machine parts

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] This invention relates to mechanical parts. [Background technology]

[0002] For example, Japanese Patent Publication No. 2013-119930 (Patent Document 1) describes a bearing component. The bearing component described in Patent Document 1 is in contact with other components on its surface. The bearing component described in Patent Document 1 is made of hardened and tempered steel. The steel contains 0.90 mass percent to 1.05 mass percent of carbon, 0.15 mass percent to 0.35 mass percent of silicon, 0.01 mass percent to 0.50 mass percent of manganese, and 1.30 mass percent to 1.65 mass percent of chromium, with the remainder being iron and unavoidable impurities.

[0003] In the bearing component described in Patent Document 1, the surface is subjected to nitrification treatment, and the nitrogen concentration on the surface is 0.25 mass percent or more. In the bearing component described in Patent Document 1, the amount of retained austenite on the surface is 6 volume percent or more and 12 volume percent or less. In the bearing component described in Patent Document 1, the static load capacity on the surface is improved by tempering at high temperature. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2013-119930 [Overview of the project] [Problems that the invention aims to solve]

[0005] Bearings are used in motors, gearboxes, differentials, and other components installed in electric vehicles (BEVs). Bearings are also used in electric VTC (variable valve timing), electric compressors, transmissions, and axles installed in plug-in hybrid vehicles (PHVs), hybrid vehicles (HVs), and conventional engine vehicles. Improving the static load capacity of bearings in these applications allows for miniaturization of the bearings themselves, and consequently, the surrounding mechanical components.

[0006] In bearings installed in hydrogen utilization equipment used in hydrogen environments such as fuel cell vehicles (FCVs) and hydrogen stations, improving static load capacity can enable miniaturization of the bearings, potentially improving the efficiency of hydrogen utilization equipment. However, the bearing component described in Patent Document 1 has room for improvement in its static load capacity on the surface.

[0007] This invention has been made in view of the problems of the prior art described above. More specifically, this invention provides a mechanical part with improved static load capacity on its surface. [Means for solving the problem]

[0008] The mechanical part of the present invention is made of steel that has a surface and has been hardened and tempered. The mechanical part has a nitrogen-filled layer formed on its surface. The steel contains 0.95 mass percent to 1.10 mass percent 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 to 1.6 mass percent of chromium, with the remainder being 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 with respect to 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 with respect to the surface may be 128° or more.

[0010] In the above mechanical part, 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 may be 2.5×10 14 m -2 or more.

[0011] In the above mechanical part, when the retained austenite amount 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 4.332 + 0.005×A - 0.580×B - 0.295×LogC ≤ 0 may be satisfied.

[0012] In the above mechanical part, 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 percentage), B (unit: mass percentage), and D (unit: m -2 ), respectively, the relationship -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] It is a cross-sectional view of the inner ring 10. [Figure 2] It is a process chart showing a method for manufacturing the inner ring 10. [Figure 3] It is a graph showing the relationship between the maximum contact surface pressure and the value obtained by dividing the indentation depth on the surfaces of Samples 1 to 6 by the diameter of the ceramic ball. [Figure 4] It is a cross-sectional view of the ball screw 20. [Figure 5] It is a cross-sectional view of the rolling bearing 100.

Embodiments for Carrying Out the Invention

[0016] Details of embodiments of the present invention will be described while referring to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions 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 circumferential surface 10c, and an outer circumferential 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 orthogonal 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. Width surface 10b is the opposite side of width surface 10a. The inner circumferential surface 10c extends along the circumferential direction. The inner circumferential surface 10c faces the central axis A. One end and the other end of the inner circumferential surface 10c in the axial direction are connected to width surfaces 10a and 10b, respectively. The inner ring 10 is fitted onto an axis (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 away from the central axis A. In other words, the outer circumferential surface 10d is the opposite surface to 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 connected to the width surfaces 10a and 10b, respectively.

[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 element. The raceway surface 10da is located in 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 of the raceway surface 10da. In a cross-sectional view perpendicular to the circumferential direction, the raceway surface 10da is, for example, partially arc-shaped.

[0023] The inner ring 10 is made of steel that has been hardened and tempered. That is, the steel constituting the inner ring 10 contains martensite and retained austenite. The surface of the inner ring 10 (width surface 10a, width surface 10b, inner circumferential surface 10c, and outer circumferential surface 10d) has been subjected to nitriding treatment. In other words, a nitriding layer 11 has been formed on the surface of the inner ring 10. Note that nitrogen is dissolved in the steel in the nitriding layer 11 and no nitrides have been formed in the nitriding layer 11.

[0024] The steel constituting the inner ring 10 contains 0.95 mass percent to 1.10 mass percent 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 to 1.6 mass percent of chromium. The remainder of the steel constituting the inner ring 10 consists of iron and unavoidable impurities. Note that the steel constituting the inner ring 10 does not necessarily have to contain silicon, manganese, and sulfur.

[0025] Specific examples of the steel that make up the inner ring 10 include bearing steel. Specific examples of bearing steel include SUJ2 specified in JIS standards, 52100 specified in ASTM standards, 100Cr6 specified in ISO standards, and GCr15 specified in GB standards.

[0026] The average nitrogen concentration on the surface of the inner ring 10 is 0.10 mass percent or higher. The average nitrogen concentration on the surface of the inner ring 10 is measured by linear analysis using an EPMA (Electron Probe Microanalyzer). A calibration curve is created using standard samples with known nitrogen concentrations.

[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. This measurement is performed using a Cr-tube X-ray diffractometer. In a Cr-tube X-ray diffractometer, the wavelength of Cr-Kα rays is 2.29093 × 10⁻¹⁰. -10 The tube voltage is set to 30kV, the tube current to 10mA, and the collimator size to 2mm x 2mm.

[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 according to the Vickers hardness test method defined in the JIS standard. The load when the hardness on the surface of the inner ring 10 is measured is 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 measured 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 30 kV, the tube current is 10 mA, and the collimator size is 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 within 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 within 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 spacing of austenite lattice planes on the surface of the inner ring 10 is calculated by applying the following Bragg equation to the austenite X-ray profile obtained by X-ray diffraction of the surface of the inner ring 10. In this equation, d is the spacing of austenite lattice planes on the surface of the inner ring 10 (unit: m), λ is the wavelength of the Cr-Kα line used in a Cr tube X-ray diffractometer (unit: m), and θ is the diffraction angle in the austenite X-ray profile obtained by X-ray diffraction of the surface of the inner ring 10 (unit: °).

[0033]

number

[0034] The dislocation density of martensite on the surface of the inner ring 10 is preferably 1.1 × 10⁻⁶. 15 m -2 That concludes the explanation. The austenite dislocation density on the surface of the inner ring 10 is preferably 2.5 × 10⁻⁶. 14 m -2 That's all.

[0035] The dislocation densities of martensite and austenite on the surface of the inner ring 10 are measured using a Co-tube X-ray diffractometer. More specifically, firstly, the X-ray profiles of martensite and austenite on the surface of the inner ring 10 are obtained using a Co-tube X-ray diffractometer. In a Co-tube X-ray diffractometer, the wavelength of Co-Kα rays is 1.7889 × 10⁻⁶. -10 The tube voltage is set to 40kV, the tube current to 50mA, and the collimator size to a diameter of 1mm. The X-ray profiles of martensite and austenite obtained by X-ray diffraction on the surface of the inner ring 10 are measured within the range of 2θ from 30° to 135°.

[0036] Secondly, after Rietveld analysis is performed, 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. Thirdly, by applying this crystallite size and strain to the following Williamson-Hall equation, the dislocation density of martensite and austenite on the surface of the inner ring 10 is obtained. In this equation, ρ is the dislocation density (unit: m -2 ) where ε is the strain mentioned above and b is the length of the Burgers vector (b = 0.25 × 10⁻¹⁰). -9 m).

[0037]

number

[0038] In the X-ray profile of martensite obtained by X-ray diffraction of the surface of the inner ring 10, the peaks of the {110}, {200}, {211}, and {220} planes are measured. In the X-ray profile of austenite obtained by X-ray diffraction of the surface of the inner ring 10, the peaks of the {111}, {200}, {220}, {311}, and {222} planes are measured. The Rietveld analysis is performed in the above to reduce the influence of the {200} plane of martensite and the {200} plane of austenite, which have different elastic moduli.

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

[0040] The static load capacity on the surface of the inner ring 10 is preferably 6.0 GPa or higher. In measuring the static load capacity on the surface of the inner ring 10, firstly, as described above, an indentation is formed on the surface of the inner ring 10 by changing the maximum contact pressure using an autograph. Secondly, for each changed maximum contact pressure, the value obtained by dividing the depth of the indentation by the diameter of the ceramic sphere is calculated. The maximum contact pressure at which the value obtained by dividing the depth of the indentation by the diameter of the ceramic sphere is 1 / 10000 is the static load capacity on the surface of the inner ring 10.

[0041] The amount of retained austenite on the surface of the inner ring 10, the average nitrogen concentration on the surface of the inner ring 10, and the austenite dislocation density on the surface of the inner ring 10 are expressed as A (unit: volume percent), B (unit: mass percent), and C (unit: m³), ​​respectively. -2 ) . In the inner ring 10, it is preferable that the relationship 4.332 + 0.005 × A - 0.580 × B - 0.295 × LogC ≤ 0 is satisfied.

[0042] The dislocation density of martensite on the surface of the inner ring 10 is given by D (unit: m -2 ) . It is preferable that the relationship -47.73 - 0.025 × A - 2.141 × B + 3.155 × LogD ≥ 0 is satisfied on the surface of the inner ring 10.

[0043] (Method for manufacturing the inner ring 10) The manufacturing method for the inner ring 10 is 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 includes a preparation step S1, a nitrogen 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 preparation step S1, the workpiece to be processed is prepared. Nitriling treatment step S2 is performed after preparation step S1. Nitriling treatment step S2 is performed by holding the workpiece to be processed at a temperature above the A1 transformation point in an atmospheric gas containing a nitrogen source. Nitriling treatment step S2 is performed so that nitrogen penetrates and diffuses to the position that will become the surface of the inner ring 10 after post-processing step S6. Quenching step S3 is performed after quenching treatment step S2. In quenching step S3, after the workpiece to be processed is held at a temperature above the A1 transformation point, M S It is cooled to a temperature below the transformation point.

[0046] The sub-zero treatment step S4 is performed after the quenching step S3. In the sub-zero treatment step S4, the workpiece is cooled to a temperature below -100°C and below room temperature. This causes some of the retained austenite formed in the steel constituting the workpiece during the quenching step S3 to transform 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 workpiece is cooled to a temperature below -100°C. This causes some of the retained austenite formed in the steel constituting the workpiece during the quenching step S3 to transform into martensite.

[0047] For example, liquid nitrogen or liquid helium can be used as the refrigerant in the sub-zero treatment step S4 and the cryo treatment step S7. Preferably, the sub-zero treatment step S4 and the cryo treatment step S7 are performed within 2 hours after the completion of the quenching step S3.

[0048] The tempering process S5 is performed after the sub-zero treatment process S4 (cryo-treatment process S7). In the tempering process S5, the workpiece is held at a temperature below the A1 transformation point. This causes a portion of the martensite in the steel that makes up the workpiece to decompose. The holding temperature in the tempering process S5 is, for example, between 180°C and 220°C. The holding time in the tempering process S5 is, for example, 2 hours.

[0049] The post-processing step S6 is performed after the tempering step S5. In the post-processing step S6, machining (e.g., grinding and polishing) is performed on the surface of the workpiece. Through these steps, the inner ring 10 with the structure shown in Figure 1 is manufactured.

[0050] (Effect of inner ring 10) The effects of the inner circle 10 are explained below.

[0051] As described in Patent Document 1, tempering at high temperatures (e.g., 230°C or higher) can be considered to improve the static load capacity on the surface of the raceway. However, in this case, the martensite formed by quenching may be excessively decomposed by tempering, reducing the hardness on the surface of the raceway and potentially resulting in insufficient static load capacity on the surface of the raceway.

[0052] On the other hand, in the inner ring 10, the surface of the inner ring 10 is solid-solution strengthened because nitrogen is solid-dissolved in the nitriding layer 11. In addition, in the inner ring 10, the sub-zero treatment process S4 (cryo-treatment process S7) is performed, which reduces the amount of retained austenite on the surface (increases the amount of martensite), and increases the dislocation density in the retained austenite and martensite on the surface. As a result, the hardness of the surface of the inner ring 10 is 850 Hv or higher. Therefore, the static load capacity on the surface of the inner ring 10 is improved compared to when tempering is performed at high temperatures.

[0053] More specifically, by ensuring that 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 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] Nitrogen is an austenite-stabilizing element, and nitrification is performed to enrich retained austenite on the surface of the orbital rings. Therefore, when nitrification is performed, sub-zero treatment (or cryo-treatment) to reduce the amount of retained austenite on the surface is not usually carried out.

[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 machine parts according to the embodiment. 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 from SUJ2 as specified in the JIS standard. Table 1 shows the details of the heat treatment for samples 1 to 6.

[0056] [Table 1]

[0057] In Sample 1, the heat treatment consisted of a nitrogen immersion process S2, a quenching process S3, a cryogenic treatment process S7, and a tempering process S5. In Samples 2 and 3, the heat treatment consisted of a nitrogen immersion process S2, a quenching process S3, a sub-zero treatment process S4, and a tempering process S5. In other words, Samples 1 to 3 are samples that mimic the machine parts according to the embodiment. In the tempering process S5 for Samples 1 to 3, the holding temperature was set to 180°C.

[0058] In Sample 4, the heat treatment consisted of nitrification (S2), quenching (S3), and tempering (S5), but the sub-zero treatment (S4) (or cryotherapy (S7)) was not performed. Furthermore, in the tempering process (S5) for Sample 4, tempering was performed at a high temperature (specifically, at 230°C).

[0059] In Sample 5, the heat treatment consisted of nitrification (S2), quenching (S3), and tempering (S5), but the sub-zero treatment (S4) (or cryogenic treatment (S7)) was not performed. For Sample 5, the holding temperature in tempering (S5) was set to 180°C. In Sample 6, the heat treatment consisted of quenching (S3) and tempering (S5), but the nitrification (S2) and sub-zero treatment (S4) (or cryogenic treatment (S7)) were not performed. For Sample 5, the holding temperature in tempering (S5) was set to 180°C.

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

[0061] The average nitrogen concentration on the surface, surface hardness, amount of retained austenite on the surface, indentation depth when a maximum contact pressure of 4.5 GPa is 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 pressure on the surface of samples 1 to 6 and the indentation depth divided by the diameter of the ceramic sphere. The diameter of the ceramic sphere was set to 3 / 8 inch (9.525 mm).

[0062] [Table 2]

[0063] Conditions 1, 2, and 3 were defined as follows: an average nitrogen concentration of 0.10 mass percent or more on the surface, a hardness of 850 Hv or more on the surface, and a retained austenite content of 20 volume percent or less on the surface. As shown in Table 2, all three conditions (1, 2, and 3) were met in samples 1 through 3. On the other hand, at least one of conditions (1, 2, and 3) was not met in samples 4 through 6.

[0064] As shown in Table 1 and Figure 3, in samples 1 to 3, the indentation depth when a maximum contact pressure of 4.5 GPa was applied to the surface 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, the indentation depth when a maximum contact pressure of 4.5 GPa was applied to the surface was greater than 0.2 μm, and the static load capacity on the surface was less than 6.0 GPa. From this comparison, it was experimentally clear that the static load capacity on the surface is improved when conditions 1, 2, and 3 are met.

[0065] Samples 1-1 to 1-3, 2-1 to 2-3, and 3-1 to 3-3 are samples that underwent the same heat treatment as Sample 1, Sample 2, and Sample 3, respectively. Samples 4-1 to 4-4 are samples that underwent the same heat treatment as Sample 4. The steel type and shape of these samples are 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, all three conditions (1, 2, and 3) were met in samples 1-1 to 1-3, samples 2-1 to 2-3, and samples 3-1 to 3-3. On the other hand, in samples 4-1 to 4-4, at least one of the three conditions (1, 2, and 3) was not met.

[0068] For samples 1-1 to 1-3, samples 2-1 to 2-3, and samples 3-1 to 3-3, the full width at half maximum (FMAX) of the martensite X-ray profile obtained by X-ray diffraction of the surface was between 7.2° and 8.0°, and the peak position indicating the {220} plane in the austenite X-ray profile obtained by X-ray diffraction of the surface was 128° or higher.

[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 of 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 of the surface was less than 128°.

[0070] This comparison revealed that when conditions 1, 2, and 3 are met, the full width at half maximum (FWHM) of the X-ray profile of martensite obtained by X-ray diffraction of the surface is between 7.2° and 8.0°, 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 greater.

[0071] From another perspective, it became clear that when the high-temperature tempering process S5 is omitted and the sub-zero treatment process S4 or cryo-treatment process S7 is performed, the dislocation density of martensite on the surface increases, increasing the full width at half maximum in the X-ray profile, and the dislocation density of austenite on the surface increases, causing the peak position showing the {220} plane to shift to the higher angle side.

[0072] As shown in Table 4, in Samples 1, 2, and 3, the full width at half maximum (FMAX) of the martensite X-ray profile obtained by X-ray diffraction of the surface was between 7.2° and 8.0°, and the peak position indicating the {220} plane in the austenite X-ray profile obtained by X-ray diffraction of the surface was 128° or higher. In addition, in Samples 1, 2, and 3, the dislocation density of martensite and austenite on the surface was 1.1 × 10⁻⁶, respectively. 15 m -2 The above and 2.5 × 10 14 m -2 That was all.

[0073] On the other hand, in Sample 4, the full width at half maximum (FWHM) of the martensite X-ray profile obtained by X-ray diffraction of the surface was less than 7.2°, and the peak position indicating the {220} plane in the austenite X-ray profile obtained by X-ray diffraction of the surface was less than 128°. In addition, in Sample 4, the dislocation density of martensite and austenite on the surface was 1.1 × 10⁻¹⁰, respectively. 15 m -2 Less than and 2.5 × 10 14 m -2 It was less than [amount missing].

[0074] [Table 4]

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

[0076] Static load capacity on the surface and amount of retained austenite on the surface (A: unit is volume percent), average nitrogen concentration on the surface (B: unit is mass percent), and dislocation density of martensite on the surface (D: unit is m -2A multiple regression analysis was performed to examine the relationship between the static load capacity on the surface (in GPa) and the given conditions. As a result of this multiple regression analysis, the static load capacity on the surface (in GPa) was estimated using the following equation: -41.73 - 0.025 × A - 2.141 × B + 3.155 × LogD. The coefficient of determination (R) of this equation was calculated using the formula R. 2 ) is 0.94. Therefore, if 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 6.0 GPa or more.

[0077] (Other examples of machine parts according to the embodiment) Examples of mechanical parts according to embodiments other than the inner ring 10 are described below.

[0078] Figure 4 is a cross-sectional view of the ball screw 20. As shown in Figure 4, the ball screw 20 has a screw shaft 21, a ball nut 22, a plurality of balls 23, and a sealing member 24. The circulation method of the balls 23 in the ball screw 20 is not particularly limited. Examples of the circulation methods of the balls 23 in the ball screw 20 include a tube type, a return tube (pipe) type, a deflector type, an end deflector type, an end cap type, a ball-type, etc.

[0079] The screw shaft 21 has an outer circumferential surface 21a. A screw groove 21b is formed on the outer circumferential surface 21a. The ball nut 22 has a hole that extends in the direction of the central axis of the screw shaft 21. The inner wall surface of this hole is the inner circumferential surface 22a of the ball nut 22. A screw groove 22b is formed on the inner circumferential surface 22a. The screw shaft 21 is inserted into the ball nut 22 so that the outer circumferential surface 21a faces the inner circumferential surface 22a. The ball 23 is positioned between the screw grooves 21b and 22b. The hole in the ball nut 22 through which the screw shaft 21 passes is closed by a sealing member 24. The screw shaft 21 also passes through a hole formed in the sealing member 24.

[0080] The screw shaft 21, ball nut 22, and ball 23 are formed from hardened and tempered steel. This steel contains 0.95 mass percent to 1.10 mass percent 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 to 1.6 mass percent of chromium. The remainder of this steel consists of iron and unavoidable impurities. This steel is, for example, bearing steel (SUJ2 as specified in JIS standards, 52100 as specified in ASTM standards, 100Cr6 as specified in ISO standards, and GCr15 as specified in GB standards, etc.).

[0081] The screw shaft 21, ball nut 22, and ball 23 have a nitrogen-infiltrating layer 11 (not shown in Figure 4) formed on their surface, with an average nitrogen concentration of 0.10 mass percent or more on the surface. The screw shaft 21, ball nut 22, and ball 23 have a surface hardness of 850 Hv or more. The amount of retained austenite on the surface of the screw shaft 21, ball nut 22, and ball 23 is 20 volume percent or less. In other words, the screw shaft 21, ball nut 22, and ball 23 are machine parts according to this embodiment.

[0082] The ball screw 20 has improved static load capacity on the surfaces of the screw shaft 21, ball nut 22, and ball 23, enabling higher load capacity and miniaturization. The miniaturization of the ball screw 20 also allows for miniaturization of surrounding components and structural members.

[0083] However, it is sufficient that at least one of the screw shaft 21, ball nut 22, and ball 23 satisfies the above-mentioned steel composition, surface hardness, nitrogen concentration on the surface, and amount of retained austenite on the surface. In other words, the ball screw 20 is fine as long as at least one of the screw shaft 21, ball nut 22, and ball 23 is a machine part according to the embodiment.

[0084] By rotating the screw shaft 21 around its central axis, the rotational power of the screw shaft 21 is transmitted to the ball nut 22 via the ball 23, causing the ball nut 22 to move along the direction of the central axis of the screw shaft 21. In other words, the ball screw 20 is a device that converts rotational motion from a motor or the like into linear motion. The ball screw 20 is used in, for example, 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) Figure 5 is a cross-sectional view of the rolling bearing 100. As shown in Figure 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 side 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 towards the central axis A and away from the central axis A, respectively. The outer ring 30 is fitted into the housing (not shown) at the outer circumferential surface 30d. The outer ring 30 is positioned radially outward of the inner ring 10 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 element 40. The raceway surface 30ca is located in 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 perpendicular to the circumferential direction, the raceway surface 30ca is, for example, partially arc-shaped. The raceway surface 30ca faces the raceway surface 10da.

[0088] The rolling elements 40 are, for example, spherical. The rolling elements 40 are arranged between the raceway surface 10da and the raceway surface 30ca. Multiple rolling elements 40 are arranged with spacing between them in the circumferential direction. The retainer 50 holds the multiple rolling elements 40 such that the spacing between two adjacent rolling elements 40 is within a certain range. The retainer 50 is arranged between the outer circumferential surface 10d and the inner circumferential surface 30c.

[0089] The outer ring 30 and rolling elements 40 are, for example, machine parts according to the embodiment. That is, the outer ring 30 and rolling elements 40 are formed from hardened and tempered steel, which contains 0.95 mass percent to 1.10 mass percent 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 to 1.6 mass percent of chromium, with the remainder being iron and unavoidable impurities. Furthermore, the outer ring 30 and rolling elements 40 have an average nitrogen concentration, hardness, and retained austenite content of 0.10 mass percent or more, 850 Hv or more, and 20 volume percent or less on their surface. However, the outer ring 30 and rolling elements 40 do not have to be machine parts according to the embodiment.

[0090] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the embodiments described above, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of symbols]

[0091] 10 Inner ring, 10a, 10b width surface, 10c inner circumferential surface, 10d outer circumferential surface, 10da raceway surface, 11 Nitrogenized layer, 20 Ball screw, 21 Screw shaft, 21a outer circumferential surface, 21b screw groove, 22 Ball nut, 22a inner circumferential surface, 22b screw groove, 23 Ball, 24 Seal member, 30 Outer ring, 30a, 30b width surface, 30c inner circumferential surface, 30ca raceway surface, 30d outer circumferential surface, 40 Rolling element, 50 Cage, A Central shaft, S1 Preparation process, S2 Nitrogenized treatment process, S3 Hardening 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 hardened and tempered, The surface is provided with a nitrogen-infiltrating layer formed thereon, The steel contains 0.95 mass percent to 1.10 mass percent 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 to 1.6 mass percent of chromium, with the remainder being iron and unavoidable impurities. The dislocation density of martensite on the aforementioned surface is 1.1 × 10¹⁵ m⁻² or greater. A mechanical part having an austenite dislocation density of 2.5 × 10¹⁴ m⁻² or greater on the aforementioned surface.

2. The amount of retained austenite on the surface, the average nitrogen concentration on the surface, and the austenite dislocation density on the surface are expressed as A (unit: volume percent), B (unit: mass percent), and C (unit: m³), ​​respectively. －２ The machine part according to claim 1, wherein the relationship 4.332 + 0.005 × A - 0.580 × B - 0.295 × Log C ≤ 0 is satisfied when this is done.

3. 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 defined as A (unit: volume percent), B (unit: mass percent), and D (unit: m³), ​​respectively. －２ The machine part according to claim 1, wherein the relationship -47.73 - 0.025 × A - 2.141 × B + 3.155 × LogD ≥ 0 is satisfied when )

4. The mechanical part according to claim 1, wherein the depth of the indentation formed on the surface when a maximum contact pressure of 4.5 GPa is applied to the surface is 0.2 μm or less.

5. The mechanical part according to claim 1, wherein the static load capacity on the surface is 6.0 GPa or more.

6. comprising 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 machine component described in any one of claims 1 to 5.

7. comprising a screw shaft, a ball nut, and a ball, A ball screw in which at least one of the screw shaft, the ball nut, and the ball is the machine component described in any one of claims 1 to 5.

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