UHSS offers the scope to save weight on electric vehicles. A report on ArcelorMittal’s updated S-in motion project with an electric car BIW exercise.

Electric vehicles (EV) present automotive designers with a whole new set of challenges. The powertrain – including the batteries – is actually heavier than conventionally-powered cars and the batteries are potentially explosive in collision situations. The challenges of extending the range of EVs to a usable level are well known and because electric power storage is relatively inefficient compared to gasoline and diesel, the need to save weight is vital.

Plastics may have been viewed in some quarters as the optimum solution but UHSS offer potential to be exploited as well. ArcelorMittal originally launched its S-in motion project as a showcase for AHSS and UHSS in conventional vehicles; it has now updated it to demonstrate possible weight savings on a conventional-size electric powered car. The original S-in motion used a range of AHSS and UHSS to achieve weight savings of around 20%. The new project used a fuel-powered C-segment vehicle as a baseline, with the stated objective of modifying it to create an electricpowered car. The project was intended from the outset to be relevant for OEMs that intended to produce electric versions of their conventional range.

Crash course
The BIW was subjected to a simulation of European NCAP’s side-impact assessment. This presented something of a challenge, as the NCAP standards require that the body deform, in order to absorb energy, but the battery compartment has to be protected from rupture, damage or intrusion.

The ArcelorMittal team reconciled these conflicting demands with the use of laser-welded blanks, made from its patented Usibor 1,500P and Ductibor 500P UHSS. Hot stamping makes Usibor hard, while Ductibor remains ductile, as its name suggests. Using the two materials together enables designers and engineers to control the body’s behaviour in a crash, with deformation resistance where it is needed and energy absorption located exactly where it is required.

Battery change
The S-in motion uses a different design for battery protection, compared with the current widespread practice of a reinforced box. Such a solution adds weight, which is the opposite of what the project is setting out to achieve. The electric version of the S-in motion uses the underbody to provide protection. The battery tray is bolted to the underside of the transmission tunnel, which has been enlarged in order to accommodate it, and under the rear seats. The result is a protective tube, and the concept has already been deployed in GM’s Chevy Volt and other vehicles.

The tunnel is formed of hot-stamped Usibor 1,500P; the battery tray is made of dual-phase DP 1,180. As the metals are high-strength steels, torsional stiffness is also improved.

More steel = less weight
The BIW was redesigned to accommodate the larger powertrain and to create the battery protection required. Increasing the use of AHSS from 35% to 58% compared with the original project, enabled the weight of the BIW to be cut by 30kg. This was achieved despite the need for additional steel to protect the battery of the electric vehicle. The weight reduction created a virtuous circle: less weight meant that less steel was required to construct the BIW. The company claims that materials cost savings achieved amounted to around 5%, although this was more than countered by higher processing and assembly costs – but they are claimed to have been kept within reasonable bounds. The total additional costs are stated to be of the order of 2%, compared with the conventionally-powered vehicle.