Large inclusions in plain-carbon steel ingots cast by bottom teeming

Inclusions in industrial-cast bottom-teemed ingots of plain carbon steel are investigated using ultrasonic detection, optical microscope observation, and SEM analysis. The composition, size distribution, entrapment locations, and sources of ingot inclusions were revealed by examining all the macro-inclusions (larger than 20μm) that were observed in 35 000 mm² of sample surface area. Based on 78 non-sulfide inclusions observed, around 3.23 × 10⁷ macro-inclusions per m³ steel exist in the ingot, with a size distribution increasing with decreasing size. Inclusions are distributed uniformly within a given horizontal section through the ingot, but with more found towards the bottom. The largest inclusions exceed 7 mm and originate from mold flux in the ingot. The largest inclusion source appears to be reoxidation, as evidenced by 59% of the ingot inclusions composed of pure alumina clusters and lumps. Eroded refractories from the ladle well block and ladle inner nozzle bricks accounted for 31% of the ingot inclusions.