Effect of the weir structure in the tundish on the cleanliness of IF steels and elimination of spot-like defects in deep drawing automobile steel sheets

The flow control devices (FCDs) in the tundish play an important role in steel cleanliness and removal of macro-inclusions. To eliminate the spot-like defects during deep-drawing process, the liquid steel samples were taken from tundish in three industrial IF steel heats, and the effect of weir structure in the tundish on the steel cleanliness were evaluated and compared with the aid of ASPEX with the total detection area of over 54 000 mm(2). The results showed that non-metallic inclusions over 5 mu m observed in the liquid tundish samples were categorized into two types: type 1: alumina based inclusions and type 2: calcium aluminates. Compared to the tundish without weir, the number density of macro-inclusions reduces by half when furnished with weir. In addition, several large sized calcium aluminates were occasionally found without weir whereas no calcium aluminates were detected with the weir. The mechanism of the inclusion distributions with the weir was proposed based on the experimental results, which shows that the weir act as a baffle that block the large sized inclusions from moving toward the mould. After using the weir in the actual casting practice, the occurrence ratio of spot-like defects was decreased by about 25%.