Micro-structure Refinement in Low Carbon High Manganese Steels through Ti-deoxidation: Austenite Grain Growth and Decomposition

Abstract

This paper investigates the effect of de-oxidation inclusions on micro-structure evolution in low-carbon steels. Low carbon (0.07 wt%), high Mn (0.9 wt%) steel in a Al2O3 or MgO crucible was deoxidized by adding either aluminum (0.05 wt%) or titanium (0.05, 0.03 or 0.015 wt%) in a 400 g-scale vacuum furnace, and cast in a Cu mold at cooling rates between 2.0–6.0 K/s. These cast samples were re-melted and cooled at various cooling rate, 1 through 100 K/s in the hot-stage of a conforcal laser scanning microscope (CSLM) in order to investigate the effect of cooling rate.Oxide inclusion sizes in all the Ti-killed steels were smaller and inclusion densities higher than those in the Al-killed steel. In Ti-killed steel, inclusion size and densities increased with increasing the oxygen content, inclusion size decreased and their densities increased with increasing the cooling rate.A Confocal Scanning Laser Microscope (CSLM) was used to study the differences in solid state micro-structural evolution between the Ti-killed and theAl-killed samples. The growth of austenite grains were studied under isothermal conditions and it was found that both grain-boundary mobility and final grain size were lower in the Ti-killed sample than for the others. With regards to austenite decomposition, during continuous cooling from a comparable austenite grain structure, the resulting austenite decomposition structure was finer for the Ti-killed sample due to a higher Widmanstätten lath density due to precipitation at. The inclusion size was found to have a significant effect on both austenite grain size and austenite decomposition structure. Different orientations of ferrite precipitates originating at inclusions were observed in the Ti-killed samples. The highest lath concentration was obtained for the sample that had the smallest average inclusion size rather than the sample with highest density of sub micro-meter inclusions.