Liquid metal embrittlement of hot stamped galvannealed boron steel sheet - Effect of heating time on crack formation

Abstract

Galvannealed (GA) boron steel sheets were subjected to direct hot stamping tests (V-bending) to investigate crack formation behavior. Specimens were heated at 1 173 K (900°C) in a combustion gas furnace and hot stamped on a cooled V-shaped die. The heating time was varied to alter the composition, phase, and microstructure of the coating layer. Generally, the reaction zone between the zinc coating layer and metal substrate could not be observed by scanning electron microscopy. For certain specimens, crack penetration occurred along primary austenitic grain boundaries of the substrate, with zinc enrichment observed at the crack sidewalls suggesting the occurrence of liquid metal embrittlement (LME) of the base metal. LME-induced cracking resulted when the coating layer corresponded to a biphasic Fe-Zn ferrite/liquid zinc structure of the binary Fe-Zn phase diagram during heating in the furnace. For cracked specimens, the maximum crack depth increased with increasing total X-ray diffraction intensity of δ and η phases in the coating layer of hot-stamped samples, being positively correlated with the amount of "liquid zinc" in the coating layer.