Papers by Keyword: Static Recrystallization

Abstract: The effect of hot deformation behavior on austenite grain size refinement of low carbon multi-microalloyed steel was investigated. The morphology of austenite grains was revealed by thermal etching and observed using optical microscope. The results showed that single pass compression can only marginally refine austenite grain size by dynamic recrystallization, even under severe plastic deformation. However, when the specimens were held for a while after hot deformation, the fine austenite grain size can be obtained due to static recrystallization behavior.

Abstract: Microstructure evolution and static recrystallization kinetics of a magnesium AZ31 alloy during annealing have been investigated. The Mg alloy sheets were rolled at 100°C at a low rolling speed of 15m/min and a high rolling speed of 1000m/min to reductions of 8%, 23% and 30%. Annealing was then conducted on the as-rolled specimens at 200°C for different times. The microstructure was characterized by optical microscopy and recrystallization kinetics were evaluated by means of Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The softening behavior was found to be divided into two annealing stages. The first stage was related to the recrystallization on high stored energy regions, such as shear bands and twins, while the second stage was associated to the recrystallization of low stored energy areas.

Abstract: Under certain conditions of extrusion temperature and strain rate Al-Mg-Si alloys produce coarse recrystallized grains at and near the surface. Current FEM models are able to analyze grain size evolution for extruded profiles, but cannot predict the coarse recrystallized grains near the surface. A new model using DEFORM 2D and local state variables such as strain, strain rate and temperature is compared with Al-Mg-Si rods extruded at 440°C and 500°C for two extremes of strain rate. The model is found to be sensitive to the processing conditions and to accurately predict the recrystallized grain size and fraction.

Abstract: he softening and static recrystallization behavior of typical Fe-Mn-Si-Al series TWIP steel between high temperature deformation passes was investigated by two-pass hot compress deformation experiments on Gleeble-3500 thermal simulated test machine. The dynamic model of static recrystallization was built according to the experimental results. The investigation of the effects of deformation temperature, deformation rate and pre-deformation shows that deformation rate is the most effective parameter, and the increase of deformation and pre-deformation can also promote the static recrystallization. The calculated static recrystallization activation energy of TWIP steel is about 147kJ/mol through the dynamic equation built. The results of model predict conform to the experimental results.

Abstract: The combined effect of Al and Nb additions on the static softening behavior of C-Mn steels was investigated. The compositions of the steels studied in this work are representative of the recently developed TRIP-assisted steels: a base composition of 0.2wt.%C, 2wt.%Mn, 50ppmN, three different Al levels, 0.03 (base steel), 1 and 2wt.%, and two Nb contents of 0.03 and 0.07wt.%. Double-hit torsion tests were performed at different deformation temperatures (925-1065oC) and pass-strains (=0.2 and 0.35). It was found that solute Al produced a significant retardation on the static softening kinetics, this effect being enhanced by the addition of Nb. Additionally, below 1000oC the addition of 2 wt.%Al promotes the γα phase transformation to occur concurrently with softening. For the Nb microalloyed steels strain induced precipitation also occurred, resulting in a complex interaction between softening, phase transformation and strain induced precipitation.

Abstract: A model describing the kinetics of static recrystallization and the kinetics of Nb and Ti precipitation has been reconsidered for modern steels. The effects of alloying and microalloying elements have been introduced into the model. The comparison of the modeling with the experimental softening shows good agreement.

Abstract: In this paper, two heat treatment routes have been carried out on a set of hot compressed 7075 aluminum alloy samples under different deforming conditions. By considering the hot deformation history the law of 7075 aluminum alloy’s microstructure evolution during these two heat treatments was studied and the microhardness was discussed. The results show that: (1) Static recrystallization in 7075 aluminum alloy was discovered during these two heat treatments (solid solution (465°C/40min) +T6 aging (120°C/24h) and solid solution (465°C/40min) + T73 aging (107°C/7h+177°C/7h)), its driving force was from stored energy which was not released in the previous thermal deformation process. Formation of recrystallized grain was due to subgrain merger mechanism. The static recrystallization became more remarkable as the hot deforming temperature decreasing. (2) The effects of strain rate on the statically recrystallized grain size of 7075 aluminum alloy were opposite under different deformation temperatures. Deformed at 450°C, during these two heat treatments the recrystallized grain size became smaller as the strain rate increasing. While deformed at 350°C, during solid solution+T73 aging treatment the recrystallized grain size increased as the strain rate increasing. (3) Under the same deformation conditions a difference existed in the microstructure of 7075 aluminum alloy after solid solution +T6 aging and solid solution + T73 aging, which was mainly due to different precipitated phases in the aging process. (4) Both two heat treatments can raise the microhardness of 7075 aluminum alloy remarkably. The hardness of 7075 aluminum alloy after solid solution+T73 aging was lower than that after solid solution+T6 aging, the main reason was the precipitated small  phase in the first stage of the T73 aging coarsen in the second stage of high-temperature aging process.

Abstract: Thin slab direct rolling (TSDR) technology has some important metallurgical differences compared to conventional routes. One of these differences deals with the combination of coarse as-cast austenite grains and the possibility of limited solubility of microalloyed addition at the entry of the rolling mill. The industrial tendency of increasing the strength and toughness of steel grades produced by TSDR route requires increased amounts of Nb addition to achieve a proper austenite conditioning prior to transformation. Nevertheless, an increase in the Nb content rises the risk of premature Nb(C,N) precipitation during early rolling stages. In order to avoid this, one possibility is to add Mo and limit the amount of Nb. This study analyzes the influence of different Mo contents on static recrystallization of Nb microalloyed steels. The analysis includes the evaluation of the incidence of the combined effect of Nb and Mo solute drag retardation in both heterogeneous recrystallization evolution and grain size distribution for the case of initial coarse austenite grain sizes. To do that, partially recrystallized microstructures have been analyzed and microstructural parameters of interest quantified.

Abstract: Under certain conditions of temperature, time and deformation, static recrystallization of austenite in microalloyed steels can be temporarily inhibited by means of the strain-induced precipitation of nanoparticles that cause a pinning effect on austenite grain boundaries in motion. This inhibition can be seen by the formation of a “plateau” in the curves of static recrystallization of austenite obtained from double-deformation tests carried out under isothermal conditions. In this work, several microalloyed steels with different compositions are studied by hot torsion tests in order to characterize the kinetics of recrystallization and its inhibition. The precipitation state in austenite is studied in several samples by means of transmission electron microscopy. The influence of the type of microalloying element (Al, Nb, V) and the mean size of the precipitates on the duration time of the plateau is studied and relationships between these variables can be obtained. Particularly, it is seen that Al-alloyed steels present a much coarser particle size and a considerably shorter plateau compared to Nb and V-microalloyed steels.

Abstract: In this study, the constitutive flow behavior and static recrystallization characteristics of a Nb-microalloyed TWIP (Fe-20Mn-1.5Al-0.1Nb) steel under hot deformation conditions have been determined and results compared with those of Fe-25Mn-Al TWIP steels. Investigations using compression testing in a Gleeble simulator, including the double-hit technique, enabled the acquisition of flow stress and recrystallization data. These were analyzed to determine the powers of strain and strain rate as well as the activation energies of deformation and recrystallization (Qdef and Qrex). For given deformation and grain size parameters, the time for 50% recrystallization (t50) of the 0.1% Nb TWIP steel was significantly longer than for the Nb-free TWIP steel: it was comparable to that of Nb-microalloyed carbon steel. Qrex was higher than that of Type 304 stainless steel that has nevertheless much longer t50 times.