The Effect of Zinc Coatings on the Formability of Automotive Sheet Steels 840370
The growth of coated sheet steels in automotive applications continues to require new application technology in joining, finishing, painting and forming.
The formability of zinc-coated steels depends on both the character of the substrate and nature of the coating. By eliminating the substrate as a variable in this study, the effect of various coatings (one-side electrogalvanized, hot-dip zinc and iron-zinc alloy) on formability was determined using simulative laboratory tests.
Under conditions of plane strain and stretch, all coated and uncoated steels performed comparably and can be considered interchangeable with each other. However, for drawing conditions, the drawability parameter, rm, of the hot-dip iron-zinc alloy coated steels was inferior to that of both free zinc coatings and uncoated steels.
In forming operations requiring both large deformations and extensive relative motion between sheet and tooling, the formability of all coatings is lower than bare, cold-rolled steels and highlights the need for judicious lubricant selection in these situations.
Increasing consumer demands for more durable vehicles have focussed the attention of automotive manufacturers on the use of zinc-coated steels as substitutes for bare cold-rolled steels to satisfy long-term corrosion considerations. This substitution presents many challenges in characterizing and understanding the behavior of coated products in terms of joining, painting, finishing and forming. The increasing application of galvanized steels and the development of novel coatings highlight the urgent need for work in these areas.
The parameters which affect the formability of coated steel include the steel substrate gauge and quality level, the type of coating and the application as defined by the strain state.
In this study, the use of a consistent AKDQ substrate eliminates gauge and quality as variables and focusses on the role of coating type and deformation mode in an attempt to develop much-needed baseline information on the formability of galvanized steels.