On the origin of micro-cracking in zinc-coated press hardened steels
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abstract
Zn-coated press hardened steels are in high demand in the automotive industry because their high strength enhances passenger safety while supplying cathodic corrosion protection. However, micro-crack formation after thermomechanical processing is an issue that limits full deployment. One of the most commonly used mechanisms for micro-cracking in Zn-coated press hardened steels (PHS) is liquid metal embrittlement (LME). During hot press forming, LME is induced by the penetration of liquid zinc along the grain boundaries of the substrate which negatively impacts mechanical properties by causing premature failure of the part. The mechanism of LME is well-established; however, it can not be applied to all cases of embrittlement in Zn-coated PHS. Therefore, the objective was to determine a new mechanism for micro-cracking in Zn-coated PHS. The focus was to address the relationship between the origin of micro-cracking and final coating microstructure created by an inward diffusion between the Zn-based overlay and underlying ferrous substrate as a function of annealing time.
Zn-coated 22MnB5 steel sheets were annealed at 900 °C for different annealing times (30 s, 60 s, 120 s, 180 s, 240 s, 300 s, 420 s, 600 s, 780 s), and were then planar or U-shaped die-quenched with an average cooling rate of 100 °Cs-1, resulting in a fully martensitic substrate microstructure. In order to precisely determine the zinc distribution and degree of Zn penetration into the bulk substrate, four sets of samples were examined. The first set of samples were annealed for 30 s (the shortest time) and 780s (the longest time) and die-quenched while the second set comprised tensile specimens from the 30 s and 780 s annealing times which were subsequently pulled to failure. The third and fourth set of samples were selected from two areas (top surface and outer
wall surface) on the U-shaped die-quenched samples that were annealed for 30 s and 780 s. U-shaped die-quenched DHPF allowed for the production of in-situ micro-cracks during forming rather and the ex-situ cracks formed during tensile testing of the planar die quench samples. The PAGBs of the substrate and GBs of the α-Fe(Zn) coating were studied before tensile testing to observe how zinc diffusion in these regions can contribute to later micro-crack formation. The micro-crack tips were investigated after tensile testing and micro-crack formation.
Scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) results showed that, by increasing annealing time, the zinc concentration in the coating decreased while the coating thickness increased parabolically. The coating microstructure underwent a transformation from a dual phase (Γ-Fe3Zn10 + α-Fe(Zn)) layer to a single-phase layer (α-Fe(Zn)) after annealing for 300s. Electron backscatter diffraction (EBSD) and scanning transmission electron microscopy couples with electron energy loss spectroscopy (STEM-EELS) were performed on four sets of samples. EBSD determined that for a α Fe(Zn) or dual-phase (Γ Fe3Zn10 + α Fe(Zn)) coating layer, a transition layer of Zn-enriched martensite (Zn-’) was present in the as-quenched samples. STEM-EELS results for both ex-situ and in-situ micro-cracking, in the absence or presence of tension, indicated zinc enrichment in the PAGBs, GBs and, at the micro-crack tip in the PAGB region for both the 30 s and 780 s DHPF samples. In addition, EELS semi-quantitative analysis of zinc concentration in the α-Fe(Zn) GBs and in the coincident PAGBs implied the presence of a thin α-Fe(Zn) layer in these regions. Based upon the current results, a new mechanism was proposed for the origin of micro-cracking in Zn-coated 22MnB5 DHPF.
