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<middle dot>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<middle dot>Al2O3 and MgO inclusions under slag basicity values of 1.7 and 2.3, correspondingly, at a temperature of 1873 K.