Speaker
Description
Thermomechanical controlled processing (TMCP) is critical for the production of state-of-the-art hot-rolled microalloyed low-carbon steels. The precise control of recrystallization and austenite grain size distribution under industrial TMCP is essential to optimize rolling schedules and product consistency.
The present study includes the characterization of austenite formation and grain growth during reheating of as-cast slabs where in Ti-Nb microalloyed steels heterogeneous grain structures with a mixture of small and very large grains were observed due to local variations in the distribution of Ti-rich carbo-nitrides. These grain structures are significantly homogenized and refined through recrystallization in hot rolling as shown with laboratory hot torsion simulations. Interrupting the deformation schedule with water quenching captured microstructural evolution during rolling simulation. Electron backscatter diffraction (EBSD) was used to characterize microstructures and reconstruct prior austenite grain (PAG) structures. Further, laser ultrasonic measurements were established to in-situ quantify recrystallization kinetics by monitoring austenite grain size evolution in hot compression testing. These in-situ measurements reduce the need for labor intensive ex-situ investigations of austenite recrystallization and provide clear evidence of the degree of recrystallization as verified with conventional double-hit tests. In particular, the conditions for partial and/or no recrystallization can be identified with laser ultrasonics. Based on these experimental studies a recrystallization model is adapted that accounts for recovery and strain-induced precipitation of Nb-rich carbo-nitrides.
The advances of laboratory hot deformation simulations in combination with EBSD-based austenite reconstruction and in-situ laser ultrasonic recording provide a powerful methodology to expedite the development of microstructure evolution models for industrially relevant hot rolling conditions.