Bake hardening behavior and microstructure evolution of hot-rolled high strength steel under different temperatures and pre-strain rates

Press forming and paint curing are essential processes in automobile manufacturing. Moreover, the pre-strain and baking effects induced by these processes on steel plates are significant. This study investigated the effects of baking temperature and pre-strain rate on bake hardening mechanism of hot-rolled steel HR420. Through an analysis of the bake hardening response and microstructural evolution in HR420, three distinct stages in the process were identified. Initially, carbon atoms diffuse to dislocations, forming Cottrell atmospheres that pin the dislocation. Subsequently, excess carbon atoms aggregate into carbon clusters or low-temperature carbides, thereby further pinning the dislocation. In the final stage, a high dislocation density facilitates rapid carbon diffusion, resulting in the precipitation of fine cementite from carbon clusters at the dislocations. Furthermore, moderate pre-strain and baking temperature enhance the bake hardening response most effectively. For as-received samples, the bake hardening value increases from 4 to 16 MPa as the baking temperature rises, accompanied by a slight decrease in elongation and an increase in embrittlement value from 3.0% to 6.0%. For samples with 2% pre-strain, the bake hardening value increases significantly from 10 to 26 MPa with rising baking temperature. These results indicate that while bake hardening improves the yield strength of HR420, it also reduces fracture elongation and overall plasticity. These findings provide critical insights into the bake hardening process, thereby enhancing the efficiency and effectiveness of HR420 steel in automotive applications.