| 摘要 |
PURPOSE: Provided is a method for manufacturing cold rolled steel sheets with improved brittleness resistance and ultra high strength, which has tensile strength over 35 kgf/mm¬2 and is suitable to deep drawing for structural steels such as automobile inner or outer panels and bumper. CONSTITUTION: The method for manufacturing cold rolled steel sheets with improved brittleness resistance and ultra high strength includes the steps of (i) finish hot rolling a steel slab comprising C 0.002% or less, Mn 1.0-1.5%, P 0.04-0.05%, S 0.006% or less, N 0.03% or less, sol-Al 0.04% or less, Ti 0.019-0.027%, Cr 0.4-0.8%, a balance of Fe and other inevitable impurities at a temperature higher than 900deg.C wherein Ti satisfies the inequality of £(48/14xN)+(48/32xS)| <= Ti <= £(48/32xS)+(48/12xC)| and Cr satisfies the inequality of 8 <= (Cr/P) <= 16; (ii) coiling the hot rolled steel sheet at a temperature of 700 to 720deg.C followed by pickling and cold rolling; and (iii) continuous annealing in the temperature range of 840 to 860deg.C.
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