Micro-crack in zinc coating layer on boron steel sheet in hot deep drawing process

Abstract

Because of the growing importance of passenger safety and fuel efficiency in the auto industry, efforts continue to apply higherstrength and lighter-weight steel to automotive parts. Because hot press forming (HPF) can potentially satisfy both the strength and weight requirements, the number of HPF parts has increased in recent years. Although bare or Al–Si-coated steel has been widely used for HPF, steel companies are trying to develop Zn-coated press-hardened steel because of its advantage of cathodic corrosion protection. However, there are hurdles that must be overcome to use Zn-coated steel in a direct HPF process, including liquid metal embrittlement, cracks, and micro-cracks. In this study, a direct HPF process was implemented through deep drawing, with experiment variables that included the heating temperature (850°C, 900°C, 950°C) and heating time (3, 5, 10 min). After the experiment, the Zncoating surface, layer, and transition of the specimen were investigated. In addition, the optimum conditions for using Zn-coated boron steel in direct HPF were studied. The Al–Si coating surface and layer were also observed under the experimental conditions for the direct hot deep drawing.