Zinc-Magnesium-Aluminium (ZM)-HDG-Coated Steel Sheet for Structural Parts to Outer Panels

Zinc-coatings with a substantial Magnesium content have been in use for over 30 years by now. Unlike the well-established Zn-Al-Mg coatings originating from Japan which have significant higher alloying contents applied mainly for building applications, this Zinc Magnesium Aluminum coating (ZM) is also specifically designed to meet the requirements of car manufacturers. The ZM coating introduced by voestalpine, corrender, is in the upper range of ZM-alloying compositions, which was set by VDA (German Association of the Automotive Industry) and SAE to be within 1.0 to 2.0 wt. % Mg and 1.0 to 3.0 wt. % Al. The properties of these “European” Zinc-Magnesium coatings are well comparable within this range. Compared to GI and GA ZM coatings exhibit significant advantages in the press shops with its excellent formability and reduced galling and powdering respectively which is a significant advantage for the forming of outer panels. Furthermore, ZM coatings excel in corrosion protection, especially cosmetic corrosion protection. The joining-process of ZM coated steel sheets is similar to the performance of GI hence different to GA. The phosphating process followed by ED-paint is very stable and equivalent to GI. ZM coatings are applied onto “endless” steel strips in state-of-the-art continuous galvanizing lines (CGL) in a continuous hot-dip-galvanizing process with several optional chemical post-treatments. The current product range covers mild steels, IF-, bake-hardening as well as micro-alloyed or (A)HSS grades. Therefore the typical applications for ZM-coated steel sheets are structural as well as exposed parts with highest surface quality appearance.