Introduction To Silicon Steel Coating Process (2)

GNEE Cold Rolled Silicon Steel

Compared with hot-rolled silicon steel sheets, cold-rolled silicon steel sheets have the obvious feature of being coated with a layer of coating on the surface during the production process. This coating not only has high insulation properties, but also has good corrosion resistance, processability and heat resistance. Coatings on the surface of oriented steel can improve magnetic properties. The interlayer insulation of the finished hot-rolled silicon steel sheet is mainly immersed in varnish, pickled and dried. Let me introduce to you the coating process of silicon steel.

The role of magnesium oxide coating

Oriented steel (including high magnetic induction oriented steel) is decarburized (or non-decarburized or nitrided) annealed and then cold rolled to the finished thickness. 2FeO· forms a SiO2 oxide film on the surface of the strip. After high-temperature annealing, a magnesium silicate bottom layer is finally formed. , in order to prevent the steel strips from adhering to each other during high-temperature annealing, an MgO isolation coating is also required on the surface of the steel strips.

The main process of magnesium oxide coating

Tap water is fed into the pure water preparation device. After preliminary filtration by the sand filter, the activated carbon removes organic matter. After the fine filter further removes impurities, it enters the cation and anion exchange tower to initially remove anions and cations in the water, and then enters the mixed ion exchange tower. Remove negative ions and positive ions from the water, turn the water into pure water, and send it to the 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 fluid, a chiller is installed in the system to control the temperature of the coating fluid 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 pumped to the water treatment station after settling in the pit; the precipitated mud is sent to the slag yard or is treated by a mud pump.

Main components and requirements of magnesium oxide coating liquid

(1) Main ingredients

(1) MgO: an alkaline oxide with a high melting point, divided into light and heavy. According to the requirements of the production process of oriented silicon steel sheets, special magnesium oxide between light and heavy is specially manufactured.

(2) TiO2: a white chemical raw material, divided into rutile type and aragonite type. Because the wencai stone type transforms into the rutile type at high temperatures and releases atomic oxygen, which has good dispersion properties in water, our factory uses wencai ore for magnesia barrier coatings.

(3) Other additives.

(2) Requirements for coating fluid

(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, good suspension must be ensured during continuous stirring;

(3) The slurry viscosity is controlled at 10.5 to 11 seconds. If it is too thin, it will affect the coating amount; if it is too thick, streaks will easily appear;

(4) Magnesium oxide is stored for a long time, and patterns or stripes are easily produced when the slurry is prepared and coated. In order to improve the uniformity of the coating, nylon filaments can be pulled on the coating roller;

(5) In order to reduce the usage 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 process requirements of moisture content;

(6) In principle, when the water content exceeds 4% or the liquid temperature exceeds 12°C, the slurry should be discarded. Generally, the storage time of slurry should not exceed 8 hours in summer and 12 hours in winter.

Main reasons affecting the quality of magnesium oxide coating liquid

(1) Effect of temperature

Generally it should be controlled below 5°C.

Due to the influence of temperature on the coating liquid, the coating system has an independent refrigeration system, which uses water as the medium and uses a circulation pump to pass cold water into each cooling point (including the circulation tank and storage tank). There are serpentine tubes distributed in the tank. Cold water is continuously circulated in the tube to maintain the temperature of the coating liquid at <20°C, thus ensuring the freshness of the coating liquid.

(2) Influence of time

When the moisture content exceeds 4% or the liquid temperature exceeds 12°C, the slurry should be discarded in principle, and the storage time of the slurry should not exceed 8 hours in summer. It should not exceed 12 hours in winter. If the liquid preparation time is too early or the slurry is stored for too long due to unit failure, the water content of the coating liquid will increase, stains or streaks will appear on the coating, and the heat preservation time of high-temperature annealing may also be extended. Influence the formation of magnesium silicate bottom layer. Due to the influence of time on the coating liquid, the coating liquid should be used up as soon as possible during the production process, and then fresh coating liquid should be added to prevent the coating liquid from deteriorating and causing poor coating. When the unit encounters a fault and needs to be shut down for a long time for maintenance, the coating fluid in the circulating tank and storage tank should be emptied in time and flushed with water to prevent the coating fluid from deteriorating and clumping to block the pump and pipelines.

Factors affecting the weight of coating per unit area of steel strip

(1) Unit speed

Since the coating roller is a following roller, the weight of the coating per unit area of the steel strip is proportional to the unit speed. The higher the speed, the thicker the coating weight per unit area of the steel strip.

(2) Reduce the usage of coating rollers

The weight of the coating per unit area of the steel strip is inversely proportional to the reduction of the coating roller. The greater the reduction, the thinner the weight of the coating per unit area of the steel strip;

(3) Roller type of coating roller

The coating roller is made of acid-resistant rubber. Roll shape factors include the hardness, diameter, angle and depth of the rubber roller grooves, as well as the number of teeth per unit length. The hardness is high, the diameter is small, the angle is large, the groove depth is large, the greater the number, the thicker the coating.

The principle of equipped with coating roller:

(1) Try to choose the same model of coating roller to adapt to the shape factor of the roller.

(2) Due to the effect of gravity, the coating amount on the lower surface is relatively thin. Therefore, a coating roller with high hardness, small diameter, and deep grooves should be used as the lower roller.

(3) When replacing the coating roller, it must be replaced in pairs.

Adjust the spray angle and spray volume of the coating fluid

The coating liquid is sprayed downward from direction A onto the steel plate. The coating liquid on the lower steel strip is half-immersed in the lower roller through the coating liquid level in the lower tank. The coating liquid is applied to the lower surface of the steel plate through the rotation of the coating roller. 

During the coating process of the coating liquid, bubbles are generated by splashing of the coating liquid. Once bubbles are formed on the surface of the steel plate, they will explode in the DF furnace, forming a round surface lacking coating, seriously affecting the coating quality. Therefore, the process requirement is to attach two fishing lines to the surface of the coating roller along the axial direction of the coating roller. Practice has proven that this measure can effectively eliminate foam in the coating fluid.