Abstract：The bulging defects were observed in inner wall of 37Mn5 steel pipe after hot rolling and straightening. In order to find out the causes of bulging, the morphology of bulge, microstructure, chemical composition, and morphology of bulge area were analyzed by means of microscope and scanning electron microscope. The results showed that the chemical composition and microstructure of the steel pipe were normal. There were many same inclusions in the steel pipe matrix and the bulging area. All inclusions contained Ca, O, Fe, Na, K, Mg and other elements, and these elements formed large-scale composite inclusions containing a variety of oxides and mold flux. It was found that the cause of the bulging defect of the steel pipe was as follows: many such compound inclusions were separated from the contact plane among and the steel pipe matrix and gradually initiated cracks in the process of pipe threading. The cracks bulged to the inner wall and formed bulging under the action of relatively high circumferential pressure.
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