rectangular observation region of 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material, For 160,000 measurement regions divided into 400 in the radial direction and 400 in the longitudinal direction, surface analysis is performed by an electron beam microanalyzer, and the Cu content in mass% in each obtained measurement region is [Cu]MA as, the [Cu] of all the measurement regions MA the arithmetic mean value of which is defined as [Cu] AVE as, the ratio of the [Cu] of each measurement region MA to the [Cu] of which AVE is defined as [Cu] S as, When a plurality of the measurement regions arranged in a line in the longitudinal direction among the plurality of the measurement regions are defined as a measurement row, in each measurement row, the ratio of the total of the [Cu] of the plurality of the measurement regions constituting the measurement row to the total number of the plurality of the measurement regions constituting the measurement row S is defined as [Cu] L as, When the sample standard deviation of the [Cu] of all the measurement rows is defined as the Cu segregation degree σ, L the Cu segregation degree σ is 0.050 or less, Steel material.

Advantages of the Invention

Advantages of the Invention

[0011] The steel material according to the present disclosure can suppress the intrusion of hydrogen.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0013] The inventors of the present invention conducted investigations and studies on steel materials capable of suppressing the intrusion of hydrogen. As a result, the following findings were obtained.

[0014] First, the inventors examined steel materials capable of suppressing hydrogen intrusion from the perspective of chemical composition. As a result, the inventors considered that it is effective to contain Cu in the steel material. The intrusion of hydrogen into the steel material is a phenomenon caused by hydrogen generated on the surface of the steel material. Cu suppresses the corrosion of the steel material. By suppressing corrosion, the generation of hydrogen on the surface of the steel material can be suppressed. Therefore, Cu suppresses the intrusion of hydrogen into the steel material.

[0015] Based on the above findings, the chemical composition of the steel material capable of suppressing hydrogen intrusion was examined. As a result, the inventors considered that if the steel material has a chemical composition of, by mass, C: 0.18 to 0.28%, Si: 0.02 to 0.50%, Mn: 0.30 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cu: 0.35 to 0.60%, Ni: 0.30 to 0.60%, B: 0.0002 to 0.0050%, Ti: 0.005 to 0.100%, Mo: 0.20 to 0.50%, Al: 0.005 to 0.060%, Sn: 0.010 to 0.500% or less, and N: 0.0100% or less, Cr: 0 to 0.10%, V: 0 to 0.10%, Nb: 0 to 0.10%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, rare earth elements: 0 to 0.0200%, and the balance being Fe and impurities, there is a possibility that hydrogen intrusion can be sufficiently suppressed.

[0016] However, it was found that even the steel material having the above chemical composition may still not be able to sufficiently suppress hydrogen intrusion. Therefore, the inventors further investigated and examined the reasons why the hydrogen intrusion amount could not be sufficiently reduced.

[0017] Here, the inventors focused on the uniformity of the Cu concentration distribution, particularly in the surface layer of the steel material. Since hydrogen penetrates from the outside, the Cu concentration distribution in the surface layer of the steel material has an impact. Even if the content of each element in the chemical composition is within the above-mentioned range, if the Cu concentration distribution in the surface layer is non-uniform, hydrogen is likely to penetrate from the region with a low Cu concentration in the surface layer. Furthermore, if the Cu concentration distribution is non-uniform, the steel material surface will be corroded non-uniformly. In this case, irregularities will occur on the steel material surface, and the surface area where the corrosion reaction occurs will increase. Therefore, the corrosion reaction will increase, and the amount of hydrogen penetration cannot be sufficiently suppressed.

[0018] Based on the above considerations, the inventors further adjusted the relationship between the Cu concentration distribution in the surface layer and the amount of hydrogen penetration. As a result, the inventors found that if the Cu segregation degree σ in the surface layer, obtained by surface analysis using the electron beam microanalyzer described later, is 0.050 or less, the Cu concentration in the surface layer will be sufficiently uniform, and as a result, the penetration of hydrogen can be sufficiently suppressed.

[0019] Based on the above findings, the steel material according to this embodiment completed has the following configuration.

[0020] [1] A steel material, by mass, C: 0.18 to 0.28%, Si: 0.02 to 0.50%, Mn: 0.30 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cu: 0.35 to 0.60%, Ni: 0.30 to 0.60%, B: 0.0002 to 0.0050%, Ti: 0.005 to 0.100%, Mo: 0.20 to 0.50%, Al: 0.005 to 0.060%, Sn: 0.010 to 0.500% or less, and, N: 0.0100% or less, containing the balance being Fe and impurities, In a cross-section including the longitudinal direction and the radial direction of the steel material, in a rectangular observation region that is 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material, For 160,000 measurement regions divided into 400 parts in the radial direction and 400 parts in the longitudinal direction, surface analysis is performed using an electron beam microanalyzer, and the Cu content in mass% in each obtained measurement region is designated as [Cu]. MA Let the arithmetic mean value of the [Cu] of all the measurement regions be designated as [Cu]. MA Let AVE the ratio of the [Cu] of each measurement region to the [Cu] be designated as [Cu]. When, among a plurality of the measurement regions, a plurality of the measurement regions arranged in a row in the longitudinal direction are defined as a measurement row, in each measurement row, the ratio of the total of the [Cu] of the plurality of measurement regions constituting the measurement row to the total number of the plurality of measurement regions constituting the measurement row is designated as [Cu]. MA Let AVE the ratio of the [Cu] of each measurement region to the [Cu] be designated as [Cu]. S Let When, among a plurality of the measurement regions, a plurality of the measurement regions arranged in a row in the longitudinal direction are defined as a measurement row, in each measurement row, the ratio of the total of the [Cu] of the plurality of measurement regions constituting the measurement row to the total number of the plurality of measurement regions constituting the measurement row is designated as [Cu]. S Let L the ratio of the total of the [Cu] of the plurality of measurement regions constituting the measurement row to the total number of the plurality of measurement regions constituting the measurement row be designated as [Cu]. When the sample standard deviation of the [Cu] of all the measurement rows is defined as the Cu segregation degree σ, L the Cu segregation degree σ is 0.050 or less. Steel material. Steel material.

[0021] [2] The steel material according to [1], further containing Cr: 0.10% or less, V: 0.10% or less, Nb: 0.10% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and rare earth elements: 0.0200% or less, containing one or more elements selected from the group consisting of Steel material.

[0022] Hereinafter, the steel material according to the present embodiment will be described in detail. Unless otherwise specified, "%" regarding an element means mass %.

[0023] [Features of the Steel Material of the Present Embodiment] The steel material of the present embodiment includes the following features. (Feature 1) The content of each element in the chemical composition is as shown in the present embodiment. (Feature 2) The Cu segregation degree σ on the surface layer of the steel material obtained by performing surface analysis using the electron beam microanalyzer described later is 0.050 or less. Hereinafter, each feature will be described.

[0024] [(Feature 1) Regarding the Chemical Composition] The chemical composition of the steel material according to the present embodiment contains the following elements.

[0025] C: 0.18 to 0.28% Carbon (C) enhances the hardenability of the steel material and increases the strength of the bolts manufactured using the steel material as a raw material. If the C content is less than 0.18%, even if the contents of other elements are within the range of the present embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.28%, even if the contents of other elements are within the range of the present embodiment, the cold forging property of the steel material deteriorates. Therefore, the C content is 0.18 to 0.28%. The preferable lower limit of the C content is 0.19%, more preferably 0.20%, and even more preferably 0.21%. The preferable upper limit of the C content is 0.27%, more preferably 0.26%, and even more preferably 0.25%.

[0026] Si: 0.02 to 0.50% Silicon (Si) increases the strength of the bolts manufactured using the steel material as a raw material by solid solution strengthening. If the Si content is less than 0.02%, even if the contents of other elements are within the range of the present embodiment, the above effects cannot be sufficiently obtained. On the one hand, if the Si content exceeds 0.50%, even if the contents of other elements are within the scope of this embodiment, the cold forging property of the steel material will deteriorate. Therefore, the Si content is 0.02 - 0.50%. The preferable lower limit of the Si content is 0.04%, more preferably 0.06%, and even more preferably 0.08%. The preferable upper limit of the Si content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0027] Mn: 0.20 - 1.20% Manganese (Mn) enhances the hardenability of the steel material and increases the strength of the bolt. If the Mn content is less than 0.20%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the one hand, if the Mn content exceeds 1.20%, even if the contents of other elements are within the scope of this embodiment, the cold forging property of the steel material will deteriorate. Therefore, the Mn content is 0.20 - 1.20%. The preferable lower limit of the Mn content is 0.25%, more preferably 0.30%. The preferable upper limit of the Mn content is 1.00%, more preferably 0.90%, and even more preferably 0.80%.

[0028] P: 0.020% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content exceeds 0%. If the P content exceeds 0.020%, even if the contents of other elements are within the scope of this embodiment, P will segregate at the grain boundaries. As a result, the hydrogen embrittlement resistance property of the bolt will deteriorate. Therefore, the P content is 0.020% or less. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the P content is 0.015%, more preferably 0.012%, still more preferably 0.010%, still more preferably 0.008%, and still more preferably 0.007%.

[0029] S: 0.020% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content exceeds 0.020%, even if the contents of other elements are within the range of this embodiment, S segregates at the grain boundaries. As a result, the hydrogen embrittlement resistance of the bolt deteriorates. Therefore, the S content is 0.020% or less. It is preferable that the S content is as low as possible. However, an extreme reduction in the S content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.002%, and still more preferably 0.003%. The preferable upper limit of the S content is 0.012%, more preferably 0.010%, and still more preferably 0.008%.

[0030] Cu: 0.35 - 0.60% Copper (Cu) suppresses the corrosion of the steel material. Thereby, the generation of hydrogen on the surface of the steel material is suppressed. As a result, the intrusion of hydrogen into the steel material is suppressed. If the Cu content is less than 0.35%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Cu content exceeds 0.60%, the steel material becomes brittle. Therefore, even if the contents of other elements are within the range of this embodiment, the hot workability and cold forging property of the steel material deteriorate. Therefore, the Cu content is 0.35 - 0.60%. The preferable lower limit of the Cu content is 0.38%, more preferably 0.40%, and still more preferably 0.42%. The preferable upper limit of the Cu content is 0.58%, more preferably 0.56%, and still more preferably 0.54%.

[0031] Ni: 0.30 - 0.60% Nickel (Ni) enhances the hardenability of the steel material and increases the strength of the bolt. Ni further enhances the corrosion resistance of the bolt. Ni is further contained together with Cu to suppress the generation of defects during hot working of the steel material. As a result, in the steel material of this embodiment containing Cu, Ni enhances the hot workability of the steel material. If the Ni content is less than 0.30%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Ni content exceeds 0.60%, the hardenability of the steel material becomes excessively high. In this case, even if the contents of other elements are within the range of this embodiment, the cold forging property of the steel material deteriorates. Therefore, the Ni content is 0.30 - 0.60%. The preferable lower limit of the Ni content is 0.32%, more preferably 0.34%, and even more preferably 0.36%. The preferable upper limit of the Ni content is 0.58%, more preferably 0.56%, and even more preferably 0.54%.

[0032] B: 0.0002 - 0.0050% Boron (B) enhances the hardenability of the steel material and increases the strength of the bolt. If the B content is less than 0.0002%, the above effects cannot be sufficiently obtained. On the other hand, if the B content exceeds 0.0050%, even if the contents of other elements are within the range of this embodiment, coarse B nitrides are generated. Coarse B nitrides become the starting points of fracture. As a result, the cold forging property of the steel material deteriorates. Therefore, the B content is 0.0002 - 0.0050%. The preferable lower limit of the B content is 0.0010%, more preferably 0.0014%, and even more preferably 0.0016%. The preferable upper limit of the B content is 0.0040%, more preferably 0.0035%.

[0033] Ti: 0.005 - 0.100% Titanium (Ti) combines with N to form Ti nitride and inhibits B from combining with N. As a result, B can enhance the hardenability of the steel material. If the Ti content is less than 0.005%, the above effect cannot be obtained sufficiently. On the other hand, if the Ti content exceeds 0.100%, even if the contents of other elements are within the scope of this embodiment, excessive amounts of Ti precipitates such as carbides and carbonitrides are generated. In this case, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Ti content is 0.005 - 0.100%. The preferable lower limit of the Ti content is 0.010%. The preferable upper limit of the Ti content is 0.060%, more preferably 0.050%.

[0034] Mo: 0.20 - 0.50% Molybdenum (Mo) enhances the hardenability of the steel and increases the strength of the bolt. Mo further forms fine carbides to increase the strength of the bolt. If the Mo content is less than 0.20%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be obtained sufficiently. On the other hand, if the Mo content exceeds 0.50%, even if the contents of other elements are within the scope of this embodiment, the steel material becomes excessively hard. In this case, the cold forging property of the steel material deteriorates. Therefore, the Mo content is 0.20 - 0.50%. The preferable lower limit of the Mo content is 0.22%, more preferably 0.24%, and even more preferably 0.26%. The preferable upper limit of the Mo content is 0.48%, more preferably 0.46%, and even more preferably 0.44%.

[0035] Al: 0.005 - 0.060% Aluminum (Al) deoxidizes the steel. If the Al content is less than 0.005%, even if the contents of other elements are within the scope of this embodiment, the deoxidation of the steel becomes insufficient. In this case, coarse oxides are generated. Therefore, the hydrogen embrittlement resistance characteristics of the bolt deteriorate. On the one hand, if the Al content exceeds 0.060%, even if the contents of other elements are within the scope of this embodiment, coarse Al nitrides will be formed. The coarse Al nitrides serve as the starting points of fracture. Therefore, the workability of the steel material deteriorates. Therefore, the Al content is 0.005 - 0.060%. The preferable lower limit of the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. The preferable upper limit of the Al content is 0.050%, more preferably 0.045%, and even more preferably 0.040%. In the chemical composition of the steel material of this embodiment, the Al content means the total Al (Total - Al) content.

[0036] Sn: 0.010 - 0.500% or less Tin (Sn) suppresses the intrusion of hydrogen into the steel material. If the Sn content is less than 0.010%, even if the contents of other elements are within the scope of this embodiment, the above - mentioned effect cannot be fully obtained. On the other hand, if the Sn content exceeds 0.500%, even if the contents of other elements are within the scope of this embodiment, Sn segregates at the grain boundaries. In this case, the hot workability and cold forging property of the steel material deteriorate. Furthermore, the hydrogen embrittlement resistance property of the bolt deteriorates. Therefore, the Sn content is 0.010 - 0.500%. The preferable lower limit of the Sn content is 0.020%, more preferably 0.040%, and even more preferably 0.060%. The preferable upper limit of the Sn content is 0.400%, more preferably 0.300%, even more preferably 0.250%, and even more preferably 0.200%.

[0037] N: 0.0100% or less Nitrogen (N) is inevitably contained. That is, the N content is more than 0%. N combines with Al or Ti to form nitrides or carbonitrides. These nitrides and carbonitrides suppress the coarsening of crystal grains due to the pinning effect. As a result, the cold forging property of the steel material is enhanced. However, if the N content exceeds 0.0100%, even if the contents of other elements are within the range of this embodiment, coarse nitrides will be formed. The coarse nitrides serve as initiation points for fracture and deteriorate the cold forging property of the steel material. Furthermore, the hydrogen embrittlement resistance property of the bolt deteriorates. Therefore, the N content is 0.0100% or less. The preferable lower limit of the N content is 0.0001%, more preferably 0.0002%, still more preferably 0.0005%, and yet more preferably 0.0010%. The preferable upper limit of the N content is 0.0080%, more preferably 0.0070%, still more preferably 0.0060%, yet more preferably 0.0050%, and even more preferably 0.0040%.

[0038] The balance of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition mean those mixed from ores, scraps, or manufacturing environments as raw materials during the industrial production of the steel material, and are permitted within a range that does not adversely affect the steel material according to this embodiment.

[0039] [Optional Elements] The steel material of this embodiment may further contain, instead of a part of Fe, Cr: 0.10% or less, V: 0.10% or less, Nb: 0.10% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and, rare earth elements: 0.0200% or less, and may contain one or more elements selected from the group consisting of. All of these elements are optional elements and may not be contained. Hereinafter, these optional elements will be described.

[0040] [Group 1: Cr] Cr: 0.10% or less Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, Cr enhances the corrosion resistance of the steel material. Even if a small amount of Cr is contained, the above effect can be obtained to some extent. However, if the Cr content is too high, the manufacturing cost will increase. Therefore, the Cr content is 0 to 0.10%, and when contained, it is 0.10% or less (more than 0 to 0.10%). The preferable lower limit of the Cr content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable upper limit of the Cr content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0041] [Group 2: V and Nb] The steel material of this embodiment may contain one or more elements selected from the group consisting of Mo, V, and Nb in place of a part of Fe. These elements are optional elements, and all of them enhance the strength of the steel material.

[0042] V: 0.10% or less Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, that is, when the V content is more than 0%, V forms V precipitates such as carbides and carbonitrides. The V precipitates enhance the strength of the bolt. Even if a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.10%, even if the contents of other elements are within the range of this embodiment, a large amount of V precipitates will be generated. In this case, the amount of hydrogen intrusion into the steel material increases. As a result, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the V content is 0 to 0.10%, and when contained, it is 0.10% or less (more than 0 to 0.10%). The preferable lower limit of the V content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable upper limit of the V content is 0.08%, more preferably 0.06%, and even more preferably 0.04%.

[0043] Nb: 0.10% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as carbides and carbonitrides. The Nb precipitates increase the strength of the bolt. Even if a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.10%, even if the contents of other elements are within the range of this embodiment, a large amount of Nb precipitates are generated. In this case, the amount of hydrogen intrusion into the steel material increases. As a result, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Nb content is 0 to 0.10%, and when contained, it is 0.10% or less (more than 0 to 0.10%). The preferable lower limit of the Nb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable upper limit of the Nb content is 0.08%, more preferably 0.06%. The preferable upper limit of the Nb content for more effectively reducing the hydrogen intrusion amount is less than 0.04%.

[0044] [Group 3: Ca, Mg and rare earth elements] The steel material of this embodiment may contain one or more elements selected from the group consisting of Ca, Mg and rare earth elements (REM) instead of a part of Fe. These elements are optional elements, and all of them refine MnS in the steel material and enhance the hydrogen embrittlement resistance characteristics of the steel material.

[0045] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When Ca is contained, that is, when Ca is more than 0%, Ca refines MnS. Therefore, the hydrogen embrittlement resistance property of the steel material is enhanced. Even if a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, even if the contents of other elements are within the range of this embodiment, coarse Ca oxides are formed. In this case, the hydrogen embrittlement resistance property of the steel material deteriorates. Therefore, the Ca content is 0 to 0.0050%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0005%. The preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%.

[0046] Mg: 0 to 0.0050% 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 Mg is more than 0%, Mg refines MnS. Therefore, the hydrogen embrittlement resistance property of the steel material is enhanced. Even if a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, even if the contents of other elements are within the range of this embodiment, coarse Mg oxides are formed. In this case, the hydrogen embrittlement resistance property of the steel material deteriorates. Therefore, the Mg content is 0 to 0.0050%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0005%. The preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%.

[0047] Rare earth element (REM): 0 to 0.0200% The rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When REM is contained, that is, when REM is more than 0%, REM refines MnS. Therefore, the hydrogen embrittlement resistance property of the steel material is enhanced. Even if a small amount of REM is contained, the above effect can be obtained to a certain extent. However, if the REM content exceeds 0.0200%, even if the contents of other elements are within the range of this embodiment, coarse oxides are formed. In this case, the hydrogen embrittlement resistance property of the steel material deteriorates. Therefore, the REM content is 0 to 0.0200%. The preferable lower limit of the REM content is 0.0001%, more preferably 0.0005%, still more preferably 0.0010%, still more preferably 0.0020%, and still more preferably 0.0050%. The preferable upper limit of the REM content is 0.0150%, more preferably 0.0100%.

[0048] In this specification, REM is one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 which are lanthanoids. The REM content in this specification is the total content of these elements.

[0049] [Method for Measuring Chemical Composition of Steel Material] The chemical composition of the steel material of this embodiment can be measured by a well-known component analysis method conforming to JIS G0321:2017. Specifically, using a drill, chips are collected from inside the steel material with a depth of 1 mm or more from the surface. The collected chips are dissolved in an acid to obtain a solution. For the solution, ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed to conduct elemental analysis of the chemical composition. The C content and the S content are determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas fusion - thermal conductivity method.

[0050] In addition, for each element content, based on the significant figures defined in this embodiment, the fractional part of the measured value is rounded off to obtain a value up to the smallest digit of each element content defined in this embodiment. For example, the C content of the steel material in this embodiment is defined by a value up to the second decimal place. Therefore, the C content is a value up to the second decimal place obtained by rounding off the third decimal place of the measured value.

[0051] Similarly, for the content of elements other than the C content of the steel material in this embodiment, the value obtained by rounding off the fractional part of the measured value to the smallest digit defined in this embodiment is taken as the content of the corresponding element.

[0052] Note that rounding off means discarding if the fractional part is less than 5 and rounding up if the fractional part is 5 or more.

[0053] [(Feature 2) Regarding the Cu segregation degree σ in the surface layer part of the steel material] In the steel material of this embodiment, further, in a rectangular observation region of 1000 μm in the radial direction (depth direction) from the surface of the steel material and 1000 μm in the longitudinal direction, among the cross-sections including the longitudinal direction and the radial direction of the steel material, for 160,000 measurement regions divided into 400 in the radial direction and 400 in the longitudinal direction, surface analysis is performed by an electron beam microanalyzer, and the Cu content in mass% in each obtained measurement region is designated as [Cu]. MA The arithmetic mean value of [Cu] for all the measurement regions is designated as [Cu]. MA The ratio of [Cu] of each measurement region to [Cu] is designated as [Cu]. AVE When a plurality of measurement regions arranged in a row in the longitudinal direction among the plurality of measurement regions are defined as a measurement row, in each measurement row, the ratio of the sum of [Cu] of the plurality of measurement regions constituting the measurement row to the total number of the plurality of measurement regions constituting the measurement row is designated as [Cu]. MA The [Cu] of each measurement region AVE to [Cu] S is designated as [Cu]. S When the specimen standard deviation of [Cu] for all the measurement rows is defined as the Cu segregation degree σ, the Cu segregation degree σ is 0.050 or less. L The [Cu] for all the measurement rows L is 0.050 or less. Hereinafter, the Cu segregation degree σ will be described.

[0054] [Regarding the Cu segregation degree σ] FIG. 1 is a schematic diagram for explaining the Cu segregation degree σ. Referring to FIG. 1, in a cross-section 10 including the longitudinal direction L and the radial direction D of the steel material, a rectangular observation region 100 with a size of 1000 μm in the radial direction D and 1000 μm in the longitudinal direction L from the surface 10S of the steel material is selected.

[0055] An enlarged view of the rectangular observation region 100 is shown in FIG. 1. In the microscopic region (rectangular observation region 100) of the steel material surface layer, when a Cu segregation region SEG exists, the Cu segregation region SEG extends in the longitudinal direction L of the steel material. Here, the Cu segregation region SEG is a region with a higher Cu concentration compared to other regions outside the Cu segregation region SEG.

[0056] Among the cross-section 10, the region from the surface 10S to a depth of 1000 μm in the radial direction D is referred to as the surface layer 10L. The rectangular observation region 100 is an arbitrary part of the surface layer 10L. When one or more Cu segregation regions SEG exist in the rectangular observation region 100, there is a variation in the Cu concentration within the rectangular observation region 100. In this case, it means that there is a variation in the Cu concentration in the surface layer 10L. When such a variation in the Cu concentration occurs within the surface layer 10L, hydrogen from the outside is likely to penetrate into the region with a lower Cu concentration in the surface layer 10L. Therefore, it is effective for suppressing hydrogen penetration that the Cu concentration distribution in the surface layer 10L is as uniform as possible.

[0057] Therefore, in the steel material of this embodiment, as an index indicating the uniformity of the Cu concentration distribution in the surface layer 10L of the steel material, the "Cu segregation degree σ" obtained by the following measurement method is used.

[0058] [Measurement method of Cu segregation degree σ] The Cu segregation degree σ of the steel material of this embodiment can be measured by the following method. Referring to FIG. 1, within the surface layer 10L, an arbitrary rectangular observation region 100 is selected. As described above, the rectangular observation region 100 is a rectangle with a size of 1000 μm in the radial direction D and 1000 μm in the longitudinal direction L from the surface 10S of the steel material.

[0059] FIG. 2 is a schematic diagram for explaining a method of measuring the Cu segregation degree σ within the rectangular observation region 100 in FIG. 1. Referring to FIG. 2, surface analysis using a field emission type electron beam microanalyzer (FE-EPMA) is performed on the rectangular observation region 100. Specifically, the rectangular observation region 100 is divided into 400 parts in the longitudinal direction L and 400 parts in the radial direction D, thereby dividing the rectangular observation region 100 into 160,000 measurement areas MA.

[0060] Elemental analysis is performed on each measurement area MA. In the elemental analysis, the acceleration voltage is 15 kV, the irradiation current is 400 nA, the beam diameter is 2 μm, and the integration time is 0.1 second. The element to be measured is Cu, and the Cu content in mass % in each measurement area MA is obtained, and this Cu content is defined as [Cu]. MA and defined as.

[0061] Using the obtained [Cu] in each measurement area MA. MA The Cu segregation degree σ in the rectangular observation region 100 is obtained by the following method.

[0062] The arithmetic mean value of the Cu content [Cu] in mass % in all measurement areas MA. MA is defined as [Cu]A. VE Furthermore, the ratio of the Cu content [Cu] in each measurement area MA to [Cu]. MA of [Cu]. AVE is defined as [Cu]. S In short, [Cu]. S means the amount of Cu in each measurement area excluding the influence of the Cu content.

[0063] Referring to FIG. 2, among the 400×400 measurement areas MA, the measurement areas arranged in a row in the longitudinal direction L are defined as "measurement rows" ML1 to ML400. Each measurement row MLj (j is an integer from 1 to 400) is composed of 400 measurement areas MA arranged in a row in the longitudinal direction L. In FIG. 2, a plurality of measurement areas MA in the region surrounded by the broken line constitute the measurement row ML1.

[0064] The [Cu] of the plurality of measurement areas MA constituting each measurement row MLj.S Define the ratio of the total of [[Cu]] to the total number of a plurality of measurement areas MA (i.e., 400) that make up the measurement row MLj. L as [Cu]. L [[Cu]] means the amount of Cu in the measurement row MLj.

[0065] [[Cu]] for all measurement rows MLj L Calculate the sample standard deviation. Define the obtained value as "Cu segregation degree σ". The significant figures of the Cu segregation degree σ are the third decimal place. That is, the Cu segregation degree σ is the value obtained by rounding the number in the fourth decimal place.

[0066] [Regarding the significance of the Cu segregation degree σ] As shown in the rectangular observation areas 100 of FIGS. 1 and 2, the Cu segregation area SEG extends in the longitudinal direction L of the steel material. Therefore, the sample standard deviation σ of the amount of Cu [[Cu]] in each measurement row MLj L is an index indicating the segregation degree of Cu.

[0067] If the Cu segregation degree σ exceeds 0.050, sufficient uniformity of the Cu concentration distribution is not obtained in the surface layer 10L. That is, the Cu concentration distribution varies excessively. In this case, even for the steel material having Feature 1, the intrusion of hydrogen cannot be sufficiently suppressed.

[0068] If the Cu segregation degree σ is 0.050 or less, the Cu concentration distribution in the surface layer 10L of the steel material having Feature 1 is sufficiently uniform. Therefore, the intrusion of hydrogen can be sufficiently suppressed.

[0069] The preferable upper limit of the Cu segregation degree σ is 0.045, more preferably 0.040, still more preferably 0.035, and even more preferably 0.030. It is preferable that the Cu segregation degree σ is as low as possible. The preferable lower limit of the Cu segregation degree σ is 0.000, more preferably 0.005, and still more preferably 0.010.

[0070] [The shape of the steel material of the present embodiment] The steel material of this embodiment is a bar or a wire rod. The bar or wire rod is a steel material extending in a bar shape. The steel material may be wound in a coil shape or may be cut to a predetermined length.

[0071] [Microstructure of the steel material of this embodiment] The microstructure of the steel material of this embodiment is not particularly limited. When the steel material of this embodiment is used as a material for bolts, if the hardness of the steel material is too high, a spheroidizing annealing treatment is carried out before the bolt manufacturing process (wire drawing process or cold forging process) is performed. The cold forging property of the spheroidized annealed steel material is enhanced. Therefore, it is possible to perform cold forging using the steel material of this embodiment as a material to manufacture bolts. Accordingly, the microstructure of the steel material of this embodiment is not particularly limited.

[0072] [Applications of the steel material of this embodiment] The steel material of this embodiment is applicable as a material for bolts, which are a type of fastening means for industrial machines, automobiles, bridges, buildings, etc. Note that the steel material of this embodiment may be used for applications other than the above.

[0073] [Manufacturing method of the steel material] An example of the manufacturing method of the steel material of this embodiment will be described. The manufacturing method of the steel material described hereinafter is an example for manufacturing the steel material of this embodiment. Accordingly, the steel material having the above-described features 1 and 2 may be manufactured by other manufacturing methods other than the manufacturing method described hereinafter. However, the manufacturing method described hereinafter is a preferred example of the manufacturing method of the steel material of this embodiment.

[0074] An example of the manufacturing method of the steel material of this embodiment includes the following steps. (Step 1) Material preparation step (Step 2) Rough rolling step (Step 3) Finish rolling step In this manufacturing method, further, in the finish rolling step, the following conditions are satisfied. Condition: The number of passes at a reduction rate of 25% or more is 2 or more within the range where the steel material temperature is 1000 to 880 °C. Hereinafter, each step will be described.

[0075] [(Process 1) Raw Material Preparation Process] In the raw material preparation process, the raw materials of the steel material of this embodiment are prepared. Specifically, molten steel with the content of each element in the chemical composition within the range of this embodiment is produced. The refining method is not particularly limited, and well-known methods can be used. For example, refining (primary refining) in a converter is carried out on the hot metal produced by a well-known method. Well-known secondary refining is carried out on the molten steel tapped from the converter. Through the above processes, molten steel with a chemical composition satisfying Feature 1 is produced.

[0076] Using the produced molten steel, raw materials are produced by a well-known casting method. For example, ingots may be produced by the ingot casting method using molten steel. Also, blooms may be produced by the continuous casting method using molten steel. Through the above methods, raw materials (ingots or blooms) are produced.

[0077] [(Process 2) Rough Rolling Process] In the rough rolling process, rough rolling is carried out on the raw materials (ingots or blooms) prepared in the raw material preparation process to produce billets.

[0078] The rough rolling process includes the following processes. (Process 21) Heating Process (Process 22) Rolling Process Hereinafter, each of Process 21 and Process 22 will be described.

[0079] [(Process 21) Heating Process] In the heating process, the raw materials are heated using a heating furnace by a well-known method. The heating temperature is not particularly limited. A well-known temperature is sufficient for the heating temperature. The heating temperature is, for example, 1000 - 1200 °C.

[0080] [(Process 22) Rolling Process] In the rolling process, the raw materials heated in the heating process are rolled (rough rolled) using a slab rolling mill, or a breakdown rolling mill and a continuous rolling mill to produce billets.

[0081] Specifically, a heated material is reversely rolled using a slab rolling mill to produce a billet. The slab rolling mill is equipped with a pair of horizontal rolls. In the slab rolling mill, reverse rolling is performed. Reverse rolling means a rolling method in which when the material passes through the slab rolling mill from upstream to downstream, it is subjected to rolling reduction from the breakdown mill, and further, when the material passes through the breakdown mill from downstream to upstream, it can also be subjected to rolling reduction from the breakdown mill.

[0082] When a continuous rolling mill is arranged downstream of the slab rolling mill, tandem rolling may be further performed on the billet after slab rolling using the continuous rolling mill to produce an even smaller-sized billet. In the continuous rolling mill, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. In the continuous rolling mill, tandem rolling is performed from upstream to downstream.

[0083] The billet produced by the above rough rolling process is air-cooled to room temperature before the finish rolling process.

[0084] [(Process 3) Finish Rolling Process] In the finish rolling process, finish rolling is performed on the billet produced in the rough rolling process to produce a steel material. Here, the steel material is a wire rod or a bar. The finish rolling process includes the following processes. (Process 31) Heating Process (Process 32) Rolling Process

[0085] In the finish rolling process, further, in the rolling process, the following conditions are satisfied. (Condition) The number of passes with a reduction rate of 25% or more is 2 or more within the range where the steel material temperature is 1000 to 880°C. Hereinafter, the heating process and the rolling process in the finish rolling process will be described.

[0086] [(Process 31) Heating Process] In the heating process, the billet cooled to room temperature is heated by a well-known method using a heating furnace. The heating temperature is not particularly limited, but for example, it is 900 to 1050°C.

[0087] [(Process 32) Rolling Process] In the rolling process, for the billet heated in the heating process, finish rolling (continuous rolling) is performed using a continuous rolling mill to manufacture steel materials. The continuous rolling mill includes a plurality of rolling stands arranged in a row from upstream to downstream. Each rolling stand includes a pair of work rolls. A caliber is formed on each roll, and a pass is formed by the calibers of the pair of rolls.

[0088] In continuous rolling using a continuous rolling mill, when the billet passes through each rolling stand from upstream to downstream, reducing the cross-sectional area of the billet at the rolling stand is defined as "one pass" rolling reduction.

[0089] Continuous rolling means reducing the cross-sectional area in a plurality of passes using a continuous rolling mill. Note that it is not necessary to reduce the cross-sectional area of the billet at all the rolling stands in the continuous rolling mill. For example, when the continuous rolling mill includes 15 rolling stands and the billet is passed through the last rolling stand without reducing its cross-sectional area, 14 passes of rolling reduction are performed.

[0090] [Regarding Conditions] In the finish rolling process, within the range of 1000 to 880 °C of the steel material temperature, the number of passes with a rolling reduction rate of 25% or more is set to 2 or more. Here, the rolling reduction rate is defined by the following formula. Rolling reduction rate = (1 - cross-sectional area perpendicular to the longitudinal direction of the billet on the outlet side of the rolling stand) / (cross-sectional area perpendicular to the longitudinal direction of the billet on the inlet side of the rolling stand) × 100

[0091] Generally, it is known to adjust the heating temperature in a heating furnace to reduce the segregation of elements such as Mn. However, in the case of Cu, the inventors' investigation has revealed that it is easier for Cu to uniformly diffuse on the surface layer of the steel material by introducing more strain through rolling reduction in a specific temperature range than by adjusting the heating temperature to make it easier to diffuse.

[0092] Specifically, when the number of passes with a reduction rate of 25% or more within the range of the steel material temperature from 1000 to 880 °C is less than 2, the amount of introduced strain is insufficient. In this case, in the finish rolling process, Cu does not diffuse sufficiently uniformly on the surface layer of the steel material. As a result, the Cu segregation degree σ exceeds 0.050.

[0093] If the number of passes with a reduction rate of 25% or more within the range of the steel material temperature from 1000 to 880 °C is 2 or more, strain is sufficiently introduced into the surface layer of the steel material in a temperature range suitable for the diffusion of Cu. In this case, Cu diffuses uniformly on the surface layer of the steel material. As a result, the Cu segregation degree σ becomes 0.050 or less.

[0094] Therefore, the number of passes is set to 2 or more with a reduction rate of 25% or more within the range of the steel material temperature from 1000 to 880 °C.

[0095] The preferable lower limit of the number of passes with a reduction rate of 25% or more within the range of the steel material temperature from 1000 to 880 °C is 3, and more preferably 4. The preferable upper limit of the number of passes with a reduction rate of 25% or more within the range of the steel material temperature from 1000 to 880 °C is not particularly limited.

[0096] Thermometers are arranged on the inlet side and / or the outlet side of each rolling stand of the continuous rolling mill. The thermometer is a well-known device, for example, a radiation thermometer, thermography, etc. The steel material temperature is measured on the inlet side and / or the outlet side at each rolling stand. Also, the reduction rate at each rolling stand is preset. Therefore, based on the steel material temperature measured on the inlet side and / or the outlet side at each rolling stand, and the reduction rate at each rolling stand, the number of passes with a reduction rate of 25% or more within the range of the steel material temperature from 1000 to 880 °C can be determined.

[0097] In the finish rolling process, the cooling method after continuous rolling is not particularly limited. The cooling method may be air cooling, slow cooling, or rapid cooling.

[0098] Through the above manufacturing process, a steel material satisfying Feature 1 and Feature 2 can be manufactured.

[0099] [Manufacturing Method of Bolt Made of Steel Material of This Embodiment] The manufacturing method of the bolt made of the steel material of this embodiment is a well-known manufacturing method. The manufacturing method of the bolt includes, for example, the following steps. ·Wire drawing process ·Cold forging process ·Quenching and tempering process Hereinafter, each process will be described.

[0100] [Wire drawing process] In the wire drawing process, a well-known wire drawing is performed on the above-mentioned steel material to manufacture a steel wire. The wire drawing may be only primary wire drawing, or multiple wire drawings such as secondary wire drawing may be performed.

[0101] [Cold forging process] In the cold forging process, a well-known cold forging is performed on the steel wire after the wire drawing process to manufacture an intermediate product in the shape of a bolt.

[0102] [Quenching and tempering process] In the quenching and tempering process, quenching and tempering are performed on the intermediate product.

[0103] [Quenching] Quenching is performed by a well-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. The intermediate product after the holding time has elapsed is rapidly cooled. Specifically, water cooling or oil cooling is performed on the intermediate product.

[0104] [Tempering] Tempering is performed on the intermediate product after quenching. The tempering temperature and the holding time at the tempering temperature are not particularly limited. The tempering temperature is, for example, 400 to 550 °C. The holding time at the tempering temperature is 30 to 360 minutes.

[0105] By the above manufacturing method, bolts made of the steel material of the present embodiment can be manufactured. The manufactured bolts have Feature 1 and Feature 2. Therefore, hydrogen intrusion in a corrosive environment is suppressed.

Example

[0106] The effects of the steel material of the present embodiment will be further specifically described by examples. The conditions in the following examples are one set of conditions adopted to confirm the feasibility and effects of the steel material of the present embodiment. Therefore, the steel material of the present embodiment is not limited to this one set of conditions.

[0107] [Material preparation process] Steel materials having the chemical compositions shown in Table 1-1 and Table 1-2 were manufactured by the following method.

[0108]

Table 1-1

[0109]

Table 1-2

[0110] In Table 1-1 and Table 1-2, "-" means that the corresponding element content is 0% in the significant figures (numerical values up to the minimum digit) specified in the embodiment. In other words, it means that when the fractional part in the significant figures (numerical values up to the minimum digit) specified in the above embodiment is rounded for the corresponding element content, it is 0%. For example, the Cr content specified in the present embodiment is defined with numerical values up to the second decimal place. Therefore, in Test No. 1 in Table 1-2, it means that when the measured Cr content was rounded to the third decimal place, it was 0%. Note that rounding means discarding if the digit below the specified minimum digit (fractional part) is less than 5, and rounding up if it is 5 or more.

[0111] [Rough rolling process] The bloom produced was subjected to a rough rolling process to produce billets. Specifically, the bloom was heated to 1100 °C using a heating furnace. The heated bloom was rolled (rough rolled) using a block rolling mill and a continuous rolling mill to produce billets. The billets produced in the rough rolling process were allowed to cool to room temperature.

[0112] [Finish rolling process] The produced billets were subjected to a finish rolling process. Specifically, the billets of each test number were heated to 950 - 1050 °C. The heated billets were subjected to finish rolling (continuous rolling) using a continuous rolling mill to produce steel materials (round bars). At this time, within the range where the steel material temperature was 1000 - 880 °C, the number of passes PN with a reduction ratio of 25% or more was as shown in Table 2.

[0113]

Table 2

[0114] The steel materials (round bars) after finish rolling were allowed to cool to room temperature. Through the above manufacturing process, steel materials (round bars) with a diameter of 10 mm for each test number were produced.

[0115] [Regarding the evaluation test] The following steel material evaluation tests (Test 1 - Test 3) were carried out on the produced steel materials of each test number. [Steel material evaluation test] (Test 1) Chemical composition measurement test of steel material (Test 2) Cu segregation degree σ measurement test (Test 3) Diffusible hydrogen concentration measurement test The following is an explanation of each test.

[0116] [(Test 1) Chemical composition measurement test of steel material] The chemical composition of the steel materials (round bars) of each test number was analyzed based on the above [Method for measuring the chemical composition of steel materials]. As a result, the chemical composition of each test number was as shown in Table 1 - 1 and Table 1 - 2.

[0117] [(Test 2) Cu Segregation Degree σ Measurement Test] For the steel materials (round bars) of each test number, the Cu segregation degree σ was determined based on the above [Measurement Method of Cu Segregation Degree σ]. The obtained results are shown in Table 2.

[0118] [(Test 3) Diffusible Hydrogen Concentration Measurement Test] For the steel materials of each test number, the following diffusible hydrogen concentration measurement test was carried out.

[0119] [Manufacture of Bolt Simulated Material] First, bolt simulated materials were manufactured using the round bars of each test number. Specifically, the following quenching treatment and tempering treatment were carried out on the round bars of each test number. The quenching treatment was carried out using a heat treatment furnace. The quenching temperature was set at 870 - 920 °C, and the holding time at the quenching temperature was 60 minutes. After the holding time elapsed, the steel material was water-cooled for quenching. In addition, the inside of the heat treatment furnace was set to a carbon potential atmosphere equivalent to the C concentration of the steel material to suppress decarburization of the steel material.

[0120] After the quenching treatment, a tempering treatment was carried out. The tempering treatment was carried out using a heat treatment furnace. In tempering, it was held for 1 - 2 hours at a tempering temperature (°C) of 430 - 520 °C so that the tensile strength of the bolt simulated material was within the range of 1200 - 1400 MPa. After the holding time elapsed, the steel material was water-cooled. Through the above manufacturing process, bolt simulated materials (round bars) were manufactured.

[0121] [Measurement Test] The bolt simulated materials (round bars) of each test number were cut perpendicular to the longitudinal direction, and a plurality of round bar test pieces with a length of 100 mm were collected. In order to exclude the influence of the scale generated during the rough rolling process, finish rolling process, and quenching and tempering during the manufacture of the bolt simulated material, the round bar test pieces were subjected to blasting treatment to remove the scale on the outermost surface of the round bar test pieces.

[0122] Using round bar test pieces after blasting, the Cyclic Artificial Acid rain Test specified in JIS H8502 (1999) was carried out. For each test number, the test cycles in the corrosion test were set to three patterns: 84 cycles, 168 cycles, and 256 cycles. Separate round bar test pieces were used for each pattern.

[0123] In the test for each cycle, after the test ended, the round bar test pieces were taken out. The taken-out round bar test pieces were subjected to blasting to remove the corrosion products generated on the surface of the round bar test pieces by the corrosion test. Using a wet cutting machine, a 30-mm central portion in the longitudinal direction of the round bar test pieces after blasting was cut out.

[0124] For the cut-out test piece portions, the diffusible hydrogen concentration was analyzed using a gas chromatograph type temperature-programmed desorption hydrogen analyzer (TDA). Specifically, the cut-out test piece portions were heated from room temperature to 400 °C at a heating rate of 100 °C / h. The amount of hydrogen released from the test piece portions to the outside due to heating was measured.

[0125] The measured amount of hydrogen was divided by the mass of the test piece portions before heating to obtain the diffusible hydrogen concentration (ppm). For each test number, the diffusible hydrogen concentration in each of the above three patterns was obtained. Among the three diffusible hydrogen concentrations, the highest value was defined as the diffusible hydrogen concentration for that test number.

[0126] [Evaluation Results] The evaluation results are shown in Table 2. For test numbers 1 to 36, the chemical composition was appropriate. Furthermore, the manufacturing conditions were also appropriate. Therefore, the Cu segregation degree was 0.050 or less. As a result, the diffusible hydrogen concentration was less than 0.05 ppm, and the intrusion of hydrogen could be suppressed.

[0127] On the other hand, for test number 37, the Cu content was too low. Therefore, the diffusible hydrogen concentration became 0.05 ppm or more, and the intrusion of hydrogen could not be sufficiently suppressed.

[0128] In Test No. 38, the Ni content was too low. Therefore, the diffusible hydrogen concentration became 0.05 ppm or more, and the intrusion of hydrogen could not be sufficiently suppressed.

[0129] In Test Nos. 39 to 42, although the chemical composition was appropriate, within the range of the steel material temperature from 1000 to 880 °C, the number of passes PN with a reduction rate of area of 25% or more was less than 2. Therefore, the Cu segregation degree σ exceeded 0.050. As a result, the diffusible hydrogen concentration became 0.05 ppm or more, and the intrusion of hydrogen could not be sufficiently suppressed.

[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 the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. A steel material, by mass%, C: 0.18 to 0.28%, Si: 0.02 to 0.50%, Mn: 0.30 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cu: 0.35 to 0.60%, Ni: 0.30 to 0.60%, B: 0.0002 to 0.0050%, Ti: 0.005 to 0.100%, Mo: 0.20 to 0.50%, Al: 0.005 to 0.060%, Sn: 0.010 to 0.500% or less, and, N: 0.0100% or less, contains, the balance consisting of Fe and impurities, in a cross-section including the longitudinal direction and the radial direction of the steel material, in a rectangular observation region of 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material, For 160,000 measurement regions divided into 400 in the radial direction and 400 in the longitudinal direction, surface analysis is performed using an electron beam microanalyzer, and the Cu content in mass % in each obtained measurement region is designated as [Cu]. ＭＡ and The [Cu] of all the said measurement regions ＭＡ is taken as the arithmetic mean value of [Cu] ＡＶＥ and is designated as [Cu]. The [Cu] in each measurement region ＭＡ The [Cu] of ＡＶＥ The ratio to [Cu] Ｓ is defined as [Cu], When a plurality of the measurement regions arranged in a row in the longitudinal direction among the plurality of the measurement regions are defined as a measurement row, in each measurement row, the [Cu] of the plurality of the measurement regions constituting the measurement row Ｓ of the total of the plurality of the measurement regions constituting the measurement row is defined as the ratio to the total number of the plurality of the measurement regions constituting the measurement row, and is denoted as [Cu]. Ｌ And The [Cu] of all the said measurement rows Ｌ When the sample standard deviation of is taken as the Cu segregation degree σ, the Cu segregation degree σ is 0.050 or less, steel material.

2. The steel material according to Claim 1, further containing, in place of a part of Fe, Cr: 0.10% or less, V: 0.10% or less, Nb: 0.10% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and, rare earth elements: 0.0200% or less, containing one or more elements selected from the group consisting of, steel material.

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