Success Story

Steels for the Automotive Industry - Deformation and Fracture of Modern High-Strength Multi-Phase Steels

The recent definition of ambitious goals for the reduction of CO2 emissions has been accompanied by legally binding guidelines for car manufacturers to reduce fuel consumption and thus also the CO2 emissions of their vehicles to a significant extent. One way to achieve this is to reduce vehicle weight, for example by using thinner body sheets, which in turn requires high-strength steels. The strength of traditional steel can usually be increased only at the expense of other important properties such as ductility and fracture toughness. These properties prevent the formation of cracks during the manufacturing of complex body parts and are necessary to meet ever more stringent crash safety requirements. In the event of an accident, the components must not break but should deform to absorb energy. The aim is thus to develop high strength steels which are both ductile and fracture tough.

These intensive efforts have led to the development of high-strength multi-phase steels such as for example DP steels (dual phase steels). They have a ferritic/martensitic microstructure and provide an excellent combination of strength and ductility. The highest-strength grades, however, are prone to cracking during the forming process, especially at sharp angles and edges. CP steels (complex phase steels) on the other hand, have a bainitic matrix with only small amounts of ferrite and martensite, and show slightly lower ductility in the tensile tests but significantly better formability at sharp angles and edges.

Two high-strength DP and CP steels with similar yield strength and the same chemical composition were investigated to obtain a better understanding of the relationships between the microstructure and deformation or fracture behaviour. Two methods were used for this purpose:

1. Adapted fracture mechanics tests were carried out to determine the fracture toughness of the steels. The two chemically identical materials showed similar ductility properties but significant differences in fracture toughness.

2. The local deformation behaviour was investigated by means of local deformation analysis. For this purpose the sheet specimens were subjected to tensile loading in a scanning electron microscope and the strains were observed in-situ. Images were taken of each step and subsequently compared using special software (Digital Image Correlation). The resulting displacement fi eld allows the strains to be determined. The microstructure components are best visible at a magnification of about 5000x. This makes it possible to distinguish the individual phases, which are only several micrometers in size, and to identify the microstructure components which experience the largest strains. It has been shown that deformation at the microstructure level is strongly inhomogeneous and that strongly deformed regions can be found next to practically undeformed regions. The degree and spatial distribution of this inhomogeneous strain distribution substantially influences subsequent damage.

The results of this analysis give an indication of where cracks originate and the most suitable phase combinations. By combining the two methods it was possible to obtain new information about the relationship between microstructure and deformation or formability which provides a sound knowledge base for the further development of sheets for lightweight and safe cars.

This project was carried out together with voestalpine Stahl GmbH and in cooperation with the Erich Schmid Institute of Materials Science (ESI) of the Austrian Academy of Sciences (ÖAW).

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