Steel material and component for mechanical structure

Abstract

Provided is a steel material having excellent cold forgeability. A steel material according to the present disclosure has the chemical composition described in the description. When ND0 is defined as the number density (molecules / mm2) of Mn sulfide, ND1 is defined as the number density (molecules / mm2) of Mn sulfide containing Cu-Ni, and ND2 is defined as the number density (molecules / mm2) of Ti-containing Mn sulfide, ND1 (molecules / mm2) is 1.00 or greater and satisfies formula (1) and formula (2).　Formula (1): ND1 + ND2 ≥ 2.00; Formula (2): (ND1 + ND2) / ND0 < 0.25

Description

Steel materials and machine structural parts

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

[0002] High strength is required for mechanical structural parts used in industrial machinery, automobiles, etc. For this reason, steel is generally used as the material for mechanical structural parts. Among mechanical structural parts, bolts in particular, cold forging is often used in the manufacturing process when steel is used as the material. This is to increase yield and reduce manufacturing costs.

[0003] The manufacturing process for a bolt by cold forging is, for example, as follows: A steel material is subjected to wire drawing; The steel material after wire drawing is subjected to heat treatment (e.g., annealing) for the purpose of softening the steel material; The heat-treated steel material is subjected to cold forging to manufacture an intermediate product in the shape of a bolt having a head and a shank; The intermediate product is quenched and tempered to manufacture the bolt.

[0004] In recent years, from the perspective of energy conservation and further reducing manufacturing costs, the omission of heat treatment before the cold forging process has been considered. In order to omit the heat treatment before the cold forging process, the steel material used to make the bolt is required to have excellent cold forgeability. Furthermore, even when the shape of the final product is complex, manufacturing costs can be reduced by performing cold forging rather than hot forging. In this case, the steel material used to make the bolt is also required to have excellent cold forgeability.

[0005] Techniques for improving the cold forgeability of steel materials used to make bolts are proposed in Japanese Patent Laid-Open No. 2006-274373 (Patent Document 1) and International Publication No. 2020 / 090149 (Patent Document 2).

[0006] The bolt steel disclosed in Patent Document 1 has a composition, by mass%, of 0.07 to 0.15% C, 0.2% or less Si, 0.5 to 2% Mn, 0.015% or less P, 0.015% or less S, 2% or less Cr, 0.005 to 0.08% Al, 0.01% or less N, and a carbon equivalent (Ceq = C + Si / 7 + Mn / 6 + Cr / 9) of 0.50% or less, with the balance being iron and unavoidable impurities. By limiting the carbon equivalent in the steel, the precipitation of cementite at grain boundaries can be suppressed, thereby reducing embrittlement of the steel. Furthermore, Patent Document 1 states that by containing the above alloying elements within appropriate ranges, excellent cold forgeability can be obtained.

[0007] The steel material for bolts disclosed in Patent Document 2 contains, in mass %, C: 0.18 to 0.24%, Si: 0.10 to 0.22%, Mn: 0.60 to 1.00%, Al: 0.010 to 0.050%, Cr: 0.65 to 0.95%, Ti: 0.010 to 0.050%, B: 0.0015 to 0.0050%, N: 0.0050 to 0.0100%, P: 0.025% or less (including 0), S: The steel material for bolts disclosed in Patent Document 2 has a composition containing 0.025% or less (inclusive), Cu: 0.20% or less (inclusive), and Ni: 0.30% or less (inclusive), in ranges satisfying 0.45≦C+Si / 24+Mn / 6+Ni / 40+Cr / 5≦0.60 and N≦0.519Al+0.292Ti, with the balance being Fe and unavoidable impurities, and has a microstructure in which bainite accounts for 95% or more in area fraction. Furthermore, the prior austenite grains in the microstructure have a grain size number of 6 or more, and a strength variation of 100 MPa or less. In the steel material, the area fraction of the bainite structure is increased and the prior austenite grains are refined. Patent Document 2 states that this results in a greater Bauschinger effect and reduces deformation resistance during cold forging when forming the bolt head.

[0008] JP 2006-274373 A International Publication No. 2020 / 090149

[0009] However, the cold forgeability may be improved by a means different from that of the steel materials disclosed in Patent Documents 1 and 2.

[0010] An object of the present disclosure is to provide a steel material having excellent cold forgeability and a machine structural part that can obtain excellent cold forgeability during the manufacturing process.

[0011] The steel material of the present disclosure has, in mass%, C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, The alloy contains V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities. The alloy has an equivalent circle diameter of 1.0 μm or more, and the number density (pieces / mm ) of Mn sulfides having an S content of 10% or more and an Mn content of 10% or more by mass. ２ ) to ND ０ The number density (pieces / mm ) of Cu-Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% or more, in mass %, is defined as ２ ) to ND １ The number density (pieces / mm ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more, is defined as ２ ) to ND ２ When defined as ND １ (pcs / mm ２ ) is 1.00 or more, and the formula (1) and the formula (2) are satisfied. １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2)

[0012] The machine structural part of the present disclosure has, in mass %, C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, The alloy contains V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities. The alloy has an equivalent circle diameter of 1.0 μm or more, and the number density (pieces / mm ) of Mn sulfides having an S content of 10% or more and an Mn content of 10% or more by mass. ２ ) to ND ０ The number density (pieces / mm ) of Cu-Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% or more, in mass %, is defined as ２ ) to ND １ The number density (pieces / mm ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more, is defined as ２ ) to ND ２ When defined as ND １ (pcs / mm ２ ) is 1.00 or more, and the formula (1) and the formula (2) are satisfied. １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2)

[0013] The steel material of the present disclosure has excellent cold forgeability. The machine structural component of the present disclosure has excellent cold forgeability in the manufacturing process.

[0014] The present inventors first investigated a steel material that can provide excellent cold forgeability from the viewpoint of chemical composition, and as a result, the present inventors found a steel material containing, in mass %, C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, and P: 0.040% or less. It was thought that excellent cold forgeability would be obtained if the chemical composition was S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, and the balance being Fe and impurities.

[0015] However, it has been found that even steel materials satisfying the above-mentioned chemical composition may have poor cold forgeability. Therefore, the inventors have investigated means for improving cold forgeability from the viewpoint of microstructure. As a result, the inventors have obtained the following findings.

[0016] (A) In the manufacturing process of bolts made from steel, a quenching process is performed to increase the strength of the bolt. Cu and Ni disperse in the matrix of the steel, improving the hardenability of the steel. Therefore, the strength of bolts manufactured from steel is further increased. However, even small amounts of Cu and Ni also increase the strength of the steel after the hot working process. As a result, the cold forgeability of the steel is reduced. On the other hand, if Mn sulfides are present near Cu and Ni, they concentrate in the Mn sulfides. Therefore, during the hot working process of the steel, the Cu and Ni concentrations locally decrease around the Mn sulfides, reducing the hardenability. As a result, the ferrite volume fraction of the steel increases, improving the cold forgeability. Here, Mn sulfides are defined as inclusions with an S content of 10% or more and an Mn content of 10% or more, as determined by element concentration analysis in mass% using EDX (energy dispersive x-ray diffraction) (described later).

[0017] (B) Cu and Ni in steel that did not concentrate into Mn sulfides during the hot working process are re-diffused during heating in the quenching process when manufacturing a bolt. If the number density of Mn sulfides in which Cu and Ni are concentrated (hereinafter referred to as "Cu-Ni-containing Mn sulfides") is already high before the quenching process, concentration of Cu and Ni into Mn sulfides is unlikely to occur during the quenching process. Therefore, localized deterioration of hardenability around the Mn sulfides is suppressed. As a result, sufficient strength can be ensured in bolts made from steel. In other words, in order to improve the cold forgeability of steel while maintaining the strength of bolts made from steel, it is effective to increase the number density of Cu-Ni-containing Mn sulfides in the steel. Here, the Cu-Ni-containing Mn sulfides are defined as inclusions having an S content of 10% or more, a total Cu and Ni content of 5% or more, and a Mn content of 10% or more, as determined by element concentration analysis in mass% using EDX, which will be described later.

[0018] (C) Mn sulfides in steel are generally coarse and elongated in the rolling direction of the steel. Such Mn sulfides become the starting point of cracks during cold forging. Among the Mn sulfides, the above-mentioned Cu—Ni-containing Mn sulfides and Ti-containing Mn sulfides, which are Mn sulfides containing Ti, are finer and less likely to elongate in the rolling direction of the steel compared to other Mn sulfides other than the Cu—Ni-containing Mn sulfides and the Ti-containing Mn sulfides (hereinafter referred to as "normal Mn sulfides"). Therefore, the Cu—Ni-containing Mn sulfides and the Ti-containing Mn sulfides are less likely to become the starting point of cracks during cold forging. In other words, by increasing the number density of the Cu—Ni-containing Mn sulfides and the number density of the Ti-containing Mn sulfides, the cold forgeability of the steel is improved. Here, the Ti-containing Mn sulfides are defined as inclusions having an S content of 10% or more, a total Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more, as determined by element concentration analysis in mass% using EDX, which will be described later.

[0019] (D) In ​​steel having the above-described chemical composition, Mn sulfides may be mainly composed of Cu-Ni-containing Mn sulfides, Ti-containing Mn sulfides, and ordinary Mn sulfides. In the Mn sulfide generation process, if the number density of Cu-Ni-containing Mn sulfides and the number density of Ti-containing Mn sulfides are high, the amount of S available for the growth of ordinary Mn sulfides decreases. In this case, the higher the number density of ordinary Mn sulfides, the more the coarsening of each ordinary Mn sulfide is suppressed. Therefore, the ordinary Mn sulfides also become finer. As a result, ordinary Mn sulfides are less likely to become the starting point of cracks during cold forging, further improving the cold forgeability of the steel.

[0020] Based on the above findings, the present inventors investigated and examined the relationship between the number density of Mn sulfides, the number density of Cu—Ni-containing Mn sulfides in the Mn sulfides, the number density of Ti-containing Mn sulfides in the Mn sulfides, and cold forgeability. As a result, the number density (numbers / mm) of Mn sulfides having a circle equivalent diameter of 1.0 μm or more was found to be 0.01 μm. ２ ) to ND ０ is defined as the number density (pieces / mm ２ ) to ND １is defined as the number density (pieces / mm ２ ) to ND ２ When defined as １ (pcs / mm ２ ) is 1.00 or more and satisfies formulas (1) and (2), a steel material having excellent cold forgeability can be obtained. １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2)

[0021] The steel material of this embodiment and the machine structural component of this embodiment have been completed based on the above technical concept, and have the following configuration.

[0022] The steel material of the first configuration has, in mass %, C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, The alloy contains V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities. The alloy has an equivalent circle diameter of 1.0 μm or more, and the number density (pieces / mm ) of Mn sulfides having an S content of 10% or more and an Mn content of 10% or more by mass. ２ ) to ND ０ The number density (pieces / mm ) of Cu-Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% or more, in mass %, is defined as ２ ) to ND １The number density (pieces / mm ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more, is defined as ２ ) to ND ２ When defined as ND １ (pcs / mm ２ ) is 1.00 or more, and the formula (1) and the formula (2) are satisfied. １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2)

[0023] The steel material of the second configuration is the steel material of the first configuration, and contains, in mass%, one or more elements selected from the group consisting of B: 0.0001 to 0.0010%, Nb: 0.001 to 0.050%, V: 0.01 to 0.15%, Sb: 0.001 to 0.050%, As: 0.001 to 0.090%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

[0024] A machine structural part of a first configuration has, in mass %, C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, The alloy contains V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities. The alloy has an equivalent circle diameter of 1.0 μm or more, and the number density (pieces / mm ) of Mn sulfides having an S content of 10% or more and an Mn content of 10% or more by mass. ２ ) to ND ０ The number density (pieces / mm ) of Cu-Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% or more, in mass %, is defined as ２ ) to ND １ The number density (pieces / mm ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more, is defined as ２ ) to ND ２ When defined as ND １ (pcs / mm ２ ) is 1.00 or more, and the formula (1) and the formula (2) are satisfied. １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2)

[0025] The mechanical structural component of the second configuration is the mechanical structural component of the first configuration, and contains, in mass%, one or more elements selected from the group consisting of B: 0.0001 to 0.0010%, Nb: 0.001 to 0.050%, V: 0.01 to 0.15%, Sb: 0.001 to 0.050%, As: 0.001 to 0.090%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

[0026] The mechanical structural component of the third configuration is a mechanical structural component of the first or second configuration, and the mechanical structural component is a bolt.

[0027] The steel material and the machine structural component of this embodiment will be described below. In the following description, "%" regarding the element content means mass % unless otherwise specified.

[0028] [Features of the Steel Material of the Present Embodiment] The steel material of the present embodiment satisfies the following features: (Feature 1) The chemical composition, in mass %, is C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: (Feature 2) The number density ND of Cu-Ni-containing Mn sulfides is 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities. １ is 1.00 pieces / mm ２ That's all. (Feature 3) Number density of Mn sulfides ND ０ (pcs / mm ２ ) and the number density ND of Cu-Ni-containing Mn sulfides １ (pcs / mm ２ ) and the number density ND of Ti-containing Mn sulfides２ (pcs / mm ２ ) satisfy the formula (1) and the formula (2). １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2) Features 1 to 3 will be explained below.

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

[0030] C: 0.04% to less than 0.20% Carbon (C) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the C content is less than 0.04%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is 0.20% or more, the strength of the steel will be excessively increased even if the contents of other elements are within the ranges of this embodiment. As a result, the cold forgeability of the steel will be reduced. Therefore, the C content is 0.04% to less than 0.20%. The preferred lower limit of the C content is 0.06%, more preferably 0.08%. The preferred upper limit of the C content is 0.19%, more preferably 0.17%, and even more preferably 0.15%.

[0031] Si: 0.01 to 0.35% Silicon (Si) solid-solution strengthens steel. Si also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. If the Si content is less than 0.01%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.35%, the strength of the steel increases excessively even if the contents of other elements are within the ranges of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Si content is 0.01 to 0.35%. The preferred lower limit of the Si content is 0.02%, more preferably 0.03%. The preferred upper limit of the Si content is 0.32%, more preferably 0.30%, and even more preferably 0.25%.

[0032] Mn: 0.20 to 1.00% Manganese (Mn) solid-solution strengthens steel. Mn also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. If the Mn content is less than 0.20%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.00%, excessive coarse Mn sulfides are formed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel is reduced. Therefore, the Mn content is 0.20 to 1.00%. The preferred lower limit of the Mn content is 0.22%, more preferably 0.25%, and even more preferably 0.30%. The preferred upper limit of the Mn content is 0.95%, more preferably 0.90%.

[0033] Al: 0.001 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.001%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.100%, coarse Al-based inclusions are generated even if the contents of other elements are within the ranges of this embodiment. The coarse Al-based inclusions become the starting points for cracks during cold forging. As a result, the cold forgeability of the steel material is reduced. Therefore, the Al content is 0.001 to 0.100%. A preferred lower limit of the Al content is 0.005%, more preferably 0.010%. A preferred upper limit of the Al content is 0.080%, more preferably 0.070%. In this embodiment, the Al content refers to the total Al (Total-Al) content.

[0034] Ti: 0.001 to 0.100% Titanium (Ti) forms Ti-containing Mn sulfides. As described above, the formation of Ti-containing Mn sulfides suppresses coarsening of Mn sulfides. Furthermore, Ti-containing Mn sulfides are less likely to elongate during processing than normal Mn sulfides. Therefore, the occurrence of cracks originating from elongated Mn sulfides in cold forged steel is suppressed. As a result, the cold forgeability of the steel is improved. If the Ti content is less than 0.001%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content exceeds 0.100%, coarse Ti carbides, Ti nitrides, and Ti carbonitrides are formed, even if the contents of other elements are within the ranges of this embodiment. These Ti inclusions or Ti precipitates become the initiation points of cracks during cold forging. As a result, the cold forgeability of the steel is reduced. Therefore, the Ti content is 0.001 to 0.100%. The lower limit of the Ti content is preferably 0.003%, more preferably 0.010%, and even more preferably 0.015%. The upper limit of the Ti content is preferably 0.080%, and even more preferably 0.070%.

[0035] Cu: 0.01 to 0.40% Copper (Cu) solid-solution strengthens steel. Cu also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. If the Cu content is less than 0.01%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cu content exceeds 0.40%, the strength of the steel becomes excessively high even if the contents of other elements are within the ranges of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Cu content is 0.01 to 0.40%. The preferred lower limit of the Cu content is 0.02%, more preferably 0.04%. The preferred upper limit of the Cu content is 0.38%, more preferably 0.34%, and even more preferably 0.30%.

[0036] Ni: 0.01 to 0.30% Nickel (Ni) solid-solution strengthens steel. Ni also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. If the Ni content is less than 0.01%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content exceeds 0.30%, the strength of the steel becomes excessively high even if the contents of other elements are within the ranges of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Ni content is 0.01 to 0.30%. The preferred lower limit of the Ni content is 0.02%, and more preferably 0.04%. The preferred upper limit of the Ni content is 0.29%, more preferably 0.27%, even more preferably 0.24%, and even more preferably 0.20%.

[0037] Cr: 0.01 to 0.30% Chromium (Cr) solid-solution strengthens steel. Cr also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. If the Cr content is less than 0.01%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 0.30%, the strength of the steel becomes excessively high even if the contents of other elements are within the ranges of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Cr content is 0.01 to 0.30%. The preferred lower limit of the Cr content is 0.02%, and more preferably 0.04%. The preferred upper limit of the Cr content is 0.27%, more preferably 0.25%, even more preferably 0.20%, and even more preferably 0.15%.

[0038] Mo: 0.001 to 0.200% Molybdenum (Mo) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the Mo content is less than 0.001%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content exceeds 0.200%, the strength of the steel becomes excessively high, even if the contents of other elements are within the ranges of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Mo content is 0.001 to 0.200%. The preferred lower limit of the Mo content is 0.005%, more preferably 0.008%, and even more preferably 0.010%. The preferred upper limit of the Mo content is 0.180%, more preferably 0.150%, and even more preferably 0.100%.

[0039] Sn: 0.001 to 0.100% Tin (Sn) segregates at the interface between the matrix and Mn sulfides, embrittling the steel material. This improves the machinability of the steel material. If the Sn content is less than 0.001%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. However, if the Sn content exceeds 0.100%, excessive Sn segregation occurs. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material decreases. Therefore, the Sn content is 0.001 to 0.100%. The preferred lower limit of the Sn content is 0.002%, and more preferably 0.004%. The preferred upper limit of the Sn content is 0.092%, more preferably 0.090%, even more preferably 0.080%, and even more preferably 0.070%.

[0040] P: 0.040% or less Phosphorus (P) is an impurity. If the P content exceeds 0.040%, P segregates excessively at grain boundaries, reducing grain boundary strength. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material decreases. Therefore, the P content is 0.040% or less. The lower the P content, the better. However, excessively reducing the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the P content is 0.035%, even more preferably 0.030%, and even more preferably 0.025%.

[0041] S: 0.040% or less Sulfur (S) is an impurity. S combines with Mn to form Mn sulfides. If the S content exceeds 0.040%, excessive coarse Mn sulfides are formed. These coarse Mn sulfides become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced. Therefore, the S content is 0.040% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the S content is 0.035%, more preferably 0.030%, and even more preferably 0.025%.

[0042] N: 0.0150% or less Nitrogen (N) is an impurity. N combines with Al, Ti, B, etc. to form nitrides. If the N content exceeds 0.0150%, excessive coarse nitrides are formed. These coarse nitrides become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced. Therefore, the N content is 0.0150% or less. The N content is preferably as low as possible. However, excessive reduction of the N content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the N content is greater than 0%, more preferably 0.0001%, even more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit of the N content is 0.0130%, even more preferably 0.0100%, and even more preferably 0.0080%.

[0043] O: 0.0030% or less Oxygen (O) is an impurity. O combines with other elements in the steel to form oxides. If the O content exceeds 0.0030%, excessive coarse oxides are formed. These coarse oxides become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel is reduced. Therefore, the O content is 0.0030% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%. The preferred upper limit of the O content is 0.0027%, more preferably 0.0024%, and even more preferably 0.0020%.

[0044] The balance of the chemical composition of the steel material of this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material of this embodiment.

[0045] [Optional Elements] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of: B: 0-0.0010%, Nb: 0-0.050%, V: 0-0.15%, Sb: 0-0.050%, As: 0-0.050%, Pb: 0-0.090%, Ca: 0-0.0050%, and Mg: 0-0.0050%. All of these elements are optional elements. These optional elements will be described below.

[0046] [First Group: B, Nb, and V] The chemical composition of the steel material of the present embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of B, Nb, and V. All of these elements increase the strength of a bolt manufactured using the steel material as a raw material.

[0047] B: 0 to 0.0010% Boron (B) is an optional element and does not necessarily need to be present. In other words, the B content may be 0%. When present, that is, when the B content exceeds 0%, boron (B) improves the hardenability of the steel. As a result, the strength of bolts manufactured using the steel increases. Even if even a small amount of B is present, the above effect can be achieved to some extent. However, if the B content exceeds 0.0010%, coarse B nitrides are formed. These coarse B nitrides become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel decreases. Therefore, the B content is 0 to 0.0010%, and if present, the B content is 0.0010% or less. The preferred lower limit of the B content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The upper limit of the B content is preferably 0.0009%, more preferably 0.0008%, and even more preferably 0.0006%.

[0048] Nb: 0 to 0.050% Niobium (Nb) is an optional element and does not necessarily need to be contained. In other words, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as carbides, nitrides, and carbonitrides. Nb precipitates increase the strength of bolts manufactured using steel materials through precipitation strengthening. Even if even a small amount of Nb is contained, the above effect can be achieved to some extent. However, when the Nb content exceeds 0.050%, coarse Nb precipitates are formed. Coarse Nb precipitates become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced. Therefore, the Nb content is 0 to 0.050%, and when contained, the Nb content is 0.050% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.005%. The upper limit of the Nb content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0049] V: 0 to 0.15% Vanadium (V) is an optional element and does not necessarily need to be contained. In other words, the V content may be 0%. When vanadium (V) is contained, that is, when the V content exceeds 0%, V forms V precipitates such as carbides and carbonitrides. V precipitates increase the strength of bolts manufactured using the steel material through precipitation strengthening. Even if even a small amount of V is contained, the above effect can be achieved to some extent. However, when the V content exceeds 0.15%, coarse V precipitates are formed. These coarse V precipitates become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced. Therefore, the V content is 0 to 0.15%, and when contained, the V content is 0.15% or less. The preferred lower limit of the V content is 0.01%, more preferably 0.02%, and even more preferably 0.04%. The upper limit of the V content is preferably 0.13%, more preferably 0.10%, and even more preferably 0.08%.

[0050] [Second Group: Sb, As, and Pb] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Sb, As, and Pb in place of a portion of Fe. All of these elements improve the machinability of the steel material.

[0051] Sb: 0 to 0.050% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. If it is contained, that is, if the Sb content exceeds 0%, Sb segregates at the interface between the matrix and Mn sulfides, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Sb is contained, the above effect can be achieved to some extent. However, if the Sb content exceeds 0.050%, Sb segregates excessively. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel deteriorates. Therefore, the Sb content is 0 to 0.050%, and if contained, the Sb content is 0.050% or less. The preferred lower limit of the Sb content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The upper limit of the Sb content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0052] As: 0 to 0.050% Arsenic (As) is an optional element and does not necessarily need to be contained. That is, the As content may be 0%. If arsenic is contained, that is, if the As content exceeds 0%, As segregates at the interface between the matrix and Mn sulfides, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of As is contained, the above effect can be achieved to some extent. However, if the As content exceeds 0.050%, As segregates excessively. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel deteriorates. Therefore, the As content is 0 to 0.050%, and if contained, the As content is 0.050% or less. The preferred lower limit of the As content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The upper limit of the As content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0053] Pb: 0 to 0.090% Lead (Pb) is an optional element and does not necessarily need to be contained. That is, the Pb content may be 0%. If it is contained, that is, if the Pb content exceeds 0%, Pb segregates at the interface between the matrix and Mn sulfides, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Pb is contained, the above effects can be achieved to some extent. However, if the Pb content exceeds 0.090%, excessive Pb segregation occurs. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel deteriorates. Therefore, the Pb content is 0 to 0.090%, and if contained, the Pb content is 0.090% or less. The preferred lower limit of the Pb content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The upper limit of the Pb content is preferably 0.080%, more preferably 0.070%, and even more preferably 0.060%.

[0054] [Third Group: Ca and Mg] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Ca and Mg in place of a portion of Fe. Any of these elements refines Mn sulfides in the steel material and improves the cold forgeability of the steel material.

[0055] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be contained. In other words, the Ca content may be 0%. When Ca is contained, that is, when the Ca content is greater than 0%, Ca refines Mn sulfides. Therefore, the cold forgeability of the steel material is improved. Even if even a small amount of Ca is contained, the above effect can be achieved to some extent. However, when the Ca content exceeds 0.0050%, coarse Ca oxides are generated. These coarse Ca oxides become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced. Therefore, the Ca content is 0 to 0.0050%, and when contained, the Ca content is 0.0050% or less. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Ca content is preferably 0.0040%, and more preferably 0.0030%.

[0056] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When Mg is contained, that is, when Mg exceeds 0%, Mg refines Mn sulfides. Therefore, the cold forgeability of the steel material is improved. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, coarse Mg oxides are generated. These coarse Mg oxides become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced. Therefore, the Mg content is 0 to 0.0050%, and when contained, the Mg content is 0.0050% or less. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Mg content is preferably 0.0040%, and more preferably 0.0030%.

[0057] [(Feature 2) Number density of Cu-Ni-containing Mn sulfides ND １As described above, in the element concentration analysis in mass% using EDX described later, inclusions (particles) having an S content of 10% or more, a Cu and Ni total content of 5% or more, and an Mn content of 10% or more are defined as Cu-Ni-containing Mn sulfides. In the steel material of this embodiment, the number density ND of Cu-Ni-containing Mn sulfides having a circle equivalent diameter of 1.0 μm or more is further defined as １ (pcs / mm ２ ) is 1.00 or more.

[0058] When normal Mn sulfides exist in the vicinity of Cu and Ni, Cu and Ni are concentrated in the normal Mn sulfides. As a result, Cu-Ni-containing Mn sulfides are formed. As described above, in order to improve the cold forgeability of steel while maintaining the strength of bolts made from steel, it is necessary to control the number density ND of Cu-Ni-containing Mn sulfides. １ (pcs / mm ２ The number density ND of Cu-Ni-containing Mn sulfides is preferably high. １ (pcs / mm ２ ) is 1.00 or more, excellent cold forgeability can be obtained on the premise that the steel material satisfies Features 1 and 3.

[0059] Number density ND １ The lower limit of the number density ND is preferably 1.10, more preferably 1.30, and even more preferably 1.50. １ When the steel material satisfies the characteristics 1 and 3, the number density ND １ The upper limit is, for example, 5.00, for example, 4.00, for example, 3.00.

[0060] [(Feature 3) Equation (1) and Equation (2)] As described above, in the element concentration analysis by mass% using EDX described later, inclusions (particles) having an S content of 10% or more and an Mn content of 10% or more are defined as Mn sulfides. In the element concentration analysis by mass% using EDX described later, inclusions (particles) having an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and an Mn content of 10% or more are defined as Ti-containing Mn sulfides. In the steel material of this embodiment, the number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more is further defined as ０(pcs / mm ２ ) and the number density ND of Cu-Ni-containing Mn sulfides having a circle equivalent diameter of 1.0 μm or more １ (pcs / mm ２ ) and the number density ND of Ti-containing Mn sulfides having a circle equivalent diameter of 1.0 μm or more ２ (pcs / mm ２ ) satisfy the formula (1) and the formula (2). １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2) Equations (1) and (2) will be explained below.

[0061] [Formula (1)] F1=ND １ +ND ２ As described above, Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides are finer than normal Mn sulfides and are less likely to elongate in the axial direction of the steel material. Therefore, Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides are less likely to become the starting point of cracks during cold forging. Furthermore, if the number density of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides is high, the amount of S used for the growth of normal Mn sulfides is reduced. Therefore, on the premise that the above-mentioned formula (2) is satisfied, coarsening of normal Mn sulfides is suppressed. As a result, the cold forgeability of the steel material is improved. Number density ND of Cu-Ni-containing Mn sulfides １ and the number density ND of Ti-containing Mn sulfides ２ When the total value F1 is 2.00 or more, the number density of the Cu-Ni-containing Mn sulfides and the Ti-containing Mn sulfides is sufficiently high, and therefore, the steel material can have excellent cold forgeability.

[0062] The lower limit of F1 is preferably 2.10, more preferably 2.20, even more preferably 2.40, and even more preferably 2.60. The upper limit of F1 is not particularly limited. When the steel material satisfies Features 1 and 2, the upper limit of F1 is, for example, 10.00, for example, 8.00, for example, 7.00, or for example, 6.50. F1 is a value obtained by rounding the obtained numerical value to two decimal places (i.e., the value to two decimal places).

[0063] [Formula (2)] F2 = (ND １ +ND ２ ) / ND ０ F2 means the ratio of the number density of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides to the number density of Mn sulfides. When the total value F1 of the number densities of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides satisfies the formula (1), the number density ND of Mn sulfides ０ If F2 is high, the number density of normal Mn sulfides is high. As described above, when the total value F1 of the number densities of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides satisfies formula (1), the amount of S available for the growth of normal Mn sulfides is reduced. If F2 is less than 0.25, the number density of normal Mn sulfides is sufficiently high in a state where the amount of S available for the growth of normal Mn sulfides is limited. Therefore, the normal Mn sulfides become sufficiently fine. Therefore, excellent cold forgeability can be obtained in steel, provided that formula (1) is satisfied.

[0064] The lower limit of F2 is not particularly limited. A preferred lower limit of F2 is 0.01, more preferably 0.03, even more preferably 0.04, and even more preferably 0.05. A preferred upper limit of F2 is 0.24, more preferably 0.23, even more preferably 0.22, and even more preferably 0.20. F2 is the value obtained by rounding the obtained numerical value to two decimal places (i.e., the value to one decimal place).

[0065] [Number density ND ０ , N.D. １ and N.D. ２ Measurement method for Mn sulfide number density ND ０ , the number density of Cu-Ni-containing Mn sulfides ND １ , and the number density ND of Ti-containing Mn sulfides ２is determined by the following method. Five test pieces are taken from a cross section including the axial and radial directions of the steel material, and have a surface including the R / 2 depth portion from the surface of the steel material. The R / 2 depth portion refers to the center of a line segment (i.e., radius R) connecting the surface of the steel material to the center position of the steel material. The size of each test piece is not particularly limited. Of the surfaces of each test piece, the surface that includes the axial and radial directions of the steel material and that includes the above-mentioned R / 2 depth portion is defined as the target surface. The R / 2 depth portion corresponds to the center position of the target surface.

[0066] The collected test specimen is embedded in resin. The target surface of the resin-embedded test specimen is polished. The observation area within the polished target surface is observed using a scanning electron microscope (SEM) equipped with composition analysis capabilities. The observation area is a rectangle of 1200 μm × 960 μm centered at the R / 2 depth. The long side of the observation area corresponds to the axial direction of the steel material. During observation, the observation area is divided into 36 non-overlapping fields of 200 μm × 160 μm, and each field is observed at a magnification of 500x.

[0067] In the observation area, particles (precipitates or inclusions) with a circle-equivalent diameter of 1.0 μm or more are identified based on the Z contrast of the backscattered electron image. In the backscattered electron image, particles appear with a darker contrast than the matrix. The circle-equivalent diameter refers to the diameter of a circle when the area of ​​a particle is converted into a circle with the same area. Each identified particle is subjected to elemental concentration analysis using energy dispersive X-ray spectroscopy (EDX) to identify Mn sulfides. The EDX analysis (elemental concentration analysis) uses the EDX-standardless method. The accelerating voltage is set to 20 kV, and the quantified elements are C, Si, Mn, P, S, Cr, Ti, Cu, Ni, Ca, N, O, and Al.

[0068] In the EDX analysis results of each particle, when the total content of the above quantified elements in mass% is taken as 100%, if the S content is 10% or more and the Mn content is 10% or more, by mass%, the particle is identified as a manganese sulfide. When the total content of the above quantified elements in mass% is taken as 100%, if the S content is 10% or more, the total content of Cu and Ni is 5% or more, and the Mn content is 10% or more, by mass%, the particle is identified as a Cu-Ni-containing manganese sulfide. When the total content of the above quantified elements in mass% is taken as 100%, if the S content is 10% or more, the total content of Cu and Ni is less than 5%, the Ti content is 10% or more, and the Mn content is 10% or more, by mass%, the particle is identified as a Ti-containing manganese sulfide.

[0069] The total number of Mn sulfides having a circle-equivalent diameter of 1.0 μm or more, which were identified in each observation region of the five test specimens, was calculated. Based on the total number of Mn sulfides obtained and the total area of ​​each observation region of the five test specimens, the number density ND of Mn sulfides having a circle-equivalent diameter of 1.0 μm or more was calculated. ０ (pcs / mm ２ ) is obtained. Similarly, the total number of Cu-Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, which are identified in each observation region of the five test specimens, is obtained. Based on the obtained total number of Cu-Ni-containing Mn sulfides and the total area of ​​each observation region of the five test specimens, the number density ND of Cu-Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more is calculated. １ (pcs / mm ２ Similarly, the total number of Ti-containing Mn sulfides having a circle-equivalent diameter of 1.0 μm or more, which are identified in each observation region of the five test specimens, is determined. Based on the obtained total number of Ti-containing Mn sulfides and the total area of ​​each observation region of the five test specimens, the number density ND of Ti-containing Mn sulfides having a circle-equivalent diameter of 1.0 μm or more is calculated. ２ (pcs / mm ２ ) is calculated. ０ , N.D. １ , and N.D. ２ is the value obtained by rounding the obtained numerical value to two decimal places (i.e., the value to two decimal places).

[0070] [Effects of the Steel Material of the Present Embodiment] As described above, the steel material of the present embodiment satisfies Features 1 to 3. Therefore, the steel material of the present embodiment has excellent cold forgeability.

[0071] [Shape of Steel Material of the Present Embodiment] The steel material of the present embodiment is a steel bar or wire rod. The steel bar or wire rod is a steel material that has a circular cross section perpendicular to the axial direction and extends in a rod shape. The steel material may be wound in a coil shape or may be cut to a predetermined length. The diameter of the cross section of the steel material is, for example, 4 to 20 mm.

[0072] [Uses of the steel material of this embodiment] The steel material of this embodiment can be used as a material for machine structural parts such as bolts. The steel material of this embodiment is particularly suitable as a material for bolts, which are fastening means for industrial machinery, automobiles, bridges, buildings, etc. The steel material of this embodiment may also be used for uses other than the above uses.

[0073] [Method for manufacturing steel material] An example of a method for manufacturing steel material according to this embodiment will be described. The method for manufacturing steel material described below is one example for manufacturing the steel material according to this embodiment. Therefore, steel material having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing steel material according to this embodiment.

[0074] An example of the method for producing a steel material according to this embodiment includes the following steps: (Step 1) Refining step, (Step 2) Casting step, and (Step 3) Hot working step.

[0075] Each step will be described below. [Step 1: Refining step] In the refining step, molten steel having a chemical composition that satisfies the above-mentioned feature 1 is produced. The refining method is not particularly limited, and a well-known method may be used. For example, molten pig iron produced by a well-known method is subjected to refining (primary refining) in a converter. Molten steel tapped from the converter is subjected to well-known secondary refining. Through the above steps, molten steel having a chemical composition that satisfies feature 1 is produced.

[0076] [(Step 2) Casting Step] In the casting step, the molten steel produced in the refining step is used to produce blooms (blooms) by continuous casting.

[0077] [(Step 3) Hot Working Step] In the hot working step, the bloom is hot worked to produce a steel material. The hot working step includes the following steps: (Step 31) Blooming Step (Step 32) Finishing Rolling Step Each step will be described below.

[0078] [(Step 31) Blooming Step] In the blooming step, the bloom is hot-rolled (blooming) using a blooming mill to produce a billet. If a continuous rolling mill having a plurality of rolling stands arranged in a line is disposed downstream of the blooming mill, the billet may be hot-rolled using the continuous rolling mill to further reduce the size of the billet. The heating temperature in the blooming step may be within a known temperature range. For example, the heating temperature is 1100 to 1300°C. The billet produced in the blooming step is allowed to cool (air-cool) to room temperature before the finish rolling step.

[0079] [(Step 32) Finish Rolling Step] In the finish rolling step, the billet is hot rolled (finish rolling) to produce the steel material of this embodiment. Specifically, the billet after the blooming step is heated using a heating furnace. The heating temperature is, for example, 1000 to 1250°C. The heated billet is hot rolled (finish rolling) using a finishing rolling mill to produce a steel material having a circular cross section with a desired diameter. The finishing rolling mill is, for example, a continuous rolling mill in which multiple rolling stands are arranged in a row so that the rolling direction of adjacent rolling stands is perpendicular.

[0080] [Manufacturing conditions in the manufacturing method of this embodiment] The manufacturing method described above satisfies the following conditions: (Condition 1) The total residence time t1 in the processes after the hot working process, where the material is heated to a temperature of 1200°C or higher, is 120 minutes or less. (Condition 2) Of the total reduction ratios TRR in the finish rolling process, the total reduction ratio RR in the finish rolling process where the steel material temperature is 980°C or lower is 120 minutes or less. ９８０ The ratio X (%) of is defined by the following formula: X = RR ９８０ / TRR×100 In this case, the following formula (A) is satisfied: X−√((Ti+Cu+Ni+1) / (2×S))≧0 (A) Here, the content of the corresponding element in mass% is substituted for each element symbol in formula (A). (Condition 3) In the finish rolling process, the maximum area reduction rate in one pass when the steel temperature is above 1000°C is defined as Y (%). In this case, the following formula (B) is satisfied: Y−15 / √(Cu+0.5×Ni+0.2)>0 (B) Here, the content of the corresponding element in mass% is substituted for each element symbol in formula (B). Conditions 1 to 3 will be explained below.

[0081] [Condition 1: Total residence time t1 at which the material is heated to 1200°C or higher] If the total residence time t1 at which the material is heated to 1200°C or higher exceeds 120 minutes in the steps subsequent to the hot working step (i.e., from the start of heating of the steel material in the blooming step, to the completion of finish rolling in the finish rolling step, and the cooling of the steel material after finish rolling to room temperature), Cu precipitates, Ni precipitates, and Ti precipitates will coarsen. The coarsening of these precipitates occurs due to Ostwald ripening of the precipitates. During the Ostwald ripening process, fine Cu precipitates, Ni precipitates, and Ti precipitates are taken up by the coarsened precipitates and disappear. Therefore, the amounts of Cu, Ni, and Ti available for forming Cu—Ni-containing Mn sulfides and Ti-containing Mn sulfides will decrease. As a result, the number density of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides decreases. In this case, the produced steel does not satisfy formula (1). Therefore, the total residence time t1 at 1200°C or higher is set to 120 minutes or less.

[0082] The preferred upper limit of the total residence time t1 at 1200°C or higher is 110 minutes, more preferably 100 minutes, and even more preferably 90 minutes. The lower limit of the total residence time t1 at 1200°C or higher is not particularly limited. The preferred lower limit is 0 minutes, but in consideration of industrial production, it is more preferably 10 minutes, even more preferably 20 minutes, and even more preferably 30 minutes. Note that if the heating temperature of the material does not reach 1200°C or higher in the processes after the hot working process, the total residence time t1 at 1200°C or higher is set to 0 minutes.

[0083] [Condition 2: Formula (A)] FA is defined as FA=X-√((Ti+Cu+Ni+1) / (2×S)). FA is an index that indicates the ease with which Mn sulfides are fragmented in the finish rolling process. In the finish rolling process, Mn sulfides tend to elongate in the rolling direction of the steel material. In the finish rolling process, Mn sulfides are likely to elongate, particularly when the steel material temperature is 980°C or lower. Furthermore, compared with Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides, normal Mn sulfides are significantly more likely to elongate.

[0084] If FA satisfies formula (A), a sufficient amount of rolling reduction is applied to the steel material at a steel material temperature of 980°C or less. In this case, in particular, Mn sulfides are usually elongated in the rolling direction and then broken into multiple pieces to be refined, reducing the number of coarse Mn sulfides that serve as crack initiation points during cold forging. As a result, the number density of Mn sulfides increases compared to the number densities of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides, and the manufactured steel material satisfies formula (2).

[0085] The lower limit of FA is preferably 0.5, more preferably 0.8, and even more preferably 1.0. The upper limit of FA is not particularly limited, but is preferably 10.0, more preferably 8.0, even more preferably 6.0, and even more preferably 5.0. Taking into account normal industrial production, the lower limit of X is, for example, 3.0%, and the upper limit of X is, for example, 15.0%.

[0086] [Condition 3: Formula (B)] FB is defined as FB=Y-15 / √(Cu+0.5×Ni+0.2). FB is an index that represents the ease with which Cu and Ni concentrate in Mn sulfides in the finish rolling process. In hot rolling using a finish rolling mill equipped with multiple rolling stands, the area reduction rate for each rolling stand of a billet passing through each rolling stand is defined as the "area reduction rate in one pass."

[0087] In the finish rolling process, when the steel temperature exceeds 1000°C, Cu and Ni in the steel are likely to diffuse. Therefore, by performing rolling at a large area reduction rate at a steel temperature exceeding 1000°C, the diffusion of Cu and Ni is further promoted, and the concentration of Cu and Ni in Mn sulfides is promoted. If FB is 0 or less, the concentration of Cu and Ni in Mn sulfides is insufficient. In this case, the steel does not satisfy Feature 2. Therefore, FB is set to be greater than 0.

[0088] The lower limit of FB is preferably 0.1, more preferably 0.2, even more preferably 0.3, and even more preferably 0.4. The upper limit of FB is not particularly limited, but is preferably 15.0, more preferably 12.0, and even more preferably 8.0. Taking into account normal industrial production, the lower limit of Y is, for example, 15.0%, and the upper limit of Y is, for example, 35.0%.

[0089] The steel material of this embodiment is manufactured by the above manufacturing process.

[0090] [Regarding the Machine Structural Component of the Present Embodiment] The machine structural component of the present embodiment is made of the steel material of the present embodiment. The machine structural component of the present embodiment is, for example, a bolt. The machine structural component of the present embodiment may also be, for example, a nut. The machine structural component of the present embodiment may also be, for example, a hollow component, or a cup-shaped component with one end open.

[0091] [Features of the machine structural component of this embodiment] The machine structural component of this embodiment satisfies the following feature: (Feature 4) The chemical composition, in mass %, is C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: (Feature 5) The number density ND of Cu-Ni-containing Mn sulfides is 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities. １ is 1.00 pieces / mm ２ That's all. (Feature 6) Number density of Mn sulfides (ND) ０ (pcs / mm ２ ) and the number density ND of Cu-Ni-containing Mn sulfides １ (pcs / mm ２ ) and the number density ND of Ti-containing Mn sulfides ２ (pcs / mm ２ ) satisfy the formula (1) and the formula (2). １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2)

[0092] Number density of Mn sulfides in machine structural parts (ND) ０ , the number density of Cu-Ni-containing Mn sulfides ND １ , and the number density ND of Ti-containing Mn sulfides ２ is the number density ND ０ , N.D. １ and N.D. ２The measurement is performed based on the method described in [Measurement method for the mechanical structural component]. However, the test piece used for the measurement is taken so that the cross section including the depth direction from the surface of the mechanical structural component is the target surface. The center position of the target surface corresponds to a position 1 mm deep from the surface of the mechanical structural component. The observation area on the target surface is a rectangle of 1200 μm × 960 μm centered at a position 1 mm deep from the surface of the mechanical structural component. The long side of the observation area is perpendicular to the depth direction from the surface of the mechanical structural component.

[0093] The function of each element in Feature 4 is the same as the function of the corresponding element in Feature 1 of the steel material of this embodiment. The technical significance of Feature 5 is the same as that of Feature 2 of the steel material of this embodiment. The technical significance of Feature 6 is the same as that of Feature 3 of the steel material of this embodiment. Therefore, the machine structural component of this embodiment, which satisfies Features 4 to 6, can achieve excellent cold forgeability in the manufacturing process.

[0094] [Method for manufacturing a machine structural component] The machine structural component of this embodiment is manufactured by a known method using the steel material of this embodiment as a raw material. The chemical composition of the machine structural component manufactured by a known method is the same as the chemical composition of the steel material used as the raw material. In addition, the number density ND of Mn sulfides in the machine structural component manufactured by a known method ０ , the number density of Cu-Ni-containing Mn sulfides ND １ , and the number density ND of Ti-containing Mn sulfides ２ Therefore, the machine structural component of this embodiment can satisfy Features 4 to 6 if it is manufactured by a known method using the steel material of this embodiment that satisfies Features 1 to 3 as a raw material.

[0095] As an example of a machine structural component of this embodiment, a manufacturing process for a bolt will be described. The manufacturing method for a bolt, which is a machine structural component of this embodiment, includes, for example, a wiredrawing process, a cold forging process, and a quenching and tempering process. In the wiredrawing process, a steel wire is manufactured by performing a well-known wiredrawing process on the steel material of this embodiment. The wiredrawing process may be only a primary wiredrawing process, or multiple wiredrawing processes, such as a secondary wiredrawing process, may be performed. In the cold forging process, a well-known cold forging (heading) is performed on the steel wire after the wiredrawing process to manufacture an intermediate product in the shape of a bolt. In the quenching and tempering process, the intermediate product is quenched and tempered. Quenching is performed by a well-known method. The quenching temperature is, for example, 840 to 970°C. The holding time at the quenching temperature is, for example, 15 to 360 minutes (6 hours). After the holding time has elapsed, the intermediate product is quenched. Specifically, the intermediate product is water-cooled or oil-cooled. The quenched intermediate product is tempered. The tempering temperature is, for example, 400 to 600° C. The holding time at the tempering temperature is, for example, 0.5 to 6.0 hours.

[0096] The bolt, which is the machine structural component of this embodiment, can be manufactured by the above-described manufacturing method. The machine structural component of this embodiment has excellent cold forgeability in the manufacturing process. Therefore, in the manufacturing process of the machine structural component of this embodiment, cracking of the material and deterioration of the mold during cold forging are suppressed even without performing heat treatment for softening before cold forging.

[0097] The effects of the steel material of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

[0098] A known refining process was carried out to produce steel materials having the chemical compositions shown in Table 1 (Tables 1A and 1B).

[0099]

[0100]

[0101] Specifically, blooms were produced using the produced molten steel by continuous casting. The produced blooms were subjected to blooming to produce billets. In the blooming process, the blooms were heated to 1100 to 1300°C and then hot rolled using a blooming mill. The produced billets were allowed to cool to room temperature. The produced billets were subjected to a finish rolling process. In the finish rolling process, the billets were heated to 1000 to 1250°C. The heated billets were hot rolled using a finish rolling mill to produce wire rods (steel materials) with a diameter of 10.0 mm. Steel materials with each test number were produced using the above production process.

[0102] The total residence time t1 (minutes) during which the temperature of the steel (bloom, billet, and steel material) was 1200°C or higher from the start of heating of the bloom in the blooming process until the steel material was cooled to room temperature after the finish rolling process was as shown in the "t1 (minutes)" column in Table 2. In the finish rolling process, the total area reduction rate RR during the finish rolling process when the steel material temperature was 980°C or lower ９８０ The ratios X (%) and FA were as shown in the "X (%)" and "FA" columns in Table 2. In the finish rolling step, the maximum area reduction rates Y (%) and FB in one pass when the billet temperature exceeded 1000°C were as shown in the "Y (%)" and "FB" columns in Table 2.

[0103]

[0104] [Evaluation Tests] The following evaluation tests were carried out on the manufactured steel materials with each test number. (Test 1) Number Density ND ０ , N.D. １ and N.D. ２ Measurement Tests (Test 2) Limit Compression Test Each test will be explained below.

[0105] [(Test 1) Number Density ND ０ , N.D. １ and N.D. ２ Measurement Test] The above-mentioned [Number Density ND ０ , N.D. １ and N.D. ２ Based on the method described in [Measurement method for the number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more]０ (pcs / mm ２ ), the number density ND of Cu—Ni-containing Mn sulfides having a circle equivalent diameter of 1.0 μm or more １ (pcs / mm ２ ), and the number density ND of Ti-containing Mn sulfides having a circle equivalent diameter of 1.0 μm or more ２ (pcs / mm ２ The results are shown in Table 2 as "ND ０ (pcs / mm ２ )” column, “ND １ (pcs / mm ２ " column and "ND ２ (pcs / mm ２ ) column.

[0106] [(Test 2) Limit Compression Test] To evaluate cold forgeability, a limit compression test was conducted to determine the limit compression ratio of the steel material. Specifically, multiple limit compression ratio measurement specimens were taken from the steel material (wire rod) of each test number. The limit compression ratio measurement specimens were cylindrical, with a diameter of 8 mm and a length of 12 mm. The longitudinal direction of the limit compression ratio measurement specimen was parallel to the axial direction of the steel material of each test number. The central axis of the limit compression ratio measurement specimen corresponded to the central axis of the steel material of each test number. A single longitudinal notch was formed on the side (circumferential surface) of the test piece. The notch angle was 30 degrees, the notch depth was 0.46 mm, and the radius of curvature of the notch tip was 0.15 mm. The notch length was 12 mm, the same as the length of the test piece. Furthermore, a recess was formed on each of the pair of end faces of the test piece to secure the test piece to an end face restraint die. The depressions were formed at the center of each end face and had a conical shape. The diameter of the opening of the depression at the end face (corresponding to the base of the cone) was 2 mm, and the angle of the apex in a cross section including the central axis of the conical depression was 120°.

[0107] A 500-ton hydraulic press was used for the limit compression test. The limit compression test was performed on the prepared limit compression ratio measurement test pieces using the following method. Each test piece was cold compressed at a speed of 15 mm / s using an end-face restraint die having a convex portion corresponding to the depression of the test piece. Compression was stopped when microcracks of 0.5 mm or more occurred at the notch bottom, and the compression ratio (%) at that time was calculated. This measurement was performed a total of five times to determine the compression ratio (%) at which the cumulative failure probability was 50%. When the limit compression ratio was 55% or more, it was evaluated as "E (Excellent)" and it was determined that excellent cold forgeability was obtained. On the other hand, when the limit compression ratio was less than 55%, it was evaluated as "NA (Not Accepted)" and it was determined that excellent cold forgeability was not obtained. The evaluation results are shown in the "Limit Compressibility" column in Table 2.

[0108] [Test Results] With reference to Tables 1A, 1B, and 2, the steel materials of Test Nos. 1 to 15 satisfied Features 1 to 3. Therefore, the steel materials of these Test Nos. obtained excellent cold forgeability.

[0109] On the other hand, in test number 16, the C content was too high, and as a result, excellent cold forgeability was not obtained.

[0110] In test number 17, the Si content was too high, and as a result, excellent cold forgeability was not obtained.

[0111] In test number 18, the Mn content was too high, and as a result, excellent cold forgeability was not obtained.

[0112] In test number 19, the Al content was too high, and as a result, excellent cold forgeability was not obtained.

[0113] In test number 20, the Ti content was too high, and as a result, excellent cold forgeability was not obtained.

[0114] In test number 21, the Cu content was too high. Furthermore, FA was too low. Therefore, F2 did not satisfy formula (2). As a result, excellent cold forgeability was not obtained.

[0115] In test number 22, the Ni content was too high. Furthermore, FA was too low. Therefore, F2 did not satisfy formula (2). As a result, excellent cold forgeability was not obtained.

[0116] In test number 23, the Cr content was too high, and as a result, excellent cold forgeability was not obtained.

[0117] In test number 24, the Mo content was too high, and as a result, excellent cold forgeability was not obtained.

[0118] In test number 25, the Sn content was too high, and as a result, excellent cold forgeability was not obtained.

[0119] In test number 26, the P content was too high, and as a result, excellent cold forgeability was not obtained.

[0120] In test number 27, the S content was too high, and as a result, excellent cold forgeability was not obtained.

[0121] In test number 28, the N content was too high, and as a result, excellent cold forgeability was not obtained.

[0122] In test number 29, the O content was too high, and as a result, excellent cold forgeability was not obtained.

[0123] In test numbers 30 and 31, the total residence time t1 at 1200°C or higher was too long, so F1 did not satisfy formula (1). As a result, excellent cold forgeability was not obtained.

[0124] In test numbers 32 and 33, FA was too low, so F2 did not satisfy formula (2), and as a result, excellent cold forgeability was not obtained.

[0125] In test numbers 34 and 35, FB was too low. １ As a result, good cold forgeability was not obtained.

[0126] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

Claims

1. In mass%, C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, The alloy contains V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities. The alloy has an equivalent circle diameter of 1.0 μm or more, and the number density (pieces / mm ) of Mn sulfides having an S content of 10% or more and an Mn content of 10% or more by mass. ２ ) to ND ０ The number density (pieces / mm ) of Cu-Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% or more, in mass %, is defined as ２ ) to ND １ The number density (pieces / mm ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more, is defined as ２ ) to ND ２ When defined as ND １ (pcs / mm ２ ) is 1.00 or more, and satisfies formula (1) and formula (2). １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2) 2. The steel material according to claim 1, containing, in mass%, one or more elements selected from the group consisting of B: 0.0001 to 0.0010%, Nb: 0.001 to 0.050%, V: 0.01 to 0.15%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Pb: 0.001 to 0.090%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

3. In mass%, C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, The alloy contains V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities. The alloy has an equivalent circle diameter of 1.0 μm or more, and the number density (pieces / mm ) of Mn sulfides having an S content of 10% or more and an Mn content of 10% or more by mass. ２ ) to ND ０ The number density (pieces / mm ) of Cu-Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% or more, in mass %, is defined as ２ ) to ND １ The number density (pieces / mm ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more, is defined as ２ ) to ND ２ When defined as ND １ (pcs / mm ２ ) is 1.00 or more, and satisfies formula (1) and formula (2). １ +ND ２ ≧2.00 (1) (ND １ +ND ２ ) / ND ０ <0.25 (2) 4. A machine structural part according to claim 3, containing, in mass%, one or more elements selected from the group consisting of B: 0.0001 to 0.0010%, Nb: 0.001 to 0.050%, V: 0.01 to 0.15%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Pb: 0.001 to 0.090%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

5. A machine structural component according to claim 3 or 4, wherein the machine structural component is a bolt.

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