High-strength cold rolled steel sheet with low material non-uniformity and excellent formability, hot dipped galvanized steel sheet, and manufacturing method therefor

A method for manufacturing a low-yield-ratio high-strength cold-rolled steel sheet with low deviation of directional material and excellent formability, the method comprising:

• manufacturing a steel slab using soft reduction and reheating the steel slab at the time of continuously casting steel using molten steel, the molten steel including, by wt %, 0.05 to 0.15% of C, 0.2 to 1.5% of Si, 2.2 to 3.0% of Mn, 0.001 to 0.10% of P, 0.010% or less of S, 0.01 to 0.10% of sol.Al, 0.010% or less of N, and the balance of Fe and impurities, satisfying a condition of Si/(Mn+Si)≦0.5;
• finish hot rolling the reheated steel slab at a temperature range of Ar3 to Ar3+50° C., and coiling the hot-rolled steel sheet at a temperature range of 600 to 750° C.;
• cold rolling the coiled steel sheet at a cold reduction ratio of 40 to 70%, and then continuously annealing the cold-rolled steel sheet at a temperature range of Ac1+30° C. to Ac3−30° C.;
• and
• primarily cooling the continuously annealed steel sheet to a temperature range of 650 to 700° C., and then secondarily cooling the primarily cooled steel sheet to a temperature range of Ms−50° C. or lower.

A method for manufacturing a low-yield-ratio high-strength hot-dipped galvanized steel sheet with low deviation of directional material and excellent formability, the method comprising:

• manufacturing a steel slab using soft reduction and reheating the steel slab at the time of continuously casting steel using molten steel, the molten steel including, by wt %, 0.05 to 0.15% of C, 0.2 to 1.5% of Si, 2.2 to 3.0% of Mn, 0.001 to 0.10% of P, 0.010% or less of S, 0.01 to 0.10% of sol.Al, 0.010% or less of N, and the balance of Fe and impurities, satisfying a condition of Si/(Mn+Si)≦0.5;
• finish hot rolling the reheated steel slab at a temperature range of Ar3 to Ar3+50° C., and coiling the hot-rolled steel sheet at a temperature range of 600 to 750° C.;
• cold rolling the coiled steel sheet at a cold reduction ratio of 40 to 70%, and then continuously annealing the cold-rolled steel sheet at a temperature range of Ac1+30° C. to Ac3−30° C.;
• and
• primarily cooling the continuously annealed steel sheet to a temperature range of 650 to 700° C., and then secondarily cooling the primarily cooled steel sheet to a temperature range of 600° C. or lower at an average cooling rate of 3 to 30° C./s;
• and
• annealing the cooled steel sheet under normal conditions, and then galvanizing the annealed steel sheet.