Effect of Electropulsing on the Evolution of MnS and Oxy-Sulfide Composite Inclusions During Solidification of 304 Stainless Steel

Manganese sulfide (MnS)inclusions can markedly influence the mechanical properties, corrosion resistance, and processing behavior of stainless steels, making their control during solidification an important issue for improving steel quality. In this work, electropulsing was applied to low-sulfur 304 austenitic stainless steel at two characteristic stages, corresponding to the onset of MnS precipitation and the post-solidification stage. A positive square-wave pulsed current was applied from 1550 °C to either 1405 °C or 1350 °C, and the evolution of MnS, oxide inclusions, and oxy-sulfide composite inclusions was characterized by SEM/EDS, quantitative image analysis, and thermodynamic calculations. At 1405 °C, MnS remained predominantly spherical or near-spherical, while the mean equivalent circular diameter of the inclusions decreased from 2.64 to 1.76 μm as the current density increased from 0 to 25.48 A·cm−2. When electropulsing was continued to 1350 °C, the mean inclusion size decreased from about 4.39 to 2.86 μm, and oxide-core/MnS-shell composite inclusions exhibited progressively thinner MnS layers with increasing current density. At the initial precipitation stage, electropulsing may modify the transport of Mn and S and thereby influence MnS nucleation and inclusion refinement. At the later stage, local current redistribution around low-conductivity inclusions and the associated electrical free-energy contribution may influence subsequent inclusion growth and the development of MnS-rich peripheral regions. These results indicate that electropulsing can influence the precipitation, growth, and composite evolution of MnS during the late solidification process of 304 stainless steel.