热浸镀Zn-11Al-3Mg镀层黑点缺陷形成机理研究

目的 研究Zn-11Al-3Mg镀层黑点缺陷位置与正常位置组织特征差异，阐明黑点缺陷的形成机理，寻找导致黑点缺陷的原因，从而控制和消除黑点缺陷。方法 以工业生产的Zn-11Al-3Mg镀层钢板表面的黑点缺陷为研究对象，综合运用光学显微镜(OM)、扫描电镜(SEM)、能谱仪(EDS)、双束聚焦离子束显微镜(FIB-SEM)等，详细对比了缺陷位置和正常位置镀层显微组织的差异，分析了引起镀层组织差异的根本原因，揭示了Zn-11Al-3Mg镀层表面黑点缺陷的形成机理。结果 Zn-11Al-3Mg镀层组织由初生Al枝晶和枝晶间第二相组成，缺陷位置和正常位置枝晶间第二相显微组织存在明显差异，宏观上表现为局部黑点缺陷。缺陷位置枝晶间第二相的平均成分与原始镀液成分接近，组织为细小的颗粒状Zn/Al/MgZn₂三元组织，未出现明显的Zn/MgZn₂二元共晶；而正常位置枝晶间第二相由大量的层片状Zn/MgZn₂二元共晶及少量Zn/Al/MgZn₂三元共晶组成。造成上述镀层组织差异的根本原因是镀后凝固过程中冷却速率不均匀...

Formation Mechanism of Dark Spot Defects on Hot-dip Galvanized Zn-11Al-3Mg Coating

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.