Experimental and numerical study on strain path and crystal orientation dependences of phase transformation behaviors of QP1180 steel

Phase transformation behaviors of a quenching & partitioning (QP) steel, QP1180, under different strain paths are investigated by both experiment and crystal plasticity simulation. Experiments of various strain paths including uniaxial tension (UT), uniaxial compression (UC), plane strain tension (PST) and equi-biaxial tension (EBT) are conducted, and the mechanical stability of retained austenite (RA) shows obvious strain path and orientation dependences. A crystal plasticity-phase transformation model with multiple-variant kinetics is proposed and used to predict the transformation behaviors. The single crystal analysis with 11 typical orientations is performed under various loading paths. Different variant types and volume fraction evolutions are observed in 11 orientations, which results in the overall path and orientation dependences of phase transformation in polycrystal. Eight major texture components of the RA grains show a similar strain path dependence and determine the transformation behaviors of the QP1180 steel. A numerical simulation comparison between single-variant and multiple-variant kinetics shows that both theories can capture strain path dependence and texture evolution in phase trans-formation of the current QP1180, but the multiple-variant kinetics can better reproduce the texture of newly formed martensite. The effects of non-proportional loadings on transformation and the related mechanism are also studied with the proposed model.