Steel used as a material for bolts

Abstract

To provide a steel material capable of preventing invasion of hydrogen.SOLUTION: There is provided a steel material which comprises, by mass%, 0.15 to 0.40% of C, 0.02 to 0.50% of Si, 0.20 to 1.20% of Mn, 0.020% or less of P, 0.020% or less of S, 0.10 to 1.20% of Cr, 0.10 to 0.35% of Cu, 0.05 to less than 0.30% of Ni, 0.0010 to 0.0050% of B, 0.010 to 0.100% of Ti, 0.005 to 0.060% of Al 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] This disclosure relates to steel materials, and more particularly to steel materials used as the material for bolts. [Background technology]

[0002] Bolts are used as fastening devices in industrial machinery, automobiles, bridges, and buildings. Among these applications, bridges and buildings may be constructed in coastal areas or cold regions. Coastal areas are corrosive environments with high salt content. In cold regions, de-icing salts and de-icing agents may be used. These corrode the steel that makes up the bolts. Therefore, cold regions are often corrosive environments as well.

[0003] In such corrosive environments, hydrogen embrittlement is likely to occur. Therefore, bolts used in corrosive environments require excellent resistance to hydrogen embrittlement.

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

[0005] The steel material disclosed in Patent Document 1 has a composition in mass%, containing C: 0.15-0.6%, Si: 0.05-0.5%, Mn and Cr: 0.5-3.5% in total, 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: 0.05-0.50%, with the remainder being Fe and impurities, and a Cu / Sn ratio of 1 or less. In this document, the inclusion of Sn suppresses the penetration of hydrogen into the steel material, thereby improving its resistance to hydrogen embrittlement. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2008-274367 [Overview of the project]

Problems to be Solved by the Invention

[0007] As disclosed in Patent Document 1, it is known that the hydrogen embrittlement resistance of steel materials can be enhanced if the intrusion of hydrogen into the steel materials can be suppressed. In order to enhance the hydrogen embrittlement resistance of steel materials, the intrusion of hydrogen into the steel materials 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 is a steel material, whose chemical composition is, in mass%, C: 0.15 to 0.40%, Si: 0.02 to 0.50%, Mn: 0.20 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cr: 0.10 to 1.20%, Cu: 0.10 to 0.35%, Ni: less than 0.05 to 0.30%, B: 0.0010 to 0.0050%, Ti: 0.010 to 0.100%, Al: 0.005 to 0.060%, and N: 0.0100% or less, and contains the balance consists of Fe and impurities, 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 among 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 and the [Cu] of all the said measurement regions MAThe arithmetic mean value of AVE is defined as [Cu] For the [Cu] in each measurement region MA The ratio of the [Cu] AVE to the above is defined as [Cu] S When a plurality of the above measurement regions arranged in a line in the longitudinal direction among the plurality of measurement regions are defined as a measurement row, in each measurement row, the [Cu] of the plurality of measurement regions constituting the measurement row S The ratio of the sum of to the total number of the plurality of measurement regions constituting the measurement row is defined as [Cu] L When the sample standard deviation of the [Cu] of all the above measurement rows is defined as the Cu segregation degree σ, L The Cu segregation degree σ is 0.050 or less. The steel material according to the present disclosure is

[0010] a steel material, having a chemical composition in mass%, C: 0.15 to 0.40%, Si: 0.02 to 0.50%, Mn: 0.20 to 1.20%, P: 0.020% or less, ​​​​​​​​​​​​​​​​​​​​​​​​Surface analysis was performed using an electron beam microanalyzer on 160,000 measurement areas, which were divided into 400 sections in the radial direction and 400 sections in the longitudinal direction. The Cu content in mass % in each measurement area was then determined, and [Cu] MA year, All of the above measurement areas [Cu] MA The arithmetic mean of [Cu] AVE year, The [Cu] in each measurement area MA The aforementioned [Cu] AVE The ratio to [Cu] S year, When a row of multiple measurement areas arranged in a single line in the longitudinal direction is defined as a measurement row, in each measurement row, the [Cu] of the multiple measurement areas constituting the measurement row S The ratio of the sum of to the total number of the multiple measurement areas constituting the measurement row is [Cu] L year, The [Cu] of all the measurement rows L When the sample standard deviation is denoted as the degree of Cu segregation σ, The degree of Cu segregation σ is 0.050 or less. [Group 1] Mo: 0.30% or less V: 0.10% or less, and, One or more selected from the group consisting of Nb: 0.10% or less. [Group 2] Sn: 0.050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and, Rare earth elements: One or more elements selected from the group consisting of 0.0200% or less. [Effects of the Invention]

[0011] The steel material described herein can suppress hydrogen intrusion. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a schematic diagram illustrating the degree of Cu segregation σ. [Figure 2] Figure 2 is a schematic diagram illustrating the method for measuring the degree of Cu segregation σ within the rectangular observation area 100 in Figure 1. [Modes for carrying out the invention]

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

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

[0015] Based on the above findings, the inventors investigated the chemical composition of steel that can suppress hydrogen intrusion. As a result, the inventors found that the following composition (in mass%) is suitable: C: 0.15~0.40%, Si: 0.02~0.50%, Mn: 0.20~1.20%, P: 0.020% or less, S: 0.020% or less, Cr: 0.10~1.20%, Cu: 0.10~0.35%, Ni: 0.05~0.30%, B: 0.0010~0.0050%, Ti: 0.010~0.100%, Al: 0.00 We considered that steel materials having a chemical composition of 5-0.060%, N: 0.0100% or less, Mo: 0-0.30%, V: 0-0.10%, Nb: 0-0.10%, Sn: 0-0.050%, Ca: 0-0.0050%, Mg: 0-0.0050%, rare earth elements: 0-0.0200%, and the remainder being Fe and impurities, could potentially sufficiently suppress hydrogen penetration.

[0016] However, it was found that even steel materials having the above-mentioned chemical composition sometimes still cannot sufficiently suppress hydrogen intrusion. Therefore, the inventors conducted further investigations and studies to determine the reasons why the amount of hydrogen intrusion 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 on the surface of the steel material has an effect. Even if the content of each element in the chemical composition is within the above range, if the Cu concentration distribution on the surface is non-uniform, hydrogen can easily penetrate from the areas on the surface with low Cu concentration. Furthermore, if the Cu concentration distribution is non-uniform, the surface of the steel material will be corroded non-uniformly. In this case, irregularities will occur on the surface of the steel material, and the surface area on which the corrosion reaction occurs will increase. As a result, the corrosion reaction will increase, and it will not be possible to sufficiently suppress the amount of hydrogen penetration.

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

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

[0020] [1] It is made of steel, The chemical composition is expressed in mass percent. C: 0.15~0.40%, Si: 0.02~0.50%, Mn: 0.20~1.20%, P: 0.020% or less, S: 0.020% or less, Cr: 0.10~1.20%, Cu: 0.10~0.35%, Ni: 0.05-0.30% B: 0.0010~0.0050%, Ti: 0.010~0.100%, Al: 0.005~0.060%, and, Contains N: 0.0100% or less, The remainder consists of Fe and impurities. In a rectangular observation area of ​​the steel material, including the longitudinal and radial directions, where the area extends 1000 μm radially and 1000 μm longitudinally from the surface of the steel material, Surface analysis was performed using an electron beam microanalyzer on 160,000 measurement areas, which were divided into 400 sections in the radial direction and 400 sections in the longitudinal direction. The Cu content in mass % in each measurement area was then determined, and [Cu] MA year, All of the above measurement areas [Cu] MA The arithmetic mean of [Cu] AVE year, The [Cu] in each measurement area MA The aforementioned [Cu] AVE The ratio to [Cu] S year, When a row of multiple measurement areas arranged in a single line in the longitudinal direction is defined as a measurement row, in each measurement row, the [Cu] of the multiple measurement areas constituting the measurement row S The ratio of the sum of to the total number of the multiple measurement areas constituting the measurement row is [Cu] L year, The [Cu] of all the measurement rows L When the sample standard deviation is denoted as the degree of Cu segregation σ, The degree of Cu segregation σ is 0.050 or less. Steel material.

[0021] [2] It is made of steel, The chemical composition is expressed in mass percent. C: 0.15~0.40%, Si: 0.02~0.50%, Mn: 0.20~1.20%, P: 0.020% or less, S: 0.020% or less, Cr: 0.10~1.20%, Cu: 0.10~0.35%, Ni: 0.05-0.30% B: 0.0010~0.0050%, Ti: 0.010~0.100%, Al: 0.005~0.060%, and, Contains N: 0.0100% or less, The aforementioned chemical composition further contains one or more elements selected from the groups consisting of Group 1 and Group 2, with the remainder being Fe and impurities. In a rectangular observation area of ​​the steel material, including the longitudinal and radial directions, where the area extends 1000 μm radially and 1000 μm longitudinally from the surface of the steel material, Surface analysis was performed using an electron beam microanalyzer on 160,000 measurement areas, which were divided into 400 sections in the radial direction and 400 sections in the longitudinal direction. The Cu content in mass % in each measurement area was then determined, and [Cu] MA year, All of the above measurement areas [Cu] MA The arithmetic mean of [Cu] AVE year, The [Cu] in each measurement area MA The aforementioned [Cu] AVE The ratio to [Cu] S year, When a row of multiple measurement areas arranged in a single line in the longitudinal direction is defined as a measurement row, in each measurement row, the [Cu] of the multiple measurement areas constituting the measurement row S The ratio of the sum of to the total number of the multiple measurement areas constituting the measurement row is [Cu] L year, The [Cu] of all the measurement rows L When the sample standard deviation is denoted as the degree of Cu segregation σ, The degree of Cu segregation σ is 0.050 or less. Steel material. [Group 1] Mo: 0.30% or less V: 0.10% or less, and, One or more selected from the group consisting of Nb: 0.10% or less. [Group 2] Sn: 0.050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and, Rare earth elements: One or more elements selected from the group consisting of 0.0200% or less.

[0022] [3] [2] The steel material described above, The chemical composition includes the first group, Steel material.

[0023] [4] The steel materials described in [2] or [3], The aforementioned chemical composition contains the second group, Steel material.

[0024] The steel material according to this embodiment will be described in detail below. Note that, unless otherwise specified, the "%" in relation to elements refers to mass percentage.

[0025] [Features of the steel material of this embodiment] The steel material of this embodiment has the following features: (Feature 1) The content of each element in the chemical composition is as shown in this embodiment. (Feature 2) Surface analysis using an electron beam microanalyzer, as described later, revealed that the degree of Cu segregation σ on the surface of the steel material is 0.050 or less. The following describes each of its features.

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

[0027] C: 0.15~0.40% Carbon (C) enhances the hardenability of steel, thereby increasing the strength of bolts manufactured using steel as a material. If the C content is less than 0.15%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the carbon content exceeds 0.40%, the cold forgeability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.15-0.40%. The preferred lower limit for the C content is 0.18%, more preferably 0.20%, and even more preferably 0.22%. The preferred upper limit for the C content is 0.38%, more preferably 0.36%, and even more preferably 0.34%.

[0028] Si: 0.02~0.50% Silicon (Si) enhances the strength of bolts manufactured from steel through solid solution strengthening. If the Si content is less than 0.02%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 0.50%, the cold forgeability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.02-0.50%. The preferred lower limit for the Si content is 0.04%, more preferably 0.06%, and even more preferably 0.08%. The preferred upper limit for the Si content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0029] Mn: 0.20~1.20% Manganese (Mn) enhances the hardenability of steel and increases the strength of bolts. If the Mn content is less than 0.20%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 1.20%, the cold forgeability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Mn content is 0.20-1.20%. The preferred lower limit of the Mn content is 0.25%, more preferably 0.28%, and even more preferably 0.30%. The preferred upper limit for the Mn content is 1.00%, more preferably 0.90%, and even more preferably 0.80%.

[0030] P:0.020% or less Phosphorus (P) is an impurity. Therefore, the lower limit for P content is greater than 0%. If the P content exceeds 0.020%, P will segregate at the grain boundaries, even if the content of other elements is within the range of this embodiment. As a result, the hydrogen embrittlement resistance of the bolt will decrease. Therefore, the P content is 0.020% or less. A low phosphorus (P) content is preferable. However, an extreme reduction in P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the P content is 0.015%, more preferably 0.012%, more preferably 0.010%, more preferably 0.008%, and more preferably 0.007%.

[0031] S: 0.020% or less Sulfur (S) is an impurity. Therefore, the lower limit for S content is greater than 0%. If the S content exceeds 0.020%, S will segregate at the grain boundaries, even if the content of other elements is within the range of this embodiment. As a result, the hydrogen embrittlement resistance of the bolt will decrease. Therefore, the sulfur content is 0.020% or less. A low sulfur (S) content is preferable. However, an extreme reduction in S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for S content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the S content is 0.012%, more preferably 0.010%, and even more preferably 0.008%.

[0032] Cr: 0.10~1.20% Chromium (Cr) enhances the corrosion resistance of steel. If the Cr content is less than 0.10%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content exceeds 1.20%, the hardenability of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the Cr content is 0.10-1.20%. The preferred lower limit for the Cr content is 0.15%, more preferably 0.20%, even more preferably 0.25%, and even more preferably 0.30%. The preferred upper limit for the Cr content is 1.10%, more preferably 1.00%, even more preferably 0.90%, and even more preferably 0.80%.

[0033] Cu: 0.10~0.35% Copper (Cu) inhibits corrosion of steel. This suppresses the generation of hydrogen on the surface of the steel. As a result, the penetration of hydrogen into the steel is suppressed. If the Cu content is less than 0.10%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cu content exceeds 0.35%, the steel becomes brittle. Therefore, even if the content of other elements is within the range of this embodiment, the hot workability and cold forgeability of the steel will decrease. Therefore, the copper content is 0.10-0.35%. The preferred lower limit for the Cu content is 0.12%, more preferably 0.14%, and even more preferably 0.16%. The preferred upper limit for the Cu content is 0.34%, more preferably 0.33%, even more preferably 0.30%, and even more preferably 0.28%.

[0034] Ni: 0.05-0.30% Nickel (Ni) enhances the hardenability of steel and increases the strength of bolts. Ni further enhances the corrosion resistance of bolts. Ni, when included together with Cu, suppresses the occurrence of defects during hot working of steel. As a result, in the steel of this embodiment that contains Cu, Ni enhances the hot workability of the steel. If the Ni content is less than 0.05%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ni content is 0.30% or higher, the hardenability of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the Ni content is less than 0.05-0.30%. The preferred lower limit for the Ni content is 0.10%, more preferably 0.15%, and even more preferably 0.20%. The preferred upper limit for the Ni content is 0.28%, more preferably 0.26%, and even more preferably 0.24%.

[0035] B: 0.0010~0.0050% Boron (B) enhances the hardenability of steel and increases the strength of bolts. If the B content is less than 0.0010%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the B content exceeds 0.0050%, coarse B nitrides will be formed even if the content of other elements is within the range of this embodiment. Coarse B nitrides become the starting point for fracture. As a result, the cold forgeability of the steel decreases. Therefore, the B content is between 0.0010% and 0.0050%. The preferred lower limit for the B content is 0.0012%, more preferably 0.0014%, and even more preferably 0.0016%. The preferred upper limit for the B content is 0.0040%, more preferably 0.0038%, and even more preferably 0.0035%.

[0036] Ti: 0.010~0.100% Titanium (Ti) combines with nitrogen to form Ti nitrides, which inhibits the bonding of boron (B) with nitrogen. This allows B to improve the hardenability of the steel. If the Ti content is less than 0.010%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ti content exceeds 0.100%, even if the content of other elements is within the range of this embodiment, an excessive amount of Ti precipitates such as carbides and carbonitrides will be formed. In this case, the cold forgeability of the steel material will decrease. Therefore, the Ti content is between 0.010% and 0.100%. The preferred lower limit of the Ti content is 0.015%, more preferably 0.020%, even more preferably 0.025%, and even more preferably 0.030%. The preferred upper limit for the Ti content is 0.065%, more preferably 0.060%, even more preferably 0.055%, and even more preferably 0.050%.

[0037] Al: 0.005~0.060% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, the deoxidation of the steel will be insufficient, even if the content of other elements is within the range of this embodiment. In this case, coarse oxides are formed. As a result, the hydrogen embrittlement resistance of the bolt decreases. On the other hand, if the Al content exceeds 0.060%, coarse Al nitrides will be formed even if the content of other elements is within the range of this embodiment. Coarse Al nitrides become the starting point for fracture. As a result, the cold forgeability of the steel decreases. Therefore, the Al content is 0.005 to 0.060%. The preferred lower limit for the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. The preferred upper limit for 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 refers to the total Al (Total-Al) content.

[0038] N: 0.0100% or less Nitrogen (N) is inevitably present. Therefore, the N content is greater than 0%. N combines with Al or Ti to form nitrides or carbonitrides. These nitrides and carbonitrides suppress grain coarsening through a pinning effect. As a result, the cold forgeability of the steel is improved. However, if the N content exceeds 0.0100%, coarse nitrides will be formed even if the content of other elements is within the range of this embodiment. These coarse nitrides become the starting point for fracture. As a result, the cold forgeability of the steel decreases. Furthermore, the hydrogen embrittlement resistance of the bolt decreases. Therefore, the N content is 0.0100% or less. The preferred lower limit of the N content is 0.0001%, more preferably 0.0002%, even more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the N content is 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0039] The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of steel material, and are acceptable within a range that does not adversely affect the steel material according to this embodiment.

[0040] [Optional Elements] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the groups consisting of Group 1 and Group 2, in place of a portion of Fe. Any of these elements are optional and may not be included. [Group 1] Mo: 0.30% or less V: 0.10% or less, and, One or more selected from the group consisting of Nb: 0.10% or less. [Group 2] Sn: 0.050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and, Rare earth elements: One or more elements selected from the group consisting of 0.0200% or less. The following describes these arbitrary elements.

[0041] [Group 1: Mo, V, and Nb] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Mo, V, and Nb in place of a portion of Fe. These elements are arbitrary and all increase the strength of bolts manufactured using the steel material.

[0042] Mo: 0.30% or less Molybdenum (Mo) is an optional element and does not need to be included. In other words, the Mo content may be 0%. When present, i.e., when the Mo content is greater than 0%, Mo enhances the hardenability of the steel and increases the strength of the bolt. Furthermore, Mo forms fine carbides, which also enhances the strength of the bolt. Even a small amount of Mo will provide some of the above effects. However, if the Mo content exceeds 0.30%, the steel becomes excessively hard, even if the content of other elements is within the range of this embodiment. In this case, the cold forgeability of the steel decreases. Therefore, the Mo content is 0-0.30%, and if present, it is 0.30% or less (i.e., greater than 0-0.30%). The preferred lower limit for the Mo content is 0.01%, more preferably 0.02%, more preferably 0.05%, and still more preferably 0.08%. The preferred upper limit for the Mo content is 0.28%, more preferably 0.26%, and even more preferably 0.24%.

[0043] V: 0.10% or less Vanadium (V) is an optional element and does not need to be present. In other words, the V content may be 0%. If V is present, that is, if the V content is greater than 0%, V forms V precipitates such as carbides and carbonitrides. V precipitates increase the strength of the bolt. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.10%, a large amount of V precipitates will be formed, even if the content of other elements is within the range of this embodiment. In this case, the amount of hydrogen penetration into the steel increases. As a result, the hydrogen embrittlement resistance of the steel decreases. Therefore, the V content is 0-0.10%, and if present, it is 0.10% or less (i.e., greater than 0-0.10%). The preferred lower limit for the V content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the V content is 0.08%, more preferably 0.06%, and even more preferably 0.04%.

[0044] Nb: 0.10% or less Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. If Nb is present, that is, if the Nb content is greater than 0%, Nb forms Nb precipitates such as carbides and carbonitrides. Nb precipitates increase the strength of the bolt. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.10%, a large amount of Nb precipitates will be formed, even if the content of other elements is within the range of this embodiment. In this case, the amount of hydrogen penetration into the steel increases. As a result, the hydrogen embrittlement resistance of the steel decreases. Therefore, the Nb content is 0-0.10%, and if present, it is 0.10% or less (i.e., greater than 0-0.10%). The preferred lower limit for the Nb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. A preferred upper limit for Nb content is 0.08%, and more preferably 0.06%. A more effective preferred upper limit for Nb content to further reduce hydrogen intrusion is less than 0.04%.

[0045] [Group 2: Sn, Ca, Mg, and rare earth elements] The chemical composition of the steel material in this embodiment may further contain, in place of some of the Fe, one or more elements selected from the group consisting of Sn, Ca, Mg, and rare earth elements (REM). These elements are arbitrary and all enhance the hydrogen embrittlement resistance of the steel material.

[0046] Sn: 0.050% or less Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. When sn is present, that is, when the sn content is greater than 0%, sn inhibits the penetration of hydrogen into the steel. As a result, the hydrogen embrittlement resistance of the steel is enhanced. Even if only a small amount of sn is present, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.050%, Sn will segregate at the grain boundaries, even if the content of other elements is within the range of this embodiment. In this case, the hot workability and cold forgeability of the steel material will decrease. Furthermore, the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the Sn content is between 0 and 0.050%, and if present, it is 0.050% or less (i.e., greater than 0 and up to 0.050%). The preferred lower limit for the Sn content is 0.001%, more preferably 0.003%, and even more preferably 0.006%. The preferred upper limit for the Sn content is 0.048%, more preferably 0.045%, even more preferably 0.040%, and even more preferably 0.035%.

[0047] Ca: 0.0050% or less Calcium (Ca) is an optional element and does not need to be included. In other words, the Ca content may be 0%. When calcium is present, i.e., when the calcium content is greater than 0%, the calcium refines the MnS molecules. This improves the hydrogen embrittlement resistance of the steel. Even a small amount of calcium can provide this effect to some extent. However, if the Ca content exceeds 0.0050%, coarse Ca oxides will be formed, even if the content of other elements is within the range of this embodiment. In this case, the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the Ca content is between 0 and 0.0050%, and if present, it is 0.0050% or less (i.e., greater than 0 to 0.0050%). The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit for the Ca content is 0.0040%, and more preferably 0.0030%.

[0048] Mg: 0.0050% or less Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. When Mg is present, that is, when the Mg content is greater than 0%, Mg refines MnS. Therefore, the hydrogen embrittlement resistance of the steel improves. Even a small amount of Mg can provide some degree of this effect. However, if the Mg content exceeds 0.0050%, coarse Mg oxide will be formed, even if the content of other elements is within the range of this embodiment. In this case, the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the Mg content is between 0 and 0.0050%, and if present, it is 0.0050% or less (i.e., greater than 0 and up to 0.0050%). The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit for the Mg content is 0.0040%, and more preferably 0.0030%.

[0049] Rare earth elements (REM): 0.0200% or less Rare earth elements (REMs) are optional elements and do not need to be included. In other words, the REM content may be 0%. When REM is present, i.e., when the REM content is greater than 0%, REM refines MnS. Therefore, the hydrogen embrittlement resistance of the steel is enhanced. Even a small amount of REM can provide some degree of this effect. However, if the REM content exceeds 0.0200%, coarse oxides will be formed even if the content of other elements is within the range of this embodiment. In this case, the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the REM content is between 0 and 0.0200%, and if present, it is 0.0200% or less (i.e., greater than 0 and up to 0.0200%). The preferred lower limit of the REM content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0050%. The preferred upper limit for the REM content is 0.0150%, and more preferably 0.0100%.

[0050] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this specification, REM content refers to the total content of these elements.

[0051] [Method for measuring the chemical composition of steel materials] The chemical composition of the steel material in this embodiment can be measured using a well-known component analysis method in accordance with JIS G0321:2017. Specifically, chips are collected from the inside 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. Elemental analysis of the chemical composition is performed on the solution using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). The C and S content is determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas melting-thermal conductivity method.

[0052] Furthermore, the content of each element shall be rounded to the minimum digit of the element content specified in this embodiment, based on the significant figures defined in this embodiment. For example, the carbon content of the steel material in this embodiment is defined to two decimal places. Therefore, the carbon content shall be the value obtained by rounding the third decimal place of the measured value to two decimal places.

[0053] Similarly, for the carbon content of the steel material in this embodiment, the content of other elements is determined by rounding the measured value to the minimum digit specified in this embodiment.

[0054] Rounding means that if the fractional part is less than 5, it is rounded down, and if the fractional part is 5 or greater, it is rounded up.

[0055] [(Feature 2) Regarding the degree of Cu segregation σ in the surface layer of the steel material] In the steel material of this embodiment, surface analysis is performed using an electron beam microanalyzer on 160,000 measurement areas, which are divided into 400 sections radially and 400 sections longitudinally within a rectangular observation area of ​​the cross-section of the steel material, including the longitudinal and radial directions, with a length of 1000 μm from the surface of the steel material. The Cu content in mass % in each measurement area obtained is then determined as [Cu] MA And [Cu] in all measurement areas MA The arithmetic mean of [Cu] AVE The [Cu] of each measurement area MA [Cu] AVE The ratio to [Cu] S Assuming that a measurement row is defined as a row of measurement areas arranged in a single line along the longitudinal direction, then in each measurement row, the [Cu] of the multiple measurement areas constituting the measurement row S The ratio of the total to the total number of measurement areas that make up the measurement row is [Cu] L And for all measurement rows, [Cu] L When the sample standard deviation is denoted as the degree of Cu segregation σ, the degree of Cu segregation σ is 0.050 or less. The degree of Cu segregation σ is explained below.

[0056] [Regarding the degree of Cu segregation σ] Figure 1 is a schematic diagram illustrating the degree of Cu segregation σ. Referring to Figure 1, a rectangular observation area 100 is selected from the cross section 10 of the steel material, which includes the longitudinal direction L and the radial direction D, with a length of 1000 μm in the radial direction D and 1000 μm in the longitudinal direction L from the surface 10S of the steel material.

[0057] Figure 1 shows an enlarged view of the rectangular observation area 100. In the microscopic region of the steel surface (rectangular observation area 100), if a Cu segregation region SEG exists, the Cu segregation region SEG extends in the longitudinal direction L of the steel. Here, the Cu segregation region SEG is a region with a higher Cu concentration compared to other regions.

[0058] In the cross-section 10, the region extending radially 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 area 100 is any part of the surface layer 10L. If one or more Cu segregation regions SEG exist in the rectangular observation area 100, then there is variation in Cu concentration within the rectangular observation area 100. In this case, it means that there is variation in Cu concentration in the surface layer 10L. When such variation in Cu concentration occurs in the surface layer 10L, hydrogen can easily penetrate from the outside into the areas of the surface layer 10L with low Cu concentration. Therefore, maintaining a uniform distribution of Cu concentration within the surface layer 10L is effective in suppressing hydrogen penetration.

[0059] Therefore, in the steel material of this embodiment, the "degree of Cu segregation σ" obtained by the following measurement method is used as an indicator of the uniformity of the Cu concentration distribution in the surface layer 10L of the steel material.

[0060] [Method for measuring Cu segregation degree σ] The degree of Cu segregation σ of the steel material in this embodiment can be measured by the following method. Referring to Figure 1, select an arbitrary rectangular observation area 100 within the surface layer 10L. As described above, the rectangular observation area 100 is a rectangle with a radius of 1000 μm in the radial direction D and a length of 1000 μm in the longitudinal direction L from the surface 10S of the steel material.

[0061] Figure 2 is a schematic diagram illustrating the method for measuring the degree of Cu segregation σ within the rectangular observation area 100 shown in Figure 1. Referring to Figure 2, surface analysis is performed on the rectangular observation area 100 using a field emission electron beam microanalyzer (FE-EPMA). Specifically, the rectangular observation area 100 is divided into 400 sections in the longitudinal direction L and 400 sections in the radial direction D, thereby dividing the rectangular observation area 100 into 160,000 measurement areas MA.

[0062] Elemental analysis will be performed on each measurement area MA. For the elemental analysis, the acceleration voltage will be 15kV, the irradiation current 400nA, the beam diameter 2μm, and the integration time 0.1 seconds. The target element will be Cu, and the Cu content in mass % will be determined for each measurement area MA. The Cu content will then be expressed as [Cu]. MA This is how it is defined.

[0063] [Cu] obtained in each measurement area MA MA Using the following method, the degree of Cu segregation σ in the rectangular observation area 100 is determined.

[0064] Cu content in mass % in all measurement areas MA [Cu] MA The arithmetic mean of [Cu] AVE This is defined as follows. Furthermore, the Cu content [Cu] in each measurement area MA MA [Cu] AVE The ratio to [Cu] S This is defined as [Cu]. In short, [Cu] S This represents the amount of Cu in each measurement area, excluding the influence of the Cu content.

[0065] Referring to Figure 2, we define a "measurement row" ML1 to ML400 as a row of measurement areas arranged in a single column along the longitudinal direction L, out of 400 x 400 measurement areas MA. Each measurement row MLj (where j is an integer from 1 to 400) consists of 400 measurement areas MA arranged in a single column along the longitudinal direction L. In Figure 2, multiple measurement areas MA within the dashed line constitute a measurement row ML1.

[0066] [Cu] of the multiple measurement areas MA that make up each measurement row MLjS The ratio of the sum of to the total number of measurement areas MA constituting the measurement row MLj (i.e., 400) is given by [Cu] L This is defined as [Cu] L This represents the amount of Cu in the measurement row MLj.

[0067] [Cu] of all measurement rows MLj L The sample standard deviation is calculated. The obtained value is defined as the "Cu segregation degree σ". The Cu segregation degree σ has three significant figures. In other words, the Cu segregation degree σ is the value obtained by rounding the number to the fourth decimal place.

[0068] [Regarding the significance of the degree 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. Therefore, the amount of Cu [Cu] in each measurement row MLj L The sample standard deviation σ is an indicator of the degree of segregation of Cu.

[0069] If the degree of Cu segregation σ exceeds 0.050, sufficient uniformity of the Cu concentration distribution is not achieved in the top 10L of the material. In other words, the Cu concentration distribution is excessively varied. In this case, even steel materials possessing characteristic 1 cannot adequately suppress hydrogen intrusion.

[0070] If the degree of Cu segregation σ is 0.050 or less, the Cu concentration distribution in the top 10L of the steel material possessing characteristic 1 is sufficiently uniform. Therefore, hydrogen intrusion can be sufficiently suppressed.

[0071] The preferred upper limit for the degree of Cu segregation σ is 0.045, more preferably 0.040, more preferably 0.035, and still more preferably 0.030. A lower degree of Cu segregation σ is preferable. The preferred lower limit for the degree of Cu segregation σ is 0.000, more preferably 0.005, and still more preferably 0.010.

[0072] [Shape of the steel material in this embodiment] The steel material of this embodiment is a steel material with a circular cross-section perpendicular to the longitudinal direction, and more specifically, a steel bar or wire with a circular cross-section perpendicular to the longitudinal direction. The steel bar or wire is a steel material that extends in a rod shape. The steel material may be wound in a coil shape or cut to a predetermined length.

[0073] [Microstructure of the steel material of this embodiment] The microstructure of the steel material in this embodiment is not particularly limited. When the steel material in this embodiment is used as a material for bolts, if the hardness of the steel material is too high, a spheroidizing annealing treatment is performed before carrying out the bolt manufacturing process (wire drawing process or cold forging process). The cold forgeability of spheroidizing annealed steel material is increased. Therefore, it is possible to manufacture bolts by performing cold forging using the steel material in this embodiment as a material. Accordingly, the microstructure of the steel material in this embodiment is not particularly limited.

[0074] [Applications of the steel material of 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, and buildings. However, the steel material of this embodiment may also be used for applications other than those described above.

[0075] [Methods for manufacturing steel materials] 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 just one example for manufacturing the steel material of this embodiment. Therefore, the steel material having the above-described features 1 and 2 may be manufactured by other manufacturing methods other than the method described below. However, the method described below is a preferred example of a method for manufacturing the steel material of this embodiment.

[0076] An example of a method for manufacturing steel materials according to this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Rough rolling process (Process 3) Finish rolling process In this manufacturing method, the following conditions are also met during the finishing rolling process. Conditions: The steel material temperature must be within the range of 1000 to 880°C, and the number of passes with a reduction ratio of 25% or more must be 2 or more. The following describes each step.

[0077] [(Process 1) Material preparation process] In the material preparation process, the material for the steel of this embodiment is prepared. Specifically, molten steel is produced in which the content of each element in the chemical composition is within the range of this embodiment. The refining method is not particularly limited, and any well-known method may be used. For example, molten iron produced by a well-known method is refined in a converter (primary refining). A well-known secondary refining is performed on the molten steel tapped from the converter. Through the above process, molten steel with a chemical composition satisfying Feature 1 is produced.

[0078] The material is manufactured using the molten steel produced by a well-known casting method. For example, an ingot may be manufactured using the ingot-making method with the molten steel. Alternatively, a bloom may be manufactured using the continuous casting method with the molten steel. The material (ingot or bloom) is manufactured by the above method.

[0079] [(Process 2) Rough rolling process] In the rough rolling process, the material (ingot or bloom) prepared in the material preparation process is subjected to rough rolling to produce a billet.

[0080] The rough rolling process includes the following steps: (Step 21) Heating step (Process 22) Rolling process The following describes each of the processes 21 and 22.

[0081] [(Step 21) Heating step] In the heating process, the material is heated using a heating furnace in a well-known manner. The heating temperature is not particularly limited; any known temperature is sufficient. For example, the heating temperature is 1000 to 1200°C.

[0082] [(Process 22) Rolling Process] In the rolling process, the material heated in the heating process is rolled (roughly rolled) using a bloc mill, or a bloc mill and a continuous mill, to produce a billet.

[0083] Specifically, a billet is manufactured by reverse rolling a heated material using a bract mill. The bract mill is equipped with a pair of horizontal rolls. Reverse rolling is performed in the bract mill. Reverse rolling refers to a rolling method in which the material is reduced by the bract mill as it passes from upstream to downstream, and again as it passes from downstream to upstream.

[0084] If a continuous rolling mill is located downstream of a bract mill, the billets after bract milling may be further subjected to tandem rolling using the continuous rolling mill to produce smaller billets. In a continuous rolling mill, horizontal stands with a pair of horizontal rolls and vertical stands with a pair of vertical rolls are arranged alternately in a row. In a continuous rolling mill, tandem rolling is performed from upstream to downstream.

[0085] The billets produced by the rough rolling process described above are allowed to cool to room temperature (air-cooled) before the finish rolling process.

[0086] [(Process 3) Finish Rolling Process] In the finishing rolling process, the billets produced in the rough rolling process are subjected to finishing rolling to produce steel materials. Here, the steel materials are wire rods or steel bars. The finishing rolling process includes the following steps: (Step 31) Heating step (Process 32) Rolling process

[0087] In the finishing rolling process, the following conditions must also be met during the rolling process. (conditions) The steel material temperature is within the range of 1000 to 880°C, and the number of passes with a reduction ratio of 25% or more is 2 or more. The heating and rolling processes in the finishing rolling process will be described below.

[0088] [(Step 31) Heating step] In the heating process, the billet, which has been cooled to room temperature, is heated in a heating furnace using a well-known method. The heating temperature is not particularly limited, but is, for example, 900 to 1050°C.

[0089] [(Process 32) Rolling Process] In the rolling process, steel is produced by performing finish rolling (continuous rolling) on ​​the billet heated in the heating process using a continuous rolling mill. The continuous rolling mill includes multiple rolling stands arranged in a line from upstream to downstream. Each rolling stand includes a pair of work rolls. Each roll has a caliber formed on it, and the calibers of the pair of rolls form a hole.

[0090] In continuous rolling using a continuous rolling mill, when a billet passes through each rolling stand from upstream to downstream, the reduction of the billet's surface area at that rolling stand is defined as "one pass" reduction.

[0091] Continuous rolling refers to the process of reducing a billet using a continuous rolling mill, involving multiple passes. It is not necessary to reduce the billet at every rolling stand in the continuous rolling mill. For example, if a continuous rolling mill has 15 rolling stands, and the billet is not reduced at the last rolling stand, then 14 rolling passes will be performed.

[0092] [Regarding conditions] In the finishing rolling process, the steel material temperature is within the range of 1000 to 880°C, and the number of passes with a reduction ratio of 25% or more is set to two or more. Here, the reduction ratio is defined by the following formula. Area reduction ratio (%) = (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) × 100

[0093] It is generally known that adjusting the heating temperature in a furnace can reduce the segregation of elements such as Mn. However, in the case of Cu, our research has revealed that introducing a large amount of strain by reducing the temperature in a specific temperature range facilitates more uniform diffusion on the surface of the steel than adjusting the heating temperature to facilitate diffusion.

[0094] Specifically, if the steel temperature is within the range of 1000-880°C and the number of passes with a reduction ratio of 25% or more is less than 2, the amount of strain introduced is insufficient. In this case, during the finish rolling process, Cu does not diffuse sufficiently uniformly in the surface layer of the steel. As a result, the degree of Cu segregation σ exceeds 0.050.

[0095] If the steel temperature is within the range of 1000 to 880°C and the number of passes with a reduction ratio of 25% or more is 2 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 degree of Cu segregation σ becomes 0.050 or less.

[0096] Therefore, the steel material temperature should be within the range of 1000 to 880°C, with a reduction ratio of 25% or more and a number of passes of 2 or more.

[0097] When the steel material temperature is in the range of 1000 to 880°C, the preferred lower limit for the number of passes with a reduction ratio of 25% or more is 3, and more preferably 4. There is no particular upper limit to the preferred number of passes with a reduction ratio of 25% or more when the steel material temperature is in the range of 1000 to 880°C.

[0098] A thermometer is placed at the inlet and / or outlet of each rolling stand in the continuous rolling mill. The thermometer is a well-known device, such as a radiation thermometer or thermograph. The steel temperature is measured at the inlet and / or outlet of each rolling stand. The reduction ratio at each rolling stand is also predetermined. Therefore, based on the steel temperature measured at the inlet and / or outlet of each rolling stand, and the reduction ratio at each rolling stand, the number of passes with a reduction ratio of 25% or more within the range of 1000 to 880°C for the steel temperature can be determined.

[0099] In the finishing rolling process, the cooling method after continuous rolling is not particularly limited. The cooling method may be air cooling, slow cooling, or rapid cooling.

[0100] Through the above manufacturing process, steel materials that satisfy features 1 and 2 can be produced.

[0101] [Method for manufacturing a bolt using the steel material of this embodiment] The method for manufacturing a bolt using steel as the material in this embodiment is a well-known manufacturing method. The method for manufacturing a bolt includes, for example, the following steps. ·Wire drawing process • Cold forging process • Quenching and tempering process The following describes each step.

[0102] [Wire drawing process] In the wire drawing process, steel wire is manufactured by performing the well-known wire drawing process on the aforementioned steel material. The wire drawing process may consist of primary drawing only, or it may involve multiple wire drawing processes such as secondary drawing.

[0103] [Cold forging process] In the cold forging process, the steel wire after the wire drawing process is subjected to the well-known cold forging method to produce bolt-shaped intermediate products.

[0104] [Quenching and tempering process] In the quenching and tempering process, quenching and tempering are performed on the intermediate product.

[0105] [Hardening] The hardening is carried out by a well-known method. The hardening temperature and the holding time at the hardening temperature are not particularly limited. For example, the hardening temperature is 840 to 970°C. The holding time at the hardening temperature is 15 to 360 minutes. After the holding time, the intermediate product is rapidly cooled. Specifically, the intermediate product is water-cooled or oil-cooled.

[0106] [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. For example, the tempering temperature is 400 to 550°C. The holding time at the tempering temperature is 30 to 360 minutes.

[0107] By the manufacturing method described above, bolts made from the steel material of this embodiment can be manufactured. The manufactured bolts possess features 1 and 2. Therefore, hydrogen intrusion in corrosive environments is suppressed. [Examples]

[0108] The effects of the steel material of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just 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.

[0109] Steel materials having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured by the following method.

[0110] [Table 1-1]

[0111] [Table 1-2]

[0112] [Material preparation process] Steel materials (blooms) having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured.

[0113] [Rough rolling process] A rough rolling process was performed on the manufactured bloom to produce billets. Specifically, the bloom was heated to 1100°C using a heating furnace. After heating, the bloom was rolled (roughly rolled) using a bract mill and a continuous mill to produce billets. The billets produced in the rough rolling process were allowed to cool to room temperature.

[0114] [Finishing Rolling Process] The manufactured billets underwent a finish rolling process. Specifically, each billet with a test number was heated to 950-1050°C. The heated billets were then subjected to finish rolling (continuous rolling) using a continuous rolling mill to produce steel bars. At this time, the number of passes PN for a reduction in surface area of ​​25% or more, while the steel temperature was within the range of 1000-880°C, is shown in Table 2.

[0115] [Table 2]

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

[0117] [About the evaluation test] The following steel evaluation tests (Tests 1 to 3) were performed on the steel materials of each test number that were manufactured. (Test 1) Chemical composition measurement test of steel material (Test 2) Measurement test of Cu segregation degree σ (Test 3) Diffusible hydrogen concentration measurement test The following describes each test.

[0118] [(Test 1) Chemical composition measurement test of steel material] The chemical composition of the steel material (round bar) for each test number was analyzed based on the [Method for Measuring the Chemical Composition of Steel Material] described above. As a result, the chemical composition of each test number was as shown in Tables 1-1 and 1-2.

[0119] [(Test 2) Measurement test of Cu segregation degree σ] For each steel material (round bar) with a given test number, the degree of Cu segregation σ was determined based on the [Method for Measuring Cu Segregation σ] described above. The results are shown in Table 2.

[0120] [(Test 3) Measurement Test of Diffusible Hydrogen Concentration] The following diffusible hydrogen concentration measurement tests were performed on the steel materials for each test number.

[0121] [Manufacturing of bolt imitation materials] First, bolt simulations were manufactured using round bars corresponding to each test number. Specifically, the following quenching and tempering treatments were performed on the round bars corresponding to each test number. The quenching treatment was carried out using a heat treatment furnace. The quenching temperature was set to 870-920°C, and the holding time at the quenching temperature was set to 60 minutes. After the holding time, the steel material was water-cooled and then quenched. In addition, the heat treatment furnace was maintained with a carbon potential atmosphere equivalent to the carbon concentration of the steel material to suppress decarburization of the steel material.

[0122] After quenching, tempering was performed. The tempering was carried out using a heat treatment furnace. During tempering, the bolt simulant material was held at a tempering temperature of 430-520°C for 1-2 hours so that its tensile strength was within the range of 1200-1400 MPa. After the holding time, the steel material was water-cooled. Bolt simulant material (round bar) was manufactured using the above manufacturing process.

[0123] [Measurement Test] Each bolt simulant (round bar) for each test number was cut perpendicular to its longitudinal direction, and multiple round bar test pieces with a length of 100 mm were taken. To eliminate the influence of scale generated during the rough rolling process, finish rolling process, and quenching and tempering process during the manufacture of the bolt simulant, the round bar test pieces were blast-treated to remove the scale from the outermost layer of the round bar test pieces.

[0124] Combined cycle corrosion tests (CCTs) as specified in JASO M609 (1991) were performed using blast-treated round bar specimens. For each test number, four test cycle patterns were used: 84 cycles, 168 cycles, 252 cycles, and 336 cycles. A separate round bar specimen was used for each pattern.

[0125] After conducting tests for each pattern, the round bar specimens were removed. The removed round bar specimens were blast-treated to remove the corrosion products that had formed on the surface of the round bar specimens during the corrosion test. Using a wet cutting machine, the central 30 mm portion of the blast-treated round bar specimens was cut out.

[0126] The diffusible hydrogen concentration in the cut-out round bar specimen was analyzed using a gas chromatograph-type thermodynamic desorption hydrogen analyzer (TDA). Specifically, the cut-out round bar specimen was heated from room temperature to 400°C at a heating rate of 100°C / h. The amount of hydrogen released from the round bar specimen to the outside was measured.

[0127] The measured amount of hydrogen was divided by the mass of the round bar test specimen 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 among the four diffusible hydrogen concentrations was defined as the diffusible hydrogen concentration for that test number.

[0128] [Evaluation Results] The evaluation results are shown in Table 2. In tests 1 through 37, the chemical composition was appropriate. Furthermore, the manufacturing conditions were also appropriate. As a result, the degree of Cu segregation was 0.050 or less. Consequently, the diffusible hydrogen concentration was less than 0.10 ppm, and hydrogen intrusion was suppressed.

[0129] On the other hand, in tests 38 and 39, the Cu content was too low. As a result, the diffusible hydrogen concentration exceeded 0.10 ppm, and hydrogen intrusion could not be adequately suppressed.

[0130] In tests 40-45, although the chemical composition was appropriate, the number of passes (PN) at a reduction ratio of 25% or more within the steel temperature range of 1000-880°C was less than 2. As a result, the degree of Cu segregation (σ) exceeded 0.050. Consequently, the diffusible hydrogen concentration exceeded 0.10 ppm, and hydrogen penetration could not be adequately suppressed.

[0131] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. It is made of steel, The chemical composition is expressed in mass percent. C: 0.15-0.40%, Si: 0.02-0.50%, Mn: 0.20-1.20%, P: 0.020% or less, S: 0.020% or less, Cr: 0.10-1.20%, Cu: 0.10 to 0.35%, Ni: 0.05% to less than 0.30% B: 0.0010 to 0.0050%, Ti: 0.010 to 0.100%, Al: 0.005–0.060%, and, Contains N: 0.0100% or less, The remainder consists of Fe and impurities. In a rectangular observation area of ​​the steel material, including the longitudinal and radial directions, where the area extends 1000 μm radially and 1000 μm longitudinally from the surface of the steel material, Surface analysis was performed using an electron beam microanalyzer on 160,000 measurement areas, which were divided into 400 sections in the radial direction and 400 sections in the longitudinal direction. The Cu content in mass % in each measurement area was then determined as [Cu] ＭＡ year, All of the above measurement areas [Cu ＭＡ The arithmetic mean of [Cu] ＡＶＥ year, The [Cu] in each measurement area ＭＡ The aforementioned [Cu] ＡＶＥ The ratio to [Cu] Ｓ year, When a row of multiple measurement areas arranged in a single line in the longitudinal direction is defined as a measurement row, in each measurement row, the [Cu] of the multiple measurement areas constituting the measurement row Ｓ The ratio of the sum of to the total number of measurement areas constituting the measurement row is [Cu] Ｌ year, The [Cu] of all the measurement rows Ｌ When the sample standard deviation is defined as the Cu segregation degree σ The degree of Cu segregation σ is 0.050 or less. Steel material.

2. It is made of steel, The chemical composition is expressed in mass percent. C: 0.15-0.40%, Si: 0.02-0.50%, Mn: 0.20-1.20%, P: 0.020% or less, S: 0.020% or less, Cr: 0.10-1.20%, Cu: 0.10 to 0.35%, Ni: 0.05% to less than 0.30% B: 0.0010 to 0.0050%, Ti: 0.010 to 0.100%, Al: 0.005–0.060%, and, Contains N: 0.0100% or less, The aforementioned chemical composition further contains one or more elements selected from the group consisting of Group 1 and Group 2, with the remainder being Fe and impurities. In a rectangular observation area of ​​the steel material, including the longitudinal and radial directions, where the area extends 1000 μm radially and 1000 μm longitudinally from the surface of the steel material, Surface analysis was performed using an electron beam microanalyzer on 160,000 measurement areas, which were divided into 400 sections in the radial direction and 400 sections in the longitudinal direction. The Cu content in mass % in each measurement area was then determined as [Cu] ＭＡ year, All of the above measurement areas [Cu ＭＡ The arithmetic mean of [Cu] ＡＶＥ year, The [Cu] in each measurement area ＭＡ The aforementioned [Cu] ＡＶＥ The ratio to [Cu] Ｓ year, When a row of multiple measurement areas arranged in a single line in the longitudinal direction is defined as a measurement row, in each measurement row, the [Cu] of the multiple measurement areas constituting the measurement row Ｓ The ratio of the sum of to the total number of measurement areas constituting the measurement row is [Cu] Ｌ year, The [Cu] of all the measurement rows Ｌ When the sample standard deviation is denoted as the degree of Cu segregation σ, The degree of Cu segregation σ is 0.050 or less. Steel material. [Group 1] Mo: 0.30% or less V: 0.10% or less, One or more selected from the group consisting of Nb: 0.10% or less. [Group 2] Sn: 0.050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Rare earth elements: One or more selected from the group consisting of 0.0200% or less.

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

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

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