Influence of the Surface Selective Oxidation of Advanced High Strength Steels (AHSS) on the Reactive Wetting in Continuous Hot Dip Galvanizing

Abstract

The formation of an external oxide layer of Mn, Si, and Al during the continuous annealing of advanced high strength steel (AHSS) usually results in the deterioration of the coating quality of the galvanized AHSS. One promising method which was proposed to improve the quality of hot dip Zn coating is to anneal the strip in a high dew point (DP) atmosphere, which results in internal, rather than external, oxidation. This method however requires a modification to the parts of the industrial lines. An alternative method involves the addition of surface active elements such as Sn and Sb to the steel composition. When these elements segregate to the free surface, they suppress the surface oxidation by occupying the oxygen adsorption surface sites. In the present work, the two methods were tested on conventional CMnSi transformation-induced plasticity (TRIP) steels by means of hot dip galvanizing (HDG) simulator tests. The surface oxides formed on the steels during the continuous annealing were analyzed in high resolution by means of transmission electron microscopy (TEM) and 3-dimensional atom probe (3D AP) tomography. The use of the DP control effectively improved the galvanizability of CMnSi TRIP steels processed in hot dip galvanizing/galvannealing lines. This is because the thickness of the surface oxides was effectively reduced by increasing the DP to activate internal selective oxidation. The addition of surface active elements also significantly improved the galvanizability of CMnSi TRIP steels. The addition of these elements changed the morphology of the surface oxides of xMnO•SiO2 from a continuous film to lens-shaped island morphology. Of the five elements, Cr, Ni, Cu, Ti, and Sn, the most significant change was observed when Sn was added. The addition of Sn suppressed the formation of the Mn-rich oxides of MnO and 2MnO•SiO2. The changes in the morphology and chemistry of the surface oxides was due to the surface segregation of Sn, which resulted in a decrease of the oxygen permeability at the surface. The formation of lens-shaped oxides improved the wettability by molten Zn. The improved wetting effect was attributed to an increased area fraction of the surface where the oxide layers were thinner, enabling a direct unhindered reaction between Fe and the Al in the liquid Zn and the formation of the inhibition layer in the initial stages of the hot dipping.