带结晶器电磁搅拌的大圆坯连铸的数值模拟

在圆坯连铸中,结晶器电磁搅拌器(M-EMS)是常见的改善钢流内流场的手段。众所周知,液芯的流动对最终产品的质量有着重要影响,用M-EMS能优化凝固前沿的流速,进一步促进柱状晶向等轴晶的转变,进而改善铸坯的表面和皮下缺陷。在奥钢联多纳维茨钢厂,绝大多数产品的生产都应用了M-EMS。由于在奥钢联多纳维茨钢厂的恶劣环境下难于进行测量,因此研究采用数值模拟的方法。连铸过程1:1比例的物理模拟也很难实现,因为水的传导率太低,且液态金属不透明或金属处理难,因此数值模拟就成为获得整个过程较好的重要手段。数值模型考虑了流场和电磁场的完全耦合,许多物理问题都用最优参数进行模拟,流场用商业有限元CFD编码软件FLUENT进行计算,电磁场用商业有限元求解器ANSYSEMAG计算。采用这种方法可以研究各种参数对流场和凝固组织的影响,也能揭示搅拌频率和搅拌强度的变化对液芯内部流场的影响。应用数值模拟能够加深对电磁场下连铸过程的认识,也能够找到优化的参数。     

基于连续模型的板坯连铸凝固过程的数值模拟

结合实际测量数据，建立了基于连续模型的板坯连铸过程流场、温度场和凝固的三维耦合数学模型。计算结果表明：该模型可用于描述连铸板坯结晶器和整个二冷喷水区的凝固进程；凝固坯壳的生长限制了流体流动的空间，加快了水口出口钢流的动量衰减；流体流动加快了铸坯内部传热机制由对流向热传导传热的转变进程，控制了两相区的发展。     

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。      

水平连铸结晶器内真实换热边界计算及传递模型

连铸结晶器内冷却水的流动、传热和金属液的传热、凝固全耦合计算时,模型计算效率低、收敛性差,而不考虑真实冷却水流场只进行金属液传热-凝固耦合时,模型计算精度较低,可将冷却水流动-传热和金属液传热-凝固过程分别建模,并基于二次开发的温度场量原位传递程序,将模型串联耦合。结果表明,通过耦合流场计算得到的真实冷却边界替代传统均匀界面换热系数的边界条件施加方式,在保证模型计算效率和计算精度的前提下,可准确模拟铸坯晶粒的尺寸、取向和数量,经试验验证,模拟结果与试验结果高度吻合。     

复合磁场作用下板坯结晶器内流场与温度场的数值模拟

提出了一种用于板坯流动控制的复合磁场,在弯月面附近施加电磁搅拌,在浸入式水口附近施加电磁制动,以形成对流场及温度场的复合控制.使用Fluent软件建立了结晶器内复合磁场作用下的钢液流场与温度场三维计算模型.通过与无磁场作用的钢液流动及传热行为进行对比,揭示了复合磁场对结晶器钢液流场和温度场的作用规律.结果表明:复合磁场在一定电磁参数条件下可以减弱水口出流主流股对窄面的冲击,冲击位置处的最大湍动能值降低62％,减少下返流侵入深度;弯月面附近得到了充分的搅拌,结晶器壁面附近的钢液流速增强30％,有利于凝固前沿的冲刷;同时结晶器壁面附近温度提高了1～2 K,均匀了结晶器内温度分布.结晶器电磁搅拌与电磁制动的流场控制优势得以同时发挥.     

影响连铸传输模型精度的热物理性质

对影响连铸过程动量、能量及溶质传输模型计算精度的几个最重要的热物理性质(即钢的密度、导热系数和溶质分配系数)在凝固过程中的变化情况进行了考虑.根据Fe-C二元平衡相图以及宏观传输-微观偏析计算获得了Q235钢连铸过程中局部热物理性质与相组成及温度之间的函数关系式,为连铸传输模型数值计算的精确度提供了保证.     

方坯连铸结晶器浸入式水口结构类型的研究

应用流场计算软件PHOENICS及水力模型,模拟了方坯连铸结晶器内钢液的流场和流动分布。在此基础上模拟计算了几种不同形式的水口对流动形态的影响,并用水力模型试验进行了验证。通过数值模拟,为优化结晶器内钢液的流场以及浸入式水口的设计提供了科学依据。     

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。     

SEN底部形状对结晶器内钢液流场和温度场的影响

针对某钢厂板坯连铸结晶器及浸入式水口(SEN)的结构参数,建立了描述结晶器内钢液流动的三维数学模型,应用FLUENT软件对结晶器内钢液的流场和温度场进行耦合计算,分析SEN底部形状对结晶器内流场和温度场的影响,并利用水模试验对钢液的流动行为进行了验证。结果表明:凸底水口出口附近钢液的速度较大,流股的冲击深度较大,使结晶器内高温区下移,不利于凝固坯壳的生长;平底水口自由面速度较大、温度较高,有利于保护渣的熔化,但易产生液面裸露现象;凹底自由面速度较小、温度较低,有利于液面稳定。     

连铸结晶器内钢液流动、凝固和夹杂物的分布

 建立并求解动量、热量和质量传输耦合模型,研究了连铸结晶器内钢液流动、传热、凝固、溶质输送和夹杂碰撞长大行为。数值结果表明,受钢液流动的影响,在连铸机内钢液温度、碳浓度和夹杂物的空间分布与钢水流动特征相似,也可分为上下两个循环区。但是其分布具有各自的特点。在涡心处,钢液温度较低,碳浓度较高,夹杂物体积浓度和数量密度较低。在弯月面处,钢液温度较低,碳浓度较高,夹杂物的体积浓度和数量密度较低。在结晶器出口处的凝固坯壳内,夹杂物的浓度和数量密度分布极不均匀,存在阶跃现象,这与冲击点处凝固坯壳的重熔有关。     

Modeling of solute redistribution in the mushy zone during solidification of aluminum-copper alloys

A mathematical model has been established to predict the formation of macrosegregation for a unidirectional solidification of aluminum-copper alloys cooled from the bottom. The model, based on the continuum formulation, allows the calculation of transient distributions of temperature, velocity, and species in the solidifying alloy caused by thermosolutal convection and shrinkage-induced fluid flow. Positive segregation in the casting near the bottom (inverse segregation) is found, which is accompanied by a moving negative-segregated mushy zone. The effects of shrinkage-induced fluid flow and solute diffusion on the formation of macrosegregation are examined. It is found that the redistribution of solute in the solidifying alloy is caused by the flow of solute-rich liquid in the mushy zone due to solidification shrinkage. A higher heat-extraction rate at the bottom increases the solidification rate, decreasing the size of the mushy zone, reducing the flow of solute-rich liquid in the mushy zone and, as a result, lessening the severity of inverse segregation. Comparisons between the theoretical predictions from the present study and previous modeling results and available experimental data are made, and good agreements are obtained.     

Mathematical simulation on coupled flow, heat, and solute transport in slab continuous casting process

A three-dimensional comprehensively coupled model has been developed to describe the transport phenomena, including fluid flow, heat transfer, solidification, and solute redistribution in the continuous casting process. The continuous casting process is considered as a solidification process in a multicomponent solid-liquid phase system. The porous media theory is used to model the blockage of fluid flow by columnar dendrites in the mushy zone. The relation between flow pattern and the shape of the solid shell is demonstrated. Double diffusive convection caused by thermal and concentration gradients is considered. The change in the liquidus temperature with liquid concentration is also considered. The formation mechanism of macrosegregation is investigated. Calculated solid shell thickness and temperature distribution in liquid core are compared with the measured quantities for validating the model.     

连铸板坯凝固末端位置的研究

 连铸坯凝固末端位置的确定方法有实验测定法和数学模型法。针对鞍钢厚板坯连铸机,采用有效比热容法建立板坯连铸二冷凝固传热数学模型来预报凝固末端的位置,用实验测定值验证了模型的可靠性。应用数学模型分析拉速对厚板坯凝固末端位置的影响,将计算结果回归得到了X65管线钢和船板钢在不同拉速范围内凝固末段位置和拉速的数学关系式,可为现场控制连铸凝固终点提供参考。     

Influence of Billet Size on Flow, Solidification and Solute Transport in Continuous Casting

ListofSymbol　　B———Buoyancy ,m·s- 2 ;　　c———Concentrationofsoluteelement ;　　Cμ———Turbulentconstant;　　D———Diffusivityofsoluteelement ,m2 ·s- 1 ;　　fl,fs———Liquidandsolidfraction ;　　fμ———Turbulentcoefficient ;　　h———Enthalpy ,J·kg- 1 ;　　k———Turbulentkineticenergy ,m2 ·s- 2 ;　　kp———Equilibriumpartitioncoefficient;　　Kp———Permeabilityofmushyzone ,m2 ;　　K0 ———Permeabilitycoefficient;　　p———Pressure ,Pa ;　　Pr———Prandtlnumber ;…     

Vibration Method to Detect Liquid–Solid Fraction and Final Solidifying End for Continuous Casting Slab

Accurate measurement of liquid–solid fraction of casting slab in the secondary cooling zone and precise prediction of final solidifying end of casting slab have momentous significances for internal defects improvement of continuous casting slabs, such as the segregation and shrinkage porosity etc. However, the on‐line detection of slab solidification processing and final solidifying end haven't accomplished, which have been acknowledged as metallurgy problem all over the world. The paper presents a novel method, which called vibration method, to detect the liquid fraction and locate the final solidifying end of continuous casting slab. A physical simulation device using the materials of organic glass and water has been set up according to the similarity theory. Basing on theoretical deduction, physical experiment and numerical simulation, the research results provide guidance and reference for the on‐line detection of liquid–solid fraction and final solidifying end of the slab.     

Modeling of the formation of under-riser macrosegregation during solidification of binary alloys

The formation of macrosegregation in a rectangular ingot with reduced cross section from the riser to the casting, chilled from the bottom, has been studied numerically. In addition to positive inverse segregation occurring near the chilled surface, very severe negative segregation around the under-riser region and moderate positive segregation near the top corners of the casting were found. Although large circulating vortexes are created by natural convection in the under-riser region during the early stage of solidification, the fluid flow in the mushy zone is dominated by solidification shrinkage. As a result, the final solute distribution in the casting is determined by the flow of solute-rich liquid in the mushy zone owing to the combined effects of solidification shrinkage and change of cross section from casting to riser. Detailed explanations regarding the effect of different flow phenomena on the formation of the segregations are provided. The effects of riser size and cooling condition at the bottom of the ingot on the formation of macrosegregation also were studied. The predicted negative and positive macrosegregations in the casting compared very well with the available experimental data.     

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.     

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.     

GCr15轴承钢大方坯凝固过程微观偏析模型及应用研究

建立了考虑δ/γ相变的GCr15轴承钢大方坯连铸凝固两相区溶质微观偏析模型,并应用于220 mm ×260 mm铸坯的凝固传热。结果表明:通过模型可以获得高碳钢精确的固液相线温度,以及温度与固相率的关系;GCr15轴承钢大方坯凝固过程仅析出γ相,凝固末期S、P和C元素的偏析严重;固相率越大,冷却速率对偏析度的影响更明显;S和P元素含量以及冷却速率对零塑性温度（ZDT）影响较大;采用基于凝固传热模型优化的连铸工艺后,铸坯中心碳偏析指数控制在0.961.05,且铸坯未产生内裂纹。