Shaft material

Abstract

To provide a shaft member in which plastic bending hardly occurs.SOLUTION: The shaft member is a steel shaft member having a surface. The steel contains carbon by 0.10 mass % or more and 0.40 mass % or less, silicon by 0.10 mass % or more and 2.50 mass % or less, manganese by 0.30 mass % or more and 1.20 mass % or less, chromium by 0.40 mass % or more and 3.00 mass % or less, and molybdenum by 1.00 mass % or less, and the balance is iron and unavoidable impurities. The distance between the surface of the shaft member and the first position where the hardness of the steel is 653Hv is 0.2 mm or more and 1.0 mm or less. The nitrogen concentration on the surface is 0.2 mass % or more and 1.2 mass % or less, and the carbon concentration on the surface is 0.6 mass % or more and 1.2 mass % or less. In the X-ray diffraction peak, the half value width of the peak of the martensite crystal (211) plane at the second position where the depth Z (unit: mm) from the surface is 0.085D with respect to the diameter D (unit: mm) of the shaft member is 6.5° or less.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a shaft member.

Background Art

[0002] Japanese Patent Application Laid-Open No. 2010-1521 (Patent Document 1) describes a shaft. The shaft described in Patent Document 1 is a pinion shaft for a planetary gear mechanism. In the manufacturing method of the shaft described in Patent Document 1, a carbo-nitriding treatment, quenching, sub-zero treatment, and tempering are sequentially performed on the surface (raceway surface). As a result, in the shaft described in Patent Document 1, the surface fatigue resistance performance is improved, and the amount of retained austenite in the core is 0% by volume, making plastic bending less likely to occur.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the shaft described in Patent Document 1, in order to suppress plastic deformation of the shaft accompanying thermal decomposition of retained austenite in the core when used in a high-temperature environment, the amount of retained austenite in the core is set to 0% by volume.

[0005] However, there is room for improvement in the plastic bending performance (difficulty in occurring plastic bending) of the shaft described in Patent Document 1.

[0006] The main object of the present invention is to provide a shaft member in which plastic bending is less likely to occur.

Means for Solving the Problems

[0007] The shaft member according to the present invention is a steel shaft member having a surface. The steel contains 0.10% to 0.40% by mass of carbon, 0.10% to 2.50% by mass of silicon, 0.30% to 1.20% by mass of manganese, 0.40% to 3.00% by mass of chromium, and 1.00% by mass of molybdenum, with the remainder being iron and unavoidable impurities. The distance between the first position P1 where the hardness of the steel is 653 Hv and the outer circumferential surface 30a is 0.2 mm to 1.0 mm. The nitrogen concentration on the surface is 0.2% to 1.2% by mass. The carbon concentration on the surface is 0.6% to 1.2% by mass. In the X-ray diffraction peak, the full width at half maximum of the peak of the martensitic crystal (211) plane at the second position, where the depth Z (unit: mm) from the surface is 0.085D relative to the diameter D (unit: mm) of the shaft member, is 6.5° or less.

[0008] In the above-mentioned shaft member, the full width at half maximum (FWHM) of the peak of the martensitic crystal (211) plane at the third position, where the depth Z (unit: mm) is 0.017D, is wider than the FWHM at the second position in the X-ray diffraction peak.

[0009] In the above-described shaft member, it is preferable that the amount of retained austenite on the surface is 25% by volume or more and 40% by volume or less, and that the amount of retained austenite in the core portion located inside the first and second positions is 0.5% by volume or more and 3.0% by volume or less.

[0010] In the above-mentioned shaft member, the diameter D may be 6 mm or more and 30 mm or less.

[0011] The shaft member described above may also be a shaft member for a planetary gearbox. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a shaft member that is less prone to plastic bending. [Brief explanation of the drawing]

[0013] [Figure 1] This is a front view of a planetary gear system. [Figure 2]This is a cross-sectional view taken along line II-II in Figure 1. [Figure 3] This is an enlarged cross-sectional view of the shaft member near its outer surface. [Figure 4] This graph shows the relationship between the depth from the outer surface of samples 1-3 and the full width at half maximum of the peak on the martensite crystal (211) plane. [Modes for carrying out the invention]

[0014] Embodiments of the present invention will be described below with reference to the drawings. In the following, the same or corresponding parts will be denoted by the same reference numerals, and redundant descriptions will not be repeated.

[0015] <Configuration of a planetary gear system> As shown in Figures 1 and 2, the shaft member 30 in this embodiment is the pinion shaft of the planetary gear system 100.

[0016] The planetary gear system 100 comprises an internal gear 10, a shaft member 20, a sun gear 21, a shaft member 30, planetary gears 31, and a cage 40. The planetary gear system 100 is used, for example, in a reduction gear for an automobile transmission. In other words, the shaft member 30 is, for example, a shaft member for a planetary reduction gear.

[0017] The internal gear 10 has an annular shape. The internal gear 10 has an inner circumferential surface and an outer circumferential surface. Multiple teeth are formed on the inner circumferential surface of the internal gear 10 along the circumferential direction of the internal gear 10. The teeth of the internal gear 10 protrude radially inward from the inner circumferential surface of the internal gear 10.

[0018] The shaft member 20 has a cylindrical shape. The position of the central axis of the shaft member 20 coincides with the position of the central axis of the internal gear 10. The sun gear 21 has an inner peripheral surface and an outer peripheral surface. A plurality of teeth are formed on the outer peripheral surface of the sun gear 21 along the circumferential direction of the sun gear 21. The teeth of the sun gear 21 project from the outer peripheral surface of the sun gear 21 toward the radially outer side of the sun gear 21. A central hole penetrating the sun gear 21 in the thickness direction is formed at the central portion of the sun gear 21. The shaft member 20 is attached to the sun gear 21 by being fitted into the central hole of the sun gear 21.

[0019] The shaft member 30 has a cylindrical shape. The shaft member 30 has an outer peripheral surface 30a. The detailed configuration of the shaft member 30 will be described later. The planetary gear 31 is disposed between the internal gear 10 and the sun gear 21.

[0020] The planetary gear 31 has an inner peripheral surface 31a and an outer peripheral surface 31b. The inner wall surface of the central hole of the planetary gear 31 is the inner peripheral surface 31a. A plurality of teeth are formed on the outer peripheral surface 31b along the circumferential direction of the planetary gear 31. The teeth of the planetary gear 31 project from the outer peripheral surface 31b toward the radially outer side of the planetary gear 31. The teeth of the planetary gear 31 mesh with the teeth of the internal gear 10 and the teeth of the sun gear 21. A central hole penetrating the planetary gear 31 in the thickness direction is formed at the central portion of the planetary gear 31.

[0021] The shaft member 30 is fitted into the central hole of the planetary gear 31. That is, the shaft member 30 is a pinion shaft. The outer diameter D of the shaft member 30 is, for example, 6.0 mm or more and 30.0 mm or less. The shaft member 30 is rotatably supported by the inner peripheral surface 31a. More specifically, a plurality of rolling elements 32 are disposed between the outer peripheral surface 30a and the inner peripheral surface 31a.

[0022] The rolling element 32 is, for example, a needle roller. The rolling element 32 has a rolling surface 32a that contacts the outer circumferential surface 30a of the shaft member 30 and the inner circumferential surface 31a of the planetary gear 31. The outer diameter of the rolling element 32 is the outer diameter d. The outer diameter d is 0.5 times or less the outer diameter D. For example, the outer diameter d is 1.5 mm or more and less than 5.0 mm.

[0023] The rolling elements 32 are made of steel. The steel constituting the rolling elements 32 is, for example, high-carbon chromium bearing steel such as SUJ2 as specified in the JIS standard (JIS G 4805:2019).

[0024] Preferably, the nitrogen concentration on the rolling surface 32a is 0.1% by mass or more, from the viewpoint of improving surface fatigue resistance by enriching the amount of retained austenite and improving the resistance to tempering softening. Preferably, the nitrogen concentration on the rolling surface 32a is 0.7% by mass or less, from the viewpoint of suppressing the decrease in surface hardness that occurs when the amount of retained austenite is excessive.

[0025] The cage 40 is positioned between the shaft member 30 and the planetary gear 31 and holds each of the multiple rolling elements 32. The material constituting the cage 40 is not particularly limited. For example, the material constituting the cage 40 may be cold-rolled steel sheet (SPC) as specified in JIS standard (JIS G 3141:2017), hardened steel (SCM415, SNCM415, etc.) as specified in JIS standard (JIS G 4053:2016), or carbon steel pipe for machine structures (STKM) as specified in JIS standard (JIS G 3445:2016).

[0026] <Detailed configuration of the shaft member> The detailed configuration of the shaft member 30 is described below.

[0027] The shaft member 30 is made of steel. The steel constituting the shaft member 30 contains 0.10% to 0.40% by mass of carbon, 0.10% to 2.50% by mass of silicon, 0.30% to 1.20% by mass of manganese, 0.40% to 3.00% by mass of chromium, and 1.00% by mass or less of molybdenum. The statement that the steel contains 1.00% by mass or less of molybdenum means that the steel does not contain molybdenum, or that the steel contains 1.00% by mass or less of molybdenum. The remainder of the steel is iron and unavoidable impurities.

[0028] The carbon content in the steel is 0.4% by mass or less, from the viewpoint of keeping the dislocation density inside the shaft member 30 low and improving the plastic bending resistance performance. From the viewpoint of keeping the dislocation density inside the shaft member 30 even lower, the carbon content in the steel constituting the shaft member 30 is preferably 0.10% by mass or more and 0.25% by mass or less.

[0029] The silicon content in the steel is 0.10% by mass or more and 2.50% by mass or less, from the viewpoint of improving tempering softening resistance and promoting nitride deposition in the surface layer. The manganese content in the steel is 0.30% by mass or more and 1.20% by mass or less, from the viewpoint of improving hardenability and stabilizing austenite. The chromium content in the steel is 0.40% by mass or more and 3.00% by mass or less, from the viewpoint of improving hardenability and improving tempering softening resistance. The molybdenum content in the steel is 1.00% by mass or less, from the viewpoint of improving hardenability and improving tempering softening resistance.

[0030] The steel constituting the shaft member 30 is, for example, chromium-molybdenum steel such as SCM420, SCM425, SCM430, and SCM435 as specified in the JIS standard (JIS G 4053:2016).

[0031] The above chemical composition of the steel constituting the shaft member 30 is measured using EPMA (Electron Probe Microanalyzer).

[0032] As shown in Figure 3, in the shaft member 30, the position where the Vickers hardness of the steel is 653 HV is defined as the first position P1. The distance between the first position P1 where the hardness is 653 HV and the outer circumferential surface 30a is between 0.2 mm and 1.0 mm. From a different perspective, the shaft member 30 has a high-hardness region 30b where the Vickers hardness is 653 HV or higher. The high-hardness region 30b is formed in the radial direction of the shaft member 30 between the first position P1 and the outer circumferential surface 30a.

[0033] The Vickers hardness of the steel constituting the shaft member 30 is measured by the Vickers hardness test method specified in the JIS standard (JIS Z 2245:2009).

[0034] The maximum contact pressure when the rolling element 32 and the outer surface 30a are in contact is, for example, between 2000 MPa and 4000 MPa. In this case, the maximum shear stress due to contact with the surface of the raceway is applied at a depth greater than 0.20 mm and less than 1.0 mm from the outer surface 30a, i.e., within the high-hardness region 30b.

[0035] The shaft member 30 further has a core portion 30c located inside the high-hardness region 30b. The core portion 30c is a region in which the nitrogen and carbon content in the steel is constant regardless of the depth from the outer surface 30a. In other words, the nitrogen and carbon content in the steel is measured sequentially along the depth direction from the outer surface 30a, and the position where the measured nitrogen and carbon content becomes constant becomes the outer edge of the core portion 30c. The high-hardness region 30b is a part of the surface layer located outside the core portion 30c.

[0036] The nitrogen concentration on the outer surface 30a is 0.2% by mass or more, from the viewpoint of improving surface fatigue resistance by enriching the amount of retained austenite and improving tempering softening resistance. The nitrogen concentration on the outer surface 30a is 1.2% by mass or less, from the viewpoint of suppressing the decrease in surface hardness that occurs when the amount of retained austenite is excessive. Preferably, the nitrogen concentration on the outer surface 30a is 0.3% by mass or more and 0.7% by mass or less. The nitrogen concentration in the core portion 30c is lower than the nitrogen concentration on the outer surface 30a.

[0037] The carbon concentration of the outer surface 30a is 0.6% by mass or more, from the viewpoint of ensuring surface hardness. The carbon concentration of the outer surface 30a is 1.2% by mass or less, from the viewpoint of suppressing the formation of abnormal structures such as reticular cementite in the surface layer. Preferably, the carbon concentration of the outer surface 30a is 0.7% by mass or more and 0.9% by mass or less. The carbon concentration of the core 30c is lower than the carbon concentration of the outer surface 30a. The nitrogen concentration and carbon concentration of the outer surface 30a are measured using EPMA.

[0038] The second position P2 is defined as the position where the depth Z (in mm) from the outer surface 30a is 0.085D relative to the diameter D (in mm) of the shaft member 30. In the X-ray diffraction peak, the full width at half maximum (FWHM) of the peak of the martensite crystal (211) plane at the second position P2 is 6.5° or less. The X-ray diffraction peak is obtained by cutting the unused shaft member 30 with a plane perpendicular to the central axis and performing X-ray diffraction on the cross-section using an X-ray residual stress measuring device. The FWHM here is the average value of the FWHM (measured values) corresponding to the crystal orientation (211) of the martensite phase, measured using, for example, the Kα rays of a Cr tube with a tube voltage of 30kV, a tube current of 10mA, and incident angles (ψ angles) of 11.8°, 28.9°, 40.7°, and 51.8°, respectively.

[0039] Preferably, the full width at half maximum of the peak of the martensitic crystal (211) plane at the second position P2 is 6.0° or less.

[0040] The second position P2 is located outside the core portion 30c. The second position P2 is, for example, deeper than the first position P1 in the radial direction. However, the second position P2 may also be shallower than the first position P1 in the radial direction.

[0041] The third position P3 is defined as the position where the depth Z (in mm) from the outer peripheral surface 30a is 0.033D relative to the diameter D (in mm) of the shaft member 30. In the X-ray diffraction peaks, the full width at half maximum (FWHM) of the martensite crystal (211) plane peak at the third position P3 is wider than the FWHM of the martensite crystal (211) plane peak at the second position P2. The FWHM of the martensite crystal (211) plane peak at the third position P3 is 6.8° or less. Preferably, the FWHM of the martensite crystal (211) plane peak at the third position P3 is 6.2° or less.

[0042] The ratio (first ratio) of the decrease in the width at half maximum (in degrees) between the outer surface 30a and the third position P3 to the radial distance (in mm) between the outer surface 30a and the third position P3 is higher than the ratio (second ratio) of the decrease in the width at half maximum (in degrees) between the third position P3 and the second position P2 to the radial distance (in mm) between the third position P3 and the second position P2. The first ratio (in degrees / mm) is, for example, between 1 and 9. The second ratio (in degrees / mm) is, for example, between 0.1 and 1.

[0043] The amount of retained austenite on the outer circumferential surface 30a of the shaft member 30 is preferably 25 volume% or more, from the viewpoint of suppressing the height of the bulge around the indentation formed on the outer circumferential surface 30a when hard foreign matter (such as wear particles) is embedded. By suppressing the height of the bulge around the indentation, the lifespan of the planetary gear unit 100 is extended. The amount of retained austenite on the outer circumferential surface 30a is preferably 40 volume% or less, from the viewpoint of suppressing a decrease in surface hardness.

[0044] The amount of retained austenite in the core portion 30c is 0.3 volume% or more. The shaft member 30 is manufactured without heat treatment, such as sub-zero treatment or tempering treatment, to reduce the amount of retained austenite in the core portion 30c to 0 volume%. From the viewpoint of suppressing plastic bending of the shaft member 30 due to creep deformation of retained austenite, the amount of retained austenite in the core portion 30c is preferably 3 volume% or less.

[0045] The amount of retained austenite in steel is measured by X-ray diffraction. More specifically, the amount of retained austenite in steel is measured by comparing the integrated intensity of the X-ray diffraction peak of austenite in the steel with the integrated intensity of the X-ray diffraction peak of other phases in the steel.

[0046] The diameter D of the shaft member 30 is, for example, 6 mm or more and 30 mm or less. The outer diameter of the rolling element 32 is, for example, 1.5 mm or more and 5.0 mm or less.

[0047] The grain size number of the prior austenite crystal grains in the high-hardness region 30b of the shaft member 30, as defined by the JIS standard (JIS G 0551), is 9 or greater. The grain size number is measured by the method specified in the JIS standard (JIS G 0551:2020).

[0048] The compressive residual stress on the outer surface 30a of the shaft member 30 is 600 MPa or more. The residual stress is obtained by X-ray diffraction using an X-ray residual stress measuring device.

[0049] <Manufacturing method for shaft members> The manufacturing method for the shaft member 30 includes a preparation step S1, a carburizing step S2, a carburizing and nitriding step S3, a quenching step S4, a tempering step S5, and a post-processing step S6. The carburizing step S2 is performed after the preparation step S1. The carburizing and nitriding step S3 is performed after the carburizing step S2. The quenching step S4 is performed after the carburizing and nitriding step S3. The tempering step S5 is performed after the quenching step S4. The post-processing step S6 is performed after the tempering step S5.

[0050] In preparation step S1, the workpiece to be processed is prepared. The workpiece is in the shape of a rod. The workpiece is prepared, for example, by machining such as forging and turning to shape the raw material into a shape similar to the shaft member 30.

[0051] In the carburizing process S2, the surface of the workpiece is subjected to a carburizing treatment. The carburizing treatment is carried out by holding the workpiece at a temperature above the A1 transformation point of the steel constituting the workpiece in a heat treatment gas. The heat treatment gas may include, for example, an endothermic transformation gas (RX gas) and an enrichment gas that serves as a carbon source (e.g., propane (C3H8) gas, butane (C4H8) gas). 10 A product to which )) has been added is used. The holding temperature in this step S2 is, for example, 850°C to 940°C.

[0052] In the carburizing and nitriding process S3, the surface of the workpiece is subjected to carburizing and nitriding treatment. Carburizing and nitriding treatment is carried out by holding the workpiece at a temperature above the A1 transformation point of the steel constituting the workpiece in a heat treatment gas. The heat treatment gas may include, for example, an endothermic transformation gas (RX gas) and an enrichment gas that serves as a carbon source (e.g., propane (C3H8) gas, butane (C4H8) gas). 10 A material is used to which a nitrogen source gas (for example, ammonia (NH3) gas) has been added. The holding temperature in this step S3 is, for example, 850°C to 940°C. When the holding temperature is 950°C or higher, the decomposition of ammonia is promoted, the amount of undecomposed ammonia decreases, and the nitrogen concentration on the outer surface 30a tends to decrease. Preferably, the holding temperature in the carburizing and nitriding step S3 is the same as the holding temperature in the carburizing step S2. In this case, the atmosphere inside the furnace in the carburizing step S2 and the carburizing and nitriding step S3 is stable, so the nitrogen concentration and carbon concentration on the outer surface 30a tend to stabilize within the above numerical range. Note that the carburizing step S2 may be omitted in the manufacturing method of the shaft member 30.

[0053] In the quenching process S4, the workpiece is quenched. The quenching process involves holding the workpiece at a temperature above the A1 transformation point of the steel that makes up the workpiece, and then quenching the M of the steel that makes up the workpiece. S This is done by rapidly cooling to a temperature below the transformation point. The hardening is performed as a whole hardening by furnace heating, not by high-frequency induction hardening. Rapid cooling of the workpiece is performed, for example, by water cooling or oil cooling.

[0054] In the tempering process S5, the workpiece is tempered. Tempering is performed by holding the workpiece at a temperature below the A1 transformation point of the steel that makes up the workpiece. In the post-treatment process S6, the workpiece is post-treated. The holding temperature in this process S5 is, for example, 160°C to 200°C. This post-treatment includes machining (grinding, polishing, etc.), cleaning, and rust prevention of the surface of the workpiece. Through these steps, the shaft member 30 shown in Figures 1 and 2 is manufactured.

[0055] <Manufacturing method for rolling elements> The manufacturing method for the rolling element 32 includes, for example, the steps of preparing a workpiece molded to a shape similar to that of the rolling element 32, performing a full-body hardening on the workpiece, and tempering the workpiece after full-body hardening (through-hardening). Unlike the shaft member 30, the rolling element 32 does not have the problem of plastic bending. According to the above manufacturing method, existing heat treatment equipment (for example, a continuous furnace) can be used, and it becomes possible to mass-produce the rolling element 32 without increasing costs.

[0056] The manufacturing method for the rolling element 32 may involve a carburizing and nitriding process instead of an overall quenching process. After carburizing and nitriding, tempering is performed to ensure a surface nitrogen concentration of 0.1% by mass or more, thereby preventing surface damage to the needle rollers.

[0057] <Effects> The effect of the shaft member 30 will be explained based on a comparison with the shaft member in the comparative example.

[0058] The shaft member in the comparative example conforms to the shaft described in Patent Document 1, and is made of steel with a relatively high carbon content of 0.3% to 0.5% by mass, which is subjected to carburizing or carbonitriding and sub-zero treatment, resulting in a surface retained austenite content of 20% by volume or more and a core retained austenite content of 0. In such a comparative example shaft member, the dislocation density increases from the surface to the core. Therefore, when used in a high-temperature environment, even if the thermal decomposition of retained austenite in the core is suppressed, plastic bending due to the movement of dislocations closer to the surface is more likely to occur than in the core.

[0059] In contrast, the carbon content of the steel constituting the shaft member 30 is relatively low, with the carbon concentration on the outer surface 30a being between 0.6% and 1.2% by mass. In such a shaft member 30, carbon is supplied through carburizing and carbonitrinization treatments, so the carbon concentration and dislocation density decrease as you approach the core portion 30c from the outer surface 30a. Furthermore, in the shaft member 30, the full width at half maximum of the peak of the martensitic crystal (211) plane at the second position, where the depth Z (unit: mm) from the outer surface 30a is 0.085D relative to the diameter D (unit: mm) of the shaft member 30, is 6.5° or less in the X-ray diffraction peak. In other words, the dislocation density at the second position of the shaft member 30 is relatively low. Therefore, because the dislocation density at the second position and deeper positions is relatively low in the shaft member 30, plastic bending is less likely to occur even when used in a high-temperature environment compared to the shaft member of the comparative example.

[0060] Furthermore, in the shaft member 30, the distance between the first position P1 where the hardness of the steel is 653 Hv and the outer circumferential surface 30a is 0.2 mm or more and 1.0 mm or less.

[0061] When the shaft member 30 is the pinion shaft of an automobile planetary gearbox, the maximum contact pressure between the outer circumferential surface 30a of the shaft member 30 and the rolling surface 32a of the rolling element 32 is typically assumed to be between 3000 MPa and 4000 MPa. In this case, the maximum shear stress generated below the contact surface during rolling is applied to the high-hardness region 30b located between the first position P1 of the shaft member 30 and the outer circumferential surface 30a in the radial direction. In other words, the hardness of the steel in the shaft member 30 is sufficiently ensured near the location where the maximum shear stress occurs, thus improving surface fatigue resistance.

[0062] Furthermore, in the shaft member 30, the nitrogen concentration on the outer surface 30a is 0.2% by mass or more and 1.2% by mass or less, and the carbon concentration on the outer surface 30a is 0.6% by mass or more and 1.2% by mass or less. As a result, the hardness of the outer surface 30a is maintained while improving the tempering softening resistance, thus improving the surface fatigue resistance.

[0063] The amount of retained austenite in the core portion 30c of the shaft member 30 is 0.3 volume% or more. The inventors have confirmed that the shaft member 30 whose half-width satisfies the above numerical range exhibits improved plastic bending resistance compared to a shaft member whose core retained austenite amount is less than 0.3 volume% but whose half-width does not satisfy the above numerical range (details will be described later).

[0064] Furthermore, the shaft member 30 is manufactured without undergoing any heat treatment, such as sub-zero treatment or tempering treatment, to reduce the amount of retained austenite in the core to 0% by volume. Therefore, the manufacturing cost of the shaft member 30 does not increase compared to the manufacturing cost of the shaft member in the comparative example.

[0065] The diameter D of the shaft member 30 may be between 6 mm and 30 mm. If the diameter D of the shaft member 30 is relatively short within the above range, the second position P2 may be located between the first position P1 and the outer circumferential surface 30a. If the diameter D of the shaft member 30 is relatively long within the above range, the second position P2 is located deeper than the first position P1. In either case, the dislocation density at the second position P2 and deeper positions has a greater impact on the plastic bending performance than the dislocation density at shallower positions than the second position P2. Therefore, the plastic bending resistance of a shaft member 30 with a suppressed dislocation density at the second position P2 can be improved regardless of the value of the diameter D.

[0066] Since the planetary gear system 100 has a shaft member 30 as a pinion shaft, the rolling fatigue life of the pinion shaft and the plastic bending of the pinion shaft are suppressed.

[0067] (Examples) To confirm the effectiveness of the shaft member according to this embodiment, static bending tests and half-width evaluation tests were performed on shaft member samples 1 to 3.

[0068] (sample) As shown in Table 1, Sample 1 was a shaft member made of SCM420, Sample 2 was a shaft member made of SCr435 as specified in JIS standard (JIS G 4053 (2016)), and Sample 3 was a shaft member made of SUJ2. Samples 1 and 2 were prepared by applying the shaft member manufacturing method according to this embodiment described above to the workpiece made of each steel type. Sample 3 was prepared by performing overall quenching on the workpiece made of SUJ2 instead of the carburizing and carburizing / nitriding processes. The tempering temperature for Sample 1 was 170°C. The tempering temperature for Samples 2 and 3 was 180°C. The diameter D of each of Samples 1 to 3 was 18 mm, and the axial length was 73.9 mm.

[0069] [Table 1]

[0070] Table 1 shows the nitrogen concentration, carbon concentration, residual austenite content on the outer surface, surface hardness, residual austenite content in the core, depth from the outer surface at position P1 where the Vickers hardness is 653 HV, residual stress, and full width at half maximum at position P1, measured for samples 1 to 3. The measurement methods for each parameter are as described above.

[0071] In sample 1, the nitrogen content of the outer surface 30a was 0.58 mass%, and the carbon content of the outer surface 30a was 0.75 mass%. In sample 2, the nitrogen content of the outer surface 30a was 0.42 mass%, and the carbon content of the outer surface 30a was 0.80 mass%. In sample 3, the nitrogen content of the outer surface 30a was 0.13 mass%, and the carbon content of the outer surface 30a was 1.00 mass%.

[0072] In sample 1, the amount of retained austenite on the outer surface was 31 vol% and the amount of retained austenite in the core was 2 vol%. In sample 2, the amount of retained austenite on the outer surface was 30 vol% and the amount of retained austenite in the core was 2 vol%. In sample 3, the amount of retained austenite on the outer surface was 20 vol% and the amount of retained austenite in the core was 10 vol%.

[0073] In sample 1, the residual stress on the outer surface was -815 MPa. In sample 2, the residual stress on the outer surface was -670 MPa. In sample 3, the residual stress on the outer surface was -575 MPa.

[0074] As shown in Table 1 and Figure 4, in sample 1, the full width at half maximum (FWHM) at the second position was 5.2° and the FWHM at the third position was 5.6°. In sample 2, the FWHM at the second position was 5.9° and the FWHM at the third position was 6.1°. In sample 3, the FWHM at the second position was 7.0° and the FWHM at the third position was 6.9°.

[0075] (Evaluation of axial bending amount after static bending test and axial bending amount after rolling fatigue life test) A static bending test was performed on samples 1-3, and the amount of bending after the test was measured.

[0076] In the static bending test, each sample was supported at its axial center while not rotating around its central axis (rotation speed 0 revolutions / minute) in a furnace maintained at a temperature of 130°C. A total radial load of 15,000 N was applied to both ends of the axial direction for 200 hours. After the static bending test, the total axial length of each sample was measured using a contour tracer (profile measuring machine) to calculate the amount of axial bending on both the loaded and unloaded sides of each sample. Furthermore, the amount of axial bending for each sample was calculated as the average of the calculated axial bending amounts on the loaded and unloaded sides. The measurement results are shown in Table 2.

[0077] Similarly, rolling fatigue life tests were performed on each of samples 1 to 3, along with the caged needle roller bearing and the outer member. In the rolling fatigue life tests, lubrication was provided using 120°C lubricating oil (automatic transmission fluid), the radial load was 6670 N, the moment load was 13.5 N·m, and the relative rotational speed of the outer member with respect to the test shaft was 9000 rotations / min. After the rolling fatigue life tests, the amount of shaft bending of each sample was measured using the method described above. The measurement results are shown in Table 2.

[0078] [Table 2]

[0079] As shown in Table 2, the axial bending of each sample, Sample 1 and Sample 2, was less than that of Sample 3, which had a relatively higher amount of retained austenite in the core. Furthermore, between Sample 1 and Sample 2, which had similar amounts of retained austenite in the core, the axial bending of Sample 1, which had a relatively narrower full width at half maximum (FWHM) at the second position, was less than that of Sample 2, which had a relatively wider FWHM at the second position. From these results, it was experimentally confirmed that plastic bending can be suppressed as the FWHM at the second position narrows.

[0080] While embodiments of the present invention have been described above, various modifications of these embodiments are possible. Furthermore, the scope of the present invention is not limited to the embodiments described above. The scope of the present invention is indicated by the claims and is intended to include all modifications within the meaning and scope equivalent to the claims. [Explanation of symbols]

[0081] 10 Internal gear, 20, 30 Shaft members, 21 Sun gear, 30a, 31b Outer surface, 30b High hardness region, 30c Core, 31 Planetary gear, 31a Inner surface, 32 Rolling element, 32a Rolling surface, 40 Cage, 100 Planetary gear assembly.

Claims

1. A steel shaft member having a surface, The steel contains 0.10% by mass or more and 0.40% by mass or less of carbon, 0.10% by mass or more and 2.50% by mass or less of silicon, 0.30% by mass or more and 1.20% by mass or less of manganese, 0.40% by mass or more and 3.00% by mass or less of chromium, and 1.00% by mass or less of molybdenum, with the remainder being iron and unavoidable impurities. The distance between the surface and the first position where the hardness of the steel is 653 Hv is 0.2 mm or more and 1.0 mm or less. The nitrogen concentration on the surface is 0.2% by mass or more and 1.2% by mass or less. The carbon concentration of the surface is 0.6% by mass or more and 1.2% by mass or less. In the X-ray diffraction peak, the full width at half maximum of the peak of the martensitic crystal (211) plane at a second position where the depth Z (unit: mm) from the surface is 0.085D relative to the diameter D (unit: mm) of the shaft member is 6.5° or less. A shaft member in which the amount of residual austenite in the core portion located inside each of the first and second positions is 3.0 volume percent or less.

2. The shaft member according to claim 1, wherein, in the X-ray diffraction peak, the full width at half maximum of the peak of the martensitic crystal (211) plane at the third position, where the depth Z (unit: mm) is 0.033D, is wider than the full width at half maximum at the second position.

3. The amount of retained austenite on the surface is 25% by volume or more and 40% by volume or less. The shaft member according to claim 1 or 2, wherein the amount of residual austenite in the core portion located inside each of the first and second positions is 0.5 volume percent or more.

4. The shaft member according to any one of claims 1 to 3, wherein the diameter D is 6 mm or more and 30 mm or less.

5. The shaft member is a shaft member for a planetary gearbox, according to any one of claims 1 to 3.

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