Characteristics of Inclusions in Low Carbon Al-Killed Steel during Ladle Furnace Refining and Calcium Treatment

A plant trial of the productions of LCAK steel was performed, and characteristics of inclusions during LF refining and calcium treatment were investigated. Besides, thermodynamic diagram among magnesium, aluminum, and oxygen as well as calcium, magnesium, aluminum, and oxygen in the steel melt were studied to understand the fundamentals of inclusion modification by calcium treatment in LCAK steel. Furthermore, the change mechanisms of oxide inclusions and the precipitation of calcium sulfide were discussed. The experimental results showed that oxide inclusions were partly changed along with the path of Al2O3→MgO·Al2O3→(MgO·)CaO·Al2O3, which was a little different from the thermodynamic calculation results due to the limited kinetic conditions. Calcium treatment somehow modified the inclusions, however, many large inclusions were generated, and the modification effect would be dramatically decreased by the formation of large amount of CaS inclusions, which appeared with three main distribution forms. The formation of inclusions after calcium treatment were discussed based on the thermodynamic analysis. In order to reach the target of modification, sulfur concertration in steel should be reduced to a small amount to decrease the formation of CaS.