Examining the unloading behavior of dual-phase steels by means of microstructure simulations

Ferritic-martensitic dual-phase (DP) steels exhibit a pronounced nonlinear unloading behavior, which is subjected to strong variations and depends on the complex interaction of parameters of the sheet metal material production process and the resulting heterogeneous microstructure. The knowledge of the unloading behavior in dependence of the governing parameters and their underlying mechanisms play a crucial role in sheet metal forming of high strength dual-phase steels. To analyze the influence of individual parameters on the unloading behavior of dual-phase steels, computational studies on the microstructure level were carried out based on the finite element method. The investigated parameters included microstructural parameters, unloading rate and thermo-mechanical deformation history caused by sheet production. In the course of the simulations, numerous different periodic volume elements were uniaxially loaded and unloaded. The computational results demonstrate that residual stresses of type II in the microstructure cannot fully explain the nonlinear unloading behavior of the steels. The reversible slip of dislocations in ferrite also plays an important role. Of all parameters examined, the martensite volume fraction and the phase strength contrast have the largest impact on the unloading behavior. Furthermore, the distribution of martensite in the microstructure significantly affects the unloading behavior, as well as residual stresses and a plastic pre-deformation both caused by the sheet production.