The causes of scab defects on Q345B hot-rolled strips were analyzed by metallography. The scab defects come from the cast slab. It is found that by the inappropriate technological operation of the continuous casting, the slab entrapped slag in the surface or nearly surface, small crack and the bubbles, the oxygenation under the high temperature happens in the crack of the slab; the reoxygenation the decarbonization and the weak oxygenation of grain boundary in the slab cracks take place in the heating furnace; the strip surface is baded further in the rolling process. The scab defects results from the above process.