Microstructural evolution and consequent strengthening through niobium-microalloying in a low carbon quenched and partitioned steel

In the present study the determining role of niobium (Nb) on significant enhancement in mechanical properties in a low-carbon quenched and partitioned steel is elucidated. The study indicates that solute drag and precipitation pinning effect of Nb suppressed the recrystallization during hot deformation, leading to grain size refinement of hot-rolled steels. The cold-rolled and final microstructure after Q-P treatment was also refined because of refined hot rolled microstructure. Additionally, the degree of refinement was enhanced with increase in Nb-content. The tensile strength of the experimental steels was increased with increase in Nb-content from 1130 MPa in steel without Nb to 1210 MPa in steel with 0.048 wt% Nb. However, the total elongation first increased to 18% followed by a small decrease to 15%. The decrease in ductility is attributed to the consumption of carbon by precipitating NbC, which decreased the enrichment of austenite by carbon during partitioning, with consequent decrease in the stability of austenite. The decrease in the stability of retained austenite ultimately reduced the volume fraction of retained austenite and led to reduction in elongation.