Effect of gas nitriding in flowing ammonia on the hot-dip galvanising of the Dual-Phase steel DP500

Dual-Phase steels are excellent candidates for applications in the automotive industry. However, because of their high content in alloying elements and the formation of the corresponding surface oxides during recrystallisation annealing, the Dual-Phase steels exhibit major problems in hot-dip galvanising. In the present work, the influence of short-time gas nitriding at recrystallisation temperature in an atmosphere containing reactive nitrogen on the galvaniseability of the Dual-Phase steel DP500 is investigated. The experiments are performed in an annealing and hot-dip simulator. Galvanising is performed in a standard zinc bath containing 0.2 wt.% Al.

After nitriding, the surface-near regions of the steel exhibit a nitrogen diffusion zone with a much finer-grained microstructure with higher hardness than the unnitrided reference samples. The density of surface oxides is substantially reduced, and the corresponding samples show a distinct improvement in their galvanising behaviour. In the microstructural investigations with transmission electron microscopy, particular attention is paid to the formation of the Fe₂Al₅ intermediate layer. After pre-nitriding, the layer becomes denser, and larger plate-like crystals develop at the interface. Fe—Zn phases were found to nucleate on the faceted Fe₂Al₅ crystals, which also contain a finite amount of Zn in solution. The results will be discussed, and a model for the evolution of the zinc coating will be presented.