High Manganese Press Hardenable Steel for Automotive Safety Applications

Abstract

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.