Galvannealed steel sheet: 10 years of experience in product and process improvement

During the last ten years galvannealing has been the central focus of resea rch and development in the field of strip galvanising. Results in this fiel d have also led Voest-Alpine Stahl GmbH to introduce new technologies and o ptimise the production process in various respects. It has been found that with comparable coating thicknesses and Fe-contents in the galvannealed lay er the powdering and adhesion behaviour of galvannealed steel sheet is main ly dependent on whether the F phase or the F, phase is formed. This, on the other hand, depends on the steel composition and the galvannealing tempera ture. The influence of the steel composition on the formation of the ZnFe-p hase has been described by a model based on the Hume-Rothery theory. The po wdering behaviour and the adhesion correlate with the galvannealing reactiv ity. The article also presents a new process enabling the decoupling of the galv annealing behaviour from the steel composition. During an operating trial i t could be proved that the galvannealing reactivity and/or the adhesion beh aviour can be controlled by applying phosphorus to the surface of the roll- hardened steel sheet. The article also describes how the deep drawing behav iour of galvannealed steel sheet can be improved by an in-line phosphate tr eatment. The reduced friction is due to the fact that after phosphating the acicular ZnFe crystals on the surface of the coating come off more easily. Besides the good welding properties, the excellent lacquer adhering perfor mance is an outstanding feature of galvannealed steel sheet, enabling direc t lacquering and providing the lacquered galvannealed steel sheet with exce llent corrosion protection properties.