Diffusion and cracking behavior involved in hot press forming of Zn coated 22MnB5

Hot press forming of Zn coated 22MnB5 is an effective way of producing ultrahigh strength structural parts for automobiles. The austenization process in hot press forming was investigated in our work. The results showed that the interfacial Fe2Al5 inhibition layer in coated steel plays a critical role in diffusion during the austenization process. We proposed three stages of diffusion based on the level of stability of the inhibition layer (i.e., the non-diffusion, inhomogeneous diffusion and homogeneous diffusion stages). The progression of diffusion in the coating through the three diffusion stages was discussed using Fe–Zn binary phase diagram and the Znliquid+α-Fe(Zn) region was considered as the susceptible area to cracking. The penetration of liquid Zn into the substrate through grain boundary (GB) diffusion was observed after the breakdown of the interfacial Fe2Al5 inhibition layer. In addition, we identified a transition diffusion layer at the interface which we assumed to be a result of the relatively long-term bulk diffusion of liquid Zn. The transition diffusion layer was analyzed following hot tensile tests. The structure of the layer was identified as body-centered cubic (BCC) α-Fe with large amounts of dissolved Zn. Although GB diffusion of liquid Zn into substrate was not observed because of the presence of the transition diffusion layer at the interface, the liquid metal induced embrittlement (LMIE) was not avoided for the Zn coated 22MnB5 in the hot tensile test. The results suggest that the BCC α-Fe phase is also sensitive to liquid Zn.