Microstructural evolution of Zn coating on 22MnB5 steel during austenitization and hot stamping processes

Abstract

Short-time austenitization of Zn-coated steels plays a critical role in the surface microstructure and corrosion resistance of the final component after hot-deformation. In this work, the evolution of the surface microstructure of the Zn coating on 22MnB5 hot stamping steel was investigated during austenitization at 900 °C for various durations. With the austenitization time increase from 300s to 400s, most of Zn in the Zn-coating diffused into the Fe matrix, forming an α-Fe(Zn) layer, while the residual Zn-rich zones discretely distributed at the interfaces of α-Fe(Zn) layers gradually disappear. Additionally, Fe2Al5Znx phase in the (Fe2Al5Znx + Mn2O3) inhibit layer of the original as-galvanized sample decomposed, resulting in the formation of a thin, increasingly continuous Al2O3 layer adjacent to the α-Fe(Zn) layer during the austenitization. After hot stamping, a multilayered Zn-coating structure was observed, containing a continuous Al2O3 layer on the surface, a continuous Г-Fe3Zn10 layer and an α-Fe(Zn) layer. Moreover, these isolated Zn-rich zones in α-Fe(Zn) layer was transformed into cellular-like (α-Fe(Zn)+Г-Fe3Zn10) structure via a non-equilibrium peritectic reaction. This transformation facilitates the consumption of liquid Zn and contributes to the prevention of crack formation during hot stamping at high temperatures.

Keywords

• Hot stamping steel
• Zn coating
• Austenitization
• Transmission electron microscopy