Morphology of Inclusions and Its Refinement Mechanism in the New P91 Heat-Resistant Pipeline Steel

Abstract

A new type of P91 heat-resistant pipeline steel ingot was prepared by feeding Al twisted wire into a steel melt through a multi-point regional micro-supply method, combined with electromagnetic stirring. The type, shape, and size of inclusions in the new P91 steel after forging were then analyzed by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscope (TEM). The results showed that four types of inclusions were detected in the P91 steel, including: spherical independent Al2O3 inclusions, irregular Al2O3-SiO2 composite inclusions, nearly spherical MgO-Al2O3 composite inclusions, and spherical (Ca, Mg, Al) (O) composite inclusions. Compared with traditional P91 steel, the inclusions in the new P91 steel were significantly refined. Refining mechanisms of inclusion showed that Al2O3 oxide particles distributed dispersedly with fine sizes could be obtained through a multi-point regional micro-supply method. Further, Al2O3 particles act as the nucleation core to form a “core-shell” structure and play the role of a heterogeneous nucleation to refine SiO2, MgO, (Ca, Mg) (O), and other inclusions in the steel.