Research on optimization of annealing process for cold-rolled strip steel in bell-type furnace

Abstract : Through the analysis of the actual production situation of the heat treatment unit, the direction of annealing process optimization is determined, and the control process of heating and cooling is optimized and adjusted. On the basis of production practice, a large number of process optimization tests have been carried out, good results have been achieved, and a relatively complete annealing system has been established.
Key words : cold-rolled strip steel ; annealing ; process optimization

45 annealing furnaces in the cold rolling plant of the Iron and Steel Group Corporation , with a designed output of 820,000 tons. The output of the cold rolling plant has reached 1.3 million tons. The heat treatment unit has become the bottleneck of cold rolling production. Increasing the production capacity of the unit has become a must. subject. The heat treatment process is an important process in the production process of Benxi Iron and Steel's cold rolling plant. It mainly performs recrystallization annealing on the cold-rolled steel coils through the bell annealing furnace to ensure the quality requirements of the cold-rolled products. Recrystallization bright annealing is carried out on the cold-rolled strip steel, and the annealing process is all automatically controlled by computer, with a high degree of automation. Its advantage is that annealing in pure hydrogen state can ensure that the steel coil will not be oxidized, and greatly shorten the annealing cycle of the furnace table. The annealing of the full hydrogen bell furnace can also improve the surface quality of the cold-rolled plate and remove the carbon deposit on the surface of the steel plate. , to achieve bright annealing. The current output of Benxi Iron and Steel Cold Rolling Plant has far exceeded the designed output, so it is necessary to increase the production capacity of the annealing process, the corresponding annealing process must be adjusted, the heating and cooling process specifications should be optimized, the annealing cycle of the hearth can be shortened, and the production capacity can be increased. .
1 Analysis of annealing process
1 1 Basic principle of bell furnace annealing
The purpose of recrystallization annealing is to eliminate the work hardening produced in the rolling process of strip steel, reduce the strength and hardness of strip steel, and improve plasticity and toughness. The bell annealing process is mainly composed of three parts: heating, heat preservation and cooling. During the heating and heat preservation process of cold-rolled steel strip, three processes of recovery, recrystallization and grain growth will successively occur. The temperature of steel coils in different positions in the bell furnace and different parts of the same steel coil are different. However, from the perspective of the overall distribution of mechanical properties after annealing, it is relatively uniform, so the direction of process optimization should be how to complete the annealing process in the shortest time and ensure product quality.
1 2 Process characteristics of bell annealing furnace
A reasonable annealing process should be set separately for different steel types, different thicknesses, different widths, and different furnace stacking quantities. At present, the products of Benxi Iron and Steel's cold rolling plant are mainly positioned at the CQ and DQ grades of ordinary cold products, and the steel types are relatively single . , the work of optimizing the annealing process must be carried out from the specifications and furnace load.
The steel coils in different stacks and different parts of the same steel coil in the bell furnace have different temperatures. This is the inherent characteristic of bell annealing. During the annealing process, the steel coil has a temperature difference between the cold spot and the hot spot, which is reasonable Controlling the temperature difference to optimize the holding time is also a method to shorten the furnace annealing cycle from the perspective of technology.
2 Annealing process optimization
2.1 Annealing process with small furnace load
it is not necessary to use controlled heating for furnaces with a small furnace capacity (generally less than 90 tons ) . In this mode, the full-speed heating mode can be fully adopted, so that the heating capacity of the unit can be maximized, the heating cycle can be effectively reduced, and the production capacity of the unit can be increased. Since the annealing process needs to volatilize all the residual emulsion on the surface of the strip steel to improve the surface finish, it is considered to ensure that the emulsion on the surface of the strip steel is fully volatilized as much as possible . Therefore, it is necessary to blow hydrogen at a large flow rate at the optimum volatilization temperature of the emulsion. The volatilization curves of rolling oils from different manufacturers are different. Generally, the optimum volatilization temperature of the emulsion is generally less than 5O0C, so it should be set larger in the full-speed heating section. The hydrogen flow rate of the flow rate strengthens the purging of the strip surface and promotes the volatilization of the emulsion. According to the test data , the heating period is generally shortened , the annealing period of four batches of cold - rolled coils of dc 01 steel with two hearths of different specifications is shortened by 120 , 52 , 33 and 33 percent in the optimized full - speed heating mode After 180 minutes, the tensile strength and elongation tests show that the mechanical properties of the product are generally good and fully meet the technical requirements. It shows that the full-speed heating mode is suitable for the furnace base with a small furnace capacity, which does not affect the product quality and directly improves the utilization rate of the furnace base.
2 . 2 Annealing process with large furnace capacity
In actual production practice, in order to ensure the stability of product performance, the process of controlling the temperature rise rate of the heating section with a large amount of furnace is fully considered. After the process is annealed, the performance is stable, the tensile strength tends to the lower limit of the process, and the elongation is also very high. Therefore, There is a possibility to reduce the holding time, and optimize the holding time on the premise of ensuring the cold spot temperature of the steel coil. When the temperature difference between the cold and hot spots of the steel coil is the shortest holding time can be shortened to h. When the temperature difference between the cold and hot spots of the steel coil is A T , the shortest holding time can be shortened to h hours. When the temperature difference between the cold and hot spots of the steel coil is A T3 , the shortest holding time can be shortened to h hours. △ T 〉 △ T2 〉 △ T3 , h 〉 h 〉 h . Through the study of the relationship between the temperature difference between the hot and cold spots of the steel coil and the mechanical properties, the holding time was optimized, and the annealing was completed with a shorter holding time, reducing the The heating cycle, the mechanical properties fully meet the technical requirements, and the production capacity is improved. Because the holding temperature and holding time have a great influence on the bonding defects. Too high holding temperature and too long holding time are not conducive to the control of bonding. Reduce heat preservation The time is equivalent to reducing the holding temperature, thereby reducing the probability of bonding defects. Table 1 is part of the test data. It can be seen from Table 1 that the effect of the process optimization test is very good.

   

---