Abstract: The open， stretch and exfoliation of cracks in the galvannealed (GA) steel sheets under different stress states (tensile or compressive stress) were studied by three point bend test， EPMA， SEM and OM. The results showed that the cracks are generated in the δ and Γ phase during the cooling process after alloying annealed. The open， stretch and exfoliation mechanism of cracks in the coating are different for the tensile and compressive stress. When the coating subjects to tensile stress， the cracks propagate perpendicularly to coating plane， then the cracks reach and propagate along the Γ-αFe interface， while contact to the second crack， the coating exfoliates. In the coating subjected to compressive stress， the cracks are generated in the δ phase and propagated at an angle inclined to coating plane in the δ phase. Then， fracture occurs within δ phase or along the Γ-αFe interface. Furthermore， the exfoliation of the coating occurs more easily in compressive deformation than in tensile deformation.

Key words: three points bend test, galvannealed coating, cracks, stress state, powdering