PINHOLE AND INCLUSION DEFECTS FORMED AT THE SUBSURFACE IN ULTRA-LOW CARBON-STEEL

Excellent surface characteristics are necessary for the production of ultra low carbon steel sheet which is used for the automobile. Crack formation is not a serious problem in continuously cast ultra low carbon steel because of good ductility at high temperatures. However, pinhole defects in slabs and sliver defects in cold sheets are often observed, and they are the most harmful defects in producing ultra low carbon steel. In this paper, the reason why pinhole defects are more likely to occur in ultra low carbon steel than other carbon steels is discussed based on the investigations of gas composition in the pinhole defects and,the relationship between pinhole defects and solidification structure. Furthermore, the origin of sliver defects with inclusions is discussed based on investigations of inclusions found inside of the pinhole in the slabs and near the sliver defects in the cold rolled sheets.