Speaker
Description
Thermomechanical processing (TMP) implies rigorous design, careful control, and optimization of microstructure through combinations of deformations and phase transformations to obtain desired final microstructure and end properties of products. Deformation operations that may include hot, warm, and cold forming can be in general combined with multiphase and single phase (e.g., recrystallization) transformations.
One of the most important characteristics of microstructure in steels is the austenite grain size that influences both deformation and transformation behaviors, as well as their coupling. In the present work, mechanisms and kinetics of grain growth and recrystallization in austenite, as well as their effects on phase transformations in cooling of several Advanced High Strength Steels were studied using Confocal Laser Scanning Microscopy (CLSM), SEM and EBSD techniques. Evolution of austenite grain structure was monitored in situ during reheating, soaking at temperatures up to 1200oC (with and without deformation) and during subsequent cooling including that under load. Depending on steel chemistry and austenite grain size distribution attained upon reheating various grain growth mechanisms were revealed, including conventional grain boundary (GB) migration, GB bulging, dissociation of lower energy GBs, especially under deformation, evolution of triple junctions, the combinations of the above, etc. Dynamic recrystallization in austenite under different deformation conditions was detected with mechanisms naturally depending on grain size prior to deformation, GB energy and steel chemistry. The impacts of above factors on types and kinetics of phase transformations and on the variability of resultant microstructure are discussed. Limitations of CLSM technique with respect to TMP design are also addressed.