Low-Temperature Direct Hot Stamping of a Zn-Coated Press-Hardening Steel with Enhanced Mechanical Properties

Direct hot press forming (DHPF) of Zn-coated press-hardening steel (PHS) has not been widely adopted by industry due to liquid metal embrittlement (LME), which occurs when coated steel is hot stamped above the Fe-Zn peritectic temperature (~782 °C). In the present study, low-temperature hot stamping was performed on a 2.0 wt% Mn PHS to avoid LME by preventing liquid zinc formation during plastic deformation while achieving target mechanical properties (yield strength (YS) ≥ 1100 MPa and ultimate tensile strength (UTS) ≥ 1500 MPa) and preserving corrosion performance. The enhanced hardenability, indicated by a critical cooling rate (CCR) of 10 °C/s, enabled a predominantly martensitic microstructure following DHPF at 550–700 °C. Tensile testing of samples extracted from U-shaped panels yielded similar results for uncoated and Zn-coated samples, with a YS of ~1170 MPa, a UTS of ~1600 MPa, a uniform elongation (UE) of 0.05, and a total elongation (TE) of 0.09, demonstrating the preservation of baseline mechanical properties in the coated samples. Microstructural analysis confirmed the absence of LME-induced substrate cracking. Additionally, XRD, SEM-BSE, and EDS analyses confirmed Γ-Fe3Zn10 formation in DHPF galvanized coatings, with volume fractions averaging ~0.6, well above the critical value of 0.15, irrespective of the DHPF temperature, demonstrating the formation of a cathodically protective coating microstructure.