Distributions of content and size of inclusions in unsteady transition slab of automotive outer panel

A statistical analysis was performed on both micro-inclusions and macro-inclusions in a 31-meter-long unsteady-state transition slab from two heats（A and B） of IF steel for automotive outer panels produced at a steel plant. Analysis of micro-inclusions larger than 5 μm revealed that the number density of inclusions increases significantly and remains elevated from 1 m before to 13 m after the start of casting heat B. Typical macro-inclusions larger than 30 μm, extracted via large-sample electrolysis, mainly consist of: blocky Al₂O₃ inclusions around 250 μm, irregular blocky mold flux inclusions around 600 μm, Al₂O₃+SiO₂ inclusions generally ranging from 50 μm to 200 μm, and spherical calcium aluminate inclusions with sizes between 30 μm and 100 μm. The content of macro-inclusions rises notably and stays high from 3 m before to 14 m after the start of casting heat B. Thus, the length of the transition slab with severely compromised molten steel cleanliness is approximately 17 meters.