Formability Characterization of 3rd Generation Advanced High-Strength Steel and Application to Forming a B-Pillar 2021-01-0267

The objective of this study was to assess the formability of two 3rd generation advanced high strength steels (3rd Gen AHSS) with ultimate strengths of 980 and 1180 MPa and evaluate their applicability to a structural B-Pillar for a mid-sized sport utility vehicle. The constitutive behavior including strain-rate effects and formability were characterized to generate the material models for use within AutoForm R8 software to design the B-pillar tooling and forming process. An extended Bressan-Williams instability model was able to deterministically predict the forming limit curves obtained using Marciniak tests. The tooling for the representative B-pillar was designed and fabricated with Bowman Precision Tooling and forming trials conducted for both 3rd Gen steels that had a thickness of 1.4 mm. The 3rd Gen 980 B-pillar was successfully formed in accordance with the predictions of the numerical models while the 3rd Gen 1180 was predicted to have significant failure based upon the in-plane FLC. Most areas of splitting were concluded to be false-positive predictions since the 3rd Gen 1180 B-pillar only fractured in one location of approximately in-plane uniaxial tension. The predicted splitting regions were in areas of local bending and tool contact that are currently not well accounted for in the traditional approach to formability evaluation using an in-plane FLC. To fully exploit the enhanced formability of 3rd Generation steels, the dynamic nature of forming limits in light of bend severity and contact pressure effects need to be considered.