Effect of metastable austenite on fracture resistance of quenched and partitioned (Q&P) sheet steels

The fracture resistance was comparatively characterized on two commercial sheet steel, quenched and partitioned (Q&P) steel and dual phase (DP) steel, with the same strength levels but differing by the phase constituents and by the ductility. In Q&P steel, transformation of martensite as well as its effect on fracture resistance by tension of double edge notched (DEN) plate specimens was investigated. The map of the distribution of transformation ratios measured locally around the notch was compared with the map of the plastic deformation zone computed by finite element simulations. The influence of stress triaxiality on austenite transformation kinetics was analyzed. Essential work of fracture of Q&P steel (290.9 kJ m−2) was relatively lower than that of DP steel (544.8 kJ m−2), however, it was found that the fracture resistance (work of fracture) of Q&P steel is effectively enhanced by the transformation of metastable austenite as stress triaxiality decreases. Through deduction, transformation of 8 vol% of austenite (ΔUT) was proved to absorb 22%–35% of the increments of the work of fracture (ΔW).