The effect of the continuous casting process on ultrasonically-tested defects in heavy plates

摘要

A plate's internal quality is very critical,especially for boilers and high-pressure vessels.The ultrasonic test(UT) is the main type of non-destructive flaw detection for heavy plates,which is important because one of the main reasons for plate defects is ultrasonic flaws.This study,based on Baosteel's practical experience in the manufacture of heavy plates,elucidates the cause of defect formation by analyzing ultrasonic flaw testing maps and using special equipment,such as the scanning electron microscope,electron probe and the optical microscope.The author puts forward the following improvement measures:① Ultrasonic flaws are caused by central porosity and segregation,[H] bubbles and inclusion in slabs.② Ultrasonic flaws are more likely to occur in the bottom and top of slabs rather than the other positions in the casting sequence.It is clear that one-quarter of the inner camber's thickness is accumulated inclusion.③ It is clear that overheating in the tundish and the flow of the casting mould have an effect on ultrasonic flaws caused by inclusions.④ Soft reduction improves central porosity and segregation,which decreases the number of ultrasonic flaws in the plate.