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The influence of alloying process of hot galvanized bake hardening steel on the GA coating microstructure and powdering resistance

A hot galvanized alloying (GA) of the bake-hardening steel (BH) is related to a concomitant slight powdering and therefore subject to enhancement of a GA coating powdering resistance. In the presented paper the coating properties and microstructure of GA of the BH steel (H220BD) have been studied by means of scanning electron microscope (SEM), glow discharge optical emission spectroscopy (GDOES) and a 60-degree V-bending test, the influence of different alloying parameters has been discussed. The results show that the higher the alloying temperature is and the longer the process is proceeding, the more comprehensively zinc and iron of the hot galvanized coating on the BH steel are spreading. At the same time, aluminum spreads into the galvanized coating gradually from the coating/base interface. The mean iron content in a coating and the powdering degree also increase progressively with temperature and time. At the alloying temperature 480 °C and duration 25 s, the coating surface represents a fine granular zinc-iron phase with small both mean iron content and coating powdering degree. These seem to be optimal alloying parameters for the bake hardening of steel H220BD.