Inclusions Size Distributions After Calcium Treatment in Low Carbon Aluminum Killed Steels

Abstract

This work addresses conflicting results shown in the literature regarding liquid inclusion agglomeration. It has been shown that liquid calcium aluminates do agglomerate and hence change in size over time. Under laboratory conditions, solid spinels and alumina inclusions were successfully modified into calcium aluminates. Those liquid inclusions exhibited a weak but statistically significant tendency to agglomerate. The main mechanisms by which liquid inclusions agglomerate is by Stokes collision and laminar fluid flow collisions when no external stirring is present. For industrial conditions, where the liquid steel is agitated via either argon bubbling or electromagnetic stirring, turbulent collisions appears be the dominant mechanism. Regarding flotation and removal of liquid calcium aluminates, similar flotation rates between liquid and solid inclusions were observed. This contradicts common sense in the industry that liquid inclusions, once formed, are more difficult to remove compared to their solid counterparts.It was observed that, immediately following CaSi2 injection, inclusions reduce their size by approximately two to three times, depending on the parent inclusion. Experimental data suggests that calcium aluminates consists of a new population of inclusions formed upon calcium addition. The presence of calcium in the liquid steel will destabilize alumina and MgO-alumina inclusions, which then dissolve into the melt. It is observed that solid spinels that remain in the steel after calcium treatment are slightly smaller than prior to Ca insertion.When floating on top the steel, liquid inclusions will appear approximately twice as large then when embedded in liquid steel. Confirmation using theoretical calculations based on literature data and experimental evidence, comparing average size of inclusions using SEM analysis and confocal microscope measurements, is provided in this work.