Conventionally, machinability-improving elements such as S, Pb, and Bi are added to improve the machinability of steels. However, adding Pb and Bi reduces the strength of the steels while little affecting the forgeability. Note that the amount of Pb used has been decreasing from the viewpoint of environmental protection.
S forms MnS (soft inclusion for cutting work) to improve the machinability. However, MnS has particles larger than those of Pb and the like, and thus, stress is more likely to concentrate on MnS. Further, in the case where MnS is drawn through forging and rolling, anisotropy occurs in the steel structure, significantly reducing the strength in a specific direction. As described above, adding the machinability-improving elements leads to a reduction in the strength, and thus, it is difficult to obtain both the strength and the machinability only by adjusting the components.
To deal with these problems, studies have been made to obtain a desired strength using high-frequency hardening, and several steels for high-frequency hardening have been proposed (see Patent Documents 1 to 5).
For example, Patent Document 4 proposes a steel material exhibiting excellent machinability and fatigue characteristics after the high-frequency hardening. This steel material contains predetermined components, and has a base structure including a ferrite and a pearlite (total of both is 90 vol % or more). Further, the maximum thickness of the ferrite in the steel material is 30 μm or less.
Patent Document 5 proposes a high-frequency-hardened steel for a pinion exhibiting excellent machinability. This steel contains predetermined components, and has an average aspect ratio of inclusions including MnS of 10 or less. This steel is subjected to a high-frequency thermal treatment to make the center portion of the steel become hard, thereby obtaining bending fatigue characteristics of bending fatigue life: 1.0×105 cycles or more with a rotary bending stress of 280 MPa.
In recent years, there has been an increasing demand for automobile parts having higher machining accuracy and improved fatigue strength. However, conventional steels for a machine structure cannot satisfy this demand.