저널

In an effort to improve the dent resistance of exterior body panels at a reduced steel thickness, the bake hardenable steels added Ti or Nb with tensile strength of 35Kgf/$ extrm{mm}^2$ were investigated. The bake hardenability increased with the annealing temperature and solute carbon content. Bake hardening of 3 to 5Kgf/$ extrm{mm}^2$ was obtained in steels with a controlled solute carbon concentration range from 6 to 10ppm. The galvannealing temperature and time had little influence on the bake hardenability. The Fe-Zn alloying reaction of 35Kgf/$ extrm{mm}^2$ BH steel was remarkably retarded due to a 0.07%P addition. The optimum galvannealing temperatures of 35Kgf/$ extrm{mm}^2$ BH steel were ranged from 520 to 56$0^{circ}C$ in view of the Fe content and powdering resistance. The cross-section and planar views of the galvannealed coatings to characterize morphology development were discussed.