To form a non-layer formed structure of nanocrystals having an average grain diameter of 100 nm in the vicinity of the surface of a material of a hard metal to be treated, by spouting fine particles with an average particle diameter of 100 μm or less and preferably 45 μm or less having the hardness exceeding 6.8 GPa by HV together with a pressurized airflow of 0.5 MPa or more to the surface of hard steel having initial hardness exceeding 7.0 GPa by HV.
The method of refining the crystal grains comprises: employing the hard steel having the initial hardness exceeding 7.0 GPa by HV as the material to be treated; and spouting the fine particles with an average particle diameter of 100 μm or less having the hardness exceeding 6.8 GPa by HV together with the pressurized airflow of 0.5 MPa or more to the surface of the material to be treated. Then, dynamic continuous recrystallization occurs in a region between the surface and an about 0.5 μm inner part in the surface of the material to be treated, and forms the nanocrystal structure of the non-layer formed structure, which is a plastic flow structure formed in the vicinity of the surface of the material to be treated through a clear border.
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TAKAGI SHINICHI
KUMAGAI MASAO
MIYASAKA YOSHIO
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FUJI KIHAN KK