AUSTENITE GRAIN SIZE CONTROL IN UPSTREAM PROCESSING OF NIOBIUM MICROALLOYED STEELS BY NANO-SCALE PRECIPITATE ENGINEERING OF TiN-NbC COMPOSITE

There is a growing demand for thicker gage pipes particularly for off-shore projects. Austenite grain size control in upstream processing before pancaking is essential to obtain excellent DBTT and DWTT properties in thicker gage product. This paper examines the basic science aspects of austenite grain size control by nano-scale precipitate engineering. Basic science aspects of pinning pressure due to TiN-NbC composite precipitates in retarding recrystallization were investigated in a previous work way back, using hot torsion simulation studies. In this case, dislocation density from deformation is the driving force for boundary movement, which is larger than the driving force from capillarity for grain coarsening of recrystallized austenite grains. The experimental observations confirmed that if the inter-particle spacing of TiN is small and the dispersion of TiN is uniform, then NbC will grow epitaxially on pre-existing TiN precipitates, thereby eliminating the need for strain induced precipitation of NbC on dislocations. The pinning pressure from NbC growing on pre-existing TiN particles is found to be adequate to retard recrystallization. The analysis showed that the Zener pinning pressure from TiN-NbC composite precipitates is about 0.1 MPa, which is adequate to prevent grain coarsening of recrystallized austenite grain of about 25 micron size by capillarity driven driving force. Thus, the core concept of austenite grain size control by nano-scale precipitate engineering is based on Zener limiting austenite grain size by engineering TiN-NbC composites, using TiN dispersion to control the interparticle spacing to be about 200 nm, and growth of NbC on pre-existing TiN particles to control the size of TiN-NbC composite precipitates to be about 30nm. Recent TEM-EELS studies have confirmed epitaxial growth of NbC on pre-existing TiN particles with an inter-particle spacing of 200 nm. Nano-scale precipitate engineering of TiN-NbC composites offers a general platform for refining austenite grain size in titanium-niobium microalloyed steels in a wide range of technological applications including super-martensitic stainless steel. The application of nano-scale precipitate engineering concept for refining austenite grain size in upstream austenite conditioning in line pipe grades will be examined and the benefits will be pointed out.