Interface migration behavior of the δ→γ interface in low carbon high manganese steel samples de-oxidized with Ti or Al

The effect of inclusions, generated during de-oxidation and solidification, of low carbon (0.07 wt%) and high manganese (0.9 wt%) steels, on the rates of migrations of the boundaries between delta-ferrite/gamma-austenite interfaces, during austenite formation was investigated. The presence of two types of de-oxidation products were investigated, namely Al₂O₃ (1-3μm) in Al-killed steel samples, and Ti-Al-(Mg)-O (0.3-1 μm) in Ti-killed steel samples. The samples were prepared in a vacuum-induction furnace and a Confocal Scanning Laser Microscope was used to image in real time the interface movement on the surface of the samples. Austenite was found to precipitate at the triple-points of the delta-ferrite grain boundaries during cooling from 1 500 to 1 460 and 1 450°C and the growth appeared to be diffusion controlled. The migration rates of delta/gamma interfacial boundaries were higher in the Al-killed steel samples than in the Ti-killed steel samples and this is attributed to the finer oxide particle population in the latter. When the samples were cooled to 1 440°C and below sword-like precipitation was observed in all samples that grew at a higher rate than what was observed at higher temperatures and the growth appeared to have linear time dependence, and thus the process appears to not be governed by long range diffusion.