On the origin of micro-cracking in zinc-coated press hardened steels

Zn-coated press hardened steels (PHS) are in high demand for automotive mass reduction and enhanced passenger safety applications while the Zn coating supplies robust cathodic corrosion protection. However, the mechanism of micro-crack formation during direct hot-press forming (DHPF) has not been adequately described. Thus, the objective of this work was to determine the mechanism for micro-crack formation in Zn-coated DHPF PHS that addressed the relationship between micro-cracking and the coating microstructure created during substrate austenitization. Zn-coated 22MnB5 steel sheets were annealed at 900 degrees C for annealing times ranging from 30 to 780 s and DHPF at 75 degrees C s(-1) to obtain a fully martensitic substrate microstructure. The inward diffusion between the Zn coating and the substrate during annealing resulted in a dual phase coating microstructure initially comprising Gamma-Fe3Zn10 + alpha-Fe(Zn), transitioning to a single phase alpha-Fe(Zn) coating after annealing for 240-420 s. Coincident coating alpha-Fe(Zn) and substrate Zn-enriched austenite (gamma-Fe(Zn)) grain boundaries became Zn-enriched, forming a thin layer of alpha-Fe(Zn) along the gamma-Fe(Zn) grain boundaries. It is proposed that coincident coating alpha-Fe(Zn) and substrate prior austenite grain boundaries (PAGBs) were weakened by this grain boundary alpha-Fe(Zn) layer. Upon the application of tensile stress, intergranular fracture occurred along the coincident coating alpha-Fe(Zn) and Zn-enriched PAGBs in the Zn-enriched martensite (M(Zn)) layer. It was further determined that crack propagation ceased and the crack tip was blunted when Zn-enrichment along the PAGBs in the M(Zn) layer was exhausted.

Automated summary

This study has determined the mechanism for micro-crack formation in Zn-coated DHPF PHS and revealed the relationship between the coating microstructure and micro-cracks.