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forming & fabricating


strength, greater tendency to retain coil set, and tighter toler- ances between yield and tensile strength. Additionally, AHSS materials are also usually thinner than traditional steels, which can present their own set of problems. Stamping AHSS typically requires increased forces, which can adversely impact tooling as well as amplifying vibra- tions that can potentially break the dies and even the press. Over the past decade, task forces have worked to develop guidelines that help stampers learn how to deal with these challenges in terms of the die and stamping process. Stampers and fabricators need to understand that to suc- cessfully process AHSS, there is a need to look at every as- pect of the stamping line—including press feeding operations and, especially, the straightening equipment and process.


Putting Experience and Science to Good Work For the past few years, COE Press Equipment (Sterling Heights, MI) has met with customers to understand their needs and to identify gaps in current product offerings, especially in terms of running the new steels. It was found that advanced steels with higher yield and tensile strengths are taking a toll on feed lines, stamping presses, and tooling. Companies are often moving these materials to larger equip- ment than normal, increasing cost and reducing effi ciency. With AHSS, there has been particular diffi culty in achiev-


Coils of AHSS are like giant coiled springs and require proper coil containment with hold-down and threading devices that control the stored energy of the coil.


The purpose of the straightening process in a typical coil feed line is to prepare the material shape so that an accept- able fi nished part can be produced from the fl attened steel. The requirements for straightening vary depending on the material type, cross section,and strength; the requirements of the die; and the fi nished part requirements. In a conventional coil feeding system straightening is accomplished by bend- ing the strip around sets of rollers to alternately stretch and compress the upper and lower surfaces to exceed its yield strength to take the material to a plastic state. Once in this state the material is reshaped so that both surfaces end up the same length after spring back, resulting in fl at material.


ing the required material fl atness during the straightening process prior to feeding the press. The historical approach of machine builders has been to design straighteners with large diameter rollers on wide center distances. As the strength of materials has increased and their thickness reduced, these convention- al straighteners can no longer effectively yield the materials and achieve acceptable fl atness tolerances. With these higher strength materials also come safety concerns. When you’re dealing with the material in coil form, it’s like a giant coiled spring. You need to con- tain the coil with hold-down and threading devices that control stored energy in the coil. The coil feeding system also requires debending and threading devices for safe and effi cient threading of the coil through the equipment into the stamping die. In general, there has been a ten- dency to overestimate the capability of the pressroom equipment in terms of


handling, straightening and feeding advanced steels. The rated capacity of most straighteners is based on processing relatively mild steel with yield strengths below 350–400 MPa. When straightening AHSS of 1000+ MPa, greater forces are required and this infl uences the straightener design including choice of components, materials and power sources.


Modifying Straightener Designs for AHSS Through targeted R&D, resulting in signifi cant straightener design modifi cations as well as successful customer applica- tions, COE has learned there are key areas of consideration when specifying a straightener to effectively process AHSS.


62 — Motorized Vehicle Manufacturing 2016


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