Mechanical behavior and microstructural evolution of a bainite-based quenching-partitioning (BQ&P) steel under high strain rates

In this paper, the mechanical behavior of a bainite-based quenching-partitioning (BQ&P) steel under high strain rates in a range of 2200–3600/s was studied using Split Hopkinson Pressure Bar (SHPB) experimental technique. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were employed to investigate the microstructural change and fracture behavior. Results show that the stress-strain relation of the steel exhibits obvious strain rate dependence, i.e. strength increases as strain rate increases. After high strain rates deformation, twisting, directional arrangement or crystal rotation appeared for lath (Bainite&Martensite) structure, and a <100> fiber texture was formed, indicating that the slip mechanism operates under high strain rates. As the strain rate increases from 2200-3000 1/s to 3001-3600 1/s, the fracture frequency of samples significantly increases from 33.3% to 77.8%. The fracture of the BQ&P steel is characterized as 45° fracture mode with dimples near impact end and shear river pattern near the other end. Most of blocky retained austenite (RA) underwent martensitic phase transformation in the form of γ→α′ and/or γ→ε→α′ during high strain rates deformation, whereas only partial RA inside a filmy RA underwent martensitic phase transformation. The calculated adiabatic temperature rise, ~100 °C is considered to increase the stability of RA, which results in the occurrence of stress-induced γ→ε transformation under high strain rates. The increased frequency of fracture by increasing strain rates can be attributed into the delayed strain response and embrittlement caused by the martensitic transformation of RA under higher strain rates.