The bake hardening behaviour of electro-galvanized cold rolled CMnSi and CMnAlSi TRIP steels

Cold rolled, intercritically annealed TRIP steels will very likely be used as electrogalvanized sheet steel for passenger safety-related automotive bo dy parts. This implies that their properties during and after manufacturing , i.e, in the deformed and paint-baked state. must be known to evaluate the ir actual in-service properties. The ageing behaviour of electrogalvanized TRIP steels and the effect of controlled ageing of the complex microstructu re by paint baking on the mechanical properties was therefore investigated. Electroplating tests showed that the H uptake during pretreatment and elec trogalvanization is limited and that the paintability of the electroplated sheet steel is unaffected even after deformation induced transformation of the austenite. In addition, a strong bake hardening effect was found in the case of the CMnAlSi TRIP steel. Internal friction measurements indicated t hat a damping peak for interstitial C could be obtained but that the free C content was less than 1 ppm prior to paint baking. A BH mechanism for TRIP steels is proposed.