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Description
Zn-coated press hardened steels are in high demand in the automotive industry because their high strength enhances passenger safety while supplying robust cathodic corrosion protection. However, micro-crack formation after thermomechanical processing is an issue that limits full deployment. Thereby, the objective of this research was to determine the mechanism for micro-cracking in Zn-coated PHS by focusing on the relationship between the origin of micro-cracks and the diffusion-driven coating microstructural evolution as a function of annealing time.
Galvanized 22MnB5 steel sheets were annealed at 900°C for a variety of annealing times ranging from 30 – 780s and were then planar die-quenched with an average cooling rate of 100°Cs-1, resulting in a fully martensitic substrate microstructure. In order to precisely determine the degree of Zn penetration into the bulk substrate, two sets of samples were examined. The first set of samples were annealed for 30s (the shortest time) and 780s (the longest time) and die-quenched while the second set comprised tensile specimens from the 30s and 780s annealing times which were subsequently pulled to failure. The substrate prior austenite grain boundaries (PAGBs) and grain boundaries (GBs) of the Zn-ferrite (α-Fe(Zn)) coating were studied before and after tensile testing to determine if zinc diffusion in these regions contributed to micro-crack formation and propagation. Scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) indicated significant zinc enrichment in the PAGBs, GBs and at the micro-crack tip in the PAGB region for both the 30s and 780s planar die DHPF samples. Based upon the mentioned results, a new micro-cracking mechanism was proposed which clarified the importance of solid-state grain boundary diffusion and zinc enrichment in micro-crack formation and propagation in Zn-coated press hardened steels.