Machinery 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] The present invention relates to a machine part. [Background technology]

[0002] For example, JP 2013-119930 A (Patent Document 1) describes a bearing part. The bearing part described in Patent Document 1 comes into contact with other parts at its surface. The bearing part described in Patent Document 1 is made of steel that has been quenched and tempered. The steel contains 0.90 to 1.05 percent by mass of carbon, 0.15 to 0.35 percent by mass of silicon, 0.01 to 0.50 percent by mass of manganese, and 1.30 to 1.65 percent by mass of chromium, with the remainder being iron and unavoidable impurities.

[0003] In the bearing part described in Patent Document 1, the surface is subjected to a nitriding treatment, and the nitrogen concentration in the surface is 0.25 mass percent or more. In the bearing part described in Patent Document 1, the amount of retained austenite in the surface is 6 volume percent or more and 12 volume percent or less. In the bearing part described in Patent Document 1, tempering is performed at a high temperature, thereby improving the static load capacity in the surface. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2013-119930 A Summary of the Invention [Problem to be solved by the invention]

[0005] Bearings are used in motors, gearboxes, differentials, etc. installed in electric vehicles (BEVs). Bearings are also used in electric VTCs (variable valve mechanisms), electric compressors, transmissions, axles, etc. installed in plug-in hybrid vehicles (PHVs), hybrid vehicles (HVs) and internal combustion vehicles. If the static load capacity of bearings is improved in these applications, it will be possible to reduce the size of the bearings and, in turn, the size of the surrounding mechanical parts.

[0006] Improving the static load capacity also enables bearings to be made smaller in size for bearings mounted on hydrogen-utilizing equipment used in hydrogen environments such as fuel cell vehicles (FCVs) and hydrogen stations, potentially improving the efficiency of the hydrogen-utilizing equipment. However, the bearing component described in Patent Document 1 leaves room for improvement in the static load capacity on the surface.

[0007] The present invention has been made in view of the above-mentioned problems in the prior art. More specifically, the present invention provides a mechanical component having an improved static load capacity on its surface. [Means for solving the problem]

[0008] The mechanical component of the present invention is made of a steel having a surface and having been quenched and tempered. The mechanical component is provided with a nitriding layer formed on the surface. The steel contains 0.95 to 1.10 mass percent carbon, less than 0.30 mass percent silicon, less than 0.50 mass percent manganese, less than 0.0080 mass percent sulfur, 1.3 to 1.6 mass percent chromium, and the balance being iron and inevitable 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 mechanical component, the half width in an X-ray profile of martensite obtained by X-ray diffraction of the surface may be 7.2° or more and 8.0° or less. The position of a peak representing a {220} plane in an X-ray profile of austenite obtained by X-ray diffraction of the surface may be 128° or more.

[0010] In the above machine part, the dislocation density of martensite on the surface is 1.1×10 15 m -2 The dislocation density of the austenite at the surface may be 2.5×10 14 m -2 It may be more than that.

[0011] In the above machine part, 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 designated as A (unit: volume percent), B (unit: mass percent), and C (unit: m -2 ), the relationship 4.332+0.005×A−0.580×B−0.295×LogC≦0 may be satisfied.

[0012] In the above machine part, 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 designated as A (unit: volume percent), B (unit: mass percent), and D (unit: m -2 ), the relationship -47.73-0.025×A-2.141×B+3.155×LogD≧0 may be satisfied.

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

[0014] The mechanical component of the present invention makes it possible to improve the static load capacity at the surface. [Brief description of the drawings]

[0015] [Figure 1] FIG. [Diagram 2] 3A to 3C are process diagrams showing a manufacturing method of the inner ring 10. [Diagram 3] 1 is a graph showing the relationship between the maximum contact pressure and the indentation depth divided by the diameter of the ceramic ball on the surfaces of Samples 1 to 6. [Figure 4] 2 is a cross-sectional view of a ball screw 20. FIG. [Diagram 5] 1 is a cross-sectional view of a rolling bearing 100. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and redundant description will not be repeated.

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

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

[0019] FIG. 1 is a cross-sectional view of an inner ring 10. As shown in FIG. 1, the inner ring 10 has a surface. More specifically, the inner ring 10 has, as its surfaces, a width surface 10a, a width surface 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d. The central axis of the inner ring 10 is defined as 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 surface 10a and the width surface 10b constitute the end surfaces of the inner ring 10 in the axial direction. The width surface 10b is the opposite surface to the width surface 10a. The inner peripheral surface 10c extends along the circumferential direction. The inner peripheral surface 10c faces the central axis A. One end and the other end of the inner peripheral surface 10c in the axial direction are connected to the width surface 10a and the width surface 10b, respectively. The inner peripheral surface 10c of the inner ring 10 is fitted onto a shaft (not shown).

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

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

[0023] The inner ring 10 is made of steel that has been hardened and tempered. That is, the steel that constitutes the inner ring 10 contains martensite and retained austenite. The surfaces of the inner ring 10 (width faces 10a, width faces 10b, inner peripheral face 10c, and outer peripheral face 10d) have been subjected to nitriding treatment. That is, a nitriding layer 11 is formed on the surface of the inner ring 10. Note that nitrogen is dissolved in the steel in the nitriding layer 11, and does not form nitrides in the nitriding layer 11.

[0024] The steel constituting the inner ring 10 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. 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 have to contain silicon, manganese, or sulfur.

[0025] Bearing steel is an example of the steel that constitutes the inner ring 10. Specific examples of bearing steel include SUJ2 defined in the JIS standard, 52100 defined in the ASTM standard, 100Cr6 defined in the ISO standard, and GCr15 defined in the GB standard.

[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 known 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. This measurement is performed using a Cr tube type X-ray diffraction device. In the Cr tube type X-ray diffraction device, the wavelength of Cr-Kα radiation 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 x 2 mm.

[0028] The hardness of the surface of the inner ring 10 is 850Hv or more. The hardness of the surface of the inner ring 10 is measured in accordance with the Vickers hardness test method defined in the JIS standard. The load used when measuring the hardness of the surface of the inner ring 10 is 300g. The hardness of 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-width in the X-ray profile of martensite obtained by X-ray diffraction on the surface of the inner ring 10 is preferably 7.2° or more and 8.0° or less. The position (2θ) of the peak showing the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction on the surface of the inner ring 10 is preferably 128° or more. The lattice spacing of the austenite on the surface of the inner ring 10 is preferably 1.275×10 -10 m or less.

[0030] The X-ray profile of martensite and the X-ray profile of austenite obtained by X-ray diffraction on the surface of the inner ring 10 are obtained by using a Cr tube type X-ray diffraction device. In the Cr tube type X-ray diffraction device, the wavelength of Cr-Kα radiation 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 on the surface of the inner ring 10 is measured within a range of 2θ of 142.75° or more and 170.8° or less, and the X-ray profile of austenite obtained by X-ray diffraction on the surface of the inner ring 10 is measured within a range of 2θ of 114.75° or more and 142.8° or less.

[0031] Background processing is performed on the X-ray profile of martensite and the X-ray profile of austenite obtained by X-ray diffraction on the surface of the inner ring 10. The position of the peak indicating the {220} plane in the X-ray profile of austenite obtained by X-ray diffraction on the surface of the inner ring 10 is determined from the center position of the half-width.

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

[0033]

number

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

[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 obtained using a Co tube type X-ray diffractometer. In the Co tube type X-ray diffractometer, the wavelength of Co-Kα radiation is 1.7889×10 -10 The tube diameter is set to 40 mm, 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 on the surface of the inner ring 10 are measured within a range of 2θ of 30° or more and 135° or less.

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

[0037]

number

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

[0039] The depth of the indentation formed when a maximum contact pressure of 4.5 GPa is applied to the surface (raceway surface 10da) of the inner ring 10 is preferably 0.2 μm or less. The depth of this indentation is measured by contacting a ceramic ball with the surface of the inner ring 10 using an autograph so that the maximum contact pressure is 4.5 GPa, and then measuring the surface of the inner ring 10 using the white light interference function of a laser microscope. The load application speed is 3 N / sec, and the load is maintained for 120 seconds after the applied load is reached.

[0040] The static load capacity on the surface of the inner ring 10 is preferably 6.0 GPa or more. In measuring the static load capacity on the surface of the inner ring 10, first, as described above, an autograph is used to change the maximum contact pressure to form an indentation on the surface of the inner ring 10. Second, for each change in maximum contact pressure, the value obtained by dividing the depth of the indentation by the diameter of the ceramic ball is calculated. The maximum contact pressure at which the value obtained by dividing the depth of the indentation by the diameter of the ceramic ball is 1 / 10,000 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 dislocation density of the austenite on the surface of the inner ring 10 are designated as A (unit: volume percent), B (unit: mass percent), and C (unit: m -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 defined as D (unit: m -2 On the surface of the inner ring 10, it is preferable that the relationship: -47.73-0.025×A-2.141×B+3.155×LogD≧0 is satisfied.

[0043] (Manufacturing method of the inner ring 10) A method for manufacturing the inner ring 10 will now be described.

[0044] Fig. 2 is a process diagram showing a manufacturing method of the inner ring 10. As shown in Fig. 2, the manufacturing method of the inner ring 10 includes 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 workpiece is prepared. The nitriding step S2 is performed after the preparation step S1. In the nitriding step S2, the workpiece is held at a temperature equal to or higher than the A1 transformation point in an atmospheric gas containing a nitrogen source. The nitriding step S2 is performed so that nitrogen penetrates and diffuses to a position that will become the surface of the inner ring 10 after the post-treatment step S6. The quenching step S3 is performed after the nitriding step S2. In the quenching step S3, the workpiece is held at a temperature equal to or higher than the A1 transformation point and then quenched with 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 above -100°C and below room temperature. As a result, part of the retained austenite formed in the steel constituting the workpiece in the quenching step S3 is transformed into martensite. Instead of the sub-zero treatment step S4, a cryo-treatment step S7 may be performed. In the cryo-treatment step S7, the workpiece is cooled to a temperature below -100°C. As a result, part of the retained austenite formed in the steel constituting the workpiece in the quenching step S3 is transformed into martensite.

[0047] The coolant used in the sub-zero treatment step S4 and the cryo-treatment step S7 is, for example, liquid nitrogen, liquid helium, etc. Preferably, the sub-zero treatment step S4 and the cryo-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 (cryo-treatment step S7). In the tempering step S5, the workpiece is held at a temperature below the A1 transformation point. This causes a portion of the martensite in the steel that constitutes the workpiece to decompose. 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 process S6 is carried out after the tempering process S5. In the post-treatment process S6, machining (for example, grinding and polishing) is carried out on the surface of the workpiece to be processed. In this manner, the inner ring 10 having the structure shown in FIG. 1 is manufactured.

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

[0051] As described in Patent Document 1, it is possible to improve the static load capacity of the surface of the raceway by tempering at high temperatures (e.g., 230°C or higher). However, in this case, the martensite formed by quenching is decomposed excessively by tempering, which reduces the hardness of the surface of the raceway, and the static load capacity of the surface of the raceway may become insufficient.

[0052] On the other hand, in the inner ring 10, nitrogen is dissolved in the nitriding layer 11, so that the surface of the inner ring 10 is solution strengthened. Also, in the inner ring 10, the sub-zero treatment step S4 (cryo-treatment step S7) is performed, so that the amount of retained austenite on the surface is reduced (martensite is increased) and the dislocation density in the retained austenite and martensite on the surface is increased. As a result, the hardness of the surface of the inner ring 10 is 850 Hv or more. Therefore, according to the inner ring 10, the static load capacity on the surface is improved compared to when tempering is performed at high temperatures.

[0053] More specifically, since the amount of retained austenite on the surface of inner ring 10 is 20 volume percent or less, the average nitrogen concentration on the surface of inner ring 10 is 0.10 mass percent or more, and the hardness on the surface of 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 inner ring 10 will be 0.2 μm or less, and the static load capacity on the surface of inner ring 10 will be 6.0 GPa or more.

[0054] Nitrogen is an austenite stabilizing element, and nitriding is performed to enrich the retained austenite on the surface of the raceway rings. Therefore, when nitriding is performed, sub-zero treatment (or cryo-treatment) to reduce the amount of retained austenite on the surface is not usually performed.

[0055] (Static load capacity and indentation depth evaluation) Samples 1 to 6 were prepared to evaluate the static load capacity on the surface of the mechanical component according to the embodiment. Samples 1 to 6 were flat plates with a diameter of 85 mm and a thickness of 5 mm. Samples 1 to 6 were made of SUJ2 as defined by the JIS standard. Table 1 shows details of the heat treatment for Samples 1 to 6.

[0056] [Table 1]

[0057] For Sample 1, the heat treatments were performed as follows: nitriding process S2, quenching process S3, cryo-treatment process S7, and tempering process S5. For Sample 2 and Sample 3, the heat treatments were performed as follows: nitriding process S2, quenching process S3, sub-zero treatment process S4, and tempering process S5. That is, Samples 1 to 3 are samples simulating the mechanical component 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 treatments were performed as follows: nitriding step S2, quenching step S3, and tempering step S5, but the sub-zero treatment step S4 (or the cryo-treatment step S7) was not performed. In addition, in the tempering step S5 for Sample 4, tempering was performed at a high temperature (specifically, at 230°C).

[0059] For sample 5, the heat treatments performed were the nitriding treatment step S2, the quenching treatment step S3, and the tempering treatment step S5, and the sub-zero treatment step S4 (or the cryo-treatment step S7) was not performed. In the tempering step S5 for sample 5, the holding temperature was set to 180°C. For sample 6, the heat treatments performed were the quenching treatment step S3 and the tempering step S5, and the nitriding treatment step S2 and the sub-zero treatment step S4 (or the cryo-treatment step S7) were not performed. In the tempering 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 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 indentation depth when a maximum contact 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 pressure on the surfaces of Samples 1 to 6 and the indentation depth divided 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 at the surface is 0.10 mass percent or more, the hardness at the surface is 850 Hv or more, and the amount of retained austenite at the surface is 20 volume percent or less, which are defined as Condition 1, Condition 2, and Condition 3, respectively. As shown in Table 2, Samples 1 to 3 all satisfied Condition 1, Condition 2, and Condition 3. On the other hand, Samples 4 to 6 did not satisfy at least any of Condition 1, Condition 2, and Condition 3.

[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 more than 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 the static load capacity on the surface was improved by satisfying Conditions 1, 2, and 3.

[0065] Samples 1-1 to 1-3, Samples 2-1 to 2-3, and Samples 3-1 to 3-3 are samples that have been subjected to the same heat treatment as Sample 1, Sample 2, and Sample 3, respectively. Samples 4-1 to 4-4 are samples that have been subjected to the same heat treatment as Sample 4. The steel type and shape of these samples are SUJ2 and a flat plate with a diameter of 85 mm and a thickness of 5 mm, respectively.

[0066] [Table 3]

[0067] As shown in Table 3, Samples 1-1 to 1-3, Samples 2-1 to 2-3, and Samples 3-1 to 3-3 all satisfied Condition 1, Condition 2, and Condition 3. On the other hand, Samples 4-1 to 4-4 did not satisfy at least any of Condition 1, Condition 2, and Condition 3.

[0068] In Samples 1-1 to 1-3, Samples 2-1 to 2-3, and Samples 3-1 to 3-3, the half-width in the X-ray profile of martensite obtained by X-ray diffraction on 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 on the surface was 128° or more.

[0069] On the other hand, in Samples 4-1 to 4-4, the half-width in the X-ray profile of martensite obtained by X-ray diffraction on 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 on the surface was less than 128°.

[0070] This comparison revealed that when Condition 1, Condition 2, and Condition 3 are satisfied, the half-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.

[0071] From another perspective, it has become clear that when the sub-zero treatment step S4 or the cryo-treatment step S7 is performed without performing the high-temperature tempering step S5, the dislocation density of martensite on the surface increases, causing an increase in the half-width in the X-ray profile, and the dislocation density of austenite on the surface increases, causing the peak position representing the {220} plane to shift to the higher angle side.

[0072] As shown in Table 4, in Samples 1, 2, and 3, the half-width in the X-ray profile of martensite obtained by X-ray diffraction on 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 on the surface was 128° or more. 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 Above 2.5×10 14 m -2 It was more than that.

[0073] On the other hand, in Sample 4, the half-width in the X-ray profile of martensite obtained by X-ray diffraction on 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 on the surface was less than 128°. In addition, in Sample 4, the dislocation density of martensite and the dislocation density of austenite on the surface were 1.1×10 15 m -2 Less than or equal to 2.5 x 10 14 m -2 It had become less than that.

[0074] [Table 4]

[0075] The depth of the indentation when a maximum contact pressure of 4.5 GPa is 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 pressure of 4.5 GPa is 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 ) 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 made 0.2 μm or less.

[0076] The static load capacity of 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 -2As 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 ) 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 made 6.0 GPa or more.

[0077] (Another example of a mechanical component according to an embodiment) Below, examples of machine components according to embodiments other than the inner ring 10 will be described.

[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 system of the balls 23 in the ball screw 20 is not particularly limited. The circulation system of the balls 23 in the ball screw 20 may be, 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.

[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 extending 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 so that the outer peripheral surface 21a faces the inner peripheral surface 22a. The ball 23 is 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 a seal member 24. The screw shaft 21 also passes through a hole formed in the seal member 24.

[0080] The screw shaft 21, the ball nut 22, and the balls 23 are made of steel that has been hardened and tempered. This steel contains 0.95 to 1.10 mass percent carbon, less than 0.30 mass percent silicon, less than 0.50 mass percent manganese, less than 0.0080 mass percent sulfur, and 1.3 to 1.6 mass percent chromium. The balance of the steel is iron and unavoidable impurities. This steel is, for example, bearing steel (SUJ2 as specified in the JIS standard, 52100 as specified in the ASTM standard, 100Cr6 as specified in the ISO standard, and GCr15 as specified in the GB standard, etc.).

[0081] The screw shaft 21, the ball nut 22, and the ball 23 have a nitriding layer 11 (not shown in FIG. 4) formed on their surfaces, and the average nitrogen concentration at the surfaces is 0.10 mass percent or more. The screw shaft 21, the ball nut 22, and the ball 23 have a hardness at their surfaces of 850 Hv or more. The screw shaft 21, the ball nut 22, and the ball 23 have an amount of retained austenite at their surfaces of 20 volume percent or less. In other words, the screw shaft 21, the ball nut 22, and the ball 23 are mechanical components according to the embodiment.

[0082] The ball screw 20 can have a high load capacity and can also be made smaller and lighter because the static load capacity of the surfaces of the screw shaft 21, the ball nut 22, and the balls 23 is improved. The miniaturization of the ball screw 20 also enables the miniaturization of peripheral parts and structural members.

[0083] However, it is sufficient that at least one of the screw shaft 21, the ball nut 22, and the balls 23 satisfy the above-mentioned steel composition, hardness at the surface, nitrogen concentration at the surface, and amount of retained austenite at the surface. From another perspective, it is sufficient that at least one of the screw shaft 21, the ball nut 22, and the balls 23 of the ball screw 20 is a mechanical component 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 balls 23, and the ball nut 22 moves along the direction of the central axis of the screw shaft 21. In other words, the ball screw 20 is a device that converts the rotational motion of a motor or the like 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, and the like.

[0085] (Rolling bearing 100 using inner ring 10) 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 peripheral surface 30c, and an outer peripheral surface 30d. The width surface 30a and the width surface 30b constitute the end surfaces of the outer ring 30 in the axial direction. The width surface 30b is the opposite surface to the width surface 30a. The inner peripheral surface 30c and the outer peripheral surface 30d extend along the circumferential direction. The inner peripheral surface 30c and the outer peripheral surface 30d face the central axis A side and the opposite side to the central axis A, respectively. The outer ring 30 is fitted into a housing (not shown) at the outer peripheral surface 30d. The outer ring 30 is disposed radially outward of the inner ring 10 so that the inner peripheral surface 30c faces the outer peripheral surface 10d.

[0087] The inner peripheral surface 30c has a raceway surface 30ca. The raceway surface 30ca is a portion of the inner peripheral surface 30c that contacts the rolling element 40. The raceway surface 30ca is located at the center of the inner peripheral surface 30c in the axial direction. The raceway surface 30ca extends along the circumferential direction. The inner peripheral surface 30c is recessed toward the outer peripheral 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 disposed between the raceway surface 10da and the raceway surface 30ca. The rolling elements 40 are arranged at intervals in the circumferential direction. The cage 50 holds the rolling elements 40 such that the interval between two adjacent rolling elements 40 is 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 from steel that has been quenched and tempered, and 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, 1.3 mass percent or more and 1.6 mass percent or less of chromium, and the balance is iron and inevitable impurities. In addition, the outer ring 30 and the rolling elements 40 have an average nitrogen concentration, hardness, and amount of retained austenite on the surface of 0.10 mass percent or more, 850 Hv or more, and 20 volume percent or less, respectively. However, the outer ring 30 and the rolling elements 40 do not have to be mechanical components according to the embodiment.

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

[0091] 10 inner ring, 10a, 10b width surface, 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 screw groove, 22 ball nut, 22a inner peripheral surface, 22b screw groove, 23 ball, 24 seal member, 30 outer ring, 30a, 30b width surface, 30c inner peripheral surface, 30ca raceway surface, 30d outer peripheral surface, 40 rolling element, 50 cage, A central shaft, S1 preparation process, S2 nitriding process, S3 hardening process, S4 sub-zero treatment process, S5 tempering process, S6 post-treatment process, S7 cryo-treatment process.

Claims

1. A machine part made of hardened and tempered steel having a surface that contacts other parts, A nitriding layer is formed on the surface, The steel contains 0.95 to 1.10 percent by weight of carbon, less than 0.30 percent by weight of silicon, less than 0.50 percent by weight of manganese, less than 0.0080 percent by weight of sulfur, and 1.3 to 1.6 percent by weight of chromium, with the balance being iron and unavoidable impurities; The half-width in an X-ray profile of martensite obtained by X-ray diffraction on the surface is 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 is 128° or more; X-ray diffraction on the surface has a wavelength of 2.29093×10 －１０ Mechanical parts are examined using Cr-Kα radiation at 1000m.

2. The dislocation density of martensite on the surface is 1.1×10 １５ m －２ That's all. The dislocation density of the austenite on the surface is 2.5×10 １４ m －２ The mechanical component according to claim 1 .

3. The amount of retained austenite on the surface, the average nitrogen concentration on the surface, and the dislocation density of the austenite on the surface are designated as A (unit: volume percent), B (unit: mass percent), and C (unit: mass percent), respectively. －２ 2. The mechanical component according to claim 1, wherein the relationship 4.332+0.005×A−0.580×B−0.295×LogC≦0 is satisfied.

4. 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 designated as A (unit: volume percent), B (unit: mass percent), and D (unit: mass percent), respectively. －２ 2.) The mechanical part according to claim 1, wherein the relationship of -47.73-0.025×A-2.141×B+3.155×LogD≧0 is satisfied.

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

6. The mechanical component according to claim 1 , wherein the static load capacity of the surface is 6.0 GPa or more.

7. 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 machine component according to any one of claims 1 to 6.

8. The present invention includes a screw shaft, a ball nut, and a ball, 7. A ball screw, wherein at least one of the screw shaft, the ball nut, and the ball is the machine component according to any one of claims 1 to 6.

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