Summary of annealing, finishing processes, and other defect treatment methods for cold-rolled strip

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Annealing process: The main purpose of annealing strip steel is to eliminate the work hardening caused by cold rolling, reduce resistance to deformation, restore plasticity, and improve the deep drawing properties of the strip steel. The main forms of steel strip annealing are bell-type annealing and continuous annealing; among them, bell-type annealing is further divided into nitrogen-hydrogen bell-type annealing and pure hydrogen bell-type annealing, depending on the type of protective gas used. The annealing process parameters for bell-type annealing furnaces mainly include heating rate, heating temperature, holding time, cooling rate, and exit temperature. The main defects in the annealing process include: 1. Adhesion: This can occur in forms such as spot adhesion, strip adhesion, and surface adhesion; when the adhesion is severe, a raised texture can be felt to the touch, and it usually occurs on the edges or in the middle of the steel strip ； Severe surface adhesion can cause tearing or hole formation during unwinding; in severe cases, the roll cannot be unwound at all, becoming a “dead roll”. Causes: (1) Excessive tension; tension is one of the main reasons for coil sticking. Tension includes the rolling tension and coiling tension of the mill. When the tension is too high, it impairs the circulation of the protective gas flow, leading to heat buildup and causing the steel plates to stick together. (2) Poor sheet shape: An inadequate sheet shape can cause the strip steel to have thicker edges and a thinner center, or thinner edges and a thicker center; it may also result in edge waves, center waves, multiple waves, or localized wave patterns around the periphery. Adhesion can occur after high-temperature annealing in all such cases. (3) During winding, uneven overflow edges occur, and the steel strip is wound unevenly; especially with thinner sheets, adhesion is likely to occur. (4) The emulsion is impure and contains contaminants; after annealing and evaporation, these remain between the steel plates. If they are not completely removed, adhesion can also occur. (5) Overheating: Regardless of the cause, if severe overheating occurs inside the furnace, it will inevitably lead to adhesion. (6) Poor gas circulation: Due to improper stacking inside the furnace, the protective gas does not circulate properly, resulting in uneven furnace temperatures. In some areas, severe heat stagnation occurs, leading to large temperature differences that cause sticking. (7) The surface roughness of the strip steel is too small. (8) The steel is too soft; it contains low levels of carbon and silicon, resulting in a high tendency to stick together. (9) The cooling rate is too fast. Prevention and elimination methods: (1) Select the correct crimping tension. (2) Control the roughness of the work roll. (3) Control the strip shape and coil shape properly. (4) Stack according to the stacking principle, placing the tower-shaped coils and the coils with overhanging edges at the top of the steel stack. (5) Follow the operating procedures and control the annealing temperature and time. (6) Ensure the integrity of the annealing equipment, the accuracy of measuring instruments, and that thermocouples are properly inserted, etc. 2. Oxidized color: The surface of the steel strip oxidizes, and its color transitions gradually from dark blue at the edges to light blue and pale yellow. Reasons: (1) During annealing, the protective cover is not sealed properly or leaks air, resulting in a chemical reaction. (2) The protective cover is removed too early, resulting in oxidation of the surface at the edges of the steel coil when it comes out of the furnace at high temperature (the temperature of the steel coil being greater than 110°C). (3) Impure composition of the shielding gas, as well as an inappropriate ratio of CO to CO2, leads to carbon deposition. (4) The pre-purge time before heating is insufficient; residual oxygen remains in the furnace, and the steel coil is annealed in an oxidizing atmosphere. Prevention and elimination methods: (1) The emulsion on the surface of the steel strip should be blown away to reduce the amount of water and residues that enter the furnace. (2) After installing the furnace cover, a seal check must be carried out; if poor sealing is detected, it should be addressed promptly. (3) Ensure an adequate pre-blowing time to thoroughly purge the air from the annealing chamber, thereby preventing oxidation of the steel coil. (4) Ensure the composition of the shielding gas and its dew point. (5) It is strictly prohibited to remove from the furnace at high temperatures. (6) When severe oxidation is detected, re-annealing for deoxidation is necessary. (7) Regularly check the operating status of control instruments and the placement position of thermocouples. 3. Inadequate performance: The annealed material does not meet the requirements regarding its microstructure after annealing, which results in properties such as yield strength, tensile strength, and hardness not being within the specified limits. This is mainly due to an unreasonable processing procedure. Reason for occurrence: (1) An error occurred in the process parameters. If the process parameters are set incorrectly during ingredient preparation, it is very likely to result in substandard performance. (2) Improper furnace loading. In the annealing furnace, if the inner diameters of the steel coils are not uniform and they press against each other, it leads to poor circulation of the protective gas, resulting in large temperature differences within the furnace. Additionally, a low airflow from the circulation fans can also cause certain performance issues. (3) Under-temperature. Heating is terminated before reaching the specified annealing temperature, which can lead to inadequate performance; this is mainly caused by faults in the control equipment. (4) Shortage of time. If the heating time is insufficient or the holding time is too short, the heating process will end prematurely, which can also affect the performance. Prevention and elimination measures: (1) Establish a proper annealing process system based on the condition of the incoming materials and the intended use of the products. (2) Strictly implement the furnace loading system and annealing process system. (3) Strengthen operational control, adjust the air-fuel ratio in a timely manner based on the calorific value of the fuel, and extend the insulation time when a temperature drop is detected. (4) Regularly calibrate detection equipment such as thermocouples. 4. Edge damage: Edge damage is most likely to occur during annealing. Before loading the material into the furnace, it is necessary to check the condition of the rolls; those with severe overhangs or a tower-shaped profile should be grouped together for loading. In principle, only one roll with an overhanging edge or a tower-shaped profile should be placed in a furnace, and it should be positioned in the upper layer. Finishing processes include rewinding, slitting, cross-cutting, straightening, and packaging. The main quality issues that arise include curling and burrs, uneven coating, uneven winding, deviations at the start and end of the roll, scoring, and wavy edges at the cut portions. 1. Burrs and rough edges: These are mainly caused by improper control of the overlap amount and gap between the cutting edges. The lateral gap between the blades depends on the thickness and strength of the steel sheet; too large a gap leads to tearing, while too small a gap increases the load on the equipment, accelerates blade wear, and results in shiny cut edges or excessive burrs. During cold shearing, the gap is set at (9~11%) of the thickness of the sheet to be sheared, while the overlap amount of the blades is determined based on the thickness of the steel plate being sheared, generally ranging from 0 to 4 mm. When the strip thickness is 1 mm, the overlap amount is about 1.25 mm. The thicker the strip, the smaller the overlap amount; when the thickness is greater than 5 mm, the overlap amount becomes negative. If curling or burrs occur, it is necessary to readjust the blade gap. 2. Uneven oiling: This is mainly caused by a malfunction in the oiling machine or blockage of the knife beam. To ensure proper equipment maintenance, clean the blade beam regularly and maintain the cleanliness of the oil. 3. Gouging defects: Gouging defects are one of the typical quality issues associated with coiled products. The root cause of these defects is interlayer sliding of the steel strip, and they are related to factors such as uncoiling tension and the speed at which the steel strip moves. Solutions: 1) Ensure that the uncoiling tension of the rewinding unit matches the coiling tension from the previous process (the uncoiling tension should be lower than the coiling tension) ； 2) Spot welding on the inner ring of the steel coil to prevent it from loosening ； 3) Appropriately reduce the uncoiling speed. 4) Slightly increase the uncoiling expansion force. Other defects: 1. Roll imprinting/indentations. Roll imprinting appears as protrusions or depressions that occur periodically along the length of the steel strip’s surface, in line with the circumference of the working roll. The characteristic of indentation marks is that the steel strip bulges on one side and is indented on the other, and they also appear periodically along the rolling direction. Reasons for roll imprint/indentation defects on the steel strip surface: (1) Protruding roll imprints are caused by indentations on the working rolls during cold rolling or leveling. (2) Dented roll printing is caused by foreign substances adhering to the working rolls during cold rolling or leveling. (3) Chromium traces are caused by protruding debris adhering to rollers such as tension rollers and pinch rollers that come into contact with the steel strip during the production process. Solutions: 1) Strengthen the inspection of the roller system, and replace them promptly if any defects are found ； 2) Strengthen the inspection, cleaning, and wiping of process channels ； 3) Ensure the surface is clean ； 4) Improve the management of the working environment to reduce environmental contamination of the steel strip. 2. Indentation: Although indentation is also caused by rollers, its manifestation differs significantly from roller imprinting. It usually appears as a continuous band on the surface of the plate, with a width similar to that of the pressure roller, and has no tactile feel ； Some indentations caused by dirty surfaces can be removed after wiping. Defects such as poor cleanliness of the incoming material surface and roller surface, excessive pressure from the pressing rollers, and stains on the incoming material can easily lead to the formation of indentations. Therefore, ensuring that the incoming materials have good surface cleanliness and adjusting the pressure of the pressing rollers in accordance with the plate thickness and material are effective ways to avoid indentations. 3. Scratches/abrasions: Scratches often occur when the strip steel curls at the head, swings at the tail, or comes to a stop in the middle. They are usually long in shape, resembling tadpoles or cat scratches, and appear in rows for the most part ； Scratches appear as grooved or linear defects on the surface of the steel plate, below the rolled surface, and are distributed continuously or intermittently over the entire or part of the steel plate. The process by which scratches are formed is also relatively easy to distinguish; scratches that occur during cold rolling turn dark and black after bell-type annealing due to the carbonization of oils within those scratches ； The scratches and abrasions that occur during leveling are relatively smooth, as rolling is carried out, so there are no burrs ； Scratches and abrasions that have not been treated with a straightening machine appear shiny and burry, while those that have been treated with a straightening machine resemble the surface after flattening. Scrapes are mainly caused by misalignment between the layers of the steel coil, while scratches are primarily resulting from friction between sharp mechanical objects or stationary hard objects and the surface of the plate. Once it is determined that the defect originated from this machine, for scratches, the main checks involve verifying whether the tension of the uncoiling machine is constant and appropriate, as well as checking for any sudden stops or reverse unwinding. 4. Lifting collisions: The multi-stage production process of cold-rolled products requires frequent handling of these products, and such handling involves using cranes for lifting and pallet trucks for transportation (a few large enterprises use traveling beam conveyors instead). During the handling and transportation of products, defects such as damage caused by collisions with lifting equipment (usually occurring at the ends) and damage caused by abrasion from saddles (resulting from foreign objects or hard spots on the saddles of flatbed trucks or those used for storing steel coils, often occurring on the outer surface of the steel coils) can arise.