In Situ Observation on the Heterogeneous Nucleation of MnS in Heavy Rail Steels with Varied Aluminum Content

In situ observation, scanning electron microscope/energy-dispersive spectrometer detection, and lattice disregistry calculation are combined to investigate the evolution mechanism of inclusions in heavy rail steels with varied aluminum contents from SiO2-Al2O3-MnO to SiO2-Al2O3-MnO-MnS at 1323 K and 1393 K. The precipitation of MnS inclusions on SiO2-Al2O3-MnO inclusions in the steel occurs when the Al2O3 content in identical oxide inclusions is 30% at 1393 K and 30%, 40%, and 59% at 1323 K. The MnS outer layer shows visible precipitation from 3 min after heating at experimental temperature. The area fraction of MnS phase in SiO2-Al2O3-MnO-MnS inclusions with varied Al2O3 content at 1323 K decreases in the following order: SiO2-Al2O3 (30%)-MnO > SiO2-Al2O3 (40%) -MnO > SiO2-Al2O3 (59%)-MnO. The precipitation of MnS is prone to occur on the Al2O3-less region of the SiO2-Al2O3-MnO inclusion. The solubility of Mn and S decreases with decreasing temperature. Lower heating temperature and lower Al2O3 content in SiO2-Al2O3-MnO oxide cores are profitable to the heterogeneous nucleation of MnS due to lower disregistry between MnS with the SiO2-Al2O3-MnO inclusion.