Lightweight Potential of Ultra High Strength Steel Tubular Body Structures

Horst Lanzerath; Markus Tuerk

doi:10.4271/2015-01-0570

Hot-formed steels, also called “Boron steels” or Ultra-High Strength Steels-UHSS, offer a great weight saving potential versus conventional cold-formed high strength steels used for crash relevant structural parts.

Boron steels allow complex shaped parts due to the hot-forming process. In the hot forming process first the sheet metal with initial yield strength of around σ_y=400 MPa is blanked and then heated in an oven up to ∼950° Celsius. In the next step the “hot” sheet metal is stamped and at the same time rapidly cooled down and quench hardened in the stamping die.

During this process the yield and ultimate tensile strength increase up to approximately σ_y>1100 MPa and UTS∼1500 MPa in the final stamped part. The enormous strength and the very good dimensional tolerances with nearly no springback result in the use of more and more hot-formed parts in the body, especially for crash relevant parts like structural reinforcements.

Hydroforming enables tubular designs and is applied in high volume production for body structure parts. Tubular designs are structurally very efficient, because of the closed section & continuous joint in the tube. But for complex shaped body parts hydroforming is nowadays limited to cold formable Advanced High Strength Steels (AHSS) with strength up to UTS∼1000 MPa.

The ACCRA process combines the benefit of hot-forming and hydroforming by enabling tubular designs with quench-hardened Boron steel mechanical properties. ACCRA is a trade name for several forming and hardening processes for tubular designs. The focus in the paper will be on the Form-Blow Hardening (FBH) process. In this process a tubular Boron steel pre-material is going through a process of hot-forming, air-pressure forming and water quenching resulting in a tubular structure with an ultimate tensile strength of UTS∼1500 MPa.

The paper will give an overview on

The ACCRA process including FBH,
The benefits of tubular UHSS designs compared to conventional stamped designs
Application samples to demonstrate the lightweight potential of Boron tubular steels designs for structural applications.

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Lightweight Potential of Ultra High Strength Steel Tubular Body Structures 2015-01-0570

SAE MOBILUS