Diamond transformation: the length of two diagonal lines on the cross section of billet is not equal, that is, the two diagonal lines on the cross section are greater than or less than 90 degrees, which is called diamond transformation, commonly known as off-square. Bulging: the phenomenon that the solidified shell on the surface of slab bulges into a convex surface due to the static pressure of molten steel. Internal crack: cracks caused by various stresses (including thermal stress and mechanical stress) acting on the fragile solidification interface. It is generally believed that internal cracks occur at the solidification front, and most of them are accompanied by segregation. According to the position of internal cracks, they can be divided into intermediate cracks, extrusion cracks, corner cracks, central cracks and triangular cracks. Central porosity: Fine dispersed pores of different degrees can be seen on the cross section of continuous casting billet, which is called porosity. There are three kinds of pores, namely, general pores scattered on the whole section, dendritic pores in dendrites and central pores along the slab axis.