| 摘要 |
<p>PURPOSE: A cold-rolled steel and a manufacturing method thereof are provided to add boron, thereby improving the moldability of bake hardened steel and having the dent resistance in entire outer panel. CONSTITUTION: A manufacturing method of a cold-rolled steel comprises the following steps of: heating a slab plate material which includes 0.005 wt% or less of carbon (C), 0.03 wt% or less of silicon (Si), 1.0 wt% or less of manganese (Mn), 0.10 wt% or less of phosphorus (P), 0.01 to 0.06 wt% of aluminum (Al), 0.003 wt% or less of nitrogen (N), and 0.0005 to 0.002 wt% or less of boron (B), further includes niobium (Nb) in a range of 0.0005 wt% <= C(wt%) - 0.13*Nb(wt%) <= 0.0015 wt%, and consists of the remainder iron (Fe) and other inevitable impurities(S110); heat-rolling the heated plate material at the finishing delivery temperature (FDT) of 860 to 900°C (S120); winding the heat-rolled plate material up to the coiling temperature (CT) of 640 to 680°C (S130); acid-pickling the wound plate material (S140); cold-annealing the plate after the acid-pickling (S150); cold-temper rolling the cold-annealed plate material (S160); and cooling the cold-temper rolled plate material (S170). [Reference numerals] (AA) Start; (BB) End; (S110) Reheating a slab (SRT: 1150-1250°C); (S120) Heat-rolling (FDT: 860-900°C); (S130) Cooling/winding (CT: 640-680°C); (S140) Acid-pickling; (S150) Cold-annealing (the temperature of a cold rolling and annealing heat treatment: 840-860°C); (S160) Cold-temper rolling the cold-annealed plate material; (S170) Cooling</p> |
