Bake hardening and uniaxial tensile behavior in a low carbon steel accompanying inhomogeneous plastic yielding

Bake hardening (BH) is a vital part of application-specific steel manufacturing. Previous studies on BH have focused on steels undergoing homogeneous yielding, which critically fails to observe and characterize various interesting phenomena that occur during inhomogeneous yielding. In this study, we investigated the BH behavior observed upon the application of inhomogeneous pre-strains. Two steels of identical composition that exhibited inhomogeneous plastic yielding during uniaxial tensile testing were prepared. One sample was composed of acicular ferrite while the other was composed of annealed polygonal ferrite. Inhomogeneous plastic yielding during uniaxial tensile testing is inevitably accompanied by the formation of a Lüders band (LB). When the LB propagated partially to the gauge section, no BH response (BHR) was observed in either steel during the re-loading of the pre-strained specimens after baking treatment (BT). However, when the LB propagated completely, BHR was exhibited by both steels. Digital image correlation analysis and macro-indentation testing confirmed that in the former cases, inhomogeneous hardening occurred due to inhomogeneous plastic yielding, whereas in the latter case, the inhomogeneous yielding was resolved, and hardening occurred uniformly throughout the gauge section. This influences not only BHR but also the overall uniaxial tensile properties such as tensile strength and total elongation. Nanoindentation and small-angle neutron scattering tests were performed on the pre-strained specimens before and after BT to determine the origin of the hardening effect. The results confirmed that nano-scale clusters/ε-precipitates can be formed at a low BT temperature if a Cottrell atmosphere is induced, thereby improving not only the BHR but also tensile strength.