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

Abstract

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 ζ‐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 Γ1‐phase containing a higher Fe content compared with the ζ‐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 Γ1‐phase. These results indicate that the performance in the galvanizing process can be controlled by Cr addition through the modification of the surface oxides.