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The effect of post heating on the powdering resistance of the galvannealed steel sheet was investigated. Four types of substrates, i.e., Al-killed, Mn and P added, interstitial-free Nb-added (Nb-IF) and interstitial-free Ti-added (Ti-IF), were used. The powdering resistance of the galvannealed steel sheet was evaluated by a draw bead tester which simulated considerably the practical press forming. It is found that relatively low temperature post heating from 250 to 420°C improves the powdering resistance of the galvannealed steel sheet. Generally the increase of iron content in the coating detriorates the powdering resistance. Although the iron content in the coating is increased by the post heating, in some cases, the powdering resistance after post heating is almost the same campared to that of the as-received specimen.

The effects of working mode, coating weight, thickness of substrate and iron content of coating on the peeling-off resistance of zinc and iron-zinc coatings have been investigated using a draw bead tester, which was designed to evaluate the peeling-off phenomena of coated materials. In the test, specimens are subjected to both bending-unbending deformation and the slide caused by the bead tip. The test was conducted under several working modes; i.e., proportion of the deformation to the slide was changed by altering draw bead conditions. The draw bead results showed that Fe-Zn coating were mainly peeled off by the deformation, while Zn coatings were removed chiefly by the slide. It was also revealed that the amount of peeling-off of the coatings was considerably affected by the working mode. The peeling-off resistance of galvannealed steel sheets was then investigated by the test.

To improve the performance of zinc-rich painted steel sheets, such as Zincrometal®, we at first developed Zn-Ni plated steel sheet with a thin (about 1μm) organic-silicate composite film entirely different from a zinc-rich paint film. This product (EZN-UC) is superior to zinc-rich painted steel sheet in corrosion resistance, anti-powdering property and weldability. This product (EZN-UC), however, is not able to provide bake hardenability due to the high curing temperature of the film. In response to increasing demand for the product with bake hardenability, we have developed new organic-silicate composite coated steel sheet (EZN-UCII), which allows the coating to be cured in the temperature range, where bake hardenability can be maintained.