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Steel for sustainable mobility
Vehicles should be light, safe, environmentally friendly and, above all, affordable. This is made possible by the development of ever-stronger but nevertheless easy-to-process steels with excellent cost/benefit ratios.
Regardless of whether a two-wheeled vehicle, a car, commercial vehicle or rail-borne vehicle, whether driven by diesel, petrol, electricity or even muscle-power – no mobility would be possible without steel. No wheel would turn without steel bearings, no combustion engine could run without steel valve springs and camshafts, and no electrical drive could operate without magnetic sheet. Aircraft could not take-off or land without steel, nor could ships leave the harbour. Steel is the most important material for our mobility.
The car is a particularly impressive example of this: an average of about 60 % of a car is made of steel – despite the increasing trend towards composite designs. Steel’s unique property profile makes it the material of choice for the body, the power-train, the suspension and the steering. No other material has a comparable range of properties, such as excellent resistance to mechanical and thermal stresses; an extraordinary combination of formability, joinability and paintability; optimum recycling capability; and high total cost-effectiveness for automotive mass production.
Steel for the car of the future
Combined with new production techniques, high-strength and ultra-high-strength steels make a major contribution towards meeting the continuously rising demands made of new vehicle models. On the one hand, better driving performance, greater comfort and improved safety are expected. On the other hand, the demand exists for greater environmental friendliness i.e. reduced fuel consumption or greater range in the case of e-vehicles, lower emission values and improved recyclability.
In addition to optimising the power-train and the driving resistances, it is weight savings that reduce a vehicle’s fuel consumption – and thus also CO2 emissions. Examined in isolation, however, light construction is not always a reasonably priced alternative – but it does open up additional secondary CO2 reduction potentials: with a light body, for example, other assemblies such as the power-train, gearbox or tank system can be adapted, further reducing consumption.
Light is not enough
For this reason, therefore, the approach of producing as many vehicle components as possible from specific lighter materials such as aluminium or carbon-fibre reinforced plastics (CFRPs) is frequently discussed. In practice, however, it has been found that the weight advantage of light construction materials cannot be carried over to a particular component. The mechanical properties of many light construction materials not only require a modified design but mostly also an enlarged cross-section. This reduces the desired weight advantage or even neutralizes it for certain individual components.
Only a holistic consideration counts
Light materials have a positive effect on emissions during the utilisation phase of a vehicle as a result of the weight savings. Whether the use of a light material is really ecologically positive is only shown when considered holistically, which also involves examining the production of the material, the production of the car or component, and subsequent recycling. It is precisely here that the material steel scores points. The CO2 emissions that are created during the primary production of a tonne of steel are considerably lower than those for aluminium or CFRP. Steel can be recycled as often as desired and without any loss of quality.
Holistic ecological assessments taking into account all life-cycle phases are even more important for future e-vehicles than for vehicles with conventional drives – because they are driven locally without emissions, i.e. strictly speaking no CO2 emissions occur during operation. These emissions are entirely to be found in the phases of material production, component or vehicle production, generation of the drive energy, and recycling.
Keeping costs under control
In addition to the ecological assessment aspects, however, cost-effectiveness continues to play a central role in the decision to use a material in mass production. Mass producers therefore continue to choose steel for the body, in particular, as the vehicle’s largest single module. Thus, for example, according to VW “extremely expensive materials such as aluminium, magnesium or even carbon-fibre materials” are out of the question if a vehicle like the Golf is to remain affordable for millions of customers. Nevertheless, VW’s engineers have succeeded in reducing the weight of the ‘body-in-white’ for the current Golf by 23 kg compared to the previous model, despite further improved crash and rigidity requirements and larger dimensions – mainly through the intensive use of high-strength and ultra-high-strength steels and innovative production processes.
While the proportion of high-strength and ultra-high-strength steels in the body already accounted for about two-thirds of all the steel used in the previous model, this figure has risen to 80 % in the new Golf – a top value, not only amongst compact cars. Whereby a large number of hot-formed components made of manganese-boron steels have been used, offering maximum crash security and low weights. The proportion of these steels has risen from just 6 % in the Golf VI to a current value of 28 %. Almost all safety-relevant structural components that form the backbone of the vehicle consist of these steels.
But the development potential of steel still offers many opportunities. New alloying and heat-treatment concepts allow increased strength with improved forming properties. In combination with new production technologies, such as flexible roller profiling, they enable more efficient material use and, often, fewer process steps. It is therefore to be assumed that light, safe and environmentally friendly vehicles will continue to be built with steel in future.