Niobium microalloying in rail steels

Rail steels rely primarily for their properties on wear and rolling-contact-fatigue resistance. These properties, together with toughness, can be optimised by implementing thermo-mechanical processing during rail manufacturing, assisted by controlled additions of strong carbide-forming microalloying elements. Niobium can be used at low concentrations, but the major difficulty is to ensure that niobium does not segregate during casting, especially when rail steels are high in carbon content. The solubility of niobium in ternary Fe-Nb-C austenite is critically assessed to address the issue of segregation. Thereafter, the effect of niobium on medium-and high-carbon (similar to 0.2-0.8 wt-% C) austenite grain-growth kinetics during reheating or recrystallisation kinetics during hot deformation, and the corresponding effect on pearlitic and bainitic microstructures and mechanical properties are reviewed.