Machine parts and rolling bearings

Abstract

To provide a mechanical component capable of suppressing a secular change of a dimension and improving an indentation start point type rolling fatigue service life on surface thereof.SOLUTION: A machine component is made of steel and has a surface. The steel has been quenched and annealed. The machine component includes a nitriding layer located on the surface and a core part of which distance from the surface is larger than that of the nitriding layer. A nitrogen concentration in the steel on the surface is 0.01 mass% or more. Hardness of the steel on the surface is 820 Hv or more. Retained austenite amount in the steel in the core part is 0.1 vol.% or more and 9 vol.% or less. Dislocation density of the retained austenite in the steel in the core part is 4.0×1014 m-2 or more. The steel is high carbon steel or bearing steel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a mechanical component and a rolling bearing, and more particularly to a mechanical component made of hardened and tempered steel and a rolling bearing including the mechanical component. [Background technology]

[0002] The electrification of automobiles is progressing, with a focus on battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and hybrid electric vehicles (HEVs). For example, electric axles (e-axles) that replace engines, electric brakes, electric VTC (variable valve timing), and electric compressors are now being applied.

[0003] In electric vehicles, in order to extend the driving distance using less electricity (improving electric fuel efficiency), weight reduction through miniaturization of the unit and higher speed and output of the motor are desired. Accordingly, various components are required to be smaller, rotate faster, and be stronger. For example, as motors become smaller and faster, the amount of heat generated in rolling bearings increases, causing the temperature of the rolling bearings to rise during use. Furthermore, to reduce the torque of rolling bearings, etc., there is a high possibility that the viscosity of lubricating oil will decrease and the amount of oil or grease will be reduced, which will further increase the amount of heat generated by the rolling bearings. Therefore, the rolling bearings used in electric vehicles must maintain dimensional stability and high hardness at high temperatures.

[0004] The structure of the constituent material of a rolling bearing that has been quenched and tempered contains martensite, retained austenite, and precipitates such as undissolved carbides and nitrides. An appropriate amount of retained austenite is said to be effective in improving clean oil rolling fatigue life and indentation-initiated rolling fatigue life.

[0005] In hardened and tempered rolling bearings, the retained austenite decomposes as the operating temperature rises. As a result, the volume expansion associated with the decomposition of the retained austenite causes dimensional changes in the components of the rolling bearing. Furthermore, if the dimensional change rate of the rolling bearing's raceways increases, problems such as creep may occur, the clearance between the raceway surface and the rolling elements may decrease, resulting in increased contact pressure and premature damage, and abnormal noise and vibration may increase due to reduced dimensional accuracy.

[0006] Japanese Patent Laid-Open Publication No. 2001-99163 (Patent Document 1) describes a raceway ring for a rolling bearing. The raceway ring described in Patent Document 1 is made of steel that has been quenched and tempered. In the raceway ring described in Patent Document 1, the amount of retained austenite in the steel is substantially zero. The raceway ring described in Patent Document 1 is suppressed from changing in dimension over time during use. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-99163 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the raceway ring described in Patent Document 1, the hardness of the steel on the surface is less than 752 Hv. When the hardness of the steel on the surface of the raceway ring is low when the amount of retained austenite is small, the indentation-initiated rolling contact fatigue life decreases. Therefore, there is room for improvement in the indentation-initiated rolling contact fatigue life of the raceway ring described in Patent Document 1.

[0009] The present invention has been made in view of the above-mentioned problems of the conventional art. More specifically, the present invention provides a mechanical component and a rolling bearing that are capable of suppressing dimensional changes over time and improving the rolling fatigue life due to indentation-initiated surface fatigue. [Means for solving the problem]

[0010] A mechanical component according to a first aspect of the present invention is made of steel and has a surface. The steel has been quenched and tempered. The mechanical component has a nitrided layer on the surface and a core that is farther from the surface than the nitrided layer. The nitrogen concentration in the steel at the surface is 0.01 mass percent or more. The hardness of the steel at the surface is 820 Hv or more. The amount of retained austenite in the steel at the core is 0.1 volume percent or more and 9 volume percent or less. The dislocation density of the retained austenite in the steel at the core is 4.0 × 10 14 m -2 That's all. The steel is high carbon steel or bearing steel.

[0011] In the mechanical component according to the first aspect of the present invention, the dislocation density of martensite in the steel in the core portion is 6.0 × 10 14 m -2 It may be more than that.

[0012] A mechanical component according to a second aspect of the present invention is made of steel and has a surface. The steel has been quenched and tempered. The mechanical component has a nitrided layer on the surface and a core that is further from the surface than the nitrided layer. The nitrogen concentration in the steel at the surface is 0.01 mass percent or more. The hardness of the steel at the surface is 820 Hv or more. The amount of retained austenite in the steel at the core is 0.1 volume percent or more and 5 volume percent or less. The dislocation density of the retained austenite in the steel at the core is 1.0 x 10 15 m -2 That's all. The steel is a low carbon steel or a carburized steel.

[0013] In the mechanical component according to the first or second aspect of the present invention, the nitrogen concentration in the steel at the surface is represented by X (unit: mass percent), and the dislocation density of martensite in the steel at the surface is represented by Y (unit: m -2 ) then 934923.48+379.96×X-330.96×Y 2 -5.41×10 4 ×logY+783.83×logX 2≧0 may be satisfied.

[0014] In the mechanical component according to the first aspect of the present invention, the steel may contain 0.77 mass percent or more of carbon, 4.0 mass percent or less of chromium, 0.10 mass percent or more and 0.70 mass percent or less of silicon, and 0.25 mass percent or less of molybdenum.

[0015] In the mechanical component according to the second aspect of the present invention, the steel may contain 0.01 mass percent or more and less than 0.77 mass percent carbon, 4.0 mass percent or less chromium, 0.10 mass percent or more and 0.70 mass percent or less silicon, and 0.25 mass percent or less molybdenum.

[0016] In the mechanical component according to the first or second aspect of the present invention, the dimensional change rate after holding at 160°C for 2500 hours is 40 × 10 -5 In the mechanical component according to the first or second aspect of the present invention, the dimensional change rate after holding at 160°C for 2500 hours may be 15×10 or less. -5 The dimensional change rate is a value obtained by subtracting the dimension of the mechanical component before the holding from the dimension of the mechanical component after the holding (the difference in the dimensions of the mechanical component before and after the holding), and dividing the result by the dimension of the mechanical component before the holding.

[0017] The rolling bearing of the present invention comprises an inner ring, an outer ring, and rolling elements, at least one of which is the above-described mechanical component. [Effects of the Invention]

[0018] According to the machine component according to the first or second aspect of the present invention and the rolling bearing of the present invention, it is possible to suppress dimensional changes over time and improve the rolling fatigue life of the surface indentation-initiated type. [Brief explanation of the drawings]

[0019] [Figure 1]FIG. 2 is a cross-sectional view of the inner ring 10. [Figure 2A] FIG. 2 is a cross-sectional view of the outer ring 30. [Figure 2B] FIG. 2 is a cross-sectional view of a rolling element 40. [Figure 2C] 1 is a cross-sectional view of a rolling bearing according to an embodiment. [Figure 3] 3A to 3C are process diagrams showing a manufacturing method of the inner ring 10. [Figure 4] 2 is a cross-sectional view of a workpiece 20. FIG. [Figure 5A] 2 is a cross-sectional view of a workpiece 50 to be processed. FIG. [Figure 5B] 2 is a cross-sectional view of a workpiece 60 to be processed. FIG. [Figure 6] 4 is a schematic graph showing the shape of the surface of a bearing ring on which an indentation is formed. [Figure 7] 1 is a graph showing the relationship between the hardness of the steel on the surface of the raceway and the rise around the indentation. DETAILED DESCRIPTION OF THE INVENTION

[0020] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and redundant description will not be repeated.

[0021] (Configuration of mechanical component according to embodiment) The configuration of a mechanical component according to an embodiment will be described below. The mechanical component according to an embodiment is, for example, a raceway ring of a rolling bearing. The mechanical component according to an embodiment may also be a rolling element of a rolling bearing. The mechanical component according to an embodiment may also be a sliding member such as a gear, an axis, a shaft, or a ball screw, or may also be a structural member such as a housing. Here, an inner ring 10 of a rolling bearing will be described as an example of a mechanical component according to an embodiment.

[0022] The mechanical component according to the embodiment may be a component of a rolling bearing used in an electric axle (such as a raceway ring (inner ring, outer ring), or rolling element (ball, roller)). The mechanical component according to the embodiment may be a gear, shaft, or other component used in an electric axle. A typical electric axle is a unit with a three-axis structure comprising a drive motor, a reducer, an inverter, and the like. Other electric axles include a coaxial unit comprising a drive motor, a planetary reducer, an inverter, and the like, or one composed of a drive motor, a CVT (Continuously Variable Transmission), an inverter, and the like. There is a possibility that foreign matter generated by friction and wear of the gears or housing may be mixed into the rolling bearing used in the reducer of an electric axle. Therefore, the components of the rolling bearing used in the reducer are required to have dimensional stability at high temperatures, a high load capacity, and to be resistant to early damage even if foreign matter is mixed in. Furthermore, to save space inside the reducer and to reduce the size of the reducer, the rolling bearing used in the reducer is required to have a high load capacity. Furthermore, for the gears, shafts, and other components used in electric axles, dimensional stability at high temperatures, resistance to foreign matter, and high load capacity are required, for the same reasons as for the rolling bearing components used in the electric axle reducer.

[0023] The mechanical component according to the embodiment may be a component of a rolling bearing or ball screw used in an electric brake. An electric brake is composed of, for example, a motor, a reduction gear, a ball screw, a cylinder, a control device, etc. A ball screw is a component composed of a shaft having a raceway surface, a nut (outer ring) having a raceway surface, rolling elements (balls) arranged between the raceway surface of the shaft and the raceway surface of the nut, a tube, a top, an end cap, etc. The rolling bearings and ball screws used in electric brakes are also required to have dimensional stability at high temperatures, resistance to foreign matter, and a high load capacity.

[0024] The electric compressor cools the interior of a vehicle, as well as the battery and on-board electronic devices that tend to become hot. The mechanical component according to the embodiment may be a rolling bearing used in the electric compressor. The rolling bearing used in the electric compressor is also required to have dimensional stability at high temperatures, resistance to foreign matter, and a high load capacity.

[0025] The applications of the machine components according to the embodiments are not limited to automobile applications. Even in other applications, sliding components such as rolling bearings, ball screws, shafts, and pins are required to have high dimensional stability and stability in geometric tolerances such as roundness, cylindricity, and coaxiality due to the harsh operating environments. For example, high precision and dimensional stability are also required for components of rolling bearings for the spindles of machine tools.

[0026] Ball screws used in electric actuators, positioning devices, electric jacks, servo cylinders, electric servo presses, mechanical presses, transmissions, electric push-button steering, electric injection molding machines, etc., also require dimensional stability at high temperatures, resistance to foreign matter, and high load capacity. Sliding parts such as shafts and pins tend to become hotter as they rotate at higher speeds, so dimensional stability at high temperatures is required.

[0027] The configuration of the inner ring 10 will be described below. Fig. 1 is a cross-sectional view of inner ring 10. As shown in Fig. 1, inner ring 10 has a first end face 10a, a second end face 10b, an inner circumferential surface 10c, and an outer circumferential surface 10d. The first end face 10a, the second end face 10b, the inner circumferential surface 10c, and the outer circumferential surface 10d form the surface of inner ring 10. Inner ring 10 is ring-shaped.

[0028] The central axis of the inner ring 10 is defined as the central axis A. The direction along the central axis A is defined as the axial direction. The direction perpendicular to the central axis A and passing through the central axis A is defined as the radial direction. The direction along the circumference of a circle centered on the central axis A is defined as the circumferential direction.

[0029] The first end face 10a and the second end face 10b are axial end faces of the inner ring 10. The second end face 10b is the surface opposite the first end face 10a in the axial direction.

[0030] The inner peripheral surface 10c extends in the circumferential direction. The inner peripheral surface 10c faces the central axis A. One end and the other end in the axial direction of the inner peripheral surface 10c are connected to the first end face 10a and the second end face 10b, respectively. Although not shown, the inner ring 10 is fitted onto a shaft at the inner peripheral surface 10c.

[0031] The outer peripheral surface 10d extends in the circumferential direction. The outer peripheral surface 10d faces away from the central axis A. In other words, the outer peripheral surface 10d is the opposite surface of the inner peripheral surface 10c in the radial direction. One end and the other end of the outer peripheral surface 10d in the axial direction are connected to the first end face 10a and the second end face 10b, respectively. The outer peripheral surface 10d has a raceway surface 10da. The raceway surface 10da is the portion of the outer peripheral surface 10d that contacts the rolling elements (not shown). The raceway surface 10da is located, for example, at the center of the outer peripheral surface 10d in the axial direction. The raceway surface 10da extends in the circumferential direction. In a cross-sectional view, the raceway surface 10da is partially arc-shaped.

[0032] The inner ring 10 is made of steel. The steel constituting the inner ring 10 is hardened and tempered. The steel constituting the inner ring 10 is, for example, high-carbon steel. High-carbon steel refers to hyper-eutectoid steel with a carbon concentration of 0.77 mass percent or more. The steel constituting the inner ring 10 may be bearing steel. Bearing steel refers to high-carbon chromium steel with a carbon concentration of 0.9 mass percent or more and 1.05 mass percent or less and a chromium concentration of 0.9 mass percent or more and 1.7 mass percent or less. The steel constituting the inner ring 10 may be low-carbon steel or carburized steel. Low-carbon steel refers to hypo-eutectoid steel with a carbon concentration of less than 0.77 mass percent. Carburized steel refers to steel containing one of chromium, molybdenum, or nickel with a carbon concentration of 0.1 mass percent or more and 0.5 mass percent or less.

[0033] Specific examples of high-carbon steel include SK85 specified in JIS. Specific examples of bearing steel include SUJ2, SUJ3, SUJ4, and SUJ5 specified in JIS, 50100, 51100, 52100, and A485 Grade 1 specified in ASTM, 100Cr6, 100CrMnSi4-4, 100CrMnSi6-4, 100CrMo7, 100CrMo7-3, and 100CrMnMoSi8-4-6 specified in ISO, 105Cr4 specified in DIN, and GCr4, GCr15, GCr15SiMn, GCrSiMo, and GCr18Mo specified in GB / T. Specific examples of low-carbon steel include S55C, S53C, S50C, S45C, S25C, and S15C specified in the JIS standard, 1045, 1046, 1050, 1053, and 1055 specified in the AISI standard, C45, C45E, C45R, C55, C55E, and C55R specified in the ISO standard, and 45, 50Mn, and 55 specified in the GB / T standard. Specific examples of carburizing steel include SCr420, SCr435, SCM420, SCM435, SNCM420, and SNCM815 specified in JIS standards; 5120, 4118, 4135, 4320, 8620, 5135, and 9315 specified in AISI standards; 20Cr4, 20CrMo4, 20NiCrMo7, 18NiCrMo14-6, 17NiCrMo6-4, 37Cr4, 25CrMo4, and 34CrMo4 specified in ISO standards; and G20CrMo and G20CrNi2Mo specified in GB / T.

[0034] The carbon concentration in the steel making up the inner ring 10 is, for example, 0.77 mass percent or more. The carbon concentration in the steel making up the inner ring 10 may be 0.01 mass percent or more and less than 0.77 mass percent. The steel making up the inner ring 10 may also contain 4.0 mass percent or less of chromium, 0.10 mass percent or more and 0.70 mass percent or less of silicon, and 0.25 mass percent or less of molybdenum. In this case, the steel making up the inner ring 10 does not have to contain chromium or molybdenum.

[0035] The surface of the inner ring 10 has been subjected to nitriding or carbonitriding treatment. That is, the inner ring 10 has a nitrided layer 11 on its surface (the surface of the inner ring 10 is formed as the nitrided layer 11). Nitrogen is dissolved in the steel in the nitrided layer 11. The portion of the inner ring 10 that is farther from the surface than the nitrided layer 11 is referred to as the core portion 12. From another perspective, the core portion 12 is the interior portion of the inner ring 10 other than the nitrided layer 11. Nitrogen is not dissolved in the steel in the core portion 12. The nitrogen concentration in the steel at the surface of the inner ring 10 is 0.01 mass percent or more. The nitrogen concentration in the steel at the surface of the inner ring 10 may be 0.10 mass percent or more. The nitrogen concentration at the surface of the inner ring 10 is measured using, for example, an EPMA (Electron Probe Micro Analyzer). When measuring using EPMA, a calibration curve is created using a standard sample with a known nitrogen concentration.

[0036] The hardness of the steel on the surface of the inner ring 10 is 820 Hv or more. The hardness of the steel on the surface of the inner ring 10 is measured using the Vickers hardness test method specified in the JIS standard (JIS Z 2244:2009). The load used when measuring the hardness of the steel on the surface of the inner ring 10 is 300 g. The hardness of the steel on the surface of the inner ring 10 is measured at at least three locations and obtained by averaging the measured values.

[0037] When the steel constituting the inner ring 10 is high carbon steel or bearing steel, the amount of retained austenite in the steel in the core portion 12 is 9 volume percent or less. In this case, the amount of retained austenite in the steel in the core portion 12 is 0.1 volume percent or more. When the steel constituting the inner ring 10 is low carbon steel or carburized steel, the amount of retained austenite in the steel in the core portion 12 is 5 volume percent or less. In this case, the amount of retained austenite in the steel in the core portion 12 is 0.1 volume percent or more.

[0038] The amount of retained austenite in the steel in the core portion 12 is measured using X-ray diffraction. When measuring the amount of retained austenite in the steel in the core portion 12 by X-ray diffraction, a Cr tube type X-ray diffractometer is used. In the Cr tube type X-ray diffractometer, the wavelength of Cr-Kα rays is 2.29093×10 -10 The tube diameter is set to 1000 mm, the tube voltage is set to 30 kV, the tube current is set to 10 mA, and the collimator size is set to 2 mm × 2 mm. When measuring the amount of retained austenite in the steel in the core portion 12, the inner ring 10 is preferably electropolished to prevent work-induced transformation of the retained austenite.

[0039] When the steel constituting the inner ring 10 is high carbon steel or bearing steel, the dislocation density of the retained austenite in the steel in the core portion 12 is 4.0 × 10 14 m -2 When the steel constituting the inner ring 10 is high carbon steel or bearing steel, the dislocation density of martensite in the steel in the core portion 12 is 6.0 × 10 14 m -2 When the steel constituting the inner ring 10 is low carbon steel or carburized steel, the dislocation density of the retained austenite in the steel in the core portion 12 is preferably 1.0×10 or more. 15 m -2 That's all.

[0040] The nitrogen concentration in the steel at the surface of the inner ring 10 is X (unit: mass percent), and the dislocation density of martensite in the steel at the surface of the inner ring 10 is Y (unit: m -2 ) and the hardness of the steel on the surface of the inner ring 10 is Z (unit: Hv). In this case, Z is calculated as follows: 935743.48 + 379.96 × X - 330.96 × Y 2 -5.41×10 4 ×logY+783.83×logX 2 It can be calculated using (Equation 1). Equation 1 can be obtained by performing multiple regression analysis after determining the value of Z when X and Y are changed through experiments. From Equation 1, in order to make the hardness of the steel on the surface of the inner ring 10 820 Hv or more, it is necessary to obtain the following: 934923.48 + 379.96 × X - 330.96 × Y 2 -5.41×10 4×logY+783.83×logX 2 ≧0 (Equation 2) is satisfied.

[0041] The dislocation density of the retained austenite in the steel in the core portion 12 and the dislocation density of the martensite in the steel on the surface of the inner ring 10 are measured using a cobalt (Co) tube X-ray diffractometer. More specifically, first, the X-ray profiles of the austenite and martensite in the core portion 12 (surface of the inner ring 10) are measured using a Co tube X-ray diffractometer. In the Co tube X-ray diffractometer, the wavelength of Co-Kα radiation is 1.7889×10 -10 m, the tube voltage is 40 kV, the tube current is 50 mA, and the collimator size is 1 mm in diameter. The X-ray profiles of austenite and martensite in the core 12 (surface of the inner ring 10) are measured within a 2θ range of 30° or more and 135° or less. Secondly, after Rietveld analysis is performed, the half-widths of the peaks in the X-ray profiles of martensite and austenite obtained by X-ray diffraction are separated into crystallite size and strain. Thirdly, the separated crystallite size and strain are applied to the following Williamson-Hall equation to obtain the dislocation densities of martensite and austenite. In this equation, ρ is the dislocation density (unit: m -2 ), where ε is the strain and b is the length of the Burgers vector (b = 0.25 × 10 -9 m).

[0042]

number

[0043] In the X-ray profile of martensite obtained by X-ray diffraction of the core portion 12 (surface of the inner ring 10), the peaks of the {110}, {200}, {211}, and {220} planes are measured. In the X-ray profile of austenite obtained by X-ray diffraction of the core portion 12 (surface of the inner ring 10), the peaks of the {111}, {200}, {220}, {311}, and {222} planes are measured. The Rietveld analysis is performed in the above manner to reduce the influence of the {200} plane of martensite and the {200} plane of austenite, which have different elastic moduli.

[0044] The dimensional change rate of the inner ring 10 after holding at 160°C for 2500 hours is 40×10 -5 Preferably, the dimensional change rate of the inner ring 10 after being held at 160°C for 2500 hours is 15×10 or less. -5 The dimensional change rate of the inner ring 10 is calculated by subtracting the dimension of the inner ring 10 before holding from the dimension of the inner ring 10 after holding, and dividing the result by the dimension of the inner ring 10 before holding.

[0045] <Modification> FIG. 2A is a cross-sectional view of the outer ring 30. FIG. 2B is a cross-sectional view of the rolling elements 40. As shown in FIG. 2A, the mechanical component according to the embodiment may be the outer ring 30 of a rolling bearing. As shown in FIG. 2B, the mechanical component according to the embodiment may be the rolling elements 40. The configurations of the outer ring 30 and the rolling elements 40 are the same as those of the inner ring 10 except for their shapes. FIG. 2C is a cross-sectional view of the rolling bearing according to the embodiment. The rolling bearing according to the embodiment (rolling bearing 100) has the inner ring 10, the outer ring 30, the rolling elements 40, and a cage 70. In the rolling bearing 100, it is sufficient that at least one of the inner ring 10, the outer ring 30, and the rolling elements 40 is the mechanical component according to the embodiment.

[0046] (Mechanical component manufacturing method according to the embodiment) A method for manufacturing the inner ring 10 will be described below.

[0047] Fig. 3 is a process diagram showing a method for manufacturing the inner ring 10. As shown in Fig. 3, the method for manufacturing the inner ring 10 includes a preparation step S1, a nitriding step S2, a quenching step S3, a cooling step S4, a tempering step S5, and a post-treatment step S6.

[0048] In the preparation step S1, a workpiece 20 is prepared. FIG. 4 is a cross-sectional view of the workpiece 20. As shown in FIG. 4, the workpiece 20 is ring-shaped and has a first end face 20a, a second end face 20b, an inner circumferential surface 20c, and an outer circumferential surface 20d. The first end face 20a, the second end face 20b, the inner circumferential surface 20c, and the outer circumferential surface 20d are surfaces that will become the first end face 10a, the second end face 10b, the inner circumferential surface 10c, and the outer circumferential surface 10d, respectively, after the post-processing step S6 is completed. The workpiece 20 is formed from the same steel as the inner ring 10.

[0049] In the nitriding step S2, the workpiece 20 is subjected to a nitriding treatment. The nitriding treatment of the workpiece 20 is performed by heating and holding the workpiece 20 in an atmospheric gas containing a nitrogen source. The heating temperature and the nitrogen concentration in the atmospheric gas in the nitriding step S2 are set so that a compound layer is not formed on the surface of the workpiece 20. By performing the nitriding step S2, nitrogen penetrates from the surface of the workpiece 20 to the interior, and the nitrogen is dissolved in the workpiece 20. Note that the nitriding step S2 is performed so that the nitrogen reaches a position deeper than the position that will become the surface of the inner ring 10 after the post-treatment step S6 is performed.

[0050] The workpiece 20 may be subjected to a carbo-nitriding process instead of the nitriding process S2. The carbo-nitriding treatment of the workpiece 20 is performed by heating and holding the workpiece in an atmospheric gas containing a nitrogen source and a carbon source. The heating temperature in the carbo-nitriding process and the carbon and nitrogen concentrations in the atmospheric gas are set so that a compound layer is not formed on the surface of the workpiece 20. By performing the carbo-nitriding process, carbon and nitrogen penetrate from the surface of the workpiece 20 to the interior, and the carbon and nitrogen are dissolved in the workpiece 20. The carbo-nitriding process is performed so that the nitrogen and carbon reach a position deeper than the position that will become the surface of the inner ring 10 after the post-treatment process S6 is performed.

[0051] In the quenching step S3, the workpiece 20 is quenched. The quenching of the workpiece 20 is performed by heating the workpiece 20 to a temperature equal to or higher than the A1 transformation point of the steel constituting the workpiece 20, and then maintaining that temperature, and then cooling it to a temperature equal to or lower than the Ms transformation point of the steel constituting the workpiece 20. By performing the quenching step S3, martensite and retained austenite are generated in the steel constituting the workpiece 20. Note that after the quenching step S3 is performed, the quenching step S3 may be repeated by heating the workpiece 20 to a temperature equal to or higher than the A1 transformation point. By performing the quenching step S3 multiple times, the crystal grains become finer, improving the effect of the cooling step S4.

[0052] In the cooling step S4, sub-zero treatment is performed on the workpiece 20. In the cooling step S4, cryo-treatment (ultra-sub-zero treatment) may be performed on the workpiece 20. In the sub-zero treatment, the workpiece 20 is cooled to a temperature above -100°C and below room temperature. In the cryo-treatment, the workpiece 20 is cooled to a temperature below -100°C. By performing the cooling step S4, some of the retained austenite in the steel that constitutes the workpiece 20 is transformed into martensite. Note that before the cooling step S4 is performed, a low-temperature tempering step or cleaning step may be performed to prevent cracking.

[0053] In the tempering step S5, the workpiece 20 is tempered. The workpiece 20 is tempered by heating the workpiece 20 to a temperature below the A1 transformation point of the steel that constitutes the workpiece 20. More specifically, the workpiece 20 is tempered by heating the workpiece 20 to a temperature of approximately 180°C. In the post-treatment step S6, the surface of the workpiece 20 is subjected to machining such as grinding and polishing. As a result, the inner ring 10 having the structure shown in FIG. 1 is manufactured. Note that when the workpiece 20 is heated to a temperature of 180°C or higher in the tempering step S5, the higher the heating temperature, the lower the dislocation density of martensite and the lower the hardness. On the other hand, if the cooling step S4 is performed, the dislocation density of martensite is less likely to be reduced by heating in the tempering step S5, and therefore, although the hardness decreases as the heating temperature increases, a higher hardness than usual can be obtained.

[0054] <Modification> Fig. 5A is a cross-sectional view of a workpiece 50. Fig. 5B is a cross-sectional view of a workpiece 60. As shown in Fig. 5A, when the mechanical component according to the embodiment is an outer ring 30, a workpiece 50 that is a ring-shaped component is used as the workpiece. As shown in Fig. 5B, when the mechanical component according to the embodiment is a rolling element 40, a workpiece 60 that is a spherical component is used as the workpiece. The workpieces 50 and 60 have the same configuration as the workpiece 20, except for their shapes.

[0055] (Effects of the mechanical component according to the embodiment) One possible way to suppress dimensional changes over time in quenched and tempered steel raceways and rolling elements is to reduce the amount of retained austenite by tempering at high temperatures. However, while high-temperature tempering can suppress dimensional changes over time by reducing the amount of retained austenite, it also reduces the hardness of the steel on the surfaces of the raceways and rolling elements.

[0056] When a foreign object becomes caught between the surface of the raceway and the rolling element, an indentation is formed on the surface of the raceway. Figure 6 is a schematic graph showing the shape of the surface of the raceway where an indentation is formed. As shown in Figure 6, the surface of the raceway bulges around the indentation. Figure 7 is a graph showing the relationship between the hardness of the steel on the surface of the raceway and the bulge around the indentation. In Figure 7, the horizontal axis represents hardness (unit: Hv), and the horizontal axis represents the amount of bulge around the indentation (unit: μm). As shown in Figure 7, the amount of bulge around the indentation increases as the hardness of the steel on the surface of the raceway decreases.

[0057] If the amount of swelling around the indentation becomes large, stress will be concentrated on the swelling around the indentation, making fatigue fracture more likely to occur from the indentation. Therefore, if tempering is performed at high temperature to suppress dimensional changes over time, the rolling fatigue life of the bearing ring may be insufficient.

[0058] When the amount of retained austenite in steel is low and the retained austenite is surrounded by martensite with a high dislocation density (i.e., poor deformability), the retained austenite is constrained or stressed by the martensite, reducing its lattice spacing (lattice constant), resulting in a high dislocation density of the retained austenite in the steel. Even when the volume of such retained austenite expands due to decomposition, it is constrained by the surrounding martensite with a high dislocation density, so that even if the retained austenite decomposes during use, the dimensional change associated with the decomposition is small.

[0059] The inner ring 10 has been subjected to sub-zero treatment or cryo-treatment to reduce the amount of retained austenite in the steel in the core portion 12. More specifically, in the case of the inner ring 10, if the steel constituting the inner ring 10 is high carbon steel or bearing steel, the amount of retained austenite in the steel in the core portion 12 is 9 volume percent or less, and if the steel constituting the inner ring 10 is low carbon steel or carburized steel, the amount of retained austenite in the steel in the core portion 12 is 5 volume percent or less.

[0060] Furthermore, since the inner ring 10 has undergone sub-zero treatment or cryo-treatment, the dislocation density of martensite in the steel in the core portion 12 is increased, and as a result, the dislocation density of the retained austenite in the steel in the core portion 12 is also increased. More specifically, in the case of the inner ring 10, when the steel constituting the inner ring 10 is high carbon steel or bearing steel, the dislocation density of the retained austenite in the steel in the core portion 12 is 4.0 × 10 14 m -2 When the steel constituting the inner ring 10 is low carbon steel or carburized steel, the dislocation density of the retained austenite in the steel in the core portion 12 is 1.0 × 10 or more. 15 m -2 That's all.

[0061] As described above, since the retained austenite is surrounded by martensite with a high dislocation density in the steel of the core portion 12, even if the retained austenite in the steel of the core portion 12 decomposes due to the rise in temperature that accompanies use of the inner ring 10, the volume expansion that accompanies the decomposition is restrained by the surrounding martensite with a high dislocation density, and dimensional change is therefore unlikely to occur. In this way, dimensional change over time that accompanies use of the inner ring 10 is suppressed.

[0062] Furthermore, because the inner ring 10 has not been tempered at high temperatures, the decomposition of martensite on the surface of the inner ring 10 progresses only slightly. Furthermore, the nitrogen concentration in the steel on the surface of the inner ring 10 is 0.01 mass percent or more, and the steel on the surface of the inner ring 10 is solid-solution strengthened. As a result, the hardness of the steel on the surface of the inner ring 10 is 820 Hv or more. As shown in FIG. 7 , when the hardness of the steel on the surface of the inner ring 10 reaches 820 Hv or more, the amount of swelling around the indentation decreases sharply. Thus, the inner ring 10 also improves the rolling fatigue life at the indentation origin. By increasing the hardness of the steel on the surface of the inner ring 10, indentations are less likely to form on the surface of the inner ring 10, improving the static load capacity of a rolling bearing using the inner ring 10.

[0063] Furthermore, the surfaces of the rolling elements may be subjected to high residual compressive stress during the pressurizing process, making them less susceptible to indentations than the surfaces of the raceways (inner and outer rings). Therefore, even if only the raceways are used as the mechanical component according to the embodiment, the indentation-initiated rolling contact fatigue life of the rolling bearing is improved.

[0064] (Hardness evaluation test) Samples 1 to 19 were prepared to evaluate the relationship between hardness, martensite dislocation density, and nitrogen concentration on the surface of quenched and tempered steel machine parts. Samples 1 to 19 were ring-shaped, with an inner diameter of 54 mm, an outer diameter of 60 mm, and a width of 15 mm. As shown in Table 1, the steel type, nitrogen concentration in the steel at the sample surface, and dislocation density of martensite in the steel at the sample surface were varied for Samples 1 to 19. The "OK" and "NG" entries in the "Fulfillment of Equation 2" column in Table 1 indicate that the above equation 2 is fulfilled and that the above equation 2 is not fulfilled, respectively.

[0065] [Table 1]

[0066] The nitrogen concentration in the steel at the surface of each sample was adjusted by changing the heating temperature and holding time in the nitriding or carbonitriding treatment, and the dislocation density of martensite in the steel at the surface of each sample was adjusted by changing the cooling temperature and holding time in the sub-zero treatment or cryo-treatment.

[0067] The above formula 2 was satisfied in Samples 1 to 7 and Samples 9 to 14. On the other hand, the above formula 2 was not satisfied in Sample 8 and Samples 15 to 19.

[0068] In Samples 1 to 7 and Samples 9 to 14, the hardness of the steel at the sample surface was 820 Hv or more. On the other hand, in Sample 8 and Samples 15 to 19, the hardness of the steel at the sample surface was less than 820 Hv. This comparison revealed that when the above formula 2 is satisfied, the hardness of the steel at the surface of the mechanical component becomes 820 Hv or more, and the rolling contact fatigue life is improved. From another perspective, it was revealed that by increasing both the nitrogen concentration in the steel at the surface of the mechanical component and the dislocation density of martensite in the steel at the surface of the mechanical component, the hardness of the steel at the surface of the mechanical component, and therefore the rolling contact fatigue life of the mechanical component, is improved.

[0069] (Evaluation test of dimensional changes over time) To evaluate the change in dimensions over time, the above Samples 3 to 7 and Samples 9 to 14 were used. Table 2 shows the amount of retained austenite in the steel at the core of Samples 3 to 7 and Samples 9 to 14, and the dislocation density in the steel at the core of Samples 3 to 7 and Samples 9 to 14.

[0070] [Table 2]

[0071] The dislocation density of martensite in the steel in the core 12 of each sample and the amount of retained austenite in the martensite in the steel in the core 12 of each sample were adjusted by changing the cooling temperature and holding time in the sub-zero treatment or cryo-treatment.

[0072] When the steel is high carbon steel or bearing steel, the dislocation density of martensite in the steel in the core portion 12 is 4.0 × 10 14 m -2 If the steel is low carbon steel or carburized steel, the dislocation density of martensite in the steel in the core portion 12 is 1.0×10 or more, then the condition A is satisfied. 15 m -2If so, condition A is satisfied.

[0073] If the steel is high carbon steel or bearing steel, condition B is met if the amount of retained austenite in the steel at the core 12 of the sample is 9 volume percent or less. If the steel is low carbon steel or carburized steel, condition B is met if the amount of retained austenite in the steel at the core 12 of the sample is 5 volume percent or less.

[0074] Conditions A and B were satisfied in Samples 3 to 7 and Samples 9 to 11. On the other hand, Conditions A and B were not satisfied in Samples 12 to 14.

[0075] In samples 3 to 7 and samples 9 to 11, the dimensional change rate after holding at 160°C for 2500 hours was 40×10 -5 On the other hand, in Samples 12 to 14, the dimensional change rate after holding at 160°C for 2500 hours was 50×10 -5 This comparison reveals that satisfying conditions A and B suppresses the dimensional changes of machine parts over time.

[0076] When the steel is a high carbon steel or a bearing steel, the dislocation density of martensite in the steel in the core 12 of the sample is 6.0 × 10 14 m -2 If this is the case, condition C is satisfied. Samples 3 to 5 satisfy condition C, and the dimensional change rate after holding at 160°C for 2500 hours is 15×10 -5 On the other hand, in Samples 9 to 11, Condition C was not satisfied, and the dimensional change rate after holding at 160°C for 2500 hours was 15×10 or less. -5 This comparison reveals that satisfying condition C further suppresses the dimensional changes of mechanical parts over time.

[0077] Although the embodiments of the present invention have been described above, the above embodiments can be modified in various ways. Furthermore, the scope of the present invention is not limited to the above embodiments. The scope of the present invention is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0078] A: central shaft, S1: preparation process, S2: nitriding process, S3: quenching process, S4: cooling process, S5: tempering process, S6: post-treatment process, 10: inner ring, 10a: first end face, 10b: second end face, 10c: inner peripheral surface, 10d: outer peripheral surface, 10da: raceway surface, 11: nitriding layer, 12: core, 20: workpiece, 20a: first end face, 20b: second end face, 20c: inner peripheral surface, 20d: outer peripheral surface, 30: outer ring, 40: rolling elements, 50, 60: workpiece, 70: cage, 100: rolling bearing.

Claims

1. A machine part made of quenched and tempered steel and having a surface, a nitriding layer on the surface in which nitrogen is dissolved, and a core located farther from the surface than the nitriding layer; The nitrogen concentration in the steel at the surface is 0.12 mass percent or more, The hardness of the steel at the surface is 820 Hv or more, the amount of retained austenite in the steel at the surface is 12.6 volume percent or less; The steel contains 0.77 mass percent or more of carbon, 4.0 mass percent or less of chromium, 0.10 mass percent or more and 0.70 mass percent or less of silicon, and 0.25 mass percent or less of molybdenum; The steel is SUJ2 as specified in the JIS standard, the dislocation density of martensite in the steel at the surface is 1.30×10 15 m −2 or more; The amount of retained austenite in the steel in the core is 9 volume percent or less, A mechanical component, wherein the dislocation density of martensite in the steel in the core portion is 6.0×10 14 m −2 or more.

2. The mechanical component according to claim 1 , wherein the amount of retained austenite in the steel in the core is 0.1 volume percent or more.

3. The dislocation density of the retained austenite in the steel in the core portion is 4.0 × 10 １４ m －２ The mechanical component according to claim 1 .

4. The dimensional change rate after holding at 160°C for 2500 hours was 40 x 10 －５ 2. The machine part according to claim 1, wherein:

5. The dimensional change rate after holding at 160°C for 2500 hours was 15 x 10 －５ 2. The machine part according to claim 1, wherein:

6. With inner circle, The outer ring and a rolling element; A rolling bearing, wherein at least one of the inner ring, the outer ring, and the rolling elements is the mechanical component according to any one of claims 1 to 5.

7. Axle and Nut and a rolling element; A ball screw, wherein at least one of the shaft, the nut, and the rolling elements is the mechanical component according to any one of claims 1 to 5.

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