Trends in the Development of Coated Steel Sheet Production Technology

1) Use high-quality substrates

The requirements for the surface, flatness, and dimensional accuracy of substrates in various coating processes are becoming increasingly high. For outdoor use, small zinc flower flat hot-dip galvanized steel coils are generally used, and it is best to use flat hot-dip galvanized steel coils without zinc flowers; in recent years, the trend of zinc alloy hot-dip coils has been increasing. For indoor use, electro-galvanized steel coils can be used. Film-coated color steel plates use cold-rolled sheets, while sanitary ware uses aluminum sheet coils.

2) Improve pretreatment processes and pretreatment solutions. A small number of devices and low costs have become mainstream processes, continuously improving the stability and corrosion resistance of pretreatment solutions. South Korea has used Cr+3 pretreatment solutions, and Japan has developed Cr+6-free pretreatment solutions to improve environmental protection.

3) Focus on the development of new coatings

Improve general polyester, polyvinylidene fluoride (PVDF), and plastic gels to achieve super color reproducibility, UV resistance, SO2 resistance, and enhanced corrosion resistance; develop functional coatings that are pollution-resistant and heat-absorbing.

4) More advanced equipment.

A new type of welding machine is adopted. Galvanized sheets have poor weldability, and most units use sewing machines. Mitsubishi Electric has developed a welding machine for galvanized sheets. The strip steel roller coater does not need to lift the coating roller, avoiding damage to the roller surface in the seam area of the strip steel and reducing the length of uncoated strip steel.

B new type of roller coater. The new single-slide rail coating machine eliminates errors between multiple roller systems. By adding doctor blades and sensors, the coating becomes more uniform. A roller curtain flow coating machine has been developed—coating material flows between two coating rollers, with the gap between the rollers determining the film thickness, which can be precisely controlled. The board surface has no roller marks, and the color-coated plate surface has a unique metallic luster. The initial coating uses roller coating, while the finishing coating uses a curtain coating machine, which flows liquid coating onto the surface of the steel strip, leaving no roller marks on the coated steel strip.

C Improve curing furnaces. Use fresh air purification technology, and the incoming fresh air must be strictly filtered. Medium-efficiency and sub-high-efficiency filters are installed at the air inlet. After filtering, the dust content of the fresh air is <1.5mg/Nm3; the indirect combustion post-combustion system uses coke oven gas or mixed gas as fuel, concentrating heat in the waste gas incineration furnace, heating the fresh air to a sufficiently high temperature, and sending it to various sections of the curing furnace. The combustion products do not enter the curing furnace; temperature control technology for sections adjusts the temperature of each section by simultaneously regulating the hot and cold air volumes, equipped with a complete hot and cold air control system to improve thermal efficiency; use the flue gas from the incineration furnace to preheat the fresh air, employing regenerative thermal oxidation (RTO) technology, with exhaust gas temperature <200℃.

5) Development of cold-pressing production technology

Cold pressing is cheaper than hot pressing and is widely used in North America. The embossing machine is similar to a two-roll leveling machine, with patterns engraved on the upper and lower rolls, deforming the color plate through pressing. By changing the rolls, textures such as leather, brick, and granite can be obtained, which are aesthetically pleasing, have a three-dimensional feel, and high strength. Cold embossing machines can be set online or offline, with a rolling force of up to 50t and a speed of up to 210m/min.

6) Focus on product diversification, functionality, and high-end quality

A deep-drawing color-coated plate. Based on detailed analysis results of the deformation state of the coating film during processing, a coating film that deforms correspondingly with the compression deformation of the steel plate has been developed, with precise resin design, resulting in deep-drawing color-coated plates with good adhesion and corrosion resistance.

B "Pomelo peel" color-coated plate. By adding several micron-level additives to the coating, they float to the surface during film formation, creating a color plate with fine bumps like pomelo peel. The surface requires precise control of the bumps to form an appropriate wavelength, used together with "post-coating" plates to achieve the same appearance. The surface of the pomelo peel color-coated plate makes defects less noticeable and very concealed.

C Anti-static color-coated plate. Widely used in office automation, video recorders, and other areas. The voltage of the charged coating film can be reduced by 1/4. significantly reducing the dust attached to the color-coated plate, and during processing and assembly, it can reduce dust wiping and other procedures while maintaining good additivity, formability, and chemical resistance.

D Pollution-resistant color-coated plate. Kitchen appliances such as refrigerators need to resist oil, seasoning, and detergent pollution. A coating film with a hard surface and flexible interior, called "inclined coating film," has been developed by combining polyester resin with hardened melamine resin to increase molecular structure density. After heating, the surface of the film hardens with melamine accumulation, making the surface of the coating film hard and difficult to pollute, while the inner layer has less melamine, making it flexible and easy to process.

E High heat-absorbing color-coated plate. The high performance and miniaturization of electrical products have increased the thermal load per unit volume of electrical appliances, requiring rapid and effective dissipation of heat inside the machine. The coating film of the heat-absorbing color-coated plate is specially designed to absorb infrared rays generated by heat through the resin, additives, and pigment molecules, and then dissipate it through the steel plate, reducing the internal temperature of the electrical appliance by about 10℃, which can eliminate the need for air cooling for heat dissipation and reduce heat dissipation holes.

Currently, the production of coated and plated sheets in China has made unprecedented progress, achieving significant results; however, there are also many problems, such as a shortage of tin-plated substrates and electro-galvanized sheet production; the hot-dip galvanized and color-coated sheets are too thin, with poor product quality and chaotic market competition. In the future, there will also be issues of relative overcapacity in hot-dip galvanized and color-coated sheet production. We can only continuously solve these problems, strengthen industry self-discipline, integrate into the international market, and seek survival and development in competition.