Successive phase and morphology evolution of galvannealed coating in hot stamping and diffusion modeling of α-Fe(Zn)/steel system considering the effect of Zn concentration

Abstract

The successive phase evolution and surface oxidation of GA coating during austenization in hot stamping were investigated. Furthermore, a diffusion model for α-Fe(Zn)/steel system was developed to evaluate the relation between the interdiffusion coefficient and Zn concentration in the α-Fe(Zn) layer. The results indicated that the phase evolution of GA coating could be divided into five successive stages, which were in sequence of Γ layer growth below 665 °C, liquid phase transforming into Γ phase followed by coexistence of dominated Γ phase and thin α-Fe(Zn) layer from 665 °C to 782 °C, transformation of liquid phase into α-Fe(Zn) phase above 782 °C, and thickening of α-Fe(Zn) coating by solid state diffusion. Surface oxidation became activated with the occurrence of liquid phase in coating. The surface oxides evolved from dispersive particles to accumulated clusters and finally into a continuous oxide layer which could retard further coating oxidation. With heating time prolonged from 300 s to 600 s, the thickness of the α-Fe(Zn) coating increased from 19.8 μm to 23.3 μm with gradually decreasing Zn concentration distributions. A concentration dependent interdiffusion coefficient could adequately describe the diffusion behavior and thickening process of the α-Fe(Zn) layer. The estimated interdiffusion coefficient in α-Fe(Zn) coating had an order of magnitude of 10−13 m2 s−1 and increased with increasing Zn concentration.