The overall objective seems rather simple – build lighter vehicles by using thinner, stronger grades of steel sheet and tubes for structural components.
The overall objective seems rather simple – build lighter vehicles by using thinner, stronger grades of steel sheet and tubes for structural components. But the path from research and development to production is quite a bit more complicated. Higher strength is usually obtained at the expense of ductility. Thinner material gauges mean reduced formability for a manufacturer of stamped or hydroformed parts and the objective must be met without compromising the vehicle’s structural integrity and “crash-worthiness,” or by adding significant cost.
Current and near-future generations of advanced high-strength steel (AHSS) grades represent the steel industry’s response to this achievable, albeit challenging, objective. Specific AHSS grades, such as dual phase (DP), transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP), reflect key microstructural features and/or deformation mechanisms that provide superior strength while maintaining ductility.
Using novel materials
Today’s vehicles already contain some AHSS components, but there’s still room for improvement through the use of novel material processing techniques and innovative manufacturing. An ongoing research project between US Steel Canada and the AUTO21 Network of Centres of Excellence has brought together at team of leading metallurgists and metal forming experts to address specific issues with each of these AHSS grades.
As the name would suggest, DP steels are comprised of a population of hard reinforcing particles (martensite) surrounded by a ductile matrix (ferrite). Accordingly, this steel behaves much like a composite material, exhibiting a combination of strength and ductility that is tailored to an application by altering the volume fraction, size and spatial distribution of the martensite particles. Recent developments with DP steels have focused on the formation of alternative matrix phases, either in place of or in addition to the ferrite. These grades are seeing more use in automobiles worldwide.
TRIP grades are similar to DP steels, with one added feature. The hard, reinforcing marteniste particles actually form while the material is being deformed so, in essence, the material automatically strengthens itself in the regions undergoing the greatest amount of deformation. This unique property is highly beneficial for the manufacture of structural components. Material failure typically occurs as a result of excessive thinning of the sheet or tube wall. This automatic strengthening effect counteracts excessive thinning in TRIP steels.
During a vehicle crash, key structural members in the crumple zone are required to undergo a lot of deformation. TRIP steels slow the advance of microscopic cracks that cause premature failure in critical components. Vehicles produced in Europe and Asia are already using the current generation while the use of these steels in North American automobiles is imminent, having been delayed by incompatibilities between current steel chemistries and the need to produce zinc coated (galvanized) products. Research in this area is ongoing.
Strength and ductility
TWIP, the third AHSS grade of interest, which has the potential to produce a combination of strength and ductility that’s significantly better than DP and TRIP grades. TWIP steels rely on the addition of high levels of alloying element to produce a fundamentally different mechanism of plastic deformation, called twinning, that inherently suppresses the material failure mechanisms observed in steels and other metal alloys. However, alloying elements used in the current generation of TWIP steels are expensive and required in large quantities, making the material prohibitively expensive for automotive applications. As a result, researchers are trying to produce the desired effect using low-cost alloying elements.
Given the continuing trend towards lighter, safer vehicles coupled with the proven and promising properties of AHSS grades, this class of steels will become the most prevalent new material in next generation automobiles.
Keith Pilkey is a professor at Queen’s University and a project leader for the AUTO21 Network of Centres of Excellence. Visit www.auto21.ca.
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