Process for producing extra high tensile steel in 1080 MPa yield strength class having excellent stress corrosion cracking resistance

摘要

An extra high tensile steel having an excellent stress corrosion cracking resistance and a yield strength of 1080 MPa or more is provided. A slab comprising, in terms of % by weight, 0.03 to 0.08% of C, 0.01 to 0. 10% of Si, 0.05 to 0.65% of Mn, 8.0 to 11.0% of Ni, 0.5 to 1. 5% of Mo, 0. 2 to 1.5% of Cr, 0.02 to 0.20% of V and 0.01 to 0.08% of Al with the balance consisting of iron and unavoidable impurities is heated to a temperature between 1000° C. and 1250° C., hot-rolled in an austenite recrystallization temperature region with a reduction ratio of 30 to 70%, subsequently rolled in an austenite nonrecrystallization temperature region with a reduction ratio of 20 to 60%, subjected to roll finishing, water-cooled from a temperature of 600° C. or above, reheated to so as to have an area ratio of non-diffusion type reverse transformed austenite grains of 40 to 80% and an area ratio of diffusion type reverse transformed austenite grains of 20 to 60%, quenched and then tempered at a temperature of A.sub.c1 point or below.