COLD ROLLED STEEL SHEET HAVING EXCELLENT SPOT WELDING PROPERTY AND DELAYED FRACTURE RESISTANCE AND METHOD FOR MANUFACTURING THE SAME

摘要

The present invention relates to an ultra-high strength cold rolled steel sheet mainly used for bumper reinforcement materials or shock absorbers in doors and various strength reinforcement parts of automobile components, in weight%, C: 0.02 to 0.22%, and Si: 2.5 to 4.0%. , Mn: 2.5 to 3.5%, Al: 0.02% or less, Cr: 0.01 to 0.1%, P: 0.02% or less, Ti: 0.005 to 0.02%, B: 5 to 20 ppm, Sb: 0.01 to 0.03%, S: 0.008 Less than or equal to N, 0.006% or less, the rest is provided with a cold-rolled steel sheet and a hot-dip galvanized steel sheet excellent in spot welding properties and delayed fracture properties composed of Fe and unavoidable impurities, 1150 steel slab that satisfies the composition Heating to a temperature range of ˜1250 ° C. to perform hot finish rolling at a temperature range of 880 ° C. to 920 ° C .; Start cooling within 1 second after the hot rolling, and then cooled to a temperature range below the bainite transformation start temperature (Bs) at a martensite transformation start temperature (Ms) + 20 ° C. or more at a cooling rate of 10 to 30 ° C./sec. Winding up; Pickling after the winding; Cold rolling after the pickling process in a range of 20 to 60% of a cold reduction rate; And after the cold rolling continuous annealing for more than 60 seconds at a temperature of 720 ~ 880 ℃, after cooling at a rate of 10 ~ 200 ℃ / sec after maintaining at a temperature of 250 ~ 350 ℃ 60 seconds or more after the final cooling It provides a cold-rolled steel sheet and hot-dip galvanized steel sheet excellent in spot welding properties and delayed fracture resistance.; AUTOMOTIVE STEEL SHEET, TRANSFORMATION INDUCED PLASTICITY, RETAINED AUSTENITE, SPOT WELDABILITY, DELAYED FRACTURE