1. What is tinned steel sheet
Tin plate (hereinafter referred to as tin plate or tinplate) due to its substrate has suitable strength, excellent weldability and stamping properties, and the surface coating has good corrosion resistance and is easy to coat and print. Therefore, it is widely used Food packaging is also used in the packaging of various forms of articles such as sprays, chemicals, paints, and the manufacture of various appliances.
The electroplated tin plate developed on the basis of hot-dip tin plate has a production history of more than 70 years. The tin-plating production lines and total output in the world are basically stable, but developing countries have built or are building some tin-plating units one after another. China’s development is particularly obvious. In terms of varieties, secondary cold-rolled tin plate and plated plate (TFS) have been greatly developed. Japan’s coated iron (TFS surface is covered with a plastic film) has developed rapidly in recent years. The production and consumption of tinplate in developing countries is increasing rapidly, and developed countries are characterized by the development of new cans, exquisite printing, and environmental protection.
The development of tinplate in China started late, and it has mainly relied on imports for a long time, with a self-sufficiency rate of less than 50%. In the 1990s, Japan, South Korea, Taiwan and Hong Kong successively built factories in the mainland, which greatly increased the production capacity of tin plate. In particular, the two tin-plating production lines of Baosteel put into production in 1998, which increased the self-sufficiency rate of China’s tin-plated sheet to about 70%. Moreover, high-difficulty products such as K-plate and DI material were also provided domestically, which changed the K-plate required by our country. (The state of plain iron is applied to the high corrosion resistance tin plate of fruit cans> The long-term dependence on imports, the production of DI material (two-piece tin plate for beverage cans) has made Baosteel one of the few iron and steel plants in the world that can produce this product. Ranks.
Metal packaging has experienced more than 20 years of development in China’s reform and opening up, and now it has formed a complete metal including printing iron, cans (two-piece cans, three-piece cans), aerosol cans, various bottle caps, steel drums, etc. The packaging industry system has become one of the important categories of China’s packaging industry.
With the rapid development of China’s economy, the total consumption of tin plate required for metal packaging is increasing at a rate of about 10% every year. It can be expected that the demand for tin plate in China will continue to develop at a relatively high speed in the next few years.
Second, the structure of tinplate
The tinplate consists of five layers, and its cross-sectional structure and thickness are shown in Figure 2-1.
Third, the performance of tin plate
1. Material characteristics
(1) The degree of quenching and tempering (expressed by the surface Rockwell hardness HR30T) The degree of quenching and tempering represents the mechanical properties of the material. Under the same manufacturing conditions, it better represents the material performance, but in the annealing process (continuous annealing or hood type) When annealing) is different, the same quenching and tempering degree shows different strength and elongation. The thickness of the substrate is different, and there is a certain difference between the measured value and the actual value due to the anvil effect. Due to aging reasons, the degree of quenching and tempering will also change.
(2) Formability Except that the material has to withstand severe deformation during drawing and other processing, general can-making and lid-making processing can withstand tin plate. Uniform performance is required, and the grain size of the material is suitable (the tin plate is generally 7# ~ 9#). With the acceleration of the can-making speed, it is required that the material performance fluctuation is small, the thickness fluctuation is small, and the plate shape is good, otherwise it is easy to cause defects such as can jam. In order to prevent ridges in can making and tensile strain during punching, the tin plate is required to have a small strain aging effect.
(3) Directionality In addition to DI materials which have strict requirements for anisotropy, in general can-making processing, the direction requirements are not too strict. For the secondary cold-rolled tin plate, the longitudinal and transverse properties are quite different (transverse σsσb). Large, low elongation), the circumferential direction of the can body is required to be consistent with the rolling direction. When the primary cold-rolled tin-plated sheet has expanded-diameter and square-shaped tensile deformation, it is better to make the circumferential direction of the can body consistent with the rolling direction, which can reduce the occurrence of defects such as cracking and local deformation.
2. Size and shape
Thickness: The thickness tolerance is required to be small, the standard is ±8.5%, the actual requirement is much lower.
Width and length: The tolerance is required to be small, the standard is -O/+3mm, the actual requirement is much smaller.
Flatness: Large warpage, medium waves, and edge waves will affect iron printing and can making, and it is required to be within 3mm.
Pinhole: In the tin plate production process, it is detected by a pinhole instrument. The leak detection device on the canning line is also very important to prevent leaking cans from leaving the factory.
3. Surface characteristics
(1) Surface roughness The surface roughness of the substrate is obtained when it is leveled. In addition to the DI material drawing process, the roughness has an important influence, and the roughness can improve the adhesion of the tin layer and the paint layer; increase Roughness, not easy to scratch or abrasion.
(2) The amount of tin plating The thicker the tin plating layer, the fewer the pores and the higher the corrosion resistance. It is necessary to study how to obtain a tin plating layer with good continuity and high corrosion resistance with a relatively small amount of tin plating. In addition to requiring a clean surface of the original plate, the electroplating process must be controlled, and the process parameters must be stable to reduce fluctuations.
(3) The tin-iron alloy layer is reflowed to obtain the required tin-iron alloy layer, which improves the adhesion of the tin-plated layer and the corrosion resistance of the tin-plated plate. The K plate requires a higher alloy layer; the box layer is hard and hard. Brittle, too thick will reduce the welding performance, but also easy to fall off. It is necessary to control the continuity of the alloy layer and the compactness of the arrangement to produce a tin plate with high corrosion resistance.
The key of the reflow process is to complete the heating according to the desired reflow heating curve. The combination of resistance heating and induction heating provides good conditions for the control of reflow.
(4) Passivation layer Obtained by chromate passivation treatment, it can effectively inhibit the oxidation of tin, improve the coating and corrosion resistance of the tin plate, and can effectively prevent the occurrence of vulcanization spots.
According to the use and different contents, different passivation layers are required, such as food cans such as oranges, cherries, pineapples, pears and peaches and 18L bamboo shoots cans, which require high power for passivation.
(5) Coat a layer of DOS oil on the oiled surface, which acts as a lubricant to prevent scratches, abrasions, abrasions, and rust prevention. But when the oil is applied too much or unevenly, it is easy to cause “eyeholes” defects in the coating film.
4. Corrosion resistance
(1) Corrosion resistance of canned plain noodles For canned food such as oranges and pineapples, plain iron is used to maintain the flavor and color of the food in the can. Most of these foods are acidic and require high corrosion resistance. The main factors related to corrosiveness are tin plate materials for can-making, types of contents, canning conditions, storage conditions, etc.
From the perspective of tin plate materials, in an acidic environment without oxygen, tin is first corroded and dissolved to protect iron. In order to improve the corrosion resistance of tin plate, strict production control is required, and high corrosion resistance plate (ie K plate) is provided to users.
(2) Corrosion resistance of painted cans In order to reduce costs and save precious tin resources, people no longer increase the corrosion resistance by adding tin coatings, but paint with lower tin coatings to improve cans. Long life; painting can also prevent sulfur-containing protein foods from sulfide blackening; flavors such as beverages, beer, etc. are sensitive to the presence of small amounts of dissolved metals, so paint cans are used.
It is necessary to ensure that the coating film is complete and free of defects. Cans no longer rely on the dissolution of tin to prevent corrosion of the substrate, and the paint layer has become the first line of defense. Someone has done a test: the shelf life of cans with defective coating films is much shorter than that of unpainted cans. Therefore, it is necessary to prevent scratches on the coating film, damage to the flanging part, or peeling of the coating film generated during the stamping of the can lid. . In order to ensure the quality of the coating film, paint is sometimes applied multiple times; for strong acid beverages, internal spraying must be carried out after the can is made to prevent rapid corrosion or even leakage of the can.
Most of the tin plate needs to be coated, and the quality of the coating will directly affect the corrosion resistance of the metal can; some also need to be color printed. Finishing properties include wettability and adhesion.
(1) Wettability: The paint does not wet or has eye holes, and it concentrates on corrosion at the defective part. Compared with unpainted, it is more prone to pitting corrosion. There are many factors that affect wettability, including paint, finishing conditions and tinplate.
The main reasons caused by the tin plate are as follows:
① Too much oil or uneven oil.
② Foreign matter adheres to the surface. Machine oil, silicone resin, fine powder, moisture, etc.
③Surface condition, aging, storage temperature, etc.
(2) Adhesion When the adhesion of the coating film is not good, cracks, peeling or peeling will occur during processing and deformation.
Adhesion is also related to paint and painting process. The factors related to tin plate are as follows:
① Through passivation treatment, the oxidation of tin is inhibited and the adhesion of paint is improved. However, the storage time and the number of repeated heating and baking affect the adhesion.
②The influence of surface roughness.
③The influence of tin layer thickness.
The three-piece can body and various barrels must be welded. Therefore, the quality of the weld is very important. It is required to adjust the welding parameters reasonably to obtain a smooth, dense and uniform weld to prevent concentrated corrosion and even leakage.
The base plate of tin plate is mostly low-carbon aluminum killed steel, and the welding performance is good. With the acceleration of can-making speed, stable welding performance is required. The relevant performance of tin plate must be guaranteed, mainly as follows:
①The steel is pure, with small thickness deviation and uniform hardness.
②The surface is clean, free of dust and garbage.
③The surface roughness is appropriate.
④The lower the tin-plated layer, the better the soldering performance. When the tin-plated layer is low, control the tin-iron alloy layer to ensure a certain pure tin layer.
⑤The welding parameters of tin plate with IF steel as the base plate should be controlled carefully.