Micro-structure Refinement in Low Carbon High Manganese Steels through Ti-deoxidation—Inclusion Precipitation and Solidification Structure

The substitution of Al by Ti as a de-oxidizing agent in a carbon (0.07 wt%) and Mn (0.9 wt%) containing steel was studied for two purposes. The first one was to establish whether inclusion precipitation during solidification (secondary de-oxidation) can be promoted. The second purpose was to investigate the influence of secondary inclusions on the subsequent evolution of the steel solidification-structure, varying initial oxygen and titanium contents along with cooling rate during solidification. In the Ti-killed steel samples, the oxide inclusions were identified as MnO–TiO2 (0.5–5 μm) and MnS (1–3 μm) in the samples with the higher initial oxygen contents (Total Oxygen (T.O)=50–80 ppm) while Ti–Al–(Mg)–O (0.3–1 μm) in the samples with the lower oxygen contents (T.O=7–10 ppm). Comparing with thermodynamic calculations, the latter inclusions are considered to be the result of solely secondary de-oxidation precipitated in the inter-dendrite regions. For the high initial oxygen content, the inclusions were found as a result of both primary and secondary de-oxidation. The influence of cooling rate during solidification was investigated by controlling the cooling rate between 3–10 K/s by using different molds in a vacuum induction furnace. In addition, cooling rates were controlled at 1.1, 14 and 84 K/s by re-melting the samples in a gold-image furnace attached to a Confocal Scanning Laser Microscope (CSLM). An increase in the cooling rate resulted in an increase in the inclusion density in the Ti-killed samples while such an effect was not observed in the Al-killed sample. The secondary particle sizes for the Ti-killed sample predicted by a solute segregation model during solidification agreed well with the observed average particle sizes. In the Ti-killed samples, the solidification structure was finer with increasing density of inclusions below 1 μm, whereas such an effect was not observed in the Al-killed samples.