Understanding the Formation and Evolution of Oxide Inclusions in Si-Deoxidized Spring Steel

To illustrate the mechanism of the formation and evolution of inclusions in Si-deoxidized spring steel, pilot trials and systematic samplings were conducted, and detailed information about the inclusion density, morphology and composition was elucidated by electron probe X-ray microanalysis. The steel samples were collected from the vacuum degassing furnace, tundish furnace and spring rods after hot rolling. Based on the relationship between the mass ratio of \( X_{{\left( {\text{CaO}} \right)}} /X_{{\left( {{\text{SiO}}_{ 2} } \right)}} \) and diameters of inclusions, CaO-SiO₂ inclusions were found to be formed by the coalescence between SiO₂ and CaO-SiO₂ inclusions. Most CaO-SiO₂ inclusions > 10 μm were located in the melilite areas with liquidus temperatures < 1400 °C. The Al₂O₃-SiO₂-CaO inclusions in the steel samples from the vacuum degassing and tundish furnaces were considered to be formed by reduction of CaO-SiO₂ inclusions by Al dissolved in steel. The increased number density of Al₂O₃-SiO₂-CaO inclusions in spring rods was primarily attributed to the entrapment of Na₂O-contained mould flux particles and inclusions crushed after hot rolling. Many Al₂O₃-SiO₂-MnO and SiO₂-MnO inclusions were observed in tundish furnace samples, which originated from inherent reactions between [Al], [Si], [Mn] and [O] in the molten steel. The SiO₂ precipitation of Al₂O₃-SiO₂-MnO inclusions was attributed to the solubility decrease during the solidification process with temperature drop.