Relation between Cracking of Intermetallic Coating Layer of Galvannealed Steel Sheet and Grain Boundary of Substrate Steel

Abstract

Multiple cracking behavior and its relation to the grain boundary of the substrate steel were investigated. The crack spacing and its distribution of the specimens strained to prescribed strains were observed with a scanning electron microscope. The crystallographic orientation of the substrate grains was estimated from the electron back scattering pattern. The images of the cracks of the coating layer and grain boundaries of the substrate steel were combined together with image processing software to know whether the cracking of coating layer initiates above the substrate grain boundaries or not. The main results are summarized as follows. (1) The cracking of the coating layer initiates in the regions just above the substrate grain boundaries. (2) The cracks that initiate above the substrate grain boundaries propagate transversely; namely they propagate into the region, below which no substrate grain boundary exists. (3) It was suggested the cracking of the coating layer initiates above the small angle grain boundaries or above the grain boundaries intact to the grains with large Schmid factors. (4) In the multiple cracking process of the coating layer, the similarity of the crack spacing distribution to the grain boundary spacing distribution is kept.