LOCAL USE OF BAKE HARDENING IN MULTIPHASE STEELS FOR IMPROVED PROPERTIES IN STRUCTURES AND JOINTS

This paper investigates processes leading to local bake hardening (BH) effects in multiphase steels. The influence of the deformation path and of the temperature and duration of thermal treatments on strengthening in multiphase steels, in regard to both local and bulk properties of steel structure are investigated. Bake hardening is the ability of a metal, to harden during an annealing after forming, for example during the paint baking process in the automobile industry, delivering a post-forming strength increase of the final component. Multiphase steels such as dual phase (DP) and complex phase (CP) steels are investigated to examine the effect of thermo mechanical processing parameters on the local bake hardening ability of hot rolled DP and CP steels. For this purpose two methods of achieving local BH (local deformation and local heat treatment) are studied. The influence of a local deformation through bending on the work hardening, and bake hardening effects is examined. By locally limited laser treatment on cold-rolled material local BH can be reached through local heat treatment. Furthermore, the ageing stability of the gain in strength is examined.