JP2006241559A - Rolling bearing unit for supporting wheel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve both of cold workability and strength after cold working in forming a hub ring and outer ring of a rolling bearing unit for supporting wheels by subjecting a stock material composed of carbon steel sheet to cold working. <P>SOLUTION: At least either of the hub ring 10 (the first member) and the outer ring 30 (the second member) is formed by cold working the stock material composed of a steel sheet of ferrite and pearlite structure of ≤82.0 in hardness HRB and ≥0.50 μm in lamellae spacing of pearlite obtained by heating and holding steel having a C content of ≥0.48 but ≤0.58mass%, an Si content of ≤0.50mass%, an Mn content of ≤0.90mass%, a Cr content of ≤0.50mass%, and the balance Fe and inevitable impurities at a temperature above the AC<SB>1</SB>transformation point below the AC<SB>3</SB>transformation point, and subjecting the steel to soft annealing treatment of cooling the steel to a temperature below the Ar<SB>1</SB>transformation point, holding the same at a temperature below the Ar<SB>1</SB>transformation point then slowly cooling the steel. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a wheel support rolling bearing unit for rotatably supporting a wheel of an automobile or the like with respect to a suspension device.

An example of a conventional wheel support rolling bearing unit is shown in FIG.
As shown in FIG. 12, the wheel support rolling bearing unit includes a hub wheel (first member) 10, an inner ring 20, an outer ring (second member) 30, and a plurality of rolling elements 40. .
A wheel mounting flange 120 is integrally formed on the outer peripheral surface of the hub wheel 10 on the outboard side (the end portion on the outer side in the vehicle width direction in the assembled state in the automobile, which indicates the left end portion in FIG. 12). ing. On the side surface of the wheel mounting flange 120, a plurality of bolt holes 120a for mounting a brake rotor and wheels (not shown) with hub bolts 60 are provided at substantially equal intervals in the circumferential direction.

On the other hand, a small-diameter step portion 130 is formed on the inboard side of the hub wheel 10 (the end portion on the inner side in the vehicle width direction in the assembled state in the automobile, which indicates the right end portion in FIG. 12). The inner ring 20 is fitted to the small diameter step portion 130. Inner ring raceway surfaces 10 a and 20 a are formed on the outer peripheral surface of the intermediate portion in the axial direction of the hub wheel 10 and the outer peripheral surface of the inner ring 20, respectively.
Further, the tip of the hub wheel 10 on the inboard side is formed in a cylindrical shape, and the cylindrical portion (clamping portion) is caulked and expanded outward in the radial direction, whereby the small-diameter step portion 130 of the hub wheel 10 is predetermined. The inner ring 20 is fixed at the position. In addition, the inner ring 20 may apply a necessary pressurizing force by tightening a nut in addition to caulking.

Double row outer ring raceway surfaces 30 a and 30 b corresponding to the inner ring raceway surfaces 10 a and 20 a of the hub wheel 10 and the inner ring 20 are formed on the inner peripheral surface of the outer ring 30. In the outer ring 30, a suspension device mounting flange 310 is integrally formed at the end of the hub wheel 10 on the side away from the wheel mounting flange 120.
A plurality of rolling elements 40 are interposed between the inner ring raceway surfaces 10 a and 10 b formed on the hub wheel 10 and the inner ring 20 and the outer ring raceway surfaces 30 a and 30 b formed on the outer ring 30 via the cage 50. It is arranged so that it can roll freely.

In addition, in FIG. 12, although the case where a ball is used as the rolling element 40 is described, a tapered roller may be used as the rolling element 40 in the case of a wheel support rolling bearing unit that is heavy.
In order to assemble the wheel bearing rolling bearing unit having the above-described configuration to the automobile, the suspension device mounting flange 310 of the outer ring 30 on the non-rotating side is fixed to the suspension device (not shown) and the wheel ring of the hub wheel 10 on the rotating side is mounted. A brake rotor and a wheel (not shown) are fixed to the flange 120 via the hub bolt 60. Thereby, a wheel can be rotatably supported with respect to a suspension apparatus.

The hub wheel 10 constituting such a wheel-supporting rolling bearing unit is formed by forming a material made of medium carbon steel such as S53C into a predetermined shape by hot forging (hot working), and then allowing to cool and performing initial analysis. It is formed by turning, grinding, drilling, etc. on the intermediate material of ferrite and pearlite structure.
Further, the inner ring raceway surfaces 10a and 20a and the outer ring raceway surfaces 30a and 30b are formed with a hardened layer T by induction hardening in order to ensure a rolling fatigue life and prevent fretting at the fitting portion of the inner ring 20.

On the other hand, most of the wheels including the wheel mounting flange 120 and the suspension device mounting flange 310 are not subjected to quenching and tempering treatment to facilitate the work of cutting the support holes and the bolt holes 120a. It is formed as an intermediate material in a cooled state.
That is, in the wheel mounting flange 120 and the suspension device mounting flange 310, hot workability when forming into a predetermined shape, cutting workability when forming the bolt hole 120a and the like, and a wheel bearing rolling bearing unit. And strength when used as a battery is required. For this reason, in the hub ring 10 and the outer ring 30 formed by hot working, means for imparting strength by increasing the thickness of the flange is used, and there is room for further improvement in terms of weight reduction. was there.

Patent Document 1 proposes that an outer ring on the rotating side is formed by subjecting a material made of a steel plate to cold forging (cold working) in order to realize both strength and weight reduction. According to this method, processing accuracy and strength can be improved by work hardening at the time of cold processing as compared with the case of forming by conventional hot processing. Further, by using a material made of a steel plate, the thickness of the flange can be reduced as compared with the case where it is formed by conventional hot working, so that the weight of the entire outer ring can be reduced. Furthermore, in the case of forming by cold working, since the support hole and bolt hole of the flange can be formed by drilling (piercing), cutting can be omitted, and it can be formed at low cost.
JP 7-317777 A

By the way, it is known that the formability during cold working (hereinafter referred to as “cold workability”) depends on the hardness of the material and the state of the metal structure before cold working. However, in Patent Document 1 described above, although a method of cold working is mentioned, no disclosure is made regarding the hardness of the material and the state of the metal structure before cold working.
Here, as a material for the hub wheel and the outer ring of the wheel bearing rolling bearing unit, medium carbon steel such as S53C is used in order to ensure the necessary strength by subjecting the raceway surface to induction hardening.

For this reason, if the carbon steel plate which consists of S53C etc. which were mentioned above is used as a raw material of the outer ring | wheel described in patent document 1, since a raw material is too hard, a cold workability will fall and a crack will arise, or at the time of cold work The life of the mold used may be reduced. On the other hand, when a material having a hardness lower than that of the carbon steel plate described above is used as the material of the outer ring described in Patent Document 1, the cold workability is improved, but the strength after the cold work becomes insufficient.
Therefore, the present invention has been made in view of such circumstances, in the case of forming the hub ring and the outer ring of the wheel bearing rolling bearing unit by subjecting a material made of a carbon steel plate to cold working, It is an object to improve both cold workability and strength after cold work.

In order to solve such a problem, the wheel support rolling bearing unit of the present invention includes a first member having a raceway surface formed on the outer peripheral surface, a second member having a raceway surface formed on the inner peripheral surface, A plurality of rolling elements disposed between the first member and the second member so as to be freely rollable, wherein one of the first member and the second member is attached to a suspension device, and the other In the wheel bearing rolling bearing unit for rotatably supporting the wheel with respect to the suspension device by attaching a wheel to the member, at least one of the first member and the second member is: C content is 0.48 mass% or more and 0.58 mass% or less, Si content is 0.50 mass% or less, Mn content is 0.90 mass% or less, and Cr content is 0.50 mass. % And the balance is Fe and inevitable impurities Respect, and heated and held at A _c1 transformation point or above A _c3 transformation point temperature, then cooled to a temperature lower than A _r1 transformation temperature and held at a temperature of less than A _r1 transformation temperature, then gradually cooled The material obtained by the softening and annealing treatment, the hardness is HRB82.0 or less, and the pearlite lamellar spacing (layer spacing of the layered pearlite) is 0.50 μm or more of a material composed of ferrite and pearlite steel. It is characterized by being formed by cold working.
That is, in the present invention, at least one of the first member and the second member constituting the wheel support rolling bearing unit is formed by subjecting a material made of the following specific steel plate to cold working.

Hereinafter, the material before cold working in the present invention will be described in detail.
[Regarding the steel plate of the material]
First, the present inventors verified a structure having both cold workability and strength after cold working out of a metal structure obtained by subjecting carbon steel to soft annealing.
First, from S55C having a hardness of HRB92 (C content: 0.53 mass%, Si content: 0.2 mass%, Mn content: 0.76 mass%, Cr content: 0.11 mass%) A hot rolled steel sheet was prepared.
Next, this steel sheet was subjected to the softening annealing process shown in FIG. 1 to obtain the ferrite and pearlite structure shown in FIG. Similarly, the steel and the spheroidized cementite structure shown in FIG. 4 were obtained by subjecting the steel sheet to a softening annealing treatment shown in FIG.

Next, cold rolling (cold rolling) is performed on the material made of the steel sheet after the above-described heat treatment under the conditions that the cold working rate calculated by the following formula (1) is 0%, 30%, and 50%. Processing).
Cold work rate (%) = (plate thickness after rolling) / (plate thickness before rolling) × 100 (1)
Next, the steel sheet after cold working was subjected to a tensile test specified in JIS Z 2241 to measure the yield stress (stress that produces a residual strain of 0.2%), and the strength of each steel sheet was evaluated. . And the graph of FIG. 5 which shows the relationship between the cold work rate and the yield stress of a steel plate was created.

  As shown in FIG. 5, it can be seen that the yield stress increases in both the material made of the steel plate of ferrite and pearlite structure and the material of the steel plate made of ferrite and spheroidized cementite structure as the cold working rate increases. . Moreover, the yield stress when the cold work rate is 0% is larger in the material made of steel sheets of ferrite and spheroidized cementite structure, and the yield stress when the cold work rate is 30% or more is the ferrite and pearlite structure. It can be seen that the material made of the steel plate is larger.

  From this, steel sheets with ferrite and pearlite structure are less deformable before cold working than steel sheets with ferrite and spheroidized cementite structure, excellent in cold workability, and strength after cold working Is high. Therefore, in order to have both the cold workability and the strength after the cold work, in the present invention, the steel before the cold work is made a steel plate of ferrite and pearlite structure.

Secondly, the present inventors have conducted intensive studies on the structure conditions that can further improve the cold workability of a material made of a steel plate having a ferrite and pearlite structure. As a result, the present inventors have found that it is sufficient to increase the pearlite lamella spacing, which affects the hardness improvement, to obtain a rough pearlite structure, and verified the optimum lamella spacing.
First, as shown in FIG. 6, a hot-rolled steel sheet made of S55C similar to that described above was heated and maintained at 760 ° C. for 4 hours, and then a predetermined temperature between 580 to 710 ° C. (580 ° C., 600 ° C., 630 ° C., 650 ° C., 680 ° C., 690 ° C., 690 ° C., 700 ° C., 710 ° C.) and held for 4 hours, followed by a soft annealing treatment that gradually cools to 550 ° C. over 10 hours. , Ferrite and pearlite structure.

Next, the hardness and the lamellar spacing of pearlite were measured in the steel plate after the heat treatment.
The hardness was measured from the surface to a depth of at least 200 μm using the Rockwell hardness test method defined in JIS Z 2245.
The pearlite lamella spacing was measured by the following procedure. First, the heat-treated steel sheet was mirror-polished, etched with a nital etchant, and observed with a 500 × optical microscope. next,
A portion having the highest pearlite density in one field of view was observed with a 1000 × optical microscope or a 2000 × scanning electron microscope, and an arbitrary five lamellar spacing was measured. This measurement was performed for five visual fields, and the average value of lamella intervals for five visual fields was calculated.

And the graph of FIG. 7 which shows the relationship between the hardness of the steel plate after heat processing, and the lamellar space | interval of a pearlite was created.
As shown in FIG. 7, it can be seen that the hardness decreases as the pearlite lamella spacing increases. In the present invention, in order to obtain a hardness (HRB82.0 or less) that can impart excellent cold workability to the hub wheel and the outer ring, the pearlite lamella spacing is set to 0. 50 μm or more, preferably 0.55 μm or more.

  On the other hand, in order to make the pearlite lamella spacing in the material made of steel sheets exceed 0.65 μm, a long-time heat treatment is required, resulting in a decrease in production efficiency and an increase in cementite spheroidization due to the long-time heat treatment. , Cold workability decreases. Therefore, it is preferable that the pearlite lamella spacing in the material made of the steel plate is 0.65 μm or less.

Next, using the specific steel shown below, soft annealing is performed to form a material composed of a steel plate of ferrite and pearlite structure having a hardness of HRB 82.0 or less and a pearlite lamellar spacing of 0.50 μm or more. The processing will be described with reference to FIG.
[About specific steel]
<C content (mass%): 0.48% or more and 0.58% or less>
C (carbon) has an effect of improving quenching hardness, and is an element necessary for curing the raceway surface by induction quenching. The C content is set to 0.48% or more in order to impart the necessary hardness (HRC 58 or more) to the raceway surface of the wheel supporting rolling bearing unit.
On the other hand, if the C content is too high, the cold workability deteriorates, so the C content is 0.58% or less, preferably 0.55% or less.

<Si content (mass%): 0.50% or less>
Si (silicon) has the effect of improving the rolling fatigue life by dissolving in the base. In order to obtain this effect, the Si content is preferably 0.1% or more. However, when the Si content is too high, Si is solid-solved in the ferrite and hardened, and the cold workability is lowered. Therefore, the Si content is set to 0.50% or less.

<Mn content (mass%): 0.90% or less>
Mn (manganese) is an element added as a deoxidizer and has the effect of improving the hardenability of steel. In order to obtain this effect, the Mn content is preferably 0.3% or more. However, if the Mn content is too high, Mn is solid-solved in the ferrite and hardens to lower the cold workability, so the Mn content is set to 0.90% or less.

<Cr content (mass%): 0.50% or less>
Cr (chromium) is a carbide forming element and has an effect of improving hardness and hardenability. In order to obtain this effect, the Cr content is preferably 0.10% or more. However, if the Cr content is too high, Cr is solid-solved in the ferrite and hardened, and the cold workability is lowered. Therefore, the Cr content is set to 0.50% or less.

[About softening and annealing treatment]
First, as shown in FIG. 8, the temperature is raised from room temperature to the austenitizing temperature range (above the A _c1 transformation point) (step 1). The temperature raising time is preferably 1 to 4 hours in order to allow soaking of the object to be heated.
Next, A _c1 transformation point or above A _c3 transformation point of austenitizing temperature range (e.g., seven hundred forty to seven hundred seventy ° C.) for heating and maintaining at (Step 2).
Here, when the A _c1 transformation point is exceeded, the metal structure of the steel becomes an austenite single phase structure, and ferrite and pearlite are precipitated from the supercooled austenite in the subsequent cooling step. However, when the austenitizing temperature is high, the degree of supercooling tends to be high and the lamella spacing of pearlite tends to be large, so the heat is held below the A _c3 transformation point.

Further, in the range from the A _c1 transformation point to the A _c3 transformation point, it becomes a two-phase region of γ-ferrite and α-ferrite, and the amount of ferrite becomes higher than the theoretical precipitation amount, so that it softens. However, just below the A _c3 transformation point, it is not suitable because the amount of ferrite precipitated becomes small. Therefore, the upper limit value of the austenitizing temperature range is preferably set to ( _Ac3 transformation point −20) ° C.
On the other hand, the lower limit of the austenitizing temperature range is preferably close to the A _c1 transformation point because the theoretical precipitation amount of ferrite increases as the degree of supercooling of austenite decreases. Considering temperature variation, it is preferable to set it to (A _c1 transformation point + 20) ° C.

The heating and holding time in the austenitizing temperature range is preferably 0.5 hours to 4 hours, more preferably 1 hour to 2 hours in consideration of production efficiency.
Next, it is cooled to less than the A _r1 transformation point (constant temperature transformation point) (step 3). This cooling time is preferably set to 1 hour or more and 4 hours or less in consideration of soaking of the object to be heated and production efficiency. In addition, the cooling rate at the time of passing through the transformation point in this range of cooling time is 10 to 60 ° C./Hr.

Next, a temperature of less than A _r1 transformation temperature (isothermal transformation point) (e.g., six hundred and eighty-five to seven hundred fifteen ° C.) and held at (step 4). In this process, austenite is transformed into pearlite from the two-phase structure of ferrite and austenite by isothermal transformation, and becomes a ferrite and pearlite structure, and the lamella spacing of pearlite is determined.
Here, if it is held at a temperature just below the A _r1 transformation point, the pearlite lamella spacing becomes the largest and softens, but a long-time heat treatment is required. On the other hand, when it is held at a temperature lower than (A _r1 transformation point −40) ° C., the degree of supercooling of austenite increases, the lamella spacing of pearlite decreases, and it hardens. Therefore, retention after cooling is preferably kept at (A _r1 transformation temperature -40) ° C. or higher (A _r1 transformation temperature -10) ° C. or lower.

If the transformation of austenite is not completely completed, pearlite with a small lamellar interval is precipitated from untransformed austenite in the next slow cooling step and is difficult to soften. _Therefore , the holding time at a temperature below the _Ar1 transformation point is A time (for example, 2 to 20 hours) in which all transformations of austenite are completed is taken.
Next, slow cooling is performed (step 5). In this step, first, the transformation is completed and the ferrite layer and the pearlite structure are cooled to a temperature of, for example, 650 to 685 ° C. over 2 to 10 hours so that the layered pearlite is divided (step 5A). Subsequently, in the ferrite and pearlite structure after the transformation is completed, in order to advance softening by precipitating carbon slightly remaining in the ferrite as pearlite, it takes 5 to 10 hours, for example, to a temperature of 500 to 580 ° C. (Step 5B).

In this annealing step, as shown in FIG. 8, it may be cooled in two stages, as shown in FIG. 9, after holding at a temperature of less than A _r1 transformation temperature of 500 to 580 ° C. You may make it cool to temperature over 5 to 15 hours.
A hub wheel is obtained by cold-working a material made of a ferrite and a pearlite steel plate having a hardness of HRB 82.0 or less and a pearlite lamella spacing of 0.50 μm or more obtained in this way. And at least one of the outer rings.
In addition, although it does not specifically limit with the cold work in this invention, For example, cold forging, cold rolling, and cold press are mentioned.

  According to the rolling bearing unit for supporting a wheel of the present invention, at least one of the hub ring and the outer ring is cold worked on a material made of a steel plate of ferrite and pearlite structure in which hardness and lamella spacing of pearlite are specified. Therefore, both the cold workability and the strength after cold working can be improved.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[First embodiment]
In the present embodiment, the hub wheel (first member) 10 of the conventional wheel support rolling bearing unit already described with reference to FIG. 12 is formed by the following procedure.
First, from S55C having a hardness of HRB97 (C content: 0.53 mass%, Si content: 0.22 mass%, Mn content: 0.76 mass%, Cr content: 0.11 mass%) A hot-rolled steel sheet (thickness: 9 mm) was prepared.

Next, the steel plate was subjected to the softening annealing treatment shown in FIG. 8 under the respective heat treatment conditions shown in Table 1.
In the steel plate thus obtained, hardness and pearlite lamella spacing were measured using the same method as described above. The results are also shown in Table 1.
Next, the obtained steel plate was punched out, and the material was subjected to cold forging (cold working) shown in FIG. 10 to complete the hub wheel 10. That is, as shown in FIG. 10, first, the material is first subjected to deep drawing and piercing in order, then formed into the final shape of the hub wheel 10, the wheel mounting flange 120 is formed by trim, and the wheel is mounted by piercing. Bolt holes 120a were formed in the flange 120 for use.
And the surface shape of the hub wheel 10 after completion was confirmed visually, and the crack generation number per 1000 pieces was calculated. The results are also shown in Table 1.

As shown in Table 1, the hardness of the material made of the steel plate before cold working and the lamella spacing of pearlite are No. in the scope of the present invention. 1-No. In No. 33, the number of cracks generated during cold working was zero.
On the other hand, no. 34-No. In No. 43, since at least one of the hardness of the material made of the steel plate before cold working and the lamella spacing of pearlite was outside the scope of the present invention, cracks during cold working occurred.
From the above results, excellent cold workability can be obtained by using a steel plate of ferrite and pearlite structure with hardness and pearlite lamella spacing within the scope of the present invention as the material of the hub wheel 10 of the wheel bearing rolling bearing unit. It turns out that it is obtained.

[Second Embodiment]
In the present embodiment, the hub wheel (first member) 10 of the wheel support rolling bearing unit already described with reference to FIG. 12 is formed by the following procedure.
First, the steels containing C, Si, Mn, and Cr having the respective contents shown in Table 2 were subjected to No. 1 shown in Table 1. The softening annealing process of 2 was given and the raw material which consists of a steel plate was formed.
In the steel plate thus obtained, the hardness and the lamella spacing of pearlite were measured using the same method as described above. The results are also shown in Table 2.
Next, the obtained steel sheet was punched out, and the above-described cold forging (cold working) shown in FIG. 10 was performed as a material to complete the hub wheel 10.
Then, in the hub wheel 10 after completion, the number of cracks generated per 1000 pieces was calculated in the same manner as described above. The results are also shown in Table 2.

As shown in Table 2, a steel plate which is a raw material before being cold worked is No. formed from steel which is within the scope of the present invention. 51-No. In No. 58, the hardness and the lamella spacing of pearlite were able to be a ferrite and pearlite structure within the range of the present invention, and the number of cracks during cold working was zero.
On the other hand, the steel plate which is a raw material before cold working is No. formed from steel outside the scope of the present invention. 59-No. No. 62, even if the pearlite lamella spacing was within the range of the present invention, the hardness could not be within the range of the present invention, and cracking occurred during cold working.
From the above results, the steel sheet as the material of the hub wheel 10 of the rolling shaft housing unit for supporting the wheel is composed of the steel of the present invention, so that the ferrite and pearlite structure whose hardness and pearlite lamellar spacing are within the scope of the present invention are obtained. It was found that excellent cold workability was obtained.

[Third embodiment]
In the present embodiment, the hub wheel (first member) 10 of the conventional wheel support rolling bearing unit already described with reference to FIG. 12 is formed by the following procedure.
First, no. 71-No. In 73, the soft annealing process shown in FIG. 8 was performed on each heat treatment condition shown in Table 3 using the hot rolled steel plate (sheet thickness: 9 mm) which consists of S55C similar to 1st embodiment.
On the other hand, no. In 74, the softening annealing process shown in FIG. 3 mentioned above was performed using the hot rolled steel plate (sheet thickness: 9 mm) made of S55C as in the first embodiment.

In the steel sheet thus obtained, the hardness of the part from the surface to a depth of at least 200 μm was measured using the same method as described above. Further, the obtained steel sheet was observed with a 500-fold optical microscope using the same method as described above, and the structural structure was confirmed. The results are also shown in Table 3. In addition, FIG. It is an optical microscope photograph of 71 steel plates.
Next, the obtained steel sheet was punched out, and the above-described cold forging (cold working) shown in FIG. 10 was performed as a material to complete the hub wheel 10.
On the side surface of the wheel mounting flange 120 of the hub wheel 10 obtained in this way, the hardness of the portion from the surface to a depth of at least 200 μm was measured by the same method as described above. The results are also shown in Table 3.

  The obtained hub wheel 10 and the brake rotor (ordinary product) were fastened to the wheel mounting flange 120 at 100 N · m via the hub bolt 60 and the nut. Thereafter, the hub bolt 60 and the nut were removed, and the deformation amount of the flange surface was measured at ± 10 mm positions inside and outside at the PCD position of the bolt hole 120a of the hub bolt 60. And the deformation resistance value of the flange surface was calculated based on the maximum deformation amount. This result is shown in No. The ratio when the deformation resistance value of 71 is 1 is also shown in Table 3.

As shown in Table 3, a material made of a steel sheet before cold working was made into a ferrite and pearlite structure. 71-No. No. 73, a ferrite and spheroidized cementite structure. It can be seen that the deformation resistance value of the flange surface is larger than 74, and the cold workability is excellent. In addition, the material before cold working was ferrite and pearlite structure No. 71-No. No. 73 is a ferrite and spheroidized cementite structure. Compared to 74, it was the same degree of hardness at the material stage, but it can be seen that the hardness after cold working is increased.
From the above results, it was found that both the cold workability and the strength after cold working can be improved by forming the hub wheel 10 by cold working a material made of a steel plate of ferrite and pearlite structure. .

It is a schematic diagram which shows an example of a softening annealing process. It is an optical microscope photograph which shows the ferrite and pearlite structure | tissue obtained by the heat processing of FIG. It is a schematic diagram which shows the other example of a softening annealing process. 4 is an optical micrograph showing the ferrite and spheroidized cementite structure obtained by the heat treatment of FIG. 3. It is a graph which shows the relationship between a cold work rate and the yield stress of a steel plate. It is a schematic diagram which shows the other example of a softening annealing process. It is a graph which shows the relationship between the lamella space | interval of a pearlite, and the hardness of a steel plate. It is a schematic diagram which shows an example of the softening annealing process which concerns on this invention. It is a schematic diagram which shows the other example of the softening annealing process which concerns on this invention. It is a schematic diagram which shows an example of cold work. No. It is an optical microscope photograph of the steel plate which makes the material of 71. It is sectional drawing which shows an example of the rolling bearing unit for wheel support.

Explanation of symbols

10 Hub wheel (first member)
10a Inner ring raceway surface (track surface)
20 Inner ring 30 Outer ring (second member)
30a Outer ring raceway surface (track surface)
40 Rolling elements

Claims (1)

A first member having a raceway surface formed on an outer peripheral surface; a second member having a raceway surface formed on an inner peripheral surface; and a plurality of members disposed so as to be freely rollable between the first member and the second member. A rolling element, wherein one member of the first member and the second member is attached to a suspension device, and a wheel is attached to the other member, whereby the wheel is rotatable with respect to the suspension device. In the wheel bearing rolling bearing unit for supporting
At least one of the first member and the second member is
C content is 0.48 mass% or more and 0.58 mass% or less, Si content is 0.50 mass% or less, Mn content is 0.90 mass% or less, and Cr content is 0.50 mass. %, With the balance being Fe and inevitable impurities, heat and hold at a temperature not lower than the A _c1 transformation point and not higher than the A _c3 transformation point, and then cooled to a temperature lower than the A _r1 transformation point to _cause the A _r1 transformation. Of ferrite and pearlite structure obtained by a softening and annealing treatment that is held at a temperature below the point and then gradually cooled, with a hardness of HRB 82.0 or less and a pearlite lamellar spacing of 0.50 μm or more. A rolling bearing unit for supporting a wheel, which is formed by subjecting a material made of steel to cold working.

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