An analysis was conducted to address the frequent deviation issues experienced by Xinyu Iron and Steel Group Corporation’s 1580mm hot rolling mill when rolling thin strip (2.2mm and below). Improvements were made to the equipment’s installation accuracy, roughing blade camber control, and centering control. This significantly reduced the occurrence of deviation, improved production stability and line availability, and now enables large-scale production of thinner strip (2.0mm and below).
The 1580mm hot rolling mill at Xinyu Iron and Steel Group Corporation’s hot rolling mill frequently experiences deviation during the production of thin-gauge (2.2mm thick) strip, particularly during the middle and late stages of the rolling process. Sometimes, tail swing occurs at the tail of the strip, causing roll scratches and indentations at the end of the strip, compromising strip quality. In severe cases, frequent work roll replacement is necessary, making it difficult to maintain production schedules. This significantly shortens roll change cycles, placing significant pressure on the roll grinding shop and impacting strip flatness quality.
Operators previously relied solely on visual inspection to determine strip camber or deviation, then adjusted the rolls to correct the problem. Clearly, this manual approach had significant limitations. First, visual inspection accuracy was insufficient, often detecting camber or deviation only when it became severe. Second, operators experienced a delay in their operations, requiring adjustments only after camber appeared, often leading to snaking. Furthermore, the adjustment amount determined by the operator based on experience was a subjective value, requiring multiple attempts to achieve the desired result.
Analysis of the causes affecting deviation
Equipment status
Xinsteel’s 1580mm hot rolling mill commenced operation in 2008. The roughing mill consists of the R1 and R2 stands, and the finishing mill comprises stands F1 through F7. The entire mill was designed by First Heavy Industries. The R2 stand in the roughing mill and all stands in the finishing mill utilize hydraulic systems. The R2 stand utilizes both electric and hydraulic reduction, while the finishing mill utilizes full hydraulic reduction, and the R1 stand utilizes only electric reduction. Side guides are installed at the front and rear of the roughing mill stands, as well as in the finishing mill. To control flatness, the finishing mill utilizes roll bending and CVC technology.
Camber of rough-rolled slab
Camber can be caused by a variety of factors, including uneven temperatures on both sides of the plate in the heating furnace, wedge-shaped incoming material, improper side guide alignment, inaccurate installation of rolling mill equipment, and poor rigidity on both sides of the rolling mill. Camber can create numerous problems in subsequent finishing operations.。
The investigation revealed that in actual 1580mm production, strips exhibited a certain degree of camber after rough rolling. Strips with severe camber were prone to lateral bending during the rolling process, even producing scrap steel, after entering the finishing mill. After the completion of the R1 rough rolling process, a certain degree of camber often occurred. The direction of this camber varied with the state of the rolls, exhibiting a certain regularity. The direction of the camber remained largely the same within a roll-changing cycle. The R2 rough rolling process typically involved three passes. During the first pass, the camber was not noticeable (not visible to the naked eye), but became more pronounced during the third pass.
During R2 rolling, since camber has already occurred during R1, operators often make appropriate adjustments based on experience in the hope of eliminating this camber. However, in actual production, even straight steel plates from R1 will still camber after R2 rolling, creating significant challenges in controlling the finishing process. Therefore, the camber that develops during roughing must first be eliminated.
Finishing rolling deviation
There are generally three reasons for deviation during the finishing mill: first, the rolls themselves are not parallel during installation, resulting in a certain difference in rigidity between the drive and operating sides of the mill; second, rolling a wedge-shaped workpiece or a roughing mill with sickle camber; and third, poor alignment, that is, a deviation between the centerline of the workpiece and the centerline of the rolls. These factors cause a difference in reduction ratio between the left and right sides of the mill. The side with higher reduction ratio has greater elongation than the other side, resulting in increased slip on the entry side, causing the strip to bend toward the side with lower elongation, resulting in deviation. The strip moves within the mill frame, which can easily lead to scrap and damage to the rolls.
Generally speaking, when rolling strip with a large crown (thick gauge strip), if camber occurs, the strip will move toward the side with a smaller compression ratio, and the center of the workpiece will move toward the center of the roll, so that the center of the workpiece and the center of the roll are aligned, and the camber can be partially or completely eliminated on its own. However, when rolling strip with a smaller crown (thin gauge strip), camber is difficult to eliminate automatically, which affects the normal production of the strip. The higher the crown accuracy of the hot-rolled strip, the more important it is to control the camber.
In actual production, for thin-gauge strip, the sharp corners at the tail clearly indicate strip deviation, as shown in Figure 1. After casting in the finishing mill’s F stand, the tail shape is essentially normal. After casting in the F2 stand, a certain sharp corner begins to appear. The sharp corners become increasingly pronounced in subsequent stands, reaching their maximum at the F stand exit, making tail swinging very likely.
The reasons for the appearance of sharp angles. First, the stiffness or reduction (incoming material wedge) on both sides of the finishing mill are inconsistent. Second, after the tail is cut, the steel is lost due to the tension of the strip, causing the tail of the strip to tilt, which in turn produces a sharp angle. The reason is the side bending caused by rough rolling. From the actual follow-up investigation results, in addition to the rough rolling sickle bend brought into the finishing rolling, the formation of sharp angles is highly correlated with the finishing rolling machinery and control. Moreover, in actual production, the direction of the sharp angle is mostly biased towards the operating side, showing a strong regularity. The strip runs off in the rolling mill, causing the center line of the strip to deflect to one side during rolling. In severe cases, tail swinging, strip breakage and other phenomena will occur, seriously affecting the stability of rolling, especially when rolling thin strips. Due to the high rolling speed, it is difficult for humans to judge the deflection direction of the strip, nor is it easy to grasp the size and timing of the adjustment. The effect of manual adjustment is poor.
Solution
To address the deviation issues on the 1580mm hot rolling mill line, Xingang organized a project team to conduct scientific and technological research. They focused on three key areas: first, improving machine installation accuracy; second, controlling camber on the roughing mill; and third, adding centering control in the finishing mill area.
Equipment installation accuracy
The accuracy of rolling mill equipment is a prerequisite. Good installation accuracy can effectively reduce the occurrence of deviation. Equipment accuracy includes machining accuracy and installation accuracy, namely, the grinding of the rolls and the installation and online adjustment of the mechanical equipment. For 1580 mm rolling mills, the accuracy of the rolling mill includes the equipment accuracy of the roughing and finishing mills. Because the camber of the roughing mill incoming material is relatively severe, improving the equipment accuracy of the roughing mill is a top priority.
In order to improve the accuracy of the equipment, the cooling time of the rolling mill, the grinding roller accuracy, and the mechanical installation are strictly controlled, which avoids the problems caused by equipment accuracy from the source and lays the foundation for the subsequent electrical model debugging.
Camber control for roughing R1 and R2
Since the roughing mill R1 only has electric reduction, which cannot achieve dynamic adjustment during the rolling process, camber control can only be achieved by adjusting the R1 side guides, not by adjusting the reduction roll gap. However, effective use of the side guides can reduce the occurrence or degree of camber. Therefore, adjustments were made to the front and rear side guides of the R1 mill to maximize their auxiliary role.
The R2 stand is equipped with hydraulic pressure, which can dynamically adjust the roll gap on both sides during production to achieve the purpose of controlling camber. Therefore, the camber generated during the R1 rolling process can only be calibrated and compensated by R2, while also ensuring that camber does not occur during the R2 rolling process. Before the project, the camber control of R2 was mainly adjusted by the operator through visual observation and manual intervention. This method often has a large lag and is unstable. Therefore, the project team adopted a dynamically set bilateral AGC model:
Centering control
Centering control is sensitive to both strip width and thickness. Narrower and thinner strips are more likely to misalign within the mill stand. Centering control can be implemented in every stand in both the roughing and finishing mills, and can be selected based on on-site equipment conditions and product specifications. Practice has shown that centering control is very effective for thin strip, significantly alleviating misalignment.
Strip wedge control
There are two scenarios for wedge control. First, the plate already has a wedge. Forcibly removing this wedge would destroy the ideal plate shape and cause camber. Second, wedges form during the rolling process, and these should be removed. For manufacturers, detecting wedges in plate is difficult, and the causes are complex and difficult to pinpoint. To achieve a good plate shape and reduce camber and runout during finishing, a wedge control block diagram can be constructed for the flat roll stand through calculation. This control ensures the wedge remains constant throughout the rolling process, preventing runout.
Implementation Effect
After several research projects, the project team significantly improved the stability of the finishing rolling process by improving equipment installation accuracy and implementing camber control models, centering control models, and camber control models. This significantly reduced the chance of deviation for strip thicknesses over 2mm. Strip thicknesses below 2mm, previously difficult to reliably roll, have now been implemented on a large scale, generating significant economic benefits for New Steel.
