12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Zinc coatings offer the advantage of a galvanic corrosion protection instead of a conventional barrier corrosion protection. Thus zinc coatings prevent the corrosion of the steel substrate even in the case of a damaged coating layer. It is therefore of interest to use zinc coatings in the automobile industry particularly for components of the body in white. These components are often produced by the direct press hardening process consisting of a hot forming and a subsequent quenching step in the die. Zinc is known to have an embrittling effect on the sheet’s base material at elevated temperatures leading to surface cracks in the steel substrate during forming. To be able to numerically simulate the press hardening process including a reliable damage indicator model it is necessary to understand the crack mechanism of zinc induced cracks in detail. To this end metallographic investigations of galvannealed press-hardening steel sheets (20MnB8) using a light optical microscope as well as a scanning electron microscope comprising Electron-Backscattering-Diffraction (EBSD) and Energy Dispersive X-Ray Analysis (EDX) were conducted on selected cracks appearing during press hardening experiments of simple hat shaped components. The crack depth and distribution along the formed component’s cross sections were determined by light optical microscopy for establishing a subsequent correlation to the simulated load and temperature conditions. The crack propagation path in the substrate was correlated to the lattice orientation obtained from EBSD measurements. Thereby it was determined if cracks propagated along prior austenite grain boundaries or martensite block boundaries to analyze whether or not zinc penetration in prior austenite grain boundaries is an influencing factor for crack formation. The presence of zinc in the vicinity of the crack was detected by means of EDX scans. The investigations showed that the cracks propagate along prior austenite grain boundaries as well as martensite block boundaries. Hence zinc penetration in prior austenite grain boundaries does not seem to be the only relevant factor to zinc induced crack formation.
Knowledge of the crack distribution, the cracks’ propagation paths and the zinc distribution in the crack near regions enables to better understand the mechanisms leading to zinc induced crack formation, which is crucial for developing a valid damage criterion taking into account the most important influencing factors.