Mark Venables discovers how OEMs are applying new high-strength steels to different areas of the vehicle structure

Volkswagen is increasingly using new high strength steel to make cars lighter and comply with strict emissions rules, confounding forecasts that aluminium would be the metal of choice for reducing weight. The German carmaker used thinner sheets of high-strength steel to help reduce the weight of its latest Golf model by about 100kg.

“We are using high-strength steels in increasing amounts; it is a very cost-effective way of reducing weight,” says Armin Plath, VW’s head of materials research and manufacturing. “Using new innovations in steel engineering, it is possible to reduce weight without the use of more costly materials such as aluminium and carbon fibre.”

The use of advanced high-strength steel (AHSS) in North America nearly doubled between 2005 and 2009 – to 150lb per vehicle – and is due to more than double again to 365lbs by 2025. In the US, Ford has opted for high-strength steels, including an alloy using boron, for more than half of the Fiesta’s body structure. Meanwhile, Nissan plans to deploy a new ultra-strong steel this year across its line-up of models, cutting at least 15kg of weight from each vehicle.

Reversing the weight spiral

As a pioneer of the self-supporting aluminium body, lightweight design has long been a top priority at Audi. “Future innovations will ensure that the brand can continue to reverse the weight spiral,” promises Wolfgang Dürheimer, Audi’s head of research and development. “Each new Audi model shall be lighter than the one it replaces.”

“When it comes to ultra-lightweight engineering,” he adds, “we do not employ just one material, but rather a multi-material mix. Our motto is ‘the right material at the right place in the volume needed’. That’s why we use high-end steels as well as aluminium, magnesium and Fibre- Reinforced Plastics (FRP).”

The hot-shaped steels that Audi uses in numerous models, from the A1 to the A8, reduce weight while providing maximum strength. Many hot-shaped steel components – most of the B-pillars, for instance – are fabricated in-house.

Two natural gas-fired furnaces, each measuring 23m long, are installed at the press shop south, in Ingolstadt. A robot gripper places the steel plate on an article carrier at the entrance to the furnaces and ceramic rollers carry the plate through them in less than four minutes, during which time the plate is heated to over 920°C.

At the furnace exit, a gripper arm rapidly places the redhot plate into a hydraulic press that works with a closing force of more than 600 tons. Cooling tubes, through which cold water flows, are cast into the die of the press and the panel is cooled to approximately 180°C.

The martensitic structure thus formed has an extremely high tensile strength of up to 1500MPa – the same as the cables of a suspension bridge. A single wire with a crosssection of one square millimetre can suspend a weight of 150kg. It is so strong that it can only be machined using a laser or a diamond-edged tool.

The B-pillars are partially hardened and tempered to purposefully influence the deformation characteristics. In a side-impact collision, the pillars deform more at the bottom than at the top in order to dissipate as much energy as possible.

“Some of the hot-shaped panels are tailored blanks,” Dürheimer adds. “These custom components are composed of different types of steel (boron alloy and micro alloy), thus they have different characteristic strength values depending on the area.”

He continues: “Audi developed this principle and fabrication process and is the first to use it. The B-pillars, for example, should undergo defined deformation in the event of a side-impact collision, with greater deformation at the bottom than at the top. This not only serves passenger safety, it also protects the roof from expensive damage in the event of a minor accident.

“Other plates, called tailored rolled blanks, are rolled to different thicknesses. Audi is a pioneer in this field as well. The engineers want to combine both technologies in the future, as they offer additional potential for reducing weight together with greater strength. The hot-shaping plant also harbours new opportunities. Clever tool and process design enables the panels to be heated and/or cooled to different levels in different zones, and thus in part hardened to different degrees.”

Focusing on weight reduction

While powertrain developments such Toyota Hybrid Synergy Drive have helped enhance fuel economy, many of the vehicles across the Toyota product line have improved their efficiency because of a focused effort on weight reduction. The 2013 Avalon is a prime example.

Avalon chief engineer Randy Stephens and his development team at Toyota Technical Centre (TTC) in Ann Arbor, Michigan, worked to make Avalon lighter, yet more rigid, to help improve overall driving performance. As a result, the new 2013 Avalon is 110lbs (50kg) lighter than the 2012 model, tipping the scales at 3,461lbs.

“By adopting a higher content of high-strength steel in the pillars and rocker panels, we were able to conserve mass while maintaining crash performance,” says Rob McConnell, principal engineer, body shell and exterior plastics.

Low mass – high quality

At GM, the Cadillac CTS sedan is longer, lighter and stronger than the previous CTS sedan structure, which enabled more precise ride and handling tuning, as well as a quieter driving experience. “The architecture helps advance its performance legacy with a tremendous feeling of balance and control that is reinforced with confidence-inspiring solidity and refinement,” says John Plonka, CTS programme engineering manager. “Its lightweight body structure is a strategic enabler for all elements of its performance, while its strength contributes to greater refinement through a smoother, quieter ride.”

Innovative applications of lightweight materials – including the first aluminium doors ever on a GM production vehicle – help the 2014 CTS weigh in with an approximate appr base curb weight of 3,600lbs. That’s about 200lbs less than the 2013 BMW 528i and approximately 7% less than the current model.

Advanced materials, including magnesium and lightweight, high-strength, low-alloy steels and bakehardenable steels, enabled engineers to tailor the CTS’s structure down to the last gram; ensuring strength and refinement were achieved without unnecessary mass.

The rear suspension cradle is made of steel, which helps quell noise and vibration while providing counterweight to the powertrain at the front of the car to help the CTS achieve its nearly 50/50 weight distribution. Also, unique lower-front chassis braces provide stiffening reinforcement to the structure to support the connected driving feel and a smoother, quieter ride.