| 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 DEG C. and 1250 DEG 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 DEG 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 Ac1 point or below.
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