On Control of Grain Coarsening of Austenite by Nano-Scale Precipitate Engineering of TiN-NbC Composite in Ti-Nb Microalloyed Steel

Quantitative analysis of hot torsion data of Ti-Nb microalloyed steel with high number density of TiN precipitates showed that pinning force exerted by TiN-NbC composite is adequate to counteract the driving force for recrystallization of 0.1 MPa from dislocation density imparted by the applied deformation strain. Further analysis showed that this pinning force exerted by TiN-NbC composite is adequate to counteract the driving force for grain coarsening of austenite refined by recrystallization and give Zener limiting austenite grain size of 30 micron.The concept of nano-scale TiN-NbC composite precipitate engineering is based on controlling inter-particle spacing of TiN precipitate to 200 nm and promoting epitaxial growth of NbC on pre-existing TiN precipitate to increase precipitate size of TiN-NbC composite to 30 nm in order to promote adequate Zener drag of 0.1 MPa to retard recrystallization. In consequence, the temperature of no recrystallization is raised significantly (about 100oC over strain induced precipitation) that enables austenite grains in the center of thick gage to be pancaked, thereby suppress the occurrence of problematic rotated cube texture. Technological application of austenite grain size control in upstream processing and texture control on the production of thick gage line pipe steel will be examined.