Use of high-strength steels in vehicle bodies

"Reaching for the stars"

In times of very demanding customers, automobile manufacturers face increasingly tough competition. Alongside brand image, equipment and design, safety aspects are decisive selling points. For customers, dynamic drive and active body control systems are visible and tangible evidence of progress made in vehicle safety.

Significant advancements have been made in vehicle crash safety over recent years. Customers are alerted to the enhancements indirectly via the EuroNCAP star rating system, which is becoming increasingly important in the battle for customers. Having identified this trend, the automobile industry has been engaged in a real "star war" in the past few years.

To meet the stringent requirements of these crash tests, vehicles need constant improvement. It's no longer enough to analyze individual components in the body structure - a holistic approach is called for. Synergies between individual components and structural groups are combined in an overall concept.

In parallel with this development in vehicle safety, demands for downweighting the BIW have steadily risen. One reason for this is the increased level of equipment which increases vehicle weight and thus fuel consumption. This has resulted in the continuous further development of lightweight designs. Reconciling weight reduction with the increased demands for passenger safety is no easy task.

Potential high strength steel applications

In the field of tension between these seemingly conflicting goals, minimum weight and high safety standards, the use of high-strength steels offers a solution to both problems.

The range of crash-safety applications for high-strength steels is vast. Their versatile combination of material properties makes them suitable for universal applications in the BIW. The wide range of crash safety requirements in the various parts of the vehicle places diverse demands on the material. In the side structure, for example, component strength is the key aspect. The material has to absorb a high volume of impact energy over a relatively short distance. On the other hand, the front and rear structures have to absorb a high volume of impact energy over a relatively long distance. This calls for a material with good formability and medium strength. By selecting the most suitable steel grade, the vehicle structure properties can be improved.

In lightweight design, high-strength steel can enhance the relationship between the weight and function of the components. Without altering the component's properties, high strength steel can be used in thinner gauges. As well as reducing the weight of the component, the lower material weight has positive cost effects.

As a result of these advantages, the use of high-strength steel grades will increase further in the coming years. Modern vehicle bodies are no longer conceivable without these materials.

To harness the full potential of high-strength steels, however, automobile manufacturers will have to undergo a process of change. This process begins in the pre-development phase and must be carried through to the production operations. In the development and design phase, extensive knowledge of the material properties and formability of high-strength steels is essential. To date there are very few specialists available for these steels, unlike conventional steels.

On the basis of many years' experience, the Bielefeld-Brackwede plant of ThyssenKrupp Umformtechnik is a leading address for the processing of high-strength steels. It has the expertise to design the basic component, develop it in line with manufacturing requirements and create a finished product suitable for volume manufacture. By being involved early in the development process, we can engineer products in line with production requirements and thus exploit the full potential of high-strength steel. ThyssenKrupp Umformtechnik has been producing door impact protection bars of martensite phase steel since 1996. With regard to cold forming, this material with its tensile strength of 1200 N/mm˛ represents a major challenge in terms of presses and dies. Forming, springback, cutting technology and process reliability are terms the Brackwede specialists understand. Alongside high-performance coil units with forces of up to 1,250 tons, a series of transfer presses with forces of up to 3,600 tons are available. In addition to cold forming, hot-worked parts are to be added to the range in the medium term. The tools used in the production of high-strength components are manufactured exclusively in the company's own plant construction shop. This ensures that knowledge gained in volume production can be used directly in component development and tool making.

This ThyssenKrupp Steel plant has the equipment to process high-strength steels - as evidenced by the millions of door impact bars and B pillar reinforcements it continues to supply to the auto industry today. The joint projects carried out with vehicle producers benefit from the Brackwede plant's expert input, encompassing component design, plant engineering and volume supply in line with its own high quality standards. By cooperating closely with the leading developer of high-strength steels, ThyssenKrupp Stahl AG, we are also well equipped to meet the challenges of the future.