For example, it is known that a machine structural component usable for a power transmission part, such as a gear for car transmission, may develop pitching damage, i.e., peel failure of a metal contact portion due to increased pressure of contacted surface during use thereof. Thus, a steel component for use in such an application includes a component prepared in such a manner that one of various types of case-hardening steel such as SCr, SCM, and SNCM is subjected to forming through hot forging and cutting, and is then subjected to surface hardening treatment such as carburizing treatment or carbonitriding treatment, and furthermore a solid lubrication film such as a molybdenum disulfide film is provided on a surface of the component.
In recent years, however, a machine structural part is increasingly demanded to have higher output power, smaller size, and lighter weight, and thus the machine structural element for use in such a power transmission part tends to increasingly receive a higher load. As a result, it is becoming difficult to achieve the required pitching resistance not only by the component including one of various types of case-hardening steel such as SCr, SCM, and SNCM subjected to surface hardening treatment, but also by the component on which the solid lubrication film is further provided.
In an electric car, which is recently increasingly produced in light of reduction in environmental load, since rotation of a motor is directly transmitted to a reduction gear, such components are subjected to higher rotation than in a gasoline-powered vehicle. Furthermore, since a lubricating oil for use in the electric car has a kinematic viscosity in use environment lower than that for use in the gasoline-powered vehicle, a thin oil film is formed on a surface of each of such steel components configuring the power transmission part. In such environment, a site having almost no oil film thereon may be locally formed. In particular, along with increase in rotation and/or slippage, oil temperature increases, and kinematic viscosity of the lubricating oil tends to be lowered, and the sites each having almost no oil film thereon further increases. In such environment, therefore, steel components easily wear due to metal contact therebetween, and each steel component is easily softened through temperature rise due to friction heat; hence, seizing is likely to occur in an early stage.
Various technologies have been proposed on steel members usable in the environment as described above. For example, PTL 1 discloses a component having high resistance to pressure of contacted surface, which is composed of steel that contains C: 0.15 to 0.40%, Si: 0.50 to 1.50%, Mn: 0.20 to 1.50%, Cr: 0.50 to 1.50%, Mo: 0.05 to 0.50%, at least one element selected from the group consisting of Ni: 0.50 to 3.50%, Ti: 0.03 to 0.20%, Nb: 0.03 to 0.15%, and Al: 0.01 to 0.10%, and P: 0.010% or less, with the remainder consisting of Fe and inevitable impurities, wherein a rolling site of a surface of the component has a carbon content of 0.8 to 1.2%.
However, in this technology, the carbon content is provided through carburizing or carbonitriding treatment without nitriding treatment. Hence, the nitrogen amount of a surface layer is small, less than 1%. Consequently, if this component is used in an environment where seizing more easily occurs, the component cannot maintain excellent seizing resistance.
PTL 2 discloses a method of manufacturing a machine structural component having excellent fatigue strength, particularly surface fatigue strength, the method being characterized in that steel is used as a material, the steel containing C: 0.4 to 0.7%, Si: 0.3% or less, Mn: 0.2 to 1%, Cr: 0.2 to 3%, Mo: 0.1 to 1%, V: 0.1 to 1%, Al: 0.01 to 0.05%, N: 0.003 to 0.02%, S: 0.07% or less, and Ti: 0.002% or less, with the remainder consisting of Fe and inevitable impurities including P and O that are controlled, respectively, to be 0.02% or less and 0.002% or less, the material is formed into a predetermined component shape, and is then subjected to nitriding treatment or nitrocarburizing treatment and successively subjected to induction hardening treatment so that nitrogen is diffused from a surface of the component, and a surface-hardened layer having a nitrogen content of 0.05% or more is formed at a depth position of at least 0.2 mm from a top surface of the component.
However, since this technology includes the diffusion treatment of surface nitrogen, the surface layer cannot maintain high nitrogen content. In addition, since carbide is not dispersed in a surface layer of the component, the component cannot exhibit good seizing resistance in a high slippage environment.
PTL 3 discloses a method of manufacturing a rotary hook of a sewing machine, the method being characterized in that case-hardening steel such as carbon case-hardening steel and chromium-molybdenum case-hardening steel is subjected to carburizing-and-quenching and tempering, and further subjected to nitrocarburizing treatment. In this technology, however, only typical carburizing treatment is performed, and carbide does not dispersedly exist in a surface layer of a component. Hence, the component cannot exhibit excellent seizing resistance in a high slippage environment.