Modeling recrystallization kinetics,grain sizes,and textures during multipass hot rolling

A physically based model for the evolution of recrystallization microstructures and textures during hot rolling of aluminum is presented. The approach taken differs from similar models developed for steels. The present model is based on recent experimental investigations directed toward identifying the nature of the nucleation sites for recrystallized grains of different crystallographic orientations. Particle stimulated nucleation (PSN) and nucleation from cube bands and grain boundary regions have been incorporated in the model. The multipass aspect complicates the modeling due to partial recrystallization between the rolling passes. Two different approaches have been suggested to handle this. The model has been applied to predictions of recrystallization kinetics, recrystallized grain sizes, and recrystallization textures during multipass hot rolling of aluminum. The predictions are reasonable compared to experimental results.     

FGH98合金的再结晶行为

利用扫描电镜和透射电镜对热挤压态和退火态FGH98合金的相析出规律及组织特征进行了分析,并对其再结晶机制进行了深入研究.结果表明:热挤压态FGH98合金在空冷过程中已经发生了少量的再结晶现象,随着挤压温度的提高,体系内位错密度下降.退火态FGH98合金中再结晶机制主要与体系内未回溶的一次γ'相有关,在一次γ'相聚集区再结晶主要以依靠亚晶的聚合和亚晶的长大,或两者的混合机制进行形核;随着一次γ'相的减少,合金还可以通过应变诱发晶界迁移、多晶粒交汇区形核、孪晶叠加等多种方式形核.需要指出的是,FGH98合金中未回溶的一次γ'相在退火处理过程中也会通过部分的回复和再结晶发生软化效应.      

Static recrystallization kinetics with homogeneous and heterogeneous nucleation using a cellular automata model

The kinetics of homogeneous and heterogeneous static recrystallization in a single-phase material were analyzed using two-dimensional (2-D) and three-dimensional (3-D) cellular automata (CA). A CA model was developed, which was then validated using the theory based on relationships developed by Johnson and Mehl, Avrami, and Kolmogorov (JMAK) for homogeneous site-saturated and constant-rate nucleation. The model was then modified for heterogeneous nucleation at grain boundaries, with either a fixed number of nuclei or a constant rate of nucleation. The fraction of boundary sites nucleated, for the case of fixed nucleation, varied from 0.006 to 0.28, resulting in Avrami exponents (k) ranging from 1.8 to 1.1 (site saturation). Site saturation with fixed nucleation produced a lamellar microstructure. The parameters of q and m, from Vandermeer’s microstructural path method, were calculated and compared with theoretical values. Constant-rate nucleation at grain boundaries between newly recrystallized grains and the unrecrystallized matrix resulted in k values of ≈1. Simulated microstructures revealed that with a low nucleation rate, recrystallized grains formed in clusters, while a high nucleation rate resulted in a necklace microstructure, with kinetics similar to those observed in dynamic recrystallization (k=1.4).     

Static recrystallization mechanisms in a coarse-grained Nb-microalloyed austenite

Static recrystallization mechanisms have been studied in a coarse-grained Nb microalloyed austenite. An austenite with a coarse grain size of 800 μm, typical of thin slab casting processes, has been deformed in torsion at a temperature of 1100 °C. After deformation, the specimens have been held for different times at this high temperature and then water quenched. The microstructural changes occurring during static recrystallization were characterized by metallographic evaluation. It has been observed that new recrystallized grains nucleate preferentially on parent austenite grain boundaries and tend to form in clusters. Once all the boundaries have been consumed, intragranular nucleation is actived at late stages of recrystallization. Clustered nucleation allows impingement to take place early during the recrystallization process, favoring grain-coarsening phenomena to occur behind the recrystallization front, which is denoted by the significant reduction in the number of grains per unit volume observed during early stages of recrystallization. Static recrystallization proceeds heterogeneously, as a result of a nonuniform distribution of stored energy in the deformed material. A continuous decrease of the average migration rate of the recrystallization front is observed, which can be ascribed to the reduction of the driving force for migration as recrystallization advances.     

The spatial distribution of nucleation sites and its effect on recrystallization kinetics in commercial aluminum alloys

Particle distributions play a major role in the processing response of aluminum alloys. While large constituent particles play an important role in the nucleation of recrystallization, small particles may heavily restrict the growth of recrystallized grains. In the present investigation, a two-dimensional (2-D) tesselation procedure has been used to characterize the particle distribution in commercial aluminum alloys and its relevance to nucleation of recrystallization. This procedure enabled the quantification of the degree of particle clustering in samples rolled to different strain levels. A characteristic aspect seems to be a transition from a rather nonuniform spatial distribution at low rolling strains, toward a more or less random distribution at high strains. Nucleation kinetics has been found to be site saturated, indicating that all nucleation events effectively occur at the start of recrystallization. A simple model is proposed, which explains the development of the spatial particle distribution as a function of rolling strain.     

Modeling recystallization kinetics in a deformed iron single crystal

The kinetics of recrystallization of a (111) { } single crystal of pure iron deformed 70 pct by rolling were characterized experimentally at temperatures between 450°C and 600°C, using quantitative metallography. The method of Laplace transforms was applied to the overall recrystallization kinetic behavior to separate nucleation from interface migration kinematics. A comprehensive nucleation and growth model was developed to explain all observations quantitatively. the model consisted of the following important features: (a) nucleation sites were distributed randomly; (b) nucleation was site-saturated and occurred with no practical incubation time at all annealing temperatures; (c) recrystallized grains grew three-dimensionally and were spheroidally shaped; (d) all recrystallized grains grew at approximatelly the same rate within experimental error; and (e) interface migration rates were not constant but decreased with time according to at ^−0.38 law at all temperatures. The time dependency of the interface migration rate was rationalized in terms of deformation-induced, nonuniform distribution of stored energy, Recovery processes competing with recrystallization were evident at long annealing times at the two lowest annealing temperatures.     

Nucleation of recrystallization in a Co- Cr- Mo alloy

Incoherent twin boundaries and twin-twin intersections have been identified as nucleation sites for recrystallization in a Co-Cr-Mo alloy. The recrystallized grain is a region of hep and has a twin orientation with respect to the fee matrix in its early stages of growth. This crystallographic relationship is discussed in terms of a martensitic embryo nucleation model for the fee → hcp transformation.     

Simulation of Dynamic Recrystallization Using Cellular Automaton Method

A new modeling approach that couples fundamental metallurgical principles of dynamical recrystallization with the cellular automaton method was developed to simulate the microstructural evolution linking with the plastic flow behavior during thermomechanieal processing. The driving force for the nucleation and growth of dynamically recrystallized grain is the volume free energy due to the stored dislocation density of a deformation matrix. The growth terminates the impingement. The model is capable of simulating kinetics, microstructure and texture evolution during recrystallization. The predictions of microstructural evolution agree with the experimental results.     

Mesoscale Modeling and Validation of Texture Evolution during Asymmetric Rolling and Static Recrystallization of Magnesium Alloy AZ31B

The focus of the present research is to develop an integrated deformation and recrystallization model for magnesium alloys at the microstructural length scale. It is known that in magnesium alloys nucleation of recrystallized grains occurs at various microstructural inhomogeneities such as twins and localized deformation bands. However, models need to be developed that can predict the evolution of the grain structure and texture developed during recrystallization and grain growth, especially when the deformation process follows a complicated deformation path such as in asymmetric rolling. The deformation model is based on a crystal plasticity approach implemented at the length scale of the microstructure that includes deformation mechanisms based on dislocation slip and twinning. The recrystallization simulation is based on a Monte Carlo technique that operates on the output of the deformation simulations. The nucleation criterion during recrystallization is based on the local stored energy, and the Monte Carlo technique is used to simulate the growth of the nuclei resulting from local stored energy differences and curvature. The model predictions are compared with experimental data obtained through electron backscatter analysis and neutron diffraction.     

Recrystallization behavior of twin roll cast low carbon steel strip

The dynamic and static recrystallization behaviors of twin roll cast low carbon steel strip were investigated with an attempt to provide guiding deformation parameters for the on line hot rolling.In order to investigate dynamic recrystallization behavior,as cast strip was reheated and soaked with austenite grain size similar to the width level of the as cast columnar structure.Tensile test was used and the deformation temperature is in the range of 900℃to 1 100℃and strain rates are 0.01 s^-1,0.1 s^-1,1 s^-1.The activation energy and stress exponent were determined as 306kJ/mol and 4.69 respectively.The ratio of critical strain to the peak strain is 0.65,and that of critical stress to the peak stress is 0.92.The dependence of the peak strain on the initial grain size and Zener - Hollomon parameters Z isε_p =9.1×10^-4×D₀^0.48Z^0.13.The kinetics of the dynamic recrystallization and recrystallized grain size was predicted using models published.The as cast coarse austenite were dramatically refined after complete dynamic recrystallization.For static recrystallization,the tensile test was carried out on Gleeble -3500 thermo - mechanical simulator.The deformation temperature is in the range of 800℃to 1 200℃with strain rate 0.01 s^-1 to 1s^-1.The pre strain is fixed at 0.04 to 0.12 and the inter-hit delay time varies from 1 s to 3 000 s.The activation energy and Avrami exponent of static recrystallization were determined as 241 kJ/mol and 0.54 respectively.A kinetics model was proposed to describe the static recrystallization kinetics.The predicted results were in good agreement with the experimental results.     

Modeling the microstructural changes during hot tandem rolling of AA5XXX aluminum alloys: Part I. Microstructural evolution

A comprehensive mathematical model of the hot tandem rolling process for aluminum alloys has been developed. Reflecting the complex thermomechanical and microstructural changes effected in the alloys during rolling, the model incorporated heat flow, plastic deformation, kinetics of static recrystallization, final recrystallized grain size, and texture evolution. The results of this microstructural engineering study, combining computer modeling, laboratory tests, and industrial measurements, are presented in three parts. In this Part I, laboratory measurements of static recrystallization kinetics and final recrystallized grain size are described for AA5182 and AA5052 aluminum alloys and expressed quantitatively by semiempirical equations. In Part II, laboratory measurements of the texture evolution during static recrystallization are described for each of the alloys and expressed mathematically using a modified form of the Avrami equation. Finally, Part III of this article describes the development of an overall mathematical model for an industrial aluminum hot tandem rolling process which incorporates the microstructure and texture equations developed and the model validation using industrial data. The laboratory measurements for the microstructural evolution were carried out using industrially rolled material and a state-of-the-art plane strain compression tester at Alcan International. Each sample was given a single deformation and heat treated in a salt bath at 400 °C for various lengths of time to effect different levels of recrystallization in the samples. The range of hot-working conditions used for the laboratory study was chosen to represent conditions typically seen in industrial aluminum hot tandem rolling processes, i.e., deformation temperatures of 350 °C to 500 °C, strain rates of 0.5 to 100 seconds and total strains of 0.5 to 2.0. The semiempirical equations developed indicated that both the recrystallization kinetics and the final recrystallized grain size were dependent on the deformation history of the material i.e., total strain and Zener-Hollomon parameter (Z), where and time at the recrystallization temperature.     

Heating Rate Effects on Recrystallized Grain Size in Two Al-Zn-Mg-Cu Alloys

A method has previously been described whereby a fine and stable grain size may be achieved in conventional, heat-treatable aluminum alloy sheet by thermomechanical processing. The present work has examined the final recrystallization stage more closely. In particular, the effects of heating rate on recrystallized grain size have been determined and explained. It has been shown that heating rates greater than about 5 K . s^-1 should be employed in the final recrystallization stage in order to obtain maximum benefit from the fine grain processing technique. The coarser recrystallized grain sizes obtained with slower heating rates are mainly due to early activation of the most highly favored nucleation sites. Thermal recovery of the matrix defect structure below the recrystallization temperature is an additional, though less significant, effect. The influence of the degree of cold work and the volume fraction of insoluble particles on recrystallized grain size is discussed in relation to the heating rate.     

Recrystallization, Precipitation Behaviors, and Refinement of Austenite Grains in High Mn, High Nb Steel

Through a series of experiments conducted on three kinds of high Mn steels with different Nb content, including stress relaxation tests, physical metallurgical modeling, and observation of prior austenite grains and precipitates, the effect of Nb on recrystallization and precipitation behaviors were investigated. The results indicate the existence of a novel deformation temperature range for grain refinement resulting from complete static recrystallization (SRX) in high Mn, high Nb steel, whereas slow SRX kinetics can be accelerated by a finer initial grain size. In this deformation temperature range, the effect of precipitation is too weak to prohibit SRX nucleation efficiently, but solute drag is still large enough to slow down growth rate. As a consequence, shorter incubation and homogeneous recrystallized nucleation can be realized at relative low temperature, and the coarsening rate of grains is much slower because of the high solute drag effect in the rolling of low C high Mn, high Nb line pipe steel.     

Simulation of recrystallization microstructures and textures: Effects of preferential growth

Three-dimensional (3-D) simulations of recrystallization were carried out using an approach where grains of different orientations are characterized by different nucleation and growth parameters. The aim was, by simulation, to study the effects of preferential growth on recrystallization under various nucleation assumptions. The simulations were based on the assumptions that (1) a material can be represented by two types of grains, one that grows 1.5 to 2.5 times faster than the other type; (2) the nucleation takes place either instantaneously when annealing is started or continuously during the annealing period; and (3) the nucleation sites are distributed either randomly or in clusters (around points, along lines, or on planes). These assumptions relate to common experimental observations. It was found that the nucleation and growth assumptions have very significant influences on the recrystallization characteristics. The preferential growth of one type of grain in particular affects the width of the recrystallized grain size distribution and the texture development. Furthermore, it was found that many different nucleation and growth assumptions can result in identical recrystallization microstructures and textures. The results of the simulations are discussed with reference to typical experimental findings.     

The Effect of the Initial Microstructure on Recrystallization and Austenite Formation in a DP600 Steel

The effects of initial microstructure and thermal cycle on recrystallization, austenite formation, and their interaction were studied for intercritical annealing of a low-carbon steel that is suitable for industrial production of DP600 grade. The initial microstructures included 50 pct cold-rolled ferrite–pearlite, ferrite–bainite–pearlite and martensite. The latter two materials recrystallized at similar rates, while slower recrystallization was observed for ferrite–pearlite. If heating to an intercritical temperature was sufficiently slow, then recrystallization was completed before austenite formation, otherwise austenite formed in a partially recrystallized microstructure. The same trends as for recrystallization were found for the effect of initial microstructure on kinetics of austenite formation. The recrystallization–austenite formation interaction accelerated austenization in all the three starting microstructures by providing additional nucleation sites and enhancing growth rates, and drastically altered morphology and distribution of austenite. In particular, for ferrite–bainite–pearlite and martensite, the recrystallization–austenite formation interaction resulted in substantial microstructural refinement. Recrystallization and austenite formation from a fully recrystallized state were successfully modeled using the Johnson–Mehl–Avrami–Kolmogorov approach.     

Static recrystallization of austenite and strain induced precipitation kinetics in titanium microalloyed steels

Using torsion tests and applying the back extrapolation method the recrystallized fraction of austenite was measured at two strains (0.20 and 0.35) and several temperature makes it possible steels. Graphical representation of the activation energy vs the inverse of the temperature makes it possible to determine the static recrystallization critical temperature (SRCT) of each steel as a function of the strain. With the results obtained, a model was constructed for the recrystallized fraction both at temperatures above and below the SRCT, and this, together with other equations already published, permits calculation of the residual strain and austenite grain size which the steel is expected to have at the end of rolling and before the γ → α transformation. Graphs of the recrystallized fraction vs time show the kinetics of strain induced precipitation and PTT diagrams (precipitation-time-temperature) were constructed for the three steels. It is noted that the degree to which the strain affects the precipitation kinetics decreases as the titanium content of the steel is increased.     

退火参数对ZE42镁合金热挤压板显微组织结构的影响

采用光学显微镜和TEM观察等方法,分析了ZE42镁合金热挤压板材在不同退火温度和时间条件下的显微组织结构。结果表明:该合金板材经过退火热处理后均发生明显的再结晶组织转变,晶粒尺寸在10～60μm之间,合金基体中存在大量含有稀土元素的第二相颗粒,这些第二相在热变形过程中破碎成细小的颗粒,促进了再结晶晶粒的形核。当退火温度一定时,随着保温时间的延长,该合金板材的平均晶粒尺寸首先增加,随后少量减小,最后又随退火时间的增加而增加;当保温时间一定时,随着退火温度的增加,晶粒平均尺寸持续增加,并且温度越高,增长速率越大。同时,通过晶粒长大动力学分析和计算建立了该合金再结晶晶粒长大的动力学模型,通过计算可知:该合金的再结晶晶界迁移激活能为15.32 kJ·mol-1。     

Quantitative Analysis of Micro‐Textures during Recrystallization in an Interstitial‐Free Steel

Two interstitial‐free steel samples were prepared by normal and by cross rolling. The effect of the resulting different deformation textures and microstructures on the subsequent recrystallization behavior was studied by micro‐texture analysis. The differences in recrystallization textures of the two differently rolled samples can be attributed to the microstructural differences in the as‐deformed state. The orientation distribution of the recrystallized grains forming at the early stages of the recrystallization dominated the final recrystallization textures, pointing to the importance of oriented nucleation in the formation of recrystallization textures of interstitial‐free steels.     

Grain refinement in 7075 aluminum by thermomechanical processing

A thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported. The process includes severe overaging, deformation, and recrystallization steps. Microstructural studies by optical and transmission electron microscopy of grain refinement in 7075 aluminum have revealed that precipitates formed during the overaging step create preferential nucleation sites for recrystallizing grains. The relationship between precipitate density following severe overaging and recrystallized grain density has been investigated; the results show that the localized deformation zones associated with particles larger than about 0.75 μ m can act at preferential nucleation sites for recrystallizing grains. The density of particles capable of producing nucleation sites for new grains is approximately ten times greater than the density of recrystallized grains. A close relationship between dislocation cell size after the deformation step and recrystallized grain density has also been established. Both quantities saturate for rolling reductions larger than approximately 85 pct. The grain size produced in 2.5 mm thick sheet by the optimum processing schedule is approximately 10 μm in longitudinal and long transverse directions and 6 μm in the short transverse direction.