How Steel Properties Influence the Roll Forming Process
You’ll find this content as part of our page on Roll Forming, but this month, we want to highlight it in our AHSS Insights blog. Thanks to Brian Oxley, Product Manager, Shape Corporation, and Dr. Daniel Schaeffler, President, Engineering Quality Solutions, Inc., and Technical Editor – Metallurgy and Forming, AHSS Application Guidelines, for this case study.
Roll forming is no longer limited to producing simple circular, oval, or rectangular profiles. Advanced cross sections, such as those shown in Figure 1, highlight some profile designs that aid in body structure stiffness and packaging space reductions.
Figure 1 – Roll forming profile design possibilities. Courtesy of Shape Corporation.
Optimizing the use of roll forming requires understanding how the sheet metal behaves through the process. Making a bend in a roll-formed part occurs only when forming forces exceed the metal’s yield strength, causing plastic deformation to occur. Higher-strength sheet metals increase forming force requirements, leading to the need to have larger shaft diameters in the roll forming mill. Each pass must have greater overbend to compensate for the increasing springback associated with the higher strength.
Although a high-strength material requires greater forming loads, grades with higher yield strength can resist stretching of the strip edge and prevent longitudinal deformations such as twisting or bowing.
Force requirements for piercing operations are a function of the sheet tensile strength. High strains in the part design exceeding uniform elongation resulting from loads in excess of the tensile strength produce local necking, representing a structural weak point. However, assuming the design does not produce these high strains, the tensile strength has only an indirect influence on the roll forming characteristics.
Yield strength and flow stress are the most critical steel characteristics for roll forming dimensional control. Receiving metal with limited yield strength variability results in consistent part dimensions and stable locations for pre-pierced features.
Flow stress represents the strength after some amount of deformation and is therefore directly related to the degree of work hardening: starting at the same yield strength, a higher work hardening steel will have a higher flow stress at the same deformation.
Two grades are shown in Figure 2: ZE 550 and CR420Y780T-DP. ZE 550, represented by the red curve, is a recovery annealed grade made by Bilstein having a yield strength range of 550 to 625 MPa and a minimum tensile strength of 600 MPa, while CR420Y780T-DP, represented by the blue curve, is a conventional dual phase steel with a minimum yield strength of 420 MPa and a minimum tensile strength of 780 MPa. For the samples tested, ZE 550 has a yield strength of approximately 565 MPa, where that for CR420Y780T-DP is much lower at about 485 MPa. Due to the higher work hardening (n-value) of the DP steel, its flow stress at 5% strain is 775 MPa, while the flow stress for the HSLA grade at 5% strain is 620 MPa.
Figure 2 – Stress-strain curves for CR420Y780T-DP (blue) and ZE 550 (red). See text for description of the grades.
In conventional stamping operations, this work hardening is beneficial to delay the onset of necking. However, the use of dual-phase steels and other grades with high n-value can lead to dimensional issues in roll-formed parts. Flow stress in a given area is a function of the local strain. Each roll station induces additional strain on the overall part, and strains vary within the part and along the edge. This strength variation is responsible for differing springback and edge wave across a roll formed part.
Unlike conventional stamping, grades with a high yield/tensile ratio where the yield strength is close to the tensile strength are better suited to produce straight parts via roll forming.
Total elongation to fracture is the strain at which the steel breaks during tensile testing, and is a value commonly reported on certified metal property documents (cert sheets). As observed on the colloquially called “banana diagram”, elongation generally decreases as the strength of the steel increases.
For lower-strength steels, total elongation is a good indicator of a metal’s bendability. Bend severity is described by the r/t ratio or the ratio of the inner bend radius to the sheet thickness. The metal’s ability to withstand a given bend can be approximated by the tensile test elongation since, during a bend, the outermost fibers elongate like a tensile test.
In higher-strength steels where the phase balance between martensite, bainite, austenite, and ferrite plays a much larger role in developing strength and ductility than in other steels, microstructural uniformity usually limits bendability. Dual-phase steels, for example, have excellent uniform elongation and resistance to necking coming from the hardness difference between ferrite and martensite. However, this large hardness difference is also responsible for relatively poor edge stretchability and bendability. In roll-forming applications, those grades with a uniform microstructure will typically have superior performance. As an example, refer to Figure 2. The dual-phase steel shown in blue can be bent to a 2T radius before cracking, but the recovery annealed ZE 550 grade with noticeably higher yield strength and lower elongation can be bent to a ½T radius.
Remember that each roll-forming station only incrementally deforms the sheet, with subsequent stations working on a different region. Roll-formed parts do not need to use grades associated with high total elongation, especially since these typically have a bigger gap between yield and tensile strength.
We encourage you to visit https://ahssinsights.org/forming/roll-forming/roll-forming/ to learn more about roll forming and the types of coil shape that influence roll forming. Thank you to Brian Oxley and Dr. Daniel Schaeffler for providing this case study.
Brian Oxley, Product Manager, Shape Corporation, is a Product Manager in the Core Engineering team at Shape Corp. Shape Corp. is a global, full-service supplier of lightweight steel, aluminum, plastic, composite and hybrid engineered solutions for the automotive industry. Brian leads a team responsible for developing next generation products and materials in the upper body and closures space that complement Shape’s core competency in roll forming. Brian has a Bachelor of Science degree in Material Science and Engineering from Michigan State University.
Danny Schaeffler is the Metallurgy and Forming Technical Editor of the AHSS Applications Guidelines available from WorldAutoSteel. He is founder and President of Engineering Quality Solutions (EQS). Danny wrote the monthly “Science of Forming” and “Metal Matters” column for Metalforming Magazine, and provides seminars on sheet metal formability for Auto/Steel Partnership and the Precision Metalforming Association. He has written for Stamping Journal and The Fabricator, and has lectured at FabTech. Danny is passionate about training new and experienced employees at manufacturing companies about how sheet metal properties impact their forming success.