| 摘要 |
PURPOSE: A method for manufacturing ultra high strength cold rolled steel sheet for automotive outer panels by adding carbon, manganese and silicon to low carbon aluminum killed steel and properly controlling addition amount and ratio of boron or molybdenum that is a special element, thereby rolling the aluminum killed steel. CONSTITUTION: The method comprises a step of homogenizing an aluminum killed steel to which 0.05 to 0.15 wt.% of carbon, 1.2 to 1.8 wt.% of manganese, 0.7 to 1.3 wt.% of silicon, 0.003 to 0.007 wt.% of boron, 0.05 to 0.3 wt.% of molybdenum, 0.02 to 0.06 wt.% of aluminum, 0.02 wt.% or less of sulfur, 0.02 wt.% or less of phosphorus and 0.004 wt.% or less of nitrogen are added, and which comprises elements inevitably contained during steelmaking in the temperature range of 1,050 to 1,300 deg.C, finish hot rolling the homogenized aluminum killed steel at a temperature of 850 to 950 deg.C that is right above Ar3 transformation point, and hot coiling the finish hot rolled steel sheet in the temperature range of 400 to 600 deg.C; a step of intercritical annealing the cold rolled steel sheet at a temperature between A1 and A3 after cold rolling the hot coiled steel sheet to a cold reduction ratio of 30 to 80%; and a step of maintaining the steel sheet for one minute or more as slowly cooling the rapidly cooled steel sheet in the temperature range of 450 to 350 deg.C after firstly slowly cooling the intercritical annealed steel sheet to a temperature of 700 to 620 deg.C in a cooling rate of -1 deg.C/sec to -7 deg.C/sec and secondly rapidly cooling the slowly cooled steel sheet to the temperature range of 450 to 350 deg.C in a cooling rate of -10 deg.C/sec or more.
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