PURPOSE: To produce a 70kgf/mm2 class steel plate having superior weldability and low yield ratio by subjecting a slab of a steel with a specific composition to hot rolling and then to heat treatments under respectively specified conditions.

CONSTITUTION: A steel plate having an yield ratio as low as ≤80% in base material and a base material strength of 70kgf/mm2 class and also having superior weldability can be produced by subjecting a slab of a steel having a composition which consists of 0.07-0.15% C, 0.05-0.50% Si, 0.30-1.80% Mn, 0.10-1.20% Cr, 0.10-1.00% Mo, 0.01-0.10% Al, 0.02-0.08% V, and the balance Fe with inevitable impurities and in which PcM represented by Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/ 0+5B(%) is regulated to ≤0.26% to hot rolling and then to heat treatments consisting of hardening, normalizing, and tempering under the following conditions: hardening temp. between the Ac3 point and 950°C, normalizing temp. between the Ac1 point and <Ac3 point, and tempering treatment of 500°C and <Ac1 point.