| 摘要 |
PROBLEM TO BE SOLVED: To impart large compressive residual stress in a short time by subjecting a thin sheet in which a fluoride layer has been formed on the surface after solution heat treatment and aging treatment to nitriding treatment in gas which contains gaseous ammonia and does not contain carbon or contains it by a trace amt. and controlling the concn. of carbon in the surface to the value equal to or below the specified one. SOLUTION: The solution treatment is executed for allowing elution atoms of Ni, Al, Ti or the like to enter into solid solution in austenite. The aging treatment is executed for precipitating alloy elements entered into solid solution in supersaturated martensite and subjecting the steel to precipitation hardening. By forming a fluoride layer on the surface of the steel, the formation of an oxidized film checking the infilatration of nitrogen in the nitriding treatment is suppressed. In the fluoriding treatment, gaseous fluorine such as NF3 is used by being diluted with inert gas. In the nitriding treatment, reaction gas contg. gaseous ammonia is used, and activated nitrogen is infiltrated from the surface of the steel. The concn. of carbon in the surface of the steel is controlled to <=2 wt.%. The content of the carbon component in the nitriding reaction gas is preferably controlled to <10 vol.%.
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