Abstract
With the development of modern automotive manufacturing technology, and the increased demands of automotive use, coated sheet steel has been applied in the manufacturing of large-scaled auto-panels more widely. It presents many challenges in characterizing, and understanding the behavior of coated products in terms of forming. In recent years, the increasing application of galvanized steels has highlighted the urgent need for work in this area in relation to both formability, and stamping technology. The forming characteristics of pre-phosphated electro-galvanized (EG) steel are analyzed in this paper, including the effect of a zinc coating on formability, zinc damage characteristics, and relationship between the coefficient of friction, and the viscosity of lubricants. In addition, the sensitivity of the material mechanical parameters of sheet steel to the stamping process of an auto-panel is discussed. It is shown that compared with other zinc-coated steel the application of pre-phosphated film to electro-galvanized steel prior to its being subjected to stamping operations will increase the formability, and reduce the coefficient of surface friction. With the increase of the viscosity of lubricants, the coefficient of friction of both pre-phosphated EG, and EG sheet steel decrease. Electro-zinc coatings have little influence on the mechanical properties, and the forming limit of sheet steel, but they will affect the strain distribution in the stamping process. Because of their relatively low hardness, the coatings of electro-galvanized, and pre-phosphated electro-galvanized steel are much easier to deform with the substrate than galvannealed steel, so that cracks occur later, and are to a lesser extent. It can be seen that the mechanical parameters, σs/σb, N value, R value, and δ, are relatively sensitive to the forming process under the bulging-deep drawing deformation of pre-phosphated electro-galvanized sheet steel.
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