Effects of niobium and heat treatment on microstructure and mechanical properties of low carbon cast steels

Abstract

The present paper describes an attempt to reveal the effects of microalloying element niobium (Nb), normalizing and tempering temperatures on microstructure, strength and especially toughness of low carbon microalloyed cast steels. Four kinds of microalloyed cast steels with different contents of niobium have been examined by optical microscopy (OM), transmission electron microscopy (TEM), and electron back scatter diffraction (EBSD), tensile testing and room temperature Charpy V-notch impact toughness testing. As compared to Nb-free steel, the grains in the Nb-microalloyed cast steels are refined and the average grain sizes are about 20.8~34.6% lower. Moreover, fine spherical NbC precipitates with a diameter of about 1–15nm are formed in the Nb-microalloyed cast steels. When normalized at 900°C and tempered at 550°C, the yield strength (YS) and ultimate tensile strength (UTS) of the microalloyed cast steel with 0.044wt.% of Nb are increased to 350MPa and 520MPa from 290MPa and 485MP, respectively, as compared to Nb-free cast steel. Meanwhile, the impact energy is improved by 25.6% with ductility remained almost the same.