Thermodynamic and Kinetic Analysis of Non-Metallic Inclusions Evolution in Si-Killed 316L Stainless Steel with Various Refining Slags

Abstract

Reactions between Si-killed 316L stainless steel (STS316L) and CaO–SiO₂–CaF₂–5%MgO–5%Al₂O₃ slag were conducted in a magnisa crucible at 1873 K to study the influences of slag binary basicity (CaO/SiO₂) ratio and CaF₂ content on the formation of inclusions in molten steel. Increasing the binary basicity from C/S = 1.0 to 2.3 while keeping CaF₂ content constant at 10 wt% led to a significant decrease in the total oxygen content in the molten steel. However, changing the CaF₂ content from 10 to 30 wt% had minimal impact on reducing the total oxygen content (at C/S = 1.7). The transformation of inclusions from initial SiO₂ and MnO–Cr₂O₃ inclusions to MgO·Al₂O₃ spinel phases and pure MgO particles occurred in the molten Si-killed STS316L when the slag had basicity values of C/S = 1.7 and 2.3, respectively. In this scenario, Al₂O₃ inclusion could serve as an intermediary product during the mentioned transformation process. However, the utilization of slags with lower basicity (C/S = 1.0–1.3) led to the occurrence of liquid inclusions instead. The presence of Al₂O₃ inclusions in the molten steel can be linked to the higher Al₂O₃ activity in extremely basic slag compositions. Furthermore, the activity of MgO in high basicity and low viscosity slags (C/S = 1.7–2.3) is nearly equal to unity, causing the transfer of Mg from the slag and subsequently resulting in the transformation of inclusions from Al₂O₃ to MgO·Al₂O₃ and MgO inclusions under slag basicity values of 1.7 and 2.3, correspondingly, at a temperature of 1873 K.

Graphical Abstract