Steel material

Abstract

Provided is a steel material that has exceptional chip disposal performance and can achieve exceptional fatigue strength when the steel material is carburized to produce machine structural parts. A steel material according to the present disclosure has a chemical composition containing, in mass%, 0.15-0.25% of C, 0.15-0.50% of Si, 0.70-1.30% of Mn, 0.030% or less of P, 0.005-0.035% of S, 1.40-1.70% of Cr, 0.020-0.060% of Al, and 0.0100-0.0200% of N, the balance comprising Fe and impurities. The steel material satisfies formulas (1) and (2), and, in a cross-section that includes the central axis of the steel material and is parallel to the central axis, the number density ND of MnS having a maximum central-axis-direction length of 1-50 μm is 100 / mm2 or greater.　Formula (1): −34.85 × (10Si + 0.54Cr / (2.5Cr +2Mn + 5Mo) + 820 ≥ 788; Formula (2): [ND] × {Cr / (Mn + Mo)}0.5 ≤ 255

Description

steel material

[0001] The present disclosure relates to steel materials, and more particularly to steel materials suitable as materials for machine structural components.

[0002] Machine structural parts are used in automobiles, construction vehicles, mining machinery, etc. Examples of machine structural parts include gears, shafts, and cogwheels. As materials for machine structural parts, alloy steel materials for machine structures, such as SCr420, SCM420, and SNCM420 specified in JIS G 4053 (2023), are used.

[0003] Machine structural components made from these steel materials are manufactured, for example, by the following manufacturing process. The steel material is subjected to hot working (hot forging), and then cutting is performed to manufacture an intermediate product of the desired shape. The intermediate product is subjected to surface hardening treatment to increase the hardness of the surface layer. Examples of surface hardening treatment include quenching and tempering, carburizing treatment, or carbonitriding treatment. Machine structural components are manufactured by the above manufacturing process.

[0004] Mechanical structural components are subjected to repeated compressive and tensile loads during use. Therefore, mechanical structural components are required to have excellent fatigue strength. Furthermore, as mentioned above, in the manufacturing process of mechanical structural components, the steel material from which the mechanical structural components are made is cut after hot working. Therefore, the steel material from which the mechanical structural components are made is required to have excellent machinability.

[0005] Japanese Patent Laid-Open Publication No. 2011-089189 (Patent Document 1) proposes a technology for obtaining excellent machinability in steel materials and excellent fatigue strength in machine structural parts manufactured using steel materials as raw materials.

[0006] The steel material disclosed in Patent Document 1 contains, by mass%, 0.15 to 0.25% C, 0.15 to less than 0.50% Si, 0.70 to 1.30% Mn, 0.015% or less S, more than 1.25% to 1.80% Cr, 0.005 to 0.035% Al, and 0.010 to 0.025% N, with the balance being Fe and impurities, fn1 = Si + Cr being 1.50 to 1.85, 95% or more of the microstructure being ferrite and pearlite, and the area fraction of ferrite being 40 to 60%. Patent Document 1 states that by adjusting the Si and Cr contents and forming the microstructure as described above, this steel material exhibits excellent machinability and excellent fatigue strength in machine structural components.

[0007] JP 2011-089189 A

[0008] As mentioned above, in the manufacturing process of machine structural parts, surface hardening is performed to increase fatigue strength. Among surface hardening processes, carburizing can increase the hardness of the surface layer by increasing the C concentration in the surface layer through carburizing, even when the C content of the steel is low, at 0.15 to 0.25%, and then quenching. Therefore, carburizing is sometimes performed in the manufacturing process of machine structural parts made from steel with a low C content.

[0009] There are two types of carburizing processes: gas carburizing and vacuum carburizing. However, vacuum carburizing requires equipment to create a vacuum inside the furnace, which increases manufacturing costs. For this reason, gas carburizing is usually used as the carburizing process.

[0010] However, when machine structural parts are manufactured by carrying out gas carburizing treatment, there are cases in which the machine structural parts do not have sufficient fatigue strength.

[0011] Furthermore, as described above, when manufacturing machine structural parts using steel as a raw material, cutting is performed on the hot-worked steel, which requires chip disposability that allows chips generated during cutting to be broken into pieces of an appropriate length.

[0012] An object of the present disclosure is to provide a steel material that has excellent chip disposal properties and that, when gas carburized to form machine structural parts, exhibits excellent fatigue strength.

[0013] The steel material of the present disclosure has a chemical composition, in mass%, of C: 0.15 to 0.25%, Si: 0.15 to 0.50%, Mn: 0.70 to 1.30%, P: 0.030% or less, S: 0.005 to 0.035%, Cr: 1.40 to 1.70%, Al: 0.020 to 0.060%, N: 0.0100 to 0.0200%, Mg: 0 to 0.0100%, Ti: 0 to 0.150%, Nb: 0 to 0.080%, W: 0 to 0.400%, Zr: 0 to 0.200%, V: 0 to 0.400%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, Sn: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, Sb: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.10%, Cu: 0 to 0.20%, and Ni: 0 to 0.20%, with the balance being Fe and impurities, and satisfying formulas (1) and (2), the number density ND of MnS having a maximum length in the central axis direction of 1 to 50 μm in a cross section that includes a central axis of the steel material and is parallel to the central axis is 100 pieces / mm ２ -34.85 × (10Si + 0.54Cr) / (2.5Cr + 2Mn + 5Mo) + 820 ≧ 788 (1) [ND] × {Cr / (Mn + Mo)} ０．５ ≦255 (2) Here, the content of the corresponding element in the chemical composition in mass % is substituted for each element symbol in formula (1) and formula (2). When an element is not contained, "0" is substituted for the corresponding element symbol. [ND] in formula (2) represents the number density ND (pieces / mm ２ ) is substituted.

[0014] The steel material according to the present disclosure has excellent chip disposal properties, and when gas carburized to form machine structural parts, it has excellent fatigue strength.

[0015] FIG. 1 is a side view of an Ono-type rotating bending fatigue test piece for evaluating fatigue strength in the examples.

[0016] The present inventors first conducted a study from the standpoint of chemical composition on steel materials that have excellent chip disposability and exhibit excellent fatigue strength when gas carburized to form machine structural parts. As a result, the inventors have found that the chemical composition is, in mass%, C: 0.15 to 0.25%, Si: 0.15 to 0.50%, Mn: 0.70 to 1.30%, P: 0.030% or less, S: 0.005 to 0.035%, Cr: 1.40 to 1.70%, Al: 0.020 to 0.060%, N: 0.0100 to 0.0200%, Mg: 0 to 0.0100%, Ti: 0 to 0.150%, Nb: 0 to 0.080%, W: 0 to 0.400%, Zr: 0 to 0.200%, V: 0 to 0.400%, Ca: 0 to 0.0100%, Te: 0 to 0. The present inventors considered that a steel material having a chemical composition containing the following elements: 0-0.0100%, B: 0-0.0050%, Sn: 0-0.0100%, rare earth elements: 0-0.0100%, Co: 0-0.0100%, Se: 0-0.0100%, Sb: 0-0.0100%, In: 0-0.0100%, Mo: 0-0.10%, Cu: 0-0.20%, and Ni: 0-0.20%, with the balance being Fe and impurities, would have excellent chip disposability and, when gas carburized to form a machine structural part, could potentially obtain excellent fatigue strength.

[0017] However, even when a steel material satisfies the above-mentioned chemical composition, there are cases in which sufficient chip disposability and fatigue strength cannot be obtained. Therefore, the present inventors have further investigated means for improving chip disposability and fatigue strength. As a result, the present inventors have obtained the following findings.

[0018] The present inventors first investigated means for improving the chip disposability of steel materials. In steel materials satisfying the above-mentioned chemical composition, multiple MnS particles are generated in the steel material. If MnS particles are present at an appropriate number density, cracks will occur at the interfaces between the multiple MnS particles and the matrix during cutting. Furthermore, cracks may propagate along the interfaces between the multiple MnS particles and the matrix, or multiple cracks generated at multiple interfaces between the MnS particles and the matrix may connect and propagate further. As a result, chips become more easily separated from the steel material.

[0019] Based on the above findings, the present inventors have investigated and considered the appropriate size and number density of MnS. As a result, the present inventors have obtained the following findings: In a steel material having the above chemical composition, the number density ND of MnS having a maximum length in the central axis direction of 1 to 50 μm in a cross section that includes the central axis of the steel material and is parallel to the central axis is 100 pieces / mm ２ If the above conditions are met, chips of an appropriate length will separate from the base material during cutting, resulting in high chip disposability of the steel material.

[0020] The present inventors then investigated means for increasing the fatigue strength of machine structural parts. When gas carburizing is performed as a surface hardening treatment for machine structural parts, grain boundary oxides may be generated. If grain boundary oxides are generated, an incompletely hardened layer is formed during quenching, resulting in a decrease in fatigue strength. Therefore, the present inventors focused on the grain boundary oxides generated during gas carburizing.

[0021] In gas carburizing, oxides of Si and Cr in the surface layer of steel are likely to form at grain boundaries as grain boundary oxides. The Si and Cr concentrations decrease around the grain boundary oxides. As a result, even after carburizing and quenching, an incompletely hardened layer is formed. In machine structural parts with an incompletely hardened layer, the strength of the surface layer is insufficient, resulting in reduced fatigue strength. Therefore, the inventors have considered that reducing the Si and Cr contents, which form grain boundary oxides during gas carburizing, is an effective way to suppress the decrease in fatigue strength due to gas carburizing.

[0022] On the other hand, Si and Cr are also elements that enhance hardenability. Therefore, simply reducing the Si and Cr contents may suppress grain boundary oxidation, but will result in a decrease in hardenability. Therefore, the present inventors focused on Mn and Mo, which are elements effective in enhancing hardenability. Mn and Mo are elements that are less susceptible to oxidation than Si and Cr. Therefore, the present inventors believed that by appropriately adjusting the relationship between the contents of Si, Cr, Mn, and Mo, it would be possible to suppress the generation of grain boundary oxides while ensuring the hardenability of the steel material.

[0023] Based on the above findings, it was discovered that if formula (1) is satisfied in a steel material having the above chemical composition, excellent fatigue strength can be obtained when gas carburizing is performed to form a machine structural part: -34.85 x (10Si + 0.54Cr) / (2.5Cr + 2Mn + 5Mo) + 820 ≥ 788 (1) Here, the content of the corresponding element in the chemical composition in mass % is substituted for each element symbol in formula (1). If an element is not contained, "0" is substituted for the corresponding element symbol.

[0024] However, even if the steel material having the above chemical composition satisfies formula (1), there are cases where excellent fatigue strength is not obtained when gas carburizing is performed to form a machine structural part. The present inventors have further investigated means for improving fatigue strength. As a result, the present inventors have obtained the following findings.

[0025] The present inventors investigated the reason why excellent fatigue strength was not obtained for machine structural components. When machine structural components for which excellent fatigue strength was not obtained were observed, it was found that cracks were generated and propagated from MnS in the machine structural components.

[0026] As mentioned above, MnS improves chip disposability. However, MnS can be the starting point for crack initiation in machine structural components. Therefore, to suppress crack initiation, it is necessary to limit the number density of MnS to some extent. Furthermore, as mentioned above, the formation of grain boundary oxides also contributes to a decrease in fatigue strength. Therefore, when adjusting the number density of MnS, it is necessary to consider the degree of grain boundary oxide formation.

[0027] As described above, Cr improves hardenability but is prone to the formation of grain boundary oxides. On the other hand, Mn and Mo improve hardenability and are less likely to form grain boundary oxides. Therefore, the inventors of the present invention thought that fatigue strength could be further improved by appropriately adjusting the relationship between the number density of MnS and the contents of Cr, Mn, and Mo.

[0028] Therefore, the present inventors have investigated the appropriate relationship between the number density ND of MnS in a steel material and the contents of Cr, Mn, and Mo. As a result, they have found that if a steel material having the above chemical composition satisfies formula (1) and also satisfies the following formula (2), excellent fatigue strength can be obtained when gas carburized to form a machine structural part: [ND] × {Cr / (Mn+Mo)} ０．５ ≦255 (2) Here, each element symbol in formula (2) is substituted with the content of the corresponding element in the chemical composition in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol. [ND] in formula (2) represents the number density ND (number / mm ２ ) is substituted.

[0029] The steel material of this embodiment has been completed based on the above technical concept, and has the following configuration.

[0030] The steel material of the first configuration has a chemical composition, in mass%, of C: 0.15 to 0.25%, Si: 0.15 to 0.50%, Mn: 0.70 to 1.30%, P: 0.030% or less, S: 0.005 to 0.035%, Cr: 1.40 to 1.70%, Al: 0.020 to 0.060%, N: 0.0100 to 0.0200%, Mg: 0 to 0.0100%, Ti: 0 to 0.150%, Nb: 0 to 0.080%, W: 0 to 0.400%, Zr: 0 to 0.200%, V: 0 to 0.400%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, Sn: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, Sb: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.10%, Cu: 0 to 0.20%, and Ni: 0 to 0.20%, with the balance being Fe and impurities, and satisfying formulas (1) and (2), the number density ND of MnS having a maximum length in the central axis direction of 1 to 50 μm in a cross section that includes a central axis of the steel material and is parallel to the central axis is 100 pieces / mm ２-34.85 × (10Si + 0.54Cr) / (2.5Cr + 2Mn + 5Mo) + 820 ≧ 788 (1) [ND] × {Cr / (Mn + Mo)} ０．５ ≦255 (2) Here, the content of the corresponding element in the chemical composition in mass % is substituted for each element symbol in formula (1) and formula (2). When an element is not contained, "0" is substituted for the corresponding element symbol. [ND] in formula (2) represents the number density ND (pieces / mm ２ ) is substituted.

[0031] The steel material of a second configuration is the steel material of the first configuration, wherein the chemical composition, in mass%, is: Mg: 0.0001 to 0.0100%, Ti: 0.001 to 0.150%, Nb: 0.001 to 0.080%, W: 0.001 to 0.400%, Zr: 0.001 to 0.200%, V: 0.001 to 0.400%, Ca: 0.0001 to 0.0100%, Te: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, Sn: 0.0001 to 0.0100%, rare earth elements: 0.0001 to 0.0100%, Co: 0.0001 to 0.0100%, Contains one or more elements selected from the group consisting of Se: 0.0001 to 0.0100%, Sb: 0.0001 to 0.0100%, In: 0.0001 to 0.0100%, Mo: 0.01 to 0.10%, Cu: 0.01 to 0.20%, and Ni: 0.01 to 0.20%.

[0032] The steel material according to this embodiment will be described in detail below. Note that "%" regarding elements means "mass %" unless otherwise specified.

[0033] [Features of the Steel Material of the Present Embodiment] The steel material of the present embodiment includes the following features: (Feature 1) A chemical composition, in mass %, of C: 0.15 to 0.25%, Si: 0.15 to 0.50%, Mn: 0.70 to 1.30%, P: 0.030% or less, S: 0.005 to 0.035%, Cr: 1.40 to 1.70%, Al: 0.020 to 0.060%, N: 0.0100 to 0.0200%, Mg: 0 to 0.0100%, Ti: 0 to 0.150%, Nb: 0 to 0.080%, W: 0 to 0.400%, Zr: 0 to 0.200%, (Feature 2) The steel material has a number density ND of 100 particles / mm2 of MnS particles having a maximum length in the central axis direction of 1 to 50 μm in a cross section that includes the central axis of the steel material and is parallel to the central axis. ２ or more. (Feature 3) The steel satisfies formula (1). -34.85 x (10Si + 0.54Cr) / (2.5Cr + 2Mn + 5Mo) + 820 ≧ 788 (1) Here, the content of the corresponding element in the chemical composition in mass % is substituted for each element symbol in formula (1). When an element is not contained, "0" is substituted for the corresponding element symbol. (Feature 4) The steel satisfies formula (2). [ND] x {Cr / (Mn + Mo)} ０．５ ≦255 (2) Here, each element symbol in formula (2) is substituted with the content of the corresponding element in the chemical composition in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol. [ND] in formula (2) represents the number density ND (number / mm ２ ) is substituted. Features 1 to 4 will be explained below.

[0034] [(Feature 1) Chemical Composition] The chemical composition of the steel material of this embodiment contains the following elements: C: 0.15 to 0.25% Carbon (C) increases the hardness of machine structural components manufactured using the steel material, thereby increasing the fatigue strength of the machine structural components. If the C content is less than 0.15%, the above effects cannot be sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.25%, the chip disposability of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.15 to 0.25%. A preferred lower limit of the C content is 0.16%, more preferably 0.17%, and even more preferably 0.18%. A preferred upper limit of the C content is 0.24%, more preferably 0.23%, and even more preferably 0.22%.

[0035] Si: 0.15 to 0.50% Silicon (Si) deoxidizes steel during the steelmaking process. Si also improves the hardenability of steel and increases the fatigue strength of machine structural components manufactured using the steel. If the Si content is less than 0.15%, 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 Si content exceeds 0.50%, grain boundary oxides are likely to form in the surface layer of the steel after gas carburizing, even if the contents of other elements are within the ranges of this embodiment. Therefore, an incompletely hardened layer is formed in the surface layer of the steel after gas carburizing. As a result, the fatigue strength of the machine structural components decreases. Therefore, the Si content is 0.15 to 0.50%. The preferred lower limit of the Si content is 0.18%, more preferably 0.20%, and even more preferably 0.25%. The preferred upper limit of the Si content is 0.47%, more preferably 0.45%, and even more preferably 0.40%.

[0036] Mn: 0.70 to 1.30% Manganese (Mn) improves the hardenability of steel and enhances the fatigue strength of machine structural components manufactured using the steel. If the Mn content is less than 0.70%, 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 1.30%, the hardness of the steel becomes excessively high, even if the contents of other elements are within the ranges of this embodiment. As a result, the chip disposability of the steel deteriorates. Therefore, the Mn content is 0.70 to 1.30%. The preferred lower limit of the Mn content is 0.73%, more preferably 0.75%, even more preferably 0.80%, even more preferably 0.85%, and even more preferably 0.90%. The preferred upper limit of the Mn content is 1.27%, even more preferably 1.25%, and even more preferably 1.20%.

[0037] P: 0.030% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is greater than 0%. If the P content exceeds 0.030%, P segregates at grain boundaries, reducing grain boundary strength. As a result, the fatigue strength of machine structural components made from steel materials is reduced. Therefore, the P content is 0.030% or less. A preferred upper limit of the P content is 0.028%, more preferably 0.025%, and even more preferably 0.020%. The P content is preferably as low as possible. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, a preferred lower limit of the P content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%.

[0038] S: 0.005 to 0.035% Sulfur (S) forms MnS, improving the chip disposability of steel. If the S content is less than 0.005%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the S content exceeds 0.035%, MnS is formed in excess, even if the contents of other elements are within the ranges of this embodiment. In this case, the fatigue strength of machine structural parts decreases. Therefore, the S content is 0.005 to 0.035%. The preferred lower limit of the S content is 0.007%, more preferably 0.010%, and even more preferably 0.015%. The preferred upper limit of the S content is 0.032%, more preferably 0.030%, and even more preferably 0.025%.

[0039] Cr: 1.40 to 1.70% Chromium (Cr) improves the hardenability of steel and enhances the fatigue strength of machine structural components manufactured using the steel. If the Cr content is less than 1.40%, 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 Cr content exceeds 1.70%, the hardness of the steel increases excessively, even if the contents of other elements are within the ranges of this embodiment. As a result, the chip disposability of the steel decreases. Furthermore, if the Cr content exceeds 1.70%, even if the contents of other elements are within the ranges of this embodiment, grain boundary oxides are likely to be generated in the surface layer of the steel after gas carburizing, even if the contents of other elements are within the ranges of this embodiment. As a result, an incompletely hardened layer is formed in the surface layer of the steel after gas carburizing. As a result, the fatigue strength of the machine structural components decreases. Therefore, the Cr content is 1.40 to 1.70%. The preferred lower limit of the Cr content is 1.43%, more preferably 1.45%, and even more preferably 1.50%. The upper limit of the Cr content is preferably 1.67%, more preferably 1.65%, and even more preferably 1.60%.

[0040] Al: 0.020 to 0.060% Aluminum (Al) deoxidizes steel. Furthermore, Al combines with N in the steel to form AlN, suppressing coarsening of austenite grains. This enhances the fatigue strength of machine structural components manufactured using the steel. If the Al content is less than 0.020%, 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 0.060%, hard oxide-based inclusions are formed, reducing the fatigue strength of machine structural components, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.020 to 0.060%. The preferred lower limit of the Al content is 0.023%, more preferably 0.025%, and even more preferably 0.030%. The preferred upper limit of the Al content is 0.057%, more preferably 0.055%, and even more preferably 0.050%.

[0041] N: 0.0100 to 0.0200% Nitrogen (N) forms nitrides and / or carbonitrides during cooling after hot working in the manufacturing process of machine structural components made from steel, thereby precipitation-strengthening the steel. As a result, the fatigue strength of the machine structural components is increased. If the N content is less than 0.0100%, the above effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0200%, the hot workability of the steel deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.0100 to 0.0200%. A preferred lower limit of the N content is 0.0110%, more preferably 0.0120%, and even more preferably 0.0130%. A preferred upper limit of the N content is 0.0190%, more preferably 0.0180%, and even more preferably 0.0170%.

[0042] The balance of the chemical composition of the steel material according to 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, scrap, or the manufacturing environment when industrially manufacturing steel material, and are acceptable within a range that does not adversely affect the steel material according to this embodiment. Impurities other than the above-mentioned impurities include, for example, oxygen (O). The upper limit of the content of O, which is an impurity, is, for example, 0.0025%. The lower limit of the content of O may be 0%.

[0043] [Optional Elements] The chemical composition of the steel material of this embodiment further includes, in place of a portion of Fe, Mg: 0 to 0.0100%, Ti: 0 to 0.150%, Nb: 0 to 0.080%, W: 0 to 0.400%, Zr: 0 to 0.200%, V: 0 to 0.400%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, Sn The alloy may contain one or more elements selected from the group consisting of: 0 to 0.0100% of rare earth elements, 0 to 0.0100% of Co, 0 to 0.0100% of Se, 0 to 0.0100% of Sb, 0 to 0.0100% of In, 0 to 0.0100% of Mo, 0 to 0.10% of Cu, and 0 to 0.20% of Ni. All of these elements are optional and do not necessarily need to be contained. These optional elements will be described below.

[0044] [First Group: Mg] The chemical composition of the steel material of this embodiment may further contain Mg in place of a portion of Fe.

[0045] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not necessarily need to be present. In other words, the Mg content may be 0%. When present, that is, when the Mg content exceeds 0%, Mg deoxidizes the steel. Even if even a small amount of Mg is present, the above effect can be achieved to some extent. However, when the Mg content exceeds 0.0100%, Mg forms coarse oxides even when the contents of other elements are within the ranges of this embodiment. The coarse oxides reduce the fatigue strength of machine structural components manufactured using the steel material. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0090%, more preferably 0.0070%, even more preferably 0.0050%, and even more preferably 0.0040%.

[0046] [Second Group: Ti, Nb, W, Zr, and V] The chemical composition of the steel material of this embodiment may further contain elements of the second group described above in place of a portion of Fe. All of these elements form precipitates to enhance the toughness of machine structural parts manufactured using the steel material. Each element of the second group will be described below.

[0047] Ti: 0 to 0.150% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When titanium (Ti) is contained, that is, when the Ti content exceeds 0%, Ti forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of a machine structural component, thereby refining the crystal grains. This increases the toughness of the machine structural component. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.150%, the above effect saturates and the manufacturing cost increases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.150%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.005%, and even more preferably 0.008%. The upper limit of the Ti content is preferably 0.140%, more preferably 0.120%, even more preferably 0.100%, even more preferably 0.050%, even more preferably 0.020%, and even more preferably 0.015%.

[0048] Nb: 0 to 0.080% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of the machine structural component, thereby refining the crystal grains. This increases the toughness of the machine structural component. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.080%, the above effect saturates and the manufacturing cost increases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.080%. The preferable lower limit of the Nb content is 0.001%, more preferably 0.005%, and even more preferably 0.008%. The upper limit of the Nb content is preferably 0.070%, more preferably 0.060%, even more preferably 0.050%, even more preferably 0.020%, and still more preferably 0.015%.

[0049] W: 0 to 0.400% Tungsten (W) is an optional element and does not necessarily need to be contained. That is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of a machine structural component, thereby refining the crystal grains. This increases the toughness of the machine structural component. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content exceeds 0.400%, the above effect saturates and the manufacturing cost increases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0 to 0.400%. The preferred lower limit of the W content is 0.001%, more preferably 0.005%, and even more preferably 0.050%. The preferred upper limit of the W content is 0.350%, more preferably 0.300%, and even more preferably 0.200%.

[0050] Zr: 0 to 0.200% Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When Zr is contained, that is, when the Zr content exceeds 0%, Zr forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of a machine structural component, thereby refining the crystal grains. This increases the toughness of the machine structural component. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.200%, the above effect saturates and the manufacturing cost increases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Zr content is 0 to 0.200%. The preferred lower limit of the Zr content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The upper limit of the Zr content is preferably 0.150%, more preferably 0.100%, even more preferably 0.050%, and still more preferably 0.010%.

[0051] V: 0 to 0.400% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When vanadium (V) is contained, that is, when the V content exceeds 0%, V forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of the machine structural component, thereby refining the crystal grains. This increases the toughness of the machine structural component. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.400%, the above effect saturates and the manufacturing cost increases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0 to 0.400%. The preferred lower limit of the V content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the V content is 0.300%, more preferably 0.200%, and even more preferably 0.150%.

[0052] [Third Group: Ca, Te, B, Sn, and Rare Earth Elements] The chemical composition of the steel material of this embodiment may further contain elements of the third group described above in place of a portion of Fe. All of these elements form precipitates to improve the machinability of the steel material. Each element of the third group will be described below.

[0053] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, Ca improves the machinability of the steel material. Even if even a small amount of Ca is contained, the above effect can be achieved to some extent. However, if the Ca content exceeds 0.0100%, coarse oxides will form even if the contents of other elements are within the ranges of this embodiment. In this case, the fatigue strength of machine structural components manufactured using the steel material will decrease. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0015%. The preferred upper limit of the Ca content is 0.0090%, more preferably 0.0070%, even more preferably 0.0050%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0054] Te: 0 to 0.0100% Tellurium (Te) is an optional element and does not necessarily need to be contained. In other words, the Te content may be 0%. When contained, that is, when the Te content exceeds 0%, Te improves the machinability of the steel material. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.0100%. The preferred lower limit of the Te content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0010%. The preferred upper limit of the Te content is 0.0090%, more preferably 0.0085%, and even more preferably 0.0080%.

[0055] B: 0 to 0.0050% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B improves the machinability of the steel material. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content exceeds 0.0050%, hot working cracks tend to occur in the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0056] Sn: 0 to 0.0100% Tin (Sn) is an optional element and does not necessarily need to be contained. In other words, the Sn content may be 0%. When contained, that is, when the Sn content exceeds 0%, Sn improves the machinability of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.0100%. The preferred lower limit of the Sn content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Sn content is 0.0095%, more preferably 0.0090%, even more preferably 0.0085%, and even more preferably 0.0080%.

[0057] Rare Earth Elements: 0 to 0.0100% Rare earth elements (REM) are optional elements and do not necessarily need to be contained. That is, the REM content may be 0%. When contained, that is, when the REM content exceeds 0%, REM improves the machinability of the steel material. Even if even a small amount of REM is contained, the above effect can be obtained to some extent. However, if the REM content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the REM content is 0.0090%, more preferably 0.0070%, and even more preferably 0.0055%.

[0058] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, the REM content in this specification refers to the total content of these elements.

[0059] [Fourth Group: Co, Se, Sb, and In] The chemical composition of the steel material according to the present embodiment may further contain elements of the fourth group described above in place of a portion of Fe. All of these elements form precipitates to suppress decarburization of the steel material. Each element of the fourth group will be described below.

[0060] Cobalt (Co) is an optional element and does not necessarily need to be contained. That is, the Co content may be 0%. When Co is contained, that is, when the Co content exceeds 0%, Co suppresses decarburization of the steel material during hot working. Even if even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0 to 0.0100%. The preferred lower limit of the Co content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Co content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0061] Se: 0 to 0.0100% Selenium (Se) is an optional element and does not necessarily need to be contained. That is, the Se content may be 0%. When contained, that is, when the Se content exceeds 0%, Se suppresses decarburization of the steel material during hot working. Even if even a small amount of Se is contained, the above effect can be obtained to some extent. However, if the Se content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Se content is 0 to 0.0100%. The preferred lower limit of the Se content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Se content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0062] Sb: 0 to 0.0100% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb suppresses decarburization of the steel material during hot working. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.0100%. The preferred lower limit of the Sb content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Sb content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0063] In: 0 to 0.0100% Indium (In) is an optional element and does not necessarily need to be contained. That is, the In content may be 0%. When contained, that is, when the In content exceeds 0%, In suppresses decarburization of the steel material during hot working. Even if even a small amount of In is contained, the above effect can be obtained to some extent. However, if the In content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the In content is 0 to 0.0100%. The preferred lower limit of the In content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the In content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0064] [Fifth Group: Mo, Cu, and Ni] The chemical composition of the steel material of this embodiment may further contain elements of the fifth group described above in place of a portion of Fe. All of these elements form precipitates and enhance the fatigue strength of machine structural components manufactured using the steel material. Each element of the fifth group will be described below.

[0065] Mo: 0 to 0.10% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo enhances the fatigue strength of machine structural components. Even if even a small amount of Mo is contained, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.10%, the hardness of the steel material increases excessively even if the contents of other elements are within the ranges of this embodiment. As a result, hot workability deteriorates. Therefore, the Mo content is 0 to 0.10%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Mo content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0066] Cu: 0 to 0.20% Copper (Cu) is an optional element and does not necessarily need to be contained. In other words, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, Cu enhances the fatigue strength of machine structural components. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.20%, the hardness of the steel material will increase excessively even if the contents of other elements are within the ranges of this embodiment. As a result, hot workability will decrease. Therefore, the Cu content is 0 to 0.20%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Cu content is 0.15%, more preferably 0.13%, and even more preferably 0.10%.

[0067] Ni: 0 to 0.20% Nickel (Ni) is an optional element and does not necessarily need to be contained. In other words, the Ni content may be 0%. When contained, that is, when the Ni content exceeds 0%, Ni enhances the fatigue strength of machine structural components. Even if even a small amount of Ni is contained, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.20%, the hardness of the steel material will increase excessively even if the contents of other elements are within the ranges of this embodiment. As a result, hot workability will decrease. Therefore, the Ni content is 0 to 0.20%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Ni content is 0.15%, more preferably 0.13%, and even more preferably 0.10%.

[0068] [(Feature 2) Regarding MnS Number Density ND] In the steel material of this embodiment, the number density ND of MnS having a maximum length in the central axis direction of 1 to 50 μm in a cross section that includes the central axis of the steel material and is parallel to the central axis is 100 pieces / mm ２ Here, in this specification, the number density ND of MnS having a maximum length in the central axis direction of 1 to 50 μm in a cross section that includes the central axis of the steel material and is parallel to the central axis is defined as "MnS number density ND".

[0069] During cutting, stress concentrates at the interface between multiple MnS and the parent phase, causing cracks to form. If multiple cracks connect, the cracks propagate. In this case, the cracks propagate and chips separate from the steel. MnS with a maximum length in the central axis direction of 1 to 50 μm tends to promote the generation and propagation of such cracks. Therefore, if the number density ND of MnS with a maximum length in the central axis direction of 1 to 50 μm is appropriate, chips of an appropriate length will separate from the parent material during cutting. As a result, high chip disposability is obtained.

[0070] MnS number density ND is 100 pieces / mm ２If the ratio is less than 1 / 2, the amount of MnS with a maximum length in the central axis direction of 1 to 50 μm is too small. In this case, cracks do not sufficiently occur at the interface between the MnS and the matrix. This makes it difficult for chips to separate from the steel material, resulting in extremely long chips during cutting. As a result, chip disposal during cutting of the steel material is reduced.

[0071] MnS number density ND is 100 pieces / mm ２ If the above conditions are met, the chips generated during cutting of the steel material will have an appropriate length. As a result, provided that Features 1, 3, and 4 are satisfied, the chip disposability of the steel material will be improved.

[0072] The preferred lower limit of the MnS number density ND is 120 pieces / mm ２ and more preferably 125 pieces / mm ２ and more preferably 130 pieces / mm ２ The upper limit of the MnS number density ND is not particularly limited, but is, for example, 270 pieces / mm ２ and more preferably 200 pieces / mm ２ is.

[0073] [Method for measuring MnS number density ND] In this embodiment, the MnS number density ND (number / mm ２ ) can be determined by the following method. A test piece is taken that includes the central axis of the steel material (steel bar) and has a cross section parallel to the central axis as the observation surface. When the cross section perpendicular to the central axis direction of the steel material is a circle with a radius R, the observation surface includes the R / 2 position as the observation reference position. When the cross section perpendicular to the central axis direction of the steel material is a rectangle with a short side length L, the observation surface includes the L / 4 depth position from the center position in the long side direction on the surface of the steel material as the observation reference position. Note that when the cross section perpendicular to the central axis direction of the steel material is a square, any side is regarded as the short side, and the side perpendicular to the short side is regarded as the long side. After mirror polishing the observation surface, a scanning electron microscope (SEM) is used to observe 20 arbitrary fields of view centered on the observation reference position in the observation surface at 1000x magnification to obtain a backscattered electron image. Each field of view is 100 μm×120 μm.

[0074] Based on the backscattered electron image obtained by SEM observation, the MnS number density is measured using a well-known particle analysis method based on image analysis. Specifically, the inclusions and precipitates (hereinafter, inclusions and precipitates are also referred to as particles) in the steel material are identified based on the interface between the matrix of the steel material and the inclusions and / or precipitates. In the backscattered electron image, the particles are displayed with a lower brightness than the matrix. The maximum length of each identified particle in the direction of its central axis is calculated using well-known image processing.

[0075] Among the identified particles, particles having a maximum length in the central axis direction of 1 to 50 μm are selected. The selected particles are subjected to component analysis using energy dispersive X-ray spectroscopy (EDX) equipped in the SEM. In this embodiment, the beam diameter of the EDX used for component analysis is not particularly limited as long as it is a beam diameter that can analyze the components of the particles to be analyzed. In component analysis using EDX, the quantified elements are N, O, Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Cr, Mn, Zr, Nb, La, and Ce. When the total content of the quantified elements in mass % is taken as 100%, particles having a total content of Mn and S of 20 mass % or more are determined to be MnS.

[0076] The above method is used to identify MnS particles with a maximum length in the central axis direction of 1 to 50 μm. The total number of MnS particles with a maximum length in the central axis direction of 1 to 50 μm identified in each field of view and the total area (0.24 mm2) of the 20 fields of view are then calculated. ２ ) and the number density per unit area of ​​MnS having a maximum length in the central axis of 1 to 50 μm (pieces / mm ２ The MnS number density is an integer value obtained by rounding the obtained value to one decimal place.

[0077] [(Feature 3) Regarding Formula (1)] The steel material of this embodiment satisfies Formula (1): -34.85 × (10Si + 0.54Cr) / (2.5Cr + 2Mn + 5Mo) + 820 ≧ 788 (1) Here, the content of the corresponding element in the chemical composition in mass % is substituted for each element symbol in Formula (1). When an element is not contained, "0" is substituted for the corresponding element symbol.

[0078] Fn1 is defined as follows: Fn1 = -34.85 x (10Si + 0.54Cr) / (2.5Cr + 2Mn + 5Mo) + 820

[0079] Fn1 is an index for improving fatigue strength when a steel material having a chemical composition satisfying Feature 1 is gas carburized to form a machine structural component. Si and Cr are likely to form grain boundary oxides during gas carburizing. Around the grain boundary oxides, the Si and Cr contents decrease, reducing hardenability. As a result, an incompletely hardened layer is formed around the grain boundary oxides. Machine structural components with an incompletely hardened layer have reduced fatigue strength. Therefore, in order to improve the fatigue strength of machine structural components, it is effective to reduce the Si and Cr contents of the steel material in order to suppress the formation of grain boundary oxides.

[0080] However, Si and Cr are also elements that enhance hardenability. Therefore, if the Si and Cr contents are reduced, the hardenability will decrease. Therefore, in order to ensure hardenability, the contents of Mn and Mo, which are elements that enhance hardenability and do not easily form grain boundary oxides, are adjusted.

[0081] The numerator of Fn1 is an index showing the ease of grain boundary oxide formation. In the numerator of Fn1, the coefficient of the Si content is larger than the coefficient of the Cr content because Si is more likely to form grain boundary oxides than Cr. On the other hand, the denominator of Fn1 indicates hardenability. The coefficients of the Cr content, Mn content, and Mo content in the denominator indicate the degree of contribution of each element to improving hardenability in a chemical composition that satisfies Feature 1. Cr is included in the denominator because, in a chemical composition that satisfies Feature 1, Cr forms grain boundary oxides but also makes a significant contribution to hardenability.

[0082] As described above, Fn1 defined by the above relational expression is an index of the fatigue strength of a machine structural part when the machine structural part is manufactured by gas carburizing a steel material having a chemical composition that satisfies Feature 1. If Fn1 is less than 788, the formation of grain boundary oxides during gas carburizing cannot be suppressed, and sufficient hardenability cannot be ensured. As a result, when the machine structural part is manufactured by gas carburizing, sufficient fatigue strength cannot be obtained.

[0083] If Fn1 is 788 or more, the generation of grain boundary oxides during gas carburizing can be sufficiently suppressed, and sufficient hardenability can be ensured. As a result, on the premise that Features 1, 2, and 4 are satisfied, when gas carburizing is performed to produce a machine structural part, excellent fatigue strength can be obtained.

[0084] The lower limit of Fn1 is preferably 790, more preferably 795, even more preferably 797, even more preferably 800, even more preferably 801, and even more preferably 802. The upper limit of Fn1 is not particularly limited. When the chemical composition of the steel satisfies Feature 1, the upper limit of Fn1 is, for example, 810. Note that Fn1 is an integer value obtained by rounding the obtained numerical value to one decimal place.

[0085] [(Feature 4) Regarding Formula (2)] The steel material of this embodiment satisfies Formula (2): [ND] × {Cr / (Mn+Mo)} ０．５ ≦255 (2) Here, each element symbol in formula (2) is substituted with the content of the corresponding element in the chemical composition in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol. [ND] in formula (2) represents the number density ND (number / mm ２ ) is substituted.

[0086] Fn2 is defined as follows: Fn2 = [ND] × {Cr / (Mn + Mo)} ０．５

[0087] Fn2 is an index for suppressing the initiation and propagation of cracks when a steel material having a chemical composition satisfying Features 1 and 3 is gas carburized to form a machine structural part. As described above, if the size and number density of MnS satisfy Feature 2, the chip disposability of the steel material is improved. On the other hand, although MnS improves chip disposability, it can also be the starting point for crack initiation in machine structural parts. Therefore, the number density of MnS is limited to a certain extent. Furthermore, as described above, the formation of grain boundary oxides is also a factor in reducing fatigue strength. Therefore, when adjusting the number density of MnS, the degree of grain boundary oxide formation must also be taken into consideration.

[0088] As described above, Cr improves hardenability but is prone to the formation of grain boundary oxides. On the other hand, Mn and Mo improve hardenability and are less likely to form grain boundary oxides. By appropriately adjusting the relationship between the MnS number density ND of the steel material and the contents of Cr, Mn, and Mo, the initiation and propagation of cracks can be suppressed when gas carburizing is performed to form machine structural parts. If Fn2 exceeds 255, the initiation and propagation of cracks in machine structural parts cannot be suppressed. As a result, sufficient fatigue strength cannot be obtained when gas carburizing is performed to form machine structural parts.

[0089] If Fn2 is 255 or less, the occurrence and propagation of cracks can be suppressed when gas carburizing is performed to form a machine structural part. As a result, on the premise that Features 1 to 3 are satisfied, excellent fatigue strength can be obtained when gas carburizing is performed to form a machine structural part.

[0090] The upper limit of Fn2 is preferably 250, more preferably 247, even more preferably 240, even more preferably 220, even more preferably 210, and even more preferably 200. The lower limit of Fn2 is not particularly limited. When the steel material satisfies Features 1 to 3, the lower limit of Fn2 is, for example, 120. Note that Fn2 is an integer value obtained by rounding the obtained numerical value to one decimal place.

[0091] [Effects of the Steel Material of the Present Embodiment] The steel material of the present embodiment satisfies Features 1 to 4. Therefore, the steel material of the present embodiment has excellent chip disposal properties. Furthermore, when gas carburizing is performed to form machine structural parts, excellent fatigue strength is obtained.

[0092] [Regarding the microstructure of the steel material] The microstructure of the steel material of this embodiment is not particularly limited. The steel material of this embodiment has excellent chip disposability, and when gas carburizing is performed to manufacture machine structural parts, it has excellent fatigue strength. These effects are properties required for machine structural parts during the manufacturing process or after manufacture. Usually, during hot working in the manufacturing process of machine structural parts or during gas carburizing, the steel material is treated with Ac ３The steel is heated to a temperature equal to or higher than the transformation point temperature. As a result, the microstructure of the steel material is transformed into austenite, and the influence of the previous structure is eliminated. Therefore, the microstructure of the steel material that is the raw material for machine structural parts is not particularly limited. As described above, as long as the above-mentioned Features 1 to 4 are satisfied, the chip disposability of the steel material in this embodiment and the fatigue strength when used as a machine structural part are sufficiently improved regardless of the microstructure of the steel material. Therefore, the microstructure of the steel material in this embodiment is not particularly limited.

[0093] [Shape of Steel Material in This Embodiment] The steel material in this embodiment is a steel bar or wire rod. The steel bar or wire rod is a steel material that extends in a rod shape. The steel material may be wound into a coil or cut to a predetermined length. The cross section perpendicular to the axial direction of the steel material is circular or rectangular. A circular shape includes not only a perfect circle, but also a substantially circular shape within a certain tolerance range. For example, in the case of a steel bar, a circular shape also includes an elliptical shape. For example, in the case of a wire rod, a circular shape also includes a circle with a radial deviation of 0.64 mm or less. The cross-sectional size of the steel material is, for example, a circle equivalent diameter of 10 to 150 mm. The circle equivalent diameter means the diameter of a circle when the cross-sectional area of ​​the steel material is replaced with a circle having the same area.

[0094] [Uses of the steel material of this embodiment] The steel material of this embodiment can be used as a material for machine structural parts used in automobiles, construction vehicles, mining machinery, etc. In particular, the steel material of this embodiment can be applied as a material for machine structural parts manufactured by carrying out gas carburizing treatment. The machine structural parts are not particularly limited, but examples include gears, cogwheels, shafts, etc. The steel material of this embodiment can also be applied to uses other than machine structural parts. In this specification, gas carburizing treatment also includes gas carbonitriding treatment.

[0095] [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.

[0096] An example of a method for producing a steel material according to an embodiment includes the following steps: (Step 1) Refining step, (Step 2) Continuous casting step, and (Step 3) Hot rolling step.

[0097] 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 Features 1 and 3 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 Features 1 and 3 is produced.

[0098] [(Step 2) Continuous Casting Step] In the continuous casting step, a bloom is produced by continuous casting using the molten steel produced in the refining step. In the continuous casting step, the following condition 1 is satisfied. (Condition 1) In the continuous casting step, the solidification cooling rate CR is set to 6.0 to 8.0°C / min. Condition 1 will be explained below.

[0099] [Regarding Condition 1] In this embodiment, the solidification cooling rate CR is set to 6.0 to 8.0°C / min. MnS crystallizes from molten steel during the continuous casting process. In this embodiment, by setting the solidification cooling rate CR to 6.0 to 8.0°C / min, it is possible to prevent the size of MnS produced by crystallization from becoming coarse, and fine MnS can be obtained.

[0100] On the other hand, when the solidification cooling rate CR exceeds 8.0°C / min, a large amount of fine MnS crystallizes. In this case, even if the conditions 2 and 3 described below are satisfied, Fn2 exceeds 255.

[0101] If the solidification cooling rate CR is less than 6.0°C / min, the size of the MnS produced by crystallization tends to become coarse. In this case, even if the conditions 2 and 3 described below are satisfied, the MnS number density may not exceed 100 particles / mm ２ Therefore, in the continuous casting process, the solidification cooling rate CR is 6.0 to 8.0°C / min.

[0102] The solidification cooling rate CR referred to here is determined by the following method. When the cross section perpendicular to the longitudinal direction of a slab produced by continuous casting has a rectangular cross section, a test piece is taken that includes the midpoint between the surface of the slab and the center of the slab on the line at the center of the slab's width. When the cross section is circular, a test piece is taken that includes the midpoint of the radius. The surface of the test piece that corresponds to the cross section perpendicular to the longitudinal direction of the slab is used as the observation surface. Within the observation surface, a 5 mm x 5 mm region centered on the midpoint is used as the observation region. The dendrite secondary arm spacing is measured at 10 locations within the observation region, and the arithmetic mean value is defined as λ (μm). The solidification cooling rate CR (°C / min) is determined using the measured dendrite secondary arm spacing λ (μm). CR = (λ / 770) －１／０．４１

[0103] [(Step 3) Hot Rolling Step] In the hot rolling step, the bloom is hot rolled to produce a steel material. The hot rolling step includes a blooming step and a finish rolling step.

[0104] [(Step 31) Blooming Process] In the blooming process, a blooming mill is used to hot roll the bloom (blooming) to produce a billet. If a continuous rolling mill with multiple rolling stands arranged in a line is located downstream of the blooming mill, the continuous rolling mill may be used to hot roll the billet to further reduce its size. In the blooming process, the bloom is heated in a heating furnace. The holding temperature and holding time are not particularly limited, but for example, the bloom is held in the heating furnace at a temperature of 1000 to 1200°C, and the holding time at the holding temperature is 60 to 300 minutes. The holding temperature here refers to the temperature (°C) of the heating furnace. The billet produced in the blooming process is allowed to cool to room temperature before the finish rolling process. In the blooming process, the following condition 2 must also be satisfied: (Condition 2) In the blooming process, the maximum area reduction rate RRmax1 in each pass is 25% or less. Condition 2 is explained below.

[0105] [Regarding Condition 2] In this embodiment, the maximum area reduction rate RRmax1 in each pass of blooming is set to 25% or less in the blooming process. Here, a "pass" refers to a single reduction (external force) from the work rolls when the bloom is transported from the entry side to the exit side (forward pass) or from the exit side to the entry side (return pass) of a blooming mill that performs reverse rolling. Therefore, if the bloom is not subjected to an external force from the work rolls on the outward or return pass of the blooming mill, that operation does not qualify as a "pass."

[0106] In the blooming process, MnS in the bloom tends to elongate in the direction of the central axis. The MnS elongated in the direction of the central axis may be partially melted by heating in a heating furnace in the finish rolling process described below. In this case, the MnS elongated in the direction of the central axis is broken down. This produces MnS with an adjusted maximum length in the direction of the central axis.

[0107] If the maximum value of the area reduction rate RRmax1 in each pass in the blooming process exceeds 25%, the MnS in the steel material will elongate excessively in the central axis direction. In this case, a large amount of MnS particles with an adjusted maximum length in the central axis direction will be produced by heating in the finish rolling process after the blooming process. This will result in an excessive increase in the MnS number density ND. As a result, even if the above-mentioned condition 1 and the condition 3 described below are satisfied, Fn2 will exceed 255. Therefore, in the blooming process, the maximum value of the area reduction rate RRmax1 in each pass is 25% or less.

[0108] The area reduction rate (%) in each pass in the blooming process is calculated by the following formula: Area reduction rate = (1 - area of ​​cross section perpendicular to the longitudinal direction of the bloom after reduction / area of ​​cross section perpendicular to the longitudinal direction of the bloom before reduction) x 100

[0109] [(Step 32) Finish Rolling Step] In the finish rolling step, hot rolling (finish rolling) is performed on the billet 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 not particularly limited, but is set to 1000 to 1200°C, for example. The heated billet is then hot rolled (finish rolling) using a continuous rolling mill. In finish rolling, the surface temperature of the billet on the outlet side of the rolling stand in the continuous rolling mill that applies the final reduction to the billet is defined as the finish rolling temperature. The finish rolling temperature is, for example, 900°C or higher. In the finish rolling step, the following condition 3 is further satisfied. (Condition 3) In the finish rolling step, the maximum value of the area reduction rate in each pass, RRmax2, is set to 25% or less. Condition 3 will be explained below.

[0110] [Regarding Condition 3] In this embodiment, in the finish rolling process, the maximum value of the area reduction rate RRmax2 in each pass in finish rolling is set to 25% or less. Here, a "pass" means that in finish rolling using a continuous rolling mill, the billet receives a single reduction (external force) from the work rolls as it passes through each rolling stand from upstream to downstream. Therefore, if no external force is received from the work rolls in finish rolling, that operation does not qualify as a "pass."

[0111] In the finish rolling, the MnS in the billet elongates in the direction of the central axis. If the maximum value RRmax2 of the area reduction rate in each pass of the finish rolling is 25% or less, the maximum length of the MnS in the steel material in the direction of the central axis will be 1 to 50 μm, provided that the conditions 1 and 2 are satisfied.

[0112] If the maximum value of the area reduction rate RRmax2 in each pass in the finish rolling process exceeds 25%, the MnS in the steel material will be too elongated in the central axis direction. In this case, even if the above-mentioned conditions 1 and 2 are satisfied, the MnS number density cannot be made 100 pieces / mm ２ Therefore, in the finish rolling process, the maximum value RRmax2 of the area reduction rate in each pass is 25% or less.

[0113] The area reduction rate (%) for each pass in the finish rolling process is calculated using the following formula: Area reduction rate = (1 - area of ​​cross section perpendicular to the longitudinal direction of the billet after reduction / area of ​​cross section perpendicular to the longitudinal direction of the billet before reduction) x 100

[0114] The steel material of this embodiment is manufactured by the above manufacturing process.

[0115] [Method for Manufacturing Machine Structural Parts] Gas carburized machine structural parts made of the steel material of this embodiment are manufactured, for example, by the following manufacturing method.

[0116] An example of a method for manufacturing a gas carburized machine structural part includes, for example, a hot working step and a gas carburizing treatment step (gas carburizing, quenching, and tempering). In the hot working step, hot working is performed on the steel material of this embodiment to manufacture an intermediate product having a predetermined shape. The hot working is, for example, hot forging. In the hot working step, the steel material is subjected to Ac ３ After heating to a temperature above this point, the steel is processed. Therefore, the microstructure of the steel is reset during the heating process. The heating temperature is, for example, 1000 to 1300°C, and the holding time at the heating temperature is 30 to 300 minutes. The finishing temperature is, for example, 900°C or higher. The intermediate product after hot working is cooled to room temperature. If necessary, cutting may be performed on the intermediate product after hot working. In other words, cutting is an optional process.

[0117] A gas carburizing process is performed on an intermediate product after hot working or cutting to manufacture a machine structural part. The gas carburizing process includes a gas carburizing quenching process and a tempering process. In the gas carburizing quenching process, the intermediate product is quenched with Ac in an atmosphere containing a known carburizing gas. ３ The intermediate product is heated to the transformation point or higher and held there, and then rapidly cooled. In the tempering process, for example, the carburized and quenched intermediate product is held at a temperature in the range of 150 to 200°C for a predetermined period of time.

[0118] 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.

[0119] Steel materials having the chemical compositions shown in Tables 1A and 1B were produced by the following method. In all test numbers, the O content in the chemical composition of the steel materials was 0.0025% or less. In the following description, Tables 1A and 1B will be collectively referred to as "Table 1."

[0120]

[0121]

[0122] Steel materials (steel bars) having the chemical compositions shown in Table 1 were produced by the following method. Specifically, blooms were produced by continuous casting in a continuous casting process using molten steel produced in a refining process. The produced blooms were subjected to blooming in a blooming process to produce billets. The holding temperature of the heating furnace in the blooming process was 1200°C, and the holding time at the holding temperature was 120 minutes. The produced billets were allowed to cool to room temperature.

[0123] The manufactured billet was subjected to a finish rolling process. The heating temperature in the heating furnace in the finish rolling process was 1100°C. The finish rolling temperature was 920°C.

[0124] Through the above steps, a steel material (steel bar) having a diameter of 50 mm was produced.

[0125] In the manufacturing process, the solidification cooling rate CR (°C / min) in the continuous casting process, the maximum value of the area reduction rate RRmax1 (%) in the blooming process, and the maximum value of the area reduction rate RRmax2 (%) in the finish rolling process were as shown in Table 2.

[0126]

[0127] [Evaluation Tests] The following evaluation tests were carried out using the steel materials with each test number: (Test 1) Measurement test of MnS number density ND (Test 2) Chip disposability evaluation test (Test 3) Fatigue strength evaluation test Each test will be described below.

[0128] [(Test 1) Measurement test of MnS number density ND] Based on the method described in the above [Method for measuring MnS number density ND], the MnS number density ND of the steel material of each test number was determined. The obtained MnS number density ND was recorded as "MnS number density (pieces / mm ２ ) column.

[0129] [(Test 2) Chip Disposability Evaluation Test] The steel material (50 mm diameter steel bar) of each test number was heated at a heating temperature of 1200°C for a holding time of 30 minutes. Then, hot working (hot forging) was performed at a finishing temperature of 950°C or higher to produce a steel bar with a diameter of 35 mm. Through the above process, intermediate cut test pieces simulating the hot working of machine structural parts were produced.

[0130] The following chip disposability evaluation test was performed on the intermediate cut test pieces of each test number. The intermediate cut test pieces of each test number were cut perpendicular to the axial direction at an arbitrary position to obtain cut test pieces with a diameter of 35 mm and a length of 100 mm. The cut test pieces were subjected to peripheral turning under the following conditions. The cutting tool was an uncoated ultra-high alloy equivalent to M20 specified in JIS B 4053 (2013). The cutting speed was 100 m / min, the feed rate was 0.20 mm / rev, and the depth of cut was 1.0 mm. Water-soluble cutting oil was used during turning. Under the above conditions, the test pieces of each test number were subjected to peripheral turning until the diameter reached 25 mm. In this example, the chip disposability evaluation test was performed using cut test pieces with a diameter of 35 mm. However, even when cut test pieces with different diameters are used, the chip disposability evaluation test can be performed appropriately by adjusting the cutting speed and total depth of cut to be similar to those in this example.

[0131] The length of the longest chip (maximum chip length L) was determined from the chips obtained by peripheral turning. The maximum chip length L was the length of the line segment (straight line) connecting the two ends of each chip. The obtained maximum chip length L is shown in the "Maximum chip length L (mm)" column in Table 2. If the maximum chip length L was 50 mm or less, it was determined that the chip disposal property was excellent.

[0132] [(Test 3) Fatigue Strength Evaluation Test] Ono-type rotating bending fatigue test specimens simulating machine structural parts were prepared from the steel material (steel bar with a diameter of 50 mm) of each test number. The shape of the Ono-type rotating bending fatigue test specimen is shown in Figure 1. The numbers in Figure 1 represent dimensions (unit: mm). "φ" in Figure 1 means diameter. "R1" means that the radius of curvature at the notch bottom is 1 mm.

[0133] Specifically, the steel material (50 mm diameter steel bar) of each test number was heated at a heating temperature of 1200°C for a holding time of 30 minutes. Then, hot working (hot forging) was performed at a finishing temperature of 950°C or higher to produce a steel bar with a diameter of 35 mm. The 35 mm diameter steel bar was machined (cut) to produce an intermediate product for the Ono-type rotary bending fatigue test.

[0134] The intermediate products of each test number were subjected to gas carburizing (gas carburizing, quenching, and tempering) to prepare the Ono-type rotating bending fatigue test specimens shown in Figure 1. In the gas carburizing treatment, the intermediate products (steel bars) of each test number were heated at 950°C for 240 minutes in an atmosphere with a carbon potential Cp1 of 0.8%. Subsequently, they were heated at 950°C for 60 minutes in an atmosphere with a carbon potential Cp2 of 0.8%. After that, they were heated at 850°C for 30 minutes and oil-quenched in 130°C oil. After oil-quenching, the intermediate products were tempered at a tempering temperature of 180°C for a holding time of 120 minutes. After the holding time had elapsed, they were air-cooled. Using the above method, Ono-type rotating bending fatigue test specimens simulating the machine structural parts of each test number were prepared.

[0135] Ono type rotating bending fatigue tests were conducted using Ono type rotating bending fatigue test specimens with each test number. Multiple test specimens were prepared for each test number. Fatigue tests were conducted by applying different stresses to each test specimen, and the fatigue test was conducted for 10 million cycles (10 ７ The highest stress at which the specimen did not break after repeated cycles of 1000 times was defined as the bending fatigue strength (MPa). In the Ono-type rotating bending fatigue test, the rotation speed was 3000 rpm and the stress ratio was alternating.

[0136] The fatigue strength (MPa) obtained for each test number is shown in the "Fatigue strength (MPa)" column in Table 2. If the fatigue strength was 800 MPa or more, it was determined that excellent fatigue strength was obtained.

[0137] [Evaluation Results] Referring to Tables 1 and 2, in test numbers 1 to 26, the steel materials satisfied characteristics 1 to 4. Therefore, excellent chip disposability was achieved, and further, the fatigue strength was 800 MPa or more, resulting in excellent fatigue strength.

[0138] On the other hand, in test numbers 27 and 28, the solidification cooling rate CR in the continuous casting process was too slow. Therefore, the MnS number density was 100 pieces / mm ２ As a result, the maximum chip length L exceeded the upper limit, and sufficient chip disposability of the steel material could not be obtained.

[0139] In test numbers 29 and 30, the solidification cooling rate CR in the continuous casting process was too fast. Therefore, Fn2 exceeded 255. As a result, the fatigue strength of machine structural parts manufactured using the steel material decreased, and sufficient fatigue strength could not be obtained.

[0140] In test numbers 31 and 32, the maximum value of the area reduction rate RRmax1 in each pass in the blooming process was too high. Therefore, Fn2 exceeded 255. As a result, the fatigue strength of machine structural parts manufactured using the steel material decreased, and sufficient fatigue strength could not be obtained.

[0141] In test numbers 33 and 34, the maximum value RRmax2 of the area reduction rate in each pass in the finish rolling process was too high. Therefore, the MnS number density was 100 pieces / mm ２ As a result, the maximum chip length L exceeded the upper limit, and sufficient chip disposability of the steel material could not be obtained.

[0142] In test numbers 35 and 36, Fn1 was too low. As a result, the fatigue strength of the machine structural parts manufactured using the steel material was reduced, and sufficient fatigue strength could not be obtained.

[0143] In test numbers 37 and 38, Fn2 was too high. As a result, the fatigue strength of the machine structural parts manufactured using the steel material was reduced, and sufficient fatigue strength could not be obtained.

[0144] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A steel material having a chemical composition, in mass%, of C: 0.15 to 0.25%, Si: 0.15 to 0.50%, Mn: 0.70 to 1.30%, P: 0.030% or less, S: 0.005 to 0.035%, Cr: 1.40 to 1.70%, Al: 0.020 to 0.060%, N: 0.0100 to 0.0200%, Mg: 0 to 0.0100%, Ti: 0 to 0.150%, Nb: 0 to 0.080%, W: 0 to 0.400%, Zr: 0 to 0.200%, V: 0 to 0.400%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, Sn: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, Sb: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.10%, Cu: 0 to 0.20%, and Ni: 0 to 0.20%, with the balance being Fe and impurities, and satisfying formulas (1) and (2), the number density ND of MnS having a maximum length in the central axis direction of 1 to 50 μm in a cross section that includes a central axis of the steel material and is parallel to the central axis is 100 pieces / mm ２ Steel material that is equal to or greater than -34.85 × (10Si + 0.54Cr) / (2.5Cr + 2Mn + 5Mo) + 820 ≧ 788 (1) [ND] × {Cr / (Mn + Mo)} ０．５ ≦255 (2) Here, the content of the corresponding element in the chemical composition in mass % is substituted for each element symbol in formula (1) and formula (2). When an element is not contained, "0" is substituted for the corresponding element symbol. [ND] in formula (2) represents the number density ND (pieces / mm ２ ) is substituted.

2. A steel material according to claim 1, wherein the chemical composition is, in mass%, Mg: 0.0001 to 0.0100%, Ti: 0.001 to 0.150%, Nb: 0.001 to 0.080%, W: 0.001 to 0.400%, Zr: 0.001 to 0.200%, V: 0.001 to 0.400%, Ca: 0.0001 to 0.0100%, Te: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, Sn: 0.0001 to 0.0100%, rare earth elements: 0.0001 to 0.0100%, Co: 0.0001 to 0.0100%, A steel material containing one or more elements selected from the group consisting of Se: 0.0001 to 0.0100%, Sb: 0.0001 to 0.0100%, In: 0.0001 to 0.0100%, Mo: 0.01 to 0.10%, Cu: 0.01 to 0.20%, and Ni: 0.01 to 0.20%.

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