PRODUCTION OF LOW YIELD RATIO HIGH TENSILE STRENGTH STEEL EXCELLENT IN WELD CRACKING SENSITIVITY

摘要

PROBLEM TO BE SOLVED: To provide a method for producing high tensile strength steel of ≤100 mm sheet thickness excellent in respective characteristics of weld cracking sensitivity, a low yield ratio and acoustic anisotropy at a low cost. ;SOLUTION: Steel having a compsn. contg., by weight, 0.06 to 0.11% C, 0.01 to 0.4% Si, 0.5 to 1.6% Mn, 0.01 to 0.1% Al, 0.1 to 1% Mo, 0.01 to 0.1% V, 0.005 to 0.05% Nb, 0.0005% Ti and 0.0003% B, also having ≥0.22 Pcm value, and the balance iron with inevitable impurities is rolled under heating at 1,000 to 1,250°C and is thereafter applied with cumulative rolling reduction of ≥20% at ≤1,000°C, hot rolling is finished at a temp. above 800°C in the central part of the sheet thickness, and it is directly quenched from the temp. at least equal to or above the Ar3 transformation point, is thereafter reheated to a two phase region temp. of the Ac1 point to the Ac3 point, is thereafter quenched and is moreover subjected to tempering treatment at the temp. equal to or below the Ac1 point, where Pcm=C%+Si%/30+Mn%/20+Cu%/20+Ni%/30+Cr%/20+Mo%/15+V%/10+5B%.;COPYRIGHT: (C)2000,JPO