PURPOSE: To produce a low yield ratio-type high tensile strength steel plate with ≥590N/mm2 tensile strength in which ≤80% yield ratio is secured and simultaneously having excellent weld crack resistance equal to or more than that of a high yield ratio-type high tensile strength steel plate produced by the conventional Q-T process.
CONSTITUTION: This steel has a compsn. contg., by mass, 0.05 to 0.15% C, 0.05 to 0.50% Si, 0.30 to 1.80% Mn, ≤0.015% P, ≤0.005% S, 0.005 to 0.10% Al and 0.005 to 0.100% Nb, and the balance Fe with inevitable impurities, contg. the carbon nitrides of Nb having ≤50nm grain size by 0.01 to 0.6% by volume fractional ratio and ferrite by ≥2% by volume fractional ratio, and in which PCM obtained by the following formula is regulated to ≤0.22%: PCM=C+Si/30+ Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×B(%). Thus, by utilizing precipitation strengthening by Nb, high strength can be obtd. while its low PCM and low yield ratio are maintained.
TAKESHITA SATOSHI
OE KENICHI
