steel

Abstract

To provide a steel capable of suppressing hydrogen infiltration.SOLUTION: A steel disclosed herein comprises, in mass%, C: 0.30-0.50% (excluding 0.50%), Si: 0.10-0.30%, Mn: 0.10-0.80%, P: 0.020% or less, S: 0.020% or less, Cr: 0.90-1.80%, Mo: 0.10-1.00%, Cu: 0.04-0.35% (excluding 0.35%), Ni: 0.04-0.30%, Al: 0.005-0.060%, N: 0.0200% or less, O: 0.0030% or less, with a balance being Fe and impurities. In a cross-section encompassing both longitudinal and radial directions of the steel, within a rectangular observation area extending 1000 μm from the steel's surface radially and 1000 μm longitudinally, a Cu segregation degree σ is equal to or lower than 0.050.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a steel material, and more particularly to a steel material that can be used as a material for bolts. [Background technology]

[0002] Bolts are used as fastening means for industrial machinery, automobiles, bridges, buildings, etc. Among these applications, bridges and buildings may be built in coastal areas or cold regions. Coastal areas are corrosive environments with high salt content. In cold regions, snow-melting salts and antifreeze agents may be used. Snow-melting salts and antifreeze agents corrode the steel that makes up the bolts. In other words, cold regions are often corrosive environments as well.

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

[0004] A technique for improving corrosion resistance and hydrogen embrittlement resistance is proposed in Japanese Patent Laid-Open Publication No. 2008-274367 (Patent Document 1).

[0005] The steel material disclosed in Patent Document 1 contains, in mass%, 0.15 to 0.6% C, 0.05 to 0.5% Si, 0.5 to 3.5% Mn and Cr in total, 0.05% or less P, 0.03% or less S, less than 0.3% Cu, less than 1% Ni, 0.01% or less O, and 0.05 to 0.50% Sn, with the balance being Fe and impurities, and has a composition in which the Cu / Sn ratio is 1 or less. In this document, the inclusion of Sn inhibits hydrogen penetration into the steel material, thereby improving hydrogen embrittlement resistance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-274367 Summary of the Invention [Problem to be solved by the invention]

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

[0008] An object of the present disclosure is to provide a steel material capable of suppressing hydrogen penetration. [Means for solving the problem]

[0009] The steel material according to the present disclosure is a steel material, The chemical composition is, in mass%, C: 0.30 to less than 0.50% Si: 0.10 to 0.30% Mn: 0.10 to 0.80% P: 0.020% or less, S: 0.020% or less, Cr: 0.90~1.80%, Mo: 0.10 to 1.00%, Cu: 0.04 to less than 0.35% Ni: 0.04 to 0.30% Al: 0.005 to 0.060%, N: 0.0200% or less, and O: 0.0030% or less, the balance being Fe and impurities; In a rectangular observation area of ​​the cross section including the longitudinal direction and radial direction of the steel material, the area is 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material. The 160,000 measurement areas, which were divided into 400 in the radial direction and 400 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% in each of the obtained measurement areas was calculated as [Cu] MA year, The [Cu] in all the measurement areas MA The arithmetic mean value of [Cu] AVE year, The [Cu] in each measurement area MA The above [Cu] AVE The ratio to [Cu] S year, When a plurality of measurement areas arranged in a row in the longitudinal direction among the plurality of measurement areas is defined as a measurement row, in each measurement row, the [Cu] of the plurality of measurement areas constituting the measurement row is S The ratio of the total number of the measurement areas constituting the measurement row to the total number of the measurement areas is defined as [Cu] L year, The [Cu] for all the measurement rows L When the sample standard deviation is the Cu segregation degree σ, The Cu segregation degree σ is 0.050 or less.

[0010] The steel material according to the present disclosure is a steel material, The chemical composition is, in mass%, C: 0.30 to less than 0.50% Si: 0.10 to 0.30% Mn: 0.10 to 0.80% P: 0.020% or less, S: 0.020% or less, Cr: 0.90~1.80%, Mo: 0.10 to 1.00%, Cu: 0.04 to less than 0.35% Ni: 0.04 to 0.30% Al: 0.005 to 0.060%, N: 0.0200% or less, and O: 0.0030% or less, the chemical composition further contains one or more elements selected from the group consisting of Group 1 and Group 2, with the remainder consisting of Fe and impurities; In a rectangular observation area of ​​the cross section including the longitudinal direction and radial direction of the steel material, the area is 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material. The 160,000 measurement areas, which were divided into 400 in the radial direction and 400 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% in each of the obtained measurement areas was calculated as [Cu]MA year, The [Cu] in all the measurement areas MA The arithmetic mean value of [Cu] AVE year, The [Cu] in each measurement area MA The above [Cu] AVE The ratio to [Cu] S year, When a plurality of measurement areas arranged in a row in the longitudinal direction among the plurality of measurement areas is defined as a measurement row, in each measurement row, the [Cu] of the plurality of measurement areas constituting the measurement row is S The ratio of the total number of the measurement areas constituting the measurement row to the total number of the measurement areas is defined as [Cu] L year, The [Cu] for all the measurement rows L When the sample standard deviation is the Cu segregation degree σ, The Cu segregation degree σ is 0.050 or less. [Group 1] V: 0.50% or less, Nb: 0.10% or less, Ti: 0.100% or less, and B: less than 0.0010% [Group 2] Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and Rare earth elements: 0.0200% or less, one or more selected from the group consisting of [Effects of the Invention]

[0011] The steel material according to the present disclosure can suppress the penetration of hydrogen. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram for explaining the Cu segregation degree σ. [Figure 2] FIG. 2 is a schematic diagram for explaining a method for measuring the Cu segregation degree σ within the rectangular observation region 100 in FIG. [Figure 3] FIG. 3 is a diagram showing the cycle pattern of one cycle in the combined cyclic corrosion test of the example. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors have investigated and studied steel materials capable of suppressing hydrogen penetration, and as a result have obtained the following findings.

[0014] First, the inventors investigated steel materials capable of suppressing hydrogen penetration from the viewpoint of chemical composition. As a result, the inventors considered that it would be effective for the steel material to contain Cu. Hydrogen penetration into steel materials is a phenomenon caused by hydrogen generated on the steel material surface. Cu suppresses corrosion of steel materials. By suppressing corrosion, it is possible to suppress the generation of hydrogen on the steel material surface. Therefore, Cu suppresses hydrogen penetration into steel materials.

[0015] Based on the above findings, the inventors investigated the chemical composition of a steel material capable of suppressing hydrogen penetration. As a result, they concluded that a steel material containing, by mass%, C: 0.30 to less than 0.50%, Si: 0.10 to 0.30%, Mn: 0.10 to 0.80%, P: 0.020% or less, S: 0.020% or less, Cr: 0.90 to 1.80%, Mo: 0.10 to 1.00%, Cu: 0.04 to less than 0.35%, Ni: 0.04 to 0.30%, Al: 0.005 to 0.060%, N: 0.0200% or less, and O: 0.0030% or less, and if any optional element is contained, further containing one or more elements selected from the group consisting of the above-mentioned first and second groups in place of a portion of Fe, with the balance consisting of Fe and impurities, may be able to sufficiently suppress hydrogen penetration.

[0016] However, it has been found that even in steel materials having the above-mentioned chemical composition, there are still cases where hydrogen penetration cannot be sufficiently suppressed. Therefore, the present inventors have further investigated and considered the reason why the amount of hydrogen penetration cannot be sufficiently reduced.

[0017] Here, the inventors focused on the uniformity of Cu concentration distribution in the steel material, particularly in the surface layer. Because hydrogen penetrates from the outside, the Cu concentration distribution in the surface layer of the steel material has an effect. 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 uneven, hydrogen will easily penetrate from areas of the surface layer with low Cu concentration. Furthermore, if the Cu concentration distribution is uneven, the steel material surface will corrode unevenly. In this case, unevenness will occur on the steel material surface, increasing the surface area where the corrosion reaction occurs. As a result, the corrosion reaction will increase, and it will become impossible to sufficiently suppress the amount of hydrogen penetration.

[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 area analysis using an electron probe microanalyzer described below, is 0.050 or less, the Cu concentration in the surface layer becomes sufficiently uniform, and as a result, hydrogen penetration can be sufficiently suppressed.

[0019] The steel material according to this embodiment, which has been completed based on the above findings, has the following configuration.

[0020] [1] A steel material, The chemical composition is, in mass%, C: 0.30 to less than 0.50% Si: 0.10 to 0.30% Mn: 0.10 to 0.80% P: 0.020% or less, S: 0.020% or less, Cr: 0.90~1.80%, Mo: 0.10 to 1.00%, Cu: 0.04 to less than 0.35% Ni: 0.04 to 0.30% Al: 0.005 to 0.060%, N: 0.0200% or less, and O: 0.0030% or less, and the balance being Fe and impurities, In a rectangular observation area of ​​the cross section including the longitudinal direction and radial direction of the steel material, the area is 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material. The 160,000 measurement areas, which were divided into 400 in the radial direction and 400 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% in each of the obtained measurement areas was calculated as [Cu] MA year, The [Cu] in all the measurement areas MA The arithmetic mean value of [Cu] AVE year, The [Cu] in each measurement area MA The above [Cu] AVE The ratio to [Cu] S year, When a plurality of measurement areas arranged in a row in the longitudinal direction among the plurality of measurement areas is defined as a measurement row, in each measurement row, the [Cu] of the plurality of measurement areas constituting the measurement row is S The ratio of the total number of the measurement areas constituting the measurement row to the total number of the measurement areas is defined as [Cu] L year, The [Cu] for all the measurement rows L When the sample standard deviation is the Cu segregation degree σ, The Cu segregation degree σ is 0.050 or less. Steel material.

[0021] [2] A steel material, The chemical composition is, in mass%, C: 0.30 to less than 0.50% Si: 0.10 to 0.30% Mn: 0.10 to 0.80% P: 0.020% or less, S: 0.020% or less, Cr: 0.90~1.80%, Mo: 0.10 to 1.00%, Cu: 0.04 to less than 0.35% Ni: 0.04 to 0.30% Al: 0.005 to 0.060%, N: 0.0200% or less, and O: 0.0030% or less, the chemical composition further contains one or more elements selected from the group consisting of Group 1 and Group 2, with the remainder consisting of Fe and impurities; In a rectangular observation area of ​​the cross section including the longitudinal direction and radial direction of the steel material, the area is 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material. The 160,000 measurement areas, which were divided into 400 in the radial direction and 400 in the longitudinal direction, were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% in each of the obtained measurement areas was calculated as [Cu] MA year, The [Cu] in all the measurement areas MA The arithmetic mean value of [Cu] AVE year, The [Cu] in each measurement area MA The above [Cu] AVE The ratio to [Cu] S year, When a plurality of measurement areas arranged in a row in the longitudinal direction among the plurality of measurement areas is defined as a measurement row, in each measurement row, the [Cu] of the plurality of measurement areas constituting the measurement row is S The ratio of the total number of the measurement areas constituting the measurement row to the total number of the measurement areas is defined as [Cu] L year, The [Cu] for all the measurement rows L When the sample standard deviation is the Cu segregation degree σ, The Cu segregation degree σ is 0.050 or less. Steel material. [Group 1] V: 0.50% or less, Nb: 0.10% or less, Ti: 0.100% or less, and B: less than 0.0010% [Group 2] Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and Rare earth elements: 0.0200% or less, one or more selected from the group consisting of

[0022] [3] [2] The steel material according to The chemical composition contains the first group. Steel material.

[0023] [4] The steel material according to [2] or [3], The chemical composition contains the second group. Steel material.

[0024] The steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0025] [Features of the steel material of this embodiment] The steel material of this embodiment satisfies the following characteristics. (Feature 1) The chemical composition is within the range of this embodiment. (Feature 2) The degree of Cu segregation σ in the surface layer of the steel material obtained by carrying out an area analysis using an electron probe microanalyzer described below is 0.050 or less. Each feature will be explained below.

[0026] [(Feature 1) Chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0027] C: 0.30 to less than 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 is 0.50% or more, 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 C content is 0.30 to less than 0.50%. The lower limit of the C content is preferably 0.34%, more preferably 0.36%, and even more preferably 0.38%. The upper limit of the C content is preferably 0.48%, more preferably 0.46%, and even more preferably 0.44%.

[0028] Si: 0.10 to 0.30% Silicon (Si) increases the strength of bolts manufactured from steel materials through solid solution strengthening. If the Si content is less than 0.10%, 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 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 0.10 to 0.30%. The lower limit of the Si content is preferably 0.14%, more preferably 0.16%, and even more preferably 0.18%. The upper limit of the Si content is preferably 0.28%, more preferably 0.26%, and even more preferably 0.24%.

[0029] Mn: 0.10 to 0.80% 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 0.80%, 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 Mn content is 0.10 to 0.80%. 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 0.75%, and more preferably 0.70%.

[0030] P:0.020% 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 0.020%, P will segregate at grain boundaries even if the contents of other elements are within the ranges specified in this embodiment. As a result, the hydrogen embrittlement resistance of the bolt will decrease. 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 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 0.015%, more preferably 0.012%, even more preferably 0.010%, even more preferably 0.008%, and even more preferably 0.007%.

[0031] S: 0.020% 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 0.020%, 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 hydrogen embrittlement resistance of the bolt will decrease. Therefore, the S content is 0.020% 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 0.012%, more preferably 0.010%, and even more preferably 0.008%.

[0032] Cr: 0.90~1.80% Chromium (Cr) enhances the corrosion resistance of steel materials. If the Cr content is less than 0.90%, 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 Cr content exceeds 1.80%, even if the contents of other elements are within the ranges specified in this embodiment, Cr will concentrate in carbides in the steel during the annealing process in the manufacturing process of a bolt made from the steel. In this case, the carbides will not easily form a solid solution in the subsequent heat treatment process. This will result in a large variation in hardness within the manufactured bolt. As a result, the hydrogen embrittlement resistance and fatigue properties of the bolt will be reduced. Therefore, the Cr content is 0.90 to 1.80%. The lower limit of the Cr content is preferably 0.95%, more preferably 1.00%, even more preferably 1.05%, and still more preferably 1.10%. The upper limit of the Cr content is preferably 1.75%, more preferably 1.70%, even more preferably 1.65%, and still more preferably 1.60%.

[0033] Mo: 0.10 to 1.00% Molybdenum (Mo) improves the corrosion resistance of steel. Mo also forms fine carbides to increase the strength of the bolt. If the Mo 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 Mo content exceeds 1.00%, even if the contents of other elements are within the ranges of this embodiment, the steel material becomes excessively hard, which reduces the cold forgeability of the steel material and increases the manufacturing cost. Therefore, the Mo content is 0.10 to 1.00%. The lower limit of the Mo content is preferably 0.15%, more preferably 0.20%, and even more preferably 0.25%. The upper limit of the Mo content is preferably 0.80%, more preferably 0.70%, and even more preferably 0.60%.

[0034] Cu: 0.04 to less than 0.35% Copper (Cu) suppresses corrosion of steel materials. This suppresses the generation of hydrogen on the surface of the steel material. As a result, the penetration of hydrogen into the steel material is suppressed. If the Cu content is less than 0.04%, 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 Cu content is 0.35% or more, the steel material becomes embrittled, and therefore the hot workability and cold forgeability of the steel material deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.04 to less than 0.35%. The lower limit of the Cu content is preferably 0.07%, more preferably 0.10%, and even more preferably 0.13%. The upper limit of the Cu content is preferably 0.34%, more preferably 0.33%, even more preferably 0.30%, and still more preferably 0.28%.

[0035] Ni: 0.04 to 0.30% Nickel (Ni) improves the hardenability of the steel material and increases the strength of the bolt. Ni also improves the corrosion resistance of the bolt. Ni, when contained together with Cu, suppresses the occurrence of defects during hot working of the steel material. As a result, in the steel material of this embodiment containing Cu, Ni improves the hot workability of the steel material. If the Ni content is less than 0.04%, 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 Ni content exceeds 0.30%, the hardenability of the steel material becomes excessively high, and in this case, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material decreases. Therefore, the Ni content is 0.04 to 0.30%. The lower limit of the Ni content is preferably 0.10%, more preferably 0.15%, and even more preferably 0.20%. The upper limit of the Ni content is preferably 0.28%, more preferably 0.26%, and even more preferably 0.24%.

[0036] Al: 0.005 to 0.060% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, the deoxidation of the steel will be insufficient even if the contents of other elements are within the ranges of this embodiment. In this case, coarse oxides will be generated, which will reduce the hydrogen embrittlement resistance of the bolt. On the other hand, if the Al content exceeds 0.060%, 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 workability of the steel material. Therefore, the Al content is 0.005 to 0.060%. The lower limit of the Al content is preferably 0.010%, more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Al content is preferably 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 content.

[0037] N: 0.0200% or less Nitrogen (N) is unavoidably contained, i.e., the N content is greater than 0%. N combines with Al or Ti to form nitrides or carbonitrides. These nitrides and carbonitrides have a pinning effect that suppresses grain coarsening, thereby improving the cold forgeability of steel. However, if the N content exceeds 0.0200%, coarse nitrides are formed even if the contents of other elements are within the ranges of this embodiment. The coarse nitrides become the starting points for fracture, reducing the cold forgeability of the steel material. Furthermore, the hydrogen embrittlement resistance of the bolt is reduced. Therefore, the N content is 0.0200% or less. The lower limit of the N content is preferably 0.0001%, more preferably 0.0010%, even more preferably 0.0030%, even more preferably 0.0050%, even more preferably 0.080%, and even more preferably 0.0100%. The upper limit of the N content is preferably 0.0180%, more preferably 0.0170%, even more preferably 0.0160%, and still more preferably 0.0150%.

[0038] O: 0.0030% or less Oxygen (O) is unavoidably contained, that is, the O content is greater than 0%. O combines with other elements to form oxides. These oxides have a pinning effect that suppresses grain coarsening, thereby improving the cold forgeability of steel. However, if the O content exceeds 0.0030%, coarse oxides are generated even if the contents of other elements are within the ranges of this embodiment. The coarse oxides become the starting points of fracture, reducing the cold forgeability of the steel material. Furthermore, the hydrogen embrittlement resistance of the bolt is reduced. Therefore, the O content is 0.0030% or less. The lower limit of the O content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.007%. The upper limit of the O content is preferably 0.0028%, more preferably 0.0025%, and even more preferably 0.0023%.

[0039] 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.

[0040] [Optional Elements] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of first and second groups. [Group 1] V: 0.50% or less, Nb: 0.10% or less, Ti: 0.100% or less, and B: less than 0.0010% [Group 2] Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and Rare earth elements: 0.0200% or less, one or more selected from the group consisting of The optional elements will be explained below.

[0041] [Group 1: V, Nb, Ti and B] The chemical composition of the steel material of this embodiment may further contain the following elements of Group 1 in place of a portion of Fe. These elements are optional elements, and all of them increase the strength of the steel material. [Group 1] V: 0.50% or less, Nb: 0.10% or less, Ti: 0.100% or less, and B: less than 0.0010%

[0042] V:0.50% or less Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms V precipitates such as carbides and carbonitrides. V precipitates increase the strength of the bolt. Even if even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, a large amount of V precipitates are formed, and in this case, the amount of hydrogen that penetrates into the steel material increases. Therefore, the V content is 0 to 0.50%, and if V is contained, it is 0.50% or less. The lower limit of the V content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the V content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[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 Nb is contained, that is, when the Nb content is more than 0%, Nb forms Nb precipitates such as carbides and carbonitrides. The Nb precipitates increase the strength of the bolt. Even if even 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 ranges of this embodiment, a large amount of Nb precipitates are formed. In this case, the amount of hydrogen that penetrates into the steel material increases. As a result, the hydrogen embrittlement resistance of the steel material decreases. Therefore, the Nb content is 0 to 0.10%, and if Nb is contained, it is 0.10% or less. The preferred lower limit of the Nb content is 0.01%. The upper limit of the Nb content is preferably 0.08%, more preferably 0.06%, and the upper limit of the Nb content, which is more effective for further reducing the amount of hydrogen penetration, is preferably less than 0.04%.

[0044] 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 exceeds 0%, Ti forms Ti precipitates such as carbides and carbonitrides. Ti precipitates increase the strength of the bolt. Even if Ti is contained even a small amount, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, excessive Ti precipitates are formed, which significantly reduces the cold forgeability of the steel material. Therefore, the Ti content is 0 to 0.100%, and if contained, it is 0.100% or less. The lower limit of the Ti content is preferably 0.004%, more preferably 0.008%, even more preferably 0.012%, and still more preferably 0.016%. The upper limit of the Ti content is preferably 0.065%, more preferably 0.060%, even more preferably 0.055%, and still more preferably 0.050%.

[0045] B: Less than 0.0010% Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel material and increases the strength of the bolt. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content is 0.0010% or more, even if the contents of other elements are within the ranges of this embodiment, when a bolt made of steel is tempered at a high temperature after quenching during the manufacturing process, coarse carbides containing B may be generated, which may make the steel more susceptible to brittle fracture. Therefore, the B content is 0 to less than 0.0010%, and if B is contained, it is less than 0.0010%. The preferred lower limit of the B content is 0.0001%. The upper limit of the B content is preferably 0.0008%, more preferably 0.0006%, and even more preferably 0.0004%.

[0046] [Group 2: Sn, Ca, Mg and rare earth elements] The chemical composition of the steel material of this embodiment may further contain the following elements of Group 2 instead of part of Fe. These elements are optional elements, and all of them suppress hydrogen penetration into the steel material and improve hydrogen embrittlement resistance. Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and Rare earth elements: 0.0200% or less, one or more selected from the group consisting of

[0047] Sn: 0.100% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When Sn is contained, that is, when the Sn content is more than 0%, Sn suppresses the penetration of hydrogen into the steel material, thereby improving the hydrogen embrittlement resistance of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, Sn segregates at grain boundaries. In this case, the hot workability and cold forgeability of the steel material deteriorate. Furthermore, the hydrogen embrittlement resistance of the bolt deteriorates. Therefore, the Sn content is 0 to 0.100%, and if Sn is contained, it is 0.100% or less. The lower limit of the Sn content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.006%. The upper limit of the Sn content is preferably 0.080%, more preferably 0.060%, even more preferably 0.040%, and still more preferably 0.020%.

[0048] Ca:0.0050% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%. When Ca is contained, that is, when the Ca content is more than 0%, Ca refines MnS, thereby improving the hydrogen embrittlement resistance of the steel. 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%, even if the contents of other elements are within the ranges of this embodiment, coarse Ca oxides are formed, which reduces the hydrogen embrittlement resistance of the steel material. Therefore, the Ca content is 0 to 0.0050%, and if contained, it is 0.0050% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Ca content is preferably 0.0040%, and more preferably 0.0030%.

[0049] 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 MnS, thereby improving the hydrogen embrittlement resistance of the steel. 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%, even if the contents of other elements are within the ranges of this embodiment, coarse Mg oxides are formed, which reduces the hydrogen embrittlement resistance of the steel material. Therefore, the Mg content is 0 to 0.0050%, and if Mg is contained, it is 0.0050% or less. 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%.

[0050] Rare earth elements (REM): 0.0200% or less Rare earth elements (REM) are optional elements and may not be contained, i.e., the REM content may be 0%. When REM is contained, that is, when the REM content is more than 0%, REM refines MnS, thereby improving the hydrogen embrittlement resistance of the steel. Even if the REM content is even a small amount, the above effect can be obtained to some extent. However, if the REM content exceeds 0.0200%, even if the contents of other elements are within the ranges of this embodiment, coarse oxides are formed, which reduces the hydrogen embrittlement resistance of the steel material. Therefore, the REM content is 0 to 0.0200%, and if contained, it is 0.0200% or less. The lower limit of the REM content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0050%. The upper limit of the REM content is preferably 0.0150%, and more preferably 0.0100%.

[0051] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In this specification, the REM content refers to the total content of these elements.

[0052] [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 in accordance with JIS G0321:2017. 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. The O content is determined by a known inert gas fusion-infrared absorption method.

[0053] 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 be a value up to two decimal places. Therefore, the C content is determined to be a value up to two decimal places obtained by rounding off the measured value to two decimal places.

[0054] 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.

[0055] Rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0056] [(Feature 2) Cu segregation degree σ in the surface layer of steel] In the steel material of this embodiment, a rectangular observation region of 1000 μm in the radial direction from the surface of the steel material and 1000 μm in the longitudinal direction in a cross section including the longitudinal direction and the radial direction of the steel material was subjected to area analysis using an electron beam microanalyzer for 160,000 measurement regions divided into 400 in the radial direction and 400 in the longitudinal direction, and the Cu content in mass% in each of the obtained measurement regions was calculated as [Cu] MA and [Cu] in all measurement areas MA The arithmetic mean value of [Cu] AVE and [Cu] in each measurement area MA [Cu] AVE The ratio to [Cu] S Among the multiple measurement areas, a row of multiple measurement areas arranged in a line in the longitudinal direction is defined as a measurement row. In each measurement row, the [Cu] of the multiple measurement areas that make up the measurement row is S The ratio of the total number of measurement areas that make up the measurement row to the total number of measurement areas is [Cu] L and [Cu] for all measurement lines L When the sample standard deviation is taken as the Cu segregation degree σ, the Cu segregation degree σ is 0.050 or less. The Cu segregation degree σ will be described below.

[0057] [Cu segregation degree σ] Fig. 1 is a schematic diagram for explaining the degree of Cu segregation σ. Referring to Fig. 1, a rectangular observation area 100 is selected from a cross section 10 of the steel material, which includes a longitudinal direction L and a radial direction D, and which is 1000 µm in the radial direction D and 1000 µm in the longitudinal direction L from a surface 10S of the steel material.

[0058] An enlarged view of a rectangular observation region 100 is shown in Fig. 1. When a Cu segregation region SEG is present in a micro region (rectangular observation region 100) of the steel material surface layer, the Cu segregation region SEG extends in the longitudinal direction L of the steel material. Here, the Cu segregation region SEG is a region having a higher Cu concentration than other regions other than the Cu segregation region SEG.

[0059] The region of the cross section 10 extending 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 any part of the surface layer 10L. If one or more Cu segregation regions SEG are present in the rectangular observation region 100, there is variation in the Cu concentration within the rectangular observation region 100. In this case, this means that there is variation in the Cu concentration within the surface layer 10L. If such variation in Cu concentration occurs within the surface layer 10L, hydrogen is more likely to penetrate from the outside into regions of the surface layer 10L with low Cu concentration. Therefore, it is effective in suppressing hydrogen penetration to have as uniform a distribution of Cu concentration as possible within the surface layer 10L.

[0060] Therefore, in the steel material of this embodiment, the "degree of Cu segregation σ" obtained by the following measurement method is used as an index showing the uniformity of the Cu concentration distribution in the surface layer 10L of the steel material.

[0061] [Method for measuring Cu segregation σ] The Cu segregation degree σ of the steel material of this embodiment can be measured by the following method. 1, an arbitrary rectangular observation area 100 is selected in the surface layer 10L. As described above, the rectangular observation area 100 is a rectangle measuring 1000 μm in the radial direction D and 1000 μm in the longitudinal direction L from the surface 10S of the steel material.

[0062] Fig. 2 is a schematic diagram for explaining a method for measuring the Cu segregation degree σ in the rectangular observation region 100 in Fig. 1. Referring to Fig. 2, an area analysis is performed on the rectangular observation region 100 using a field emission electron probe microanalyzer (FE-EPMA). Specifically, the rectangular observation region 100 is divided into 400 sections in the longitudinal direction L and 400 sections in the radial direction D, thereby dividing the rectangular observation region 100 into 160,000 measurement areas MA.

[0063] Elemental analysis is performed on each measurement area MA. In elemental analysis, the acceleration voltage is 15 kV, the probe current is 400 nA, the beam diameter is 2 μm, and the integration time is 0.1 seconds. The element to be measured is Cu, and the Cu content in mass% in each measurement area MA is calculated and the Cu content is defined as [Cu] MA It is defined as:

[0064] [Cu] in each measurement area MA MA Using this, the Cu segregation degree σ in the rectangular observation region 100 is determined by the following method.

[0065] Cu content in mass% in all measurement areas MA [Cu] MA The arithmetic mean value of [Cu] AVE Furthermore, the Cu content in each measurement area MA [Cu] MA [Cu] AVE The ratio to [Cu] S In other words, [Cu] S means the amount of Cu in each measurement area, excluding the effect of the Cu content.

[0066] Referring to Fig. 2, of the 400 x 400 measurement areas MA, a row of measurement areas arranged in a line in the longitudinal direction L is defined as a "measurement row" ML1 to ML400. Each measurement row MLj (j is an integer from 1 to 400) is made up of 400 measurement areas MA arranged in a line in the longitudinal direction L. In Fig. 2, the multiple measurement areas MA in the area surrounded by the dashed line make up measurement row ML1.

[0067] [Cu] of the multiple measurement areas MA that make up each measurement row MLj S The ratio of the total of the measurement areas MA constituting the measurement row MLj (i.e., 400) to the total number of measurement areas MA constituting the measurement row MLj is defined as [Cu] L Defined as [Cu] L means the amount of Cu in the measurement row MLj.

[0068] [Cu] for all measurement rows MLj L The sample standard deviation of is calculated. The obtained value is defined as the "Cu segregation degree σ." The significant figure of the Cu segregation degree σ is three decimal places. In other words, the Cu segregation degree σ is the value obtained by rounding off the number to four decimal places.

[0069] [Significance of Cu segregation σ] As shown in the rectangular observation area 100 in Figures 1 and 2, the Cu segregation region SEG extends in the longitudinal direction L of the steel material. L The sample standard deviation of is an index showing the degree of Cu segregation σ.

[0070] If the Cu segregation ratio σ exceeds 0.050, the Cu concentration distribution is not sufficiently uniform in the surface layer 10L. In other words, the Cu concentration distribution is excessively varied. In this case, even if the steel material has the characteristic 1, it cannot sufficiently suppress hydrogen penetration.

[0071] 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 the characteristic 1 is sufficiently uniform, and therefore, the penetration of hydrogen can be sufficiently suppressed.

[0072] The upper limit of the Cu segregation degree σ is preferably 0.045, more preferably 0.040, even more preferably 0.035, and even more preferably 0.030. The Cu segregation degree σ is preferably as low as possible. The lower limit of the Cu segregation degree σ is preferably 0.000, more preferably 0.005, and even more preferably 0.010.

[0073] [Shape of steel material in this embodiment] The steel material in this embodiment is a steel bar or a wire rod. The steel bar or wire rod is a steel material that extends in a rod shape. The steel material may be wound in a coil shape or may be cut to a predetermined length.

[0074] [Microstructure of the steel material according to 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 bolt material, if the hardness of the steel material is too high, spheroidizing annealing is performed before the bolt manufacturing process (wire drawing process or cold forging process) is performed. The cold forgeability of the spheroidizing annealed steel material is improved. Therefore, it is possible to manufacture a bolt by performing cold forging using the steel material of this embodiment as a material. Therefore, the microstructure of the steel material of this embodiment is not particularly limited.

[0075] [Use of the steel material according to this embodiment] The steel material of this embodiment can be used as a material for bolts, which are a type of fastening means for industrial machinery, automobiles, bridges, buildings, etc. The steel material of this embodiment may also be used for applications other than those mentioned above.

[0076] [Steel manufacturing method] An example of a method for manufacturing the steel material of this embodiment will be described. The method for manufacturing the steel material described below is one example for manufacturing the steel material of this embodiment. Therefore, the steel material having the above-mentioned Features 1 and 2 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 the steel material of this embodiment.

[0077] An example of the method for manufacturing the steel material according to this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Rough rolling process (Process 3) Finishing rolling process Furthermore, in the present manufacturing method, the following conditions are satisfied in the finish rolling step. Conditions: The steel temperature must be within the range of 1000 to 880°C, and the number of passes must be two or more with an area reduction rate of 25% or more. Each step will be described below.

[0078] [(Process 1) Material preparation process] In the material preparation step, a material for the steel material of this embodiment is prepared. Specifically, molten steel is produced in which the content of each element in the chemical composition falls within the range of this embodiment. The refining method is not particularly limited, and any known method may be used. For example, molten pig iron produced by a known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to known secondary refining. Through the above steps, molten steel with a chemical composition that satisfies Feature 1 is produced.

[0079] The produced molten steel is used to produce a material by a well-known casting method. For example, an ingot may be produced by an ingot casting method using the molten steel. Alternatively, a bloom may be produced by a continuous casting method using the molten steel. By the above methods, a material (ingot or bloom) is produced.

[0080] [(Process 2) Rough rolling process] In the rough rolling step, the material (ingot or bloom) prepared in the material preparation step is subjected to rough rolling to produce a billet.

[0081] The rough rolling process includes the following steps. (Step 21) Heating step (Step 22) Rolling process Each of steps 21 and 22 will be explained below.

[0082] [(Step 21) Heating step] In the heating step, the material is heated using a heating furnace by a well-known method. The heating temperature is not particularly limited. Any well-known temperature is sufficient. The heating temperature is, for example, 1000 to 1200°C.

[0083] [(Step 22) Rolling process] In the rolling step, the material heated in the heating step is rolled (roughly rolled) using a blooming mill, or a blooming mill and a continuous rolling mill, to produce a billet.

[0084] Specifically, a heated material is reverse-rolled using a blooming mill to produce a billet. The blooming mill is equipped with a pair of horizontal rolls. Reverse rolling is performed in the blooming mill. Reverse rolling refers to a rolling method in which the material is subjected to reduction by the blooming mill as it passes from upstream to downstream through the blooming mill, and can also be subjected to reduction by the blooming mill as it passes from downstream to upstream through the blooming mill.

[0085] When a continuous rolling mill is disposed downstream of the blooming mill, the billets after blooming may be further subjected to tandem rolling using the continuous rolling mill to produce even smaller billets. In the continuous rolling mill, horizontal stands each having a pair of horizontal rolls and vertical stands each 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.

[0086] The billet produced by the above rough rolling process is allowed to cool (air-cool) to room temperature before the finish rolling process.

[0087] [(Process 3) Finishing rolling process] In the finish rolling process, the billet produced in the rough rolling process is subjected to finish rolling to produce a steel material. Here, the steel material is a wire rod or a steel bar. The finish rolling process includes the following steps. (Step 31) Heating step (Step 32) Rolling process

[0088] In the finish rolling process, the following conditions are further satisfied in the rolling process. (conditions) The steel temperature is within the range of 1000 to 880°C, and the number of passes is set to 2 or more with an area reduction rate of 25% or more. The heating step and rolling step in the finish rolling step will be described below.

[0089] [(Step 31) Heating process] In the heating step, the billet cooled to room temperature is heated in a heating furnace by a known method. The heating temperature is not particularly limited, but is, for example, 900 to 1050°C.

[0090] [(Process 32) Rolling process] In the rolling process, the billet heated in the heating process is subjected to finish rolling (continuous rolling) using a continuous rolling mill to produce steel. The continuous rolling mill includes multiple rolling stands arranged in a row from upstream to downstream. Each rolling stand includes a pair of work rolls. Each roll is formed with a caliber, and the calibers of the pair of rolls form a groove.

[0091] In continuous rolling using a continuous rolling mill, the reduction of the area of ​​a billet in each rolling stand as the billet passes from upstream to downstream is defined as "one pass."

[0092] Continuous rolling refers to rolling a billet through a plurality of passes using a continuous rolling mill. It is not necessary to reduce the billet in all of the rolling stands in the continuous rolling mill. For example, if the continuous rolling mill includes 15 rolling stands and the billet passes through the last rolling stand without being reduced, 14 passes of reduction are performed.

[0093] [Conditions] In the finish rolling process, the steel material temperature is in the range of 1000 to 880° C., and the number of passes with an area reduction rate of 25% or more is set to 2 or more. Here, the area reduction rate is defined by the following formula. Area reduction rate = (1 - cross-sectional area perpendicular to the longitudinal direction of the billet at the exit side of the rolling stand) / (cross-sectional area perpendicular to the longitudinal direction of the billet at the entry side of the rolling stand) x 100

[0094] It is generally known that the segregation of elements such as Mn can be reduced by adjusting the heating temperature in a heating furnace. However, in the case of Cu, the inventors have found through their investigation that the introduction of a large amount of strain by rolling within a specific temperature range facilitates uniform diffusion in the surface layer of the steel material, rather than adjusting the heating temperature to facilitate diffusion.

[0095] Specifically, when the steel temperature is in the range of 1000 to 880°C and the number of passes is less than two with an area reduction of 25% or more, the amount of strain introduced is insufficient. In this case, Cu does not diffuse sufficiently uniformly in the steel surface layer during the finish rolling process. As a result, the Cu segregation degree σ exceeds 0.050.

[0096] If the steel temperature is in the range of 1000 to 880°C and the number of passes is two or more with an area reduction of 25% or more, sufficient strain is introduced into the surface layer of the steel in a temperature range suitable for Cu diffusion. In this case, Cu diffuses uniformly into the surface layer of the steel. As a result, the Cu segregation ratio σ becomes 0.050 or less.

[0097] Therefore, the steel temperature is set to a range of 1000 to 880°C, the area reduction rate is set to 25% or more, and the number of passes is set to 2 or more.

[0098] The lower limit of the number of passes at a steel material temperature in the range of 1000 to 880°C and an area reduction rate of 25% or more is preferably 3, and more preferably 4. There is no particular upper limit to the preferable number of passes when the steel material temperature is in the range of 1000 to 880°C and the area reduction rate is 25% or more.

[0099] A thermometer is arranged at the entry side and / or the exit side of each rolling stand of the continuous rolling mill. The thermometer is a well-known device, such as a radiation thermometer or a thermograph. The steel material temperature is measured at the entry side and / or the exit side of each rolling stand. The area reduction rate of each rolling stand is set in advance. Therefore, based on the steel material temperature measured at the entry side and / or the exit side of each rolling stand and the area reduction rate of each rolling stand, the number of passes at an area reduction rate of 25% or more when the steel material temperature is in the range of 1000 to 880°C can be determined.

[0100] In the finish rolling step, the cooling method after continuous rolling is not particularly limited, and the cooling method may be natural cooling, slow cooling, or rapid cooling.

[0101] Through the above manufacturing process, a steel material satisfying Features 1 and 2 can be manufactured.

[0102] [Method for manufacturing a bolt using steel material according to this embodiment] The method for manufacturing the bolt made of steel according to this embodiment is a well-known manufacturing method, and includes, for example, the following steps. ·Wire drawing process Cold forging process ·Quenching and tempering process Each step will be described below.

[0103] [Wire drawing process] In the wiredrawing process, the steel material is subjected to a well-known wiredrawing process to manufacture a steel wire. The wiredrawing process may be a primary wiredrawing only, or multiple wiredrawing processes such as a secondary wiredrawing may be performed. Furthermore, a softening treatment, typically spheroidizing annealing, may be performed before or after the wiredrawing process.

[0104] [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.

[0105] [Quenching and tempering process] In the quenching and tempering step, the intermediate product is quenched and tempered.

[0106] [Quenching] Quenching is carried out 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. After the holding time has elapsed, the intermediate product is quenched. Specifically, the intermediate product is water-cooled or oil-cooled.

[0107] [Tempering] The intermediate product after quenching is tempered. The tempering temperature and the holding time at the tempering temperature are not particularly limited. The tempering temperature is, for example, 500 to 650°C. The holding time at the tempering temperature is 30 to 360 minutes.

[0108] By the above manufacturing method, a bolt can be manufactured using the steel material of this embodiment. The manufactured bolt has Features 1 and 2. Therefore, hydrogen penetration in a corrosive environment is suppressed. [Example]

[0109] 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.

[0110] [Material preparation process] Steel materials having the chemical compositions shown in Tables 1-1 and 1-2 were produced by the following method.

[0111] [Table 1-1]

[0112] [Table 1-2]

[0113] [Rough rolling process] The produced bloom was subjected to a rough rolling process to produce a billet. Specifically, the bloom was heated to 1100°C using a heating furnace. The heated bloom was rolled (rough rolling) using a blooming mill and a continuous rolling mill to produce a billet. The billet produced in the rough rolling process was allowed to cool to room temperature.

[0114] [Finishing rolling process] The produced billets were subjected to a finish rolling process. Specifically, the billets of each test number were heated to 950 to 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, the number of passes (PN) at which the steel material temperature was in the range of 1000 to 880°C and the area reduction rate was 25% or more was as shown in Table 2.

[0115] [Table 2]

[0116] The steel material (round bar) after finish rolling was allowed to cool to room temperature. Through the above manufacturing process, steel material (round bar) having a diameter of 10 mm and each test number was manufactured.

[0117] [About the evaluation test] The following steel evaluation tests (Tests 1 to 3) were carried out on the manufactured steel materials with each test number. [Steel evaluation test] (Test 1) Chemical composition measurement test of steel material (Test 2) Cu segregation σ measurement test (Test 3) Diffusible hydrogen concentration measurement test Each test will be explained below.

[0118] [(Test 1) Steel chemical composition measurement test] The chemical composition of the steel material (round bar) of each test number was analyzed based on the above-mentioned "Method for measuring the chemical composition of steel material." As a result, the chemical composition of each test number was as shown in Table 1-1 and Table 1-2.

[0119] [(Test 2) Cu segregation σ measurement test] The Cu segregation degree σ was determined for each steel material (round bar) of each test number based on the above-mentioned [Method for measuring Cu segregation degree σ]. The results are shown in Table 2.

[0120] [(Test 3) Diffusible hydrogen concentration measurement test] The following diffusible hydrogen concentration measurement test was carried out on the steel material with each test number.

[0121] [Manufacturing of bolt imitation materials] First, bolt simulants were manufactured using the round bars of each test number. Specifically, the round bars of each test number were subjected to the following quenching and tempering treatments. The quenching process was carried out using a heat treatment furnace. The quenching temperature was set to 870-950°C, and the holding time at the quenching temperature was set to 60 minutes. After the holding time had elapsed, the steel was water-cooled to perform quenching. The inside of the heat treatment furnace was set to a carbon potential atmosphere equivalent to the C concentration of the steel, to prevent decarburization of the steel.

[0122] After the quenching treatment, a tempering treatment was carried out. The tempering treatment was carried out using a heat treatment furnace. In the tempering, the bolt simulant was held at a tempering temperature (°C) of 500 to 650°C for 1 to 2 hours so that the tensile strength of the bolt simulant would be in the range of 1200 to 1400 MPa. After the holding time had elapsed, the steel material was water-cooled. A bolt simulant (round bar) was produced using the above manufacturing process.

[0123] [Measurement test] The bolt simulant (round bar) of each test number was cut perpendicular to the longitudinal direction to obtain multiple round bar test pieces with a length of 100 mm. In order to eliminate the influence of scale formed during the rough rolling process, finish rolling process, and quenching and tempering process in manufacturing the bolt simulant, the round bar test pieces were subjected to a blasting treatment to remove the scale from the outermost surface layer of the round bar test pieces.

[0124] The following cyclic corrosion test (CCT) was performed using the blast-treated round bar test specimens. Figure 3 shows the cycle pattern of one cycle in the cyclic corrosion test. Referring to Figure 3, one cycle was performed as follows: First, the round bar test specimens were held in a low-temperature environment at -20°C for four hours (low-temperature environment holding step S1). Then, the round bar test specimens were held in a salt-water spray environment in which 5% by mass of salt water was sprayed at 20°C and 70% relative humidity for four hours (salt-water spray environment holding step S2). Then, the round bar test specimens were held in a dry environment at 70°C and 60% relative humidity for four hours (dry environment holding step S3). Then, the round bar test specimens were held in a humid environment at 50°C and 95% relative humidity for four hours (humid environment holding step S4). The temperature and / or humidity adjustment time required to transition from a given step to the next step was two hours. Specifically, the time required to transition from step S1 to the temperature and relative humidity of step S2 was two hours. The transition time for other steps was also two hours.

[0125] For each test number, the test cycle in the cyclic corrosion test was performed in four patterns: 7 cycles, 14 cycles, 28 cycles, and 56 cycles. A separate round bar test specimen was used for each pattern.

[0126] After each test pattern, a round bar test specimen was removed. The removed round bar test specimen was subjected to a blasting treatment to remove corrosion products formed on the surface of the round bar test specimen during the corrosion test. A wet cutting machine was used to cut out a 30 mm long central portion of the round bar test specimen in the longitudinal direction.

[0127] The diffusible hydrogen concentration of the cut-out test piece was analyzed using a gas chromatograph-type thermal desorption hydrogen analyzer (TDA). Specifically, the cut-out test piece was heated from room temperature to 400°C at a heating rate of 100°C / h. The amount of hydrogen released from the test piece to the outside due to heating was measured.

[0128] The measured amount of hydrogen was divided by the mass of the test piece before heating to determine the diffusible hydrogen concentration (ppm). For each test number, the diffusible hydrogen concentration was determined for each of the four patterns described above. The highest value of the four diffusible hydrogen concentrations was defined as the diffusible hydrogen concentration for that test number.

[0129] [Evaluation results] The evaluation results are shown in Table 2. In test numbers 1 to 35, 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 0.50 ppm, and hydrogen penetration was suppressed.

[0130] On the other hand, in test number 36, the Cr content was too low, so the diffusible hydrogen concentration was 0.50 ppm or more, and the penetration of hydrogen could not be sufficiently suppressed.

[0131] On the other hand, in test number 37, the Mo content was too low, so the diffusible hydrogen concentration was 0.50 ppm or more, and the penetration of hydrogen could not be sufficiently suppressed.

[0132] On the other hand, in test number 38, the Cu content was too low, so the diffusible hydrogen concentration was 0.50 ppm or more, and the penetration of hydrogen could not be sufficiently suppressed.

[0133] In test numbers 39 to 42, although the chemical composition was appropriate, the steel temperature was in the range of 1000 to 880°C, and the number of passes (PN) with an area reduction of 25% or more was less than 2. Therefore, the Cu segregation ratio (σ) exceeded 0.050. As a result, the diffusible hydrogen concentration was 0.50 ppm or more, and hydrogen penetration could not be sufficiently suppressed.

[0134] 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, The chemical composition, in mass%, is C: 0.30 to less than 0.50% Si: 0.10-0.30%, Mn: 0.10-0.80%, P: 0.020% or less, S: 0.020% or less, Cr: 0.90-1.80%, Mo: 0.10-1.00%, Cu: 0.04 to less than 0.35%; Ni: 0.04-0.30%, Al: 0.005-0.060%, N: 0.0200% or less, and O: 0.0030% or less, and the balance being Fe and impurities, In a rectangular observation region of the cross section including the longitudinal direction and the radial direction of the steel material, the region is 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material, The 160,000 measurement areas divided into 400 in the radial direction and 400 in the longitudinal direction were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% in each of the obtained measurement areas was calculated as [Cu] ＭＡ year, The [Cu] in all the measurement areas ＭＡ The arithmetic mean value of [Cu] ＡＶＥ year, The [Cu] in each measurement area ＭＡ The [Cu] ＡＶＥ The ratio to [Cu] Ｓ year, When a plurality of measurement areas arranged in a row in the longitudinal direction among the plurality of measurement areas is defined as a measurement row, in each measurement row, the [Cu] of the plurality of measurement areas constituting the measurement row is Ｓ The ratio of the sum of the measurement areas constituting the measurement row to the total number of the measurement areas is defined as [Cu] Ｌ year, The [Cu] of all the measurement rows Ｌ When the sample standard deviation is the Cu segregation degree σ, The Cu segregation degree σ is 0.050 or less. Steel material.

2. A steel material, The chemical composition, in mass%, is C: 0.30 to less than 0.50% Si: 0.10-0.30%, Mn: 0.10-0.80%, P: 0.020% or less, S: 0.020% or less, Cr: 0.90-1.80%, Mo: 0.10-1.00%, Cu: 0.04 to less than 0.35%; Ni: 0.04-0.30%, Al: 0.005-0.060%, N: 0.0200% or less, and O: 0.0030% or less, the chemical composition further contains one or more elements selected from the group consisting of Group 1 and Group 2, with the balance consisting of Fe and impurities; In a rectangular observation region of the cross section including the longitudinal direction and the radial direction of the steel material, the region is 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material, The 160,000 measurement areas divided into 400 in the radial direction and 400 in the longitudinal direction were subjected to area analysis using an electron beam microanalyzer, and the Cu content in mass% in each of the obtained measurement areas was calculated as [Cu] ＭＡ year, The [Cu] in all the measurement areas ＭＡ The arithmetic mean value of [Cu] ＡＶＥ year, The [Cu] in each measurement area ＭＡ The [Cu] ＡＶＥ The ratio to [Cu] Ｓ year, When a plurality of measurement areas arranged in a row in the longitudinal direction among the plurality of measurement areas is defined as a measurement row, in each measurement row, the [Cu] of the plurality of measurement areas constituting the measurement row is Ｓ The ratio of the sum of the measurement areas constituting the measurement row to the total number of the measurement areas is defined as [Cu] Ｌ year, The [Cu] of all the measurement rows Ｌ When the sample standard deviation is the Cu segregation degree σ, The Cu segregation degree σ is 0.050 or less. Steel material. [Group 1] V: 0.50% or less, Nb: 0.10% or less, Ti: 0.100% or less, and B: less than 0.0010%. [Group 2] Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and Rare earth elements: 0.0200% or less, one or more selected from the group consisting of

3. The steel material according to claim 2, the chemical composition contains the first group; Steel material.

4. The steel material according to claim 2, The chemical composition contains the second group. Steel material.

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