Body-in-white: Material world
All-aluminium structures are established in the luxury and sportscar segments but OEMs need to save weight in every model they make. AE asks materials suppliers about the latest developments
- Published in Focus.
Cutting carbon: Composites can offset electric vehicles’ extra weight
Jean-Luc Thirion
Head of automotive R&D, ArcelorMittal
Our highest performing products have already reached 1,500MPa, not only for hot stamping steels but also martensitic grades intended for cold stamping. We are currently working on 1,700MPa and our ambition is to reach 2,000MPa in the near future.
The full metallurgical limits of steel are not yet known. Processes used by OEMs – stamping, joining, painting – are in some ways obstacles to steel’s development. When OEMs adopt more sophisticated assembly practices, this has the knock-on effect of opening up new possibilities in terms of steel innovation. Hot-stamping is an example – it has enabled significant weight savings.
High strength is only one factor. In-use properties and dimensional capabilities are constantly extending the potential applications of advanced high-strength steels (AHSS). New coating technologies that promise enhanced corrosion protection and cost-efficiency are another focus for our research.
Transformation-induced plasticity (TRIP) steels are designed to improve the compromise between high strength and formability. Even though these variants are not yet as popular as dual-phase (DP) or hot stamping steels, we’re seeing increasing demand for grades that enable the stamping of complex components at levels of 700-780MPa.
Twinning-induced plasticity (TWIP) steels go further, creating new opportunities for components integration. These steels retain a significant proportion of their ductility after stamping, which may prove invaluable in the event of a crash.
The high percentage of alloying elements required for TWIP steels makes them relatively expensive compared with other steels at present – this restricts their use. While TWIP products are not yet fully mature, the increasing demand for high strength is likely to boost uptake in future.
We’re seeing a boom in demand for hot-stamping steels, especially because these combine ultra-high strength with elimination of springback. Our R&D division is investigating their full potential, in terms of increased strength and residual ductility.
Delivering varying thicknesses and mechanical properties to specific areas of the body-in-white (BIW) is an ever more important consideration as OEMs look to reduce weight while also improving crash resistance. A key element is the use of ultra-high strength steels (UHSS) and hot-stamping for critical parts.
Partial hardening is an effective means through which to retain “soft” areas in parts where residual elongation is required – the bottom of B-pillars for example. Our subsidiary ArcelorMittal Tailored Blanks has succeeded in developing tailored welded blanks that use a patented process to combine UHSS hot-stamping grades with milder hot-stamping grades for such applications.
We’re fully aware that alternative materials enable weight and CO2 reductions, and we’re integrating some of this thinking into our own strategy.
However, we’re convinced that steel will remain by far the preferred choice for the majority of applications, owing to its unrivalled balance between performance and cost.
