Abstract: The chemical composition and microstructure near surface net cracks of continuously cast slabs and hot rolled plates were analyzed. The results showed that there exist Cu and Cr near the cracksand the cracks propagate along the vicinities of grain boundary. The reason of net cracks formation can be concluded that abrasion of the mold coating causes liquid copper to penetrate into the Fe matrix along the grain boundary ofaustenite and enriched copper will deteriorate the hot ductility of steel tremendously, on the basis of which, some measures to control surface net cracks were put forward.