Carburized machine structural component

Abstract

To provide a carburized component for a machine structure which has excellent surface fatigue strength and in which heat treatment strain is suppressed.SOLUTION: A carburized mechanical structural component according to the present invention satisfies the chemical composition described in the specification, and 1A defined by Formula (Fn1A) is 0.27 to 0.35, and Fn1A and Fn2A defined by Formula (2A) satisfy Formula (3A). Fn1A = exp (-0.01 * (170 - 45 * Mn - 15 * (Cr + Mo))) (1A) Fn2A = 20.0 * C + 10.0 * Si + 30.0 * Mn - 10.0 * Cr -0.5 * Mo + 2000.0 * N (2A) Fn1A * Fn2A ≥ 20.1 (3A) SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a carburized machine structural component, which is a machine structural component manufactured by carrying out a carburizing and quenching treatment. [Background technology]

[0002] Mechanical structural parts are used in automobiles, construction vehicles, mining machinery, etc. Mechanical structural parts include gears, shafts, and cogwheels. As materials for mechanical structural parts, alloy steels for mechanical structures, such as SCr420, SCM420, and SNCM420 specified in JIS G 4053 (2016), are used.

[0003] Machine structural parts made from these steel materials are manufactured, for example, by the following manufacturing process. The steel material is subjected to hot working (hot forging), and then cutting is performed as necessary to produce an intermediate product of the desired shape. The intermediate product is subjected to heat treatment (quenching and tempering, or carburizing and quenching treatment, etc.) to adjust the hardness and microstructure of the intermediate product. Machine structural parts are manufactured by the above manufacturing process.

[0004] Machine structural components used in the above applications are required to have high surface fatigue strength. Carburizing and quenching are known as a method for increasing the surface fatigue strength of machine structural components. In this specification, carburizing and quenching include carbonitriding and quenching. In carburizing and quenching, a hardened layer (carburized layer or carbonitriding layer) is formed on the surface of the machine structural component. This hardened layer improves the surface fatigue strength of the machine structural component. In the following description, machine structural components that have been subjected to carburizing and quenching are also referred to as carburized machine structural components.

[0005] An example of a carburizing and quenching process is vacuum carburizing and quenching. However, when vacuum carburizing and quenching is performed, the carburized machine structural parts are prone to deformation. In this specification, the deformation that occurs in the carburized machine structural parts due to the influence of heat during vacuum carburizing and quenching is referred to as heat treatment distortion. Heat treatment distortion distorts the shape of the carburized machine structural parts. The distortion of the shape of the carburized machine structural parts causes noise and vibration when the automobile or the like is operating.

[0006] In conventional automobiles and other vehicles powered by internal combustion engines, the noise emitted by the engine was louder than the noise emitted by gears, cogwheels, shafts, and other mechanical structural parts. For this reason, the noise of mechanical structural parts has not received much attention. However, with the recent trend toward electrification of automobiles and other vehicles, the noise emitted from the power source has been significantly reduced, and as a result, the noise emitted by gears and other mechanical structural parts has become more noticeable. Therefore, there has been a recent demand for carburized mechanical structural parts that can suppress heat treatment distortion during vacuum carburizing and quenching.

[0007] A technique for suppressing heat treatment distortion is proposed in International Publication No. 2014 / 038548 (Patent Document 1).

[0008] The steel material disclosed in Patent Document 1 contains, by mass%, 0.20-0.30% C, 0.10-1.50% Si, 0.10-1.20% Mn, 0.030% or less P, 0.030% or less S, 1.30-2.50% Cr, 0.30% or less Cu, 0.008-0.300% Al, 0.0030% or less O, 0.0020-0.0300% N, and the balance being Fe and unavoidable impurities. This steel material has a martensitic transformation start temperature (Ms point) of 460°C or less. Furthermore, when the hardness measured by the Jominy end-quenching method at a position 1.5 mm from the quenched end of a steel material is defined as J1.5, the hardness at a position 9 mm from the quenched end as J9, and the hardness at a position 11 mm from the quenched end as J11, (J9 / J1.5) is 0.70 to 0.85, and (J11 / J1.5) is 0.67 to 0.78. This steel material has a low Ms point and its hardness is adjusted to a predetermined range. Patent Document 1 states that this suppresses deformation during heat treatment of the steel material. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2014 / 038548 Summary of the Invention [Problem to be solved by the invention]

[0010] However, heat treatment distortion may be suppressed in the vacuum carburizing and quenching process during the manufacturing process of carburized machine structural components by means other than that disclosed in Patent Document 1.

[0011] An object of the present disclosure is to provide a carburized machine structural component that has excellent surface fatigue strength and suppresses heat treatment strain. [Means for solving the problem]

[0012] The carburized machine structural component of the present disclosure comprises: A hardened layer; a core portion located inside the hardened layer, The C concentration in the hardened layer is 0.60% by mass or more, The chemical composition of the core is, in mass%, C: 0.13~0.30%, Si: 1.50-2.00%, Mn: 0.60-1.45% P: 0.015% or less, S: 0.025% or less, Cr: 0.06 to 0.40%, Mo: 0.35% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, the balance being Fe and impurities, Fn1A defined by formula (1A) is 0.27 to 0.35, The Fn1A and Fn2A defined by the formula (2A) satisfy the formula (3A). Fn1A=exp(-0.01×(170-45×Mn-15×(Cr+Mo))) (1A) Fn2A=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+2000.0×N (2A) Fn1A×Fn2A≧20.1 (3A) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (2A).

[0013] The carburized machine structural component of the present disclosure comprises: A hardened layer; a core portion located inside the hardened layer, The C concentration in the hardened layer is 0.60% by mass or more, The chemical composition of the core is, in mass%, C: 0.13~0.30%, Si: 1.50-2.00%, Mn: 0.60-1.45% P: 0.015% or less, S: 0.025% or less, Cr: 0.06 to 0.40%, Mo: 0.35% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, Fn1B defined by formula (1B) is 0.27 to 0.35, The Fn1B and Fn2B defined by formula (2B) satisfy formula (3B). [Group 1] Cu: 0.20% or less, and Ni: 0.30% or less, one or more selected from the group consisting of [Group 2] V: 0.15% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.15% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, and Mg: 0.010% 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: 0.050% or less, and Sb: 0.050% or less, one or more selected from the group consisting of Fn1B=exp(-0.01×(170-45×Mn-15×(Cr+Mo+Ni))) (1B) Fn2B=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+500.0×Nb+2000.0×N (2B) Fn1B×Fn2B≧20.1 (3B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1B) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol. [Effects of the Invention]

[0014] The carburized machine structural component of the present disclosure has excellent surface fatigue strength and suppresses heat treatment strain. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a side view of a small roller test piece for a surface fatigue strength evaluation test. [Figure 2] FIG. 2 is a schematic diagram of the roller pitting test carried out in the surface fatigue strength evaluation test. [Figure 3] FIG. 3 is a front view of the large roller test piece in FIG. [Figure 4] FIG. 4 is a perspective view of a test piece for evaluating heat treatment distortion used in a heat treatment distortion evaluation test. [Figure 5] FIG. 5 is a view showing a surface (measurement surface) perpendicular to the longitudinal direction of the test piece for evaluating heat treatment distortion shown in FIG. [Figure 6] FIG. 6 is a schematic diagram in which a circle is approximated by the least squares method based on the measurement points in FIG. [Figure 7] FIG. 7 is a schematic diagram for explaining a method for determining the amount of bending based on the center positions obtained on the three measurement surfaces in the heat treatment distortion evaluation test. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present inventors have investigated carburized machine structural parts that can provide excellent surface fatigue strength from the viewpoint of chemical composition, and have found the following.

[0017] Carburized machine structural parts may slide against other parts during use. During sliding, the sliding surfaces generate heat, causing the C that had been dissolved in the crystal grains to precipitate. As a result, the strength of the sliding surfaces decreases, accelerating fatigue failure. In this way, the surface fatigue strength of carburized machine structural parts decreases. In other words, by increasing the tempering softening resistance of carburized machine structural parts, it is possible to suppress the decrease in strength due to heat generation at the sliding surfaces and increase the surface fatigue strength.

[0018] Silicon is known to increase temper softening resistance. However, it is known that an increase in the Si content in the steel used to make carburized machine structural parts inhibits gas carburizing. However, in vacuum carburizing, the carburizing inhibition caused by an increase in the Si content in the steel is minimal. Therefore, in carburized machine structural parts manufactured using vacuum carburizing, it is effective to increase the Si content in order to obtain excellent surface fatigue strength.

[0019] Based on the above findings, the inventors have concluded that excellent contact fatigue strength can be obtained in a carburized machine structural component comprising a hardened layer and a core portion deeper than the hardened layer, wherein the C concentration in the hardened layer is 0.60% or more by mass, and the chemical composition of the core portion is, by mass, C: 0.13 to 0.30%, Si: 1.50 to 2.00%, Mn: 0.60 to 1.45%, P: 0.015% or less, S: 0.025% or less, Cr: 0.06 to 0.40%, Mo: 0.35% or less, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, and when optional elements are contained, further containing one or more elements selected from the group consisting of the above-mentioned Groups 1 to 4 in place of a portion of Fe, with the balance being Fe and impurities.

[0020] Therefore, the present inventors further investigated a technique capable of suppressing heat treatment distortion in carburized machine structural parts having the above-mentioned chemical composition.

[0021] Initially, the present inventors attempted to reduce heat treatment strain by lowering the Ms point, as in Patent Document 1. However, in carburized machine structural components having the above-mentioned chemical composition, there were cases where heat treatment strain could not be sufficiently suppressed even when the contents of each element were adjusted to lower the Ms point.

[0022] Therefore, the present inventors attempted to suppress the heat treatment distortion from a different perspective, rather than suppressing the heat treatment distortion by lowering the Ms point.

[0023] First, the inventors investigated the main factors that cause heat treatment strain during vacuum carburizing and quenching. As a result of the investigation, it was found that, among the various strains that constitute the heat treatment strain during vacuum carburizing and quenching, the main factors are transformation expansion strain and transformation plastic strain. Therefore, the inventors thought that if both transformation expansion strain and transformation plastic strain could be reduced, heat treatment strain could be suppressed.

[0024] Transformation expansion strain is caused by volume expansion due to martensitic transformation during vacuum carburizing and quenching. Therefore, transformation expansion strain can be reduced by suppressing excessive martensitic transformation during vacuum carburizing and quenching. In order to suppress excessive martensitic transformation, it is effective to increase the stability of the austenite phase.

[0025] On the other hand, if the stability of the austenite phase is excessively high, the amount of retained austenite after vacuum carburizing and quenching increases. As a result, the surface fatigue strength of the carburized machine structural component may decrease. In other words, in order to reduce the transformation expansion strain while suppressing the decrease in the surface fatigue strength of the carburized machine structural component, it is important to appropriately adjust the stability of the austenite phase.

[0026] Therefore, the present inventors have investigated the relationship between the chemical composition of the core and the stability of the austenite phase, and have found that when the chemical composition of the core consists of only essential elements, Fn1A defined by formula (1A) is an index of the stability of the austenite phase, and when the chemical composition of the core contains essential elements and optional elements, Fn1B defined by formula (1B) is an index of the stability of the austenite phase. Fn1A=exp(-0.01×(170-45×Mn-15×(Cr+Mo))) (1A) Fn1B=exp(-0.01×(170-45×Mn-15×(Cr+Mo+Ni))) (1B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (1B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0027] The transformation plastic strain ε, another major factor in heat treatment strain, is expressed by the following formula, where K is the transformation plasticity coefficient and σ is the stress applied by the transformation during vacuum carburizing and quenching. Note that the formula below shows the transformation plastic strain ε after complete transformation (i.e., after 100% of the phase transformation has progressed). ε=Kσ

[0028] Referring to the above formula, if the transformation plasticity coefficient K can be reduced, the transformation plastic strain ε can be reduced. Therefore, the present inventors have investigated means for reducing the transformation plasticity coefficient K from the viewpoint of the chemical composition of the core.

[0029] In the above-mentioned chemical composition, C, Si, Mn, Nb, and N (available 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. Therefore, by appropriately adjusting the contents of C, Si, Mn, Nb, and N and the contents of Cr and Mo within the above-mentioned chemical composition range, the transformation plasticity coefficient can be lowered.

[0030] Therefore, the present inventors have investigated the relationship between the contents of C, Si, Mn, N, Cr, Mo, and Nb and the transformation plasticity coefficient. As a result, it has been found that the transformation plasticity coefficient can be reduced by increasing Fn2A defined by formula (2A) when the chemical composition of the core consists of essential elements, or by increasing Fn2B defined by formula (2B) when the chemical composition of the core contains essential elements and optional elements. Fn2A=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+2000.0×N (2A) Fn2B=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+500.0×Nb+2000.0×N (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2A) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0031] Based on the above findings, the inventors further investigated the relationship between Fn1A and Fn1B, Fn2A and Fn2B, and contact fatigue strength and heat treatment strain. As a result, they found that when the chemical composition of the core consists of essential elements, if Fn1A is 0.27 to 0.35 and Fn1A and Fn2A satisfy formula (3A), excellent contact fatigue strength can be obtained and heat treatment strain can be sufficiently suppressed, and when the chemical composition of the core contains essential elements and optional elements, if Fn1B is 0.27 to 0.35 and Fn1B and Fn2B satisfy formula (3B), excellent contact fatigue strength can be obtained and heat treatment strain can be sufficiently suppressed. Fn1A×Fn2A≧20.1 (3A) Fn1B×Fn2B≧20.1 (3B)

[0032] The carburized machine structural part of this embodiment has been completed based on the above technical concept and has the following configuration.

[0033] The carburized machine structural component of the first configuration is A hardened layer; a core portion located inside the hardened layer, The C concentration in the hardened layer is 0.60% by mass or more, The chemical composition of the core is, in mass%, C: 0.13~0.30%, Si: 1.50-2.00%, Mn: 0.60-1.45% P: 0.015% or less, S: 0.025% or less, Cr: 0.06 to 0.40%, Mo: 0.35% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, the balance being Fe and impurities, Fn1A defined by formula (1A) is 0.27 to 0.35, The Fn1A and Fn2A defined by the formula (2A) satisfy the formula (3A). Fn1A=exp(-0.01×(170-45×Mn-15×(Cr+Mo))) (1A) Fn2A=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+2000.0×N (2A) Fn1A×Fn2A≧20.1 (3A) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (2A).

[0034] The second configuration of the carburized mechanical structural part is A hardened layer; a core portion located inside the hardened layer, The C concentration in the hardened layer is 0.60% by mass or more, The chemical composition of the core is, in mass%, C: 0.13~0.30%, Si: 1.50-2.00%, Mn: 0.60-1.45% P: 0.015% or less, S: 0.025% or less, Cr: 0.06 to 0.40%, Mo: 0.35% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, Fn1B defined by formula (1B) is 0.27 to 0.35, The Fn1B and Fn2B defined by formula (2B) satisfy formula (3B). [Group 1] Cu: 0.20% or less, and Ni: 0.30% or less, one or more selected from the group consisting of [Group 2] V: 0.15% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.15% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, and Mg: 0.010% 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: 0.050% or less, and Sb: 0.050% or less, one or more selected from the group consisting of Fn1B=exp(-0.01×(170-45×Mn-15×(Cr+Mo+Ni))) (1B) Fn2B=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+500.0×Nb+2000.0×N (2B) Fn1B×Fn2B≧20.1 (3B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1B) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0035] The third configuration of carburized mechanical structural parts is A carburized machine structural component of a second configuration, The chemical composition of the core contains the first group.

[0036] The fourth configuration of carburized mechanical structural parts is: A carburized machine structural part having the second or third configuration, The chemical composition of the core contains the second group.

[0037] The fifth configuration of carburized mechanical structural parts is: A carburized machine structural part having any one of the second to fourth configurations, The chemical composition of the core contains the third group.

[0038] The sixth configuration of carburized mechanical structural parts is: A carburized machine structural part having any one of the second to fifth configurations, The chemical composition of the core contains the fourth group.

[0039] The carburized machine structural component according to this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0040] [Configuration of the carburized machine structural component of this embodiment] In this specification, "carburized machine structural parts" refers to machine structural parts that have been carburized and quenched. Examples of carburized and quenched parts include vacuum carburized gears, vacuum carburized gear wheels, and vacuum carburized shafts.

[0041] The carburized machine structural component of this embodiment includes a hardened layer and a core portion located inside the hardened layer. The hardened layer is formed on the surface of carburized machine structural parts and is a layer that is hardened by the penetration of carbon during carburizing and quenching. The hardened layer is also called the carburized layer. The hardened layer is formed from the surface of the carburized machine structural parts to a specified depth. The microstructure of the hardened layer is mainly composed of martensite.

[0042] The carbon concentration in the hardened layer of the carburized machine structural component of this embodiment is 0.60% by mass or more. The upper limit of the carbon concentration in the hardened layer is not particularly limited, but is, for example, 1.50%.

[0043] The core is a region located inside the hardened layer and is not affected by the penetration and diffusion of C caused by carburizing and quenching. The hardness of the core is lower than that of the hardened layer. It is a technical matter well known to those skilled in the art that the hardened layer and the core can be easily distinguished by well-known microstructural observation.

[0044] [Method for measuring carbon concentration in hardened layer] The carbon concentration in the hardened layer is determined using an electron probe microanalyzer (EPMA). Specifically, a test piece is taken from the surface of the carburized machine structural part, with the observation surface covering a depth of 60 μm. The size of the test piece is not particularly limited, as long as the observation surface covers a 50 μm x 50 μm measurement area centered at a depth of 60 μm. The observation surface is parallel to the depth direction from the surface of the carburized machine structural part.

[0045] The carbon concentration (mass%) is measured on the observation surface of the obtained test piece using an EPMA. Specifically, an area analysis of the above-mentioned measurement area of ​​the observation surface is performed. The arithmetic mean value of the carbon concentration (mass%) in the obtained measurement area is defined as the carbon concentration (mass%) in the hardened layer. In the EPMA area analysis, the acceleration voltage is 15 kV, the irradiation current is 500 nA, the electron beam diameter is 3 μm, and the measurement pitch is 3 μm.

[0046] [Characteristics of the carburized machine structural part of this embodiment] The carburized machine structural component of this embodiment has the following features. (Feature 1) The chemical composition of the core is, in mass%, C: 0.13 to 0.30%, Si: 1.50 to 2.00%, Mn: 0.60 to 1.45%, P: 0.015% or less, S: 0.025% or less, Cr: 0.06 to 0.40%, Mo: 0.35% or less, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, O: 0.0015% or less, Cu: 0 to 0.20%, Ni : 0-0.30%, V: 0-0.15%, Nb: 0-0.100%, Ti: 0-0.100%, W: 0-0.15%, Ca: 0-0.0050%, Mg: 0-0.010%, Te: 0-0.050%, Bi: 0-0.100%, Pb: 0-0.09%, Sn: 0-0.050%, and Sb: 0-0.050%, with the remainder being Fe and impurities. (Feature 2) When the chemical composition of the core consists of essential elements, Fn1A defined by formula (1A) is 0.27 to 0.35, and when the chemical composition of the core contains essential elements and optional elements, Fn1B defined by formula (1B) is 0.27 to 0.35. Fn1A=exp(-0.01×(170-45×Mn-15×(Cr+Mo))) (1A) Fn1B=exp(-0.01×(170-45×Mn-15×(Cr+Mo+Ni))) (1B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (1B), and when an element is not contained, "0" is substituted for the corresponding element symbol. (Feature 3) When the chemical composition of the core consists of essential elements, Fn1A and Fn2A defined by formula (2A) satisfy formula (3A), and when the chemical composition of the core contains essential elements and optional elements, Fn1B and Fn2B defined by formula (2B) satisfy formula (3B). Fn2A=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+2000.0×N (2A) Fn1A×Fn2A≧20.1 (3A) Fn2B=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+500.0×Nb+2000.0×N (2B) Fn1B×Fn2B≧20.1 (3B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2A) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol. Features 1 to 3 will be explained below.

[0047] [(Feature 1) Chemical composition of the core] The chemical composition of the core of the carburized machine structural component of this embodiment contains the following elements.

[0048] C: 0.13 to 0.30% Carbon (C) improves hardenability and increases the surface fatigue strength of carburized machine structural parts. If the C content is less than 0.13%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.30%, coarse precipitates may form. These coarse precipitates are likely to become crack initiation points during use of carburized machine structural parts. As a result, the surface fatigue strength of the carburized machine structural parts decreases. Therefore, the C content is 0.13 to 0.30%. The lower limit of the C content is preferably 0.15%, and more preferably 0.18%. The upper limit of the C content is preferably 0.28%, more preferably 0.25%, and further preferably 0.23%.

[0049] Si: 1.50 to 2.00% Silicon (Si) improves hardenability. Si also increases the temper softening resistance of the hardened layer of the carburized machine structural component. As a result, the contact fatigue strength is increased. If the Si content is less than 1.50%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 2.00%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used to make the carburized machine structural parts becomes excessively high, and the hot workability deteriorates. Therefore, the Si content is 1.50 to 2.00%. The lower limit of the Si content is preferably 1.55%, more preferably 1.60%, and even more preferably 1.65%. The upper limit of the Si content is preferably 1.95%, more preferably 1.90%, and even more preferably 1.85%.

[0050] Mn: 0.60 to 1.45% Manganese (Mn) improves hardenability and increases the surface fatigue strength of carburized machine structural parts. Mn also suppresses heat treatment strain. If the Mn content is less than 0.60%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.45%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used to make the carburized machine structural component becomes excessively high, and the hot workability deteriorates. Therefore, the Mn content is 0.60 to 1.45%. The lower limit of the Mn content is preferably 0.65%, more preferably 0.70%, even more preferably 0.75%, and still more preferably 0.80%. The upper limit of the Mn content is preferably 1.40%, more preferably 1.35%, even more preferably 1.30%, even more preferably 1.25%, and still more preferably 1.20%.

[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 ranges of this embodiment, P will segregate excessively at the grain boundaries, reducing the grain boundary strength. As a result, the surface fatigue strength of the carburized machine structural component will decrease. Therefore, the P content is 0.015% or less. The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The upper limit of the P content is preferably 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%, coarse sulfide-based inclusions are formed even if the contents of other elements are within the ranges of this embodiment. As a result, the surface fatigue strength of the carburized machine structural component decreases. Therefore, the S content is 0.025% or less. The S content is preferably as low as possible. However, excessive reduction in the S content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the S content is preferably 0.020%, more preferably 0.010%, and even more preferably 0.005%.

[0053] Cr: 0.06 to 0.40% Chromium (Cr) improves hardenability and surface fatigue strength of carburized machine structural parts. If the Cr content is less than 0.06%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, Cr increases the transformation plasticity coefficient. If the Cr content exceeds 0.40%, even if the contents of other elements are within the ranges of this embodiment, the transformation plasticity coefficient increases excessively, and the heat treatment strain during vacuum carburizing and quenching increases. Therefore, the Cr content is 0.06 to 0.40%. The lower limit of the Cr content is preferably 0.08%, more preferably 0.10%, and even more preferably 0.13%. The upper limit of the Cr content is preferably 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] Mo: 0.35% or less Molybdenum (Mo) improves hardenability and increases the surface fatigue strength of carburized machine structural parts. Even if the content of Mo is small, these effects can be obtained to some extent. On the other hand, if the Mo content exceeds 0.35%, even if the contents of other elements are within the ranges of this embodiment, the transformation plasticity coefficient increases excessively, and the heat treatment strain during vacuum carburizing and quenching increases. Therefore, the Mo content is 0.35% or less. The lower limit of the Mo content is preferably more than 0%, more preferably 0.01%, even more preferably 0.03%, and still more preferably 0.05%. The upper limit of the Mo content is preferably 0.33%, more preferably 0.30%, and even more preferably 0.25%.

[0055] Al: 0.005 to 0.100% Aluminum (Al) combines with N to form AlN, which has a pinning effect that suppresses grain coarsening during heating in the carburizing process. This increases the surface fatigue strength of carburized machine structural components. If the Al content is less than 0.005%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, coarse clustered oxides are formed, and the coarse clustered oxides reduce the contact fatigue strength of the carburized machine structural component. Therefore, the Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably 0.008%, and more preferably 0.010%. The upper limit of the Al content is preferably 0.080%, more preferably 0.070%, and even more preferably 0.060%.

[0056] N: 0.0020~0.0300% Nitrogen (N) reduces the transformation plasticity coefficient, thereby reducing heat treatment strain during vacuum carburizing and quenching. If the N content is less than 0.0020%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0300%, the hot workability of the steel material used to make the carburized machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.0020 to 0.0300%. The lower limit of the N content is preferably 0.0021%, and more preferably 0.0022%. The upper limit of the N content is preferably 0.0280%, more preferably 0.0250%, and even more preferably 0.0200%.

[0057] O: 0.0015% or less Oxygen (O) is an impurity. O combines with other elements to form coarse oxide-based inclusions. The coarse oxide-based inclusions reduce the contact fatigue strength of carburized machine structural components. If the O content exceeds 0.0015%, the contact fatigue strength of the carburized machine structural component will be significantly reduced, even if the contents of other elements are within the ranges of this embodiment. Therefore, the O content is 0.0015% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the O content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the O content is preferably 0.0013% or less, more preferably 0.0011%, and even more preferably 0.0009%.

[0058] The remainder of the chemical composition of the core of the carburized machine structural component of this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the carburized machine structural component, and are acceptable within a range that does not adversely affect the carburized machine structural component of this embodiment.

[0059] [About optional elements] The chemical composition of the core of the carburized machine structural component of this embodiment may further contain one or more elements selected from the group consisting of first to fourth groups, in place of a portion of Fe. [Group 1] Cu: 0.20% or less, and Ni: 0.30% or less, one or more selected from the group consisting of [Group 2] V: 0.15% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.15% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, and Mg: 0.010% 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: 0.050% or less, and Sb: 0.050% or less, one or more selected from the group consisting of Each of the elements in the first to fourth groups will be explained below.

[0060] [Group 1: Cu and Ni] The chemical composition of the core of the carburized machine structural component of this embodiment may further contain elements of Group 1 in place of part of Fe. These elements are optional elements, and all of them improve the hardenability of the carburized machine structural component. Each element of Group 1 will be described below.

[0061] Cu: 0.20% or less 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 is more than 0%, Cu improves the hardenability and the surface fatigue strength of the carburized machine structural part. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 0.20%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used to make the carburized machine structural parts becomes excessively high, and the machinability deteriorates. Therefore, the Cu content is 0 to 0.20%, and when Cu is contained, the Cu content is 0.20% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.05%. The upper limit of the Cu content is preferably 0.15%, more preferably 0.13%, and even more preferably 0.10%.

[0062] 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 improves the hardenability and the surface fatigue strength of the carburized machine structural parts. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content exceeds 0.30%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used to make the carburized machine structural parts becomes excessively high, and the machinability deteriorates. Therefore, the Ni content is 0 to 0.30%, and when Ni is contained, the Ni content is 0.30% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.05%. The upper limit of the Ni content is preferably 0.20%, more preferably 0.12%, even more preferably 0.10%, and still more preferably 0.05%.

[0063] [Group 2: V, Nb, Ti, and W] The chemical composition of the core of the carburized machine structural component of this embodiment may further contain elements of Group 2 in place of part of Fe. These elements are optional elements, and all of them form precipitates to increase the surface fatigue strength of the carburized machine structural component. Each element of Group 2 will be described below.

[0064] 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 V is contained, that is, when the V content exceeds 0%, V forms V precipitates such as V carbides and V carbonitrides. V precipitates have a pinning effect that suppresses grain coarsening during carburizing. This increases the surface fatigue strength of carburized machine structural parts. Even if even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.15%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used to make the carburized machine structural parts becomes excessively high, and the machinability deteriorates. Therefore, the V content is 0 to 0.15%, and if V is contained, the V content is 0.15% or less. The lower limit of the V content is preferably 0.01%, more preferably 0.03%, even more preferably 0.05%, and still more preferably 0.07%. The upper limit of the V content is preferably 0.12%, and more preferably 0.08%.

[0065] Nb: 0.100% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as Nb carbides and Nb carbonitrides. Nb precipitates have a pinning effect, which suppresses grain coarsening during carburizing. This increases the surface fatigue strength of carburized machine structural parts. Nb also reduces the transformation plasticity coefficient. This reduces heat treatment strain during vacuum carburizing and quenching. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the Nb precipitates become coarse. In this case, the coarsening of crystal grains during the carburizing treatment cannot be sufficiently suppressed. As a result, the surface fatigue strength of the carburized machine structural component decreases. Therefore, the Nb content is 0 to 0.100%, and when Nb is contained, the Nb content is 0.100% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.010%, even more preferably 0.030%, and still more preferably 0.050%. The upper limit of the Nb content is preferably 0.090%, more preferably 0.075%, and even more preferably 0.060%.

[0066] Ti:0.100% or less Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content exceeds 0%, Ti forms Ti precipitates such as Ti carbide and Ti carbonitride. The Ti precipitates have a pinning effect, which suppresses the coarsening of crystal grains during carburizing. This increases the surface fatigue strength of carburized machine structural parts. Even if Ti is contained even a small amount, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the Ti precipitates become coarse. In this case, the coarsening of crystal grains during the carburizing treatment cannot be sufficiently suppressed. As a result, the surface fatigue strength of the carburized machine structural component decreases. Therefore, the Ti content is 0 to 0.100%, and when Ti is contained, the Ti content is 0.100% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.010%, even more preferably 0.030%, and still more preferably 0.050%. The upper limit of the Ti content is preferably 0.090%, more preferably 0.075%, and even more preferably 0.060%.

[0067] W: 0.15% or less Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W forms W precipitates such as W carbides and W carbonitrides. The W precipitates have a pinning effect, which suppresses the coarsening of crystal grains during carburizing. This increases the surface fatigue strength of carburized machine structural parts. Even if even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content exceeds 0.15%, W precipitates become coarse even if the contents of other elements are within the ranges of this embodiment. In this case, the coarsening of crystal grains during carburizing cannot be sufficiently suppressed. As a result, the surface fatigue strength of the carburized machine structural component decreases. Therefore, the W content is 0 to 0.15%, and when W is contained, the W content is 0.15% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the W content is preferably 0.12%, and more preferably 0.10%.

[0068] [Group 3: Ca and Mg] The chemical composition of the core of the carburized machine structural component of this embodiment may further contain elements of Group 3 in place of part of Fe. These elements are optional elements, and all of them refine and spheroidize sulfides, thereby increasing the surface fatigue strength of the carburized machine structural component. Each element of Group 3 will be described below.

[0069] 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 is greater than 0%, Ca refines sulfides. Furthermore, Ca promotes the spheroidization of sulfides. This increases the surface fatigue strength of carburized machine structural parts. Even if even a small amount of Ca is contained, the above effects can be achieved to some extent. However, if the Ca content exceeds 0.0050%, the formation of coarse Ca oxides is promoted even if the contents of other elements are within the ranges of this embodiment, and in this case, the surface fatigue strength of the carburized machine structural component decreases. Therefore, the Ca content is 0 to 0.0050%, and when Ca is contained, the Ca content is 0.0050% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and still more preferably 0.0015%. The upper limit of the Ca content is preferably 0.0045%, more preferably 0.0040%, and even more preferably 0.0030%.

[0070] Mg: 0.010% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When Mg is contained, that is, when the Mg content is more than 0%, Mg refines sulfides. Furthermore, Mg promotes the spheroidization of sulfides. This increases the surface fatigue strength of carburized machine structural parts. Even if even a small amount of Mg is contained, the above effects can be achieved to some extent. However, if the Mg content exceeds 0.010%, the formation of coarse Mg oxides is promoted even if the contents of other elements are within the ranges of this embodiment, and in this case, the surface fatigue strength of the carburized machine structural component decreases. Therefore, the Mg content is 0 to 0.010%, and when Mg is contained, the Mg content is 0.010% or less. The lower limit of the Mg content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Mg content is preferably 0.008%, more preferably 0.006%, and even more preferably 0.005%.

[0071] [Group 4: Te, Bi, Pb, Sn, and Sb] The chemical composition of the core of the carburized machine structural component of this embodiment may further contain elements of Group 4 in place of part of Fe. These elements are optional elements, and all of them improve the machinability of the steel material that is the raw material for the carburized machine structural component. Each element of Group 4 will be described below.

[0072] 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 is more than 0%, Te improves the machinability of the steel material that is used to make carburized machine structural parts. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.050%, the hot workability of the steel material used to make the carburized machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.050%, and when Te is contained, the Te content is 0.050% or less. The lower limit of the Te content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.010%. The upper limit of the Te content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0073] 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 is more than 0%, Bi improves the machinability of the steel material that is used to make carburized machine structural parts. Even if even a small amount of Bi is contained, this effect can be obtained to some extent. However, if the Bi content exceeds 0.100%, the hot workability of the steel material used to make the carburized machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Bi content is 0 to 0.100%, and when Bi is contained, the Bi content is 0.100% or less. The lower limit of the Bi content is preferably 0.001%, more preferably 0.002%, even more preferably 0.005%, and still more preferably 0.010%. The upper limit of the Bi content is preferably 0.090%, more preferably 0.080%, even more preferably 0.070%, even more preferably 0.060%, and still more preferably 0.050%.

[0074] 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 Pb is contained, that is, when the Pb content is more than 0%, Pb improves the machinability of the steel material that is used to make carburized machine structural parts. Even if even a small amount of Pb is contained, this effect can be obtained to some extent. However, if the Pb content exceeds 0.09%, the hot workability of the steel material used to make the carburized machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.09%, and if Pb is contained, the Pb content is 0.09% or less. The lower limit of the Pb content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Pb content is preferably 0.08%, more preferably 0.07%, even more preferably 0.06%, and still more preferably 0.05%.

[0075] Sn: 0.050% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When Sn is contained, that is, when the Sn content is more than 0%, Sn improves the machinability of the steel material that is used to make carburized machine structural parts. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.050%, the hot workability of the steel material used to make the carburized machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.050%, and when Sn is contained, the Sn content is 0.050% or less. The lower limit of the Sn content is preferably 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Sn content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0076] Sb: 0.050% or less Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0%. When Sb is contained, that is, when the Sb content is more than 0%, Sb improves the machinability of the steel material that is used to make carburized machine structural parts. Even if even a small amount of Sb is contained, this effect can be obtained to some extent. However, if the Sb content exceeds 0.050%, the hot workability of the steel material used to make the carburized machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.050%, and when Sb is contained, the Sb content is 0.050% or less. The lower limit of the Sb content is preferably 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Sb content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0077] [(Feature 2) Fn1A and Fn1B] In the carburized mechanical structural component of this embodiment, when the chemical composition of the core consists of essential elements, Fn1A defined by formula (1A) is 0.27 to 0.35, and when the chemical composition of the core contains essential elements and optional elements, Fn1B defined by formula (1B) is 0.27 to 0.35. Fn1A=exp(-0.01×(170-45×Mn-15×(Cr+Mo))) (1A) Fn1B=exp(-0.01×(170-45×Mn-15×(Cr+Mo+Ni))) (1B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (1B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0078] Fn1A and Fn1B are indicators of the stability of the austenite phase. The higher the Fn1A and Fn1B, the higher the stability of the austenite phase. Therefore, martensitic transformation during vacuum carburizing and quenching is suppressed, and transformation expansion strain, which is one of the main causes of heat treatment strain, is reduced. If Fn1A or Fn1B is less than 0.27, the stability of the austenite phase is too low. In this case, excessive martensitic transformation occurs during vacuum carburizing and quenching, which increases transformation expansion strain. As a result, heat treatment strain during vacuum carburizing and quenching cannot be suppressed.

[0079] On the other hand, if Fn1A or Fn1B exceeds 0.35, the stability of the austenite phase becomes excessively high. In this case, martensitic transformation during vacuum carburizing and quenching does not proceed sufficiently, resulting in a large amount of retained austenite. As a result, the hardness of carburized machine structural parts decreases, and excellent surface fatigue strength cannot be obtained.

[0080] When Fn1A or Fn1B is 0.27 to 0.35, the stability of the austenite phase is appropriate. Therefore, it is possible to reduce transformation expansion strain while suppressing a decrease in contact fatigue strength. As a result, assuming that the carburized machine structural component satisfies Features 1 and 3, excellent contact fatigue strength is obtained and heat treatment strain is suppressed.

[0081] The lower limit of Fn1A and Fn1B is preferably 0.28, more preferably 0.29, and even more preferably 0.30. The upper limit of Fn1A and Fn1B is preferably 0.34, more preferably 0.33, and even more preferably 0.32. Note that Fn1A and Fn1B are values ​​obtained by rounding off the obtained numerical value to two decimal places.

[0082] [(Feature 3) Regarding Formula (3A) and Formula (3B)] Furthermore, in the carburized mechanical structural component of this embodiment, when the chemical composition of the core portion consists of essential elements, Fn1A and Fn2A defined by formula (2A) satisfy formula (3A), and when the chemical composition of the core portion contains essential elements and optional elements, Fn1B and Fn2B defined by formula (2B) satisfy formula (3B). Fn2A=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+2000.0×N (2A) Fn1A×Fn2A≧20.1 (3A) Fn2B=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+500.0×Nb+2000.0×N (2B) Fn1B×Fn2B≧20.1 (3B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2A) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0083] As described above, the main factors of heat treatment strain are transformation expansion strain and transformation plastic strain. By sufficiently reducing both the transformation expansion strain and the transformation plastic strain, the heat treatment strain can be suppressed. In other words, even if Fn1A or Fn1B is sufficiently high and the transformation expansion strain is reduced, the heat treatment strain cannot be suppressed unless the transformation plastic strain is sufficiently reduced.

[0084] Fn2A and Fn2B are indicators of the transformation plasticity coefficient. If the transformation plasticity coefficient can be reduced, the transformation plastic strain will be reduced. Of the elements mentioned above in the chemical composition, C, Si, Mn, N, Cr, Mo, and Nb affect the transformation plasticity coefficient. Specifically, C, Si, Mn, Nb, 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. The coefficients of Fn2A and Fn2B for each element differ depending on the degree of influence each element has on the transformation plasticity coefficient. The higher the Fn2A or Fn2B, the lower the transformation plasticity coefficient. As a result, the transformation plastic strain is reduced.

[0085] Furthermore, Fn3A and Fn3B are defined as follows: Fn3A=Fn1A×Fn2A Fn3B=Fn1B×Fn2B Fn3A and Fn3B are indices that represent the degree of suppression of heat treatment strain. If Fn3A or Fn3B is 20.1 or higher, both Fn1A or Fn1B and Fn2A or Fn2B are sufficiently high. In this case, both transformation expansion strain and transformation plastic strain are sufficiently reduced. As a result, assuming that the carburized machine structural component satisfies Features 1 and 2, heat treatment strain is suppressed during vacuum carburizing and quenching.

[0086] The lower limit of Fn3A and Fn3B is preferably 20.5, more preferably 21.0, and even more preferably 22.0. There are no particular limitations on the upper limits of Fn3A and Fn3B. When the chemical composition of the core satisfies Features 1 and 2, the upper limits of Fn3A and Fn3B are, for example, 42.5. Note that Fn3A and Fn3B are values ​​obtained by rounding off the obtained numerical values ​​to one decimal place.

[0087] [Effects of the carburized machine structural parts of this embodiment] The carburized machine structural component of this embodiment satisfies Features 1 to 3. Therefore, the carburized machine structural component of this embodiment has excellent surface fatigue strength and suppresses heat treatment strain. Therefore, the carburized machine structural component of this embodiment is suitable for carburized gears, carburized shafts, carburized bearing parts, etc. used in automobiles, construction vehicles, mining machinery, etc.

[0088] [Manufacturing method for carburized machine structural parts] An example of a method for manufacturing a carburized machine structural component of this embodiment will be described below. The method for manufacturing a carburized machine structural component described below is one example for manufacturing a carburized machine structural component of this embodiment. Therefore, a carburized machine structural component having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a carburized machine structural component of this embodiment.

[0089] An example of the method for manufacturing the carburized machine structural component of this embodiment includes the following steps. (Process 1) Steel material preparation process (Process 2) Hot processing process (Process 3) Vacuum carburizing and quenching process Each step will be described below.

[0090] [(Process 1) Steel material preparation process] In the steel preparation step, a steel material is prepared as the raw material for the carburized machine structural component of this embodiment. Specifically, molten steel having a chemical composition that satisfies Features 1 to 3 is produced. The refining method is not particularly limited, and any known method may be used. For example, molten pig iron produced by a known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to known secondary refining. In the secondary refining, alloy elements are added to the molten steel to adjust the composition, and molten steel having a chemical composition that satisfies Features 1 to 3 is produced.

[0091] The molten steel produced by the above-described refining method is used to produce a material by a known casting method. For example, an ingot may be produced by an ingot casting method using the molten steel. Alternatively, a bloom may be produced by a continuous casting method using the molten steel. The produced material (ingot or bloom) is subjected to known hot working to produce a steel material of a predetermined shape. For example, hot forging may be performed as the hot working. Alternatively, finish rolling using a continuous rolling mill may be performed after blooming or hot forging. The continuous rolling mill has a plurality of rolling stands arranged in a row. The heating temperature during hot working is not particularly limited, but is, for example, 1000 to 1300°C. The shape of the steel material produced in this way is, for example, a steel bar.

[0092] [(Process 2) Hot processing process] In the hot working step, hot working is performed on the prepared steel material to produce an intermediate product having a predetermined shape. The hot working is, for example, hot forging. The heating temperature in the hot working step is, for example, 1000 to 1300°C. The intermediate product after hot working is cooled to room temperature. If necessary, cutting may be performed on the intermediate product after hot working. In other words, cutting is an optional step.

[0093] [(Process 3) Vacuum carburizing and quenching process] In the vacuum carburizing and quenching process, intermediate products after hot working or cutting are subjected to vacuum carburizing and quenching to manufacture carburized machine structural parts. The vacuum carburizing and quenching process includes a vacuum carburizing and quenching step and a tempering step.

[0094] In the vacuum carburizing and quenching process, for example, a heating process, a soaking process, a carburizing process, a diffusion process, and a quenching process are performed in this order. In the heating step, the intermediate product is placed in a heat treatment furnace and heated to a carburizing temperature. At this time, the pressure inside the furnace is set to, for example, 10 Pa or less. The carburizing temperature is, for example, 900 to 1100°C. In the soaking step, the intermediate product is held at the carburizing temperature for a predetermined time to perform soaking treatment. The pressure inside the furnace during soaking treatment may be 10 Pa or less, or a nitrogen gas atmosphere of 1000 Pa or less may be created by simultaneously introducing nitrogen gas and evacuating with a vacuum pump. In the carburizing process, the intermediate product is held at the carburizing temperature for a predetermined time. A well-known carburizing gas is used in the carburizing process. Examples of the carburizing gas include hydrocarbon gases such as acetylene, propane, and ethylene. The carburizing gas pressure is set to a predetermined value depending on the type of carburizing gas. When acetylene is used as the carburizing gas, the carburizing gas pressure is set to, for example, 10 to 1000 Pa. When propane is used as the carburizing gas, the carburizing gas pressure is set to, for example, 200 to 3000 Pa. After the carburizing process, a diffusion process is carried out. In the diffusion process, the intermediate product is held at the carburizing temperature for a predetermined time. The pressure inside the furnace during the diffusion process may be 100 Pa or less to remove any residual gas from the carburizing process. Alternatively, nitrogen gas may be introduced and evacuated using a vacuum pump simultaneously to create a nitrogen gas atmosphere of 1000 Pa or less. After the diffusion process, the intermediate product is subjected to a quenching process. A known cooling method may be used for the quenching. The cooling method is oil cooling or water cooling. Specifically, the intermediate product, which has been maintained at the quenching temperature, is immersed in a cooling bath containing oil or water as a cooling medium to be rapidly cooled.

[0095] The intermediate product after the quenching step is subjected to a tempering step at a tempering temperature of, for example, 100 to 200°C.

[0096] The carburized machine structural component of this embodiment is manufactured by the above steps. Note that the steel material may be subjected to normalizing treatment or spheroidizing annealing treatment as needed. [Example]

[0097] Steel materials with each test number were manufactured as materials for carburized machine structural parts having the chemical compositions shown in Tables 1A and 1B. The steel material with test number REF was a reference steel material and had a chemical composition equivalent to SCr420 specified in JIS G 4053 (2016).

[0098] [Table 1A]

[0099] [Table 1B]

[0100] Specifically, molten steel was used to carry out continuous casting to produce slabs. The produced slabs were heated and subjected to blooming and subsequent continuous rolling to produce 160 mm x 160 mm billets. The heating temperature of the slabs during blooming was 1000°C to 1300°C. The produced billets were then heated again and hot rolled to produce steel products (steel bars) with a diameter of 50 mm. The heating temperature of the billets at this time was 1000°C to 1300°C. The produced steel products were cooled to room temperature in the atmosphere. Steel products with a diameter of 50 mm and each test number were produced using the above production process.

[0101] [About the evaluation test] The manufactured steel materials were subjected to the following evaluation tests. (Test 1) Surface fatigue strength evaluation test (Test 2) Heat treatment distortion evaluation test Test 1 and Test 2 will be explained below.

[0102] [(Test 1) Surface fatigue strength evaluation test] From the steel material (steel bar with a diameter of 50 mm) of each test number, several small roller test pieces 1 were prepared as carburized machine structural parts. The shape of the small roller test piece 1 is shown in Figure 1. The numbers in Figure 1 represent dimensions (unit: mm). "φ" in Figure 1 means diameter.

[0103] Specifically, the steel material (50 mm diameter steel bar) for each test number was heated at a heating temperature of 1200°C for a holding time of 30 minutes. The bar was then hot-worked (hot-forged) to a finishing temperature of 950°C or higher to a diameter of 40 mm, and then cooled in air to obtain a steel bar. The 40 mm diameter steel bar was machined (cut) to produce an intermediate product with the shape shown in Figure 1. The intermediate product for each test number underwent a vacuum carburizing and quenching process (vacuum carburizing and quenching process).

[0104] In the vacuum carburizing and quenching process, the pressure inside the furnace was kept below 10 Pa. In the heating process, the intermediate products (steel bars) of each test number were heated to the carburizing temperature of 950°C. After the heating process, a soaking process was carried out. In the soaking process, the intermediate products were held at the carburizing temperature of 950°C for 60 minutes.

[0105] After the soaking process, the carburizing process was carried out. In the carburizing process, acetylene was supplied as the carburizing gas into the vacuum carburizing furnace. The carburizing gas pressure in the carburizing process was kept below 1000 Pa. In the carburizing process, the carburizing temperature was held at 950°C for 40 minutes. After the carburizing process, the diffusion process was carried out. The carburizing gas pressure in the diffusion process was kept below 5 hPa. In the diffusion process, the carburizing temperature was held at 950°C for 70 minutes.

[0106] After the diffusion process, the steel temperature was furnace cooled to 850°C. Then, a quenching process was carried out. In the quenching process, the intermediate product was held at a quenching temperature of 850°C for 30 minutes. It was then quenched in oil at 130°C. After the quenching process, a tempering process was carried out on the intermediate product. The tempering temperature was 180°C, and the holding time at the tempering temperature was 120 minutes. After the holding time had elapsed, the product was cooled in the atmosphere.

[0107] After the tempering process, the outer circumferential surface of the cylindrical portion of each intermediate product, which originally had a diameter of 26.0 mm, was ground to restore the diameter of the cylindrical portion, which had expanded due to heat treatment strain. The grinding process was performed so that the outer circumferential surface of the cylindrical portion with a diameter of 26.0 mm after grinding had an arithmetic mean roughness Ra of 0.6 to 0.8 μm and a maximum height Rz of 2.0 to 4.0 μm, in accordance with JIS B 0601 (2001).

[0108] In this manner, small roller test pieces 1 for each test number were prepared. Here, the carbon concentration in the hardened layer of the small roller test piece 1 of each test number was determined based on the method described above in [Method for measuring carbon concentration in hardened layer]. The center position of the measurement area in the test piece of each test number corresponded to a depth of 60 μm from the outer surface of the cylindrical portion of the small roller test piece 1 with a diameter of 26.0 mm. As a result of the measurement, the carbon concentration in the hardened layer of the small roller test piece 1 of each test number was 0.60% by mass or more.

[0109] Using the small roller test piece 1, a roller pitting test (two-cylinder rolling fatigue test) shown in Fig. 2 was carried out to evaluate the surface fatigue strength of the small roller test piece 1, which is a carburized machine structural part. The test machine used was a roller pitting test machine "RP201" manufactured by Komatsu Engineering Co., Ltd.

[0110] Specifically, referring to Fig. 2, the small roller test piece 1 was rotated while the large roller test piece 2 was pressed against the small roller test piece 1. The large roller test piece 2 was prepared by the following method. A 140 mm diameter cylindrical steel material with a chemical composition equivalent to SUJ2 as specified in JIS G 4805 (2019) was machined to produce the intermediate product shown in Figure 3. The numerical values ​​in Figure 3 represent dimensions (unit: mm). "φ" in Figure 3 refers to the diameter at the center of thickness. "R150" in Figure 3 indicates a crowning radius of 150 mm on the outer periphery. The intermediate product underwent quenching and tempering processes. In the quenching process, the intermediate product was heated and held at 830°C for 60 minutes in an atmosphere with a carbon potential Cp of 0.6%, and then rapidly cooled in oil at 130°C. The intermediate product after the quenching process was then tempered. The tempering temperature was 180°C, and the holding time at the tempering temperature was 120 minutes. After the holding time, it was cooled in air. After the tempering process, the outer periphery of the intermediate product, which originally had a diameter of 130.0 mm at the center of thickness, was ground. This was done to correct the diameter at the center of the thickness of the intermediate product, which had expanded due to heat treatment strain, back to 130.0 mm. The grinding process was performed so that the outer surface of the intermediate product after grinding had an arithmetic mean roughness Ra of 0.6 to 0.8 μm and a maximum height Rz of 2.0 to 4.0 μm, in accordance with JIS B 0601 (2001). Using the above method, a large roller test piece 2 with the shape shown in Figure 3 was produced.

[0111] The small roller test piece 1 was rolled while in contact with the central position of the outer circumferential surface (the outer peripheral portion of the 130 mm diameter) of the large roller test piece 2. The Hertzian surface pressure during contact was 2.0 to 3.7 GPa. The rotation speed of the small roller test piece 1 was 2000 rpm. The circumferential speed directions of both roller test pieces (small roller test piece 1 and large roller test piece 2) at the contact area were aligned, and the slip ratio was set to -40% (the circumferential speed of the large roller test piece 2 at the contact area was 40% higher than that of the small roller test piece 1). During the test, lubricating oil was supplied to the contact area between the small roller test piece 1 and the large roller test piece 2. The lubricating oil was automatic transmission fluid, and the oil temperature was 90°C and the oil flow rate was 2.0 L / min. The number of repetitions (cycles) during the roller pitting test was 2.0 x 10 7The number of repetitions was 2.0 × 10 7 The maximum surface pressure (GPa) reached in the test was taken as the fatigue limit (GPa) of the small roller test piece 1. The occurrence of pitting damage was detected using a vibration meter attached to the testing machine. After vibration detection, the rotation of both the small roller test piece 1 and the large roller test piece 2 was stopped, and the presence or absence of pitting damage in the small roller test piece 1 was visually confirmed. If pitting damage was confirmed, the number of repetitions at the time of vibration detection was taken as the number of repetitions at which pitting damage occurred. If pitting damage was not confirmed, the roller pitting test was repeated from the beginning at that surface pressure using another small roller test piece 1.

[0112] The ratio of the fatigue limit of each test number to the fatigue limit of the reference steel (test number REF) was defined as the fatigue limit ratio. That is, the fatigue limit ratio was calculated using the following formula. Fatigue limit ratio = Fatigue limit (GPa) for the test number / Fatigue limit (GPa) for test number REF

[0113] If the fatigue limit ratio obtained was 1.10 or more, it was determined that excellent contact fatigue strength was obtained (indicated by "○" in the "Contact fatigue strength" column in Table 2). On the other hand, if the fatigue limit ratio was less than 1.10, it was determined that excellent contact fatigue strength was not obtained (indicated by "×" in the "Contact fatigue strength" column in Table 2).

[0114] [Table 2]

[0115] [(Test 2) Heat treatment distortion evaluation test] The heat treatment strain of carburized machine structural components manufactured using the steel with each test number was determined using the following method. First, a heat treatment strain evaluation test specimen was prepared as a carburized machine structural component from the steel with each test number (a 50 mm diameter steel bar). The shape of the heat treatment strain evaluation test specimen is shown in Figure 4. The numbers in Figure 4 represent dimensions (unit: mm). "φ" in Figure 4 refers to the diameter. In the following explanation, we will assume that the heat treatment strain evaluation test specimen is positioned in a three-dimensional Cartesian coordinate system (xyz coordinate system). The longitudinal direction of the heat treatment strain evaluation test specimen is defined as the y direction, the height direction of the heat treatment strain evaluation test specimen is defined as the z direction, and the direction perpendicular to the z and y directions is defined as the x direction.

[0116] The steel material for each test number was heated at a temperature of 1200°C for a holding time of 30 minutes. It was then hot-worked (hot-forged) to a finishing temperature of 950°C or higher to a diameter of 25 mm, and then cooled in air to obtain a steel bar. The 25 mm diameter steel bar was machined (cut) to produce an intermediate specimen for the heat treatment strain evaluation, measuring 10.0 mm in diameter and 100.0 mm in length. A groove measuring 2.5 mm deep, 4.0 mm wide, and 100.0 mm long was machined (cut) into the side of the intermediate specimen to produce the heat treatment strain evaluation specimen shown in Figure 4.

[0117] The straightness of the test piece for evaluating heat treatment distortion before vacuum carburizing and quenching was measured by the following method. Referring to FIG. 4, the outer circumferential shape of the test specimen was measured on a plane (a plane on the x-z plane) perpendicular to the longitudinal direction (y direction) at measurement positions A1 and A2 at both ends in the longitudinal direction and at measurement position A3, which was the center position in the longitudinal direction. Specifically, a CNC three-dimensional measuring machine (product name: Crystal-Apex) manufactured by Mitutoyo Corporation was used as the three-dimensional measuring machine. As shown in FIG. 5, the test specimen for evaluating heat treatment strain was positioned so that the grooves of the test specimen were positioned downward on a plane perpendicular to the longitudinal direction of the test specimen (hereinafter referred to as the measurement plane). With the CNC three-dimensional measuring machine, the coordinates (x, y, z) of measurement points P1 to P7 at 45° intervals around the outer periphery were measured at each of measurement positions A1 to A3, with the apex of the measurement plane set to 0°. Note that the 180° position, when the apex of the measurement plane is set to 0°, corresponds to the grooves, so the coordinates at the 180° position were not measured. That is, the coordinates were determined at seven measurement points P1 to P7 on the outer periphery of each measurement surface.

[0118] As shown in Figure 6, on each measurement surface, the coordinates of the seven measurement points P1 to P7 were used to approximate a circle C0 using the least squares method. From the obtained circle C0, the three-dimensional coordinates (x, y, z) of the center position CP of the circle C0 were calculated. In the following explanation, the center position of the circle C0 at measurement position A1 is defined as CP1. Similarly, the center position of the circle C0 at measurement position A2 is defined as CP2, and the center position of the circle C0 at measurement position A3 is defined as CP3.

[0119] As shown in Fig. 7, based on the coordinates of the obtained three center positions CP1 to CP3, the distance D in the x direction between the line segment SG connecting the center positions CP1 and CP2 and the center position CP3 was calculated, and the calculated distance D was defined as the amount of bending D. The amount of bending D is an index that indicates the degree of bending of the test piece.

[0120] After determining the amount of bending D of the heat treatment strain evaluation test piece before vacuum carburizing and quenching, the heat treatment strain evaluation test piece was subjected to vacuum carburizing and quenching under the same conditions as in Test 1. The amount of bending D of the heat treatment strain evaluation test piece after vacuum carburizing and quenching was determined using the same method as for the heat treatment strain evaluation test piece before vacuum carburizing and quenching. The difference ΔD between the amount of bending D of the heat treatment strain evaluation test piece after vacuum carburizing and quenching and the amount of bending D of the heat treatment strain evaluation test piece before vacuum carburizing and quenching was determined. The obtained difference ΔD is an index that shows the amount of heat treatment strain introduced by vacuum carburizing and quenching.

[0121] Using the difference value ΔD obtained for test number REF and the difference value ΔD for each test number, the bending amount ratio of the heat treatment strain evaluation test piece for each test number was calculated according to the following formula. Bending ratio = difference value ΔD at the test number / difference value ΔD at test number REF If the obtained bending ratio was 0.8 or less, it was determined that the heat treatment distortion was suppressed (indicated by "〇" in the "Heat Treatment Distortion" column in Table 2). On the other hand, if the bending ratio was more than 0.8, it was determined that the heat treatment distortion was not sufficiently suppressed (indicated by "×" in the "Heat Treatment Distortion" column in Table 2).

[0122] [Test Results] The test results are shown in Table 2. Referring to Table 1 (Table 1A and Table 1B) and Table 2, the carburized machine structural parts with test numbers 1 to 17 satisfied features 1 to 3. Therefore, excellent surface fatigue strength was obtained. Furthermore, heat treatment strain was sufficiently suppressed.

[0123] On the other hand, in test numbers 18 and 19, the Si content was too low, and therefore excellent contact fatigue strength was not obtained.

[0124] In test number 20, Fn1A was too low, so the heat treatment distortion was not sufficiently suppressed.

[0125] In test number 21, Fn1B was too low, so the heat treatment distortion was not sufficiently suppressed.

[0126] In test number 22, Fn1A was too high, and therefore excellent surface fatigue strength was not obtained.

[0127] In test number 23, Fn1B was too high, and therefore excellent surface fatigue strength was not obtained.

[0128] In test number 24, Fn3A was too low, so the heat treatment distortion was not sufficiently suppressed.

[0129] In test number 25, Fn3B was too low, so the heat treatment distortion was not sufficiently suppressed.

[0130] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A hardened layer; a core portion located inside the hardened layer, The carbon concentration in the hardened layer is 0.60% by mass or more, The chemical composition of the core is, in mass %, C: 0.13-0.30%, Si: 1.50-2.00%, Mn: 0.60-1.45%, P: 0.015% or less, S: 0.025% or less, Cr: 0.06-0.40%, Mo: 0.35% or less, Al: 0.005-0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, the balance being Fe and impurities; Fn1A defined by formula (1A) is 0.27 to 0.35, The Fn1A and Fn2A defined by formula (2A) satisfy formula (3A), Carburized mechanical structural parts. Fn1A=exp(-0.01×(170-45×Mn-15×(Cr+Mo))) (1A) Fn2A=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+2000.0×N (2A) Fn1A×Fn2A≧20.1 (3A) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (2A).

2. A hardened layer; a core portion located inside the hardened layer, The carbon concentration in the hardened layer is 0.60% by mass or more, The chemical composition of the core is, in mass %, C: 0.13-0.30%, Si: 1.50-2.00%, Mn: 0.60-1.45%, P: 0.015% or less, S: 0.025% or less, Cr: 0.06-0.40%, Mo: 0.35% or less, Al: 0.005-0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities; Fn1B defined by formula (1B) is 0.27 to 0.35, The Fn1B and Fn2B defined by formula (2B) satisfy formula (3B). Carburized mechanical structural parts. [Group 1] Cu: 0.20% or less, and Ni: 0.30% or less, one or more selected from the group consisting of [Group 2] V: 0.15% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.15% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, and Mg: 0.010% 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: 0.050% or less, and Sb: 0.050% or less, one or more selected from the group consisting of Fn1B=exp(-0.01×(170-45×Mn-15×(Cr+Mo+Ni))) (1B) Fn2B=20.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+500.0×Nb+2000.0×N (2B) Fn1B×Fn2B≧20.1 (3B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1B) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

3. The carburized machine structural component according to claim 2, the chemical composition of the core portion contains the first group; Carburized mechanical structural parts.

4. The carburized machine structural component according to claim 2, The chemical composition of the core portion contains the second group. Carburized mechanical structural parts.

5. The carburized machine structural component according to claim 2, The chemical composition of the core portion contains the third group. Carburized mechanical structural parts.

6. The carburized machine structural component according to claim 2, The chemical composition of the core portion contains the fourth group. Carburized mechanical structural parts.

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