This higher yield strength of AHSS presents challenges to metal stampers in terms of formability and a greater tendency to retain coil set. These materials also have wider tolerances on their yield strength, yet narrower tolerances between their yield and tensile strength. Therefore, the force required to yield and form the material cannot be excessive because of the potential to take the material past its tensile strength.
Stamping AHSS typically requires increased forces, which can adversely affect tooling as well as amplify vibrations that can potentially damage the dies and even the press. Over the past decade, task forces have been formed to develop guidelines that help stampers gain the knowledge they need to help deal with these challenges in terms of the die and stamping process.
However, to successfully process AHSS, stampers need to look at every aspect of the stamping line, including the press feeding operations and, in particular, the straightening equipment and process.
Traditional Straightening Technologies Overview
In a typical coil feed line, the purpose of straightening is to prepare the material shape so that an acceptable finished part can be produced from the flattened steel. Straightening requirements vary depending on the material type, material strength, and the finished part. Straightening is accomplished by bending the strip or coil around sets of rollers to stretch and compress the upper and lower surfaces alternately to exceed its yield and remove coil set (see Figure 1). Then it is re-formed so that both surfaces end up the same length, accounting for springback. This yields flat material.
A basic principle of straightening is that thick materials require fewer rollers with more spacing between them than thin materials do, but that as material thickness increases, the roller diameters and support journal diameters must increase also. The work rolls must be capable of withstanding the forces required to reverse-bend the material without excessive deflection across their width.
Conversely, thin materials require a greater number of relatively smaller-diameter rollers, with the spacing of the rollers relatively close, to effectively stretch and compress the material. That is because as material and machine widths increase, the small-diameter rollers tend to flex and deflect more. Deflection of the straightening roll or journals can lead to material defects such as wavy edges, and machine problems such as premature gear wear and broken journals.
Design Considerations for Straightening AHSS
Stampers have a tendency to overestimate the pressroom equipment’s ability to handle AHSS. The rated capacity of most straighteners is based on processing relatively mild steel with yield strengths below 340 MPa [50,000 pounds per square inch (PSI)]. Greater forces are required to straighten AHSS, and this influences the straightener design, including choice of components, materials, and power sources. Some key areas of consideration when specifying a straightener to effectively process AHSS are work rollers, system rigidity, component strength, and horsepower (see Figure 2).
Work Rollers. In terms of design, bigger does not mean better. With traditional straighteners, conventional wisdom held that processing stiff materials required large-diameter rolls with wide roll spacing. Actually high-tensile-strength materials such as AHSS require more small-diameter work rollers than mild steel does to effectively stretch and compress the material. This is because AHSS has to be bent more severely around a smaller radius to exceed its higher yield point.
Also, work roller center spacing must be closer for AHSS than for mild steel. Closer spacing means that more force is required to reverse-bend the material and greater power is required to process it. However, tight work roller center spacing leaves little room for the work roller force delivery mechanism. Conventional off-the-shelf screw-jack designs cannot provide adequate force in this reduced space. This has led to the development of custom worm gear modules, which can be designed to fit the available space and deliver the high forces required to effectively yield AHSS.
The straightener must also be designed so that there is adequate travel, or penetration, of the upper adjustable rolls and lower fixed rolls to effectively yield AHSS materials. This distance can be as much as 60 percent greater than what is required for conventional straighteners designed for mild cold-rolled steel.
System Rigidity. The greater force required to bend AHSS requires more support. So the machine width, roller, and gearing journals must be supported better, and backup structural bridges must be more rigid.
Backup Rollers. Here’s where “wider and stronger” comes into play. Given the smaller roll diameters and great force requirements for straightening AHSS, work roll deflection becomes a much bigger concern. As material and machine widths increase, the tendency for the small-diameter rollers to flex and deflect also increases. Excessive work roll deflection results in undesirable side effects, such as material edge wave, increased journal stresses, and premature gear failures. Larger-than-usual backup rollers often are required at the center and intermediate positions of the straightener roll to prevent work roll deflection.
Backup Roller Support. Backup roller support is vital to ensuring the effectiveness of the backup rollers to straighten AHSS and prevent work roll deflection. Backup roller face width and the cross-section of the support structure are key to providing proper work roll support. Simply adding screw jacks to narrow crossbars will not provide the backup roller support necessary to achieve a good result.
Journal Support. Outboard journal support is critical to providing the rigidity necessary to withstand higher horsepower and torque requirements of AHSS and improve the system gear ratings. Deflection of the straightening rolls or journals can cause machine problems such as broken journals and excessive gear wear.
Robust Components. Although the work rollers for AHSS are smaller than for mild steel, there is a tendency for many of the support mechanisms to be larger. In addition to large outboard journal support, wide gear faces and idler shafts are needed to produce high gear power ratings.
In addition, large journal diameters with big radii and large bearing capacity are needed to withstand the great roll force required to straighten high-strength material using the closer roller center spacing.
Overall, very durable materials are needed in all of the components. Rollers should be produced from high-strength steels, and special heat treatments should be applied to ensure that they can withstand extreme stresses for long periods without experiencing fatigue failure. Gears should be produced from heat-treated high-strength metals.
Motor. In general, processing AHSS requires greater motor power and torque capability to effectively pull the material through the straightener. One size does not fit all. Calculating the correct power is based on many factors, including:
- Material characteristics (thickness, width, and yield strength)
- Maximum coil weight
- Required processing speed
- Response/acceleration time
Each application has unique combinations of these variables, which can result in an almost infinite number of outcomes.