Materials Science Forum Vols. 715-716

Abstract: Recent observations show that the strain reversal affects significantly and in a complex way both the static recrystallisation (SRX) and strain-induced precipitation (PPT) kinetics in Nb-microalloyed steel. It is already known that the recrystallisation stagnation is a consequence of the competition between the driving pressure for recrystallisation and the pinning pressure caused by the strain-induced precipitation of Nb (C,N) precipitates. Both of these parameters depend in turn on the local dislocation density. Thus, it is expected that a variation of the local dislocation density due to reversal of the strain will affect at the same time the local driving and the pinning pressures, which will cause the difference in the hardening levels. In the present paper, the influence of strain path change on microstructure evolution and mechanical behaviour in Nb-microalloyed steel (API X-70 grade) was studied. The deformation schedules were designed in order to investigate an effect of strain reversal on both static recrystallisation and strain-induced precipitation process kinetics. Flow curves recorded during deformation of X-70 steel showed clear influence of applied strain path on both static recrystallisation kinetics and strain-induced precipitation process.

Abstract: Recrystallization and austenite formation in a TRIP-assisted steel during conventional and ultra fast reheating for intercritical annealing are studied with the purpose to clarify the possibility for grain refinement. Partially recrystallized (or transformed) samples were prepared by reheating and water quenching to temperatures between 650 and 1050°C at reheating rates of 10, 50, and 3000 °C/s, respectively, without isothermal soaking from 95% cold rolled steel sheet with ferrite-pearlite microstructure. By monitoring the hardness and microstructure, it was shown that the increase of the reheating rate from 10 to 3000°C/s causes grain refinement from 5µm to 1µm in diameter and the final ferrite grain size depends significantly on both reheating temperature and reheating rate. It was observed that after an extreme reheating rate of 3000°C/s the α-γ phase transformation starts before the completion of the recrystallization. This opens up possibilities for further structural refinement and alternative texture control.

Abstract: In the present study, monotonic and cyclical torsional deformations of an X-70 microalloyed steel were conducted at austenite temperatures below the recrystallisation-stop temperature (T_5%). The austenite deformation is followed by accelerated continuous cooling to allow the investigation of the strain reversal effect on the subsequent phase transformation mechanisms. The transformation behaviours were studied by a dilatometry method, and the microstructures of the transformed products have been analysed using electron back scatter diffraction (EBSD). The results of this study shows that although subjected to the same total cumulative strain and the same cooling rate, strain path reversal by cyclical torsion produces lower temperature transformation products involving mainly a displacive mechanism, comparing to simple strain path deformation which leads to higher temperature transformation by a reconstructive mechanism.

Abstract: The relevance of the solute drag phenomenon to the hot rolling of modern steel grades is outlined. An overview of our present knowledge of solute drag in grain growth and recrystallisation in austenite is presented and recommendations for subsequent research are given.

Abstract: The effects of 0.74wt% Cr addition on the recrystallisation kinetics of low carbon (LC) steel was investigated using Vickers hardness testing, optical metallography and Electron Back-Scattering Diffraction (EBSD). Compared to the unalloyed steel, Cr addition significantly delays the time for recrystallisation and increases the JMAK pre-exponential factor (k) by an order of magnitude. During initial softening, EBSD returned similar orientations for nucleated and growing grain fractions in both steels. However with longer annealing times the micro-texture of recrystallised grains differs from their earlier counterparts. Overall, the effect of fine Cr carbides on grain boundary mobility in the LC-Cr steel causes variations in recrystallisation kinetics, grain morphology and micro-textures.

Abstract: Recrystallization was investigated in the context of its effect on the roping phenomenon in a 6000 series Al alloy. The findings suggest that, in general, the recrystallized grain size affects the material's susceptibility to roping more than the actual area fraction of specific texture components. For example, bands of Cube grains were typically observed in the final processed samples in cases which had coarse, recrystallized grain sizes from the earlier stages of processing. In contrast, bands were not observed for cases having fine, intermediate recrystallized grain sizes. Although microstructure and texture are highly intertwined, these findings suggest that the microstructural characteristics, such as the recrystallized grain size, are more important than those related to the texture.

Abstract: The Dutta-Sellars equations for predicting recrystallisation and precipitation in microalloyed steels have been assessed for accuracy over a range of strain levels (0.15 to 0.45) during hot deformation (975-1075 °C) of a homogenised 0.045 wt % Nb steel. It has been found that the model predicts the deformation state well at a strain of 0.3 whereas at both lower and higher strains i.e. 0.15 and 0.45, the agreement is less good. The differences between prediction and experimental measurement have been related to solute drag and precipitate potential of Nb.

Abstract: A Monte Carlo model is presented, in which the interface anisotropy is primarily inclination dependent. A dual grid method is used to determine boundary inclination in discretized digital microstructures. Evolution of the grain boundary character distribution (GBCD) in polycrystalline systems, from an initially random distribution, is inversely correlated with the anisotropy in interfacial energy, as expected.

Abstract: t is well known that alumina ceramics undergo a time dependent wear transition during sliding wear. The transition, which is associated with 1-2 orders of magnitude increase in specific wear rate, involves a change from mild wear to intergranular fracture. The transition is strongly grain size dependent, with the time to the transition decreasing with grain size. However, there is a minimum grain size that can be achieved in fully dense alumina using commercially viable processing. Alternative strategies for reducing grain size and increasing toughness are through the addition of a fine second phase, with SiC and ZrO₂ being the most promising. The resultant composite not only has finer grain size, but also exhibits additional toughening mechanisms. This paper reports on the microstructural control in alumina, zirconia toughened alumina and alumina-silicon carbide composites. The grain size and residual stress distribution are related to the damage accumulation mechanism that occur during frictional contact, in particular the surface specific dislocation activity.

Abstract: Thermomechanical processes based on direct charging routes combined with near net shape technologies have become one of the main industrial production routes. The singularity of the coarse as cast initial austenite grain size, combined with the limited total applied strain during hot working, requires a tailored design of the composition and deformation schedules in order to achieve the required mechanical properties. This becomes more and more complex as higher steel grades combined with thicker sections are incorporated into production. This paper reviews the role played by the interaction of dynamic-metadynamic-static recrystallisation and strain induced precipitation on achieving the finest and most homogeneous austenite microstructures as possible, prior to transformation in the case of Nb, Nb-Mo and Ti microalloyed steels. Special emphasis will be put on the relevance of the kinetics of combined postdynamic softening mechanisms before a complete stop of recrystallisation due to precipitation occurs.