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Analysis on the Quality of Hot Rolled Coils

The main process equipment of 1580 hot-rolled strip production line are: two three-stage regenerative walking furnaces, post-furnace dephosphorization, one four-high reversible roughing mill, coil box, fine phosphorus removal, and seven four-high finishing mills , Layer cooling roller table, three downcoilers with three auxiliary rollers, walking beam transport area. As the market competition becomes more and more intense, the user’s requirements for roll shape are becoming more and more strict [1]. In order to improve the control ability of the hot-rolled production line on the coil shape, improve the yield of strip products, and fully meet the needs of downstream customers, we continue to find and summarize in the daily

production process, and gradually improve the quality of the coil shape.

 

Roll Type

 

In the hot rolling production process, the coil shape mainly reflects two types in appearance: tower shape and stacking fault.

 

 

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Influencing factors and control measures

 

(1) The shape of the strip itself. From the daily production experience, the influence of the strip shape on the finished coil is fatal. Although the coiler will play a certain role in correcting the strip, if the strip itself is irregular in shape, or if it travels Serious deviation from the rolling centerline, the coiling equipment cannot make up for it in a short time, resulting in coil-like defects. Plate-shaped defects are mainly manifested in sickle bending, rolling, edge wave, middle wave and so on.

 

First, cambering and rolling can seriously affect the short stroke control of the side guides. The short stroke of the side guide is divided into two stages. After the strip head reaches the parallel section, the side guide starts to perform the first short stroke. When the pinch roller bites the steel, the side guide starts to perform the second short stroke. After the second expansion of the mandrel is completed, the side guide plate will continue to close due to the opening degree compensation value until it touches the strip steel. At this time, the transmission side performs pressure control and automatically opens and closes according to the detected pressure. If the strip is cambered or rolled, it will disturb the detection of the edge of the strip by the side guide, so that the side guide will correct the strip incorrectly. In the subsequent coiling process, there will be excessive pressure on one side, and the side guide will be adjusted again, trying to make the strip return to the rolling center line, which will cause a tower shape. In the same way, if there are such defects in the steel strip, the side guides will be continuously and repeatedly corrected during the coiling process, thereby forming stacking fault defects.

 

The wave defect is mainly caused by the shaking phenomenon during the strip coiling process. During the coiling process, the pinch rollers are also controlled by pressure. When the strip has an edge or middle wave shape, the pressure detected by the pinch rollers will change accordingly. At this time, the pinch rollers will The adjustment of the roll gap and the uneven force along the direction of the roll body will cause the strip steel to be subjected to a certain force in the horizontal direction perpendicular to the rolling center line, which will cause the strip steel to sway from side to side in the horizontal direction, which is very easy to cause layer faults. Because the wave shape generally occurs at the end of the strip, the control of the pinch roller is optimized for the new production line of Legang, that is, the pinch roller is changed from the original pressure control to the position control when the distance from the end of the belt is 9m (this value can be adjusted). , thereby reducing the left and right shaking of the strip to a certain extent.

 

Control measures: There are many factors that affect the shape of strip steel. 1580 has made improvements in the production process from steel temperature, rolling force distribution, looper control, transition material matching, cooling water use and rough rolling incoming material shape. At present, good results have been received in the thinning and rolling of tinplate.

 

(2) Accuracy of side guides. The accuracy of the side guides is mainly reflected in three aspects: parallelism, counterweight and liner wear.

 

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As shown in the figure above, the parallelism is relative to the rolling center line, and its value is m1=d1-d3; m2=d2-d4. According to the 1580 coiling post regulations, the values ​​of m1 and m2 should be ≤ 3mm. This regulation is just for the consideration of coil shape control, because if the parallelism of both sides is poor, then the side guide plate and the strip steel cannot be completely contacted, but the inlet or outlet is partially in contact, which greatly reduces the side guide plate and the strip steel. Therefore, it can not play a good clamping effect on the strip.

 

In actual production, the parallelism of the side guide plates cannot be maintained well, which is mainly affected by the design structure of the side guide plates of the 1580 production line and the impact of the strip during production. The 1580 production line has a hydraulic cylinder in the middle of both sides of the parallel section of the side guide plate. The outlet and inlet are designed with rack and pinion, and the hydraulic cylinder is connected with synchronizing shafts on both sides. When the strip head hits one side of the side guide, under the huge impact force, the inlet or outlet of one side of the side guide will be forced to deviate, and finally the single side of the side guide is no longer parallel to the rolling centerline.

 

Control measures: Use the jack to adjust the parallelism of the side guide plate during roller change or maintenance, and ensure that the connection of the synchronous shaft does not rotate relative to each other. The centering degree of the side guide plate is n1=d1-d2; n2=d3-d4. It is stipulated in the 1580 coiling post regulations that the values ​​of n1 and n2 should be ≤ 5mm. If the alignment of the side guides is poor, even if the side guides can hold the strip perfectly, because the strip deviates from the rolling centerline during the clamping process, coiling defects in the inner or outer tower will eventually occur. Control measures: After the side guide plate is replaced, it should be carefully calibrated, and the parallelism value should be strictly controlled within the standard. In the production process, the wear of the side guide plate lining is inevitable, but excessive wear will affect the pressure control of the side guide plate, because in the program, in order to avoid the side guide plate under pressure control due to abnormal pressure detection, it has been closed all the time. phenomenon, the side guide will set itself a maximum closing amount after pressure control, that is, the side guide will automatically realize position control after the closing amount under pressure control exceeds a certain set value. Due to the excessive wear of the side guide liner, the actual closing amount of the side guide may exceed the set value, and the side guide will remain in its current position. At this time, the side guide plate has a great hidden danger to the coiling process, because the slight fluctuation of the strip width will cause the steel to be clamped at the side guide plate, which may cause steel piles in severe cases. As an operator, the abnormal condition of the side guide plate must be detected in time, and the opening function of the side guide plate must be immediately intervened. However, the rear side guide plate cannot have the original clamping effect on the strip, and the strip may sway from side to side. Eventually a stacking fault occurs. Control measures: Use the roller changing time to carefully check the wear of the side guide plate. If the wear amount is more than 5mm, it should be replaced in time. In the new production line of Legang, in order to solve the problem of rapid wear of the side guide plate, the side guide plate is specially designed with a water cooling structure, which will greatly improve the hardness of the contact part between the side guide plate and the strip steel, thereby reducing the impact of the edge of the strip steel on it. wear.

 

(3) Accuracy of pinch rollers. The pinch roll itself plays the role of bending and guiding the strip head, and provides a certain tension for the strip coiling through the lead rate and the lag rate during the coiling process. The accuracy of the pinch roller is mainly reflected in two aspects, one is the wear amount in the length direction of the roller body, and the other is its levelness. During the long-term contact with the strip steel, the surface of the pinch roller will be worn to a certain extent. In addition to the usual grinding of the roller surface of the pinch roller, the wear amount of the pinch roller along the length of the roller body is often uneven. This will cause itself to not be in good contact with the strip steel, and it is likely to generate a lateral force in the horizontal direction perpendicular to the rolling center line, so that the strip steel can not be stable in the horizontal direction, but sway left and right . Controls: Periodic replacement of pinch rollers in accordance with design standards. The standard for the levelness of the pinch roll is 0.1mm/m. If it exceeds the standard, then one side of the pinch roll means that the reduction is higher than the other side, resulting in uneven pressure on both sides, so that the strip rolls in the coil. The lateral force during the pick-up process will also accelerate the wear of one side of the pinch roller. Control measures: On the premise of ensuring the levelness of the lower pinch roll, use a feeler gauge to measure the roll gap on both sides of the pinch roll regularly, and notify the mechanic to make adjustments if necessary. At present, the design of the pinch roll of Legang production line has added the function of “leveling”, that is, the leveling function can be jogged within the range of 2.5mm/m, which provides a considerable amount of adjustment for the levelness of the upper pinch roll. convenient.

 

(4) The use of pinch rollers. The pinch roll is a follow-up roll located in front of the pinch roll. When the head of the strip passes through the pinch roll and reaches the pinch roll, the pinch roll starts to press down and maintains a certain distance from the strip. Its main function is to During the coiling process, the up and down fluctuation of the strip steel can be slowed down, so that the strip steel can reach a relatively stable state before entering the pinch roll, so that the pinch roll can have a good control of the strip steel, and finally form a Better roll shape. The 1580 production line was designed with pinch rollers, but for various reasons, the pinch rollers were not used effectively. The investigation and analysis show that the pinch roller has a certain positive effect on the roll shape control of the thin gauge with poor sheet shape and the thick gauge ≥ 7mm. Control measures: It is recommended to use pinch rollers for thin and thick products with poor shape and ≥ 7mm in the production process, and regularly check the surface of the pinch rollers, and grind or replace them if necessary.

 

(5) The precision of the rollers of the unloading car. The phenomenon of the outer tower caused by the idler of the unloading car mainly exists within 3 or 3 turns of the outer ring of the steel coil. Locked, when the unloading car is in contact with the outer ring of the steel coil, the idler has a lateral extrusion on the strip, causing the outer ring of the steel coil to gradually overflow. Control measures: Regularly check the parallelism of the idler and the locking pin, and deal with any abnormality in time.

 

Conclusion

 

Through years of production practice and continuous exploration, the 1580 production line’s control of roll shape has been greatly improved. However, the control of the coil shape involves many factors and multiple levels. Currently, Legang is in the stage of equipment installation, and the control of the coil shape has a long way to go.

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