Annealing process of bell type annealing furnace

The annealing process of cold rolled strip is mainly determined by the chemical composition of the steel, the technical standard of the product, the size of the strip and the coil weight. The annealing furnace process system must ensure that there is no bond between rolls and no oxidation on the surface.

A. The determination of heating speed is mainly determined by the thermal conductivity of steel.

Carbon content and alloy content in steel have great influence on heat conduction. If their content is high, the thermal conductivity is small, and the heating speed should be slow, so as to avoid the uneven structure and performance caused by the large temperature difference between the inside and outside. From room temperature to 400 ℃, the heating speed is generally unlimited. According to the principle of the crystallization process, there is no significant change in the internal structure of the strip from room temperature to 400 ℃, the elongated grains have just been recovered during the rolling process, and no recrystallization has been formed, so the faster the heating speed of the steel in this region, the better.

The heating speed has a great influence on the performance and surface quality of the strip from 400 ℃ to the holding temperature. Generally, the heating speed is 30-50 ℃ / h. The period from 400 ℃ to 723 ℃ is the recrystallization stage, so the heating speed must be controlled in this temperature range.

The heat transfer in the inner cover of the full hydrogen annealing furnace is mainly convection heat transfer. The ratio of the heat flow density of convection heat transfer and radiation heat transfer in the full hydrogen annealing furnace.

At the beginning and near the end of the whole annealing process, due to the low temperature of steel coil and inner cover, the radiation heat transfer condition is very poor, the heat flow density of convection heat transfer in the furnace can reach more than 250 times of the radiation heat transfer density, and the above ratio also remains more than 20 times in the soaking stage. Therefore, the convection heat transfer plays a decisive role in the heat transfer in the furnace.

B. The recrystallization temperature of steel is not a fixed temperature, which is related to the internal structure of strip steel. The recrystallization temperature in actual production is selected according to the product in the range of 570-720 ℃. The heat preservation temperature and time are mainly determined according to the product standards, technical conditions, steel grades and strip thickness. The heat preservation time and temperature are also related to the coil weight and strip thickness. If the coil is heavy and the steel plate is thick, the heat preservation temperature is high and the heat preservation time is long. For the steel and thin specification strip which are easy to produce interlayer bonding defects, the heat preservation temperature can be lower and the heat preservation time can be shorter.

C. Bright annealing must be carried out to make the strip steel free of decarburization and oxidation.

The key problem to prevent oxidation of annealed steel coil in annealing furnace is to make the pressure in the protective cover meet the process requirements. In addition, we should do a good job in cold and hot blowing. The purpose of cold and hot blowing is to use the protective gas to drive away the air in the inner cover and the oil and gas moisture brought in by the steel coil. The function of hot blowing is to further drive out the residual gas in the inner cover, and more importantly, to remove and blow away all the harmful substances such as oil smoke and water vapor produced by the emulsion brought by the coil, so as to avoid polluting the surface of the steel plate and reducing the surface quality of the steel plate.

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