Papers by Keyword: Static Recrystallization

Abstract: The main purpose of this study is to develop a numerical model to describe the static recrystallization kinetics in X70 pipeline steel. In order to investigate the static softening behavior, double-hit compression tests were performed on a Gleeble-3500 thermomechanical simulator over a temperature range of 950-1150°C. The interpass time varies from 1-500s. Different values were applied to estimate effects of strain rate and deformation on static recrystallization. The results show that factors such as high temperature, long holding time and lower strain rate will lead to the inclination of softening behavior. The static softening activation energy was obtained. The good agreement between the predicted values and experimental results indicated the validation of the developed model of this study.

Abstract: Recrystallization kinetics of Mg alloy has been investigated in this study. Mg alloys such as Mg-3Al and Mg-6Al in weight percents were cast into rectangular shaped ingots of 20mm thickness. Solution treatment at 400°C for 24 hrs has been carried out on these ingots and pure Mg for comparison. Heat treated ingots including pure Mg were rolled at room temperature by thickness reductions of 10, 20, and 30 percents. Annealing treatment for recrystallization has been conducted on these cold-rolled plates at temperatures of 200, 300, and 400°C for various times from 1 min to 24 hrs. The microstructure observation and hardness measurement conducted on the recrystallized specimens revealed that static recrystallization at 200°C was very slow and expedited with increasing temperature, regardless of Al contents. While recrystallization behavior of Mg at 300°C appeared to be retarded by increase in Al contents, that of Mg was not affected at 400°C.

Abstract: In this paper, people mainly take three two-dimensional arrays State, Energy and Ori (Ori is the positive integer of 1 ≤Ori≤Ori_max ) to perform the molecular states and build models. According to the simulation results of molecular automaton, people have a research on the change and distribution of the grain diameter and the number of side during the process of static recrystallization, and conclude as follows, the relationship between the disappearing rate of individual deformation grain and the growth rate of individual recrystallization grain is characterized by randomness during the whole recrystallization process; the more the deformation, the smaller the mean diameter of grain is, at the finished moment of recrystallization; the grain diameters are not homogeneous during the static recrystallization process and at the finished moment of static recrystallization.

Abstract: Avoiding recrystallization of austenite in hot strip rolling of steels is highly important for enhancing mechanical properties of hot rolled products. The present work focuses on computation of incubation time t_inc for static recrystallization using laboratory hot deformation data and on extrapolation the results to industrial conditions. The computations are done based on application of critical conditions for initiation of dynamic recrystallization to the static case. No-recrystallization temperature in hot strip rolling is determined by setting t_inc equal to interpass time. Simulations allow for prediction of the onset of austenite static recrystallization after individual rolling passes during industrial hot rolling and evaluation of the effects of strip thickness, rolling speeds, etc.

Abstract: In recent past, a linear regression model to predict the activation energy (Qrex) and kinetics of static recrystallisation for hot-deformed austenite was developed based on stress relaxation test results of over 40 different carbon steels. The model is able to predict satisfactorily the static recrystallisation (SRX) kinetics of common carbon steel grades (including microalloyed steels) and also several special steel grades. In this study, the main effects of seven alloying elements, viz., C, Mn, Cr, Ni, Mo, Nb and V, on the activation energy of recrystallisation were further examined by using eight experimental steels based on an orthogonal Taguchi L8 matrix. All steels contained constant additions of B, Ti and Si. Even though originally intended for studies on phase transformation characteristics and hardenability under direct quenching conditions, the L8 matrix steels were suitably employed for further validation of the SRX regression model. In addition, the SRX characteristics and kinetics of a set of new steel compositions including C-Mn, C-Mn-Nb and C-Mn-Nb-Ti types were examined in the light of model equations, which further confirm the suitability of the regression model.

Abstract: A three dimensional phase field model has been developed to simulate the texture formed during the static recrystallisation of FCC metals with medium or high stacking fault energy, such as aluminium, copper and nickel. Before recrystallisation the deformation texture as well as the stored energy was simulated using a three dimensional crystal plasticity finite element model. This output calculated on the distorted finite element mesh was first mapped onto the regular grid of the phase field model using a linear interpolation method and then used as initial condition for the subsequent recrystallisation texture modelling. This model has successfully predicted the typical recrystallisation texture components: cube {001}<100>, R {124}<211> in the aluminium alloy. In addition, the softening fraction and three dimensional microstructure produced during static recrystallisation have also been simulated by this model.

Abstract: Quantitative prediction of grain size and recrystallized volume fraction is still a real challenge for many alloys, and even for simple materials when subjected to complex thermal/mechanical histories, as in multi-pass (industrial) processing. A first step is therefore taken in the direction of multiscale modelling of recrystallization, by considering digital polycrystalline microstructures. These synthetic mesoscopic microstructures are meshed adaptively and anisotropically, with refinement close to the grain boundaries. Crystal plasticity finite element (CPFEM) simulations are combined with a level set framework to model primary recristallization, following plastic deformation. In the level set method, the kinetic equation describing interface motion uses the calculated stored energy field provided by CPFEM calculations, and works on the same mesh. Discontinuous dynamic recrystallization can be modelled within the same approach, effectively coupling plastic deformation with nucleation and growth processes. Parallel to the finite element approach, a mean field model is developed in the general context of multi-pass processing. The model considers categories of grains based on two state variables : grain size and total dislocation density. As opposed to the finite element approach, there is no crystallographic or topological information. It is computationally much cheaper and therefore suitable for direct coupling at the scale of forming processes, for industrial applications. The parameters of the model can be identified from inverse analysis, using experimental stress-strain curves, recrystallized volume fractions, and grain sizes. Mean field and finite element models are compared, and it is shown that the detailed information provided by finite element simulations can be used to calibrate or optimize the mean field method.

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.

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: the hot deformation behaviors of 97# High Strength Rod was investigated through double-hit compression experiments using Gleeble 3500 thermal-mechanical similar within the temperature range of 850~1100°C, the strain rate of 5 s-1 and the interval range of 1-100s, the softening fractiong at different pass interval and deforming temperature was determined and analyzed. The results show that when pass intervals is the same, as deformation temperature increase, the volume fraction of static recrystallization of 97# High Strength Rod increases and the recrystallization process is enchanced. Activation energy of austenite static recrystallization of 97# High Strength Rod is 100.476 kJ/mol. The kinetic equation of static recrystallization of 97# High Strength Rod by avrami equation wan obtained.