Effect of secondary Al alloying on non-metallic inclusions in Ca-treated high-Al steel

Abstract

Lab-scale experiments and thermodynamic calculations were conducted to investigate the evolution of inclusions in Ca-treated high-Al steel during secondary Al alloying process. By adjusting the ratio of the first and second Al addition, different paths of inclusion evolution were obtained. When the first Al addition was greater than 0.1 wt-%, the inclusions in steel matrix were mainly Al₂O₃, while when the first addition of Al was as low as 0.01 wt-%, the composition of the main inclusions was Mn–Al–O(–Si). Following the Ca treatment, Al₂O₃ inclusions were modified into composite inclusions of calcium aluminate and CaS. In contrast, Mn–Al–O(–Si) type inclusions did not act as a good Ca stabiliser in the steel bath, resulting in a low Ca yield. After the second Al addition, the inclusion types remained unchanged, and only the CaO component in the liquid inclusions could be partially reduced by Al. With a certain amount of dissolved Ca, keeping the inclusions in liquid state before the second Al addition is beneficial to ultimately obtain the most concentrated liquid inclusions. Compared with the heat without secondary Al alloying, the optimised method can significantly reduce the solid calcium aluminates and CaS in high-Al steel.