Coil Processing: Straightening and Leveling

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Steel production and processing are continuous operations where the last step is coiling. Steelmakers and processors use tension when coiling to avoid producing “soft” or collapsing coils. Coiling induces tensile and compressive stresses into the strip, and these stresses can contribute to blank or part distortion in subsequent processes. Unless sufficient winding tension adjustments are made, the degree of these stresses change throughout the coil – whereas the outer laps of the coil may be on the order of 6 feet (1800 mm) in diameter, the inner laps typically are wound on a 20 inch to 24 inch (500 mm to 600 mm) diameter mandrel. In addition, the magnitude of these stresses increases with higher strength products, leading to coil shape imperfections like coil set and crossbow.

Coil set is a bow condition parallel with the rolling direction, and curves downward in the same direction as the upper outside lap of an overwound coil (Figure 1a).  Here, the top surface of the coil or strip is stretched more than the bottom surface, and typically becomes more severe as the coil is processed and the lap diameter decreases.  Crossbow is a bow condition perpendicular to the rolling direction, and curves downward in the same direction as the upper outside lap of an overwound coil, with the center portion of the sheet raised a measurable amount above the sheet edges (Figure 1b).

The first operation when unwinding a coil is some type of shape correction to ensure flatness before further processing. There are two main types of equipment used to create a flat coil – a straightener and a precision leveler. While these two types of equipment are similar, a precision leveler has additional capabilities. Both bend the coil back and forth over a series of work rolls to alternately stretch and compress the upper and lower surfaces (Figure 2).  Critical equipment parameters include roll diameter, roll spacing, backup rolls, roll material type, gear design, backup rolls, overall system rigidity, and power requirements.  The amount of force required to relieve the residual stresses is a function of the sheet thickness and yield strength. Equipment sufficient for shape correction on conventional grades may not be sufficient to completely flatten the advanced steel grades available now and in the future.

Straighteners and levelers have a series of rolls that progressively flex the strip to remove the residual stresses. Each successive roll pair has an adjustable gap to deform the sheet to a targeted amount with the goal of resulting in a flat coil once the steel passes through all the rolls. The entry end has the smallest gap, putting in the most deformation. The last pair of rolls has the largest gap, usually set for metal thickness. The gap profile varies based on thickness, yield strength, and equipment (Figure 3). Many equipment manufacturers have generated tables to guide the operator as to the best settings for various yield strength/thickness combinations.

Removing coil set requires permanent yielding in the outer 20 percent of the top and bottom surfaces of the metal. The central 80 percent of the thickness remains unchanged.T-14  Straighteners are appropriate for this type of shape correction (Figure 4).  Only end bearings support the simplest straighteners, with no backup rolls used.  Closing the entry roll gap risks deflection of the unsupported center, potentially leading to creating edge waves in the coil.

Eliminating crossbow and other shape imperfections like buckles or waves requires permanent yielding in the outer 80 percent of the top and bottom surfaces, with only the central core — 20 percent — remaining in the elastic range.T-14  Precision levelers, which applies tension to the strip as it bends around more smaller diameter rolls, can achieve this deformation (Figure 5).  While this deformation can get the coil shape closer to flat, it also reduces the inherent formability of the grade.  Processors should use only the least amount of deformation necessary to correct the shape to retain sufficient formability for stamping or other operations.

Yield point elongation (YPE), Lüders lines, and stretcher strains are names describing the same phenomenon seen in some annealed or aged metals. A related defect called fluting occurs in V-bending. Leveling at-risk coils with repeated cycles of bending and unbending, like shown in Figure 3, may be an effective way to minimize stretcher strains or fluting. However, process control is critical, since excessive leveling work hardens the coil and results in increased strength and reduced ductility. On the other hand, insufficient leveling does not address the defects related to the yield-point phenomenon. 

Recent studies K-24, K-48, K-49 describe the importance of sufficient leveling, using real-world examples as well as simulation to model the phenomena and show potential corrective actions, as shown in the following animations.K-50

Figure 6 shows an animation of V-bending without any roller leveling. The fluting defect occurs, since the formed panel shape does not conform to the punch. Figure 7 is an animation of leveling with roller penetration deep enough to produce deformation equivalent to an 85% plastic fraction. Figure 8 presents a closer view of the V-bending, highlighting improved formed panel shape conformance to the punch. The references cited above detail the simulation methodology.

Design and Processing Implications

The progressively higher yield strengths for AHSS are challenging the capabilities of straighteners and precision levelers that were not designed for flattening these high strength materials. Equipment manufacturers have been studying and developing solutions to address this issue. There are a series of factors related to the design of straighteners and precision levelers affected by advanced steel grades:

Roll Diameter – Leveling rolls for AHSS generally are smaller in diameter than those used for mild steel, providing a smaller radius around which to bend the material. This is because exceeding the higher yield strength of Advanced High Strength Steels requires a more aggressive bend.

Roll Spacing – Work roll center-spacing will be closer for AHSS than for comparable mild steels. Closer spacing leads to the requirement of more force to reverse-bend the material, resulting in greater power requirements for processing.

Roll Support – Larger journal diameters with larger radii and bearing capacity will withstand the greater forces and higher power required to straighten AHSS.

Roll Depth Penetration – The upper rolls must have enough travel to be able to penetrate the lower fixed rolls sufficiently so the deformation exceeds the yield strength of the AHSS grade. This penetration may need to be as much as 50 to 60 percent greater than for mild steels.

Roll Deflection – Given the greater force requirements for straightening AHSS, work roll deflection becomes a concern especially with smaller-diameter rolls more likely to flex and deflect. Processing wider sheet also increases the deflection risk. Excessive work roll deflection results in undesirable side effects such as edge waves, increased journal stresses and premature gear failure. Backup rollers prevent excessive work roll deflection.

Roll Material – Higher strength materials and special heat treatment should be employed to ensure rolls can withstand greater stresses for longer periods without experiencing fatigue failure.

Gear Materials – Gears that drive the rolls should be produced from heat treated high strength materials to produce smooth running, chatter free roll drive for long life under high loads.

Gear Positioning – Closer roll center spacing requires higher power transmission and results in a smaller gear-pitch ratio, which reduces gear power ratings.

Gear Sizes – To compensate for the gear positioning issue, flattening AHSS grades requires wider gear faces as well as stronger outboard support of journals and idler shafts to produce higher gear power ratings.

Frame Rigidity – The higher strength of advanced steels results in stresses throughout all the components of the processing unit. Frame rigidity is vital to prevent work roll deflection.

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Equipment manufacturers have also developed design solutions that address processing of AHSS. As an example, several manufacturers have designed equipment with removable cartridges allowing for swapping between sets containing differently sized rolls, gears, and support structures. As they switch jobs from AHSS to conventional steels, they swap in the appropriate cartridge. This also allows for off-line roll cleaning and maintenance.

Remember that the likelihood of coil set and residual stresses in the coil increases with strength. Operators must take proper precautions when cutting the strapping banks used in coil shipment to avoid “clock-springing.”

Newer processing equipment may contain additional hold-down arms or other features to protect both plant personnel and equipment from damage.E-11

Material Handling Considerations When Working With Higher Strength Steels (U-13)

Stamping AHSS materials can affect the size, strength, power and overall configuration of every major piece of the press line, including material-handling equipment, coil straighteners, feed systems and presses.

Higher-strength materials, due to their greater yield strengths, have a greater tendency to retain coil set. This requires greater horsepower to straighten the material to an acceptable level of flatness. Straightening higher-strength coils requires larger-diameter rolls and wider roll spacing in order to work the stronger material more effectively. But increasing roll diameter and center distances on straighteners to accommodate higher-strength steels limits the range of materials that can effectively be straightened. A straightener capable of processing 600-mm-wide coils to 10 mm thick in mild steel may still straighten 1.5-mm-thick material successfully. But a straightener sized to run the same width and thickness of DP steel might only be capable of straightening 2.5 mm or 3.0-mm thick mild steel. This limitation is primarily due to the larger rolls and broadly spaced centers necessary to run AHSS materials. The larger rolls, journals and broader center distances safeguard the straightener from potential damage caused by the higher stresses.

Stamping manufacturers vary in many ways, but they share in the struggle to find skilled labor to fill open manufacturing positions.

Filling and retaining people for these positions can be tough, especially as baby boomers retire from the workforce. Many of the new generation of workers want remote or flexible work schedules, which is difficult when stampers need an on-site operator to run equipment.

The good news is that interest in manufacturing as a career is growing. In the 2019 Leading2Lean Manufacturing Index, 27% of Generation Z and 26% of millennials have considered manufacturing as a potential career versus only 17% of Generation X. The caveat to this good news is that most of them need training in the fundamentals of coil processing.

Coil Training Topics: Equipment Basics

Much of what is involved in coil processing equipment operation has not changed significantly over the years. Technology upgrades have been evolutionary rather than revolutionary.

Uncoiling, straightening, and feeding material to the press certainly are the mainstays of coil processing. Training starts with becoming familiar with the equipment basics—the reel/uncoiler, straightener, and feeder—componentry, and purpose. Next, trainees learn how to operate the equipment and the processes they perform.

Short, informative videos can be useful for connecting to and teaching the basics of stamping and coil processing to employees and potential employees. Training materials have typically existed only in the form of slides, presentations, and publication articles, but in the age of TikTok and Instagram, videos can supplement them. Videos can resonate with the new generations entering the workforce. “A picture is worth a thousand words” is especially meaningful in manufacturing in which processes can be complex and verbal instructions difficult to understand and remember.

Coil Reel/Uncoiler. The process starts with the uncoiling of material from a coil reel. The coil is loaded either directly onto the mandrel, which is a rotating spindle the coil is placed on for unwinding, or onto a mandrel.

Coil reel machines come in two versions—an unpowered pull-off, or a motorized version. Pull-off reels usually have only a short, fixed-speed threading drive and rely on a power straightener. Alternatively, they can have a set of pinch rolls that pull the material off the coil during an automatic operation. Powered reels are motorized with a loop control for payoff into a slack loop and are generally used in applications that don’t require straightening, although they also can be used with a pull-through straightener powered by a feeder at the press. Either style of reel can handle a variety of coil widths and weights.

Containing the Coil. Coiled metal stores energy, similar to how a spring does, so it must be contained during storage with retaining bands. Before removing these metal bands, some other means of coil containment must be in place to prevent clock-springing from occurring. Most coil reels are equipped with hold-down or hold-up arms that contain the outer diameter of the material while the operator removes the retaining bands.

Coil keepers are another device used to contain the coil. They prevent the coil from telescoping. This occurs when the outer coil shifts out of line with the inner coil.

Straightening. The next step of the process is flattening the metal using a straightener. The steel- and aluminum-making processes inherently impart tensions and stresses into the coil. These create slight variations within the coil called crossbow, set, edge wave, buckle, camber, and twist. Typically, the purpose of a straightener in a coil feed line is to prepare the material so that it can pass freely through the die and produce an acceptable part. Straightening is executed by bending the strip around sets of rollers that stretch and compress the upper and lower surfaces alternately. By doing this, they exceed the coil’s yield point so that both surfaces end up the same length after springback, which results in flat material.

Straighteners typically are available in models with seven to 11 work rolls. The roll diameters and center distances vary, depending on material thickness and width. Generally, they are equipped with fairly large-diameter, widely spaced rollers.

The growing use of advanced high-strength steels (AHSS) has required changes to straightener designs to effectively flatten these high-tensile materials.

Processing AHSS materials correctly demands a higher number of small rollers with closer spacing than are needed for conventional steels or aluminum. This roll configuration enables the material to be stretched and compressed more effectively. However, the smaller radii and closer spacing require that the straightener be built with stronger construction materials and greater depth penetration into the material.

Feeding. Once the material is straightened, it is fed to the roll feed which delivers the material in precise lengths to a stamping press or shear. Roll feeds are driven by servomotors that allow for precise control of angular position, velocity, and acceleration. The feed must not only move the proper amount of material into the tool, it must position the material correctly into the die—front to back, side to side, and square with the tool. If the material is not positioned correctly, the result is slippage.

The Pilot Release. One of the most important aspects of running a press feed operation is making sure that the pilot release is properly adjusted. Pilot release is the act of momentarily releasing the strip to allow it to be aligned by pilot pins in a progressive or blanking die. It gives stampers the ability to position their part on a strip in a consistent manner. The pilot pins in the die will correct slight misfeeds by pulling the material into final position for forming or blanking. This momentary release helps to relieve built-up stress and binding of the strip through the feed and die caused by misalignment or camber. In addition, it prevents the coil strip from walking as it moves through the die.

Simplifying Operations With Advanced Controls

Another way to help address the labor issues facing manufacturing is by simplifying the process through software. A lot of coil processing machinery is equipped with software that provides prompts and cues to expedite tasks such as automating line setup. It may help the operator to quickly determine optimal speed, running parameters, and acceleration values for a job. Usually, these values can be saved immediately to the controller’s memory for the next time the job is run. By automating repetitive tasks, software can help employees focus on more value-added activities, such as problem-solving and product development.

A learning curve is involved in any job, but it is especially prevalent on the manufacturing floor. Delivering easy-to-digest, accessible content can help shorten the learning curve, supplement traditional employee and customer classroom instruction, or serve as a resource for those who want to learn more about these topics.

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