Role of solute Nb in altering phase transformations during continuous cooling of a low-carbon steel

Abstract

It has been known that Nb precipitate strengthened steels attain an excellent combination of strength and ductility. However, the effect of solute Nb on phase transformations, during continuous cooling, has not yet been fully understood. This limits the realization of the benefit of Nb addition on mechanical properties. Therefore, in the present investigation, a low-carbon Nb-rich alloy has been designed using thermodynamic calculations to understand the phase transformation phenomena. The austenitizing conditions (holding time and temperature) were varied such that entire Nb can either be in the solid solution of austenite or in the precipitate form (NbCN). The microstructural analysis showed the occurrence of a wide range of phase transformations during continuous cooling, at different cooling rates, after austenitization for various time-temperature combinations. Interestingly, a bainitic transformation was observed when the entire Nb was in the solid solution of austenite. This is in contrast to ferrite + pearlite formation when Nb was present in the precipitate form. This change in phase transformation (bainite or ferrite + pearlite) was also confirmed by dilatometry. The steel with bainitic microstructure showed higher hardness and strength, with a slight reduction in ductility, in comparison to ferrite + pearlite microstructure in the same alloy. The reduction in the ductility could be associated with the presence of martensite between bainite-ferrite laths and the coarse bainite laths.