Rolling bearing and rolling component for rolling bearing

Abstract

A rolling component which comprises a planetary gear (1) corresponding to the outer ring of a rolling bearing, a shaft (2) corresponding to the inner ring, and rolling elements (3) is configured such that: the micro-Vickers hardness from the surface to a depth of 25 μm is 800-930 Hv0.1; the absolute value of a compressive residual stress on the surface is 850 MPa or more; and the full width at half maximum of the surface X-ray diffraction is 6.0-6.7°. This rolling bearing is configured to use said rolling component.

Description

Rolling bearings and rolling parts for rolling bearings

[0001] The present invention relates to a rolling bearing and a rolling part included in the rolling bearing as a component thereof.

[0002] Generally, durability, i.e., a long life, is required of rolling bearings. However, to extend the life of a rolling bearing that rotates at high speeds under specific lubrication environments, it is necessary to overcome problems specific to the outer ring, inner ring, shaft, or rolling elements depending on the usage conditions.

[0003] For example, rolling bearings used in e-axles, which combine major components such as motors used in automobiles, and in multi-stage automatic transmissions (ATs), are expected to be used at high speeds, and in order to keep torque loss low, they are required to use as little low-viscosity lubricating oil as possible.Moreover, there are cases in which the bearings are used in conditions where the entire bearing is at a high temperature or where foreign matter has been mixed into the lubricating oil.

[0004] The surfaces of the steel rolling components of rolling bearings used under such harsh conditions may experience spalling. This spalling is not caused by internal spalling originating from inclusions contained in the steel components, but rather by spalling originating from micro-spalling on the surface. Micro-spalling often occurs in the rolling components of rolling bearings that rotatably support the planetary gear shafts in planetary gear mechanisms commonly used in e-axles, automatic transmissions, and the like, when the inner diameter surface roughness of the planetary gears is high.

[0005] Japanese Patent No. 6211051 (Patent Document 1) describes a rolling bearing part in which the hardness at a depth of 40 μm from the surface of the part is limited to 870 HV0.3 to 1000 HV0.3 by nitriding treatment, so that the mechanical elements used in aircraft can be used safely and the part will have a long life and be able to withstand a certain degree of damage, thereby minimizing the spread of damage when a rolling load is applied.

[0006] Patent Document 1 also describes that there is an "edge zone" in which the nitrogen content decreases from the outer portion near the surface of the rolling bearing component toward the inner portion, and a "core zone" having a nearly constant hardness, and that the hardness at a depth of 40 μm is 870 to 1000 HV0.3, and that the hardness at a depth of 300 μm is at most 250 HV0.3 less than the hardness at a depth of 40 μm, and that the absolute value of the compressive residual stress at the surface is 500 to 1000 MPa, and that the compressive residual stress decreases from the outside to the inside in the edge zone.

[0007] Patent No. 6211051

[0008] As described above, in Patent Document 1, the hardness of the outer ring, inner ring or rolling element of a rolling bearing is increased by nitriding at a depth of 40 μm from the surface. However, since micro-spalling occurs in the above-mentioned parts used under harsh conditions such as high temperature, high rotational speed and lubrication with foreign matter contamination, it is a phenomenon that occurs even at positions shallower than 40 μm from the surface of the part, and it has been difficult to sufficiently prevent the occurrence of micro-spalling.

[0009] Therefore, an object of the present invention is to solve the problems of the prior art described above, to suppress the occurrence of micro-spalling even in portions shallower than 40 μm from the surface of the rolling components of a rolling bearing, and to extend the life of rolling bearings used in lean lubrication environments, such as in increasingly electrified devices such as e-axle accelerators and decelerators, and in multi-stage automatic transmissions (ATs), etc. Also, the present invention aims to provide a rolling component, or a rolling bearing using such a rolling component, that can withstand the load of the rolling bearing and have a long life even when used in harsh lubrication environments where the rotational speed of the rolling bearing is 9000 rpm or higher and the kinematic viscosity of the lubricating oil used is 13 cSt (40°C) or less or 4 cSt (100°C) or less, and even under conditions where sudden acceleration and deceleration are expected.

[0010] To solve the above problems, the present invention provides rolling components constituting a rolling bearing, each of which has a micro Vickers hardness of 800 to 930 Hv0.1 from the surface to a depth of 25 μm, an absolute value of the residual compressive stress at the surface of 850 MPa or more, and a half-width of the X-ray diffraction peak at the surface of 6.0 to 6.7°. The rolling components for a rolling bearing referred to here refer to, for example, an inner ring, an outer ring, rolling elements, a shaft that slides on the inner ring, or components integrated with these that are used in a rolling bearing. Furthermore, the rolling components for a rolling bearing preferably include at least the rolling elements, among the inner ring, a shaft integrated with the inner ring, an outer ring, a component equivalent to the outer ring such as a planetary gear, and rolling elements.

[0011] The rolling bearing component of the present invention configured as described above has the micro Vickers hardness from the surface to a depth of 25 μm, the residual compressive stress at the surface, and the half-value width of the X-ray diffraction peak specified within predetermined ranges, thereby making it possible to prevent micro-spalling from occurring at a position shallower than 40 μm from the component surface.

[0012] In other words, the rolling parts for rolling bearings of the present invention specify the hardness of the outermost surface layer to a predetermined range of 800 to 930 Hv0.1, which was not anticipated in conventional technology, and by work-hardening the surface to densify the crystal grains so that the compressive residual stress on the surface of the rolling parts is equal to or greater than a predetermined value and the half-width of the X-ray diffraction peak is within a predetermined range, it is possible to prevent the occurrence of micro-spalling in parts very close to the surface of the rolling parts for rolling bearings.

[0013] Furthermore, by setting the amount of retained austenite in the surface layer portion from the surface of the rolling component to a depth of 50 μm to 15 to 35 volume %, stress is alleviated in a predetermined region from the surface (contact surface) to a depth of 25 μm, and the effect of suppressing the occurrence of micro-spalling is further enhanced.

[0014] Furthermore, when micro-spalling occurs, it is preferable to work-harden the surface so that the grain size number (JIS G0551) of the prior austenite grain boundary in the surface layer portion up to 50 μm from the surface of the rolling component is 9 to 11, in order to retard the progression of cracks from the micro-spalling to the periphery.

[0015] A rolling bearing equipped with rolling components configured in this manner and exhibiting the above-described effects is work-hardened at a depth of less than 40 μm from the surface of the rolling components, resulting in hardened crystal grains in a densified state, which sufficiently prevents the occurrence of micro-spalling. This makes it possible to sufficiently extend the life of rolling bearings that rotate at high speeds, and the above-described desired effects are also exhibited when applied to rolling bearings that support planetary gears (planetary pinions) of planetary gear mechanisms commonly used in e-axles, automatic transmissions, etc.

[0016] This invention provides a rolling component for a rolling bearing in which the micro Vickers hardness from the surface of the rolling component to a depth of 25 μm, the residual compressive stress on the surface, and the half-width of the X-ray diffraction peak on the surface are specified to fall within predetermined ranges. Therefore, the occurrence of micro-spalling is suppressed even in a portion shallower than 40 μm from the surface of such a rolling component, and a rolling bearing incorporating this has the advantage of being able to achieve a longer life when used in a lean lubrication environment such as an e-Axle or a multi-stage automatic transmission (AT).

[0017] Schematic diagram of a planetary gear mechanism. A perspective view showing a rolling bearing and its rolling components according to an embodiment, with a part of the planetary gear cut away. A cross-sectional view showing a rolling bearing according to another embodiment.

[0018] An embodiment of the present invention will be described below with reference to the accompanying drawings. As shown in Figures 1 and 2, this embodiment is a rolling bearing and its rolling components that rotatably support the shaft 2 of a planetary gear 1 incorporated in a planetary gear mechanism A. The planetary gear 1, shaft 2, and rolling elements (needle rollers) 3 of the rolling components have a micro Vickers hardness of 800 to 930 Hv0.1 from their surfaces to a depth of 25 μm, an absolute value of the residual compressive stress at the surface of 850 MPa or more, and a half-value width of the X-ray diffraction peak at the surface of 6.0 to 6.7°. Incidentally, the inner diameter of a rolling bearing that supports a planetary gear of a planetary gear mechanism is generally φ7 to 50 mm.

[0019] As shown in FIG. 1, the planetary gear mechanism A includes a ring gear 4 having internal teeth surrounding the outer periphery, a sun gear (sun gear) 5 having external teeth and disposed at the center of the ring gear 4, and a plurality of planetary gears 1 having external teeth and disposed between the ring gear 4 and the sun gear 5 and assembled so as to mesh with each other.

[0020] As shown in Figure 2, the shaft 2 of the planetary gear 1 is a rolling part that corresponds to the inner ring of a general rolling bearing, and is rotatably supported relative to the planetary gear 1, which corresponds to the outer ring, by the planetary gear 1 and rolling elements 3 that are rotatably held in a cage-type retainer 6. The end of the shaft 2 is connected to a carrier (not shown), and the rotational force required for the revolution of the planetary gear mechanism A is input and output from this carrier, thereby driving the planetary gear mechanism.

[0021] The rolling parts of the embodiment are rolling parts made of steel, and the rolling elements 3 consisting of needle rollers are made of high carbon chromium bearing steel (SUJ material). For example, SUJ2 is a steel material that is highly wear resistant and easily available.

[0022] It should be noted that steel materials with compositions similar to SUJ materials specified in global standards (ASTM, GB, DIN, etc.) can also be used as steel materials for rolling components. Incidentally, cages, which are not included in the rolling components of this invention, are generally made of carbon steel for machine structures.

[0023] A specific example of the composition of the steel used for the steel rolling part is a steel containing 0.93 to 1.10 mass% carbon, 0.5 mass% or less manganese, 0.025 mass% or less sulfur, 0.15 to 0.35 mass% silicon, 0.90 to 1.65 mass% chromium, and 0.30 mass% or less nickel, with the balance being iron and impurities. The elements and composition of the steel can be detected by spectroscopic analysis such as EPMA.

[0024] As in the above composition example, by setting the carbon content of the steel to 0.93 to 1.10 mass%, it is possible to significantly affect the hardness and carbide amount of the rolling part after quench hardening. In other words, by setting the carbon content of the steel to 0.93 mass% or more, sufficient hardness and carbide amount can be ensured without introducing a large amount of carbon into the steel during heat treatment. Furthermore, by setting the carbon content of the steel to 1.10 mass% or less, the risk of large carbides forming during the steel manufacturing stage is reduced.

[0025] Furthermore, by setting the silicon content of the steel to 0.35 mass% or less, the increase in the amount of hydrogen absorbed in the steel is suppressed, reducing the risk of peeling due to hydrogen embrittlement, and by setting the silicon content to 0.15 mass% or more, nitrides are more likely to precipitate during heat treatment, increasing hardness.

[0026] Furthermore, by limiting the sulfur content to 0.025 mass% or less, the risk of chemically bonding with Mn and other elements to form non-metallic inclusions such as manganese sulfide is reduced. By limiting the Mn content to 0.50 mass% or less, the material hardness before heat treatment can be controlled low, improving workability in the cold process. Furthermore, by limiting the chromium content to 0.90 to 1.65 mass%, the hardenability of the steel is improved.

[0027] Another example of the composition of the steel material is a steel material containing 0.95 to 1.10 mass% carbon, 0.90 to 1.15 mass% manganese, 0.025 mass% or less sulfur, 0.40 to 0.70 mass% silicon, 0.90 to 1.20 mass% chromium, and 0.25 mass% or less nickel, with the remainder being iron and unavoidable impurities.

[0028] The rolling component having the above composition has a micro Vickers hardness of 800 to 930 HV0.1 in a region extending 25 μm from the surface (contact surface) that comes into contact with other components, an absolute value of compressive residual stress on the surface of 850 MPa or more, and a half-width of an X-ray diffraction peak on the surface of 6.0 to 6.7°. The hardness of the rolling component extending 40 μm from the contact surface, which is the surface that comes into contact with other components, is 800 HV0.1 or more.

[0029] The amount of retained austenite in the surface layer portion of the rolling component extending from the surface to 50 μm is preferably 15 to 35% by volume. The amount of retained austenite can be measured using an X-ray stress measurement device.

[0030] By setting the amount of retained austenite to 15% by volume or more as described above, it is possible to suppress the propagation of cracks due to spalling, even when foreign matter gets caught in a rolling bearing under operating conditions with contaminated lubrication. This is because stress is alleviated at a predetermined position 50 μm from the surface (contact surface) in a rolling component having relatively soft retained austenite. Furthermore, by setting the amount of retained austenite at the predetermined position to 35% by volume or less, it is appropriate to prevent excessive reduction in hardness of the surface layer.

[0031] In order to obtain the above-described effect, the amount of retained austenite in the entire surface layer portion (all regions) is preferably 15 to 35% by volume, but the average amount of retained austenite in the entire surface layer portion may be 15 to 35% by volume.

[0032] The grain size number of the prior austenite crystals in the surface layer portion up to 50 μm from the surface of the rolling component is preferably 9 or more and 11 or less (JIS G0551). By adjusting the crystal grains of the needle roller to fall within the above grain size number range and densifying them, it is possible to delay the propagation of cracks when micro-spalling occurs.

[0033] The grain size number of the prior austenite crystals can be determined by corroding the rolling part with an ethanol solution of nitric acid or an ethanol solution of picric acid to reveal the crystal grains, observing them with an optical microscope, etc., and measuring the grain size. When measuring, the grain size number of the prior austenite crystals is measured at a position 50 μm deep from the revealed surface (contact surface), but even if the specified grain size number is measured at any position shallower than the depth of 50 μm, this does not affect the determination.

[0034] A method for manufacturing a rolling element, taking a needle roller (diameter 1.5 to 5.5 mm) as a representative example of an embodiment of the present invention, will be described below.

[0035] First, a high carbon chromium bearing steel (SUJ material) for forming needle rollers is used as the steel material, and wire material is prepared by being subjected to wire drawing multiple times. This wire material is then subjected to processes such as cutting, forging, and turning, and the steel material formed into the general shape of the needle roller is then subjected to heat treatment.

[0036] The heat treatment process for steel material to be made into needle rollers begins with a preparatory heating step in which the steel material is heated to 850°C or higher and 940°C or lower, and then, as the overall heat treatment step, the steel material is heated to and maintained at the A1 transformation point or higher in a heat treatment gas.

[0037] Specifically, an enriched gas such as propane gas or butane gas serving as a carbon source is added to an endothermic converted gas (RX gas) as a base, and the steel material is heated to 850°C or higher and 940°C or lower in such an atmosphere. The entire heat treatment process may be a nitriding process.

[0038] This is followed by the quenching process, in which the steel material, which has been kept at or above the A1 transformation point, is immersed in oil (oil cooling) to be rapidly cooled to a temperature below the Ms point (the martensitic transformation starting point).

[0039] In the quenching process, instead of oil cooling, the steel may be cooled by immersing it in water (water cooling). This is followed by a tempering process. In the tempering process, the steel that has been quench-hardened in the quenching process is heat-treated at a temperature below the A1 transformation point (160 to 200°C). By holding the steel at this temperature for a predetermined time and then cooling it in air at room temperature, toughness can be improved. This is followed by a grinding process, whereby needle rollers with the final dimensions and shape are obtained.

[0040] Thereafter, work hardening treatment such as barrel processing is carried out to increase the hardness near the surface (region from the surface to a depth of 25 μm) and improve micro-spalling resistance.

[0041] Barrel processing can be performed using a conventional method using a well-known barrel finishing machine, and the tank of the machine is filled with an appropriate amount of media (grinding stones and abrasives), liquid compound, water, rollers for the workpiece, etc.

[0042] In barrel finishing, a method of polishing while applying centrifugal force is preferred to increase the surface hardness of the workpiece. To achieve this, a barrel finishing machine can be used that rotates multiple tanks while revolving them as a whole, and centrifugal barrel finishing is preferably performed by applying centrifugal force to each tank due to high-speed rotation. By performing barrel finishing in this manner, the efficiency of work hardening of the surface layer of the rolling part can be improved, and the hardness and densification of crystal grains near the part surface can be efficiently improved.

[0043] In addition, rolling parts other than needle rollers can also be manufactured using almost the same process as above, including cutting, forging, turning, heat treatment, grinding, and near-surface work hardening treatment (barrel processing, etc.).

[0044] As shown in FIG. 3 , another embodiment of a rolling bearing incorporating the above-described rolling parts is a general-purpose rolling bearing having an outer ring 7, an inner ring 8, and rolling elements 10 (rollers or balls) interposed between them and rotatably held by a cage 9.

[0045] A rolling bearing of such an embodiment or an embodiment comprising the planetary gear 1 (see FIG. 2) described above can be used under harsh conditions, such as a rotational speed of 9000 rpm or more and a lubricating oil kinematic viscosity of 13 cSt (40°C) or 4 cSt (100°C) or less, and even under conditions where sudden acceleration and deceleration are expected, and is a rolling component of a rolling bearing that can be used in a reducer of an e-Axle, or a rolling bearing using the same.

[0046] The type of rolling bearing in the embodiment described above is exemplified as a needle bearing (needle roller bearing), but is not limited to this and may be a cylindrical roller bearing, a tapered roller bearing, a self-aligning roller bearing, a ball bearing, etc.

[0047] Example 1 Needle rollers, a typical example of rolling parts that can be incorporated into rolling bearings that support planetary gears of a planetary gear mechanism, were manufactured by the following process: A high-carbon chromium bearing steel (SUJ2) for forming needle rollers was used as the steel material, and a wire rod that had been subjected to multiple wiredrawing processes was prepared, and then the rough shape was formed by cutting, forging, and turning.

[0048] In the heat treatment process (deep hardening), the steel material was first heated to 850°C or higher and 940°C or lower, and then, in the overall heat treatment process, enriched gas was added to an endothermic converted gas (RX gas) as a base, and the steel material was heated to 850°C or higher and 940°C or lower.

[0049] In the subsequent quenching process, the steel material maintained at or above the A1 transformation point was immersed in oil (oil cooling) to be rapidly cooled to a temperature below the Ms point (martensitic transformation starting point).

[0050] In the tempering process, the steel material that had been quench-hardened in the quenching process was heat-treated at a temperature below the A1 transformation point (160-200°C), held for a predetermined time, and then cooled in air at room temperature. The hardness of the surface vicinity (the region from the surface to a depth of 25 μm) was then increased by grinding and centrifugal barrel processing.

[0051] [Example 2] A needle roller (Example 2) was manufactured in exactly the same manner as in the manufacturing process of Example 1 described above, except that the heat treatment process was a nitriding quenching process. In the nitriding quenching process, nitrogen gas was used instead of the enriched gas used in the manufacturing process of Example 1 to dissolve and precipitate nitrogen, and then the quenching process was carried out. [Comparative Example 1] A needle roller (Comparative Example 1) was manufactured in exactly the same manner as in Example 2, except that barrel processing was not performed in the manufacturing process of Example 2 described above.

[0052] The hardness at a predetermined depth of the obtained rolling elements (needle rollers) of Examples 1 and 2 and Comparative Example 1 was measured as follows: <Hardness Measurement Test> The needle rollers were cut (sliced ​​into cross sections) in a direction perpendicular to the axial direction, the cut surfaces were mirror-polished, and then a load of 100 g was applied using a Vickers hardness tester to measure the diagonal length of the indentation, thereby measuring the micro Vickers hardness (Hv0.1) at a depth of 25 μm from the surface of the rolling element.

[0053] The compressive residual stress and the half width were measured by X-ray diffraction using an X-ray stress measurement device, and measurements were taken using Kα rays from a Cr tube under conditions of a tube voltage of 40 kV, a tube current of 40 mA, and incident angles (ψ angles) of 11.8°, 28.9°, 40.7°, and 51.8°. The measured values ​​were obtained by examining the average values ​​of the half widths (measured values) measured for the crystal orientation of the marnsite phase.

[0054] The results of the hardness measurement test were evaluated as poor (×) when the measured value was less than 800 HV0.1, and as good (◯) when it was 800 to 930 HV0.1, and these results are shown by symbols in Table 1. Furthermore, it was confirmed that in Examples 1 and 2, the absolute value of the residual compressive stress on the surface was 850 MPa or more, and the half-width of the X-ray diffraction peak on the surface was 6.0 to 6.7°.

[0055] <Spalling Resistance Evaluation Test> Assuming a usage state of a planetary gear mechanism (FIGS. 1 and 2) incorporating planetary gears equipped with the rolling elements (rollers) of Examples 1 and 2 and Comparative Example 1, spalling resistance was evaluated using a radial load tester under the following test conditions.

[0056] Radial load: 6670N Moment load: 13.5N m Outer ring rotation speed: 9000 rpm Lubricating oil: Clean oil A high kinematic viscosity oil (kinematic viscosity at 100°C: 7 cSt) or Clean oil B low kinematic viscosity oil (kinematic viscosity at 100°C: 3 cSt) Lubrication conditions: Circulating oil supply

[0057] This spalling resistance evaluation test was carried out using two types of clean oils A and B with different kinematic viscosities. The life (spalling occurrence) of the rolling elements using clean oil A (high kinematic viscosity oil) was used as the standard for the conventional product, and if the life when clean oil B (low kinematic viscosity oil) was used, it was judged to be good (high spalling resistance), and if the life was shorter than the standard (low spalling resistance), it was judged to be good (high spalling resistance), and these results are shown in Table 1.

[0058]

[0059] As is clear from the results shown in Table 1, the rolling elements of Examples 1 and 2, as rolling components for rolling bearings, had a hardness of 800 to 930 Hv0.1 at a depth of 25 μm from the surface in a hardness test, and the compressive residual stress and the half-value width of the X-ray diffraction peak were within predetermined ranges. Therefore, good results were obtained in a spalling resistance evaluation test using low-viscosity oil, and it was confirmed that they have a longer life than conventional products as rolling bearings that support planetary gears incorporated into a planetary gear mechanism.

[0060] The present invention can be applied to rolling parts used in rolling bearings that are lubricated with an insufficient amount of lubricant or a low-viscosity liquid lubricant, particularly rolling bearings that rotate at high speeds, and further rolling bearings for which it is necessary to minimize the rotational torque and heat generation of the bearing, as well as to rolling bearings incorporated into reduction mechanisms and transmission mechanisms such as e-axles, automatic transmissions (ATs), and continuously variable transmissions (CVTs) in automobiles, and various types of industrial machinery such as high-speed rotating machine tools, and can be used in a wide range of industrial fields.

[0061] A Planetary gear mechanism 1 Planetary gear 2 Shaft 3, 10 Rolling element 4 Ring gear 5 Sun gear 6, 9 Cage 7 Outer ring 8 Inner ring

Claims

1. A rolling component for a rolling bearing, having a micro Vickers hardness of 800 to 930 Hv0.1 from the surface to a depth of 25 μm, an absolute value of the residual compressive stress on the surface of 850 MPa or more, and an X-ray diffraction peak half-width on the surface of 6.0 to 6.7°.

2. A rolling component for a rolling bearing according to claim 1, wherein the amount of retained austenite in the surface layer portion of said rolling component up to 50 μm from the surface is 15 to 35% by volume.

3. A rolling component for a rolling bearing according to claim 1 or 2, wherein the grain size number (JIS G0551) of the prior austenite grain boundary in the surface layer portion of said rolling component up to 50 μm from the surface is 9 to 11.

4. A rolling bearing equipped with the rolling component for a rolling bearing according to any one of claims 1 to 3.

5. A rolling bearing according to claim 4, wherein said rolling bearing is a rolling bearing that supports a planet gear of a planetary gear mechanism.

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