Abstract: The problem of low qualification rate of AK-B3 ball mill steel had been appeared, the microstructure, inclusions and precipitation around defects of flaw detection were detected and analyzed by OM, SEM and metal in situ analyzer, the segregation of chemical elements of continuous casting billets was analyzed. The results show that there were strip shaped cracks, micro-cracks and micro-viods distributed along rolling direction. The side morphology of cracks were cleavage fracture, inclusion of MnS and TiN occurred around cracks. Chemical elements of Mn, P and S of continuous casting billet exhibited obviously segregation, which was the main reason for the low qualification rate of flaw detection. The qualification rate of flaw detection increased from 85% to 94% through improving smelting and continuous casting processes, enhancing the cleanliness of molten steel and the internal quality of casting billets, optimizing processes of slow cooling of billets and heating process of furnace diminution thermal stress and increasing rolling reduction of continuous casting billets.