Study on the bake hardening behavior and tensile properties of ultra-low carbon automotive BH steel

Bake-hardening steel (BH steel) is widely used in automotive manufacturing, where its bake-hardening properties significantly impact production costs and safety, making it a subject of considerable research importance. In this work, the texture evolution, tensile properties and bake-hardening behavior of an ultra-low carbon (ULC) BH steel after hot-rolling, cold-rolling, continuous annealing and baking treatment at different temperatures (100-200 °C) was systematically investigated. It is demonstrated that both hot-rolled and cold-rolled specimens exhibit typical α-texture. Neither the annealed nor baked specimens underwent significant γ-texture transformation but evolved into a characteristic dominated by small-angle deviation of α-texture, with the preferential growth mechanism driven by energy storage differences. The activation of the (111)[110] slip system promoted grain rotation toward the γ-texture during the tensile deformation process, leading to γ-texture significant re-strengthening prior to fracture. The (Nb,Ti,Mo) C-type carbides precipitated in the annealed specimens,which effectively inhibited the slip of dislocations during tensile deformation. 170°C baked specimen exhibited the highest BH value of 67 MPa. The diffusion rate of carbon atoms in the solid solution accelerates with baking temperature increasing, which is theoretically expected to promote the formation of Cottrell atmospheres that pin dislocations, thereby significantly enhancing the BH value. However, at excessively higher baking temperatures, the material exhibits over-aging, leading to reduced stability and pinning effectiveness of Cottrell atmosphere, which consequently lowers the BH value.