Effect of Annealing Atmosphere on the Galvanizing Behaviour of a Dual Phase Steel

Abstract

<p> The selective surface oxidation of alloying elements such as Mn can cause dual phase (DP) steel wettability problems by liquid Zn during continuous galvanizing. It is well known that process parameters, such as the annealing atmosphere %H2 and dew point, can affect surface and subsurface oxidation. The purpose of this research was to study the effect of the annealing atmosphere to determine the optimum DP steel surface that would result in better reactive wetting by zinc. In particular, the evolution of the surface phases and structures during the continuous galvanizing annealing cycle were studied. It was shown that the internal I external oxidation behavior of the alloying elements of DP steel (e.g. Mn and Mo) at the surface and subsurface can be controlled by changing process parameters (dew point and H2/N2 ratio) and that some segregation of elements is unavoidable but can result in good reactive wetting by liquid galvanizing alloys. A transition from external to internal oxidation was observed when the oxidation potential (pH20ipH2) of the annealing atmosphere was increased from 0.00844 to 0.03451. Despite the presence of 9-19 wt% Mn as MnO in the pre-dipped steel surface, the coatings exhibited good adhesion and a well developed Fe2Als inhibition layer at the coating I substrate interface for all experimental annealing atmospheres as a result of reactive wetting. This is attributed to aluminothermic reduction of manganese oxide by aluminum present in the liquid galvanizing alloy. </p>