| 摘要 |
PURPOSE: A method for manufacturing high tensile steel product for line pipes having superior impact toughness at low temperature is provided to secure superior impact toughness at low temperature and reduce cost by replacing vanadium (V) with chromium (Cr) in constituents of a conventional high tensile steel product for line pipes. CONSTITUTION: The method comprises the processes of spheroidizing annealing the molten steel using an inclusion of Ca-Si when vacuum degassing molten steel comprising 0.04 to 0.07 wt.% of C, 1.40 to 1.70 wt.% of Mn, 0.15 to 0.25 wt.% of Si, 0.010 wt.% or less of P, 0.003 wt.% or less of S, 0.040 to 0.060 wt.% of Nb, 0.010 to 0.025 wt.% of Ti, 0.10 to 0.30 wt.% of Mo, 0.20 to 0.35 wt.% of Ni, 0.15 to 0.25 wt.% of Cr and a balance of Fe and other inevitable impurities; and heating, rolling and cooling the spheroidizing-annealed slab under the conditions that a reheating temperature is 1,160 to 1,185 deg.C, a reduction ratio is 25% or more during rough rolling at final pass, a bar thickness is 45 to 55 mm, a rough rolling finishing temperature is 880 to 920 deg.C, a finishing temperature of finish rolling is 780 to 820 deg.C, and a coiling temperature is 540 to 590 deg.C.
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