Online Lecture by prof. em. Wolfgang Bleck

Some particular findings on microstructures and mechanical properties of advanced high strength steels

Speaker: prof. em. Wolfgang Bleck, Steel Institute of RWTH Aachen University, Germany.

Title:
Some particular findings on microstructures and mechanical properties of advanced high strength steels.

Abstract:
Optimizing the balance of conflictive mechanical properties like strength, toughness, fatigue, formability is the key issue of current steel and process development. Therefore, new steel design concepts use high Mn contents for controlling low temperature phase transformations, for stabilizing the fcc phase, and for adjusting the stacking fault energy. By this, extremely fine microstructures down to the nm-level could be achieved. Heterogeneous microstructure combines various phases, including metastable phases, enable new combinations of mechanical properties, especially when phenomena such as the TRIP, TWIP, or MBIP effects are triggered. Materials that have one or more of these special features are summarized under the term Advanced High Strength Steels (AHSS).

The talk will report on recent results for various steels. This will be done in the order of increasing manganese content as the characteristic alloying element for AHSS. In complex phase steels the challenging description of the heterogeneous microstructure will be discussed and the competition of dual phase and complex phase steel with respect to global and local formability will be elucidated. For air hardening steels, the control of low temperature phase transformations is required for improved toughness and fatigue behaviour. With higher Mn contents in medium Mn steels metastable phases become interesting; this applies equally to the microstructure setting and to the mechanical properties. The interaction of alloying elements can no longer be neglected; special challenges in process technology now also have to be considered. This will be highlighted, as an example, for continuous casting and for continuous annealing.

The lecture will look at developments in the recent past and show possible areas of application for the newly developed steels. Conclusions about the importance of new design principles are drawn.

This session was chaired by prof. Erik Offerman (TU Delft).