PURPOSE: To reduce a carbon equivalent, to improve a weldability and to ensure a prescribed strength by executing accelerated cooling on line under the prescribed condition after rolling the steel with prescribed chemical components.
CONSTITUTION: A steel slab having a composition consisting of, by weight, 0.03-0.20% C, 0.05-0.50% Si, 0.30-2.50% Mn, 0.005-0.10% Al, and the balance Fe with inevitable impurities is hot-rolled, and then subjected to accelerated cooling with ≥10°C/sec cooling speed and ≥200°C cooling stop temp. after cooled to the prescribed temp. in the range of 500-800°C. Successively, it is subjected to annealing as necessary, at a temp. between 400°C and the Ac1 point. By this means, the carbon equivalent is reduced and a high tensile strength steel plate over 490N/mm2 class with good weldability is obtained. Also, a prescribed quantity of one or more kinds among Cu, Ni, Cr, Mo, V, Nb, Ti, and Ca are contained in the steel, as necessary.
IWAI KIYOSHI
OKANO SHIGEO
