| 摘要 |
PROBLEM TO BE SOLVED: To industrially and stably produce a coating/baking curable type cold rolled steel sheet excellent in aging resistance by specifying the composition of a steel sheet and also specifying the value determined from the relation between the aging index of the steel sheet and the aging index of the heat treated steel sheet. SOLUTION: A composition, consisting of, by weight, <=0.005% C, <=1.0% Si, <=3.0% Mn, <=0.15% P, <=0.05% S, 0.01-0.20% Al, <=0.01% N, 0.01-0.2% Ti, and the balance Fe with inevitable impurities, is provided. Further, the value of A represented by equation is >=0.4, and the value of AIQUENCH is >=30, where the value of AIQUENCH is an aging index (MPa) after 500 deg.C×40 sec heating and water quenching treatment is applied to the steel sheet and AI is an aging index (MPa) of the cold rolled and annealed sheet. Moreover, the aging index is the amount of increase (MPa) of yield stress, as a difference between the amounts of yield stress before and after the undermentioned heat treatment, in the case where 7.5% tensile prestrain is applied to the steel sheet and then heat treatment is applied at 100 deg.C for 30 min.
|
