Analysis and control of corrugation defects of cold rolled strip after leveling
Abstract：This paper analyzes the causes of corrugation defects in the rolling process of annealed strip steel, introduces the measures of controlling the rolling temperature, optimizing the rolling process and reforming the driving mode of roll system and their effects.
The cold rolling flattening process is the key process to determine the surface quality and mechanical properties of cold rolled products. Its purpose is to use a small deformation (0.3% ~ 3.0%) to eliminate the yield platform of the steel coil after cap annealing, improve the sheet shape of the strip, and adjust the mechanical properties of the strip to obtain good subsequent processing properties. However, in the recent production process, when the 1550mm leveling unit of our factory rolled the strip steel with a thickness of > 1.0mm, the probability of plate crease defects at the end of the steel coil was higher, which seriously affected the rolling hit rate and the yield of cold rolled products. Therefore, the factory organized force to carry out on-site tracking analysis of the causes of corrugations, and put forward the corresponding solution measures are summarized in this paper.
2、Panel crease and its analysis
The crease defect of annealed strip steel produced in the process of flat rolling runs through the whole rolled plate, expands from the middle to both sides and the trace gradually fades. The appearance of the defects on the upper and lower surfaces is the same, and the appearance is similar to the vibration pattern, and the rolling direction is about 90°, and the distribution of irregular transverse stripes is interwoven with each other, and there is no obvious boundary. Its texture is similar to the "bark" shape, with a slight feel or even no feel, which is obviously visible to the naked eye.
The defects all appear in the area from the middle to the tail of the flat unwinding, with a length of tens of meters and a length of hundreds of meters. And the higher the rolling temperature is, the more obvious the corrugation is. The thicker the strip, the more serious the folding before rolling, and the greater the chance of creasing the plate surface after rolling.
It is generally believed that the crease defect of the plate is that the annealed low carbon steel sheet in the production and processing, due to the local sudden yield uneven deformation, resulting in the "Luders band" phenomenon, and the transverse texture on the upper and lower surfaces of the steel plate.
According to the data, after recrystallization annealing, low carbon steel has obvious upper and lower yield limits, and the lower yield limits have a long yield platform. When the local strip steel is subjected to the stress exceeding the yield limit, there will be a sudden transformation from the elastic zone to the plastic zone. Both the stress under the strip steel exceeds its elastic limit, resulting in yield. Cause uneven and local flow deformation on the strip section, then there will be wrinkles.
Analysis of factors leading to crease：
(1) Poor quality of raw material core. When the strip is cold rolled, if the shape of the lead part is not well controlled, the edge wave and the middle wave are obvious during rolling, resulting in uneven distribution of stress in the width direction during the winding process, and the stress is released after the cover annealing furnace, which makes the state of the coil deteriorate, especially the quality of the coil deteriorate. At the same time, under the action of the tightening force of the core shaft of the uncoiler, the strip is extruded at the edge of the sector block of the uncoiler to produce continuous transverse folding, and wrinkles are formed after flat rolling.
(2) Improper annealing process. When the strip is annealed in the cover furnace, if the thermocouple is faulty, the temperature is too high, the holding time is too long, the temperature is too high during annealing, and the cooling rate is too fast, the interlayer bonding phenomenon will occur. If the end face of the steel coil is mechanically damaged before loading, and the spokes of the convection plate and the final cooling plate are too thin, the bond between the end face of the steel coil will also be caused. When the strip is bonded between layers, the strip no longer opens along the tangential direction of the steel coil due to the influence of the bonding force, but produces a certain arc, and part of the bonding force becomes the component force that makes the strip bend. When this component force exceeds the yield limit of the strip, the arc bending perpendicular to the length direction of the strip is formed.
(3) Smooth rolling temperature. According to the field tracking, it is found that in summer production, when the temperature of steel coil before leveling is > 45 ℃, the strip steel is more prone to wrinkle defects, especially the steel coil that has just been transferred from the annealing terminal cold table. The main reason is that when the temperature of the steel coil is > 45℃, the metal molecules are more active, and the strip is easy to jump from the elastic deformation zone to the plastic deformation zone when the strip is subjected to tension, and the slip line is generated on the strip surface, resulting in wrinkling defects on the strip surface.
(4) Smooth roller system operation. The transmission mode of the levelling unit is designed in the form of two 600kW DC motors respectively driving the upper and lower work rollers through the reducer. The advantage of this double work roll transmission mode is that the transmission mode is stable and can provide a larger rolling moment, but it has higher requirements for the precision of the roll system, the rolling gap of the rolling mill and the drive shaft. However, for the equipment of the whole machine, due to the large assembly gap of the roll system, the uneven roll system and the beating of the transmission system, the rolling phenomenon of the work roll during decelerating rolling is caused: ① The work roll bearing seat is nested in the bending roll block, and the sliding block on the side of the bearing seat causes uneven wear when the work roll changes the roll, and the upper and lower work roll assembly will tilt and cross in the bending roll block, and the rolling phenomenon will occur when the double roll drive rolling. (2) During the rolling acceleration process and high speed uniform process, the work roll and support roll are offset to the rolling exit direction to eliminate the gap between the roll system and the arch under the action of rolling moment, and the transmission system also eliminates the mechanical gap under the action of rolling moment. At this time, the rolling process is stable and the strip surface can obtain better surface quality. Once continuous deceleration rolling or stage deceleration rolling, the transmission gap and roll assembly gap are gradually amplified under the action of rolling inertia and rolling torque reduction, so that the stability of the roll system is broken, and the roll system produces vibration and flickering phenomenon.
3.1、Control coil temperature before leveling
The production mode of the hood type annealing furnace is that the furnace is released in the shift to the final cooling table, and the cooling is finished in the corresponding shift the next day, and the steel coil is transferred to the flat production. In theory, the air cooling time on the final cooling table is 24h, due to the product specifications and the inconsistent temperature, the specific time of the shift is different, the steel coil group on the final cooling table is not orderly, the cooling wind circulation difference, the production environment temperature and other factors, resulting in the cooling effect of the steel coil on the final cooling table is quite different. At this time, if the flat rolling is carried out directly, it will lead to the increase of the dispersion of the steel coil properties and the defects of the plate surface.
Therefore, it is necessary to formulate a system to standardize the flat feeding operation, from the final cold table to the flat steel coil, must be placed in the front warehouse for 8 to 24 hours, to ensure that the heat of the steel coil from inside to outside uniform diffusion, if necessary, with the fan to assist the acceleration of air convection in the warehouse, in order to achieve the purpose of rapid cooling. When the steel coil reaches the storage time, the end surface temperature is strictly controlled within 45℃, and for the steel coil with a thickness greater than 1.0mm, the feeding temperature should be controlled within 40℃.
3.2、Adjusting rolling process
In order to improve the surface quality of strip steel, corrugation can be prevented or eliminated by increasing elongation setting value, increasing uncoiling and coiling tension and adopting constant rolling force mode at the end.
3.3、Increase rolling speed
When the rolling temperature and deformation amount are known, the deformation rate can be improved by increasing the production speed. Practice has proved that for plate shape, roll shape and other reasons caused by the opening of the cross fold printing, under the premise of ensuring safe production, the speed of high-speed rolling is not less than 600m/min, improve the slope of acceleration and deceleration, to avoid a long time of low speed rolling, it is very beneficial to control plate crease defects.
3.4、Improve the work roll driving mode
On the basis of the original control mode, a new single roll drive control program is added to make it have two modes of double drive of work roll and single drive of lower roll. The practice shows that the rolling of single roll drive is stable, the rolling phenomenon of roll system is eliminated, and the surface quality of strip steel is better than that of double roll drive.
There are many reasons for the corrugation of plate surface, which are related to the rolling condition, rolling temperature and rolling system movement. The corrugation of plate surface can be effectively suppressed by optimizing the rolling process, implementing the transformation of single roll drive, strictly controlling the feeding temperature and adopting the constant rolling force mode at the tail.
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