The 45 steel rolled plate with 120 mm thickness was roughly-treated and tempered.After heat treatment,the defects in the middle part of the thickness direction of the steel plate were found by ultrasonic inspection.The results of failure analysis show that the defects in the steel plate are banded structure and distributed along the deformation direction,and some of the banded structures have intergranular cracks.The mechanism of appearance of banded structure and crack is as follows:the central part of the ingot casting process appear dendritic segregation,and the segregation zone is lengthened during the rolling process to form the segregation zone.Because of the high hardness of martensite structure forms in the strip zone,the microstructure of martensite strip is cracked along the intergranular brittleness under the action of microstructural stress and thermal stress during the tempering treatment.