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).
Figure 1: Coil shape imperfections – A) Coil set and B) CrossbowA-30
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.
Figure 2: Alternately stretching and compressing the upper and lower sheet surfaces by passing the coil through work rolls.C-8
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.
Figure 3: The severity of the bending and unbending around the work changes with the roll gap, roll diameter, and roll spacing.
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.
Figure 4: Computer generated analysis showing a straightener roll working the outer 20% of the steel strip. The different colors indicate the bending force, which is symmetrical the neutral fiber (the center part that is neither compressed nor stretched). Red areas indicate stresses beyond the yield point, and yellow areas indicate material is at the yield point. Other areas are in the elastic range.T-15
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.
Figure 5: Computer generated analysis showing a leveler roll working the outer 80% of the steel strip while it is under tension. The stress pattern is not symmetrical, with higher stresses seen on the outside of each bend. Passing through the multiple small-diameter rolls under tension results in stresses exceeding the yield point through most or all of the cross section.T-15
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.
Figure 6: V-bending without roller leveling leads to fluting.K-50
Figure 7: Leveling to produce 85% plastic fraction.K-50
Figure 8: V-bending after leveling to produce 85% plastic fraction minimizes the fluting defect.K-50
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.
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.