Mechanisms and Modeling of Bake-Hardening Steels: Part I. Uniaxial Tension

A physically based model for bake-hardening (BH) steels is developed suitable to predict the BH as well as the macroscopic behavior of strain-aged steels in tensile tests, such as the lower yield stress and the yield point elongation or Lüders strain. A description of the strain aging kinetics is given by considering two aging steps: Cottrell atmospheres formation and precipitation of coherent carbides. The modeling includes the effect of solute carbon content, aging time, temperature, and prestrain. Then, a numerical approach of Lüders phenomenon using finite element (FE) method codes is conducted. The strain aging model is eventually coupled with the previous numerical study thanks to a local mechanical behavior that schematically describes the local dislocation behavior. Simulations of tensile tests are performed and agree well with experiments carried out on aluminum-killed (AlK) and ULC BH steels, in terms of lower yield stress and yield point elongation. Effects of aging treatment, grain size, and strain rate on the macroscopic behavior are particularly enlightened.