Optimization of scarfing depth for high quality automotive external panel steel slabs

Scarfing is commonly employed to improve the surface quality of interstitial-free (IF) steel slabs used for automotive external panels, and usually the steelworkers tend to scarf deeper to ensure surface quality even at the cost of losing yield. However, this may not work and sometimes the surface quality becomes even worse. In the present work, slab scarfing experiments were designed to study the effect of slab scarfing depths on the density of sliver defects on the cold rolled sheets and the distribution of nonmetallic macro inclusions (NMIs) in the slab surface were evaluated using automatic scanning electron microscopy (SEM). Results show that deeper scarfing cannot guarantee high surface quality, and the number of alumina-based sliver defects after slab scarfing 6–11 mm for a single side was much larger than those slabs with scarfing less than 6 mm and over 11 mm. In addition, inclusion aggregation zones (IAZs) existed at 7∼12 mm, and 7∼11 mm from the loose and fixed slab surface, respectively. A stagnant zone under the submerged entry nozzle (SEN) bottom with the solidifying shell thickness of 8∼13 mm may account for the existence of IAZs. Accordingly, a scarfing depth of 3 mm is recommended to balance removing hooks and ensure inclusions will not be exposed to the surface.