Investigation of microstructural evolution and crack extension in a quenching and partitioning (Q & P) steel at different strain rates

This paper presents a detailed investigation of the microstructure evolution and crack propagation in quenching and partitioning (Q&P) steel at different strain rates. The results showed that for an increased strain rate from 0.01 s−1 to 500 s−1, the tensile strength increases from 1490 MPa to 1866 MPa, and the elongation increased from 10.22% to 15.61%. Furthermore, at high strain rates, the lath martensite underwent plastic deformation, and the dislocation density rose markedly. Also the process of low angle grain boundary (LAGB)-to-high angle grain boundary (HAGB) was blocked, which enhanced the line density of LAGB and the strength of the Q&P steel. Moreover, the texture of the Q&P steel changed clearly at different strain rates. It changed from Goss {110}<001> texture at low strain rates to Rotated Goss {110}<110> texture at high strain rates, and the latter type of texture is more conducive to plastic deformation due to its higher Taylor factor. In addition, at higher strain rates, the TRIP effect of the retained austenite (RA) was delayed due to the adiabatic temperature rise effect. In this case, the variants selection tends to the variant 13 (V13), variant 15(V15), and variant 17(V17) of the CP3 group, which inhibited the crack propagation and enhanced the plasticity. Furthermore, at different strain rates, the fracture mode of the Q&P steel was a mixed fracture mechanism containing both toughness and brittleness. However, the fracture mode was more inclined to a ductile fracture at higher strain rates.