Manufacturing method of retained austenite contained cold rolled steel sheets with good ductility

摘要

PURPOSE: A manufacturing method of residual austenite contained cold rolled steel sheet having better ductility compared to existing method by controlling cooling of the annealed steel sheet after titrating chemical constituents of high strength cold rolled steel sheet having 50 to 80 kg/mm¬2 class tensile strength and annealing the steel sheet is provided. CONSTITUTION: The method comprises a process of hot rolling a steel slab under the condition that finish rolling temperature is A3 transformation point or more, wherein the steel slab comprises 0.05 to 0.20 wt.% of C, 0.7 to 1.5 wt.% of Si, 1.5 to 1.8 wt.% of Mn, 0.005 wt.% or less of S, 0.003 to 0.007 wt.% or less of N, 0.01 to 0.1 wt.% of Mo, 0.0005 to 0.002 wt.% of B, 0.01 to 0.1 wt.% of P, 0.02 to 0.5 wt.% of Al, 0.02 to 0.10 wt.% of Cr, 0.1 to 0.5 wt.% of Ni and a balance of Fe and inevitable impurities, and Si, P, Al, Cr and Ni satisfy the following relational expression: Si%+9.5P%+0.45Al%+0.3Cr%+0.6Ni%=1.0; a process of cold rolling the coiled steel slab to a reduction ratio of 30 to 80% after coiling the hot rolled steel slab at a temperature of 630 to 680 deg.C; a process of maintaining the cold rolled steel sheet at a temperature of 780 to 810 deg.C for 5 to 180 seconds; and annealing process comprising the steps of first cooling the steel sheet to a temperature of 620 to 680 deg.C at a cooling rate of 5 deg.C/sec or less, second cooling the first cooled steel sheet to a temperature of 350 to 450 deg.C, maintaining the second cooled steel sheet at a constant temperature for 1 to 20 minutes and cooling the resulting steel sheet to an ordinary temperature, wherein an average cooling rate (CRtot) of the first and second cooling rates in the annealing process satisfies the following relational expression: CRmax£3.0-(Mn+0.3Si%+2.7P%)|=log(CRtot, deg.C/sec)=CRmin£2.5-(Mn+0.3Si%+2.7P%)|, where the CRmax is the maximum cooling rate, the CRtot is the total cooling rate, and the CRmin is the minimum cooling rate.