Abstract: After low carbon aluminum-killed steel was annealed in the bell annealing furnace,the filiform blackening surface of the cold-rolled steel sheet was investigated,and its micro-morphology,elemental composition,structure and generation mechanism was analyzed and discussed.The studies show that the phe- nomenon is the result of that carbon was crystallized on the sheet surface during the recrystallization annealing. The black substance causing the surface blackening is graphite.