Q345B中厚钢板在拉伸试验后断口出现分层现象,选择分层缺陷严重的部位,采用光学显微镜、扫描电镜、能谱仪等对其显微组织、缺陷形态和微区成分进行了分析。结果表明：Q345B钢板拉伸试样分层的主要原因是元素偏析;连铸时，硫、锰、铌和钛等元素在钢带心部偏析形成了硫化锰、碳化铌和碳化钛等夹杂物;轧制过程中,这些夹杂物使心部组织变形成条状或带状组织，从而引起了钢板的分层。

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.