PURPOSE: To prevent the production of strain at hardening time, by cooling rapidly after heating at the temperature between Ac3 point of soft iron and Ac3 point of thermal refining high tensile steel, at the time of hardening joining steel sheet of soft steel and thermal refining high tensile steel.

CONSTITUTION: Clad steel sheet is formed by soft steel contining 0.05% of C and thermal refining high tensile steel containing 0.12% of C and the above formed sheet is hardened. On this occasion, the above sheet is heated at the middle temperature of 897°C of Ac3 transformation point of soft iron and 837°C of Ac3 transformation point of thermal refining high tensile steel and is tempered at 620°C after carrying out water hardening. By the above method, deformation on the basis of transformation expansion at the time of transforming from austenite to ferrite, is prevented.