Influence of Cr addition on hot-dip galvanizing reaction of Mn containing sheet steel

To clarify the influence of Cr on the galvanizability of a sheet steel containing Mn, a Cr-added Mn steel and a Cr-free Mn steel were compared in terms of selective surface oxidation and interfacial reaction during galvanizing with a molten Zn bath containing Al. The surface oxides formed by selective oxidation on the Cr-added steel were identified as Mn-Cr spinel in addition to MnO, whereas only MnO was found on the Cr-free steel. Analyses of the coating interface of the galvanized samples revealed that a columnar zeta-phase Fe-Zn intermetallic compound was formed on both of the Cr-added and Cr-free steels. Furthermore, a granular Fe-Zn intermetallic compound was found anomalously on the Cr-added steel, which was (inverted right perpendicular)(1)-phase containing a higher Fe content compared with the zeta -phase. The most remarkable feature was that Al from the molten Zn preferentially reacted with the Mn-Cr spinel more than the Fe of the substrate steel on the Cr-added steel during the galvanizing. This reaction could suppress the formation of the Fe-Al intermetallic compound, which is generally formed and inhibit a reaction between Zn and Fe in a normal galvanizing process, and instead lead to the formation reaction of the Fe-Zn intermetallic compound to make the Fe content higher to form (inverted right perpendicular)(1)-phase. These results indicate that the performance in the galvanizing process can be controlled by Cr addition through the modification of the surface oxides.