Materials Science Forum Vols. 715-716

Abstract: Numerical and analytic models for diffusion creep have commercial and geological uses. For single phase polycrystals, numerical models of interface diffusion creep illustrate how grains rotate and what the relative contributions of grain shape change and grain boundary sliding are to the overall strain. In particular they shows that an equi-axed starting material will initially show large grain angular velocities but that these slow down as grain become slightly elongate. A steady state microstructure with some grain elongation and little or no grain rotation is reached. Consequently the equi-axed grain shapes seen in superplastic deformation require additional processes for a full explanation. For two phase aggregates, the mathematical framework cannot be simply extended it breaks down as the system becomes mathematically overdetermined. Further work is required to solve this problem. If the second phase is insoluble, the mathematics can, though, be extended successfully, paving the way for models of diffusion creep with insoluble second phase particles.

Abstract: Conventional controlled rolling is normally used in a hot strip mill in order to refine the microstructure and, consequently, to meet a good combination of mechanical strength and toughness in low carbon steels. Nb microalloying are used for this purpose once this rolling technique requires the occurrence of strain induced precipitation before that any recrystallization process takes place. In a hot strip mill, the transfer time between roughing and finishing steps is considered fast enough to keep all Nb, previously solubilized in plate furnace, in solid solution just before finishing. As a consequence, the thermodynamic driving force required for strain induced precipitation remains high, accelerating its kinetics and preventing the occurrence of recrystallization. On the other hand, in a seamless tube rolling, the transfer time between roughing and finishing steps is about 20 times longer than in a hot strip mill. As a consequence, Nb can precipitate in the recrystallized austenite after roughing, decreasing the driving force for strain induced precipitation at finishing. Transmission electron microscopy investigations on samples simulating the material just before finishing rolling in a seamless tube mill show that both grain boundary precipitation and co-precipitation on TiN particles takes place during transfer time. Based on these findings, this paper discusses the effect of long times after roughing steps on the driving force for strain induced precipitation at finishing rolling in seamless tube. Thermo-Calc software was used to evaluate the possible amount of Nb that remains in solid solution just before finishing. Due to previous precipitation, the driving force to be considered in the Dutta-Sellars equations for onset of precipitation decreases and a comparison between recrystallization and precipitation curves shows that recrystallization takes place before the start of strain induce precipitation.

Abstract: The present work deals with the evolution of mechanical properties and structure of low-carbon Fe-1,12Mn-0,08V-0,07Ti-0,1C (wt.%) steel after severe plastic deformation (SPD) and high-temperature annealing. Steel in initial ferritic-pearlitic state was deformed by equal channel angular pressing (ECAP) at T=200°C and high pressure torsion (HPT) at room temperature. The evolution of ultrafine grained structure and its thermal stability were investigated after annealing at 400-700°C for 1 hour. The results shown that SPD leads to formation of structure with an average size of (sub-) grain of 260 nm after ECAP and 90 nm after HPT. Ultrafine grained structures produced by SPD reveal a high thermal stability up to 500°C after ECAP and 400°C after HPT. At higher annealing temperatures a growth of structural elements and a decrease in microhardness were observed.