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 results of X-ray diffraction analysis reveal that X-ray diffraction peaks of the post heated specimens become sharper compared to those of the as-received specimen. This phenomenon suggests that the internal stress of the coating induced by high temperature galvannealing treatment in the conventional continuous galvanizing line, is decreased by the post heating at rather low temperature.