Abstract: It is identified via relevant the micro-morphology and dynamic analyses that the lamination defect of the Φ374.65 mm×18.65 mm 110S casing is caused by the excessive content of the Al₂O₃-SiO₂-CaO complex inclusion. And it is further identified via the inversion check of the process that the said excessive inclusion content is caused by poor effect of the ladle soft argon blowing, and the ladle slag discharge. Accordingly the inclusion qualification rate is increased from 91.34% up to higher than 98% by means of actions aiming at systematically controlling the steel inclusion, including assuring proper ladle argon blowing effect, remaining steel operation, regulating the long nozzle inserting procedure and the slag overflowing procedure, and optimizing the tundish, etc. As a result the lamination defect of the steel pipe has never occurred.