Roll Forming takes a flat sheet or strip and feeds it longitudinally through a mill containing several successive paired roller dies, each of which incrementally bend the strip into the desired final shape. The incremental approach can minimize strain localization and compensate for springback. Therefore, roll forming is well suited for generating many complex shapes from Advanced High-Strength Steels, especially from those grades with low total elongation such as martensitic steel. The following video, kindly provided by Shape Corp.S-104, highlights the process which can produce either open or closed (tubular) sections.
The number of pairs of rolls depends on the sheet metal grade, finished part complexity, and the design of the roll forming mill. A roll forming mill used for bumpers may have as many as 30 pairs of roller dies mounted on individually driven horizontal shafts.A-32
Roll forming is one of the few sheet metal forming processes requiring only one primary mode of deformation. Unlike most forming operations which have various combinations of forming modes, the roll forming process is nothing more than a carefully engineered series of bends. In roll forming, metal thickness does not change appreciably except for a slight thinning at the bend radii.
Roll forming is appropriate for applications requiring high-volume production of long lengths of complex sections held to tight dimensional tolerances. The continuous process involves coil feeding, roll forming and cutting to length. Notching, slotting, punching, embossing, and curving combine with contour roll forming to produce finished parts off the exit end of the roll forming mill. In fact, companies directly roll form automotive door beam impact bars to the appropriate sweep and only need to weld on mounting brackets prior to shipment to the vehicle assembly line.A-32 Figure 1 shows example automotive applications that are ideal for the roll forming process.
Roll forming can produce AHSS parts with:
- Steels of all levels of mechanical properties and different microstructures.
- Small radii depending on the thickness and mechanical properties of the steel.
- Reduced number of forming stations compared with lower strength steel.
However, the high sheet-steel strength means that forces on the rollers and frames in the roll forming mill are higher. A rule of thumb says that the force is proportional to the strength and thickness squared. Therefore, structural strength ratings of the roll forming equipment must be checked to avoid bending of the shafts. The value of minimum internal radius of a roll formed component depends primarily on the thickness and the tensile strength of the steel (Figure 2).
As seen in Figure 2, roll forming allows smaller radii than a bending process. Figure 3 compares CR1150/1400-MS formed with air-bending and roll forming. Bending requires a minimum 3T radius, but roll forming can produce 1T bends.S-30
The main parameters having an influence on the springback are the radius of the component, the sheet thickness, and the strength of the steel. As expected, angular change increases for increased tensile strength and bend radius (Figure 4).
Figure 5 shows a profile made with the same tool setup for three steels at the same thickness having tensile strength ranging from 1000 MPa to 1400 MPa. Even with the large difference in strength, the springback is almost the same.
Citation A-33 provides guidelines for roll forming High-Strength Steels:
- Select the appropriate number of roll stands for the material being formed. Remember the higher the steel strength, the greater the number of stands required on the roll former.
- Use the minimum allowable bend radius for the material in order to minimize springback.
- Position holes away from the bend radius to help achieve desired tolerances.
- Establish mechanical and dimensional tolerances for successful part production.
- Use appropriate lubrication.
- Use a suitable maintenance schedule for the roll forming line.
- Anticipate end flare (a form of springback). End flare is caused by stresses that build up during the roll forming process.
- Recognize that as a part is being swept (or reformed after roll forming), the compression of metal can cause sidewall buckling, which leads to fit-up problems.
- Do not roll form with worn tooling, as the use of worn tools increases the severity of buckling.
- Do not expect steels of similar yield strength from different steel sources to behave similarly.
- Do not over-specify tolerances.
Guidelines specifically for the highest strength steelsA-33:
- Depending on the grade, the minimum bend radius should be three to four times the thickness of the steel to avoid fracture.
- Springback magnitude can range from ten degrees for 120X steel (120 ksi or 830 MPa minimum yield strength, 860 MPa minimum tensile strength) to 30 degrees for M220HT (CR1200/1500-MS) steel, as compared to one to three degrees for mild steel. Springback should be accounted for when designing the roll forming process.
- Due to the higher springback, it is difficult to achieve reasonable tolerances on sections with large radii (radii greater than 20 times the thickness of the steel).
- Rolls should be designed with a constant radius and an evenly distributed overbend from pass to pass.
- About 50 percent more passes (compared to mild steel) are required when roll forming ultra high-strength steel. The number of passes required is affected by the number of profile bends, mechanical properties of the steel, section depth-to-steel thickness ratio, tolerance requirements, pre-punched holes and notches.
- Due to the higher number of passes and higher material strength, the horsepower requirement for forming is increased.
- Due to the higher material strength, the forming pressure is also higher. Larger shaft diameters should be considered. Thin, slender rolls should be avoided.
- During roll forming, avoid undue permanent elongation of portions of the cross section that will be compressed during the sweeping process.
Roll forming is applicable to shapes other than long, narrow parts. For example, an automaker roll forms their pickup truck beds allowing them to minimize thinning and improve durability (Figure 6). Reduced press forces are another factor that can influence whether a company roll forms rather than stamps truck beds.
Traditional two-dimensional roll forming uses sequential roll stands to incrementally change flat sheets into the targeted shape having a consistent profile down the length. Advanced dynamic roll forming incorporates computer-controlled roll stands with multiple degrees of freedom that allow the finished profile to vary along its length, creating a three-dimensional profile. The same set of tools create different profiles by changing the position and movements of individual roll stands. In-line 3D profiling expands the number of applications where roll forming is a viable parts production option.
One such example are the 3D roll formed tubes made from 1700 MPa martensitic steel for A-pillar / roof rail applications in the 2020 Ford Explorer and 2020 Ford Escape (Figure 7). Using this approach instead of hydroforming created smaller profiles resulting in improved driver visibility, more interior space, and better packaging of airbags. The strength-to-weight ratio improved by more than 50 percent, which led to an overall mass reduction of 2.8 to 4.5 kg per vehicle.S-104
In summary, roll forming can produce AHSS parts with steels of all levels of mechanical properties and different microstructures with a reduced R/T ratio versus conventional bending. All deformation occurs at a radius, so there is no sidewall curl risk and overbending works to control angular springback.
Traditional roll forming creates products with essentially uniform cross sections. A newer technique called Roll Stamping enhances the ability to create shapes and features which are not in the rolling axis.
This approach can be applied to a conventional roll forming line. In the example of an automotive door impact beam, the W-shaped profile in the central section and the flat section which attaches to the door inner panel are formed at the same time, without the need for brackets or internal spot welds (Figure 9).
A global automaker used this method to replace a three-piece door impact beam made with a 2.0 mm PHS-CR1500T-MB press hardened steel pipe requiring 2 end brackets to attach it to the door frame with a one-piece roll stamped CR900Y1180T-CP complex phase steel impact beam. This substitution resulted in a 10% weight savings and 20% cost savings.