Steel material used as the material of the bolt

Abstract

To provide a steel having excellent corrosion resistance and hydrogen embrittlement resistance.SOLUTION: A steel comprises, in mass%, C: 0.15-0.30%, Si: 0.01-0.50%, Mn: 0.50-1.20% (exclusive), P: 0.020% or less, S: 0.020% or less, Cu: 0.10-0.60%, Ni: 0.10-0.60%, Cr: 0-0.20%, Mo: 0.01-0.60%, Sn: 0.010-0.500%, Al: 0.005-0.060%, N: 0.0010-0.0080%, B: 0.0002-0.0050%, Ti: 0.005-0.100%, with the balance being Fe and impurities, where Y1 defined by the formula (1) is 10.00-70.00. Y1=X1 / X2 (1) with X1=(Cu+4Sn)-Cr / 2-Mn / 10 and X2=V+10 Nb+(Ti-3.4 N). In the X1 and X2, each element symbol represents the content of the corresponding element in mass%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to steel materials, and more particularly to steel materials used as materials for bolts.

Background Art

[0002] Bolts are used as fastening means for industrial machines, automobiles, bridges, buildings, etc. In recent years, with the improvement of the performance of industrial machines and automobiles, and the enlargement of buildings, etc., large-diameter bolts with a diameter exceeding 20 mm have been frequently used.

[0003] Among the above applications, bridges, buildings, etc. may be built in coastal areas or cold regions. Coastal areas are corrosive environments with a lot of salt. Also, in cold regions, snow melting salts and antifreezing agents may be used. Snow melting salts and antifreezing agents corrode the steel materials constituting the bolts. That is, cold regions are often corrosive environments. Therefore, bolts used in these corrosive environments are required to have excellent corrosion resistance.

[0004] Furthermore, in the above corrosive environments, hydrogen embrittlement is likely to occur. Therefore, bolts used in corrosive environments are required to have not only excellent corrosion resistance but also excellent hydrogen embrittlement resistance characteristics.

[0005] Technologies related to improving corrosion resistance and hydrogen embrittlement resistance characteristics have been proposed in Japanese Patent Application Laid-Open No. 2008-274367 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2020-180325 (Patent Document 2).

[0006] The steel material disclosed in Patent Document 1 contains, by mass%, C: 0.15 to 0.6%, Si: 0.05 to 0.5%, Mn and Cr: a total of 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 has a composition with a Cu / Sn ratio of 1 or less. In this document, the hydrogen embrittlement resistance characteristics are enhanced by containing Sn. Further, in order to suppress the decrease in the corrosion resistance effect of Cu by Sn, Cu / Sn is set to 1 or less. It is described in this document that the corrosion resistance can be enhanced thereby.

[0007] The steel material disclosed in Patent Document 2 contains, by mass%, C: 0.15% or more and 0.25% or less, Si: 0.05% or more and 0.30% or less, Mn: 0.50% or more and 1.80% or less, P: 0.002% or more and 0.030% or less, S: 0.0005% or more and 0.0200% or less, Al: 0.010% or more and 0.065% or less, Cu: 0.01% or more and 0.48% or less, Nb: 0.005% or more and 0.030% or less, Sn: 0.005% or more and 0.200% or less, Ti: 0.005% or more and 0.200% or less, B: 0.0001% or more and 0.0050% or less, N: 0.0020% or more and 0.0100% or less, and O: 0.0025% or less, with the balance having a component composition of iron and inevitable impurities. In this document, it is described that the corrosion resistance is enhanced by containing Cu, Nb, and Sn.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] The steel materials for bolts disclosed in Patent Documents 1 and 2 enhance corrosion resistance and / or hydrogen embrittlement resistance. However, corrosion resistance and hydrogen embrittlement resistance may also be enhanced by other means.

[0010] An object of the present disclosure is to provide a steel material having excellent corrosion resistance and excellent hydrogen embrittlement resistance.

Means for Solving the Problems

[0011] The steel material according to the present disclosure has the following configuration.

[0012] By mass%, C: 0.15 to 0.30%, Si: 0.01 to 0.50%, Mn: less than 0.50 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cu: 0.10 to 0.60%, Ni: 0.10 to 0.60%, Cr: 0 to 0.20%, Mo: 0.01 to 0.60%, Sn: 0.010 to 0.500%, Al: 0.005 to 0.060%, N: 0.0010 to 0.0080%, B: 0.0002 to 0.0050%, Ti: 0.005 to 0.100%, V: 0 to 0.050%, Nb: 0 to 0.030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Rare earth elements: 0 to 0.0200%, and, The balance: consisting of Fe and impurities, On the premise that the content of each element satisfies the above range, Y1 defined by the formula (1) is 10.00 to 70.00, Steel material. Y1 = X1 / X2 (1) X1 = (Cu + 4Sn) - Cr / 2 - Mn / 10 X2 = V + 10Nb + (Ti - 3.4N) Here, for each element symbol in X1 and X2, the content in mass % of the corresponding element is substituted.

Advantages of the Invention

[0013] The steel material according to the present disclosure has excellent corrosion resistance and excellent hydrogen embrittlement resistance characteristics.

Brief Description of the Drawings

[0014]

Figure 1

Modes for Carrying Out the Invention

[0015] The present inventors conducted investigations and studies on steel materials having excellent corrosion resistance and excellent hydrogen embrittlement resistance characteristics. As a result, the following matters were found.

[0016] In order to enhance the hydrogen embrittlement resistance characteristics, it is effective to reduce the Cr content in the steel material as much as possible. Furthermore, it is effective to contain Cu and Sn. On the other hand, when the Cr content is reduced, the hardenability of the steel material decreases. As described above, in the case of bolts manufactured using the steel material, the diameter of the shaft portion may exceed 20 mm. It is necessary to ensure the hardenability of the steel material so that sufficient strength can be obtained even for such thick bolts. Therefore, it is effective to contain Mo and B as a substitute for Cr. Furthermore, Cu, Ni, and Sn are effective for improving corrosion resistance.

[0017] Based on the examination from the perspective of the above chemical composition, the inventors of the present invention studied the chemical composition of steel materials having excellent corrosion resistance and excellent hydrogen embrittlement resistance. As a result, the inventors found that, in mass%, C: 0.15 to 0.30%, Si: 0.01 to 0.50%, Mn: less than 0.50 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cu: 0.10 to 0.60%, Ni: 0.10 to 0.60%, Cr: 0 to 0.20%, Mo: 0.01 to 0.60%, Sn: 0.010 to 0.500%, Al: 0.005 to 0.060%, N: 0.0010 to 0.0080%, B: 0.0002 to 0.0050%, Ti: 0.005 to 0.100%, V: 0 to 0.050%, Nb: 0 to 0.030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, rare earth elements: 0 to 0.0200%, and the balance: Fe and impurities. If it is a chemical composition consisting of these, it was considered that excellent corrosion resistance and excellent hydrogen embrittlement resistance can be obtained.

[0018] However, it was found that even if it is a steel material having the above chemical composition, although it has excellent corrosion resistance, the hydrogen embrittlement resistance may still be low. Therefore, the inventors of the present invention further studied the hydrogen embrittlement resistance of the steel material having the above chemical composition, particularly from the perspective of the amount of hydrogen ingress. As a result, the following matters were found.

[0019] In a corrosive environment, hydrogen embrittlement occurs when hydrogen penetrates into the steel material. Even in the same corrosive environment, the amount of hydrogen ingress changes depending on the chemical composition of the steel material. If the amount of hydrogen ingress into the steel material is large, the hydrogen embrittlement resistance of the steel material becomes low. Among the above chemical compositions, Cr and Mn promote the ingress of hydrogen into the steel material in a corrosive environment. On the other hand, Cu and Sn suppress the ingress of hydrogen into the steel material in a corrosive environment. Therefore, Cu, Sn, Cr, and Mn are an element group that is mutually related to the degree of hydrogen ingress of the steel material in a corrosive environment.

[0020] Furthermore, among the above chemical compositions, V, Nb, and Ti are elements that can form carbides and / or carbonitrides (hereinafter referred to as carbides, etc.). When these elements form carbides, etc., the carbides of these elements promote hydrogen intrusion into the steel material. Therefore, V, Nb, and Ti are elements related to the degree of hydrogen intrusion into the steel material as precipitates in a corrosive environment.

[0021] Based on the above examination results, the inventors considered that the element group consisting of Cu, Sn, Cr, and Mn and the element group consisting of V, Nb, and Ti are related to the degree of hydrogen intrusion into the steel material in a corrosive environment. Therefore, further investigation was conducted on the relationship between these element groups and the amount of hydrogen intrusion in a corrosive environment. As a result, the inventors found that, on the premise that the content of each element in the chemical composition of the steel material is within the above range, if Y1 defined by formula (1) is 10.00 or more, hydrogen intrusion into the steel material can be sufficiently suppressed in a corrosive environment, and as a result, excellent hydrogen embrittlement resistance characteristics can be obtained. Y1 = X1 / X2 (1) X1 = (Cu + 4Sn) - Cr / 2 - Mn / 10 X2 = V + 10Nb + (Ti - 3.4N) Here, the content in mass% of the corresponding element is substituted for each element symbol in X1 and X2.

[0022] Based on the above findings, the steel material according to the present embodiment completed has the following configuration.

[0023] [1] In mass%, C: 0.15 to 0.30%, Si: 0.01 to 0.50%, Mn: less than 0.50 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cu: 0.10 to 0.60%, Ni: 0.10 to 0.60%, Cr: 0 to 0.20%, Mo: 0.01 to 0.60%, Sn: 0.010 to 0.500%, Al: 0.005 to 0.060%, N: 0.0010 to 0.0080%, B: 0.0002 to 0.0050%, Ti: 0.005 to 0.100%, V: 0 to 0.050%, Nb: 0 to 0.030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Rare earth elements: 0 to 0.0200%, and, The balance consists of Fe and impurities, On the premise that the content of each element satisfies the above range, Y1 defined by formula (1) is 10.00 to 70.00, Steel material. Y1 = X1 / X2 (1) X1 = (Cu + 4Sn) - Cr / 2 - Mn / 10 X2 = V + 10Nb + (Ti - 3.4N) Here, in each element symbol in X1 and X2, the content in mass% of the corresponding element is substituted.

[0024] [2] The steel material according to [1], V: 0.001 to 0.050%, Nb: 0.001 to 0.030%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, and, Contains one or more elements selected from the group consisting of rare earth elements: 0.0001 to 0.0200%, Steel material.

[0025] [3] The steel material according to [1] or [2], and further, On the premise that the content of each element satisfies the above range, Y2 defined by formula (2) is 60.00 or more, Steel material. Y2 = (8.5×√C)×(1 + 3.1Mn)×(1 + 0.3Cu)×(1 + 0.2Ni)×(1 + 5.0Cr)×(1 + 3.1Mo)×(1 + 1.5×(0.9 - C)) (2) Here, in each element symbol in Y2, the content in mass % of the corresponding element is substituted.

[0026] [4] The steel material according to any one of [1] to [3], further satisfying assuming that each of the above element contents satisfies the above range, Y2 defined by formula (2) is 120.00 or less, in the microstructure, the total area ratio of the primary ferrite and the hard phase composed of bainite and / or martensite is 90% or more, when the diameter of the steel material is D, the Vickers hardness HV at the D / 4 position is 190 to less than 260, Steel material. Y2 = (8.5×√C)×(1 + 3.1Mn)×(1 + 0.3Cu)×(1 + 0.2Ni)×(1 + 5.0Cr)×(1 + 3.1Mo)×(1 + 1.5×(0.9 - C)) (2) Here, in each element symbol in Y2, the content in mass % of the corresponding element is substituted.

[0027] Hereinafter, the steel material according to this embodiment will be described in detail. Note that, unless otherwise specified, "%" regarding an element means mass %.

[0028] [Essential features of the steel material of this embodiment] The steel material of this embodiment has the following essential features 1 and 2. (Essential feature 1) The content of each element in the chemical composition is as shown below. (Essential feature 2) Assuming that the content of each element in the chemical composition is within the following range, Y1 defined by formula (1) is 10.00 to 70.00. Y1 = X1 / X2 (1) X1 = (Cu + 4Sn) - Cr / 2 - Mn / 10 X2 = V + 10Nb + (Ti - 3.4N) Here, in each element symbol in X1 and X2, the content in mass % of the corresponding element is substituted. Hereinafter, each feature will be described.

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

[0030] C: 0.15 to 0.30% Carbon (C) enhances the hardenability of the steel material and increases the strength of bolts manufactured using the steel material. If the C content is less than 0.15%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.30%, even if the contents of other elements are within the range of this embodiment, the cold forging property of the steel material deteriorates. Therefore, the C content is 0.15 to 0.30%. The preferable lower limit of the C content is 0.18%, more preferably 0.20%. The preferable upper limit of the C content is 0.28%, more preferably 0.25%.

[0031] Si: 0.01 to 0.50% Silicon (Si) increases the strength of bolts manufactured using the steel material by solid solution strengthening. If the Si content is less than 0.01%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Si content exceeds 0.50%, even if the contents of other elements are within the range of this embodiment, the cold forging property of the steel material deteriorates. Therefore, the Si content is 0.01 to 0.50%. The preferable lower limit of the Si content is 0.02%, more preferably 0.03%. The preferable upper limit of the Si content is 0.30%, more preferably 0.15%, still more preferably 0.10%. The preferable upper limit of the Si content for more effectively enhancing the cold forging property is less than 0.05%.

[0032] Mn: 0.50 to less than 1.20% Manganese (Mn) increases the hardenability of the steel material and enhances the strength of the bolt. If the Mn content is less than 0.50%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mn content is 1.20% or more, even if the contents of other elements are within the range of this embodiment, the cold forging property of the steel material deteriorates. Therefore, the Mn content is 0.50 to less than 1.20%. The preferable lower limit of the Mn content is 0.55%, more preferably 0.60%. The preferable upper limit of the Mn content is 1.10%, more preferably 1.00%, still more preferably 0.90%.

[0033] P: 0.020% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is more than 0%. If the P content exceeds 0.020%, even if the contents of other elements are within the range of this embodiment, P segregates at the grain boundaries. As a result, the hydrogen embrittlement resistance property of the bolt deteriorates. Therefore, the P content is 0.020% or less. The lower the P content, the better. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, still more preferably 0.003%. The preferable upper limit of the P content is 0.015%, more preferably 0.010%.

[0034] S: 0.020% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content exceeds 0.020%, even if the contents of other elements are within the range of this embodiment, S segregates at the grain boundaries. As a result, the hydrogen embrittlement resistance property of the bolt deteriorates. Therefore, the S content is 0.020% or less. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the S content is 0.018%, more preferably 0.010%.

[0035] Cu: 0.10 - 0.60% Copper (Cu) enhances the corrosion resistance of steel materials in a corrosive environment. Cu further suppresses the intrusion of hydrogen into the steel material. Therefore, Cu enhances the corrosion resistance and hydrogen embrittlement resistance characteristics of bolts made of steel materials. If the Cu content is less than 0.10%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Cu content exceeds 0.60%, the steel material becomes brittle. Therefore, even if the contents of other elements are within the scope of this embodiment, the hot workability and cold forging properties of the steel material deteriorate. Therefore, the Cu content is 0.10 - 0.60%. The preferable lower limit of the Cu content is 0.20%, more preferably 0.25%, even more preferably 0.30%, and even more preferably 0.35%. The preferable upper limit of the Cu content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0036] Ni: 0.10 - 0.60% Nickel (Ni) enhances the hardenability of steel materials and increases the strength of bolts. Ni further enhances the corrosion resistance of bolts. Ni is further contained together with Cu to suppress the generation of defects during hot working of steel materials. As a result, in the steel material of this embodiment containing Cu, Ni enhances the hot workability of the steel material. If the Ni content is less than 0.10%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the one hand, if the Ni content exceeds 0.60%, the hardenability of the steel material becomes excessively high. In this case, even if the contents of other elements are within the range of this embodiment, the cold forging property of the steel material deteriorates. Therefore, the Ni content is 0.10 to 0.60%. The preferable lower limit of the Ni content is 0.20%, more preferably 0.30%, and even more preferably 0.35%. The preferable upper limit of the Ni content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0037] Preferably, the ratio of the Cu content to the Ni content (i.e., Cu / Ni) is made greater than 1.0. In this case, the hot workability of the steel material can be sufficiently maintained. The upper limit of Cu / Ni is not particularly limited. The preferable upper limit of Cu / Ni is 5.0, more preferably 4.0, and even more preferably 3.0.

[0038] Cr: 0 to 0.20% In the steel material of this embodiment, Cr is an impurity. In this embodiment, the Cr content is defined by a numerical value up to the second decimal place. When the Cr content is defined by this significant figure, the Cr content may be 0%. Cr promotes the intrusion of hydrogen into the steel material. Therefore, the hydrogen embrittlement resistance property of the bolt deteriorates. If the Cr content exceeds 0.20%, even if the contents of other elements are within the range of this embodiment, the hydrogen embrittlement resistance property of the bolt significantly deteriorates. Therefore, the Cr content is 0 to 0.20%. The lower the Cr content, the better. However, an extreme reduction in the Cr content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the Cr content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable upper limit of the Cr content is 0.15%, more preferably 0.10%, and even more preferably 0.07%. The preferable upper limit of the Cr content for further enhancing the hydrogen embrittlement resistance property is less than 0.05%.

[0039] Mo: 0.01 - 0.60% Molybdenum (Mo) increases the hardenability of steel and enhances the strength of bolts. For bolts used in civil engineering and construction applications, the diameter may exceed 20 mm. To increase the strength of such thick bolts, it is necessary to enhance the hardenability of the steel material. Mo easily increases the hardenability of steel. Mo, like Ti, V, and Nb, further forms fine carbides to enhance the hydrogen embrittlement resistance of bolts. Moreover, compared with Ti, V, and Nb, Mo does not promote the intrusion of hydrogen into steel in a corrosive environment. Therefore, Mo not only increases the strength of bolts but also enhances their hydrogen embrittlement resistance. If the Mo content is less than 0.01%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Mo content exceeds 0.60%, even if the contents of other elements are within the scope of this embodiment, the steel becomes overly hard. In this case, the cold forging property of the steel deteriorates. Therefore, the Mo content is 0.01 - 0.60%. The preferable lower limit of the Mo content is 0.05%, more preferably 0.10%, even more preferably 0.15%, and even more preferably 0.20%. The preferable upper limit of the Mo content is 0.50%, more preferably 0.45%, and even more preferably 0.40%.

[0040] Sn: 0.010 - 0.500% Tin (Sn), like Cu, suppresses the intrusion of hydrogen into steel in a corrosive environment and enhances the hydrogen embrittlement resistance of steel. If the Sn content is less than 0.010%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Sn content exceeds 0.500%, even if the contents of other elements are within the scope of this embodiment, Sn segregates at the grain boundaries. In this case, the hot workability and cold forging property of the steel deteriorate. Furthermore, the hydrogen embrittlement resistance of the bolts deteriorates. Therefore, the Sn content is 0.010 - 0.500%. The preferable lower limit of the Sn content is 0.030%, more preferably 0.050%, and even more preferably 0.100%. The preferable upper limit of the Sn content is 0.400%, more preferably 0.300%, even more preferably 0.200%, and even more preferably 0.150%.

[0041] Al: 0.005 - 0.060% Aluminum (Al) deoxidizes the steel. If the Al content is less than 0.005%, even if the contents of other elements are within the range of this embodiment, the deoxidation of the steel will be insufficient. In this case, coarse oxides are generated. Therefore, the hydrogen embrittlement resistance property of the bolt deteriorates. On the other hand, if the Al content exceeds 0.060%, even if the contents of other elements are within the range of this embodiment, coarse Al nitrides are generated. Coarse Al nitrides become the starting points of fracture. Therefore, the workability of the steel material deteriorates. Therefore, the Al content is 0.005 - 0.060%. The preferable lower limit of the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. The preferable upper limit of the Al content is 0.050%, more preferably 0.045%, and even more preferably 0.040%. In the chemical composition of the steel material of this embodiment, the Al content means the total Al (Total - Al) content.

[0042] N: 0.0010 - 0.0080% Nitrogen (N) combines with Al or Ti to form nitrides or carbonitrides. These nitrides and carbonitrides suppress the coarsening of crystal grains by the pinning effect. As a result, the cold forging property of the steel material is enhanced. If the N content is less than 0.0010%, even if the contents of other elements are within the range of this embodiment, the above - mentioned effect cannot be sufficiently obtained. On the one hand, if the N content exceeds 0.0080%, even if the contents of other elements are within the range of this embodiment, coarse nitrides will be generated. The coarse nitrides serve as fracture initiation points, reducing the cold forging property of the steel material. Furthermore, the hydrogen embrittlement resistance property of the bolt deteriorates. Therefore, the N content is 0.0010 - 0.0080%. The preferable lower limit of the N content is 0.0015%, more preferably 0.0020%, and even more preferably 0.0025%. The preferable upper limit of the N content is 0.0070%, more preferably 0.0065%, even more preferably 0.0060%, even more preferably 0.0055%, and even more preferably 0.0050%.

[0043] B: 0.0002 - 0.0050% Boron (B) enhances the hardenability of the steel material and increases the strength of the bolt. As described above, in the steel material of this embodiment, the Cr content is suppressed to suppress the intrusion of hydrogen into the steel material in a corrosive environment. In civil engineering and construction applications, bolts with a large diameter exceeding 20 mm may be used. B, together with Mo, enhances the hardenability of the steel material and increases the strength of the large - diameter bolt as an alternative to Cr. If the B content is less than 0.0002%, the above effects cannot be fully obtained. On the other hand, if the B content exceeds 0.0050%, even if the contents of other elements are within the range of this embodiment, coarse B nitrides will be generated. The coarse B nitrides serve as fracture initiation points. As a result, the cold forging property of the steel material deteriorates. Therefore, the B content is 0.0002 - 0.0050%. The preferable lower limit of the B content is 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%. The preferable upper limit of the B content is 0.0045%, more preferably 0.0040%, even more preferably 0.0030%, and even more preferably 0.0025%.

[0044] Ti: 0.005 - 0.100% Titanium (Ti) combines with N to form Ti nitride and suppresses the combination of B with N. As a result, B can enhance the hardenability of the steel material. If the Ti content is less than 0.005%, the above effect cannot be sufficiently obtained. On the other hand, if the Ti content exceeds 0.100%, even if the contents of other elements are within the range of this embodiment, excessive amounts of Ti precipitates such as carbides and carbonitrides are generated. In this case, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Ti content is 0.005 to 0.100%. The preferable lower limit of the Ti content is 0.010%, more preferably 0.015%, and even more preferably 0.018%. The preferable upper limit of the Ti content is 0.080%, more preferably 0.060%, even more preferably 0.040%, and even more preferably 0.030%.

[0045] The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition refer to those mixed in from ores, scraps, or the manufacturing environment as raw materials during the industrial production of the steel material, and are those allowed within a range that does not adversely affect the steel material according to this embodiment.

[0046] [Optional Elements] The steel material of this embodiment may further contain one or more elements selected from the following element groups in place of a part of Fe. V: 0 to 0.050%, Nb: 0 to 0.030%, Ca: 0 to 0.0050% Mg: 0 to 0.0050% Rare earth elements (REM): 0 to 0.0200% These elements are optional elements. Each element will be described below.

[0047] [Group 1: V and Nb] The steel material of this embodiment may contain one or more elements selected from the group consisting of V and Nb in place of a part of Fe. These elements are optional elements, and all of them increase the strength of the steel material.

[0048] V: 0 to 0.050% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, that is, when the V content exceeds 0%, V forms V precipitates such as carbides and carbonitrides. The V precipitates increase the strength of the bolt. If even a little V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.050%, even if the contents of other elements are within the range of this embodiment, a large amount of V precipitates are generated. In this case, the amount of hydrogen intrusion into the steel material increases. As a result, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the V content is 0 to 0.050%. The preferable lower limit of the V content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the V content is 0.030%, more preferably 0.020%, and even more preferably 0.010%. The preferable upper limit of the V content for more effectively reducing the hydrogen intrusion amount is less than 0.005%.

[0049] Nb: 0 to 0.030% Niobium (Nb) is an optional element and may not 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 and carbonitrides. The Nb precipitates increase the strength of the bolt. If even a little Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.030%, even if the contents of other elements are within the range of this embodiment, a large amount of Nb precipitates are generated. In this case, the amount of hydrogen intrusion into the steel material increases. As a result, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Nb content is 0 to 0.030%. The preferable lower limit of the Nb content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the Nb content is 0.020%, more preferably 0.010%. The preferable upper limit of the Nb content for more effectively reducing the hydrogen ingress amount is less than 0.005%.

[0050] As described above, the steel material of the present embodiment may contain one or more elements selected from the above-described first group of element groups. That is, the steel material of the present embodiment may contain one or more elements selected from the group consisting of V: 0.001 to 0.050% and Nb: 0.001 to 0.030%.

[0051] [Group 2: Ca, Mg and rare earth elements] The steel material of the present embodiment may contain one or more elements selected from the group consisting of Ca, Mg and rare earth elements (REM) in place of a part of Fe. These elements are optional elements, and all of them refine MnS in the steel material and enhance the hydrogen embrittlement resistance characteristics of the steel material.

[0052] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When Ca is contained, that is, when Ca is more than 0%, Ca refines MnS. Therefore, the hydrogen embrittlement resistance characteristics of the steel material are enhanced. Even if a little Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0050%, coarse Ca oxides are generated even if the contents of other elements are within the range of the present embodiment. In this case, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Ca content is 0 to 0.0050%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%.

[0053] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When Mg is contained, that is, when Mg is more than 0%, Mg refines MnS. Therefore, the hydrogen embrittlement resistance property of the steel material is enhanced. Even if a little Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, even if the contents of other elements are within the range of this embodiment, coarse Mg oxides are formed. In this case, the hydrogen embrittlement resistance property of the steel material deteriorates. Therefore, the Mg content is 0 to 0.0050%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%.

[0054] Rare earth element (REM): 0 to 0.0200% The rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When REM is contained, that is, when REM is more than 0%, REM refines MnS. Therefore, the hydrogen embrittlement resistance property of the steel material is enhanced. Even if a little REM is contained, the above effect can be obtained to some extent. However, if the REM content exceeds 0.0200%, even if the contents of other elements are within the range of this embodiment, coarse oxides are formed. In this case, the hydrogen embrittlement resistance property of the steel material deteriorates. Therefore, the REM content is 0 to 0.0200%. The preferable lower limit of the REM content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0050%. The preferable upper limit of the REM content is 0.0150%, more preferably 0.0100%.

[0055] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71, which are lanthanoids. The REM content in this specification refers to the total content of these elements.

[0056] As described above, the steel material of this embodiment may contain one or more elements selected from the above-described group of elements in the second group. That is, the steel material of this embodiment may contain one or more elements selected from the group consisting of Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, and rare earth elements: 0.0001 to 0.0200%.

[0057] [Method for Measuring Chemical Composition of Steel Material] The chemical composition of the steel material of this embodiment can be measured by a well-known component analysis method. Specifically, using a drill, chips are collected from inside the steel material at a depth of 1 mm or more from the surface. The collected chips are dissolved in an acid to obtain a solution. For the solution, ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed to conduct elemental analysis of the chemical composition. The C content and S content are determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas fusion - thermal conductivity method.

[0058] Note that for each element content, based on the significant figures defined in this embodiment, the fractional digits of the measured value are rounded off to the numerical value up to the minimum digit of each element content defined in this embodiment. For example, the C content of the steel material of this embodiment is defined as a numerical value up to the second decimal place. Therefore, the C content is the numerical value up to the second decimal place obtained by rounding off the third decimal place of the measured value.

[0059] Similarly, for the other element contents other than the C content of the steel material of this embodiment, the value obtained by rounding off the fractional digits of the measured value to the minimum digit defined in this embodiment is taken as the element content.

[0060] Note that rounding means rounding down if the fractional part is less than 5 and rounding up if the fractional part is 5 or more.

[0061] [(Required Feature 2) Regarding the Ingress Hydrogen Quantity Suppression Index Y1] On the premise that the content of each element of the steel material of this embodiment is within the range of this embodiment, Y1 defined by formula (1) is 10.00 to 70.00. Y1 = X1 / X2 (1) X1 = (Cu + 4Sn) - Cr / 2 - Mn / 10 X2 = V + 10Nb + (Ti - 3.4N) Here, for each element symbol in X1 and X2, the content in mass % of the corresponding element is substituted.

[0062] As described above, among the elements in the steel material, Cu and Sn are elements that suppress the ingress of hydrogen into the steel material in a solid solution state. On the other hand, Cr and Mn are elements that promote the ingress of hydrogen into the steel material. Therefore, X1 is an index indicating the degree of suppression of hydrogen ingress into the steel material by the solid solution elements.

[0063] Furthermore, among the elements in the steel material, V, Nb, and Ti are elements that can form carbides and the like. When these elements form carbides and the like, the carbides and the like of these elements promote the ingress of hydrogen into the steel material. Therefore, X2 is an index indicating the degree of promotion of hydrogen ingress into the steel material by V carbides, Nb carbides, and Ti carbides. Therefore, Y1 composed of X1 and X2 is an index regarding the amount of hydrogen ingress into the steel material.

[0064] On the premise that the content of each element in the steel material is within the range of this embodiment, when Y1 is less than 10.0, the ingress of hydrogen into the steel material cannot be sufficiently suppressed. Therefore, the hydrogen embrittlement resistance characteristics of the steel material (bolt) deteriorate.

[0065] Therefore, Y1 is 10.00 or more. The preferable lower limit of Y1 is 15.00, more preferably 20.00, still more preferably 25.00, and even more preferably 30.00.

[0066] On the other hand, if Y1 becomes excessively high, although the amount of hydrogen intrusion into the steel material can be suppressed, the hot workability of the steel material deteriorates. In particular, if Y1 exceeds 70.00, the hot workability of the steel material deteriorates.

[0067] Therefore, Y1 is 70.00 or less. The preferable upper limit of Y1 is 68.00, more preferably 66.00, still more preferably 60.00, and even more preferably 55.00.

[0068] As described above, the steel material of the present embodiment has the content of each element in the chemical composition within the range of the present embodiment (essential feature 1), and on the premise that the content of each element in the chemical composition is within the range of the present embodiment, Y1 is 10.00 to 70.00. Therefore, the steel material of the present embodiment is excellent in corrosion resistance. Further, since the steel material of the present embodiment can suppress the amount of hydrogen intrusion, it is excellent in hydrogen embrittlement resistance characteristics. Further, the steel material of the present embodiment is excellent in hot workability.

[0069] [Regarding the preferable features of the steel material of the present embodiment] The steel material of the present embodiment may have the above-described essential features 1 and 2, and may further have the following preferable feature 1 and / or preferable feature 2. [Preferable feature 1] On the premise that the content of each element in the chemical composition is within the range of the present embodiment and Y1 is 10.00 to 70.00, Y2 defined by formula (2) is 60.00 or more. Y2 = (8.5×√C)×(1 + 3.1Mn)×(1 + 0.3Cu)×(1 + 0.2Ni)×(1 + 5.0Cr)×(1 + 3.1Mo)×(1 + 1.5×(0.9 - C)) (2) Here, the content in mass% of the corresponding element is substituted for each element symbol in Y2. [Preferable feature 2] On the premise that the content of each element in the chemical composition is within the range of this embodiment and Y1 is 10.00 to 70.00, further, Y2 is 120.00 or less, and in the microstructure, the total area ratio of the primary ferrite and the hard phase composed of bainite and / or martensite is 90% or more, and the Vickers hardness HV at the D / 4 position is 190 to less than 260. Hereinafter, these preferable features 1 and 2 will be described.

[0070] [Regarding preferable feature 1] Preferably, in the steel material of this embodiment, on the premise that the content of each element in the chemical composition is within the range of this embodiment and Y1 is 10.00 to 70.00, Y2 defined by formula (2) is 60.00 or more. Y2 = (8.5×√C)×(1 + 3.1Mn)×(1 + 0.3Cu)×(1 + 0.2Ni)×(1 + 5.0Cr)×(1 + 3.1Mo)×(1 + 1.5×(0.9 - C)) (2) Here, the content in mass% of the corresponding element is substituted into each element symbol in Y2.

[0071] Y2 is an index for achieving both high strength and hydrogen embrittlement resistance characteristics on the premise that the content of each element in the chemical composition is within the range of this embodiment and Y1 is 10.00 to 70.00. If Y2 defined by formula (2) is 60.00 or more, it is possible to achieve both excellent hydrogen embrittlement resistance characteristics and a tensile strength of 1100 MPa or more in bolts made of the steel material of this embodiment.

[0072] A more preferable lower limit of Y2 is 63.00, more preferably 66.00, more preferably 69.00, and even more preferably 75.00.

[0073] [Regarding preferable feature 2] The microstructure of the steel material of this embodiment is not particularly limited. When the steel material of this embodiment is used as the material for bolts, if the hardness of the steel material is too high, spheroidizing annealing treatment is performed. The cold forging property of the spheroidized annealed steel material is enhanced. Therefore, it is possible to perform cold forging using the steel material of this embodiment as the material to manufacture bolts. Therefore, the microstructure of the steel material of this embodiment is not particularly limited.

[0074] Preferably, in the steel material of this embodiment, on the premise that the content of each element in the chemical composition is within the range of this embodiment and Y1 is 10.00 to 70.00, the following requirement 1 and requirement 2 are further satisfied. (Requirement 1) Y2 defined by formula (2) is 120.00 or less. (Requirement 2) In the microstructure of the steel material, the total area ratio of the primary ferrite and the hard phase composed of bainite and / or martensite is 90% or more, and when the diameter of the steel material is D, the Vickers hardness HV at a depth of D / 4 in the radial direction from the surface in the cross section perpendicular to the longitudinal direction of the steel material is 190 to less than 260.

[0075] In this case, the cold forging property of the steel material of this embodiment is enhanced. As described above, when manufacturing bolts using the steel material as the material, the steel material is cold forged. If the cold forging property of the steel material is high, cold forging can be performed without performing spheroidizing annealing treatment on the steel material. Hereinafter, each requirement will be described.

[0076] [Regarding Requirement 1 of Preferred Feature 2] As described above, Y2 is an index of strength. Y2 also becomes an index of cold forging property. Specifically, if Y2 is 120.00 or less, high cold forging property can be obtained on the premise of satisfying Requirement 2.

[0077] A more preferable upper limit of Y2 is 110.00, more preferably 105.00, more preferably 100.00, more preferably 95.00, and more preferably 90.00.

[0078] [Regarding Requirement 2 of Desirable Feature 2] In the case of the chemical composition of the steel material of this embodiment, the microstructure may contain primary ferrite, a hard phase, and pearlite. Here, pearlite causes variations in the hardness of the steel material. Therefore, when the area ratio of pearlite is high, the cold forging property of the steel material deteriorates. On the other hand, in ferrite and the hard phase, variations in hardness are less likely to occur compared to pearlite.

[0079] Therefore, preferably, in the microstructure of the steel material, the total area ratio of primary ferrite and the hard phase is 90% or more. In this case, the pearlite in the microstructure is 10% or less. Therefore, the hardness variations caused by pearlite can be reduced, and the cold forging property of the steel material can be improved.

[0080] Here, even if the total area ratio of primary ferrite and the hard phase in the microstructure of the steel material is 90% or more, if the area ratio of martensite in the hard phase is too high, the hardness of the entire steel material becomes excessively high. In this case, although the hardness variations are suppressed, since the hardness itself is high, ultimately, the cold forging property of the steel material becomes low.

[0081] Therefore, preferably, in the steel material of this embodiment, when the diameter of the steel material is D, the Vickers hardness HV at a depth of D / 4 in the radial direction from the surface in a cross-section perpendicular to the longitudinal direction of the steel material is less than 190 to 260.

[0082] In the microscopic structure observation by an optical microscope described later, it is possible to distinguish primary ferrite, pearlite, and the hard phase according to the contrast. However, it is extremely difficult to distinguish between bainite and martensite in the hard phase by contrast. On the other hand, the hardness of primary ferrite, bainite, and martensite is different respectively. Therefore, the Vickers hardness of the steel material reflects the area ratios of primary ferrite, bainite, and martensite in the microstructure of the steel material.

[0083] If the total area ratio of proeutectoid ferrite and hard phase in the microstructure is 90% or more, and further, if the Vickers hardness HV at the D / 4 depth position is less than 190 to 260, it means that the bainite area ratio and martensite area ratio in the hard phase in the microstructure are within an appropriate range. Therefore, on the premise that the content of each element in the chemical composition is within the range of this embodiment, Y1 is 10.00 to 70.00, and Y2 is 120.00 or less, the cold forging property of the steel material is further enhanced.

[0084] The preferable lower limit of the total area ratio of proeutectoid ferrite and hard phase is 92%, more preferably 95%, and even more preferably 97%. The preferable lower limit of the Vickers hardness HV at the D / 4 depth position is 195, more preferably 200, even more preferably 205, and even more preferably 210. The preferable upper limit of the Vickers hardness HV at the D / 4 depth position is 245, more preferably 240, and even more preferably 235.

[0085] [Method for measuring the total area ratio of proeutectoid ferrite and hard phase] The total area ratio of proeutectoid ferrite and hard phase can be obtained by the following method. In a cross-section perpendicular to the longitudinal direction of the steel material, a surface including the D / 4 depth position in the radial direction from the surface of the steel material is defined as the observation surface. A sample including the observation surface is taken. The observation surface of the sample is mirror-polished. Etching is performed on the mirror-polished observation surface using 3% nitric acid alcohol (nital etching solution). Among the etched observation surfaces, an arbitrary observation field (0.5 mm × 0.5 mm) is observed with a 500-fold optical microscope.

[0086] In the observation field, each of proeutectoid ferrite, pearlite, and hard phase has a different contrast for each phase. Specifically, proeutectoid ferrite is observed as white. The hard phase is observed darker than proeutectoid ferrite. In pearlite, a lamellar structure is observed. Therefore, proeutectoid ferrite, hard phase, and pearlite can be easily distinguished based on the contrast.

[0087] In the observation field of view, determine the total area of the primary ferrite and the hard phase. Based on the area of the observation field of view and the total area of the primary ferrite and the hard phase, determine the total area ratio (%) of the primary ferrite and the hard phase.

[0088] [Method for Measuring Vickers Hardness HV at D / 4 Depth Position] The Vickers hardness HV at the D / 4 depth position can be measured by the following method. Collect a sample with a cross-section perpendicular to the longitudinal direction of the steel material as the observation surface. The observation surface of the sample is the entire cross-section perpendicular to the longitudinal direction of the steel material. That is, the diameter of the observation surface of the sample is D. Polish the observation surface to a mirror finish. On the observation surface after mirror polishing, take an arbitrary measurement position at a depth of D / 4 in the radial direction from the surface of the steel material as the measurement position P1. From the measurement position P1, determine the measurement positions P2 to P12 at a pitch of 30° around the center of the observation surface of the sample (i.e., corresponding to the center of the cross-section of the steel material). Measure the Vickers hardness at the 12 measurement positions P1 to P12, which are at the D / 4 depth position.

[0089] At each measurement point P1 to P12, conduct a Vickers hardness test in accordance with JIS Z 2244:2009. The test force is 0.98 N. Measure the two-point Vickers hardness at each measurement position, and define the arithmetic mean value of the two Vickers hardnesses as the Vickers hardness HV at that measurement position.

[0090] When the steel material of this embodiment has the preferable feature 2, all of the obtained Vickers hardnesses HV at the 12 positions are included within the range of 190 to less than 260.

[0091] [Applications of the Steel Material of this Embodiment] The steel material of this embodiment is applicable as a material for bolts, which are a type of fastening means for industrial machines, automobiles, bridges, buildings, etc. In particular, it is suitable as a material for bolts with a large diameter exceeding 20 mm. Note that the steel material of this embodiment may also be used for applications other than the above.

[0092] [Manufacturing Method of Steel Material] An example of the method for manufacturing the steel material of the present embodiment will be described. The method for manufacturing the steel material to be described hereinafter is an example for manufacturing the steel material of the present embodiment. Therefore, the steel material having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method to be described hereinafter. However, the manufacturing method to be described hereinafter is a preferred example of the method for manufacturing the steel material of the present embodiment.

[0093] An example of the method for manufacturing the steel material of the present embodiment includes the following steps. (Step 1) Step of preparing a raw material (raw material preparation step) (Step 2) Step of hot-working the raw material to manufacture a steel material (hot-working step) Hereinafter, each step will be described.

[0094] [(Step 1) Raw material preparation step] In the raw material preparation step, a raw material for the steel material of the present embodiment is prepared. Specifically, a molten steel is produced in which the content of each element in the chemical composition is within the range of the present embodiment and Y1 is 10.00 to 70.00. The refining method is not particularly limited, and a well-known method may be used. For example, refining (primary refining) in a converter is performed on hot metal produced by a well-known method. Well-known secondary refining is performed on the molten steel tapped from the converter. In the secondary refining, the content of alloy elements in the molten steel is adjusted to produce a molten steel having a chemical composition in which the content of each element is within the range of the present embodiment and Y1 is 10.00 to 70.00.

[0095] Using the produced molten steel, a raw material is manufactured by a well-known casting method. For example, an ingot may be manufactured by an ingot-making method using molten steel. Also, a bloom or a billet may be manufactured by a continuous casting method using molten steel. By the above methods, a raw material (ingot, bloom or billet) is manufactured.

[0096] [(Step 2) Hot-working step] In the hot working process, hot working is performed on the material (ingot, bloom, or billet) prepared in the material preparation process to produce the steel material of the present embodiment. The shape of the steel material is not particularly limited. For example, it may be a bar or a wire rod. In the following description, as an example, the case where the steel material is a wire rod will be described. However, even if the steel material is a bar, it can be manufactured in the same hot working process.

[0097] The hot working process includes the following processes. The main manufacturing conditions are also described for each process. (Process 21) Block rolling process (Process 22) Finish rolling process Preferred heating temperature T1: 900 - 1000 °C Preferred finish temperature FT: 800 - less than 900 °C (Process 23) Cooling process Preferred average cooling rate CR1: 0.6 - 1.8 °C / second Hereinafter, each process will be described.

[0098] [(Process 21) Block rolling process] In the block rolling process, the material is hot rolled to produce a billet. Specifically, in the block rolling process, hot rolling (block rolling) is performed on the material using a block rolling mill to produce a billet. When a continuous rolling mill is arranged downstream of the block rolling mill, further hot rolling may be performed on the billet after block rolling using the continuous rolling mill to produce a smaller-sized billet. In the continuous rolling mill, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. As described above, in the block rolling process, a billet is produced from the material using a block rolling mill or using a block rolling mill and a continuous rolling mill.

[0099] The heating temperature in the block rolling process may be within a well-known temperature range. The heating temperature is, for example, 1100 - 1300 °C. The billet produced by the block rolling process is air-cooled to room temperature before the finish rolling process.

[0100] [(Process 22) Finishing Rolling Process] In the finishing rolling process, first, the billet cooled to room temperature is heated using a heating furnace. Hot rolling is performed on the heated billet using a continuous rolling mill to produce wire rods, which are steel materials.

[0101] The preferred conditions in the finishing rolling process are as follows. Preferred heating temperature T1: 900 - 1000 °C Preferred finishing temperature FT: 800 - less than 900 °C

[0102] [Regarding the preferred heating temperature T1] The heating temperature T1 in the heating furnace in the finishing rolling process may generally be within a well-known range.

[0103] Preferably, the heating temperature T1 is set to 900 - 1000 °C. If the heating temperature T1 is 900 °C or higher, on the premise of satisfying other preferred conditions (preferred finishing rolling temperature FT, preferred average cooling rate CR1), in the microstructure of the steel material after manufacturing, the total area ratio of primary ferrite and hard phases composed of bainite and / or martensite is 90% or more, and the Vickers hardness HV at the D / 4 position is 190 - less than 260.

[0104] On the other hand, if the heating temperature T1 is 1000 °C or lower, coarsening of austenite grains is suppressed in the steel material during finishing rolling. In this case, the formation of martensite is suppressed in the steel material after manufacturing. Therefore, on the premise of satisfying other preferred conditions (preferred finishing rolling temperature FT, preferred average cooling rate CR1), the Vickers hardness HV at the D / 4 position of the steel material after manufacturing is 190 - less than 260.

[0105] [Regarding the preferred finishing temperature FT] In the finish rolling process, hot rolling (finish rolling) is performed using a continuous rolling mill equipped with a plurality of rolling stands arranged in a row. In hot rolling using a continuous rolling mill, the temperature of the steel material on the outlet side of the stand that finally reduces the thickness of the steel material is defined as the finish temperature FT (°C). Note that the steel material temperature means the surface temperature of the steel material. The finish temperature FT may be within a well-known range in principle.

[0106] Preferably, the finish temperature FT is set to be 800 to less than 900 °C. If the finish temperature FT is 800 °C or higher, on the premise of satisfying other preferable conditions (preferable heating temperature T1, preferable average cooling rate CR1), in the microstructure of the steel material after manufacturing, the total area ratio of the primary ferrite and the hard phase is 90% or more.

[0107] If the finish temperature FT is less than 900 °C, on the premise of satisfying other preferable conditions (preferable finish rolling temperature FT, preferable average cooling rate CR1), coarsening of austenite grains in the steel material during finish rolling is suppressed. In this case, generation of martensite in the steel material after manufacturing is suppressed. Therefore, on the premise of satisfying other preferable conditions (preferable finish rolling temperature FT, preferable average cooling rate CR1), the Vickers hardness HV at the D / 4 position of the steel material after manufacturing is 190 to less than 260.

[0108] [(Process 23) Cooling Process] In the cooling process, the steel material after the finish rolling process is cooled. The preferable manufacturing conditions in the cooling process are as follows. Preferable average cooling rate CR1: 0.6 to 1.8 °C / second Here, the average cooling rate CR1 means the average value of the cooling rates from the finish temperature FT to 300 °C.

[0109] The average cooling rate CR1 in the cooling process may be within a well-known range.

[0110] Preferably, the average cooling rate CR1 is set to 0.6 to 1.8 °C / second. If the average cooling rate CR1 is 0.6 °C / second or more, generation of pearlite in the steel material can be suppressed during the cooling process. Therefore, on the premise of satisfying other preferable conditions (preferable finish rolling temperature FT, preferable average cooling rate CR1), the total area ratio of the primary ferrite and the hard phase composed of bainite and / or martensite becomes 90% or more, and further, the Vickers hardness HV at the D / 4 position of the steel material after manufacturing is less than 190 to 260.

[0111] Furthermore, if the average cooling rate CR1 is 1.8 °C / second or less, generation of martensite in the steel material can be suppressed during the cooling process. In this case, on the premise of satisfying other preferable conditions (preferable heating temperature T1, preferable finish rolling temperature FT), the total area ratio of the primary ferrite and the hard phase composed of bainite and / or martensite becomes 90% or more, and the Vickers hardness HV at the D / 4 position of the steel material after manufacturing is less than 190 to 260.

[0112] In addition, in the hot working process of the above manufacturing process, the finish rolling process may be carried out without carrying out the block rolling process. That is, the block rolling process is an optional process. For example, when a billet is prepared in the material preparation process, the block rolling process may be omitted and the finish rolling process may be carried out.

[0113] [Manufacturing method of bolt using the steel material of the present embodiment] The manufacturing method of a bolt using the steel material of the present embodiment is a well-known manufacturing method. The manufacturing method of a bolt includes, for example, the following steps. (Step 31) Wire drawing process (Step 32) Cold forging process (Step 33) Quenching and tempering process Hereinafter, each step will be described.

[0114] [(Step 31) Wire drawing process] In the wire drawing process, a well-known wire drawing is performed on the above-described steel material to produce a steel wire. The wire drawing may be only primary wire drawing, or multiple wire drawings such as secondary wire drawing may be performed.

[0115] [(Process 32) Cold Forging Process] In the cold forging process, well-known cold forging is performed on the steel wire after the wire drawing process to produce an intermediate product in the shape of a bolt.

[0116] [(Process 33) Quenching and Tempering Process] In the quenching and tempering process, quenching and tempering are performed on the intermediate product.

[0117] [Quenching] Quenching is performed by a well-known method. The quenching temperature and the holding time at the quenching temperature are not particularly limited. The quenching temperature is, for example, 840 to 970 °C. The holding time at the quenching temperature is, for example, 15 minutes to 360 minutes (6 hours). After the holding time has elapsed, the intermediate product is rapidly cooled. Specifically, water cooling or oil cooling is performed on the intermediate product.

[0118] [Tempering] Tempering is performed on the intermediate product after quenching. The tempering temperature and the holding time at the tempering temperature are not particularly limited. The tempering temperature is, for example, 400 to 550 °C. The holding time at the tempering temperature is 0.5 to 6.0 hours.

[0119] By the above manufacturing method, a bolt made of the steel material of the present embodiment can be manufactured. The manufactured bolt is excellent in corrosion resistance. Furthermore, since hydrogen intrusion in a corrosive environment is suppressed, it is excellent in hydrogen embrittlement resistance characteristics.

Example

[0120] The effects of the steel material of the present embodiment will be further specifically described by way of examples. The conditions in the following examples are one example of the conditions adopted to confirm the feasibility and effects of the steel material of the present embodiment. Therefore, the steel material of the present embodiment is not limited to this one example of conditions.

[0121] [Raw material preparation process] Molten steel having the chemical composition shown in Table 1 was produced. Using the molten steel, a raw material (cast slab) was produced by the continuous casting method.

[0122]

Table 1

[0123] In Table 1, “-” means that the corresponding element content is 0% in the significant figures (numerical values up to the minimum digit) defined in the embodiment. In other words, it means that when the fractional part in the significant figures (numerical values up to the minimum digit) defined in the above embodiment is rounded off for the corresponding element content, it is 0%. For example, the V content defined in this embodiment is defined with numerical values up to the third decimal place. Therefore, in Test No. 1 in Table 1, it means that when the measured V content was rounded off to the fourth decimal place, it was 0%. Also, the Ca content defined in this embodiment is defined with numerical values up to the fourth decimal place. Therefore, in Test No. 1 in Table 1, it means that when the measured Ca content was rounded off to the fifth decimal place, it was 0%. Also, the Cr content defined in this embodiment is defined with numerical values up to the second decimal place. Therefore, in Test No. 19, it means that when the measured Cr content was rounded off to the third decimal place, it was 0%. Note that rounding off means that if the digit below the defined minimum digit (fractional part) is less than 5, it is truncated, and if it is 5 or more, it is rounded up.

[0124] [Hot working process] A billet was produced by performing a block rolling process on the produced raw material. In the block rolling process, after heating the raw material to 1100 - 1300 °C, hot rolling was performed using a block rolling mill and a continuous rolling mill. The billet produced by the block rolling process was air-cooled to room temperature.

[0125] For the manufactured billets, a finishing rolling process was carried out. In the finishing rolling process, the billets were heated at the heating temperature T1 (°C) shown in Table 2. For the heated billets, hot rolling was carried out using a continuous rolling mill to produce steel materials (round bars) with a diameter of 21.5 mm. The finishing temperature FT (°C) in hot rolling was as shown in Table 2.

[0126]

Table 2

[0127] [Cooling process] The steel materials (bar steels) after hot rolling were cooled. At this time, the average cooling rate CR1 when the steel material temperature was from the finishing temperature FT to 300 °C was as shown in Table 2.

[0128] Through the above manufacturing process, steel materials (round bars) with a diameter of 21.5 mm for each test number were manufactured.

[0129] [Regarding the evaluation test] For the manufactured steel materials of each test number, the following steel material evaluation tests (Test 1 to Test 5) were carried out. [Steel material evaluation test] (Test 1) Chemical composition measurement test of steel material (Test 2) Microstructure observation test (Test 3) Vickers hardness test (Test 4) Hot workability evaluation test (Test 5) Cold forging property evaluation test

[0130] Furthermore, using a bolt simulation material assuming bolts manufactured from the steel material, the following bolt evaluation tests (Test 6 to Test 9) were carried out. [Bolt evaluation test] (Test 6) Tensile strength measurement test (Test 7) Corrosion resistance evaluation test (Test 8) Ingress hydrogen amount measurement test (Test 9) Hydrogen embrittlement resistance property evaluation test The following will explain each test.

[0131] [Regarding the Steel Material Evaluation Test] The above Tests 1 to 5 were conducted in the following manner.

[0132] [(Test 1) Chemical Composition Measurement Test of Steel Material] For the steel materials of each test number, the chemical composition was analyzed based on the above [Method for Measuring the Chemical Composition of Steel Material]. As a result, the chemical composition of each test number was as shown in Table 1.

[0133] [(Test 2) Microstructure Observation Test] For the steel materials of each test number, based on the method described in the above [Method for Measuring the Total Area Ratio of Proeutectoid Ferrite and Hard Phase], the total area ratio (%) of proeutectoid ferrite and hard phase at the D / 4 depth position was determined. The obtained total area ratio is shown in the column of "Proeutectoid F + Total Area Ratio of Hard Phase (%)" in Table 2.

[0134] [(Test 3) Vickers Hardness Test] For the steel materials of each test number, based on the method described in the above [Method for Measuring the Vickers Hardness HV at the D / 4 Depth Position], the Vickers hardness HV at the measurement positions P1 to P12 at the D / 4 depth position was determined. Based on the measured Vickers hardness HV at 12 locations, it is expressed as follows in the column of "D / 4 Vickers Hardness (HV)" in Table 2. E: All of the Vickers hardness HV at 12 locations are less than 190 to 260. B1: Among the Vickers hardness HV at 12 locations, at least one is 260 or more. B2: Among the Vickers hardness HV at 12 locations, at least one is less than 190.

[0135] [(Test 4) Hot Workability Evaluation Test] The hot workability of the steel materials of each test number was evaluated by the following test. First, test pieces were taken from the center position in the cross-section perpendicular to the longitudinal direction of the steel material. The test pieces were round bar test pieces with a diameter of 10 mm and a length of 120 mm. The longitudinal direction of the test pieces was parallel to the longitudinal direction of the steel material. The central axis of the test pieces was coaxial with the steel material.

[0136] Using test pieces, a hot tensile test was carried out with a hot tensile testing machine. For the hot tensile test, a Gleeble testing machine manufactured by Fuji Denpa Co., Ltd. was used. Specifically, the test pieces were heated to 1100 °C and held for 5 minutes. Then, the test pieces were cooled to 900 °C at a cooling rate of 1 °C / second. After that, a tensile test was carried out at a strain rate of 1 / second until fracture. After the fractured test pieces were cooled to room temperature, the reduction value was measured from the cross-sectional area of the test pieces. Based on the obtained reduction value, the hot workability was evaluated.

[0137] The evaluation results are shown in the "Hot Workability" column in Table 2. When the reduction value is 75% or more, it is judged that the hot workability is excellent (indicated by "E" (Excellent) in Table 2). On the other hand, when the reduction value is less than 75%, it is judged that the hot workability is low (indicated by "B" (Bad) in Table 2).

[0138] [(Test 5) Cold Forging Property Evaluation Test] The cold forging properties of the steel materials with each test number were evaluated by a limiting compression test. First, a plurality of limiting compression test pieces were sampled from the steel materials with each test number. The diameter of the limiting compression test pieces was 10 mm and the length was 15 mm. The longitudinal direction of the limiting compression test pieces was parallel to the longitudinal direction of the steel materials with each test number. An annular notch was formed in the circumferential direction at the central position in the longitudinal direction of the limiting compression test pieces. The notch angle was 30°, the notch depth was 0.8 mm, and the curvature radius of the notch bottom was 0.15 mm.

[0139] For the limiting compression test, a 500-ton hydraulic press was used. The following method was used to carry out the limiting compression test on the prepared limiting compression test pieces. For each limiting compression test piece, cold compression was carried out at a speed of 10 mm / min using a restraining die. Compression was stopped when microcracks of 0.5 mm or more occurred near the notch, and the compression ratio (%) at that time was calculated. This measurement was carried out a total of 10 times, and the compression ratio (%) at which the cumulative fracture probability became 50% was obtained. The obtained compression ratio was taken as the limiting compression ratio (%).

[0140] The evaluation results are shown in the "Cold Forging Property" column in Table 2. When the limiting compression ratio is 38% or more, it was judged that the cold forging property was excellent (indicated by "E" in Table 2). On the other hand, when the limiting compression ratio was less than 38%, it was judged that the cold forging property was low (indicated by "B" in Table 2).

[0141] [Bolt Evaluation Test] Assuming bolts manufactured from the steel materials of each test number as the raw materials, the following bolt simulation materials for each test number were manufactured.

[0142] [Manufacture of Bolt Simulation Materials] For the steel materials (round bars with a diameter of 21.5 mm) of each test number, the following quenching treatment and tempering treatment were carried out. The quenching treatment was carried out using a heat treatment furnace. The quenching temperature was set at 880 °C, and the holding time at the quenching temperature was 60 minutes. After the holding time elapsed, the steel material was water-cooled for quenching. In addition, the inside of the heat treatment furnace was filled with an Ar gas atmosphere to suppress decarburization of the steel material.

[0143] After the quenching treatment, a tempering treatment was carried out. The tempering treatment was carried out using a heat treatment furnace. In tempering, it was held for 0.5 to 4.0 hours at the tempering temperature (°C) shown in Table 2 so that the tensile strength of the bolt simulation material was within the range of 1000 to 1200 MPa. After the holding time elapsed, the steel material was water-cooled. Through the above manufacturing process, bolt simulation materials were manufactured.

[0144] Among the bolt evaluation tests, in Tests 6, 8, and 9, the tests were carried out using the bolt simulation materials of each test number. On the other hand, in the corrosion resistance evaluation test of Test 7, instead of the bolt simulation material in the shape of a round bar (round bar), a steel plate was used as a substitute material for the round bar, a plate-shaped bolt simulation material was manufactured, and the following corrosion resistance evaluation test was carried out using the plate-shaped bolt simulation material.

[0145] [(Test 6) Tensile Strength Measurement Test] The tensile strength of each test number was measured by the following method. First, specimens were taken from the center position in a cross-section perpendicular to the longitudinal direction of the bolt simulation material. The length of the specimen was 80 mm, the length of the parallel part was 40 mm, and the diameter was 6.0 mm. The central axis of the specimen was coaxial with the bolt simulation material. Two specimens were prepared for each test number.

[0146] In accordance with JIS Z2241:2011, a tensile test was carried out at normal temperature in the atmosphere to obtain the tensile strength (MPa). The arithmetic mean value of the two tensile strengths was defined as the tensile strength (MPa) of the bolt simulation material made of the steel material of that test number.

[0147] The tensile strength of each test number was within the range of 1100 - 1200 MPa, except for the steel materials of test numbers 26 and 27. On the other hand, for test numbers 26 and 27, Y2 was small. Therefore, the tensile strength of these test numbers was within the range of less than 1000 - 1100 MPa. Specifically, the tensile strength of test number 26 was 1045 MPa. The tensile strength of test number 27 was 1059 MPa.

[0148] [(Test 7) Corrosion resistance evaluation test] The corrosion resistance of the bolt simulation materials of each test number was evaluated by the following test. In the corrosion resistance evaluation test, considering the ease of evaluating corrosion resistance, steel plates of each test number that replaced the round bar were manufactured in the following method instead of using the round bar (round steel) as the material. Specifically, for the material having the chemical composition shown in Table 1, a finish rolling process and a cooling process were carried out to manufacture a steel plate with a thickness of 20 mm. The heating temperature T1 (°C) of the material, the finish temperature FT (°C), and the average cooling rate CR1 in the cooling process were as shown in Table 2.

[0149] Using the manufactured steel plates, bolt simulation materials of each test number were manufactured. Specifically, for the steel plates of each test number, the quenching treatment described in the above [Manufacture of bolt simulation material] was carried out. Further, for the steel plate after quenching, tempering was carried out by holding at the tempering temperature shown in Table 2 for 1.0 hour to manufacture a bolt simulation material. Through the above manufacturing process, a bolt simulation material (steel plate) for the corrosion resistance evaluation test was manufactured.

[0150] A plate-shaped test piece with dimensions of 100 mm × 60 mm × 3 mm in thickness was sampled from the manufactured bolt simulation material (steel plate). Shot blasting was performed on the surface of the sampled plate-shaped test piece, and on the surface of the plate-shaped test piece, it was adjusted so that the ten-point average roughness Rzjis conforming to JIS B0601:2001 was 75 μm.

[0151] Coating was performed on the surface of the plate-shaped test piece after shot blasting, and a coating film composed of an undercoat with a thickness of 120 μm (product name: Neo Gosei #2300PS manufactured by Shinto Paint Co., Ltd.), an intermediate coat with a thickness of 30 μm (product name: Sintoflon #100 manufactured by Shinto Paint Co., Ltd.), and a topcoat with a thickness of 25 μm (product name: Sintoflon #100 manufactured by Shinto Paint Co., Ltd.) was formed.

[0152] Coating film defects reaching the substrate (steel plate) were formed using a cutter. The total length of the coating film defects was set to 500 mm.

[0153] Using the plate-shaped test piece with coating film defects, a corrosion test conforming to the US standard SAE J2334 was carried out with a dry-wet cycling tester capable of salt water immersion. Specifically, a test with the following three steps (total 24 hours) as one cycle was carried out. (Step 1: Wetting process) The plate-shaped test piece is held in an environment of 50 °C and 100% RH relative humidity for 6 hours. (Step 2: Salt water immersion process) The plate-shaped test piece after Step 1 is immersed in an aqueous solution with a pH of 8 containing 0.5% NaCl, 0.1% CaCl2, and 0.075% NaHCO3 for 15 minutes. (Step 3: Drying process) The plate-shaped test piece after Step 2 is held in an environment of 60 °C and 50% RH for 17.75 hours. The plate-shaped test piece after holding is dried.

[0154] The above Steps 1 to 3 were taken as one cycle, and 80 cycles were carried out.

[0155] Among the coating films of the plate-shaped test pieces after 80 cycles of implementation, the coating film portion that had peeled off from the surface of the test piece starting from a coating film defect (hereinafter referred to as the coating film peeling portion) was removed with a cutter. After removing the coating film peeling portion, an image of the coating film of the plate-shaped test piece was generated in a plan view. By image processing, the area where the coating film remained and the area where the steel plate was exposed (coating film peeling portion) on the surface of the plate-shaped test piece were distinguished. And the total area (peeling area) of the coating film peeling portion was determined.

[0156] Furthermore, the corrosion products generated in the coating film defects by the above test were mechanically removed with a hammer and a scraper, etc. Furthermore, the plate-shaped test piece was immersed in a mixed solution of a 10 mass% aqueous solution of diammonium hydrogen citrate and 2 mass% Ibitt manufactured by Asahi Chemical Co., Ltd. for 24 hours for pickling to remove the corrosion products adhering to the surface.

[0157] After physically and chemically removing the corrosion products, the coating film peeling portion was divided into 20 sections approximately evenly. The maximum corrosion depth for each section was determined with a point micrometer. The arithmetic mean value of the obtained maximum corrosion depths was defined as the corrosion depth (mm) of that test number.

[0158] Based on the peeling area and the corrosion depth, the corrosion resistance was evaluated. The evaluation results are shown in the "Corrosion Resistance" column in Table 2. When the peeling area was 30% or less and the corrosion depth was 0.2 mm or less, it was evaluated that the corrosion resistance was excellent (indicated by "E" in Table 2). On the other hand, when the peeling area exceeded 30% or the corrosion depth exceeded 0.2 mm, it was evaluated that the corrosion resistance was low (indicated by "B" in Table 2).

[0159] [(Test 8) Ingress Hydrogen Amount Measurement Test] The ingress hydrogen amount for each test number was measured by the following method. First, the test piece was sampled from the center position in the cross-section perpendicular to the longitudinal direction of the bolt simulation material. The test piece was a round bar test piece with a diameter of 7 mm and a length of 100 mm. The central axis of the test piece was coaxial with the bolt simulation material. Two test pieces were prepared for each test number.

[0160] Under the same conditions as in Test 7, a corrosion test in accordance with the US standard SAE J2334 was conducted, and the amount of hydrogen ingress into each test piece after the corrosion test was measured.

[0161] Specifically, taking Steps 1 to 3 as one cycle, the amount of hydrogen ingress into the test pieces after 56 cycles of testing was measured. To prevent the hydrogen that had entered the test pieces after the corrosion test from escaping, the test pieces after the corrosion test were immersed in liquid nitrogen until immediately before measuring the amount of hydrogen ingress. Before measuring the amount of hydrogen ingress, the corrosion products adhering to the surface of the test pieces were completely removed using sandblasting. The amount of hydrogen ingress into the test pieces from which the corrosion products had been removed was measured using a temperature-programmed desorption analyzer. Specifically, the amount of diffusible hydrogen detected by the desorption reaction from room temperature to 200 °C by the temperature-programmed desorption analyzer was measured and taken as the amount of hydrogen ingress. The arithmetic mean value of the amounts of hydrogen ingress of the two obtained test pieces was defined as the amount of hydrogen ingress He (ppm) of the steel material of that test number. The obtained amounts of hydrogen ingress were shown in the "Amount of hydrogen ingress" column in Table 2.

[0162] [(Test 9) Hydrogen embrittlement resistance property evaluation test] The hydrogen embrittlement resistance properties of the steel materials of each test number were evaluated by the following test. First, test pieces were taken from the center position in the cross-section perpendicular to the longitudinal direction of the bolt mock-up. The test pieces were annular notch round bar test pieces with a diameter of 7 mm and a length of 70 mm. An annular notch was formed at the central position in the longitudinal direction of the test pieces. The depth of the notch was 1.4 mm, the notch angle was 60°, and the radius of curvature at the bottom of the notch was 0.175 mm.

[0163] Using the prepared annular notch round bar test pieces, an SSRT (Slow Strain Rate Technique) test was conducted. Specifically, a hydrogen charging solution was prepared by adding 3 g / L of NH4SCN to a 3% NaCl solution. With the annular notch round bar test pieces immersed in the hydrogen charging solution, the current density applied to the test pieces was adjusted to adjust the amount of hydrogen ingress into the test pieces.

[0164] The round bar specimens with circular notches charged with hydrogen at each current density were subjected to plating treatment so that hydrogen would not desorb. The round bar specimens with circular notches after the plating treatment were left at room temperature for 8 hours or more. Then, a tensile test was carried out at a speed of 0.005 mm / min to break the round bar specimens with circular notches. After fracture, the amount of hydrogen absorbed (ppm) in the round bar specimens with circular notches was measured using a temperature-programmed desorption analyzer.

[0165] Based on the above tests, a graph of the amount of hydrogen absorbed (ppm) and the fracture load (kN) as illustrated in Fig. 1 was created. Based on the created graph, the fracture load (σ 2He ) at the hydrogen amount (2He) where the amount of hydrogen absorbed becomes twice the amount of hydrogen absorbed (He) determined in [(Test 8) Hydrogen Absorption Amount Measurement Test] was obtained.

[0166] Furthermore, a tensile test was carried out at a speed of 0.005 mm / min on the round bar specimens with circular notches without hydrogen charging for each test number to break the round bar specimens with circular notches, and the fracture load (σ0) was obtained.

[0167] The fracture load σ 2He when hydrogen was charged was divided by the fracture load (σ0) when hydrogen was not charged to obtain the fracture load ratio (σ 2He / σ0).

[0168] Based on the fracture load ratio (σ 2He / σ0), the hydrogen embrittlement resistance characteristics were evaluated. The evaluation results are shown in the "Hydrogen Embrittlement Resistance Characteristics" column in Table 2. If the fracture load ratio was 0.8 or more, it was judged that the hydrogen embrittlement resistance characteristics were excellent (indicated by "E" in Table 2). If the fracture load ratio was less than 0.8, it was judged that the hydrogen embrittlement resistance characteristics were low (indicated by "B" in Table 2).

[0169] [Evaluation Results] The evaluation results are shown in Table 2. In Test Nos. 1 to 32, the chemical composition was appropriate and Y1 satisfied Formula (1). Therefore, excellent corrosion resistance was obtained for these test numbers. Furthermore, the amount of hydrogen intrusion was as low as 0.050 ppm or less. Therefore, excellent hydrogen embrittlement resistance characteristics were obtained. Also, all of these test numbers were excellent in hot workability.

[0170] Moreover, in Test Nos. 1 to 25 and 28 to 32, Y2 was 60.00 or more. Therefore, for these test numbers, not only were the hydrogen embrittlement resistance characteristics excellent, but the tensile strength was 1100 MPa or more. Note that in Test No. 26, since Y2 was less than 60.00, although the hydrogen embrittlement resistance characteristics were excellent, the tensile strength was 1045 MPa, which was 1000 MPa or more and less than 1100 MPa.

[0171] Also, in Test Nos. 1 to 26, Y2 was 120.00 or less, and the heating temperature T1, finishing temperature FT, and average cooling rate CR1 in the finishing rolling process were all appropriate. Therefore, in the microstructure of the steel materials of these test numbers, the total area ratio of primary ferrite and hard phases was 90% or more, and the Vickers hardness HV at the D / 4 position was less than 190 to 260. Therefore, excellent cold forgeability was shown in the cold forging property evaluation test.

[0172] Note that in Test No. 27, Y2 was less than 60.00, and furthermore, in the microstructure of the steel material, the total area ratio of primary ferrite and hard phases composed of bainite and / or martensite was less than 90%. Therefore, although it was excellent in corrosion resistance, hydrogen embrittlement resistance characteristics, and hot workability, the tensile strength was 1059 MPa, which was 1000 MPa or more and less than 1100 MPa. Also, the cold forgeability was lower than that of Test Nos. 1 to 25.

[0173] In Test No. 28, Y2 exceeded 120.00. Therefore, although it was excellent in hydrogen embrittlement resistance characteristics and hot workability, the cold forgeability was lower than that of Test Nos. 1 to 26.

[0174] In addition, in Test No. 29, the heating temperature T1 and the finishing temperature FT in the finish rolling process were low. Therefore, among the Vickers hardness HV at 12 locations at the D / 4 position, at least one was 260 or more. Therefore, although it was excellent in corrosion resistance, hydrogen embrittlement resistance, and hot workability, the cold forging property was lower than that of Test Nos. 1 to 26.

[0175] In Test No. 30, the heating temperature T1 and the finishing temperature FT in the finish rolling process were high. Therefore, among the Vickers hardness HV at 12 locations at the D / 4 position, at least one was 260 or more. Therefore, although it was excellent in corrosion resistance, hydrogen embrittlement resistance, and hot workability, the cold forging property was lower than that of Test Nos. 1 to 26.

[0176] In Test No. 31, the average cooling rate CR1 in the cooling process after the finish rolling process was fast. Therefore, among the Vickers hardness HV at 12 locations at the D / 4 position, at least one was 260 or more. Therefore, although it was excellent in corrosion resistance, hydrogen embrittlement resistance, and hot workability, the cold forging property was lower than that of Test Nos. 1 to 26.

[0177] In Test No. 32, the average cooling rate CR1 in the cooling process after the finish rolling process was slow. Therefore, in the microstructure of the steel material, the total area ratio of the primary ferrite and the hard phase composed of bainite and / or martensite was less than 90%. Furthermore, among the Vickers hardness HV at 12 locations, at least one was less than 190. Therefore, although it was excellent in corrosion resistance, hydrogen embrittlement resistance, and hot workability, the cold forging property was lower than that of Test Nos. 1 to 26.

[0178] On the other hand, in Test No. 33, the Cu content was low. Therefore, the amount of hydrogen intrusion exceeded 0.050 ppm, and the corrosion resistance and hydrogen embrittlement resistance were low.

[0179] In Test No. 34, the Cu content was high. Therefore, the hot workability and cold forging property were low.

[0180] In Test No. 35, the Ni content was low. Therefore, the corrosion resistance and hot workability were low.

[0181] In Test No. 36, the Cr content was high. Therefore, the amount of hydrogen ingress exceeded 0.050 ppm, and the hydrogen embrittlement resistance characteristics were low.

[0182] In Test No. 37, the Sn content was low. Therefore, the amount of hydrogen ingress exceeded 0.050 ppm, and the hydrogen embrittlement resistance characteristics were low.

[0183] In Test No. 38, the Sn content was high. Therefore, the hot workability, cold forging property, and hydrogen embrittlement resistance characteristics were low.

[0184] In Test No. 39, the V content was high. Therefore, the amount of hydrogen ingress exceeded 0.050 ppm. Therefore, the hydrogen embrittlement resistance characteristics were low.

[0185] In Test No. 40, the Nb content was high. Therefore, the amount of hydrogen ingress exceeded 0.050 ppm. Therefore, the hydrogen embrittlement resistance characteristics were low.

[0186] In Test Nos. 41 to 43, Y1 was low. Therefore, the amount of hydrogen ingress exceeded 0.050 ppm. As a result, the hydrogen embrittlement resistance characteristics were low.

[0187] In Test Nos. 44 to 46, Y1 was high. Therefore, the hot workability was low.

[0188] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. By mass percentage, C: 0.15 to 0.30%, Si: 0.01 to 0.50%, Mn: less than 0.50 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cu: 0.10 to 0.60%, Ni: 0.10 to 0.60%, Cr: 0 to 0.20%, Mo: 0.01 to 0.60%, Sn: 0.010 to 0.500%, Al: 0.005 to 0.060%, N: 0.0010 to 0.0080%, B: 0.0002 to 0.0050%, Ti: 0.005 to 0.100%, V: 0 to 0.050%, Nb: 0 to 0.030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Rare earth elements: 0 to 0.0200%, and The balance: Fe and impurities, and On the premise that the content of each element satisfies the above range, Y1 defined by formula (1) is 10.00 to 70.00, Steel material. Y1 = X1 / X2 (1) X1 = (Cu + 4Sn) - Cr / 2 - Mn / 10 X2 = V + 10Nb + (Ti - 3.4N) Here, in each element symbol in X1 and X2, the content in mass percentage of the corresponding element is substituted.

2. The steel material according to Claim 1, wherein V: 0.001 to 0.050%, Nb: 0.001 to 0.030%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, and Rare earth elements: containing one or more elements selected from the group consisting of 0.0001 to 0.0200%, Steel material.

3. The steel material according to Claim 1 or Claim 2, further comprising On the premise that the content of each element satisfies the above range, Y2 defined by formula (2) is 60.00 or more, Steel material. Y2 = (8.5 × √C) × (1 + 3.1Mn) × (1 + 0.3Cu) × (1 + 0.2Ni) × (1 + 5.0Cr) × (1 + 3.1Mo) × (1 + 1.5 × (0.9 - C)) (2) Here, in each element symbol in Y2, the content in mass percentage of the corresponding element is substituted.

4. The steel material according to any one of Claims 1 to 3, wherein The steel material is a round bar, and further On the premise that the content of each element satisfies the above range, Y2 defined by formula (2) is 120.00 or less, In the microstructure, the total area ratio of primary ferrite and a hard phase composed of bainite and / or martensite is 90% or more, When the diameter of the steel material is D, the Vickers hardness HV at the D / 4 position is 190 to less than 260, Steel material. Y2 = (8.5 × √C) × (1 + 3.1Mn) × (1 + 0.3Cu) × (1 + 0.2Ni) × (1 + 5.0Cr) × (1 + 3.1Mo) × (1 + 1.5 × (0.9 - C)) (2) Here, in each element symbol in Y2, the content in mass % of the corresponding element is substituted.

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