2005 Volume 91 Issue 3 Pages 313-319
The investigation on GA coating properties and in-situ SEM observation in three-point bend test has been carried out. The intention of this study is to characterize the fracture mechanism in galvannealed coatings on interstitial free steel sheet. Several cracks in coating are induced during heating and cooling process. The segment of the pre-existing cracks is related to by an outburst reaction. The crack spaces are dominated by grain size and divided into segments of approximately 20 μm. Ductility in GA coating is inhomogeneous by the difference of diffusion rate between Fe and Zn. Ductility in GA coating decreases with increasing Fe content in the coating. Therefore, the cracks take place easily in the coating as the product of an outburst reaction. These cracks propagate in a direction perpendicular to the coating phase under tensile loading condition. Generated crack relaxes applied tensile loading and then deformation of steel substrate takes place. Therefore, decohesion at steel-coating interfaces hardly takes place at the tensile loading side. The generation of the crack at the compression loading side is slower than the tensile side because the tensile residual stress in the coating is released by compressive loading. Elastic energy of the compression is then accumulated in the coating and when the stress at steel-coating interface reaches the critical decohesion shear stress, the coating divides the steel substrate. Based on the above results, the optimum coating structure is discussed.