Effect of microstructure on mechanical properties of quenching & partitioning steel

The microstructure and mechanical properties of a low-carbon steel produced via the quenching & partitioning (Q&P) heat treatment was investigated, with particular focus on the hole expansion ratio, which is an index of the stretch-flange-formability. 0.19mass%C-1.5mass%Si-2.9mass%Mn steel was annealed at 850 ˚C, then cooled to 150~400 ˚C (QT: quench temperature), followed by holding at 400˚C for 1100 s. Yield strength and hole expansion ratio drastically increased when the QT was below the Ms (martensite start) temperature. The steel with QT of 300 ˚C exhibited not only a higher elongation, which has been well documented, but also a higher hole expansion ratio, when compared to the conventional TRIP steel with QT of 400 ˚C having equal tensile strength around 1200 MPa. The micro-void formation during deformation was suppressed in the steel with QT of 300 ˚C due to the smaller volume fraction of large blocky martensite compared to the TRIP steel. These excellent mechanical properties are attributed to its unique microstructure consisting of a certain amount of tempered martensite, lath-shaped retained austenite and bainitic ferrite, which was generated via the Q&P heat treatment.