At the Lightweight Design Summit in Wurzburg, Germany, Lothar Patberg of Thyssenkrupp Steel Europe said, "Steel is still the most useful lightweight material in automotive design." The company has developed a new steel grade that not only has higher strength and stiffness, but also is easier to form and lighter.
Lothar Patberg said that almost three-quarters of all new cars in 2018 were made of steel. Hybrid structures and pure aluminum bodies account for only 15% and 12%, respectively. This is because steel has high strength, stable manufacturing and processing methods, and cost advantages, making it a very reliable material.
However, the framework conditions for electric vehicles have changed. The primary goal of maximizing battery life is primarily through optimizing the electrical and electronic components of the car. According to calculations by ThyssenKrupp Steel, a weight reduction of 100 kilograms can only increase mileage by 8 kilometers. In addition, the body of future electric vehicles will use simple geometric parts with the best bending performance, reducing the use of complex geometric materials with ideal deep drawing performance. The electric drive puts forward new requirements for vehicle body safety. The increase in weight leads to higher energy consumption. The increase in weight and driving force will also increase the load on the chassis. Based on these new circumstances, ThyssenKrupp Steel concluded that customers will place more emphasis on functionality than materials for automobiles, and that light-weight steel structures with low cost will be more widely used due to the high cost of electric transportation.
ThyssenKrupp complements steel grades and forming processes in a targeted manner. For example, a new type of hot formed steel with a special oxide layer can reduce the hydrogen absorption rate during hot forming by more than 40%. Patberg said that this not only reduces the risk of material damage, but also achieves "sustainable cost and energy savings."
Another innovation is a new manufacturing process for high-strength cold-formed steels with a strength of up to 1200 MPa. The first step is to make prefabricated parts with simple tools, without considering springback deformation. Then use the calibration tool to accurately measure the rebound by compressive stress superposition, and finally make the preform into a geometric shape with precise dimensions. According to the different parts, this multi-stage forming process can reduce the use of raw materials by 15%, and the repeatability is good, and no subsequent processing is required.
The company uses this new steel to develop a virtual electric vehicle body that meets both safety requirements and cost requirements. It includes accessories and battery trays and weighs only 430 kilograms. More than two-thirds of the shell is made of high-strength cold-formed steel, with a strength of about 600 to 1200 MPa, and only about 11% is a high-temperature complex-shaped high-strength steel. In addition, ThyssenKrupp produces battery trays using very strong mixed steel.
For other battery tray projects, the company has developed a reinforced deep-drawn steel plate that costs about half the cost of similar aluminum and adds only 7% in weight. According to Patberg, these examples prove that steel still has potential for lightweight design. More importantly, the perfect body structure can be obtained with optimized cost.