In recent years, there has been an increase in the use of press hardened steel (PHS) in the body-in-white of automobiles, namely in parts such as side impact beams, roof rails, engine firewalls, and the floor area. As these parts are expected to possess corrosion resistance, Al-Si coatings are utilized on them. The implementation of Zn coatings is limited, despite possessing improved corrosion resistance attributed to cathodic corrosion protection, due to the detrimental effects of liquid metal embrittlement (LME) and microcracking. LME can be mitigated if stamping occurs at a temperature lower than the Fe-Zn peritectic temperature of 782 °C, as this ensures that the conditions of LME are not met and the cathodically-protective Γ-Fe3Zn10 phase is formed. The objective of this work was to determine a process window for stamping at lower temperatures for a GA80-coated prototype steel (steel K) with the composition of 0.19C-1.92Mn-0.20Si-0.003B-0.03Ti (wt.%). The target mechanical properties in this process window were UTS ≥ 1400 MPa, and YS ≥ 1000 MPa. To achieve robust cathodic corrosion protection, more than 15 vol% of Γ-Fe3Zn10 in the coating was desired. Furthermore, it was aimed to determine the robustness of this process and the industrial feasibility of it through pilot-scale trials for the GA80-coated prototype steel. To this end, a bare prototype steel (steel I) with the composition of 0.20C-1.96Mn-0.25Si-0.003B-0.01Ti was compared to GA-coated steel K. It was found that stamping a direct hot press forming (DHPF) temperature of 650 °C produced similar mechanical properties such as tensile strengths and ductilities. Microstructural analysis of laboratory-scale DHPF for steel K showed that both the coating and the substrate microstructures were not a strong function of the DHPF temperatures of 550 – 700 °C. The substrate predominantly consisted of martensite with small amounts of ferrite and the coating consisted of α-Fe(Zn) and ≥ 15 vol.% Γ-Fe3Zn10. Based on the fraction of Γ-Fe3Zn10 and the coating thickness, robust cathodic corrosion protection is expected from GA80-coated steel K. Furthermore, no evidence of LME or severe microcracking was observed in the microstructure for DHPF temperatures of 550–700 °C for steel K and ductile fracture was observed in tensile coupons. Tensile testing for laboratory-scale DHPF for steel K showed that the mechanical properties such as YS, UE and PUE were not significantly affected by DHPF temperatures of 550-700 °C. Moreover, the targets of UTS ≥ 1400 MPa, and YS ≥ 1000 MPa were met for all DHPF temperatures of 550 – 700 °C. Through pilot-scale tensile testing of steel K, it was determined that the targets for tensile strengths (UTS and YS), were achieved across various conditions involving austenitization at 890 °C for 60 – 240 s and DHPF temperatures ranging from 550 – 700 °C. These findings strongly suggest that industrial feasibility is attainable for DHPF of GA-coated steel K at lower temperatures, enabling the attainment of desirable mechanical properties and robust corrosion protection. Thus, it can be concluded that the process window yielding desirable properties for steel K was determined to be austenitization at 890 °C for 60 – 240 s and DHPF temperatures between 550 – 700 °C. / Thesis / Master of Applied Science (MASc) / The forming process of the steels used in the automotive industry is not suitable for zinc coatings because it requires pressing at high temperatures to ensure obtaining strong steels. Thus, zinc coatings cannot be utilized despite the desirable cathodic corrosion protection properties they can provide. This study aimed to determine a suitable process window for a zinc-coated prototype steel that would have both high strength and desirable corrosion protection. The zinc-coated prototype steel was pressed at lower temperatures. It was observed that the target mechanical properties were met for all the temperatures tested. Moreover, robust cathodic corrosion protection is expected for all the temperatures. Furthermore, pilot-scale tests yielded comparable results to those obtained in the laboratory, indicating that this process can be successfully applied in the industry as it possesses a sufficiently large process window.

Despite the high demand for press-hardenable steel (PHS) with coatings that provide sacrificial cathodic protection, Zn-based coatings have experienced limited use due to the significant challenges associated with avoiding liquid metal embrittlement (LME) while maintaining robust cathodic protection when using conventional PHS materials and processing techniques. The present research addresses these challenges by reducing the conventional direct hot press forming (DHPF) temperature to between 600–700 °C, such that forming and quenching occurs well below the Zn(Fe)liq → Г-Fe3Zn10 peritectic temperature of 782 °C, thereby removing the conditions necessary for LME to occur while allowing for formation of the cathodically-protective Г-Fe3Zn10 phase. The objective of this work was to define a process window for two galvanized prototype PHS alloys with compositions of 0.20C-2.01Mn-0.26Si-0.005B and 0.19C-2.5Mn-0.26Si-0.005B (wt%) that would result in fully martensitic microstructures, tensile strengths (TS) ≥ 1500 MPa, and robust cathodic protection, defined as attaining ≥ 15 vol% Г-Fe3Zn10 in the coating microstructure, while avoiding LME. Accomplishing this task involved characterizing both grades as a function of austenization time, stamping temperature, and strain imposed by the forming process in order to define process windows that resulted in parts that met the aforementioned property requirements. It was found that the approach of increasing the Mn content relative to conventional PHS grades was successful in improving the hardenability sufficiently to enable the formation of fully martensitic microstructures despite the lower effective cooling rates associated with the reduced DPHF temperatures. Microstructural imaging and tensile testing demonstrated that, for both prototype PHS grades, a process window exists for the production of parts that satisfy the targets of the formation of fully martensitic microstructures and TS ≥ 1500 MPa while exhibiting uniform elongation of about 0.05 followed by significant post-uniform elongation. The effect of DHPF temperature and strain imposed by the forming process on mechanical properties was found to be negligible. Tensile tests and fractography revealed that reducing the DHPF temperature to between 600–700 °C was successful in preventing LME, thereby allowing samples to fracture in a ductile manner. Micro-cracking in the coating of the DHPF part was observed; however, these cracks were arrested at the coating-substrate interface. For all tested conditions, the coating met the target of ≥ 15 vol% Г-Fe3Zn10, implying that robust cathodic protection is expected. Based on the results of these experiments, it was concluded that DHPF process windows that meet all of the property targets include austenization times and DHPF temperatures of 120–180 s and 600–700 °C, respectively, for the 2Mn grade, and 60–180 s and 600–700 °C, respectively, for the 2.5Mn grade. / Thesis / Master of Science (MSc)