| 摘要 |
PURPOSE: Provided is a manufacturing method of a cold-rolled steel with good formability and tensile strength higher than 450MPa. Obtained cold-rolled steel has a plastic deformation ratio(rm) higher than 1.5, and superior adhesiveness between steel sheet and zinc layer after galvanizing. CONSTITUTION: The method includes the steps of: (i) reheating an extra-low carbon steel slab comprising C 0.0050wt.% or less, Mn 1.3-1.7wt.%, P 0.07-0.12wt.%, Si 0.15-0.4wt.%, V 0.15-0.45wt.%, B 0.0005-0.0020wt.%, N 0.0050wt.% or less, S 0.015wt.% or less, Ti (C÷12 + N÷14)x48, sol-Al 0.025-0.06wt.%, balance Fe, and other inevitable impurities; (ii) finish hot rolling the reheated extra-low carbon steel slab at higher than Ar3 transformation temperature; (iii) coiling in the temperature range of 450 to 700°C; (iv) pickling the hot coil followed by cold rolling; and (v) annealing the pickled and cold rolled coil at a temperature between recrystallization temperature and Ac1 temperature followed by over-aging.
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