Evolution of MnS and MgO·Al₂O₃ inclusions in AISI M35 steel during electroslag remelting

Thermodynamics and kinetics of dissociation and precipitation of MnS inclusions, as well as the effect of reoxidation in liquid steel on MgO·Al₂O₃ inclusions in AISI M35 steel during electroslag remelting (ESR) process were investigated. The inclusions found in the consumable electrode were MnS, MgO·Al₂O₃ and MnS adhering to MgO·Al₂O₃. MnS inclusions were nearly spherical and ellipse in morphology, and most of them were less than 2 μm in size. MgO·Al₂O₃ inclusions were polygonal and nearly spherical and most about 1–4 μm in size. The inclusions in ESR ingot observed by scanning electron microscopy–energy-dispersive X-ray spectrometer were polygonal and nearly spherical MgO·Al₂O₃. MnS inclusions in the consumable electrode were completely dissociated before the liquid film dripping into molten slag pool. The controlling step of MnS inclusions dissociation was the mass transfer of [Mn] in the liquid steel. During the solidification process, the thermodynamic driving force could not meet MnS inclusions precipitation before the solid fraction exceeds 0.996, and the kinetics condition is too poor for the growth of MnS inclusions in the steel when the solid fraction is larger than 0.996. MgO·Al₂O₃ inclusions in ESR ingot originated from the remained MgO·Al₂O₃ inclusions in consumable electrode and the fresh ones formed by the reaction between dissolved magnesium, oxygen and aluminum in liquid steel.