Abstract: A Q235 square steel tube fractured during the transit, and the fracture reasons were analyzed by means of chemical compositions analysis, mechanical properties test, metallographic examination, fracture macro and micro analysis, Auger electron spectroscopy (AES) analysis and so on. The results indicate that the main cause for fracture of the square steel tube was related to the improper hot working process which caused carbides precipitating along the grain boundary and phosphorus element segregating at the grain boundary and induced material embrittlement, so brittle fracture happened under the action of impact load during the transit.