Improvements in Formabibity and Spot Weldability of Galvannealed Steel Sheet 940539
Modifications in the properties of galvannealed steel sheet have been made to meet the needs of the automotive industry. The press formability (powdering and flaking) of the galvannealed steel sheet is very sensitive to the phase composition and coating weight of the coating. The allowable coatings for a satisfactory anti-powdering property may fail to provide the anti-flaking property, showing a high coefficient of friction due to the existence of a soft phase, ζ, at the surface. In this study, the effects of an inorganic film on the surface of the galvannealed coating were studied improving press formability. It was found that a thin borax film was effective to minimize surface friction and to improve unti-flaking properties.
Short electrode life is another problem, as it limits continuous spot welding of the galvannealed steel sheet. During welding, electrode tips (Cu) are consumed by an interaction with the coating and the steel substrate. A small amount of B addition in the ultra-low carbon steel substrate was found to be effective in increasing the electrode life by reducing this interaction. Thus, galvannealed steel sheet becomes one of the best balanced materials for automotive panels when used with a borax coating and B addition to the ultra-low carbon steel substrate.