A multiscale study on the novel strain hardening behavior of DP steels

The uniaxial tensile behavior of materials can be described by the flow stress model, which is the basic constitutive model. In this paper, the flow stress models were divided into full strain (FS) and plastic strain (PS) flow stress models. Based on the extension of n & K values in the classical Hollomon equation, a “new” physical-based flow stress model was proposed, whose forms of FS and PS were named as Hollomon-var and Hollomon-varp equations, respectively. It was well evaluated for approximating the stress-strain curves of DP780, DP980, and DP1180 steel sheets. And various materials were used to test the applicability of the newly proposed equation. The flow stress curves of DP steel during large deformation were obtained by a multi-scale method and verified by the pure shear's finite element simulation in reverse. The real microstructure-based RVE model was built and the strain-hardening behavior of ferrite and martensite was determined and verified. The mesoscale strain-hardening behavior and deformation incompatibility of DP steels with different martensite volume fraction (MVF) was analyzed. The results show that the newly proposed equations show better fitting accuracy and wider application range for the description of strain-hardening behavior with relatively few parameters. The novel nonlinear hardening behavior of DP steel is mainly due to the high yield stress of martensite. Based on the trend of q value, the NISM-var method was proposed and verified. As the MVF increases, the strain in ferrite and martensite both gets greater, and the deformation incompatibility in ferrite intensifies although the deformation in martensite becomes more consistent. For the overall DP steel, the tensile strength increases linearly with MVF, while the uniform deformation and the corresponding plastic work per unit volume are the opposite. These findings provide guidance for engineering application of DP steel.