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

Galvanized press hardened steel (PHS) is in serial production more than 15 years. Zinc provides excellent cathodic corrosion protection for PHS parts, but can cause Liquid Metal Embrittlement (LME) and micro cracks. One can dispose of this LME risk by using the indirect hot forming process and avoiding tensile stress at high temperatures. To eliminate LME for the direct hot forming process there are two approaches. One approach is to use thin zinc coatings so that the whole coating after the PHS annealing furnace is pure zinc ferrite but the lower zinc content in the coating also reduces the cathodic corrosion protection. Another approach is pre-cooling (< 650°C) to avoid liquid zinc due to transformation to ZnFe-intermetallic phases before starting hot forming. Serial production of several hundred thousand tons has proved that all this countermeasures are successful.
In contrary to micro cracks caused by liquid zinc and forming at high temperatures, (depths up to several 100 µm) at forming temperatures below 650 °C and under special conditions small micro cracks (depths up to several 10 µm) may occur. Those micro cracks are also known as 2nd order micro cracks. They sometimes arise in parts with high frames and are often located in the frame near to the flange. Several mechanisms have been discussed about the origin of those 2nd order micro cracks. The weakening of austenitic grain boundaries near to the interface by zinc penetration and LME by residues of liquid zinc or LME by friction induced hot spot temperatures, were the most intensively discussed explanations so far. We could prove by several experiments resp. lab investigations and real part production, that 2nd order micro cracks are due to vapor metal embrittlement (VME). This results from the high zinc vapor 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 immediately zinc oxide, 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 2nd 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. Measures to eliminate also these small micro cracks are proposed.