Design, Development and Characterization of Advanced High Strength Steels (AHSS)
Fuel saving and safety aspects are the driving forces behind the steadily increasing use of high-strength steels in the automotive industry. Research is currently being conducted on the third generation of Advanced High Strength Steels, where a significant amount of retained austenite is stabilized by carbon redistribution in a martensitic or bainitic matrix. Under load, the metastable retained austenite transforms into martensite, thus increasing the formability. This effect is deployed in the production of components with complex geometry or in the event of a crash to absorb energy. A basic understanding of the relationship between chemical composition, heat treatment, microstructure and mechanical properties is essential to further improve the performance of these high strength steels.
A promising heat treatment concept of the third generation Advanced High Strength Steels is the so-called "Quenching and Partitioning" (Q&P). After austenitization, the steel is quenched to a temperature between martensite start and finish temperature to generate a defined fraction of martensite. In a second step, the carbon is redistributed from the supersaturated martensite into the remaining austenite, thereby stabilizing it. In order to gain a basic understanding of the relationship between heat treatment, phase evolution and carbon redistribution, in-situ heat treatments were carried out in the synchrotron at DESY in Hamburg. Recording the diffraction patterns during the entire heat treatment cycle enables a time-resolved determination of the phase fractions and their carbon content. These findings were subsequently correlated with the mechanical properties and provide the basis for optimizing the heat treatment parameters.