Role of heating conditions on microcrack formation in zinc coated 22MnB5

Zinc coated steels for hot press forming are intended to offer active corrosion resistant ultrahigh strength steel for applications in the automotive industry. During hot-press forming, zinc can infiltrate the underlying grain boundaries during the heat and forming step, leading to intergranular cracking of the base metal. In this study, samples of coated 22MnB5 steel were heated in a furnace at temperatures of 880, 900 and 920°C for 240 to 600 s and subsequently hot-formed into U-shaped profiles. Microstructure, morphology and depth of cracks were analysed using conventional optical and scanning electron microscopy, energy dispersive spectroscopy and focused ion beam imaging. The distribution of Zn in the heated coatings and underlying substrate was measured in detail by energy dispersive spectroscopy in transmission electron microscopy with samples taken out from the coating/base metal interfaces applying an in-plane focused ion beam lift-out method. In general the heating conditions do not appear to have effect on the density and spatial distribution of cracking in the top-wall and in the side-wall, but concerning the depth of penetration into the martensite/prior austenite substrate there is clear evidence that with extended heating time and temperature the depth of the penetration into the substrate and also the amount of penetrating cracks inside the top-wall is reduced. However full understanding of the role of heating condition on microcrack formation in zinc coated 22MnB5 has not been achieved at this stage yet; further work is underway and will be published later.