连铸结晶器内大方坯的热力耦合分析

针对攀钢大方坯连铸机投产初期铸坯表面角部纵裂缺陷,建立了大方坯连铸结晶器内铜板与铸坯问的热力耦合模型,应用模型分析了大方坯连铸结晶器内的传热过程和坯壳的应力分布.在传热模型中,以稳态模型分析结晶器的传热过程,以瞬态模型分析铸坯的传热过程;在力学模型中,考虑铸坯和结晶器的接触边界以处理结晶器角部的气隙,以热弹塑性模型分析铸坯的变形和应力场.2种结构的连铸结晶器中大方坯温度场和应力场计算结果表明,结晶器倒角从25 mm×45°变为12 mm×45°时,可改善铸坯角部的传热条件,降低凝固坯壳角部温度,增加凝固坯壳厚度,有利于减轻和防止铸坯角部裂纹.     

连铸结晶器凝固传热模型研究

以连铸结晶器内凝固传热过程作为研究对象,结合高温钢液在结晶器内的实际热传输过程,建立连铸结晶器凝固传热通用仿真模型.模型充分考虑凝固铸坯与结晶器间的缝隙对传热的影响.利用Visual Basic 6.0程序开发出相应的结晶器凝固传热仿真软件,计算得到铸坯表面温度,坯壳厚度,结晶器锥度等与生产相关数据,并利用钢厂连铸机结晶器的生产数据对模型进行验证,所得结果均与实测值相符.该模型能够为连铸生产工艺的确定和调整提供理论依据.     

连铸应用凝固传热计算的几个问题

1.连铸坯在结晶器内的凝固传热计算 通过对连铸坯凝固传热过程的研究,可以得到铸坯在冷却凝固过程中的温度场、坯     

连铸结晶器动态热流及其影向

本文从接触一间隙传热原理出发建立起结晶器动态热流数学模型,它与铸坯凝固传热计算相结合,预报了连续铸坯钢的过程。首次得到了气隙、接触压力、结晶器热流,铸坯温度场、坯壳厚度等状态变量沿铸坯横向和纵向的分布规律。预报值与现场实测值一致。该研究使得对连铸热过程的认识更加深入。它为开发新型连铸机和解决与结晶器有关的铸坯质量问题提供了依据。     

板坯连铸过程数值模拟分析

基于传热学基本原理、凝固理论和有限单元法,建立了凝固传热有限差分数学模型,对连铸凝固全过程进行模拟分析,结果表明,拉速越大,铸坯中心及表面温度越高,出结晶器坯壳厚度越薄;过热度增大,铸坯中心及表面温度均上升,出结晶器坯壳厚度减薄;冷却水量相对增大时,铸坯出结晶器坯壳厚度增大,二冷区温度下降较快。连铸坯凝固模型可用来确定常规拉速范围及不同拉速下的凝固壳厚度、凝固末端位置以及铸坯表面温度分布。     

宝钢厚板连铸机结晶器凝固传热模型的研究和开发

以宝钢厚板连铸机结晶器一冷传热过程为研究对象,结合高温铸坯在结晶器内的实际热量传输规律,建立了宝钢厚板连铸机结晶器凝固和传热模型.结晶器内凝固传热过程分为凝固坯壳传热、缝隙间传热和结晶器铜板传热,其中结晶器缝隙问传热模型综合考虑了气隙、保护渣和振痕对传热的影响.利用Fortran语言对模型进行编程,开发出相应的结晶器凝固和传热仿真软件Moheat.结合厚板连铸机结晶器生产数据,对模型进行了验证.所得计算结果符合实际测量值.利用该软件能够对不同生产工艺下的凝固坯壳厚度、坯壳表面温度、结晶器铜板温度、冷却水温差以及结晶器理想锥度等进行计算,分析和优化结晶器一冷制度,指导连铸生产.     

水平连铸坯的热应力计算分析

应用铸坯在凝固过程中的热应力模型，对水平连铸坯进行了传热、应力和应米的计算分析，表明铸坯出结晶器后温度回升太大和凝固末期中心温度的迅速下降，是产生中心裂纹和中间裂纹的可能原因。     

水平连铸结晶器的传热特性研究

采用数模计算与实际热试测量相结合的方法,研究了马钢水平连铸结晶器的传热特性(平均热流、热流分布、铸坯凝固传热等)。初步明确了马钢水平连铸结晶器的特点及其热流分布的基本规律。     

板坯连铸结晶器倒角结构和锥度研究

针对板坯连铸铸坯常出现角部横裂纹缺陷,建立了板坯连铸结晶器与铸坯二维瞬态热力耦合有限元模型,研究了板坯结晶器窄面铜板在不同倒角结构和不同锥度情况下铸坯的凝固收缩行为,计算了铸坯在结晶器内的温度和应力分布情况。模型较全面地考虑了保护渣和气隙对传热的影响。数值模拟结果表明：结晶器窄面铜板倒角过大或过小,都不利于铸坯温度的均匀分布;对断面厚度为230mm的铸坯,窄面铜板采用20mm～25mm ×45°;倒角及抛物线锥度时,铸坯表面温度分布最均匀,最大平面主应力分布较合理,角部出现横裂纹的可能性会大大降低。     

基于反算热流的结晶器内流动--传热--凝固耦合模拟

基于Navier-Stokes动量方程和湍流低雷诺数k-ε方程,综合考虑能量守恒和钢液凝固与糊状区对流动过程的影响,建立了描述结晶器内钢液流动、传热及凝固过程的三维耦合数学模型.以实测温度和结晶器反问题模型计算出的热流为边界条件,模拟计算了结晶器内钢水的流动、传热和凝固行为.钢液流动决定结晶器内的温度和热流分布,铸坯凝固受钢液流动和结晶器热流双重因素的影响.建立的模型以及由此得到的铸坯凝固非均匀特征可为进一步考察浇铸过程中纵裂和其他表面缺陷提供借鉴和参考.      

Combination of inverse problem and neural network for thermal behaviour calculation of mould process based on temperature measurements in plant trial

Abstract

Heat transfer between mould and strand has a critical influence on billet quality, caster productivity and operating safety. It is very important to obtain the correct distributions of temperature and heat flux, and many studies are made on the calculation methods of heat transfer between strand and mould, aiming to reduce the computation time and improve the calculation accuracy. In the present paper, based on measured data of temperature and heat flux during round billet continuous casting, the calculation method which combines the online measurement data and numerical simulation was investigated. Through identifying the local thermal resistance and its distribution between the mould and the strand by an inverse heat transfer model, the heat flux and shell thickness profiles were calculated. To avoid the iterative solution by inverse model, a faster alternative model using an artificial neural network was developed to predict the thermal resistance from the measured temperature. After training, there is an exact correspondence between the observed temperature values and the thermal resistance. The calculation results obtained by the combination of neural network and numerical simulation can correctly reflect the characteristics of non-uniform heat transfer around the mould circumference, which provides a worthwhile and applicable method for online calculation and visual technology of heat transfer and solidification in continuous casting mould.     

Modeling of the solidification process in a continuous casting installation for steel slabs

The development of a computational simulation system for modeling the solidification process in a continuous casting facility for steel slabs is discussed. The system couples a module for solving the direct problem (the calculation of temperatures in the steel strand) with an inverse analysis module that was developed for evaluating the steel/mold heat fluxes from the information provided by thermocouples installed in the continuous casting mold copper plates. In order to cope with the non-uniqueness of the inverse analysis, a priori information on the solution, based on the consideration of the problem physics, is incorporated. The stability of the system predictions are analyzed and the influence of the first trial used to start the evaluation procedure is discussed. An industrial case is analyzed.     

Investigation of mould thermal behaviour by means of mould instrumentation

Abstract

The thermal behaviour of the continuous casting mould has a critical influence on strand surface quality, casting productivity and operating safety. voestalpine Stahl GmbH has long been interested in the field of research into mould behaviour, and began with the investigation of heat flux in 1995, and then of thermal variability in 1999. Since 2000, an online mould thermal monitoring system for heat flux density, thermal variability and friction has been installed. The heat flow density in the mould is determined by measuring the inflow and outflow temperatures and the throughflow volume of the primary cooling water. Temperature as measured by a thermocouple based breakout detection system in the mould copper plate is used to investigate the thermal variability. These online measured values have been employed to examine the influence of casting parameters, steel analysis and casting powder on the heat flux and thermal variability, and the relationship between these variables and cracking. The knowledge gained through these wide ranging plant based investigations has been used as a major tool in the diagnosis of problems (such as breakout and sticking), for optimisation of the process, particularly in the field of casting powders, and for control of slab quality. In particular, this knowledge has been incorporated into online mould thermal monitoring.     

Heat Transfer and Its Effect on Solidification in Combined Mould

SymbolList cp,cps,cpl———Specificheat,specificheatofsolidsteel andmoltensteel,(J·kg-1·K-1);C2———EmpiricalcoefficientoflowReynoldsκεturbu lencemodel;Dl———Darcyconstant;fl,fs———Fractionofmoltensteelandsolidsteel;f2———Empiricalcoefficientre     

Abnormal transient phenomena in the continuous casting process: Part 2

Abstract

This paper is the second of a series of two describing abnormal transient phenomena observed during online monitoring of a billet continuous casting machine. Special attention is paid in Part 2 to in mould solidification. A mould heat flux drift phenomenon (HFD) has been detected, but only for mould powder basicities larger than 0·8. The HFD is related to a decrease of the heat flux in the lower part of the mould and an increase in both the billet-mould friction force and mould thermocouple variability. Results of tests changing the mould powder grade during casting have provided help in explaining the HFD. The probable reason for the HFD is crystallisation of the glassy slag layer. The heat flux ratio parameter (HFR), defined as the ratio between the heat flux in the lower part of the mould and the heat flux in the upper part of the mould, has proved to be a good tool for judging the casting performance of a mould powder.     

Optimization of Oscillation Model for Slab Continuous Casting Mould Based on Mould Friction Measurements in Plant Trial

Lubrication and friction between the mould and strand are strongly influenced by mould oscillation, and play an important role in slab quality and operating safety during continuous casting processes. Investigation of mould oscillation is therefore essential for getting a better online control of the mould processes. A feasible approach for the development and optimization of mould oscillation was put forward, which combined online measurement of mould friction, design of negative oscillating parameters and evaluation for powder consumption. Three different control models including sinusoidal and non-sinusoidal oscillation for mould oscillations were developed to investigate and evaluate the effects of oscillation on mould friction and powder lubrication. For the purpose of investigating mould friction between mould and strand, online measurement was carried out on a slab continuous caster equipped with a hydraulic oscillator. Also the comparison of the mould friction in sinusoidal and non-sinusoidal mould oscillation was made for subsequent analysis. The industrial experiment result shows that the combination of inverse control model and non-sinusoidal oscillation mode will contribute to the proper powder consumption, leading to a suitable effect of friction force on strand surface, especially for high speed continuous casting. The proposed method provides reliable basis for guiding and optimizing mould oscillation among control models, sinusoidal oscillation and non-sinusoidal oscillation.     

Estimating the Effective Metal-Mould Interfacial Heat Transfer Coefficient via Experimental-Simulated Cooling Curve Convergence

The design of improved casting systems requires accurate modeling of metal cooling processes. This can only be accomplished after determining the interfacial heat transfer coefficient (IHTC) between a solidifying casting and its mould. In the current work, a simple and robust inverse heat conduction technique was applied for the estimation of the effective IHTC between an aluminum alloy casting and a steel permanent mould during solidification. The solidification of the alloy at varying mould preheating temperatures was monitored using a thermocouple, and the experimental cooling curves were compared with curves simulated by casting solidification modeling software. The IHTC value applied to the software was varied until its output converged with the experimental data, leading to an estimation of 6000 W/m²K for this system. This technique is useful as a preliminary tool in materials modeling, and it will promote the development of improved casting processes without the need for excessive experimentation.     

Real-Time,Model-Based Spray-Cooling Control System for Steel Continuous Casting

This article presents a new system to control secondary cooling water sprays in continuous casting of thin steel slabs (CONONLINE). It uses real-time numerical simulation of heat transfer and solidification within the strand as a software sensor in place of unreliable temperature measurements. The one-dimensional finite-difference model, CON1D, is adapted to create the real-time predictor of the slab temperature and solidification state. During operation, the model is updated with data collected by the caster automation systems. A decentralized controller configuration based on a bank of proportional-integral controllers with antiwindup is developed to maintain the shell surface-temperature profile at a desired set point. A new method of set-point generation is proposed to account for measured mold heat flux variations. A user-friendly monitor visualizes the results and accepts set-point changes from the caster operator. Example simulations demonstrate how a significantly better shell surface-temperature control is achieved.     

Solidification and Particle Entrapment during Continuous Casting of Steel

Avoiding particle entrapment into the solidifying shell of a steel continuous caster is important to improve the quality of the continuous cast product. Therefore, the fluid flow dynamics in the steel melt and mushy zone, heat transfer and solidification of the steel shell, as well as the motion and entrapment of inclusion particles during the casting process were investigated using computational models. Solidification of the strand shell is modelled with an enthalpy‐formulation by assuming a columnar morphology in the mushy zone. The motion of particles is tracked with a Lagrangian approach. When the particles reach the solidification front, they can be entrapped/engulfed into the solid shell or pushed away from the solidification front, depending on the mushy zone morphology and the forces acting on them. The current paper focuses on the mould region at a steel continuous caster, including the submerged entry nozzle (SEN) and 1.2 m length of the strand. The results are validated with plant measurements and demonstrate the potential of the model to predict fluid flow, shell growth and the positions and the amount of entrapped/engulfed particles in the solidifying strand.     

Microstructure and cooling conditions of steel solidified in the continuous casting mould

Importance of uniform shell growth and fine microstructure on breakout safety and strand surface quality. Analysis of heat flow and shell growth in the mould for a 0.62 wt% C-steel, and correlation with the microstructure by comparison of calculated with measured dendrite arm spacing. Proposal of metallographic standards for the subsurface microstructure in order to check the intensity and uniformity of solidification conditions in the mould.