One-step quenching and partitioning treatment of a commercial low silicon boron steel

In the current study, the microstructure evolution and mechanical behaviour in a commercial low silicon boron steel were investigated after one-step quenching and partitioning (Q&P) treatment under different isothermal temperatures in the range 260-320 °C. This processing approach resulted in a complex microstructure consisting of a tempered martensite matrix with bainitic ferrite, fresh martensite, and retained austenite, offering a superior combination of mechanical properties compared to the directly water-quenched condition, where a fully martensitic structure was exhibited. The Q&P processed steel isothermally held at a high temperature of 320 °C showed lower work-hardening, which was mainly associated with the lower fraction of initially formed martensite and the corresponding tempering responses, compared to the steels held at lower temperatures. The phase transformation behaviour throughout the heat treatment was studied by dilatometry, regarding the martensitic transformation upon initial cooling, carbon partitioning combined with the martensite tempering and austenite decomposition during isothermal holding, as well as the fresh martensite formation on the final cooling. Additionally, the microstructure characteristics were examined using X-ray diffraction, scanning electron microscope, and electron backscatter diffraction techniques.