JP5298683B2 - Rolling bearing and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolling bearing in which the starting point in the inner part is hardly broken dowm and which has a long service life, and a manufacturing method therefor. <P>SOLUTION: An inner ring 1 and an outer ring 2 of the cylindrical rolling bearing are constituted of steel having &ge;0.7 mass% carbon content and a rolling body 3 is constituted of a bearing steel of SUJ3, etc. The inner ring 1 and the outer ring 2 are subjected to a heat-treatment including a high-frequency induction-hardening, and the hardened layers hardened by the heat-treatment are formed on orbital surfaces 1a, 2a. The hardened layer is composed of a martensitic structure, but in the inside of the hardened layer, a pearlitic structure is formed by the heat-treatment and further, in the inside of the avove inside, a core part composed of a spheroidized structure not being hardened is formed. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

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

Rolling bearings require life and toughness. In particular, regarding industrial rolling bearings that are often subjected to high loads or impact loads, the balance between them is regarded as important.
Separation that causes the rolling life of the rolling bearing is roughly classified into internal origin type separation and surface origin type separation. Since the former is based on non-metallic inclusions contained in the steel, the life can be extended by a method of reducing the oxygen content of the steel material. On the other hand, the latter is caused by stress concentration at the edge of the indentation caused by the inclusion of foreign matter such as metal powder contained in the lubricant, so the life is extended by controlling the amount of retained austenite and relaxing the stress concentration. Can be achieved.

  In general, since surface-origin type peeling has an apparently short life compared to internal origin-type peeling, development of a long-life rolling bearing is often performed by suppressing surface-origin type peeling. However, in order to deposit a large amount of retained austenite, it is necessary to form a carbon or nitrogen-enriched region on the surface, which requires quenching in a special gas atmosphere such as carburizing or carbonitriding. It becomes. In addition, the precipitation of a large amount of retained austenite results in the reduction of the surface hardness that is most necessary for rolling bearings, so this must be supplemented with hard carbonitrides, for which expensive alloy elements such as molybdenum are added. There is also a case. Therefore, there is a possibility that a problem of an increase in production cost occurs.

  On the other hand, regarding toughness, there is a trade-off between the hardness of the material. Therefore, in order to improve toughness, basically, it is necessary to secure as many regions with low hardness as possible. Based on this concept, carburized bearings have been developed in which only the surface is hardened by carburizing or carbonitriding the low / medium carbon steel. However, carburized steel is often used for relatively large rolling bearings such as rolling bearings for steel machines. In addition, in order to ensure hardenability, the addition of relatively expensive alloy elements such as nickel, molybdenum, chromium, etc. is the mainstream, leading to an increase in production cost in combination with the complexity of heat treatment such as carburizing.

  On the other hand, in recent years, high-frequency heat treatment that hardens and hardens only the surface portion that requires hardness (see Patent Documents 1 and 2). This method is a method of achieving both life and toughness by making a hardened and hardened surface portion and a high toughness core portion that is not hardened in one part. In addition, by having a core portion that is not quenched, compressive residual stress is imparted to the hardened and hardened surface portion, which is effective in improving the life and suppressing the occurrence of cracks.

  Furthermore, since the hardness can be controlled by the presence or absence of heat treatment, high carbon steel represented by general-purpose bearing steel having excellent cleanliness can be used instead of high alloy low carbon steel. Furthermore, as described above, the retained austenite is effective against surface damage. However, the current density is high on the surface due to the characteristics of induction hardening, so if the carbon concentration of the base material is about 0.7% by mass, the extreme surface layer Only retained austenite comparable to carburized steel can be secured.

A large amount of retained austenite can cause dimensional changes, but when viewed in the depth direction, the amount of retained austenite decreases rapidly, so the total amount is low while being present in a large amount on the surface side than the maximum shear stress depth. There is also an advantage that it can be suppressed. That is, by subjecting the bearing steel to induction hardening, it is possible to obtain a rolling bearing having an excellent life with respect to both internal fatigue and surface fatigue, and having excellent crack resistance.
Japanese Patent Laid-Open No. 11-37163 Japanese Patent Laid-Open No. 14-256336

However, in the induction hardening, the hardness gradient in the boundary region between the hardened surface portion and the core portion that is not hardened becomes sharper than in the carburizing treatment. There is a problem in that internal origin destruction (so-called case crash) occurs due to the action.
In other words, industrial rolling bearings are often subjected to higher loads than automotive rolling bearings, etc., and since shear stress acts on the inside, reducing the hardness of the core to ensure toughness Further, it becomes necessary to increase the depth of the hardened layer more than necessary, and there is a possibility that the amount of the core part for obtaining sufficient toughness cannot be secured.
Accordingly, an object of the present invention is to solve the above-described problems of the prior art, and to provide a long-life rolling bearing and a method for manufacturing the same that are less likely to cause internal origin fracture.

  In order to solve the above problems, the present invention has the following configuration. That is, the rolling bearing of claim 1 according to the present invention includes an inner ring having a raceway surface, an outer ring having a raceway surface facing the raceway surface of the inner ring, and a plurality of rolling bearings arranged between the raceway surfaces. And at least one of the inner ring and the outer ring is made of steel having a carbon content of 0.7% by mass or more, and the raceway surface includes heat treatment including induction hardening. A hardened layer formed by hardening is formed, and a pearlite structure layer is formed inside the hardened layer made of martensite structure by the heat treatment, and further, it is not hardened inside. A core portion made of a spheroidized structure is formed.

According to a second aspect of the present invention, there is provided the rolling bearing according to the first aspect, wherein the hardness of the hardened layer is between the Hv650 portion and the core portion of the Hv300 portion. The value of the hardness gradient in the depth direction obtained by the least square method is 95 or less. Furthermore, the rolling bearing according to claim 3 according to the present invention is the rolling bearing according to claim 1 or 2, wherein the amount of retained austenite of the raceway surface subjected to the heat treatment is 26% by volume or more. Features.
Furthermore, the rolling bearing of Claim 4 which concerns on this invention is a rolling bearing as described in any one of Claims 1-3. Residual of the rolling direction of the said rolling element in the said raceway surface where the said heat processing was performed. The stress is −204 MPa or less.

Et al is, the production method of a rolling bearing according to claim 5 of the present invention, when producing a rolling bearing according to claim 1, after performing spheroidizing annealing, required hardening Heating to A1 transformation point or more to a deeper part than the layer depth and slow cooling, then induction heating to A1 transformation point or more to the required hardened layer depth to perform the induction hardening.

  The rolling bearing of the present invention has a long service life that hardly causes internal origin fracture. Moreover, the manufacturing method of the rolling bearing of this invention can manufacture a long-life rolling bearing which is hard to produce internal origin destruction.

DESCRIPTION OF EMBODIMENTS Embodiments of a rolling bearing and a manufacturing method of the rolling bearing according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a partial longitudinal sectional view showing a structure of a cylindrical roller bearing which is an embodiment of a rolling bearing according to the present invention.
This cylindrical roller bearing has an inner ring 1 having a raceway surface 1a on its outer peripheral surface, an outer ring 2 having a raceway surface 2a opposite to the raceway surface 1a of the inner ring 1 on its inner peripheral surface, and rolling between both raceway surfaces 1a and 2a. A plurality of rolling elements (cylindrical rollers) 3 arranged freely, and a cage 4 that holds the rolling elements 3 between the inner ring 1 and the outer ring 2, both raceway surfaces 1 a and 2 a and the rolling elements 3 are provided. Lubricating with the rolling surface 3a is performed by a lubricant (not shown) such as grease or lubricating oil. In addition, the holder | retainer 4 does not need to be provided. Moreover, you may provide sealing devices, such as a seal and a shield.

  In this cylindrical roller bearing, the inner ring 1 and the outer ring 2 are made of steel having a carbon content of 0.7% by mass or more (for example, bearing steel such as SUJ2 and SUJ3), and the rolling element 3 is made of SUJ3 or the like. It is made of bearing steel. The inner ring 1 and the outer ring 2 are subjected to heat treatment including induction hardening, and hardened layers (not shown) cured by the heat treatment are formed on the raceway surfaces 1a and 2a. This hardened layer is composed of a martensite structure, and a pearlite structure layer is formed by heat treatment on the inner side of the hardened layer, and further on the inner side, an unhardened core (the core is made of a spheroidized structure). Formed).

  As described above, the inner ring 1 and the outer ring 2 have a three-layer structure in which a medium-hardness pearlite structure layer is sandwiched between a hard hardened layer and a soft core portion. Because the hardness gradient in the boundary area with the part is moderate, both life and high toughness are compatible. Cylindrical roller bearings are less susceptible to internal origin failure even when used in applications where high stress acts. Long life. Therefore, this cylindrical roller bearing is suitable for an industrial rolling bearing, and is particularly suitable for a large-sized rolling bearing having an outer diameter of the outer ring of 200 mm or more.

  It should be noted that the hardness gradient in the depth direction between the portion of the hardened layer having a hardness of Hv650 and the portion of the core having a hardness of Hv300 is 95 or less. Preferably it is. By doing so, the hardness gradient becomes gentler or similar to the distribution of shear stress acting on the inner ring 1 and the outer ring 2 (shear stress gradient), so that both life and high toughness can be achieved.

  The amount of retained austenite on the raceway surfaces 1a and 2a is preferably 26% by volume or more. Furthermore, in order to suppress the generation and propagation of cracks, it is preferable that the residual stress in the rolling direction of the rolling elements 3 on the raceway surfaces 1a and 2a is −204 MPa or less (residual compressive stress is 204 MPa or more). Furthermore, in order to suppress the occurrence and development of cracks, the residual stress in the depth direction (radial direction) in the portion up to the maximum shear stress depth of the hardened layer should be −210 MPa or less (residual compressive stress is 210 MPa or more). Is preferred.

As the heat treatment for manufacturing such a cylindrical roller bearing, for example, after spheroidizing annealing is performed on a steel material, it is gradually cooled (for example, heated to a portion deeper than the required hardened layer depth to the A1 transformation point or more). Then, heat treatment is performed such that the air is heated to high frequency above the A1 transformation point to the required hardened layer depth, rapidly cooled, and quenched.
Starting from a spheroidized structure, heat to a point deeper than the required hardened layer depth (for example, 1.2 to 2.5 times the required hardened layer depth) above the A1 transformation point by a method such as high frequency heating. Then, when it is gradually cooled, the heated portion becomes pearlite. The core that is not heated remains a spheroidized structure.

Next, when induction heating is performed by induction heating above the A1 transformation point to the required hardening layer depth and quenching, induction hardening is formed on the surface. As a result, a martensite structure, a pearlite structure, and a spheroidized structure are formed in this order from the surface side, and the hardness gradient in the boundary region between the hardened layer and the core portion becomes gentle.
The pearlite structure has a hardness almost intermediate between the martensite structure and the spheroidized structure. In addition, since the pearlite structure has a shorter distance between carbides than the spheroidized structure, if the previous structure when forming a hardened layer by induction hardening is a pearlite structure, carbon diffuses quickly even with short heating. Thus, it is easy to obtain a uniform structure while securing retained austenite.

A tempering treatment, which is a process of quenching and tempering the entire area, is effective as a heat treatment to moderate the hardness gradient in the boundary region, but the amount of deformation increases because the whole is heated to a temperature above the phase transformation point. It has the problem of being easy.
The rolling elements 3 are not particularly limited, and general ones can be used without problems. For example, a heat treatment including a carbonitriding process or a nitriding process is performed, and a nitride layer formed by the heat treatment is formed on the rolling surface 3a.

  In addition, this embodiment shows an example of this invention and this invention is not limited to this embodiment. For example, in the present embodiment, a cylindrical roller bearing has been described as an example of a rolling bearing, but the present invention can be applied to various types of rolling bearings. For example, radial type rolling bearings such as deep groove ball bearings, angular contact ball bearings, self-aligning ball bearings, self-aligning roller bearings, tapered roller bearings and needle roller bearings, and thrust types such as thrust ball bearings and thrust roller bearings This is a rolling bearing.

〔Example〕
A cylindrical roller bearing (nominal number NU2210ET) having a configuration substantially similar to that of the cylindrical roller bearing of the present embodiment was prepared, and its life was evaluated. However, since the present invention is suitable for industrial large bearings having an outer diameter of 200 mm or more, the inner diameter of the inner ring is 30 mm and the outer diameter is 60 mm in order to reproduce the average thickness of the large bearing. The wall thickness (the width in the radial direction) was 15 mm. First, the manufacturing method of the cylindrical roller bearing used for the test is demonstrated.

The inner ring is made of SUJ3 spheroidized annealed material into a predetermined shape by roughing, then heated to a depth of 5mm or 10mm from the surface to the A1 transformation point or higher by air heating, and the heated part is made pearlite. did. In addition, the pearlite depth was calculated | required by structure | tissue observation mentioned later. Further, the conditions for the high-frequency heating are a frequency of 10 kHz, a heating time of 20 to 60 seconds, and a work rotation speed of 60 min ⁻¹ .
Next, induction hardening and tempering were performed, and further post-processing such as polishing was performed to complete the inner ring. The conditions for this induction hardening are a frequency of 100 kHz, a heating time of 5 to 200 seconds, a work rotation speed of 60 min ⁻¹ , and fixed hardening. Moreover, the conditions of tempering are that it cools after hold | maintaining at 180 degreeC.

In Comparative Example 1, a spheroidized annealed material made of SUJ3 was formed into a predetermined shape by roughing, and then subjected to sub-quenching to produce an inner ring. In Comparative Example 2, a spheroidized annealing material made of SUJ3 was formed into a predetermined shape by roughing, and then subjected to induction hardening without performing pearlite to form a hardened layer on the raceway surface. is there.
Here, the inner ring thus obtained was analyzed. First, hardness Hv (hardness profile) at various depths from the surface (orbital surface) was measured. The results are shown in Table 1 and the graph of FIG. The graph of FIG. 2 shows the distribution of shear stress generated when a radial load of 84 kN (P / C = 0.6) is applied, converted to durable hardness Hv.

And the hardness gradient of the depth direction between the part of hardness Hv650 and the part of hardness Hv300 was computed by the least square method. The results are shown in Table 2. When the hardness of the core exceeds Hv300, the hardness gradient in the depth direction between the portion of hardness Hv650 and the total hardened layer depth by induction hardening is calculated by the least square method.
As can be seen from the graph of FIG. 2, the hardness from the raceway surface to the depth of 11 mm is hardly changed in Comparative Example 1, whereas Examples 1, 2 and Comparative Example 2 are gradually decreased. doing. It can be seen that the gradient is the steepest in Comparative Example 2 and the slowest in Example 2.

  Moreover, the residual austenite amount and the residual stress in the rolling direction (circumferential direction) of the rolling elements were measured by an X-ray diffraction method. At that time, the surface (track surface) of the inner ring was electropolished to remove the work-affected layer, and measurement was performed. Further, the inner ring was cut, the cut surface was mirror-finished and subjected to corrosion treatment, and then the structure was identified by microscopic observation of the cut surface. Moreover, the residual stress in the depth direction was measured by the X-ray diffraction method using the sample subjected to the structure observation. That is, the residual stress was measured for a circular range having a diameter of 0.1 mm centering on a portion having a depth of 0.15 mm. The results are shown in Table 2 and FIG.

  FIG. 3 schematically shows the structure of each sample from the surface (orbital surface) to a depth of 10 mm. Comparative Example 1 has a martensite structure as a whole, and Comparative Example 2 has a martensite structure from the surface to about 2 mm, and the core has a spheroidized structure. On the other hand, Examples 1 and 2 have a martensite structure, a pearlite structure, and a spheroidized structure in order from the surface, and the pearlite structure layer is thicker in Example 2 than in Example 1. The hardness profiles in Table 1 and FIG. 2 well reflect such a structure.

  A cylindrical roller bearing was manufactured by assembling the inner ring obtained as described above, a general outer ring made of SUJ2, and rolling elements. The rotation test of these cylindrical roller bearings is conducted under a clean lubrication environment in which no foreign matter is mixed in the lubricant (hereinafter referred to as a clean lubrication environment) and in a lubrication environment in which foreign matter is mixed in the lubricant (hereinafter referred to as a clean lubrication environment). This was carried out in each of the above-mentioned cases, and the time until peeling occurred was measured.

Then, to create a Weibull plot subjected to seven rotation test per one bearing obtains the L ₁₀ life From the results of Weibull distribution, which was used as a lifetime. The results are shown in Table 2. In addition, the lifetime of Table 2 is shown by the relative value when the lifetime of the comparative example 1 in each lubrication environment is set to 1. The conditions of the rotation test are as follows.
(Rotation test under clean lubrication environment)
Radial load: 84kN (P / C = 0.6)
Rotational speed: 1000 min ^-1
Lubricant: Lubricating oil whose ISO viscosity grade is ISO VG68 (Rotation test in a contaminated lubricating environment)
Radial load: 25kN (P / C = 0.3)
Rotational speed: 1000 min ^-1
Lubricant: Lubricating oil whose ISO viscosity grade is ISO VG68 Foreign matter: Instead of mixing foreign matter into the lubricant, 8 pseudo indentations are formed in the center of the raceway surface of the inner ring in the width direction using a Rockwell hardness meter. did.

  First, the results of the rotation test in a clean lubrication environment will be described. In Comparative Example 2, destruction from the core occurred in about half of the samples, resulting in a short life. As can be seen from FIG. 2, this is considered to have occurred because the stress applied to the core part exceeded the hardness of the material. On the other hand, in Examples 1 and 2, the destruction from the inside as seen in Comparative Example 2 was not observed. In order to sufficiently suppress the destruction from the inside, it is necessary to satisfy the relationship that the hardness of the material is larger than the stress applied. For that purpose, the portion of hardness Hv650 and the portion of hardness Hv300 It was found that the hardness gradient in the depth direction between and the value obtained by the least square method needs to be 95 or less. Moreover, it is preferable that the surface hardness of the raceway surface at this time is Hv650 or more.

  Furthermore, Examples 1 and 2 had a longer life than Comparative Example 1. This is presumably because, under a clean lubrication environment, peeling occurs starting from inclusions present near the surface to which high stress is applied (region within a depth of 1.5 mm). When the damaged product after the rotation test of Comparative Example 1 was investigated, it was found that cracks in the direction parallel to the raceway surface were generated from the inclusions, which caused peeling. In other words, in Examples 1 and 2, the radial residual compressive stress suppresses the generation of cracks parallel to the raceway surface, which is considered to have a long life. And in order to suppress generation | occurrence | production of the crack parallel to a track surface, it turns out that it is preferable that the residual compressive stress of radial direction is 210 Mpa or more.

  Next, the results of the rotation test in a foreign matter mixed lubrication environment will be described. Residual austenite is effective for extending the life in a foreign matter-contaminated lubrication environment, but Examples 1 and 2 are considered to have a longer life than Comparative Example 1 because the amount of retained austenite exceeds 26% by volume. It is done. The reason why a large amount of retained austenite was easily obtained in Examples 1 and 2 was because it was pearlite once, so that the carbide was finer than the spheroidized carbide, and chromium that suppresses dissolution of the carbide, etc. It is conceivable that the alloy element is not as concentrated as the spheroidized carbide.

  On the other hand, regarding the peeling from the indentation by the foreign matter, the crack is generated in a direction orthogonal to the circumferential direction. That is, when compressive residual stress is generated in the circumferential direction, generation of cracks is suppressed. When attention is paid to Comparative Example 2, the amount of retained austenite is not much different from that of Comparative Example 1, but it is considered that a long life was obtained due to the occurrence of residual compressive stress in the circumferential direction. And in order to suppress generation | occurrence | production of a crack, it turns out that it is preferable that the circumferential direction residual compressive stress is 204 Mpa or more.

Next, a crushing test for evaluating toughness will be described. A pre-crack was formed on the raceway surface of the inner ring to a depth of 1 mm by wire cutting, the direction in which the pre-crack extends was horizontal, the inner ring was mounted on a crushing test apparatus, and a load was applied from above to compress the inner ring. And the load which the crack propagated from the pre-crack was made into crushing strength. The results are shown in Table 2. The crushing strength in Table 2 is shown as a relative value when the crushing strength of Comparative Example 1 is 1.
Examples 1 and 2 and Comparative Example 2 were superior in crushing strength to Comparative Example 1. Factors affecting the crushing strength are considered to be the presence of a high toughness core and the effect of circumferential residual compressive stress, and the circumferential residual compressive stress is thought to be effective in suppressing cracking, Since pre-cracks were introduced in this test, it is considered that the crushing strength was determined mainly by the influence of the core.

It is a fragmentary longitudinal cross-section which shows the structure of the cylindrical roller bearing which is one Embodiment of the rolling bearing which concerns on this invention. It is a graph which shows the measurement result of a hardness profile. It is the figure which showed typically the structure | tissue structure of the inner ring.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Inner ring 1a Raceway surface 2 Outer ring 2a Raceway surface 3 Rolling element 3a Rolling surface

Claims (5)

In a rolling bearing comprising an inner ring having a raceway surface, an outer ring having a raceway surface facing the raceway surface of the inner ring, and a plurality of rolling elements arranged to be freely rollable between the both raceway surfaces,
At least one of the inner ring and the outer ring is made of steel having a carbon content of 0.7 mass% or more, and a hardened layer formed by heat treatment including induction hardening is formed on the raceway surface. And
A pearlite structure layer is formed by the heat treatment on the inner side of the hardened layer made of martensite structure, and further, a core part made of a spheroidized structure that is not hardened is formed on the inner side. Characteristic rolling bearing.   The hardness gradient in the depth direction between the portion of the hardened layer having a hardness of Hv650 and the portion of the core having a hardness of Hv300 is a value obtained by a least square method of 95 or less. The rolling bearing according to claim 1, wherein   The rolling bearing according to claim 1 or 2, wherein the amount of retained austenite of the raceway surface subjected to the heat treatment is 26% by volume or more.   The rolling bearing according to claim 1, wherein a residual stress in a rolling direction of the rolling elements on the raceway surface subjected to the heat treatment is −204 MPa or less. In producing the rolling bearing according to any one of claims 1 to 4 , after subjecting to spheroidizing annealing, it is gradually cooled by heating above the A1 transformation point to a portion deeper than the required hardened layer depth, Next, the method for producing a rolling bearing is characterized in that the induction hardening is performed by induction heating above the A1 transformation point to the required hardened layer depth.

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