Advanced high strength thin sheet grades: improvement of properties by microalloying assisted microstructure control

The mechanical properties of materials are generally controlled by their microstructure. Therefore, a thorough knowledge of the impact of alloying elements and process parameters on the resulting microstructure is essential. For mild and conventional high strength thin sheet grades microalloying additions such as Nb are the basis to control the microstructure and therefore, the mechanical properties. However, for advanced high strength steel (AHSS) grades such as dual phase, partial martensitic and TRIP grades, the microstructure is mainly determined by alloying additions controlling the phase transformations during cooling. For this purpose Mn, Cr, Mo, Si, and Al are the most important alloying elements. However, the addition of microalloying elements results in a remarkable refinement of the microstructure of AHSS grades and a further improvement seems possible. In this contribution, the decisive impact of Nb on the microstructure and mechanical properties of mild and conventional high strength steel grades is shown. Then, the most important alloying and thermal treatment concepts for processing AHSS steel grades are demonstrated. In the final part, the very promising impact of microstructure control based on microalloying additions is shown for AHSS steel grades. This contribution also highlights the necessity for further research in this topic. (AU)