Formation Mechanism of Large-size CaO–Al<sub>2</sub>O<sub>3</sub>–MgO–SiO<sub>2</sub> Inclusions in High Carbon Chromium Bearing Steel

Abstract

Industrial experiments and thermodynamic analyses were carried out to investigate the formation mechanism of large-size (>30 µm) CaO–Al2O3–MgO–SiO2 (CAMS) inclusions in high carbon chromium bearing steel. It was found that the large-size CAMS inclusions existed during the whole refining process, most of which compositions were located in the liquid region. The average content of SiO2 in CAMS inclusions decreased from 27.5 mass% at argon-blowing station to 3.0 mass% in the hot-rolled bars. The results calculated by Factsage 7.3 indicated that the CAMS inclusions were originated from slag entrapment. During BOF tapping, the low basicity slag with 60 mass% SiO2 was entrapped into steel and combined with the deoxidation product Al2O3, forming a large amount of liquid CAMS inclusions. During LF refining process, [Al], [Ca] and [Mg] in molten steel were affected by the activities of corresponding slag components. The reaction between these three elements and SiO2 in CAMS inclusions originating from slag lead to the decrease of SiO2 in the inclusions. Due to the low interfacial energy between liquid CAMS inclusions and steel, a few large-size inclusions may be inherited to hot-rolled bars. In light of this, several optimization steps were conducted during BOF tapping and argon-blowing station. After the optimization, large-size CAMS originated form BOF tapping were effectively removed.