Evolution of microstructure of zinc-nickel alloy coating during hot stamping of boron added steels

Abstract

The environmental regulations and safety requirements have enhanced the requirement of advanced and ultra-high strength steels. The hot forming process is known to deliver ultra-high strength steels. The boron added steels are heated beyond the austenitization temperature and subsequently deformed and quenched to form deformed components having fully martensitic structure. However, the bare steel suffers from oxidation and decarburization if no coating is applied. The conventional zinc based coatings like, galvanised, galvannealed are not suitable as zinc or iron-zinc phases melt at high temperature of austenitization and give rise to microcrakcs. The present work deals with a new zinc based coating system formed by a combination of electrodeposition and galvanising incorporating nickel in the coating. The coating microstructure is comprised of different nickel-zinc phases having difference in melting points as well. The evolution of coating microstructure at different soaking temperature and time is studied. The coating microstructure and compositions are observed by optical and confocal laser scanning microscope along with Secondary Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS). The different coating phases undergo melting and there is homogeneous nucleation of iron-rich solids in the coating. The composition and phase fractions of iron and zinc-rich phases are verified from ThermoCalc calculations. The coating transforms to complete iron-rich solid after 300 s of soaking at the austenitizing temperature. However, the initial zinc content largely determines the final fraction of zinc-rich liquid phases. The hot deformation study shows microcrack free regions at different areas of the hat sections.