When flat is where it’s at …

Heavy-duty straighteners have been engineered to tackle the processing of AHSS materials, such as those with 1,000 megapascal (MPa) tensile strengths.

As automakers increasingly rely on lightweighting and advanced structural materials to achieve vehicular fuel efficiency goals, the use of materials with tensile strengths of more than 1,000 megapascal (MPa) continues to grow. Research shows advanced high-strength steels (AHSS) to be the fastest-growing material in automotive applications. Forecasts indicate that these steels will comprise 40 percent of vehicle content on body and closures by 2025.

However, the complex steel chemistries and thermomechanical properties of AHSS come with formability challenges. These high-strength metals have a greater tendency to retain coil set (the residual curve that results from having been wound into a coil) and have closer yield and tensile strength ratings than milder steels. This results in a finer line between forming the material and splitting it. The American Iron & Steel Institute (AISI) has published guidelines to help with stamping and forming AHSS materials, but the challenge of attaining the flatness required to properly feed AHSS into the press has been largely ignored.

Stamping manufacturers’ struggles with uncoiling, straightening, and feeding high-tensile-strength steels have increased over the past five years as the materials have gotten stronger. Coil feed equipment older than five years typically does not deliver enough force to the work rolls to adequately counterbend coil-set AHSS material. Furthermore, older equipment typically is not designed with the structural features required to process these new materials effectively.

Straightening materials with yield strengths of more than 1,000 MPa requires greater forces than for those with lower strengths and, therefore, a significantly different straightener design to provide sufficient roll support. In addition, the straighteners require gear train reliability because the roll force delivery needed to feed AHSS causes roll deflection, which, in turn, causes gear wear and the journal ends to snap.

New Straightener-Feeder Design

A new generation of heavy-duty straighteners have been designed with significant design modifications to the straightener heads for better processing of AHSS materials (see lead photo). These straighteners also are designed to process a wider variety of materials—both thick and thin—than conventional straighteners used in the metal stamping industry.

Whereas conventional straighteners typically incorporate a series of seven to 11 large-diameter work rolls, the new generation of heavy-duty straighteners have borrowed innovations from precision straightener technology by incorporating a series of relatively small-diameter work rolls.

Other design enhancements include high-strength construction frame materials, tighter straightener roll spacing, and increased roll depth penetration. The new straightener heads also are equipped with increased roll force delivery, stronger gears and bearings, and an improved straightener roller backup mechanism to provide the rigidity needed to process AHSS materials effectively.

Heavy-duty Straightener in Action

Tier 2 automotive supplier Nahanni Steel Products, Brampton, Ont., encountered challenges while processing thin-gauge, high-tensile steel and other materials (see Figure 1). One of the key problems Nahanni faced with AHSS was trying to obtain the required material flatness before feeding it into the press. In addition to typical coil set, the company encountered consistent crossbow, in which the material curves across the coil width. Overall, the increasing use of AHSS was taking a toll on Nahanni’s feed lines and tooling.

The company sought technologies to help overcome its processing challenges. Its wish list of improvements included better safety, throughput, quality, and setup times, as well as a reduced footprint.

Nahanni installed Coe’s patent-pending SpaceMaster^® Series 4 Compact Coil line. This one piece of equipment unwinds, straightens, and feeds coil stock in yield strengths up to 1,000 MPa.

Figure 1
Tier 2 automotive supplier Nahanni Steel Products encountered problems with coil set and crossbow while trying to obtain the required material flatness before feeding it into the press. The entire coil feed system had been redesigned to stand up to advanced high-strength metals.

The system features a pull-off roll that maintains tension on the coil during the pull-off process and helps feed slack material at a constant speed. The feeder-straightener also is equipped with a servo-eccentric pilot release mechanism.

All of these features improved the material flatness as well as operator safety.

Flatness for Fineblanking

Although fineblanking equipment has been on the market for more than 40 years and fineblanked components are common in vehicles, it is not an easy process. Fineblanking requires particularly tight tolerances in two aspects: the part’s external dimensions and flatness.

UTIL, Concord, Ont., is a global manufacturer of backing plates and brake shoes in vehicle braking systems. Its customers demanded tighter and tighter tolerances, especially in braking system components because deviations hinder caliper performance (see Figure 2). The manufacturer looked for a new straightener that would improve flatness and output. The crew was having to stop the line to adjust the material. It needed a system that would straighten the coil material consistently to stabilize production and eliminate disruptions.

UTIL contacted Coe and provided a list of materials it was running, including AISI 3040, 4040, and 5149 (at 80 millimeters thick) steel grades, and the feed and speed parameters the company required from new straightener equipment.

The manufacturer installed Coe’s Fineblanking Edition HD straightener. The heavy-duty straightener can process coil widths from 1 to 36 inches; thickness up to 0.500 in.; weights up to 40,000 pounds; and yield strengths up to 1,000 MPa while holding flatness tolerances of 0.003-in. total indicator reading per square foot.

Fineblanked parts often are stamped out of thick, high-strength material, which can be dangerous to handle manually. To ensure operator safety, the coil lines’ reels are equipped with multiple hydraulic hold-downs with motorized end wheels to contain the coils. The mandrel and motorized end wheels rotate in unison for effective threading and rewinding. Additionally, Coe’s pivoting “alligator-style” peeler-debender is a hands-free, modular approach to threading.

Since its installation in early 2017, the equipment is proving its ability to deliver precision and safety.

Bruce Grant is R&D manager for Coe Press Equipment, 586-979-4400, www.cpec.com.

Figure 2
Global manufacturer of backing plates and brake shoes in vehicle braking systems UTIL looked for a new straightener that would improve flatness and output to meet increasing demands for tight tolerances.Global manufacturer of backing plates and brake shoes in vehicle braking systems UTIL looked for a new straightener that would improve flatness and output to meet increasing demands for tight tolerances.