Continuous Hot‐Dip Galvanizing of Medium‐Manganese Third‐Generation Advanced High‐Strength Steels

Abstract

The effects of process atmosphere oxygen partial pressure (pO2) on the preimmersion surface structures, interfacial reactive wetting products, and reactive wetting mechanisms of two prototype (0.15–0.20)C–(5.6–5.9)Mn–(0.4–1.9)Al–(1.1–1.5)Si–(0–0.6)Cr (wt%) third‐generation advanced high‐strength steels during continuous hot‐dip galvanizing are determined. In this study, the two‐stage thermal processing routes employed comprise an austenitization anneal followed by flash pickling and an intercritical anneal. All annealing treatments are conducted in a N2–5 vol% H2 process atmosphere with a controlled dew point. The substrates are austenitized at dew points of –30 or –10 °C, intercritically annealed at a dew point of –30, –10, or +5 °C, and then galvanized using a conventional 0.2 wt% Al (dissolved) bath. The preimmersion surfaces comprise a near‐pure Fe layer with dispersed nanoscaled oxide nodules. The primary reactive wetting mechanism is the direct wetting of the surface Fe, while oxide wetting, oxide cracking and liftoff, and oxide bridging by the liquid metal bath are secondary reactive wetting mechanisms. Zn ingress into the substrate via pores in the oxide network from selective dissolution during flash pickling is also noted. A well‐developed Fe2Al5–xZnx interfacial layer is observed for all metallic coating conditions. The resultant coatings show outstanding adherence after ASTM three‐point bend testing.