Papers by Keyword: Recrystallization Kinetics

Abstract: The recrystallization behavior of the cold-rolled AA3003 aluminum alloy with the reduction rate of 20%, 50% and 90% during annealing at the temperature ranging from 300°C to 400°C was investigated. As increasing reduction rate, the cold rolled specimens exhibit deformation bands with elongated grain microstructure consisting of straight grain boundary parallel to rolling direction. Therefore, large density of nucleation sites for recrystallization would be expected with increase of strain energy. The grain size of the cold-rolled specimens decreased with increase of reduction rate, c.f., as the rolling reduction increased to 90%, grain size along the direction normal to the sheet decreased to about 8μm in thick. When the sample annealed at 350°C for 5s, the first recrystallized grains were observed in the vicinity of the grain boundary. The relaxation and recrystallization kinetics under different annealing conditions were characterized in terms of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. The apparent activation energies of recrystallization for the cold-rolled specimens with reduction rate of 20%, 50% and 90% were determined as 332 kJ/mol, 239 kJ/mol and 115 kJ/mol, respectively. XRD analysis by using modified Williamson–Hall plots revealed that the tendency of the change in dislocation density is varied depending on reduction rate. These results indicate that the apparent activation energy for recrystallization and the crystallites size decrease with increase of the reduction rate, which leads to a decrease in the size of the recrystallized grains.

Abstract: The technological and metallurgical advancements of value-added niobium (Nb) microalloyed thermo-mechanical controlled process (TMCP) plate steels continue to be developed for more demanding end user requirements. The market demand for reduced fuel consumption and CO₂ emissions in the automotive and construction sectors have further increased the demand for these new and advanced higher quality Nb-bearing steel grades. Often, the transition from laboratory melted and TMCP hot rolled heats to the production scale requires some continuous casting, thermal and mechanical metallurgy adjustments from the laboratory results in order to accomplish proper industrial continuous casting and hot rolling processes. These advanced high strength steels are microalloyed with Nb, Mo and/or other elements which affect the austenite-ferrite transformation. Niobium enables achievement of substantial grain refinement when the plate is rolled with the proper reduction and thermal schedule. The effects of these microalloying elements on the continuous cooling transformation behavior must be carefully controlled during the reheating and rolling process to successfully achieve the desired mechanical properties. TMCP applications have been successfully developed in numerous product sectors with thickness exceeding 120 mm. Since the very fine grained microstructure improves toughness and increases the yield strength, this TMCP process enables the required tensile properties with the growing trend to leaner chemical composition designs (less than 0.10%C) and excellent toughness properties. The consequence of leaner chemical compositions, especially lower carbon content and lower carbon equivalent enhances mechanical properties, fabrication and weldability.

Abstract: Investigation of the microstructural evolution of Al-Mn-Fe-Si alloys during annealing after cold rolling has been carried out. The effect of microchemistry state in terms of Mn in solid solution, constituents, size and volume fraction of dispersoids, introduced prior to cold rolling through different homogenization treatments, on microstructural evolution is compared during subsequent isothermal and non-isothermal heating experiments, with focus on the dependency of the amount solute (potential for concurrent precipitation) and pre-existing particles prior to deformation. It is clearly demonstrated that the actual kinetics and final microstructure are the result of a delicate balance between processing conditions and microchemistry state. In general, non-isothermal annealing treatments produce inhomogeneous microstructure, as compared to isothermal annealing. Moreover, more and finer particles (resulting from low-temperature homogenization) tend to hinder boundary motion, leading to even slower recrystallization kinetics and coarse non-equiaxed grains with a strong P texture component.

Abstract: Effects of local variations in the deformation microstructure on subsequent recrystallization are discussed and illustrated by three examples. The three examples consider local variations on different length scales and are: 1. Effects of local variations in the deformation microstructure on the formation of protrusions on migrating boundaries. 2. Effects of an inhomogeneous spatial distribution of second phase particles on growth. 3. Effects of stored energy and orientation variations on recrystallization kinetics.

Abstract: The recrystallization behavior of 50% cold rolled Fe-22%Mn-0.376%C alloy during annealing at 560°C, 630°C and 700°C was investigated. Microhardness tests were applied for characterization of the recrystallization kinetics, X-ray diffraction and EBSD measurements were utilized to characterize the crystallographic texture and the grain microstructure. The obtained experimental data were evaluated in terms of the JMAK model. The obtained values of the Avrami exponent varied in the range between 0.70 and 1.37. The inhomogeneous grain microstructure after recrystallization is interpreted in terms of non-randomly distributed nuclei. Shear bands, lamellar lines intersecting with mechanical twins and grain boundaries with localized high misorientation gradients were identified to be preferential nucleation sites. No pronounced texture was observed after annealing at 630°C.

Abstract: New in-situ 3DXRD results obtained since the last Rex&GG conference are presented and discussed. This includes: Documentation of the formation of nuclei with new orientations, determination of apparent activation energies for individual bulk grains during recrystallization and evolution in the 3D microstructure during grain growth.

Abstract: The recrystallization behavior of a 50% cold rolled Fe-23.2Mn-0.57C alloy was investigated during annealing at temperatures between 560°C and 670°C. The recrystallization kinetics were characterized by microhardness tests. X-ray diffractions and EBSD measurements were used to characterize the grain microstructure and texture evolution during annealing. The obtained experimental data were evaluated in terms of the JMAK model. The annealing texture in the investigated temperature range was very weak.

Abstract: The metallurgical equations have been implemented into the finite difference model to predict the microstructure evolution at different locations in the plate cross-section. Recrystallization kinetics and grain size distributions instead of average grain size values were computed for different rolling schedules. For 20mm plate, the austenite grain sizes at the surface are smaller than at the center, with the exception of the conner where there are the largest grain sizes in throughout cross-section, and the smallest grain sizes can be found near the end of the horizontal central line. The fine austenite grain size and relatively high retained strain could be obtained by modifying rolling practice, such as changing the temperature and thickness at the entrance of finishing rolling and adopting intermediate water cooling. The ferrite grain size and its distribution have a good agreement with the measurements.

Abstract: The recrystallization kinetic of 17Cr 1Mo ferritic steel was studied using 60% cold rolled samples. The recrystallization was carried out at 750 , 800 and 9000C for 3, 5, 15, 30 and 60 minuets. The volume fraction of the recrystallized grains was used as a kinetic parameter. The magnitude of time exponent “n” was much less than the ideal value because of the heterogeneous recrystallization. The estimated activation energy for recrystallization “Q” was more as compared to those for pure metals.

Abstract: Effect of deformation temperature on subsequent recrystallization characteristics of commercial purity titanium (CP-Ti) was investigated using EBSD and transmission electron microscopy. CP-Ti plates were rolled to at various temperatures between 25°C and 600°C to achieve 60% reduction, and were subsequently annealed at 600°C for various time periods. Increase in rolling temperature resulted in more refined microstructures, which could be attributed to coactivation of secondary slip systems such as basal slip or pyramidal slip in addition to prism slip. During subsequent annealing, a large number of recrystallization nuclei formed in the warm-worked (below 450°C) CP-Ti while nucleation of recrystallization was slower for coldrolled one. This led to the faster recrystallization kinetics and a finer microstructure in asrecrystallized state for warm-worked CP-Ti. Meanwhile, the rate of recrystallization in CP-Ti rolled at 600°C was significantly retarded because a good portion of strain energy stored in deformed matrix was released by dynamic recovery. In the present work, we show that the widely accepted explanation for the influence of the deformation temperature on the recrystallization characteristics, i.e., increase of the deformation temperature retards the recrystallization process, is not always true for the hcp metals since deformation at elevated temperature activates secondary slip systems, providing more sites for nucleation of recrystallization.