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 Al 2 O 3 ·SiO 2 ·CaO and Al 2 O 3 ·SiO 2 ·CaO·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 Al 2 O 3 ·SiO 2 ·CaO·MgO inclusions and Al 2 O 3 ·SiO 2 ·CaO·inclusions dotted with·CaS inclusions after the rolling. The typical inclusions in the different steelmaking stages are mainly composed of CaS, Al 2 O 3 ·(SiO 2 ), CaO·(SiO 2 ), MnS·(TiN), Al 2 O 3 ·SiO 2 ·CaO·(CaS)·(MnS), Al 2 O 3 ·SiO 2 ·CaO·MgO·(MnO), Al 2 O 3 ·SiO 2 ·CaO·MgO·(CaS)·(MnS), etc. In the billet, the average sizes of Al 2 O 3 ·SiO 2 ·CaO‐based and Al 2 O 3 ·SiO 2 ·CaO·MgO‐based inclusions are much larger than those of the other types of inclusions. Part of SiO 2 in the deoxidized products SiO 2 can be reduced by [Al], resulting in the formation of the Al 2 O 3 ·SiO 2 composite inclusions. The SiO 2 in Al 2 O 3 ·SiO 2 inclusions can continuously be reduced by the dissolved [Ca] to form the Al 2 O 3 ·SiO 2 ·CaO composite inclusions. The SiO 2 in the Al 2 O 3 ·SiO 2 ·CaO inclusions can be reduced by the dissolved [Mg] to form the Al 2 O 3 ·SiO 2 ·CaO·MgO composite inclusions. Another formation process of Al 2 O 3 ·SiO 2 ·CaO·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.