INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-162065 filed on Aug. 25, 2017 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a rolling sliding member and a method of producing the same, and a rolling bearing including the rolling sliding member.

2. Description of Related Art

For rolling sliding members used in bearings and the like, carburized steels such as chromium molybdenum steel (SCM steel) and nickel chromium molybdenum steel (SNCM steel) are widely used in consideration of ease of quenching.

The rolling sliding member is in contact with a counterpart member on a rolling sliding surface. In consideration of the influence of this contact on the product lifespan, a technique in which, in the rolling sliding member, a carburizing treatment and the like are performed on a molding raw material containing a carburized steel, and a hard layer (hereinafter referred to as a surface layer) is provided on a surface part thereof is known (for example, Japanese Unexamined Patent Application Publication No. 2007-308792 (JP 2007-308792 A)).

SUMMARY

In the above carburized steels such as SCM steel or SNCM steel, a carbon content is relatively low. In order to obtain a surface layer that can contribute to improvement of the product lifespan, it is necessary to perform a carburizing treatment for a long time. When the carburizing treatment is performed for a long time, the production costs may be increasing.

In addition, when a time required for the carburizing treatment is long, crystal grains are coarsened. Thus, it is necessary to perform secondary quenching in order to adjust the size of crystal grains to a desired size. When secondary quenching is performed in addition to the carburizing treatment being performed for a long time, the above production costs further increase.

When a time required for the carburizing treatment is long, formation of a grain boundary oxide layer on a surface of a molding raw material is facilitated. The developed grain boundary oxide layer reduces the strength of the surface layer. Therefore, when the developed grain boundary oxide layer remains on the rolling sliding member, cracks are likely to occur in the grain boundary oxide layer part when a load is applied to the surface, and thus the crack resistance is reduced. Moreover, the grain boundary oxide layer is likely to fall off. When the grain boundary oxide layer falls off, fallen material, as a foreign substance, may be caught in the inside of a product such as a bearing. As a result, the foreign substances may have an adverse effect on the product lifespan.

The present disclosure provides a rolling sliding member and a method of producing the same and a rolling bearing including the rolling sliding member through which it is possible to reduce the production costs and it is possible to contribute to improvement of the product lifespan.

A first aspect of the present disclosure is a rolling sliding member. The rolling sliding member includes a rolling sliding surface that is in contact with a counterpart member in a relative manner. The rolling sliding member includes a base part and a surface layer. The base part has a composition including 0.30 mass % to 0.45 mass % of carbon, 0.15 mass % to 0.45 mass % of silicon, 0.40 mass % to 1.50 mass % of manganese, 0.60 mass % to 2.00 mass % of chromium, 0.10 mass % to 0.35 mass % of molybdenum, 0.20 mass % to 0.40 mass % of vanadium, and 0.005 mass % to 0.100 mass % of aluminum and a remainder of iron and inevitable impurities. The surface layer is positioned around the base part and includes the rolling sliding surface. The surface layer has a Vickers hardness of 700 to 800. The surface layer has a retained austenite content of 25 volume % to 50 volume %. The thickness of a grain boundary oxide layer in the surface layer satisfies the following formula (I) expressed using an equivalent diameter of the rolling sliding member.
Thickness of grain boundary oxide layer≤equivalent diameter of rolling sliding member×1.4×10−3.  Formula (I):

The base part has the same composition as a molding raw material having the composition as mentioned above. According to this configuration, the rolling sliding member is formed of a molding raw material having a carbon content of 0.30 mass % or more. The rolling sliding member is formed of a steel material having a relatively high carbon content for a base compared to SCM steel and SNCM steel which are widely used for rolling sliding members in the related art. Thus, in the rolling sliding member, even if a time required for a carburizing treatment and the like is shortened, a hard and tough surface layer is obtained. In the rolling sliding member, since a treatment time can be shortened, it is possible to reduce the production costs. In the rolling sliding member, the surface layer has the above Vickers hardness and retained austenite content. The surface layer is hard and also tough. According to the rolling sliding member, the rolling fatigue lifespan is prolonged. In addition, since the grain boundary oxide layer is formed only to the extent that its thickness satisfies Formula (I), the rolling sliding member has favorable crack resistance. According to the rolling sliding member, since the grain boundary oxide layer is unlikely to fall off, formation of foreign substances that could be caught between the rolling sliding member and a counterpart member is suppressed. Thus, the rolling sliding member can contribute to improvement of the product lifespan.

In the rolling sliding member, the surface layer may be either a carburized layer or a carbonitrided layer. In this case, in the rolling sliding member, a surface layer which is sufficiently hard and tough is obtained.

A second aspect of the present disclosure is a rolling bearing. The rolling bearing includes an outer ring, an inner ring, and a plurality of rolling elements. The outer ring includes a rolling sliding surface on an inner circumference of the outer ring. The inner ring includes a rolling sliding surface on an outer circumference of the inner ring. The plurality of rolling elements is disposed between the first rolling sliding surface and the second rolling sliding surface. At least one of the outer ring, the inner ring, and a rolling element is the rolling sliding member according to the first aspect.

In the rolling bearing of the present disclosure, since at least one of the outer ring, the inner ring and the rolling element is formed of the above rolling sliding member, it is possible to reduce the production costs. In addition, according to the rolling bearing, since it is possible to effectively prevent a foreign substance from entering between counterpart members, it is possible to improve the product lifespan.

A third aspect of the present disclosure is a method of producing a rolling sliding member including a rolling sliding surface that is in contact with a counterpart member in a relative manner. The production method includes: obtaining a molding raw material from a steel material having a composition which includes 0.30 mass % to 0.45 mass % of carbon, 0.15 mass % to 0.45 mass % of silicon, 0.40 mass % to 1.50 mass % of manganese, 0.60 mass % to 2.00 mass % of chromium, 0.10 mass % to 0.35 mass % of molybdenum, 0.20 mass % to 0.40 mass % of vanadium, and 0.005 mass % to 0.100 mass % of aluminum, and a remainder of iron and inevitable impurities; obtaining an intermediate material by heating the molding raw material while a temperature of 900° C. to 980° C. is maintained under an atmosphere in which a carbon potential is set in a range of 0.9 to 1.4; quenching the intermediate material by cooling the intermediate material from a temperature of 820° C. to 900° C.; and tempering the quenched intermediate material.

In the method of producing a rolling sliding member of the present disclosure, heating is performed under an atmosphere in which a carbon potential is adjusted for a molding raw material containing a steel material having a relatively high carbon content for a base, and quenching and tempering are performed on an intermediate material obtained by the heating. Therefore, in the production method of the present disclosure, a desired surface layer is obtained within a shorter treatment time than in a production method of the related art. In other words, it is possible to shorten a time required for formation of the surface layer. In the production method, it is possible to reduce the production costs. In addition, since shortening of a treatment time suppresses formation of the grain boundary oxide layer, it is possible to obtain a rolling sliding member having excellent crack resistance according to the production method. According to the rolling sliding member obtained in the production method, since the grain boundary oxide layer is unlikely to fall off, formation of foreign substances that could be caught between the rolling sliding member and a counterpart member is suppressed. The rolling sliding member can also contribute to improvement of the product lifespan.

As can be clearly understood from the above description, the rolling sliding member and the method of producing the same of the present disclosure have effects of reducing the production costs and contributing to improvement of the product lifespan. That is, according to the present disclosure, it is possible to obtain a rolling sliding member and a method of producing the same and a rolling bearing including the rolling sliding member through which it is possible to reduce the production costs and it is possible to contribute to improvement of the product lifespan.

