Simulation of Thermal Insulation of Top-Surface Slag Layers in Continuous Casting Mold

 A computational model of thermal behavior of the top-surface slag layers in continuous casting mold was applied to interpret the thermal insulation of mold powder. The temperature drop of liquid steel caused by heat removal at the interface of molten steel and slag in mold was proposed to evaluate the thermal insulation of mold slag. The calculation results show that slag consumption is one of important factor influencing the temperature drop, while the casting speed has no obvious effect on it. With the increase of slag consumption, the temperature drop is increased.     

Simulation of thermal behavior during peritectic steel solidification in slab continuous casting mold

Thermal behavior of the solidifying shell in continuous casting mold is very important to final steel products.In the present work,one two-dimension transient thermal-mechanical finite element model was developed to simulate the thermal behavior of peritectic steel solidifying in slab continuous casting mold by using the sequential coupling method.In this model,the steel physical properties at high temperature was gotten from the micro-segregation model withδ/γtransformation in mushy zone,and the heat flux was obtained according to the displacement between the surface of solidifying shell and the hot face of mold as solidification contraction,the liquid-solid structure and distribution of mold flux,and the temperature distribution of slab surface and mold hot face,in addition,the rate-dependent elastic-viscoplastic constitutive equation was applied to account for the evolution of shell stress in the mold.With this model,the variation characteristics of surface temperature,heat flux, and growth of the solidifying shell corner,as well as the thickness distribution of the liquid flux,solidified flux,air gap and the corresponding thermal resistance were described.     

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.     

Thermomechanical finite-element model of shell behavior in continuous casting of steel

A coupled finite-element model, CON2D, has been developed to simulate temperature, stress, and shape development during the continuous casting of steel, both in and below the mold. The model simulates a transverse section of the strand in generalized plane strain as it moves down at the casting speed. It includes the effects of heat conduction, solidification, nonuniform superheat dissipation due to turbulent fluid flow, mutual dependence of the heat transfer and shrinkage on the size of the interfacial gap, the taper of the mold wall, and the thermal distortion of the mold. The stress model features an elastic-viscoplastic creep constitutive equation that accounts for the different responses of the liquid, semisolid, delta-ferrite, and austenite phases. Functions depending on temperature and composition are employed for properties such as thermal linear expansion. A contact algorithm is used to prevent penetration of the shell into the mold wall due to the internal liquid pressure. An efficient two-step algorithm is used to integrate these highly nonlinear equations. The model is validated with an analytical solution for both temperature and stress in a solidifying slab. It is applied to simulate continuous casting of a 120 mm billet and compares favorably with plant measurements of mold wall temperature, total heat removal, and shell thickness, including thinning of the corner. The model is ready to investigate issues in continuous casting such as mold taper optimization, minimum shell thickness to avoid breakouts, and maximum casting speed to avoid hot-tear crack formation due to submold bulging.     

厚板坯连铸二次冷却传热数学模拟

在考虑二冷边界换热的条件下,建立了与厚板坯连铸机相适应的传热数学模型。用远红外测温仪测试X65管线钢230 mm×1650 mm铸坯表面温度,实验结果同模拟结果吻合较好。应用数学模型,对不同拉速下管线钢的连铸凝固过程进行了仿真计算,分析了拉速对出结晶器坯壳厚度、铸坯表面温度和液芯长度的影响,得出在给定的二冷条件下,为得到合理的铸坯表面温度,管线钢的拉速应为0.9～1.2m/min。     

Surface Tension and Temperature Effect on Entrapment of Bubbles and Particles in Continuous Casting of Steel

The entrapment behavior of Ar bubbles onto the solidifying front of molten steel in the continuous casting mold was investigated. The dynamic model of bubble and particle entrapment was developed in order to consider the effect of surface tension gradient induced forces(Marangoni force) due to the gradient of sulfur concentration and temperature. The numerical analysis and water model experiment were performed to apply the present model for various conditions. The calculation result is compared with experimental results and plant data in continuous casting mold as well. It shows that the thermal Marangoni force could play an important role and this model predicts the bubble behavior in the vicinity of solid/liquid interface more precisely.     

基于有限元法优化低碳钢板坯连铸机结晶器锥度的设计

钢厂1^#连铸机以0.95 m/min拉速生产SS400,D36和X70钢150~180 mm连铸板坯时,易产生角部纵裂纹,发生率最高可达5%。利用商业有限元软件ANSYS,建立了板坯连铸结晶器二维切片式凝固传热数学模型,并采用传热和应力/应变直接耦合的方法对连铸过程结晶器内凝固传热进行计算,分析了各钢种在0.95~1.05m/min拉速下铸坯温度分布以及温度分布不均引起的热应力。工业试验结果表明,根据优化计算结果,将板坯结晶器窄面的锥度系数由原来的1.00%改进为1.10%后,有效地消除了1^#连铸机板坯的角部纵裂纹。     

板坯连铸结晶器内钢液表面流速的水模型研究

以承德钢铁厂板坯连铸结晶器为原型,采用1∶1的水模型进行试验,研究了拉速、浸入式水口出口角度、水口浸入深度、水口底面结构及结晶器断面宽度等工艺参数对板坯结晶器内表面流速的影响。结果表明:拉速对表面流速的影响最大,随着拉速的提高,结晶器内钢液表面流速明显增大,当断面宽度为1 650 mm,拉速由0.7 m/min提高到1.4 m/min,表面流速由0.04 m/s提高到0.1 m/s;波浪面结构的浸入式水口表面流速效果最优。     

The Thermal Distortion of a Funnel Mold

This article investigates the thermal distortion of a funnel mold for continuous casting of thin slabs and explores the implications on taper and solidification of the steel shell. The three-dimensional mold temperatures are calculated using shell-mold heat flux and cooling water profiles that were calibrated with plant measurements. The thermal stresses and distorted shape of the mold are calculated with a detailed finite-element model of a symmetric fourth of the entire mold and waterbox assembly, and they are validated with plant thermocouple data and measurements of the wear of the narrow-face copper mold plates. The narrow-face mold distorts into the typical parabolic arc, and the wide face distorts into a ??W?? shape owing to the large variation in bolt stiffnesses. The thermal expansion of the wide face works against the applied narrow-face taper and funnel effects, so the effect of thermal distortion must be considered to accurately predict the ideal mold taper.     

Improvement of Castability and Surface Quality of Continuously Cast TWIP Slabs by Molten Mold Flux Feeding Technology

An innovative continuous casting process named POCAST (POSCO’s advanced CASting Technology) was developed based on molten mold flux feeding technology to improve both the productivity and the surface quality of cast slabs. In this process, molten mold flux is fed into the casting mold to enhance the thermal insulation of the meniscus and, hence, the lubrication between the solidifying steel shell and the copper mold. Enhancement of both the castability and the surface quality of high-aluminum advanced high-strength steel (AHSS) slabs is one of the most important advantages when the new process has been applied into the commercial continuous casting process. A trial cast of TWIP steel has been carried out using a 10-ton scale pilot caster and 100-ton scale and 250-ton scale commercial casters. The amount of mold flux consumption was more than 0.2 kg/m² in the new process, which is much larger than that in the conventional powder casting. Trial TWIP castings at both the pilot and the plant caster showed stable mold performances such as mold heat transfer. Also, cast slabs showed periodic/sound oscillation marks and little defects. The successful casting of TWIP steel has been attributed to the following characteristics of POCAST: dilution of the reactant by increasing the slag pool depth, enlargement of channel for slag film infiltration at meniscus by elimination of the slag bear, and decrease of apparent viscosity of the mold slag at meniscus by increasing the slag temperature.     

板坯连铸结晶器非稳态流场的模拟研究

本文运用数值模拟研究方法,研究高拉速厚板坯连铸非稳态结晶器流动特性,研究浸入式水口堵塞、水口不对中对结晶器流场、液面流速和对初生坯壳的影响。高拉速厚板坯连铸,铸坯质量下降,90%表面缺陷集中在铸坯边角区域,最严重的缺陷是铸坯中心和角部纵裂。非稳态工况对结晶器的流场影响因素更为显著,研究发现水口堵塞程度、水口出口流速、流量分配比是结晶器液面流速不对称、液面波动的主要影响因素,水口不对中是钢液流股对结晶器初生坯壳局部热冲击的主要因素,因此高拉速连铸应尽量避免非稳态工况操作,确保产品质量和效率的双赢。     

采用薄板坯连铸生产高表面质量冷轧钢板的可行性分析

薄板坯连铸由于拉速高,结晶器容量小,结晶器钢水液面波动高度和表面流速显著高于传统连铸,因此容易造成保护渣卷渣,这是薄板坯连铸生产高表面质量冷轧钢板钢种的主要困难所在。NUCOR、蒂森一克鲁伯等企业采用薄板坯连铸连轧工艺生产冷轧钢板的实践也表明,在表面质量方面与传统工艺产品尚有较大的差距。采用薄板坯连铸工艺生产优质冷轧钢种,应适当增加铸坯厚度,以降低拉速和增加结晶器对钢水流的缓冲作用,可采用120mm厚铸坯(结晶器出口),3m／min左右拉速。为了减少结晶器保护渣卷渣,应对中等厚度薄板坯连铸结晶器内钢水流动控制(SEN结构参数、SEN浸入深度、拉速等)、电磁制动、保护渣等开展深入的试验研究。     

连铸坯凝固钩形成机理与控制技术综述

对凝固钩的形成机理与控制技术进行了文献综述.凝固钩的研究方法主要有金相观察法和数值模拟法,两者共同加深了对凝固钩形成机理的认识.在铸坯宽面中心、宽面和窄面边缘处凝固钩平均长度要大于其他部位,凝固钩尺寸随着振痕深度的增加而增大.凝固钩的平均长度随钢中碳含量的降低而增大,超低碳钢和低碳钢中凝固钩问题尤为突出.减小凝固钩尺寸...     

电磁搅拌作用下水口深度对液面波动的影响

 结晶器内液面波动会影响连铸坯的质量,施加电磁搅拌使钢液的液面呈旋转抛物面。电磁搅拌电流过大或拉速过高会造成保护渣卷渣现象,对铸坯质量造成不利的影响。以某钢厂[?]250 mm连铸圆坯结晶器电磁搅拌为研究对象,采用电磁-流体单相耦合的方式及流体体积函数VOF模型,建立描述结晶器电磁搅拌作用下液面波动的数学模型,研究电磁搅拌作用下浸入式水口深度对液面波动的影响。研究表明,通过增大水口深度,能够改善因电磁搅拌强度过大或拉速过大造成的卷渣现象,减小水口附近的液面波动。     

An asymptotic model of the mold region in a continuous steel caster

A model is developed to simulate the solidification of the steel shell in the mold region of the continuous casting process. Conduction-dominated temperature fields in the mold, mold flux, steel shell, and molten steel regions are determined through the development of an evolution equation for the solidifying front. This equation is derived in the limit of small aspect ratio, mold width to height, using asymptotic methods. These results are coupled with a lubrication-theory model for the mold flux region. This model assumes a temperature-dependent viscosity for the mold flux and allows for solidification of the flux at temperatures below a critical value. System response to changing casting speeds, superheat, mold wall temperatures, and mold flux properties is investigated.     

大断面圆坯连铸温度场的数值模拟

通过大型通用有限元软件ANSYS建立铸坯凝固过程有限元仿真分析模型,在拉速0.25～0.35m/min,钢水过热度20℃的条件下,对20钢Φ中600mm和40Cr钢Φ500 mm圆坯连铸过程进行了计算和分析,得出距液面0～32 m时铸坯表面温度变化曲线。计算结果表明,当20钢Φ600 mm圆坯的拉速为0.3 m/min时,结晶器出口坯壳厚度为30.9 mm,结晶器出口铸坯温度为1050℃,二冷区表面最低温度978℃铸坯在距液面19.71 mm处完全凝固。Φ600 mm圆坯连铸机20钢生产实践表明,拉速0.25 m/min,结晶器出口铸坯表面温度为1048℃,二冷区表面最低温度为918℃,与模拟结果相似。     

高速连铸结晶器内凝固传热行为及其均匀性控制

从分析高拉速包晶钢板坯连铸结晶器内凝固传热行为特征入手,首先阐明拉速对结晶器内的界面热阻、凝固坯壳的温度与应力分布的影响规律,研究发现拉速超过1.6 m·min?1时,界面热阻明显增加,拉速由1.4 m·min?1提升至1.6 m·min?1和1.8m·min?1时,出结晶器坯壳厚度相应减少约10%,其发生漏钢的危险不断增加;在此基础上,阐述了结晶器的内腔结构、保护渣、振动与液面控制等控制结晶器内坯壳凝固均匀性的相关技术。要实现高速连铸,首要应考虑结晶器内腔结构的优化设计,使其能更好地迎合凝固坯壳的生长,研制适合包晶钢等凝固特点的专用连铸保护渣至关重要,铸坯鼓肚控制也是保障高拉速液面稳定的关键。      

Measurement and modeling of heat transfer across interfacial mold flux layers

Surface quality problems in continuous cast steel are greatly affected by heat transfer across the interfacial layers in the gap between the solidifying steel shell and the mold. An experimental apparatus has been constructed to measure temperatures in the steel, mold flux layers, and copper under conditions approximating those in continuous casting. The flux solidified in multiple layers similar to those observed from continuous casting molds and contained many gas bubbles. Flux conductivities average about 1.0 W/m·K and appear to evolve with time. Contact resistances at both interfaces are significant and average about 0.0015 m²·K/W. Flux crystallization appears to be the only significant effect of flux composition. The one glassy flux tested had much greater thermal conductivities, presumably due to radiation transport. Temperature and gap thickness had a negligible effect on the properties. These properties depend on the model used to extract them. They are being implemented into a mathematical model to simulate heat transfer in the mold, interface, and solidifying shell of a continuous slab-casting machine.     

Metal flow in the mold of a continuous-casting machine

Analysis of the flow of liquid metal in the mold during continuous casting is a challenging mathematical problem. Nevertheless, precise solutions have bene found for some cases. Such analytical solutions may be used to verify numerical solutions. In the present work, the melt flow in the mold is studied numerically on the basis of the finite-difference approximation of the initial system of equations. This method is relatively universal: it has been successfully used in continuum mechanics, in mathematical modeling of the stress–strain state of shells in casting, and in other industrial contexts. In the present work, it is applied to the hydrodynamic and thermal fluxes of liquid metal in steel casting in a rectangular mold in a continuous-casting machine. The three-dimensional mathematical model that is obtained describes the liquid-metal fluxes in the mold. The processes that accompany the filling of the mold with melt are simulated by means of Odissei software. The calculations are based on the fundamental hydrodynamic equations, a formula from mathematical physics (the heat-conduction equation with allowance for mass transfer), and a familiar numerical method. The resulting system of differential equations is solved numerically. The region investigated is divided into finite elements. For each element, the system of equations is written in finite-difference form. Solution yields the field of flow velocities of the metal and the temperature field within the mold. The algebraic equations obtained by this means are solved by means of numerical algorithms. On that basis, a program is written in Fortran-4. The mathematical model permits variation of the mold dimensions and the cross section of the metal outlet from the submersible nozzle. It may also assist in understanding the motion of the cast metal, which affects the heat transfer by the mold walls, and in finding the optimal parameters of the liquid metal as it leaves the submersible nozzle. As an example, steel casting in a rectangular mold (height 100 cm) is considered. In casting, the metal leaves the submersible nozzle symmetrically on both sides, in the horizontal plane. The results are graphically displayed. The motion of the metal flux is shown in different cross sections of the mold. Regions of circular flow are identified, as well as regions of vertical motion in the mold. The magnitude and intensity of these regions are determined. The temperature field indicates a local hot zone at the mold wall. That may be attributed to the direct flux of hot metal from the aperture in the submersible nozzle.