| 摘要 |
PURPOSE: A high strength cold rolled steel sheet with superior formability and weldability, and a method for manufacturing thereof are provided. The high strength cold rolled steel sheet has tensile strength of 70 to 90 kgf/mm¬2 and exhibits excellent impact absorbency. CONSTITUTION: The method includes the steps of reheating a steel slab comprising C 0.15 to 0.25 wt.%, Si 0.5 to 1.2 wt.%, Mn 1.0 to 2.0 wt.%, 0.25 wt.% or less of P, 0.020 wt.% or less of S, Al 0.015 to 0.050 wt.%, N 0.008 to 0.026 wt.%, a balance of Fe and incidental impurities at 1050 to 1300°C, wherein Si and P contents satisfies below relation: 1.5<=Si£wt.%|+50/8P£wt.%|<=2.0; hot rolling the steel slab at final delivery temperature of 890 to 940 deg.C; coiling the hot rolled steel sheet at 600 to 700 deg.C, followed by pickling; cold rolling the hot rolled steel sheet at a reduction ratio of 40 to 70 %; continuous annealing a cold rolled steel sheet at a temperature range defined as follow: 563+651C£wt.%|+42Si£wt.%|+18Mn£wt.%|<=annealing temperature£°C|<=850°C for more than 50 sec; rapid cooling the cold rolled steel sheet to the austenite initiating temperature ranging from 400 to 450 deg.C at a rate of 20 to 100°C/sec; and austempering the cold rolled steel sheet in the temperature range of 350 to 400°C for 200 to 500 sec.
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