| 摘要 |
PURPOSE: Provided is a method for manufacturing high strength cold rolled steel sheet with excellent strain aging resistance at room temperature and bake hardenability by grain size refinement. CONSTITUTION: The method comprises the steps of soaking an ultra low carbon/nitrogen Al-killed steel sheet comprising C 0.004 to 0.006 wt.%, 0.02 wt.% or less of Si, Mn 0.07 to 0.2 wt.%, P 0.05 to 0.05 wt.%, 0.008 wt.% or less of S, sol.Al 0.02 to 0.06 wt.%, 0.003 wt.% or less of N, a balance of Fe and incidental impurities at higher than 1200 deg.C; finish hot rolling the ultra low carbon/nitrogen Al-killed steel sheet at 900 to 950°C; coiling the hot rolled Al-killed steel sheet at 700 to 750°C; cold rolling it at a reduction ratio of 70 to 80%; continuous annealing it at 830 to 850°C; and temper rolling it at a reduction ratio of 0.8 to 1.2%.
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