Cutting weight, gaining strength
Why the future looks bright for AHSS and UHSS.
New grades of innovative, high-performance steels are evolving at a rapid pace. Besides being stronger and showing improved safety performance in some applications over mild steel, they contribute to lighter constructions that result in less fuel consumption and ultimately a cleaner world.
Advanced high-strength steel (AHSS) and ultra-high-strength steel (UHSS) are helping the auto industry in particular, and Docol® steel is taking part in this. In modern cars from nearly all of the world’s major OEMs, it can be found in an increasing number of components — from seat frames and protective side-impact and roof beams all the way to the bumper.
The strength of Docol® AHSS enables designers to use less material without compromising structural performance. Dual phase and martensitic steels are available with very high initial yield strengths. They also exhibit a pronounced work-hardening effect when formed and a bake-hardening effect from the painting process.
The work-hardening effect is typically about 150 MPa at a strain of 2%, and the corresponding bake-hardening effect another 50 MPa for a typical painting process.
In addition to the work- and bake-hardening effects, AHSS exhibits a strain-rate hardening effect, sustaining higher stresses at a higher deformation speed. This effect corresponds to an increase of about 100 MPa for the high-strain rates that are involved locally in a car crash.
The high strength of AHSS can help reduce the weight of body-in-white (BIW), bumper systems, door side-impact beams, seat structures and more. To highlight the potential more fully, the figures below show two theoretical examples. If a pure membrane stress state is present in the sheet (Fig. 1a), the stress is linear proportional to 1 over the thickness. If a pure bending stress state is present, the stress is proportional to 1 over the square of the thickness (Fig. 1b).
Figure 1: Plate and skin action
By introducing the material that sustains greater stresses, the thickness can be reduced, maintaining a performance that is equivalent. Figure 2 shows the potential for weight reduction for the two theoretical cases.
In true safety components, the possible weight reduction is in many cases between these two extremes. However, the bending case is not a conservative lower limit because local buckling may occur in slender designs.
As Figure 2 shows, AHSS puts increased focus on measures to avoid local bending in order to use the full potential of the material. Measures must be taken to avoid a pure bending stress state. A global bending moment is best transmitted by using beams. Roll-formed profiles or tubes are excellent for designing and producing an optimal geometry that sustains bending moments.
Figure 2: Weight reduction by using AHSS for membrane and bending stress states. The yield strength of the indicated steel grades corresponds to the yield strength at 2% work-hardening and after painting.