Evolution of Complex Oxide Inclusions During the Smelting Process of Oxide Metallurgical Steel and Their Effect on Acicular Ferrite Nucleation

By sampling the important nodes in the smelting process of oxide metallurgical steel, the evolution of number density, size and type of complex oxide inclusions, and the effect of inclusions on prior austenite grain size and acicular ferrite (AF) nucleation were studied. The sampling nodes are before ladle furnace (LF) refining, add Ti, add Al, add Mg, add Ca, after Ruhrstahl-Heraeus (RH) refining, respectively. The results show that with the addition of Ti\Al\Mg\Ca, the core of inclusion gradually becomes Al₂O₃, Al–Ti–O, Al–Ti–Mg–O, Al–Ti–Mg–Ca–O, and a small amount of MnS is wrapped in the outer layer. After RH refining, the core of inclusion is Al–Mg–Ca–O, and a small amount of Ti–O–N and Al–Mg–O are enriched in the outer layer. These inclusions effectively induced AF nucleation. As the smelting process progresses, the average size of inclusions gradually decreases from 0.55 to 0.2 μm, and the number density and percentage of small size inclusions (0.1 to 0.2 μm) gradually increase. After RH refining, the nucleation probability of AF (N_AF) of small size inclusions is 7.8 pct, and the N_AF of inclusions in the range of 0.4 to 0.8 μm is greater than 70 pct. The small size inclusions are an important part of inducing AF nucleation, which can also effectively pin the prior austenite grain boundaries, and the larger the proportion of small size inclusions, the finer the prior austenite grains.