Formation Mechanism of CaO-SiO₂-Al₂O₃-(MgO) Inclusions in Si-Mn-Killed Steel with Limited Aluminum Content During the Low Basicity Slag Refining

Pilot trails were carried out to study the formation mechanism of CaO-SiO₂-Al₂O₃-(MgO) inclusions in tire cord steel. 48 samples were taken from 8 heats of liquid steel during secondary refining, which were subsequently examined by an automatic scanning electron microscope with energy dispersive spectrometer (SEM–EDS). Characteristics of thousands of oxide inclusions at different refining stages were obtained, including their compositions, sizes, morphologies, etc. Based on the obtained information of inclusions, details during formation of CaO-SiO₂-Al₂O₃-(MgO) inclusions were revealed and a new mechanism was proposed, including their origin, formation, and evolution during the refining process. It was found that CaO-SiO₂-Al₂O₃-(MgO) inclusions were initially originated from the CaO-SiO₂-MnO-(MgO) inclusions, which were formed during BOF tapping by the coalescence between MnO-SiO₂ deoxidation products and the emulsified slag particles because of violent flow of steel. This can be well confirmed by the evaluation of the formation thermodynamics of CaO-SiO₂-MnO-(MgO) inclusions, which was proved very difficult to be produced by intrinsic reactions inside liquid steel. Because of chemical reactions between CaO-SiO₂-MnO-(MgO) inclusions and molten steel, they were mainly changed into CaO-SiO₂-MnO-Al₂O₃-(MgO) and partially into CaO-SiO₂-Al₂O₃-(MgO), which may be detrimental to the cold drawing ability of coils. Based on this finding, improvements were made in industrial production during BOF tapping and secondary refining. The results indicated that such (CaO-SiO₂)-based inclusions existed in conventional process were effectively decreased after the improvements.