Cleanliness Evaluation and Control for Transitional IF Steel Slabs During Ladle Exchange Process

To improve the quality of transitional slabs, the cleanliness of the transitional slabs cast between the fourth and fifth heats in an IF steel casting sequence was systematically examined using a 22‐ton steel‐remaining practice to exclude the contamination of vortex ladle slag entrapment. Thirty‐seven slab samples were cut along the casting direction covering the whole unsteady events and their cleanliness were evaluated by T.O. measurements combined with large‐area scanning analysis (ASPEX) methods, covering a total inspected area of 48 214 mm 2 . A total of 9389 inclusions over 5 μm in the slabs were detected and two types were identified: alumina‐based inclusions (9380) and calcium aluminates (9). A cleanliness‐affected zone (CAZ) for transitional slabs was observed, extending about 7 m along the slab and corresponding to the teeming period between 6.1 and 13.7 min after teeming start of the 5th heat. Within this zone, the T.O. contents, alumina‐based inclusions, and calcium aluminates all increased to relatively high levels. A unique morphology with iron core surrounded by spherical calcium aluminates was detected exclusively within the CAZ. The re‐oxidation from ladle sand and the entrainment of tundish slag into the molten steel pool are identified as the main factors degrading cleanliness.