Machine parts, rolling bearings and ball screws

Abstract

To provide a machine component which can secure the hardness of a surface while improving a static load capacity of the surface.SOLUTION: A machine component is formed of steel which is applied with quenching and tempering, and comprises a carburized / nitrided layer on a surface. The steel includes C of 0.13 mass% or higher and 0.58 mass% or lower, Si of 0.15 mass% or higher and 0.35 mass% or lower, Mn of 0.6 mass% or higher and 1.1 mass% or lower, P of lower than 0.025 mass ppm, S of lower than 0.025 mass ppm, Cr of 1.2 mass% or lower, Mo of 0.3 mass% or lower, V of 0.25 mass% or lower, and Ni of 0.25 mass% or lower. A remaining part of the steel is composed of Fe and inevitable impurities. An average carbon concentration of the steel at the surface is 0.6 mass% or higher. An average nitrogen concentration of the steel at the surface is 0.10 mass% or higher.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a machine component, a rolling bearing, and a ball screw. [Background technology]

[0002] In recent years, electrification has progressed, centering on battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and hybrid electric vehicles (HEVs). For example, hydraulic brakes have been replaced with electric brakes, and engines and transmissions have been replaced with electric axles (E-Axles) that consist of drive motors, reduction gears, and inverters. In addition, electric VTCs (variable valve timing), electric compressors, and electric CVTs that combine drive motors and CVTs (Continuously Variable Transmissions) have also come into use.

[0003] Rolling bearings and ball screws are used in the above-mentioned electric motor components. In a rolling bearing, the rolling elements contact the raceway surfaces of the inner and outer rings. In a ball screw, the rolling elements contact the raceway surfaces of the shaft and outer ring (nut).

[0004] Static load capacity is the maximum contact pressure between the rolling elements and the raceway surface when the depth of the indentation formed on the raceway surface divided by the diameter of the rolling elements is 1 / 10000.Static load rating is the static load capacity converted into a load.

[0005] In electric vehicles, weight reduction is important to improve the electric cost. Therefore, there is a demand for miniaturization of rolling bearings and ball screws. When a load exceeding the static load rating is applied to a rolling bearing or ball screw, an indentation is formed on the raceway surface, and the indentation may cause early damage and an increase in abnormal noise and vibration. Since the static load rating increases with the size of the rolling bearing or ball screw, when a large load is applied to the rolling bearing or ball screw, the formation of an indentation on the raceway surface can be suppressed by increasing the size of the rolling bearing or ball screw. However, doing so increases the size of the electric components, making it impossible to achieve weight reduction of the electric vehicle.

[0006] On the other hand, the static rated capacity can also be increased by increasing the static load capacity. Therefore, if the static load capacity can be increased, it is possible to suppress the formation of indentations on the raceway surface without increasing the size of the rolling bearings or ball screws, and ultimately the size of the rolling bearings or ball screws can be reduced.

[0007] For example, JP 2013-119930 A (Patent Document 1) describes how tempering at high temperatures (temperatures of 240°C or higher and 300°C or lower) reduces the volume ratio of retained austenite in the steel on the raceway surface, thereby improving the static load capacity of the raceway surface. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2013-119930 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, when tempering is performed at high temperatures as described in Patent Document 1, the hardness of the steel on the raceway surface decreases. If the hardness of the steel on the raceway surface decreases, there is concern that the rolling fatigue life may decrease due to surface-originating, internal-originating, indentation-originating, or hydrogen embrittlement-originating spalling, and that the wear resistance of the raceway surface may decrease.

[0010] The present invention has been made in consideration of the above-mentioned problems of the conventional techniques, and more specifically, it is an object of the present invention to provide a mechanical component, a rolling bearing, and a ball screw that can ensure the hardness of the surface while improving the static load capacity of the surface. [Means for solving the problem]

[0011] The mechanical component according to the present invention is made of steel that has been quenched and tempered. The mechanical component has a carbo-nitrided layer on the surface. The steel contains 0.13 to 0.58 mass percent carbon, 0.15 to 0.35 mass percent silicon, 0.6 to 1.1 mass percent manganese, and 0.025 mass percent Cr. ppm Less than 0.025 mass percent phosphorus ppm The steel comprises less than 1.2 mass percent sulfur, 1.2 mass percent or less chromium, 0.3 mass percent or less molybdenum, 0.25 mass percent or less vanadium, 0.25 mass percent or less nickel, and the balance being iron and unavoidable impurities. The average carbon concentration in the steel at the surface is 0.6 mass percent or more. The average nitrogen concentration in the steel at the surface is 0.10 mass percent or more. The hardness of the steel at the surface is 800 Hv or more. The volume ratio of retained austenite in the steel at the surface is 22 percent or less. The machine component is in contact with a rolling element at least on a part of its surface. The hardness of the steel at a position whose depth from the surface is 0.02 to 0.03 times the diameter of the rolling element is 750 Hv or more. The volume ratio of retained austenite in the steel at a position whose depth from the surface is 0.02 to 0.03 times the diameter of the rolling element is 20 percent or less.

[0012] In the mechanical component, when the rolling element comes into contact with the surface, an indentation formed on the surface may have a depth of 0.5 μm or less.In the mechanical component, the static load capacity may be 5.3 GPa or less.

[0013] A rolling bearing according to the present invention includes an inner ring, an outer ring, and rolling elements. At least one of the inner ring, the outer ring, and the rolling elements are the above-mentioned mechanical components. A ball screw according to the present invention includes a screw shaft, a ball nut, and balls. At least one of the screw shaft, the ball nut, and the balls are the above-mentioned mechanical components. Effect of the Invention

[0014] The machine component, rolling bearing, and ball screw according to the present invention can improve the static load capacity of the surface while ensuring the hardness of the surface. [Brief description of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view of a rolling bearing 100. FIG. [Diagram 2] 2 is a manufacturing process diagram of the rolling bearing 100. FIG. [Diagram 3] 1 is a graph showing the relationship between the indentation depth based on Estimation Formula 1 when the maximum contact pressure is 4.5 GPa and the actually measured indentation depth when the maximum contact pressure is 4.5 GPa. [Figure 4] 13 is a graph showing the relationship between the static load capacity based on estimation formula 2 and the actual measured value of the static load capacity. [Diagram 5] 2 is a cross-sectional view of a ball screw 110. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The details of the embodiment will be described with reference to the drawings. In the following drawings, the same or corresponding parts are given the same reference characters, and overlapping descriptions will not be repeated. The rolling bearing according to the embodiment is referred to as a rolling bearing 100.

[0017] (Configuration of rolling bearing 100) Fig. 1 is a cross-sectional view of a rolling bearing 100. As shown in Fig. 1, 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 cage 40. The central axis of the inner ring 10 is defined as central axis A. The direction of central axis A is defined as the axial direction. The direction along the circumference centered on central axis A when viewed along the axial direction is defined as the circumferential direction. The direction passing through central axis A and perpendicular to central axis A is defined as the radial direction.

[0018] The inner ring 10 is ring-shaped. The inner ring 10 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 end surfaces of the inner ring 10 in the axial direction. The width surface 10a faces one side in the axial direction (the right side in FIG. 1). The width surface 10b is the opposite surface to the width surface 10a in the axial direction. The width surface 10b faces the other side in the axial direction (the left side in FIG. 1).

[0020] The inner diameter surface 10c extends in the circumferential direction. One end and the other end in the axial direction of the inner diameter surface 10c are connected to the width surface 10a and the width surface 10b, respectively. The inner diameter surface 10c faces inward in the radial direction. 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 to the inner diameter surface 10c in the radial direction. In other words, the outer diameter surface 10d faces outward in the radial direction.

[0022] The inner ring 10 is fitted to a shaft (not shown) at an inner diameter surface 10c. An outer diameter surface 10d has a raceway surface 10da. The outer diameter surface 10d contacts the rolling elements 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 perpendicular to the circumferential direction, the raceway surface 10da has, for example, a partial arc shape.

[0023] The outer ring 20 is ring-shaped. The outer ring 20 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 surface opposite 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 in the axial direction of the inner diameter surface 20c are connected to the width surface 20a and the width surface 20b, respectively. The inner diameter surface 20c faces inward in the radial direction. 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 to the inner diameter surface 20c in the radial direction. In other words, the outer diameter surface 20d faces outward in the radial direction.

[0027] The outer ring 20 is fitted into a housing (not shown) at an 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 perpendicular to the circumferential direction, the raceway surface 20ca is, for example, partially arc-shaped. The outer ring 20 is disposed radially outside the inner ring 10 so that the inner diameter surface 20c faces the outer diameter surface 10d with a gap therebetween in the radial direction (so that the raceway surface 20ca faces the raceway surface 10da with a gap therebetween in the radial direction).

[0028] The rolling elements 30 are, for example, spherical. The rolling elements 30 are disposed between the raceway surface 10da and the raceway surface 20ca. The rolling elements 30 are arranged in the circumferential direction between the raceway surface 10da and the raceway surface 20ca. The surfaces of the rolling elements 30 contact the raceway surface 10da and the raceway surface 20ca. The cage 40 holds the rolling elements 30 so that the circumferential interval between two adjacent rolling elements 30 is within a certain range.

[0029] The inner ring 10, the outer ring 20, and the rolling elements 30 are made of hardened and tempered steel. The steels constituting the inner ring 10, the outer ring 20, and the rolling elements 30 have the compositions 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 elements 30 do not have to be identical to 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 elements 30 has a carbon content of 0.13 mass percent or more and 0.58 mass percent or less, a silicon content of 0.15 mass percent or more and 0.35 mass percent or less, a manganese content of 0.6 mass percent or more and 1.1 mass percent or less, and a manganese content of 0.025 mass percent or less. ppm Less than 0.025 mass percent phosphorus ppmless than 1.2 mass percent sulfur, less than 1.2 mass percent chromium, less than 0.3 mass percent molybdenum, less than 0.25 mass percent vanadium, and less than 0.25 mass percent nickel. The remainder of the steel constituting the inner ring 10, the outer ring 20, and the rolling elements 30 consists of iron and unavoidable impurities.

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

[0033] Specific examples of steels shown in Table 1 include S15C, S25C, S45C, S50C, S53C, S55C, SCM415, SCM418, SCM420, SCM435, SCM440, SCM445, SCr415, SCr420, SCr435, and SNCM420 specified in the JIS standard, and W1-8, 1015, 1025, 1045, 1060, 1080, 1090, 1100, 1115, 1120, 1130, 1140, 1150, 1160, 1170, 1180, 1200, 1220, 1240, 1260, 1280, 1290, 1300, 1310, 1320, 1330, 1340, 1350, 1360, 1370, 1380, 1400, 1420, 1440, 1450, 1460, 1470, 1480, 1500, 1520, 1560, 1580, 1590, 1610, 1620, 1630, 1640, 1650, 1660, 1700, 1710, 1720, 1730, 1740, 1750, 1760, 1770, 1780, 1790, 1800, 1810, 1820, 1830, 1840, 1850, 1860, 1870 050, 4130, 4135, 4140, 4145, 4161, 4320, 5120, 5130, 5135, and 5140 specified in the ISO standard, C15, C25, C50, 20Cr4, 37Cr4, 18CrMo4, 34CrMo4, 42CrMo4, 22CrMoS35, and 60CrMo32 specified in the GB standard, and G20CrMo, 50Mn, and 45 specified in the GB standard.

[0034] A carbo-nitrided layer 50 is formed on the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30. In the carbo-nitrided layer 50, the carbon concentration and nitrogen concentration are higher than in portions where the carbo-nitrided layer 50 is not formed. In the carbo-nitrided layer 50, carbon and nitrogen are preferably dissolved in iron and do not form compounds. The average carbon concentration in the steel on the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30 is 0.6 mass percent or more. The average nitrogen concentration in the steel on the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30 is 0.10 mass percent 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 an EPMA (Electron Probe Micro Analyzer). At this time, a calibration curve is created using a standard sample whose carbon concentration and nitrogen concentration are known.

[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 elements 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 elements 30 is measured with a Vickers hardness tester specified in the JIS standard under 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 elements 30 is measured at least at three points, and the average value of these measured values ​​is used.

[0037] The position where the depth from the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling elements 30 is 0.02 to 0.03 times the diameter of the rolling elements 30 is defined as the maximum internal stress position. The maximum internal stress position corresponds to the position where the internal stress is maximum when the maximum contact pressure between the raceway surface 10da (raceway surface 20ca) and the surface of the rolling elements 30 is 4.5 GPa. The hardness of the steel at the maximum internal stress position is 750 Hv 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 elements 30.

[0038] The volume ratio of the 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 elements 30 is 22 percent or less. The volume ratio of the retained austenite in the steel at the position of maximum internal stress is 20 percent or less. The volume ratio of the retained austenite in the steel is measured using a Cr tube type X-ray diffraction device.

[0039] In this measurement, the inner ring 10, the outer ring 20, and the rolling elements 30 are electrolytically polished so as not to cause processing-induced transformation of the steel structure. -10 The conditions used are: 1000 nm, tube voltage 30 kV, tube current 10 mA, collimator size 2 mm x 2 mm. X-ray profiles of martensite are measured over the 2θ range of 142.75° to 170.8°. X-ray profiles of austenite are measured over the 2θ range of 114.75° to 142.8°. These X-ray profiles are background processed. Based on these X-ray profiles, the volume fraction of retained austenite is determined.

[0040] The depth of the indentation formed on the raceway surface 10da (raceway surface 20ca) when the raceway surface 10da (raceway surface 20ca) and the surface of the rolling element 30 are in contact with each other at a maximum contact pressure of 4.5 GPa is preferably 0.5 μm or less. The depth of the indentation formed on the raceway surface 10da (raceway surface 20ca) when the raceway surface 10da (raceway surface 20ca) and the surface of the rolling element 30 are in contact with each other at a maximum contact pressure of 4.5 GPa is measured by the following method.

[0041] First, the rolling element 30 is pressed against the raceway surface 10da (raceway surface 20ca) using an autograph. The load applied at this time is adjusted so that the maximum contact pressure between the raceway surface 10da (raceway surface 20ca) and the surface of the rolling element 30 is 4.5 GPa. The rolling element 30 used at this time is a ceramic ball so that the rolling element 30 does not deform. The loading speed at this time is set to 3 N / sec, and the load is maintained for 120 seconds after the desired load is reached. Second, the depth of the indentation formed on the raceway surface 10da (raceway surface 20ca) is measured using the white light interference function of a laser microscope.

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

[0043] First, the depth of the indentation formed on the raceway surface 10da (raceway surface 20ca) is sequentially measured by an autograph while sequentially changing the maximum contact pressure between the raceway surface 10da (raceway surface 20ca) and the surface of the rolling element 30. Second, the maximum contact pressure between the raceway surface 10da (raceway surface 20ca) and the surface of the rolling element 30 is determined when the depth of the indentation measured in this manner becomes 1 / 10,000 of the diameter of the rolling element 30. This maximum contact pressure becomes the static load capacity of the raceway surface 10da (raceway surface 20ca).

[0044] (Method of manufacturing rolling bearing 100) A method for manufacturing the rolling bearing 100 will now be described.

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

[0046] In the preparation step S1, a workpiece is prepared. The workpiece is made of steel having the composition shown in Table 1. The workpiece for the inner ring 10 and the outer ring 20 is ring-shaped, and the workpiece for the rolling element 30 is spherical.

[0047] The carbo-nitriding process S2 is carried out after the preparation process S1. The carbo-nitriding process S2 is carried out by heating and holding the workpiece in an atmospheric gas containing a carbon source and a nitrogen source. The heating temperature in the carbo-nitriding process S2 is a temperature equal to or higher than the A1 transformation point of the steel constituting the workpiece. This heating temperature is, for example, 800°C or higher. By holding the heat in the carbo-nitriding process S2, carbon and nitrogen penetrate from the surface of the workpiece and are dissolved in the steel near the surface of the workpiece. The carbo-nitriding process S2 is carried out so that the carbon and nitrogen diffuse to positions that will become the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30 after the post-treatment process S6.

[0048] The quenching step S3 is performed after the carbonitriding step S2. The quenching step S3 is a step of quenching the workpiece from a temperature equal to or higher than the A1 transformation point to a temperature of M S The cooling step S4 is performed by cooling the workpiece to a temperature below the transformation point. The cooling step S4 is performed after the quenching step S3. The cooling step S4 is a sub-zero treatment or a cryo-treatment. In the sub-zero treatment, the workpiece is cooled to a temperature above -100°C and below room temperature. In the cryo-treatment, the workpiece is cooled to a temperature below -100°C.

[0049] By carrying out the sub-zero treatment and the cryo-treatment, the martensite formation in the steel progresses and the volume ratio of the retained austenite in the steel decreases. Note that, since the carbo-nitriding treatment step S2 is usually a treatment for increasing the volume ratio of the retained austenite in the steel, when the carbo-nitriding treatment step S2 is carried out, the sub-zero treatment or the cryo-treatment is usually not carried out.

[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). As a result, the inner ring 10, the outer ring 20, and the rolling elements 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 elements 30 are assembled together with the cage 40 to form the rolling bearing 100 having the structure shown in FIG. 1.

[0052] (Effect of Rolling Bearing 100) The effects of the rolling bearing 100 will be described below.

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

[0054] Also, the sub-zero treatment or cryo-treatment increases the dislocation density of martensite and retained austenite in the steel at the surface. Furthermore, the carbonitriding treatment step S2 is performed when forming the inner ring 10, the outer ring 20, and the rolling elements 30, thereby solid-solution strengthening the steel at the surface. In this way, the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30 can ensure hardness (specifically, 800 Hv or more) while ensuring static load capacity.

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

[0056] [Table 2]

[0057] Samples 1 to 7 are flat plate-shaped members with a diameter of 85 mm and a thickness of 5 mm. In Samples 1 to 7, the steel type, the average carbon concentration at the surface, the average nitrogen concentration at the surface, the volume ratio of retained austenite in the steel at the surface, and the hardness of the steel at the surface were changed. 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 the carbonitriding treatment step S2, the quenching step S3, and the tempering step S5. Samples 1, 3, and 6 were subjected to a cryo-treatment as the cooling step S4, and Samples 2 and 4 were subjected to a sub-zero treatment as the cooling step S4. On the other hand, Samples 5 and 7 were not subjected to the cooling step S4.

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

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

[0061] A ceramic ball with a diameter of 3 / 8 inch was pressed against the surfaces of Samples 1 to 7, and the depth of the indentation formed when a maximum contact pressure of 4.5 GPa was applied and the static load capacity was measured. "OK" in Table 1 indicates that the static load capacity was 5.3 GPa or more and the indentation depth was 0.5 μm or less. Also, "NG" in Table 1 indicates that the static load capacity was less than 5.3 GPa or the indentation depth was more than 0.5 μm.

[0062] In Samples 1 to 4, the depth of the indentation formed when a maximum contact pressure of 4.5 GPa was applied and the static load capacity was evaluated as good. On the other hand, in Samples 5 to 7, the depth of the indentation formed when a maximum contact pressure of 4.5 GPa was applied and the static load capacity was evaluated as poor. From this comparison, it became clear that the static load capacity (indentation resistance) was improved by satisfying all of Conditions A to F.

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

[0064] FIG. 3 is a graph showing the relationship between the indentation depth based on the estimation formula 1 when the maximum contact pressure is 4.5 GPa and the measured indentation depth when the maximum contact pressure is 4.5 GPa. As shown in FIG. 3, the contribution of the estimation formula 1 (R 2 ) was 0.81. This shows that by making the hardness of the steel at the position of maximum internal stress 750 Hv or more and making the volume ratio of retained austenite at the position of maximum internal stress 20 percent or less, it is possible to make the indentation depth at a maximum contact pressure of 4.5 GPa 0.5 μm or less.

[0065] A multiple regression analysis was performed to investigate the relationship between the static load capacity and the hardness in the steel at the maximum internal stress position and the volume ratio of retained austenite in the steel for samples 1 to 7. The result was: static load capacity (GPa) = -2.15 + 9.88 x 10 -3 × Hardness of steel at maximum internal stress point (Hv) + 2.25 × 10 -3 × the volume ratio (%) of retained austenite in steel at the maximum internal stress position was obtained (Estimated Equation 2).

[0066] FIG. 4 is a graph showing the relationship between the static load capacity based on the estimation formula 2 and the actual measured value of the static load capacity. As shown in FIG. 4, the contribution of the estimation formula 2 (R 2) was 0.90. This shows that the static load capacity can be increased to 5.3 GPa or more by making the hardness of the steel at the maximum internal stress position 750 Hv or more and making the volume ratio of the retained austenite at the maximum internal stress position 20 percent or less.

[0067] (Modification) Although an example has been shown above in which the carbo-nitrided layer 50 is formed on all of the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30, it is sufficient that the carbo-nitrided layer 50 is formed on the surface of any one of the inner ring 10, the outer ring 20, and the rolling elements 30. Although an example has been shown above in which the inner ring 10, the outer ring 20, and the rolling elements 30 are all made of steel having the composition shown in Table 1, it is not necessary for any one of the inner ring 10, the outer ring 20, and the rolling elements 30 to be made of steel having the composition shown in Table 1. Although an example has been shown above in which conditions A to F are satisfied on all of the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30, it is sufficient that conditions A to F are satisfied on the surface of any one of the inner ring 10, the outer ring 20, and the rolling elements 30.

[0068] In the above, the rolling bearing 100 has been described as an example, but the above configuration can be applied to, for example, a ball screw. The ball screw according to the embodiment is a ball screw 110. FIG. 5 is a cross-sectional view of the ball screw 110. As shown in FIG. 5, the ball screw 110 has a screw shaft 61, a ball nut 62, a plurality of balls 63, and a seal 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 (pipe) type, a deflector type, an end deflector type, an end cap type, a top type, or the like.

[0069] The screw shaft 61 has an outer peripheral surface 61a. A screw groove 61b is formed on the outer peripheral surface 61a. The ball nut 62 has a hole extending along the direction of the central axis of the screw shaft 61. The inner wall surface of this 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 so that the outer peripheral surface 61a faces the inner peripheral surface 62a. The ball 63 is disposed 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 closed by a seal member 64. The screw shaft 61 also passes through a hole formed in the seal member 64.

[0070] By rotating the screw shaft 61 around its central axis, the rotational power of the screw shaft 61 is transmitted to the ball nut 62 via the balls 63, and the ball nut 62 moves along the direction of the central axis of the screw shaft 61. In other words, the ball screw 110 is a device that converts the rotational motion of a motor or the like into 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 balls 63 are made of steel that has been quenched and tempered. The steels constituting the screw shaft 61, the ball nut 62, and the balls 63 have the compositions shown in Table 1. However, any one of the steels constituting the screw shaft 61, the steel constituting the ball nut 62, and the steel constituting the balls 63 does not have to have the compositions shown in Table 1. Although not shown, a carbo-nitrided layer 50 is formed on the surface of at least one of the screw shaft 61, the ball nut 62, and the balls 63. Furthermore, the conditions A to F are satisfied on the surface of at least one of the screw shaft 61, the ball nut 62, and the balls 63.

[0072] The screw shaft 61, the ball nut 62, and the balls 63 are formed by carrying out a preparation step S1, a carbo-nitriding treatment step S2, a quenching step S3, a cooling step S4, a tempering step S5, and a post-treatment step S6. However, the shapes of the workpieces to be processed for forming the screw shaft 61, the ball nut 62, and the balls 63 are different from the shapes of the workpieces to be processed for forming the inner ring 10, the outer ring 20, and the rolling elements 30.

[0073] In the ball screw 110 as well, it is possible to ensure the hardness of the surfaces of the screw shaft 61, the ball nut 62 and the balls 63 while improving the static load capacity of the surfaces of the screw shaft 61, the ball nut 62 and the balls 63.

[0074] In the above, an example was shown in which the heating temperature in the carbonitriding treatment step S2 was 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 a decrease in hardness and static load capacity due to coarsening of crystal grains.

[0075] Although the embodiment of the present invention has been described above, the above-mentioned embodiment can be modified in various ways. The scope of the present invention is not limited to the above-mentioned embodiment. 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]

[0076] 100 rolling bearing, 10 inner ring, 10a width surface, 10b width surface, 10c inner diameter surface, 10d outer diameter surface, 10da raceway surface, 20 outer ring, 20a width surface, 20b width surface, 20c inner diameter surface, 20ca raceway surface, 20d outer diameter surface, 30 rolling element, 40 retainer, 50 carburized 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 seal member, 110 ball screw, A central shaft, S1 preparation process, S2 carburized nitriding process, S3 quenching process, S4 cooling process, S5 tempering process, S6 post-treatment process, S7 assembly process.

Claims

1. A machine part made of steel that has been quenched and tempered, It has a carbonitriding layer on the surface, The steel is any one of S53C, SCM420, and SCM435 specified in the JIS standard, The average carbon concentration in the steel at the surface is 0.6 mass percent or more; The average nitrogen concentration in the steel at the surface is 0.10 mass percent or more; The hardness of the steel at the surface is 800 Hv or more; The volume ratio of the retained austenite in the steel at the surface is 22 percent or less; the mechanical component is in contact with a rolling element on at least a portion of the surface; the hardness of the steel at a position where the depth from the surface is 0.02 to 0.03 times the diameter of the rolling element is 750 Hv or more; A mechanical component, wherein a volume ratio of retained austenite in the steel at a position whose depth from the surface is 0.02 to 0.03 times the diameter of the rolling element is 20 percent or less.

2. The mechanical component according to claim 1 , wherein a depth of an indentation formed on said surface when said rolling element comes into contact with said surface is 0.5 μm or less.

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

4. The mechanical component according to claim 3 , wherein a depth of an indentation formed on said surface when said rolling element comes into contact with said surface is 0.5 μm or less.

5. The bearing comprises an inner ring, an outer ring, and rolling elements, 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 4.

6. The ball bearing includes a screw shaft, a ball nut, and a ball. At least one of the screw shaft, the ball nut, and the ball is the mechanical component according to any one of claims 1 to 4.

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