JPH11101247A - Rolling bearing part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolling bearing part to be excellent in cold machinability and provide a long fatigue life and a crack fatigue life. SOLUTION: This bearing part is produced by carbonitriding a steel containing 0.4-0.8 wt.% carbon, 0.15-1.1 wt.% silicon, and 0.3-1.5 wt.% manganese. In this case, a carbonitrided layer contains 0.6-1.3 wt.% carbon and 0.25-0.5 wt.% nitrogen, has a compression residual stress of 150 MPa, or more, residual austenite is 20-35 vol.%, the maximum grain size of a carbon nitride is 8 μm or less, and Vickers hardness is 700 or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling bearing component and, more particularly, to a rolling bearing component used under high load and lubricating conditions in which foreign matter is mixed, such as a transmission of an automobile.

[0002]

2. Description of the Related Art Conventionally, materials for rolling bearing parts include:
Three types are known: high carbon chromium bearing steel, low carbon case hardened steel and medium carbon steel. At present, these steels are properly used according to their uses.

[0003] Bearings used in transmissions of automobiles and the like are used under severe load conditions and in a sealed environment without replacing the lubricating oil, so that hard foreign matters such as gears and other debris mixed into the lubricating oil are used. Is likely to enter between the bearing and the rolling element. Since the bearing is used in a state in which foreign matter is caught in this way, an indentation is generated on the rolling contact surface of the bearing, which tends to cause damage and shorten the life of the bearing. Specifically, in rolling bearings manufactured by soaking and quenching high-carbon chromium bearing steel, there is a problem that, since tensile stress is generated on the surface around the indentation, damage is likely to occur from the indentation generated on the contact surface. . Even if this high carbon chromium bearing steel is carbonitrided to give a compressive residual stress to its surface, the grinding life is poor, and if not treated in a controlled heat treatment atmosphere, giant carbonitrides will be generated, resulting in crack life. There was a concern that it would be shorter.

On the other hand, if low-carbon case-hardened steel is used as a bearing for a transmission, a compressive stress is generated on the surface of the bearing, so that it is unlikely to be broken. It is necessary to harden to the depth, and the time required for the hardening process, that is, the carburizing or carbonitriding process becomes longer, and there is a problem that the manufacturing efficiency is reduced.

[0005] In addition, it is conceivable to use medium-carbon steel subjected to induction hardening as a rolling bearing, but in this case, there is a problem that the fatigue life is not prolonged.

[0006]

SUMMARY OF THE INVENTION Therefore, the present invention
The present invention has been made in order to solve the above problems, and has as its object to provide a rolling bearing component having excellent cold workability, a high fatigue life, and a high crack fatigue strength.

[0007]

Means for Solving the Problems The present inventors have studied the cold workability, fatigue life and crack fatigue strength of rolling bearing parts and found the following.

First, in order to improve cold workability,
The amount of additional elements such as carbon, silicon, manganese, and chromium in steel has a great effect, and reducing or optimizing them improves cold workability.

In order to extend the fatigue life under the lubrication condition in which foreign matter is mixed, an optimum amount of austenite is distributed to the surface layer of the component by carbonitriding to impart high hardness, toughness and heat resistance and to lose toughness. It is effective to make the internal hardness of the part higher than that of the carburized and quenched product within the range not present.

[0010] In order to improve the crack fatigue strength, it is effective to impart an appropriate amount of residual austenite and residual compressive stress by carbonitriding and to optimize the particle size of carbonitride. Further, the residual stress is largely affected mainly by a difference in carbon content between the surface layer and the inside of the component and a difference in hardness between the surface layer and the inside.

[0011] The rolling bearing part of the present invention, which has been made based on the above-described results, contains 0.4 to 0.8% by weight of carbon.
0.15 to 1.1% by weight of silicon and 0.3 to manganese
It is obtained by carbonitriding a steel containing 1.5% by weight, and the carbonitrided surface layer contains 0.6 to 1.3% by weight of carbon and 0.25 to 0.5% by weight of nitrogen. In addition, the compression residual stress is 150 MPa or more, the retained austenite is 20 to 35% by volume, the maximum particle size of carbonitride is 8 μm or less, and the Vickers hardness is 700 or more. In addition, the rolling bearing component means an outer ring, an inner ring, and a rolling element for a rolling bearing.

[0012] Carbon is added to steel as a raw material in order to increase the hardness of the steel. 0.8% by weight carbon content
When the content exceeds 0.8% by weight, the difference in hardness between the surface layer and the inner layer is less likely to occur even when carbonitriding occurs, and it is difficult to apply compressive residual stress to the surface layer. This is because a possibility of exceeding 8 μm appears.

The reason why the carbon content is set to 0.4% by weight or more is that if the carbon content is less than 0.4% by weight, the internal hardness is too low.

The addition of silicon and manganese to steel is for improving hardenability. The content of silicon and manganese is set to 1.1% by weight or less and 1.5% by weight or less because, when the content exceeds this range, the cold workability is greatly reduced. The reason why the contents of silicon and manganese are set to 0.15% by weight and 0.3% by weight or more is that if the content is less than this range, hardenability is reduced and the inside is not sufficiently quenched. .

The reason why the carbon content of the carbonitrided surface layer portion is set to 0.6 to 1.3% by weight is that the carbon content is 1.
If it exceeds 3%, a large amount of large carbides will be generated, so that the crack fatigue strength may decrease. Also, 0.
The reason why the content is set to 6% by weight or more is that if it is less than 0.6% by weight, sufficient hardness and the amount of retained austenite cannot be secured.

The reason why the nitrogen content is set to 0.25% by weight to 0.5% by weight is that sufficient heat resistance cannot be obtained if the content is less than 0.25% by weight. This is because a large amount of ε-carbide that does not contribute to the properties is generated, and the necessary carbon is taken into the ε-carbide, thereby deteriorating the hardenability and shortening the life.

The amount of retained austenite is 20 to 35% by volume.
The reason for this is that if the content is less than 20% by volume, the effect of improving the life by the retained austenite cannot be sufficiently obtained, and if it exceeds 35% by volume, the hardness of the surface layer is reduced, and the life is shortened.

The reason why the maximum particle size of the carbonitride is 8 μm or less is that if it exceeds 8 μm, the carbonitride tends to be a starting point of cracking.

The Vickers hardness is set to 700 or more because
If it is less than 700, the surface is liable to be worn, and a sufficient fatigue life cannot be obtained.

In such a rolling bearing component, the amount of the added element, the residual compressive stress, the residual austenite, the maximum particle size of the carbonitride and the surface hardness are optimized.
A rolling bearing component having excellent cold workability and a high fatigue life and a high crack fatigue strength can be obtained. Furthermore, since the carbon content of steel as a raw material is higher than that of low-carbon steel, the time required for carbonitriding is reduced, and the production efficiency is improved.

Further, the steel preferably contains 1.0% by weight or less of chromium. In this case, since chromium forms a carbide, the hardness of the surface layer is easily improved. The addition of chromium also improves the hardenability. The reason why the content of chromium is less than 1.0% by weight is that when the content exceeds 1.0% by weight, the cold workability decreases, and even when added over 1.0% by weight, the effect is small. That's why.

Further, it is preferable that the Vickers hardness of the non-carbonitrided portion is 600 or more. In this case, the reason why the Vickers hardness is set to 600 or more is that the life is shortened when the Vickers hardness of the non-carbonitrided portion is less than 600, that is, when the internal hardness is low.

[0023]

Embodiments of the present invention will be described below in detail.

First, steels having various contents of carbon, silicon, manganese and chromium as shown in Table 1 were prepared.

[0025]

[Table 1]

The steel was carbonitrided under the conditions shown in Tables 2 to 6 to vary the carbon and nitrogen contents of the surface layer, compressive residual stress, residual austenite, maximum carbonitride grain size, and Vickers hardness. Sample No. 1 to 46 were prepared. For comparison, bearing steel (SUJ
The carbonitrided material 2) was prepared. Sample N
o. Table 2 shows the characteristics of Nos. 1 to 9.

[0027]

[Table 2]

The "retained austenite amount (% by volume)" in Table 2 was measured by irradiating the surface of the part with X-rays. The “maximum particle size of carbonitride” was obtained by etching the carbonitrided layer and visually measuring the maximum particle size of the carbonitride crystal with an optical microscope. Sample No. 1 to 9 and conventional products were formed into tapered roller bearings (Model No. 30206), and life tests were performed under the following conditions.

Test conditions Load: Fr = 17.64 kN (load in radial direction) Rotation speed: 2000 rpm (inner ring rotation) Lubrication: Turbine 56 oil bath lubrication Calculated life: 169 hours (depending on the condition that no foreign matter is mixed) Foreign matter amount 1 g / 1000 cc The results of this test are shown in FIG.

FIG. 1 shows that when the amount of retained austenite is less than 20% by volume, the life is greatly reduced, and the effect of improving the life of the retained austenite is not sufficiently obtained. When the amount of retained austenite exceeds 35% by volume, it can be seen that the life is slightly shortened due to a decrease in surface hardness. Therefore, it can be seen that a longer life than the conventional product is obtained when the amount of retained austenite is 20 to 35% by volume.

Next, as shown in Table 3, carbonitriding treatment was performed under various conditions to obtain sample Nos. 10-18 were obtained. For comparison, a conventional product (SUJ2) was prepared. Table 3 shows the characteristics of these samples.

[0032]

[Table 3]

Sample No. 10 to 18 and the conventional product were also molded into bearings (model number 30206). The life was measured under the same test conditions as in Examples 1 to 9. FIG. 2 shows the test results.

FIG. 2 shows that when the internal hardness is reduced, the life is uniformly reduced. When the internal hardness (HV) is less than 600, the life is shorter than that of the conventional product.

Next, as shown in Table 4, the samples were subjected to carbonitriding treatment under various conditions. 19-25 were obtained.
For comparison, a conventional product (SUJ2) was prepared. Table 4 shows these characteristics.

[0036]

[Table 4]

Sample Nos. Shown in Table 4 For 19 to 25 and the conventional product, cylindrical test pieces of φ60 × φ45 × t15 were prepared, and the crack fatigue strength was measured under the following conditions.

Contact load: 9.8 kN Maximum stress: about +633 to -412 Mpa on inner diameter surface Load speed: 8000 cpm Lubrication: turbine 68 Drop lubrication The results of the test are shown in FIG.

FIG. 3 shows that the larger the compressive residual stress, the higher the crack fatigue strength, and the lower the compressive residual stress becomes, the lower the crack fatigue strength becomes. It can be seen that strength higher than that of the conventional one is obtained when the compressive residual stress is 150 MPa or more.

Next, carbonitriding was performed under various conditions shown in Table 5, 26-37 were obtained. For comparison, a conventional product (SUJ2) was prepared. Table 5 shows these characteristics.

[0041]

[Table 5]

Sample No. 26-37 and conventional products were also molded into bearings (Model No. 30206). The life was measured under the same test conditions as in Examples 1 to 9.

FIG. 4 shows the results of the test. FIG. 4 shows that the life is long when the carbon content of the surface layer is 0.6% by weight to 1.3% by weight, and the life is short when the carbon content is out of this range. This is because if the content of carbon is less than 0.6% by weight, sufficient hardness and the amount of retained austenite cannot be secured.
It is considered that when the content exceeds 1.3% by weight, a large amount of large carbides is generated.

Next, carbonitriding treatment was performed under various conditions shown in Table 6 to obtain a sample No. 38-46 were obtained. For comparison, a conventional product (SUJ2) was prepared. Table 6 shows these characteristics.

[0045]

[Table 6]

Sample No. 38-46 and conventional products were also molded into bearings (Model No. 30206). The life was measured under the same test conditions as in Examples 1 to 9.

FIG. 5 shows the results of the test. FIG. 5 shows that the life is long when the nitrogen content of the surface layer is 0.25 to 0.5% by weight, and that the life is short outside this range. If the nitrogen content is less than 0.25% by weight, sufficient heat resistance cannot be obtained, and if the nitrogen content exceeds 0.5% by weight, a large amount of ε carbide not contributing to hardenability is generated, and the necessary carbon is removed. It is considered that the quenching property is deteriorated because of this.

The embodiment disclosed this time is to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

[0049]

The rolling bearing component according to the present invention is obtained by subjecting a low-alloy steel having a reduced content of alloying elements affecting workability to a carbonitriding treatment in a short period of time equivalent to ordinary hardening. Therefore, it is possible to improve the cold workability and to reduce the cost more than the current bearing steel without lowering the rolling fatigue strength and the crack strength under the lubricating condition in which the foreign matter is mixed.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the ratio of retained austenite and life.

FIG. 2 is a diagram showing a relationship between internal hardness and life.

FIG. 3 is a diagram showing a relationship between residual stress of a surface layer and crack life.

FIG. 4 is a diagram showing the relationship between the carbon content of the surface layer and the life.

FIG. 5 is a diagram showing the relationship between the nitrogen content of the surface layer and the life.

Claims (3)

[Claims] 1. Rolling obtained by carbonitriding steel containing 0.4 to 0.8% by weight of carbon, 0.15 to 1.1% by weight of silicon and 0.3 to 1.5% by weight of manganese. A bearing component, wherein the carbonitrided surface layer contains 0.6 to 1.3% by weight of carbon and 0.25 to 0.5% by weight of nitrogen, and has a compressive residual stress of 150 MPa or more and a residual austenite. A rolling bearing component having a volume fraction of 20 to 35% by volume, a maximum particle size of carbonitride of 8 µm or less, and a Vickers hardness of 700 or more. 2. The rolling bearing component according to claim 1, wherein the steel contains up to 1.0% by weight of chromium. 3. The rolling bearing component according to claim 1, wherein the non-carbonitrided portion has a Vickers hardness of 600 or more.