Steel material, and component for gas carburization machine construction

Abstract

To provide a steel material of which heat treatment stress during carburization hardening is reduced.SOLUTION: A steel material of the present embodiment contains, in mass percentage, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, O: 0.0015% or less, with the remainder consisting of Fe and impurities, and satisfies the equations (1A), (2), and (3). 63.0<90.0×C+10.0×Si+30.0×Mn-10.0×Cr+2000.0×N (1A), 0.60<Si+Cr<1.17 (2), Al / N≥2.00(3), here, in equations (1A), (2), and (3), the content in mass% of the correspondent element is substituted.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a steel material suitable for a material of a gas carburized mechanical structure part, which is a mechanical structure part manufactured by performing a gas carburizing treatment, and a gas carburized mechanical structure part.

Background Art

[0002] Mechanical structure parts are used in automobiles, construction vehicles, mining machines, etc. Mechanical structure parts are, for example, gears, shafts, gears, etc. As materials for mechanical structure parts, alloy steel materials for mechanical structures typified by SCr420, SCM420, and SNCM420 defined in JIS G 4053 (2016) are used.

[0003] Mechanical structure parts made of these steel materials are manufactured, for example, in the following manufacturing process. Hot working (hot forging) is performed on the steel material as the material, and then, if necessary, cutting is performed to manufacture an intermediate product having a desired shape. Heat treatment (quenching and tempering, carburizing treatment, or carbonitriding treatment, etc.) is performed on the intermediate product to adjust the hardness and microstructure of the intermediate product. Through the above manufacturing process, mechanical structure parts are manufactured.

[0004] As described above, cutting may be performed on the steel material during the manufacturing process of the mechanical structure part. Therefore, high machinability is required for the steel material used as the material of the mechanical structure part.

[0005] In recent years, with the electrification of automobiles and the like, the weight reduction and miniaturization of mechanical structure parts have been progressing. Therefore, excellent bending fatigue strength is required for mechanical structure parts.

[0006] As techniques for increasing the bending fatigue strength of parts for mechanical structures, gas carburizing treatment and gas carbonitriding treatment are known. In gas carburizing treatment and gas carbonitriding treatment, a hardened layer (carburized layer or carbonitrided layer) is formed on the surface layer of parts for mechanical structures. This hardened layer increases the bending fatigue strength of parts for mechanical structures. In the following description, parts for mechanical structures subjected to gas carburizing treatment are also referred to as gas carburized parts for mechanical structures.

[0007] By the way, when gas carburizing treatment (gas carburizing treatment and gas carbonitriding treatment) is carried out, gas carburized parts for mechanical structures are likely to deform. In this specification, the deformation that occurs in gas carburized parts for mechanical structures due to the influence of heat during gas carburizing treatment is referred to as heat treatment strain. Due to heat treatment strain, the shape of gas carburized parts for mechanical structures is distorted. The distortion of the shape of gas carburized parts for mechanical structures causes noise and vibration during the operation of automobiles and the like.

[0008] In the case of conventional automobiles and the like using an internal combustion engine as a power source, the sound emitted by the internal combustion engine was louder than the sound emitted by parts for mechanical structures such as gears, sprockets, and shafts. Therefore, the sound of parts for mechanical structures has not been noticed. However, due to the electrification of recent automobiles and the like, the sound emitted from the power source has been greatly reduced. As a result, the sound emitted by parts for mechanical structures such as gears has become prominent. Therefore, recently, there has been a demand for steel materials that can suppress heat treatment strain when gas carburizing treatment is carried out.

[0009] A technique related to the suppression of heat treatment strain has been proposed in International Publication No. 2014 / 038548 (Patent Document 1).

[0010] The steel material disclosed in Patent Document 1 contains, by mass%, C: 0.20 to 0.30%, Si: 0.10 to 1.50%, Mn: 0.10 to 1.20%, P: 0.030% or less, S: 0.030% or less, Cr: 1.30 to 2.50%, Cu: 0.30% or less, Al: 0.008 to 0.300%, O: 0.0030% or less, N: 0.0020 to 0.0300%, and the balance consists of Fe and unavoidable impurities. In this steel material, the martensite transformation start temperature (Ms point) is 460°C or lower. Further, when the hardness at a position 1.5 mm from the quenched end of the steel material measured by the Jominy end-quenching method is defined as J1.5, the hardness at a position 9 mm from the quenched end is defined as J9, and the hardness at a position 11 mm from the quenched end is defined as J11, (J9 / J1.5) is 0.70 to 0.85, and (J11 / J1.5) is 0.67 to 0.78. In this steel material, the Ms point is set low and the hardness is adjusted within a predetermined range. According to Patent Document 1, this suppresses the heat treatment deformation of the steel material.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0012] However, by means other than Patent Document 1, sufficient bending fatigue strength can be obtained in the gas carburized mechanical structure parts after gas carburizing treatment, and heat treatment strain may be suppressed in the gas carburizing treatment during the manufacturing process of the gas carburized mechanical structure parts.

[0013] The object of the present disclosure is to provide a steel material that has excellent machinability, provides excellent bending fatigue strength in gas carburized machine structural parts used as materials and manufactured by gas carburizing treatment, and suppresses heat treatment strain in the gas carburizing treatment during the manufacturing process of gas carburized machine structural parts, and a gas carburized machine structural part.

Means for Solving the Problems

[0014] The steel material of the present disclosure contains, by mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, and the balance consists of Fe and impurities, and satisfies Formula (1A), Formula (2), and Formula (3). 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, in Formula (1A), Formula (2), and Formula (3), the content of each corresponding element in mass% is substituted for each element symbol.

[0015] The steel material of the present disclosure contains, by mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and containing 0.0015% or less of O, further containing one or more selected from the group consisting of Group 1 to Group 4, the balance consisting of Fe and impurities, satisfying Formula (1B), Formula (2) and Formula (3). [Group 1] Cu: 0.40% or less, Ni: 0.30% or less, Mo: 0.35% or less, B: 0.0050% or less, and Co: one or more selected from the group consisting of 0.50% or less [Group 2] V: 0.15% or less, Nb: 0.050% or less, Ti: 0.050% or less, and W: one or more selected from the group consisting of 0.50% or less [Group 3] Ca: 0.0050% or less, and Mg: one or more selected from the group consisting of 0.005% or less [Group 4] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.10% or less, and Sb: one or more selected from the group consisting of 0.05% or less 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[effective N] (1B) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, in Formula (1B), Formula (2) and Formula (3), the content in mass% of the corresponding element is substituted for each element symbol, and when the element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, [effective N] in Formula (1B) is defined as follows. When Fn = N - Ti / 3.40 - B / 0.77 > 0: [Effective N]=N - Ti / 3.40 - B / 0.77 When Fn ≤ 0: [Effective N]=0

[0016] The component for the gas carburizing machine structure of the present disclosure has a hardened layer, and a core part inside the hardened layer, and the chemical composition of the core part is, by mass%, C: 0.18 - 0.30%, Si: 0.22 - 0.79%, Mn: 0.85 - 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 - 0.40%, Al: 0.005 - 0.100%, N: 0.0020 - 0.0300%, and O: 0.0015% or less, and contains the balance consists of Fe and impurities, and satisfies Formula (1A), Formula (2) and Formula (3). 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, for each element symbol in Formula (1A), Formula (2) and Formula (3), the content in mass% of the corresponding element is substituted.

[0017] The component for the gas carburizing machine structure of the present disclosure has a hardened layer, and a core part inside the hardened layer, and the chemical composition of the core part is, by mass%, C: 0.18 - 0.30%, Si: 0.22 - 0.79%, Mn: 0.85 - 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 - 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, containing furthermore, containing one or more selected from the group consisting of the first group to the fourth group, the balance consisting of Fe and impurities, satisfies formula (1B), formula (2) and formula (3). [First group] Cu: 0.40% or less, Ni: 0.30% or less, Mo: 0.35% or less, B: 0.0050% or less, and Co: 0.50% or less, one or more selected from the group consisting of [Second group] V: 0.15% or less, Nb: 0.050% or less, Ti: 0.050% or less, and W: 0.50% or less, one or more selected from the group consisting of [Third group] Ca: 0.0050% or less, and Mg: 0.005% or less, one or more selected from the group consisting of [Fourth group] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.10% or less, and Sb: 0.05% or less, one or more selected from the group consisting of 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[Effective N] (1B) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, in formula (1B), formula (2) and formula (3), the content in mass% of the corresponding element is substituted for each element symbol, and when the element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, [Effective N] in formula (1B) is defined as follows. If Fn=N-Ti / 3.40-B / 0.77>0: [Effective N] = N-Ti / 3.40-B / 0.77 If Fn≦0: [Valid N]=0 Effect of the Invention

[0018] The steel material disclosed herein has excellent machinability, and when used as a raw material and subjected to gas carburizing treatment to manufacture a gas carburized machine structural component, it has excellent bending fatigue strength, and heat treatment strain is suppressed in the gas carburizing treatment during the manufacturing process of the gas carburized machine structural component. The gas carburized machine structural component disclosed herein has excellent bending fatigue strength, and heat treatment strain is suppressed in the gas carburizing treatment during the manufacturing process. [Brief description of the drawings]

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

[0020] The inventors first examined, from the perspective of chemical composition, a steel material that has excellent machinability and, when used as a material for gas carburized machine structural parts manufactured by performing gas carburizing treatment, can obtain excellent bending fatigue strength. As a result, in terms of mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, are contained. When containing optional elements, further, instead of a part of Fe, one or more selected from the group consisting of the above-mentioned Group 1 to Group 4 are contained, and if the balance consists of Fe and impurities, excellent machinability can be obtained, and it was considered that excellent bending fatigue strength can be obtained in gas carburized machine structural parts manufactured by performing gas carburizing treatment using it as a material.

[0021] Therefore, regarding the technology capable of suppressing the heat treatment strain in the gas carburizing treatment during the manufacturing process of gas carburized machine structural parts in the steel material having the above-mentioned chemical composition, the inventors further examined.

[0022] First, similar to Patent Document 1, the inventors tried to reduce the heat treatment strain by lowering the Ms point of the steel material. However, in the steel material having the above-mentioned chemical composition, even when adjusting the content of each element so that the Ms point becomes low, there were cases where the heat treatment strain could not be sufficiently suppressed.

[0023] Therefore, the inventors tried to suppress the heat treatment strain from other viewpoints, rather than suppressing the heat treatment strain by lowering the Ms point.

[0024] First, the inventors examined the main factors of the heat treatment strain generated in the gas carburizing treatment. As a result of the examination, among the various strains constituting the heat treatment strain generated in the gas carburizing treatment, it was found that the main factor is transformation plastic strain. Therefore, the inventors considered that if the transformation plastic strain can be reduced, the heat treatment strain can be suppressed.

[0025] The transformation plastic strain ε is expressed by the following equation, where K is the transformation plastic coefficient and σ is the stress applied by transformation during gas carburizing treatment. Note that the following equation represents the transformation plastic strain ε after complete transformation (i.e., after the phase transformation has progressed 100%). ε = Kσ

[0026] Therefore, if the transformation plastic coefficient can be lowered, the transformation plastic strain is reduced, and as a result, the heat treatment strain is also suppressed. Thus, the inventors of the present invention examined means for reducing the transformation plastic coefficient from the viewpoint of chemical composition. As a result, the inventors obtained the following findings.

[0027] In the above chemical composition, C, Si, Mn, and N (effective N) have the effect of lowering the transformation plastic coefficient. On the other hand, Cr and Mo have the effect of increasing the transformation plastic coefficient. Therefore, within the range of the above chemical composition, by appropriately adjusting the C, Si, Mn, and N contents and the Cr and Mo contents, the transformation plastic coefficient can be lowered.

[0028] Based on the above findings, the inventors of the present invention examined the relationship between the C, Si, Mn, N, Cr, and Mo contents, the transformation plastic coefficient, and the heat treatment strain. As a result, the inventors found that when the chemical composition consists of essential elements, the heat treatment strain in gas carburizing treatment can be reduced by satisfying Equation (1A), and when the chemical composition contains essential elements and optional elements, the heat treatment strain can be reduced by satisfying Equation (1B). 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[effective N] (1B) Here, for each element symbol in Equation (1A) and Equation (1B), the content in mass% of the corresponding element is substituted. When the element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, [effective N] in Equation (1B) is defined as follows. When Fn = N - Ti / 3.40 - B / 0.77 > 0: [Effective N]=N - Ti / 3.40 - B / 0.77 When Fn ≤ 0: [Effective N]=0

[0029] However, even for a steel material that satisfies the above chemical composition and satisfies Formula (1A) or Formula (1B), there are cases where sufficient machinability, sufficient bending fatigue strength in a gas carburized mechanical structure part manufactured by performing gas carburizing treatment using the material, and sufficient reduction of heat treatment strain in the gas carburizing treatment cannot be obtained.

[0030] Therefore, the present inventors conducted further investigations. As a result, in a steel material having the above chemical composition and satisfying Formula (1A) and Formula (1B), if further satisfying the following Formulas (2) and (3), it has excellent machinability, and excellent bending fatigue strength can be obtained in a gas carburized mechanical structure part manufactured by performing gas carburizing treatment using the material, and the present inventors have found that heat treatment strain is suppressed in the gas carburizing treatment during the manufacturing process of the gas carburized mechanical structure part. 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, in each element symbol in Formulas (2) and (3), the content in mass% of the corresponding element is substituted.

[0031] The steel material and the gas carburized mechanical structure part of the present embodiment are completed based on the above technical idea and have the following configuration.

[0032] [1] By mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and, O: 0.0015% or less, and contains The remainder consists of Fe and impurities, and satisfies Formula (1A), Formula (2), and Formula (3), steel material. 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Herein, for each element symbol in Formula (1A), Formula (2), and Formula (3), the content in mass % of the corresponding element is substituted.

[0033] [2] In mass %, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and, O: 0.0015% or less, and contains further, one or more selected from the group consisting of the first group to the fourth group, the remainder consists of Fe and impurities, and satisfies Formula (1B), Formula (2), and Formula (3), steel material. [First group] Cu: 0.40% or less, Ni: 0.30% or less, Mo: 0.35% or less, B: 0.0050% or less, and, Co: 0.50% or less, one or more selected from the group consisting of [Second group] V: 0.15% or less, Nb: 0.050% or less, Ti: 0.050% or less, and, W: 0.50% or less, one or more selected from the group consisting of [Third group] Ca: 0.0050% or less, and one or more selected from the group consisting of Mg: 0.005% or less [Group 4] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.10% or less, and one or more selected from the group consisting of Sb: 0.05% or less 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[Effective N] (1B) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, for each element symbol in Formula (1B), Formula (2), and Formula (3), the content in mass % of the corresponding element is substituted. When the element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, [Effective N] in Formula (1B) is defined as follows. When Fn = N - Ti / 3.40 - B / 0.77 > 0: [Effective N] = N - Ti / 3.40 - B / 0.77 When Fn ≤ 0: [Effective N] = 0

[0034] [3] The steel material according to [2], containing the said Group 1 Steel material.

[0035] [4] The steel material according to [2] or [3], containing the said Group 2 Steel material.

[0036] [5] The steel material according to any one of [2] to [4], containing the said Group 3 Steel material.

[0037] [6] The steel material according to any one of [2] to [5], containing the fourth group, Steel material.

[0038] [7] A hardened layer, and a core part inside the hardened layer, The chemical composition of the core part is, by mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and, O: 0.0015% or less, contains, the balance being composed of Fe and impurities, satisfies formula (1A), formula (2) and formula (3), Parts for gas carburized mechanical structures. 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, in formula (1A), formula (2) and formula (3), the content in mass% of the corresponding element is substituted for each element symbol.

[0039] [8] A hardened layer, and a core part inside the hardened layer, The chemical composition of the core part is, by mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, containing further containing one or more selected from the group consisting of the first group to the fourth group, the balance consisting of Fe and impurities, satisfying formula (1B), formula (2) and formula (3), parts for gas carburized mechanical structures. [First Group] Cu: 0.40% or less, Ni: 0.30% or less, Mo: 0.35% or less, B: 0.0050% or less, and Co: 0.50% or less, one or more selected from the group consisting of [Second Group] V: 0.15% or less, Nb: 0.050% or less, Ti: 0.050% or less, and W: 0.50% or less, one or more selected from the group consisting of [Third Group] Ca: 0.0050% or less, and Mg: 0.005% or less, one or more selected from the group consisting of [Fourth Group] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.10% or less, and Sb: 0.05% or less, one or more selected from the group consisting of 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[effective N] (1B) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, for each element symbol in formula (1B), formula (2) and formula (3), the content in mass% of the corresponding element is substituted. When the element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, [Effective N] in formula (1B) is defined as follows. When Fn = N - Ti / 3.40 - B / 0.77 > 0: [Effective N] = N - Ti / 3.40 - B / 0.77 When Fn ≤ 0: [Effective N] = 0

[0040] [9] The component for a gas carburizing machine structure according to [8], containing the first group, a component for a gas carburizing machine structure.

[0041]

[10] The component for a gas carburizing machine structure according to [8] or [9], containing the second group, a component for a gas carburizing machine structure.

[0042]

[11] The component for a gas carburizing machine structure according to any one of [8] to

[10] , containing the third group, a component for a gas carburizing machine structure.

[0043]

[12] The component for a gas carburizing machine structure according to any one of [8] to

[11] , containing the fourth group, a component for a gas carburizing machine structure.

[0044]

[13] The component for a gas carburizing machine structure according to any one of [7] to

[12] , When the average C concentration in mass % in the region between the surface of the component for a gas carburizing machine structure and the position 0.1 mm deep from the surface is defined as the hardened layer average C concentration [C] s and the average C concentration in mass % in the region between the position 1.9 mm deep from the surface and the position 2.0 mm deep from the surface is defined as the core average C concentration [C] c then the hardened layer average C concentration [C]s and the average C concentration [C] of the core part c satisfy the formula (4), Parts for gas carburizing machine structures. [C] s -[C] c ≦0.55 (4)

[0045] Hereinafter, the steel material according to the present embodiment and the parts for gas carburizing machine structures manufactured using the steel material as a raw material will be described in detail. In addition, "%" regarding an element means mass % unless otherwise specified.

[0046] [Features of the steel material of the present embodiment] The steel material of the present embodiment includes the following features. (Feature 1) The chemical composition satisfies the range described in the present embodiment. (Feature 2) When the chemical composition consists of essential elements, it satisfies the formula (1A), and when the chemical composition contains essential elements and optional elements, it satisfies the formula (1B). 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[effective N] (1B) Here, in each element symbol in the formula (1A) and the formula (1B), the content in mass % of the corresponding element is substituted, and when the element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, [effective N] in the formula (1B) is defined as follows. When Fn = N - Ti / 3.40 - B / 0.77 > 0: [effective N] = N - Ti / 3.40 - B / 0.77 When Fn ≦ 0: [effective N] = 0 (Feature 3) The chemical composition satisfies the formula (2). 0.60 < Si + Cr < 1.17 (2) Here, in each element symbol in the formula (2), the content in mass % of the corresponding element is substituted. (Feature 4) The chemical composition satisfies formula (3). Al / N ≥ 2.00 (3) Here, for each element symbol in formula (3), the content in mass % of the corresponding element is substituted. Hereinafter, Features 1 to 4 will be described.

[0047] [(Feature 1) Regarding the chemical composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0048] C: 0.18 to 0.30% Carbon (C) increases the hardenability of the steel material and increases the bending fatigue strength of the gas carburized mechanical structure parts manufactured using the steel material. If the C content is less than 0.18%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.30%, coarse precipitates may be generated. Coarse precipitates are likely to be the starting points of cracks during the use of gas carburized mechanical structure parts. Therefore, the bending fatigue strength of gas carburized mechanical structure parts decreases. Therefore, the C content is 0.18 to 0.30%. The preferable lower limit of the C content is 0.19%, more preferably 0.20%, and even more preferably 0.21%. The preferable upper limit of the C content is 0.28%, more preferably 0.27%, even more preferably 0.26%, even more preferably 0.25%, and even more preferably 0.23%.

[0049] Si: 0.22 to 0.79% Silicon (Si) increases the hardenability of the steel material and increases the bending fatigue strength of the gas carburized mechanical structure parts manufactured using the steel material. Si further increases the tempering softening resistance of the hardened layer of the gas carburized mechanical structure parts manufactured using the steel material. If the Si content is less than 0.22%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the one hand, if the Si content exceeds 0.79%, even if the contents of other elements are within the scope of this embodiment, the intergranular oxidation layer after gas carburizing treatment becomes excessively deep. In this case, the bending fatigue strength of the gas carburized mechanical structure parts decreases. Therefore, the Si content is 0.22 - 0.79%. The preferable lower limit of the Si content is 0.23%, more preferably 0.25%, and even more preferably 0.30%. The preferable upper limit of the Si content is 0.78%, more preferably 0.77%, and even more preferably 0.75%.

[0050] Mn: 0.85 - 1.85% Manganese (Mn) enhances the hardenability of the steel material and increases the bending fatigue strength of the gas carburized mechanical structure parts manufactured from the steel material. Mn further suppresses the heat treatment strain. If the Mn content is less than 0.85%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Mn content exceeds 1.85%, even if the contents of other elements are within the scope of this embodiment, the formation of Mn oxide is promoted on the surface layer of the steel material during gas carburizing treatment. In this case, the intergranular oxidation layer of the gas carburized mechanical structure parts becomes excessively deep. Therefore, the bending fatigue strength of the gas carburized mechanical structure parts decreases. Therefore, the Mn content is 0.85 - 1.85%. The preferable lower limit of the Mn content is 0.86%, more preferably 0.88%, and even more preferably 0.90%. The preferable upper limit of the Mn content is 1.84%, more preferably 1.82%, even more preferably 1.80%, and even more preferably 1.75%.

[0051] P: 0.015% or less Phosphorus (P) is an impurity. If the P content exceeds 0.015%, even if the contents of other elements are within the scope of this embodiment, P segregates excessively at the grain boundaries, and the grain boundary strength decreases. As a result, the bending fatigue strength of the gas carburized mechanical structure parts made of the steel material decreases. Therefore, the P content is 0.015% or less. The P content is preferably as low as possible. However, excessive reduction of the P content increases the manufacturing cost. 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 0.012%, more preferably 0.010%, and even more preferably 0.008%.

[0052] S: 0.025% or less Sulfur (S) is an impurity. If the S content exceeds 0.025%, even if the contents of other elements are within the scope of this embodiment, coarse sulfide-based inclusions are generated. Therefore, the bending fatigue strength of the gas carburized mechanical structure parts made of steel is reduced. Therefore, the S content is 0.025% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases the manufacturing cost. 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 0.020%, more preferably 0.010%, and even more preferably 0.005%.

[0053] Cr: 0.13 - 0.40% Chromium (Cr) increases the hardenability of steel and the bending fatigue strength of gas carburized mechanical structure parts made of steel. If the Cr content is less than 0.13%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, Cr increases the transformation plasticity coefficient. If the Cr content exceeds 0.40%, even if the contents of other elements are within the scope of this embodiment, the transformation plasticity coefficient of the steel increases excessively, and the heat treatment strain of the steel during gas carburizing treatment increases. Therefore, the Cr content is 0.13 - 0.40%. The preferable lower limit of the Cr content is 0.15%, more preferably 0.17%, and even more preferably 0.19%. The preferable upper limit of the Cr content is 0.39%, more preferably 0.38%, even more preferably 0.37%, even more preferably 0.36%, even more preferably 0.35%, even more preferably 0.34%, and even more preferably 0.33%.

[0054] Al: 0.005 - 0.100% Aluminum (Al) combines with N in the steel to form AlN, and due to the pinning effect, it suppresses the coarsening of crystal grains during the heating of the gas carburizing process. Therefore, the bending fatigue strength of gas carburized mechanical structure parts made of the steel is increased. If the Al content is less than 0.005%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Al content exceeds 0.100%, even if the contents of other elements are within the scope of this embodiment, clustered coarse oxides are generated. The clustered coarse oxides reduce the bending fatigue strength of gas carburized mechanical structure parts. Therefore, the Al content is 0.005 - 0.100%. The preferable lower limit of the Al content is 0.008%, more preferably 0.010%. The preferable upper limit of the Al content is 0.080%, more preferably 0.070%, and even more preferably 0.060%.

[0055] N: 0.0020 - 0.0300% Nitrogen (N) reduces the transformation plasticity coefficient of the steel. Therefore, the heat treatment strain of the steel during the gas carburizing process is reduced. If the N content is less than 0.0020%, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the N content exceeds 0.0300%, even if the contents of other elements are within the scope of this embodiment, the hot workability of the steel is reduced. Therefore, the N content is 0.0020 - 0.0300%. The preferable lower limit of the N content is 0.0021%, more preferably 0.0022%. The preferable upper limit of the N content is 0.0280%, more preferably 0.0250%, still more preferably 0.0200%.

[0056] O: 0.0015% or less Oxygen (O) is an impurity. O combines with other elements in the steel material to form coarse oxide-based inclusions. Coarse oxide-based inclusions reduce the bending fatigue strength of parts for gas carburized mechanical structures. If the O content exceeds 0.0015%, even if the contents of other elements are within the scope of this embodiment, the bending fatigue strength of parts for gas carburized mechanical structures will decrease significantly. Therefore, the O content is 0.0015% or less. It is preferable that the O content is as low as possible. However, excessive reduction of the O content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the O content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, still more preferably 0.0005%. The preferable upper limit of the O content is 0.0013% or less, more preferably 0.0011%, still more preferably 0.0009%.

[0057] The balance of the chemical composition of the steel material of this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition refer to those mixed in from ore, scrap, or the manufacturing environment as raw materials during the industrial production of the steel material, which are contained unintentionally and are allowed within the range that does not adversely affect the steel material of this embodiment.

[0058] [Regarding Optional Elements] The chemical composition of the steel material of this embodiment may further contain one or more selected from the group consisting of the first group to the fourth group in place of a part of Fe. [The First Group] Cu: 0.40% or less, Ni: below 0.30%, Mo: below 0.35%, B: below 0.0050%, and, Co: below 0.50%, one or more selected from the group consisting of [Group 2] V: below 0.15%, Nb: below 0.050%, Ti: below 0.050%, and, W: below 0.50%, one or more selected from the group consisting of [Group 3] Ca: below 0.0050%, and, Mg: below 0.005%, one or more selected from the group consisting of [Group 4] Te: below 0.050%, Bi: below 0.100%, Pb: below 0.09%, Sn: below 0.10%, and, Sb: below 0.05%, one or more selected from the group consisting of Hereinafter, each element of Group 1 to Group 4 will be described.

[0059] [Group 1: Cu, Ni, Mo, B, and, Co] The chemical composition of the steel material of this embodiment may further contain Group 1 in place of a part of Fe. These elements are optional elements, and all of them enhance the hardenability of the steel material. Hereinafter, each element of Group 1 will be described.

[0060] Cu: below 0.40% Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, Cu enhances the hardenability of the steel material and increases the bending fatigue strength of the gas carburized mechanical structure parts. If even a little Cu is contained, the above effects can be obtained to a certain extent. However, if the Cu content exceeds 0.40%, even if the contents of other elements are within the range of this embodiment, the hardness of the steel material becomes excessively high and the machinability of the steel material deteriorates. Therefore, the Cu content is 0 to 0.40%, and when contained, it is 0.40% or less. The preferable lower limit of the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.05%. The preferable upper limit of the Cu content is 0.20%, more preferably 0.10%, and even more preferably 0.05%.

[0061] Ni: 0.30% or less Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, that is, when the Ni content exceeds 0%, Ni increases the hardenability of the steel material and increases the bending fatigue strength of the parts for gas carburized mechanical structures. Even if a little Ni is contained, the above effects can be obtained to a certain extent. However, if the Ni content exceeds 0.30%, even if the contents of other elements are within the scope of this embodiment, the hardness of the steel material becomes excessively high and the machinability of the steel material decreases. Therefore, the Ni content is 0 to 0.30%, and when contained, it is 0.30% or less. The preferable lower limit of the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.05%. The preferable upper limit of the Ni content is 0.20%, more preferably 0.10%, and even more preferably 0.05%.

[0062] Mo: 0.35% or less Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo increases the hardenability of the steel material and increases the bending fatigue strength of the parts for gas carburized mechanical structures. Even if a little Mo is contained, the above effects can be obtained to a certain extent. However, if the Mo content exceeds 0.35%, even if the contents of other elements are within the scope of this embodiment, the hardness of the steel material becomes excessively high and the machinability of the steel material decreases. Therefore, the Mo content is 0 to 0.35%, and when contained, it is 0.35% or less. The preferable lower limit of the Mo content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferable upper limit of the Mo content is 0.32%, more preferably 0.30%, and even more preferably 0.27%.

[0063] B: 0.0050% or less Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B increases the hardenability of the steel material and increases the bending fatigue strength of the parts for gas carburized mechanical structures. If even a little B is contained, the above effects can be obtained to a certain extent. However, if the B content exceeds 0.0050%, even if the contents of other elements are within the range of this embodiment, the hardness of the steel material becomes excessively high. Therefore, the machinability of the steel material deteriorates. Therefore, the B content is 0 to 0.0050%, and when contained, it is 0.0050% or less. The preferable lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The preferable upper limit of the B content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0064] Co: 0.50% or less Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When contained, that is, when Co exceeds 0%, Co increases the hardenability of the steel material and increases the bending fatigue strength of the parts for gas carburized mechanical structures. If even a little Co is contained, the above effects can be obtained to a certain extent. However, if the Co content exceeds 0.50%, even if the contents of other elements are within the range of this embodiment, the hardness of the steel material becomes excessively high. Therefore, the machinability of the steel material deteriorates. Therefore, the Co content is 0 to 0.50%, and when contained, it is 0.50% or less. The preferable lower limit of the Co content is 0.01%, more preferably 0.02%, still more preferably 0.05%, and still more preferably 0.07%. The preferable upper limit of the Co content is 0.49%, more preferably 0.48%, still more preferably 0.47%, and still more preferably 0.45%.

[0065] [Group 2: V, Nb, Ti, and W] The chemical composition of the steel material of the present embodiment may further contain Group 2 in place of a part of Fe. These elements are optional elements, and all of them form precipitates to increase the bending fatigue strength of the gas carburized mechanical structure parts made of the steel material. Hereinafter, each element of Group 2 will be described.

[0066] V: 0.15% or less Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, that is, when the V content exceeds 0%, V forms V precipitates such as V carbide and V carbonitride. The V precipitate suppresses the coarsening of crystal grains in the steel material during the gas carburizing treatment due to the pinning effect. Therefore, the bending fatigue strength of the gas carburized mechanical structure parts is increased. If even a little V is contained, the above effects can be obtained to a certain extent. However, if the V content exceeds 0.15%, even if the contents of other elements are within the range of the present embodiment, the hardness of the steel material becomes excessively high. Therefore, the machinability of the steel material deteriorates. Therefore, the V content is 0 to 0.15%, and when contained, the V content is 0.15% or less. The preferable lower limit of the V content is 0.01%, more preferably 0.03%, still more preferably 0.05%, and still more preferably 0.07%. The preferable upper limit of the V content is 0.13%, more preferably 0.10%, still more preferably 0.07%, and even more preferably 0.05%.

[0067] Nb: 0.050% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as Nb carbides and Nb carbonitrides. The Nb precipitates suppress the coarsening of crystal grains in the steel material during the gas carburizing treatment due to the pinning effect. Therefore, the bending fatigue strength of the parts for gas carburized mechanical structures is increased. Even if a little Nb is contained, the above effects can be obtained to a certain extent. However, if the Nb content exceeds 0.050%, even if the contents of other elements are within the range of this embodiment, the Nb precipitates coarsen. In this case, the coarsening of crystal grains during the gas carburizing treatment cannot be sufficiently suppressed. Therefore, the bending fatigue strength of the parts for gas carburized mechanical structures decreases. Therefore, the Nb content is 0 to 0.050%, and when contained, it is 0.050% or less. The preferable lower limit of the Nb content is 0.001%, more preferably 0.010%, still more preferably 0.030%, and even more preferably 0.050%. The preferable upper limit of the Nb content is 0.045%, more preferably 0.040%, still more preferably 0.030%, and even more preferably 0.020%.

[0068] Ti: 0.050% or less Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, that is, when the Ti content exceeds 0%, Ti forms Ti precipitates such as Ti carbides and Ti carbonitrides. The Ti precipitates suppress the coarsening of crystal grains in the steel material during the gas carburizing treatment due to the pinning effect. Therefore, the bending fatigue strength of the parts for gas carburized mechanical structures is increased. Even if a little Ti is contained, the above effects can be obtained to a certain extent. However, if the Ti content exceeds 0.050%, even if the contents of other elements are within the scope of this embodiment, the Ti precipitates will coarsen. In this case, the coarsening of crystal grains during the gas carburizing treatment cannot be sufficiently suppressed. Therefore, the bending fatigue strength of the parts for gas carburized mechanical structures decreases. Therefore, the Ti content is 0 to 0.050%, and when contained, it is 0.050% or less. The preferable lower limit of the Ti content is 0.001%, more preferably 0.010%, still more preferably 0.030%, and even more preferably 0.050%. The preferable upper limit of the Ti content is 0.045%, more preferably 0.040%, still more preferably 0.030%, and even more preferably 0.020%.

[0069] W: 0.50% or less Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, that is, when the W content exceeds 0%, W forms W precipitates such as W carbides and W carbonitrides. The W precipitates suppress the coarsening of crystal grains in the steel during the gas carburizing treatment due to the pinning effect. Therefore, the bending fatigue strength of the parts for gas carburized mechanical structures is increased. If even a small amount of W is contained, the above effects can be obtained to a certain extent. However, if the W content exceeds 0.50%, even if the contents of other elements are within the scope of this embodiment, the W precipitates will coarsen. In this case, the coarsening of crystal grains during the gas carburizing treatment cannot be sufficiently suppressed. Therefore, the bending fatigue strength of the parts for gas carburized mechanical structures decreases. Therefore, the W content is 0 to 0.50%, and when contained, it is 0.50% or less. The preferable lower limit of the W content is 0.01%, more preferably 0.05%, and still more preferably 0.10%. The preferable upper limit of the W content is 0.45%, more preferably 0.40%, and still more preferably 0.35%.

[0070] [Group 3: Regarding Ca and Mg] The chemical composition of the steel material of the present embodiment may further contain Group 3 in place of a part of Fe. These elements are optional elements, and all of them refine and spheroidize sulfides to increase the bending fatigue strength of gas carburized mechanical structure parts made of the steel material. Hereinafter, each element of Group 3 will be described.

[0071] Ca: 0.0050% or less Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, Ca refines sulfides in the steel material. Further, Ca promotes the spheroidization of sulfides in the steel material. Therefore, the bending fatigue strength of gas carburized mechanical structure parts made of the steel material increases. If even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0050%, even if the contents of other elements are within the range of the present embodiment, the formation of coarse Ca oxides in the steel material is promoted. In this case, the bending fatigue strength of gas carburized mechanical structure parts decreases. Therefore, the Ca content is 0 to 0.0050%, and when contained, it is 0.0050% or less. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0005%, still more preferably 0.0010%, and still more preferably 0.0015%. The preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%, and still more preferably 0.0025%.

[0072] Mg: 0.005% or less Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, that is, when the Mg content exceeds 0%, Mg refines sulfides in the steel material. Further, Mg promotes the spheroidization of sulfides in the steel material. Therefore, the bending fatigue strength of gas carburized mechanical structure parts made of the steel material increases. Even if a little Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.005%, even if the contents of other elements are within the range of this embodiment, Mg promotes the formation of coarse oxides in the steel material. In this case, the bending fatigue strength of gas carburized mechanical structure parts decreases. Therefore, the Mg content is 0 to 0.005%, and when contained, it is 0.005% or less. The preferable lower limit of the Mg content is 0.001%, and more preferably 0.002%. The preferable upper limit of the Mg content is 0.004%, and more preferably 0.003%.

[0073] [Group 4: Te, Bi, Pb, Sn, and Sb] The chemical composition of the steel material of this embodiment may further contain Group 4 in place of a part of Fe. These elements are optional elements, and all of them improve the machinability of the steel material. Hereinafter, each element of Group 4 will be described.

[0074] Te: 0.050% or less Tellurium (Te) is an optional element and may not be contained. That is, 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 a little Te is contained, the above effects can be obtained to some extent. However, if the Te content exceeds 0.050%, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the Te content is 0 to 0.050%, and when contained, it is 0.050% or less. The preferable lower limit of the Te content is 0.001%, more preferably 0.003%, and even more preferably 0.010%. The preferable upper limit of the Te content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0075] Bi: 0.100% or less Bismuth (Bi) is an optional element and may not be contained. That is, the Bi content may be 0%. When contained, that is, when the Bi content exceeds 0%, Bi improves the machinability of the steel material. If even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.100%, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the Bi content is 0 to 0.100%, and when contained, it is 0.100% or less. The preferable lower limit of the Bi content is 0.001%, more preferably 0.002%. The preferable upper limit of the Bi content is 0.080%, more preferably 0.050%.

[0076] Pb: 0.09% or less Lead (Pb) is an optional element and may not be contained. That is, the Pb content may be 0%. When contained, that is, when the Pb content exceeds 0%, Pb improves the machinability of the steel material. If even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.09%, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the Pb content is 0 to 0.09%, and when contained, the Pb content is 0.09% or less. The preferable lower limit of the Pb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable upper limit of the Pb content is 0.08%, more preferably 0.07%, even more preferably 0.06%, and even more preferably 0.05%.

[0077] Sn: Below 0.10% Tin (Sn) is an optional element and may not be contained. That is, the Sn content may be 0%. When contained, that is, when the Sn content exceeds 0%, Sn improves the machinability of the steel material. If Sn is contained even slightly, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.10%, the hot workability of the steel material deteriorates even if the contents of other elements are within the range of this embodiment. Therefore, the Sn content is 0 to 0.10%, and when contained, the Sn content is 0.10% or less. The preferable lower limit of the Sn content is 0.01%, and more preferably 0.03%. The preferable upper limit of the Sn content is 0.08%, and more preferably 0.05%.

[0078] Sb: 0.05% or less Antimony (Sb) is an optional element and may not be contained. That is, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb improves the machinability of the steel material. If Sb is contained even slightly, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.05%, the hot workability of the steel material deteriorates even if the contents of other elements are within the range of this embodiment. Therefore, the Sb content is 0 to 0.05%, and when contained, the Sb content is 0.05% or less. The preferable lower limit of the Sb content is 0.01%. The preferable upper limit of the Sb content is 0.04%.

[0079] [(Feature 2) Regarding Formula (1A) and Formula (1B)] In the steel material of this embodiment, further, when the chemical composition consists of essential elements, Formula (1A) is satisfied, and when the chemical composition contains essential elements and optional elements, Formula (1B) is satisfied. 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[Effective N] (1B) Here, for each element symbol in Formula (1A) and Formula (1B), the content in mass % of the corresponding element is substituted. When the element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, [Effective N] in Formula (1B) is defined as follows. When Fn = N - Ti / 3.40 - B / 0.77 > 0: [Effective N] = N - Ti / 3.40 - B / 0.77 When Fn ≤ 0: [Effective N] = 0 Hereinafter, Formula (1A) and Formula (1B) will be described.

[0080] Define F1A and F1B as follows. F1A = 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N F1B = 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[Effective N]

[0081] F1A and F1B are indicators of the transformation plasticity coefficient. When the transformation plasticity coefficient decreases, the heat treatment strain decreases. Among the above-mentioned elements in the chemical composition, C, Si, Mn, Cr, N, and Mo affect the transformation plasticity coefficient. Specifically, C, Si, Mn, and N have the effect of lowering the transformation plasticity coefficient. On the other hand, Cr and Mo have the effect of increasing the transformation plasticity coefficient. In F1A and F1B, the coefficients of each element are different according to the degree of influence of each element on the transformation plasticity coefficient.

[0082] Note that N in F1A and [effective N] in F1B mean the amount of N effective for reducing the transformation plasticity coefficient. When the chemical composition of the steel consists of essential elements and does not contain optional elements, N in the steel substantially contributes to the reduction of the transformation plasticity coefficient. Therefore, N is defined as a term in F1A.

[0083] On the other hand, when the chemical composition of the steel contains Ti and / or B as optional elements, the influence of N on the transformation plasticity coefficient varies according to the Ti content and the B content. Specifically, the higher the Ti content and the B content, the lower the influence amount of N on the transformation plasticity coefficient. Therefore, when Ti and / or B are contained, the effective N content for reducing the transformation plasticity coefficient is defined as [effective N]. When Fn > 0, [effective N] = Fn. On the other hand, when Fn ≤ 0, since there is no N contributing to the reduction of the transformation plasticity coefficient, 0 is substituted into [effective N].

[0084] If F1A and F1B are 63.0 or less, a sufficiently low transformation plasticity coefficient cannot be obtained in the steel. Therefore, the heat treatment strain of the steel in the gas carburizing treatment increases. If F1A and F1B are higher than 63.0, assuming that the steel satisfies Feature 1, Feature 3, and Feature 4, a sufficiently low transformation plasticity coefficient can be obtained in the steel. Therefore, the heat treatment strain of the steel in the gas carburizing treatment can be sufficiently suppressed.

[0085] Note that the upper limits of F1A and F1B are not particularly limited. However, when the chemical composition of the steel satisfies Feature 1, the upper limits of F1A and F1B are, for example, 149.1.

[0086] The preferable lower limit of F1A and F1B is 64.0, more preferably 66.0, still more preferably 68.0, still more preferably 69.0, and still more preferably 70.0. The preferable upper limit of F1A and F1B is 145.0, more preferably 140.0, and still more preferably 135.0.

[0087] [Regarding Formula (2) of (Feature 3)] In the steel material of the present embodiment, the chemical composition further satisfies the formula (2). 0.60 < Si + Cr < 1.17 (2) Here, in the element symbols in the formula (2), the content in mass % of the corresponding element is substituted.

[0088] Define F2 = Si + Cr. F2 is an index of the strength of the core part of a gas carburized mechanical structure part made of the steel material. Even if the steel material satisfies the characteristics 1, 2, and 4, if F2 is 0.60 or less, sufficient strength of the core part of the gas carburized mechanical structure part manufactured from the steel material cannot be obtained. In this case, the bending fatigue strength of the gas carburized mechanical structure part decreases. On the other hand, even if the steel material satisfies the characteristics 1, 2, and 4, if F2 is 1.17 or more, the hardness of the steel material becomes excessively high. In this case, the machinability of the steel material decreases. If F2 is higher than 0.60 and lower than 1.17, sufficient strength can be obtained at the core part of the gas carburized mechanical structure part manufactured from the steel material. Therefore, sufficient bending fatigue strength can be obtained in the gas carburized mechanical structure part. Furthermore, since it is possible to suppress the hardness of the steel material from becoming excessively high, sufficient machinability can be obtained in the steel material.

[0089] The preferable lower limit of F2 is 0.62, more preferably 0.64, and even more preferably 0.66. The preferable upper limit of F2 is 1.15, more preferably 1.13, and even more preferably 1.11.

[0090] [(Characteristic 4) Regarding formula (3)] In the steel material of the present embodiment, the chemical composition further satisfies the formula (3). Al / N ≧ 2.00 (3) Here, in the element symbols in the formula (3), the content in mass % of the corresponding element is substituted.

[0091] Define F3 as Al / N. F3 is an index for suppressing the coarsening of crystal grains in steel during the heating of gas carburizing treatment in the manufacturing process of gas carburized mechanical structure parts made of steel. Even if the steel satisfies Features 1 to 3, if F3 is less than 2.00, a sufficient amount of Al nitride will not be generated in the steel. In this case, during the heating of the gas carburizing treatment, the pinning effect of Al nitride is not fully exerted, and the crystal grains coarsen. When the crystal grains coarsen, the transformation plastic strain increases, and as a result, the heat treatment strain cannot be sufficiently suppressed.

[0092] If F3 is 2.00 or more, a sufficient amount of Al nitride will be generated in the steel. Therefore, during the heating of the gas carburizing treatment, the pinning effect of Al nitride is exerted, and the coarsening of crystal grains is sufficiently suppressed. Therefore, in gas carburized mechanical structure parts, the heat treatment strain is sufficiently suppressed.

[0093] The preferable lower limit of F3 is 2.10, more preferably 2.30, still more preferably 2.50, still more preferably 2.75, still more preferably 2.80, and still more preferably 2.85. Note that the upper limit of F3 is not particularly limited. The upper limit of F3 is, for example, 50.00.

[0094] [Effect of the steel of the present embodiment] The steel of the present embodiment satisfies Features 1 to 4. Therefore, the steel of the present embodiment has excellent machinability, and excellent bending fatigue strength can be obtained in gas carburized mechanical structure parts manufactured by using the steel as a material and performing gas carburizing treatment, and the heat treatment strain is suppressed in the gas carburizing treatment during the manufacturing process of gas carburized mechanical structure parts.

[0095] [Regarding the microstructure of the steel] The microstructure of the steel according to the present embodiment is not particularly limited. Heat treatment strain occurs when a phase transformation occurs. For example, during the heating in the carburizing treatment in the manufacturing process of gas carburized mechanical structure parts made of steel, the steel as the material is A c3It is heated to a temperature above a certain point. During the carburizing and quenching process, the surface layer of the steel material undergoes a phase transformation from austenite to martensite. At this time, heat treatment strain is likely to occur. As described above, regardless of the microstructure of the steel material itself serving as the raw material, as long as Features 1 to 4 are satisfied, the heat treatment strain can be sufficiently suppressed regardless of the microstructure of the steel material. Therefore, the microstructure of the steel material of the present embodiment is not particularly limited.

[0096] [Shape of the steel material of the present embodiment] The steel material of the present embodiment is a bar steel or a wire rod. A bar steel or a wire rod is a steel material extending in a bar shape, for example, a round steel. The steel material may be wound in a coil shape or may be cut to a predetermined length.

[0097] [Use of the steel material of the present embodiment] The steel material of the present embodiment can be used as a raw material for mechanical structure parts used in automobiles, industrial machines, etc. In particular, the steel material of the present embodiment is applicable as a raw material for gas carburized mechanical structure parts manufactured by performing gas carburizing treatment. The gas carburized mechanical structure parts are not particularly limited, but for example, gas carburized gears, gas carburized sprockets, gas carburized shafts, etc. Note that the steel material of the present embodiment is also applicable to uses other than mechanical structure parts.

[0098] [Manufacturing method of the steel material] An example of the manufacturing method of the steel material of the present embodiment will be described. The manufacturing method of the steel material described hereinafter is an example for manufacturing the steel material of the present embodiment. Therefore, the steel material having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method described hereinafter. However, the manufacturing method described hereinafter is a preferred example of the manufacturing method of the steel material of the present embodiment. In the present embodiment, as an example of the steel material, the manufacturing method of a bar steel will be described.

[0099] An example of the manufacturing method of the steel material of the present embodiment includes the following steps. (Step 1) Raw material preparation step (Step 2) Hot working step Hereinafter, each step will be described.

[0100] [(Process 1) Material Preparation Process] In the material preparation process, the material of the steel material of the present embodiment is prepared. Specifically, molten steel whose chemical composition satisfies Features 1 to 4 is produced. The refining method is not particularly limited, and a well-known method may be used. For example, refining (primary refining) in a converter is performed on hot metal produced by a well-known method. Secondary refining is performed on the molten steel tapped from the converter. In the secondary refining, alloying elements are added to the molten steel to adjust the components, and molten steel having a chemical composition satisfying Features 1 to 4 is produced.

[0101] Using the molten steel produced by the above refining method, a material is produced by a well-known casting method. For example, an ingot is produced by an ingot-making method using molten steel. Alternatively, a bloom may be produced by a continuous casting method using molten steel. By the above methods, a material (ingot or bloom) is produced.

[0102] [(Process 2) Hot Working Process] The produced material is hot-worked to produce a steel material. Usually, in the hot working process, one or more hot working operations are performed. When performing multiple hot working operations, the first hot working is, for example, rolling using slab rolling or hot forging, and the subsequent hot working may be rolling using a continuous rolling mill. The continuous rolling mill includes a plurality of rolling stands arranged in a row. The steel material after hot working is cooled to room temperature. By slab rolling and rolling using a continuous rolling mill, a billet is produced, and then the billet is reheated and further finish rolling using a continuous rolling mill is performed to produce a steel material of a desired size. Alternatively, a steel material may be produced from the material only by hot forging. The heating temperature of the material during hot working is not particularly limited, but is, for example, 1000 to 1300°C.

[0103] [Regarding the Parts for Gas Carburizing Machine Structure of the Present Embodiment] In this specification, the parts for gas carburizing machine structure mean the parts for machine structure that have been subjected to gas carburizing treatment. As described above, the parts for gas carburizing machine structure of the present embodiment are, for example, gas carburized gears, gas carburized sprockets, gas carburized shafts, etc.

[0104] The component for the gas carburizing machine structure of this embodiment includes a hardened layer and a core part inside the hardened layer. The hardened layer is formed on the surface layer of the component for the gas carburizing machine structure, and is a layer hardened by the intrusion of C through gas carburizing treatment. The hardened layer is also called the carburized layer. The hardened layer is formed from the surface of the component for the gas carburizing machine structure to a predetermined depth. The core part is the part inside the hardened layer and is a region not affected by the intrusion and diffusion of C by gas carburizing treatment. The hardness of the core part is lower than that of the hardened layer. It is a well-known technical matter for those skilled in the art that the hardened layer and the core part can be easily distinguished by well-known microstructure observation.

[0105] The component for the gas carburizing machine structure of this embodiment includes the following features. (Feature 5) The chemical composition of the core part contains, by mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, and the balance consists of Fe and impurities. When the chemical composition of the core part contains optional elements, the chemical composition of the core part contains, by mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, and further contains one or more selected from the group consisting of the above-mentioned Group 1 to Group 4, and the balance consists of Fe and impurities. (Feature 6) When the chemical composition of the core part consists of essential elements, it satisfies formula (1A), and when the chemical composition of the core part contains essential elements and optional elements, it satisfies formula (1B). 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[Effective N] (1B) Here, for each element symbol in Formula (1A) and Formula (1B), the content in mass % of the corresponding element is substituted. When an element is not contained, “0” is substituted for the corresponding element symbol. Furthermore, [Effective N] in Formula (1B) is defined as follows. When Fn = N - Ti / 3.40 - B / 0.77 > 0: [Effective N] = N - Ti / 3.40 - B / 0.77 When Fn ≤ 0: [Effective N] = 0 (Feature 7) The chemical composition of the core satisfies Formula (2). 0.60 < Si + Cr < 1.17 (2) Here, for each element symbol in Formula (2), the content in mass % of the corresponding element is substituted. (Feature 8) The chemical composition of the core satisfies Formula (3). Al / N ≥ 2.00 (3) Here, for each element symbol in Formula (3), the content in mass % of the corresponding element is substituted.

[0106] Regarding the actions of the elements in Feature 5, they are the same as the actions of the corresponding elements described in Feature 1 of the steel material of the present embodiment. Also, the technical significance of Formulas (1A), (1B), (2), and (3) in Features 6 to 8 is the same as the content described in Features 2 to 4 of the steel material of the present embodiment.

[0107] [Effects of the Gas Carburized Machine Structural Parts of the Present Embodiment] As described above, the gas carburized machine structural parts of the present embodiment include Features 5 to 8. Therefore, the gas carburized machine structural parts of the present embodiment can obtain excellent bending fatigue strength, and heat treatment strain is suppressed in the gas carburizing treatment during the manufacturing process.

[0108] [Preferred Forms of the Gas Carburized Machine Structural Parts of the Present Embodiment] Preferably, the component for the gas carburizing machine structure of the present embodiment includes the above-described features 5 to 8, and further includes the following feature 9. (Feature 9) The average C concentration in mass % in the region between the surface of the component for the gas carburizing machine structure (hereinafter also referred to as the component surface) and the position 0.1 mm deep from the component surface is defined as the hardened layer average C concentration [C]. s And The average C concentration in mass % in the region between the position 1.9 mm deep from the component surface and the position 2.0 mm deep from the component surface is defined as the core average C concentration [C]. c When defined as such, The hardened layer average C concentration [C] s And the core average C concentration [C] c Satisfy Equation (4). [C] s - [C] c ≦0.55 (4) Hereinafter, Feature 9 will be described.

[0109] [Regarding Equation (4) of (Feature 9)] F4 = [C] s - [C] c Is defined as. F4 is an index representing the difference between the C concentration of the hardened layer and the C concentration of the core of the component for the gas carburizing machine structure. The smaller F4 is, the smaller the difference between the C concentration of the hardened layer and the C concentration of the core, and the smaller the stress σ added by the transformation in the gas carburizing treatment. As described above, the transformation plastic strain ε is represented by the product of the transformation plastic coefficient K and the stress σ added by the transformation from austenite to martensite. In order to further reduce the transformation plastic strain ε, it is preferable to reduce not only the transformation plastic coefficient K but also the stress σ. The smaller F4 is, the lower the stress σ. Therefore, the transformation plastic strain ε is further suppressed.

[0110] If F4 is 0.55 or less, the stress σ added by the transformation in the gas carburizing treatment is sufficiently small. Therefore, the heat treatment strain in the gas carburizing treatment can be further suppressed.

[0111] A more preferable upper limit of F4 is 0.53, more preferably 0.51, still more preferably 0.49, and even more preferably 0.47. The lower limit of F4 is not particularly limited. The preferable lower limit of F4 is, for example, 0.25, more preferably 0.30, still more preferably 0.35, even more preferably 0.40, and even more preferably 0.45.

[0112] [Calculation method of F4] F4 can be obtained by the following method. First, the average C concentration [C] of the hardened layer s and the average C concentration [C] of the core part c are determined by an electron probe micro analyzer (EPMA: Electron Probe Micro Analyser). Specifically, a test piece with a cross-section perpendicular to the surface of the gas carburizing machine structure part as the observation surface is taken. The observation surface includes at least the surface of the gas carburizing machine structure part (part surface) and a region with a depth of 2.0 mm or more in the depth direction from the part surface. On the observation surface, from an arbitrary position on the part surface, a line analysis of the C concentration distribution is carried out using EPMA on a line segment with a depth of 2.0 mm in the depth direction. In EPMA, the measurement pitch is 0.005 mm, the acceleration voltage is 15 kV, the irradiation current is 500 nA, and the electron beam diameter is 3 μm. The above-mentioned line analysis is carried out at five arbitrary measurement locations on the surface of the gas carburizing machine structure part. Among the C concentrations obtained from the line analysis at all measurement locations, the arithmetic mean value in mass% of the C concentrations measured from the part surface to a depth position of 0.1 mm is determined. The obtained arithmetic mean value of the C concentration is taken as the average C concentration [C] of the hardened layer s (mass%). Note that the average C concentration [C] of the hardened layer s is rounded off to the second decimal place by rounding the third decimal place of the obtained numerical value. Among the C concentrations obtained from the line analysis at all measurement locations, the arithmetic mean value in mass% of the C concentrations measured from a depth position of 1.9 mm to a depth position of 2.0 mm from the part surface is determined. The obtained arithmetic mean value of the C concentration is taken as the average C concentration [C] of the core part c(in mass %). The average C concentration [C] of the hardened layer s is obtained by rounding off the third decimal place of the obtained numerical value to the value of the second decimal place. The obtained average C concentration [C] of the hardened layer s and the average C concentration [C] of the core part c are used to obtain F4.

[0113] [Manufacturing method of parts for gas carburizing machine structure of the present embodiment] The parts for gas carburizing machine structure made of the steel material of the present embodiment are manufactured, for example, by the following manufacturing method.

[0114] An example of the manufacturing method of the parts for gas carburizing machine structure includes, for example, a hot working process and a gas carburizing treatment process (gas carburizing quenching and tempering). In the hot working process, hot working is performed on the steel material of the present embodiment to manufacture an intermediate product having a predetermined shape. The hot working is, for example, hot forging. In the hot working process, after heating the steel material to A c3 point or higher, the steel material is processed. Therefore, the microstructure of the steel material is reset during heating in the hot working process. The heating temperature is, for example, 1000 to 1300 °C. The intermediate product after hot working is cooled to room temperature. If necessary, machining may be performed on the intermediate product after hot working. That is, the machining is an optional process.

[0115] A gas carburizing treatment process is performed on the intermediate product after hot working or after machining to manufacture parts for gas carburizing machine structure. The gas carburizing treatment process includes gas carburizing quenching and tempering. In gas carburizing quenching, in an atmosphere containing a well-known carburizing conversion gas, the intermediate product is heated and held above the A c3 transformation point and then rapidly cooled. The carburizing conversion gas means a well-known endothermic conversion gas (RX gas). The RX gas is a gas obtained by mixing hydrocarbon gases such as butane and propane with air and passing the mixture through a heated Ni catalyst for reaction, and is a mixed gas containing CO, H2, N2, etc. In the tempering treatment, the intermediate product carburized and quenched is held within a temperature range of 150 to 200 °C for a predetermined time. By adjusting the heat treatment temperature, the holding time at the heat treatment temperature, and the carbon potential Cp in the atmosphere during gas carburizing quenching, the average C concentration [C] of the hardened layer s can be adjusted.

[0116] Through the above steps, gas carburized machine structural parts made of the steel material of this embodiment are manufactured. Note that normalizing treatment or spheroidizing annealing treatment may be performed on the steel material after the hot working process as necessary. In the gas carburized machine structural parts manufactured by the manufacturing process including the gas carburizing process using the steel material of this embodiment, sufficient bending fatigue strength is obtained, and the heat treatment strain is sufficiently suppressed.

Example

[0117] Steel materials having the chemical compositions shown in Table 1-1 and Table 1-2 were manufactured.

[0118]

Table 1-1

[0119]

Table 1-2

[0120] In Table 1-2, "-" means that the corresponding element content is at the impurity level.

[0121] Continuous casting was carried out using molten steel to produce a slab. The produced slab was heated and subjected to cogging rolling and subsequent continuous rolling to produce a 160 mm × 160 mm billet. Note that the heating temperature of the slab during cogging rolling was 1000°C to 1300°C. Further, after reheating the produced billet, hot rolling was carried out to produce a steel material (bar) with a diameter of 50 mm. The heating temperature of the billet at this time was 1000°C to 1300°C. The produced steel material was air-cooled to room temperature. Through the above manufacturing process, steel materials of each test number with a diameter of 50 mm were manufactured.

[0122] [Regarding the evaluation test] The following evaluation tests were carried out on the manufactured steel materials. (Test 1) Machinability evaluation test (Test 2) Test for measuring the C concentration distribution in the depth direction (Test 3) Bending fatigue strength evaluation test (Test 4) Heat treatment strain evaluation test The following describes Tests 1 to 4.

[0123] [(Test 1) Machinability evaluation test] The machinability of the steel materials with each test number was evaluated by the following method. For the steel materials (round bars with a diameter of 50 mm) with each test number, external turning was carried out to evaluate the tool life. Specifically, external turning was carried out on the steel materials with each test number under the following conditions. The cutting tool used was a cemented carbide corresponding to P10 specified in JIS B 4053 (2013). The cutting speed was 150 m / min, the feed rate was 0.15 mm / rev, and the depth of cut was 1.0 mm. No lubricant was used during turning.

[0124] Under the above cutting conditions, external turning was carried out, and the time until the flank wear amount of the cutting tool reached 0.2 mm was defined as the tool life (Hr). Based on the tool life of the reference steel material with test number REF1, the tool life ratio for each test number was obtained by the following formula. The reference steel material with test number REF1 had a chemical composition corresponding to SCM440 specified in JIS G 4053:2016. Tool life ratio = Tool life (Hr) of the steel material with the test number / Tool life (Hr) of the reference steel material with test number REF1

[0125] If the obtained tool life ratio was 1.0 or more, it was judged that sufficient machinability was obtained (indicated by "○" in the "Machinability" column in Table 2). On the other hand, if the tool life ratio was less than 1.0, it was judged that sufficient machinability was not obtained (indicated by "×" in the "Machinability" column in Table 2).

[0126]

Table 2

[0127] [(Test 2) Test for Measuring C Concentration Distribution in Depth Direction] From the steel materials (round bars with a diameter of 50 mm) for each test number, simulated gas carburized mechanical structure parts were fabricated. Specifically, the steel materials (round bars with a diameter of 50 mm) for each test number were heated under the conditions of a heating temperature of 1200 °C and a holding time of 30 minutes. Then, with the finishing temperature set at 950 °C or higher, hot working (hot forging) was carried out to produce round bars with a diameter of 35 mm as intermediate products. Gas carburizing treatment (gas carburizing quenching process and tempering process) was performed on the intermediate products to fabricate simulated gas carburized mechanical structure parts.

[0128] In the gas carburizing treatment, the intermediate products (round bars) for 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%. Then, they were heated at 850 °C for 30 minutes and oil-cooled in oil at 130 °C. Tempering was carried out on the oil-cooled intermediate products at a tempering temperature of 180 °C and a holding time of 120 minutes. After the holding time elapsed, they were air-cooled. Simulated gas carburized mechanical structure parts for each test number were fabricated by the above method.

[0129] Based on the method described in the above [Calculation Method of F4], the average C concentration [C] of the hardened layer of the simulated gas carburized mechanical structure parts fabricated from the steel materials for each test number s and the average C concentration [C] of the core part c were determined. Based on the obtained average C concentration [C] of the hardened layer s and the average C concentration [C] of the core part c , F4 was determined. The average C concentration [C] of the hardened layer s , the average C concentration [C] of the core part c , and F4 are shown in Table 2.

[0130] [(Test 3) Bending Fatigue Strength Evaluation Test] Ono-type rotating bending fatigue test specimens simulating gas carburized machine structural parts were prepared from the steel material (steel bars with a diameter of 50 mm) with each test number. The shape of the Ono-type rotating bending fatigue test specimen is shown in Figure 1. The numbers in Figure 1 indicate the dimensions (unit: mm). "φ" in Figure 1 means diameter. "R1" means that the radius of curvature at the bottom of the notch is 1 mm.

[0131] Specifically, the steel material (steel bar with a diameter of 50 mm) for each test number was heated at a heating temperature of 1200°C for a holding time of 30 minutes. After that, hot processing (hot forging) was performed with a finishing temperature of 950°C or higher to produce a steel bar with a diameter of 35 mm. The steel bar with a diameter of 35 mm was machined (cut) to produce intermediate products for the Ono-type rotating bending fatigue test specimens. The intermediate products for each test number were subjected to gas carburizing treatment (gas carburizing quenching process and tempering process) in the same manner as in Test 2 to produce the Ono-type rotating bending fatigue test specimens shown in Figure 1.

[0132] Furthermore, a steel material (a steel bar having a diameter of 50 mm) with test number REF2 was prepared as a reference steel material for the bending fatigue strength evaluation test. The reference steel material with test number REF2 had a chemical composition equivalent to SMn420 specified in JIS G 4053:2016.

[0133] Ono-type rotating bending fatigue tests were conducted using Ono-type rotating bending fatigue test pieces with each test number. Multiple test pieces were prepared for each test number. Fatigue tests were conducted by changing the stress applied to each test piece, and the fatigue life was 10 million times (10 7 The highest stress at which the specimen did not break after repeated bending tests was taken 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 reversed.

[0134] The ratio of the bending fatigue strength of each test number to the bending fatigue strength of the reference steel material (test number REF2) was defined as the bending fatigue strength ratio. That is, the bending fatigue strength ratio was calculated by the following formula. Flexural fatigue strength ratio = (Flexural fatigue strength of each test number (MPa) / Flexural fatigue strength of reference steel material with test number REF2 (MPa))

[0135] If the obtained flexural fatigue strength ratio was 1.00 or more, it was determined that sufficient flexural fatigue strength was obtained (indicated by "○" in the "Flexural Fatigue Strength" column in Table 2). On the other hand, if the flexural fatigue strength ratio was less than 1.00, it was determined that the flexural fatigue strength was low (indicated by "×" in the "Flexural Fatigue Strength" column in Table 2).

[0136] [(Test 4) Heat Treatment Strain Evaluation Test] The heat treatment strain of the steel materials with each test number was determined by the following method. First, heat treatment strain evaluation test pieces simulating gas carburized machine structural parts were produced from the steel materials (round bars with a diameter of 50 mm) with each test number. The shape of the heat treatment strain evaluation test piece is shown in Figure 2. The numerical values in Figure 2 represent dimensions (unit: mm). "φ" in Figure 2 means diameter. In the following description, it is assumed that the heat treatment strain evaluation test piece is arranged in a three-dimensional orthogonal coordinate system (xyz coordinate system) for explanation. Note that the longitudinal direction of the heat treatment strain evaluation test piece is defined as the y direction, the height direction of the heat treatment strain evaluation test piece is defined as the z direction, and the direction perpendicular to the z direction and the y direction is defined as the x direction.

[0137] As the reference steel material for the heat treatment strain evaluation test, a steel material (round bar with a diameter of 50 mm) with test number REF3 was prepared. The reference steel material with test number REF3 had a chemical composition corresponding to SCM420 specified in JIS G 4053:2016.

[0138] The steel materials with each test number were heated under the conditions of a heating temperature of 1200 °C and a holding time of 30 minutes. Then, hot working (hot forging) was performed with a finishing temperature of 950 °C or higher to produce round bars with a diameter of 25 mm. The round bars with a diameter of 25 mm were machined (cutting) to process intermediate products of heat treatment strain evaluation test pieces with a diameter of 10 mm and a length of 100 mm. A groove with a depth of 2.5 mm, a width of 4 mm, and a length of 100 mm was machined (cutting) on the side surface of the intermediate product to produce the heat treatment strain evaluation test piece shown in Figure 2.

[0139] The straightness of the heat treatment strain evaluation test piece before gas carburizing treatment was measured by the following method. Referring to FIG. 2, the outer peripheral shape of the test piece in a plane perpendicular to the longitudinal direction (y direction) (a plane on the xz plane) was obtained at the measurement positions A1 and A2 at both ends in the longitudinal direction and the measurement position A3 at the center position in the longitudinal direction. Specifically, as the three-dimensional dimension measuring machine, a CNC three-dimensional dimension measuring machine (product name: Crysta-Apex) manufactured by Mitutoyo Corporation was used. As shown in FIG. 3, in a plane perpendicular to the longitudinal direction of the heat treatment strain evaluation test piece (hereinafter referred to as the measurement plane), the heat treatment strain evaluation test piece was arranged so that the groove portion of the test piece was arranged downward. In the CNC three-dimensional dimension measuring machine, at each of the measurement positions A1 to A3, with the top of the measurement plane being 0°, the coordinates (x, y, z) of the measurement points P1 to P7 with a 45° pitch around the outer periphery were obtained. Since the 180° position corresponds to the groove portion when the top of the measurement plane is 0°, the coordinates at the 180° position were not measured. That is, the coordinates were obtained at the seven measurement points P1 to P7 on the outer periphery of each measurement plane.

[0140] As shown in FIG. 4, in each measurement plane, a circle C0 was approximated by the least squares method using the coordinates of the seven obtained measurement points P1 to P7. From the obtained circle C0, the three-dimensional coordinates (x, y, z) of the center position CP of the circle C0 were obtained. In the following description, the center position of the circle C0 at the measurement position A1 is defined as CP1. Similarly, the center position of the circle C0 at the measurement position A2 is defined as CP2, and the center position of the circle C0 at the measurement position A3 is defined as CP3.

[0141] As shown in FIG. 5, based on the coordinates of the three obtained center positions CP1 to CP3, the distance D in the x direction between the line segment SG connecting the center position CP1 and the center position CP2 and the center position CP3 was obtained, and the obtained distance D was defined as the amount of bend D. The amount of bend D is an index indicating the degree of bending of the test piece.

[0142] After obtaining the amount of bending D of the heat treatment strain evaluation test piece before the gas carburizing treatment, the gas carburizing treatment was carried out on the heat treatment strain evaluation test piece under the same conditions as in Test 2. For the heat treatment strain evaluation test piece after the gas carburizing treatment, the amount of bending D was obtained in the same manner as the heat treatment strain evaluation test piece before the gas carburizing treatment. The difference value ΔD between the amount of bending D of the heat treatment strain evaluation test piece after the gas carburizing treatment and the amount of bending D of the heat treatment strain evaluation test piece before the gas carburizing treatment was obtained. The obtained difference value ΔD is an index indicating the amount of heat treatment strain introduced by the gas carburizing treatment.

[0143] Using the difference value ΔD obtained from the reference steel material of test number REF3 and the difference value ΔD of each test number, the bending amount ratio of the heat treatment strain evaluation test piece of each test number was obtained by the following formula. Bending amount ratio = difference value ΔD of the test number / difference value ΔD of the reference steel material of test number REF3 If the obtained bending amount ratio was more than 0.7 and less than or equal to 0.8, it was judged that the heat treatment strain was sufficiently suppressed (indicated by "○" in the "heat treatment strain" column in Table 2). If the bending amount ratio was less than or equal to 0.7, it was judged that the heat treatment strain was further suppressed (indicated by "◎" in the "heat treatment strain" column in Table 2). On the other hand, if the bending amount ratio was more than 0.8, it was judged that the heat treatment strain was not sufficiently suppressed (indicated by "×" in the "heat treatment strain" column in Table 2).

[0144] [Test results] The test results are shown in Table 2. Referring to Table 2, the steel materials of test numbers 1 to 49 satisfied Features 1 to 4, and the simulated gas carburized mechanical structure parts made from the steel materials satisfied Features 5 to 8. Therefore, sufficient machinability and sufficient bending fatigue strength were obtained. Furthermore, the heat treatment strain could be sufficiently suppressed.

[0145] For the steel materials of test numbers 3 to 49, in addition to Features 5 to 8, the simulated gas carburized mechanical structure parts made from the steel materials satisfied Feature 9. Therefore, the heat treatment strain could be further suppressed.

[0146] On the one hand, in Test No. 50, the Mn content was too low. Therefore, sufficient bending fatigue strength could not be obtained. Furthermore, the heat treatment strain was not sufficiently suppressed.

[0147] In Test No. 51, the Cr content was too high. Therefore, the heat treatment strain was not sufficiently suppressed.

[0148] In Test No. 52, the N content was too low. Therefore, the heat treatment strain was not sufficiently suppressed.

[0149] In Test No. 53, F1A was too low. Therefore, the heat treatment strain was not sufficiently suppressed.

[0150] In Test Nos. 54 to 56, F1B was too low. Therefore, the heat treatment strain was not sufficiently suppressed.

[0151] In Test No. 57, F2 was too low. Therefore, sufficient bending fatigue strength could not be obtained.

[0152] In Test No. 58, F2 was too high. Therefore, sufficient machinability could not be obtained.

[0153] In Test No. 59, F3 was too low. Therefore, the heat treatment strain was not sufficiently suppressed.

[0154] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. By mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and, O: 0.0015% or less, contains, the balance consists of Fe and impurities, satisfies formula (1A), formula (2) and formula (3), steel material. 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, for each element symbol in formula (1A), formula (2) and formula (3), the content in mass% of the corresponding element is substituted.

2. By mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and, O: 0.0015% or less, contains, further contains one or more selected from the group consisting of the first group to the fourth group, the balance consists of Fe and impurities, satisfies formula (1B), formula (2) and formula (3), steel material. [First group] Cu: 0.40% or less, Ni: 0.30% or less, Mo: 0.35% or less, B: 0.0050% or less, and, Co: 0.50% or less, one or more selected from the group consisting of [Second group] V: 0.15% or less, Nb: 0.050% or less, Ti: 0.050% or less, and, W: 0.50% or less, one or more selected from the group consisting of [Third group] Ca: 0.0050% or less, and, Mg: 0.005% or less, one or more selected from the group consisting of [Fourth group] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.10% or less, and, Sb: 0.05% or less, one or more selected from the group consisting of 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[effective N] (1B) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, for each element symbol in formula (1B), formula (2) and formula (3), the content in mass% of the corresponding element is substituted, and when the element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, [Effective N] in formula (1B) is defined as follows. When Fn = N - Ti / 3.40 - B / 0.77 > 0: [Effective N] = N - Ti / 3.40 - B / 0.77 When Fn ≤ 0: [Effective N] = 0

3. The steel material according to claim 2, containing the first group, steel material.

4. The steel material according to claim 2, containing the second group, steel material.

5. The steel material according to claim 2, containing the third group, steel material.

6. The steel material according to claim 2, containing the fourth group, steel material.

7. A hardened layer and, a core part inside the hardened layer, comprising: The chemical composition of the core part is, by mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and, O: 0.0015% or less, containing, the balance consisting of Fe and impurities, satisfying formula (1A), formula (2), and formula (3), a component for gas carburized mechanical structures. 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr + 2000.0×N (1A) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, for each element symbol in formula (1A), formula (2), and formula (3), the content in mass% of the corresponding element is substituted.

8. A hardened layer and, a core part inside the hardened layer, comprising: The chemical composition of the core part is, by mass%, C: 0.18 to 0.30%, Si: 0.22 to 0.79%, Mn: 0.85 to 1.85%, P: 0.015% or less, S: 0.025% or less, Cr: 0.13 to 0.40%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and, O: 0.0015% or less, containing, furthermore, containing one or more selected from the group consisting of the first group to the fourth group, the balance consisting of Fe and impurities, satisfying formula (1B), formula (2), and formula (3), a component for gas carburized mechanical structures. [The first group] Cu: 0.40% or less, Ni: 0.30% or less, Mo: 0.35% or less, B: 0.0050% or less, and, Co: 0.50% or less, one or more selected from the group consisting of [The second group] V: 0.15% or less, Nb: 0.050% or less, Ti: 0.050% or less, and, W: 0.50% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, and Mg: 0.005% or less, one or more selected from the group consisting of [Group 4] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.10% or less, and Sb: 0.05% or less, one or more selected from the group consisting of 63.0 < 90.0×C + 10.0×Si + 30.0×Mn - 10.0×Cr - 0.50×Mo + 2000.0×[Effective N] (1B) 0.60 < Si + Cr < 1.17 (2) Al / N ≥ 2.00 (3) Here, in formula (1B), formula (2), and formula (3), the content in mass% of the corresponding element is substituted for each element symbol. When an element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, [Effective N] in formula (1B) is defined as follows. When Fn = N - Ti / 3.40 - B / 0.77 > 0: [Effective N] = N - Ti / 3.40 - B / 0.77 When Fn ≤ 0: [Effective N] = 0

9. The component for a gas carburized mechanical structure according to claim 8, containing the said Group 1, a component for a gas carburized mechanical structure.

10. The component for a gas carburized mechanical structure according to claim 8, containing the said Group 2, a component for a gas carburized mechanical structure.

11. The component for a gas carburized mechanical structure according to claim 8, containing the said Group 3, a component for a gas carburized mechanical structure.

12. The component for a gas carburized mechanical structure according to claim 8, containing the said Group 4, a component for a gas carburized mechanical structure.

13. The component for a gas carburized mechanical structure according to any one of claims 7 to 12, Define the average C concentration in mass % in the region between the surface of the parts for the gas carburizing machine structure and the position 0.1 mm deep from the surface as the hardened layer average C concentration [C]. ｓ and When the average C concentration in mass % in the region between the position 1.9 mm deep from the said surface and the position 2.0 mm deep from the said surface is defined as the core average C concentration [C] ｃ then The average C concentration [C] of the hardened layer ｓ and the average C concentration [C] of the core part ｃ satisfy formula (4). a component for a gas carburized mechanical structure. [C] ｓ -[C] ｃ ≤0.55 (4)

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