Introduction

Coated steel sheets are also called organic coated steel sheets or pre-coated steel sheets. They are made of metal coils (cold-rolled steel sheets, hot-dip galvanized steel sheets, aluminum-coated steel sheets, high aluminum alloy steel sheets, stainless steel sheets, etc.) as the base material, with various organic coatings or plastic films coated or laminated on the surface. Since they are coils that have been coated on the surface in metallurgical factories and can be directly processed into products by users, they are also called pre-coated coils  [1].。

Development of coated sheet industry

In 1935, the United States established the first continuous coating steel plate line. In the past 20 years, the color-coated plate production industry has developed rapidly with the construction industry as a breakthrough, and the output has continued to increase. There are more than 400 color-coated plate production lines in the world, with a production capacity of more than 18 million tons. Color-coated plates have been widely used in the fields of construction, household appliances, furniture and office equipment. This product has attracted attention in China in the early 1960s. After years of efforts, there are nearly 30 production lines in Baosteel, Wuhan Iron and Steel, Anshan Iron and Steel, etc. In addition, a number of large-scale color-coated plate production lines such as Shougang, Benxi Steel, Ma Steel, and Tangshan Iron and Steel have been completed and put into production. It has become another climax of the development of China's pre-coated coil industry, which will have a certain impact on the industry itself and the entire national economy.

Foreign situation

Coated steel sheets were produced in the United States in the mid-1930s. Initially, they were narrow strip steel coated sheets, mainly used in the blinds and thin iron products industries. In 1955, the United States built the first batch of wide strip steel organic coating production lines, and Japan, West Germany, the United Kingdom, France and other countries successively built a number of production lines. After the 1960s, especially in the past two decades, Belgium, Sweden, Norway, Finland, Brazil and some Eastern European countries have also successively built a number of production lines.

Organic coated sheets have developed rapidly in Europe, America and Japan, especially in the United States. In 1960, the output of the United States was less than 100,000 tons, which increased to 1.2 million tons in 1966 and reached 3 million tons in 1978 (of which about 650,000 tons were aluminum strip coating). In the late 1970s, the United States put into operation a state-of-the-art production line with a process speed of 240m/min, with an annual production capacity of more than 300,000 tons  [2].。

Domestic situation

In the 1960s, some factories and research institutes in China began to develop organic coated plates. For example, Shanghai No. 3 Iron and Steel Plant built a "plastic laminate" production line with 1 x 2m hot-rolled thin plates as the original plates in 1969. Anshan Iron and Steel Company designed, manufactured and put into production a "plastic laminate" continuous production line with cold-rolled strip steel as the original plate in 1981. In 1979, with the support of Shanghai No. 3 Steel Plant, Liuhe Town Oxidation Plastic Coating Plant in Jiangsu Province also built a single plastic film laminated aluminum plate production line similar to the production line of Shanghai No. 3 Steel Plant and put it into production. Beijing Iron and Steel Research Institute built a simulated strip steel organic coating test unit in 1982.

Some enterprises in my country with better conditions, such as Wuhan Iron and Steel and Baosteel, have introduced and put into operation two-coating and two-baking organic coated plate production lines. Wuhan Iron and Steel introduced a production line with an annual output of 640,000 tons from the UK. Baosteel introduced a production line with an annual output of 2.27 million tons of organic coated plates to provide organic coated plates to the market.

Classification of coated sheets

The coated plate has the advantages of both organic polymer and steel plate. It has the good coloring, formability, corrosion resistance and decorativeness of organic polymer, and the high strength and easy processing of steel plate. It can be easily processed by punching, bending, deep drawing, welding, etc. This makes the products made of organic coated steel plate have excellent practicality, decorativeness, processing and durability.

There are many types of coated sheets, about more than 600, but there is still no unified classification method, because it is difficult to find a standard to distinguish between the numerous varieties, and each manufacturer has its own unique equipment, process, and product variety. Now it is only classified according to general habits.

1. Classification by substrate used. The substrates commonly used for organic coated steel sheets are: cold rolled sheets, hot dip galvanized sheets, electro galvanized sheets, alloyed sheets, galvanized aluminum sheets, electro zinc alloy sheets, tinned sheets, chrome-plated sheets, aluminum sheets, etc.

2. Classification by processing technology. According to different coating methods, there are organic coated steel plates made by roller coating, spraying, powder coating, laminating, printing, etc. According to different coating times, there are organic coated steel plates made by different coating processes such as one coating and one baking, two coatings and two bakings, three coatings and three bakings, and four coatings and four bakings. According to embossing or printing processing, it can be subdivided into coated steel plates such as wood grain boards, marble grains, leather grains, and cement mortar grains.

3. Classification by coating type. Usually classified by the coating used to make organic coatings, there are polyvinyl chloride coated steel plates, polycool coated steel plates, acrylic coated steel plates, fluorocarbon coated steel plates, silicone polycool coated steel plates, etc.

4. Classification by coating performance. According to the performance characteristics of organic coatings, there are high weather-resistant coating plates, high temperature resistant coating plates, self-extinguishing coating plates, sterilization coating plates, non-stick snow coating plates, fingerprint-resistant coating plates and self-lubricating coating plates.

5. Classification by special coating and purpose. Such as epoxy zinc-rich coating , organic coated steel plate with cold-rolled steel plate as substrate, surface flocked steel plate and some special machine-coated composite plates, such as wood film board, sandwich panel, shockproof board, etc.  [3]。

Typical production process of coated panels

The whole unit consists of the following four parts: inlet section, pretreatment section, coating section, and outlet section. The common process flow of the two-coating and two-baking color coating production line is: unwinding → sewing → inlet looping → cleaning → pretreatment → primary (fine) coating → primary (fine) curing → cooling → outlet looping → shearing → winding.

Entrance section: includes equipment such as uncoiling, trimming, seaming and storage loops. Its function is to loosen the raw material coils and connect them so as to continuously and evenly supply metal sheets to the unit.

Pretreatment section: The coated substrate is cleaned and surface treated to improve corrosion resistance and adhesion to the upper paint film.

Painting section: It is the core part of the whole unit. Usually, the two-coating and two-baking process is adopted, i.e., primer coating (primary coating) → baking and curing → cooling → topcoat coating (finishing coating) → baking and curing → cooling. Depending on the product requirements, it can also be only one coating and one baking or only one side. When painting, reverse or forward roller coating can be used. The furnace temperature is controlled in sections, and the temperature of each section depends on the type of paint used, the thickness of the base plate and the time it passes through the oven.

Exit section: Divide the product into rolls (or cut into sheets according to the required size).

Summarize

The domestic color coating industry is facing a situation where low-cost production and high-tech products coexist, and the color coating product market is mixed. It is imperative to standardize industry standards, improve product quality, and develop high-tech products. It is most conducive to the development of the color coating industry if users and the market can deeply understand color plates, paint manufacturers can consciously abide by industry standards, and manufacturers can truly provide the most suitable products according to the needs of users and the market.