A mathematical model of the heat and fluid flows in direct-chill casting of aluminum sheet ingots and billets

A finite-element method model for the time-dependent heat and fluid flows that develop during direct-chill (DC) semicontinuous casting of aluminium ingots is presented. Thermal convection and turbulence are included in the model formulation and, in the mushy zone, the momentum equations are modified with a Darcy-type source term dependent on the liquid fraction. The boundary conditions involve calculations of the air gap along the mold wall as well as the heat transfer to the falling water film with forced convection, nucleate boiling, and film boiling. The mold wall and the starting block are included in the computational domain. In the start-up period of the casting, the ingot domain expands over the starting-block level. The numerical method applies a fractional-step method for the dynamic Navier-Stokes equations and the “streamline upwind Petrov-Galerkin” (SUPG) method for mixed diffusion and convection in the momentum and energy equations. The modeling of the start-up period of the casting is demonstrated and compared to temperature measurements in an AA1050 200×600 mm sheet ingot.     

A numerical simulation of the D.C. continuous casting process including nucleate boiling heat transfer

The steady-state thermal problem associated with the direct-chill continuous casting of A6063 aluminum cylindrical ingots is solved using the numerical finite element technique. Excellent correlation is demonstrated between the numerical model and experimental data from ingots cast at two different speeds. By application of the model, effective heat transfer coefficients are calculated as a function of vertical position on the outside surface of the ingot. It is shown that direct application of these coefficients to the modeling of different casting situations will produce substantial errors in the region in which heat transfer is by nucleate boiling. Using theories of nucleate boiling with forced convection and film cooling, a method is developed to calculate the external boundary conditions in the submold region of the ingot, thus making it possible for the first time to define explicitly all of the thermal boundary conditions associated with this casting configuration. These theories are incorporated into the numerical model, and a subsequent simulation shows excellent agreement with experimental data from a third ingot.     

Heat-flow-based analysis of surface crack formation during the start-up of the direct chill casting process: Part II. experimental study of an AA5182 rolling ingot

The flow of heat during the start-up of the direct chill (DC) casting process has been studied with the aim of determining the factors that make this phase of the process prone to face crack generation. Measurements have been made on an AA5182 rolling ingot instrumented with embedded thermocouples placed at key locations in the vicinity of the ingot face near its base. The resulting temperature data have been input to a two-dimensional (2-D) inverse heat-transfer model, developed in part I of this two part study, in order to calculate heat fluxvs surface temperature curves in the direct water impingement regime. The findings indicate that the flow of heat is influenced by changing surface morphology and water flow conditions during the start-up phase. A finite element based simulation of the cast start, employing the calculated flux/surface temperature relations, reveals that the ingot shell at the point of water contact reaches a maximum thickness early in the casting process. The location of this maximum was found to coincide with the position where surface cracks are routinely found to initiate. Further, this maximum was found to also coincide with position at which the rate of deflection of the base of the ingot (“butt-curl”) begins to slow. Based on the heat-flow analysis, it is believed that the face cracks form due to an excessive shell thickness during transient start-up conditions and that their occurrence could be reduced by an optimal combination of water flow rate and casting speed during start-up.     

Film Boiling and Water Film Ejection in the Secondary Cooling Zone of the Direct-Chill Casting Process

Accurate thermal modeling of the direct-chill casting process relies nowadays on increasingly complex boundary conditions for the secondary cooling zone. A two-dimensional axisymmetric finite-element model of the direct-chill casting process was developed to quantify the importance of secondary cooling at the surface compared with internal heat conduction within the billet. Boiling water heat transfer at the surface was found to dominate and be the governing factor only when stable film boiling or water film ejection take place; all other cases were dominated by internal heat conduction. The influence of various parameters (casting speed, cooling water flow rate, and thermophysical properties of the cast material) on the occurrence of water film ejection was analyzed. An exponential relationship was found between the cooling water flow rate and the minimum casting speed at which water film ejection takes place.     

The use of water cooling during the continuous casting of steel and aluminum alloys

In both continuous casting of steel slabs and direct chill (DC) casting of aluminum alloy ingots, water is used to cool the mold in the initial stages of solidification, and then below the mold, where it is in direct contact with the newly solidified surface of the metal. Water cooling affects the product quality by (1) controlling the heat removal rate that creates and cools the solid shell and (2) generating thermal stresses and strains inside the solidified metal. This work reviews the current state-of-the-art in water cooling for both processes, and draws insights by comparing and contrasting the different practices used in each process. The heat extraction coefficient during secondary cooling depends greatly on the surface temperature of the ingot, as represented by boiling water-cooling curves. Thus, the heat extraction rate varies dramatically with time, as the slab/ingot surface temperature changes. Sudden fluctuations in the temperature gradients within the solidifying metal cause thermal stresses, which often lead to cracks, especially near the solidification front, where even small tensile stresses can form hot tears. Hence, a tight control of spray cooling for steel, and practices such as CO₂ injection/pulse water cooling for aluminum, are now used to avoid sudden changes in the strand surface temperature. The goal in each process is to match the rate of heat removal at the surface with the internal supply of latent and sensible heat, in order to lower the metal surface temperature monotonically, until cooling is complete.     

Heat-Transfer Measurements in the Primary Cooling Phase of the Direct-Chill Casting Process

Thermal modeling of the direct-chill casting process requires accurate knowledge of (1) the different boundary conditions in the primary mold and secondary direct water-spray cooling regimes and (2) their variability with respect to process parameters. In this study, heat transfer in the primary cooling zone was investigated by using temperature measurements made with subsurface thermocouples in the mold as input to an inverse heat conduction algorithm. Laboratory-scale experiments were performed to investigate the primary cooling of AA3003 and AA4045 aluminum alloy ingots cast at speeds ranging between 1.58 and 2.10 mm/s. The average heat flux values were calculated for the steady-state phase of the casting process, and an effective heat-transfer coefficient for the global primary cooling process was derived that included convection at the mold surfaces and conduction through the mold wall. Effective heat-transfer coefficients were evaluated at different points along the mold height and compared with values from a previously derived computational fluid dynamics model of the direct-chill casting process that were based on predictions of the air gap thickness between the mold and ingot. The current experimental results closely matched the values previously predicted by the air gap models. The effective heat-transfer coefficient for primary cooling was also found to increase slightly with the casting speed and was higher near the mold top (up to 824 W/m²·K) where the molten aluminum first comes in contact with the mold than near the bottom (as low as 242 W/m²·K) where an air gap forms between the ingot and mold because of thermal contraction of the ingot. These results are consistent with previous studies.     

Modeling of ingot distortions during direct chill casting of aluminum alloys

A comprehensive three-dimensional (3-D) mathematical model based upon the ABAQUS software has been developed for the computation of the thermomechanical state of the solidifying strand during direct chill (DC) casting of rolling sheet ingots and during subsequent cooling. Based upon a finiteelement formulation, the model determines the temperature distribution, the stresses, and the associated deformations in the metal. For that purpose, the thermomechanical properties of the alloy have been measured up to the coherency temperature using creep and indentation tests. The thermophysical properties as well as the boundary conditions associated with the lateral water spray have been determined using inverse modeling. The predicted ingot distortions, mainly, “butt curl,” “butt swell,” and lateral faces pull-in, are compared with experimental measurements performed during solidification and after complete cooling of the ingot. Particular emphasis is placed on the nonuniform contraction of the lateral faces. The influence of the mold shape and the contributions to this contraction are assessed as a function of the casting conditions.     

圆盘铸锭机锌锭模的改进

通过对锭模的改进设计,规范造型工艺,提高了圆盘铸锭的冷却效率和锌锭的表面质量,并解决了商标清晰度的问题。     

New shape of forging ingot for making hollow forged products

Replacement of the conventional hot top — which accumulates heat from the melt inside an ingot mold — by a cooling element that is also placed on top of the mold has made it possible to obtain ingots with a narrow primary shrinkage cavity that extends along their axis. Such ingots are well-suited for the production of hollow forgings. When a standard hot top is used, the transverse dimensions of the region of the ingot affected by axial porosity correspond to as much as 15% of the ingot’s diameter. The same region accounts for just 8.5% of the ingot diameter when the above-mentioned cooling element is used instead. This makes it easier to remove the defective region by piercing or drilling it out during the forging conversion.     

A Simple Model of the Mold Boundary Condition in Direct-Chill (DC) Casting of Aluminum Alloys

An accurate thermofluids model of aluminum direct-chill (DC) casting must solve the heat-transfer equations in the ingot with realistic external boundary conditions. These boundary conditions are typically separated into two zones: primary cooling, which occurs inside the water-cooled mold, and secondary cooling, where a film of water contacts the ingot surface directly. Here, a simple model for the primary cooling boundary condition of the steady-state DC casting process was developed. First, the water-cooled mold was modeled using a commercial computational fluid dynamics (CFD) package, and its effective heat-transfer coefficient was determined. To predict the air-gap formation between the ingot and mold and to predict its effect on the primary cooling, a simple density-based shrinkage model of the solidifying shell was developed and compared with a more complex three-dimensional (3-D) thermoelastic model. DC casting simulations using these two models were performed for AA3003 and AA4045 aluminum alloys at two different casting speeds. A series of experiments was also performed using a laboratory-scale rectangular DC caster to measure the thermal history and sump shape of the DC cast ingots. Comparisons between the simulations and experimental results suggested that both models provide good agreement for the liquid sump profiles and the temperature distributions within the ingot. The density-based shrinkage model, however, is significantly easier to implement in a CFD code and is more computationally efficient.     

喷淋结晶器传热特性的实验研究

喷淋结晶器是一种新型结晶器，它是用特殊的喷嘴将水喷淋到结晶器的铜板表面上进行铸坯冷却的，这种冷却方式具有较高的传热速度，便于调节结晶器的热流密度，可以提高拉坯速度和铸坯质量，喷淋结晶器的传热现象是很复杂的，本文通过建立传热实验台，对喷淋冷却过程进行较细致的实验研究，找出喷水量和换热系数的关系等，为设计喷淋冷却结晶器提供必要的参数。     

Theoretical analysis of the possibility of controlling the solidification conditions in a continuously cast ingot

It is shown that the solidification conditions in an entire continuously cast ingot cannot be controlled by varying the thermal conditions of cooling the ingot surface. The methods that can intensify the heat transfer in the solidifying melt should be applied in the most problematic axial zone of the ingot.     

竖模在金锭浇铸工艺中的试验应用

针对某黄金冶炼厂原铸锭工艺的不足,提出竖模连续浇铸工艺,并对模具尺寸、温度、坩埚等影响因素进行了试验研究。通过批量试验结果表明:采用竖模连续浇铸工艺生产的金锭符合上海黄金交易所的质量要求,且有效提高了生产效率,降低了生产成本,改善了工人作业环境。     

Modeling of ingot development during the start-up phase of direct chill casting

Direct chill (DC) casting is a core primary process in the production of aluminum ingots. However, its operational optimization is still under investigation with regard to a number of features, one of which is the issue of curvature at the base of the ingot. Analysis of these features requires a computational model of the process that accounts for the fluid flow, heat transfer, solidification phase change, and thermomechanical anlaysis. This article describes an integrated approach to the modeling of all the preceding phenomena and their interactions     

Metal cooling and solidification in a continuous-casting mold

A mathematical model is proposed for cooling a metal solidifying in a continuous-casting mold. In this model, heat exchange is related to solidification; therefore, the thermal resistance of the gap between the ingot and the work mold surface, which is the main component of the total thermal resistance to heat transfer from the ingot to cooling water, can be calculated. This mathematical model is applied to the cooling and solidification of an ingot in a continuous-casting mold, and some numerical-calculation results for the case of a medium-carbon steel are presented.     

Influence of oxide scales on heat transfer in secondary cooling zones in the continuous casting process,part 1: heat transfer through hot-oxidized steel surfaces cooled by spray-water

For the cooling of steels in the continuous casting process it is necessary to know the heat transfer from the solidifying strand to the cooling water to enable calculation of the secondary cooling zone. Previous investigations have only determined this variable for non-oxidizing metallic surfaces. For many steels cast in practice, however, the formation of oxide layers prevents a direct transfer of the previous results. In the present research the influence of the oxide layers on the heat transfer has been investigated for spay-water cooling. Results have shown that heat transfer in the range of stable film boiling is determined for a constant spray-water temperature in the same way as for non-oxidizing metals, i.e. using the water mass flux density ·_s only. The changed surface qualities resulting from the oxide formation cause the Leidenfrost temperature, however, to shift considerably to higher values.     

超声处理对2219大规格铝锭微观组织与宏观偏析的影响

在半连续铸造过程中施加超声,成功制备了φ1250 mm 2219铝合金铸锭.利用光学显微镜、扫描电镜、能谱仪及直读光谱仪等仪器对铸锭的组织与成分分布进行检测与分析,探究超声对铸锭组织与偏析的内在作用机制.研究结果表明:超声振动引起的空化和声流效应能明显均匀组织结构,细化晶粒,尤其是心部晶粒细化率达到39.6%.超声促进铸锭晶间第二相呈枝丫状断续分布,晶内析出物点状弥散分布.同时,超声有效减小近表面负偏析,降低边部与心部之间的溶质浓度差异,弱化整个横截面的浓度波动,从而改善宏观偏析.      

Optimization of the mold parameters for casting of flat copper ingots in semicontinuous casting machines

A new mold design is proposed to decrease the number of defects in flat copper ingots cast in a semicontinuous casting machine. This design implies 1.5- to 2.5-mm concavity of the internal surface of wide plate faces in the upper part of the mold that decreases to zero in its bottom rectangular part.     

冷却工艺对高速连铸结晶器传热的影响

 为了分析冷却水的供水工艺对结晶器铜壁和冷却水温度场的影响,基于结晶器铜壁热电偶实测温度,构建铸坯/铜壁传热反问题和铜壁/冷却水正问题数学模型,采用ANSYS建立铸坯/铜壁/冷却水数值分析模型,对薄板坯结晶器温度场进行耦合传热分析,解析不同冷却工艺对高速薄板坯连铸结晶器内传热行为的影响。结果表明,水缝内冷却水流动方向对铜壁温度场具有显著影响,采用自上而下“反向供水”,比常规冷却工艺时铜壁热面温度峰值降低117 ℃,铜壁侧冷却水最高温度降低24 ℃,有效改善铜板工作状态,抑制冷却水局部沸腾趋势。提高冷却水速度可以进一步降低铜壁和冷却水温度,冷却水温度对铜壁温度场影响较小。     

Investigation of the effect of argon protection behavior in the ingot casting process

With the continuous improvement of product quality and function,the quality control of mold steel is becoming increasingly stricter. Argon protection is essential for ensuring casting quality during ingot casting. The development of argon protection in ingot casting and the production process of enclosed argon protection in 40 t line is discussed,w ith particular focus given the factors affecting the flow of oxygen in the argon protection cover are discussed. The influence of some related factors on the oxygen content is analyzed. On the basis of the online measurements of the oxygen content,the optimized operational approaches for improving the effect of argon protection are developed. This can decrease the liquid steel via secondary oxidation,and improve the quality of the ingots.