Avoiding Zinc Induced Cracking in Hot Forming

In direct hot forming or press forming of galvanized steel sheets especially at high forming temperatures, micro cracks may occur especially at high temperatures. Small micro cracks known as second order micro cracks (with depths up to several 10 µm) may even occur at forming temperatures well below 650 °C. They sometimes arise in parts with high frames and are often located in the frame near the flange. So far several mechanisms have been discussed about the origin of these second order micro cracks, but none was satisfactory. We could prove with different experiments and lab investigations and real part production that second order micro cracks are due to vapour metal embrittlement (VME). This results from the high zinc vapour pressure of the ZnFe-intermetallic phases. The cracking of the ZnFe-coating during hot forming generates high moveable Zn. If there is not sufficient oxygen to form zinc oxide immediately, there is a chance that Zn atoms reach the steel surface and weaken the austenite grain and/or austenite grain boundary. Special hot forming conditions, for example close local contact between part and tool together with high tensile stress, may cause second order micro cracks. Because of the limited amount of zinc atoms created by this mechanism, these micro cracks are limited to several 10 µm in depth. We also propose measures to eliminate also these small micro cracks.