Transient Evolution of Inclusions during Calcium Modification in Linepipe Steels

The laboratory experiments of Al2O3 inclusions modified by calcium treatment in linepipe steels with the S content of 30 ppm and 310 ppm were performed at 1 873 K. Particularly, samples were taken before calcium treatment (1 minute before the calcium addition) and at various times (typically 1 minute, 10 minutes and 30 minutes) after calcium treatment to study the transient inclusions evolution during calcium modification. Traditional modification mechanism of Al2O3 inclusions is that Al2O3 inclusion is directly modified by dissolved calcium or indirectly modified via CaO formed from calcium and oxygen. In the current study, after Ca-Si alloy power addition, CaS outer layer were promptly formed on the angular Al2O3 inclusion. With the reaction of the CaS outer layer and the Al2O3 core or [O], the Al2O3 inclusion was modified to a spherical liquid calcium aluminate. Moreover, the equilibrium curve of Ca-O and Ca-S, the stability diagram of inclusions and equilibrium precipitation of inclusions during solidification in linepipe steels were calculated to study the formation of inclusions.