| PENG Jun,JIN Xin-yan,QIAN Hong-wei.Formation Mechanism of Dark Spot Defects on Hot-dip Galvanized Zn-11Al-3Mg Coating[J],52(7):208-216 |
| Formation Mechanism of Dark Spot Defects on Hot-dip Galvanized Zn-11Al-3Mg Coating |
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| DOI：10.16490/j.cnki.issn.1001-3660.2023.07.018 |
| KeyWord:zinc aluminum magnesium dark spot defect coating microstructure cooling rate hot-dip galvanizing |
| Author | Institution |
| PENG Jun |
Baosteel Cold Rolling Plant, Shanghai , China |
| JIN Xin-yan |
Baosteel Research Institute, Shanghai , China;State Key Laboratory of Development and Application Technology of Automotive Steels, Baosteel, Shanghai , China |
| QIAN Hong-wei |
Baosteel Cold Rolling Plant, Shanghai , China |
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| Abstract: |
| Hot-dip galvanized zinc aluminum magnesium (ZnAlMg) coated steel sheets have been developed rapidly and applied widely in different industries in recent years due to their high corrosion resistance. However, it is still a great challenge to make ZnAlMg coated steel sheets with high surface quality. Different surface defects frequently appear on the industrially produced hot-dip galvanized ZnAlMg coated steel sheets. The work aims to study the difference between the microstructures of dark spot defect and normal area of Zn-11Al-3Mg coating, clarify the formation mechanism of the dark spot defects, and find out the corresponding causes, so as to control and eliminate the dark spot defects. Dark spot defects were collected and the as received coating surface, polished coating surface, and cross section of the coating were analyzed. Detailed characterizations of the coating microstructure were carried out by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and dual-beam focused ion beam microscopy (FIB-SEM). The differences in the microstructures at spot defect and normal area were compared in detail and the formation mechanism of different coating microstructures was identified. It was found that the Zn-11Al-3Mg coating was composed of primary Al dendrites and interdendritic secondary phases. The microstructure of the interdendritic secondary phases in the spot defects was obviously different from those in the normal coating. The average composition of the interdendritic secondary phases in the spot defect was very similar to the composition of the original molten bath, the microstructure of which was composed of fine granular Zn/Al/MgZn2 ternary phases, and no obvious Zn/MgZn2 binary eutectic phases were observed. However, the interdendritic secondary phases in the surrounding normal coating were composed of a large amount of lamellar Zn/MgZn2 binary eutectic phases and a small amount of Zn/Al/MgZn2 ternary eutectic phases. The above microstructure difference between the spot defect and normal coating was along the entire coating thickness. Based on the coating microstructure analysis and the metastable phase diagram calculation of Zn-11%Al-3%Mg system, the root cause of the above differences in the coating microstructure was attributed to the uneven cooling rate during the post-galvanizing cooling process, which determined the solidification path of the coating and final coating microstructure. A formation mechanism of this kind of dark spot defects on the surface of hot-dip galvanized Zn-11Al-3Mg coated steel sheet was proposed. When the cooling rate is locally high after the primary Al dendrites have been formed, the residual liquid phase is quenched to a temperature lower than the ternary eutectic reaction temperature and the rapid solidification leads to the formation of fine interdendritic secondary phases having a similar composition to the molten bath. However, in the majority areas with lower post-galvanizing cooling rate, the following reactions including primary Al dendrite precipitation, binary eutectic reaction, and ternary eutectic reaction occur in a normal sequence, resulting in the formation of different coating microstructures from the spot defects. Therefore, it is necessary to precisely control the post-galvanizing cooling rate and keep the gas pipe clean to get a homogeneous coating microstructure and avoid the dark spot defects. |
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