The bell-type annealing furnace for cold rolled strip steel coils is a batch production method. If you want to place multiple steel coils in the furnace, you need to place intermediate convection plates between the steel coils. So what kind of structure is the convection plate? Let's introduce to you today:

1. Types of convection plates for bell type annealing furnaces

The convection plate is divided into bottom convection plate, middle convection plate, and top convection plate.

1) Bottom convection plate of bell type annealing furnace

The bottom convection plate is installed on the diverter plate of the furnace platform and bears the entire mass of the steel coil stack, so it is relatively thick, with a guide strip on the top and a flat plate on the bottom.

2) Intermediate convection plate of bell type annealing furnace

The intermediate convection plate is located between the steel coils, with guide strips on both sides, and a flat plate sandwiched in the middle. Its main function is to heat or cool the shielding gas flow through the ends of the steel coils, which is an important means of heat exchange. Secondly, the two steel coils are isolated to prevent the end of the steel coils from sticking during the annealing process.

3) Top convection plate

The top convection plate is placed on the top of the steel coil stack, with a flat plate on top, a hole in the middle, and a guide strip underneath. Its main function is to prevent all protective gas flows from passing through the middle cavity of the steel coil stack, affecting furnace temperature uniformity and annealing ability. The top convection plate is designed in various bell type furnaces in China, but most of them are not used in production. The reason is that the diameter of the top convection plate is smaller than the outer diameter of the steel coil, and it is difficult and unsafe to cover or lift it onto the top of the steel coil stack. Currently, the design of the top convection plate has been gradually phased out.

The bell type annealing furnace production line, Cibao New Sheet Metal (Jiangsu) Co., Ltd. currently has 90 sets of high-temperature bell type annealing furnaces with an annual output of 60000 tons

The bell type annealing furnace is a cold rolled strip steel annealing process system that is mainly determined based on factors such as the chemical composition of the steel, product technical standards, strip size, and coil weight. The process system must ensure that there is no adhesion between the coils and no oxidation occurs on the surface during production.

Annealing Process of Bell Type Annealing Furnace

A. Determination of the heating speed of a bell type annealing furnace The heating speed of steel mainly depends on the magnitude of the thermal conductivity of the steel. The carbon content and alloy content in steel have a significant impact on heat conduction. "If their content is high, their thermal conductivity is low, and the heating speed should be appropriately slow to avoid excessive temperature differences between the inside and outside, resulting in uneven structure and performance.". From room temperature to 400 ℃, the heating speed is generally unlimited. According to the principle of crystallization process, there is no significant change in the internal structure of the strip steel from room temperature to 400 ℃. During the rolling process, the elongated grains have just recovered and have not yet formed recrystallization. Therefore, the faster the heating speed of the steel in this interval, the better. The strip steel is heated from 400 ℃ to the holding temperature, and the heating speed has a considerable impact on the performance and surface quality of the strip steel. Generally, the appropriate heating rate is 30 to 50 ℃/h. During the period when the strip steel is heated from 400 ℃ to the holding temperature below 723 ℃, it is the stage of recrystallization formation, so the heating rate must be controlled within this temperature range.

Annealing Process of Bell Type Annealing Furnace

B. Determination of Holding Temperature and Holding Time for Bell Type Annealing Furnace The recrystallization temperature of steel is not a fixed temperature, but is related to the internal microstructure of the strip steel. In actual production, the recrystallization temperature is selected based on the product in the range of 570-720 ℃. The heat preservation temperature and time are mainly determined based on product standards, technical conditions, and steel grade and strip thickness. The holding time and temperature are also related to the coil weight and the thickness of the strip steel. If the coil weight and the steel plate thickness are large, the holding temperature is high and the holding time is longer. For steel and thin gauge strip steels prone to interlayer bonding defects, the insulation temperature can be appropriately lower and the insulation time can be shorter.

Annealing Process of Bell Type Annealing Furnace

C. Bright annealing in a bell type annealing furnace must be carried out to ensure that the strip steel is free of decarburization and oxidation. The key issue in preventing oxidation of annealed steel coils is to ensure that the pressure inside the protective cover meets the process requirements. In addition, it is necessary to carefully do a good job of cold blowing and hot blowing. The purpose of cold blowing and hot blowing is to use protective gas to drive away the air in the inner cover and the oil, gas, and moisture brought in by the steel coil. The role of heat blowing is to further exhaust the residual gas in the inner cover, and more importantly, to remove and blow away all harmful substances such as oil smoke and water vapor generated by the emulsion brought by the plate roll, to avoid contaminating the surface of the steel plate and reducing the surface quality of the steel plate.