Influence of niobium microalloying on the microstructure and mechanical properties of high carbon nano bainitic steel

Carbide free nano bainitic steels have recently become a promising material for several applications such as rails, armour etc. because of their impressive combination of mechanical properties. Very high strengths up to 2500 MPa have been achieved because of the very fine nano-scale microstructure obtained through transformations at low austempering temperatures. However, the low ductility and impact toughness are the limiting factors which prevent their use in several applications. It is well known that alloying elements such as Nb and V are added in very small quantity in order to improve strength and ductility of low and medium carbon steels through Nb, V (C, N) precipitates. Influence of these microalloying elements on the mechanical properties of nano bainitic steel is not well understood. In the present study, mechanical properties of two high carbon nano bainitic steels with 0.037 wt% Nb and without Nb addition have been studied. Nb addition results in a substantial increase in ductility (total % elongation) from 16 % to 28 %. Effect of Nb addition on transformation kinetics, microstructure and mechanical properties is also discussed.