Roller levelling is a forming process used to remove the residual stresses and imperfections of metal strips by means of plastic deformations. During the process the metal fibres are subjected to cyclic tension-compression deformations leading to achieve flat product. The process is especially important to avoid final geometrical errors when coils are cold formed or when thick plates are cut by laser. In the last years, and due to the appearance of high strength materials such as Ultra High Strength Steels, machine design engineers are demanding a reliable tool for the dimensioning of the levelling facilities. In response to this demand, Finite Element Analysis and Analytical methods are becoming an important technique able to lead engineers towards facilities optimization through a deeper understanding of the process. Aiming to this study two different models have been developed to analyze the roll levelling operations: an analytical model and a finite element model. The FE-analysis was done using 2D-modelling assuming plane strain conditions. Differing settings, roller leveling configuration and materials were investigated. The one-dimensional analytical roller levelling model is based on classical beam theory to calculate the induced strain distribution through the strip, and hence the evolving elastic/plastic stress distribution. Both models provide a useful guide to process-sensitivities and are able to identify causes of poor roller leveling performance.
The modulus of elasticity (MOE) indicates a strip’s amount of stretch, or elasticity. For example, the MOE for aluminum is three times higher than that for ferrous materials. Aluminum is forced much more before it will lose its springback than the same thickness in steel. This suggests that more horsepower is required for aluminum than for steel or stainless steel. If a leveler is intended to work with both types of material, its horsepower must be rated for aluminum.
Stage No. 1. Roller leveling is achieved by precisely bending metal strip back and forth as it’s passed through a series of small-diameter offset rolls. The gap between the rolls is set independently on a leveler’s entry and exit. To roller leveling, deeply nest the entry rolls. This forces the material to pass through extreme angles to erase memory caused by trapped internal stresses. It’s also called the plunge, a technique for removing strip memory.
Stage No. 2. A leveler uses adjustable pressure points called flights under the rolls to raise and lower them to a precise position. By adjusting a work roll’s shape, you can alter the material path length through the leveler. A longer path length allows material to be stretched more because more work is being performed on it as it passes through the rolls.
Outer edges can be wavy with longer material lengths. To keep them from stretching further, choose the short path for the outer edge flights. To achieve flatness, the strip’s center must be stretched. Adjust the central flights upward to nest the rolls deeply, forcing the strip through the long path. If all material is equal length, the strip will be flat when it exits the leveling rolls.
If center buckle is present, the center is long. The flights need to be adjusted higher on the edges, making a longer path for the edges of the strip. It’s human instinct to want to work the material where it looks the worst. But, in fact, you should always work the material where it is flat and tight because it’s shortest and needs stretching.
Stage No. 3. Finally, it’s time to reset the strip’s memory to flat as it leaves the leveler. The appearance of the material shipped to the end user is achieved in the final three roll clusters. If the roll gap is set too deep, the material will be forced upward by the last roll, creating up-bow. It’s normal to set the leveler’s exit gap near the material’s gauge, a simple but important rule of thumb.
It’s usually passed from one experienced operator to another. In the past roller leveling was mostly done in service centers and toll processing houses. Today levelers are fine-tuning product quality for end users with great success. The cost of this roller leveling operation usually is offset by eliminating the middle man or by reducing internal scrap. A better understanding of these basic concepts will help you achieve a closer degree of flatness in your process or product.