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Fe-Zn binary phase diagram indicated with temperature composition range in which LME is likely to occur during the hot stamping process.
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The hot stamping process is beneficial for fabricating high strength automotive parts without spring back. To suppress high temperature oxidation and decarburization, it is necessary to coat the hot stamping steel. In the present work, the performance of galvannealed coated (GA) hot stamped steel was evaluated. During cyclic corrosion tests, the st...
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Citations
... Over 750 • C mainly super-saturated α(Fe) solid solution was observed in the coating. Very similar conclusions were reached by Hwang et al. [2] and Lee et al. [3]. At the same time, signs of liquid metal embrittlement (LME) must not be observed on the interface between the coating and the substrate. ...
... As summarised by Chakraborty [4], this relates to multiple processes using elevated temperatures where Fe-Zn phases can exist in a semi-molten state [5]. Wang et al. [1] and Hwang et al. [2] studied this issue more closely, specifically during hot-stamping of GA-coated steel parts. ...
... Galvannealing at a temperature of 500 • C supports the formation of ζ, δ and Γ phases at the expense of the η(Zn) layer [2,3,13,16]. Exposing such a GA-coated steel to the austenitising temperature leads to the formation of an α(Fe) solid solution with increased Zn content. ...
The potential of using a Zn-based, hot-dip coating to limit steel scale formation was investigated. The phase evolution within a pure Zn and a Zn0.1Al coating on a medium-carbon (0.5 wt.% C, 0.25 wt.% Si) steel sheet during a series of heat treatment steps was investigated. Such Zn-based coatings react with the steel substrate depending on the actual heat treatment condition. A series of expected intermetallic phases was observed via SEM/EDX and XRD techniques, such as ζ, δ and Γ phases along the η(Zn) phase. The η(Zn) phase was transformed to mainly δ and Γ phases during galvannealing (500 °C). The rapid quenching from 850 °C enabled the formation of the supersaturated α-(Fe) solid solution with increased Zn content. A continuous, intact, ~20 µm thick coating was observed after the final step of the heat treatment procedure, while signs of liquid metal embrittlement (LME) were not observed near the coating/steel interface. This will ensure reliable protection against heavy scale formation on heat-treated steel parts.
... Materials 2020, 13, 3379 2 of 15 melting temperatures, are favored to avoid the liquid metal embrittlement (LME) phenomenon [19] and localized corrosion attack caused by the selective dissolution of these alloying elements [6,20]. ...
The effects of the combined addition of Zn and Mg on the corrosion resistance of AlSi-based coating for automotive steel sheets were investigated using a variety of analytical and electrochemical techniques. The preferential dissolution of Mg and Zn from MgZn2/Mg2Si phases occurred on the AlSi-based coating that had been alloyed with a smaller portion of Zn and Mg, which contributed to the rapid surface coverage by corrosion products with a protective nature, reducing the corrosion current density. On the other hand, localized corrosion attacks caused by the selective dissolution of Mg were also observed in the AlSi-based coating with a smaller portion of Zn and Mg. Such alloying can also worsen its corrosion resistance when coated additionally with electrodeposited paint. The mechanistic reasons for these conflicting results are also discussed.
Hot stamping technology has been steadily developed because it provides both excellent formability and high strength. With the development of TWB hot stamping technology, it is now possible to freely apply the required strength and thickness in the right place. In this study, the microstructure and collision performance of TWB hot stamped parts were evaluated according to their combined strength. Through dilatometry analysis, the hot stamping heat treatment temperatures of 22MnB5 steel and 30MnB5 steel were set at 950 oC and 870 oC. The 5MnB8 steel was composed of Bainite + Martensite when heat-treated at 950 oC and provided a strength of 980 MPa grade. When heat-treated at 870 oC, it was composed of Ferrite + Bainite + Martensite and provided a strength of 780 MPa grade. Simulated rear impact testing showed the 30MnB5- 5MnB8 TWB combination had the best performance, because the 5MnB8 part of the 780 MPa grade absorbed enough energy and the 30MnB5 part of the 1.8 GPa grade fully served as an anti-intrusion.
As environmental regulations have been tightened around the world, domestic and foreign automobile industries are increasing the proportion of hydrogen and electric automotive components using lightweight materials to reduce exhaust gas emissions and improve fuel efficiency. In particular, hot-stamped boron steel is being used as an alternative material for frame weight reduction due to thickness reduction, excellent workability, and good elongation. However, surface coating is essential to prevent surface decarburization phenomena and oxidation scale formation between hot stamping processes of boron steel. The coating layer causes many problems in the weldability such as a narrow weldable current range and deterioration of mechanical properties during resistance welding. Therefore, in this study, we introduce research performed to improve the weldability of Al-Si coated hot-stamped boron steel in terms of the material and process.
This study evaluated the effect of preheating on early stage melting behavior of a Al-Si coated hot stamped boron steel bolt during projection welding. A large amount of heat was generated in the early stage of projection welding. Because of the large heat generation, a rapid collapse of the projection occurred and a molten coating layer remained on the interface of the welded part. This caused welding defects such as expulsion and porosity. However, preheating helped remove the molten Al-Si coating layer by pushing it out toward the outer edge of the molten pool. This suggests that preheating can effectively minimize or remove the molten coating layer within the weld. Preheating also prevented the rapid collapse of the projection by partially melting the projection, and thus improving the contact area. These phenomena can prevent the concentration of current density at the weld interface and hence decrease heat generation. Finally, the preheating current improved nugget quality by promoting the stable growth of the melted metal and by preventing expulsion and porosity.
The effects of process temperatures for warm stamping, such as annealing (Tann), austenitizing (Taus), and stamping temperatures (Ts), on the tensile properties of Nb-bearing medium-Mn (5.9 wt pct) steel were investigated. The cold-rolled tensile specimens were first annealed at 650 °C to 750 °C, austenitized at 650 °C to 900 °C, held at Ts (500 °C to 700 °C), and then air-cooled to room temperature. Of the three process temperatures, only Taus significantly influenced the tensile properties of medium-Mn steel; in the Taus range of 650 °C to 900 °C, the amount of variation in yield strength (YS), ultimate tensile strength (UTS), and total elongation (TE) were 555 MPa, 570 MPa, and 20 pct, respectively. Particularly, when the specimens were austenitized at 730 °C to 790 °C, high YS (1060 to 1100 MPa), UTS (1760 to 1795 MPa), and TE (10.0 to 11.7 pct) were obtained due to fine-grained martensite embedded with nano-sized Nb carbides. The Nb-bearing medium-Mn specimen was successfully warm-stamped into a T-part of a B-pillar prototype at 600 °C without any cracks after austenitized at 750 °C for 5 minutes, and then air-cooled to room temperature. The warm-stamped Nb-bearing medium-Mn steel revealed the higher TE (~ 9.3 pct) than the hot-stamped 30MnB5 steel (5.3 pct) at the similar level of UTS (1855 to 1900 MPa).
