Abstract: Main scab defects arising in a hot rolling line were studied through metallographic microscope,scanning electron microscope and energy spectrometer.The results show that the formation of scab defects can be attributed to press-in of welding tumor,entanglement of converter alkaline slag and refining slag into slab,and bubbles in continuously cast strand.After causes and influencing factors of defects were analyzed,countermeasures were put forward,including installing burring device,renovating converter steelmaking and refining process and tundish smelting system,promoting inclusion floating and making reasonable baking mechanism for ladle and tundish,and taking such preventive measure as checking the mold for seepage.