Lightweight Construction Summit 2019 Lightweight Design with Steel: Promising Combination
“Steel is still the most successful lightweight design material in automotive engineering," Lothar Patberg of Thyssenkrupp Steel Europe stated at the lightweight design summit in Würzburg. His company has therefore developed new steel grades for greater strength, rigidity, better formability and reduced weight.
Almost three quarters of all new cars were constructed with a steel body in 2018, says Lothar Patberg. Mixed construction methods and pure aluminum bodies were well behind with 15 and 12 % respectively. Not least because steel, with its high strength, established manufacturing and processing methods and also favorable costs, is a very reliable material.
However, electromobility changes some framework conditions. Thus, the primary goal of maximum battery ranges is addressed mainly through optimizations in vehicle electrics and electronics. According to calculations by Thyssenkrupp Steel, however, a weight reduction of 100 kilograms only extends the range by eight kilometers.
Simpler Component Geometries
Additionally, the bodies of future electric vehicles will be characterized by simple component geometries with optimum bending properties and less by complex geometries and materials with ideal deep-drawing properties. Furthermore, the electric drive places new demands on body safety, higher energy absorption due to increased mass, and higher loads in the chassis due to more mass and driving dynamics.
Based on these new conditions, Thyssenkrupp Steel concludes that the pressure on lightweight construction will tend to decline. Customers would attach more importance on functionality than on the materials of a vehicle, and the high cost pressure in e-mobility would tend to favor economical lightweight steel construction, the company states.
Under these premises, Thyssenkrupp Steel has supplemented its portfolio of steel grades and forming processes in a targeted way: One example is a new hot-formed steel with a special oxide layer that reduces hydrogen absorption in the hot-forming process by 40 % and more. This not only reduces the risk of material failure, but also results in "sustainable cost and energy savings," says Patberg.
New Process for High-Strength Cold Forming Steels
Another innovation is a new manufacturing process for high-strength cold forming steels up to a strength of 1,200 megapascals. The first step is to produce a preform in a simple tool without taking springback into account. The preform is then brought to the dimensionally accurate final geometry in the calibration tool, because the springback can be precisely defined by a compressive stress superposition. Depending on the component, this multi-stage forming process requires up to 15 % less material, is highly reproducible and requires no post-processing.
Using the latest steel materials, Thyssenkrupp Steel has virtually developed a body for a modern electric vehicle that meets both safety and cost requirements and weighs only 430 kilograms including attachments and battery tray. More than two thirds of the shell consist of high-strength cold-formed steels with a strength of 600 to 1,200 megapascals, only about 11 % of the structure consists of high-strength steels that are hot-formed in a complex way. Thyssenkrupp Steel uses a very strong hybrid steel material for the battery tray.
Cheaper Lightweight Design
For other battery tray projects the automotive supplier has developed a deep-drawn steel tub with reinforcing profiles, which costs about half as much as a comparable aluminum tub with only 7 % more weight. According to Patberg, these examples prove the still existing lightweight design potential of modern steel materials. Much more important for electromobility, however, is to achieve a sufficiently rigid body structure in a cost-optimized manner, which, in his opinion, could be achieved with steel materials.
This article was first published by Automobil Industrie.
Original by Thomas Günnel / Translation by Alexander Stark