Influence of Substrate Surface Structure on the Galvanizability of Fe-16Mn-0.7C-1.5Al TWIP Steel Sheet

Twinning-induced plasticity (TWIP) steels with a high Mn content show an advanced combination of strength and formability among the commercially available advanced high-strength steels for automotive applications. However, applying zinc coatings on TWIP steels using the continuous hot-dip galvanizing process remains a great challenge owing to the selective oxidation of Mn that occurs during continuous annealing before hot dipping. In this study, a potential procedure for improving the galvanizability of TWIP steels is developed and its mechanism is discussed. Both as-received cold-rolled and pretreated 16%Mn-0.7%C-1.5%Al (mass fraction) TWIP steel sheets were galvanized using the hot-dip process in a laboratory, and the influence of the substrate surface structure on the galvanizability of the TWIP steel sheets was studied. The wettability of molten zinc on the TWIP steel sheets was examined, and the coating adhesion was tested by bending at 180 degrees. The elemental depth profiles of both the annealed and galvanized panels were analyzed via glow discharge optical emission spectroscopy, and the surface and cross-sectional morphologies were observed via SEM. Results indicated that a thin layer of fine ferrite grains produced using the pretreatment process could effectively improve the galvanizability of the TWIP steel. When the as-received cold-rolled TWIP steel was galvanized using the hot-dip process, the dominant external oxidation of Mn was observed on the steel surface before hot dipping, which prevented the formation of an Fe-Al inhibition layer and further resulted in poor galvanizability and deteriorated coating adhesion. When a thin layer of fine ferrite grains covered the TWIP steel surface, the galvanizability was considerably improved even though the ferrite layer thickness was less than 1 mu m. The presence of surface ferrite grains almost completely suppressed the external oxidation of Mn during the annealing process, resulting in a clean surface similar to that of an interstitial-free or bake-hardened steel. Therefore, the wettability of molten zinc on the TWIP steel sheet improved considerably and a sufficient Fe-Al inhibition layer was formed. The formation of a thin layer of surface ferrite grains on the 16%Mn0.7%C-1.5%Al TWIP steel facilitates a novel technique for addressing problems associated with galvanizability and coating adhesion.