[China Aluminum Industry Network] 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 the corrosion resistance and weather resistance of materials, in order to satisfy users. This demand has led to the development of hot-dipped Zn-Al series alloy coated plates, of which the representative and widely used type is 55% Al-Zn-1.6% Si (Galvalume) developed by Bethlehem Steel Corporation in the 1960s. ) Zn-5% Al-0.1% Ce, La (Galfan), organized and developed by the International Lead-Zinc Association 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-dip galvanized sheet, the production of aluminum-zinc alloy plated steel plate has more stringent requirements for substrate cleanliness.
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 crystal is The particles should be small and finely dispersed in the plating layer. After 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 application The surface of hot-dip galvanized aluminum alloy coated plate is smooth, silver gray, metallic luster, and its surface is evenly distributed Al-Zn alloy crystal flower.
The surface of the coating is solid dendritic crystals, its dendrite part is an aluminum-rich α-phase solid solution, between the dendrites is a zinc-rich pseudo-eutectic β phase, the inner layer is Al-Fe-Si-Zn intermetallic compound layer.
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.
