Abstract: Q345B medium steel plate had a layered fracture after tension test. Microstructure,defect morphologies and micro zone compositions of the steel plate at the severe lamination zone were analyzed by optical microscope,scanning electron microscope and energy spectrum analysis. The results show that the elements segregation was the main reason for the lamination defect. During the continuous casting process,inclusions of MnS,TiC,NbC were formed because of elements segregation of S,Mn,Nb,Ti at the center of the steel. Then during the following rolling process,these inclusions resulted in the formation of banded structure at the center of the steel plate,which led to the lamination defect.