Shaft material

Abstract

To provide a shaft member capable of suppressing creep deformation and improving a rolling fatigue life under a high-temperature environment where a moment load is applied.SOLUTION: The shaft member is made of steel, and provided with an outer peripheral surface which comes into contact with a rolling element. In a region where a distance from the outer peripheral surface in the radial direction of the shaft member is equal to or smaller than a first distance, hardness is 653 Hv or higher. In a region where the distance from the outer peripheral surface in the radial direction of the shaft member is equal to or larger than a second distance, a retained austenite amount is 7 volume percent or lower. The first distance is larger than that in the radial direction from the outer peripheral surface to a position where maximum shearing stress becomes 650 MPa when the rolling element comes into contact with the outer peripheral surface. The second distance is 1.5 times or less of the distance in the radial direction from the outer peripheral surface to the position where maximum shearing stress becomes 650 MPa when the rolling element comes into contact with the outer peripheral surface.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

[0002] Patent Document 1 (JP 2010-1521 A) describes a pinion shaft. The pinion shaft described in Patent Document 1 has a core with 0 volume percent retained austenite. The pinion shaft described in Patent Document 1 has an outer peripheral surface with a hardness of 650 Hv or more.

[0003] Pinion shafts are used in high-temperature environments where moment loads are applied. As a result, pinion shafts can undergo creep deformation with use. In the pinion shaft described in Patent Document 1, the amount of retained austenite in the core is 0 volume percent, which suppresses the occurrence of creep deformation with use. In addition, in the pinion shaft described in Patent Document 1, the hardness of the outer peripheral surface is 650 Hv or more, which improves the rolling fatigue life. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-1521 Summary of the Invention [Problem to be solved by the invention]

[0005] However, after extensive research by the present inventors, it was found that there is room for improvement in the creep deformation resistance and rolling contact fatigue life of the pinion shaft described in Patent Document 1. In other words, in the pinion shaft described in Patent Document 1, the depth of the surface-hardened layer present on the outer peripheral surface and the distance from the outer peripheral surface to the core are not specified, which may result in insufficient creep deformation resistance and rolling contact fatigue life.

[0006] The present invention has been made in consideration of the above-mentioned problems of the prior art. More specifically, the present invention provides a shaft member that can suppress creep deformation and improve rolling fatigue life in a high-temperature environment where a moment load is applied. [Means for solving the problem]

[0007] A shaft member according to a first aspect of the present invention is made of steel and has an outer peripheral surface that contacts rolling elements. In a region of the shaft member that is a first distance or less from the outer peripheral surface in the radial direction, the hardness is 653 Hv or more. In a region of the shaft member that is a second distance or more in the radial direction, the amount of retained austenite is 7 volume percent or less. The first distance is greater than the radial distance from the outer peripheral surface to a position where the maximum shear stress when the rolling elements contact the outer peripheral surface is 650 MPa. The second distance is 1.5 times or less the radial distance from the outer peripheral surface to a position where the maximum shear stress when the rolling elements contact the outer peripheral surface is 650 MPa.

[0008] A shaft member according to a second aspect of the present invention is made of steel and has an outer circumferential surface that contacts rolling elements. In a region of the shaft member that is a first distance or less from the outer circumferential surface in the radial direction, the hardness is 653 Hv or more. In a region of the shaft member that is a second distance or more in the radial direction, the amount of retained austenite is 7 volume percent or less. The first distance is 0.038 times or more the diameter of the shaft member when the diameter is less than 12 mm, 0.03 times or more the diameter when the diameter is 12 mm or more but less than 16 mm, 0.025 times or more the diameter when the diameter is 16 mm or more but less than 20 mm, and 0.02 times or more the diameter when the diameter is 20 mm or more. The second distance is not more than 0.04 times the diameter when the diameter is less than 12 mm, not more than 0.038 times the diameter when the diameter is 12 mm or more but less than 16 mm, not more than 0.03 times the diameter when the diameter is 16 mm or more but less than 20 mm, and not more than 0.025 times the diameter when the diameter is 20 mm or more.

[0009] A shaft member according to a third aspect of the present invention is made of steel and has an outer circumferential surface that contacts rolling elements. In a region of the shaft member that is a first distance or less from the outer circumferential surface in the radial direction, the hardness is 653 Hv or more. In a region of the shaft member that is a second distance or more in the radial direction, the amount of retained austenite is 7 volume percent or less. The first distance is 0.02 times or more the diameter of the shaft member when the diameter is 12 mm or less. The first distance is 0.015 times or more the diameter of the shaft member when the diameter is greater than 12 mm. The second distance is 0.025 times or less the diameter of the shaft member when the diameter is 12 mm or less. The second distance is 0.02 times or less the diameter of the shaft member when the diameter is greater than 12 mm.

[0010] In the above shaft member, the steel may contain 0.10 weight percent or more and 0.40 weight percent or less of carbon, 0.10 weight percent or more and 2.50 weight percent or less of silicon, 0.30 weight percent or more and 1.20 weight percent or less of manganese, 1.20 weight percent or less of chromium, and 0.30 weight percent or less of molybdenum.

[0011] In the above shaft member, the steel may contain 0.90 weight percent or more and 1.20 weight percent or less of carbon, 0.10 weight percent or more and 2.50 weight percent or less of silicon, 0.50 weight percent or less of manganese, 1.20 weight percent or more and 1.70 weight percent or less of chromium, and 0.08 weight percent or less of molybdenum.

[0012] In the above shaft member, the steel may contain 0.90 weight percent to 1.20 weight percent carbon, 0.10 weight percent to 2.50 weight percent silicon, 0.80 weight percent to 1.30 weight percent manganese, 0.80 weight percent to 1.30 weight percent chromium, and 0.08 weight percent or less molybdenum.

[0013] The shaft member may have an outer peripheral surface that has been subjected to carburizing or carbo-nitriding. The amount of retained austenite on the outer peripheral surface may be 10 volume percent or more and 40 volume percent or less. The shaft member may be a pinion shaft for a planetary gear device. [Effects of the Invention]

[0014] According to the shaft members according to the first and second aspects of the present invention, it is possible to suppress creep deformation and improve rolling fatigue life in a high temperature environment where a moment load is applied. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a front view of the planetary gear device 100. [Figure 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Figure 3] 3 is an enlarged cross-sectional view of the shaft member 30 in the vicinity of the outer peripheral surface 30a. [Figure 4] 10 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.015 times the outer diameter D1. [Figure 5] 10 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.02 times the outer diameter D1. [Figure 6] 10 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.025 times the outer diameter D1. [Figure 7] 10 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.03 times the outer diameter D1. [Figure 8] 10 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.038 times the outer diameter D1. [Figure 9] 3A to 3C are process diagrams showing a manufacturing method of the shaft member 30. [Figure 10] FIG. 1 is a schematic diagram for explaining the details of a four-point bending test. [Figure 11] 1 is a graph showing the relationship between the amount of retained austenite in a test piece W and the amount of warpage of the test piece W in a four-point bending test. [Figure 12] 1 is a graph showing the relationship between the change in the amount of retained austenite in a test piece W before and after a four-point bending test and the amount of warpage of the test piece W. [Figure 13] 10 is a graph showing the relationship between the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 and the maximum shear stress. DETAILED DESCRIPTION OF THE INVENTION

[0016] The details of the embodiment will be described with reference to the drawings. Here, the same or corresponding parts are denoted by the same reference numerals, and redundant description will not be repeated.

[0017] (Configuration of planetary gear device according to embodiment and shaft member according to embodiment) The configurations of the planetary gear device according to the embodiment (hereinafter referred to as "planetary gear device 100") and the shaft member according to the embodiment (hereinafter referred to as "shaft member 30") will be described below.

[0018] Fig. 1 is a front view of a planetary gear device 100. Fig. 2 is a cross-sectional view taken along line II-II in Fig. 1. As shown in Figs. 1 and 2, the planetary gear device 100 has an internal gear 10, a shaft member 20, a sun gear 21, a shaft member 30, planetary gears 31, and a carrier 40 (not shown in Fig. 1). The planetary gear device 100 is used, for example, in a reducer for an automobile transmission.

[0019] The internal gear 10 has an annular shape. The internal gear 10 has an inner peripheral surface and an outer peripheral surface. A plurality of teeth are formed on the inner peripheral surface of the internal gear 10 along the circumferential direction of the internal gear 10. The teeth of the internal gear 10 protrude from the inner peripheral surface of the internal gear 10 toward the radially inward direction of the internal gear 10.

[0020] 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 protrude from the outer peripheral surface of the sun gear 21 toward the radially outward direction of the sun gear 21. A central hole is formed in the center of the sun gear 21, penetrating the sun gear 21 in the thickness direction. The shaft member 20 is attached to the sun gear 21 by being fitted into the central hole of the sun gear 21.

[0021] The shaft member 30 has a cylindrical shape. The shaft member 30 has an outer peripheral surface 30a. A 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. A central hole is formed in the center of the planetary gear 31, penetrating the planetary gear 31 in the thickness direction. The outer diameter of the shaft member 30 is an outer diameter D1.

[0022] The planetary gear 31 has an inner peripheral surface 31a and an outer peripheral surface 31b. The inner wall surface of the center 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 protrude 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.

[0023] The shaft member 30 is inserted into the center hole of the planetary gear 31. In other words, the shaft member 30 is a so-called pinion shaft. The shaft member 30 is rotatably supported by the inner peripheral surface 31a. More specifically, a plurality of rolling elements 32 are arranged between the outer peripheral surface 30a and the inner peripheral surface 31a. From another perspective, the outer peripheral surface 30a is the surface of the shaft member 30 that contacts the rolling elements 32. The rolling elements 32 are, for example, needle rollers. The outer diameter of the rolling elements 32 is an outer diameter D2. The outer diameter D2 is 0.5 times or less the outer diameter D1. The carrier 40 is fixed to one axial end of the shaft member 30.

[0024] By rotating the shaft member 20, which serves as the input shaft, about its central axis, the sun gear 21 rotates about the central axis of the shaft member 20. Because the teeth of the planetary gears 31 mesh with the teeth of the sun gear 21 and the teeth of the internal gear 10, the planetary gears 31 revolve around the sun gear 21 as the sun gear 21 rotates. The revolution of the planetary gears 31 is transmitted to the carrier 40 via the shaft member 30, causing an output shaft (not shown) fixed to the carrier 40 to rotate about its central axis. In this way, according to the planetary gear set 100, the rotation of the input shaft is decelerated and then transmitted to the output shaft.

[0025] <Detailed configuration of shaft member according to embodiment> The detailed configuration of the shaft member 30 will be described below.

[0026] The shaft member 30 is made of steel. The steel constituting the shaft member 30 is preferably steel having the following first composition, second composition or third composition.

[0027] As shown in Table 1, the steel of the first composition contains 0.10 to 0.40 weight percent carbon, 0.10 to 2.50 weight percent silicon, 0.30 to 1.20 weight percent manganese, 1.20 weight percent or less chromium, and 0.30 weight percent or less molybdenum. Note that the steel of the first composition may not contain chromium or molybdenum. The remainder of the steel of the first composition is iron and unavoidable impurities.

[0028] [Table 1]

[0029] As shown in Table 2, the steel of the second composition contains 0.90 to 1.20 weight percent carbon, 0.10 to 2.50 weight percent silicon, 0.50 to 0.50 weight percent manganese, 1.20 to 1.70 weight percent chromium, and 0.08 to 0.08 weight percent molybdenum. Note that the steel of the second composition may not contain molybdenum. The remainder of the steel of the second composition is iron and unavoidable impurities.

[0030] [Table 2]

[0031] As shown in Table 3, the steel of the second composition contains 0.90 to 1.20 weight percent carbon, 0.10 to 2.50 weight percent silicon, 0.80 to 1.30 weight percent manganese, 0.80 to 1.30 weight percent chromium, and 0.08 weight percent or less molybdenum. Note that the steel of the third composition does not necessarily contain molybdenum. The balance of the steel of the third composition is iron and unavoidable impurities.

[0032] [Table 3]

[0033] Fig. 3 is an enlarged cross-sectional view of the shaft member 30 near the outer circumferential surface 30a. Fig. 3 shows a cross section passing through the central axis of the shaft member 30. As shown in Fig. 3, the shaft member 30 has a first region 30b and a second region 30c.

[0034] The first region 30b is located on the outer peripheral surface 30a. The first region 30b is a region that is a distance L1 or less from the outer peripheral surface 30a in the radial direction of the shaft member 30. The hardness of the first region 30b is 653 Hv or more (58 HRC or more).

[0035] The position where the maximum shear stress when the rolling element 32 and the outer peripheral surface 30a come into contact is 650 MPa is defined as position P. Distance L1 is greater than the radial distance of the shaft member 30 from the outer peripheral surface 30a to position P. The maximum contact surface pressure when the rolling element 32 and the outer peripheral surface 30a come into contact is, for example, 2000 MPa or more and 4000 MPa or less.

[0036] The maximum shear stress when the rolling element 32 comes into contact with the outer peripheral surface 30a is a function of the distance from the outer peripheral surface 30a by substituting equations (2) and (3) into equation (1). Therefore, the position P can be determined based on this function.

[0037]

number

[0038] The second region 30c is a region that is located at a distance L2 or more from the outer peripheral surface 30a in the radial direction of the shaft member 30. In the second region 30c, the amount of retained austenite is 7 volume percent or less. The amount of retained austenite in the second region 30c may be 0 volume percent.

[0039] The distance L2 is, for example, equal to or greater than the distance L1. However, the distance L2 may be smaller than the distance L1. From another perspective, the first region 30b and the second region 30c may overlap each other in the radial direction of the shaft member 30. If the distance L2 is too large, the second region 30c becomes too thin, making it impossible to suppress creep deformation. Therefore, the distance L2 is 1.5 times or less the distance from the outer peripheral surface 30a to the position P in the radial direction of the shaft member 30. The distance L2 is preferably 1.3 times or less the distance from the outer peripheral surface 30a to the position P in the radial direction of the shaft member 30.

[0040] Fig. 4 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.015 times the outer diameter D1. Fig. 5 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.02 times the outer diameter D1. Fig. 6 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.025 times the outer diameter D1.

[0041] Fig. 7 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.03 times the outer diameter D1. Fig. 8 is a graph showing the relationship between the maximum shear stress and the outer diameter D1 at a position where the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 is 0.038 times the outer diameter D1.

[0042] In calculating the graphs shown in Figures 4 to 8, the values ​​shown in Table 4 were applied to the maximum contact pressure between the rolling element 32 and the outer peripheral surface 30a, the outer diameter D1, the outer diameter D2, the contact width between the rolling element 32 and the outer peripheral surface 30a, Young's modulus, Poisson's ratio, and the friction coefficient.

[0043] [Table 4]

[0044] Taking into consideration the graphs shown in FIGS. 4 to 8 and the variations in the heat treatment for forming the first region 30b, exemplary minimum values ​​of the distance L1 and maximum values ​​of the distance L2 are calculated for each of the outer diameters D1 and D2, as shown in Tables 5 and 6.

[0045] [Table 5]

[0046] [Table 6]

[0047] The outer peripheral surface 30a is preferably subjected to carburizing treatment. The outer peripheral surface 30a may also be subjected to carbo-nitriding treatment. The carburizing treatment is preferably performed so that the carbon concentration in the outer peripheral surface 30a is 0.7 weight percent or more. The carbo-nitriding treatment is preferably performed so that the carbon concentration and nitrogen concentration in the outer peripheral surface 30a are 0.7 weight percent or more and 0.3 weight percent or more, respectively.

[0048] The amount of retained austenite on the outer peripheral surface 30a is preferably 10 to 40 volume percent. Note that if the amount of retained austenite in a region from the outer peripheral surface 30a to a distance of 0.01 times the outer diameter D1 is 10 to 40 volume percent, the statement "the amount of retained austenite on the outer peripheral surface 30a is 10 to 40 volume percent" is satisfied.

[0049] The hardness of the first region 30b is measured by the Vickers hardness test method defined in the JIS standard (JIS Z 2244:2009). The amount of retained austenite in the first region 30b and the second region 30c is measured by X-ray diffraction. That is, the amount of retained austenite can be obtained by comparing the integrated intensity of the X-ray diffraction peak of austenite with the integrated intensity of the X-ray diffraction peak of a phase other than austenite in the steel.

[0050] <Method of manufacturing shaft member according to embodiment> Fig. 9 is a process diagram showing a method for manufacturing the shaft member 30. As shown in Fig. 9, the method for manufacturing the shaft member 30 includes a preparation step S1, a carbo-nitriding treatment step S2, a quenching step S3, a tempering step S4, and a post-treatment step S5.

[0051] In the preparation step S1, a workpiece is prepared. In the carbo-nitriding step S2, the outer peripheral surface of the workpiece is carbo-nitrided. The carbo-nitriding is performed by heating and holding the workpiece in an atmosphere containing carbon and nitrogen. The atmospheric gas used for the carbo-nitriding includes, for example, RX gas, enriched gas, and ammonia gas. The heating temperature during the carbo-nitriding is, for example, a temperature equal to or higher than the A1 transformation point of the steel constituting the workpiece.

[0052] In the quenching step S3, first, the workpiece is heated and held at a temperature equal to or higher than the A1 transformation point. This generates austenite in the steel constituting the workpiece. In the quenching step S3, second, the workpiece is heated and held at a temperature equal to or higher than the A1 transformation point. S The steel is rapidly cooled to a temperature below the transformation point, whereby part of the austenite produced by the above-mentioned heating and holding process becomes martensite, and the remainder becomes retained austenite.

[0053] In the tempering step S4, the workpiece is heated and maintained at a temperature below the A1 transformation point. This decomposes some of the retained austenite remaining after the quenching step S3. In the post-treatment step S5, the workpiece is subjected to finishing (grinding, polishing, etc.) and cleaning. Through the above steps, a shaft member 30 having the structure shown in FIG. 3 is manufactured.

[0054] If it is desired to further reduce the amount of retained austenite in the workpiece, a deep-freezing step S6 may be further performed after the quenching step S3 and before the tempering step S4. f The steel sheet is cooled to a temperature equal to or lower than the transformation point, whereby a portion of the retained austenite remaining after the quenching step S3 is transformed into martensite.

[0055] The distances L1 and L2 are adjusted by appropriately adjusting the heating temperature and holding time in the tempering step S4 and the cooling temperature in the deep-cooling treatment step S6.

[0056] (Effects of the planetary gear device according to the embodiment and the shaft member according to the embodiment) The effects of the planetary gear device 100 and the shaft member 30 will be described below.

[0057] If the distance L1 is smaller than the radial distance of the shaft member 30 from the outer peripheral surface 30a to the position P, sufficient hardness cannot be ensured at the position P. As a result, the rolling fatigue life of the shaft member 30 may be insufficient. However, in the shaft member 30, the distance L1 is larger than the radial distance of the shaft member 30 from the outer peripheral surface 30a to the position P, so sufficient hardness (more specifically, a hardness of 653 Hv or more) can be ensured at the position P. Therefore, the rolling fatigue life of the shaft member 30 can be improved.

[0058] Fig. 10 is a schematic diagram for explaining the details of the four-point bending test. As shown in Fig. 10, the four-point bending test is performed by applying loads at positions P3 and P4 to a test piece W supported at positions P1 and P2.

[0059] The dimensions of the test piece W are 140 mm in length, 20 mm in width, and 3 mm in thickness. Positions P1 and P2 are positioned symmetrically with respect to the center of the test piece W in the longitudinal direction. The distance between positions P1 and P2 is 120 mm. The distance between positions P3 and P4 is 60 mm. Positions P3 and P4 are positioned symmetrically with respect to the center of the test piece W in the longitudinal direction. A load is applied to the test piece W so that the maximum bending stress applied to the test piece W is 200 MPa. This load is applied from the second surface Wb side of the test piece W toward the first surface Wa side. With the above load applied, the test piece W is held in the air at 130°C for 50 hours. The amount of warpage of the test piece W is the distance between the end of the second surface Wb in the longitudinal direction of the test piece W and the position on the second surface Wb at which the distance from the end is greatest.

[0060] Fig. 11 is a graph showing the relationship between the amount of retained austenite in the test specimen W in the four-point bending test and the amount of warpage of the test specimen W. As shown in Fig. 11, the smaller the amount of retained austenite in the test specimen W before the four-point bending test, the smaller the amount of warpage of the test specimen W. Therefore, by setting the amount of retained austenite in the second region 30c to 7 volume percent or less, creep deformation of the shaft member 30 can be suppressed.

[0061] However, if the distance L2 is greater than 1.5 times the radial distance of the shaft member 30 from the outer peripheral surface 30a to the position P, the second region 30c becomes too thin, and the effect of suppressing creep deformation achieved by setting the amount of retained austenite to 7 volume percent or less may be insufficient. In the shaft member 30, the distance L2 is 1.5 times or less the radial distance of the shaft member 30 from the outer peripheral surface 30a to the position P, so creep deformation is sufficiently suppressed.

[0062] Fig. 12 is a graph showing the relationship between the change in the amount of retained austenite in the test specimen W before and after the four-point bending test and the amount of warpage of the test specimen W. As shown in Fig. 12, there is no particular correlation between the change in the amount of retained austenite in the test specimen W before and after the four-point bending test and the amount of warpage of the test specimen W.

[0063] 11, when the test piece W is made of SUJ3 steel as specified in the JIS standard (JIS G 4805:2019), the amount of warpage of the test piece W is smaller than when the test piece W is made of SUJ2 steel as specified in the JIS standard. In other words, the alloying elements in the steel that constitutes the shaft member 30 affect the creep deformation of the shaft member 30.

[0064] SUJ3 contains relatively more silicon and manganese than SUJ2. The steel of the first composition has a relatively higher silicon content than SUJ2 and SUJ3. The steel of the second composition and the third composition have relatively higher silicon and manganese contents than SUJ2 and SUJ3. Therefore, by forming the shaft member 30 from steel of the first composition to the third composition, creep deformation is further suppressed.

[0065] When the carbon concentration in steel is low, the dislocation density in the martensite formed by quenching is low, and dislocations are less likely to move when held at high temperatures. The carbon concentration of the steel of the third composition is relatively low compared to SUJ2 and SUJ3. Therefore, by forming the shaft member 30 from the steel of the third composition, creep deformation is further suppressed.

[0066] Increasing the chromium content in steel promotes creep deformation of the steel. The steels of the first to third compositions have a relatively low chromium content, which can suppress creep deformation. Furthermore, the reduced chromium content in the steel allows for reduction in steel costs.

[0067] When the outer peripheral surface 30a is subjected to carburizing or carbo-nitriding, the hardness of the outer peripheral surface 30a of the shaft member 30 increases, further improving the rolling fatigue life of the shaft member 30. When the amount of retained austenite on the outer peripheral surface 30a is 10% or more and 40% or less, the presence of the retained austenite suppresses stress concentration and the occurrence of indentations, further improving the rolling fatigue life in an environment containing foreign matter.

[0068] The planetary gear device 100 has the shaft member 30 as a pinion shaft, so that the rolling fatigue life of the pinion shaft and creep deformation of the pinion shaft are suppressed.

[0069] (Variation 1) From the standpoint of production efficiency, etc., it may be difficult to change the distances L1 and L2 for each of the outer diameters D1 and D2 as shown in Tables 5 and 6. In this case, the same effect can be obtained by changing the distances L1 and L2 for each of the outer diameters D1. More specifically, in the shaft member 30, when the outer peripheral surface 30a and the rolling elements 32 contact with each other at a maximum contact pressure of 4000 MPa, the distances L1 and L2 may be set for each of the outer diameters D1 as shown in Table 7.

[0070] [Table 7]

[0071] (Variation 2) Fig. 13 is a graph showing the relationship between the distance from the outer peripheral surface 30a in the radial direction of the shaft member 30 and the maximum shear stress. In calculating the graph of Fig. 13, the outer diameter D1 was set to 18 mm, and the outer diameter D2 was set to 3.5 mm. In calculating the graph of Fig. 13, the maximum contact surface pressure between the outer peripheral surface 30a and the rolling elements 32 was set to 2164 MPa.

[0072] As shown in FIG. 13, depending on the outer diameter D1, the outer diameter D2, and the maximum contact pressure between the outer peripheral surface 30a and the rolling elements 32, the maximum shear stress applied to the shaft member 30 may fall below 650 MPa. Even in such cases, consideration must be given to preventing damage originating from the surface. In this case, too thin a second region 30c makes it difficult to suppress creep deformation. From this perspective, when the outer peripheral surface 30a and the rolling elements 32 contact each other at a maximum contact pressure such that the maximum shear stress is 650 MPa or less, the distances L1 and L2 may be set for each outer diameter D1 as shown in Table 8.

[0073] [Table 8]

[0074] Although the embodiments of the present invention have been described above, the above-described embodiments can be modified in various ways. Furthermore, the scope of the present invention is not limited to the above-described embodiments. The scope of the present invention is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. [Industrial Applicability]

[0075] This embodiment is particularly advantageously applied to a pinion shaft and a planetary gear device using a pinion shaft. [Explanation of symbols]

[0076] 10 Internal gear, 20 Shaft member, 21 Sun gear 30 Shaft member, 30a Outer surface, 30b 1st region, 30c 2nd region, 31 Planetary gear, 31a Inner surface, 31b Outer surface, 32 Rolling element, 40 Carrier, 100 Planetary gear unit, D1 Outer diameter, D2 Outer diameter, P position, P1 Position, P2 position, P3 position, P4 position, L1 distance, L2 distance, S1 preparation process, S2 carbo-nitriding process, S3 quenching process, S4 tempering process, S5 post-treatment process, S6 deep cooling process, W test piece.

Claims

1. A steel shaft member, having an outer circumferential surface that contacts the rolling elements, the steel comprises from 0.10 to 0.40 weight percent carbon, from 0.10 to 2.50 weight percent silicon, from 0.30 to 1.20 weight percent manganese, from 1.20 to 1.20 weight percent chromium, and from 0.30 to 1.20 weight percent molybdenum, with the balance being iron and unavoidable impurities; When a distance in the radial direction of the shaft member from the outer peripheral surface to a portion where the hardness is 653 Hv is defined as a first distance, and a distance in the radial direction from the outer peripheral surface to a portion where the amount of retained austenite is 7 volume percent is defined as a second distance, the first distance is greater than a distance in the radial direction from the outer circumferential surface to a position where a maximum shear stress is 650 MPa when the rolling element contacts the outer circumferential surface, the second distance is 1.5 times or less of a distance in the radial direction from the outer circumferential surface to a position where a maximum shear stress when the rolling element and the outer circumferential surface come into contact with each other is 650 MPa, A shaft member, wherein the radial distance from the outer peripheral surface to a position where the maximum shear stress when the rolling element and the outer peripheral surface come into contact is 650 MPa is z obtained from Equation 1. [Equation 1]

2. A steel shaft member, having an outer circumferential surface that contacts the rolling elements, the steel comprises from 0.10 to 0.40 weight percent carbon, from 0.10 to 2.50 weight percent silicon, from 0.30 to 1.20 weight percent manganese, from 1.20 to 1.20 weight percent chromium, and from 0.30 to 1.20 weight percent molybdenum, with the balance being iron and unavoidable impurities; When a distance in the radial direction of the shaft member from the outer peripheral surface to a portion where the hardness is 653 Hv is defined as a first distance, and a distance in the radial direction from the outer peripheral surface to a portion where the amount of retained austenite is 7 volume percent is defined as a second distance, the first distance is equal to or greater than 0.038 times the diameter of the shaft member when the diameter is less than 12 mm, equal to or greater than 0.03 times the diameter when the diameter is 12 mm or greater and less than 16 mm, equal to or greater than 0.025 times the diameter when the diameter is 16 mm or greater and less than 20 mm, and equal to or greater than 0.02 times the diameter when the diameter is 20 mm or greater; The second distance is not more than 0.04 times the diameter when the diameter is less than 12 mm, not more than 0.038 times the diameter when the diameter is 12 mm or more and less than 16 mm, not more than 0.03 times the diameter when the diameter is 16 mm or more and less than 20 mm, and not more than 0.025 times the diameter when the diameter is 20 mm or more, in a shaft member.

3. A steel shaft member, having an outer circumferential surface that contacts the rolling elements, the steel comprises from 0.10 to 0.40 weight percent carbon, from 0.10 to 2.50 weight percent silicon, from 0.30 to 1.20 weight percent manganese, from 1.20 to 1.20 weight percent chromium, and from 0.30 to 1.20 weight percent molybdenum, with the balance being iron and unavoidable impurities; When a distance in the radial direction of the shaft member from the outer peripheral surface to a portion where the hardness is 653 Hv is defined as a first distance, and a distance in the radial direction from the outer peripheral surface to a portion where the amount of retained austenite is 7 volume percent is defined as a second distance, the first distance is equal to or greater than 0.02 times the diameter of the shaft member when the diameter is equal to or less than 12 mm, and is equal to or greater than 0.015 times the diameter of the shaft member when the diameter is greater than 12 mm; The second distance is equal to or less than 0.025 times the diameter when the diameter is equal to or less than 12 mm, and equal to or less than 0.02 times the diameter when the diameter is greater than 12 mm.

4. The outer circumferential surface is subjected to carburizing or carbonitriding treatment, The shaft member according to any one of claims 1 to 3, wherein the amount of retained austenite on the outer peripheral surface is 10 volume percent or more and 40 volume percent or less.

5. The shaft member according to any one of claims 1 to 4, which is a pinion shaft for a planetary gear device.

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