Formation and Evolution Mechanism of the Inclusions of Al2O3•SiO2•CaO and Al2O3•SiO2•CaO•MgO in Seamless Steel Tube Steel

Herein, the formation and evolution mechanism of inclusions of Al2O3 center dot SiO2 center dot CaO and Al2O3 center dot SiO2 center dot CaO center dot MgO in seamless steel tube steel are investigated. In the long strip defects on the longitudinal cross section of the steel tube after the rolling bar piercing, the defect is mainly formed by Al2O3 center dot SiO2 center dot CaO center dot MgO inclusions and Al2O3 center dot SiO2 center dot CaO center dot inclusions dotted with center dot CaS inclusions after the rolling. The typical inclusions in the different steelmaking stages are mainly composed of CaS, Al2O3 center dot(SiO2), CaO center dot(SiO2), MnS center dot(TiN), Al2O3 center dot SiO2 center dot CaO center dot(CaS)center dot(MnS), Al2O3 center dot SiO2 center dot CaO center dot MgO center dot(MnO), Al2O3 center dot SiO2 center dot CaO center dot MgO center dot(CaS)center dot(MnS), etc. In the billet, the average sizes of Al2O3 center dot SiO2 center dot CaO-based and Al2O3 center dot SiO2 center dot CaO center dot MgO-based inclusions are much larger than those of the other types of inclusions. Part of SiO2 in the deoxidized products SiO2 can be reduced by [Al], resulting in the formation of the Al2O3 center dot SiO2 composite inclusions. The SiO2 in Al2O3 center dot SiO2 inclusions can continuously be reduced by the dissolved [Ca] to form the Al2O3 center dot SiO2 center dot CaO composite inclusions. The SiO2 in the Al2O3 center dot SiO2 center dot CaO inclusions can be reduced by the dissolved [Mg] to form the Al2O3 center dot SiO2 center dot CaO center dot MgO composite inclusions. Another formation process of Al2O3 center dot SiO2 center dot CaO center dot MgO inclusions is the entrapment of ladle slag in the vacuum degassing (VD) stage, due to the strong agitation of the rising Ar bubbles in the vacuum condition of the VD stage.