A coupled model on fluid flow,heat transfer and solidification in continuous casting mold

Fluid flow, heat transfer and solidification of steel in the mold are so complex but crucial, determining the surface quality of the continuous casting slab. In the current study, a 2D numerical model was established by Fluent software to simulate the fluid flow, heat transfer and solidification of the steel in the mold. The VOF model and k-ε model were applied to simulate the flow field of the three phases(steel, slag and air), and solidification model was used to simulate the solidification process. The phenomena at the meniscus were also explored through interfacial tension between the liquid steel and slag as well as the mold oscillation. The model included a 20 mm thick mold to clarify the heat transfer and the temperature distribution of the mold. The simulation results show that the liquid steel flows as upper backflow and lower backflow in the mold, and that a small circulation forms at the meniscus. The liquid slag flows away from the corner at the meniscus or infiltrates into the gap between the mold and the shell with the mold oscillating at the negative strip stage or at the positive strip stage. The simulated pitch and the depth of oscillation marks approximate to the theoretical pitch and measured depth on the slab.     

Casting/mold thermal contact heat transfer during solidification of Al-Cu-Si alloy (LM 21) plates in thick and thin molds

Heat flow at the casting/mold interface was assessed and studied during solidification of Al-Cu-Si (LM 21) alloy in preheated cast iron molds of two different thicknesses, coated with graphite and alumina based dressings. The casting and the mold were instrumented with thermocouples connected to a computer controlled temperature data acquisition system. The thermal history at nodal locations in the mold and casting obtained during experimentation was used to estimate the heat flux by solving the one-dimensional inverse heat conduction problem. The cooling rate and solidification time were measured using the computer-aided cooling curve analysis data. The estimated heat flux transients showed a peak due to the formation of a stable solid shell, which has a higher thermal conductivity compared with the liquid metal in contact with the mold wall prior to the occurrence of the peak. The high values of heat flux transients obtained with thin molds were attributed to mold distortion due to thermal stresses. For thin molds, assumption of Newtonian heating yielded reliable interfacial heat transfer coefficients as compared with one-dimensional inverse modeling. The time of occurrence of peak heat flux increased with a decrease in the mold wall thickness and increase in the casting thickness.     

Heat transfer behavior of top side-pouring twin-roll casting

The interfacial heat transfer between the casting and the substrate from liquid/solid contact to solid/solid contact with pressure was investigated using a set of equipment designed according to the characteristics of the top side-pouring twin-roll casting process. The interfacial heat transfer behavior of this process consists of 4 stages: chilling, solidification shrinkage, compression and cooling. High values of the IHTC ranging from 50,000 to 90,000 W/m² °C were detected in the chilling stage, followed by a sharp decrease in solidification shrinkage stage (4000–8000 W/m² °C). Due to the pressure, which modeled the effect of rolling in twin-roll casting, the IHTC bounced back to 6000–20000 W/m² °C, according to different conditions. The influence of process variables such as pressure magnitude, compress speed, pouring temperature, surface roughness and alloy composition had been discussed. Because of the compress action, the influence of these variables performed in a different way, but it was concluded that the way to improve the contact conditions always accompanied with an increase in the IHTC.     

双辊薄带连铸凝固组织模拟微观模型的验证

以双辊薄带连续铸轧工艺凝固过程为基础,基于金属凝固的基本原理,并运用现代计算机仿真技术建立了双辊薄带连铸凝固过程的异质形核、修正的枝晶尖端生长动力学、柱状晶向等轴晶生长转变(CET)的解析模型以及基于元胞自动机(CA)的双辊薄带连续铸轧凝固组织的仿真模型,为双辊薄带连续铸轧凝固组织形成的仿真模拟奠定了基础,同时利用铝合金双辊薄带连续铸轧组织凝固过程验证了数学模拟的可行性。结果表明:可用建立的数学模型模拟预测铸轧工艺参数对双辊连续铸轧薄带凝固组织的影响。     

薄板坯连铸凝固状态分析与结晶器长度设计

推导了薄板坯连铸过程中钢液在结晶器内的凝固传热有限元方程,开发了钢液凝固状态及温度场分布的模拟分析软件,探讨了拉坯速度及结晶器长度对结晶器出口坯壳厚度的影响。     

Effect of electromagnetic stirring on solidification structure of austenitic stainless steel in horizontal continuous casting

An investigation on the influence of low frequency rotary electromagnetic stirring on solidification structure of austenitic stainless steel in horizontal continuous casting was experimentally conducted and carried out on an industrial trial basis. The results show that application of appropriate electromagnetic stirring parameters can obviously improve the macrostructure of austenitic stainless steel, in which both columnar and equiaxed grains can be greatly refined and shrinkage porosity or cavity zone along centerline can be remarkably decreased due to eliminating intracrystalline and enlarging equiaxed grains zone. The industrial trials verify that the electromagnetic stirring intensity of austenitic stainless steel should be higher than that of plain carbon steel. Electromagnetic stirring has somewhat affected the macrostructure of austenitic stainless steel even if the magnetic flux density of the electromagnetic stirring reaches 90 mT (amplitude reaches 141 mT ) in average at frequency f=3-4Hz, which provides a reference for the optimization of design and process parameters when applying the rotary electromagnetic stirrer     

矩形连铸坯凝固传热的变间距有限差分模拟

针对用传统有限差分法模拟计算矩形连铸坯凝固传热时在整个计算区域上离散网格大小需均匀一致且要用直角来代替铸坯圆角这一缺点,提出了变间距有限差分法.变间距有限差分能够在计算区域上灵活地配置大小不同的离散网格,也能够准确地离散铸坯的角部形状,而且直观、简单,计算工作量小,与有限单元法的计算结果几乎相同.模拟计算结果与实测值吻合.     

A thermal system model for a radiant-tube continuous reheating furnace

A thermal system mathematical model developed for a gas-fired radiant-tube continuous reheating furnace is discussed. The mathematical model of the furnace integrates submodels for combustion and heat transfer within the radiant tube with models for the furnace enclosure. The transport processes occurring in the radiant tube are treated using a one-dimensional scheme, and the radiation exchange between the load, the radiant-tube surfaces, and the furnace refractories are analyzed using the radiosity method. The continuous furnace operation is simulated under steady-state conditions. Model simulations of load surface temperature variation compare well with measurements in an industrial galvannealing furnace. The scope and flexibility of the model are assessed by performing extensive parametric studies using furnace geometry, material properties, and operating conditions as input parameters in the model and predicting the thermal performance of the furnace. The various parameters studied include the effects of load and refractory emissivities, load velocities, properties of the stock material, and variations in the radiant-tube designs.     

Casting/mould interfacial heat transfer during solidification in graphite,steel and graphite lined steel moulds

Heat flow between the casting and the mould during solidification of three commercially pure metals, in graphite, steel and graphite lined steel moulds, was assessed using an inverse modelling technique. The analysis yielded the interfacial heat flux (q), heat transfer coefficient (h) and the surface temperatures of the casting and the mould during solidification of the casting. The peak heat flux was incorporated as a dimensionless number and modeled as a function of the thermal diffusivities of the casting and the mould materials. Heat flux transients were normalised with respect to the peak heat flux and modeled as a function of time. The heat flux model proposed was used to estimate the heat flux transients during solidification in graphite lined copper composite moulds.     

铝合金连续铸造喷水冷却的换热系数

基于边界条件替换法建立了铝合金连续铸造喷水冷却过程的换热系数计算模型.采用实验测量铸锭冷却过程的表面温度和温度场数值计算相结合的方法,确定了铸锭表面温度为100～500 ℃和喷水密度为11.3～27.8     

高碳钢连铸保护渣的性能

为了研究高碳钢保护渣在连铸过程中的匹配性,对典型工业高碳钢保护渣的熔化、润湿、黏度、渣膜分布,以及传热性能进行研究。结果表明,4个高碳钢保护渣的开始熔化温度范围为1 110~1 129 ℃,润湿角范围为30.1°~37.8°,黏度范围为0.210~0.312 Pa·s,转折温度范围为1 046~1 130 ℃,渣膜的液态层比例为14.7%~18.9%。其中,1号高碳钢保护渣熔化温度较低(熔化区间1 110～1 345 ℃)、黏度较低(0.264 Pa·s)、渣膜液态层较高(比例为18.9%)、转折温度(1 059 ℃)和控热能力均适宜,表明该渣在高碳钢连铸结晶器中可以迅速熔化,形成足够的液态渣,并从弯月面渗入渣道,形成均匀的渣膜,从而润滑铸坯,避免黏结漏钢和裂纹等缺陷,保障高碳钢连铸的顺行。     

高拉速薄板坯连铸中碳钢保护渣开发与应用

针对在高拉速情况下薄板坯连铸过程中频繁出现"冷齿"、黏结以及铸坯表面纵裂纹较多等问题,依据中碳钢凝固收缩特性并结合现场实际生产情况,开发了一种适合在高拉速情况下薄板坯中碳钢连铸用保护渣。生产实践表明,在拉速提高后,使用新型保护渣基本避免了铸坯表面纵裂纹的产生,也无"冷齿"、黏结等报警现象出现,铸坯质量显著提高,完全满足生产要求。     

薄板坯连铸漏斗型结晶器内流动、传热和凝固行为的数值研究

针对薄板坯连铸漏斗型结晶器,建立了钢液流动与凝固传热的三维耦合模型,在不改变造型复杂的水口和漏斗型结晶器的尺寸和结构的前提下,计算分析了不同操作参数——拉坯速度和浇注温度对结晶器内流场、温度场和凝壳的影响,同时采用了第二类和第三类两种不同的热边界条件进行了流场和温度场的分析。计算结果表明,钢液凝固壳及糊状区对结晶器内钢液的流动有很大影响,采用第二类热边界条件能够更真实地反映出结晶器内流动对传热的影响。     

管坯电磁连铸内结晶器侧换热系数的模拟与研究

将实验得到的初始凝固坯壳的形状与数值模拟结果相结合的反推法确定了内结晶器冷却水量与内结晶器侧壁换热系数之间的关系,模拟了冷却水流量为80、100、120 L/s时凝固坯壳的形状和位置;在模拟得到的固-液界面位置附近,即距液面5～20 mm处施加了频率为2 500Hz的电磁场.结果表明:与传统连铸方法相比,施加电磁场后空心管坯的质量得到了明显的改善.     

Heat transfer coefficient on external mold surface at high pressure and application to metal-matrix composite casting

A relationship is established between the heat transfer coefficient on the external surface of a mold and the ambient temperature and pressure within the operating range typical of pressure-infiltration casting of metal-matrix composites. This relationship is based on existing data and correlation for the thermo-physical properties of the ambient nitrogen. The predicted temperature profiles are in good agreement with the experimental data obtained for a metal-matrix composite casting process. The application of the established relationship is carried beyond the experimental evidence to predict the solidification pattern in the pressure-casting system.     

双辊薄带铸轧低碳钢薄板的组织及力学性能

对双辊薄带连铸(TRC)工艺条件下低碳钢铸带坯、热轧板的显微组织及力学性能进行研究,并与传统生产工艺(TP)的冷轧板产品进行了对比。铸带坯主要由200~300μm的不规则的多边形铁素体组成,并伴随着少量20μm左右的多边形铁素体。经一道次热轧后,厚度方向上组织严重不均:上下表层为细晶区,平均晶粒尺寸为10μm左右;中部为粗晶区,平均晶粒尺寸为40μm左右。粗晶区宽度约占整个板厚的50%。传统生产工艺的冷轧退火板的组织则较为均匀,平均晶粒尺寸约为15μm。薄带连铸热轧板与传统冷轧退火板相比具有较强的α织构和较弱的γ织构。薄带连铸热轧板的屈服强度、抗拉强度与传统工艺的冷轧退火板相当。但是,两者的拉伸曲线显著不同,前者表现为连续屈服,后者出现了屈服平台。     

The deformation of the chill in experiments to determine the interfacial heat transfer coefficient during casting solidification

Interfacial heat transfer coefficients during casting solidification are often measured in experiments in which unidirectional heat transfer is assumed. Finite element modelling of the chill in these experiments has shown that the chill surface deforms elastically into a convex shape, the extent of which generally decreases with time. Examples from several different unidirectional solidification experiments are given. The deformation of the chill surface was also calculated with the assumption of nonuniform temperature boundary conditions. The deformation of the chill could be such that only in the central region of the casting-chill interface would the two surfaces be in contact, with a localised gap between them at their periphery. The extent of this deformation could be sufficient that heat transfer through the interface may not be unidirectional as assumed in the experiment and, depending on the location of the thermocouples placed close to the interface, the accuracy of the heat transfer coefficients calculated from the data collected at these points may therefore be affected.     

A mathematical model of unidirectional solidification in cooled mold

For the unidirectional solidification in a mold cooled by air or water, several mathematical models have been proposed. However, some are complex series solutions, and some are good approximations only when solidification time is sufficiently small. Therefore, our research team proposes the present mathematical model, which is a good approximation when solidification time is sufficiently large. In order to estimate the convergency characteristics of the present model, numerical solutions were calculated with mold/solid heat transfer coefficients, which are experimentally determined by a previous and the present model.     

Microstructure and corrosion resistance of 304 stainless steel electroslag strip cladding

The 304 stainless steel strips were deposited one layer on carbon steel base metal by electroslag strip cladding(ESC) and submerged arc cladding(SAC),respectively.The solidification microstructure of ESC metal was analyzed by the optical microscopy,scanning electron microscope and energy dispersive spectroscopy.The corrosion resistance studies of strip cladding metals were carried out in 10%oxalic acid electrolytic etching test.The results showed that the cladding metal obtained by ESC presented low content of C,high content of Cr and enough alloying element of Ni in the chemical composition.The transition zone of ESC with small width was almost parallel with the base metal,leading to a lower dilution.There are three types of solidification modes(A→AF→FA) occurred in the ESC metal due to the decrease of cooling rate and degree of dilution from the transition zone to the top of ESC metal.As a result,the microstructure of ESC metal exhibited mainly austenite with a small amount offerrite,contributing to achievement of better corrosion resistance.     

薄板坯连铸保护渣的研制

通过对保护渣理化性能、熔化特性的基础研究，确定了适合高拉速薄板坯连铸用保护渣的组成及成分范围。利用数值仿真建立的传热数学模型确定的保护渣熔化温度、粘度被实践证明是合理的，这为保护渣的研究提供了理论依据。通过工业性试验的应用证明所研制的保护渣系能满足薄板坯连铸的需要，使用效果良好。