Improved formability prediction by modeling evolution of anisotropy of steel sheets

Depending on the experimental observations, the Marciniak–Kuckzinki (MK) model was modified incorporating the evolution in the Yld2000-2d anisotropic yield function as a function of plastic work for two ferritic stainless steel sheets. The numerically estimated FLDs were validated with the strain limits evaluated by stretch forming experiments. Moreover, FE simulations were conducted to predict limiting dome heights (LDH) and limiting drawing ratios (LDR) by both models with and without considering the anisotropy evolutions. Additional formability performances such as surface strain distributions over the deformed cup surfaces and earing profile were also compared with experimental results. It was observed that the accuracy of predictions for formability could be significantly improved in the FE simulations when both initial and its subsequent evolution in yield function were included in the modeling. Further, microstructural analysis of parent sheets and stretch formed cups were performed to understand the effect of microstructure change on the anisotropy and formability. It was found that the preferred orientations along rolling and transverse directions changed differently with deformation. Orientation distribution function and Taylor factor maps were analyzed to confirm non-proportional evolution in stress directionality for both the materials.