Formation and evolution of non-metallic inclusions in X70MS pipeline steel during LF-RH-CC process

Abstract

The formation and evolution of non-metallic inclusions in X70MS pipeline steel during LF-RH-CC process were investigated by industrial trials, and the evolution mechanism of non-metallic inclusions was analyzed and discussed with thermodynamic calculations. The results show that after RH vacuum refining, the inclusions in molten steel are mainly spherical CaO-MgO-Al2O3 and CaO-Al2O3. After calcium treatment, the inclusions are transformed into spherical CaS-CaO-Al2O3 and CaO-CaS with finer size. During calcium treatment, it is difficult for [Ca] and [S] in the molten steel to react directly to form CaS, and CaS is mainly generated at the edge of liquid calcium aluminate. When w[Al] in molten steel is 0.032%, w[Ca] is controlled above 0.0035%, which is more favorable to avoid the generation of liquid calcium aluminate. Calcium treatment to the tundish reaches the casting liquid level, the molten steel occurred a continuous reoxidation, reoxidation of inclusions composition transformation is mainly determined by the [Ca] and T[O] contents of the molten steel, this process inclusions are transformed from CaO-CaS to liquid CaO-MgO-Al2O3. During cooling and solidification of the molten steel, the CaS content in inclusions increases sharply, the CaO content decreases sharply, and the changes in inclusions are mainly affected by the reaction between the molten steel and the inclusions and the precipitation of new phases.