Effect of Ni addition on the mechanical behavior of quenching and partitioning (Q&P) steel

The effects of Ni addition on the austenite stability and mechanical properties of quenched and partitioned Fe-0.3%C-1.5%Mn-1.5%Si steel were investigated by means of micrograph observation, x-ray analysis, tensile testing, and hardness measurement. The Ni addition lowered the martensite start temperature and the partitioning temperature to achieve the maximum retained austenite fraction. The increase in Ni content brought about grain refinement and an increase of the retained austenite fraction in the quenched and partitioned specimens. A simultaneous improvement of both tensile strength and total elongation was observed in the Ni-added specimens. The stability of the retained austenite increased with the Ni addition, resulting in retarded kinetics of the strain-induced martensite transformation during deformation. The variation of the retained austenite fraction as a function of the tensile strain was quantitatively evaluated by using an equation considering the austenite stability factors.