With the increasingly harsh global environment, in order to save energy and extend the service life of materials, users in the construction, automotive and other industries have put forward higher requirements on the corrosion resistance and weather resistance of materials, in order to meet the needs of users, research and development. The hot-dip Zn-Al alloy alloy plated plate, which is representative and widely used, is 55% Al-Zn-1.6% Si (Galvalume) developed by the American Bethlehem Steel Company in the 1960s and the International Lead and Zinc Association. Developed in the 1980s, Zn-5% Al-0.1% Ce, La (Galfan). Galvalume has been commercialized since 1972 in Bethlehem Steel. Up to now, 42 companies have obtained production licenses for this product. At present, its annual production capacity has reached 12 million tons. The products are widely used in construction, automobiles, etc. industry. China's research, production and application of hot-dip aluminum-zinc alloy coated plates can be said to be still blank. At present, only three lines of Wuxi Xindazhong, Baosteel and Panzhihua Iron and Steel Co., Ltd., and Panzhihua Iron and Steel Co., Ltd. The production line has just entered the trial production stage. Therefore, it is of great practical significance to discuss the production process technology and product quality characteristics of hot-dip aluminum-zinc alloy coated plates, and the production of hot-dip aluminum-zinc alloy coated plates in China and the rational use of the products by users.
1. Production process characteristics
Cleaning: Compared with ordinary hot-dip galvanized sheet production, the production of aluminum-zinc alloy coated steel sheets has stricter requirements on substrate cleanliness. This is because when the aluminum-zinc alloy is hot-plated, the aluminum content of the plating solution is high, the bath temperature is high, the aluminum is easily oxidized, and zinc vapor is easily generated, thereby easily deteriorating the adhesion of the plating layer. Therefore, in order to ensure the adhesion of the coating, the hot-dip aluminum-zinc alloy plated plate production generally needs to be cleaned outside the furnace, that is, an electrolytic degreasing cleaning section is arranged before the annealing furnace.
Annealing: Annealing of hot-dip aluminized zinc-alloyed plate has two problems compared with ordinary hot-dip galvanized sheet: First, how to achieve online over-aging treatment of LCAK steel, because the optimum temperature for overaging is 300-350 °C, and the temperature of the steel into the zinc pot is greater than 550 ° C; the second is because the temperature of the plating solution is higher than the melting point of zinc by about 200 ° C, which easily causes the evaporation of zinc at the furnace nose, which is attached to the strip. This will cause leakage defects in the product, so it is necessary 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.
Hot plating: Because the melting point of Al-Zn alloy is higher, the bath temperature is higher than ordinary hot-dip galvanizing temperature, and the optimum 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 dendritic structure of aluminum-rich, the volume fraction of zinc-rich crystal grains between dendrites is reduced to improve the corrosion resistance of the coating (the aluminum-rich dendrites are fine, the intercrystalline spaces are small, and the intergranular zinc-rich crystals are formed). The particles are less and finely dispersed in the plating layer. After the plating, rapid cooling is required, and the cooling rate is generally required to be 10 to 30 ° C / s.
Post-plating treatment: As with ordinary hot-dip galvanizing, hot-dip aluminum-zinc alloy coated sheets are also generally subjected to chromic acid passivation. However, in recent years, in order to improve the formability and environmental friendliness of the hot-dip aluminum-zinc alloy plated plate, the roll-coated acrylic acid process has gradually replaced the traditional chromic acid passivation, and the chromium-free organic passivation has been widely concerned.
In addition, in the case of hot-dip aluminum-zinc alloy coated plates used as color-coated substrates, the surface is cleaner than ordinary hot-dip galvanized sheets, and sensitive to pretreatment, excessive pre-cleaning, which will lead to blackening of the coating surface and reduction of coating. Adhesion, so its pretreatment requirements are milder.
2, product quality characteristics and applications
The surface of the hot-dip galvanized aluminum alloy plate is flat, silver-gray, metallic luster, and the surface is uniformly distributed Al-Zn alloy crystal flower.
The surface layer of the coating is a solid dendritic crystal, the dendritic part of which is an aluminum-rich α phase solid solution, the zinc intercalation is a zinc-rich pseudo-eutectic β phase, and the inner layer is an Al-Fe-Si-Zn intermetallic compound layer.
The microstructure of the coating of aluminum-zinc alloy plated plate determines its quality characteristics:
1) Due to the combination of non-dynamic protection of aluminum and sacrificial anode protection of zinc, the remarkable performance characteristics of hot-dip galvanized aluminum alloy plate (GL) is excellent corrosion resistance, hot-dip aluminum-zinc alloy coating The corrosion resistance of the steel plate coating is 2 to 6 times that of the ordinary hot-dip galvanized sheet. In addition, the GL plate exhibits excellent cut corrosion resistance at the middle and the end of use.
2) GL plate has excellent high temperature corrosion resistance due to its high melting point and strong oxidation resistance and heat reflection. The GL board allows long-term use in a 300 ° C working environment and can be used for a short period of time at a high temperature of 500 to 600 ° C. The operating temperature of the GI is generally only allowed to be room temperature, and the limit operating temperature is 230 °C.
3) The surface of the GL board is flat, silver-gray, metallic luster, and the surface is uniformly distributed Al-Zn alloy crystal flower, so it has good appearance decoration.
4) Since the aluminum-zinc alloy coating has a porosity of at least 2%, the GL plate is prone to corrosion during inventory and transportation, and causes defects such as white rust and black spots.
In addition, the GL board also has good coating properties, processability and soldering properties to meet the requirements of users in the construction, home appliance 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-dip galvanized sheet, such as roof, wall, waterproof board, etc.
2) Automotive industry: instead of aluminum plate and aluminum plate, such as bottom plate, molded inner layer plate, oil filter tube, exhaust system components and noise elimination system components, etc.
3) Household appliance industry: bread oven, refrigerator, air conditioner, stove, oven, heat exchanger parts, etc.
4) Industrial instruments: heaters, vending machines, combustion components and industrial freezers, etc.
3, Pangang hot-dip aluminum-zinc alloy unit and products
In order to meet the demand of hot-dip galvanizing and hot-dip aluminum-zinc alloy coated plates for users in the domestic high-end building and home appliance industry, Pangang invested 600 million yuan to build No. 2 continuous hot-dip galvanizing unit (2#CGL), which was in 2004. It was completed and put into production in May. The 2#CGL unit adopts the world's first-class equipment and process technology, and has the following characteristics:
1) A special degreasing cleaning section is adopted, which adopts multi-stage composite strengthening cleaning means such as lye cleaning, brush roller cleaning and electrolytic degreasing, so that the adhesion of the coating and the surface quality of the product are greatly improved.
2) The French STEIN vertical annealing furnace is used, and the full-radiation tube type is indirectly heated, which not only improves the adhesion of the product coating and the shape of the plate, but also makes the mechanical properties of the product more stable, and can provide ultra-deep-stage to the needs of users. Hot-dip galvanized products of different strength grades. The hot tensioning roller is arranged at the exit section of the annealing furnace, which can realize the segmental control of the tension of the zinc pot section in the furnace and the furnace, which is beneficial to improve the uniformity of the coating thickness and the surface quality of the coating.
3) Using a double zinc pot system, one set of zinc pots produces hot-dip aluminum-zinc alloy coating products, and another set of zinc pots produces ordinary hot-dip pure zinc-plated products. The zinc pot is an American AJAX electric induction heating ceramic zinc pot, which is advantageous for reducing the amount of dross generated compared with the iron zinc pot, thereby contributing to the improvement of the surface quality of the coating.
4) Adopting lead-free hot-dip galvanizing and chromium-free passivation process, the coating does not contain lead, chromium and other toxic substances, which is beneficial to environmental protection.
5) DAK air knife manufactured by IMSA of Mexico with dynamic automatic control system to ensure uniformity of coating thickness and surface quality of coating.
6) Adopt French CLECIM four-roll finishing machine and its six-roll stretch bending straightening machine to ensure the shape and surface quality of the strip, and to improve the stamping and forming performance of the product.
7) Adopt French STEIN vertical roll coater to coat thin organic coating to produce environmentally friendly products such as fingerprint resistant and self-lubricating board.
Therefore, Pangang No. 2 continuous hot-dip galvanizing unit has laid a solid foundation for the production of high-quality, environmentally-friendly hot-dip aluminum-zinc alloy coating products with world-class equipment.
4 Conclusion
The quality performance of hot-dip aluminized zinc-alloy plated plates combines the advantages of hot-dip galvanized sheets and hot-dip aluminized sheets, and is popular among users in the high-end construction, home appliance and automotive industries. However, it is still a new technology and a new type of material in China. There is still a process for users to know and recognize it. Therefore, each manufacturer should carry out research on the production technology and application technology of hot-dip aluminum-zinc alloy plated plate. Great publicity efforts, strengthen mutual exchanges, and jointly promote the progress of the production technology of hot-dip aluminum-zinc alloy plated plates and the application of their products in China. The continuous hot-dip aluminum-zinc unit of Panzhihua Iron & Steel Co., Ltd. has reached the world-class level. Its leading products are aluminum-zinc alloy, lead-free small zinc flower, fingerprint-resistant, self-lubricating, chrome-free passivation and other environmentally friendly products to meet high-end buildings and automobiles. The needs of users in the home appliance industry.
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Tags: hot-dip galvanized sheet hot-dip galvanizing unit small zinc flower solid solution organic coating
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