Papers by Keyword: Static Recrystallization

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Authors: Mayu Muramatsu, Yuichi Tadano, Kazuyuki Shizawa
Abstract: A new recrystallization phase-field method is proposed, in which the three stages of recrystallization phenomena, i.e., recovery, nucleation and nucleus growth are sequentially taken into account in a computation. From the information of subgrain patterns and crystal orientations in a polycrystal that are obtained by a dislocation-crystal plasticity FE analysis based on a reaction-diffusion model, subgrain groups surrounded by high angle boundary are found out. Next, subgrains in the group are coalesced into a nucleus by rotation of crystal orientation and migration of subgrain boundaries through a phase-field simulation. Then a computation of nucleus growth is performed also using the phase-field method on account of an autonomic incubation period of nucleation, in which stored dislocation energy assumes a role of driving force. It is shown that the present method can numerically reproduce the three stages of recrystallization as a sequence of computational procedure.
Authors: Mahesh C. Somani, L. Pentti Karjalainen
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.
Authors: L. Sanz, B. Pereda, B. López
Abstract: Semi-empirical models for predicting the austenite static recrystallization behavior are widely used in designing thermomechanical treatments to improve final mechanical properties. However, a problem with these models is that their utility can be limited to the range of deformation conditions and chemical compositions they were developed for. This work focuses on the study of the applicability of current recrystallization models to the range of low strain conditions and/or high Nb microalloying additions (≈0.1%). To do so, the recrystallization behavior of two low carbon Nb-Ti microalloyed steels (0.04 and 0.11% Nb and ≈0.01% Ti) has been investigated by torsion tests. Experimental results for recrystallization time and recrystallized grain size have been compared to previously developed equations. It has been observed that at low strains (ε = 0.1) the predictions fail. A dependence of the n Avrami exponent both on temperature and applied strain was also found.
Authors: L. Pentti Karjalainen, Mahesh C. Somani, Atef S. Hamada
Abstract: Processing of a large number of novel steel types, such as DP, TRIP, CP and TWIP, and high-strength low-carbon bainitic and martensitic DQ-T steels, have been developed based on physical simulation and modelling studies. Among stainless steels, guidelines for processing of ultra-fine grained austenitic stainless steels have been created. Physical simulation has been used by employing a Gleeble thermo-mechanical simulator to reveal the phenomena occurring in the hot rolling stage (the flow resistance, recrystallization kinetics and microstructure evolution), and in the cooling stage (CCT diagrams) for carbon steels and in short-term annealing of cold rolled metastable austenitic steels. Connecting these data with microstructures examined in optical and electron microscopes and resultant mechanical properties have improved the understanding on complex phenomena occurring in the processing of these steels and the role of numerous process variables in the optimization of enhanced mechanical properties.
Authors: J.M. Rodriguez-Ibabe, Beatriz López
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.
Authors: Xiang Wei Kong, Tian Zhong Sui, Zhi Yong Hu
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.
Authors: Kirill Khlopkov, Georg Paul, Thomas Pretorius
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.
Authors: Manuel Gómez, S.F. Medina, Pilar Valles, Alberto Quispe
Abstract: Excellent mechanical properties (high strength and toughness) of microalloyed steels are mainly caused by induced precipitation during thermomechanical treatment (TMT) and grain refinement. It has been recently found that TMT of Nb-microalloyed steels can give rise to two different kinds of precipitates, manifested by the double plateau in the statically recrystallised fraction (Xa) against time curves. This work presents an electron diffraction study performed in a transmission electron microscope, equipped with an EDS analytical system. Lattice parameters of a great deal of particles, smaller than 200 nm and with face cubic centred structure, have been measured. Frequency distribution of the values of lattice parameters shows that these are grouped in two sets whose mean values are close. Comparison of these values with those found in the literature for carbides, nitrides and carbonitrides usually present in microalloyed steels demonstrates that they are Nb carbonitrides with slight stoichiometric differences (NbCxNy).
Authors: Cheng Wu Zheng, Na Min Xiao, Dian Zhong Li, Yi Yi Li
Abstract: The kinetics and microstructure evolution during static recrystallization (SRX) of hot-deformed austenite in a low carbon steel are simulated by coupling a cellular automaton (CA) model with a crystal plasticity finite element model (CPFEM). The initial deformed characteristics, which include the stored energy of deformation and the crystallographic orientation induced by a plane strain hot compression are simulated using a crystal plasticity finite element model. These data are mapped onto the CA regular lattices as the initial parameters for SRX simulation. The coupled simulation results reveal that the heterogeneous distribution of the stored energy of deformation results in non-uniform nucleation and a slower kinetics. The influence of non-uniform distribution in stored energy on the SRX kinetics and microstructure evolution is discussed based on a microstructural path (MP) analysis.
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