12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Direct hot press forming (DHPF) of Zn-coated steels presents significant challenges associated with avoiding liquid metal embrittlement (LME) while maintaining robust cathodic protection. As identified by previous studies, a minimum amount of the Γ-Fe3Zn10 phase (15 vol%) is necessary to provide robust cathodic protection for Zn-coated press hardening steels (PHS). The Γ-Fe3Zn10 phase is liquid at typical forming temperatures, and the combination of a liquid metal phase present on the surface of the steel and an applied strain creates conditions known to cause LME. Therefore, the dual objectives of avoiding LME and providing robust cathodic protection are ostensibly incompatible under typical hot stamping conditions.

To this end, two prototype PHS steels which have increased manganese contents relative to 22MnB5 were created to enable stamping below the peritectic temperature of 783°C, thus potentially eliminating the liquid phase essential for LME. Experiments have shown that generating fully martensitic microstructures is possible while avoiding the presence of Zn-rich liquids during deformation and providing a sufficient fraction of the  Fe3Zn10 phase to provide robust cathodic protection. The microstructural development, mechanical testing results, coating analysis and fractography of these novel direct press-hardening steels will be discussed as a function of the imposed processing routes.