Relation between microstructure and formability of quenching-partitioning-tempering martensitic steel

Investigation of the mechanical properties and forming limit diagrams (FLDs) of Fe-0.20C-1.49Mn-1.52Si-0.58Cr-0.05Nb (wt%) steel treated by conventional quenching and tempering (Q&T) and a novel quenching-partitioning-tempering (Q-P-T) process has revealed that the latter produces more retained austenite (10.8%) than the former (less than 3%) and improves the formability. The better formability of the Q-P-T martensitic steel is attributed to a higher strain hardening exponent (n¯=0.0958) and true uniform elongation (ε¯u=10.3%) than the Q&T martensitic steel (n¯=0.0410,ε¯u=3.6%), which leads to a lower yield ratio (σs/σb) and higher plastic strain ratio (r). The high value of n is found through microstructural characterization to stem from the high strain hardening rate of the retained austenite during deformation, as well as a low dislocation density in the martensitic matrix prior to deformation due to carbon partitioning into the retained austenite during the Q-P-T process. The high εu is attributed to the dislocation absorption by retained austenite (DARA) effect, which effectively enhances the deformability of the martensitic matrix, as well as the transformation induced plasticity of retained austenite during deformation. Since there is no carbon-depletion in the single-phase martensitic matrix of the Q&T steel, it exhibits reduced formability due to a lower value of n and εu.