Causes and Control Measures of Surface Defects in the Heat Treatment Unit of Benxi Iron and Steel Cold Rolling Plant

Analysis of the causes of surface defects in a hood type annealing unit
Author Introduction: Han Ping (1981~), male, heat treatment engineer, graduated from Jilin University with a major in Metal Materials Engineering in 2004.
At present, the surface quality defects in the hood furnace area mainly include oxidation color and adhesion.
1.1 Analysis of Reasons for Oxidation Color Defects
Oxidation color defect: mainly refers to the presence of light yellow and light blue oxidation stripes on the surface of the steel coil after it is taken out of the furnace. Severe oxidation of the entire steel coil results in a deep blue color, and oxide adheres to the surface of the steel strip, seriously affecting its surface quality and mechanical properties (as shown in Figure 1).
After analyzing and studying a large number of oxidation color defects, several reasons for the occurrence of oxidation color defects have been identified:
(1) Oxidation caused by equipment malfunction
1) In order to fully ensure safety, the bell type furnace unit of this steel plant is programmed to generate emergency nitrogen purging in the furnace platform system when the pressure inside the furnace is below 2.5mbar. During production, due to frequent leaks in the furnace platform system, the pressure inside the hood is insufficient, causing emergency purging. The emergency purging replaces the hydrogen gas inside the furnace with nitrogen. Due to the high dew point and residual oxygen content of nitrogen, the temperature at which emergency purging occurs is generally high, which can easily lead to oxidation. After being discharged from the furnace, the surface of the steel coil turns blue and the oxidation is more severe.
2) In order to improve the heat exchange rate inside the furnace, the bell type furnace unit of this steel adopts a circulating fan to accelerate the circulation of protective gas inside the furnace and improve the heat exchange efficiency. Due to the failure of the circulating fan in the cooling section, it causes low speed and non rotation, resulting in uneven temperature distribution of the controlled atmosphere inside the furnace. The temperature of the steel coil near the upper part of the inner hood is higher than the actual displayed temperature, and oxidation is prone to occur when it is discharged from the furnace.
3) The bell type furnace unit of this steel adopts thermocouples and temperature transmitters to monitor the temperature changes inside the furnace. Generally, thermocouples are used to display the temperature when exiting the furnace. When thermocouples and temperature transmitters malfunction or have inaccurate readings, there may be a certain range of deviation between the actual atmosphere temperature inside the annealing furnace and the displayed temperature of the thermocouple. When the actual temperature is higher than the displayed temperature of the thermocouple, oxidation is prone to occur after exiting the furnace.
1.1.2 Oxidation caused by production processes and operations
1) During the furnace loading process, the alignment of the steel coils and the state of the steel coil core are often disrupted in actual production due to uneven stacking of steel coils or severe bending of the steel coil core. In the actual annealing process, the circulating fan cannot effectively function, and the protective atmosphere inside the furnace is severely affected, resulting in low heat exchange efficiency. The actual temperature of the steel coil deviates too much from the displayed temperature, and the actual atmosphere temperature at the time of discharge is significantly higher than the thermocouple display temperature, leading to oxidation.
2) Annealing and cooling cycle of furnace platform
The cooling cycle of the furnace table is too short, which will cause a large difference in the cooling rate of the hot and cold spots of the steel coil. The cooling rate of the hot spot is much higher than that of the cold spot, resulting in a large temperature difference between the hot and cold spots after the steel coil is taken out of the furnace, and the temperature of the cold spot is significantly higher than that of the hot spot, causing oxidation of the cold spot part of the steel coil. Under normal cooling cycle of the steel coil, although the cooling rate of the hot spot is greater than that of the cold spot, the temperature difference between the hot and cold spots of the steel coil is not significant when it is discharged from the furnace, and the probability of oxidation is very small.
3) Residual emulsion on the surface of cold-rolled strip steel
Due to the lack of degreasing treatment on the rolled steel coil, if the emulsion blowing effect of the rolling mill process is not good, a large amount of uncleaned emulsion will remain on the surface of the strip steel. During the annealing process, the steel coil cannot be blown clean by the emulsion after being loaded into the furnace, and the H20 contained in the emulsion will oxidize the steel coil.
4) The influence of the weight of the first steel coil loaded onto the furnace platform
If the weight of the first coil of steel loaded onto the furnace platform is too low, the temperature of the cold end face of the coil cannot be detected by the detection part of the furnace platform thermocouple during annealing, and the detected temperature is close to the hot spot of the coil, causing temperature distortion. The actual cold point temperature during cooling is higher than the displayed value of the thermocouple, resulting in oxidation of the coil when it is taken out of the furnace.
1.2 Analysis of Reasons for Adhesive Defects
Adhesive defects: Cold rolled strip steel can undergo recrystallization bright annealing in a hood furnace to remove work hardening and internal stress caused by cold processing deformation, resulting in good mechanical properties and surface quality of the strip steel. However, during the annealing process, adhesive defects such as local adhesion between the strip steel can easily occur, making it difficult or impossible to unwind smoothly. Even after smooth unwinding production, there are still a large number of folds on the surface of the strip steel, causing partial or whole roll degradation products (as shown in Figure 2). After studying a large number of bonding defects, the influencing factors of bonding defects can be basically determined. The main reason for bonding defects is the increase in pressure between the steel strips during annealing, which causes atomic diffusion and penetration between the steel plates, resulting in bonding. The main factors that lead to an increase in pressure between the strip steel are as follows:
1.2.1 Coiling tension of rolling mill
The magnitude of the coiling tension of the rolling mill is a very important factor in the occurrence of bonding defects. The greater the coiling tension of the rolling mill, the greater the pressure between the steel strips, and the greater the tendency for bonding to occur.
1.2.2 Heating temperature and time during annealing process
The diffusion ability of atoms mainly depends on temperature, and at the same time, an increase in temperature causes thermal expansion of the steel plate, thereby affecting the contact pressure of the steel plate. The higher the temperature, the stronger the diffusion ability of atoms, and the easier it is to produce bonding; The distance of atomic diffusion, which refers to the degree of local welding between two steel plates, is mainly influenced by temperature and time. The longer the time, the greater the distance of atomic diffusion, and the easier it is to produce bonding.
1.2.3 Plate shape of strip steel
After cold rolling, the strip steel exhibits varying degrees of plate shape defects due to various reasons, such as bulges, waves, and bends. Due to the poor flatness of the strip steel, the pressure between the steel plates is uneven, and in some areas, the pressure is extremely high. During the annealing process, areas with high pressure are prone to bonding.
1.2.4 Effects of Cooling System
The annealing and insulation temperature of the hood furnace is usually between 680 ℃ and 720 ℃. It cools rapidly above the recrystallization temperature, with a large temperature gradient between the hot and cold spots. This increases the tendency for diffusion and permeation between atoms and the original material, making it prone to bonding defects.
1.2.5 Equipment failures caused by annealing reasons
Equipment failure is also one of the causes of bonding defects in cold-rolled annealed coils. Due to equipment failure, heating delays or repeated shutdowns occur in the heating and insulation section, resulting in uneven heating of the steel coil and easy local bonding. In addition, the temperature measurement system is not accurate. If the actual temperature of the steel coil is much higher than the detection temperature, the steel coil will be in a softened state, and atomic penetration is very likely to cause bonding defects.
2. Control measures for defects
2.1 Measures for controlling oxidation color defects
1) Strengthen the leak point inspection of the furnace platform system, repair any leaks in a timely manner. In the event of an emergency purge of the system, while ensuring safety, the furnace platform system can be replaced with hydrogen and annealed in a hydrogen state. This measure can basically eliminate the oxidation color defects caused by the emergency purge. If hydrogen cannot be replaced, the furnace temperature needs to be lowered accordingly. However, if conditions permit, the dew point and residual oxygen content of nitrogen must be controlled within the process requirements.
2) Strengthen the maintenance of the circulating fan to avoid low-speed or locked situations. If there is a malfunction of the circulating fan, the outlet temperature must be reduced accordingly.
3) Regularly verify the accuracy of the furnace table thermocouple, replace any faulty thermocouple in a timely manner, and correspondingly reduce the temperature of the furnace outlet.
4) The production operation process must ensure that the steel coils are aligned and straight during the furnace loading process, and the core of the steel coil cannot be folded, to ensure smooth flow of protective gas inside the furnace.
5) Strictly control the cooling cycle of the furnace platform, ensuring that the cooling cycle of the furnace platform reaches at least 16 hours under the condition of meeting the exit temperature. Ensure that the temperature inside the steel coil meets the required temperature for exiting the furnace.
6) The rolling mill process must strictly control the residual emulsion on the surface of the steel coil to ensure its cleanliness.
7) During the furnace loading process, it is necessary to ensure that the first roll on the furnace platform is a large roll with a tonnage of 27 tons or more, to ensure the accuracy of the temperature measurement of the steel coil.
8) For the oxidation color defects that have already occurred, the method of re annealing can be used for repair, and basically all of them can be completely repaired.
2.2 Control measures for bonding defects
1) Minimize the coiling tension of the rolling mill as much as possible while ensuring the normal execution of the process.
2) Reasonably optimize the annealing process system, achieve stable heating and cooling rates, and minimize heating and holding time while meeting product performance requirements.
3) The front process must effectively reduce the number of defects in the plate shape, such as the bulging of thin materials and the buckling and wavy shapes of thick materials.
4) Properly adding a cooling hood in process control to reduce the cooling rate in the high-temperature section and minimize the occurrence of bonding.
5) Effectively reduce equipment downtime during operation, ensuring stable execution of annealing processes.
6) Maximize the surface roughness of the strip steel while meeting user requirements. Experimental measurements show that the larger the Ra value of the surface roughness of the rolled strip steel, the more favorable it is for avoiding adhesion. This is because the roughness value increases the atomic bonding resistance between the annealed steel coil layer and the interlayer interface.
7) Although bonding defects have already occurred during the annealing process in the hood furnace, they only form transverse bends during the flattening and unwinding process. Proper control measures can be used to salvage some of the lighter bonding during flattening. The main control measures include appropriate unwinding angle, high unwinding speed, and high flatness elongation. Appropriately increasing the unwinding angle and speed can help reduce bonding when determining severe bonding in steel coils. Due to the requirements for the shape, performance, and surface quality of the strip steel, which have already been specified for coiling tension, annealing process, and flat elongation, the above measures can only be adopted under the process conditions.