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A galvannealed steel sheet (GA) with a Ni-base inorganic lubricant film has been developed to meet the needs of the automotive industry, especially for improved press formability. This paper describes the results of tests performed to evaluate formability, spot weldability, phosphatability, paintability and corrosion resistance of the GA with the Ni-base lubricant film. The coefficient of friction as measured by flat sliding test decreases with increasing Ni content in the lubricant film. The spot weldability evaluated by electrode tip life test is slightly improved after the application of the lubricant film. Phosphatability, paintability and corrosion resistance of the GA with the lubricant film are equivalent to those of GA without the lubricant film.

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 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.