The first operation when unwinding a coil is straightening to ensure flatness before further processing. There are two main types of equipment used to create a flat coil – a straightener and a precision leveller. In some respects, these two types of equipment are similar, but a precision leveller has additional capabilities.
Straighteners – All coils must be wound under tension to avoid “soft” or collapsed coils. As a result, tension and compression stresses are induced into the strip during the coiling process, and these can contribute to blank or part distortion in subsequent processes. Straighteners are designed to eliminate these stresses and create flat material; the objective is to relieve the residual stresses in the outer 20% of the strip.
Straightening of coils is accomplished by bending the strip around a set of rolls to alternately stretch and compress the upper and lower surfaces of the steel to erase coil set or crossbow. Critical parameters include roll diameter, roll spacing, backup roll configuration, roll material type, gear design, overall system rigidity, and power requirements. The higher the material’s yield strength, the greater the force that’s required to relieve those residual stresses. Thus, equipment capability needs to be considered when processing AHSS products. Figure 1 shows a schematic of a straightener working the outer 20% of the steel strip. Figure 2 depicts coil set and crossbow conditions.
When utilizing a straightener, the two critical steel variables that impact the process are the yield strength and the thickness. Straighteners have a series of rolls that progressively flex the strip to remove the residual stresses. These rolls have an entry and exit gap that need to be adjusted when thickness and yield strengths vary.
Figure 3 shows a schematic of a typical set of straightener rolls required to obtain a flat strip. These gaps should be adjusted based on the yield strength and thickness of the material. Many equipment manufacturers have generated tables to guide the operator as to the best settings for various yield strength-thickness combinations.1
Precision Levellers – Although precision levellers can perform all the functions of a straightener, they have additional capabilities. They can remove coil set and crossbow conditions and are capable of correcting other shape issues in coils that can create manufacturing problems in subsequent operations, for example, edge waves and center buckles.
Correcting edge waves and distortion requires exceeding the yield strength of the steel strip at levels of 50% or more of the cross section of the strip; this may increase the potential for work hardening the steel strip, negatively affecting formability. Figure 4 shows a schematic of a precision levelling roll working 50% of the cross section of the steel strip, which is the typical process for removing edge waves and center buckles.
Design and Processing Implications
The progressively higher yield strengths for AHSS are challenging the capabilities of straighteners and precision levellers 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 components related to the design of straighteners and precision levellers that need to be considered:
Roll Diameter – Straightener rolls for AHSS generally may need to be smaller in diameter than those used for mild steel, providing a smaller radius around which to bend the material. This is because AHSS must be bent more severely to exceed its higher yield strength.
Roll Spacing – Work roll center-spacing may need to be closer for AHSS than for comparable mild steels. Closer spacing means that more force is required to back-bend the material, requiring greater power for processing.
Roll Support – Larger journal diameters with larger radii and bearing capacity may be needed to withstand the greater forces and higher power required to straighten AHSS.
1 Courtesy of Peter Mooney, 3S-Superior Stamping Solutions, LLC