steel

Abstract

To provide a steel material that can suppress hydrogen intrusion.SOLUTION: The steel material according to the present disclosure is a steel material that is rod-shaped or line-shaped and whose cross section perpendicular to the longitudinal direction is circular, and contains, in mass%, C: 0.50 to 0.80%, Si: 0.010 to 0.500%, Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.080%, Ti: 0.005 to 0.100%, B: 0.0003 to 0.0050%, Cu: 0.02 to 0.50%, Ni: 0.01 to 0.50%, N: 0.0150% or less, and O: 0.0100% or less, and the balance consists of Fe and impurities, and in which the microstructure mainly comprises bainite, and in a cross section including the longitudinal and radial directions of the steel material, the Cu segregation degree σ is 0.050 or less in a rectangular observation area of 1000 μm in the radial direction and 1000 μm in the longitudinal direction from the surface of the steel material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to steel products. [Background technology]

[0002] Machine parts such as bolts are used in 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 machine parts. In other words, cold regions are often corrosive environments as well.

[0003] Hydrogen embrittlement is likely to occur in such corrosive environments, so mechanical parts used in such environments are required to have excellent resistance to hydrogen embrittlement.

[0004] A technique for improving 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] Recently, non-heat treated machine parts have been proposed, which omit the heat treatment (quenching and tempering) in the manufacturing process of machine parts such as bolts, etc. Therefore, there is a demand for steel materials that can be used as raw materials for such non-heat treated machine parts.

[0009] An object of the present disclosure is to provide a steel material capable of suppressing hydrogen penetration. [Means for solving the problem]

[0010] The steel material according to the present disclosure has the following configuration.

[0011] A steel material that is rod-shaped or wire-shaped and has a circular cross section perpendicular to the longitudinal direction, In mass%, C: 0.50~0.80%, Si: 0.010 to 0.500%, Mn: 0.50 to 2.00%, P:0.030% or less, S: 0.030% or less, Al: 0.005 to 0.080%, Ti: 0.005 to 0.100%, B: 0.0003~0.0050%, Cu: 0.02 to 0.50% Ni: 0.01 to 0.50% N: 0.0150% or less, and O: 0.0100% or less, the balance being Fe and impurities, When a diameter of the cross section perpendicular to the longitudinal direction of the steel material is defined as D, in a microstructure at a depth position of D / 4 from the surface of the steel material, an area ratio of bainite is 100%, or an area ratio of bainite is 95% or more and the remainder is composed of ferrite and / or pearlite, In a rectangular observation area of ​​the cross section of the steel material including the longitudinal direction and the radial direction, 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. [Effects of the Invention]

[0012] The steel material according to the present disclosure can suppress the penetration of hydrogen. [Brief explanation of the drawings]

[0013] [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 in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present inventors have investigated and studied steel materials capable of suppressing hydrogen penetration, and have obtained the following findings.

[0015] 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.

[0016] Based on the above findings, the inventors investigated the chemical composition of steel that can suppress hydrogen penetration. As a result, they found that the composition is, in mass%, C: 0.50-0.80%, Si: 0.010-0.500%, Mn: 0.50-2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.005-0.080%, Ti: 0.005-0.100%, B: 0.0003-0.0050%, Cu: 0.02-0.50%, Ni: 0.01-0.50%, N: 0. It was thought that hydrogen penetration could be sufficiently suppressed if the steel had a chemical composition of: Cr: 0-1.50%, Mo: 0-0.50%, Nb: 0-0.050%, V: 0-0.20%, Ca: 0-0.0100%, Mg: 0-0.0100%, and Sn: 0-0.0020%, with the balance being Fe and impurities.

[0017] Furthermore, if the steel material is used as a material for non-thermal treated machine parts, the manufactured non-thermal treated machine parts are required to have high strength. Furthermore, in the cold working process during the manufacturing process of the non-thermal treated machine parts, the steel material is required to have excellent cold workability. In order to obtain sufficient strength in the non-thermal treated machine parts and sufficient cold workability of the steel material during the manufacturing process of the non-thermal treated machine parts, it is preferable that the microstructure of the steel material having the above-mentioned chemical composition is a structure mainly composed of bainite.

[0018] However, it has been found that even in steel materials having the above-mentioned chemical composition and microstructure, 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.

[0019] 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.

[0020] Therefore, the inventors further adjusted the relationship between the Cu concentration distribution in the surface layer and the amount of hydrogen penetration, and 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.

[0021] The steel material according to this embodiment, which has been completed based on the above findings, has the following configuration.

[0022] [1] A steel material that is rod-shaped or wire-shaped and has a circular cross section perpendicular to the longitudinal direction, In mass%, C: 0.50~0.80%, Si: 0.010 to 0.500%, Mn: 0.50 to 2.00%, P:0.030% or less, S: 0.030% or less, Al: 0.005 to 0.080%, Ti: 0.005 to 0.100%, B: 0.0003~0.0050%, Cu: 0.02 to 0.50% Ni: 0.01 to 0.50% N: 0.0150% or less, and O: 0.0100% or less, the balance being Fe and impurities, When a diameter of the cross section perpendicular to the longitudinal direction of the steel material is defined as D, in a microstructure at a depth position of D / 4 from the surface of the steel material, an area ratio of bainite is 100%, or an area ratio of bainite is 95% or more and the remainder is composed of ferrite and / or pearlite, In a rectangular observation area of ​​the cross section of the steel material including the longitudinal direction and the radial direction, 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 SThe 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.

[0023] [2] A steel material that is rod-shaped or wire-shaped and has a circular cross section perpendicular to the longitudinal direction, In mass%, C: 0.50~0.80%, Si: 0.010 to 0.500%, Mn: 0.50 to 2.00%, P:0.030% or less, S: 0.030% or less, Al: 0.005 to 0.080%, Ti: 0.005 to 0.100%, B: 0.0003~0.0050%, Cu: 0.02 to 0.50% Ni: 0.01 to 0.50% N: 0.0150% or less, and O: 0.0100% or less, Further, it contains one or more elements selected from the group consisting of Groups 1 to 3, with the balance being Fe and impurities, When a diameter of the cross section perpendicular to the longitudinal direction of the steel material is defined as D, in a microstructure at a depth position of D / 4 from the surface of the steel material, an area ratio of bainite is 100%, or an area ratio of bainite is 95% or more and the remainder is composed of ferrite and / or pearlite, In a rectangular observation area of ​​the cross section of the steel material including the longitudinal direction and the radial direction, 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] MAyear, 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] Cr: 1.50% or less, Mo: 0.50% or less Nb: 0.050% or less, and V: 0.20% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0100% or less, and Mg: 0.0100% or less, one or more selected from the group consisting of [Group 3] Sn: 0.0020% or less

[0024] [3] [2] The steel material according to containing the first group, Steel material.

[0025] [4] The steel material according to [2] or [3], containing the second group, Steel material.

[0026] [5] The steel material according to any one of [2] to [4], containing the third group, Steel material.

[0027] The steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0028] [Features of the steel material of this embodiment] The steel material of this embodiment includes the following features. (Feature 1) The chemical composition is, in mass%, C: 0.50 to 0.80%, Si: 0.010 to 0.500%, Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.080%, Ti: 0.005 to 0.100%, B: 0.0003 to 0.0050%, Cu: 0.02 to 0.50%, N i: 0.01 to 0.50%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 1.50%, Mo: 0 to 0.50%, Nb: 0 to 0.050%, V: 0 to 0.20%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, and Sn: 0 to 0.0020%, and the balance is Fe and impurities. (Feature 2) When the diameter of a steel material having a circular cross section perpendicular to the longitudinal direction is defined as D, in the microstructure at a depth of D / 4 from the surface of the steel material, the area ratio of bainite is 100%, or the area ratio of bainite is 95% or more with the remainder consisting of ferrite and / or pearlite. (Feature 3) In a cross section including the longitudinal and radial directions of a steel material whose cross section perpendicular to the longitudinal direction is circular, in a rectangular observation area of ​​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 analyzed by an electron beam microanalyzer, and the Cu content in mass% in each measurement area was calculated as [Cu] MA year, [Cu] in all measurement areas MA The arithmetic mean value of [Cu]AVE year, [Cu] in each measurement area MA [Cu] AVE The ratio to [Cu] S year, When a row of measurement areas is defined as a row of measurement areas arranged in a line in the longitudinal direction of the steel material, the [Cu] of the measurement areas that make up each row is calculated as follows: 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 year, [Cu] for all measurement rows L When the sample standard deviation is the Cu segregation degree σ, The Cu segregation degree σ is 0.050 or less. Each feature will be explained below.

[0029] [(Feature 1) Chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0030] C: 0.50 to 0.80% Carbon (C) increases the tensile strength of mechanical parts manufactured from steel materials. If the C content is less than 0.50%, 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 C content exceeds 0.80%, the cold workability 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 0.50 to 0.80%. The lower limit of the C content is preferably 0.51%, more preferably 0.52%, even more preferably 0.56%, and still more preferably 0.61%. The upper limit of the C content is preferably 0.78%, more preferably 0.75%, and even more preferably 0.73%.

[0031] Si: 0.010 to 0.500% Silicon (Si) increases the tensile strength of mechanical parts manufactured from steel materials through solid solution strengthening. If the Si content is less than 0.010%, 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 0.500%, even if the contents of other elements are within the ranges of this embodiment, the ductility during hot rolling of the steel material may decrease, which may cause defects. Furthermore, the workability may decrease, making the steel more susceptible to work cracks. Therefore, the Si content is 0.010 to 0.500%. The lower limit of the Si content is preferably 0.012%, more preferably 0.022%, and even more preferably 0.051%. The upper limit of the Si content is preferably 0.460%, more preferably 0.430%, and even more preferably 0.400%.

[0032] Mn: 0.50 to 2.00% Manganese (Mn) promotes the bainite transformation of steel. As a result, the tensile strength of mechanical parts manufactured using the steel is increased. If the Mn content is less than 0.50%, 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 2.00%, Mn segregation occurs in the steel material even if the contents of other elements are within the ranges of this embodiment. In this case, martensite is formed locally in the steel material. As a result, the cold workability of the steel material deteriorates. Therefore, the Mn content is 0.50 to 2.00%. The lower limit of the Mn content is preferably 0.54%, more preferably 0.57%, and even more preferably 0.60%. The upper limit of the Mn content is preferably 1.90%, more preferably 1.70%, and even more preferably 1.50%.

[0033] P:0.030% or less Phosphorus (P) is an impurity. If the P content exceeds 0.030%, P segregates at grain boundaries even if the contents of other elements are within the ranges of this embodiment. As a result, the hydrogen embrittlement resistance of the mechanical part decreases. If the P content exceeds 0.030%, the cold workability of the steel material also decreases. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the P content is preferably more than 0% (more than 0.000%), more preferably 0.001%, even more preferably 0.002%, and still more preferably 0.005%. The upper limit of the P content is preferably 0.024%, more preferably 0.019%, and even more preferably 0.014%.

[0034] S: 0.030% or less Sulfur (S) is an impurity. If the S content exceeds 0.030%, S segregates at grain boundaries even if the contents of other elements are within the ranges of this embodiment. As a result, the hydrogen embrittlement resistance of the mechanical component decreases. If the S content exceeds 0.030%, the cold workability of the steel material also decreases. Therefore, the S content is 0.030% or less. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the S content is preferably more than 0% (more than 0.000%), more preferably 0.001%, even more preferably 0.002%, and still more preferably 0.005%. The upper limit of the S content is preferably 0.024%, more preferably 0.019%, and even more preferably 0.014%.

[0035] Al: 0.005 to 0.080% Aluminum (Al) combines with N to form Al nitrides. The Al nitrides act as pinning particles and refine the crystal grains. As a result, the cold workability of the steel is improved. Furthermore, by forming Al nitrides, Al reduces the amount of solute N in the steel. As a result, the deterioration of the cold workability of the steel due to dynamic strain aging is suppressed. If the Al content is less than 0.005%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.080%, even if the contents of other elements are within the ranges of this embodiment, coarse Al oxides are generated, which become the starting points for fracture, thereby deteriorating the cold workability of the steel material. Therefore, the Al content is 0.005 to 0.080%. The lower limit of the Al content is preferably 0.006%, more preferably 0.008%, and even more preferably 0.010%. The upper limit of the Al content is preferably 0.075%, more preferably 0.070%, and even more preferably 0.060%.

[0036] Ti: 0.005 to 0.100% Titanium (Ti) combines with N to form Ti nitrides. These Ti nitrides act as pinning particles and refine the crystal grains. As a result, the cold workability of the steel is improved. Furthermore, by forming Ti nitrides, Ti reduces the amount of solute N in the steel. As a result, the deterioration of the cold workability of the steel due to dynamic strain aging is suppressed. If the Ti content is less than 0.005%, the above effects cannot be fully achieved. On the other hand, if the Ti content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, excessive Ti inclusions are formed in the steel material, which reduces the cold workability of the steel material. Therefore, the Ti content is 0.005 to 0.100%. The lower limit of the Ti content is preferably 0.006%, more preferably 0.007%, and even more preferably 0.010%. The upper limit of the Ti content is preferably 0.080%, more preferably 0.060%, and even more preferably 0.040%.

[0037] B: 0.0003 to 0.0050% Boron (B) promotes bainite transformation and increases the tensile strength of machine parts. If the B content is less than 0.0003%, the above effects cannot be sufficiently obtained. On the other hand, if the B content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, coarse B nitrides and Fe borocarbides are formed. The coarse B nitrides and Fe borocarbides become the starting points of fracture. As a result, the cold workability of the steel material is reduced. Therefore, the B content is 0.0003 to 0.0050%. The lower limit of the B content is preferably 0.0005%, more preferably 0.0007%, and even more preferably 0.0010%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0038] Cu: 0.02 to 0.50% Copper (Cu) suppresses corrosion of steel materials. This suppresses hydrogen generation on the surface of the steel material. As a result, hydrogen penetration into the steel material is suppressed. If the Cu content is less than 0.02%, 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 exceeds 0.50%, the steel material becomes embrittled, and therefore the hot workability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.02 to 0.50%. The lower limit of the Cu content is preferably 0.03%, more preferably 0.04%, and even more preferably 0.05%. The upper limit of the Cu content is preferably 0.46%, more preferably 0.43%, and even more preferably 0.40%.

[0039] Ni: 0.01 to 0.50% Nickel (Ni) is contained together with Cu to suppress the deterioration of hot workability of the steel material when Cu is contained. If the Ni content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content exceeds 0.50%, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0.01 to 0.50%. The lower limit of the Ni content is preferably 0.02%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Ni content is preferably 0.46%, more preferably 0.43%, and even more preferably 0.40%.

[0040] N: 0.0150% or less Nitrogen (N) is an impurity. If the N content exceeds 0.150%, the cold workability of the steel material will be reduced due to dynamic strain aging, even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.0150% or less. The N content is preferably as low as possible. However, an extreme reduction in the N content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the N content is preferably more than 0% (more than 0.0000%), more preferably 0.0001%, even more preferably 0.0002%, and still more preferably 0.0005%. The upper limit of the N content is preferably 0.0120%, more preferably 0.0100%, even more preferably 0.0080%, and still more preferably 0.0040%.

[0041] O: 0.0100% or less Oxygen (O) is an impurity. If the O content exceeds 0.0100%, oxide-based inclusions are formed in the steel material even if the contents of other elements are within the ranges of this embodiment. In this case, the cold workability of the steel material is reduced. Therefore, the O content is 0.0100% or less. The O content is preferably as low as possible. However, an extreme reduction in the O content significantly increases production costs. Therefore, in consideration of industrial production, the lower limit of the O content is preferably more than 0% (more than 0.0000%), more preferably 0.0001%, even more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the O content is preferably 0.0080%, more preferably 0.0060%, and even more preferably 0.0050%.

[0042] 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 or scrap or the manufacturing environment during industrial production of the steel material, and are unintentionally contained, but are acceptable within a range that does not adversely affect the steel material according to the present embodiment.

[0043] [Optional Elements] The steel material of this embodiment may further contain one or more elements selected from the group consisting of first to third groups in place of a portion of Fe. [Group 1] Cr: 1.50% or less, Mo: 0.50% or less Nb: 0.050% or less, and V: 0.20% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0100% or less, and Mg: 0.0100% or less, one or more selected from the group consisting of [Group 3] Sn: 0.0020% or less These elements are optional and may not be contained. These optional elements will be described below.

[0044] [Group 1: Cr, Mo, Nb and V] The steel material of the present embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cr, Mo, Nb, and V. These elements are optional elements, and all of them promote bainite transformation and increase the tensile strength of mechanical components manufactured using the steel material as a raw material.

[0045] Cr:1.50% or less Chromium (Cr) is an optional element and may not be contained, that is, the Cr content may be 0% (0.00%). When contained, that is, when the Cr content is more than 0% (more than 0.00%), Cr promotes bainite transformation and increases the tensile strength of mechanical parts manufactured using the steel material. Even if the Cr content is even a small amount, the above effects can be obtained to some extent. However, if the Cr content exceeds 1.50%, even if the contents of other elements are within the ranges of this embodiment, Cr segregates and martensite forms locally in the steel material, which reduces the cold workability of the steel material. Therefore, the Cr content is 0 (0.00) to 1.50%, and when contained, the Cr content is 1.50% or less. The lower limit of the Cr content is preferably more than 0% (more than 0.00%), more preferably 0.01%, even more preferably 0.02%, and still more preferably 0.05%. The upper limit of the Cr content is preferably 1.40%, more preferably 1.30%, and even more preferably 1.20%.

[0046] Mo: 0.50% or less Molybdenum (Mo) is an optional element and may not be contained, that is, the Mo content may be 0% (0.00%). When contained, that is, when the Mo content is more than 0% (more than 0.00%), Mo promotes bainite transformation and increases the tensile strength of mechanical parts manufactured using the steel material. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. However, if the Mo content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, Mo segregates and martensite forms locally in the steel material, which reduces the cold workability of the steel material. Therefore, the Mo content is 0 (0.00) to 0.50%, and when Mo is contained, the Mo content is 0.50% or less. The lower limit of the Mo content is preferably more than 0% (more than 0.00%), more preferably 0.01%, even more preferably 0.03%, and still more preferably 0.05%. The upper limit of the Mo content is preferably 0.46%, more preferably 0.43%, and even more preferably 0.40%.

[0047] Nb: 0.050% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0% (0.000%). When Nb is contained, that is, when the Nb content is more than 0% (more than 0.000%), Nb promotes bainite transformation and increases the tensile strength of mechanical parts manufactured using the steel material. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.050%, even if the contents of other elements are within the ranges of this embodiment, the workability of the steel material decreases and surface defects tend to occur. If surface defects occur, the cold workability of the steel material further decreases. Therefore, the Nb content is 0 (0.000) to 0.050%, and when Nb is contained, the Nb content is 0.050% or less. The lower limit of the Nb content is preferably more than 0% (more than 0.000%), more preferably 0.001%, even more preferably 0.003%, and still more preferably 0.005%. The upper limit of the Nb content is preferably 0.046%, more preferably 0.043%, and even more preferably 0.040%.

[0048] V: 0.20% or less Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0% (0.00%). When contained, that is, when the V content is more than 0% (more than 0.00%), V promotes bainite transformation and increases the tensile strength of mechanical parts manufactured using the steel material. 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 0.20%, even if the contents of other elements are within the ranges of this embodiment, the precipitation strengthening due to V precipitates becomes excessively large, and therefore the cold workability of the steel material deteriorates. Therefore, the V content is 0 (0.00) to 0.20%, and when V is contained, the V content is 0.20% or less. The lower limit of the V content is preferably more than 0% (more than 0.00%), more preferably 0.01%, even more preferably 0.02%, and still more preferably 0.05%. The upper limit of the V content is preferably 0.18%, more preferably 0.16%, and even more preferably 0.15%.

[0049] [Group 2: Ca and Mg] The steel material of this embodiment may contain one or more elements selected from the group consisting of Ca and Mg in place of a portion of Fe. These elements are optional elements, and both spheroidize MnS in the steel material, improving the cold workability and machinability of the steel material.

[0050] Ca:0.0100% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0% (0.0000%). When Ca is contained, that is, when the Ca content is more than 0% (more than 0.0000%), Ca spheroidizes MnS, thereby improving the cold workability and machinability of the steel. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0100%, even if the contents of other elements are within the ranges of this embodiment, coarse Ca-based inclusions are formed, which deteriorate the cold workability of the steel material. Therefore, the Ca content is 0 (0.0000) to 0.0100%, and when Ca is contained, the Ca content is 0.0100% or less. The lower limit of the Ca content is preferably more than 0% (more than 0.0000%), more preferably 0.0001%, even more preferably 0.0002%, and still more preferably 0.0005%. The upper limit of the Ca content is preferably 0.0080%, more preferably 0.0060%, and even more preferably 0.0050%.

[0051] Mg: 0.0100% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0% (0.0000%). When Mg is contained, that is, when the Mg content is more than 0% (more than 0.0000%), Mg spheroidizes MnS, thereby improving the cold workability and machinability of the steel. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.0100%, even if the contents of other elements are within the ranges of this embodiment, coarse Mg-based inclusions are formed, which deteriorate the cold workability of the steel material. Therefore, the Mg content is 0 (0.0000) to 0.0100%, and when Mg is contained, the Mg content is 0.0100% or less. The lower limit of the Mg content is preferably more than 0% (more than 0.0000%), more preferably 0.0001%, even more preferably 0.0002%, and still more preferably 0.0005%. The upper limit of the Mg content is preferably 0.0080%, more preferably 0.0060%, and even more preferably 0.0050%.

[0052] [Group 3: Sn] Sn: 0.0020% or less The steel material of this embodiment may contain tin (Sn) instead of part of Fe. Sn is an optional element and may not be contained. In other words, the Sn content may be 0% (0.0000%). When Sn is contained, that is, when the Sn content is more than 0% (more than 0.0000%), Sn suppresses hydrogen penetration into the steel material. Therefore, the hydrogen embrittlement resistance of the steel material is further improved. 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 0.0020%, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 (0.0000) to 0.0020%, and when Sn is contained, the Sn content is 0.0020% or less. The lower limit of the Sn content is preferably more than 0% (more than 0.0000%), more preferably 0.0001%, even more preferably 0.0003%, and still more preferably 0.0005%. The upper limit of the Sn content is preferably 0.0018%, more preferably 0.0016%, and even more preferably 0.0014%.

[0053] [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.

[0054] 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.

[0055] 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.

[0056] Rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0057] [(Feature 2) Microstructure] The diameter of a circular cross section perpendicular to the longitudinal direction of the steel material is defined as D. In the microstructure at a depth of D / 4 from the surface of the steel material, the area ratio of bainite is 100%, or the area ratio of bainite is 95% or more, with the remainder consisting of ferrite and / or pearlite.

[0058] The steel material of this embodiment may be applied as a material for a non-heat treated machine part. In other words, when the steel material of this embodiment is used as a material for a non-heat treated machine part, the steel material is not quenched or tempered during the manufacturing process of the machine part.

[0059] Bainite is a structure that combines high strength and excellent workability. The D / 4 depth position from the surface of a steel material is defined as the "internal position." The microstructure at the internal position of the steel material in this embodiment represents the microstructure of the entire steel material. If the bainite area ratio is 95% or more in the microstructure at the internal position of the steel material, the microstructure will be homogeneous, with almost no microstructure other than bainite. Even when used as a raw material for non-thermal treated machine parts, the steel material will have high cold workability during the manufacturing process of the non-thermal treated machine parts. Furthermore, the manufactured non-thermal treated machine parts will have sufficient strength. On the other hand, if the microstructure of the steel material is predominantly bainite, but the bainite area ratio is less than 95% and the remainder is ferrite and / or pearlite, the resulting microstructure will be heterogeneous, with ferrite and / or pearlite mixed within the bainite structure. In this case, the cold workability of the steel material may be reduced. Furthermore, when the bainite area fraction is low and the microstructure of the steel is mainly composed of pearlite, the cold workability of the steel may also be reduced. When the bainite area fraction is low and the microstructure of the steel is mainly composed of ferrite and pearlite, the non-thermal-treated machine parts manufactured using the steel may not have sufficient tensile strength. Even if the bainite area fraction is 95% or more, the cold workability of the steel may be reduced if the remaining microstructure contains martensite.

[0060] The lower limit of the bainite area fraction in the microstructure at the internal position is preferably 96%, more preferably 97%, and even more preferably 98%. As described above, the bainite area fraction in the microstructure at the internal position may be 100%.

[0061] When the bainite area fraction in the microstructure at an internal position is 95% or more but less than 100%, the remainder of the microstructure consists of ferrite and / or pearlite. When the bainite area fraction is 95% or more but less than 100% and the remainder other than bainite consists of ferrite and / or pearlite, the steel material has sufficient cold workability. Furthermore, non-thermal-treated machine parts manufactured using the steel material have sufficient tensile strength and sufficient hydrogen embrittlement resistance.

[0062] [Method for observing the microstructure of steel materials] The microstructure at an internal position of the steel material of this embodiment is observed by the following method. A test piece is taken from a circular cross section perpendicular to the longitudinal direction of the steel material, including a D / 4 depth position from the steel surface. Of the surfaces of the taken test piece, the surface that is a cross section perpendicular to the longitudinal direction of the steel material and includes a D / 4 depth position is designated as the "observation surface."

[0063] The observation surface is mirror-polished. After mirror polishing, the observation surface is etched using picral (a mixed solution of 4 g of picric acid per 100 ml of ethanol) to reveal the microstructure. After etching, four observation fields are selected from the D / 4 depth position at 90° intervals around the circumference of the observation surface. For each selected observation field, an SEM image (secondary electron image) is generated at 1000x magnification using a field emission scanning electron microscope (FE-SEM). The area of ​​each observation field (SEM image) is 112 μm × 84 μm.

[0064] The microstructure is identified in the SEM image of each observation field. The structures of each phase other than bainite (ferrite, pearlite, martensite) can be identified from contrast and morphology. In this embodiment, pseudo-pearlite (pearlite in which the lamellar structure in the structure is broken down and carbides are precipitated in a separated state) is included in bainite. Therefore, the structures other than bainite (ferrite, pearlite, martensite) in the SEM image are identified. Then, the total area of ​​ferrite, the total area of ​​pearlite, the total area of ​​bainite, and the total area of ​​martensite are calculated for all four observation fields. The area ratio of bainite (%) is then calculated based on the following formula (A). The area ratio of bainite is an integer obtained by rounding off the first decimal place.

[0065] Bainite area ratio (%) = {Total area of ​​four observation fields - (Total area of ​​ferrite + Total area of ​​pearlite + Total area of ​​martensite)} / Total area of ​​four observation fields × 100

[0066] [(Feature 3) Cu segregation 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 (depth direction) and 1000 μm in the longitudinal direction from the surface of the steel material in a cross section including the longitudinal direction and radial direction of the steel material was further subjected to area analysis using an electron beam microanalyzer for 160,000 measurement areas 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 areas 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 linesL 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.

[0067] [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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] [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.

[0072] 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.

[0073] 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:

[0074] [Cu] obtained 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.

[0075] Cu content in mass% in all measurement areas MA [Cu] MA The arithmetic mean value of [Cu]A VE 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 normalized Cu amount in each measurement area, excluding the effect of Cu content.

[0076] 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.

[0077] [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 arithmetic mean value of the normalized Cu amount in the measurement row MLj.

[0078] [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.

[0079] [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 σ of is an index showing the degree of Cu segregation.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] [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 in a rod or wire shape. The steel material may be wound in a coil shape or may be cut to a predetermined length.

[0084] [Use of the steel material according to this embodiment] The steel material of this embodiment satisfies Features 1 to 3, and thus can sufficiently suppress the penetration of hydrogen. Therefore, the steel material of this embodiment can be used as a material for machine parts such as industrial machinery, automobiles, bridges, and buildings. As described above, in the steel material of this embodiment, the bainite area ratio is 95% or more in the microstructure at internal positions. Therefore, the steel material of this embodiment is suitable as a material for non-heat treated machine parts. An example of a non-heat treated machine part is a non-heat treated bolt. Note that the steel material of this embodiment may also be used for applications other than the above applications.

[0085] [Steel manufacturing method] An example of a method for manufacturing a steel material according to this embodiment will be described below. The method for manufacturing a steel material described below is one example for manufacturing the steel material according to this embodiment. Therefore, a steel material having the above-mentioned Features 1 to 3 may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel material according to this embodiment.

[0086] 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 (Process 4) Tissue adjustment process In one example of the present manufacturing method, the following condition 1 is satisfied in the finish rolling step, and the following conditions 2 and 3 are satisfied in the structure adjusting step. Condition 1: The steel temperature is within the range of 1000 to 880°C, and the number of passes is 2 or more with an area reduction rate of 25% or more. Condition 2: The average cooling rate CR of the steel material in the range of 800 to 500°C is set to 10°C / second or more. Condition 3: An isothermal transformation treatment is carried out by immersing the material in a thermostatic bath at a holding temperature T1 of 450 to 500°C for a holding time t1 of 45 seconds or more. Each step will be described below.

[0087] [(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.

[0088] 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.

[0089] [(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.

[0090] 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.

[0091] [(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.

[0092] [(Process 22) Rolling process] In the rolling process, the material heated in the heating process is rolled (roughly rolled) using a blooming mill, or a splitting mill and a continuous rolling mill, to produce a billet.

[0093] 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 a reduction from a splitting mill as it passes from upstream to downstream through the blooming mill, and can also be subjected to a reduction from the splitting mill as it passes from downstream to upstream through the splitting mill.

[0094] If a continuous rolling mill is located 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. The continuous rolling mill includes multiple rolling stands. Each rolling stand includes a pair of work rolls. Each work roll is formed with a caliber, and the calibers of the pair of work rolls form a groove. In the continuous rolling mill, tandem rolling is performed from upstream to downstream.

[0095] The billet produced by the above rough rolling process is allowed to cool (air-cool) to room temperature before the finish rolling process.

[0096] [(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

[0097] In the finish rolling process, the following conditions are further satisfied in the rolling process. Condition 1: The steel temperature is within the range of 1000 to 880°C, and the number of passes is 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.

[0098] [(Step 31) Heating step] 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.

[0099] [(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 work roll is formed with a caliber, and the calibers of the pair of work rolls form a groove.

[0100] 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."

[0101] 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.

[0102] [Regarding Condition 1] 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

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] [(Step 4) Tissue adjustment step] In the structure adjusting step, the steel material that has been finish-rolled in the finish rolling step is subjected to cooling and isothermal transformation treatment to make the microstructure of the steel material a structure mainly composed of bainite. The structure adjusting step includes the following steps. (Step 41) Cooling step (Step 42) Isothermal transformation treatment step Each step will be described below.

[0110] [(Step 41) Cooling step] The steel material after finish rolling is wound into a coil and then placed on a conveying path for transport. In the cooling process, the steel material after finish rolling is cooled. In the cooling process, the following condition 2 is satisfied. Condition 2: The average cooling rate CR in the range of 800 to 500°C is set to 10°C / sec or more.

[0111] [Regarding Condition 2] When the average cooling rate CR is less than 10°C / sec, the cooling rate is too slow. In this case, in the continuous cooling transformation diagram (CCT diagram) of the steel satisfying Feature 1, the cooling curve intersects with a ferrite nose or a pearlite nose. In this case, excessive ferrite and / or pearlite are formed in the microstructure of the steel. As a result, the bainite area fraction in the microstructure of the steel becomes less than 95%.

[0112] If the average cooling rate CR is 10°C / sec or more, the cooling rate until the steel temperature reaches 800 to 500°C is appropriate. Therefore, assuming that condition 3 described below is satisfied, in the microstructure at an internal position of the steel, the bainite area fraction is 100%, or the bainite area fraction is 95% or more with the remainder being ferrite and / or pearlite. The average cooling rate CR (°C / sec) can be determined based on the time measured until the steel temperature falls to 800 to 500°C.

[0113] [(Step 42) Isothermal transformation treatment step] The steel material after the cooling process is immediately subjected to an isothermal transformation process. In the isothermal transformation process, the steel material is subjected to isothermal transformation treatment at a holding temperature T1 (°C) for a holding time t1 (seconds). The isothermal transformation process is carried out, for example, by immersing the steel material in an immersion bath maintained at the above-mentioned temperature T1. The immersion bath may be a molten salt bath, a lead bath, or a fluidized bed. The steel material after the isothermal transformation process is cooled. The cooling may be water cooling or natural cooling. The isothermal transformation process satisfies the following condition 3. Condition 3: The holding temperature T1 is set to 450 to 500° C., and the holding time t1 is set to 45 seconds or longer.

[0114] [Regarding condition 3] If the holding temperature T1 is less than 450°C, the time required for the bainite transformation to be completed in the steel material becomes excessively long. In this case, the steel material is cooled before the bainite transformation is completed. As a result, martensite is formed in the steel material, and the cold workability of the steel material is reduced. Similarly, if the holding time t1 is less than 45 seconds, the steel material is cooled before the bainite transformation is completed. As a result, martensite is formed in the steel material, and the cold workability of the steel material is reduced.

[0115] On the other hand, if the holding temperature T1 exceeds 500°C, pearlite may be excessively formed in the steel material, which may result in a decrease in cold workability.

[0116] If the holding temperature T1 is 450 to 500°C and the holding time t1 is 45 seconds or longer, assuming that condition 2 is satisfied, the area ratio of bainite in the microstructure at an internal position of the steel material will be 100%, or the area ratio of bainite will be 95% or more, with the remainder being a structure consisting of ferrite and / or pearlite.

[0117] Through the above manufacturing process, a steel material satisfying Features 1 to 3 can be manufactured.

[0118] [Method for manufacturing non-heat treated machine parts using the steel material of this embodiment] The method for manufacturing a non-heat treated machine part made of the steel material of this embodiment is a well-known manufacturing method. As an example of a non-heat treated machine part, a method for manufacturing a non-heat treated bolt will be described. The method for manufacturing a non-heat treated bolt includes, for example, the following steps. Steel wire manufacturing process Cold working processes (heading processes, rolling processes) Brewing process Plating process Baking process Of the above steps, the bluing step, plating step, and baking step are optional steps, that is, the bluing step, plating step, and baking step do not have to be performed. Each step will be described below.

[0119] [Steel wire manufacturing process] In the steel wire manufacturing process, first, a well-known lubrication treatment is performed on the steel material. Specifically, a well-known lubrication treatment is performed on the steel material to form a well-known lubricating film on the surface of the steel material. The lubricating film is, for example, a well-known phosphate film or a well-known soap lubricating film. The steel material on which the lubricating film has been formed is subjected to a well-known wiredrawing process to manufacture the steel wire. The wiredrawing process may be only a primary wiredrawing process, or multiple wiredrawing processes such as a secondary wiredrawing process may be performed. The total area reduction rate in the wiredrawing process is not particularly limited, but is, for example, 5 to 65%.

[0120] [Cold working process] In the cold working process, the steel wire is subjected to a known heading process and a rolling process to produce a bolt-shaped intermediate product. Specifically, a known heading process is performed to form a thread shape in the steel wire. Furthermore, a known rolling process is performed to form a thread on the shank of the bolt. Through these processes, the bolt-shaped intermediate product is produced.

[0121] [Brewing process] The bluing process is an optional process. In other words, it does not have to be performed. If performed, the intermediate product is held in the bluing process at a temperature range of 200 to 600°C for 10 to 300 minutes. If the bluing process is performed, the tensile strength and yield ratio of the non-heat-treated bolt will increase.

[0122] [Plating process] The plating process is an optional process. In other words, it does not have to be performed. If performed, the plating process involves performing a well-known plating process on the intermediate product to form a plating layer on the surface of the intermediate product. The formation of the plating layer improves the corrosion resistance of the untempered bolt. The plating layer is not particularly limited, but examples include a zinc plating film (JIS B1044:2001, JIS B1048:2007) and a zinc flake film (JIS B1046:2020).

[0123] [Baking process] The baking process is an optional process. In other words, it does not have to be performed. If hydrogen penetrates into the intermediate product during the plating process, the baking process is also performed. In the baking process, the intermediate product after the plating process is held at a temperature range of 150 to 250°C for 60 to 480 minutes. The baking process releases hydrogen that penetrated into the steel during the plating process to the outside.

[0124] The above manufacturing method allows the manufacture of a non-heat treated bolt using the steel material of this embodiment as its raw material. In the above manufacturing process, no heat treatment (quenching and tempering) is performed. Therefore, the microstructure of the steel material, which is the raw material, does not transform in the above bolt manufacturing process. Therefore, the microstructure of the manufactured non-heat treated bolt is substantially the same as the microstructure of the steel material, and is a structure mainly composed of bainite. And, like the steel material, it satisfies Features 1 to 3. Therefore, the non-heat treated bolt suppresses hydrogen penetration in a corrosive environment. [Example]

[0125] 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.

[0126] [Material preparation process] Steel materials having the chemical compositions shown in Tables 1-1 and 1-2 were produced by the following method.

[0127] [Table 1-1]

[0128] [Table 1-2]

[0129] In Table 1-2, "-" means that the content of the corresponding element is 0% in significant figures (numerical values ​​to the least significant digit) as specified in the embodiment. In other words, the content of the corresponding element is 0% when rounded to the nearest significant digit as specified in the embodiment. For example, the Cr content specified in this embodiment is specified as a numerical value up to two decimal places. Therefore, in the case of Test No. 1 in Table 1-2, when the measured Cr content is rounded to two decimal places, it means that it is 0%. Rounding off means that if the digit (fraction) below the specified minimum digit is less than 5, it is rounded down, and if it is 5 or more, it is rounded up.

[0130] [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.

[0131] [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.

[0132] [Table 2]

[0133] The steel material (round bar) after finish rolling was subjected to a structure adjustment process. In the cooling step of the structure adjustment process, the average cooling rate CR in the steel material temperature range of 800 to 500°C was as shown in Table 2.

[0134] The steel material after the initial cooling step was immediately subjected to an isothermal transformation treatment step. The holding temperature T1 (°C) and holding time t1 (seconds) in the isothermal transformation treatment step were as shown in Table 2. After the isothermal transformation treatment step, the steel material was water-cooled to 220°C, and then allowed to cool naturally.

[0135] Through the above manufacturing process, steel materials (round bars) having a diameter of 10.0 mm were manufactured for each test number.

[0136] [About the evaluation test] The following steel evaluation tests (Tests 1 to 6) 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) Microstructure observation test of steel material (Test 3) Cu segregation σ measurement test (Test 4) Diffusible hydrogen concentration measurement test (Test 5) Tensile strength evaluation test (Test 6) Cold workability evaluation test Each test will be explained below.

[0137] [(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.

[0138] [(Test 2) Microstructure observation test of steel material] For each steel material (round bar) with each test number, the bainite area fraction (%) in the microstructure at the D / 4 depth position was determined based on the above-mentioned "Method for Observing the Microstructure of Steel Materials." If the bainite area fraction was less than 100%, the remaining structure was identified. The obtained bainite area fraction (%) and remaining structure are shown in the "Bainite Area Fraction (%)" and "Remaining Structure" columns of Table 2. In the "Remaining Structure" column, "F" means ferrite, "P" means pearlite, and "M" means martensite. When "F,P" is written in the column, it means that the remaining structure is a structure consisting of ferrite and pearlite.

[0139] [(Test 3) 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.

[0140] [(Test 4) Diffusible hydrogen concentration measurement test] The following diffusible hydrogen concentration measurement test was carried out on the steel material with each test number.

[0141] [Manufacturing of untreated mechanical part simulants] First, simulated non-heat treated machine parts were manufactured using the steel materials of each test number. Specifically, a known lubrication treatment was performed on the steel materials of each test number to form a lubricating film (phosphate film and soap lubricating film) on the surface of the steel material. The lubrication treatment conditions were the same for each test number. Then, a known wire drawing process was performed on the steel materials with the lubricating film formed. The total area reduction rate of the wire drawing process was 36%. Using the above manufacturing process, simulated non-heat treated machine parts (round bars) were manufactured.

[0142] [Measurement test] The unheat-treated machine part simulants of each test number were cut perpendicular to the longitudinal direction to obtain multiple 100 mm long round bar test pieces. In order to eliminate the influence of the lubricating film formed on the surface of the steel wire before wire drawing, the round bar test pieces were subjected to a blasting treatment to remove the lubricating film from the surface of the round bar test pieces.

[0143] The blasted round bar test specimens were subjected to a combined cyclic corrosion test (CCT) as specified in JASO M609 (1991). For each test number, the test cycle in the corrosion test was set to four patterns: 21 cycles, 42 cycles, 84 cycles, and 126 cycles. A separate round bar test specimen was used for each pattern.

[0144] After each test cycle, a round bar test specimen was removed and 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 section from the center of the longitudinal direction of the round bar test specimen after the blasting treatment.

[0145] The diffusible hydrogen concentration of the cut-out test piece was analyzed using a gas chromatograph-type thermal desorption analysis (TDA). Specifically, the cut-out test piece was heated from room temperature to 200°C at a heating rate of 100°C / hr. The amount of hydrogen released from the test piece to the outside due to heating was measured.

[0146] The measured amount of hydrogen was divided by the mass of the test piece before heating to determine the diffusible hydrogen concentration (unit: mass 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. The obtained diffusible hydrogen concentrations are shown in Table 2 under "Diffusible hydrogen concentration (mass ppm)."

[0147] [(Test 5) Tensile Strength Evaluation Test] As an example of a non-heat treated machine part made from the steel material of each test number, a non-heat treated bolt was manufactured by the following manufacturing method.

[0148] A well-known lubrication treatment was carried out on the steel material (round bar) of each test number to form a lubricating film (phosphate film and soap lubricating film) on the surface of the steel material. The lubrication treatment conditions were the same for each test number. Then, the steel material with the lubricating film formed thereon was subjected to a well-known wire drawing process. The total area reduction rate of the wire drawing process was 36.0%. Steel wire was produced using the above manufacturing process.

[0149] The steel wire was subjected to well-known cold heading to produce intermediate products in the shape of M8 flanged hex bolts as specified in JIS B1189:2014. The shank diameter (D) of the intermediate products was 8.0 mm. The cold heading conditions were the same for each test number. The intermediate products after cold heading were subjected to well-known cold rolling to form a threaded portion with a nominal thread size (M) of 8.0 mm and a pitch (P) of 1.25 mm on a portion of the shank of the intermediate products. The cold rolling conditions were the same for each test number.

[0150] The intermediate product after cold rolling was subjected to a bluing treatment in which it was held at a temperature of 350°C for one hour.Untreated bolts were manufactured using the above manufacturing process.

[0151] The manufactured non-heat treated bolts were subjected to a tensile test in accordance with JIS B 1051:2014 at room temperature in the atmosphere to measure the tensile strength (MPa). The crosshead displacement rate during the tensile test was 3.0 mm / min. The obtained tensile strength is shown in the "Tensile strength (MPa)" column in Table 2. A tensile strength of 1100 MPa or more was considered to be sufficient strength.

[0152] [(Test 6) Cold workability evaluation test] During the manufacturing process for the untempered bolts described above, the intermediate products after cold forging were visually inspected for cracks in the area corresponding to the bolt head or the transition area between the bolt head and shank. If no cracks longer than 0.5 mm were observed, it was determined that sufficient cold workability had been achieved (indicated by a "○" in the "Cold Workability" column in Table 2). On the other hand, if cracks longer than 0.5 mm were observed, it was determined that sufficient cold workability had not been achieved (indicated by a "×" in the "Cold Workability" column in Table 2). If it was difficult to determine the presence of cracks visually, the relevant area was observed using a 10x magnification loupe to confirm the presence or absence of cracks. For test numbers where it was determined that sufficient cold workability had not been achieved, the manufacturing process after cold forging was discontinued, and no tensile tests were performed (indicated by a "-" in the "Tensile Strength (MPa)" column in Table 2).

[0153] [Evaluation results] The evaluation results are shown in Table 2. Test Nos. 1 to 32 had appropriate chemical compositions. Furthermore, the manufacturing conditions were also appropriate. Therefore, the bainite area ratio in the microstructure at the internal position of the steel was 95% or more, with the remainder being ferrite and / or pearlite, and the Cu segregation degree was 0.050 or less. As a result, the diffusible hydrogen concentration was less than 0.30 mass ppm, and hydrogen penetration was suppressed. Furthermore, the tensile strength was 1100 MPa or more, and sufficient strength was obtained for non-thermal-treated machine parts. Furthermore, sufficient cold workability was also obtained.

[0154] On the other hand, in test number 33, the Mn content was too high, and martensite was observed in the microstructure of the steel material, resulting in insufficient cold workability.

[0155] In test number 34, the Cu content was too low, so the diffusible hydrogen concentration was 0.30 mass ppm or more, and hydrogen penetration could not be sufficiently suppressed.

[0156] In test numbers 35 and 36, although the chemical composition was appropriate, the steel temperature was in the range of 1000 to 880°C, and the number of passes (PN) for 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 0.30 mass ppm or more, and hydrogen penetration could not be sufficiently suppressed.

[0157] In test number 37, the average cooling rate (CR) was too slow due to natural cooling. As a result, the microstructure of the steel material was composed of ferrite and pearlite. As a result, the tensile strength of the untempered bolt was low.

[0158] In test number 38, the average cooling rate CR was too slow. As a result, the area ratio of bainite in the microstructure of the steel was less than 95%, with the remainder consisting of ferrite. As a result, the microstructure was inhomogeneous and sufficient cold workability was not obtained.

[0159] In test number 39, the holding temperature T1 in the isothermal transformation treatment process was too low. As a result, the area ratio of bainite in the microstructure of the steel was less than 95%, with the remainder consisting of ferrite and martensite. As a result, sufficient cold workability was not obtained.

[0160] In test number 40, the holding temperature T1 in the isothermal transformation treatment process was too high. As a result, the microstructure of the steel material was made of pearlite. As a result, sufficient cold workability was not obtained.

[0161] In test number 41, the holding time t1 in the isothermal transformation treatment was too short. As a result, the area ratio of bainite in the microstructure of the steel was less than 95%, with the remainder consisting of ferrite and martensite. As a result, sufficient cold workability was not obtained.

[0162] 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 that is rod-shaped or wire-shaped and has a circular cross section perpendicular to the longitudinal direction, In mass%, C: 0.50-0.80%, Si: 0.010-0.500%, Mn: 0.50-2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.005-0.080%, Ti: 0.005-0.100%, B: 0.0003 to 0.0050%, Cu: 0.02 to 0.50%, Ni: 0.01-0.50%, N: 0.0150% or less, and O: 0.0100% or less, the balance being Fe and impurities; When a diameter of the cross section perpendicular to the longitudinal direction of the steel material is defined as D, in a microstructure at a depth position of D / 4 from the surface of the steel material, an area ratio of bainite is 100%, or an area ratio of bainite is 95% or more and the remainder is composed of ferrite and / or pearlite, In a rectangular observation region of the cross section of the steel material including the longitudinal direction and the radial direction, the rectangular observation 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 that is rod-shaped or wire-shaped and has a circular cross section perpendicular to the longitudinal direction, In mass%, C: 0.50-0.80%, Si: 0.010-0.500%, Mn: 0.50-2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.005-0.080%, Ti: 0.005-0.100%, B: 0.0003 to 0.0050%, Cu: 0.02 to 0.50%, Ni: 0.01-0.50%, N: 0.0150% or less, and O: 0.0100% or less, Further, it contains one or more elements selected from the group consisting of Groups 1 to 3, with the balance being Fe and impurities; When a diameter of the cross section perpendicular to the longitudinal direction of the steel material is defined as D, in a microstructure at a depth position of D / 4 from the surface of the steel material, an area ratio of bainite is 100%, or an area ratio of bainite is 95% or more and the remainder is composed of ferrite and / or pearlite, In a rectangular observation region of the cross section of the steel material including the longitudinal direction and the radial direction, the rectangular observation 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] Cr: 1.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, and V: 0.20% or less, and one or more selected from the group consisting of [Group 2] Ca: 0.0100% or less, and Mg: 0.0100% or less, one or more selected from the group consisting of [Group 3] Sn: 0.0020% or less

3. The steel material according to claim 2, containing the first group, Steel material.

4. The steel material according to claim 2, containing the second group, Steel material.

5. The steel material according to claim 2, containing the third group, Steel material.

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