Silicon steel coating process introduction (2)

27 Nov 2023
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Compared with hot-rolled silicon steel sheets, cold-rolled silicon steel sheets have an obvious feature that a layer of coating is applied on the surface during the production process. This layer of coating not only has high insulation performance, but also has good corrosion resistance, processing performance and heat resistance. The coating on the surface of oriented steel can improve the magnetic properties. The interlayer insulation of the finished hot-rolled silicon steel sheet is mainly completed by dipping in varnish and drying after pickling. Let me tell you about the coating process of silicon steel.
The role of magnesium oxide coating
After decarburization (or non-decarburization or nitriding) annealing of oriented steel (including high magnetic induction oriented steel) cold-rolled to the finished thickness, a 2FeO·SiO2 oxide film is formed on the surface of the strip steel, and a magnesium silicate bottom layer is finally formed after high-temperature annealing , and to prevent the steel strips from sticking to each other during high temperature annealing, it is also necessary to carry out MgO isolation coating on the surface of the strip steel.
oriented silicon steel
The main process of magnesium oxide coating
The tap water is sent to the pure water preparation device, and after the initial filtration of the sand filter, the removal of organic matter by the activated carbon, and the further removal of impurities by the fine filter, it enters the cation and anion exchange columns to initially remove the anions and cations in the water, and then enters the mixed ion exchange column The negative and positive ions in the water are further removed, and the water is made into pure water, which is sent to a pure water storage tank for storage. Send the raw materials and pure water into the coating liquid preparation tank, stir evenly and then flow into the coating liquid circulation tank. During normal operation, the circulating coating liquid is supplied to the coating machine by the coating liquid circulating pump.
In order to ensure the quality of the coating liquid, a chiller is installed in the system, and the temperature of the coating liquid is controlled below a certain temperature through the serpentine cooling pipes installed in each tank. The waste coating liquid is discharged into the waste liquid pit, and the supernatant liquid is sent to the water treatment station by pump after sedimentation in the pit; the settled mud is sent to the slag yard or water treatment by mud pump.
The main components and requirements of magnesium oxide coating solution
⑴ Main ingredients
① MgO: A basic oxide with a high melting point, which is divided into light and heavy. According to the requirements of the production process of oriented silicon steel sheet, special magnesium oxide between light and heavy is specially manufactured.
② TiO2: A white chemical raw material, which is divided into rutile type and arasite type. Because the arasite type is converted into rutile type at high temperature and releases atomic oxygen, and has good dispersion performance in water, our factory uses arasite Mine is used in magnesia barrier coatings.
③ Other additives.
⑵ Requirements for coating liquid
① When the water temperature is lower than 5°C, add MgO half an hour before use;
② During the preparation, storage and use of magnesium oxide slurry, it must be in the process of continuous stirring to ensure good suspension;
③ The viscosity of the slurry is controlled at 10.5 to 11 seconds. If it is too thin, it will affect the coating amount, and if it is too thick, streaks will easily appear;
④ Magnesium oxide has been stored for a long time, and when the slurry is prepared and coated, it is easy to produce patterns or stripes. In order to improve the uniformity of the coating, a nylon filament can be drawn on the coating roller;
⑤ In order to reduce the use temperature of the coating liquid, two coating liquid circulation tanks can be connected in series to fully cool the coating liquid, lower the liquid temperature, reduce hydration, and meet the technological requirements of moisture content;
⑥ When the water content exceeds 4% or the liquid temperature exceeds 12°C, the slurry should be discarded in principle. Generally, the slurry should be stored for no more than 8 hours in summer and no more than 12 hours in winter.
non oriented silicon steel

 
Main reasons affecting the quality of magnesium oxide coating solution
① Influence of temperature
Generally, it should be controlled below 5°C.
Due to the influence of temperature on the coating liquid, the coating system has a separate refrigeration system, using water as the medium, and using a circulation pump to pass cold water into each cooling point (including the circulation tank and storage tank), and there are serpentine pipes distributed in the tank , cold water circulates continuously in the tube to keep the temperature of the coating solution at <20°C, so as to ensure the freshness of the coating solution
② Influence of time
When the water content exceeds 4% or the liquid temperature exceeds 12°C, the slurry should be discarded in principle, and the storage time of the slurry in summer should not exceed 8 hours. It should not exceed 12 hours in winter. If the liquid preparation time is too early or the storage time of the slurry is too long due to the failure of the unit, the moisture content of the coating liquid will increase, and the coating will produce stains or streaks, and it may also prolong the holding time of high temperature annealing. Affects the formation of the bottom layer of magnesium silicate. Due to the influence of time on the coating liquid, the coating liquid should be used up as soon as possible during production, and then fresh coating liquid should be added to prevent the coating liquid from deteriorating and causing poor coating. When the unit encounters failures and the coating system needs to be stopped for a long time for maintenance, the coating liquid in the circulation tank and storage tank should be emptied in time and rinsed with water to prevent the coating liquid from deteriorating and agglomerating to block the pump and pipeline.

Factors Affecting Coating Weight per Unit Area of Steel Strip
⑴ Unit speed
Since the coating roller is a follower roller, the weight of the coated coating per unit area of the steel strip is proportional to the speed of the unit. The higher the speed, the thicker the weight of the coated coating per unit area of the steel strip.
⑵ Reduction amount of coating roller
The weight of the coated coating on the steel strip per unit area is inversely proportional to the reduction of the coating roller. The greater the reduction, the thinner the weight of the coating on the steel strip per unit area;
⑶ Roller type of coating roller
The coating roller is made of acid-resistant rubber. The roller shape factors include the hardness, diameter, angle and depth of the groove of the rubber roller, and the number of teeth per unit length. The hardness is large, the diameter is small, the angle is large, the groove depth, The more the number, the thicker the amount of coating.
grain oriented silicon steel


The principle of equipped with coating rollers:
① Try to choose the same type of coating roller for the roller shape factor.
② Due to gravity, the amount of coating on the lower surface is relatively thin. Therefore, the coating roller with high hardness, small diameter and deep groove should be used as the lower roller.
③ When replacing the coating rollers, they must be replaced in pairs.
Adjustment of spray angle and spray volume of coating liquid
The coating liquid is sprayed down from the direction of A to the steel plate, and the coating liquid of the lower steel strip is half submerged in the lower roller by the liquid level of the coating liquid in the lower tank, and the coating liquid is applied to the lower surface of the steel plate by the rotation of the coating roller.
non grain oriented silicon steel
During the coating process of the coating liquid, the splash of the coating liquid will generate bubbles. Once the bubbles are formed on the surface of the steel plate, the bubbles will explode in the DF furnace, and a circular missing coating surface will be formed, which will seriously affect the coating quality. Therefore, the process requirements are in On the coating roller, two fishing lines are attached to the surface of the coating roller along the axial direction of the coating roller. Practice has proved that this measure can effectively eliminate the foam of the coating liquid.




 
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