Effect of High Ti Contents on Austenite Microstructural Evolution During Hot Deformation in Low Carbon Nb Microalloyed Steels

This work has focused on the study of hot working behavior of Ti-Nb microalloyed steels with high Ti contents (> 0.05%). The role of Nb during the hot deformation of low carbon steels is well known: it mainly retards austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, to design rolling schedules that result in properly conditioned austenite microstructures, it is necessary to develop models that take into account the effect of high Ti concentrations on the microstructural evolution of austenite. To that end, in this work torsion tests were performed to investigate the microstructural evolution during hot deformation of steels microalloyed with 0.03% Nb and different high Ti concentrations (0.05%, 0.1%, 0.15%). It was observed that the 0.1% and 0.15% Ti additions resulted in retarded softening kinetics at all the temperatures. This retardation can be mainly attributed to the solute drag effect exerted by Ti in solid solution. The precipitation state of the steels after reheating and after deformation was characterized and the applicability of existing microstructural evolution models was also evaluated. Determined recrystallization kinetics and recrystallized grain sizes reasonably agree with those predicted by equations previously developed for Nb-Ti microalloyed steels with lower Ti concentrations (<0.05%).

Microstructural Analysis of Type 304 Metal Sleeve

In order to form thin metal sleeve with the thickness of 0.03 mm, type 304 austenitic steel sheet was deeply drawn to a cup and spinning method applied to its body. The sleeve shows high strength with a dual-phase microstructure of fine austenite and transformed martensite. Pancaked austenite and martensite grains were highly elongated along RD (drawing direction) in the layer structure, and their grain width was about 100 nm. Dynamically recovered austenite grains were highly aligned from {101} to {101}. The strain-induced martensite grains mainly showed two components of {001} and {111}. Recover and recrystallization of the sleeve appeared at the temperature from 873 K to 1073 K. Annealed at 1073 K the austenite grains were mostly recrystallized with intensifying {101}, and the martensite grains were also reverse-transformed to austenite.