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Galvannealed steel sheets containing 10wt%Fe in coated layer were prepared in various galvannealing temperature and cooling rate. Powdering resistances were evaluated by a draw bead test. X-ray diffraction examination revealed an attribution of the coating structure to powdering resistance. Higher galvannealing temperature(500∼550°C) deteriorates powdering resistance, but slower cooling from higher temperature improves it. At low galvannealing temperature(440°C), powdering resistance is rather good independent of cooling rate. Г1-phase(Fe5Zn21) is formed along the interface between coating and steel when samples are galvannealed at lower temperature(≦500°C) or cooled at slower cooling rate from higher galvannealing temperature. The results suggest that the increase of Г1 ratio [Г1/(Г+Г1)] in the coating is inclined to improve powdering resistance.

The formability of electrodeposited Zn-SiO2 composite coating has been studied. Flaking of the coating and die pick-up were hardly observed during the draw bead simulation test. Powdering resistance of the coating were also better than that of galvannealed steel. Good formability of Zn-SiO2 composite coatings is attributed, primarily to the low friction coefficient of the coating, and secondly to the microcracking effect which release the forming stress.

Corrosion resistance of Zn-SiO2 composite coated steel has been investigated both in the wet/dry cyclic corrosion test and in the accelerated atmospheric corrosion test (modified Volvo test). In this investigation, perforation corrosion test were carried out using cathodic electrocoated lapped-panel models. Zn-SiO2 composite coated steel has shown good perforation resistance as well as good cosmetic corrosion resistance. Polarization measurements for the Zn-SiO2 composite coating have indicated the inhibition of both anodic and cathodic reaction. The inhibition of these reactions becomes more marked as corrosion of the coating progresses. From the microstructural analysis of the coating after the corrosion tests, excellent corrosion resistance of Zn-SiO2 composite coated steel is attributed to the chemically stable corrosion product consisting of zinc hydroxide and SiO2 particles.

In response to the increasing demand for organic-silicate composite coated steel sheet with bake hardenability, we developed EZN-UCII, which allowed the coating to be cured in the temperature range where bake hardenability could be maintained. It was said that improved perforation corrosion resistance of automotive body panels was achieved through the development of EZN-UCII. In recent years, the most important objective with respect to the corrosion resistance of automotive body panels has been to improve cosmetic corrosion resistance of the outer panel. The cosmetic corrosion resistance can be improved by applying an organic-silicate composite coating on Zn-Ni plated steel sheet. EZN-UCII, however, could not be applied to the automotive outer panel because the inferior appearance of the cationic e-coat made it worthless for use on the exposed side of the outer panel.

Increase of load capacity for trucks by weight reduction of the body has been required to cut down transportation costs. The weight reduction of truck body is feasible in terms of the application of high-strength steel sheet to body. Especially, the application to truck frame is important because it is heavy. The required properties of steel sheet for truck frame use are press formability and fatigue strength. Press formability can be optimized based on the previous investigation results. On the other hand, investigations on fatigue strength are relatively few, and the value of fatigue strength is not sufficient. Accordingly, the fatigue strength of high-strength steel sheet for truck frame use was investigated in this study. As for fatigue strength, the fatigue strength of steel sheet with scale is significant because the steel sheet is painted without pickling to remove the surface scale.