LAUSR

The CaO–Al2O3 inclusion is occasionally observed during the secondary refining of Al-killed steel.1–8) Todoroki et al.1) studied the change in composition of the inclusions in Al-killed 18%Cr-8%Ni stainless steel through a reaction with the top slag of a CaO–Al2O3–MgO–F system. They found that alumina or MgO·Al2O3 spinel inclusions were formed immediately after the addition of Al. The amounts of MgO and CaO in the slag were then reduced owing to the presence of Al, and the concentrations of Mg and Ca in the steel increased. CaO-Al2O3-type inclusions, which are considered the most stable based on a thermodynamic calculation, formed during the final stage of refining when the Ca concentration reached 1 mass ppm or higher. A similar phenomenon in which an Al2O3 inclusion was changed into a CaO-Al2O3-type inclusion through a reaction between Al in the steel and the CaO-containing slag has also been observed.2–7) However, because the equilibrium relation with Ca and oxygen differs widely in individual studies,9–14) the most stable oxide in steel containing a small concentration of Ca (few ppm) remains unclear. Harada et al.8) Change in Composition of Inclusions through the Reaction between Al-killed Steel and the Slag of CaO and MgO Saturation