Residual stress of the Fe-Zn coating layer on GA (galvannealed) steel was analyzed by means of the finite element analysis. The experimentally observed result that the coating layer has been fractured in the as supplied samples was accounted for by the analyzed high tensile residual stress introduced in the Fe-Zn coating layer due to the difference in coefficient of thermal expansion between the coating layer and the substrate steel during cooling from the galvannealing temperature. The multiple fracture phenomenon of the coating layer under the thermally induced residual stress was explained as the stress-relaxation process. From the stress analysis for the measured average crack spacing in the coating layer on the IF (Interstitial Free) and SPCC (Steel Plate Cold Commercial) steel substrates, the tensile strength of the coating layer was estimated to be 260-270 MPa. Concerning the influence of the coating layer thickness on the multiple cracking, it was revealed that the crack spacing increases with increasing thickness of the coating layer and it is proportional to the thickness of the coating layer if the strength of the coating layer is insensitive to length and thickness. Concerning the influence of the stress-strain behavior of the substrate on the multiple cracking, it was shown that the multiple cracking progresses more when the substrate steel has higher yield- and plastic flowstress.
