| The purpose of this study was to develope IF(interstitial free) steels for the application to automotive body panels. IF steels containing less than 20 wt ppm of C and N, and 1.2 to 2.8 wt% of Mn were prepared to investigate the effect of Mn on the microstructure and mechanical properties. Microstructure observation of IF steels annealed at austenitic region revealed network type polygonal ferrite due to Mn gradient for the steels containing 1.23% Mn(A) and 1.94% Mn(B), and granular bainitic ferrite with high dislocation density for the steel containing 2.76%Mn(C). Tensile strength of A. B and C after annealing (800-920℃) was measured as about 400, 450 and 500 ㎫, respectively Tensile strength decreased slightly for A and B, butincreased for C with increasing annealing temperature. Granular bainitic ferrite of C might induce the strengthening effect at a high annealing temperature. (Fe. Ti) P precipitates formed when IF steels were annealed at ferritic region, which lead to the shortage of effective Ti content to remove solute carbon. The major texture of IF steels was found to be (554)[225￣] that is favorable to improve the formability. However, intensity of (554)[225￣] was decreased with increasing Mn content. |
|
