Why does the cold rolled strip annealed in bell type annealing furnace bond? How to solve it? Suzhou bell type furnace manufacturer - Suzhou jinruier Electric Furnace Co., Ltd. Xiaobian shares with you why the cold rolled strip annealed in bell type annealing furnace is bonded? How to solve it?
At present, all hydrogen bell type annealing furnace is an ideal annealing equipment for cold rolled strip. After the cold rolled strip passes through the bell type annealing furnace, the hardening and internal stress caused by cold rolling deformation are eliminated, so that the strip has good mechanical properties and bright surface. However, after annealing in the bell type furnace, the cold rolled strip may appear local bonding when it is flattened and uncoiled, resulting in bulge deformation or tearing at the bonding place, thus reducing the yield of the strip. So what is the cause of local bond of strip steel? How to solve it?
In fact, in the process of strip rolling, the surface of strip steel not only adsorbs some emulsion and anti-corrosion oil, but also adsorbs a large number of iron particles produced by strip steel and roll extrusion wear. In the whole annealing process, the coil generally takes 24 to 36 hours, and the higher temperature of the coil can reach 690 ℃. The emulsion and anticorrosive oil will vaporize during the annealing heating process, and different gases will be formed after vaporization, including methane, acetylene, carbon dioxide, carbon monoxide and water vapor, etc. these gases and protective gas hydrogen will vaporize during the whole annealing process During the heating process, a series of chemical reactions will be formed at different temperatures. Through the analysis of chemical reaction in four temperature ranges during annealing process, it is found that the iron oxide will be reduced to iron in the temperature range of 420 to 620 ℃. Where the iron is regenerated and the density is high, under the action of compressive stress, the surface of the coil layer and the layer will be welded together to form a bond. The condition of bonding is the redox reaction of iron under suitable temperature and pressure. Therefore, by analyzing the changes of temperature and compressive stress in each stage of coil annealing, we can find a way to avoid bonding.
According to the analysis of the annealing process curve of the strip steel, the bonding is easy to occur in the primary stage of coil cooling, and the cleanliness of the strip steel surface and the purity of the shielding gas are also the key factors for the bonding. Therefore, the following methods can be selected to avoid the bonding:
1. To improve the annealing process, in the cooling stage of annealing process, the faster the cooling speed is, the greater the temperature difference between the inside and outside of the coil is, and the greater the compressive stress is formed, especially at the beginning of cooling. Therefore, it is easy to form bonding in the temperature range of 420 to 620 ℃. Therefore, when the strip temperature is higher than 420 ℃ at the beginning of cooling, the cooling speed should not be too fast. Use the heating cover to cool to 420 ℃, and then change the cooling cover for air cooling and water cooling.
2. Clean the surface of the strip steel to reduce the residual substances. The residual chemical reaction substances mainly come from the iron oxide powder, oil and residual emulsion on the surface of the strip steel. These substances will produce a series of chemical reactions in the bell type furnace, which will have oxidation-reduction reaction with iron, and then weld together under the action of compressive stress to form a bond. Therefore, it is necessary to improve the cleanliness of the strip surface, reasonably select the emulsion and set the appropriate injection pressure.
3. The use of high purity hydrogen, improve the efficiency of purging, improve the purity of shielding gas and thoroughly purge the inner cover can effectively reduce the oxidation and bonding of the strip surface.
