Development of a New Bake-Hardenable Galvannealed Sheet Steel for Automotive Exposed Panels

Various types of bake-hardenable steels have been developed and applied to automotive body panels for a weight reduction since 1981. Recently for the improvement in the cosmetic corrosion resistance of the exposed panels, an ultra-low carbon bake-hardenable galvannealed steel was developed and has been produced in a large quantity. The main metallurgical features of the ultra-low carbon bake-hardenable steel are follows,

1. (1)
   adjustment of bake-hardenability by controlling the total carbon content between 15 and 25 wt.ppm,
2. (2)
   suppression of TiC formation by controlling Ti content between 48/14 ·N and 48·(N/14+S/32).
3. (3)
   optimisation of temper rolling condition to eliminate the stretcher strain.