Behaviour of oxide inclusions and sulphur in ‘two-stage basicity control’ refining method of Si-killed spring steel

Abstract

A ‘two-stage basicity control’ refining method was carried out that high basicity (R > 1.5) top slag was used in the LF refining and then low basicity (R = 0.8–1.0, Al₂O₃< 6%) top slag was used in VD refining. This method can obtain inclusions with better deformability and make the top slag have good desulphurization ability. The average sulphur removal percentage of the refining is 47.22%. There were mainly three types of inclusions classified as Al₂O₃–SiO₂, CaO–Al₂O₃–SiO₂, and MgO–CaO–Al₂O₃–SiO₂ inclusions in the LF refining. In the VD refining, the number of Al₂O₃–SiO₂ and CaO–Al₂O₃–SiO₂ inclusions reduced obviously. The main type of inclusions was MgO–CaO–Al₂O₃–SiO₂ and their melting points reduced from about 1873 K to less than 1673 K during the VD refining. The results of thermodynamic calculations show that the mass fraction of [Al] should be controlled less than 20×10⁻⁶ to obtain low-melting point inclusions (<1673 K).