| 摘要 |
PURPOSE: A high strength thin strip with ultra-high formability is provided which has superior formability compared with conventional high strength steel with high formability by properly controlling ratio of alloy constituents and manufacturing conditions of hot rolling-cold rolling-continuous annealing, and a method for manufacturing the same is provided. CONSTITUTION: The method comprises a step of hot rolling a steel comprising 0.003 wt.% or less of C, 0.02 wt.% or less of Si, 0.6 to 1.5 wt.% of Mn, 0.04 to 0.15 wt.% of P, 0.006 wt.% or less of S, 0.003 wt.% or less of N, 0.01 to 0.03 wt.% of Ti, 0.005 to 0.02 wt.% of Nb, 0.002 wt.% or less of B and a balance of Fe and other inevitable impurities so that the steel satisfies hot rolling roll reduction rate distribution coefficient (Y1) of the following relational expression 5 at a rolling temperature of 1,020 deg.C or more and satisfies distribution coefficient (Y2) of the following relational expression 6 at a temperature less than the rolling temperature of 1,020 deg.C: Y1=2.86(n)+(8.93 to 12.2), Y2=-2.27(n)+(62.5 to 63.4), where n is the number of stands of rolling rolls of hot rolling mill; a step of hot coiling the hot rolled steel sheet in the temperature range of 680 to 720 deg.C; a step of cold rolling the coiled steel sheet at a cold reduction ratio of 77% or more; a step of continuously annealing the cold rolled steel sheet at a temperature from 850 deg.C to less than 880 deg.C; and a step of hot dip galvanizing or hot dip galvannealing the surface of the annealed steel sheet.
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