In order to reduce the car weight, various steels with excellent formability and high strength have been developed for the last 10 years at POSCO. The steels include interstitial free (IF) steels, solution and precipitation hardened steels, and transformation for manufacture of hot strips. The steels either have the fine bainite matrix or demonstrate the transformation induced plasticity (TRIP) behavior, exhibiting an excellent strenght-formability combiantion. IF steels are most widely used for cold strips and surface treated steels due to their excellent deep drawability. Reduction of impurity contents and high temperature annealing were found critical to obtain superior deep drawability. Solution hardening was the major strenghening tool for high strength IF steels. The TRIP behaivor is also utilized for cold strips because of their high strength and elongation combiantion. The volume fraction of the retained austenite was carefully controlled depending on their applications. Both hot dip and electrogalvanized steels were developed to improve the corrosion resistance. Applications of new grade steels togther with new forming technologies contribute significantly to the reductionin the weight of atuomotives.
展开▼
