Study of cracks propagation inside the steel on press hardened steel zinc based coatings

Abstract

Al-Si coating is the most suitable solution for main Hot-Forming applications, particularly in terms of process
window for the hot stampers and in perforating corrosion resistance after austenitization. But for some specific
cases, a few customers require galvanic edge protection. So, in order to satisfy this requirement zinc based
coatings were developed for Hot-Forming.
On these Zn based coatings a full microstructural characterization was carried out on the coating influence on
the steel/coating interface during the hot stamping. It appears that some cracks propagation is always
observed inside the steel with Zn based coatings. Two separate cases corresponding to two mechanisms have to
be distinguished: Macro and Micro-cracks. The MACRO-cracks propagation is related to a liquid zinc
penetration inside the previous austenitic steel grains boundaries. This is encountered for areas showing a
high level of tensile stress with remaining liquid Fe-Zn phases in the coating during the deformation. Thus, a
cold deformation is a preliminary step for GI coating. The MICRO-cracks propagation is related to a friction
issue between the coating surface and the tools at high temperature. The higher micro-cracks density is
consequently observed on areas more sensitive to friction. The phases inside the steel responsible for this
propagation have been identified. Some solutions to avoid these phenomena are proposed, particularly in the
case of the micro-cracks for Direct Hot-Forming applications (GA coatings).