Steel material and bolt

Abstract

Provided is a steel material with which it is possible to obtain exceptional hydrogen embrittlement resistance in a bolt manufactured using the steel material as a raw material. A steel material according to the present disclosure has the chemical composition set forth in the description, has an equivalent circle diameter of at least 1.0 μm, and is such that: the median value of the aspect ratio of MnS inclusions having an Mn content of at least 10% by mass and an S content of at least 5% by mass is 20.0 or lower; the number ratio of Ti-containing MnS inclusions having a Ti content of at least 5% by mass, from among MnS inclusions having an aspect ratio of at least 5.0, is at least 30.0%; and the number ratio of CuNiTi-containing MnS inclusions in which the sum of the Cu content and the Ni content is at least 1% by mass, from among the Ti-containing MnS inclusions having an aspect ratio of at least 5.0, is at least 50.0%.

Description

Steel and bolts

[0001] The present disclosure relates to a steel material and a bolt made from the steel material.

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

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

[0004] Techniques for improving the hydrogen embrittlement resistance of steel materials used to make bolts are proposed in Japanese Patent Laid-Open No. 2008-274367 (Patent Document 1) and Japanese Patent Laid-Open No. 2012-017519 (Patent Document 2).

[0005] The steel material disclosed in Patent Document 1 contains, by 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 consisting of Fe and impurities, and has a composition in which the Cu / Sn ratio is 1 or less. In this steel material, the inclusion of Sn suppresses the penetration of hydrogen into the steel material, resulting in increased hydrogen embrittlement resistance of the steel material.

[0006] The steel material for bolts disclosed in Patent Document 2 consists, by mass%, of 0.15 to 0.30% C, 2.0% or less Si, 1.5% or less Mn, 2.5 to 5.0% Cr, 0.0005 to 0.01% B, 0.1% or less Ti, 1.0% or less Al, and the balance being Fe and unavoidable impurities. This steel material for bolts has improved resistance to hydrogen embrittlement due to the inclusion of the above alloy elements in appropriate ranges.

[0007] JP 2008-274367 A JP 2012-017519 A

[0008] In Patent Documents 1 and 2, hydrogen embrittlement resistance is improved by appropriately adjusting the chemical composition of the steel material. However, hydrogen embrittlement resistance may be improved by a means different from that of the steel material disclosed in Patent Documents 1 and 2.

[0009] An object of the present disclosure is to provide a steel material that can be used as a material to produce a bolt that exhibits excellent hydrogen embrittlement resistance, and a bolt that can exhibit excellent hydrogen embrittlement resistance.

[0010] The steel material of the present disclosure contains, in mass %, C: 0.20 to less than 0.40%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, sol. containing Al: 0.005 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 1.50%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.015% or less, S: 0.015% or less, N: 0.0150% or less, O: 0.0030% or less, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, The remainder consists of Fe and impurities, the median aspect ratio of MnS inclusions having an equivalent circle diameter of 1.0 μm or more and having a Mn content of 10% or more and an S content of 5% or more, by mass%, is 20.0 or less, the proportion by number of Ti-containing MnS inclusions having a Ti content of 5% or more, by mass%, is 30.0% or more, and the proportion by number of CuNiTi-containing MnS inclusions having a sum of Cu content and Ni content, by mass%, of the Ti-containing MnS inclusions having an aspect ratio of 5.0 or more is 50.0% or more.

[0011] The bolt of the present disclosure comprises a head, a shank, and a neck portion connecting the head and the shank, and the chemical composition of the bolt is, in mass%, C: 0.20 to less than 0.40%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, sol. containing Al: 0.005 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 1.50%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.015% or less, S: 0.015% or less, N: 0.0150% or less, O: 0.0030% or less, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, the remainder consisting of Fe and impurities, and in the shaft portion, the median aspect ratio of MnS inclusions having an equivalent circle diameter of 1.0 μm or more and an Mn content of 10% or more and an S content of 5% or more, expressed in mass%, is 20.0 or less, the proportion by number of Ti-containing MnS inclusions having a Ti content of 5% or more, expressed in mass%, is 30.0% or more, and the proportion by number of CuNiTi-containing MnS inclusions having a sum of Cu content and Ni content of 1% or more, expressed in mass%, is 50.0% or more, among the Ti-containing MnS inclusions having an aspect ratio of 5.0 or more.

[0012] In the steel material of the present disclosure, excellent hydrogen embrittlement resistance can be obtained in a bolt manufactured using the steel material of the present disclosure.In the bolt of the present disclosure, excellent hydrogen embrittlement resistance can be obtained.

[0013] Fig. 1 is a side view showing an example of a bolt according to the present embodiment, and Fig. 2 is a perspective view of a tightening jig used in a tightening acid immersion test.

[0014] The present inventors first investigated, from the standpoint of chemical composition, steel materials that would provide excellent hydrogen embrittlement resistance in bolts manufactured from steel materials, and found that the steel materials contained, in mass %, C: 0.20 to less than 0.40%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, sol. Al: 0.005-0.100%, Cu: 0.01-0.40%, Ni: 0.01-0.30%, Cr: 0.01-1.50%, Mo: 0.001-0.200% , Sn: 0.001 to 0.100%, P: 0.015% or less, S: 0.015% or less, N: 0.0150% or less, O: 0.0030% or less, Nb: 0 to 0.050 %, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, and the balance being Fe and impurities, the inventors considered that a bolt manufactured using a steel material would have excellent hydrogen embrittlement resistance.

[0015] However, it has been found that even steel materials having the above-mentioned chemical composition still do not always have excellent hydrogen embrittlement resistance. Therefore, the present inventors have investigated means for further improving hydrogen embrittlement resistance by means other than the chemical composition.

[0016] A bolt is composed of a head, a shank, and a lower neck. Delayed fracture due to hydrogen embrittlement is particularly likely to occur around the lower neck, which is the part connecting the head and shank. Delayed fracture tends to propagate radially rather than axially around the shank. Therefore, the inventors investigated the metal structure, particularly around the lower neck of the bolt. As a result, the following was discovered.

[0017] Steel materials used to make bolts contain multiple MnS inclusions. In the steel material, the MnS inclusions are elongated in the axial direction of the steel material. Delayed fracture due to hydrogen embrittlement tends to propagate along the interface between the elongated MnS inclusions and the matrix. The MnS inclusions are also susceptible to corrosion. When the MnS inclusions are corroded, they dissolve, creating voids, which form corrosion pits. The corrosion pits tend to become sources of stress concentration, making delayed fracture due to hydrogen embrittlement more likely to occur.

[0018] As mentioned above, MnS inclusions in steel extend in the axial direction of the steel. Therefore, in bolts manufactured using steel, MnS inclusions also extend in the axial direction of the bolt shank. Therefore, it seems that MnS inclusions do not affect delayed fracture in the radial direction of the shank around the neck portion.

[0019] However, in the vicinity of the underneck at the boundary between the head and shank of the bolt, the elongation direction of MnS inclusions may differ from the elongation direction of MnS inclusions in the shank. Specifically, in the manufacturing process of a bolt, the raw steel material is usually subjected to forging (upset forging). During this forging, the end of the steel material protrudes radially to form the underneck and head. At this time, the MnS inclusions at the end are inclined radially from the axial direction of the shank of the bolt. As a result, the MnS inclusions are elongated radially around the underneck. Therefore, if the steel material contains many MnS inclusions with a large aspect ratio and these MnS inclusions are susceptible to corrosion, it is thought that a bolt manufactured using that steel material will be more susceptible to radial delayed fracture around the underneck.

[0020] Based on the above findings, the inventors of the present invention have considered that, in order to improve the hydrogen embrittlement resistance of a bolt, it is effective to reduce the aspect ratio of MnS inclusions in the steel material from which the bolt is made. They also considered that it is effective to suppress corrosion of MnS inclusions with large aspect ratios as much as possible. When MnS inclusions contain Ti, their elongation and corrosion are suppressed. Therefore, increasing the proportion of Ti-containing MnS inclusions among the MnS inclusions is effective in improving the hydrogen embrittlement resistance of the bolt.

[0021] In order to further improve the hydrogen embrittlement resistance of bolts, the inventors further investigated the relationship between the chemical composition of MnS inclusions and corrosion resistance. As a result, the inventors found that Ti-containing MnS inclusions in which Cu and / or Ni are concentrated have extremely high corrosion resistance. Therefore, by increasing the proportion of CuNiTi-containing MnS inclusions in which Cu and / or Ni are concentrated among the Ti-containing MnS inclusions, the hydrogen embrittlement resistance of bolts can be further improved.

[0022] Based on the above findings, the present inventors further investigated the relationship between the morphology (aspect ratio and chemical composition) of MnS inclusions in steel and the hydrogen embrittlement resistance of bolts. As a result, the inventors have found that excellent hydrogen embrittlement resistance can be obtained in a bolt manufactured using a steel material if the steel has an equivalent circle diameter of 1.0 μm or more, and the MnS inclusions have a Mn content of 10% or more and an S content of 5% or more, and the median aspect ratio of these MnS inclusions is 20.0 or less in an element concentration analysis by mass using EDS described below, and further, among the MnS inclusions having an aspect ratio of 5.0 or more, the number ratio of Ti-containing MnS inclusions having a Ti content of 5% or more is 30.0% or more in an element concentration analysis by mass using EDS described below, and among the Ti-containing MnS inclusions having an aspect ratio of 5.0 or more, the number ratio of CuNiTi-containing MnS inclusions having a sum of Cu and Ni contents, by mass%, of 1% or more is 50.0% or more in an element concentration analysis by mass using EDS described below.

[0023] The steel material and the bolt of this embodiment have been completed based on the above technical concept, and have the following configuration.

[0024] The steel material of the first configuration contains, in mass %, C: 0.20 to less than 0.40%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, sol. containing Al: 0.005 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 1.50%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.015% or less, S: 0.015% or less, N: 0.0150% or less, O: 0.0030% or less, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, The remainder consists of Fe and impurities, the median aspect ratio of MnS inclusions having an equivalent circle diameter of 1.0 μm or more and having a Mn content of 10% or more and an S content of 5% or more, by mass%, is 20.0 or less, the proportion by number of Ti-containing MnS inclusions having a Ti content of 5% or more, by mass%, is 30.0% or more, and the proportion by number of CuNiTi-containing MnS inclusions having a sum of Cu content and Ni content, by mass%, of the Ti-containing MnS inclusions having an aspect ratio of 5.0 or more is 50.0% or more.

[0025] The steel material of the second configuration is the steel material of the first configuration, and contains, in mass%, one or more elements selected from the group consisting of Nb: 0.001 to 0.050%, V: 0.01 to 0.15%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Pb: 0.001 to 0.090%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

[0026] A bolt of a first configuration includes a head, a shank, and a neck portion connecting the head and the shank, and the chemical composition of the bolt is, in mass %, C: 0.20 to less than 0.40%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, sol. containing Al: 0.005 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 1.50%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.015% or less, S: 0.015% or less, N: 0.0150% or less, O: 0.0030% or less, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, the remainder consisting of Fe and impurities, and in the shaft portion, the median aspect ratio of MnS inclusions having an equivalent circle diameter of 1.0 μm or more and an Mn content of 10% or more and an S content of 5% or more, expressed in mass%, is 20.0 or less, the proportion by number of Ti-containing MnS inclusions having a Ti content of 5% or more, expressed in mass%, is 30.0% or more, and the proportion by number of CuNiTi-containing MnS inclusions having a sum of Cu content and Ni content of 1% or more, expressed in mass%, is 50.0% or more, among the Ti-containing MnS inclusions having an aspect ratio of 5.0 or more.

[0027] The bolt of the second configuration is the bolt of the first configuration, wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of Nb: 0.001 to 0.050%, V: 0.01 to 0.15%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Pb: 0.001 to 0.090%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

[0028] The steel material and the bolt of this embodiment will be described below. In the following description, "%" regarding the element content means mass % unless otherwise specified.

[0029] [Features of the Steel Material of the Present Embodiment] The steel material of the present embodiment satisfies the following features: (Feature 1) The chemical composition is, in mass %, C: 0.20 to less than 0.40%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, sol. Al: 0.005 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 1.50%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.015% or less, S: 0.015% or less, N: 0.0150% or less, O: 0.0030% or less, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, and the balance being Fe and impurities. (Feature 2) The median aspect ratio of MnS inclusions having an equivalent circle diameter of 1.0 μm or more, a Mn content of 10% or more, and an S content of 5% or more, expressed in mass%, is 20.0 or less. (Feature 3) Of the MnS inclusions having an equivalent circle diameter of 1.0 μm or more and an aspect ratio of 5.0 or more, the number ratio of Ti-containing MnS inclusions having a Ti content of 5% or more, expressed in mass%, is 30.0% or more. (Feature 4) Of the Ti-containing MnS inclusions having an equivalent circle diameter of 1.0 μm or more and an aspect ratio of 5.0 or more, the number ratio of CuNiTi-containing MnS inclusions having a sum of Cu and Ni contents of 1% or more, expressed in mass%, is 50.0% or more. Features 1 to 4 will be described below.

[0030] [(Feature 1) Chemical Composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0031] C: 0.20 to less than 0.40% Carbon (C) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the C content is less than 0.20%, the above effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is 0.40% or more, the hydrogen embrittlement resistance of bolts manufactured using the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.20 to less than 0.40%. A preferred lower limit of the C content is 0.21%, more preferably 0.23%, and even more preferably 0.24%. A preferred upper limit of the C content is 0.39%, more preferably 0.38%, and even more preferably 0.36%.

[0032] Si: 0.01 to 0.50% Silicon (Si) increases the strength of bolts manufactured using steel as a raw material through solid solution strengthening. If the Si 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 Si content exceeds 0.50%, the hydrogen embrittlement resistance of bolts manufactured using steel as a raw material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.01 to 0.50%. A preferred lower limit of the Si content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. A preferred upper limit of the Si content is 0.44%, more preferably 0.40%, and even more preferably 0.30%.

[0033] Mn: 0.30 to 1.50% Manganese (Mn) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the Mn content is less than 0.30%, the above effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.50%, the hydrogen embrittlement resistance of bolts manufactured using the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.30 to 1.50%. The preferred lower limit of the Mn content is 0.35%, more preferably 0.40%, and even more preferably 0.44%. The preferred upper limit of the Mn content is 1.45%, more preferably 1.40%, and even more preferably 1.38%.

[0034] Ti: 0.005 to 0.100% Titanium (Ti) combines with N to form Ti nitrides, which inhibit B from combining with N. Therefore, Ti promotes the improvement of the hardenability of the steel material due to B. Ti also forms Ti-containing MnS inclusions. Ti-containing MnS inclusions are resistant to elongation and corrosion during processing. Therefore, Ti-containing MnS inclusions are less likely to become the initiation point of delayed fracture and to propagate delayed fracture. As a result, the hydrogen embrittlement resistance of bolts manufactured using the steel material is improved. If the Ti content is less than 0.005%, the above effect cannot be fully achieved. On the other hand, if the Ti content exceeds 0.100%, Ti nitrides become coarse. Coarse Ti nitrides become the initiation point of delayed fracture. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance of bolts manufactured using the steel material is reduced. Therefore, the Ti content is 0.005 to 0.100%. A preferable lower limit of the Ti content is 0.007%, more preferably 0.010%, even more preferably 0.015%, and still more preferably 0.018%. A preferable upper limit of the Ti content is 0.090%, more preferably 0.080%, even more preferably 0.060%, even more preferably 0.050%, and still more preferably 0.040%.

[0035] B: 0.0005 to 0.0050% Boron (B) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the B content is less than 0.0005%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content exceeds 0.0050%, coarse B nitrides are formed. These coarse B nitrides become the starting point for delayed fracture. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance of bolts manufactured using the steel is reduced. Therefore, the B content is 0.0005 to 0.0050%. The preferred lower limit of the B content is 0.0007%, more preferably 0.0010%, even more preferably 0.0015%, and even more preferably 0.0020%. The upper limit of the B content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0030%, and still more preferably 0.0025%.

[0036] Sol. Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the sol. Al content is less than 0.005%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the sol. Al content exceeds 0.100%, coarse Al nitrides are formed. These coarse Al nitrides become the starting point for delayed fracture. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance of bolts manufactured using this steel material is reduced. Therefore, the sol. Al content is 0.005 to 0.100%. The preferred lower limit of the sol. Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. The preferred upper limit of the sol. Al content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0037] In this embodiment, the sol. Al content refers to the content of acid-soluble Al. The sol. Al content is the Al content (mass%) obtained by performing ICP atomic emission spectroscopy on the steel material as specified in JIS G 1258-1:2014. Aqua regia is used as the acid for dissolving the steel material during the analysis. Note that in the steel material of this embodiment, the sol. Al content and the total. Al (total Al) content are approximately equal.

[0038] Cu: 0.01 to 0.40% Copper (Cu) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. Cu also concentrates in Ti-containing MnS inclusions, further increasing the corrosion resistance of the Ti-containing MnS inclusions. As a result, the hydrogen embrittlement resistance of bolts manufactured using the steel is improved. If the Cu content is less than 0.01%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cu content exceeds 0.40%, the steel becomes embrittled. In this case, the hot workability and cold forgeability of the steel are reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.01 to 0.40%. The preferred lower limit of the Cu content is 0.02%, more preferably 0.04%. The preferred upper limit of the Cu content is 0.37%, more preferably 0.34%, and even more preferably 0.30%.

[0039] Ni: 0.01 to 0.30% Nickel (Ni) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. Ni also concentrates in Ti-containing MnS inclusions, further increasing the corrosion resistance of the Ti-containing MnS inclusions. As a result, the hydrogen embrittlement resistance of bolts manufactured using the steel is improved. If the Ni content is less than 0.01%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content exceeds 0.30%, the hardenability of the steel becomes excessively high. In this case, the cold forgeability of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0.01 to 0.30%. The preferred lower limit of the Ni content is 0.02%, more preferably 0.04%. The preferred upper limit of the Ni content is 0.27%, more preferably 0.24%, and even more preferably 0.20%.

[0040] Cr: 0.01 to 1.50% Chromium (Cr) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the Cr content is less than 0.01%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 1.50%, the strength of the steel becomes excessively high even if the contents of other elements are within the ranges of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Cr content is 0.01 to 1.50%. The preferred lower limit of the Cr content is 0.02%, more preferably 0.04%, and even more preferably 0.07%. The preferred upper limit of the Cr content is 1.40%, more preferably 1.20%, even more preferably 1.10%, and even more preferably 1.00%.

[0041] Mo: 0.001 to 0.200% Molybdenum (Mo) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the Mo content is less than 0.001%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content exceeds 0.200%, the strength of the steel becomes excessively high, even if the contents of other elements are within the ranges of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Mo content is 0.001 to 0.200%. The preferred lower limit of the Mo content is 0.002%, more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Mo content is 0.190%, even more preferably 0.180%, even more preferably 0.170%, and even more preferably 0.150%.

[0042] Sn: 0.001 to 0.100% Tin (Sn) segregates at the interface between the matrix and inclusions, embrittling the steel material. This improves the machinability of the steel material. If the Sn content is less than 0.001%, the above effects are not fully achieved, even if the contents of other elements are within the ranges of this embodiment. However, if the Sn content exceeds 0.100%, excessive Sn segregation occurs. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced. Therefore, the Sn content is 0.001 to 0.100%. The preferred lower limit of the Sn content is 0.001%, more preferably 0.002%, even more preferably 0.004%, and even more preferably 0.010%. The preferred upper limit of the Sn content is 0.095%, even more preferably 0.090%, even more preferably 0.080%, and even more preferably 0.070%.

[0043] P: 0.015% or less Phosphorus (P) is an impurity. If the P content exceeds 0.015%, P segregates at grain boundaries. Therefore, even if the contents of other elements are within the ranges of this embodiment, the strength of bolts manufactured using the steel material will decrease. Therefore, the P content is 0.015% or less. The lower the P content, the better. However, excessively reducing the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is greater than 0%, more preferably 0.001%, even more preferably 0.002%, even more preferably 0.003%, and even more preferably 0.007%. The preferred upper limit of the P content is 0.012%, even more preferably 0.010%, and even more preferably 0.008%.

[0044] S: 0.015% or less Sulfur (S) is an impurity. S combines with Mn to form MnS inclusions. If the S content exceeds 0.015%, the MnS inclusions become coarse. These coarse MnS inclusions serve as initiation sites for delayed fracture. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance of bolts manufactured using this steel material is reduced. Therefore, the S content is 0.015% or less. The lower the S content, the better. However, excessively reducing the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is greater than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.006%. The preferred upper limit of the S content is 0.013%, more preferably 0.010%, and even more preferably 0.008%.

[0045] N: 0.0150% or less Nitrogen (N) is an impurity. N combines with Al, Ti, B, etc. to form nitrides. If the N content exceeds 0.0150%, the nitrides become coarse. These coarse nitrides become the starting point for delayed fracture. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance of bolts manufactured using this steel material is reduced. Therefore, the N content is 0.0150% or less. The N content is preferably as low as possible. However, excessive reduction of the N content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the N content is greater than 0%, more preferably 0.0001%, even more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit of the N content is 0.0145%, even more preferably 0.0130%, even more preferably 0.0100%, and even more preferably 0.0080%.

[0046] O: 0.0030% or less Oxygen (O) is an impurity. O combines with other elements in the steel to form oxides. If the O content exceeds 0.0030%, the oxides become coarse. These coarse oxides become the starting point for delayed fracture. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance of bolts manufactured using the steel material is reduced. Therefore, the O content is 0.0030% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%. The preferred upper limit of the O content is 0.0027%, more preferably 0.0024%, and even more preferably 0.0020%.

[0047] The balance of the chemical composition of the steel material of this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material of this embodiment.

[0048] [Optional Elements] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%. All of these elements are optional elements. These optional elements will be described below.

[0049] [First Group: Nb and V] The chemical composition of the steel material of the present embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Nb and V. Any of these elements forms precipitates and increases the strength of a bolt manufactured using the steel material as a raw material through precipitation strengthening.

[0050] Nb: 0 to 0.050% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as carbides, nitrides, and carbonitrides. Nb precipitates increase the strength of bolts manufactured using the steel material through precipitation strengthening. Even if even a small amount of Nb is contained, the above effect can be achieved to some extent. However, if the Nb content exceeds 0.050%, coarse precipitates are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance of bolts manufactured using the steel material will decrease. Therefore, the Nb content is 0 to 0.050%, and when contained, the Nb content is 0.050% or less. The preferred lower limit of the Nb content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The upper limit of the Nb content is preferably 0.046%, more preferably 0.040%, and even more preferably 0.035%.

[0051] V: 0 to 0.15% Vanadium (V) is an optional element and does not necessarily need to be contained. That is, the V content may be 0%. When vanadium (V) is contained, that is, when the V content exceeds 0%, V forms V precipitates such as carbides, nitrides, and carbonitrides. V precipitates increase the strength of bolts manufactured using the steel material through precipitation strengthening. Even if even a small amount of V is contained, the above effect can be achieved to some extent. However, if the V content exceeds 0.15%, coarse precipitates are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance of bolts manufactured using the steel material decreases. Therefore, the V content is 0 to 0.15%, and when contained, the V content is 0.15% or less. The preferred lower limit of the V content is 0.01%, more preferably 0.02%, and even more preferably 0.04%. The upper limit of the V content is preferably 0.13%, more preferably 0.10%, and even more preferably 0.08%.

[0052] [Second Group: Sb, As, and Pb] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Sb, As, and Pb in place of a portion of Fe. All of these elements improve the machinability of the steel material.

[0053] Sb: 0 to 0.050% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. If it is contained, that is, if the Sb content exceeds 0%, Sb segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Sb is contained, the above effects can be achieved to some extent. However, if the Sb content exceeds 0.050%, Sb segregates excessively. In this case, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance and low-temperature toughness of bolts manufactured using the steel material will be reduced. Therefore, the Sb content is 0 to 0.050%, and if contained, the Sb content is 0.050% or less. The preferred lower limit of the Sb content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The upper limit of the Sb content is preferably 0.044%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0054] As: 0 to 0.050% Arsenic (As) is an optional element and does not necessarily need to be contained. That is, the As content may be 0%. If arsenic is contained, that is, if the As content exceeds 0%, As segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of As is contained, the above effects can be achieved to some extent. However, if the As content exceeds 0.050%, As segregates excessively. In this case, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance and low-temperature toughness of bolts manufactured using the steel material will be reduced. Therefore, the As content is 0 to 0.050%, and if contained, the As content is 0.050% or less. The preferred lower limit of the As content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The upper limit of the As content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0055] Pb: 0 to 0.090% Lead (Pb) is an optional element and does not necessarily need to be present. In other words, the Pb content may be 0%. If Pb is present, i.e., if the Pb content exceeds 0%, Pb segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Pb is present, the above effects can be achieved to some extent. However, if the Pb content exceeds 0.090%, there is a risk of environmental impact. Therefore, the Pb content is 0 to 0.090%, and if present, the Pb content is 0.090% or less. The preferred lower limit of the Pb content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The preferred upper limit of the Pb content is 0.088%, more preferably 0.080%, even more preferably 0.070%, and even more preferably 0.060%.

[0056] [Third Group: Ca and Mg] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Ca and Mg, instead of a portion of Fe. Any of these elements refines MnS inclusions in the steel material, thereby improving the hydrogen embrittlement resistance of the steel material.

[0057] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. That is, the Ca content may be 0%. When Ca is contained, that is, when the Ca content exceeds 0%, Ca refines MnS inclusions. Therefore, the hydrogen embrittlement resistance of bolts manufactured using the steel material is improved. Even if even a small amount of Ca is contained, the above effect can be achieved to some extent. However, if the Ca content exceeds 0.0050%, coarse Ca oxides are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance and low-temperature toughness of bolts manufactured using the steel material will be reduced. Therefore, the Ca content is 0 to 0.0050%, and if contained, the Ca content is 0.0050% or less. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Ca content is preferably 0.0047%, more preferably 0.0040%, and still more preferably 0.0030%.

[0058] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be contained. That is, the Mg content may be 0%. When Mg is contained, that is, when Mg exceeds 0%, Mg refines MnS inclusions. Therefore, the hydrogen embrittlement resistance of bolts manufactured using the steel material is improved. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, coarse Mg oxides are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the hydrogen embrittlement resistance and low-temperature toughness of bolts manufactured using the steel material are reduced. Therefore, the Mg content is 0 to 0.0050%, and when contained, the Mg content is 0.0050% or less. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Mg content is preferably 0.0040%, and more preferably 0.0030%.

[0059] [(Feature 2) Median value MV of aspect ratios of MnS inclusions] The steel material of this embodiment further has a median value MV of aspect ratios of MnS inclusions having an equivalent circle diameter of 1.0 μm or more among inclusions contained in the steel material, of 20.0 or less. In element concentration analysis in mass% using EDS, which will be described later, inclusions having a Mn content of 10% or more and an S content of 5% or more are defined as MnS inclusions.

[0060] The more elongated the MnS inclusions in the steel material are, the more likely they are to become the starting point of delayed fracture and promote the propagation of cracks. Therefore, it is preferable that there are as few excessively elongated MnS inclusions as possible.

[0061] If the median aspect ratio MV of MnS inclusions having an equivalent circle diameter of 1.0 μm or more is 20.0 or less, the proportion of excessively elongated MnS inclusions in the steel material is sufficiently small. Therefore, on the premise that the steel material satisfies Features 1, 3, and 4, a bolt manufactured using the steel material of this embodiment as a raw material can obtain excellent hydrogen embrittlement resistance.

[0062] The upper limit of the median MV is preferably 19.0, more preferably 17.0, even more preferably 15.0, even more preferably 13.0, even more preferably 11.0, and even more preferably 10.0. The lower limit of the median MV is not particularly limited. When a steel material satisfies Features 1, 3, and 4, the lower limit of the median MV is, for example, 2.0, and in consideration of industrial production, the lower limit is preferably 3.5, even more preferably 5.0, and even more preferably 7.0. The median MV of the aspect ratios of MnS inclusions having an equivalent circle diameter of 1.0 μm or more is the value obtained by rounding the obtained value to one decimal place (i.e., the value to one decimal place).

[0063] [(Feature 3) Number Proportion P1 of Ti-Containing MnS Inclusions Among MnS Inclusions Having an Equivalent Circle Diameter of 1.0 μm or More and an Aspect Ratio of 5.0 or More] MnS inclusions having an equivalent circle diameter of 1.0 μm or more and an aspect ratio of 5.0 or more are defined as elongated MnS inclusions. Furthermore, among MnS inclusions, MnS inclusions containing 5% or more of Ti in element concentration analysis by mass using EDS, which will be described later, are defined as Ti-containing MnS inclusions. In the steel material of this embodiment, the number proportion P1 of Ti-containing MnS inclusions among the elongated MnS inclusions is 30.0% or more.

[0064] If MnS inclusions are susceptible to corrosion, they will dissolve and pits will be more likely to form. Pits become sources of stress concentration. Therefore, pits promote the occurrence of delayed fracture and the propagation of the crack. On the other hand, even if elongated MnS inclusions have a large aspect ratio, if they are resistant to corrosion, pit formation will be suppressed. As a result, delayed fracture will be suppressed. If the elongated MnS inclusions contain 5% or more Ti by mass, corrosion of the elongated MnS inclusions will be sufficiently suppressed.

[0065] If the proportion P1 of Ti-containing MnS inclusions among the elongated MnS inclusions is 30.0% or more, corrosion of the elongated MnS inclusions can be sufficiently suppressed. Therefore, provided that the steel material satisfies Features 1, 2, and 4, a bolt manufactured using the steel material of this embodiment as its raw material can obtain excellent hydrogen embrittlement resistance.

[0066] The preferred lower limit of the number proportion P1 is 31.0%, more preferably 32.0%, even more preferably 34.0%, and even more preferably 36.0%. The upper limit of the number proportion P1 is not particularly limited. When the steel material satisfies Features 1, 2, and 4, the upper limit of the number proportion P1 is, for example, 100.0%, and in consideration of industrial production, the preferred upper limit is 80.0%, and even more preferably 60.0%. The number proportion P1 is a value obtained by rounding the obtained numerical value to one decimal place (i.e., the value to one decimal place).

[0067] [(Feature 4) Number Proportion P2 of CuNiTi-containing MnS Inclusions Among Ti-containing MnS Inclusions Having an Equivalent Circle Diameter of 1.0 μm or More and an Aspect Ratio of 5.0 or More] Among Ti-containing MnS inclusions, Ti-containing MnS inclusions having a sum of Cu and Ni contents of 1% or more in element concentration analysis by mass using EDS, which will be described later, are defined as CuNiTi-containing MnS inclusions. In the steel material of this embodiment, the number proportion P2 of CuNiTi-containing MnS inclusions among Ti-containing MnS inclusions having an equivalent circle diameter of 1.0 μm or more and an aspect ratio of 5.0 or more is 50.0% or more.

[0068] Cu and Ni concentrate in the Ti-containing MnS inclusions, further enhancing the corrosion resistance of the Ti-containing MnS inclusions. Specifically, if the sum of the Cu content and Ni content in the Ti-containing MnS inclusions is 1% by mass or more, Ti-containing MnS inclusions (CuNiTi-containing MnS inclusions) with even higher corrosion resistance can be obtained. Therefore, increasing the number of CuNiTi-containing MnS inclusions among the elongated MnS inclusions can further enhance the hydrogen embrittlement resistance of the bolt.

[0069] If the number proportion P2 of CuNiTi-containing MnS inclusions among the Ti-containing MnS inclusions having an aspect ratio of 5.0 or more is 50.0% or more, then excellent hydrogen embrittlement resistance can be obtained in a bolt manufactured using the steel material of this embodiment as a raw material, provided that the steel material satisfies Features 1 to 3.

[0070] The preferred lower limit of the number proportion P2 is 53.0%, more preferably 55.0%, even more preferably 57.0%, even more preferably 60.0%, even more preferably 65.0%, and even more preferably 70.0%. The upper limit of the number proportion P2 is not particularly limited. When the steel material satisfies Features 1 to 3, the upper limit of the number proportion P2 is, for example, 100.0%, and in consideration of industrial production, the preferred upper limit is 90.0%, even more preferably 80.0%. The number proportion P2 is the value obtained by rounding the obtained numerical value to one decimal place (i.e., the value to one decimal place).

[0071] [Method of measuring median value MV, number percentage P1, and number percentage P2] Median value MV, number percentage P1, and number percentage P2 are determined by the following method. A test piece is taken, the target surface of which is a cross section parallel to the axial direction of the steel material and passing through the central axis of the steel material. The target surface is a square with one side equal to the diameter of the steel material. The size of the test piece is not particularly limited as long as it has a target surface.

[0072] The collected test piece is embedded in resin. The target surface of the resin-embedded test piece is polished. The type of each particle on the polished target surface is identified. Specifically, an observation area within the target surface is observed at 500x magnification using a scanning electron microscope (SEM-EDS device) equipped with composition analysis capabilities. The observation area is a square with a side length of (steel diameter - 2 mm). The observation area is set so that its center is positioned on the central axis of the steel on the target surface.

[0073] The observation area may be divided into multiple observation fields, and each observation field may be observed at 500x magnification. In the observation area (or in all observation fields when the observation area is divided into multiple observation fields), particles (precipitates or inclusions) with an equivalent circle diameter of 1.0 μm or more are identified based on contrast. Note that in the observation field, particles appear with a dark contrast compared to the matrix. The equivalent circle diameter refers to the diameter of a circle when the area of ​​a particle is converted into a circle with the same area. Each identified particle is subjected to elemental concentration analysis using an EDS (energy dispersive X-ray spectroscopy) attached to the SEM to identify MnS inclusions. The EDS analysis (elemental concentration analysis) uses the EDS-standardless method. The accelerating voltage is set to 20 kV, and the quantified elements are C, Si, Mn, P, S, Cr, Ti, Cu, Ni, B, Ca, N, O, and Al.

[0074] In the EDS analysis of each particle, when the total mass of the above-quantified elements is taken as 100%, the particle is judged to be an MnS inclusion if the Mn content is 10% or more and the S content is 5% or more, by mass. Furthermore, when the elemental analysis of a particle is when the total mass of the above-quantified elements is taken as 100%, the particle is judged to be a Ti-containing MnS inclusion if the elemental analysis of a particle is when the total mass of the above-quantified elements is taken as 100%, the Mn content is 10% or more, the S content is 5% or more, and the Ti content is 5% or more, by mass. Furthermore, when the total mass of the above-quantified elements is taken as 100%, the particle is judged to be a CuNiTi-containing MnS inclusion if the elemental analysis of a particle is when the total mass of the above-quantified elements is taken as 100%, the Mn content is 10% or more, the S content is 5% or more, the Ti content is 5% or more, and the sum of the Cu content and the Ni content is 1% or more, by mass.

[0075] The aspect ratio of each identified MnS inclusion is determined. Specifically, the length of the longest line segment connecting any two points on the interface between the MnS inclusion and the matrix is ​​defined as the major axis (μm) of the MnS inclusion. The length of the longest line segment connecting any two points on the interface between the MnS inclusion and the matrix is ​​defined as the minor axis (μm) of the MnS inclusion. The aspect ratio (major axis / minor axis) of each MnS inclusion is determined based on the major axis and minor axis of the MnS inclusion thus determined.

[0076] In the observation area, the median value MV of the aspect ratios of the MnS inclusions is determined based on the aspect ratios of all the MnS inclusions identified.

[0077] Furthermore, from the identified MnS inclusions, elongated MnS inclusions with an aspect ratio of 5.0 or more are selected, and the total number of elongated MnS inclusions is calculated. Furthermore, from the elongated MnS inclusions, Ti-containing MnS inclusions are selected. The selected Ti-containing MnS inclusions with an aspect ratio of 5.0 or more are referred to as elongated Ti-containing MnS inclusions. The total number of elongated Ti-containing MnS inclusions is calculated. Based on the total number of elongated MnS inclusions and the total number of elongated Ti-containing MnS inclusions, the number percentage P1 (%) is calculated using the following formula: Number percentage P1 = total number of elongated Ti-containing MnS inclusions / total number of elongated MnS inclusions × 100

[0078] Furthermore, CuNiTi-containing MnS inclusions are selected from the elongated Ti-containing MnS inclusions. The selected CuNiTi-containing MnS inclusions with an aspect ratio of 5.0 or more are referred to as elongated CuNiTi-containing MnS inclusions. The total number of elongated CuNiTi-containing MnS inclusions is calculated. Based on the total number of elongated Ti-containing MnS inclusions and the total number of elongated CuNiTi-containing MnS inclusions, the number proportion P2 (%) is calculated using the following formula: Number proportion P2 = total number of elongated CuNiTi-containing MnS inclusions / total number of elongated Ti-containing MnS inclusions × 100

[0079] [Effects of the Steel Material of the Present Embodiment] As described above, the steel material of the present embodiment satisfies Features 1 to 4. Therefore, bolts manufactured using the steel material of the present embodiment as a raw material can obtain excellent hydrogen embrittlement resistance.

[0080] [Shape of Steel Material of the Present Embodiment] The steel material of the present embodiment is a steel bar or wire rod. The steel bar or wire rod is a steel material that has a circular cross section perpendicular to the axial direction and extends in a rod shape. The steel material may be wound in a coil shape or may be cut to a predetermined length. The diameter of the cross section of the steel material is, for example, 4 to 20 mm.

[0081] [Applications of the Steel Material of the Present Embodiment] The steel material of the present embodiment can be used as a material for bolts, which are a type of fastening means for industrial machinery, automobiles, bridges, buildings, etc. The steel material of the present embodiment may also be used for applications other than the above applications.

[0082] [Method for manufacturing steel material] An example of a method for manufacturing steel material according to this embodiment will be described. The method for manufacturing steel material described below is one example for manufacturing the steel material according to this embodiment. Therefore, steel material having the above-described configuration 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 steel material according to this embodiment.

[0083] An example of the method for producing a steel material according to this embodiment includes the following steps: (Step 1) Refining step, (Step 2) Casting step, and (Step 3) Hot working step.

[0084] Each step will be described below. [Step 1: Refining step] In the refining step, molten steel having a chemical composition that satisfies the above-mentioned feature 1 is produced. The refining method is not particularly limited, and a well-known method may be used. For example, molten pig iron produced by a well-known method is subjected to refining (primary refining) in a converter. Molten steel tapped from the converter is subjected to well-known secondary refining. Through the above steps, molten steel having a chemical composition that satisfies feature 1 is produced.

[0085] [(Step 2) Casting Step] In the casting step, the molten steel produced in the refining step is used to produce a bloom by continuous casting.

[0086] [(Step 3) Hot Working Step] In the hot working step, the bloom is hot worked to produce a steel material. The hot working step includes the following steps: (Step 31) Blooming Step (Step 32) Finishing Rolling Step Each step will be described below.

[0087] [(Step 31) Blooming Step] In the blooming step, the bloom is heated in a heating furnace. The heated bloom is then hot-rolled (blooming) using a blooming mill to produce a billet. If a continuous rolling mill having a plurality of rolling stands arranged in a line is disposed downstream of the blooming mill, the billet may be hot-rolled using the continuous rolling mill to further reduce the size of the billet. The heating temperature in the heating furnace in the blooming step may be within a known temperature range. For example, the heating temperature is 1050 to 1300°C. The billet produced in the blooming step is allowed to cool (air-cool) to room temperature before the finish rolling step.

[0088] [(Step 32) Finish Rolling Step] In the finish rolling step, the billet is hot rolled (finish rolling) to produce the steel material of this embodiment. Specifically, the billet after the blooming step is heated using a heating furnace. The heating temperature is, for example, 1000 to 1250°C. The heated billet is hot rolled (finish rolling) using a finishing rolling mill to produce a steel material having a circular cross section with a desired diameter. The finishing rolling mill is, for example, a continuous rolling mill in which multiple rolling stands are arranged in a row so that the rolling direction of adjacent rolling stands is perpendicular.

[0089] [Manufacturing conditions in the manufacturing method of this embodiment] The manufacturing method described above satisfies the following conditions: (Condition 1) The total residence time (minutes) t1 of the workpiece (bloom or billet) at a temperature of 1,200°C or higher in the hot working process and subsequent processes is 120 minutes or less. (Condition 2) The total reduction ratio RR in the finish rolling process at a billet temperature of 980°C or lower, out of the total reduction ratio TRR in the finish rolling process, is 120 minutes or less. ９８０ The ratio X (%) of is defined by the following formula: X = RR ９８０ / TRR×100 In this case, the following formula (A) is satisfied: √(Ti / S)−X>0 (A) In this case, the contents of the corresponding elements in mass% are substituted for S and Ti in formula (A). (Condition 3) In the finish rolling process, the maximum area reduction rate in one pass when the billet temperature is above 1000°C is defined as Y (%). In this case, the following formula (B) is satisfied: Y−15 / √(Cu+0.5×Ni+0.2)>0 (B) In this case, the contents of the corresponding elements in mass% are substituted for Cu and Ni in formula (B). Conditions 1 to 3 will be explained below.

[0090] [Condition 1: Total residence time t1 at which the temperature of the workpiece is 1200°C or higher] If the total residence time (minutes) t1 at which the temperature of the workpiece is 1200°C or higher in the processes subsequent to the hot working process (i.e., from the start of heating of the steel in the blooming process until finish rolling in the finish rolling process and cooling the steel after finish rolling to room temperature) exceeds 120 minutes, TiN will coarsen. As TiN coarsens, the amount of solute Ti in the steel will decrease. Therefore, the proportion of MnS inclusions containing Ti among the MnS inclusions will decrease. In this case, a steel material that satisfies Feature 3 will not be obtained. Therefore, the total residence time t1 at 1200°C or higher is set to 120 minutes or less.

[0091] The upper limit of the total residence time t1 at 1200° C. or higher is preferably 110 minutes, more preferably 100 minutes, and even more preferably 90 minutes. The lower limit of the total residence time t1 at 1200° C. or higher is not particularly limited, but the lower limit is preferably 30 minutes, more preferably 40 minutes, and even more preferably 50 minutes.

[0092] [Condition 2: Formula (A)] FA is defined as √(Ti / S)-X. FA is an index that represents the resistance of MnS inclusions to elongation in the finish rolling process. In the finish rolling process, MnS inclusions are elongated in the rolling direction of the billet (steel material). If the billet temperature during hot rolling in the finish rolling process is 980°C or lower, MnS inclusions are more likely to elongate. If FA is 0 or lower, MnS inclusions are excessively elongated in the hot working process. In this case, the steel material does not satisfy Feature 2, and bolts manufactured using the steel material do not have excellent hydrogen embrittlement resistance. Therefore, FA is set to be greater than 0.

[0093] The lower limit of FA is preferably 0.1, more preferably 0.2, even more preferably 0.3, and even more preferably 0.4. The upper limit of FA is not particularly limited, but is preferably 1.0, more preferably 1.2, and even more preferably 3.0. Taking into consideration normal industrial production, the lower limit of X is, for example, 0.5%, and the upper limit of X is, for example, 5.0%.

[0094] [Condition 3: Formula (B)] FB is defined as FB=Y-15 / √(Cu+0.5×Ni+0.2). FB is an index that represents the ease with which Cu and Ni concentrate in Ti-containing MnS inclusions in the finish rolling process. In hot rolling using a finish rolling mill equipped with multiple rolling stands, the area reduction rate of a billet passing through each rolling stand is defined as the "area reduction rate in one pass."

[0095] In the finish rolling process, if the billet temperature exceeds 1000°C, Cu and Ni in the steel material are likely to diffuse. Therefore, by performing rolling at a billet temperature exceeding 1000°C and a large area reduction, the diffusion of Cu and Ni is further promoted, and the concentration of Cu and Ni in the Ti-containing MnS inclusions is promoted. If FB is 0 or less, the concentration of Cu and Ni in the Ti-containing MnS inclusions is insufficient. In this case, the steel material does not satisfy Feature 4, and a bolt manufactured using the steel material does not have excellent hydrogen embrittlement resistance. Therefore, FB is set to be greater than 0.

[0096] The lower limit of FB is preferably 3.0, more preferably 4.0, and even more preferably 5.0. The upper limit of FB is not particularly limited, but is preferably 12.0, more preferably 11.0, and even more preferably 10.0. Taking into consideration normal industrial production, the lower limit of Y is, for example, 17.0%, and the upper limit of Y is, for example, 38.0%.

[0097] The steel material of this embodiment is manufactured by the above manufacturing process.

[0098] [Configuration of the bolt of this embodiment] The shape of the bolt of this embodiment has a known structure. Fig. 1 is a side view showing an example of a bolt of this embodiment. Referring to Fig. 1, the bolt of this embodiment includes a head 10, a shank 11, and a neck portion 12. The neck portion 12 is a portion that connects the head 10 and the shank 11. The diameter of the neck portion 12 continuously decreases from the head 10 side toward the shank 11 side. Therefore, the surface of the neck portion 12 is curved. The shank 11 extends from the neck portion 12 in the direction of the central axis of the bolt. A thread is formed on at least a portion of the circumferential surface of the shank 11.

[0099] [Features of the Bolt of the Present Embodiment] The bolt of the present embodiment satisfies the following features: (Feature 5) The chemical composition is, in mass %, C: 0.20 to less than 0.40%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, sol. Al: 0.005 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 1.50%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.015% or less, S: 0.015% or less, N: 0.0150% or less, O: 0.0030% or less, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, and the balance being Fe and impurities. (Feature 6) In the shank portion, the median aspect ratio of MnS inclusions having an equivalent circle diameter of 1.0 μm or more and an Mn content of 10% or more and an S content of 5% or more, expressed in mass%, is 20.0 or less. (Feature 7) In the shank portion, among the MnS inclusions having an equivalent circle diameter of 1.0 μm or more and an aspect ratio of 5.0 or more, the number ratio of Ti-containing MnS inclusions having a Ti content of 5% or more, expressed in mass%, is 30.0% or more. (Feature 8) In the shank portion, among the Ti-containing MnS inclusions having an equivalent circle diameter of 1.0 μm or more and an aspect ratio of 5.0 or more, the number ratio of CuNiTi-containing MnS inclusions having a sum of Cu and Ni contents of 1% or more, expressed in mass%, is 50.0% or more.

[0100] The function of each element in Feature 5 is the same as the function of the corresponding element in Feature 1 of the steel material of this embodiment. The technical significance of Feature 6 is the same as that of Feature 2 of the steel material of this embodiment. The technical significance of Feature 7 is the same as that of Feature 3 of the steel material of this embodiment. The technical significance of Feature 8 is the same as that of Feature 4 of the steel material of this embodiment. Therefore, the bolt of this embodiment, which satisfies Features 5 to 8, can obtain excellent hydrogen embrittlement resistance.

[0101] [Method for manufacturing the bolt of this embodiment] The bolt of this embodiment is manufactured by a well-known method using the steel material of this embodiment as the starting material. The chemical composition of a bolt manufactured by a well-known method is the same as the chemical composition of the steel material used as the starting material. Furthermore, the morphology of MnS inclusions in the shank of a bolt manufactured by a well-known method is almost unchanged from that of the steel material used as the starting material. Therefore, if the bolt of this embodiment is manufactured by a well-known method using the steel material of this embodiment that satisfies Features 1 to 4 as the starting material, it can satisfy Features 5 to 8.

[0102] The manufacturing method for the bolt of this embodiment includes, for example, a wiredrawing process, a cold forging process, and a quenching and tempering process. In the wiredrawing process, a steel wire is manufactured by performing a known wiredrawing process on the steel material of this embodiment. The wiredrawing process may be only a primary wiredrawing process, or multiple wiredrawing processes such as a secondary wiredrawing process may be performed. In the cold forging process, a known cold forging (heading) is performed on the steel wire after the wiredrawing process to manufacture an intermediate product in the shape of a bolt including a head, a shank, and a neck portion. In the quenching and tempering process, the intermediate product is quenched and tempered. Quenching is performed by a known method. The quenching temperature is, for example, 840 to 970°C. The holding time at the quenching temperature is, for example, 15 to 360 minutes (6 hours). After the holding time has elapsed, the intermediate product is quenched. Specifically, the intermediate product is water-cooled or oil-cooled. The quenched intermediate product is tempered. The tempering temperature is, for example, 400 to 550° C. The holding time at the tempering temperature is, for example, 0.5 to 6.0 hours.

[0103] 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.

[0104] A known refining process was carried out to produce steel materials having the chemical compositions shown in Table 1 (Tables 1A and 1B).

[0105]

[0106]

[0107] Specifically, blooms were produced using the produced molten steel by continuous casting. The produced slabs were subjected to blooming to produce billets. In the blooming process, the blooms were heated to 1050 to 1300°C and then hot rolled using a blooming mill. The produced billets were allowed to cool to room temperature. The produced billets were subjected to a finish rolling process. In the finish rolling process, the billets were heated to 1000 to 1250°C. The heated billets were hot rolled using a finish rolling mill to produce steel bars (steel products) with a diameter of 16.5 mm. Steel products with each test number were produced using the above production process.

[0108] The total residence time t1 (minutes) during which the temperature of the steel (bloom, billet, and steel material) was 1200°C or higher from the start of heating of the bloom in the blooming process until the steel material was cooled to room temperature after the finish rolling process was as shown in the "t1 (minutes)" column in Table 2. In the finish rolling process, the total area reduction rate RR in the finish rolling process when the billet temperature was 980°C or lower ９８０ The ratios X (%) and FA were as shown in the "X (%)" and "FA" columns in Table 2. In the finish rolling step, the maximum area reduction rates Y (%) and FB in one pass when the billet temperature exceeded 1000°C were as shown in the "Y (%)" and "FB" columns in Table 2.

[0109]

[0110] [Evaluation Tests] The following evaluation tests were carried out on the produced steel materials with each test number: (Test 1) Measurement test of the median aspect ratio MV of MnS inclusions, the number proportion P1 of Ti-containing MnS inclusions, and the number proportion P2 of CuNiTi-containing MnS inclusions (Test 2) Evaluation test of hydrogen embrittlement resistance Each test will be described below.

[0111] [(Test 1) Measurement test of the median aspect ratio MV of MnS inclusions, the number proportion P1 of Ti-containing MnS inclusions, and the number proportion P2 of CuNiTi-containing MnS inclusions] The median aspect ratio MV of MnS inclusions having an equivalent circle diameter of 1.0 μm or more was determined based on the method described above in [Methods for measuring median MV, number proportion P1, and number proportion P2]. The results are shown in the "Median MV" column in Table 2. The number proportion P1 (%) of Ti-containing MnS inclusions among elongated MnS inclusions was also determined based on the method described above in [Methods for measuring median MV, number proportion P1, and number proportion P2]. The results are shown in the "Number proportion P1 (%)" column in Table 2. Furthermore, the number proportion P2 (%) of CuNiTi-containing MnS inclusions among Ti-containing MnS inclusions having an aspect ratio of 5.0 or more was determined based on the method described above in [Methods for measuring median value MV, number proportion P1, and number proportion P2]. The results obtained are shown in the "Number proportion P2 (%)" column in Table 2.

[0112] [(Test 2) Hydrogen Embrittlement Resistance Evaluation Test] M16 bolts (hexagonal bolts) conforming to JIS B 1180:2014 were manufactured from the steel materials of each test number. Specifically, the steel materials (steel bars with a diameter of 16.5 mm) of each test number were subjected to wire drawing under the same conditions to manufacture steel wires. Thereafter, the steel wires of each test number were subjected to cold forging under the same conditions to manufacture the above-mentioned bolts.

[0113] The bolts of each test number were subjected to quenching and tempering processes. Specifically, the bolts of each test number were heated at 880°C for 1 hour (60 minutes) and then oil-cooled in 60°C oil. The bolts after oil-cooling were then tempered at a tempering temperature of 420 to 550°C for a holding time of 1 hour. The bolts (hexagonal bolts) of each test number were manufactured using the above manufacturing process. Each bolt had a total length under the neck of 80 mm, a thread length of 48 mm, and a pitch of 2.00 mm.

[0114] (Tightening Acid Immersion Test) The following tightening acid immersion test was carried out using the manufactured bolts. The bolts with each test number after the quenching and tempering processes were tightened to the tightening jig 100 shown in FIG. 2. The tightening jig 100 had a cube shape with sides of 55 mm. The tightening jig 100 had three through holes 200 with a diameter of 18.0 mm that penetrated three pairs of opposing faces perpendicularly. The three through holes 200 intersected each other at the center of the tightening jig 100. The specific tightening method was as follows. First, a bolt was inserted into one of the three through holes 200 of the tightening jig 100. Thereafter, the bolt was tightened to the tightening jig 100 using a nut and a washer by rotation angle method tightening in accordance with JIS B 1083:2008 so that the tightening axial force was the ultimate tightening axial force (also referred to as the maximum axial force). Hereinafter, the bolt tightened to the tightening jig 100 will be referred to as the tightening sample.

[0115] The fastened sample of each test number was immersed in hydrochloric acid of 2N normality for 96 hours. ２ ) The specific liquid volume of 2N hydrochloric acid is 12 ml / cm ２ The tightened samples after immersion for 96 hours were visually observed to check for the presence or absence of bolt fracture. When no bolt fracture was confirmed, the sample was rated "E (Excellent)", and when bolt fracture was confirmed, the sample was rated "NA (Not Accepted)". When the sample was rated "E", it was determined that excellent hydrogen embrittlement resistance was obtained. On the other hand, when the sample was rated "NA", it was determined that excellent hydrogen embrittlement resistance was not obtained. The evaluation results are shown in the "Hydrogen embrittlement resistance" column in Table 2.

[0116] [Test Results] With reference to Tables 1A, 1B, and 2, the steel materials of Test Nos. 1 to 12 satisfied Features 1 to 4. Therefore, the bolts manufactured using the steel materials of these test numbers as raw materials exhibited excellent hydrogen embrittlement resistance.

[0117] On the other hand, in test numbers 13 and 14, the Ti content was too low. As a result, the median aspect ratio MV of the MnS inclusions was too large. Furthermore, the number fraction P1 of Ti-containing MnS inclusions was too low. As a result, excellent hydrogen embrittlement resistance was not obtained.

[0118] In test numbers 15 and 16, the total residence time t1 at 1,200°C or higher was too long, and therefore the proportion P1 of the number of Ti-containing MnS inclusions was too low, resulting in failure to obtain excellent hydrogen embrittlement resistance.

[0119] In test numbers 17 and 18, FA was too low, and therefore the median aspect ratio MV of the MnS inclusions was too large, resulting in failure to obtain excellent hydrogen embrittlement resistance.

[0120] In test numbers 19 and 20, FB was too low, and therefore the proportion P2 of the number of CuNiTi-containing MnS inclusions was too low, resulting in failure to obtain excellent hydrogen embrittlement resistance.

[0121] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

Claims

1. In mass%, C: 0.20 to less than 0.40%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, sol. containing Al: 0.005 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 1.50%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.015% or less, S: 0.015% or less, N: 0.0150% or less, O: 0.0030% or less, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, A steel material comprising the remainder consisting of Fe and impurities, wherein the MnS inclusions have an equivalent circle diameter of 1.0 μm or more and, by mass%, a Mn content of 10% or more and a S content of 5% or more, the median aspect ratio of which is 20.0 or less, and among the MnS inclusions having an aspect ratio of 5.0 or more, the number ratio of Ti-containing MnS inclusions having a Ti content of 5% or more by mass is 30.0% or more, and among the Ti-containing MnS inclusions having an aspect ratio of 5.0 or more, the number ratio of CuNiTi-containing MnS inclusions having a sum of Cu content and Ni content of 1% or more by mass is 50.0% or more.

2. The steel material according to claim 1, containing, in mass%, one or more elements selected from the group consisting of Nb: 0.001 to 0.050%, V: 0.01 to 0.15%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Pb: 0.001 to 0.090%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

3. A bolt comprising a head, a shank, and a neck portion connecting the head and the shank, the chemical composition of the bolt being, in mass %, C: 0.20 to less than 0.40%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, sol. containing Al: 0.005 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 1.50%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.015% or less, S: 0.015% or less, N: 0.0150% or less, O: 0.0030% or less, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, A bolt having a shaft portion in which the remainder consists of Fe and impurities, and in which: MnS inclusions have an equivalent circle diameter of 1.0 μm or more and a Mn content, by mass, of 10% or more and an S content of 5% or more, the median aspect ratio of which is 20.0 or less; of the MnS inclusions having an aspect ratio of 5.0 or more, the number ratio, by mass%, of Ti-containing MnS inclusions having a Ti content of 5% or more is 30.0% or more; and of the Ti-containing MnS inclusions having an aspect ratio of 5.0 or more, the number ratio of CuNiTi-containing MnS inclusions having a sum of Cu content and Ni content, by mass%, of 1% or more is 50.0% or more.

4. A bolt according to claim 3, wherein the chemical composition contains, in mass %, one or more elements selected from the group consisting of Nb: 0.001 to 0.050%, V: 0.01 to 0.15%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Pb: 0.001 to 0.090%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

Images