Fe-Zn phase evolution and cracking behavior in Zn-coated press-hardened steel

Abstract

Zinc coated press-hardened steel (PHS) sheet used for the production of strong, corrosion resistant parts is of interest to the automotive community, but concerns about liquid metal embrittlement (LME) remain. Specifically, mitigation of the cracking associated with LME, via better understanding of the alloyed coating microstructural features and interaction with the substrate sheet during deformation, is desired. The objective of this work was to relate the microstructural evolution in the Fe Zn coatings heat treated under single or two step thermal processing to the cracking response in specimens deformed in uniaxial tension at elevated temperature. A Gleeble® 3500 was used to heat treat galvanized 22MnB5 samples at hold times and temperatures relevant to new processes recently implemented in some hot stamping lines. The alloying achieved during heating and isothermal holding of specimens was assessed, and used to interpret the cracking response in the coating (and substrate) of specimens deformed at high temperature. Heat treatments using systematic heating rates were conducted to investigate the Fe Zn phase development during heating to elevated temperatures relevant to press-hardening. The specimen heated at the slowest rate (2 ºC/s) was comprised of the most Fe rich phases (Γ and αFe(Zn)). Comparison of the compositions of these phases to the Fe Zn phase diagram at elevated temperature (775 ºC), suggested that this coating was mostly solid at elevated temperature, and thus may also have had little Zn rich liquid upon reaching target temperature. The minimized Zn-rich liquid at elevated temperature indicated that minimal soak time would have been necessary to eliminate Zn rich liquid in the coating prior to deformation at this temperature. Specimens were also heat treated via single or two step isothermal profiles (with and without deformation), and the phases were identified and quantified to understand the state of the coating microstructure at each room and elevated temperature. Fractions of the phases developed in the alloyed coating were measured using quantitative image analysis. The fraction of δ (most Zn rich phase identified in the coatings) appeared to decrease, while the fraction of Γ and αFe(Zn) appeared to increase, with increased soak time and temperature. The δ phase was not observed in specimens heat treated at the highest time/temperature combinations (850 °C for 60-120 s), suggesting that the greatest degree of alloying occurred in these specimens. The fraction of the δ+Γ1 phases present in each specimen are suggested to contribute to the amount of Zn rich liquid present at the highest deformation temperature (700 °C), and thereby used to estimate the amount of Zn rich liquid available during elevated temperature tensile tests. The thickness of the αFe(Zn) layer also increased with increased hold time and temperature. The αFe(Zn) layer thickness in undeformed versus deformed specimens and composition gradient across the layer were measured and compared to cracking behavior of the layer. At the highest deformation temperature (700 °C), the αFe(Zn) layer exhibited cracks after intermediate and extended hold times (60 and 120 s); these specimens also exhibited the higher αFe(Zn) layer thicknesses and slightly narrower composition gradients compared to specimens deformed at the same temperature after shorter soak times. The cracking behavior observed in the deformed specimens was related to the microstructural evolution of the coating based on the inferred amount of liquid present at elevated temperature and the relative plasticity of the αFe(Zn) layer. In most cases, deep cracking into the substrate sheet associated with LME was avoided. Crack mitigation during deformation was proposed to be a result of: reduction of Zn rich liquid via increased alloying or reduced deformation temperature, or retention of a continuous αFe(Zn) layer between the alloyed coating and substrate, that served as a barrier layer to prevent contact between the Zn rich liquid and Fe-substrate.