Abstract: The delamination phenomenon appears in the middle of the 700MPa beam steel plate during slitting and cutting, and the delamination defect is more obvious after cold bending. The delamination was detected and analyzed by metallography, scanning electron microscopy and energy dispersive spectrometer. The results show that there are a large number of nonmetallic inclusions on the layered surface of the steel plate, and the inclusions divide the steel plate into three layers in the thickness. The banded structure of the steel plate is serious, and the grains in the banded structure are larger than those in the matrix. Under the comprehensive action of these factors, serious delamination defects occur in the 700MPa grade beam steel. It is suggested to strictly control the purity of steel during steelmaking, especially to prevent the floating of large foreign inclusions. By means of homogenizing chemical composition and adjustiny rolling process parameters, microstructure segregation can be reduced and the mixed crystal can be improved.