S-95. V. Savic, L. Hector, H. Singh, M. Paramasuwom, U. Basu, A. Basudhar, and N. Stander, “Development of a Lightweight Third-Generation Advanced High-Strength Steel (3GAHSS) Vehicle Body Structure,” SAE Int. J. Mater. Manf. 11(4):303-313, 2018, doi.org/10.4271/2018-01-1026.
Several recent studies are forecasting that; “Within the next 10 to 15 years, urban transportation will be dominated by Electric and Automated vehicles”.B-50 Meaning most of us will be driving Battery Electric Vehicles (BEVs) in the not-distant future. In 2011, just eight years ago, there were only three BEVs on the market with 70 to 80 miles range on a single charge. These were the first generation BEVs. Since then, the number of EVs on the market has increased, with significant improvements in range (now approaching 300 miles). BEV 2020 vehicles cover all current segments, from small cars to SUV’s and trucks (Figure 1). These vehicles will be available from most OEMs as well as several new start-up companies. The construction material for body structures of these vehicles is predominantly steel, while some of the premium vehicles ($60,000 to $100,000) are aluminium. And the prevailing OEM message seems to be “anything TESLA can do, we can do better”.
So how will this change the vehicle body structure design, choice of construction material, its implications for manufacturing and assembly, and ultimately, the impact on automotive steel?
Figure 1: Electric Vehicle Boom – Models by Style and Range Available Through 2020.B-50 CHART SUMMARY: a) Covers all current segments, b) Structures predominantly Steel, c)Some premium vehicles highlight Aluminium, d)Products from most OEMs as well as several new start-up companies.
The driver for this electrification boom is increasing affordability. The upfront cost of BEVs will become competitive on an unsubsidized basis starting in 2024.F-38 By 2030 in the U.S., almost all light duty vehicle segments will reach cost parity as battery prices continue to fall.B-73 Forecasters, such as McKinsey, Morgan Stanley and Bloomberg, predict that about half of all new vehicle production will be electric somewhere between 2035 and 2040. However, Tesla’s CEO Elon Musk’s prediction is much more aggressive. He expects more than half of new vehicles in the U.S. will be electric within the next 10 years, roughly 10 to 15 years ahead of most other predictions.
The Main Drivers of BEV Cost Reduction
- Lithium-ion battery prices have fallen 75% since 2013, hitting $176/kWh in 2018 (Figure 2). Industry-wide prices fell due to the adoption of new cell designs and the availability of higher energy-density cathodes. Prices are expected to drop further in coming years to below $100 per kWh. Besides the reduction in cost, packaging efficiency and the cell energy density also is improving.
- Package space required by other BEV powertrain systems also is being optimized, e.g., motor, transmission, differential and power electronics. This is yielding significant weight and cost reductions, which are then directly reinvested into lower-cost structural materials, such as Advanced High-Strength Steels (AHSS) versus higher cost Aluminium, to keep the overall price of the vehicle low.
Figure 2: BEV Price Parity with Gas-powered Cars by 2024 – Main Drivers.B-74
BEV to ICE Vehicle Structural Differences and Advantages for Steel
Figure 3: BEV to ICE Vehicle Structural Differences.M-64
BEV packaging differences compared with ICE Vehicles are shown in Figure 3, and include:
- Narrower and compact transverse electric powertrains, leading to shorter front end, with increased occupant space for same size vehicle and larger/efficient front crash rails.
- Lack of an exhaust system eliminates the need for the tunnel, allowing straighter/ efficient cross-members.
- No fuel tank/filler leads to more efficient rear rail load path.
- High voltage electric powertrain and large (300 litres, 500 kg) under-floor battery pack crash protection requirements result in higher safety requirements for BEV front and side structures.
- Safety. The BEV body structure load path requirements are ideal for AHSS application. The floor cross members, without the presence of the tunnel, are straight and can use very high-strength martensitic roll formed sections. Cross members can be stamped from 3rd Generation Steels offering Giga-Pascal strength and over 20% elongation. For frontal crash load management and to minimize passenger/battery compartment intrusions for increased safety, 3rd Generation steels offer the most mass/cost efficient solution. The very high strengths offered by AHSS and UHSS for the safety-critical structural members such as the rocker, rails, cross members and pillars, greatly enhance the required protection of the BEV powertrain and high energy/voltage battery systems. The battery enclosure construction greatly benefits from AHSS usage, providing protection from road-debris impacts from below the vehicle, along with fire protection into the passenger compartment. Advanced steels also enable reduced section sizes for the occupant compartment, required for improved panoramic visibility, without compromising occupant safety and comfort.
- Cost. For widespread adoption of BEVs to occur, the overall cost of the vehicle must be affordable, and its range must be above the ‘range anxiety limit’ of most drivers. Various surveys indicate this range to vary greatly from 75 miles to over 400 miles. Using steel for the vehicle structure leads to the lowest cost BEV, just as with ICE-based vehicles. The vehicle range can be increased through lightweighting and/or by increasing the size of the battery; a cost comparison of these two options is shown in Figure 4. With battery cost reduction approaching $100 per kWh, lightweighting is cost effective at approximately US$2.00 per kg saved. Lightweighting is still very important and the latest steel grades, in particular 3rd Generation steels, offer the most cost-effective lightweighting option. In comparison, if we consider lightweighting with aluminium, the cost is typically in the order of US$6.00 per kg saved. This could be cost effective if the battery cost is over $250 per kWh, which was the case a decade ago. We can see the evidence of this in OEM decisions at that time. For example, the 2011 Nissan Leaf BEV closures were aluminium; but the latest 2019 Nissan leaf BEV closures are steel.
Figure 4: BEV Range Increase – Lightweighting Cost versus Battery Cost 2020 – 2022.M-64
Battery Electric Vehicles – Boom or Bust for AHSS?
For the increased safety required for BEVs to protect the high voltage systems, the structural load paths are ideally suited for the Giga Pascal level strengths offered by AHSS and UHSS. The Battery Enclosure structure offer an additional 85 kg per vehicle opportunity, an increase of approximately 10% sheet metal over ICE vehicles. Also, using advanced steels the BEV structure can take full advantage of well-established body shop practices for manufacturing and assembly, such as stamping, roll forming and spot welding. With future increased focus on BEV affordability, safety and sustainability, steel offers the best solutions and flexibility to address these key challenges.