Abstract: The causes of star cracks of SA-210C steel tube during flaring process were analyzed through macroscopic inspection, chemical composition, metallographic analysis and energy spectrum analysis. The results show that the refractory of the tundish nozzle fell off during the casting process, and formed the inclusions in the steel tube. During the flaring process, the crack formed and expanded in the inclusion area under the action of the tensile stress, and eventually became the macroscopic star crack.