During the reduction of iron ores, substantial amounts of S, originating from the coke, dissolve in the molten iron. S poses several problems during the casting and processing of steels, such as hot tearing or surface defects, respectively. Residual amounts of S are typically bound by adding Mn to the steel to avoid the formation of low-melting phases. Mn is also an important alloying element for large variety of steel grades. To track the steel quality during the casting process, online quality prediction systems are currently under development in numerous steel plants. Thermodynamic data of the steel grade are combined with solidification calculations and kinetic models to describe the casting process as a function of the casting parameters, whereby the thermodynamic information is obtained from the CALPHAD approach.
In the present work, experiments in the systems Fe-Mn and Fe-Mn-S with very low amount of C (~ 150 ppm) were performed using Differential Scanning Calorimetry (DSC) and Differential Thermal Analysis (DTA). The Fe-Mn-Clow phase diagram at high temperatures was experimentally reconstructed up to 40 mass pct. Mn. [1] Previous thermodynamic assessments [2,3] showed noticeable deviation from the measured peritectic phase equilibria, which required further investigation and re-optimization of the thermodynamic database. Hence, a CALPHAD-type thermodynamic modeling of the Fe-Mn and Fe-Mn-C system using FactSage thermochemical software [4] was performed to improve the prediction of solid/liquid phase equilibrium temperatures. For the liquid phase, the Modified Quasichemical Model (MQM) was used, solid solutions were described by the Compound Energy Formalism (CEF) and several compounds with constant composition were treated as stoichiometric. In the second part, the experimental and computational approach is combined and applied to the ternary Fe-Mn-S system, as shown exemplary in Figure 1 (a). Two isopleth sections of 0.5 and 2.0 mass pct. Mn with up to 0.3 mass pct. S were studied.
The DSC technique enabled also to analyze the dissolution of manganese sulfides (Mn,Fe)S, as can be seen in Figure 1 (b), which was additionally in situ by high temperature laser scanning confocal microscopy (HT-LSCM). Hence, the proper evaluation of the DSC signals could be confirmed. Though a significant improvement was obtained for calculating the Fe-C-Mn system, the previous evaluation of the Fe-Mn-S system, which in this case was selected from the studies of Kang and coworkers [2,3], already led to excellent results.

References:
[1] Presoly, P., private communication, Montanuniversitaet Leoben, 2023
[2] Y.-B. Kang, Critical evaluations and thermodynamic optimizations of the Mn–S and the Fe–Mn–S systems. Calphad, 34 (2010), 2, pp. 232–244.
[3] M.-S. Kim and Y.-B. Kang, Thermodynamic Modeling of the Fe-Mn-C and the Fe-Mn-Al Systems Using the Modified Quasichemical Model for Liquid Phase. Journal of Phase Equilibria and Diffusion, 36 (2015), 5, pp. 453–470.
[4] C.W. Bale et al., FactSage thermochemical software and databases, 2010–2016. Calphad, 54 (2016), pp. 35–53.