Analysis of Linear Defects on Hot-dip Galvanized IF Steel Surface

The microstructure of single-long line and double-edged line defects on the surface of hot-dip galvanized sheets and hot-rolled sheets produced in the same time were analyzed by means of scanning electron microscope(SEM) and metallography.The results show that the entrapment of protective slag caused by crystallizer level fluctuation during continuous casting is the fundamental cause of such defects.By reducing the casting speed and increasing the insertion depth of the submerged nozzle properly,the slag entrapment can be effectively controlled by controlling the fluctuation of the liquid level within ±3 mm.At the same time,surface flame cleaning was used to handle those slabs with liquid level fluctuation exceeding ±5 mm during continuous casting.The occurrence rate of such edge linear defects can be greatly reduced.