Making adjustments for stronger steel
December 13, 2005
With its higher tensile and yield strengths, HSS is stronger at any given gauge than conventional "mild" steels. Initially, lighter-gauge HSS was used to replace heavier-gauge mild steel to reduce weight in many automotive parts, and this continues to be the most widely used application for HSS. However, as a new trend to run thicker and thicker gauges of HSS continues, stamping fabricators are evaluating their press feeding and coil handling equipment, as well as press tonnage capacities and die designs to ensure that these harder parts can be formed effectively. The dynamics of HSS have a bearing on feeding, straightening, and coil handling equipment.
High-strength steel (HSS) rapidly is becoming a popular material for the automotive industry. Why?
With its higher tensile and yield strengths, HSS is stronger at all gauges than conventional mild steels. Initially light-gauge HSS was used to replace heavy-gauge mild steel to reduce weight in many automotive parts, and this continues to be the most widely used application for HSS. However, a growing trend is to run thicker and thicker gauges of HSS.
As this trend continues, stamping fabricators are evaluating their current press tonnages and die designs to ensure that these thicker-gauge parts can be formed effectively. Not to be overlooked is the need to carefully review the press feeding and coil handling equipment either currently installed or being specified for a new project. The dynamics of HSS have a bearing on feeding, straightening, and coil handling equipment.
High-strength steel affects the size and configuration of four major pieces of coil feed line equipment —the feeder, straightener, feeder/straightener combination units, and coil handling equipment.
Feeder. Because HSS is harder than its mild steel relative, the amount of space for the slack loop behind the feeder may need to be increased. Therefore, it may be necessary to review the torque the servo feeder generates and to ask some questions to determine if adjustments need to be made: Will more material be in the loop? Will the loop be too tight and put pressure on the feed rolls when they are closed? If the answer to either or both of these questions is yes, it would be wise to ensure there is adequate torque in the feeder to pull this heavier and tighter loop. The feed equipment manufacturer can supply the current torque output of the feeder and make appropriate recommendations.
Straightener. Nowhere else in a coil feeding system does HSS require more attention than in the straightener. High-strength steel cannot be expected to run with the same straightener roll diameters and roll spacing as conventional mild steels for a given material's width and thickness. The harder the material, the bigger the rolls and the wider the spacing need to be between the rolls.
As a rule of thumb, HSS requires straightener equipment that can process mild steel that is 50 percent thicker than the HSS.
For example, if the application calls for 24-inch-wide by 0.250-in.-thick material at 80,000 pounds per square inch (PSI), then the straightener should be sized as though it were going to be processing 24-in. by 0.375-in.-thick mild steel. This calculation will allow the journals, or axles, of the straightener rolls to be proportionally larger as well. This is necessary to prevent the journals from being overly stressed by the added pressure caused by penetrating the harder material. What's more, deeper roll penetration often is required to remove coil set from HSS (curvature in the coil induced during the slitting process). The larger rolls, larger journals, and broader center distances safeguard the straightener from potential damage from these additional stresses.
A drawback to increasing the roll size and center distance on straighteners to accommodate HSS is that doing so alters the range of materials that can be straightened effectively. Whereas a typical straightener running 24-in. by 0.250-in. mild steel might still be able to straighten 0.060-in. material effectively, a straightener sized to run the same width and thickness of HSS might be able to effectively straighten only 0.100-in.- to 0.125-in.-thick material. This limitation is due to the larger rolls and broadly spaced centers that are necessary to run the HSS. To counteract this, conventional feed lines can have installed a smaller pull-through straightener behind the feeder, in addition to the larger high-powered straightener sized for the thicker HSS.
Feeder/Straightener Combination Unit. An alternative to a separate feeder and straightener is a feeder/ straightener combination unit because of the space it can save. This combination unit is available in two basic styles: a servo feed with a pull-through straightener (straightener rolls are nondriven) and a servo feeder/straightener combination (all feeder and lower straightener rolls). The same considerations that apply to the stand-alone feeder and straightener apply to both of these styles.
The torque requirements for a feeder/straightener always are greater than they are for only the feeder because of the increased power required to move the material through many more rolls and to straighten it. This torque requirement is even greater for HSS. Care should be exercised when comparing systems to ensure the proper torque requirements have been calculated for the application, especially when stamping HSS.
Pilot release is another issue for feeder/straightener combination units. If the application calls for running progressive dies with pilots and the material thickness exceeds 0.080 in., the pilot release for all upper rolls should be given extra attention. Because HSS is so much harder than mild steel, the straightener rolls generally penetrate more deeply into the material. Without a pilot release of all upper rolls (including straightener rolls), it is difficult for the pilots to register properly if the feeder is not perfectly accurate. Even with a very accurate servo feed, side-to-side adjustment for camber will be hindered if the upper rolls do not release.
Coil Handling Equipment. Two types of coil handling equipment designs are reels and cradles.
Reels offer greater versatility than cradles and are better for surface-critical materials because they engage the coil by expanding on its inner diameter. A cradle's nest roll engages the coil on its outer diameter, which can scuff the coil's surface.
However, with a reel, safety is a concern. Heavy-duty hold-down and threading devices need to be used with HSS, especially with gauges thicker than 0.125 in. If the material exceeds 0.250 in. thick, hydraulically actuated (rather than pneumatic) hold-down arms, peelers, and lead-edge debenders should be used to help keep the operator safe. In addition, many of these devices make hands-free threading possible, which is desirable with HSS. Space-saving designs employing reels also should have safety containment and threading devices.
Cradles are a simpler coil handling device. The coil is loaded onto nest rolls that decoil the material, either with a set of pinch rolls or a powered straightener. The advantage of the cradle is that the coil can clockspring, or unravel, when the bands are cut within the confines of the cradle's side plates. With very thick and hard materials, a hold-down arm is also used with cradles, as are peelers and debenders. A drawback to using cradles is that they usually are quite difficult to rewind a partially used coil because of the coil's clockspringing during band cutting and threading.
In the final analysis, each HSS application must be reviewed carefully.
If existing equipment is used to run HSS, care should be exercised to ensure that the equipment is built properly for the HSS application. Most capacity tags on feeds and straighteners have been calculated for mild steel, so they should not be used as a guideline for running HSS in the press. Discussions with the OEM will prove beneficial in converting capacities from mild steel to HSS.
For new-equipment specifications, it is paramount to relay the correct width and thickness ranges for the HSS, along with the maximum hardness of the HSS, to the prospective coil equipment suppliers. Only then can the necessary adjustments be applied to ensure that the equipment will provide years of fine service. Stampers should be sure to ask the prospective builder for the modifications anticipated to accommodate the HSS in the specification.
To be sure, HSS can offer great weight and cost savings to stampers and contract manufacturers. By exercising a bit of foresight and care when specifying new equipment and carefully considering the capacities of existing equipment, most metal stamping suppliers will experience fewer maintenance headaches, more uptime, and enhanced profits.