Method of manufacutring a high-strength cold rolled steel sheet having high hole expansion ratio, and highstrength hot-dip galvanized steel sheet

A method of manufacturing a cold rolled steel sheet, the method comprising:

• preparing a slab including, by weight percent (wt %), 0.17 to 0.21% of carbon (C), 0.3 to 0.8% of silicon (Si), 2.7 to 3.3% of manganese (Mn), 0.3 to 0.7% of chromium (Cr), 0.01 to 0.3% of aluminum (Al), 0.01 to 0.03% of titanium (Ti), 0.001 to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.02% or less of sulfur(S), 0.01% or less of nitrogen (N), balance of iron (Fe), and other inevitable impurities, wherein the contents of carbon (C), silicon (Si), and aluminum (Al) satisfy Equation 1:
• [C]+([Si]+[Al])/5≤0.35% wherein [C], [Si], and [Al] in Equation 1 refer to weight percent of C, Si, and Al, respectively;
• heating the slab to a temperature in a range of 1,150° C. to 1,250° C.;
• finish hot rolling the heated slab within a finish delivery temperature (FDT) range of 900° C. to 980° C.;
• cooling the slab at an average cooling rate of 10° C./s to 100° C./s after the finish hot rolling;
• winding the slab in a temperature in a range of 500° C. to 700° C.;
• cold rolling the slab at a cold-rolling reduction ratio of 30% to 60% to obtain a cold rolled steel sheet;
• continuous annealing the cold rolled steel sheet at a temperature in a range of (Ae3+30° C. to Ae3+80° C.) to obtain a continuously annealed steel sheet;
• primarily cooling the continuously annealed steel sheet at an average cooling rate of 10° C./s or less to a temperature in a range of 560° C. to 700° C. and secondarily cooling the steel sheet at an average cooling rate of 10° C./s or more to a temperature in a range of 270° C. to 330° C. to obtain a cooled steel sheet;
• and
• reheating the cooled steel sheet at a temperature increase rate of 5° C./s or lower to a temperature in a range of 380° C. to 460° C.;
• wherein the cold rolled steel sheet has a microstructure consisting of:
• by area fraction, 3 to 7% of retained austenite, 5 to 15% of fresh martensite, and 5% or less including 0% of ferrite;
• by volume fraction, 1 to 3% of a cementite phase as a precipitate;
• and a balance of bainite or tempered martensite, wherein the cementite phase is distributed at least one of between laths of the bainite, in laths of the tempered martensite, and in a grain boundary of the tempered martensite.