High-strength cold rolled steel sheet having excellent ductility, hot-dip galvanized steel sheet and method for manufacturing same

A method for manufacturing a high-strength cold-rolled steel sheet having excellent ductility and a tensile strength of 750 MPa or more comprising:

• reheating a steel slab at 1,000° C. to 1,300° C., wherein the steel slab comprises:
• by wt %, carbon (C):
• 0.1% to 0.3%, silicon (Si):
• 0.1% to 2.0%, aluminum (Al):
• 0.005% to 1.5%, manganese (Mn):
• 1.5% to 3.0%, phosphorus (P):
• 0.04% or less (excluding 0%), sulfur (S):
• 0.015% or less (excluding 0%), nitrogen (N):
• 0.02% or less (excluding 0%), and a remainder of iron (Fe) and inevitable impurities, wherein a sum of Si and Al (Si+Al) (wt %)satisfies 1.0% or more;
• manufacturing a hot-rolled steel sheet by hot finishing rolling the reheated steel slab at 800° C. to 950° C.;
• coiling the hot-rolled steel sheet at 750° C. or less;
• manufacturing a cold-rolled steel sheet by cold rolling the coiled hot-rolled steel sheet;
• performing a first annealing operation of annealing and cooling the cold-rolled steel sheet at a temperature of Ac3 or more;
• and
• performing a second annealing operation of heating and maintaining the cold-rolled steel sheet at a temperature of Ac1 to Ac3 after the first annealing operation, cooling the cold-rolled steel sheet at a temperature of Ms to Mf at a cooling rate of 20 C/s or more, reheating the cold-rolled steel sheet at a temperature of Ms or more, maintaining the cold-rolled steel sheet for 1 second or more, and cooling the cold-rolled steel sheet,
• wherein a microstructure comprises:
• by area fraction, 5% or less of polygonal ferrite having a minor axis to major axis ratio of 0.4 or greater, 70% or less (excluding 0%) of acicular ferrite having a minor axis to major axis ratio of 0.4 or less, 25% or less (excluding 0%) of acicular retained austenite, and a remainder of martensite.