Effect of Ferrosilicon Deoxidation on the Evolution and Removal of Oxide Inclusions in 55SiCr Spring Steel

Abstract

The effects of pure silicon deoxidation (Si group) and ferrosilicon deoxidation (FeSi group) on the evolution of oxide inclusions in 55SiCr spring steel are methodically examined using OTSInca, SEM-EDS, and FactSage 8.0. Compared with the Si group, the FeSi group is more favorable for controlling the total oxygen content in steel, and the total oxygen content in ingots is only 0.0012 pct. The average equivalent diameter of inclusions in the Si group continuously lessens with the melting process, while it is exactly the opposite in the FeSi group, and the average equivalent diameter of the inclusions in the ingots reaches 3.64 μm. The evolution of inclusions in the Si group is essentially provided by SiO₂ → SiO₂–MnO–Al₂O₃ → SiO₂–MnO–Al₂O₃–MgO, whereas the evolution process of inclusions in the FeSi group is mainly characterized by Al₂O₃–CaO → Al₂O₃–CaO–SiO₂ → Al₂O₃–CaO–SiO₂–MgO. However, MgO–Al₂O₃ and MgO–SiO₂ inclusions are precipitated in the inclusions because of the uneven distribution of inclusions in the ingot. In continuing, the Gibbs free energy of chemical reaction is utilized to explain the evolution of inclusions. FactSage calculation results reveal that the main inclusions in the Si group under equilibrium solidification conditions are 2Al₂O₃·2MgO·5SiO₂, 5Al₂O₃·4MgO·2SiO₂, and Al₂O₃·SiO₂. Additionally, the inclusions in the FeSi group are obtained as CaO·2MgO·8Al₂O₃, MgO·Al₂O₃, and 2CaO·MgO·2SiO₂. The deformability of inclusions in the FeSi group is not as good as in the Si group. The calculated results of the complete melting temperature and Young’s modulus of inclusions indicate that reducing the proportion of Al₂O₃ and MgO in inclusions leads to the improvement of the deformability of inclusions. This study is aimed to provide a fairly solid reference for controlling and removing inclusions in spring steel.