Change of the Non-metallic Inclusions during the Secondary Refining of Low Oxygen Alloyed Structural Steels

Slag of high basicity and high Al₂O₃ content was used in secondary refining of alloyed structural steels aiming at decreasing total oxygen contents(T[O]) of steel and forming relatively lower melting point inclusions to improve the anti-fatigue properties of the steels.It was found that T[O] at the end of LF and RH refining both decreased with the increase of slag basicity.When Al₂O₃ in the slag was less than 25%,T[O] decreased with the decrease of Al₂O₃ content of slag.While,when Al₂O₃ in the slag was more than 25%, T[O] decreased with the increase of Al₂O₃ content of slag.In the refining,compositions of the non-metallic inclusions changed in order of "inclusions of Al₂O₃ → inclusions of MgO-Al₂O₃ system → inclusions of CaO-MgO-Al₂O₃ system",among which,the change of inclusions of MgO-Al₂O₃ system to was inwardly from outside surface and the reaction speed was relatively slower.Thus,at the end of the ladle furnace refining(LF),there were still many unchanged MgO-Al₂O₃ system inclusions.The T[O] content of the steels significantly affects the composition change of the inclusions and decreasing T[O] content could help obtain inclusions of CaO-MgO-Al₂O₃ system with low melting point.