| 摘要 |
PURPOSE: A cold-rolled steel sheet and a manufacturing method thereof are provided to increase the fraction of martensite and the refinement of ferrite grains by forming an austenite nucleus on a sub-grain boundary of ferrite before recrystallization is completed. CONSTITUTION: A method for manufacturing a cold-rolled steel sheet is as follows: a step for reheating a slab within 1150-1250 deg. C.(S105); a step for winding by cooling the hot-rolled slab within 550-700 deg. C.(S120); a step for cold-rolling the wound slab after pickling treatment(S140); a step for annealing the cold-rolled slab after rapidly heating within Ac1-(Ac1+100) deg. C. at a speed of 15-30 deg. C./sec(S150). The slab is composed of 0.15 wt.% or less of carbon, 3.0 wt.% or less of manganese, 0.1 wt.% or less of molybdenum, and the rest of Fe and inevitable impurities. The slab is additionally composed of at least one of 0.8 wt.% or less of silicon and 0.4 wt.% or less of chrome. [Reference numerals] (AA) Start; (BB) End; (S105) Reheating slab(SRT: 1150-1250°C); (S110) Hot rolling(FDT: 850-950°C); (S120) Cooling/coiling(CT: 550-700°C); (S130) Pickling; (S140) Cold rolling; (S150) Annealing(heating finish temperature: Ac1 to Ac1+100°C) |
