Abstract
Experimental and thermodynamic considerations on the transformation of inclusions during cooling process of pipeline steel were carried out. In plant trials, CaO-Al2O3 type inclusions in molten steel were fully or partially transformed into MgO-CaO-Al2O3-CaS type in a slab depending on the size. The transformation details were revealed by thermodynamic calculations. The deviations between experimental and calculated results are discussed.