Search

Optimization of the galvannealing for cutting-edge 3rd generation AHSS requires detailed structural and chemical characterization. Alloying elements such as Si dramatically modify the steel surface, forming an oxide layer during pre-annealing, which suppresses the desired interfacial interaction. Progress is impossible without understanding the phases' interplay at the steel-coating interface. We investigated this area in high-Si (1.49 wt%) galvanized AHSS at different annealing times via TEM and revealed that while hot-dip galvanizing at 460 °C the surface oxide prevents the desired Fe-Al reaction and liquid Zn penetrating gaps in the oxide membrane forms a δ phase layer underneath. Located between the steel substrate and the oxide layer, the δ phase can grow during subsequent annealing due to incoming liquid Zn, while Fe diffusion into the coating is suppressed. The oxide film remains stable even after long-term annealing (120 s at 480 °C) separating the coating from the reaction zone. The growing δ layer interacting with the steel substrate consumes dissolved Si. Reaching a certain threshold concentration, supersaturated δ decomposes forming a δ matrix with Fe-Si-Al-based nanoprecipitates. Using EDX data, phase configuration was refined via Thermo-Calc by the Fe–Zn–Si–Al system simulation suggesting that nanoprecipitates are based on Al- and Si-rich type of α-Fe.

Designing Advanced High-Strength Steels assumes fine-tuning of alloying elements to enhance performance properties. This affects the kinetics of Fe-Zn interaction during further galvannealing. A key role here belongs to the inhibition layer at the steel/coating interface. Applying mutually-reinforcing TEM techniques, we traced the evolution of Zn-Fe phases in low- and high-Si steels during galvannealing. It has been shown that conventional room temperature Ga FIB for TEM sample preparation is applicable only to annealed low-Si steel, a coating of which comprises equilibrium Zn-Fe compounds. The phases were validated via electron microscopy and diffraction methods. In contrast, even relatively long annealing of the high-Si steel was not sufficient to form stable Zn-Fe phases pointing to a hampering of the interphase reactions. As-galvanized and annealed high-Si specimens contain almost pure Zn and are affected by Ga via a eutectic reaction during FIB preparation. Thus, low-temperature FIB has been applied for reliable Zn-coated steel preparation. We refined with HRTEM the inhibition layer structures specific for low- and high-Si steel grades. While the typical Fe2Al5-xZnx interface layer has been found in the low-Si samples, for high-Si ones the formation of a quite stable Si-Mn mixed oxide membrane, which drastically hinders Fe-Al reaction, has been disclosed.