Experimental characterization and thermodynamic study of (Mn,Fe)S stability in low-carbon steel

The formation of manganese sulfides, (Mn,Fe)S, during solidification of steel and subsequent cooling is a critical factor influencing the quality of continuously cast products, generally impacting hot tearing sensitivity and surface cracking on the strand. Understanding and predicting the formation of sulfides in the process is, therefore, essential for steelmakers. In this study, phase equilibria in two isopleth sections in the systems Fe-0.02C-0.50Mn-S and Fe-0.02C-2Mn-S (in wt.-%, S = 0.002 - 0.30 wt.-%) were experimentally characterized between 700 °C and 1550 °C. Alloys were produced in a high-frequency remelting (HFR) furnace. The morphology and distribution of (Mn,Fe)S inclusions in selected HFR samples were then examined via Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-Ray Analysis (EDX). High-temperature phase equilibria were analyzed by Differential Scanning Calorimetry (DSC) over the temperature range of 700 to 1550 °C. To further elucidate the dissolution behavior of (Mn,Fe)S during the melting, phase transformations in a selected sample were observed in situ using High-Temperature Laser Scanning Confocal Microscopy (HT-LSCM). A CALPHAD-type thermodynamic database for the Fe-C-Mn-S system was created based on literature assessments and used to perform thermodynamic calculations of (Mn,Fe)S stability, showing excellent agreement with the DSC data. The critically evaluated database was used to derive the analytical equilibrium constant for the stability of stoichiometric manganese sulfide, (MnS) → [Mn] + [S], in equilibrium with the liquid, δ-ferrite (BCC), and austenite (FCC) phases.