steel

Abstract

To provide a steel material that can suppress intrusion of hydrogen.SOLUTION: A steel material according to the present disclosure has a chemical composition as described in the specification, a tensile strength of 1100 MPa or less, and satisfies formulas (1) and (2) when electron backscatter diffraction is performed at a depth position of D (diameter) / 4 from a surface of the steel material to determine a crystal orientation difference of grain boundaries, the class width is set to 2.5°, and a grain boundary length ratio, which is the ratio of the total length of the grain boundaries belonging to each class to the total length of the grain boundaries having a crystal orientation difference of 5.0° or more and less than 65.0°is determined for each class in the range of crystal orientation difference of 5.0°or more and less than 65.0°, and the grain boundary length ratio of the class having the maximum crystal orientation difference in the range of 5.0°or more and less than 15.0° is defined as F1, the grain boundary length ratio of the class having the minimum crystal orientation difference in the range of 15.0°or more and less than 55.0°is defined as F2, and the grain boundary length ratio of the class having the maximum crystal orientation difference in the range of 55.0°or more and less than 65.0°is defined as F3. F1 / F2≥2.5 (1) F3 / F1≥1.8 (2)SELECTED DRAWING: Figure 1

Description

【Technical Field】 【0001】 This disclosure relates to steel materials. 【Background Art】 【0002】 Mechanical parts typified by bolts and the like are used in industrial machines, automobiles, bridges, buildings, and the like. Among these applications, bridges and buildings may be built in coastal areas or cold regions. Coastal areas have a corrosive environment with a lot of salt. Also, in cold regions, snow melting salts and antifreeze agents may be used. Snow melting salts and antifreeze agents corrode the steel materials constituting the mechanical parts. That is, cold regions are often corrosive environments as well. 【0003】 In such corrosive environments, hydrogen embrittlement is likely to occur. Therefore, excellent hydrogen embrittlement resistance is required for mechanical parts used in corrosive environments. 【0004】 A technique regarding improvement of hydrogen embrittlement resistance has been proposed in Japanese Patent Application Laid-Open No. 2008-274367 (Patent Document 1). 【0005】 The steel material disclosed in Patent Document 1 contains, in mass%, C: 0.15 to 0.6%, Si: 0.05 to 0.5%, Mn and Cr: in total 0.5 to 3..5%, P: 0.05% or less, S: 0.03% or less, Cu: less than 0.3%, Ni: less than 1%, O: 0.01% or less, and Sn: 0.05 to 0.50%, with the balance being composed of Fe and impurities and having a composition in which the Cu / Sn ratio is 1 or less. In this document, by containing Sn, the intrusion of hydrogen into the steel material is suppressed, and as a result, the hydrogen embrittlement resistance is enhanced. 【Prior Art Documents】 【Patent Documents】 【0006】 【Patent Document 1】 Japanese Patent Application Laid-Open No. 2008-274367 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0007】 As disclosed in Patent Document 1, it is known that suppressing the penetration of hydrogen into steel materials increases the hydrogen embrittlement resistance of the steel materials. In order to improve the hydrogen embrittlement resistance of steel materials, the penetration of hydrogen into steel materials may be suppressed by means other than those disclosed in Patent Document 1. 【0008】 Furthermore, in recent years, non-heat-treated machine parts have been proposed, which omit heat treatment (quenching and tempering) in the manufacturing process of machine parts such as bolts. Therefore, there is a need for steel materials that can be used as the material for such non-heat-treated machine parts. 【0009】 The purpose of this disclosure is to provide a steel material capable of suppressing hydrogen intrusion. [Means for solving the problem] 【0010】 The steel materials disclosed herein are A steel material that is rod-shaped or linear, with a circular cross-section perpendicular to the axial direction, In mass%, C: 0.20~0.45%, Si: 0.02~0.50%, 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%, N: 0.0150% or less, and, It contains less than 0.0100% O, with the remainder consisting of Fe and impurities. The tensile strength is 1100 MPa or less. The diameter of the cross-section is defined as D, and electron backscatter diffraction is performed at a depth of D / 4 from the surface of the steel material to determine the crystal orientation difference of the grain boundaries. With a class width of 2.5°, the grain boundary length ratio, which is the ratio of the total length of the grain boundaries belonging to each class to the total length of the grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0°, is determined for each class within the range of crystal orientation differences of 5.0° or more and less than 65.0°. The grain boundary length ratio of the class in which the crystal orientation difference is maximized within the range of 5.0° to less than 15.0° is defined as F1. The grain boundary length ratio of the class in which the crystal orientation difference is smallest within the range of 15.0° to less than 55.0° is defined as F2. When F3 is defined as the grain boundary length ratio of the class in which the crystal orientation difference is greatest in the range of 55.0° or more and less than 65.0°, equations (1) and (2) are satisfied. F1 / F2≧2.5 (1) F3 / F1≧1.8 (2) 【0011】 The steel materials disclosed herein are A steel material that is rod-shaped or linear, with a circular cross-section perpendicular to the axial direction, In mass%, C: 0.20~0.45%, Si: 0.02~0.50%, 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%, N: 0.0150% or less, and, Contains O: 0.0100% or less, Furthermore, it contains one or more elements selected from the groups consisting of Groups 1 to 3, with the remainder being Fe and impurities. The tensile strength is 1100 MPa or less. Define the diameter of the cross-section as D, perform electron backscatter diffraction at a depth of D / 4 from the surface of the steel material to obtain the crystal orientation difference of grain boundaries, set the class width to 2.5°, and calculate the grain boundary length ratio, which is the ratio of the total length of the grain boundaries belonging to each class to the total length of the grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0°, for each class within the range of the crystal orientation difference of 5.0° or more and less than 65.0°. Define the grain boundary length ratio of the class with the maximum value within the range where the crystal orientation difference is 5.0° or more and less than 15.0° as F1. Define the grain boundary length ratio of the class with the minimum value within the range where the crystal orientation difference is 15.0° or more and less than 55.0° as F2. When the grain boundary length ratio of the class with the maximum value within the range where the crystal orientation difference is 55.0° or more and less than 65.0° is defined as F3, the following equations (1) and (2) are satisfied. [Group 1] Cr: 1.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, and V: 0.20% or less, selected from one or more of the group consisting of [Group 2] Ca: 0.0100% or less, and Mg: 0.0100% or less, selected from one or more of the group consisting of [Group 3] Cu: 0.35% or less, Ni: 0.35% or less, and Sn: 0.0400% or less, selected from one or more of the group consisting of F1 / F2 ≥ 2.5 (1) F3 / F1 ≥ 1.8 (2) [Advantages of the Invention] 【0012】 The steel material according to the present disclosure can suppress the intrusion of hydrogen. [Brief Description of the Drawings] 【0013】 [Figure 1] Figure 1 is a graph showing the relationship between F1 / F2 and the diffusible hydrogen concentration in a steel material whose chemical composition is within the range of the present embodiment and satisfies equation (2). [Figure 2] Figure 2 is a graph showing the relationship between F3 / F1 and diffusible hydrogen concentration in steel materials whose chemical composition is within the range of this embodiment and satisfies formula (1). [Figure 3] Figure 3 is an example of a histogram showing the grain boundary length ratio fi for each class, which is the ratio of the total length GLi (where i is 1 to 24) of grain boundaries belonging to each class with a class width of 2.5° to the total length GL0 of grain boundaries in steel with a crystal orientation difference of 5.0° or more and less than 65.0°. [Figure 4] Figure 4 is an example of a histogram showing the grain boundary length ratio fi for each class, which is the ratio of the total length GLi (where i is 1 to 24) of grain boundaries belonging to each class with a class width of 2.5° to the total length GL0 of grain boundaries in steel with a crystal orientation difference of 5.0° or more and less than 65.0°. [Modes for carrying out the invention] 【0014】 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. 【0015】 First, the inventors investigated steel materials capable of suppressing hydrogen intrusion from the perspective of chemical composition. As a result, the inventors found that, in mass%, C: 0.20~0.45%, Si: 0.02~0.50%, 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%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 We considered that steel materials having a chemical composition of ~1.50%, Mo:0~0.50%, Nb:0~0.050%, V:0~0.20%, Ca:0~0.0100%, Mg:0~0.0100%, Cu:0~0.35%, Ni:0~0.35%, Sn:0~0.0400%, with the remainder being Fe and impurities, could potentially sufficiently suppress hydrogen penetration. 【0016】 However, even steel materials that satisfy the above-mentioned chemical composition sometimes still fail to adequately suppress hydrogen intrusion. Therefore, the inventors investigated means of suppressing hydrogen intrusion from the viewpoint of microstructure. 【0017】 Here, the inventors focused on the difference in crystal orientation at grain boundaries and investigated the relationship between the difference in crystal orientation and the ease with which hydrogen penetrates. As a result, they obtained the following findings. The diameter of the cross-section (circular shape) perpendicular to the axial direction of the steel material is defined as D. Electron backscatter diffraction (EBSD) is performed at a depth of D / 4 from the surface of the steel material to determine the crystal orientation difference of the grain boundaries. Based on the obtained crystal orientation of the grain boundaries, with a class width of 2.5°, the grain boundary length ratio, which is the ratio of the total length of grain boundaries belonging to each class to the total length of grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0°, is determined for each class in the range of crystal orientation differences from 5.0° to less than 65.0°. The grain boundary length ratio of the class with the largest crystal orientation difference in the range of 5.0° to less than 15.0° is defined as F1. The grain boundary length ratio of the class with the smallest crystal orientation difference in the range of 15.0° to less than 55.0° is defined as F2. The grain boundary length ratio of the class with the largest crystal orientation difference in the range of 55.0° to less than 65.0° is defined as F3. In this case, the grain boundary length ratios F1 to F3 satisfy equations (1) and (2). F1 / F2≧2.5 (1) F3 / F1≧1.8 (2) 【0018】 Figure 1 is a graph showing the relationship between F1 / F2 and diffusible hydrogen concentration in a steel material whose chemical composition is within the range of this embodiment and satisfies formula (2). Figure 2 is a graph showing the relationship between F3 / F1 and diffusible hydrogen concentration in a steel material whose chemical composition is within the range of this embodiment and satisfies formula (1). 【0019】 Referring to Figure 1, when F1 / F2 is less than 2.5, the diffusible hydrogen concentration is high. On the other hand, when F1 / F2 is 2.5 or higher, the diffusible hydrogen concentration decreases significantly. Similarly, referring to Figure 2, when F3 / F1 is less than 1.8, the diffusible hydrogen concentration is high. On the other hand, when F3 / F1 is 1.8 or higher, the diffusible hydrogen concentration decreases significantly. 【0020】 As described above, the inventors have found that when F1 / F2 is 2.5 or higher and F3 / F1 is 1.8 or higher, the diffusible hydrogen concentration decreases significantly, and hydrogen intrusion can be effectively suppressed, as shown in Figures 1 and 2. The reason for this is not entirely clear, but the following reason is possible: The distribution of crystal orientation differences at grain boundaries is related to the ease with which hydrogen can intrude. In the distribution of crystal orientation differences at grain boundaries, the proportion of grain boundaries on the low orientation difference side is larger than the proportion of grain boundaries on the high orientation difference side. By using a steel material with a distribution of crystal orientation differences at grain boundaries such that the proportion of grain boundaries on the high orientation difference side is larger than the proportion of grain boundaries on the low orientation difference side, hydrogen intrusion can be suppressed. It is possible that hydrogen intrusion is suppressed for reasons other than those stated above. However, the suppression of hydrogen intrusion by having the above-described chemical composition and satisfying formulas (1) and (2) has been demonstrated in the examples described later. 【0021】 Based on the above findings, the steel material according to this embodiment has the following configuration. 【0022】 [1] A steel material that is rod-shaped or linear, with a circular cross-section perpendicular to the axial direction, In mass%, C: 0.20~0.45%, Si: 0.02~0.50%, 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%, N: 0.0150% or less, and, It contains less than 0.0100% O, with the remainder consisting of Fe and impurities. The tensile strength is 1100 MPa or less. The diameter of the cross-section is defined as D, and electron backscatter diffraction is performed at a depth of D / 4 from the surface of the steel material to determine the crystal orientation difference of the grain boundaries. With a class width of 2.5°, the grain boundary length ratio, which is the ratio of the total length of the grain boundaries belonging to each class to the total length of the grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0°, is determined for each class within the range of crystal orientation differences of 5.0° or more and less than 65.0°. The grain boundary length ratio of the class in which the crystal orientation difference is maximized within the range of 5.0° to less than 15.0° is defined as F1. The grain boundary length ratio of the class in which the crystal orientation difference is smallest within the range of 15.0° to less than 55.0° is defined as F2. When F3 is defined as the grain boundary length ratio of the class in which the crystal orientation difference is greatest in the range of 55.0° or more and less than 65.0°, the following conditions satisfy equations (1) and (2): Steel material. F1 / F2≧2.5 (1) F3 / F1≧1.8 (2) 【0023】 [2] A steel material that is rod-shaped or linear, with a circular cross-section perpendicular to the axial direction, In mass%, C: 0.20~0.45%, Si: 0.02~0.50%, 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%, N: 0.0150% or less, and, Contains O: 0.0100% or less, Furthermore, it contains one or more elements selected from the groups consisting of Groups 1 to 3, with the remainder being Fe and impurities. The tensile strength is 1100 MPa or less. The diameter of the cross-section is defined as D, and electron backscatter diffraction is performed at a depth of D / 4 from the surface of the steel material to determine the crystal orientation difference of the grain boundaries. With a class width of 2.5°, the grain boundary length ratio, which is the ratio of the total length of the grain boundaries belonging to each class to the total length of the grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0°, is determined for each class within the range of crystal orientation differences of 5.0° or more and less than 65.0°. The grain boundary length ratio of the class in which the crystal orientation difference is maximized within the range of 5.0° to less than 15.0° is defined as F1. The grain boundary length ratio of the class in which the crystal orientation difference is smallest within the range of 15.0° to less than 55.0° is defined as F2. When F3 is defined as the grain boundary length ratio of the class in which the crystal orientation difference is greatest in the range of 55.0° or more and less than 65.0°, the following conditions satisfy equations (1) and (2): Steel material. [Group 1] Cr: 1.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, and, One or more selected from the group consisting of V: ​​0.20% or less. [Group 2] Ca: 0.0100% or less, One or more selected from the group consisting of Mg: 0.0100% or less. [Group 3] Cu: 0.35% or less, Ni: 0.35% or less, and, One or more selected from the group consisting of Sn: 0.0400% or less. F1 / F2≧2.5 (1) F3 / F1≧1.8 (2) 【0024】 [3] [2] The steel material described above, The above group 1 contains, Steel material. 【0025】 [4] The steel materials described in [2] or [3], The following include the second group: Steel material. 【0026】 [5] Steel materials as described in any one of items [2] to [4], The third group contains, Steel material. 【0027】 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. 【0028】 [Features of the steel material of this embodiment] The steel material of this embodiment has the following features: (Feature 1) The chemical composition, in mass%, is as follows: C: 0.20-0.45%, Si: 0.02-0.50%, 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%, N: 0.0150% or less. O: 0.0100% or less, Cr: 0-1.50%, Mo: 0-0.50%, Nb: 0-0.050%, V: 0-0.20%, Ca: 0-0.0100%, Mg: 0-0.0100%, Cu: 0-0.35%, Ni: 0-0.35%, Sn: 0-0.0400%, and the remainder consists of Fe and impurities. (Feature 2) The tensile strength is 1100 MPa or less. (Feature 3) The diameter of the cross-section (circular shape) perpendicular to the axial direction of the steel material is defined as D. Electron backscatter diffraction is performed at a depth of D / 4 from the surface of the steel material to determine the crystal orientation difference of the grain boundaries. With a class width of 2.5°, the grain boundary length ratio, which is the ratio of the total length of grain boundaries belonging to each class to the total length of grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0°, is determined for each class within the range of crystal orientation differences between 5.0° and less than 65.0°. We define F1 as the grain boundary length ratio of the class in which the crystal orientation difference is greatest within the range of 5.0° to less than 15.0°. We define F2 as the grain boundary length ratio of the class in which the crystal orientation difference is smallest within the range of 15.0° to less than 55.0°. When F3 is defined as the grain boundary length ratio of the class in which the crystal orientation difference is greatest in the range of 55.0° or more and less than 65.0°, equations (1) and (2) are satisfied. F1 / F2≧2.5 (1) F3 / F1≧1.8 (2) The following describes each of its features. 【0029】 [(Feature 1) Regarding chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements: 【0030】 C: 0.20~0.45% Carbon (C) increases the tensile strength of machine parts manufactured using steel as a material. If the C 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 carbon content exceeds 0.45%, the cold workability 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 0.20-0.45%. The preferred lower limit for the C content is 0.21%, more preferably 0.24%, and even more preferably 0.26%. The preferred upper limit for the C content is 0.43%, more preferably 0.41%, and even more preferably 0.39%. 【0031】 Si: 0.02~0.50% Silicon (Si) enhances the tensile strength of machine parts 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%, even if the content of other elements is within the range of this embodiment, the ductility of the steel during hot rolling will decrease, and defects may occur. Furthermore, the workability will decrease, making it more susceptible to processing cracks. Therefore, the Si content is 0.02 to 0.50%. The preferred lower limit for the Si content is 0.03%, more preferably 0.05%, and even more preferably 0.07%. The preferred upper limit for the Si content is 0.44%, more preferably 0.37%, and even more preferably 0.29%. 【0032】 Mn: 0.50~2.00% Manganese (Mn) increases the tensile strength of machine parts manufactured using steel as a material. If the Mn content is less than 0.50%, 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 2.00%, Mn segregation will occur in the steel material even if the content of other elements is within the range of this embodiment. In this case, the cold workability of the steel material will decrease. Therefore, the Mn content is 0.50-2.00%. The preferred lower limit for the Mn content is 0.53%, more preferably 0.57%, and even more preferably 0.62%. The preferred upper limit for the Mn content is 1.78%, more preferably 1.52%, and even more preferably 1.26%. 【0033】 P:0.030% or less Phosphorus (P) is an impurity. If the P content exceeds 0.030%, 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 machine parts will decrease. If the P content exceeds 0.030%, the cold workability of the steel will further decrease. Therefore, the P content is 0.030% 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 of P content is greater than 0%, more preferably 0.002%, even more preferably 0.004%, and even more preferably 0.006%. The preferred upper limit for the P content is 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 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 machine parts will decrease. If the S content exceeds 0.030%, the cold workability of the steel will further decrease. Therefore, the sulfur content is 0.030% 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 of S content is greater than 0%, more preferably 0.002%, even more preferably 0.004%, and even more preferably 0.006%. The preferred upper limit for the S content is 0.024%, more preferably 0.019%, and even more preferably 0.014%. 【0035】 Al: 0.005~0.080% Aluminum (Al) combines with nitrogen to form Al nitrides. Al nitrides act as pinning particles, refining the crystal grains. As a result, the cold workability of the steel is improved. Furthermore, by forming Al nitrides, Al reduces the amount of dissolved nitrogen in the steel. As a result, the decrease in 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 fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.080%, coarse Al oxides will be formed even if the content of other elements is within the range of this embodiment. Coarse Al oxides become the starting point for fracture. As a result, the cold workability of the steel material decreases. Therefore, the Al content is between 0.005% and 0.080%. The preferred lower limit for the Al content is 0.008%, more preferably 0.012%, and even more preferably 0.021%. The preferred upper limit for the Al content is 0.068%, more preferably 0.058%, and even more preferably 0.046%. 【0036】 Ti: 0.005~0.100% Titanium (Ti) combines with nitrogen (N) to form Ti nitrides. These Ti nitrides act as pinning particles, refining the crystal grains. As a result, the cold workability of the steel is improved. Furthermore, by forming Ti nitrides, Ti reduces the amount of dissolved nitrogen in the steel. Consequently, the decrease in cold workability of the steel due to dynamic strain aging is suppressed. If the Ti content is less than 0.005%, the above effects are not fully achieved. 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, excessive Ti inclusions will be formed in the steel. In this case, the cold workability of the steel will decrease. Therefore, the Ti content is between 0.005% and 0.100%. The preferred lower limit for the Ti content is 0.006%, more preferably 0.008%, and even more preferably 0.011%. The preferred upper limit for the Ti content is 0.080%, more preferably 0.058%, and even more preferably 0.040%. 【0037】 B: 0.0003~0.0050% Boron (B) increases the tensile strength of machine parts. If the B content is less than 0.0003%, the above effect will not be fully obtained. On the other hand, if the B content exceeds 0.0050%, coarse B nitrides and Fe carbonides will be formed, even if the content of other elements is within the range of this embodiment. These coarse B nitrides and Fe carbonides become the starting point for fracture. As a result, the cold workability of the steel material decreases. Therefore, the B content is between 0.0003% and 0.0050%. The preferred lower limit for the B content is 0.0005%, more preferably 0.0007%, and even more preferably 0.0011%. The preferred upper limit for the B content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0029%. 【0038】 N: 0.0150% or less Nitrogen (N) is an impurity. If the N content exceeds 0.0150%, the cold workability of the steel will decrease due to dynamic strain aging, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.0150% or less. A low N content is preferable. However, an extreme reduction in N content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of N content is greater than 0%, more preferably 0.0002%, even more preferably 0.0008%, and even more preferably 0.0017%. The preferred upper limit for the N content is 0.0120%, more preferably 0.0100%, even more preferably 0.0080%, and even more preferably 0.0040%. 【0039】 O: 0.0100% or less Oxygen (O) is an impurity. If the O content exceeds 0.0100%, oxide inclusions will form in the steel, even if the content of other elements is within the range of this embodiment. In this case, the cold workability of the steel will decrease. Therefore, the O content is 0.0100% or less. A low oxygen content is preferable. However, an extreme reduction in oxygen content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of oxygen content is greater than 0%, more preferably 0.0002%, more preferably 0.0005%, and more preferably 0.0008%. The preferred upper limit for the O content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0040%. 【0040】 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 unintentionally included during the industrial production of steel material, such as ore, scrap, or the manufacturing environment, and are acceptable as long as they do not adversely affect the steel material according to this embodiment. 【0041】 [Optional Elements] The steel material of this embodiment may further contain one or more elements selected from the groups consisting of Group 1 to Group 3 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, One or more selected from the group consisting of V: ​​0.20% or less. [Group 2] Ca: 0.0100% or less, One or more selected from the group consisting of Mg: 0.0100% or less. [Group 3] Cu: 0.35% or less, Ni: 0.35% or less, and, One or more selected from the group consisting of Sn: 0.0400% or less. These elements are all optional and do not need to be included. The following describes these optional elements. 【0042】 [Group 1: Cr, Mo, Nb, and V] The steel material of this embodiment may contain one or more elements selected from the first group described above in place of some of the Fe. These elements are arbitrary and all increase the tensile strength of machine parts manufactured using the steel material. The elements of the first group will be described below. 【0043】 Cr:1.50% or less Chromium (Cr) is an optional element and does not need to be included. In other words, the Cr content may be 0%. When present, i.e., when the chromium content is greater than 0%, chromium increases the tensile strength of machine parts manufactured from steel. Even a small amount of chromium content will provide some degree of this effect. However, if the Cr content exceeds 1.50%, even if the content of other elements is within the range of this embodiment, Cr segregates, reducing the cold workability of the steel. Therefore, the Cr content is between 0% and 1.50%, and if present, the Cr content is 1.50% or less. The preferred lower limit for the Cr content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and still more preferably 0.06%. The preferred upper limit for the Cr content is 1.40%, more preferably 1.20%, and even more preferably 1.05%. 【0044】 Mo: 0.50% 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 increases the tensile strength of machine parts manufactured from steel. Even a small amount of Mo can provide some degree of this effect. However, if the Mo content exceeds 0.50%, even if the content of other elements is within the range of this embodiment, Mo will segregate, reducing the cold workability of the steel material. Therefore, the Mo content is 0-0.50%, and if present, the Mo content is 0.50% or less. The preferred lower limit of the Mo content is greater than 0%, more preferably 0.01%, more preferably 0.03%, and still more preferably 0.06%. The preferred upper limit for the Mo content is 0.46%, more preferably 0.43%, and even more preferably 0.40%. 【0045】 Nb: 0.050% or less Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. When Nb is present, that is, when the Nb content is greater than 0%, Nb increases the tensile strength of machine parts manufactured using steel as a material. Even a small amount of Nb will provide some degree of the above effect. However, if the Nb content exceeds 0.050%, even if the content of other elements is within the range of this embodiment, the workability of the steel material will decrease and surface defects will be more likely to occur. If surface defects occur, the cold workability of the steel material will further decrease. Therefore, the Nb content is 0-0.050%, and if present, the Nb content is 0.050% or less. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.001%, more preferably 0.003%, and more preferably 0.005%. The preferred upper limit for the Nb content is 0.046%, more preferably 0.043%, and even more preferably 0.040%. 【0046】 V: 0.20% or less Vanadium (V) is an optional element and does not need to be present. In other words, the V content may be 0%. When present, i.e., when the V content is greater than 0%, V increases the tensile strength of machine parts manufactured from steel. Even a small amount of V will provide some degree of the above effect. However, if the V content exceeds 0.20%, even if the content of other elements is within the range of this embodiment, the strengthening due to precipitation of V precipitates becomes excessively large. As a result, the cold workability of the steel material decreases. Therefore, the V content is 0-0.20%, and if present, the V content is 0.20% or less. The preferred lower limit of the V content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.05%. The preferred upper limit for the V content is 0.18%, more preferably 0.16%, and even more preferably 0.15%. 【0047】 [Group 2: Ca and Mg] The steel material of this embodiment may contain one or more elements selected from the second group described above in place of a portion of the Fe. These elements are arbitrary and all of them spheroidize the MnS in the steel material, thereby improving the cold workability and machinability of the steel material. The elements of the second group will be described below. 【0048】 Ca:0.0100% 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 (Ca) is present, that is, when the Ca content is greater than 0%, the Ca spheroidizes the MnS. This improves the cold workability and machinability of the steel. Even a small amount of Ca can provide some degree of this effect. However, if the Ca content exceeds 0.0100%, coarse Ca-based inclusions will form even if the content of other elements is within the range of this embodiment. Coarse Ca-based inclusions reduce the cold workability of the steel material. Therefore, the Ca content is between 0 and 0.0100%, and if present, the Ca content is 0.0100% or less. The preferred lower limit of the Ca content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0005%. The preferred upper limit for the Ca content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0040%. 【0049】 Mg: 0.0100% 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%, the Mg spheroidizes MnS. This improves the cold workability and machinability of the steel. Even a small amount of Mg can provide some degree of this effect. However, if the Mg content exceeds 0.0100%, coarse Mg-based inclusions will form even if the content of other elements is within the range of this embodiment. Coarse Mg-based inclusions reduce the cold workability of the steel material. Therefore, the Mg content is between 0 and 0.0100%, and if present, the Mg content is 0.0100% or less. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0005%. The preferred upper limit for the Mg content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0040%. 【0050】 [Group 3: Cu, Ni, and Sn] The steel material of this embodiment may contain one or more elements selected from the third group described above in place of a portion of the Fe. These elements are arbitrary and all suppress hydrogen intrusion into the steel material. The elements of the third group will be described below. 【0051】 Cu: 0.35% or less Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. When copper (Cu) is present, that is, when the Cu content is greater than 0%, the Cu inhibits corrosion of the steel. This suppresses the generation of hydrogen on the steel surface, and as a result, inhibits the penetration of hydrogen into the steel. Even a small amount of Cu can provide some degree of the above effect. However, 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 of the steel decreases. Therefore, the Cu content is 0-0.35%, and if present, the Cu content is 0.35% or less. The preferred lower limit of the Cu content is greater than 0%, more preferably 0.03%, more preferably 0.04%, and still more preferably 0.05%. The preferred upper limit for the Cu content is 0.33%, more preferably 0.31%, and even more preferably 0.30%. 【0052】 Ni: 0.35% or less Nickel (Ni) is an optional element and does not need to be included. In other words, the Ni content may be 0%. Ni, when present together with Cu, suppresses the decrease in hot workability of steel materials when Cu is greater than 0%. Even a small amount of Ni can provide the above effect to some extent. However, if the Ni content exceeds 0.35%, the cold workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Ni content is 0-0.35%, and if present, the Ni content is 0.35% or less. The preferred lower limit of the Ni content is greater than 0%, more preferably 0.02%, more preferably 0.03%, and still more preferably 0.05%. The preferred upper limit for the Ni content is 0.33%, more preferably 0.30%, and even more preferably 0.28%. 【0053】 Sn: 0.0400% or less Tin (Sn) is an optional element and does not need to be present. In other words, the Sn content may be 0%. If Sn is present, that is, if the Sn content is greater than 0%, Sn suppresses hydrogen penetration into the steel. Therefore, the hydrogen embrittlement resistance of the steel is further 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 0.0400%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is between 0 and 0.0400%, and if present, the Sn content is 0.0400% or less. The preferred lower limit for the Sn content is greater than 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0005%. The preferred upper limit for the Sn content is 0.0200%, more preferably 0.0100%, and even more preferably 0.0050%. 【0054】 [Method for measuring the chemical composition of steel materials] The chemical composition of the steel material in this embodiment can be measured by 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. The O content is determined using a well-known inert gas melting-infrared absorption method. 【0055】 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. 【0056】 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. 【0057】 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. 【0058】 [(Feature 2) Regarding Tensile Strength] The tensile strength of the steel material in this embodiment is 1100 MPa or less. If the tensile strength exceeds 1100 MPa, the cold workability of the steel material will decrease during the manufacturing process of machine parts. Therefore, the tensile strength of the steel material in this embodiment is 1100 MPa or less. 【0059】 The preferred upper limit of the tensile strength is 1050 MPa, more preferably 950 MPa, and even more preferably 850 MPa. The lower limit of the tensile strength is not particularly limited, but for example, it is 550 MPa. 【0060】 [Method for measuring the tensile strength of steel materials] The tensile strength of the steel material in this embodiment is determined by the following method. A tensile test specimen is taken from the steel material. The shape of the tensile test specimen is, for example, type 14A as specified in Annex D of JIS Z 2241:2011. The length of the parallel section of the type 14A specimen should be set within the range of 5.5 to 7.0 times the diameter of the parallel section of the specimen, as specified in JIS Z 2241:2011. The radius of the shoulder of the type 14A specimen is 15 mm. The length direction of the tensile test specimen is parallel to the axial direction of the steel material. The tensile test specimen is taken from the center of the cross section perpendicular to the axial direction of the steel material. 【0061】 Tensile strength (MPa) will be measured by performing a tensile test on the collected tensile specimens in accordance with JIS Z 2241:2011 at room temperature and in air. The crosshead displacement rate during the tensile test will be 8.0 mm / min. 【0062】 [(Feature 3) Regarding equations (1) and (2)] In the steel material of this embodiment, the diameter of the cross-section perpendicular to the axial direction of the steel material is defined as D, electron backscatter diffraction is performed at a depth of D / 4 from the surface of the steel material to determine the crystal orientation difference of the grain boundaries, and with a class width of 2.5°, the grain boundary length ratio, which is the ratio of the total length of grain boundaries belonging to each class to the total length of grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0°, is determined for each class in the range of crystal orientation differences of 5.0° or more and less than 65.0°, and the grain boundary length ratio of the class with the maximum in the range of crystal orientation differences of 5.0° or more and less than 15.0° is defined as F1, the grain boundary length ratio of the class with the minimum in the range of crystal orientation differences of 15.0° or more and less than 55.0° is defined as F2, and the grain boundary length ratio of the class with the maximum in the range of crystal orientation differences of 55.0° or more and less than 65.0° is defined as F3, thereby satisfying equations (1) and (2). F1 / F2≧2.5 (1) F3 / F1≧1.8 (2) The following explains this point. 【0063】 As described above, in steel materials, the difference in crystal orientation at grain boundaries affects the ease with which hydrogen penetrates the steel. Here, the difference in crystal orientation refers to the relative difference in crystal orientation between two adjacent crystal grains separated by a grain boundary. The difference in crystal orientation is expressed in degrees (°). In this specification, a region enclosed by grain boundaries with a difference in crystal orientation of 5.0° or more is defined as a crystal grain. 【0064】 Here, X1 and X2 are defined as follows: X1 = F1 / F2 X2 = F3 / F1 Referring to Figure 1, when F1 / F2 is less than 2.5, the diffusible hydrogen concentration is high. On the other hand, when F1 / F2 is 2.5 or higher, the diffusible hydrogen concentration decreases significantly. Similarly, referring to Figure 2, when F3 / F1 is less than 1.8, the diffusible hydrogen concentration is high. On the other hand, when F3 / F1 is 1.8 or higher, the diffusible hydrogen concentration decreases significantly. As described above, from Figures 1 and 2, when F1 / F2 is 2.5 or higher and F3 / F1 is 1.8 or higher, the diffusible hydrogen concentration decreases significantly, and hydrogen intrusion can be effectively suppressed. 【0065】 A preferred lower limit for X1 is 2.6, more preferably 2.9, even more preferably 3.2, and even more preferably 3.5. The upper limit for X1 is not particularly limited. For example, the upper limits for X1 are 10.0 and 9.0. A preferred lower limit for X2 is 1.9, more preferably 2.0, and even more preferably 2.2. The upper limit for X2 is not particularly limited. For example, the upper limits for X2 are 8.0 and 6.0. 【0066】 [Regarding the measurement method for grain boundary length ratios F1-F3] The grain boundary lengths F1 to F3 are measured by the following method. A test specimen containing the D / 4 depth position is taken from the surface of the steel material. Of the surface of the taken test specimen, the surface perpendicular to the axial direction of the steel material and containing the D / 4 depth portion is designated as the "observation surface". The observation surface of the test specimen is wet polished using #400 to #1500 emery paper (silicon carbide waterproof abrasive paper). After wet polishing, the observation surface is further polished to a mirror finish with an abrasive cloth soaked in a diamond suspension (a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in alcohol or pure water). After that, colloidal silica polishing is performed on the observation surface. At the D / 4 depth portion of the polished observation surface, four observation fields are selected in the circumferential direction of the observation surface. The area of ​​each observation field is a rectangle with a short side of 60 μm and a long side of 180 μm. 【0067】 Each observation area is divided into regular hexagonal pixels, and the crystal orientation of each pixel is determined by the EBSD method. The crystal structure of the observation area is assumed to be a BCC structure. An EBSD analyzer equipped with a FE-SEM (Field Emission-Scanning Electron Microscope) and an EBSD detector is used to measure the crystal orientation at each pixel. For example, the FE-SEM used is the JSM-7100F manufactured by JEOL Ltd. For example, the EBSD detector used is the DVC5 detector manufactured by TLC Solutions Co., Ltd. For example, the measurement software used for EBSD analysis is OIM Data Collection (trademark) ver7.3.1 manufactured by TLC Solutions Co., Ltd. The vacuum level inside the EBSD analyzer is set to 9.6 × 10⁻⁶. -5 The pressure shall be less than or equal to Pa. The acceleration voltage of the electron beam shall be 15kV. The irradiation current level of the electron beam shall be 14, and the irradiation interval of the electron beam shall be 1.0μm. The direction of incidence of the electron beam shall be inclined 70 degrees from the axial direction of the steel material with respect to the observation surface, and the working distance shall be 15mm. The length of one side of a regular hexagonal pixel shall be (1 / √(3)) times the irradiation interval. 【0068】 Using the crystal orientation at each measurement location, grain boundaries are identified in the following way: The difference in crystal orientation between adjacent pixels is determined. If the crystal orientation difference is 5.0° or greater, the interface between adjacent pixels that form a closed region is recognized as a grain boundary, and the closed region enclosed by the recognized grain boundary is recognized as a crystal grain. In other words, even if the crystal orientation difference is 5.0° or greater, interfaces that are interrupted without forming a closed region are not recognized as grain boundaries. The grain boundaries in the observation area are identified using the above method. Note that if the recognized crystal grain is 2 pixels or less, the grain boundary surrounding that crystal grain is excluded as an outlier. In other words, grain boundaries constituting crystal grains of 2 pixels or less are not recognized as grain boundaries. 【0069】 Based on the crystal orientation difference of the grain boundaries determined by the above method, the total length GL0 of grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0° is determined. Furthermore, with a class width of 2.5°, the total length GLi (i=1 to 24) of grain boundaries belonging to each class i (i=1 to 24) is determined for each class. There are 24 classes. Specifically, the total length GL1 of grain boundaries is the total length of grain boundaries with a crystal orientation difference of 5.0° or more and less than 7.5°. The total length GL2 of grain boundaries is the total length of grain boundaries with a crystal orientation difference of 7.5° or more and less than 10.0°. The total length GLi of grain boundaries is the total length of grain boundaries with a crystal orientation difference of {5.0 + 2.5(i-1)}° or more and less than (5.0 + 2.5i)°. 【0070】 The ratio of the total grain boundary length GLi in each class to the total grain boundary length GL0 is defined as the grain boundary length ratio fi (i=1~24). Figure 3 is an example of a histogram showing the grain boundary length ratio fi for each class, and Figure 4 is an example of a different histogram showing the grain boundary length ratio fi for each class. Referring to Figures 3 and 4, the grain boundary length ratios F1~F3 in each class are defined as follows. Grain boundary length ratio F1: The maximum grain boundary length ratio among grain boundary length ratios f1 to f3 within the range of crystal orientation difference of 5.0° or more and less than 15.0°. Grain boundary length ratio F2: The smallest grain boundary length ratio among the grain boundary length ratios f4 to f20 within the range of crystal orientation difference between 15.0° and less than 55.0°. Grain boundary length ratio F3: The maximum grain boundary length ratio among grain boundary length ratios f21 to f24 within the range of crystal orientation difference between 55.0° and less than 65.0°. 【0071】 The grain boundary length ratios F1 to F3 are determined for each of the four observation fields. The arithmetic mean of the four obtained grain boundary length ratios F1 is defined as the grain boundary length ratio F1 for that steel material. The arithmetic mean of the four obtained grain boundary length ratios F2 is defined as the grain boundary length ratio F2 for that steel material. The arithmetic mean of the four obtained grain boundary length ratios F3 is defined as the grain boundary length ratio F3 for that steel material. 【0072】 The above-mentioned calculation of crystal orientation differences and identification of grain boundaries can be performed using known analysis software included with the EBSD analyzer. For example, the analysis software used is OIM Analysis (trademark) ver7.3.1, manufactured by TSL Solutions Co., Ltd. 【0073】 [Shape of the steel material in this embodiment] The steel material in this embodiment is a steel bar or wire. The steel bar or wire is a steel material in the shape of a bar or wire. The steel material may be wound into a coil or cut to a predetermined length. 【0074】 [Applications of the steel material of this embodiment] The steel material of this embodiment can sufficiently suppress hydrogen intrusion by satisfying features 1 to 3. Therefore, the steel material of this embodiment can be applied as a material for machine parts in industrial machinery, automobiles, bridges, and buildings. Non-heat-treated machine parts include, for example, non-heat-treated bolts. 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 manufacturing method for the steel material of this embodiment will be described. The steel material satisfying the above-described features 1 to 3 may be manufactured by other manufacturing methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a manufacturing method for 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 (Step 4) Tissue adjustment step 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. Specifically, 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. 【0080】 Billets are manufactured by rolling (rough rolling) heated material using a bloc mill, or a bloc mill and a continuous mill. Specifically, billets are manufactured by reverse rolling heated material using a bloc mill. A bloc mill is equipped with a pair of horizontal rolls. Reverse rolling is performed in the bloc mill. Reverse rolling refers to a rolling method in which the material is reduced by the bloc mill as it passes from upstream to downstream, and again as it passes from downstream to upstream. 【0081】 If a continuous rolling mill is located downstream of the bract mill, the billets after bract rolling may be further subjected to tandem rolling using the continuous rolling mill to produce smaller billets. The continuous rolling mill includes multiple rolling stands. Each rolling stand includes a pair of work rolls. Each work roll has a caliber formed on it, and the calibers of the pair of work rolls form a hole. In the continuous rolling mill, tandem rolling is performed from upstream to downstream. The billets produced by the above rough rolling process are allowed to cool to room temperature (air-cooled) before the finish rolling process. 【0082】 [(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 The heating and rolling processes in the finishing rolling process will be described below. 【0083】 [(Step 31) Heating process] 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. 【0084】 [(Process 32) Rolling Process] In the rolling process, continuous rolling (finish rolling) is performed on the heated billet using a continuous rolling mill in which multiple rolling stands are arranged in a row. In continuous rolling using a continuous rolling mill, when the billet passes through each rolling stand from upstream to downstream, reducing the billet at that rolling stand is defined as "1 pass" reduction. Continuous rolling means reducing the billet using a continuous rolling mill in multiple passes. Note that it is not necessary to reduce the billet at all rolling stands in the continuous rolling mill. For example, if a continuous rolling mill has 15 rolling stands, and the billet passes through the last rolling stand without being reduced, then 14 passes of reduction are performed. The rolling process satisfies the following conditions. 【0085】 Condition 1: The processing speed at three rolling stands (for the final three passes of reduction) out of multiple rolling stands is set to over 15.0 / second. Condition 2: The finishing rolling temperature shall exceed 850°C. Here, the processing speed is defined by the following equation. Machining speed={-LN(1-R)} / t Here, R is the cumulative reduction rate (%) at rolling stands STe, STe-1, and STe-2. t is the time (seconds) from the start of reduction at rolling stand STe-2 to the end of reduction at rolling stand STe. 【0086】 If either condition 1 or condition 2 is not met, X2 will be less than 1.8 in the steel material. If conditions 1 and 2 are met, and condition 3 in the isothermal transformation treatment described below is also met, X2 will be 1.8 or greater. 【0087】 [(Step 4) Tissue adjustment step] In the microstructure adjustment process, the steel material that has been finish-rolled in the finish-rolling process is subjected to cooling and isothermal transformation treatment to make the microstructure of the steel material satisfy formulas (1) and (2). The microstructure adjustment process includes the following steps. (Step 41) Cooling step (Step 42) Constant Temperature Transformation Process The following describes each step. 【0088】 [(Step 41) Cooling step] After finish rolling, the steel material is wound into a coil and then placed on a transport path for transport. In the cooling process, the steel material is cooled after finish rolling. In the cooling process, the average cooling rate CR in the range of 800 to 500°C is set to 10°C / second or higher. 【0089】 [(Step 42) Constant Temperature Transformation Process] A constant-temperature transformation treatment is performed immediately on the steel material after the cooling process. In the constant-temperature transformation treatment, the steel material is subjected to a constant-temperature transformation treatment at a holding temperature T1 (°C) and a holding time t1 (seconds). The constant-temperature transformation treatment is performed, for example, by immersing the steel material in an immersion tank maintained at the above temperature T1. The immersion tank may be a molten salt tank, a lead bath, or a fluidized bed. The steel material is cooled after the constant-temperature transformation treatment. Cooling may be done by water cooling or by air cooling. The constant-temperature transformation treatment process satisfies the following condition 3. Condition 3: The holding temperature T1 is set to 450-500°C, and the holding time t1 is set to 45 seconds or more. 【0090】 If the holding temperature T1 is less than 450°C, the tensile strength exceeds 1100 MPa. On the other hand, if the holding temperature T1 is greater than 500°C, X1 will be less than 2.5. Also, if the holding time t1 is less than 45 seconds, the tensile strength may exceed 1100 MPa. If the holding temperature T1 is between 450 and 500°C, assuming that conditions 1 and 2 are met and the holding time t1 is 45 seconds or more, a microstructure is obtained in which the tensile strength of the steel is 1100 MPa or less and equations (1) and (2) are satisfied. 【0091】 Through the above manufacturing process, steel materials that satisfy features 1 to 3 can be produced. 【0092】 [Method for manufacturing non-heat-treated machine parts using steel material according to this embodiment] The method for manufacturing non-heat-treated machine parts using steel as a material in this embodiment is a well-known manufacturing method. As an example of a non-heat-treated machine part, the 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 (forging process, rolling process) • Bluing process • Plating process • Baking process Of the above steps, the bluing, plating, and baking steps are optional. In other words, the bluing, plating, and baking steps do not have to be performed. The following describes each step. 【0093】 [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 its surface. The lubricating film is, for example, a well-known phosphate film and a well-known soap lubricating film. A well-known wire drawing process is then performed on the steel material on which the lubricating film has been formed to manufacture steel wire. The wire drawing process may consist of primary drawing only, or multiple drawing processes such as secondary drawing may be performed. The total area reduction rate in the wire drawing process is not particularly limited, but for example, it is 5 to 65%. 【0094】 [Cold working process] In the cold working process, the steel wire is subjected to the well-known forging and rolling processes to produce an intermediate bolt-shaped product. Specifically, the steel wire is given a threaded shape by the well-known forging process. Furthermore, the threads are formed on the shaft of the bolt-shaped product by the well-known rolling process. Through these processes, an intermediate bolt-shaped product is manufactured. 【0095】 [Bluing process] The bluing process is optional; in other words, it does not have to be performed. If performed, the intermediate product is held at a temperature range of 200-600°C for 10-300 minutes. When the bluing process is performed, the tensile strength and yield ratio of the non-heat-treated bolts increase. 【0096】 [Plating process] The plating process is optional. In other words, the plating process does not have to be performed. If performed, the plating process involves applying a well-known plating treatment to the intermediate product to form a plating layer on its surface. The formation of the plating layer improves the corrosion resistance of the non-heat-treated bolts. The plating layer is not particularly limited, but examples include zinc plating (JIS B1044:2001, JIS B1048:2007) and zinc flake coating (JIS B1046:2020). 【0097】 [Baking process] The baking process is optional; in other words, it does not have to be performed. However, if hydrogen penetrates 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 150-250°C for 60-480 minutes. The baking process releases the hydrogen that has penetrated the steel material during the plating process. 【0098】 By the manufacturing method described above, non-heat-treated bolts can be produced using the steel material of this embodiment. In the above manufacturing process, no heat treatment (quenching and tempering) is performed. Therefore, the microstructure of the steel material does not change during the bolt manufacturing process described above. Consequently, the microstructure of the produced non-heat-treated bolt is substantially the same as that of the steel material. And, like the steel material, it satisfies features 1 to 3. Therefore, hydrogen intrusion in corrosive environments is suppressed in non-heat-treated bolts. [Examples] 【0099】 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. 【0100】 [Material preparation process] Steel materials having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured by the following method. 【0101】 [Table 1-1] 【0102】 [Table 1-2] 【0103】 [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. 【0104】 [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. The processing speed ( / second) and finish rolling temperature (°C) are shown in Table 2. 【0105】 [Table 2] 【0106】 [Tissue adjustment process] A microstructure adjustment process was performed on the steel material (round bar) after finish rolling. During the cooling process in the microstructure adjustment stage, the average cooling rate CR was 10°C / second or higher in the range of steel material temperature from 800°C to 500°C. 【0107】 The steel material was immediately subjected to a constant-temperature transformation process after the initial cooling process. The holding temperature T1 (°C) and holding time t1 (seconds) during the constant-temperature transformation process are shown in Table 2. After the constant-temperature transformation process, the steel material was water-cooled to 220°C, and then allowed to cool. 【0108】 Through the manufacturing process described above, steel bars (round bars) with a diameter of 11.0 mm were produced for each test number. 【0109】 [About the evaluation test] The following steel evaluation tests (Tests 1 to 6) 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 grain boundary length ratio F1-F3 in steel materials (Test 3) Evaluation test of the tensile strength of steel materials (Test 4) Diffusible hydrogen concentration measurement test (Test 5) Tensile strength evaluation test of non-heat-treated machine parts (Test 6) Cold workability evaluation test The following describes each test. 【0110】 [(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. 【0111】 [(Test 2) Measurement test of grain boundary length ratio F1~F3 in steel materials] For each test number of steel material (round bar), the grain boundary length ratios F1 to F3 were determined based on the above-mentioned [Measurement Method for Grain Boundary Length Ratios F1 to F3]. The FE-SEM used was the JSM-7100F manufactured by JEOL Ltd. The EBSD detector used was the DVC5 type detector manufactured by TLC Solutions Co., Ltd. The measurement and analysis software used was OIM Data Collection / Analysis (trademark) ver7.3.1 manufactured by TLC Solutions Co., Ltd. The obtained F1 to F3, and X1 (=F1 / F2), X2 (=F3 / F1) are shown in the "F1", "F2", "F3", "X1 (=F1 / F2)", and "X2 (=F3 / F1)" columns of Table 3, respectively. 【0112】 [Table 3] 【0113】 [(Test 3) Evaluation test of tensile strength of steel material] For each steel material (round bar) with a given test number, the tensile strength (MPa) was determined based on the [Method for Measuring the Tensile Strength of Steel Materials] described above. The No. 14A test specimen used had a parallel section diameter of 7.0 mm, a parallel section length of 49.0 mm, and a shoulder radius of 15 mm. The obtained tensile strengths are shown in the "Tensile Strength of Steel Materials (MPa)" column in Table 3. 【0114】 [(Test 4) Measurement Test of Diffusible Hydrogen Concentration] The following diffusible hydrogen concentration measurement tests were performed on the steel materials for each test number. 【0115】 [Manufacturing of non-heat-treated machine part imitation materials] First, non-heat-treated machine part simulants were manufactured using steel materials of each test number. Specifically, a well-known lubrication treatment was performed on the steel materials of each test number to form a lubricating film (phosphate film and soap lubrication film) on the surface of the steel material. The lubrication treatment conditions were the same for each test number. Subsequently, a well-known wire drawing process was performed on the steel materials on which the lubricating film had been formed. The total reduction ratio of the wire drawing process was 47%. Through the above manufacturing process, non-heat-treated machine part simulants (round bars) were manufactured. 【0116】 [Measurement Test] For each test number, a non-heat-treated simulated machine part was cut perpendicular to its length, and multiple 100mm long round bar test pieces were taken. To eliminate the influence of the lubricating film formed on the surface of the steel wire before drawing, the round bar test pieces were blast-treated to remove the lubricating film from the surface of the round bar test pieces. 【0117】 Combined cycle corrosion tests (CCTs) as specified in JASO M609 (1991) were performed using blast-treated round bar specimens. For each test number, the number of test cycles in the corrosion test was set to 21, 42, 84, and 126 cycles. A separate round bar specimen was used for each pattern. 【0118】 In each test cycle, the round bar specimen was removed after the test was completed. The removed round bar specimen was subjected to blasting to remove the corrosion products that had formed on the surface of the round bar specimen during the corrosion test. Using a wet cutting machine, the central 30 mm portion in the longitudinal direction of the blasted round bar specimen was cut out. 【0119】 The diffusible hydrogen concentration in the excised test specimen was analyzed using a thermal desorption analysis (TDA) gas chromatograph. Specifically, the excised test specimen was heated from room temperature to 200°C at a heating rate of 100°C / hr. The amount of hydrogen released from the test specimen to the outside was measured. 【0120】 The measured amount of hydrogen was divided by the mass of the test specimen 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 among the four diffusible hydrogen concentrations was defined as the diffusible hydrogen concentration for that test number. The obtained diffusible hydrogen concentrations are shown in the "Diffusible Hydrogen Concentration (mass ppm)" column in Table 3. If the diffusible hydrogen concentration was 0.20 mass ppm or less, it was judged that hydrogen penetration was sufficiently suppressed. 【0121】 [(Test 5) Tensile strength evaluation test of non-heat-treated machine parts] As an example of non-heat-treated machine parts made from the steel materials of each test number, non-heat-treated bolts were manufactured using the following manufacturing method. 【0122】 For each test number of steel material (round bar), a well-known lubrication treatment was performed to form a lubricating film (phosphate film and soap lubrication film) on the surface of the steel material. The lubrication treatment conditions were the same for each test number. Subsequently, a well-known wire drawing process was performed on the steel material on which the lubricating film had formed. The total reduction ratio of the wire drawing process was 47%. Steel wire was manufactured using the above manufacturing process. 【0123】 Intermediate bolts with the shape of an M8 flanged hexagonal bolt as specified in JIS B1189:2014 were manufactured by cold heading of steel wire using a well-known method. The shaft diameter (D) of the intermediate bolts was set to 8.0 mm. The cold heading conditions were the same for each test number. After cold heading, cold rolling was performed on the intermediate bolts to form a threaded section on a portion of the shaft of the intermediate bolts, with a nominal thread size (M) of 8.0 mm and a pitch (P) of 1.25 mm. The cold rolling conditions were the same for each test number. 【0124】 After cold rolling, the intermediate product underwent a bluing process, which involved holding it at a temperature of 350°C for one hour. Through this manufacturing process, non-heat-treated bolts were produced. 【0125】 Tensile tests were conducted on the manufactured non-heat-treated bolts in accordance with JIS B 1051:2014 at room temperature and in air to measure their tensile strength (MPa). The crosshead displacement rate during the tensile test was set to 3.0 mm / min. The obtained tensile strengths are shown in the "Tensile Strength of Machine Parts (MPa)" column of Table 3. A tensile strength of 1000 MPa or higher was considered to indicate sufficient strength. 【0126】 [(Test 6) Cold Workability Evaluation Test] In the manufacturing process of the non-heat-treated bolts described above, the presence or absence of cracks was visually inspected for the bolt head portion or the transition area between the bolt head and the shaft portion of the intermediate product after cold heading. If no cracks longer than 0.5 mm were found, it was determined that sufficient cold workability had been achieved (indicated by "○" in the "Cold Workability" column of Table 3). On the other hand, if cracks longer than 0.5 mm were found, it was determined that sufficient cold workability had not been achieved (indicated by "×" in the "Cold Workability" column of Table 3). If it was difficult to visually determine whether a crack was present, the relevant portion 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 heading was stopped, and the tensile strength evaluation test of the non-heat-treated machine parts was not performed (indicated by "-" in the "Tensile Strength of Machine Parts (MPa)" column of Table 3). 【0127】 [Evaluation Results] The evaluation results are shown in Table 3. In tests 1-30, the steel material met characteristics 1-3. Therefore, the diffusible hydrogen concentration was 0.20 mass ppm or less, suppressing hydrogen intrusion. Furthermore, the tensile strength was 1000 MPa or higher, providing sufficient strength for non-heat-treated machine parts. In addition, sufficient cold workability was achieved. 【0128】 In tests 31-34, although the chemical composition met characteristic 1, the processing speed in the finish rolling process was less than 15.0 / second. Therefore, X2 did not satisfy equation (2). As a result, the diffusible hydrogen concentration exceeded 0.20 mass ppm, and hydrogen intrusion could not be sufficiently suppressed. 【0129】 In tests 35 and 36, although the chemical composition met characteristic 1, the finishing rolling temperature was below 850°C. Therefore, X2 did not satisfy equation (2). As a result, the diffusible hydrogen concentration exceeded 0.20 mass ppm, and hydrogen intrusion could not be sufficiently suppressed. 【0130】 In tests 37-39, the holding temperature T1 during the isothermal transformation process was too low. As a result, the tensile strength of the steel exceeded 1100 MPa. Consequently, sufficient cold workability could not be obtained. 【0131】 In tests 40-42, the holding temperature T1 during the constant-temperature transformation process was too high. As a result, X1 did not satisfy equation (1). Consequently, the diffusible hydrogen concentration exceeded 0.20 mass ppm, and hydrogen intrusion could not be sufficiently suppressed. 【0132】 In tests 43-46, the holding time t1 during the isothermal transformation process was too short. As a result, the tensile strength of the steel exceeded 1100 MPa. Consequently, sufficient cold workability could not be obtained. 【0133】 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

[Claim 1] A steel material that is rod-shaped or linear, with a circular cross-section perpendicular to the axial direction, In mass percent, C: 0.20-0.45%, Si: 0.02-0.50%, 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%, N: 0.0150% or less, and It contains O: 0.0100% or less, with the remainder consisting of Fe and impurities. The tensile strength is 1100 MPa or less. The diameter of the cross-section is defined as D, and electron backscatter diffraction is performed at a depth of D / 4 from the surface of the steel material to determine the crystal orientation difference of the grain boundaries. With a class width of 2.5°, the grain boundary length ratio, which is the ratio of the total length of the grain boundaries belonging to each class to the total length of the grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0°, is determined for each class within the range of a crystal orientation difference of 5.0° or more and less than 65.0°. The grain boundary length ratio of the class in which the crystal orientation difference is greatest within the range of 5.0° to less than 15.0° is defined as F1. The grain boundary length ratio of the class in which the crystal orientation difference is smallest within the range of 15.0° to less than 55.0° is defined as F2. When F3 is defined as the grain boundary length ratio of the class in which the crystal orientation difference is greatest in the range of 55.0° or more and less than 65.0°, the following conditions satisfy equations (1) and (2): Steel material. F1 / F2≧2.5 (1) F3 / F1≧1.8 (2) [Claim 2] A steel material that is rod-shaped or linear, with a circular cross-section perpendicular to the axial direction, In mass percent, C: 0.20-0.45%, Si: 0.02-0.50%, 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%, N: 0.0150% or less, and O: Contains 0.0100% or less, Furthermore, it contains one or more elements selected from the groups consisting of Group 1 to Group 3, with the remainder being Fe and impurities. The tensile strength is 1100 MPa or less. The diameter of the cross-section is defined as D, and electron backscatter diffraction is performed at a depth of D / 4 from the surface of the steel material to determine the crystal orientation difference of the grain boundaries. With a class width of 2.5°, the grain boundary length ratio, which is the ratio of the total length of the grain boundaries belonging to each class to the total length of the grain boundaries with a crystal orientation difference of 5.0° or more and less than 65.0°, is determined for each class within the range of a crystal orientation difference of 5.0° or more and less than 65.0°. The grain boundary length ratio of the class in which the crystal orientation difference is greatest within the range of 5.0° to less than 15.0° is defined as F1. The grain boundary length ratio of the class in which the crystal orientation difference is smallest within the range of 15.0° to less than 55.0° is defined as F2. When F3 is defined as the grain boundary length ratio of the class in which the crystal orientation difference is greatest in the range of 55.0° or more and less than 65.0°, the following conditions satisfy equations (1) and (2): Steel material. [Group 1] Cr: 1.50% or less, Mo: 0.50% or less Nb: 0.050% or less, V: One or more selected from the group consisting of 0.20% or less. [Group 2] Ca: 0.0100% or less, One or more selected from the group consisting of Mg: 0.0100% or less. [Group 3] Cu: 0.35% or less, Ni: 0.35% or less, One or more selected from the group consisting of Sn: 0.0400% or less. F1 / F2≧2.5 (1) F3 / F1≧1.8 (2) [Claim 3] The steel material according to claim 2, The above group 1 contains, Steel material. [Claim 4] The steel material according to claim 2, The following include the second group: Steel material. [Claim 5] The steel material according to claim 2, The following contain the third group: Steel material.

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