JP7363663B2 - Rolling bearing and its manufacturing method - Google Patents

Description

The present invention relates to a rolling bearing and a manufacturing method thereof.

It is known that when a rolling bearing receives an excessive load while it is stationary, Hertz contact occurs between the outer raceway ring, the inner raceway raceway, and the rolling elements, leaving permanent deformation (Brinell impression). The presence of such impressions affects acoustic and vibration characteristics when the rolling bearing is used. For example, in rolling bearings used in applications that rotate at high speeds such as machine tools, even a minute indentation of about 1 μm causes abnormal noise and vibration, which poses a serious problem. For this reason, when designing rolling bearings, the static limit load (basic static load rating) is determined by contact stress, and JIS B 1519 (2009) specifies, for example, thrust ball bearings and self-aligning ball bearings It is specified that the contact stress of radial ball bearings, excluding the following, shall be 4.2 GPa.

In addition, as rolling bearings become smaller due to lower fuel consumption in automobiles and the like, they need to have plastic deformation resistance that can withstand excessive loads. Conventionally, in order to improve the plastic deformation resistance of rolling bearings, it is important to balance the hardness of the outer ring raceway and inner ring raceway of the rolling bearing with the amount of retained austenite. Efforts are being made to improve permanent deformation resistance and indentation resistance by reducing retained austenite, which is the soft structure of steel.

For example, Patent Document 1 describes a technique in which high carbon chromium bearing steel is carbonitrided and tempered. Further, Patent Document 2 describes a technique in which sub-zero treatment is applied to raceway surfaces of inner and outer rings of a bearing.

JP 2015-200351 Publication Japanese Patent Application Publication No. 2000-274440

However, special heat treatment such as carbonitriding requires a long time and requires a separate process, which increases manufacturing costs. In addition, there is a concern that sub-zero treatment may reduce toughness, and a separate process is required, which increases manufacturing costs.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a rolling bearing with improved indentation resistance at low cost and a method for manufacturing the same.

As described above, if an attempt is made to improve the indentation resistance of a rolling bearing only by heat treatment and component adjustment, other functions such as cracking resistance will be degraded, and productivity will be reduced. Therefore, it is necessary to solve the problem without impairing the function of the rolling bearing.

On the other hand, the formation of indentations in rolling bearings is due to plastic deformation of the raceway surface due to contact between the rolling elements and the raceway ring, and can be explained as a simple material yield phenomenon. There are various methods for strengthening materials, and in addition to heat treatment and component adjustment, a method called work hardening is known. The present inventors have discovered that the indentation resistance of a rolling bearing can be further improved by machining hardened steel that has been hardened to the maximum extent through heat treatment. Various methods are known for machining rolling bearings, such as burnishing and shot peening, and have been realized industrially. When performing such machining on a rolling bearing, the greater the machining force, the better the resulting indentation resistance. As the machining force increases, the mechanism that applies the load becomes complicated and large, and the tools used to process the raceway surface become more worn out, so it is preferable to obtain a large amount of hardening with as little machining force as possible. .

When high carbon steel used in rolling bearings is heat treated, it becomes a structure in which a soft structure called retained austenite is mixed with a hard structure called martensite. Soft retained austenite is a factor that reduces indentation resistance, but it has the property of transforming into martensite when large stress is applied during machining. The transformation to martensite has the characteristic that it can greatly improve the hardness of rolling bearings. Therefore, by performing machining on a starting material in which retained austenite remains in advance, large work hardening can be obtained and the amount of retained austenite that is harmful to indentation resistance can be reduced.

The present invention is based on such knowledge, and provides rolling bearings shown in (1) and (2) below in order to solve the above problems.

(1) In a rolling bearing in which a plurality of rolling elements are held between an inner ring and an outer ring so as to be able to roll freely,
A region on the raceway surface of at least one of the inner ring and the outer ring that is not affected by machining and has a residual austenite amount of 10% or more is a first region,
In the raceway surface that contacts the rolling elements, a second region is defined as a region in which the amount of retained austenite is smaller than the amount of retained austenite in the first region, and the hardness is higher than the hardness of the first region. Characteristic rolling bearings.

(2) The second region is
(a) the amount of retained austenite is 78% or less of the amount of retained austenite in the first region;
(b) The rolling bearing as described in (1) above, wherein the hardness is 104% or more than the hardness of the first region.

Moreover, the above-mentioned problem is solved by the following method for manufacturing a rolling bearing related to the present invention.

(3) The method for manufacturing a rolling bearing according to (1) or (2) above,
After quenching and tempering the inner ring and outer ring,
Machining is performed on the raceway surface of at least one of the inner ring and the outer ring under the condition that the maximum contact surface pressure with the processing jig is 8.2 GPa or more, which is calculated on the assumption that only elastic deformation occurs. Characteristic manufacturing method for rolling bearings.

According to the present invention, for example, in a bearing such as a machine tool where a static load is applied to the bearing ring and Brinell impressions are likely to occur, it is possible to suppress the formation of minute impressions of about 1 μm that occur in the bearing ring. Further, separate processes such as carbonitriding treatment and sub-zero treatment are not required, and manufacturing costs do not increase.

FIG. 1 is a partially cutaway perspective view showing a radial ball bearing, which is an example of a rolling bearing according to the present invention.

Embodiments of the present invention will be specifically described below. However, the present invention is not limited to the following embodiments, and can be modified and applied as appropriate without changing the gist of the present invention.

In the present invention, there are no restrictions on the type or configuration of the rolling bearing, and for example, a radial ball bearing shown in FIG. 1 may be used. As shown in the figure, the radial ball bearing 1 includes an outer ring 3 having an outer ring raceway surface 2 on its inner peripheral surface, an inner ring 5 having an inner ring raceway surface 4 on its outer peripheral surface, and the outer ring raceway surface 2 and the inner ring raceway surface 4. A plurality of balls 6, each of which is a rolling element, are provided between the balls. These balls 6 are rotatably held by a cage 7 in a state where they are arranged at equal intervals in the circumferential direction.

There are no restrictions on the materials of the outer ring 3, inner ring 5, and balls 6, and general bearing steel materials such as SUJ2 and SUJ3 can be used. Therefore, there is no need to adjust the additive elements of the steel material, and material costs can be reduced.

The above-mentioned radial ball bearing 1 is manufactured by first subjecting an annular material to a turning process to produce an outer ring blank material and an inner ring blank material having a predetermined shape. Next, each blank material is subjected to quenching and tempering treatment, and the outer ring raceway surface 2 and the inner ring raceway surface 4 are finished to a predetermined precision by polishing. After that, the outer ring raceway surface 2 and the inner ring raceway surface 4 are machined and finally finished. Each process up to machining and finishing can be performed according to conventional methods.

The machining process is not limited as long as it applies compressive stress, but burnishing and shot peening are preferred.

Burnishing is a process in which a processing jig with a spherical tip and a high-hardness component is pressed against the outer ring raceway surface 2 or inner ring raceway surface 4, and the outer ring 3 or inner ring 5 is centered on its own axis. This is a processing method in which compressive stress is applied by rotating the material. Further, shot peening is a processing method in which a highly hard and substantially spherical shot material is injected onto the outer ring raceway surface 2 and the inner ring raceway surface 4. By adjusting processing conditions such as the size, material, and jetting speed of the approximately spherical shot material, it is possible to achieve the same quality as burnishing processing.

By this machining, it is preferable that the amount of retained austenite in the second region is smaller than the amount of retained austenite in the first region, specifically, 78% or less of the amount of retained austenite in the first region. Since retained austenite is soft, it is a factor that reduces indentation resistance, but it is transformed into martensite by machining, improving hardness. Therefore, by setting the amount of retained austenite in the second region to 78% or less of the amount of retained austenite in the first region, hardness can be improved and retained austenite, which is a soft structure, can be reduced, and sufficient indentation resistance can be achieved. You can realize your sexuality.

Through this machining, it is preferable that the hardness of the second region is higher than that of the first region, specifically, 104% or more of the hardness of the first region. By making the hardness of the second region 104% or more of the hardness of the first region, sufficient indentation resistance can be achieved.

Note that the first area, which is deeper than the second area and is not affected by machining such as the core, is the so-called "unprocessed area", and the area affected by machining is the "processed area". ”. This "processing area" also includes the second area.

In order to obtain such an amount of retained austenite, it is preferable to increase the amount of retained austenite in the first region by quenching and tempering to 10% or more, preferably 13% or more, so that a large amount of retained austenite exists before machining.

In addition, when manufacturing the outer ring 3 and inner ring 5 in machining, the relationship between the processing jig and the processing jig is calculated assuming that only elastic deformation occurs after the outer ring 3 and inner ring 5 are quenched and tempered. It is preferable that the maximum contact pressure is 8.2 GPa or more. The larger the maximum contact surface pressure, the more the retained austenite in the processed area undergoes martensitic transformation, and the retained austenite decreases. Therefore, if the maximum contact surface pressure is less than 8.2 GPa, the increase in hardness is small and a large amount of retained austenite is present, making it difficult to obtain sufficient indentation resistance. In addition, it is more preferable that the maximum contact surface pressure is 9.2 GPa or more.

(Preliminary test)
In this example, burnishing was employed as the machining process, and the processing conditions were studied. First, a bearing ring of a predetermined shape was produced using bearing steel (SUJ2 steel) as a raw material, and was subjected to quenching and tempering treatment. Thereafter, the bearing rings were subjected to finishing processing, and then burnishing processing was performed under three different conditions to produce three types of thrust ball bearings with different specifications.

In addition, in the burnishing processing device, the tip shape of the burnishing tool is φ3 mm, the sliding rate is 100%, the circumferential speed is 100 m/min, the feed rate of the burnishing tool is 0.05 mm/rev, and the pushing amount of the burnishing tool is 0.3 mm. be. Also, during machining, filtered working fluid was supplied. Then, the maximum contact pressure (P) between the burnishing tool and the raceway surface during the burnishing process was calculated on the assumption that only elastic deformation occurs. The maximum contact pressures (P) of the various rolling bearings produced in the preliminary tests were P=5.8GPa, P=7.3GPa, and P=8.2GPa, respectively.

Subsequently, Table 1 shows the results of measuring the residual stress in the depth direction from the surface of the raceway surface of the raceway ring using the X-ray diffraction method for each maximum contact surface pressure (P). Note that an area with a depth of 250 μm or more is defined as an unprocessed area (first area), and this area is an area that is not affected by the burnishing process. Further, a region less than 250 μm in depth from the surface (depth 0 μm) is defined as a processed region (second region), and this region is a region affected by the burnishing process.

The reason why the amount of retained austenite at the surface (depth 0 μm) is small is because the amount of retained austenite was reduced to about 5% in the polishing process before the burnishing process.

From the results in Table 1, it can be seen that the larger the burnishing load, that is, the maximum contact pressure (P), the more the residual austenite in the processed area decreases through deformation-induced martensitic transformation, and when the maximum contact pressure is 8.2 GPa or more, It can be seen that if there is, the amount of retained austenite in the raceway surface surface layer, which is the processed region (second region), can be reduced to at least 78% or less. In other words, by setting the amount of retained austenite in the second region to 78% or less of the amount of retained austenite in the first region, hardness is improved, retained austenite, which is a soft structure, is reduced, and sufficient indentation resistance is achieved. You can realize your sexuality.

(Example 1)
A test piece was prepared by forming a flat plate using bearing steel, quenching at 840°C, tempering at 180°C, and performing burnishing after flat finishing. The maximum contact surface pressure in the burnishing process was determined to be 9.2 GPa because it is sufficient to have 8.2 GPa or more from the preliminary test described above. Note that other processing conditions were the same as above.

(Comparative example 1)
A test piece was produced in the same manner as in Example 1, except that it was quenched at 840°C and tempered at 300°C. That is, Example 1 and Comparative Example 1 were made to have different amounts of retained austenite by changing the quenching and tempering conditions.

(Comparative example 2)
Bearing steel was used to form a flat plate, which was quenched at 840°C and tempered at 180°C, and then flat-finished to give a test piece. That is, in Comparative Example 2, no burnishing process was performed.

(Comparative example 3)
A test piece was prepared in the same manner as in Comparative Example 2, except that it was quenched at 840°C and tempered at 300°C. That is, in Comparative Example 3, no burnishing process was performed.

Table 2 shows the results of measuring the amount of retained austenite in the depth direction from the raceway surface in Example 1 and Comparative Example 1 by X-ray diffraction. Since Example 1 and Comparative Example 1 are hard-baked products, from the results of the unprocessed area, the amount of retained austenite in the raceway surface layer (corresponding to the processed area) of Example 1 and Comparative Example 1 before burnishing is as follows: They are about 13% and about 1%, respectively. For bearing steel that has been subjected to standard heat treatment, the amount of retained austenite is about 10±5%, and from the results of measuring three types of thrust ball bearings shown in Table 1 (preliminary test), it is preferably about 10%. As shown in Example 1 of Table 2, it is more preferably 13% or more. The reason for this is that, as shown in Table 5 below, the indentation resistance of Example 1 of 13% is almost twice as good as that of the other Examples. Further, in Example 1, the amount of retained austenite in the burnishing region was reduced to 78% or less compared to the untreated region, and the amount of soft retained austenite was significantly reduced.

Subsequently, Tables 3 and 4 show the measurement results of the hardness in the depth direction from the raceway surface in Example 1 and Comparative Examples 1 to 3, and it can be seen that the hardness was work hardened by burnishing. In Example 1, the hardness of the processed area is increased by 104% or more compared to the unprocessed area. Furthermore, in Example 1, the amount of work hardening in the processed region was greater than in Comparative Example 1. From this, it can be seen that by performing burnishing in a state where a large amount of retained austenite remains, the amount of deformation-induced martensitic transformation of the retained austenite increases, and the hardness increases significantly.

Further, Table 5 shows the results of the indentation test for Example 1 and Comparative Examples 1 to 3. In the indentation test, 3/8-inch steel balls (rolling elements) made by heat-treating bearing steel were used to test Example 1 and Comparative Example, as well as flat test pieces simulating the bearing rings of rolling bearings. After applying a load to each test piece prepared in steps 1 to 3 so that the maximum contact surface pressure was 5.0 GPa, 5.5 GPa, and 6.0 GPa, a three-dimensional surface texture measuring machine manufactured by Taylor Hobson Co., Ltd. ( CCI) was used to measure the depth of an indentation formed on the surface of a flat test piece. From the results in Table 5, it can be seen that Example 1 has the shallowest indentation depth and is the most excellent in indentation resistance.

1 Radial ball bearing 2 Outer ring raceway surface 3 Outer ring 4 Inner ring raceway surface 5 Inner ring 6 Ball 7 Cage

Claims (3)

In a rolling bearing that has a plurality of rolling elements held between an inner ring and an outer ring so as to be able to roll freely,
A region on the raceway surface of at least one of the inner ring and the outer ring that is not affected by machining and has a residual austenite amount of 10% or more is a first region,
In the raceway surface that contacts the rolling elements, a second region is defined as a region in which the amount of retained austenite is lower than the amount of retained austenite in the first region, and the hardness is higher than the hardness of the first region. Characteristic rolling bearings. The second region is
(a) the amount of retained austenite is 78% or less of the amount of retained austenite in the first region;
The rolling bearing according to claim 1, wherein (b) the hardness is 104% or more than the hardness of the first region. A method for manufacturing a rolling bearing according to claim 1 or 2, comprising:
After quenching and tempering the inner ring and outer ring,
Machining is performed on the raceway surface of at least one of the inner ring and the outer ring under the condition that the maximum contact surface pressure with the processing jig is 8.2 GPa or more, which is calculated on the assumption that only elastic deformation occurs. Characteristic manufacturing method for rolling bearings.

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