Effect of magnesium deoxidation on the evolution of non-metallic inclusions in austenitic stainless steel: Applications to ultra-high cleanliness products

The effect of magnesium deoxidation on the evolution behavior of non-metallic inclusions (NMIs) in molten austenitic stainless steel (ASS) was investigated at 1823 K using an alumina crucible. To confirm the deoxidizing capability, the initial oxygen content was adjusted to about 200 ppm by adding Fe 2 O 3 powder, and the magnesium deoxidizer was added in the form of Ni-6wt%Mg alloy. Fe 2 O 3 powder and Ni-6wt%Mg alloy were wrapped in iron foil to prevent scattering and then were added to the molten steel. As the [Mg] content and reaction time increased, the [O] content in the molten steel decreased, while the [Al] content increased due to the reaction of dissolved Mg in the molten steel with Al 2 O 3 crucible. The number density and size of inclusions decreased with increasing content of [Mg] and reaction time. The initial inclusions in the molten steel were relatively small SiO 2 -MnO-Cr 2 O 3 system liquid phase due to the reaction between [O] with [Si], [Mn] and [Cr] in the molten steel. After the addition of Ni-Mg alloy, the [Mg] and [Al] dissolved from the crucible reacted with the initial inclusions, resulting in an increase of (MgO) and (Al 2 O 3 ) contents of NMI. This reaction continued as a function of reaction time, and hence the inclusions transformed into MgO-containg inclusions olivine, spinel phases. The inclusions grew and formed clusters, after which they floated up and accumulated on the surface of the molten steel or on top of the interface between the crucible and molten steel to form an inclusion layer. Consequently, the cleanliness of molten ASS was dramatically improved by Mg deoxidation practice.