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| Microstructure characterization of hot-dip Zn-Al-Mg coating of low alloy high strength steel |
| CUI Gui-bin, JU Xin-hua, YAN Chun-lian, MA Ze-jun |
| Research Institute of Technology, Shougang Group Co., Ltd., Beijing 100043, China |
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Abstract Hot-dip Zn-Al-Mg coating is widely used in automobiles and home appliances because of its better notch protection performance and corrosion resistance,and the corrosion resistance of Zn-Al-Mg coated steel sheet is closely related to its phase composition. Therefore,in order to better study the microstructure and phase structure composition of the surface and cross-section of the Zn-Al-Mg coating on the hot-dip low alloy high strength steel. Micro-machining and fine polishing of the coating section by focused ion beam (FIB),a flat,bright and stress-free surface to be observed is prepared. Firstly,the microstructure,morphology and composition of the surface and cross-section of the Zn-Al-Mg coating were preliminarily analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS),secondly,the phase structure and composition of the Zn-Al-Mg coating were accurately determined by means of electron backscatter diffraction (EBSD) Kikuchi pattern calibration. The research shows that the phase composition of the surface of the Zn-Al-Mg coating is the same as that of the cross-section,which is composed of a coarse dendritic primary Zn-rich phase,a relatively fine fishbone-like or dendritic Zn-Mg binary eutectic phase,and more fine grained or striped Zn-Mg-Al ternary eutectic phase. In addition,a large number of nano-precipitated Al-rich phases are uniformly dispersed in the bulk primary Zn-rich phase. The phase structure of the hot-dip Zn-Al-Mg coating was accurately determined by the EBSD Kikuchi pattern calibration. The Zn-Mg binary eutectic phase is Zn-MgZn2 with the same close-packed hexagonal structure,and the finer granular or strip-shaped Zn-Mg-Al ternary eutectic phase is Zn-MgZn2-Al. In addition,the nano-scale Al-rich phase particles precipitated in the primary Zn phase can effectively improve the surface hardness of the coating. At the same time,the edges of the primary Zn phase and the eutectic phase are covered with a layer of MgZn2 phase,which can effectively delay the corrosion process and improve the corrosion resistance of the coating.
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Received: 29 June 2022
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2023, Vol. 58 
