It is low production manner null of the 2 class steel sheet on resistance weld crack Characteristic is superior yield ratio

摘要

PURPOSE:To produce a steel plate excellent in welding crack resistance and reduced in yield ratio by controlling chemical components in a steel plate for construction use, reducing segregation, and properly regulating rolling conditions. CONSTITUTION:A slab of a steel having a composition which consists of, by weight, 0.10-0.18% C, 0.05-0.50% Si, 0.7-1.50% Mn, 0.010% P, 0.002% S, 0.005-0.030% Nb, 0.0010-0.0030% Ca, 0.30% Cu, 0.50% Ni, 0.30% Cr, 0.30% Mo, 0.05% V, and the balance Fe and in which carbon equivalent (Ceq.) specified by equation I is regulated to =0.45% is heated to 1150-1250 deg.C and held for (2.5XT)min (where T means the thickness of the steel slab expressed in millimeters), and a total reduction of area of =60% is secured in an austenite recrystallization temp. region and rolling is completed in this recrystallization temp. region. Subsequently, the resulting plate is cooled from a temp. not lower than the Ar3 point down to =400 deg.C at 5-30 deg.C/sec cooling rate, heated further to a temp. in the range between the Ac3 point and the Ac1 point, water-quenched, and successively tempered at a temp. not higher than the Ac1 point. By this method, the 60kgf/mm2 class steel plate excellent in welding crack resistance and reduced in yield ratio can be produced.