JP2024046662A - Machine parts and rolling bearings - Google Patents

Abstract

[Problem] To provide a machine component capable of suppressing dimensional changes over time and improving the rolling fatigue life of indentation-initiated type on the surface. [Solution] The machine component is made of steel and has a surface. The steel has been quenched and tempered. The machine component has a nitriding layer on the surface and a core that is further from the surface than the nitriding 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 x 1014 m-2 or more. The steel is high carbon steel or bearing steel. [Selected Figure] Figure 1

Description

The present invention relates to mechanical parts and rolling bearings. More specifically, the present invention relates to a mechanical component made of hardened and tempered steel and a rolling bearing equipped with the mechanical component.

The electrification of automobiles is progressing, centering on battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and hybrid electric vehicles (HEVs). For example, electric axles (e-axles), electric brakes, electric VTCs (variable valve mechanisms), electric compressors, and the like are being used instead of engines.

In electric vehicles, in order to extend the driving distance with less electricity (improving electric fuel efficiency), it is desirable to reduce the weight by making the unit smaller, and to make the motor faster and more powerful. Accordingly, various parts 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, and the temperature of the rolling bearings during use rises. In addition, there is a high possibility that the viscosity of lubricating oil will decrease and the amount of oil or grease will decrease in order to reduce the torque of rolling bearings, etc., which will further increase the amount of heat generated by the rolling bearings. For this reason, rolling bearings used in electric vehicles are required to maintain dimensional stability and high hardness at high temperatures.

The structure of the constituent materials of rolling bearings that have 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 impression-initiated rolling fatigue life.

In rolling bearings that have been quenched and tempered, residual austenite is decomposed when the operating temperature increases. As a result, the volumetric expansion accompanying the decomposition of retained austenite causes dimensional changes in the components of the rolling bearing. Furthermore, when the dimensional change rate of the raceway ring of a rolling bearing increases, creep occurs, the contact surface pressure increases due to a decrease in the gap between the raceway surface and the rolling elements, and this causes early damage and dimensional accuracy. Problems such as abnormal noise and increased vibration may occur as the temperature decreases.

JP-A-2001-99163 (Patent Document 1) describes a bearing ring for a rolling bearing. The bearing ring described in Patent Document 1 is made of steel that has been hardened and tempered. In the bearing ring described in Patent Document 1, the amount of retained austenite in the steel is substantially zero. According to the bearing ring described in Patent Document 1, changes in dimensions over time due to use are suppressed.

Japanese Patent Application Publication No. 2001-99163

However, in the raceway 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 is low with a small amount of retained austenite, the rolling fatigue life of the indentation-initiated type is reduced. Therefore, there is room for improvement in the rolling fatigue life of the raceway described in Patent Document 1.

The present invention has been made in view of the problems of the prior art as described above. 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 of an indentation originating type on the surface.

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 nitriding layer on the surface and a core that is farther from the surface than the nitriding 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 x 10 ¹⁴ m ^-2 or more. The steel is high carbon steel or bearing steel.

In the machine component according to the first aspect of the present invention, a dislocation density of martensite in the steel in the core portion may be 6.0×10 ¹⁴ m ⁻² or more.

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 nitriding layer on the surface and a core that is farther from the surface than the nitriding 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 1.0×10 ¹⁵ m ^-2 or more. The steel is low carbon steel or carburized steel.

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

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.

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.

In the mechanical component according to the first aspect or the second aspect of the present invention, the dimensional change rate after being held at 160° C. for 2500 hours may be 40×10 ⁻⁵ or less. In the mechanical component according to the first aspect or the second aspect of the present invention, the dimensional change rate after being held at 160° C. for 2500 hours may be 15×10 ⁻⁵ or less. The dimensional change rate is the value obtained by subtracting the dimension of the machine part before the above holding from the dimension of the machine part after the above holding (the difference in the dimensions of the machine part before and after the above holding). This is the value divided by the dimensions of the mechanical parts.

The rolling bearing of the present invention comprises an inner ring, an outer ring, and rolling elements. At least one of the inner ring, the outer ring, and the rolling elements is the above-mentioned mechanical component.

According to the mechanical component according to the first aspect or the second aspect of the present invention and the rolling bearing of the present invention, it is possible to suppress changes in dimensions over time, and it is also possible to improve the rolling contact fatigue life of the indentation originating type on the surface.

FIG. FIG. 3 is a cross-sectional view of the outer ring 30. FIG. FIG. 4 is a cross-sectional view of a rolling element 40. FIG. FIG. 2 is a cross-sectional view of a rolling bearing according to an embodiment. 4A to 4C are process diagrams showing a manufacturing method of the inner ring 10. FIG. 2 is a cross-sectional view of a workpiece member 20. FIG. It is a sectional view of a workpiece member 50. FIG. 6 is a cross-sectional view of a workpiece member 60. FIG. 4 is a schematic graph showing the shape of the surface of a bearing ring on which an indentation is formed. 1 is a graph showing the relationship between the hardness of the steel on the surface of the race and the rise around the indentation.

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 will be given the same reference symbols, and redundant explanations will not be repeated.

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

The mechanical parts according to the embodiment may be components of a rolling bearing used in an electric axle (races (inner and outer rings), rolling elements (balls, rollers), etc.). The mechanical parts according to the embodiment may be components such as gears, shafts, and shafts used in an electric axle. A typical electric axle is a unit with a three-axis structure composed of a drive motor, a reducer, an inverter, and the like. In addition, some electric axles are composed of a coaxial structure unit composed of a drive motor, a planetary reducer, an inverter, and the like, or 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 gears and housings may be mixed into the rolling bearings used in the reducer of an electric axle. Therefore, in addition to dimensional stability and high load capacity at high temperatures, components of the rolling bearing used in the reducer are required to not be damaged early even if foreign matter is mixed in. In addition, in order to save space in the reducer and to reduce the size of the reducer, the rolling bearings used in the reducer are required to have a high load capacity. Furthermore, for the gears, axles, shafts, etc. used in electric axles, dimensional stability, foreign matter resistance, and high load capacity at high temperatures are required for the same reasons as for the rolling bearing components used in the reduction gears of electric axles.

The mechanical part according to the embodiment may be a component part 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.

Electric compressors are used to cool indoor rooms, batteries, and in-vehicle electronic devices that tend to reach high temperatures. The mechanical component according to the embodiment may be a rolling bearing used in an electric compressor. Rolling bearings used in electric compressors are also required to have dimensional stability at high temperatures, foreign object resistance, and high load capacity.

The use of the mechanical component according to the embodiment is not limited to automobile use. In other applications, sliding parts such as rolling bearings, ball screws, shafts, and pins are subject to harsh operating environments, resulting in high dimensional stability, stability against geometric tolerances such as roundness, cylindricity, and coaxiality. Sex is required. For example, components of rolling bearings for machine tool spindles require high precision and dimensional stability.

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.

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

The center axis of the inner ring 10 is defined as a center 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 centered on the central axis A is defined as the circumferential direction.

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

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

The outer peripheral surface 10d extends along the circumferential direction. The outer circumferential surface 10d faces the opposite side to the central axis A. That is, the outer circumferential surface 10d is a surface opposite to the inner circumferential surface 10c in the radial direction. One end and the other end in the axial direction of the outer peripheral surface 10d are connected to the first end surface 10a and the second end surface 10b, respectively. The outer peripheral surface 10d has a raceway surface 10da. The raceway surface 10da is a portion of the outer circumferential surface 10d that contacts a rolling element (not shown). The raceway surface 10da is located, for example, at the center in the axial direction of the outer circumferential surface 10d. The raceway surface 10da extends in the circumferential direction. In cross-sectional view, the raceway surface 10da has a partially arcuate shape.

The inner ring 10 is made of steel. The steel constituting the inner ring 10 is quenched and tempered. The steel constituting the inner ring 10 is, for example, high carbon steel. High carbon steel refers to hypereutectoid 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 to 1.05 mass percent and a chromium concentration of 0.9 mass percent to 1.7 mass percent. The steel constituting the inner ring 10 may be low carbon steel or carburized steel. Low carbon steel refers to hypoeutectoid steel with a carbon concentration of less than 0.77 mass percent. Carburized steel refers to steel containing any of chromium, molybdenum, and nickel with a carbon concentration of 0.1 mass percent to 0.5 mass percent.

Specific examples of high carbon steel include SK85 as specified in the JIS standard.Specific examples of bearing steel include SUJ2, SUJ3, SUJ4, and SUJ5 as specified in the JIS standard, 50100, 51100, 52100, and A485 Grade 1 as specified in the ASTM standard, 100Cr6, 100CrMnSi4-4, 100CrMnSi6-4, 100CrMo7, 100CrMo7-3, and 100CrMnMoSi8-4-6 as specified in the ISO standard, 105Cr4 as specified in DIN, and GCr4, GCr15, GCr15SiMn, GCrSiMo, and GCr18Mo as 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 GB/T. Specific examples of carburized steel include SCr420, SCr435, SCM420, SCM435, SNCM420, and SNCM815, as specified in the JIS standard; 5120, 4118, 4135, 4320, 8620, 5135, and 9315, as specified in the AISI standard; 20Cr4, 20CrMo4, 20NiCrMo7, 18NiCrMo14-6, 17NiCrMo6-4, 37Cr4, 25CrMo4, and 34CrMo4, as specified in the ISO standard; and G20CrMo and G20CrNi2Mo, as specified in GB/T.

The carbon concentration in the steel constituting the inner ring 10 is, for example, 0.77 mass percent or more. The carbon concentration in the steel constituting the inner ring 10 may be 0.01 mass percent or more and less than 0.77 mass percent. The steel constituting 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 constituting the inner ring 10 does not have to contain chromium and molybdenum.

The surface of the inner ring 10 is subjected to nitriding or carbonitriding treatment. That is, the inner ring 10 has a nitriding layer 11 on its surface (the surface of the inner ring 10 is the nitriding layer 11). In the nitriding layer 11, nitrogen is dissolved in the steel. The part of the inner ring 10 that is farther from the surface than the nitriding layer 11 is called the core 12. From another perspective, the core 12 is the inner part of the inner ring 10 other than the nitriding layer 11. In the core 12, nitrogen is not dissolved in the steel. 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, for example, using an EPMA (Electron Probe Micro Analyzer). When measuring with EPMA, a calibration curve is created using a standard sample with a known nitrogen concentration.

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 determined by the Vickers hardness test method specified in the JIS standard (JIS Z 2244:2009). Note that the load 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 obtained by measuring at least three locations and averaging the obtained measured values.

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 percent by volume 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 12 is 5 percent by volume 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.

The amount of retained austenite in the steel in the core portion 12 is measured using an X-ray diffraction method. When measuring the amount of retained austenite in the steel in the core portion 12 using the X-ray diffraction method, a Cr tube type X-ray diffraction device is used. In the Cr tube type X-ray diffractometer, the wavelength of the Cr-Kα ray is 2.29093 x 10 ^-10 m, the tube voltage is 30 kV, the tube current is 10 mA, and the collimator size is 2 mm x 2 mm. Ru. When measuring the amount of retained austenite in the steel in the core portion 12, the inner ring 10 is preferably electrolytically polished so that the retained austenite does not undergo deformation-induced transformation.

When the steel constituting the inner ring 10 is high carbon steel or bearing steel, the dislocation density of retained austenite in the steel in the core portion 12 is 4.0×10 ¹⁴ m ⁻² or more. 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 preferably 6.0×10 ¹⁴ m ⁻² or more. When the steel constituting the inner ring 10 is low carbon steel or carburized steel, the dislocation density of retained austenite in the steel in the core 12 is 1.0×10 ¹⁵ m ⁻² or more.

Let the nitrogen concentration in the steel on the surface of the inner ring 10 be X (unit: mass percent), let the dislocation density of martensite in the steel on the surface of the inner ring 10 be Y (unit: m ^-2 ), and let the steel on the surface of the inner ring 10 be Let the hardness of be Z (unit: Hv). In this case, Z can be calculated by 935743.48+379.96×X−330.96×Y ² −5.41×10 ⁴ ×logY+783.83×logX ² (Formula 1). Equation 1 is obtained by performing multiple regression analysis after determining the value of Z when changing X and Y through experiments. From formula 1, in order to make the hardness of the steel on the surface of the inner ring 10 820Hv or more, 934923.48+379.96×X-330.96×Y ² −5.41×10 ⁴ ×logY+783.83×logX ² It is sufficient if the relationship ≧0 (Formula 2) is satisfied.

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 type X-ray diffraction device. 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 type X-ray diffraction device. In the Co tube type X-ray diffraction device, the wavelength of Co-Kα rays is set to 1.7889×10 ⁻¹⁰ m, the tube voltage is set to 40 kV, the tube current is set to 50 mA, and the collimator size is set to a diameter of 1 mm. The X-ray profiles of the austenite and martensite in the core portion 12 (surface of the inner ring 10) are measured within a range of 2θ of 30° or more and 135° or less. Secondly, after Rietveld analysis, 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 density of martensite and the dislocation density of austenite. In this equation, ρ is the dislocation density (unit: m ⁻² ), ε is the above strain, and b is the length of the Burgers vector (b=0.25×10 ⁻⁹ m).

In the X-ray profile of martensite obtained by X-ray diffraction of the core 12 (the 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 12 (the 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 in order to reduce the influence of the {200} plane of martensite and the {200} plane of austenite, which have different elastic moduli.

The dimensional change rate of the inner ring 10 after being held at 160° C. for 2500 hours is 40×10 ⁻⁵ or less. Preferably, the dimensional change rate of the inner ring 10 after being held at 160° C. for 2500 hours is 15×10 ⁻⁵ or less. 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, divided by the dimension of the inner ring 10 before holding.

<Modified example>
FIG. 2A is a cross-sectional view of the outer ring 30. FIG. 2B is a cross-sectional view of the rolling element 40. As shown in FIG. 2A, the mechanical component according to the embodiment may be an outer ring 30 of a rolling bearing. As shown in FIG. 2B, the mechanical component according to the embodiment may be a rolling element 40. Note that the configurations of the outer ring 30 and the rolling elements 40 are similar to the configuration of the inner ring 10 except for the shape. FIG. 2C is a cross-sectional view of the rolling bearing according to the embodiment. The rolling bearing (rolling bearing 100) according to the embodiment includes an inner ring 10, an outer ring 30, rolling elements 40, and a cage 70. In the rolling bearing 100, at least one of the inner ring 10, the outer ring 30, and the rolling elements 40 may be a mechanical component according to the embodiment.

(Method for manufacturing mechanical parts according to embodiment)
A method of manufacturing the inner ring 10 will be described below.

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

In the preparation step S1, the 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 peripheral surface 20c, and an outer peripheral surface 20d. The first end face 20a, the second end face 20b, the inner peripheral surface 20c, and the outer peripheral surface 20d are surfaces that become the first end face 10a, the second end face 10b, the inner peripheral surface 10c, and the outer peripheral 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.

In the nitriding step S2, the workpiece 20 is nitriding treated. 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 inside, and the nitrogen is dissolved in the workpiece 20. The nitriding step S2 is performed so that the nitrogen reaches a position inside the workpiece 20 that will become the surface of the inner ring 10 after the post-treatment step S6 is performed.

The workpiece 20 may be subjected to a carbo-nitriding process instead of the nitriding process S2. The carbo-nitriding process of the workpiece 20 is performed by heating and holding the workpiece in an atmosphere 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 atmosphere 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 inside, 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 inside the workpiece 20 that will become the surface of the inner ring 10 after the post-treatment process S6 is performed.

In the hardening step S3, the workpiece member 20 is hardened. The workpiece member 20 is hardened by heating and holding the workpiece member 20 to a temperature equal to or higher than the _A1 transformation point of the steel that constitutes the workpiece member 20, and then forming the workpiece member 20. This is carried out by cooling the steel to a temperature below the Ms transformation point. By performing the quenching step S3, martensite and retained austenite are generated in the steel that constitutes the workpiece member 20. In addition, after the quenching process S3 is performed, the quenching process S3 may be repeated by heating the workpiece 20 again to the _A1 transformation point or higher. By performing the quenching process S3 multiple times, the crystal grains become finer and the effect of the cooling process S4 is improved.

In the cooling step S4, sub-zero processing is performed on the workpiece 20. In the cooling step S4, cryo processing (ultra-subzero processing) may be performed on the workpiece member 20. In the sub-zero process, the workpiece 20 is cooled to a temperature above -100° C. and below room temperature. In the cryo process, the workpiece 20 is cooled to a temperature of −100° C. or lower. By performing the cooling step S4, a portion of the residual austenite in the steel constituting the workpiece member 20 is transformed into martensite. Note that before the cooling step S4 is performed, a low-temperature tempering step or a cleaning step may be performed to prevent cracking.

In the tempering step S5, the workpiece 20 is tempered. The workpiece 20 is tempered by heating the workpiece 20 to a temperature lower than the _A1 transformation point of the steel constituting the workpiece 20. More specifically, the workpiece 20 is tempered by heating the workpiece 20 to a temperature of about 180°C. In the post-treatment step S6, the surface of the workpiece 20 is subjected to machining such as grinding and polishing. In this manner, the inner ring 10 having the structure shown in FIG. 1 is manufactured. When the workpiece 20 is heated at 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, when the cooling step S4 is performed, the dislocation density of martensite is less likely to be reduced by heating in the tempering step S5, so that although the hardness decreases with an increase in the heating temperature, a higher hardness than usual is obtained.

<Modified example>
FIG. 5A is a cross-sectional view of the workpiece 50. FIG. 5B is a cross-sectional view of the workpiece 60. As shown in FIG. 5A, when the mechanical component according to the embodiment is the outer ring 30, a workpiece member 50, which is a ring-shaped member, is used as the workpiece member. As shown in FIG. 5B, when the mechanical component according to the embodiment is the rolling element 40, a workpiece member 60, which is a spherical member, is used as the workpiece member. The member to be processed 50 and the member to be processed 60 have the same configuration as the member to be processed 20 except for the shape.

(Effects of mechanical parts according to embodiment)
As a measure to suppress dimensional changes over time in steel bearing rings and rolling elements that have been quenched and tempered, it is possible to reduce the amount of retained austenite by performing tempering at a high temperature. However, when tempering is performed at a high temperature, although it becomes possible to suppress changes in dimensions over time due to a decrease in the amount of retained austenite, the hardness of the steel on the surfaces of the raceway rings and rolling elements decreases.

When foreign matter gets caught between the surface of the raceway and the rolling elements, impressions are formed on the surface of the raceway. FIG. 6 is a schematic graph showing the shape on the surface of the bearing ring on which indentations are formed. As shown in FIG. 6, the surface of the raceway is raised around the indentation. FIG. 7 is a graph showing the relationship between the hardness of steel on the surface of the bearing ring and the bulge around the indentation. In FIG. 7, the horizontal axis represents hardness (unit: Hv), and the horizontal axis represents the amount of swelling around the indentation (unit: μm). As shown in FIG. 7, as the hardness of the steel on the surface of the bearing ring decreases, the amount of swelling around the indentation increases.

If the amount of swelling around the indentation becomes large, stress will be concentrated in the swelling around the indentation, making fatigue failure more likely to occur starting from the indentation. Therefore, if tempering is performed at high temperatures to suppress dimensional changes over time, there is a risk that the rolling fatigue life of the raceway will be insufficient.

When the amount of retained austenite in steel is low and the retained austenite is surrounded by martensite with a high dislocation density (that is, with poor deformability), the retained austenite is constrained or stressed by the martensite and the lattice plane As a result of the spacing (lattice constant) becoming smaller, the dislocation density of retained austenite in the steel increases. This kind of retained austenite is restrained by the surrounding martensite, which has a high dislocation density, even when the volume expands due to decomposition, so even if the retained austenite is decomposed with use, the dimensional change due to the decomposition is small. Become.

The inner ring 10 is subjected to sub-zero treatment or cryo-treatment to reduce the amount of retained austenite in the steel in the core 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 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 12 is 5 volume percent or less.

Furthermore, because the inner ring 10 has been subjected to sub-zero treatment or cryo-treatment, the dislocation density of the martensite in the steel in the core portion 12 is high, and as a result, the dislocation density of the retained austenite in the steel in the core portion 12 is also high. More specifically, in 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 or more, and 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 ¹⁵ m ^-2 or more.

From the above, the retained austenite in the steel in the core part 12 is surrounded by martensite with a high dislocation density, so the retained austenite in the steel in the core part 12 is Even if decomposition occurs, the volumetric expansion accompanying the decomposition is restrained by the surrounding martensite with a high dislocation density, so dimensional changes are unlikely to occur. In this way, in the inner ring 10, changes in dimensions over time due to use are suppressed.

Further, since the inner ring 10 is not tempered at a high temperature, the progress of decomposition of martensite on the surface of the inner ring 10 is slight. Further, 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 becomes 820 Hv or more, the amount of swelling around the indentation rapidly decreases. In this way, according to the inner ring 10, the rolling fatigue life of the indentation starting point is also improved. By increasing the hardness of the steel on the surface of the inner ring 10, indentations are less likely to be formed on the surface of the inner ring 10, so that the static load capacity of the rolling bearing using the inner ring 10 is improved.

The surfaces of the rolling elements may be subjected to high residual compressive stress during the pressurization 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 mechanical components in the embodiment, the indentation-initiated rolling fatigue life of the rolling bearing is improved.

(Hardness evaluation test)
Samples 1 to 19 were prepared to evaluate the relationship between the hardness, dislocation density of martensite, and nitrogen concentration on the surface of a steel mechanical part that had been quenched and tempered. 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, the nitrogen concentration in the steel on the surface of the sample, and the dislocation density of martensite in the steel on the surface of the sample were changed in Samples 1 to 19. "OK" and "NG" in the "Satisfaction of Formula 2" column in Table 1 mean that the above formula 2 is satisfied and that the above formula 2 is not satisfied, respectively.

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

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

In Samples 1 to 7 and Samples 9 to 14, the hardness of the steel on 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 on the sample surface was less than 820 Hv. From this comparison, it became clear that by satisfying the above formula 2, the hardness of the steel on the surface of the machine component was 820 Hv or more, and the rolling fatigue life was improved. From another perspective, it became clear that by increasing both the nitrogen concentration in the steel on the surface of the machine component and the dislocation density of martensite in the steel on the surface of the machine component, the hardness of the steel on the surface of the machine component, and therefore the rolling fatigue life of the machine component, was improved.

(Evaluation test of dimensional change over time)
In order 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.

Note that 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 change the cooling temperature and holding time in sub-zero treatment or cryo treatment. Adjusted by.

When the steel is high carbon steel or bearing steel, condition A is satisfied if the dislocation density of martensite in the steel in the core 12 is 4.0×10 ¹⁴ m ⁻² or more. Furthermore, if the steel is low carbon steel or carburized steel, condition A is satisfied if the dislocation density of martensite in the steel in the core 12 is 1.0×10 ¹⁵ m ⁻² or more. do.

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

In samples 3 to 7 and samples 9 to 11, conditions A and B were satisfied. On the other hand, in samples 12 to 14, conditions A and B were not satisfied.

Samples 3 to 7 and Samples 9 to 11 had dimensional change rates of 40×10 ⁻⁵ or less after being held at 160° C. for 2500 hours. On the other hand, in samples 12 to 14, the dimensional change rate after being held at 160° C. for 2500 hours exceeded 50×10 ⁻⁵ . From this comparison, it has become clear that when conditions A and B are satisfied, changes over time in the dimensions of mechanical parts are suppressed.

In the case where the steel is a high carbon steel or a bearing steel, condition C is satisfied if the dislocation density of martensite in the steel in the core 12 of the sample is 6.0×10 ¹⁴ m ^-2 or more. Samples 3 to 5 satisfy condition C, and the dimensional change rate after holding at 160° C. for 2500 hours was 15×10 ^-5 or less. On the other hand, samples 9 to 11 do not satisfy condition C, and the dimensional change rate after holding at 160° C. for 2500 hours exceeded 15×10 ^-5 . From this comparison, it became clear that the dimensional change over time of the mechanical component is further suppressed by satisfying condition C.

Although the embodiment of the present invention has been described above, the above embodiment can be modified in various ways. Furthermore, the scope of the present invention is not limited to the above embodiment. The scope of the present invention is indicated by the claims, and is intended to include all modifications that are equivalent in meaning and scope to the claims.

A: center 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: retainer; 100: rolling bearing.

Claims (9)

A mechanical part made of hardened and tempered steel and having a surface,
a nitrided layer located on the surface and in which nitrogen is dissolved as a solid solution, and a core located at a position 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 on the surface is 820Hv or more,
The amount of retained austenite in the steel in the core is 0.1 volume percent or more and 9 volume percent or less,
The dislocation density of retained austenite in the steel in the core portion is 4.0×10 ¹⁴ m ⁻² or more,
A mechanical component, wherein the steel is a high carbon steel or a bearing steel. The mechanical component according to claim 1, wherein the dislocation density of martensite in the steel in the core portion is 6.0×10 ¹⁴ m ⁻² or more. A machine part having a surface made of hardened and tempered steel,
A nitriding layer is provided on the surface and contains nitrogen as a solid solution, and a core is provided at a position farther from the surface than the nitriding 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 in the core portion is 0.1 volume percent or more and 5 volume percent or less,
The dislocation density of the retained austenite in the steel in the core portion is 4.0×10 ¹⁵ m ⁻² or more;
The machine part, wherein the steel is a low carbon steel or a carburized steel. When the nitrogen concentration in the steel at the surface is X (unit: mass percent) and the dislocation density of martensite in the steel at the surface is Y (unit: m-2), 934923.48 + 379.96 The mechanical component according to claim 1, wherein the following relationship is satisfied: ×X−330.96×Y ² −5.41×104×log Y+783.83×log ^{X 2} ≧0. The mechanical part according to claim 1, wherein 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 contains 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 silicon, and 0.25 mass percent. 3. A machine component according to claim 2, containing less than or equal to % molybdenum. The mechanical component according to claim 1, having a dimensional change rate of 40×10 ⁻⁵ or less after being held at 160° C. for 2500 hours. The mechanical part according to claim 1, having a dimensional change rate of 15×10 ⁻⁵ or less after being held at 160° C. for 2500 hours. Inner circle and
outer ring and
Equipped with a rolling element,
A rolling bearing, wherein at least one of the inner ring, the outer ring, and the rolling element is the mechanical component according to any one of claims 1 to 8.

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