PURPOSE: To reduce the carbon equivalent in a steel sheet and to attain the improvement of its weldability and to maintain the prescribed strength by rolling steel having specified chemical components, thereafter executing heating and air cooling under specified conditions and subsequently subjecting it to hardening and tempering treatment.
CONSTITUTION: A slab constituted of 0.05 to 0.20% C, 0.05 to 0.50% Si, 0.30 to 1.80% Mn and 0.005 to 0.10% Al, and the balance Fe with inevitable impurities is subjected to hot rolling. It is heated to the Ac3 point to <980°C, is subjected to air cooling to a prescribed temp. between 400 to 700°C and is thereafter hardened. Successively, it is subjected to tempering treatment at 400°C to less than the Ac1 point. In this way, the heat treated high tensile strength steel sheet of ≥570N/mm2 class excellent in weldability can be obtd. Moreover, this steel is incorporated with prescribed amounts of one or more kinds among Cu, Ni, Cr, Mo, V, Nb, B, Ti and Ca according to necessary.
IWAI KIYOSHI
OKANO SHIGEO
