Evolving EVs demand a passive safety rethink

Worldwide, ArcelorMittal expects that some 25% of new vehicles will be at least partially electrified by 2025. Lightweighting remains important – but it’s not the only consideration, writes Xavier Boucherat

It is hard to understate the weight of electric vehicle (EV) batteries. Take the 85 kWh pack used in the Tesla Model S, which weighs in at around 540kg (1200lbs) – that’s slightly heavier than the average fully grown Kodiak grizzly bear. Smaller models which can sacrifice some lithium-ion cell capacity aren’t quite so hefty, but still considerable. At 436kg, the Chevrolet Bolt battery pack is around the weight of a standard grand piano, and still results in an overall vehicle weight increase compared with an internal combustion engine (ICE) equivalent.

A consequence of this is that in the event of a crash, the car has more energy to absorb. Furthermore, the design of EVs is such that the way it absorbs that energy is also different. A battery pack sits on the bottom of a vehicle, lowering the centre of gravity, whilst an electric motor is smaller than that of an ICE, changing the way space is distributed up front.

The traditional ideas which have thus far guided passive safety design have therefore changed, and certain solutions need a rethink. That’s according to Jean Luc Thirion, General Manager, Global R&D for Automotive at ArcelorMittal. Once upon a time, it was generally thought that aluminium was the obvious choice for making EVs, with its weight-saving potential enabling acceptable ranges. This, suggests Thirion, is changing, as automakers accept that some extra weight when compared with ICEs is unavoidable, and with the advent of advanced high strength steels (AHSS) and ultra high strength steels (UHSS).

“Progressive improvements in battery technology will see weight become less of a problem,” he says. “Consider that a weight saving of 100kg only gives around 11 or 12km more range, which isn’t so high. I believe that passive safety, along with cost, are becoming the most important considerations for EV manufacturers.”

Certain developments back Thirion up. Tesla upset the aluminium sector when it announced it would move away from the material for Model 3 production. The Nissan Leaf, the Renault Zoe and the Chevrolet Bolt have all also followed a largely steel-based strategy.

Taking a battering

The most immediate concern when it comes to EV safety is the battery itself. Whilst there is little to suggest that an EV is more likely to catch fire than an ICE, the consequences of lithium-ion cell fires have made themselves known in recent years. Fire services have had to develop special protocols to deal safely with high-voltage blazes, and packs involved in crashes have been known to spontaneously reignite days after the fact.

At 436kg, the Chevrolet Bolt battery pack is around the weight of a standard grand piano

“There’s not much freedom to play with the shape or design of a battery box, because it’s rectangular,” he explains. “The main freedom is the material itself, and so it’s clear that roll-forming will become one of the manufacturing technologies used to produce such framing. We produce martensitic grades with very high strengths, up to 1500 megapascals (MPa).” Customers are now requesting strengths of 1700MPa, he says, and are even exploring options for 2000MPa. “This is a real trend,” he adds. “Strengths will get higher and higher to protect battery cells.”

Throughout the vehicle

But beyond the battery, Thirion expects there will be a general rework of safety throughout the car, and as such, martensitic steels could find themselves used to make other parts of the vehicle. Shield panels fitted to the underside of the vehicle will be required to mitigate the risk of stones, water and other roadside debris from rupturing the battery. Meanwhile, further body-in-white (BIW) protection could reinforce the battery, and ultra-high strength side seals and rocker panels (fitted between the front and rear wheels which sit beneath the doors at the level of the battery) of up to 2000MPa could become a requirement for customers.

“It’s different from conventional ICE vehicles,” says Thirion, “because whilst ideally an automaker doesn’t want penetration in the event of a crash, they can accept a certain amount of deformation. Batteries raise the anti-intrusion requirement, and roll-formed or press-hardened martensitic steels are an excellent solution.”

The effect on production

Do these new requirements present manufacturing challenges? Over the last five years, says Thirion, investment from ArcelorMittal in new capacities has been sizeable. First came investment in upstream facilities, to ensure access to steels with excellent levels of cleanliness. Next, a mix of new hot-rolling and cold-rolling capabilities were needed to produce high-strength steels which are sufficiently thin, to save weight. Finally, specific facilities were required to create annealing capacity, in which heated metals can be quenched extremely quickly to create very high strength grades.

Passive safety, along with cost, are becoming the most important considerations for EV manufacturers

“Today, it’s no longer a matter of investment,” he says. “For hot stamping grades and third-generation AHSS for cold stamping, we are already at industrial levels in Europe and North America. In the mid-term we want to be able to produce these products across all four regions, including China and South America.”

In an example of how EVs are effecting change in steel production, Thirion points to ArcelorMittal’s developments in the field of laser-welded blanks for press-hardened steels. A standard B-pillar, he explains, is made by welding an upper part of 1500MPa and a lower part of 500MPa.

New grades have boosted these to 2000MPa and 1000MPa respectively. Using higher strengths might normally present risks: the traditional process uses ‘ablation’, in which the aluminium coating used on top of advanced steels is removed via laser to ensure it can’t enter the weld and make it brittle. However, removing it altogether can cause corrosion along the weld. Simultaneous welding and partial ablation is now possible at ArcelorMittal’s line in Uckange, France, following an investment of €7m (US$7.98M).

“With this new technology,” says Thirion, “we can create products in the 2000MPa range without any risk, thus protecting batteries and passengers against intrusion, particularly in the case of side-crashes.”

The time is now

ArcelorMittal’s current analysis suggests that electric vehicles, including all types of hybrids, could represent 25% of new vehicle sales worldwide by 2025, and this figure will be higher in specific markets. As such, new grades of EV-suitable steel will become key offerings to ArcelorMittal’s customers. Along with lighter parts that can still offer adequate safety performance, Thirion points out that electrical steels used in electric motors and elsewhere will be an important area for the manufacturer. Normally thinner, this too has required investment in production and innovation.

“It will be a challenge for all steelmakers to follow this incredible growth,” concludes Thirion. “It’s probably the highest growth rate the industry will know for electrical steels, which in themselves are a very interesting products.” The debate around EV manufacturing has moved far beyond how light you can make it, and moving forward, steel manufacturers can expect requirements to transform as fast as the industry itself.

This article appeared in the Q1 2019 issue of M:bility | Magazine. Follow this link to download the full issue.