PURPOSE: To obtain a carbon steel having the high strength, high toughness and wear resistance equiv. to those of an alloy steel by subjecting a carbon steel material which is subjected to cold plastic working to have the increased electric resistivity to high-frequency surface hardening.

CONSTITUTION: The carbon steel material 11, when subjected to cold plastic working, is improved in strength and remarkably improved in the electric resistivity by work hardening. Such steel material 1 is put into a coil 2 and high-frequency current is passed in the coil 2, then the width of a quench hardened layer 11 is increased and a fine crystal grain layer 13 is formed by the recrystallization effect on the inside of the quench hardened layer 11. Then, the quench hardened layer 11 shares the wear resistance and strength, the fine crystal grain layer 13 shares the toughness and the remaining core part 12 shares the strength. The carbon steel material 1 having the performance equiv. to the performance of the alloy steel is thus obtd.