1 Introduction
With the worsening of the global environment, in order to save energy and prolong the service life of materials, users of construction, automotive and other industries have put forward higher requirements for corrosion resistance and weather resistance of materials. To meet this demand of users, R&D The hot-dipped Zn-Al series alloy plate, among which the representative and widely used variety is 55% Al-Zn-1.6% Si (Galvalume) and international lead-zinc association developed by Bethlehem Steel Corporation in the 1960s. Zn-5% Al-0.1% Ce, La (Galfan) was developed in the 1980s. Galvalume began commercial production at the Bethlehem Steel Company in 1972. Nowadays, 42 foreign companies have obtained production licenses for this product. Currently, the annual production capacity has reached 12 million tons, and the products are widely used in construction, automobiles, etc. industry. China's research, production and application of hot-dip aluminum-zinc alloy coated plate can be said to be still a blank. At present, only three lines of Wuxi Xindazhong, Baosteel and Panzhihua Iron and Steel are established, and Panzhihua Iron and Steel and Baosteel hot-dip aluminum-zinc alloy plating plate The production line has just entered the trial production stage. Therefore, the production process technology and product quality characteristics of hot-dip aluminum-zinc alloy coated plate are discussed. It is of great practical significance to the production of hot-dip aluminum-zinc alloy plate in China and the rational use of the product by users.
2 production process characteristics
2.1 Cleaning: Compared with the production of ordinary hot galvanized sheet, the production of aluminum-zinc alloy coated steel sheet has more stringent requirements on the cleanliness of the substrate. This is because when the hot-dip aluminum-zinc alloy is used, the aluminum content of the plating solution is high, the temperature of the plating solution is high, aluminum is easily oxidized, zinc vapor is easily generated, and the adhesion of the plating layer is easily deteriorated. Therefore, in order to ensure the adhesion of the plating layer, the production of hot-dip aluminum-zinc alloy plating plates generally needs to be cleaned outside the furnace, that is, an electrolytic degreasing cleaning section is set in front of the annealing furnace.
2.2 Annealing: Annealing of hot-dip aluminum-zinc alloy plate has two problems compared with ordinary hot-dip galvanized plate. The first is how to achieve over-ageing treatment of LCAK steel. The better temperature for over-aging is 300～ 350 °C, while the strip into the zinc pot temperature requirements greater than 550 °C; Second, because the bath temperature is higher than the melting point of zinc is about 200 °C, easily lead to the evaporation of zinc at the nose, this zinc vapor attached to the strip , It will cause the product to leak plating defects, so need to ensure a strong reducing atmosphere in the furnace and strictly control the zero temperature at the nose. In view of this, some foreign manufacturers have increased the H2 content in the furnace to 40% to 60%, and the zero temperature at the furnace nose is controlled below -50°C. 2.3 Hot-dip coating: As the Al-Zn alloy has a higher melting point, the bath temperature is higher than the normal hot-dip galvanizing temperature. The preferred temperature range is 620-650°C. In order to obtain better coating adhesion, the temperature of the strip into the zinc pot is about 50°C lower than the bath temperature. In order to refine the Al-rich dendritic structure and reduce the volume fraction of the interdendritic zinc-rich grains, the corrosion resistance of the coating is improved (the aluminum-rich dendrite is small, the inter-crystal gap is small, and the interdendritic zinc-rich crystals are The particles should be small and finely dispersed in the plating layer. After the plating, rapid cooling is required. The cooling rate is generally required to be 10 to 30°C/s.
2.4 Post-plating treatment: As with ordinary hot-dip galvanizing, hot-dip aluminum-zinc alloy plating plates are generally subjected to chromate passivation post-treatment. However, in recent years, in order to improve the formability and environmental protection of the hot-dip aluminum-zinc alloy plate, the roll-coating acrylic process has gradually replaced the traditional chromate passivation, and the chromium-free organic passivation has gradually attracted widespread attention. In addition, the use of hot-dip galvannealed plate as a color coated substrate, due to its surface is more clean than the ordinary hot galvanized sheet, and sensitive to pretreatment, excessive pre-cleaning will cause the surface of the coating to become dark and reduce the coating. Adhesion, so its pretreatment requirements are milder.
3 Product Quality Characteristics and Applications
Hot dip galvanized aluminum alloy plated surface is smooth, silver gray, metallic luster, its surface is uniformly distributed Al-Zn alloy crystal flower. The surface of the coating is a solid dendritic crystal, its dendrite part is an aluminum-rich α-phase solid solution, the interdendritic is a zinc-rich pseudo-eutectic β phase, and the inner layer is an Al-Fe-Si-Zn intermetallic layer (such as Figure 2 and Table 1).
The organizational structure of the aluminum-zinc alloy coating plate coating determines that it has the following quality characteristics:
1) Due to the combination of non-dynamic protection of aluminum and sacrificial anode protection of zinc, the outstanding performance characteristics of hot-dip galvanized aluminum alloy coated plate (abbreviated as GL) are excellent corrosion resistance, hot-dip aluminum-zinc alloy coating The corrosion resistance of steel plate coating is 2-6 times that of ordinary hot galvanized steel. In addition, the GL board also exhibits excellent incision corrosion resistance in the middle and later stages of use.
2) GL plate has excellent high-temperature corrosion resistance due to its high melting point and strong resistance to oxidation and heat reflection. The GL board allows long-term use in a working environment of 300°C and can be used in a short time at a high temperature of 500-600°C. The working ambient temperature of the GI is generally only allowed at room temperature, and the extreme operating temperature is 230°C.
3) The surface of the GL plate is smooth, silver-gray, metallic luster, and the Al-Zn alloy crystal is evenly distributed on the surface of the GL plate, and thus has a good appearance and decorativeness.
4) Since the porosity of the aluminum-zinc alloy coating is at least 2%, the GL plate is prone to corrosion during inventory and transportation, resulting in defects such as white rust and black spots. In addition, GL board also has good coating property, processing formability and welding performance, and can meet the requirements of users in the construction, household appliances and automotive industries.
The quality characteristics of the GL board determine its wide application in the construction, automotive and other industries:
1) Construction industry: use instead of hot galvanized sheet, such as roof, wall, waterproof sheet, etc.
2) Automobile industry: instead of aluminum plate and aluminum plate, such as bottom plate, molded inner plate, oil filter tube, exhaust system components, and silencer system components, etc.
3) Household Appliance Industry: Bakery ovens, refrigerators, air conditioners, stoves, ovens, heat exchanger components, etc.
4) Industrial Instrumentation: Heaters, Vending Machines, Combustion Components, Industrial Refrigerated Cabinets, etc.
4 Panzhihua hot-dip aluminum-zinc alloy units and products
In order to meet the demand of customers in the high-end construction and household appliances industry in China for hot-dip galvanizing and hot-dip aluminum-zinc alloy plating, Pangang has invested RMB 600 million to build No. 2 continuous hot-dip galvanizing unit (2#CGL), which was already in 2004. May was completed and put into production. The #2 CGL unit adopts the world's first-class equipment and process technology and has the following features:
1) Specialized degreasing and cleaning section is provided, and multi-stage compound enhanced cleaning means such as alkali cleaning, brush roller scrubbing and electrolytic degreasing are used, thereby greatly improving the adhesion of the coating and the surface quality of the product.
2) Using France STEIN vertical annealing furnace, full-radiant tube indirect heating, not only can improve the product coating adhesion and plate shape, but also can make the product more stable mechanical properties, according to the needs of users and provide ultra-deep leveling to Full hard grade hot dip galvanized products with different strength levels. The annealing section at the outlet of the annealing furnace is provided with a heat tensioning roller, which can realize the subsection control of the tension in the furnace and the zinc pot section at the back of the furnace, which helps to improve the uniformity of the coating thickness and the surface quality of the coating.
3) Double zinc pot system, one set of zinc pot to produce hot-dip aluminum-zinc alloy coating products, and the other set of zinc pots to produce ordinary hot-dipped pure zinc coating products. The zinc pot is the United States AJAX electric induction heating ceramic zinc pot, which is more conducive to reducing the generation of zinc slag than the iron zinc pot, which is conducive to the improvement of the surface quality of the coating.
4) Using lead-free hot-dip galvanizing and chromium-free passivation technology, the coating is free of lead, chromium and other toxic substances, which is conducive to environmental protection.
5) The DAK air knife manufactured by Mexico IMSA, with a dynamic automatic control system, ensures the uniformity of the coating thickness and the surface quality of the coating.
6) Using the French CLECIM four-roll mill and its six-roll stretch bending leveler to ensure the strip shape and surface quality, and is conducive to the improvement of product stamping performance.
7) Using the French STEIN vertical roll coater to coat thin organic coatings to produce environmentally friendly products such as fingerprint resistant, self-lubricating plates. Therefore, the Panzhihua Iron and Steel No. 2 continuous hot dip galvanizing line has laid a solid foundation for the production of high-quality, environment-friendly hot-dip aluminum-zinc alloy coating products (as shown in Table 2) with world-class equipment.
5 Conclusion
The quality and performance of hot-dip aluminum-zinc alloy coated plates combine the advantages of hot-dip galvanized plates and hot-dipped aluminum plates, and are welcomed by users in the high-end construction industry, home appliance and automotive industries. However, it still belongs to new technologies and new materials in the country, and there is still a process for users to understand and recognize them. Therefore, all manufacturers must carry out the research on production technology and application technology of hot-dip aluminum-zinc alloy plating plates, and add Great propaganda, strengthen exchanges between each other, and jointly promote the progress of production technology of hot-dip aluminum-zinc alloy coated plate and its application in China. Pangang continuous hot-dip galvanizing line equipment has reached the world-class level, its leading products are aluminum-zinc alloy, lead-free small spangle, fingerprint-resistant, self-lubricating, non-chrome passivation and other environmentally friendly products to meet the high-end construction and automotive , household appliances industry demand.