Cause Analysis and Control Measures of Annealing Adhesive Defects in Cold Rolled Strip

Abstract : Aiming at the problem of sticking after bell-type annealing of cold-rolled steel strip and reducing the yield, the reasons for sticking after bell-type annealing of cold-rolled steel strip were analyzed . Measures such as work roll roughness , optimized annealing and tempering process have effectively reduced the annealing bond defect rate of cold-rolled strip steel .
Keywords : cold rolled strip steel ; annealing ; bonding

Bell furnace recrystallization bright annealing can eliminate the work hardening and internal stress produced in the cold deformation process of cold-rolled strip steel, optimize the microstructure of the material, and make the strip steel have excellent mechanical properties and bright surface . However, since the strip steel coil is easily bonded during the annealing process in the bell furnace, the surface of the strip steel will form a horseshoe-shaped imprint after being flattened. In severe cases, the strip steel will be torn and deformed to produce waste products . The bonding defects of cold-rolled coils produced by Zhejiang Longsheng Sheet Co., Ltd. have always been serious, reducing the yield . Aiming at this problem, this paper analyzes its causes and proposes a solution from the perspective of actual production .

During the process, a radial temperature gradient is formed, resulting in compressive stress between the inner and outer layers and the core, and under the action of compressive stress, the local bulge between the coil layers (mainly caused by the wave shape of the strip or the excessive difference between the same plate ) produces Plastic deformation increases the close contact area between layers . Due to the lattice distortion caused by the deformation, the defects such as vacancies and dislocations increase, the energy of the interface area increases, and the iron atoms are in an activated state, which is easy to diffuse and form a metal bond connection . At the same time, under the effect of high temperature, the diffusion speed of iron atoms is accelerated. When the diffusion layer reaches a certain depth, the metal bond connection becomes a metallurgical connection, that is, a bond is formed .

Analysis of Influencing Factors of Annealing Bonding Defects in Cold Rolled Strip
Influence of Chemical Composition of Steel Strip on Bonding Defects
other non-iron elements ( mainly carbon ) in the steel strip increases, the non-iron atoms occupy more vacancies, which reduces the concentration of vacancies for the diffusion of iron atoms and reduces the self-diffusion coefficient, thus making the strip steel bond
53

Tendency to weaken . In addition, non-ferrous elements will also increase the high temperature yield limit of the strip, and reduce the plastic deformation between the strip coils, which is beneficial to reduce the bonding .
2.2 Influence of strip geometry on bonding defects
2 2 1 Influence of Coil Diameter on Bonding Defects
According to the statistics of production data, the bonding probability of a steel strip with a width of 1250 mm and a thickness of 0.5 mm and a steel coil weighing 15 to 20 t is 0.86 % , while a steel coil weighing more than 20 t has a bonding probability of 0.86%.

1. 03 % . From this it can be seen that Coil diameter is an important factor affecting the bonding rate . Because the larger the diameter of the steel coil , the larger the radial temperature gradient , the greater the interlayer compressive stress of the steel coil , and the more serious the bonding tendency .
2. 2.2 Effect of strip thickness on bonding defects

According to the law of diffusion [ 1 ] :
Corpse — D Yan (1)
dx
In the formula, J is the diffusion flux; D is the diffusion coefficient; dc/dx represents the concentration gradient .
It can be seen from formula (1) , The amount of iron atoms passing through a unit area perpendicular to the plate thickness direction per unit time is proportional to the concentration gradient of iron atoms in this direction , Therefore, the smaller the rolling thickness , The greater the concentration gradient of iron atoms , the greater the diffusion flux of iron atoms , and the more serious the bonding tendency .
2 2 3 Effect of plate shape on bonding defects
and uneven cooling of the strip steel during the rolling process , so the stress is not completely uniformly distributed along the width direction of the strip steel , resulting in poor plate shape , and due to the existence of the plate convexity during the annealing process , The radial stress of the steel coil becomes one of the factors leading to bonding .
2.3 Effect of steel strip surface quality on bonding
2 3 1 The effect of surface cleanliness on bonding Since the bonding is the result of the interpenetration of atoms between the steel coil layer and the layer at high temperature , The cleanliness of the surface thus greatly influences this permeation ability of the atoms . On the one hand , there are residues on the surface of the steel sheet that can hinder the interpenetration of atoms between the steel coil layer and the interlayer ; On the other hand , There are residues on the strip surface after cold rolling , A series of chemical reactions will take place in the annealing furnace , The residual iron powder will be reduced into fine pure iron particles under the high-temperature reduction atmosphere of full hydrogen , The welding of two layers that are tightly pressed together forms a bond . Therefore , appropriately reducing the amount of residual substances on the surface , especially the amount of residual iron powder on the surface , can effectively reduce the occurrence of bonding .
2 3 2 Influence of roughness on bonding
the surface roughness Ra of the strip , the better it is to avoid bonding . This is because the roughness increases , which increases the bonding resistance between the annealed steel coil layer and the interlayer interface atoms. However , the test shows that as the surface roughness increases , it will be at the cost of destroying the surface cleanliness , which will have an adverse effect on the control of bonding and other surface indicators of the strip . Therefore, the original surface roughness of the work rolls cannot be increased too much .
2.4 Effect of coiling tension on bonding
The coiling tension is one of the important factors that form the compressive stress between coil layers when the steel coil is cooled at high temperature . According to the theory of elastic mechanics , the value of radial stress should be equal to the value of coiling tension . The greater the coiling tension in cold tandem rolling , the greater the hot compressive stress between layers during annealing and cooling . Therefore , reducing the rolling tension value of the rolled steel coil can reduce the bonding caused by the interpenetration of edge atoms between the steel coil layers and layers in the high-temperature annealed state .
2.5 Influence of Bell Furnace Annealing on Strip Adhesion
The main factors affecting the bonding of annealing process conditions are : stacking mode and annealing process system ( including heating rate , cooling rate , holding temperature , holding time, etc. ).
2 5 1 Influence of stacking mode on strip bonding
During the production process , it was found that the steel coils of the same specification installed in the same annealing furnace rarely bonded at the top of the steel coils , and the closer the steel coils were to the bottom , the more serious the bonding was , because the lower the bottom , the heavier the weight of the steel coils. The larger the , the more serious the radial bending and the greater the interlaminar compressive stress . The thinner the steel strip , the more obvious the bending tendency of the external force . Therefore , the thinner the steel coil at the same position , the more obvious the bonding tendency . Therefore, the thinner steel coil should be installed on the top of the annealing furnace as much as possible . The wider the width of the strip , the greater the degree of bending when subjected to the same pressure , and the more obvious the bonding tendency is , so the wide coil should be placed on the top of the annealing furnace as much as possible .
2 5 2 The effect of the process system on the bonding of the steel strip There is no relative position change of the steel coil during the annealing cycle , so it can be considered as a whole . The radial stress of steel coil 6” is the direct cause of bonding , and its value is [ 3 ] :
6=6 ” + 6 (2) In the formula, 6 ” is thermal stress; 6 is elastic stress.
The elastic stress 6 1 is related to the strip composition , coiling tension and other factors , it can be considered as a constant during annealing , so the radial stress 6 ” on the strip is only related to the thermal stress 6 ” . 6 ” is a function of the temperature and temperature distribution of the steel coil , as shown in formula (3) :
6 『 = f ( 6 ” ) = F ( Ti , 1 2 , 1 3 , ” ) (3) In the formula , T is the inner temperature of the steel coil ; T 2 is the outer temperature of the steel coil ; 1 3 is the coil core temperature ( cold spot temperature ) ; ” 1 is the inner radius of the steel coil ; ” 2 is the outer radius of the steel coil ; ” 3 is the cold spot radius .
For mass-produced steel coils, the dimensions of H , E , and center are basically the same, and the main influencing factors are the sizes of T i , T 2 , T3 and their differences . Generally speaking , T and T 2 have little difference , and the difference between T 2 and T 3 , that is, the temperature difference between the cold spot and the hot spot is relatively large , which has a great impact on thermal stress and radial stress .
During the annealing and heating stage of the bell furnace , The way of heat transfer is : heating mantle f inner mantle - hydrogen in the furnace T ) - along the radial direction to the middle of the steel coil ( cold spot ) T3 . According to the principle of heat transfer , T 2 is always greater than T 3 , and the heating rate of T 2 is always greater than T 3 . The faster the annealing temperature rises , the larger the temperature difference between T2 and T3 will be, and the greater the thermal stress on the steel coil will be . The sticking tendency is more obvious , Therefore, during bell furnace annealing , Excessive heating speed and exorbitant holding temperature should not be used . During the annealing and holding stage of the bell furnace , the higher the temperature , The more favorable the penetration of atoms , the more severe the bonding . Similarly , the longer the holding time , the more atomic penetration between the strip layers , and the more serious the bonding . In the annealing and cooling stage of the bell furnace , the heat transfer path is : the middle of the steel coil ( cold spot ) f the hydrogen in the furnace T 2 - the inner cover f the cooling system ( spray water ). According to the principle of heat transfer , T 2 is always smaller than T 3 , The cooling rate of T2 is also always greater than T3 , the faster the annealing cooling rate, the greater the temperature difference between T2 and T3 , the greater the thermal stress on the steel coil, the more obvious the bonding tendency, so in the bell type When furnace annealing, do not take too fast cooling rate .
2- 6 Effect of leveling process on strip bonding
The influence of tempering process on strip bonding mainly includes : uncoiling angle , uncoiling speed and leveling elongation, etc.
During flat uncoiling , there is an interaction force between the bonded layers and the large normal stress of the steel coil will cause the surface of the bonded strip to be strained and serious bonding defects will appear . In order to reduce the normal stress as much as possible , the strip steel should be opened tangentially along the steel coil as much as possible , that is , the appropriate uncoiling angle should be controlled to make the tangential component of the tensile stress larger , thereby reducing bonding .
The uncoiling speed is fast , which can make the strip strength quickly pass through the yield zone , thereby avoiding or reducing the occurrence of bonding defects .
Controlling with a constant elongation , especially a larger elongation , can reduce the occurrence of bonding defects .
3 Measures to reduce bonding defects of cold-rolled strip steel
In order to reduce the occurrence of bonding defects , Improve product yield , Several measures have been taken :
( 1 ) Reduce the content of iron powder and ash in the emulsion , increase the purge pressure after the finished product pass , so as to improve the cleanliness of the board surface and effectively reduce the occurrence of bonding defects .
( 2 ) The finished product passes are rolled by textured rolls , The rolls on the finished product frame are roughened with a shot blasting machine, and the roughness Ra is controlled in the range of 2.0 to 2.5 mm , which increases the surface roughness of the strip after rolling and avoids the occurrence of bonding defects .
( 3 ) Appropriately reduce the coiling tension value of rolling steel , The tensile stress per unit area of the cross-section of the strip is reduced from the original 40 Nmm 2 to 32 Nmm 2 , thereby reducing the radial stress value of the steel coil and reducing the occurrence of bonding .
( 4 ) On the premise of ensuring that the strip meets the performance requirements , the annealing temperature of some steel types should be appropriately lowered to reduce the occurrence of bonding .
(5) Increase the uncoiling speed to 10 m/s when the coil is flattened to reduce the deformation of the bonded place .
(6) Under the premise of ensuring that the strip steel can meet the stamping performance requirements , appropriately increase the flattening elongation and eliminate the bonding marks as much as possible .
epilogue
By taking effective measures , Effectively reduce the bonding defects of steel coils after bell annealing , The bonding defect rate of cold-rolled strip was changed from 0 . 95 % down to 0 . 48 %, The cleanliness of the board surface and the product yield are improved .

---