Wheel bearing device and manufacturing method for same

Abstract

This wheel bearing device has an inner member 2 including a hub ring 4 having a wheel mounting flange 42, an outer member 1 mounted on a vehicle body, and double rows of rolling elements 3 disposed between the inner member 2 and the outer member 1. The hub ring 4 is formed of carbon steel having a carbon content of 0.45-0.75 mass %. At least a part of the surface layer of the hub ring 4 comprises a pearlite structure P in which the area ratio of proeutectoid ferrite is 1% or less.

Description

Wheel bearing device and method for manufacturing the same

[0001] The present invention relates to a wheel bearing device and a method for manufacturing the same.

[0002] As shown in FIG. 10, for example, the wheel bearing device includes an outer member 101 attached to a vehicle body, an inner member 102 attached to a wheel, and a double row of rolling elements 103, 103 arranged therebetween. The inner member 102 includes a hub ring 104 having a wheel mounting flange 142 and an inner ring 105 fixed to the outer periphery of the hub ring 104.

[0003] In the wheel bearing device, the root portion 149 on the outboard side (left side in the figure) of the wheel mounting flange 142 of the hub ring 104, specifically, the curved surface portion extending from the inner diameter end of the end face 142a on the outboard side of the wheel mounting flange 142 toward the outer peripheral surface of the pilot portion 143, becomes the weakest portion against rotational bending stress. Therefore, it is necessary to increase the strength of this portion.

[0004] Among the hub ring 104, the raceway surface 144a in contact with the rolling element 103 and the inner ring fixing portion 145 to which the inner ring 105 is press-fitted are subjected to high-frequency quenching to increase the hardness (see the scattered dot region in the figure). For example, by also subjecting the root portion 149 on the outboard side of the wheel mounting flange 142 to high-frequency quenching, it is conceivable to increase the strength of the root portion 149. However, when high-frequency quenching is performed on the root portion 149, the range of high-frequency quenching increases, resulting in higher costs. In addition, there is a risk of melting of the thin pilot portion 143 and deterioration of the surface accuracy of the end face 142a on the outboard side of the wheel mounting flange 142 to which the brake rotor is attached. In addition, although it is also conceivable to increase the strength by adding alloy elements such as Si, Mn, and V to the carbon steel forming the hub ring 104, an increase in cost is inevitable. Therefore, various methods for increasing the strength of the root portion 149 on the outboard side of the wheel mounting flange 142 of the hub ring 104 by methods other than the above have been studied.

[0005] For example, Patent Document 1 below describes a method for setting the hardness of a hub wheel to 23 HRC or more and 35 HRC or less by controlling the cooling of the hub wheel after forging so that it forms a troostite structure, a sorbite structure, or a mixed structure of troostite and sorbite structures. This method is said to increase the strength of the outboard base portion (corner portion 21) of the wheel mounting flange.

[0006] Patent Document 2, described below, describes a technique for improving the fatigue strength of the base of the flange on the outboard side by making the structure of the hub ring consist of 7% or less protereminate ferrite and the remainder ferrite and spheroidized carbides, and setting the hardness to HRC 20 to 30.

[0007] Patent Document 3, described below, describes a method for obtaining a non-standard structure on the surface of a hub wheel, while the base material portion of the hub wheel retains the standard structure, by cooling a portion of the hub wheel by exposing it to a coolant or controlling the cooling rate and temperature during or at the end of the hot forging process of the hub wheel. Examples of non-standard structures shown include a fine ferrite-pearlite structure.

[0008] Japanese Patent Publication No. 2008-207586, Japanese Patent Publication No. 2005-308152, Japanese Patent Publication No. 2007-24273

[0009] However, the above method may not be sufficient to adequately increase the strength of the hub wheel.

[0010] Therefore, the present invention aims to increase the strength of a predetermined portion of the hub wheel of a wheel bearing device.

[0011] To solve the aforementioned problems, the present invention provides a wheel bearing device comprising a rotating body including a hub ring having a flange for wheel mounting, a stationary body attached to a vehicle body, and a double row of rolling elements disposed between the rotating body and the stationary body, wherein the hub ring is formed of carbon steel with a carbon content of 0.45 to 0.75 mass%, and at least a portion of the surface layer of the hub ring is made of a pearlite structure with an area ratio of protoprecipitation ferrite of 1% or less.

[0012] Thus, in this invention, at least a portion of the surface layer of the hub ring consists of a pearlite structure with an area ratio of protereminate ferrite of 1% or less. By making the surface layer of the hub ring substantially composed of only pearlite, the occurrence of cracks originating from the soft ferrite structure can be prevented, thereby improving the fatigue strength in this part and increasing its overall strength. Protereminate ferrite is a ferrite structure that occurs at the grain boundaries of the pearlite structure and is distinct from ferrite present within the grains of the pearlite structure. Protereminate ferrite is prone to becoming a crack initiation point, while ferrite within the grains of the pearlite structure rarely becomes a crack initiation point.

[0013] When the surface of a hub ring is provided with a hardened portion that has been subjected to high-frequency induction hardening and an unhardened portion that has not been subjected to high-frequency induction hardening, it is preferable that at least a part of the unhardened portion be made into the pearlite structure.

[0014] For example, the pearlite structure can be applied to a range from the surface of the hub ring to a depth of twice the average grain size of the pearlite structure.

[0015] It is preferable that the weakest part of the hub wheel, the base of the wheel mounting flange on the outboard side, be formed in the portion made of the pearlite structure.

[0016] The outboard mounting flange of the hub wheel can have a cross-sectional arc shape, for example. In this case, it is preferable that the pearlite structure be applied to the area from the surface of the mounting area to a depth of four times the radius of curvature of the mounting area.

[0017] The pearlite structure can be obtained by controlling the cooling rate after heating the hub ring. Specifically, in a method for manufacturing a wheel bearing device having a rotating body including a hub ring having a flange for mounting a wheel, a stationary body attached to a vehicle body, and a double row of rolling elements disposed between the rotating body and the stationary body, the pearlite structure can be obtained by a method for manufacturing a wheel bearing device having the following steps: the hub ring is made of carbon steel with a carbon content of 0.45 to 0.75% by weight, the hub ring is heated to a predetermined temperature in the austenite region, and at least a portion of the hub ring heated to the predetermined temperature is cooled from the start of cooling to the martensitic transformation point + 100°C in 10 seconds to 100 seconds.

[0018] The hub ring may be formed by hot forging while heating to the predetermined temperature, and then cooled.

[0019] It is preferable to perform the cooling in the region of the hub wheel that includes the outboard base portion of the wheel mounting flange.

[0020] As described above, according to the present invention, it is possible to increase the strength of a predetermined part of the hub wheel of a wheel bearing device.

[0021] This is an axial cross-sectional view of a wheel bearing device according to one embodiment of the present invention. This is a cross-sectional view showing the surface structure of the hub ring of the wheel bearing device. This is an axial cross-sectional view of the hub ring near the base of the wheel mounting flange on the outboard side. This is a graph showing the change in temperature over time during the cooling process of the hub ring. This is a graph showing the change in temperature over time during the cooling of each test piece. This is a photograph of the cross section of Comparative Example 1 (magnification 500x). This is a photograph of the cross section of Comparative Example 1 (magnification 2000x). This is a photograph of the cross section of Comparative Example 2 (magnification 500x). This is a photograph of the cross section of the embodiment (magnification 500x). This is a photograph of the cross section of the embodiment (magnification 2000x). This is a graph showing the results of a rotational bending fatigue test. This is an axial cross-sectional view of a conventional wheel bearing device.

[0022] Hereinafter, embodiments of the present invention will be described based on the drawings.

[0023] As shown in Figure 1, a wheel bearing device according to one embodiment of the present invention comprises an outer member 1, an inner member 2, and a double row of rolling elements 3, 3 rotatably housed between them. When the wheel bearing device is attached to the vehicle body, the outer side in the vehicle width direction (left side in Figure 1) is called the outboard side, and the central side in the vehicle width direction (right side in Figure 1) is called the inboard side.

[0024] The outer member 1 is substantially cylindrical and has a vehicle body mounting flange (not shown) for attachment to the vehicle body. Outer raceway surfaces 13a and 13b are formed on the inner circumferential surface of the outer member 1.

[0025] The inner member 2 has a hub ring 4 and an inner ring 5 fixed to the outer circumference of the hub ring 4. The hub ring 4 has a shaft portion 41, a wheel mounting flange 42 protruding outward from the shaft portion 41, and a cylindrical pilot portion 43 provided on the outboard side of the wheel mounting flange 42. The shaft portion 41 has a through hole that penetrates axially, and a female spline 50 is formed in this through hole. The outer circumferential surface of the shaft portion 41 is provided with one inner raceway surface 44a (outboard side) and an inner ring fixing portion 45. The inner ring 5 is press-fitted and fixed to the inner ring fixing portion 45 with a predetermined overlap. The outer circumferential surface of the inner ring 5 has the other inner raceway surface 44b (inboard side).

[0026] Double rows of rolling elements 3, 3 are arranged between the outer raceway surfaces 13a, 13b of the outer member 1 and the inner raceway surfaces 44a, 44b of the inner member 2. Each row of rolling elements 3 is held rotatably by a retainer 6. In the illustrated example, the rolling elements 3 are balls. Rollers (tapered rollers or cylindrical rollers) may also be used as rolling elements 3.

[0027] Seal members 7 and 8 are installed at the axial openings on both sides of the annular space formed between the outer member 1 and the inner member 2, preventing leakage of lubricating grease sealed inside the bearing and preventing rainwater, dust, etc. from entering the bearing from the outside. The seal members 7 and 8 are fixed to the inner circumferential surface of the outer member 1. The seal member 7 on the outboard side slides against a seal surface 46 provided on the outer circumferential surface of the shaft portion 41 of the hub ring 4, and the seal member 8 on the inboard side slides against a seal surface 47 provided on the outer circumferential surface of the inner ring 5.

[0028] The hub wheel 4 is formed from carbon steel with a carbon content of 0.45 to 0.75% by weight. Examples of such carbon steels include S50C, S53C, S55C, and SAE1070 as specified in JIS G 4051:2023, ISO 683-1:2016, and ISO 683-3:2022, as well as C56E2 (equivalent to S55C) and 70Mn4 (similar to SAE1070) as specified in ISO 683-17:2014. It is more preferable that the hub wheel 4 be formed from carbon steel with a carbon content of 0.45 to 0.60% by weight.

[0029] On the surface of the hub wheel 4, the area including the inner raceway surface 44a, the inner ring fixing portion 45, and the sealing surface 46 is provided with a hardened portion H by high-frequency induction hardening (see the scattered dot area in Figure 1). In this embodiment, the hardened portion H is provided on the entire circumference of the surface from the inboard side end of the outer circumferential surface of the shaft portion 41 of the hub wheel 4 to the inboard side end face of the wheel mounting flange 42, specifically, the area near the bolt hole 48 into which the hub bolts are inserted. The area of ​​the surface of the hub wheel 4 other than the hardened portion H is the unhardened portion S, which has not been subjected to high-frequency induction hardening.

[0030] Of the hub wheel 4, the weakest point is the base portion 49 on the outboard side of the wheel mounting flange 42, more specifically, the curved portion extending from the inner diameter end of the outboard end face 42a of the wheel mounting flange 42 toward the outer circumferential surface of the pilot portion 43. The base portion 49 is provided in the non-hardened portion S.

[0031] At least a portion of the surface layer of the hub wheel 4 consists of a pearlite structure (hereinafter referred to as "pure pearlite structure P") in which the area ratio of protereminate ferrite in a cross-section perpendicular to the surface is 1% or less. In this embodiment, as shown in Figure 2, at least a portion of the unhardened portion S consists of the pure pearlite structure P, and the entire area of ​​the unhardened portion S consists of the pure pearlite structure P (see Figure 1). The pure pearlite structure P may contain components other than pearlite and protereminate ferrite, but such components are present in trace amounts, and the area ratio of pearlite in the pure pearlite structure P is, for example, 99% or more.

[0032] In this embodiment, the pure pearlite structure P extends from the surface of the hub ring 4 to a depth of twice the average grain size D of the pearlite structure, preferably to a depth of four times the average grain size D of the pearlite structure (see Figure 2). The average grain size D of the pearlite structure is measured by the method described in JIS G 0551:2021 (ISO 643:2012). For example, if the grain size number measured by the above method is 5, i.e., the average grain size D is 0.062 mm, the pure pearlite structure P extends from the surface of the non-hardened portion S to a depth of 0.125 mm, preferably to a depth of 0.250 mm.

[0033] In this embodiment, as shown in Figure 1, the surface layer of the hub ring 4 has a quenched portion H consisting of a martensitic or bainite structure and a pure pearlite structure P consisting substantially only of pearlite. The interior (core) of the hub ring 4 has a mixed structure P+F of pearlite and protereminate ferrite. The area ratio of protereminate ferrite in the mixed structure P+F is greater than 1%. In Figure 1, the boundary between the pure pearlite structure P and the mixed structure P+F is shown by a dotted line.

[0034] In this embodiment, the outboard base portion 49 of the wheel mounting flange 42 of the hub wheel 4 is provided on the pure pearlite structure P. In the illustrated example, in the axial cross-section shown in Figure 3, the base portion 49 has a concave arc shape with a radius of curvature R, and at least the area from the surface of the base portion 49 to a depth of 4R consists of the pure pearlite structure P.

[0035] As described above, by having the surface layer of the non-hardened portion S consist substantially only of a pearlite structure, the occurrence of cracks originating from protereminate ferrite can be prevented. This makes it possible to increase the strength of the non-hardened portion S of the hub wheel 4, in particular the strength of the outboard base portion 49 of the wheel mounting flange 42.

[0036] The following describes the manufacturing method of the above-mentioned wheel bearing device, focusing on the manufacturing method of the hub wheel 4.

[0037] The hub wheel 4 is formed by hot forging a cylindrical billet made of carbon steel with a carbon content of 0.45 to 0.75% by weight into a shape that is approximately the same as the finished product (without bolt holes or female splines). Hot forging is carried out while heating the workpiece at a predetermined forging temperature in the austenite region (for example, 900 to 1300°C). Hot forging of the hub wheel 4 is usually carried out in multiple stages.

[0038] Conventionally, after the hot forging process, the hub ring was air-cooled, resulting in a relatively slow cooling process, as shown in Figure 4 (1). In this case, even after 100 seconds from the start of cooling, the temperature does not drop to the martensitic transformation point + 100°C. As a result, the surface of the hub ring becomes a mixed structure of pearlite and ferrite (proecution ferrite) precipitated at the grain boundaries of the pearlite structure. Specifically, the area ratio of proecution ferrite in the cross-section of the hub ring's surface is greater than 1%, usually 2% or more. Such a mixed structure is prone to cracks originating from proecution ferrite, which may lead to insufficient fatigue strength.

[0039] In this embodiment, as shown in Figure 4 (2) or (3), the cooling rate of the hub wheel is made slightly faster than in the conventional (1). Specifically, when cooling a hub wheel formed by hot forging, the cooling conditions are adjusted so that the time it takes to decrease from the start of cooling to the martensitic transformation point (Ms) + 100°C is between 10 seconds and 100 seconds. For example, if the forging temperature (≒ temperature of the hub wheel at the start of cooling) is 1100°C and the martensitic transformation point is 300°C, the cooling rate from 1100°C to 400°C should be between 7°C / second and 80°C / second. Specifically, for example, the hub wheel after hot forging is immersed in a molten salt bath at the martensitic transformation point + 100°C for 200 seconds, then removed from the molten salt bath and left to cool in the air. As the molten salt bath, for example, a mixture of sodium nitrite and potassium nitrate (also called a salt bath heat treatment agent or salt) can be used.

[0040] In this way, by making the cooling rate of the hub wheel 4 after hot forging faster than in (1), the surface layer of the hub wheel 4 can be made into a pure pearlite structure P with an area ratio of protereminate ferrite of 1% or less. As a result, the occurrence of cracks originating from protereminate ferrite is suppressed, and the fatigue strength can be increased compared to a mixed structure of protereminate ferrite and pearlite structure like in (1).

[0041] The line labeled (4) in Figure 4 shows the case where the hub ring 4 is rapidly cooled after hot forging. In the illustrated example, the hub ring 4 is cooled to the martensitic transformation point (approximately 300°C) in about 2 to 3 seconds from the start of cooling. When the hub ring 4 is rapidly cooled in this way, the surface layer becomes a martensitic or bainite structure. These structures are very hard (HV 700 to 800), so the fatigue strength is high, but it becomes difficult to form bolt holes and female splines afterward. In contrast, as shown in (2) or (3) in Figure 4, if the surface layer of the hub ring 4 is made into a pure pearlite structure P, it can be made harder than the mixed structure of ferrite and pearlite as in (1), and still be hard enough to form bolt holes and female splines. Specifically, for example, the surface hardness of the hub ring 4 can be made to HRC 25 to 35 (≒ HV 267 to 343).

[0042] Subsequently, the hub ring 4 is turned, and bolt holes and female splines are formed. Then, a portion of the surface of the hub ring 4 is subjected to high-frequency induction hardening to form a hardened portion H (see the scattered dot area in Figure 1). After that, the inner raceway surface 44a of the hub ring 4 is ground to complete the hub ring 4. Finally, the hub ring 4, inner ring 5, rolling elements 3, cage 6, and outer member 1 are assembled to complete the wheel bearing device shown in Figure 1.

[0043] The present invention is not limited to the embodiments described above. Other embodiments of the present invention will be described below, but redundant explanations of points similar to those described above will be omitted.

[0044] In the above-described embodiment, the case where the entire hub ring 4 after hot forging is immersed in a molten salt bath to make the entire surface of the hub ring 4 have a pure pearlite structure P has been shown, but it is not limited thereto. For example, by controlling the cooling rate of a part of the hub ring 4 after hot forging, only a part of the surface of the hub ring 4 may have a pure pearlite structure P. In this case, it is preferable that the surface layer of the region that becomes the non-quenched part H where high-frequency quenching is not performed thereafter, particularly the region including the attachment root 49 on the outboard side of the wheel mounting flange 42, has a pure pearlite structure P. In this case, the non-quenched part S is provided with a part whose surface layer is composed of a pure pearlite structure P and a part whose surface layer is composed of a mixed structure P+F of primary ferrite and a pearlite structure. <> <>

[0045] In the above-described embodiment, the case where the outboard-side inner raceway surface 44a is formed on the hub ring 4 and the inboard-side inner raceway surface 44b is formed on the inner ring 5 has been shown, but it is not limited thereto. For example, a pair of inner rings may be fixed to the outer periphery of the hub ring 4, and inner raceway surfaces may be formed on the outer peripheries of the respective inner rings. <> <>

[0046] In the above-described embodiment, the case where the outer member 1 is the fixed side and the inner member 2 is the rotating side has been shown, but it is not limited thereto, and the present invention is also applicable to a wheel bearing device in which the outer member is the rotating side and the inner member is the fixed side. In this case, the outer member is provided with a hub ring having a wheel mounting flange, and at least a part of the surface layer of this hub ring (for example, the non-quenched part, particularly the attachment root on the outboard side of the wheel mounting flange) has a pure pearlite structure P. <> <>

[0047] In order to confirm the effects of the present invention, a rotating bending fatigue test was conducted. Table 1 below shows the material, heat treatment, structure, ferrite area ratio, and hardness of each test piece (Example, Comparative Example 1, Comparative Example 2), and Table 2 below shows the cooling rate of each test piece. Also, FIG. 5 shows the temperature change during cooling of each test piece. <> <>

[0048] <> <> <> <>

[0049] Comparative Examples 1 and 2 required more than 100 seconds to cool from the start of cooling (1050°C) to the martensitic transformation point Ms + 100°C (approximately 400°C) (see Figure 5), and the average cooling rate was slower than 7°C / second (see Table 2). In contrast, the Example required 10 seconds to 100 seconds to cool from the start of cooling to the martensitic transformation point + 100°C (approximately 400°C) (see Figure 5), and the average cooling rate was between 7°C / second and 80°C / second (see Table 2).

[0050] Cross-sectional photographs of each test specimen after cooling are shown in Figures 6 to 8. Comparative Examples 1 and 2, which underwent heat treatment similar to that of the prior art, showed protereminate ferrite at the grain boundaries of the pearlite structure, as shown in Figures 6 and 7, and the area ratio of each ferrite was greater than 1% (see Table 1). In contrast, the example cooled by the method of the present invention, as shown in Figure 8, consisted almost entirely of pearlite structure, with almost no protereminate ferrite observed at the grain boundaries of the pearlite structure, and the area ratio of ferrite was 1% or less (see Table 1).

[0051] As a result of rotational fatigue testing performed on each test specimen, as shown in Figure 9, the fatigue limits of Comparative Examples 1 and 2, which had a mixed structure of protereminate ferrite and pearlite, were approximately 300 MPa, while the fatigue limit of the Example, which had an area ratio of protereminate ferrite of 1% or less and consisted substantially only of pearlite, was 400 MPa or higher. From these results, it was confirmed that fatigue strength is improved by making the surface substantially only of pearlite.

[0052] The area of ​​the ferrite structure in the cross-section is measured using the following procedure: (1) Etch the cross-section of the specimen with 3% nital to reveal the microstructure. (2) Take a photograph of the cross-section with a metallurgical microscope (magnification 200 to 500x). When using coaxial incident illumination, pearlite appears brown and ferrite appears white. (3) Input the captured image into binarization software. (4) Set RGB thresholds and calculate the area ratio of proecution ferrite at the grain boundaries and ferrite within the pearlite. As thresholds, for example, R:G:B = 165:180:200 for ferrite and R:G:B = 108:100:100 for pearlite.

[0053] In this case, the captured image shows both protereminate ferrite occurring at the grain boundaries and ferrite within the pearlite grains, and it is difficult to distinguish between them through imaging processing (binarization). Therefore, the ferrite area ratio in Table 1 is the sum of protereminate ferrite and ferrite within the pearlite grains. If this value is 1% or less, it can be said that the area ratio of protereminate ferrite is 1%. Alternatively, the area ratio of protereminate ferrite alone can be calculated by selecting only the protereminate ferrite at the grain boundaries in the captured image and calculating their total area.

[0054] 1 Outer member 2 Inner member 3 Rolling element 4 Hub ring 5 Inner ring 6 Cage 7 Seal member 8 Seal member 41 Shaft portion 42 Wheel mounting flange 43 Pilot portion 45 Inner ring fixing portion 49 Base portion 50 Female spline H Hardened portion S Unhardened portion P Pure pearlite structure (pearlite structure with a proecution ferrite area ratio of 1% or less) P+F Pearlite-ferrite mixed structure (pearlite structure with a proecution ferrite area ratio of more than 1%)

Claims

1. A wheel bearing device comprising a rotating body including a hub ring having a flange for wheel mounting, a stationary body attached to a vehicle body, and a double row of rolling elements disposed between the rotating body and the stationary body, wherein the hub ring is formed of carbon steel with a carbon content of 0.45 to 0.75 mass%, and at least a portion of the surface layer of the hub ring is made of a pearlite structure with an area ratio of protoprecipitation ferrite of 1% or less.

2. The wheel bearing device according to claim 1, wherein the surface layer of the hub ring is provided with a hardened portion that has been subjected to high-frequency induction hardening and a non-hardened portion that has not been subjected to high-frequency induction hardening, and at least a part of the non-hardened portion is made of the pearlite structure.

3. The wheel bearing device according to claim 1 or 2, wherein the pearlite structure is present in the area from the surface of the hub ring to a depth of twice the average grain size of the pearlite structure.

4. The wheel bearing device according to claim 1 or 2, wherein the outboard base portion of the wheel mounting flange of the hub wheel is formed in the portion made of the pearlite structure.

5. The wheel bearing device according to claim 4, wherein the base portion has a circular arc cross-section, and the pearlite structure is present in the area from the surface of the base portion to a depth of four times the radius of curvature of the base portion.

6. A method for manufacturing a wheel bearing device comprising a rotating body including a hub ring having a flange for mounting a wheel, a stationary body attached to a vehicle body, and a double row of rolling elements disposed between the rotating body and the stationary body, wherein the hub ring is formed of carbon steel with a carbon content of 0.45 to 0.75% by weight, and the method for manufacturing a wheel bearing device comprises the steps of: heating the hub ring to a predetermined temperature in the austenite region; and cooling at least a portion of the hub ring heated to the predetermined temperature to the martensitic transformation point + 100°C in 10 seconds or more and within 100 seconds from the start of cooling.

7. The method for manufacturing a wheel bearing device according to claim 6, wherein the hub ring is formed by hot forging while being heated to the predetermined temperature, and then cooled.

8. The method for manufacturing a wheel bearing device according to claim 6 or 7, wherein the cooling is performed on the region of the hub wheel that includes the outboard base portion of the wheel mounting flange.

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