Study of combined effect of zirconium and titanium on microstructure and mechanical properties of micro-alloyed cast steels

Abstract

Aim of the present work is to study the influence of zirconium as well as titanium in combinations on the microstructures and mechanical properties of low carbon micro-alloyed cast steels and to compare with non micro-alloyed low carbon steels. The quantum of research work on the zirconium bearing micro-alloyed steels with respect to grain refinement and mechanical properties is scanty. The micro-alloyed steels with 0.05 wt% to 0.10 wt% Zr and Ti were melted by a conventional air induction melting furnace and they were further subjected to normalizing at a temperature of 1000 °C for 1 h followed by cooling in atmospheric air. The metallurgical properties of the heat treated samples were observed by optical microscope and TEM. It is observed that steel with Zr and Ti shows the presence of acicular ferrite with a very fine grain size of around 5–10 microns as compared to the non micro-alloyed steel. In addition, the inter-lamellar spacing of pearlite as well as presence of intermetallic compounds were seen clearly in the heat treated samples by TEM. It is also observed that the presence of precipitates accounts for the improvement of strength and toughness of steels containing micro alloying content in the heat treated condition. The steel with 0.1 wt% Zr + 0.1 wt% Ti content showed tensile strength with nearly two times higher in comparison to steel without micro alloy addition in steel. However the elongation of both steels is almost equal (percentage of elongation is found to be around 36%). This can be attributed due to the presence of zirconium carbo-nitrides causing the formation of fine grains in steel with Zr and Ti contents.