The soul of electricity—cold-rolled oriented silicon steel

Silicon steel is a silicon alloy steel with a silicon content of 1.0 to 4.5% and a very low carbon content. It has the characteristics of high magnetic permeability, low coercivity, and high resistivity. It is used in transformers, motors, and generators. Among them, the transformer core requires directional magnetization, and oriented silicon steel with a silicon content of about 3% is used; non-oriented silicon steel with isotropic magnetic properties (low silicon content) is used in motors and generators that perform rotating motions. .

Whether it is oriented silicon steel or non-oriented silicon steel, high-performance silicon steel sheets require thin thickness. This is because the current in the coil will induce eddy currents in the silicon steel sheet, causing power loss. One of the important means to reduce eddy current losses is to reduce the thickness of silicon steel sheets, thereby limiting the occurrence of eddy currents. At present, the world's thinnest oriented silicon steel is only 0.18 mm thick and was created by China Baowu Steel Group Co., Ltd., a leading steel company in my country. Such thin steel plates can only be produced by cold rolling.

The raw material for cold rolling is hot-rolled steel coils with a thickness of several millimeters obtained by hot rolling. The heating temperature before hot rolling of ordinary steel is about 1200°C, and the final rolling temperature (the temperature when leaving the last rolling mill) is 800-850°C; but the heating temperature before hot rolling of oriented silicon steel should be increased to about 1400°C, and the temperature entering the rolling mill About 1200℃, the final rolling temperature is controlled above 950℃, and then quickly sprayed with water to cool to below 550℃ and coiled into steel coils. High-temperature heating is to completely dissolve the second phase particles of MnS and AlN in the steel billet, precipitate uniform and fine (tens of nanometers) MnS particles during the hot rolling process, and then rapidly cool to inhibit the precipitation of AlN.

The important links that follow are all centered around MnS and AlN, which "turn decay into magic". S is the original impurity element in steel, Mn is the original element or the additional element added in steelmaking, and Al and N are the elements in steelmaking. For additional elements added, another process is to increase the N content by nitriding during the continuous annealing process.

After the hot-rolled silicon steel coils as raw materials are sent to the silicon steel plant, they are re-unrolled and welded into continuous steel strips. The first step is normalization treatment, which is heated to about 1100~900°C to precipitate during the coiling process after hot rolling. The AlN is completely solid solution; then cooled to 550°C within a few seconds (the picture above shows a test of the aerosol cooling technology developed by MCC Southern Thermal Co., Ltd., which relies on compressed air to atomize cooling water into tiny droplets to achieve silicon steel High-speed, uniform cooling) to re-precipitate small, uniform and stable AlN particles.

The normalized steel strip is pickled to remove the oxide film on the surface and cold rolled to the specified thickness (the picture above shows the 1.7-meter cold rolling unit of Wuhan Iron and Steel Co., Ltd.).

When I introduced the 1.7-meter rolling mill project before, I mentioned that high-end cold-rolled steel plates must undergo recrystallization annealing to improve the plasticity of the cold-rolled plates, thereby facilitating subsequent processing and use. This process is mainly performed in a continuous annealing furnace. ongoing. Cold-rolled grain-oriented silicon steel also requires continuous annealing, but for a different purpose.

The rolling process of silicon steel requires a certain amount of carbon, but carbon is detrimental to the magnetic properties of silicon steel. Silicon steel is decarburized at around 850°C in the absence of oxygen. The decarburizing agent used is water. At this temperature, the cementite (Fe3C) in the steel decomposes, and the solid solution carbon atoms diffuse to the surface of the steel strip and react with water vapor (C + H2O → CO + H2), reducing the carbon content of the silicon steel to 30ppm the following. During hot rolling, carbon element can promote the formation of austenite, while silicon element can stabilize ferrite. For example, a carbon content of 0.07% causes 10% to 25% austenite to appear in silicon steel at the hot rolling temperature, preventing the growth of ferrite grains, thus making the grains obtained by hot rolling more uniform and smaller. However, the introduction of carbon will reduce the magnetic permeability of silicon steel and increase hysteresis loss. It will also form austenite during the recrystallization annealing process and hinder the recrystallization process of ferrite. Therefore, it must be fully decarburized after rolling. .

The deformed grains in cold-rolled silicon steel also recrystallize during the annealing process. However, since MnS and AlN are distributed at the grain boundaries, they strongly inhibit the growth of recrystallized grains. Therefore, a large number of tens of microns are obtained by continuous annealing. size of small grains.

The continuous silicon steel strip passes through the 300-meter-long continuous annealing furnace, undergoes the annealing and cooling process, and is coated with magnesium oxide coating (a magnesium silicate glass coating is formed on the surface of the silicon steel to prevent the silicon steel from adhering during high-temperature annealing, improving the Silicon steel surface resistance), re-cut and coiled into steel coils. These steel coils are placed on the annular annealing furnace, covered with an inner cover (which is filled with nitrogen, hydrogen and other protective gases to prevent the steel from being oxidized), and wait for the final transformation.

The trolley carries the inner cover and steel coils into the annular annealing furnace, passes through different temperature zones in the furnace, and slowly rotates once to complete high-temperature annealing.

At the high temperature annealing temperature of 1250°C, silicon steel undergoes secondary recrystallization.

At such a high temperature, MnS and AlN precipitates distributed at the grain boundaries of grains with {110}<001> orientation (the {110} plane is parallel to the surface of the steel strip, and the <100> direction is along the rolling direction) are prone to occur. When the particles grow up or form a solid solution, the effect of inhibiting grain growth is lost, but a strong inhibitory effect is still maintained on the grain boundaries of other oriented grains. Therefore, the {110}<001> oriented grains "eat" the surrounding grains with different orientations and grow abnormally. The {110}<001> orientation is precisely the orientation with the highest magnetic properties of silicon steel. It was first developed by American Goss, so it is called Goss orientation.

After several days of high-temperature annealing, the Gaussian-oriented grains dominate the silicon steel and grow in size from the initial 20 microns to 30-50 mm. Mottled patterns can be seen from the surface of the removed silicon steel sheet (picture below), where each pattern is a huge grain with almost the same orientation. Silicon steel with a Gaussian orientation greater than 95% has the best performance and is called high magnetic induction oriented silicon steel. It is a high-end product among oriented silicon steels.

Finally, after processes such as hot stretching annealing (to eliminate deformation caused by high-temperature annealing) and insulating coating, the cold-rolled oriented silicon steel coil products are tightly packaged and sent to downstream manufacturers.

The production process of high-magnetic cold-rolled oriented silicon steel is interlocking and very difficult. It is a high-end symbol of the steel industry. No wonder it is called an "artwork" in the steel industry.