Characterization of 22MnB5 steel metallic coated with either hot-dip AlSi or electroplated ZnNi before and after hot stamping 2018-36-0074

The number of hot stamped components has been steadily increasing in the automotive industry. Press hardened steels (PHS) are generally used in hot stamping because, at the end of the process, the steel may achieve a tensile strength about 1,500 MPa. The hot stamping process consists of heating the steel blank at total austenitization temperatures and transferring it into the press tooling for forming and quenching simultaneously, and then to obtain a fully martensitic transformation. This transference step, from the furnace to the press, is a critical stage since it might promote steel oxidation. The application of metallic coatings avoids this event. The AlSi coating has been the most applied system on steel. However, alternative coatings, as ZnNi, are under development to keep up with the high demand. This study had the objective of characterizing and comparing the surface and transverse section morphologies in AlSi and ZnNi coatings, with scanning electron microscopy - SEM and energy dispersive spectroscopy - EDS techniques, before and after the hot stamping, and then, evaluating the effect of this process on coating morphologies. After the hot stamping, the coating layers change completely, due to the formation of different phases as a result of chemical element diffusion from the coating into the steel substrate and vice-versa. Cracks were seen in both AlSi and ZnNi coatings, but the cracks in the AlSi coating can be more critical due to the lack of cathodic protection.