Microstructure of liquid metal embrittlement cracks on Zn-coated 22MnB5 press-hardened steel

Zn-coated press-hardened steel, used for ultrahigh-strength structural parts of passenger cars, is sensitive to liquid-metal-induced embrittlement cracking during die quenching. The microstructure of the intergranular liquid-metal-induced cracks was analyzed in detail. A new model for crack formation is proposed. Intergranular cracking is due to the grain boundary penetration of a liquid Zn alloy phase along a crack tip propagating on prior austenite grain boundaries weakened by the Zn diffusion-mitigated phase transformation to ferrite.