JP3084421B2 - Rolling bearing made of carburized steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling bearing having improved rolling fatigue life when used in a lubricating environment containing oil mixed with foreign matter.

[0002]

2. Description of the Related Art Rolling bearings formed of carburized steel are known to exhibit excellent rolling fatigue life due to the compressive residual stress of the carburized hardened layer formed on the surface and the presence of residual austenite. Longer life than high carbon chromium bearing steel bearings.

[0003] The residual compressive stress of the carburized hardened layer offsets the stress concentration around non-metallic inclusions and micro cracks on the surface to increase the fatigue strength of the carburized hardened layer, and the residual austenite reduces the stress concentration by plastic deformation. It is effective as a moderator.

[0004] With the development of clean steel in recent years, the quality of high carbon chromium bearing steel has been remarkably improved with the reduction of oxygen content in steel and the reduction and miniaturization of nonmetallic inclusions. Under low lubrication conditions, especially under lubricating conditions due to foreign matter mixed oil, for example, under conditions where wear powder due to sliding contact of gears such as automobile transmissions is mixed in oil, rolling bearings made of carburized steel have the above-mentioned characteristics. Sufficient long life cannot be exhibited for bearing steel bearings.

[0005] The cause of the shortened life under such conditions is as follows.
It is thought to be due to the effect of compressive stress generated and accumulated on the surface layer during use of the rolling bearing. That is, a large compressive residual stress is formed on the surface layer of the rolling contact surface due to the foreign material being caught, and in the case of a high load, the generation of the compressive residual stress extends to a considerably deep portion of the hardened layer. It was recognized that the life was shortened.
Therefore, the present inventors have already set the surface hardened layer depth to a ratio of 0.05 or more for the raceway ring and 0.07 or more for the rolling element to the diameter of the rolling element, and set the core hardness to H
By setting the _{RC to} 48 or more, the service life of a carburized steel rolling bearing is extended (Japanese Patent Application Laid-Open No. 62-132031).

[0006]

The contaminants in the lubricating oil vary in the constituent material, hardness, grain shape, etc., depending on the operating environment of the bearing. However, the contaminants in the hermetically sealed lubricating oil are reduced as the bearing environment is improved. It tends to be finer, and the particle size is about 20 μm when exemplified by gear wear powder.

[0007] Under such a tendency of miniaturization of contaminants, peeling, which is a group of minute cracks, occurs, and flaking, which is the original form of fatigue of the bearing, starts from the peeling. Newly arising,
In the conventional technology, it is not possible to cope with the improvement of the life simply by regulating the hardened layer depth and the soft core hardness of the raceway and the rolling surface surface layer of the rolling element, and it is necessary to improve the microstructure of the carburized hardened layer. Has arisen.

[0008] Conventionally, in the case of performing nitriding (carbonitriding) after carburizing, there has been a problem that the amount of retained austenite is excessively increased to be soft and the wear resistance is reduced.

In view of the above problems, the present invention provides a carburized steel rolling bearing in which the carburizing treatment and the nitriding treatment after the carburizing treatment adjust the fine structure of the carburized hardened layer and provide excellent lubrication conditions under the contaminant contamination. An object of the present invention is to provide a rolling bearing capable of exhibiting a long service life.

[0010]

A rolling bearing according to the present invention has a carbon content of 0.1.
A bearing containing from 5 to 0.40% of the bearing ring and rolling elements formed of carburized steel, the bearing rings and rolling elements, and Rockwell C hardness H _RC of carbon content 0.80% or more 58
A surface hardened layer is at least a core portion core hardness is less than Rockwell C hardness H _RC 48 or H _RC 58, made of,
The ratio of the depth of the surface hardened layer to the diameter of the rolling element is adjusted to 0.05 or more in the race and to 0.07 or more in the rolling element, and the hardened layer has a residual austenite amount of 25 to 35% and a residual austenite structure. Size of 5μm
Or less, and the residual carbide amount is 10% or less.

[0011] The steel includes structural carbon steel or structural low alloy steel containing 0.15 to 0.40% C (SCr430 class,
SCM430 class) clean steel is used, and after being molded into the above-mentioned rolling bearing part, it is carburized to 0.80% C or more, and
A carburized layer for obtaining the above-mentioned surface hardened layer characteristics is formed by quenching and tempering treatment described later.

After carburizing, quenching and tempering are performed to form a hardened surface layer having a retained austenite content of 25 to 35%, a retained austenite structure of 5 μm or less, and a carbonized material of 10%.
Make the following adjustments and precision polishing to finish the raceway.

[0013]

The surface hardened layer is composed of a tempered martensite phase, a residual austenite phase and a residual carbide. The amount of retained austenite is expressed in terms of volume%, and the reason why the present invention sets the amount of retained austenite to 25 to 35% is to impart toughness to a hardened surface hardened layer and to perform surface press-in of solid foreign matter of lubricating oil. Buffer the generation of stress against plastic deformation due to If the amount of retained austenite is less than 25%, it is not sufficient to buffer the stress generation due to plastic deformation. If the amount of retained austenite exceeds 35%, plastic deformation is large and surface roughness is deteriorated, which is not appropriate.

The size of the retained austenite structure is a size represented by the diameter of a circle having an area equivalent to the area of one austenite structure on the polished and etched surface of the sample under microscopic observation. The thickness of 5 μm or less ensures the number of retained austenite phases contained in the foreign matter indentation to cope with the incorporation of minute foreign matter, thereby relieving the stress of foreign matter press-fitting and preventing the surface layer from cracking.

The residual carbide is mainly a quenched residue of a carbide not dissolved in the austenite phase at the time of quenching and heating. The reason why the present invention regulates the amount of residual carbide to 10% or less in the present invention is to increase the amount of solute carbon in the tempered martensite phase, thereby increasing the strength of the matrix and affecting the inside of the surface layer under the pressure of foreign matter. The purpose of the present invention is to reduce the influence of compressive stress and to provide tempering resistance to the matrix to prevent the hardened layer from softening due to a rise in temperature during use, thereby ensuring the rolling life under severe use conditions.

Such a structure of the surface hardened layer is formed by heat treatment after carburization. In order to adjust the carburized layer to 0.8% C or more, the carbon potential in the carburized atmosphere is set to 0.
Carburize at a predetermined time by setting it to 8% or more. In the carburizing step, after heating in a carburizing atmosphere (usually involving diffusion treatment), quenching in oil cooling (carburizing quenching) is performed, and then secondary quenching and tempering are performed. The secondary quenching temperature is 820-870
Temper at a temperature of 250 ° C. or lower, especially 200 ° C. or lower.

[0017] In addition to the chemical composition of the material, C and N in the carburized layer that can be provided in the heat treatment step contribute to stabilization of the retained austenite in the martensitic transformation process during the secondary quenching. , Austenitizing temperature contributes. On the other hand, the carbide after the martensitic transformation is exclusively undissolved cementite at the austenitizing temperature, and therefore, the amount of carbide is reduced to Fe- at the austenitizing temperature.
It is roughly determined by the excess carbon amount from the carbon amount on the Fe ₃ C phase diagram Acm line, and is determined by C in the carburized layer and the austenitizing temperature. In other elements, N reduces the amount of carbide.

When the carburized layer is selected to be 0.8 to 1.0% C, 2%
Adjust the secondary quenching temperature to the range of 820 to 870 ° C.
By tempering at a temperature of 00 ° C. or less, a retained austenite amount of 25 to 35% and a size of a retained austenite structure of 5 μm or less are obtained. Since the higher the secondary quenching temperature is, the more austenite is stabilized, if it exceeds 870 ° C., the amount of retained austenite increases and the retained austenite structure becomes coarse. On the other hand, when the temperature is 820 ° C. or less, the amount of retained austenite is reduced to 25% or less.

The hardened surface layer of the present invention can also be realized by performing a carbonitriding treatment on the carburized layer. In this case, after carburizing and quenching, instead of the above-mentioned secondary quenching, quenching is performed immediately after carbonitriding. An increase in the nitrogen in the carburized layer increases the amount of solute carbon and thus reduces the amount of residual carbide, but stabilizes austenite, so that the quenching temperature immediately after carbonitriding is reduced to 800 to 840 ° C. The size and amount of the retained austenite structure are adjusted to the above-mentioned predetermined range. Thus, the austenitizing temperature is set to 80
Even if the temperature is reduced to 0 to 840 ° C., an increase in nitrogen in the carburized layer increases the amount of dissolved carbon in austenite and decreases the amount of residual carbide, so that the amount of residual carbide can be reduced to 10% or less. As a method for adjusting the amount of retained austenite, sub-zero treatment or high-temperature tempering may be performed.

[0020]

[Example] Carburized steel (steel type SCr435, 0.0015%
O), a bearing ring and a rolling element of a bearing (model number 30206) were manufactured, carburizing treatment and subsequent carbonitriding, or secondary quenching and tempering were performed, and the fine structure of the carburized layer was adjusted to complete the bearing. .

Carburizing and quenching are carried out in a gas carburizing furnace so that the carbon potential CP (%) is in the range of 0.8 to 1.25.
Carbide treatment at 940 ° C x 8h, followed by 870 ° C x 30m
It was quenched in oil from the quenching temperature of in. Secondary quenching is 80
After heating at 0 to 840 ° C for 1 hour, the mixture was rapidly cooled in oil, and all tempering was performed at 180 ° C for 2 hours.

The sample to be subjected to the carbonitriding treatment was subjected to a carbon potential C
P (%) was in the range of 0.9 to 1.2, and ammonia was added to the carburizing gas in an amount of 7%, and carbonitriding was performed at 820 to 860 ° C for 2 hours, followed by quenching in oil. Table 1 shows the processing conditions.

[0023]

[Table 1]

The bearing was subjected to microscopic observation of the carburized layer structure of the cross section and a rolling life test. The results are shown in Table 2.

[0025]

[Table 2]

The life test was carried out under the following conditions under lubricating conditions in which 0.5 g / l of iron powder having a particle size of 20 μm was added as foreign matter to the lubricating oil turbine oil 56. Bearing number 30206 Load Fr 1800kgf Number of revolutions N 2000rpm

In Table 2, the bearings of Test Nos. 1 and 8 are examples of the present invention, and when the rolling life is evaluated at 10% life, excellent rolling fatigue life is shown.

Test No. 2 was a case where tempering treatment was performed without carburizing after carburizing and quenching, and no carbide remained in the carburized layer, but the residual austenite structure became coarse and the life was shortened. I have. Tests Nos. 3, 4, 5, and 7 had too little retained austenite and had a short life with increasing residual carbides. Test No. 6 had a low carbon content in the carburized layer, Sex and lack.

Test Nos. 1, 8, 9 and 10 were performed after carburizing.
Rolling bearings of Test Nos. 9 and 10 having a large amount of residual carbides and a coarse retained austenite structure have a short life.

[0030]

The rolling bearing of the present invention has a long service life even when used under lubricating conditions of clean oil, because the surface hardened layer and the core hardness are secured high by clean carburized steel. In particular, under lubricating conditions due to foreign matter contamination, the resistance to plastic deformation of the surface layer due to the generation of residual compressive stress and the decrease in critical shear strength due to temperature rise, etc., is increased to extend the life, but the microstructure of the surface hardened layer is further increased. By modifying the structure to refine the appropriate amount of retained austenite structure and reducing the amount of carbide to increase the amount of carbon solid solution in the structure matrix, This also makes it possible to develop an excellent rolling fatigue life.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ identification code FI F16C 33/34 F16C 33/34 33/62 33/62 (58) Investigated field (Int.Cl. ⁷ , DB name) F16C 33 / 32 F16C 33/34 F16C 33/62

Claims (1)

(57) [Claims] 1. A rolling bearing including a race and a rolling element formed of carburized steel having a carbon content of 0.15 to 0.40%, wherein the race and the rolling element have a carbon content of 0.80% or more. in and Rockwell C hardness H and the surface hardened layer is _RC 58 or more, the core portion hardness Rockwell C hardness H _RC 48 or H _RC 5
And a ratio of the depth of the surface hardened layer to the diameter of the rolling element is 0.05 or more in the bearing ring and 0.07 or more in the rolling element, and the surface hardened layer is Retained austenite amount 25-35
%, The size of residual austenite structure is 5 μm or less, and the amount of residual carbide is 10% or less.