For years, high - strength low - alloyed (HSLA) steels have been state of the art when steels are needed that combine high strength and toughness. Thanks to a combination of thermomechanical controlled processing (TMCP) and the addition of microalloying elements, such as Ti, Nb and V, a fine - grained microstructure in the as - rolled condition is obtained, which positively influences these properties. Due to the usually low C content of less than 0.1 wt%, these steels also show excellent weldability. In terms of strength and hardenability, however, steels with a low carbon content often do not meet the desired requirements, which is why the trend is going towards higher C contents. For this reason, this doctoral thesis deals with the influence of microalloying elements and TMCP on the recrystallization behavior and microstructure development of HSLA steels with C contents above 0.2 wt%. Through a combination of deformation dilatometer experiments and high - resolution characterization methods, the influence of the microalloying elements, both in dissolved form and in the form of precipitates, was investigated. A new method was developed which has the future potential to automatically determine the degree of recrystallization of microalloyed HSLA steels using light optical microscopy and analysis of grain elongation. The combined addition of Nb and Ti, as well as the role of various deformation parameters was investigated with regard to the recrystallization behavior and microstructure evolution. A right balance must be found when choosing the Ti content in Nb and Ti microalloyed steels to simultaneously obtain the positive effects of Ti against grain coarsening and the positive effects of Nb on the recrystallization behavior. Finally, new insights into the formation mechanism of dynamic strain - induced transformed ferrite could be found by atom probe tomography and electron backscatter diffraction. The results suggest that the formation is a displacive mechanism, although accompanied by the diffusion of C during the formation.
|Translated title of the contribution||Einfluss von Mikrolegierungselementen und Umformparametern auf das Rekristallisationsverhalten und die Gefügeentwicklung von HSLA-Stählen|
|Publication status||Published - 2022|
Bibliographical noteno embargo
- HSLA steels
- thermomechanical controlled processing
- microalloyed steels
- DSIT ferrite