连铸三维宏观传输模型—枝晶凝固微观特性的耦合研究

根据所建立的连铸三维宏观传输数学模型中，对枝晶凝固微观结构参数的影响效果进行了充分地耦合研究，包括：枝晶臂间距，枝晶凝固模式和糊状区渗透特性．宏观模型中，采用微观偏析半解析模型来对浇铸钢种的非平衡凝固路径进行近似确定，其中枝晶臂间距是重要的影响参数，即所谓的一次枝晶臂间距和二次枝晶臂间距，必须考虑后者随凝固过程的变化情况；连铸凝固过程中，两相糊状区的渗透特性对于铸坯的流场以及温度场和溶质浓度场具有重要的影响作用，模型中采用复合理论方法来对其加以了描述，即根据临界的固相分数值将糊状区分别视为非牛顿半固相流体和多孔介质；在确定多孔介质的渗透率时，考虑了枝晶凝固模式的影响效果，等轴晶区对应于各向同性渗透率，柱状晶区则对应于各向异性渗透率．采用该连铸三维耦合模型，并结合早前建立的连铸二维传热模型，针对某钢厂1号方坯连铸机进行了复合数值模拟研究，且研究成果已投入到铸机的实际生产运行中．工厂现场的生产状况表明，数值仿真结果具有较好的合理性和实用性，说明该三维耦合模型具有良好的仿真精度， 可广泛应用于实际连铸过程的数值仿真研究中．     

多元合金两相区凝固过程的数值仿真研究

基于微观枝晶计算域内的溶质质量守恒关系，推导出适用于枝晶凝固方式的二元合金微观偏析半解析数学模型，并根据简单加合原理对模型进行了多元化扩展，应用扩展后的微观偏析模型针对浇铸钢种（视为Fe—C多元合金）的两相区凝固过程进行了数值仿真计算，通过仿真获得了连铸凝固传热计算中所需要的钢种的非平衡凝固路径及实际固相线温度，研究结果表明，本文所建立的多元合金两相区凝固计算微观偏析数学模型及其仿真程序具有较好的合理性和广泛的适用性，可以方便地在连铸过程静态及动态仿真计算中加以耦合运用。     

板坯连铸初拉阶段工艺的计算机仿真

建立了板坯连铸初拉阶段的三维非稳定态传热凝固模型，且实现了该过程的数值仿真。模型中考虑了浇铸过程的影响；建立了铸坯凝固壳厚度与热流率之间的关系，用以表达铸坯在不同位置与结晶器间的换热边界条件。用直接差分法得到不同高度断面外宽面和窄面的凝固壳厚度，及不同拉速和不同过热度下晶出口处凝固的变化。模拟结果与生产实际结果基本吻合。     

板坯连铸结晶器中三维凝固壳厚度分布的数值模拟及实验验证

建立了连铸结晶器中三维凝固壳厚度分布的计算模型和计算方法，提出了非耦合计算时流动计算域与凝固传热计算域的衔接问题，以及铸坯表面换热系数确定的方法。针对２５０ｍｍ×１３００ｍｍ板坯连铸实际工况，用所建立的模型放处理方法数值模拟了其结晶器中的三维温度场和三维凝固壳厚度分布。同时用实测的凝固壳厚度分布数据验证了本计算听模型、边界条件和计算方法。本采用的方法可满足工程精度、并较简便实用。     

连铸坯枝晶凝固的重要微观结构特征参数的研究进展

连铸钢水凝固时结晶方式包括柱状晶和等轴晶对铸坯质量有显著影响。连铸过程合金成分、铸坯过热度、尺寸，冷却速度和温度梯度等技术参数均影响一次、二次枝晶臂间距、柱状晶至等轴晶转变边界位置和糊状区渗透率等结构特征参数。在已有研究的基础上总结了微观结构特征参数的半解析计算公式，在对连铸时宏观流场、温度场和浓度场分布的微观．宏观耦合数值模拟时可采用合适的半解析计算公式计算这些特征参数。     

取向硅钢连铸过程的凝固偏析模型计算

以Clyne-Kurz方程为基础建立了取向硅钢连铸过程的微观偏析模型,研究了取向硅钢连铸凝固过程中溶质元素在液相中的偏析规律。通过比较连铸凝固过程中锰和硫实际活度积和平衡活度积的关系,确定了MnS的析出条件。结果表明:二次枝晶间距随着冷却速率的增加而减小,柱状晶区域二次枝晶间距为191.3μm,元素的分配系数越小,其偏析程度越大,并且溶质偏析程度随着固相率的增加而增大,当凝固分数达到0.997时,MnS开始析出。     

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。     

强制对流影响下Fe?C合金定向凝固微观组织的相场法研究

定向凝固技术能够获得特定柱状晶结构,对于优化合金轴向力学性能具有非常显著的效果。本文采用耦合流场的相场模型模拟了定向凝固过程中枝晶的生长过程,研究了各向异性系数、界面能对定向凝固枝晶生长的影响以及强制对流作用下枝晶的生长行为。数值求解过程中,选用基于均匀网格的有限差分方法对控制方程进行离散,实现了格子中标记点算法（MAC）和相场离散计算方法的联合求解。处理微观速度场和压力场耦合时,采用MAC算法求解Navier-Stokes方程和压力Poisson方程,采用交错网格法处理复杂的自由界面。结果表明:随着各向异性系数的增大,枝晶尖端生长速度增大,曲率半径减小,枝晶根部溶质浓度逐渐降低;随着界面能的增大,枝晶尖端曲率半径增大,当界面能为最大（0.6 J·m?2）时,凝固呈现平界面的凝固方式向前推进;强迫对流对定向凝固枝晶生长方向影响较大,上游方向定向凝固枝晶粗大且生长速度更快,其现象随流速的增大而愈加明显。      

CAFE模型机理及应用

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,并应用CAFE法模拟了易切削钢9SMn28的三维微观组织,优化了9SMn28的成分.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耦合是通过FE节点和CA元胞之间的插值实现的.易切削钢9SMn28微观组织模拟结果与实验吻合较好,确定的碳、磷、锰、硅、硫的最佳质量分数分别为0.15%、0.10%、1.2%、0.08%、0.36%,并对优化结果进行了模拟,有效地改善了9SMn28的凝固组织.     

连铸坯枝晶臂间距检测方法的优化与应用实践

介绍了连铸坯凝固组织中树枝晶臂间距检测方法的优化试验及其应用效果。通过对连铸坯凝固冷却过程的分析及批量数据的采集,确定了测定连铸坯树枝晶臂间距的代表性区域,建立了一种科学测量枝晶臂间距的方法。将该方法应用于连铸圆坯生产检验中,能快速准确地找出铸坯枝晶臂间距随连铸二冷强度变化的规律,采取调整连铸二冷强度的措施,来控制一、二次枝晶臂间距大小,达到了减轻或消除铸坯开裂缺陷、提高连铸质量的目的。     

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.     

A Three-Dimensional Cellular Automata Model for Dendrite Growth with Various Crystallographic Orientations During Solidification

A three-dimensional (3-D) cellular automata (CA) model coupled with the finite-element (FE) method has been proposed to simulate dendrite growth with various crystallographic orientations during solidification. The model introduces a new tracking neighborhood method to resolve the mesh dependency caused by the cubic lattice in the CA model for simulating 3-D dendrite growth. The migration of the solid–liquid (SL) interface is associated with the dendritic preferential orientation and the driving force for the phase transition. The latter is obtained from a thermodynamics database. The local curvature and anisotropy of the surface energy are also incorporated to describe the growth kinetics of the SL interface. The solute transfer is calculated using the FE method. A numerical simulation has been performed on a Fe-1.5 wt pct C alloy. The grain morphologies with various crystallographic orientations and the solute distribution during isothermal solidification are studied and discussed. The simulation results are compared with analytical solutions and experimental results, which are in good agreement.     

关于连铸坯凝固传热有限元数值模拟的研究

建立了结晶器内连铸坯凝固传热的有限元数值计算模型，并就模型中两种凝固潜热处理方法，两种三点时间格式以及两种热容矩阵的应用进行了比较，同时研究了时间步长和计算网格大小对数值模拟结果的影响。     

基于无网格伽辽金法的连铸坯凝固计算方法

为考察无网格方法求解铸坯凝固过程的可行性,本文依据移动最小二乘和变分原理,推导并建立了基于无网格伽辽金法的结晶器内铸坯凝固过程二维非稳态传热/凝固数学模型。以小方坯凝固过程为对象,分别采用节点均匀布置、加密布置、随机布置方式,模拟分析了小方坯凝固过程的温度场变化,并将计算结果与参考解、有限元法数值解进行了对比,结果证实无网格伽辽金法在计算精度、自适应性、网格依赖性等方面均优于有限元法。研究结果为无网格方法应用于连铸过程的传热、凝固以及应力/应变行为的数值计算提供参考。      

Thermomechanics of the cooling stage in casting processes: Three-dimensional finite element analysis and experimental validation

A thermomechanical three-dimensional (3-D) finite element analysis of solidification is presented. The heat transfer model is based on a multidomain analysis accounting for noncoincident meshes for the cast part and the different mold components. In each subdomain, a preconditioned conjugate gradient solver is used. The mechanical analysis assumes the mold is rigid. A thermoelastic-viscoplastic rheological model is used to compute the constrained shrinkage of the part, resulting in an effective local air gap width computation. At each time increment, a weak coupling of the heat transfer and mechanical analyses is performed. Comparisons of experimental measurements and model predictions are given in the case of a hollow cylindrical aluminum alloy part, showing a good quantitative agreement. An application to an industrial aluminum casting is presented, illustrating the practical interest of thermomechanical computations in solidification analysis.     

连铸流动与凝固耦合模拟中糊状区系数的表征及影响

分析提出了连铸流动与凝固耦合数值模拟中, 钢液在两相区流动时的糊状区系数(A_mush)与渗透率的关系; 通过建立大方坯连铸结晶器三维耦合数值模型, 揭示了不同糊状区系数对钢液流动、传热与凝固进程的影响, 以及早期相关研究结果差异的源头.结果表明: 糊状区系数越大, 钢液在糊状区内的流动阻力越强, 凝固时钢液流动速度降低越快.采用较大的糊状区系数时, 糊状区呈较窄的"带状"分布在固液相之间; 当糊状区系数较小时, 糊状区范围变大, 钢液在结晶器内温降过快, 自由液面处出现过冷现象, 凝固坯壳局部发生重熔.结合实验数据验证与模型分析, 认为糊状区系数取值1×108~5×108 kg·m-3·s-1可以较可靠地揭示连铸结晶器内的实际凝固现象.      

Study on the Macrosegregation Behavior for the Bloom Continuous Casting: Model Development and Validation

With the aid of a coupling electromagnetic-thermal-solute transportation model validated by the industrial investigation, a three-dimensional (3-D) plus two-dimensional (2-D) hybrid modeling method has been presented for the exploration of the macrosegregation and macroscale transport phenomena in the bloom continuous casting (CC) processes of high-carbon GCr15-bearing steel. The evolution and characteristics of solute distribution and its influence on the porosity formation in the strand during CC process have been revealed. Solute segregation degree changes from a positive to a negative value with distance from strand surface in the region of initial solidification shell within thickness of 25 mm, which can be attributed to the circulation flow ahead the solidification front and the floatation of solute-rich molten steel at the upper part of the mold. The discontinuous, nonfrozen band induced by the zigzag solute distribution is proven to be the main reason that leads to the porosity formation in the final solidification stage of the CC strand. As the solidification proceeds, the segregation degree of C at the strand center is increased from 1.0 to 1.2, while the melt liquidus temperature is reduced from 1726 K (1453 °C) to 1706.91 K (1433.91 °C) during the CC process. Moreover, with the action of gravity and thermosolutal convection, a negative segregation region in the concave shape and an irregular positive segregation zone are produced in the fixed and loosened side of shell, respectively.     

软接触电磁连铸过程中结晶器及初生坯壳的传热

为了解高频磁场对软接触电磁连铸结晶器及初生坯壳传热行为的影响,用有限元二维数值模拟方法计算了软接触连铸过程中结晶器及初生坯壳的传热.得知在高频磁场（f=20 kHz）作用下,电磁场的感应加热会减少连铸初生坯壳的厚度、提高连铸坯的表面温度,并大幅度提高分瓣结晶器铜壁的温度.     

Multiphase Flow and Thermo-Mechanical Behaviors of Solidifying Shell in Continuous Casting Mold

 The metallurgical phenomena occurring in the continuous casting mold have a significant influence on the performance and the quality of steel product. The multiphase flow phenomena of molten steel, steel/slag interface and gas bubbles in the slab continuous casting mold were described by numerical simulation, and the effect of electromagnetic brake (EMBR) and argon gas blowing on the process were investigated. The relationship between wavy fluctuation height near meniscus and the level fluctuation index F, which reflects the situation of mold flux entrapment, was clarified. Moreover, based on a microsegregation model of solute elements in mushy zone with δ/γ transformation and a thermo-mechanical coupling finite element model of shell solidification, the thermal and mechanical behaviors of solidifying shell including the dynamic distribution laws of air gap and mold flux, temperature and stress of shell in slab continuous casting mold were described.     

降低高碳钢连铸小方坯中心偏析的研究

通过建立连铸小方坯凝同传热的数学模型，对实际生产中的小方坯凝固状况进行了模拟，发现原配水条件下铸坯表面温度波动大，并针对生产中连铸坯中心碳偏析较大的问题，对二冷水进行了优化。优化配水后的工业试验结果表明，通过凝固模型计算优化后的二冷配水可以降低中心碳偏析，提高连铸坯低倍质量。