Abstract
Ultra-high strength press hardening steels (UHS-PHS) are finding increased use in automotive passenger safety applications such as roof beams, side impact beams, passenger cage beams and, ore recently, battery rack protection in electric vehicles (EVs). Currently, these steels frequently use an (Al-Si)-based coating system which transforms into a series of Fe-Al-Si intermetallics upon austenitization of the steel at 850-900℃ prior to direct die quenching to a fully martensitic microstructure. However, this coating system provides only barrier protection against corrosion of the underlying steel.
Zn-based coatings provide significantly potential to provide robust cathodic and barrier protection against aqueous corrosion, but present several challenges. The first of these is that of liquid metal embrittlement (LME), where press hardening of Zn-based coatings above the G‑Fe3Zn10 peritectic temperature of ~780℃ has potential to have significant residual Zn-based liquid being present during die quenching. Recently, the risk of LME during direct die quenching has been mitigated with the McDermid group development of a series of prototype Fe-xC-2Mn-ySi-0.003B substrates which can be direct die quench at 550℃ - 700℃ to yield LME immune steels with ultimate tensile strengths (UTS) of 1500 – 2000 MPa. Concurrently with austenization of the prototype PHS, the Zn-based coating transforms from nearly pure Zn to a coating comprising mixtures of G‑Fe3Zn10 and Zn-saturated ferrite (i.e. α‑Fe(Zn)), both of which have significantly different electrochemical properties and, therefore, different potentials to provide robust cathodic protection to the underlying steel. The present contribution will outline the development cycle for the Zn-coated UHS-PHS-coating systems, focussing on the microstructural and electrochemical properties of the Zn-based coating systems employed and how these fit within the overall process map required to provide a direct die quenching PHS with robust Zn-based cathodic protection.