Role of Metastable Austenite on Crack Resistance of Quenching and Partitioning Sheet Steels

The controversial phase, metastable austenite, is deliberately retained in advanced quenching and partitioning (Q&P) sheet steels. Superficially, the plasticity of Q&P steels is enhanced through the transformation induced plasticity (TRIP) effect to a large extent. However, the role of retained austenite on the crack resistance of Q&P sheet steels is ambiguous to date. Tension of double edge notched (DEN) specimens, with different notch radii, was conducted to investigate the role of retained austenite on crack resistance. The fracture toughness of Q&P steels, critical J-integral values Jc, were 402.97 kJ·m−2 (notch radius = 0.18 mm) and 584.11 kJ·m−2 (notch radius = 1 mm). The increase rate in the plastic deformation zone (PDZ) at notch ahead modeled by finite element (FE) methods dramatically decreased with the notch root radius ρ. It reflects a relatively high sensitivity of notch ductility of Q&P steels in relation to notch radius. Propagating microcracks, regularly initiated at phase boundaries in Q&P steels, were found to be effectively impeded by adjacent retained austenite through energy absorption in the form of strain induced martensite transformation (SIMT).