Surface processing method for a steel member and surface processed steel

Abstract

A surface hardening method, in which a surface layer on a thin steel plate member is stably melted and solidified to provide a high surface resistance against wear includes irradiation with a high density energy beam to melt a process surface portion of the steel member followed by rapid cooling. The melted surface portion includes a fully melted layer and an adjacent imperfectly or partially melted layer. At least the cooling to solidify the molten steel is performed in a short time period less than an austenitic transformation completion time at temperatures below melting, e.g., one-twentieth of the normal austenitic transformation time, and preferably both heating to melt the surface layer and cooling to solidify and harden the surface layer are performed in a time period less than the normal austenitic transformation completion time period. The high density energy beam can be divided to irradiate plural regions to melt the surfaces of the plural regions at the same time. The thermal capacity (or thickness) of the entire steel member can be designed to be as small as four times the thermal capacity of the process portion (or depth of melted portion) and still allow self cooling of the steel member to produce surface hardening. Additionally the melted-hardened surface has improved smoothness such as exhibited by a roughness which is only sixty percent or less of the roughness of non-processed surfaces.