The automobile industry is currently facing enormous challenges. The EU limits for CO2 emissions in motor vehicles are forcing manufacturers to introduce extensive measures, as further reductions in fuel consumption for all vehicles’ are to be achieved by 2020.
A significant factor influencing fuel consumption is vehicle weight. The rule of thumb is simple: less weight leads to lower consumption and consequently to lower CO2 emissions. There is, however, some controversy about whether car bodies made of carbon fiber and aluminum really do improve vehicle CO2 emissions. Use of these materials in doors and the passenger compartment only amounts to about a quarter of a car’s total weight. And besides, though less weight really does reduce CO2 emissions, the high energy requirement for producing the low-weight materials needed actually cancels out the benefit.
There are great hopes linked with the topics of electro-mobility, lightweight construction and autonomous driving, and not just in political circles. In the wake of the diesel scandal, many people would like to see transport policy turn towards completely mission-free cars. The manufacturers are now going down that road and registrations of electric vehicles are steadily increasing. For long-distance HGV traffic and the heavier private cars, however, a more effective replacement for the diesel engine will not be appearing on the horizon any time soon. This highly efficient engine type – in future probably driven by regenerative fuels – and the electric motor are by no means opponents. Both are needed if climate targets are to be achieved. Steel is still relevant for car manufacturing. Important criteria in vehicle development are the safety requirements, as well as recycling considerations and environmental protection. An aluminum body does weigh a few percentage points less than a steel one and melting down and reusing the material is extremely energy-efficient.
For primary production of aluminum, however, much more electricity is required than for steel. Taking the entire life cycle of steel into consideration, from the mine to the car and finally to the recycling plant, 2.6 % less CO2 is generated compared to aluminum. Overall resource utilization also speaks in steel’s favor, as it can be almost 100 % recycled. The composite material carbon fiber has serious disadvantages. This light, corrosion resistant, long-lasting and strong material generates the highest production costs, is difficult to process and is therefore largely used in building racing car prototypes. Experts see the greatest potential for lightweight construction in the bodywork, drive units and chassis. Car manufacturers’ research is focusing on how each of these materials can be used to conform to their particular strengths. For example, traction forces can be absorbed better by composites; pressure by metals.
The steel industry is expecting stronger demand for battery powered vehicles. A current survey by the Indian steel giant Tata Steel shows demand being significantly boosted by e-mobility. Up to the year 2050, Tata is expecting an increase in orders amounting to 4.2 million tons (+ 25 %) from the European automobile industry alone. It’s believed the boost will mainly come from high-tensile, lightweight steels, which are especially hard but nevertheless sufficiently formable. The growing demand for lighter automobiles and the expected increase in battery production are promising signals for a rise in demand for special steels. Steel’s potential as a lightweight construction material is often underestimated. Increasingly tough steels ensure that cars are not only getting lighter but safer too. A particularly important role here is played by ultra-high strength hot-formed steels. During manufacturing, the plates of these steels are heated up to around 950° C and given their required shape. Precisely controlled cooling in the tool then results in the material’s extreme hardness being achieved. As car components have to achieve certain hardness levels, these steels make it possible to continually reduce material thickness, as higher strengths guarantee thinner steel sheets. Hot-formed steels are used especially for manufacturing safety-relevant components, such as side impact and bumper cross beams. By using these steels, the body shell weight of the VW Passat, for instance, could be reduced by 20 kilos, compared to the use of conventional high-tensile formed steels. An outstanding feature of all steel solutions for lightweight construction is their high material and energy efficiency. Even after a long service life, these components can be recycled without any loss in quality. Compared to using aluminum or carbon fiber, manufacturing highly efficient components from steel is cheaper. Achieving a weight reduction of 1 kilogram using steel saves about two Euros, whereas the same weight reduction using aluminum costs around 10 to 12 Euros more, and when using carbon-fiber reinforced plastic, it even costs 50 Euros more. So, steel is still and will remain an vogue. Current development seems to moving not towards an either/or situation, but to a multimaterial design. The success of the car’s “slimming programme” would seem to be best served by removing nonsensical comfort features.