| 摘要 |
PURPOSE: A method for manufacturing high strength cold rolled steel sheet having low yield ratio and superior elongation by adding proper amount of B and Mo instead of lowering content of Mn and optimizing cold reduction ratio and cooling method after annealing is provided. CONSTITUTION: The method comprises the processes of finish hot rolling to a temperature of Ar3 transformation point or more a steel slab comprising 0.12 to 0.17 wt.% of C, 0.2 to 0.5 wt.% of Si, 3.5 to 4.5 wt.% of Mn, 0.02 wt.% or less of P, 0.005 wt.% or less of S, 0.02 to 0.10 wt.% of Al, 0.005 wt.% or less of N, 0.0001 to 0.002 wt.% of B, 0.01 to 0.10 wt.% of Mo, and a balance f Fe and other inevitable impurities; coiling the finish hot rolled steel slab to a temperature of 560 to 680 deg.C; cold rolling the coiled steel sheet to a reduction ratio of 30 to 60%; annealing the cold rolled steel sheet in the temperature range of 650 to 680 deg.C for 30 minutes to 5 hours; and forcibly cooling the annealed steel sheet at a cooling rate of 100 to 600 deg.C/hr.
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