In order to find out the unqualified reason of Q345B steel plate with 20 ~ 60mm specifications by ultrasonic testing, the microstructure and composition of unqualified parts of the plate were analyzed by means of metallographic scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the main reasons for the unqualified flaw detection are the segregation of strip manganese sulfide and manganese elements in the microstructure and the microcracks caused by abnormal granular bainite.
In view of the above reasons, some improvement measures are discussed, such as increasing steel purity, controlling pouring temperature and casting speed, increasing specific water content properly, reducing manganese content, adding silicon-calcium wire, implementing rapid stacking cooling of slabs and Q345B steel plates, prolonging heating time of slabs and implementing small reduction rolling in the last four passes. The qualification rate of Q345B steel ultrasonic inspection for the specification can be increased from 65.8% before to 91.7%.
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