Experimental Study of the Micromechanical Behavior of Ferrite in DP Steel Under Various Stress States

The low yield ratio and continuous yielding behavior of ferrite-martensite dual-phase (DP) steels are determined by their production process and microstructure. In this work, high-resolution in situ electron backscatter diffraction (EBSD) experiments were performed to study the deformation and hardening behavior of DP steel in uniaxial tensile, plane strain and shear stress states. In the EBSD experiments, we first collected the data for different deformation stages from the same area for each sample (loading was paused during EBSD data collection). Subsequently, we analyzed the strain contouring maps, geometrically necessary dislocation (GND) distribution and ferrite deformation of all the samples based on the postprocessing of the EBSD data. The results showed that (i) the volume expansion accompanying the austenite-to-martensitic transformation during annealing gives rise to ferrite plastic deformation, and the residual deformation after overaging of the fine ferrite grains near the martensite group is the largest; (ii) the high-density GND bands of the shear sample is the same as the shear stress/strain direction, and the high-density GND bands of the uniaxial tensile and plane strain samples are perpendicular to their maximum stress/strain directions; and (iii) in the shear state, the average strain of ferrite grains in each grain size range is similar; but the deformation tends to aggregate toward large-size grains in the uniaxial tensile and plane strain samples.