Dynamic Recrystallization of Hot Deformed 3Cr2NiMnMo Steel: Modeling and Numerical Simulation

Hot compression tests of 3Cr2NiMnMo steel were performed at temperatures in the range of 850 to 1100 °C and with strain rates of 10^?2s^?1 to 1s^?1. Both the constitutive equations and the hot deformation activation energy were derived from the correlativity of flow stress, strain rate and temperature. The mathematical models of the dynamic recrystallization of 3Cr2NiMnMo steel, which include the dynamic recrystallization kinetics model and the crystallization grain size model, are based on Avrami's law and the results of thermosimulation experiments. By integrating derived dynamic recrystallization models with the thermal-mechanical coupled finite element method, the microstructure evolution in hot compressive deformation was simulated. The distribution of dynamic recrystallization grains and grain sizes were determined through a comparison of the simulation results with the experimental results. The distribution of strain and dynamic recrystallization grain is also discussed. The similarity between the experimental results and the simulated results indicates that the derived dynamic recrystallization models can be applied effectively to predict and analyze the microstructure evolution in hot deformed 3Cr2NiMnMo steel.     

Multistage High‐speed Compression Test to obtain Material Data for the Kinetics of Microstructure Change in Microscale analysis of Large‐strain Working Technologies

A new analytical model for predicting microstructure change is proposed, and the actual steel microstructure changes that evolve during multistage and single‐stage high‐speed compression are analysed by EBSP (Electron Back Scattering Pattern). Severe plastic deformation induces evolution of various microstructure changes. Prediction of the changes requires the micro‐scale analysis of large‐strain working technologies and accurate material data, which are usually collected by conducting experiments such as compression tests. The analytical model uses the residual dislocation density and austenite grain size as parameters, and can be used to analyse the ferrite nucleation and transformation inside the grains. The compression tests were performed using a newly developed machine that can realize multistage forming at high strain rates. The precision of the data from the tests can be expected to be higher than that from conventional tests. Through the investigation, it becomes clear that multistage high‐speed forming can produce ultrafine‐grain steel whose chemical composition is the same as plain carbon steel, when applying the kinetics of microstructure change shown in the analytical model.     

Hot Deformation and Dynamic Recrystallization Behaviour of Medium Carbon Steel in Austenite Region

The hot deformation behaviour of a 0.47%C (JIS‐S45C) steel in the stable austenite region was systematically investigated under various deformation conditions to collect fundamental data on its high‐temperature deformation and microstructure evolution. The medium carbon steel showed dynamic recrystallization in a wide range of temperatures (850°C～1150°C) and strain rates (10^‐3 s^‐1～100 s^‐1) in the stable austenite region. The dynamically recrystallized grain size was monotonically decreasing with increasing steady state stress. The minimum grain size obtained through dynamic recrystallization was 8.3 μm when the S45C specimen was deformed at 850°C and 1 s^‐1. The stress‐strain relationships were formularized based on a phenomenological model. The stress‐strain curves estimated by the obtained equation were in good agreement with the experimental results.     

Multi‐scale Finite Element Cellular Automata Simulation of Multi‐step Cold Forging Operations

The application of new materials to produce forged connecting parts is presented in this work. Particular attention is put on modern bainitic steels due to their increased ductile and strength properties, which influence the behaviour of final products under further exploitation conditions. Bainitic steels do not require a series of thermo‐mechanical operations to obtain these elevated properties, which is one of the advantages of this material. Experimental analysis and numerical simulations of steel behaviour during multi‐step cold forging operations are described in this paper. Since it is one of the possible fracture initiation mechanisms, strain localization development during cold forging is investigated in detail. Conventional constitutive models used in finite element programs have limitations in modeling stochastic and discontinuous phenomena that are responsible for strain localization. The cellular automata model is used as constitutive law in this work to overcome these difficulties and investigate material flow during multi‐stage cold forging operations. Connection of the cellular automata (CA) and finite element (FE) methods creates a so‐called multi‐scale CAFE model. The main aspects of the model are described briefly in this paper. The experimental part of this work supports the numerical investigation. Comparison of the parameters measured and predicted by the CAFE model is presented and discussed as well.     

低碳钢热变形奥氏体的再结晶行为

对热变形奥氏体的再结晶动力学和微观组织演变进行了模拟计算,对晶粒尺寸的模拟值和实测值作了比较,分析了化学成分对动态再结晶率的影响以及残余应变与变形温度的关系.结果表明:在温度较高、应变速率较低的条件下容易发生动态再结晶,随着变形温度的降低,发生动态再结晶的几率减小,而静态再结晶在前几道次进行得比较充分,随后进行得不充分,增加碳和锰的含量可以促进动态再结晶的发生,残余应变随变形温度的降低而增大,晶粒尺寸的模拟值和实测值吻合较好,表明所选用的模型有一定的参考价值.     

Three-dimensional Numerical Simulation and Experimental Analysis of Austenite Grain Growth Behavior in Hot Forging Processes of 300M Steel Large Components

The microstructure models were integrated into finite element (FE)code,and a three-dimensional (3D) FE analysis on the entire hot forging processes of 300M steel large components was performed to predict the distri-butions of effective strain,temperature field and austenite grain size.The simulated results show that the finest grains distribute in the maximum effective strain region because large strain induces the occurrence of dynamic re-crystallization.However,coarse macro-grains appear in the minimum effective strain region.Then,300M steel forg-ing test was performed to validate the results of FE simulation,and microstructure observations and quantitative analysis were implemented.The average relative difference between the calculated and experimental austenite grain size is 7.5 6%,implying that the present microstructure models are reasonable and can be used to analyze the hot forging processes of 300M steel.     

CAFE模型机理研究及应用

 摘 要：研究和分析了CAFE法模拟凝固过程微观组织的物理本质、数值计算方法。在CAFE模型中,形核密度用高斯分布来描述;枝晶尖端生长动力学用KGT模型进行计算;枝晶生长的择优取向是<100>方向,并可实现枝晶生长的竞争机制;FE与CA的耦合是通过FE节点和CA元胞之间的插值实现的。应用CAFE法模拟了易切削钢9SMn28的三维微观凝固组织,模拟结果与实验吻合较好。对易切削钢9SMn28进行了成分优化,并对优化结果进行了模拟,有效的改善了9SMn28的凝固组织。     

Grain Refinement in a Low Carbon Steel Through Multidirectional Forging

 Grain refinement is one of the successful and low-cost methods to develop metals having excellent combination of strength and ductility. Low carbon steel was deformed by using multidirectional forging (MDF) technique at room temperature. The influence of strain amount and annealing process on the microstructure and mechanical properties of investigated steel was studied. The grain refinement mechanism was studied by the microstructure observation. The results showed that the grain refinement was attained by multidirectional forging technique. The initial coarser grains of average 38 μm size fragmented into very fine ferrite with grain sizes of about 1.2 μm. After MDF, the strength properties improved significantly, although uniform elongation and elongation decreased with increasing strain.     

基于元胞自动机有限元法对方坯凝固组织的数值模拟

 基于元胞自动机有限单元法(CAFE)对国内某钢厂220mm×220mm方坯的三维显微凝固组织进行模拟,分析了CAFE法模拟凝固过程显微组织的物理本质,对形核密度、枝晶尖端生长动力学、枝晶生长的择优取向以及FE与CA耦合的实现分别进行了探讨。用该方法对方坯的三维显微组织进行模拟,并对结晶器出口处方坯的角部温度、中心表面温度及坯壳厚度进行了计算。模拟结果表明：当拉速为0. 85m/min,浇铸温度为1535℃,浇钢过热度为30℃时,结晶器出口处方坯角部温度在850~950℃之间,中心表面温度在1050~1170℃之间,坯壳厚度在15mm左右,铸坯柱状晶发达,等轴晶比率较小。模拟的铸坯组织的等轴晶比例与低倍试验结果吻合较好,可以很好地预测方坯实际凝固组织。     

A three-dimensional cellular automation-finite element model for the prediction of solidification grain structures

A three-dimensional (3-D) model for the prediction of dendritic grain structures formed during solidification is presented. This model is built on the basis of a 3-D cellular automaton (CA) algorithm. The simulation domain is subdivided into a regular lattice of cubic cells. Using physically based rules for the simulation of nucleation and growth phenomena, a state index associated with each cell is switched from zero (liquid state) to a positive value (mushy and solid state) as solidification proceeds. Because these physical phenomena are related to the temperature field, the cell grid is superimposed to a coarser finite element (FE) mesh used for the solution of the heat flow equation. Two coupling modes between the microscopic CA and macroscopic FE calculations have been designed. In a so-called “weak” coupling mode, the temperature of each cell is simply interpolated from the temperature of the FE nodes using a unique solidification path at the macroscopic scale. In a “full” coupling mode, the enthalpy field is also interpolated from the FE nodes to the CA cells and a fraction of solid increment is computed for each mushy cell using a truncated Scheil microsegregation model. These fractions of solid increments are then fed back to the FE nodes in order to update the new temperature field, thus accounting for a more realistic release of the latent heat (i.e., the solidification path is no longer unique). Special dynamic allocation techniques have been designed in order to minimize the computation costs and memory size associated with a very large number of cells (typically 10⁷ to 10⁸). The potentiality of the CAFE model is demonstrated through the predictions of typical grain structures formed during the investment casting and continuous casting processes.     

Tube Twist Pressing (TTP) as a New Severe Plastic Deformation Method

Tube twist pressing (TTP) as a new severe plastic deformation method for processing tubular parts was presented. The commercially pure aluminum tubes successfully were processed by TTP method. Microstructural examination by XRD analysis of the processed tubes revealed the formation of fine grains in the average size of 1.1 μm after four TTP passes. Also, the obtained results of mechanical tests showed a notable increase in microhardness, yield and ultimate strengths. The capabilities of TTP method were verified via comparison of the obtained results with the results of other SPD processes. To further investigate the TTP method, FE modeling was carried out using the Abaqus/Explicit to study the macroscopic deformation and microstructural evolution (the evolution of dislocation density and grain size) during TTP via continuous dynamic recrystallization. In the FE model, the strain hardening behavior of the material was related to microstructure quantities based on the micromechanical constitutive model. The FEM simulated grain refinement behavior was consistent with the experimentally obtained results.     

Numerical simulation of solidification structure of high carbon SWRH77B billet based on the CAFE method

Abstract

Based on the coupled method of cellular automaton (CA) and finite element (FE), the solidification structure of 160×160 mm cast billet of high carbon SWRH77B steel was simulated. The nucleation density, kinetics of the dendrite tip growth, crystallographic orientation and coupling of CA and FE methods are discussed. In the current study, the influence of superheat on the solidification structure of the billet is researched in detail. The results show that for an increase in superheat extent from 20 to 30°C, the density of grain in billet decreased from 4·662×10⁶ to 3·087×10⁶ m^?2, and the grain mean radius increased from 295·1 to 346·3 μm. The three-dimensional microstructure of high carbon SWRH77B steel was simulated by the CAFE method, and there was good agreement with the results from industrial billet.     

Prediction of grain structures in various solidification processes

Grain structure formation during solidification can be simulatedvia the use of stochastic models providing the physical mechanisms of nucleation and dendrite growth are accounted for. With this goal in mind, a physically based cellular automaton (CA) model has been coupled with finite element (FE) heat flow computations and implemented into the code3- MOS. The CA enmeshment of the solidifying domain with small square cells is first generated automatically from the FE mesh. Within each time-step, the variation of enthalpy at each node of the FE mesh is calculated using an implicit scheme and a Newton-type linearization method. After interpolation of the explicit temperature and of the enthalpy variation at the cell location, the nucleation and growth of grains are simulated using the CA algorithm. This algorithm accounts for the heterogeneous nucleation in the bulk and at the surface of the ingot, for the growth and preferential growth directions of the dendrites, and for microsegregation. The variations of volume fraction of solid at the cell location are then summed up at the FE nodes in order to find the new temperatures. This CAFE model, which allows the prediction and the visualization of grain structures during and after solidification, is applied to various solidification processes: the investment casting of turbine blades, the continuous casting of rods, and the laser remelting or welding of plates. Because the CAFE model is yet two-dimensional (2-D), the simulation results are compared in a qualitative way with experimental findings.     

Deformation Behaviour and Microstructural Evolution During Hot Compression of AISI 904L

The hot deformation behaviour and microstructural evolution of AISI 904L super‐austenitic steel has been investigated by means of hot compression tests. The tests were carried out on a Gleeble 1500D thermo‐mechanical simulator in the temperature range from 850 °C to 1150 °C and at strain rates range from 0.001 s^?1 to 5 s^?1. The microstructure evolution was examined by means of light optical microscopy (LOM). The results show that after an initial deformation hardening, softening mechanisms occur. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in the hyperbolic‐sine equation with the activation energy for deformation of 463 kJ/mol. The steady state was achieved at maximum strain of 0.9 only at the lower strain rates (under 1 s^?1) and the higher temperatures (above 1100 °C). Microstructural analyses showed a gradual increase in the dynamically recrystallized area with an increase of the temperature and a decrease of the strain rate. The grain size did change, as expected, correlating to the deformation conditions.     

The combined effect of grain size and strain rate on the dislocation substructures and mechanical properties in pure aluminum

The effect of grain size on the development of dislocation substructures has been studied as a function of strain rate. Pure aluminum rods with grain diameters of 70, 278, and 400 μm were deformed in tension at room temperature to various percent strains at strain rates of 0.01, 0.25, 2.5, and 5/min. It has been confirmed that the smaller grain size results in higher flow stress in this strain-rate range. The cell size strengthening described by the modified Hall-Petch (MHP) equation is applicable to samples with 70 and 278 μm grain sizes at all four strain rates used in this study, while 400 μm grain sizes show deviation from this because of inhomogeneities developed in the microstructure. The influence of strain rate on the slope of the MHP plots, for a grain size of 70 μm, is such that at lower strain rates, the slope does not change much, but at higher strain rates, there is an increase in the slope value. At all strain rates, the values of slopes from the MHP plots of the smaller grains are higher than for the larger grains.     

Modeling the kinetics of grain-boundary-nucleated recrystallization processes after cold deformation

A physical model to predict the recrystallization kinetics of single-phase polycrystalline metals, based on a single grain representation of deformed microstructure (characterized by a mean subgrain size and mean misorientation of subgrain boundaries), is presented. The model takes into account the grain geometry, the position, and the density of the nucleation sites. The selected geometry is a regular tetrakaidecahedron, combining topological features of a random Voronoi-distribution characteristic for polycrystalline material with the advantages of a single grain calculation. The model employs empirically determined relationships from existing literature to describe the deformed microstructure and, in so doing, facilitates the prediction of the recrystallization behavior when only the deformation strain and the recrystallization temperature are known. The boundary mobility and the driving force, as well as the nucleation density, are related to the true plastic strain of deformation through the microstructure. The model also describes the effects of concurrent recovery on the overall recrystallization kinetics.     

Tube Reversing and Extrusion (TRE) as a Novel Method for Producing UFG Thin Tubes

In the present research, a new severe plastic deformation method has been introduced for producing thin walled tubes with ultrafine grained substructure. The tube reversing and extrusion (TRE) technique was applied to a CP-aluminum and thin walled tubes with ultra-fine grained microstructure were successfully processed. The obtained results from tensile tests at room temperature showed the significant increase in mechanical properties of TRE processed thin walled tubes including yield and ultimate strengths and micro-hardness due to grain refinement. The microstructure evolution and deformation behaviour of commercially pure aluminium under TRE processing was simulated by the constitutive model as a micromechanical approach implemented in the finite element framework. The continuous dynamic recrystallization (the evolution of dislocation density and grain size) of aluminum tubes during TRE was considered as the main grain refinement mechanism in micromechanical constitutive model. Also, the flow stress of material in macroscopic scale was related to microstructure quantities. This was in contrast to the previous approaches in FEM simulations of SPD methods where the microstructure parameters such as grain size were not considered at all. The FEM simulated grain refinement behavior was consistent with the experimentally obtained results.     

粗晶Mg-3Gd-1Zn合金高温压缩变形过程中的动态再结晶

研究了粗晶Mg-3Gd-1Zn合金在723 ～823 K,应变速率0.100 ～0.001s-1条件下单轴压缩变形过程中的动态再结晶行为.研究结果表明,其热压缩曲线为典型的动态再结晶型,峰值流变应力和稳态流变应力随温度的升高而减小,随应变速率的增大而增大;在该实验温度范围内其变形激活能约为140 kJ·mol-1;再结晶晶粒尺寸lnd与lnZ参数偏离线性关系,且变形温度对再结晶晶粒尺寸的影响比应变速率更大.利用金相和电子背散射技术(EBSD)对773 K,0.010 s-1条件下压缩不同变形量的Mg-3Gd-1Zn合金进行了组织表征,发现其动态再结晶大都发生在孪晶界及其与原始晶界的交叉处,主要为孪生诱发动态再结晶形核(TDRX)机制.再结晶形核初期形状不规则,晶界倾向于呈直角,随着应变量的增大,由于晶界的局部迁移,再结晶晶粒逐渐转变为稳定的等轴晶.