Predictive Roll Alignment in the Reversing Stand of a Plate Mill

Abstract

Better quality of rolling is a key requirement to plate mills, including Mill 5000 operated by Magnitogorsk Iron and Steel Works (PAO MMK). A rolling-produced workpiece might have V-shaped cross-section at the site of deformation, a defect resulting from the asymmetry (misalignment, or skew) of the roll gaps. Consequently, the workpiece leaving the stand is cambered while also displaced laterally when entering the stand. Significant contortion might damage the equipment on the line or cause the plate to deform or break close to the end of the cycle. This is why addressing this problem is urgent. This paper dwells upon the physical processes associated with asymmetrical deformation of metal pieces. If edges have different temperatures, or the workpiece is V-shaped, the driven side (DS) and the undriven opposite side (OS) will differ in pressure sustained by the rolls of hydraulic cylinders. This entails asymmetric changes in specific pressure; as a result, the metal workpiece exits the stand at different speeds, leading to defects. This paper analyzes the known technical solutions that seek to improve the accuracy of compensating the V-shape by adjusting the hydraulic cylinder pressure. Implementing such methods requires installing additional workpiece-edge thickness or temperature sensors, which might be challenging to do in a real-world industrial setting. The article details upon an automated roll alignment controller (RAC) for Mill 5000. The paper shows oscillograms collected when the system was configured by default. In that by-default configuration, the system performed poorly, the DS screwdowns moved inconsistently with their OS counterparts (and vice-a-versa), and thickness varied inherently across the plate, becoming worse and worse after each pass. Oscillograms proved this to lead to considerable transverse thickness variation (up to 2.5 mm), which could cause an emergency. The authors hereof developed a method to align the roll gaps so as to compensate the inherent thickness variation. The method boils down to storing the DS and OS roll gap values in the memory after each pass so as to forcibly reverse the skew when the plate travels backwards. The paper proves a proportional-derivative RAC a feasible performance boost. It further shows oscillograms collected at Mill 5000 after implementing the developed solutions. Those are compared against the oscillograms collected when the roll alignment control system was configured by default. The approach is proven herein to effectively address the misalignment of DS and OS gaps while evening out the thickness of the workpiece. This indirectly confirms a further reduction in camber, making accidents less likely to occur.