Thermodynamic mechanism of non-metallic inclusions modification in recycled low-alloyed cast steel using CaCO₃–CaSiO₃ composite mineral fluxes

Abstract

In this study, the effects of CaCO₃–CaSiO₃ mineral fluxes on the evolution of non-metallic inclusions and phase transformations during the remelting of SCMnCr3 cast steel produced from secondary metal scrap were investigated. To determine the optimum slag composition, four fluxes containing 50–80 wt.% CaCO₃ were evaluated.

The evolution of the microstructure was characterized using optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), elemental mapping, and X-ray diffraction (XRD). Thermodynamic analysis was performed to explain the mechanisms responsible for inclusion transformation. The obtained results showed that the flux consisting of 70 wt.% CaCO₃ and 30 wt.% CaSiO₃ (F3) produced the cleanest microstructure. Under these conditions, the amount of large and irregularly shaped inclusions was significantly reduced, while they were replaced by fine spherical inclusions uniformly distributed throughout the matrix.

SEM–EDS and elemental mapping confirmed the uniform distribution of Fe, Mn, Cr, Si, Ca, and O within the investigated regions. This indicates effective inclusion modification and stable interaction between the slag and the molten metal. XRD analysis confirmed the formation of thermodynamically stable calcium silicate phases.

The addition of an excessive amount of CaCO₃ (80 wt.%) resulted in excessive CO₂ gas evolution during its decomposition. This intensified secondary oxidation and reduced the metallurgical cleanliness of the steel.