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(CaO)/X(SiO2) and diameters of inclusions, CaO-SiO inclusions were found to be formed by the coalescence between SiO and CaO-SiO inclusions. Most CaO-SiO inclusions > 10\ua0μm were located in the melilite areas with liquidus temperatures < 1400\ua0°C. The AlO-SiO-CaO inclusions in the steel samples from the vacuum degassing and tundish furnaces were considered to be formed by reduction of CaO-SiO inclusions by Al dissolved in steel. The increased number density of AlO-SiO-CaO inclusions in spring rods was primarily attributed to the entrapment of NaO-contained mould flux particles and inclusions crushed after hot rolling. Many AlO-SiO-MnO and SiO-MnO inclusions were observed in tundish furnace samples, which originated from inherent reactions between [Al], [Si], [Mn] and [O] in the molten steel. The SiO precipitation of AlO-SiO-MnO inclusions was attributed to the solubility decrease during the solidification process with temperature drop