Conflicting behavior between powdering and flaking resistance under skin pass mill process in galvannealed interstitial free steel

The failure of galvannealed (GA) coatings during press forming is an important issue for steel companies, because it results in a deteriorated product quality and reduced productivity. Powdering and flaking are thought to be the main failure modes in GA steel. However, these two modes currently lack a clear distinction, despite their different failure types. Therefore, in this study, we demonstrate that the different behaviors of these two failure modes are generated by the skin pass mill (SPM) condition and we discuss the underlying mechanism in detail using microstructural and simulation analyses. With the increase in steel elongation from 0% to 4.0% under milling force from 0 to 6 ton, a high compressive stress is produced up to −380 MPa on the surface of the steel sheet and the interface is correspondingly flattened from 0.96 to 0.53 μm in Ra. This flattening weakens the mechanical interlocking effect for adhesive bonding, deteriorating the flaking resistance from 41.1 to 65.2 hat-bead contrast index (hci). In addition, the GA coating layer becomes uniformly densified via the filling of pores under compressive stress in the layer. Furthermore, the ζ phase exhibits significant plastic deformation, leading to a uniform coverage of the coating surface; this helps to suppress crack propagation. Accordingly, the powdering resistance gradually improves from 4.2 to 3.5 mm. Consequently, with the increase in SPM-realized steel sheet elongation, the powdering resistance improves whilst the flaking resistance deteriorates. Significantly for the literature, this implies that the two failure modes occur via different mechanisms and it indicates the possibility of controlling the two coating failure modes via the SPM conditions.