Machine component, rolling bearing, and ball screw

Abstract

The machine component is made of quenched and tempered steel, and the surface of the machine component is provided with a carburizing and nitriding layer. The steel contains 0.13 mass% or more and 0.58 mass% or less of C, 0.15 mass% or more and 0.35 mass% or less of Si, 0.6 mass% or more and 1.1 mass% or less of Mn, less than 0.025 mass ppm of P, less than 0.025 mass ppm of S, 1.2 mass% or less of Cr, 0.3 mass% or less of Mo, 0.25 mass% or less of V, and 0.25 mass% or less of Ni, with the remainder being Fe and unavoidable impurities. The average carbon concentration in the steel on the surface is 0.6 mass% or more. The average nitrogen concentration in the steel on the surface is 0.10 mass% or more.

Description

Technical Field

[0001] The invention relates to a mechanical component, a rolling bearing and a ball screw. Background Art

[0002] In recent years, electrification has been developing, centered on battery electric vehicles (BEV), plug-in hybrid electric vehicles (PHEV) and hybrid electric vehicles (HEV). For example, hydraulic brakes are replaced by electric brakes, and the electric axle (E-Axle) composed of a drive motor, a speed reducer and an inverter replaces the engine and the transmission. In addition, electric CVTs composed of electric VTCs (variable valve trains), electric compressors, drive motors and CVTs (Continuously Variable Transmission) can also be used.

[0003] The motorized components described above use rolling bearings or ball screws. In rolling bearings, the rolling elements are in contact with the raceway surfaces of the inner ring and the outer ring. In ball screws, the rolling elements are in contact with the raceway surfaces of the shaft and the outer ring (nut).

[0004] The static load capacity is the maximum contact surface pressure between the rolling element and the raceway surface when the depth of the indentation formed on the raceway surface is divided by the diameter of the rolling element and the value is 1 / 10000. The rated static load is the result of converting the static load capacity into load.

[0005] For electric vehicles, lightweighting is important in order to improve power consumption. Therefore, miniaturization is required for rolling bearings and ball screws. If a load exceeding the rated static load is applied to a rolling bearing or a ball screw, an indentation will be formed on the track surface, which sometimes causes early damage, abnormal noise or increased vibration. Since the rated static load increases as the size of the rolling bearing or ball screw increases, increasing the size of the rolling bearing or ball screw can suppress the formation of indentations on the track surface when a large load is applied to the rolling bearing or ball screw. However, if this is done, the electrified components will become large, and it will not be possible to achieve lightweight electric vehicles.

[0006] On the other hand, increasing the static load capacity can also increase the static rated capacity. Therefore, if the static load capacity can be increased, the formation of indentations on the raceway surface can be suppressed without increasing the size of the rolling bearing or ball screw, thereby miniaturizing the size of the rolling bearing or ball screw.

[0007] For example, Japanese Patent Application Laid-Open No. 2013-119930 (Patent Document 1) describes that the volume ratio of retained austenite in the steel of the raceway surface can be reduced and the static load capacity of the raceway surface can be improved by tempering at a high temperature (temperature of 240° C. to 300° C.). Prior art literature Patent Literature

[0008] Patent Document 1: Japanese Patent Application Publication No. 2013-119930 Summary of the invention Technical problem to be solved by the invention

[0009] However, when tempering is performed at a high temperature as described in Patent Document 1, the hardness of the steel on the track surface decreases. Furthermore, if the hardness of the steel on the track surface decreases, there is a risk of a decrease in rolling fatigue life or a decrease in wear resistance of the track surface due to surface-starting type, internal-starting type, indentation-starting type, or hydrogen embrittlement-type peeling.

[0010] The present invention has been made in view of the above-mentioned problems of the prior art. More specifically, the present invention provides a mechanical component, a rolling bearing, and a ball screw capable of improving the static load capacity of the surface while ensuring the hardness of the surface. Solutions to technical problems

[0011] The mechanical part of the present invention is a mechanical part made of steel that has been quenched and tempered. The mechanical part has a carburizing and nitriding layer on the surface. The steel contains carbon of more than 0.13 mass% and less than 0.58 mass%, silicon of more than 0.15 mass% and less than 0.35 mass%, manganese of more than 0.6 mass% and less than 1.1 mass%, phosphorus of less than 0.025 mass ppm, sulfur of less than 0.025 mass ppm, chromium of less than 1.2 mass%, molybdenum of less than 0.3 mass%, vanadium of less than 0.25 mass%, and nickel of less than 0.25 mass%, and the remainder is composed of Fe and unavoidable impurities. The average carbon concentration in the steel on the surface is more than 0.6 mass%. The average nitrogen concentration in the steel on the surface is more than 0.10 mass%. The hardness of the steel on the surface is more than 800 Hv. The volume ratio of the retained austenite in the steel on the surface is less than 22%. The mechanical part contacts the rolling element at least partially on the surface. The hardness of the steel at a depth of 0.02 to 0.03 times the diameter of the rolling element from the surface is 750 Hv or more. The volume ratio of retained austenite in the steel at a depth of 0.02 to 0.03 times the diameter of the rolling element from the surface is 20% or less.

[0012] In the above-mentioned mechanical component, the depth of the indentation formed on the surface when the rolling element contacts the surface may be 0.5 μm or less. The static load capacity of the above-mentioned mechanical component may be 5.3 GPa or less.

[0013] The rolling bearing of the present invention comprises an inner ring, an outer ring and a rolling element. At least one of the inner ring, the outer ring and the rolling element is the above-mentioned mechanical component. The ball screw of the present invention comprises a screw shaft, a ball nut and a ball. At least one of the screw shaft, the ball nut and the ball is the above-mentioned mechanical component. Effects of the Invention

[0014] According to the mechanical component, the rolling bearing, and the ball screw of the present invention, it is possible to improve the static load capacity of the surface while ensuring the hardness of the surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a cross-sectional view of the rolling bearing 100 . Figure 2 It is a manufacturing process diagram of the rolling bearing 100 . Figure 3 : is a graph showing the relationship between the indentation depth when the maximum contact surface pressure is 4.5 GPa based on the estimation formula 1 and the actually measured value of the indentation depth when the maximum contact surface pressure is 4.5 GPa. Figure 4 This is a diagram showing the relationship between the static load capacity based on the estimation formula 2 and the static load capacity actual measurement value. Figure 5 It is a cross-sectional view of the ball screw 110 . DETAILED DESCRIPTION

[0016] The details of the embodiment are described below with reference to the accompanying drawings. In the following drawings, the same or corresponding parts are marked with the same drawing face, and repeated descriptions are omitted. The rolling bearing of the embodiment is a rolling bearing 100.

[0017] (Structure of rolling bearing 100) Figure 1 is a cross-sectional view of the rolling bearing 100. Figure 1 As shown, the rolling bearing 100 is, for example, a deep groove ball bearing. The rolling bearing 100 has an inner ring 10, an outer ring 20, a plurality of rolling elements 30, and a retainer 40. The center axis of the inner ring 10 is the center axis A. The direction of the center axis A is the axial direction. When viewed in the axial direction, the direction along the circumference centered on the center axis A is the circumferential direction. The direction passing through the center axis A and orthogonal to the center axis A is the radial direction.

[0018] The inner ring 10 is annular and has a width surface 10a, a width surface 10b, an inner diameter surface 10c, and an outer diameter surface 10d. The width surface 10a, the width surface 10b, the inner diameter surface 10c, and the outer diameter surface 10d are sometimes collectively referred to as the surface of the inner ring 10.

[0019] The width surface 10a and the width surface 10b are the end surfaces of the inner ring 10 in the axial direction. The width surface 10a faces one side in the axial direction ( Figure 1 The width surface 10b is the opposite side of the width surface 10a in the axial direction. The width surface 10b faces the other side in the axial direction ( Figure 1 on the left side of the image).

[0020] The inner diameter surface 10c extends in the circumferential direction. One end and the other end of the inner diameter surface 10c in the axial direction are connected to the width surface 10a and the width surface 10b respectively. The inner diameter surface 10c faces radially inward. That is, the inner diameter surface 10c faces the central axis A side.

[0021] The outer diameter surface 10d extends in the circumferential direction. One end and the other end of the outer diameter surface 10d in the axial direction are connected to the width surface 10a and the width surface 10b respectively. The outer diameter surface 10d is the opposite surface of the inner diameter surface 10c in the radial direction. That is, the outer diameter surface 10d faces radially outward.

[0022] The inner ring 10 is fitted with a shaft (not shown) at the inner diameter surface 10c. The outer diameter surface 10d has a raceway surface 10da. The outer diameter surface 10d contacts the rolling element 30 at the raceway surface 10da. The raceway surface 10da is located at the center of the outer diameter surface 10d in the axial direction. In a cross-sectional view orthogonal to the circumferential direction, the raceway surface 10da is, for example, in the shape of a partial arc.

[0023] The outer ring 20 is annular and has a width surface 20a, a width surface 20b, an inner diameter surface 20c, and an outer diameter surface 20d. The width surface 20a, the width surface 20b, the inner diameter surface 20c, and the outer diameter surface 20d are sometimes collectively referred to as the surface of the outer ring 20.

[0024] The width surface 20a and the width surface 20b are end surfaces of the outer ring 20 in the axial direction. The width surface 20a faces one side in the axial direction. The width surface 20b is the opposite side of the width surface 20a in the axial direction. The width surface 20b faces the other side in the axial direction.

[0025] The inner diameter surface 20c extends in the circumferential direction. One end and the other end of the inner diameter surface 20c in the axial direction are connected to the width surface 20a and the width surface 20b respectively. The inner diameter surface 20c faces radially inward. That is, the inner diameter surface 20c faces the central axis A side.

[0026] The outer diameter surface 20d extends in the circumferential direction. One end and the other end of the outer diameter surface 20d in the axial direction are connected to the width surface 20a and the width surface 20b respectively. The outer diameter surface 20d is the opposite surface of the inner diameter surface 20c in the radial direction. That is, the outer diameter surface 20d faces the outer side in the radial direction.

[0027] The outer ring 20 is fitted with a housing (not shown) at the outer diameter surface 20d. The inner diameter surface 20c has a raceway surface 20ca. The inner diameter surface 20c contacts the rolling element 30 at the raceway surface 20ca. The raceway surface 20ca is located at the center of the inner diameter surface 20c in the axial direction. In a cross-sectional view orthogonal to the circumferential direction, the raceway surface 20ca is, for example, partially arc-shaped. The outer ring 20 is arranged on the outer side of the inner ring 10 in the radial direction in such a manner that the inner diameter surface 20c is spaced apart from the outer diameter surface 10d in the radial direction (such that the raceway surface 20ca is spaced apart from the raceway surface 10da in the radial direction).

[0028] The rolling element 30 is, for example, spherical. The rolling element 30 is disposed between the track surface 10da and the track surface 20ca. A plurality of rolling elements 30 are arranged circumferentially between the track surface 10da and the track surface 20ca. The surface of the rolling element 30 contacts the track surface 10da and the track surface 20ca. The retainer 40 retains the plurality of rolling elements 30 in such a manner that the interval between two adjacent rolling elements 30 in the circumferential direction is within a certain range.

[0029] The inner ring 10, the outer ring 20 and the rolling element 30 are made of quenched and tempered steel. The steel constituting the inner ring 10, the outer ring 20 and the rolling element 30 has the composition shown in Table 1. The composition of the steel constituting the inner ring 10, the composition of the steel constituting the outer ring 20 and the steel constituting the rolling element 30 may be different from each other.

[0030] Table 1

[0031] As shown in Table 1, the steel constituting the inner ring 10, the outer ring 20, and the rolling element 30 contains 0.13 mass % or more and 0.58 mass % or less of carbon, 0.15 mass % or more and 0.35 mass % or less of silicon, 0.6 mass % or more and 1.1 mass % or less of manganese, less than 0.025 mass ppm of phosphorus, less than 0.025 mass ppm of sulfur, less than 1.2 mass % of chromium, less than 0.3 mass % of molybdenum, less than 0.25 mass % of vanadium, and less than 0.25 mass % of nickel. The remainder of the steel constituting the inner ring 10, the outer ring 20, and the rolling element 30 is composed of iron and unavoidable impurities.

[0032] In addition, the steels shown in Table 1 may not contain at least any one of chromium, molybdenum, vanadium and nickel. The lower limits of the contents of chromium, molybdenum, vanadium and nickel are preferably 0.35 mass%, 0.15 mass%, 0.10 mass% and 0.10 mass%, respectively. In addition, the steels shown in Table 1 may not contain at least any one of phosphorus and sulfur.

[0033] Specific examples of the steel shown in Table 1 include S15C, S25C, S45C, S50C, S53C, S55C, SCM415, SCM418, SCM420, SCM435, SCM440, SCM445, SCr415, SCr420, SCr435 and SCM420 specified in the JIS standard, and W1-8, 1015, 1025, 1040, 1060, 1080, 1120, 1130, 1140, 1150, 1160, 1170, 1180, 1190, 1200, 1210, 1220, 1230, 1240, 1250, 1260, 1270, 1280, 1290, 1300, 1310, 1320, 1330, 1340, 1350, 1360, 1370, 1380, 1390, 1410, 1420, 1430, 1440, 1450, 1460, 1470, 1480, 1490, 1500 5. 1050, 4130, 4135, 4140, 4145, 4161, 4320, 5120, 5130, 5135 and 5140, C15, C25, C50, 20Cr4, 37Cr4, 18CrMo4, 34CrMo4, 42CrMo4, 22CrMoS35 and 60CrMo32 specified in ISO standard and G20CrMo, 50Mn and 45 specified in GB standard.

[0034] A carburizing and nitriding layer 50 is formed on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30. In the carburizing and nitriding layer 50, the carbon concentration and the nitrogen concentration are higher than those in the portion where the carburizing and nitriding layer 50 is not formed. In the carburizing and nitriding layer 50, it is preferred that carbon and nitrogen are dissolved in iron without forming a compound. The average carbon concentration in the steel on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30 is 0.6 mass% or more. The average nitrogen concentration in the steel on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30 is 0.10 mass% or more.

[0035] The average carbon concentration and average nitrogen concentration in the steel on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30 are measured by line analysis using EPMA (Probe Micro Analyzer). At this time, a calibration curve is drawn using standard samples with clear carbon and nitrogen concentrations.

[0036] The hardness of the steel on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30 is 800 Hv or more. The hardness of the steel on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30 is measured using a Vickers hardness tester specified in the JIS standard with a load of 300 g. The hardness of the steel on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30 is measured at least at three points, and the average value of these measured values ​​is used.

[0037] The position at a depth of 0.02 times or more and 0.03 times or less of the diameter of the rolling element 30 from the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30 is taken as the maximum internal stress position. The maximum internal stress position corresponds to the position where the internal stress reaches the maximum when the maximum contact surface pressure between the track surface 10da (track surface 20ca) and the surface of the rolling element 30 is 4.5GPa. The hardness of the steel at the maximum internal stress position is 750Hv or more. The hardness of the steel at the maximum internal stress position is measured by the same method as the hardness of the steel at the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30.

[0038] The volume ratio of retained austenite in the steel on the surface of the inner ring 10, the surface of the outer ring 20 and the surface of the rolling element 30 is less than 22%. The volume ratio of retained austenite in the steel at the position of maximum internal stress is less than 20%. The volume ratio of retained austenite in the steel is measured using a Cr tube sphere type X-ray diffraction device.

[0039] During the measurement, the inner ring 10, the outer ring 20 and the rolling element 30 were electrolytically polished in such a way that the structure of the steel was not subjected to a processing-induced phase change. During the measurement, the Cr tube ball type X-ray diffraction device was used at a wavelength of 2.29093×10 -10 m, tube voltage of 30 kV, tube current of 10 mA, and collimator size of 2 mm × 2 mm. The X-ray morphology of martensite was measured in the range of 2θ of 142.75° to 170.8°. The X-ray morphology of austenite was measured in the range of 2θ of 114.75° to 142.8°. These X-ray morphologies were background processed. The volume ratio of retained austenite was determined based on these X-ray morphologies.

[0040] When the track surface 10da (track surface 20ca) contacts the surface of the rolling element 30 with a maximum contact surface pressure of 4.5 GPa, the depth of the indentation formed on the track surface 10da (track surface 20ca) is preferably less than 0.5 μm. When the track surface 10da (track surface 20ca) contacts the surface of the rolling element 30 with a maximum contact surface pressure of 4.5 GPa, the depth of the indentation formed on the track surface 10da (track surface 20ca) is measured by the following method.

[0041] First, using an automatic recorder (オートグラフ), the rolling element 30 is pressed onto the track surface 10da (track surface 20ca). The load at this time is adjusted to a maximum contact surface pressure of 4.5 GPa between the track surface 10da (track surface 20ca) and the surface of the rolling element 30. The rolling element 30 used at this time is a ceramic ball so that the rolling element 30 does not deform. In addition, the load application speed at this time is 3N / second, and it is maintained for 120 seconds after reaching the desired applied load. Second, using the white interference function of the laser microscope, the depth of the indentation formed on the track surface 10da (track surface 20ca) is measured.

[0042] The static load capacity of the track surface 10da and the track surface 20ca is preferably above 5.3 GPa. The static load capacity of the track surface 10da (track surface 20ca) is the maximum contact surface pressure between the track surface 10da (track surface 20ca) and the surface of the rolling element 30 when the value obtained by dividing the depth of the indentation formed on the track surface 10da (track surface 20ca) by the diameter of the rolling element 30 is 1 / 10000. The static load capacity of the track surface 10da (track surface 20ca) is measured by the following method.

[0043] First, the maximum contact surface pressure between the track surface 10da (track surface 20ca) and the surface of the rolling element 30 is sequentially varied using an automatic recorder, and the depth of the indentation formed on the track surface 10da (track surface 20ca) is sequentially measured. Second, the maximum contact surface pressure between the track surface 10da (track surface 20ca) and the surface of the rolling element 30 is determined when the depth of the indentation thus measured is 1 / 10000 of the diameter of the rolling element 30. This maximum contact surface pressure is the static load capacity of the track surface 10da (track surface 20ca).

[0044] (Method of Manufacturing Rolling Bearing 100) Next, a method for manufacturing the rolling bearing 100 will be described.

[0045] Figure 2 1 is a manufacturing process diagram of the rolling bearing 100. Figure 2 As shown, the method for manufacturing the rolling bearing 100 includes a preparation step S1 , a carburizing and nitriding treatment step S2 , a quenching step S3 , a cooling step S4 , a tempering step S5 , a post-treatment step S6 , and an assembling step S7 .

[0046] In the preparation step S1, the processing target components are prepared. The processing target components are formed of steel having the composition shown in Table 1. The processing target components for the inner ring 10 and the outer ring 20 are ring-shaped, and the processing target component for the rolling element 30 is spherical.

[0047] The carburizing and nitriding treatment step S2 is performed after the preparation step S1. The carburizing and nitriding treatment step S2 is performed by heating and maintaining the processing object component in an atmosphere gas containing a carbon source and a nitrogen source. The heating temperature in the carburizing and nitriding treatment step S2 is a temperature above the A1 transformation point of the steel constituting the processing object component. The heating temperature is, for example, above 800°C. By performing the heating and maintaining in the carburizing and nitriding treatment step S2, carbon and nitrogen invade from the surface of the processing object component and are dissolved in the steel near the surface of the processing object component. In addition, after the carburizing and nitriding treatment step S2 is performed to the post-processing step S6, carbon and nitrogen diffuse to the positions that become the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling element 30.

[0048] The quenching step S3 is performed after the carburizing and nitriding step S2. The quenching step S3 is performed by cooling the workpiece from a temperature above the A1 transformation point to a temperature of M S The cooling step S4 is performed at a temperature below the phase transformation point. The cooling step S4 is performed after the quenching step S3. The cooling step S4 is a sub-zero treatment or a super-sub-zero treatment. In the sub-zero treatment, the processing object is cooled to a temperature below the room temperature greater than -100°C. In the super-sub-zero treatment, the processing object is cooled to a temperature below -100°C.

[0049] By performing the sub-zero treatment and the super-sub-zero treatment, martensite formation in the steel is advanced, and the volume ratio of the retained austenite in the steel is reduced. In addition, the carburizing and nitriding treatment step S2 is generally a treatment for increasing the volume ratio of the retained austenite in the steel, so when the carburizing and nitriding treatment step S2 is performed, the sub-zero treatment or the super-sub-zero treatment is generally not performed.

[0050] The tempering step S5 is performed after the cooling step S4. The tempering step S5 is performed by heating and holding the workpiece at a temperature lower than the A1 transformation point. The heating temperature in the tempering step S5 is, for example, 180°C.

[0051] The post-processing step S6 is performed after the tempering step S5. In the post-processing step S6, the surface of the workpiece is machined (grinded, polished). Thus, the inner ring 10, the outer ring 20, and the rolling element 30 are formed. The assembly step S7 is performed after the post-processing step S6. In the assembly step S7, the inner ring 10, the outer ring 20, and the rolling element 30 are assembled with the retainer 40 to form a Figure 1 A rolling bearing 100 of the structure shown.

[0052] (Effect of rolling bearing 100) Next, the effects of the rolling bearing 100 will be described.

[0053] When the raceway ring or rolling element is formed by tempering at high temperature (e.g., above 240°C), the static load capacity is improved by reducing the volume ratio of austenite in the surface steel, but the hardness of the surface steel decreases. On the other hand, the inner ring 10, the outer ring 20, and the rolling element 30 are subjected to sub-zero treatment or super-sub-zero treatment in the cooling step S4, so while the volume ratio of retained austenite in the surface steel decreases, the volume ratio of martensite in the surface steel increases, so the surface hardness increases.

[0054] In addition, the dislocation density of martensite and retained austenite in the steel on the surface increases with the sub-zero treatment or super-sub-zero treatment. Furthermore, by performing the carburizing and nitriding treatment step S2 when forming the inner ring 10, the outer ring 20 and the rolling element 30, the steel is solid-solution strengthened at the surface. In this way, the hardness (specifically, above 800 Hv) can be ensured while ensuring the static load capacity on the surface of the inner ring 10, the surface of the outer ring 20 and the surface of the rolling element 30.

[0055] (Example) In order to evaluate the relationship between the average carbon concentration in the steel surface, the average nitrogen concentration in the steel surface, the hardness of the steel surface, and the volume ratio of retained austenite in the steel surface and the sub-zero treatment or super-sub-zero treatment, samples 1 to 7 were prepared. The details of samples 1 to 7 are shown in Table 2.

[0056] Table 2

[0057] Samples 1 to 7 are flat plate-shaped parts with a diameter of 85 mm and a thickness of 5 mm. The steel type, the average carbon concentration on the surface, the average nitrogen concentration on the surface, the volume ratio of retained austenite in the steel on the surface, and the hardness of the steel on the surface were changed for Samples 1 to 7. The steel types of Samples 1 to 7 are all included in the steels with the compositions shown in Table 1.

[0058] Samples 1 to 7 were subjected to carburizing and nitriding step S2, quenching step S3, and tempering step S5. Samples 1, 3, and 6 were subjected to super-freezing treatment as cooling step S4, and samples 2 and 4 were subjected to sub-freezing treatment as cooling step S4. On the other hand, samples 5 and 7 were not subjected to cooling step S4.

[0059] Condition A: The average carbon concentration on the surface is 0.6 mass % or more. Condition B: The average nitrogen concentration on the surface is 0.10 mass % or more. Condition C: The volume ratio of retained austenite in the steel on the surface is 22% or less. Condition D: The hardness of the steel on the surface is 800 Hv or more.

[0060] The hardness of the steel at the maximum internal stress position is 750 Hv or more as condition E. The volume ratio of retained austenite in the steel at the maximum internal stress position is 20% or less as condition F. Samples 1 to 4 satisfy all of conditions A to F. Sample 5 does not satisfy conditions C, D, and E. Sample 6 does not satisfy condition E. Sample 7 does not satisfy conditions C, D, E, and F.

[0061] The depth of the indentation and the static load capacity formed by pressing a ceramic ball with a diameter of 3 / 8 inch on the surface of Samples 1 to 7 to apply a maximum contact surface pressure of 4.5 GPa were measured. "OK" in Table 1 means that the static load capacity is above 5.3 GPa and the depth of the indentation is below 0.5 μm. "NG" in Table 1 means that the static load capacity is less than 5.3 GPa or the indentation depth exceeds 0.5 μm.

[0062] Samples 1 to 4 were evaluated well for the depth of the indentation formed when the maximum contact surface pressure of 4.5 GPa was applied and the static load capacity. However, samples 5 to 7 were evaluated poorly for the depth of the indentation formed when the maximum contact surface pressure of 4.5 GPa was applied and the static load capacity. From this comparison, it can be seen that by satisfying all conditions A to F, the static load capacity (ability to resist indentation formation) is improved.

[0063] Multiple regression analysis was performed on samples 1 to 7 to study the relationship between the surface indentation depth and the hardness of the steel at the maximum internal stress position and the volume ratio of retained austenite in the steel. The result was that the indentation depth (μm) when the maximum contact surface pressure was 4.5 GPa = 2.04 + 9.57 × 10 -3 × Hardness of steel at the location of maximum internal stress (Hv) - 2.33 × 10 -3 × Estimation formula for the volume ratio (%) of retained austenite in steel at the maximum internal stress position (Estimation formula 1).

[0064] Figure 3 : is a graph showing the relationship between the indentation depth when the maximum contact surface pressure is 4.5 GPa based on the estimation formula 1 and the measured value of the indentation depth when the maximum contact surface pressure is 4.5 GPa. Figure 3 As shown, the goodness of fit (R 2 ) is 0.81. It can be seen that by making the hardness of the steel at the maximum internal stress position greater than 750Hv and the volume ratio of the retained austenite at the maximum internal stress position less than 20%, the indentation depth at the maximum contact surface pressure of 4.5GPa can be made less than 0.5μm.

[0065] Multiple regression analysis was performed on samples 1 to 7 to study the relationship between the static load capacity and the hardness of the steel at the maximum internal stress position and the volume ratio of retained austenite in the steel. The result was that the static load capacity (GPa) = -2.15 + 9.88 × 10 -3 × Hardness of steel at the location of maximum internal stress (Hv) + 2.25 × 10 -3 × Estimation formula for the volume ratio (%) of retained austenite in steel at the maximum internal stress position (Estimation formula 2).

[0066] Figure 4 This is a graph showing the relationship between the static load capacity based on the estimated formula 2 and the actual value of the static load capacity. Figure 4 As shown, the goodness of fit (R 2 ) is 0.90. It can be seen from this that the static load capacity can be made 5.3 GPa or more by making the hardness of the steel at the maximum internal stress position 750 Hv or more and the volume ratio of the retained austenite at the maximum internal stress position 20% or less.

[0067] (Variation Example) The above description shows an example in which the carburized nitriding layer 50 is formed on all surfaces of the inner ring 10, the outer ring 20, and the rolling element 30, but the carburized nitriding layer 50 only needs to be formed on any one surface of the inner ring 10, the outer ring 20, and the rolling element 30. The above description shows an example in which the inner ring 10, the outer ring 20, and the rolling element 30 are all made of the steel composition shown in Table 1, but any one of the inner ring 10, the outer ring 20, and the rolling element 30 may not be made of the steel composition shown in Table 1. The above description shows an example in which all surfaces of the inner ring 10, the outer ring 20, and the rolling element 30 meet conditions A to F, but any surface of the inner ring 10, the outer ring 20, and the rolling element 30 only needs to meet conditions A to F.

[0068] The above description has been made by taking the rolling bearing 100 as an example, but the above structure can also be applied to a ball screw, for example. The ball screw of the embodiment is a ball screw 110 . Figure 5 1 is a cross-sectional view of the ball screw 110. Figure 5 As shown, the ball screw 110 has a screw shaft 61, a ball nut 62, a plurality of balls 63 and a sealing member 64. The circulation method of the balls 63 in the ball screw 110 is not particularly limited. The circulation method of the balls 63 in the ball screw 110 is, for example, a tube type, a return tube (tube) type, a deflector type, an end deflector type, an end cover type, a gyro type, etc.

[0069] The screw shaft 61 has an outer peripheral surface 61a. A screw groove 61b is formed on the outer peripheral surface 61a. A hole extending along the central axis direction of the screw shaft 61 is formed on the ball nut 62. The inner wall surface of the hole is the inner peripheral surface 62a of the ball nut 62. A screw groove 62b is formed on the inner peripheral surface 62a. The screw shaft 61 is inserted into the ball nut 62 in a manner that the outer peripheral surface 61a and the inner peripheral surface 62a face each other. The ball 63 is arranged between the screw groove 61b and the screw groove 62b. The hole of the ball nut 62 through which the screw shaft 61 passes is sealed by a sealing component 64. The screw shaft 61 also passes through the hole formed in the sealing component 64.

[0070] By rotating the screw shaft 61 around its central axis, the rotational power of the screw shaft 61 can be transmitted to the ball nut 62 via the ball 63, and the ball nut 62 moves along the direction of the central axis of the screw shaft 61. That is, the ball screw 110 is a device that converts the rotational motion of a motor or the like into a linear motion. The ball screw 110 is used, for example, in electric actuators, positioning devices, electric jacks, servo cylinders, electric servo presses, mechanical presses, electric brake devices, transmissions, electric power steering devices, electric injection molding machines, and the like.

[0071] The screw shaft 61, the ball nut 62, and the ball 63 are formed of quenched and tempered steel. The steel constituting the screw shaft 61, the ball nut 62, and the ball 63 has the composition shown in Table 1. However, any one of the steel constituting the screw shaft 61, the steel constituting the ball nut 62, and the steel constituting the ball 63 may not have the composition shown in Table 1. Although not shown in the figure, a carburized and nitrided layer 50 is formed on at least one surface of the screw shaft 61, the ball nut 62, and the ball 63. In addition, at least one surface of the screw shaft 61, the ball nut 62, and the ball 63 satisfies conditions A to F.

[0072] The screw shaft 61, the ball nut 62, and the ball 63 are formed by performing a preparation step S1, a carburizing and nitriding step S2, a quenching step S3, a cooling step S4, a tempering step S5, and a post-processing step S6. However, the shape of the processing object parts for forming the screw shaft 61, the ball nut 62, and the ball 63 is different from the shape of the processing object parts for forming the inner ring 10, the outer ring 20, and the rolling element 30.

[0073] The ball screw 110 can also improve the static load capacity of the surfaces of the screw shaft 61 , the ball nut 62 , and the balls 63 while ensuring the hardness of the surfaces of the screw shaft 61 , the ball nut 62 , and the balls 63 .

[0074] The above description shows an example in which the heating temperature in the carburizing and nitriding treatment step S2 is 800° C. or higher, but from the viewpoint of shortening the holding time, the heating temperature may be 1000° C. or higher. In this case, secondary quenching may be performed to prevent the hardness and static load capacity from decreasing due to grain coarsening.

[0075] The embodiments of the present invention are described above, but various modifications can be made to the above embodiments. In addition, the scope of the present invention is not limited to the above embodiments. The scope of the present invention is shown by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. Explanation of symbols

[0076] 100 rolling bearing, 10 inner ring, 10a width surface, 10b width surface, 10c inner diameter surface, 10d outer diameter surface, 10da track surface, 20 outer ring, 20a width surface, 20b width surface, 20c inner diameter surface, 20ca track surface, 20d outer diameter surface, 30 rolling element, 40 retainer, 50 carburizing and nitriding layer, 61 screw shaft, 61a outer peripheral surface, 61b screw groove, 62 ball nut, 62a inner peripheral surface, 62b screw groove, 63 ball, 64 sealing component, 110 ball screw, A center shaft, S1 preparation process, S2 carburizing and nitriding treatment process, S3 quenching process, S4 cooling process, S5 tempering process, S6 post-treatment process, S7 assembly process.

Claims

1. A mechanical component made of quenched and tempered steel, It has a carburizing and nitriding layer on the surface. The steel contains 0.13 mass % or more and 0.58 mass % or less of carbon, 0.15 mass % or more and 0.35 mass % or less of silicon, 0.6 mass % or more and 1.1 mass % or less of manganese, less than 0.025 mass ppm of phosphorus, less than 0.025 mass ppm of sulfur, 1.2 mass % or less of chromium, 0.3 mass % or less of molybdenum, 0.25 mass % or less of vanadium, and 0.25 mass % or less of nickel, with the remainder being Fe and unavoidable impurities, The average carbon concentration in the steel at the surface is 0.6 mass % or more, The average nitrogen concentration in the steel at the surface is 0.10 mass % or more, The hardness of the steel on the surface is above 800 Hv, The volume ratio of the retained austenite in the steel at the surface is less than 22%, The mechanical component is in contact with the rolling element at least in part of the surface, The hardness of the steel at a position at a depth from the surface that is 0.02 times or more and 0.03 times or less of the diameter of the rolling element is 750 Hv or more, A volume ratio of retained austenite in the steel at a position where the depth from the surface is 0.02 times or more and 0.03 times or less of the diameter of the rolling element is 20% or less.

2. The mechanical component according to claim 1, wherein: The depth of the indentation formed on the surface when the rolling element contacts the surface is less than 0.5 μm.

3. The mechanical component according to claim 1, wherein: The static load capacity of the mechanical component is below 5.3 GPa.

4. The mechanical component according to claim 3, wherein: The depth of the indentation formed on the surface when the rolling element contacts the surface is less than 0.5 μm.

5. A rolling bearing comprising an inner ring, an outer ring and rolling elements, At least any one of the inner ring, the outer ring, and the rolling element is the machine component according to any one of claims 1 to 4.

6. A ball screw comprising a screw shaft, a ball nut and balls, At least any one of the screw shaft, the ball nut, and the ball is the machine component according to any one of claims 1 to 4.

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