Defects in cold rolling annealing process

Annealing process

The main purpose of strip annealing is to eliminate the work hardening caused by cold rolling, reduce deformation resistance, restore plasticity, and improve the deep drawing performance of the strip. The main forms of strip annealing are hood annealing and continuous annealing. Hood annealing is further divided into nitrogen-hydrogen hood annealing and full hydrogen hood annealing depending on the type of shielding gas.

The annealing process of the bell annealing furnace mainly includes heating rate, heating temperature, holding time, cooling rate, furnace discharge temperature, etc. The main defects of the annealing process include:

1. Bonding:

There are some forms of sticking, strip sticking and surface sticking. When the sticking is serious, you can feel the bumps when touching, which are mostly distributed on the edge or middle of the strip. When the surface sticking is serious, the strip will be torn or have holes when unrolling, or even cannot be unrolled, becoming a “dead roll”.

Cause:

(1) Excessive tension. Tension is one of the main reasons for steel coil adhesion. Tension includes rolling tension and coiling tension of the rolling mill. When the tension is too large, the airflow circulation of the protective gas will be poor, resulting in thermal blockage, causing the steel plate to stick.(2) Poor plate shape. Poor plate shape will cause the strip to be thick on both sides and thin in the middle, or thin on both sides and thick in the middle, or edge waves, middle waves, multiple waves, and local waves on the periphery. After high-temperature annealing, adhesion can occur.

(3) Uneven overflow edges appear during coiling. Uneven coiling of the strip, especially thinner plates, is prone to adhesion.

(4) Impure emulsion. There are impurities in the emulsion. After annealing and evaporation, the impurities remain between the steel plates. If they are not blown away, adhesion can also occur.

(5) Overheating. Regardless of the cause, if the furnace is seriously overheated, adhesion will inevitably occur.

(6) Poor gas circulation. Due to the inappropriate stacking of the furnace, the protective gas does not circulate well in the furnace, resulting in uneven furnace temperature, severe thermal stagnation in some parts, and large temperature differences, which can cause adhesion.

(7) The surface roughness of the strip is too small.

(8) The steel is too soft, with low carbon and silicon content, resulting in a high tendency to adhesion.

(9) The cooling rate is too fast.

Prevention and elimination methods:

(1) Correctly select the coiling tension.
(2) Control the roughness of the working roll.
(3) Control the plate shape and coil shape.
(4) Stack according to the stacking principle, placing the tower coil and the overflow coil on 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 the detection instrument, and the correct insertion and placement of the thermocouple.

2. Oxidation color:

The surface of the steel strip is oxidized, and its color gradually transitions from dark blue at the edge to light blue and light yellow.

 

Cause:

(1) The protective cover is not sealed tightly or leaks during annealing, causing a chemical reaction.
(2) The protective cover is lifted too early, and the steel coil is taken out of the furnace at high temperature (the steel coil temperature is greater than 110°C), causing oxidation on the edge surface.
(3) The protective gas composition is impure, and the improper ratio of CO and CO2 causes carbon deposition.
(4) The pre-purge time before heating is insufficient, residual oxygen exists 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 strip should be blown clean to reduce the amount of water and residue entering the furnace.
(2) After installing the furnace cover, the sealing must be checked. If the sealing is not good, it should be handled promptly.
(3) Ensure the pre-purge time and blow as much air as possible in the annealing space to avoid oxidation of the steel coil.
(4) Ensure the composition and dew point of the protective gas.
(5) It is strictly forbidden to remove the steel coil from the furnace at high temperature.
(6) If severe oxidation is found, re-annealing and deoxidation must be carried out.
(7) Regularly check the working status of the control instrument and the position of the thermocouple.

3. Incompatible performance:

After annealing, the annealing structure requirements are not met, resulting in the yield strength, tensile strength, hardness and other properties not meeting the requirements. This is mainly caused by unreasonable process system.

Cause:

(1) Errors in the process system. If the process system is incorrectly set when preparing the materials, it is very easy to cause unqualified performance.

(2) Improper loading of the furnace. If the inner diameters of the steel coils in the annealing furnace are not aligned and the pressure is misaligned, it will cause poor circulation of the protective gas, resulting in a large temperature difference in the furnace. In addition, the air volume of the circulating fan is small, which will also cause some unqualified performance.

(3) Underheating. Heating is terminated before the rated annealing temperature is reached, which is likely to cause unqualified performance. This is mainly caused by the failure of the metering and control equipment.

(4) Undertime. Heating is terminated before the heating time is reached or the holding time is too short, which is also likely to affect the performance.

Prevention and elimination measures:

(1) Develop the correct annealing process system based on the incoming material conditions and product usage.
(2) Strictly implement the furnace loading system and annealing process system.
(3) Strengthen operational control, adjust the air-fuel ratio in a timely manner according to the fuel calorific value, and extend the holding time if temperature drops.
(4) Regularly calibrate thermocouples and other testing equipment.

4. Bump (neck):

Edge damage is most likely to occur during annealing. Check the shape of the incoming coil before loading it into the furnace. For coils with serious overflowing edges or tower-shaped coils, they should be loaded into the furnace in reasonable batches. In principle, only one coil with overflowing edges or tower-shaped coils can be loaded into a furnace, and it should be placed on the upper layer.

Finishing process

The finishing process includes re-coiling, longitudinal cutting, cross-cutting, straightening and packaging. The main quality problems caused are: curling and burrs, uneven oiling, uneven coiling, head and tail tolerances, gnawing, and wavy cut edges.

1. Curling and burrs:

This is primarily caused by improper control of blade overlap and clearance. Blade side clearance depends on the thickness and strength of the steel plate. Excessive clearance can cause tearing, while too little increases equipment load, accelerates blade wear, and results in shiny or excessive burrs on the cut edge.During cold shearing, the clearance is 9-11% of the strip being cut. Blade overlap is selected based on the thickness of the steel plate being cut, generally between 0 and 4 mm. For a 1 mm strip thickness, the overlap is approximately 1.25 mm. Thicker strips have less overlap, and for thicknesses greater than 5 mm, the overlap becomes negative. If curling or burrs occur, the blade clearance must be readjusted.

2. Uneven oiling:

This is mainly caused by a malfunction of the oiling machine or a blockage of the knife beam. It is necessary to carry out spot inspections of the equipment, clean the knife beam regularly, and ensure the cleanliness of the oil.

3. Gnawing wounds:

Gnawing defect is one of the representative quality defects of rewinding line. Its root cause is interlayer slippage of strip steel, and is related to factors such as unwinding tension and strip steel running speed.

Solution:

1) Ensure the unwinding tension during rewinding matches the winding tension during the previous process (the unwinding tension should be less than the winding tension);
2) Spot weld the inner ring of the steel coil to prevent loosening;
3) Appropriately reduce the unwinding speed.
4) Appropriately increase the unwinding expansion force.

Other defects

1. Roller marks/spot marks

Roll marks appear as raised or recessed areas that occur periodically along the length of the strip, coinciding with the circumference of the work rolls. Slot marks are characterized by raised areas on one side and recessed areas on the other, also occurring periodically along the rolling direction.

Causes of roller marks/spots on the strip surface:

(1) Raised roll marks are caused by indentations on the working roll during the cold rolling or tempering process.
(2) Concave roll marks are caused by foreign matter adhering to the working roll during the cold rolling or tempering process.
(3) Chromium marks are caused by raised foreign matter adhering to rollers such as tension rollers and pinch rollers that come into contact with the strip during the production process.

Solution:

1) Strengthen inspection of the roller system and promptly replace any defects;
2) Strengthen inspection, cleaning, and wiping of process channels;
3) Ensure the cleanliness of the plate surface;
4) Strengthen work environment management to reduce environmental pollution to the steel strip.

2. Indentation

Although indentations are also caused by rollers, their appearance is very different from roller marks. They usually appear as continuous strips on the board surface, with a width similar to the length of the roller, and are not noticeable. Some indentations caused by dirty board surfaces can be removed by wiping.

Poor cleanliness of the incoming sheet and roller surfaces, excessive roller pressure, or defects on the incoming material can easily cause indentations.Therefore, ensuring good surface cleanliness of the incoming material and adjusting roller pressure according to the thickness and material of the sheet are effective ways to avoid indentations.

3. Abrasions/scratches

Scratches often occur when the strip curls at the head, whips at the tail, or stops mid-roll. They typically appear as long, tadpole-like or cat-scratch-like strips, often forming rows. Scratches appear as grooves or linear defects on the steel plate surface that are below the rolling surface and can be continuous or intermittent, covering the entire or partial area of ​​the plate.

The process of causing scratches is relatively easy to distinguish. Scratches and scratches from cold rolling appear dark and black after bell annealing due to carbonization of the oil in the scratches. Scratches and scratches from flattening are relatively shiny, free of burrs due to rolling. Scratches and scratches that have not been treated by a straightening machine are shiny and burred, while those treated by a straightening machine are similar to those from flattening.Scratches are primarily caused by interlayer misalignment, while scratches are primarily caused by friction between sharp mechanical objects or stationary hard objects and the plate surface. Once the defect is determined to be caused by the unit, the main inspection for scratches is to ensure that the uncoiler has constant and appropriate tension, and whether there are any sudden stops or rewinding.

4. Hoisting collision

The multi-step production process of cold-rolled products necessitates frequent product turnover, which requires crane hoisting and flatbed truck transport (a few large companies use walking beam transport). This process can lead to defects such as hoisting equipment scratches (usually occurring at the ends) and saddle scratches (caused by foreign objects or hard spots on the flatbed truck saddle or coil storage saddle, often occurring on the outer ring of the coil).