Microstructure based prediction of strain hardening behavior of dual phase steels

In the automotive industries, dual phase (DP) steels have increasingly used for various car body parts due to their excellent combination of high strength and good formability. The microstructure of DP steel basically consists of a matrix of ferrite embedded by martensitic islands. For the mechanical and fracture behaviors of the DP steel, effects of martensite phase fraction, morphology, and phase distribution play an important role. In this work, dual phase steel sheets with different martensite contents were produced by intercritical annealing process. Subsequently, a Finite Element (FE) based modeling using Representative Volume Elements (RVEs) approach was proposed for predicting overall stress–strain behavior of the investigated DP steels. Two dimensional RVE models were generated from micrographs of the DP steels on the microstructure level. For the individual single phases flow curves based on dislocation theory and local chemical composition were applied. Additionally, Geometrically Necessary Dislocations (GNDs), which accumulates at the phase boundaries due to the austenite–martensite transformation during quenching process, was taken into account. A local hardening effect due to these phase boundary dislocations was applied at the interface layer between ferrite and martensite. The calculated stress–strain responses of the DP steels were verified with experimental results determined from tensile tests. The micromechanics model could be then used to describe the local stress and strain evolution of the individual phases in the DP microstructures.

► DP steels with different MPFs produced by intercritical annealing were examined.
► Strain hardening of DP steels was predicted by a microstructure based RVE approach.
► GNDs at phase boundaries due to the martensitic transformation were considered.
► Local stress–strain distributions in the DP microstructures were characterized.