Evolution Behavior of Non-metallic Inclusions in High-Al or Si-Alloyed Steel Reacting With CaO–Al2O3–FeO–SiO2–MgO Slag

The effect of the reaction between alloy elements and slag on the evolution behavior of non-metallic inclusions in high-Al or Si-alloyed steel melts was investigated at 1873 K (1600 °C). Aluminum (1.5 mass pct) added to the molten steel produces an alumina-rich inclusion by reaction with oxygen, and it reduces FeO by the interfacial reaction with the CaO–Al2O3–FeO–SiO2–MgO slag, resulting in an increase of the alumina content of the slag. On the other hand, silicon (4.0 mass pct) added to the molten steel increases the silica content of the slag by the interfacial reactions with the slag and causing aluminum pick-up from the liquid slag into the molten steel, which changed the existing silica-rich inclusion into aluminum-rich inclusions. The addition of each of the two alloying elements produced alumina-rich inclusions, and the two series of existing alumina inclusions were modified into MgAl2O4 spinel inclusions by picking up magnesium from the slag into the molten steel. Through a population density function (PDF) analysis of inclusion particles, we confirmed that inclusion growth occurred by diffusion of magnesium and aluminum in steel with 1.5 pct Al or 4.0 pct Si. The supersaturation degree for alumina inclusions in the 1.5 pct Al-added steel and 4.0 pct Si-added steel continued to decrease as the reaction time increased. As a result, dendritic or spherical alumina inclusions were observed in both series at 1 minute of alloying, and small, independently present polygonal-shaped MgAl2O4 spinel inclusions were mainly observed at 30 minutes of alloying.