Addressing springback when simulating and forming ultra-high strength automotive steels

Types of AHSS/UHSS springback

There are two types of AHSS/UHSS springback: one that occurs in the radius during forming and another that occurs in plane, creating “sidewall curl.” The amount of springback is influenced by the strength of the steel, its Young modulus, and its strain hardening.

When running a simulation of strain in radius, you cannot only use the forming limit diagram – it plots the middle layer of the steel and therefore can be quite low, e.g., 1% or 2%. By plotting all layers, including the outer layers, you can discover plastic deformation that is much higher, e.g., 11% or more.

To compare simulated AHSS/UHSS springback results with scans of an actual formed part, SSAB created a bumper design for the purposes of testing different material models. Using a simple Hill 90 isotropic hardening model, the predicted springback of the bumper correlated quite well with the actual formed part. A similar result happened when using a BBC2005 isotropic hardening model (good correlation with actual part). If you use kinematic hardening models, it is important to remember that their parameters will profoundly influence springback behavior: with the right parameters, kinematic models will deliver a very good correlation with the actual part. So it is especially important to use very good measured data for higher strength AHSS/UHSS materials when doing simulations.

Part geometry has a major influence on springback. For example, a single hat profile has much higher springback (sidewall curl) than a double hat profile, prior to any springback compensation for either design. This is because the double hat has radii going in opposite directions, so the springbacks roughly “cancel out” each other.

To compensate for springback on a single hat profile, if you’re using AHSS that is, for example, dual phase 600MPa or 800MPa, you could try to stretch out the wall of the part, then use a sharper radius, and therefore better influence the deformation of the wall. But on higher strength gigapascal steel grades (1000MPa or more) you need another solution. You could have a cam inside the tool to change the forming direction and then vary the friction, bringing the part's variation from, for example, in excess of 0.7mm to a maximum of 0.5mm. The Springback Webinar shows a video of this multi-step forming process.

SSAB’s BendCalc app and the material model for estimating springback in pure bending

SSAB's phone app, BendCalc, is the first software to predict real springback in the pure bending of AHSS/UHSS materials. Settings include selecting your Docol^® steel grade, your final desired angle, your geometry, friction level, and other setup conditions. BendCalc is free to download from the App Store or Google Play.

The theoretical model behind BendCalc is as follows. First, SSAB made bending tests under friction-free conditions, similar to the VDA 238-100 bending test procedure, to plot the force curve versus the stroke length (or position). These data were then transposed to cross-section moment and bending angle, carefully taking notes of the geometry for the bending setup.

By applying the estimated moment it is also possible to continuously calculate the current shape of the curvature angle of the part's flanges – and the angle of contact between the blank and the knife. All of these individual angles are separately influencing the final amount of AHSS/UHSS springback. The BendCalc model has good agreement with a huge number of actual SSAB bending tests internally performed, allowing you to enter your geometry setup to estimate the springback, stroke length, maximum force, and other parameters.

Stabilizing AHSS/UHSS forming via recommended tool grades

SSAB has over 40 years of experience in tool steel solutions for HSS. Our know-how is based on real-world production results from our customers, as well as attending different R&D projects studying long-run series. We encourage you to download our 40-page Tooling Solutions for Advanced High Strength Steels booklet for specific tool steel recommendations.

In forming operations, tooling failure mechanisms can include galling, wear, and plastic deformation, while blanking operations can also include chipping and cracking failures. With non-optimal tool steel methods, friction can build up over time, increasing the springback effect and putting your parts out of tolerance. Another problem can be chipping of the tool during blanking, resulting in poor cutting edges that lead to sudden risk of fractures in flanges.

SSAB provides very specific guidance for tool steel grades for the stamping of its Docol^® AHSS/UHSS steel grades, with hardnesses ≥ 60 HRC to prevent plastification and typically recommending surface treatments (coatings). The initial higher costs for these high performance tool steels will be more than offset by a reduction in production stoppages and tooling refurbishments/maintenance costs.

Stamping higher strength AHSS/UHSS grades means higher pressures and an increased risk for galling. To ensure stable friction conditions, design a tool insert and use PVD/CVD or Duplex treated tooling steel. Before adding the coating, the polishing process is very essential. Polish the surface finish to at least R_a< 0.2 µm and, at very critical areas, down to R_a 0.05 µm to avoid cracking, galling, and increased friction.

For cold forming and blanking of AHSS/UHSS grades, consider SSAB's Toolox^® tooling steel, with a surface-hardening capability for PVD, laser, induction, and nitriding. Toolox^® steel has less friction due to its high level of micro cleanliness, a very high dampening ratio because of its vibration resistance, high fatigue strength, and high resistance to chipping and cracking.