Effect of Ti De-oxidation on solidification and post-solidification microstructure in low carbon high manganese steel

This paper investigates the effect of de-oxidation inclusions on micro-structure 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 400g-scale vacuum furnace, and cast in a Cu mold at cooling rates between 2.0-6.0 K/s. The oxide inclusions were identified as Al₂O₃ (1-3 μm) in the Al-killed steel and Ti-Al-(Mg)-O (0.3-0.5 μm) in the Ti-killed steel. Oxide inclusion sizes in all the Ti-killed steels were smaller and inclusion densities higher than those in the Al-killed steel. Solidification structure, defined as the density of primary dendrite arms within a defined region was finer with increasing inclusion density and as a result, the solidification structure of the Ti-killed steel was finer than that of the Al-killed steel. A Confocal Scanning Laser Microscope (CSLM) and a Differential Scanning Calorimeter (DSC) were used to study the differences in solid state micro-structural evolution between the Ti-killed, Al-killed and the non de-oxidized 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 particles in addition to grain boundaries.