摘要
纯锌镀层热成形钢主要应用于汽车结构件,其热处理后的涂装性能是制约其应用的原因之一。为使得该材料热处理后的表面状态满足涂装质量要求,结合涂装前处理反应机理和电泳性能要求,采用喷砂等方法改变其表面状态进行分析。研究结果如下:材料热处理后,材料表面最表层到铁基体依次为氧化层,Г相,α-Fe(Zn)相和铁基体,其中氧化层为一层含有Al_(2)O_(3)、MnO、Fe_(2)O_(3)和ZnO厚度2~4μm的复合化合物。该氧化层会抑制镀层在磷化液中的溶解过程,导致磷化膜覆盖率不足,进而引起耐蚀性能下降。延长涂装前处理脱脂工序时间来溶解去除氧化层中的两性氧化物,没有获得磷化膜覆盖率的有效提升,需要喷砂去除氧化层才能在镀层表面形成均匀、致密、覆盖率100%的磷化膜,电泳漆膜杯突和耐蚀性能才会获得明显提升。因此,材料热处理后生成的氧化层是导致镀层磷化性能和电泳耐蚀性能下降根本原因。通过分析纯锌镀层热成形钢热处理后的表面状态对涂装性能的影响,明确氧化层对磷化膜和电泳耐蚀的影响机理,说明去除氧化层是提升该材料涂装质量的重要手段,为纯锌镀层热成形钢进一步推广应用提供支撑。
Hot-formed steels consist of uncoated and coated hot-formed steels.Among the coated hot-formed steels,aluminum-silicon(Al-Si)coated and zinc-based coated steels are predominant,with Al-Si coated steels being the most widely used at present.Zinc-based coatings are restricted because of zinc volatilization and liquid metal embrittlement,and its surface properties have adverse effects on coating and welding,thereby preventing their widespread application.Currently,most research focuses on the first of the two issues,while fewer scholars have investigated the effect of zinc-based coatings on painting performance.Existing studies provide only a simple evaluation of painting quality.The effect of surface condition after heat treatment on paint quality,combined with the reaction mechanism of pre-treatment for painting and the requirements of electrophoretic properties were investigated using scanning electron microscopes,X-ray photoelectron spectroscopy analyzers,electrochemical workstations,forming testers,and cyclic corrosion testers to analyze the structural composition of the coating,corrosion resistance,morphology of the phosphating film,growth process of the phosphating film,and performance of the electrophoretic paint film.The research results are as follows:Before the heat treatment of the steel plate,the microscopic morphology of the coating is a typical morphology of galvanized sheets;there are obvious skin pass marks on the surface;the average values of roughness Ra and RPc are 0.973μm and 93.2 cm^(−1)respectively;the thickness of the coating was about 10μm;and more than 99.8%of the components of the coating was zinc,with a small amount of aluminum and iron.After heat treatment,obvious alloying occurred in the coating and the surface became rough and uneven.The roughness Ra and RPc were 1.415μm and 110.3 cm^(−1),respectively.The thickness of the coating increased to 25-28μm and was divided into four layers.The outermost layer of the coating to the iron substrate included the oxide layer,Γphase,α-Fe(Zn)phase,and iron substrate,in sequence.Among them,the oxide layer was a composite compound containing Al_(2)O_(3),MnO,Fe_(2)O_(3),and ZnO with a thickness of 2-4μm.In a 3.5%NaCl solution,the corrosion resistance of the coating with the oxide layer was better than that of the coating without the oxide layer.However,the oxide layer inhibited the formation of the phosphating film,leading to a phosphating film coverage rate below 100%.Extending the degreasing time to dissolve amphoteric oxides in the oxide layer did not effectively improve the phosphating film coverage.Removing the oxide layer by sandblasting was necessary to form a uniform,dense,and 100%coverage phosphating film on the coating surface,which significantly improved its corrosion resistance and was superior to that obtained without removing the oxide layer.The test of the electrophoretic paint film showed that removing the oxide layer had a minor impact on the cross-cut adhesion(rated as 0);however,it significantly affected the cupping performance of the paint film,increasing the cupping fracture height from 6 mm to 7 mm after the oxide layer was removed.The under-film expansion corrosion performance also improved,reducing the one-sided expansion width from 1.78 mm to 0.98 mm.The oxide layer generated during heat treatment obviously caused the decline of the phosphating performance of the coating and the performance of the electrophoretic paint film.Analyzing the effect of the surface condition of pure zinc-coated hot-formed steel after heat treatment on paint performance helped clarify the mechanism of how the oxide layer affects the phosphating film and electrophoretic corrosion resistance.Thus,removing the oxide layer is an important method to improve the painting quality of this material.
作者
龙袁
李学涛
郝玉林
张博明
徐德超
刘华赛
王松涛
LONG Yuan;LI Xuetao;HAO Yulin;ZHANG Boming;XU Dechao;LIU Huasai;WANG Songtao(Technology Research Institute of Shougang Group Co.,Ltd.,Beijing 100043,China;Beijing Key Laboratory of Green Recyclable Process for Iron&Steel Production Technology,Beijing 100043,China;Manufacturing Department,Shougang Jingtang United Iron and Steel Co.,Ltd.,Tangshan 063210,China)
出处
《中国表面工程》
北大核心
2025年第4期402-413,共12页
China Surface Engineering
关键词
热成形钢
纯锌镀层
氧化层
磷化
电泳
耐蚀
hot-formed steel
GI coating
oxide layer
phosphating
electrophoresis
corrosion resistance
