Machine component

Abstract

To provide a machine component having an improved static load capacity in a surface.SOLUTION: A machine component is made of hardened and tempered steel and has a surface. The machine component comprises a nitriding layer formed on the surface. The steel contains 0.95 mass% to 1.10 mass% of carbon, less than 0.30 mass% of silicon, less than 0.50 mass% of manganese, less than 0.0080 mass% of sulfur, and 1.3 mass% to 1.6 mass% of chromium, and the balance comprising iron and inevitable impurities. The average nitrogen concentration in the surface is 0.10 mass% or more. The hardness in the surface is 850 Hv or more. The retained austenite amount in the surface layer is 20 vol% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to machine parts.

Background Art

[0002] For example, Japanese Patent Application Laid-Open No. 2013-119930 (Patent Document 1) describes a bearing part. The bearing part described in Patent Document 1 contacts other parts on the surface. The bearing part described in Patent Document 1 is made of steel that has been quenched and tempered. The steel contains 0.90 mass% or more and 1.05 mass% or less of carbon, 0.15 mass% or more and 0.35 mass% or less of silicon, 0.01 mass% or more and 0.50 mass% or less of manganese, and 1.30 mass% or more and 1.65 mass% or less of chromium, and the balance consists of iron and inevitable impurities.

[0003] In the bearing part described in Patent Document 1, a nitriding treatment is performed on the surface, and the nitrogen concentration on the surface is 0.25 mass% or more. In the bearing part described in Patent Document 1, the amount of retained austenite on the surface is 6 volume% or more and 12 volume% or less. In the bearing part described in Patent Document 1, static load capacity on the surface is improved by performing tempering at a high temperature.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Bearings are used in motors, gearboxes, differentials, etc. mounted on battery electric vehicles (BEVs). Bearings are also used in electric VTCs (variable valve mechanisms), electric compressors, transmissions, axles, etc. mounted on plug-in hybrid vehicles (PHVs), hybrid vehicles (HVs), and engine vehicles. When the static load capacity of the bearings is improved in these applications, it becomes possible to reduce the size of the bearings and, by extension, the size of the surrounding mechanical components.

[0006] Even for bearings mounted on hydrogen utilization devices used in hydrogen environments such as fuel cell vehicles (FCVs) and hydrogen stations, improving the static load capacity makes it possible to reduce the size of the bearings, and there is a possibility of improving the efficiency of the hydrogen utilization devices. However, there is room for improvement in the static load capacity on the surface of the bearing parts described in Patent Document 1.

[0007] 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 with an improved static load capacity on the surface.

Means for Solving the Problems

[0008] The mechanical component of the present invention has a surface and is made of steel that has been quenched and tempered. The mechanical component is provided with a nitrided layer formed on the surface. The steel contains 0.95 mass percent or more and 1.10 mass percent or less of carbon, less than 0.30 mass percent of silicon, less than 0.50 mass percent of manganese, less than 0.0080 mass percent of sulfur, and 1.3 mass percent or more and 1.6 mass percent or less of chromium, and the balance consists of iron and unavoidable impurities. The average nitrogen concentration on the surface is 0.10 mass percent or more. The hardness on the surface is 850 Hv or more. The amount of retained austenite on the surface is 20 volume percent or less.

[0009] In the above mechanical part, the half-value width in the X-ray profile of martensite obtained by X-ray diffraction on the surface may be 7.2° or more and 8.0° or less. The peak position indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction on the surface may be 128° or more.

[0010] In the above mechanical part, the dislocation density of martensite on the surface may be 1.1×10 15 m -2 or more. The dislocation density of austenite on the surface may be 2.5×10 14 m -2 or more.

[0011] In the above mechanical part, when the amount of retained austenite on the surface, the average nitrogen concentration on the surface, and the dislocation density of austenite on the surface are A (unit: volume percentage), B (unit: mass percentage), and C (unit: m -2 ), respectively, the relationship of 4.332 + 0.005×A - 0.580×B - 0.295×LogC ≤ 0 may be satisfied.

[0012] In the above mechanical part, when the amount of retained austenite on the surface, the average nitrogen concentration on the surface, and the dislocation density of martensite on the surface are A (unit: volume percentage), B (unit: mass percentage), and D (unit: m -2 ), respectively, the relationship of -47.73 - 0.025×A - 2.141×B + 3.155×LogD ≥ 0 may be satisfied.

[0013] In the above mechanical part, when a maximum contact surface pressure of 4.5 GPa is applied to the surface, the depth of the indentation formed on the surface may be 0.2 μm or less. In the above mechanical part, the static load capacity on the surface may be 6.0 GPa or more.

Advantages of the Invention

[0014] According to the mechanical part of the present invention, the static load capacity on the surface can be improved.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0016] Details of embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and duplicate explanations will not be repeated.

[0017] The mechanical part according to the embodiment is, for example, a raceway ring of a rolling bearing. The mechanical part according to the embodiment may be a rolling element, a shaft, a sliding member such as a ball screw, etc. of a rolling bearing. The mechanical part according to the embodiment is not limited to these, but in the following, the inner ring 10 of a deep groove ball bearing will be described as an example of the mechanical part according to the embodiment.

[0018] (Configuration of the inner ring 10) The configuration of the inner ring 10 will be described below.

[0019] FIG. 1 is a cross-sectional view of the inner ring 10. As shown in FIG. 1, the inner ring 10 has a surface. More specifically, the inner ring 10 has, as surfaces, a width surface 10a, a width surface 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d. Let the central axis of the inner ring 10 be the central axis A. The direction along the central axis A is defined as the axial direction. The direction perpendicular to the axial direction 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.

[0020] The width surfaces 10a and 10b constitute the end faces of the inner ring 10 in the axial direction. The width surface 10b is the opposite surface of the width surface 10a. The inner circumferential surface 10c extends along the circumferential direction. The inner circumferential surface 10c faces the central axis A side. One end and the other end of the inner circumferential surface 10c in the axial direction are respectively continuous with the width surfaces 10a and 10b. The inner ring 10 is fitted onto a shaft (not shown) at the inner circumferential surface 10c.

[0021] The outer circumferential surface 10d extends along the circumferential direction. The outer circumferential surface 10d faces the side opposite to the central axis A. That is, the outer circumferential surface 10d is the opposite surface of the inner circumferential surface 10c in the radial direction. One end and the other end of the outer circumferential surface 10d in the axial direction are respectively continuous with the width surfaces 10a and 10b.

[0022] The outer circumferential surface 10d has a raceway surface 10da. The raceway surface 10da is the portion of the outer circumferential surface 10d that contacts the rolling elements. The raceway surface 10da is at the center of the outer circumferential surface 10d in the axial direction. The raceway surface 10da extends along the circumferential direction. The outer circumferential surface 10d is recessed toward the inner circumferential surface 10c side at the raceway surface 10da. In a cross-sectional view orthogonal to the circumferential direction, the raceway surface 10da is, for example, in a partial arc shape.

[0023] The inner ring 10 is made of steel that has been quenched and tempered. That is, the steel constituting the inner ring 10 contains martensite and retained austenite. Nitrocarburizing treatment is performed on the surfaces of the inner ring 10 (the width surfaces 10a, 10b, the inner circumferential surface 10c, and the outer circumferential surface 10d). That is, a nitrocarburized layer 11 is formed on the surface of the inner ring 10. Note that nitrogen is dissolved in the steel in the nitrocarburized layer 11 and no nitrides are formed in the nitrocarburized layer 11.

[0024] The steel constituting the inner ring 10 contains carbon of 0.95 mass percent or more and 1.10 mass percent or less, silicon of less than 0.30 mass percent, manganese of less than 0.50 mass percent, sulfur of less than 0.0080 mass percent, and chromium of 1.3 mass percent or more and 1.6 mass percent or less. The balance of the steel constituting the inner ring 10 consists of iron and unavoidable impurities. Note that the steel constituting the inner ring 10 may not contain silicon, manganese, and sulfur.

[0025] Specific examples of the steel constituting the inner ring 10 include bearing steel. Specific examples of bearing steel include SUJ2 defined in JIS standard, 52100 defined in ASTM standard, 100Cr6 defined in ISO standard, GCr15 defined in GB standard, etc.

[0026] The average nitrogen concentration on the surface of the inner ring 10 is 0.10 mass percent or more. The average nitrogen concentration on the surface of the inner ring 10 is measured by line analysis using an EPMA (Electron Probe Micro Analyzer). At this time, a calibration curve is created using a standard sample with a clear nitrogen concentration.

[0027] The amount of retained austenite on the surface of the inner ring 10 is 20 volume percent or less. The amount of retained austenite on the surface of the inner ring 10 is measured by X-ray diffraction method. This measurement is performed using a Cr tube spherical X-ray diffractometer. In the Cr tube spherical X-ray diffractometer, the wavelength of the Cr-Kα line is 2.29093×10 -10 m, the tube voltage is 30 kV, the tube current is 10 mA, and the collimator size is 2 mm×2 mm.

[0028] The hardness on the surface of the inner ring 10 is 850 Hv or more. The hardness on the surface of the inner ring 10 is measured in accordance with the Vickers hardness test method defined in the JIS standard. The load when measuring the hardness on the surface of the inner ring 10 is set to 300 g. The hardness on the surface of the inner ring 10 is measured at three or more different points, and the average value of these measurement values is used.

[0029] The half-value width in the X-ray profile of martensite obtained by X-ray diffraction for the surface of the inner ring 10 is preferably 7.2° or more and 8.0° or less. The position (2θ) of the peak indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction for the surface of the inner ring 10 is preferably 128° or more. The lattice plane spacing of austenite on the surface of the inner ring 10 is preferably 1.275×10 -10 m or less.

[0030] The X-ray profiles of martensite and austenite obtained by X-ray diffraction for the surface of the inner ring 10 are obtained by using a Cr tube type X-ray diffractometer. In the Cr tube type X-ray diffractometer, the wavelength of the Cr-Kα ray is 2.29093×10 -10 m, the tube voltage is set to 30 kV, the tube current is set to 10 mA, and the collimator size is set to 2 mm × 2 mm. The X-ray profile of martensite obtained by X-ray diffraction for the surface of the inner ring 10 is measured in the range where 2θ is 142.75° or more and 170.8° or less, and the X-ray profile of austenite obtained by X-ray diffraction for the surface of the inner ring 10 is measured in the range where 2θ is 114.75° or more and 142.8° or less.

[0031] The X-ray profiles of martensite and austenite obtained by X-ray diffraction for the surface of the inner ring 10 are background-processed. The position of the peak indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction for the surface of the inner ring 10 is determined from the center position of the half-value width.

[0032] The lattice plane spacing of austenite on the surface of the inner ring 10 is calculated by applying the following Bragg's equation to the X-ray profile of austenite obtained by X-ray diffraction with respect to the surface of the inner ring 10. In this equation, d is the lattice plane spacing of austenite on the surface of the inner ring 10 (unit: m), λ is the wavelength of Cr-Kα rays used in a Cr tube-type X-ray diffractometer (unit: m), and θ is the diffraction angle in the X-ray profile of austenite obtained by X-ray diffraction with respect to the surface of the inner ring 10 (unit: °).

[0033]

Equation

[0034] The dislocation density of martensite on the surface of the inner ring 10 is preferably 1.1×10 15 m -2 or more. The dislocation density of austenite on the surface of the inner ring 10 is preferably 2.5×10 14 m -2 or more.

[0035] The dislocation density of martensite and the dislocation density of austenite on the surface of the inner ring 10 are measured using a Co tube-type X-ray diffractometer. More specifically, first, the X-ray profiles of martensite and austenite on the surface of the inner ring 10 are acquired using a Co tube-type X-ray diffractometer. In the Co tube-type X-ray diffractometer, 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 martensite and austenite obtained by X-ray diffraction with respect to the surface of the inner ring 10 are measured within the range where 2θ is 30° or more and 135° or less.

[0036] Second, after performing Rietveld analysis, the full-width at half-maximum of the peaks of the X-ray profiles of martensite and austenite obtained by X-ray diffraction on the surface of the inner ring 10 is separated into crystallite size and strain. Third, by applying this crystallite size and strain to the following Williamson-Hall equation, the dislocation density of martensite and the dislocation density of austenite on the surface of the inner ring 10 are obtained. In this equation, ρ is the dislocation density (unit: m -2 ) and ε is the above-mentioned strain, and b is the length of the Burgers vector (b = 0.25 × 10 -9 m).

[0037]

Equation

[0038] In the X-ray profile of martensite obtained by X-ray diffraction on the surface of the inner ring 10, the peaks of the {110}, {200}, {211}, and {220} planes are the measurement targets. Also, in the X-ray profile of austenite obtained by X-ray diffraction on the surface of the inner ring 10, the peaks of the {111}, {200}, {220}, {311}, and {222} planes are the measurement targets. The reason for performing Rietveld analysis above is to reduce the influence of the {200} plane of martensite and the {200} plane of austenite with different elastic moduli.

[0039] When a maximum contact surface pressure of 4.5 GPa is applied to the surface of the inner ring 10 (raceway surface 10da), the depth of the indentation formed is preferably 0.2 μm or less. This indentation depth is measured using the white interference function of a laser microscope on the surface of the inner ring 10 after bringing a ceramic ball into contact with the surface of the inner ring 10 using an autograph so that the maximum contact surface pressure becomes 4.5 GPa. In addition, the load application speed at this time is set to 3 N / second, and the load is held for 120 seconds after the load reaches the load.

[0040] The static load capacity on the surface of the inner ring 10 is preferably 6.0 GPa or more. In the measurement of the static load capacity on the surface of the inner ring 10, first, as described above, using an autograph, the maximum contact surface pressure is changed to form an indentation on the surface of the inner ring 10. Second, for each changed maximum contact surface pressure, a value obtained by dividing the depth of the indentation by the diameter of the ceramic ball is calculated. The maximum contact surface pressure when the value obtained by dividing the depth of the indentation by the diameter of the ceramic ball becomes 1 / 10,000 is the static load capacity on the surface of the inner ring 10.

[0041] Let the retained austenite amount on the surface of the inner ring 10, the average nitrogen concentration on the surface of the inner ring 10, and the dislocation density of austenite on the surface of the inner ring 10 be A (unit: volume percent), B (unit: mass percent), and C (unit: m -2 ) respectively. In the inner ring 10, it is preferable that the relationship of 4.332 + 0.005×A - 0.580×B - 0.295×LogC ≤ 0 is satisfied.

[0042] Let the dislocation density of martensite on the surface of the inner ring 10 be D (unit: m -2 ) respectively. On the surface of the inner ring 10, it is preferable that the relationship of -47.73 - 0.025×A - 2.141×B + 3.155×LogD ≥ 0 is satisfied.

[0043] (Manufacturing method of the inner ring 10) The manufacturing method of the inner ring 10 will be described below.

[0044] Figure 2 is a process diagram showing the manufacturing method of the inner ring 10. As shown in Figure 2, the manufacturing method of the inner ring 10 has a preparation step S1, a nitriding treatment step S2, a quenching step S3, a sub-zero treatment step S4, a tempering step S5, and a post-treatment step S6.

[0045] In the preparation step S1, a member to be processed is prepared. The nitriding treatment step S2 is performed after the preparation step S1. In the nitriding treatment step S2, the member to be processed is held at a temperature equal to or higher than the A1 transformation point in an atmosphere gas containing a nitrogen source. The nitriding treatment step S2 is performed so that nitrogen penetrates and diffuses to a position that becomes the surface of the inner ring 10 after the post-treatment step S6. The quenching step S3 is performed after the nitriding treatment step S2. In the quenching step S3, after the member to be processed is held at a temperature equal to or higher than the A1 transformation point, it is cooled to a temperature below the M S transformation point.

[0046] The sub-zero treatment step S4 is performed after the quenching step S3. In the sub-zero treatment step S4, the member to be processed is cooled to a temperature above -100°C and below room temperature. As a result, a part of the retained austenite formed in the steel constituting the member to be processed in the quenching step S3 transforms into martensite. Instead of the sub-zero treatment step S4, a cryogenic treatment step S7 may be performed. In the cryogenic treatment step S7, the member to be processed is cooled to a temperature of -100°C or lower. As a result, a part of the retained austenite formed in the steel constituting the member to be processed in the quenching step S3 transforms into martensite.

[0047] As the refrigerant in the sub-zero treatment step S4 and the cryogenic treatment step S7, for example, liquid nitrogen, liquid helium, etc. are used. Preferably, the sub-zero treatment step S4 and the cryogenic treatment step S7 are performed within 2 hours after the end of the quenching step S3.

[0048] The tempering step S5 is performed after the sub-zero treatment step S4 (cryogenic treatment step S7). In the tempering step S5, the member to be processed is held at a temperature below the A1 transformation point. As a result, a part of the martensite in the steel constituting the member to be processed is decomposed. The holding temperature in the tempering step S5 is, for example, 180°C or higher and 220°C or lower. The holding time in the tempering step S5 is, for example, 2 hours.

[0049] The post-treatment step S6 is performed after the tempering step S5. In the post-treatment step S6, machining (for example, grinding and polishing) is performed on the surface of the member to be processed. Thus, the inner ring 10 having the structure shown in FIG. 1 is manufactured.

[0050] (Effect of the inner ring 10) The effect of the inner ring 10 will be described below.

[0051] As described in Patent Document 1, in order to improve the static load capacity on the surface of the raceway ring, it is conceivable to perform tempering at a high temperature (for example, 230 °C or higher). However, in this case, the martensite formed by quenching may be excessively decomposed by tempering, resulting in a decrease in the hardness on the surface of the raceway ring and an insufficient static load capacity on the surface of the raceway ring.

[0052] On the other hand, in the inner ring 10, since nitrogen is dissolved in the nitrided layer 11, the surface of the inner ring 10 is solid-solution strengthened. Further, in the inner ring 10, due to the sub-zero treatment step S4 (cryogenic treatment step S7) being performed, the amount of retained austenite on the surface is decreased (martensite is increased), and the dislocation density in the retained austenite and martensite on the surface is increased. As a result, in the inner ring 10, the hardness on the surface is 850 Hv or more. Therefore, according to the inner ring 10, the static load capacity on the surface is improved as compared with the case where tempering is performed at a high temperature.

[0053] More specifically, when the amount of retained austenite on the surface of the inner ring 10 is 20 volume percent or less, the average nitrogen concentration on the surface of the inner ring 10 is 0.10 mass percent or more, and the hardness on the surface of the inner ring 10 is 850 Hv or more, the depth of the indentation when a maximum contact surface pressure of 4.5 GPa is applied to the surface of the inner ring 10 becomes 0.2 μm or less, and the static load capacity on the surface of the inner ring 10 becomes 6.0 GPa or more.

[0054] Note that nitrogen is an austenite stabilizing element, and the nitriding treatment is performed to enrich the retained austenite on the surface of the track ring. Therefore, when the nitriding treatment is performed, it is not usually done to perform sub-zero treatment (or cryogenic treatment) to reduce the amount of retained austenite on the surface.

[0055] (Evaluation of Static Load Capacity and Indentation Depth) Samples 1 to 6 were prepared to evaluate the static load capacity on the surface of the mechanical part according to the embodiment. The shapes of Samples 1 to 6 are flat plates with a diameter of 85 mm and a thickness of 5 mm. Samples 1 to 6 are formed of SUJ2 defined by JIS standards. Table 1 shows the details of the heat treatment for Samples 1 to 6.

[0056]

Table 1

[0057] In Sample 1, as the heat treatment, a nitriding treatment step S2, a quenching step S3, a cryogenic treatment step S7, and a tempering step S5 were performed. In Samples 2 and 3, as the heat treatment, a nitriding treatment step S2, a quenching step S3, a sub-zero treatment step S4, and a tempering step S5 were performed. That is, Samples 1 to 3 are samples that mimic the mechanical part according to the embodiment. In the tempering step S5 for Samples 1 to 3, the holding temperature was 180°C.

[0058] In Sample 4, as the heat treatment, a nitriding treatment step S2, a quenching step S3, and a tempering step S5 were performed, and the sub-zero treatment step S4 (or the cryogenic treatment step S7) was not performed. Also, in the tempering step S5 for Sample 4, tempering at a high temperature (specifically, at 230°C) was performed.

[0059] In Sample 5, as the heat treatment, a nitriding process step S2, a quenching process step S3, and a tempering process step S5 were performed, and a sub-zero treatment process step S4 (or a cryogenic treatment process step S7) was not performed. In the tempering process step S5 for Sample 5, the holding temperature was set to 180°C. In Sample 6, as the heat treatment, a quenching process step S3 and a tempering process step S5 were performed, and a nitriding process step S2 and a sub-zero treatment process step S4 (or a cryogenic treatment process step S7) were not performed. In the tempering process step S5 for Sample 5, the holding temperature was set to 180°C.

[0060] For Samples 1 to 6, measurements were made of the average nitrogen concentration on the surface, the hardness on the surface, the amount of retained austenite on the surface, the depth of the indentation when a maximum contact surface pressure of 4.5 GPa was applied to the surface, and the static load capacity on the surface.

[0061] The measurement results of the average nitrogen concentration on the surface, the hardness on the surface, the amount of retained austenite on the surface, the depth of the indentation when a maximum contact surface pressure of 4.5 GPa was applied to the surface, and the static load capacity on the surface are shown in Table 2. Figure 3 is a graph showing the relationship between the maximum contact surface pressure on the surfaces of Samples 1 to 6 and the value obtained by dividing the indentation depth by the diameter of the ceramic ball. The diameter of the ceramic ball was 3 / 8 inch (9.525 mm).

[0062]

Table 2

[0063] The average nitrogen concentration on the surface being 0.10 mass percent or more, the hardness on the surface being 850 Hv or more, and the amount of retained austenite on the surface being 20 volume percent or less are set as Condition 1, Condition 2, and Condition 3, respectively. As shown in Table 2, in Samples 1 to 3, all of Condition 1, Condition 2, and Condition 3 were satisfied. On the other hand, in Samples 4 to 6, at least one of Condition 1, Condition 2, and Condition 3 was not satisfied.

[0064] As shown in Table 1 and FIG. 3, in Samples 1 to 3, when a maximum contact surface pressure of 4.5 GPa was applied to the surface, the depth of the indentation was 0.2 μm or less, and the static load capacity on the surface was 6.0 GPa or more. On the other hand, in Samples 4 to 6, when a maximum contact surface pressure of 4.5 GPa was applied to the surface, the depth of the indentation exceeded 0.2 μm, and the static load capacity on the surface was less than 6.0 GPa. From this comparison, it was experimentally clarified that when Conditions 1, 2, and 3 were satisfied, the static load capacity on the surface was improved.

[0065] Samples 1-1 to 1-3, Samples 2-1 to 2-3, and Samples 3-1 to 3-3 were samples that had been heat-treated in the same manner as Samples 1, 2, and 3, respectively. Samples 4-1 to 4-4 were samples that had been heat-treated in the same manner as Sample 4. The steel types and shapes of these samples were SUJ2 and flat plates with a diameter of 85 mm and a thickness of 5 mm, respectively.

[0066]

Table 3

[0067] As shown in Table 3, in Samples 1-1 to 1-3, Samples 2-1 to 2-3, and Samples 3-1 to 3-3, all of Conditions 1, 2, and 3 were satisfied. On the other hand, in Samples 4-1 to 4-4, at least one of Conditions 1, 2, and 3 was not satisfied.

[0068] In Samples 1-1 to 1-3, Samples 2-1 to 2-3, and Samples 3-1 to 3-3, the half-value width in the X-ray profile of martensite obtained by X-ray diffraction with respect to the surface was 7.2° or more and 8.0° or less, and the peak position indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction with respect to the surface was 128° or more.

[0069] On the other hand, in Samples 4-1 to 4-4, the full width at half maximum in the X-ray profile of martensite obtained by X-ray diffraction with respect to the surface was less than 7.2°, and the peak position indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction with respect to the surface was less than 128°.

[0070] From this comparison, it was revealed that when Conditions 1, 2, and 3 are satisfied, the full width at half maximum in the X-ray profile of martensite obtained by X-ray diffraction with respect to the surface is 7.2° or more and 8.0° or less, and the peak position indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction with respect to the surface is 128° or more.

[0071] In other words, when the subzero treatment step S4 or the cryogenic treatment step S7 is performed without performing the tempering step S5 at a high temperature, the dislocation density of martensite increases on the surface and the full width at half maximum in the X-ray profile increases, and the dislocation density of austenite increases on the surface and the peak position indicating the {220} plane shifts to the high angle side.

[0072] As shown in Table 4, in Samples 1, 2, and 3, the full width at half maximum in the X-ray profile of martensite obtained by X-ray diffraction with respect to the surface was 7.2° or more and 8.0° or less, and the peak position indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction with respect to the surface was 128° or more. Also, in Samples 1, 2, and 3, the dislocation density of martensite and the dislocation density of austenite on the surface were 1.1×10 15 m -2 or more and 2.5×10 14 m -2 or more, respectively.

[0073] On the other hand, in Sample 4, the half-value width in the X-ray profile of martensite obtained by X-ray diffraction with respect to the surface was less than 7.2°, and the peak position indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction with respect to the surface was less than 128°. Also, in Sample 4, the dislocation density of martensite and the dislocation density of austenite on the surface were less than 1.1×10 15 m -2 and 2.5×10 14 m -2 respectively.

[0074] [Table 4]

[0075] Multiple regression analysis was performed on the relationship between the depth of the indentation when a maximum contact surface pressure of 4.5 GPa was applied to the surface, the amount of retained austenite on the surface (A: unit is volume percent), the average nitrogen concentration on the surface (B: unit is mass percent), and the dislocation density of austenite on the surface (C: unit is m -2 ). As a result of this multiple regression analysis, the depth of the indentation (unit: μm) when a maximum contact surface pressure of 4.5 GPa was applied to the surface was estimated by the formula 4.532 + 0.005×A - 0.580×B - 0.295×LogC. The coefficient of determination (R 2 ) of this formula is 0.94. Therefore, when the relationship 4.332 + 0.005×A - 0.580×B - 0.295×LogC ≤ 0 is satisfied, the depth of the indentation when a maximum contact surface pressure of 4.5 GPa is applied to the surface can be made 0.2 μm or less.

[0076] The static load capacity on the surface, the amount of retained austenite on the surface (A: unit is volume percent), the average nitrogen concentration on the surface (B: unit is mass percent), and the dislocation density of martensite on the surface (D: unit is m -2) A multiple regression analysis was performed on the relationship with []. As a result of this multiple regression analysis, the static load capacity (unit: GPa) on the surface was estimated by the formula -41.73 - 0.025×A - 2.141×B + 3.155×LogD. The coefficient of determination (R 2 ) of this formula is 0.94. Therefore, when the relationship -47.73 - 0.025×A - 2.141×B + 3.155×LogD ≥ 0 is satisfied, the static load capacity on the surface can be set to 6.0 GPa or more.

[0077] (Another example of a mechanical part according to the embodiment) Examples of mechanical parts according to embodiments other than the inner ring 10 will be described below.

[0078] FIG. 4 is a cross-sectional view of the ball screw 20. As shown in FIG. 4, the ball screw 20 has a screw shaft 21, a ball nut 22, a plurality of balls 23, and a seal member 24. The circulation method of the balls 23 in the ball screw 20 is not particularly limited. The circulation method of the balls 23 in the ball screw 20 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, etc.

[0079] The screw shaft 21 has an outer peripheral surface 21a. A screw groove 21b is formed on the outer peripheral surface 21a. The ball nut 22 has a hole formed to extend along the direction of the central axis of the screw shaft 21. The inner wall surface of this hole is the inner peripheral surface 22a of the ball nut 22. A screw groove 22b is formed on the inner peripheral surface 22a. The screw shaft 21 is inserted into the ball nut 22 such that the outer peripheral surface 21a faces the inner peripheral surface 22a. The balls 23 are disposed between the screw groove 21b and the screw groove 22b. The hole of the ball nut 22 through which the screw shaft 21 passes is closed by the seal member 24. The screw shaft 21 also passes through the hole formed in the seal member 24.

[0080] The screw shaft 21, ball nut 22, and balls 23 are formed of quenched and tempered steel. This steel contains carbon in an amount of 0.95 mass percent or more and 1.10 mass percent or less, silicon in an amount of less than 0.30 mass percent, manganese in an amount of less than 0.50 mass percent, sulfur in an amount of less than 0.0080 mass percent, and chromium in an amount of 1.3 mass percent or more and 1.6 mass percent or less. The balance of this steel consists of iron and inevitable impurities. This steel is, for example, bearing steel (SUJ2 defined in JIS standard, 52100 defined in ASTM standard, 100Cr6 defined in ISO standard, GCr15 defined in GB standard, etc.).

[0081] The screw shaft 21, ball nut 22, and balls 23 have a nitrided layer 11 (not shown in FIG. 4) formed on the surface, and the average nitrogen concentration on the surface is 0.10 mass percent or more. The screw shaft 21, ball nut 22, and balls 23 have a surface hardness of 850 Hv or more. The screw shaft 21, ball nut 22, and balls 23 have a retained austenite amount on the surface of 20 volume percent or less. That is, the screw shaft 21, ball nut 22, and balls 23 are mechanical parts according to the embodiment.

[0082] The ball screw 20 can have a higher load capacity and can also be reduced in size and weight because the static load capacity on the surfaces of the screw shaft 21, ball nut 22, and balls 23 is improved. By reducing the size of the ball screw 20, it is also possible to reduce the size of peripheral parts and structural members.

[0083] However, it is sufficient that at least one of the screw shaft 21, ball nut 22, and balls 23 satisfies the above-described steel composition, the hardness on the above-described surface, the nitrogen concentration on the above-described surface, and the retained austenite amount on the above-described surface. From another perspective, the ball screw 20 only needs to have at least any one of the screw shaft 21, ball nut 22, and balls 23 as a mechanical part according to the embodiment.

[0084] By rotating the screw shaft 21 about its central axis, the rotational power of the screw shaft 21 is transmitted to the ball nut 22 via the balls 23, and the ball nut 22 moves along the direction of the central axis of the screw shaft 21. That is, the ball screw 20 is a device that converts rotational motion, such as that of a motor, into linear motion. The ball screw 20 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, etc.

[0085] (Rolling bearing 100 using inner ring 10) FIG. 5 is a cross-sectional view of the rolling bearing 100. As shown in FIG. 5, the rolling bearing 100 has an inner ring 10, an outer ring 30, a plurality of rolling elements 40, and a cage 50.

[0086] The outer ring 30 has a width surface 30a, a width surface 30b, an inner circumferential surface 30c, and an outer circumferential surface 30d. The width surfaces 30a and 30b constitute the end faces of the outer ring 30 in the axial direction. The width surface 30b is the opposite surface of the width surface 30a. The inner circumferential surface 30c and the outer circumferential surface 30d extend along the circumferential direction. The inner circumferential surface 30c and the outer circumferential surface 30d face the central axis A side and the side opposite to the central axis A, respectively. The outer ring 30 is fitted into a housing (not shown) on the outer circumferential surface 30d. The outer ring 30 is disposed outside the inner ring 10 in the radial direction such that the inner circumferential surface 30c faces the outer circumferential surface 10d.

[0087] The inner circumferential surface 30c has a raceway surface 30ca. The raceway surface 30ca is the portion of the inner circumferential surface 30c that contacts the rolling elements 40. The raceway surface 30ca is at the center of the inner circumferential surface 30c in the axial direction. The raceway surface 30ca extends along the circumferential direction. The inner circumferential surface 30c is recessed toward the outer circumferential surface 30d at the raceway surface 30ca. In a cross-sectional view orthogonal to the circumferential direction, the raceway surface 30ca is, for example, in the shape of a partial arc. The raceway surface 30ca faces the raceway surface 10da.

[0088] The rolling elements 40 are, for example, spherical. The rolling elements 40 are disposed between the raceway surface 10da and the raceway surface 30ca. A plurality of rolling elements 40 are arranged at intervals in the circumferential direction. The cage 50 holds the plurality of rolling elements 40 such that the intervals between two adjacent rolling elements 40 are within a certain range. The cage 50 is disposed between the outer peripheral surface 10d and the inner peripheral surface 30c.

[0089] The outer ring 30 and the rolling elements 40 are, for example, mechanical components according to the embodiment. That is, the outer ring 30 and the rolling elements 40 are formed of quenched and tempered steel, and the steel contains 0.95 mass% or more and 1.10 mass% or less of carbon, less than 0.30 mass% of silicon, less than 0.50 mass% of manganese, less than 0.0080 mass% of sulfur, and 1.3 mass% or more and 1.6 mass% or less of chromium, and the balance consists of iron and unavoidable impurities. Further, the outer ring 30 and the rolling elements 40 have an average nitrogen concentration, a hardness, and a retained austenite amount on the surface of 0.10 mass% or more, 850 Hv or more, and 20 volume% or less, respectively. However, the outer ring 30 and the rolling elements 40 may not be mechanical components according to the embodiment.

[0090] It should be considered that all the embodiments disclosed this time are illustrative in all respects and not restrictive. The scope of the present invention is shown not by the above embodiments but by the claims, and it is intended that all modifications within the meaning and scope equivalent to the claims are included.

Explanation of Reference Numerals

[0091] 10 Inner ring, 10a, 10b Width surfaces, 10c Inner peripheral surface, 10d Outer peripheral surface, 10da Raceway surface, 11 Nitriding layer, 20 Ball screw, 21 Screw shaft, 21a Outer peripheral surface, 21b Thread groove, 22 Ball nut, 22a Inner peripheral surface, 22b Thread groove, 23 Ball, 24 Seal member, 30 Outer ring, 30a, 30b Width surfaces, 30c Inner peripheral surface, 30ca Raceway surface, 30d Outer peripheral surface, 40 Rolling element, 50 Cage, A Central axis, S1 Preparation process, S2 Nitriding treatment process, S3 Quenching process, S4 Sub-zero treatment process, S5 Tempering process, S6 Post-treatment process, S7 Cryogenic treatment process.

Claims

1. A steel machine part having a surface and having been quenched and tempered, comprising a nitrocarburized layer formed on the surface, wherein the steel contains 0.95 mass% or more and 1.10 mass% or less of carbon, less than 0.30 mass% of silicon, less than 0.50 mass% of manganese, less than 0.0080 mass% of sulfur, and 1.3 mass% or more and 1.6 mass% or less of chromium, and the balance consists of iron and inevitable impurities, the average nitrogen concentration on the surface is 0.10 mass% or more, the hardness on the surface is 850 Hv or more, and the retained austenite amount on the surface is 20 volume% or less. A machine part.

2. The half-value width in the X-ray profile of martensite obtained by X-ray diffraction of the surface is 7.2° or more and 8.0° or less, and the peak position indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction of the surface is 128° or more. The machine part according to Claim 1.

3. The dislocation density of martensite on the surface is 1.1×10 １５ m －２ or more, The dislocation density of austenite on the surface is 2.5×10 １４ m －２ or more. The machine part according to claim 1.

4. When the amount of retained austenite on the surface, the average nitrogen concentration on the surface, and the dislocation density of austenite on the surface are A (unit: volume percentage), B (unit: mass percentage), and C (unit: m －２ ), respectively, the mechanical part according to claim 1, wherein the relationship of 4.332 + 0.005 × A - 0.580 × B - 0.295 × Log C ≤ 0 is satisfied.

5. When the retained austenite amount on the surface, the average nitrogen concentration on the surface, and the dislocation density of martensite on the surface are A (unit: volume percent), B (unit: mass percent), and D (unit: m －２ ), respectively, the mechanical part according to claim 1, wherein the relationship of -47.73 - 0.025 × A - 2.141 × B + 3.155 × Log D ≧ 0 is satisfied.

6. When a maximum contact surface pressure of 4.5 GPa is applied to the surface, the depth of the indentation formed on the surface is 0.2 μm or less. The machine part according to Claim 1.

7. The static load capacity on the surface is 6.0 GPa or more. The machine part according to Claim 1.

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