Exploring of crack initiation mechanisms and process parameter interactions in press hardened boron steel components actively protected by Zn-based coatings (ZINCOBOR) : final report

The desire for press hardened components in heavy-duty corrosion applications led to the development of Zn-based coatings with active corrosion protection. Unfortunately these coatings show cracks after press hardening with the risk of propagation into the steel. This project aimed at increasing knowledge about the crack initiation and growth in press hardened Zn coated components in order to develop solutions to this issue. During the 42 months of project, the possible factors leading to crack formation have been isolately studied: heat treatment and quenching, coating composition, tensile elongation, deformation and friction. Complex combination of these factors has also been studied in WP4. The most relevant parameters have been found to be the temperature at the moment of forming and the geometry of the tool. Temperature at the moment of forming can be directly related to the presence of liquid phases at the moment of deformation: in this sense, formation of deep cracks is tied to the phenomenon of Liquid Metal Embrittlement. The mere presence of liquid phases appeared to cause severe cracks, with independence to the amount of liquid present. The contribution of tool geometry was analyzed with the aid of Finite Element Modelling. This study resulted in a criterion for crack prediction, based on the reversal of stresses during forming. The stress reversal criterion was used to accurately predict the zones most sensitive to crack formation in a B-pillar component. Finally, the observed dependency between forming temperature and crack formation was used to demonstrate a solution for preventing cracks on components, based in local pre cooling prior to forming.