| 摘要 |
<p>The present invention relates to a cold rolled steel sheet having the tensile strength (TS) of 637-670 MPa, the yield strength (YP) of 350-387 MPa, the elongation (EI) of 29.3-32.9%, and the yield rate (YR) of 60% or more by ensuring the fraction of ferrite and martensite by adding more silicon (Si) and manganese (Mn) in the existing manufacture condition of which the length of the over aging section is markedly long; and a method of manufacturing the same. According to the present invention, the method of manufacturing the steel sheet comprises: a step of re-heating a steel slab comprising 0.055-0.08 wt% of carbon (C), 0.6-0.8 wt% of silicon (Si), 1.9-2.1 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P), 0.05 wt% or less of sulfur (S), 0.03 wt% or less of aluminum (Al), 0.04-0.07 wt% of molybdenum (Mo), 0.25-0.45 wt% of chrome (Cr), and the remainder consisting of Fe and inevitable impurities at 1180-1220°C which is a slab reheating temperature (SRT); a step of hot rolling the re-heated slab at 840-900°C which is a finishing delivery temperature (FDT); a step of coiling the slab after cooling the hot rolled slab; a step of cold rolling the slab at the reduction rate of 40-80% after pickling the coiled slab; and a step of annealing and heat treating the cold rolled slab to 790-830°C.</p> |
