连续铸钢结晶器温度场的试验研究及其数值模拟

连铸钢坯过程中,结晶器处于高温钢液和高速冷却水的综合作用下,结晶器温度场的合理分布是保证连铸正常进行的关键.为得到结晶器内壁界面温度分布规律,设计了模拟结晶器工作过程的试验装置,进行了动态水流和静态水流对结晶器壁温度影响的测试试验.结果表明,结晶器内壁温度趋近于冷却水温度.结合试验数据推导了结晶器界面等效导热系数,用等效导热系数处理钢液与结晶器内壁的边界传热,对连铸钢坯结晶器温度场进行数值模拟,模拟结果与有关研究结果符合.     

板坯连铸结晶器传热系数

采用数学解析方法对连铸结晶器壁的热流量及结晶器和冷却水之间的传热进行了详细的分析，对炼钢厂连铸过程计算机(DKS)中现有换热系数的计算公式的来源进行了详细阐述和推导，并对现有计算公式作了改进．     

Heat transfer and solidification microstructure evolution of continuously cast steel by non-steady physical simulation

The heat transfer and solidification microstructure evolution during continuous casting were experimentally studied in this work. A new approach to physically simulate the steel solidification behavior during continuous casting was developed. Six steel grades with different solidification mode were introduced to elucidate the carbon equivalent dependent mold heat flux, prior austenite grain size and secondary dendrite arm spacing. It is found that the non-steady mold heat fluxes in the experiment against time for all steel grades are comparative to that versus distance in practical continuous casting. Due to the occurrence of L→L+δ→δ+γ→γ transformation with the largest amount of volume contraction in hypo-peritectic steel, it shows the lowest mold heat flux among these six steel grades. It is also demonstrated from the solidification microstructure results that the prior austenite grain size and secondary dendrite arm spacing in the physical simulation are in good agreement with those in continuously cast strand. In addition, the steel with a higher temperature for the onset of δ→γ transformation reveals the larger prior austenite grains resulted from the higher grains growth rate in the post solidification process.     

H型钢坯水冷金属型铸造充型凝固过程数值模拟

提出了处理水冷铸型边界条件的等效换热系数法，该法综合考虑了铸件与铸型问的辐射换热、间隙热阻及水冷铸型边界条件。获得了H型钢的充型流动过程及凝固过程的温叟场，并采用Xue判据预测了H型钢的缩松缺陷位置。为进一步改进H型钢水冷金属型铸造工艺提供了理论依据。     

软接触电磁连铸方坯结晶器的传热分析

概述了钢的软接触电磁连铸技术的发展现状及存在的问题.根据实际生产中软接触电磁连铸的结晶器的设计参数,通过建立凝固传热过程的数学模型,分析了钢液在切缝式软接触结晶器内的凝固过程.计算了结晶器水孔内水的流速、坯壳厚度、坯壳外壁的温度.计算了钢液在结晶器内单位时间放出的总热量和平均热流,并对结晶器各壁温度进行了校验,使软接触电磁连铸结晶器满足实际生产要求.     

基于BP_GA算法的铸造有效应力预测与优化

为控制铸件凝固过程中的有效应力大小,避免热裂发生,利用有限元分析软件ProCAST对ZL205A铝合金牵引结构件低压铸造过程进行温度场模拟与有效应力预测,选择浇注温度、模具预热温度、传热系数和模具壁厚等影响铸造应力的因素作为设计参数。结合有效应力预测结果,构建4-7-1-1型神经网络和遗传算法以优化铸造工艺。结果表明,神经网络预测平均相对误差为1.45%,预测精度较高。通过遗传寻优方法,发现了最佳工艺参数组合:浇注温度为688℃,模具预热温度为291℃,模具壁厚为150mm,传热系数为1 284W/(m^2·K),并进行试验验证,获得品质较好的铸件。     

软接触结晶器电磁连铸中初始凝固的基础研究

通过实验和数值模拟研究了软接触结晶器电磁连铸传热凝固特点,测定了金属Ｓｎ在连铸中的温度场,初始凝固点和坯壳厚度,得到了电磁场影响它们的基本规律。在数学模型中考虑了高频电磁场对金属的传热凝固的以下影响;（１）对结晶器的感应加热;（２）对金属的加热;（３）电磁力推斥液力金属,减少金属与结晶器壁接触的影响。     

铝铸锭凝固边界热交换规律及温度场模拟

研究铝铸锭凝固边界热交换的变化规律及数学模型,并对不同浇注温度下凝固过程的温度场进行模拟。利用实时数据采集系统获得凝固过程中铸锭和金属模温度变化历史数据,采用非线性反算法和一维传热差分法对试验数据处理,建立界面换热模型并将其应用于凝固温度场模拟中。结果表明：在铸锭表面凝固前后凝固界面热流密度可分段用指数函数来描述其变化规律,而所建立的热交换系数与边界温度的对应关系可更好地反映实际的传热情况。模拟结果与实验测温结果相符,验证了该铸件/铸型边界热交换规律的可靠性。     

Derivation and application of time step model in solidification process simulation

The heat transfer during the casting solidification process includes the heat radiation of the high temperature casting and the mold, the heat convection between the casting and the mold, and the heat conduction inside the casting and from the casting to the mold. In this paper, a formula of time step in simulation of solidification is derived, considering the heat radiation, convection and conduction based on the conservation of energy. The different heat transfer conditions between the conventional sand casting and the permanent mold casting are taken into account in this formula. The characteristics of heat transfer in the interior and surface of the casting are also considered. The numerical experiments show that this formula can avoid computational dispersion, and improve the computational efficiency by about 20% in the simulation of solidification process.     

连铸凝固传热全过程数值模拟与控制

基于传热学、凝固理论、熔渣理论,建立了凝固传热有限差分数值模型,对连铸全过程进行热状态、凝固状态分析及工艺控制,模型考虑了钢的热物性参数随温度变化的关系.并根据连铸冶金准则和目标温度控制进行二冷优化,获得合理的温度分布和二冷水量分布,实现最佳铸坯品质,同时针对不同钢种的凝固特点,对保护渣性能进行设计.计算值同现场实测数据进行对比,有较好的一致性.最后研究了浇注温度、拉坯速度、二冷配水等工艺参数对铸坯表面温度、液芯长度和凝固坯壳厚度的影响,以及浇注工艺对连铸保护渣指标的影响.     

保护渣碱度对薄板坯结晶器平均热流量的影响

在钢的连铸过程中,钢水在结晶器内的凝固对铸坯的产量和质量均有很大影响,几乎所有的铸坯表面缺陷均形成于结晶器内。近年来,随着连铸拉速的增加及对铸坯表面质量要求的提高,有关结晶器冷却、传热对钢水的初始凝固及表面纵裂纹影响的研究成为连铸科学研究的重点。结晶器壁热流不均是纵裂纹产生的有利环境,保护渣控制传热为常用的措施。薄板坯浇铸时由于拉速高,为获得表面无缺陷铸坯,对保护渣控制传热的要求更高,同时也需协调保护渣的润滑功能。通过生产试验,研究比较3种碱度保护渣（CaO/SiO2分别为1.06、1.26和1.48）对薄板坯结晶器平均热流量的影响,发现与低碱度保护渣相比,使用高碱度保护渣时,结晶器热流量最低,有利于实现弱冷却,形成均匀凝固坯壳,在一定拉速条件下浇铸裂纹敏感钢种时有助于获得良好表面质量的铸坯。     

低压铸造薄壁件温度场的动态测试

采用自制的计算机数据采集系统,测试了低压铸造薄壁件从开始充型到凝固结束全过程的温度变化,分析了薄板件的温度变化规律及分布特点。研究表明,薄壁件的充型形态对温度分面影响很大。测试结果反映了铸件的充填顺序及充型过程中的凝固现象。温度场测试结果可用于验证充型过程流场温度耦合程度的正确性。     

连铸Ag-28Cu温度场非稳态过程的数值模拟

采用Procast软件中的Mile算法对Ag-28Cu合金连铸凝固过程中温度场的非稳态变化进行了模拟,研究了不同拉速、过热度和换热系数对温度场分布、凝固速率和凝固前沿温度梯度变化的影响。模拟结果表明:随拉速的增大,铸件中心区域的凝固速率加快,凝固前沿温度梯度变化范围减小。换热系数的改变在靠近铸件表面区域对凝固速率基本没有影响,但靠近铸件中心区域后,其凝固速率随换热系数开始大幅度增大。在整个凝固过程中,换热系数越大,凝固前沿温度梯度越大。随浇注温度的提高,其凝固速率呈振荡式增大,凝固前沿温度梯度呈振荡式减小。     

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。     

连铸结晶器振动下弯月面处温度波动的模拟实验

设计了连铸结晶器弯月面处传热模拟实验，测量了在结晶器振动情况下弯月面处的温度，发现该处的温度随着结晶器的振动而产生周期性的变化，实验结果表明，结晶器振动频率越大，振动幅值，结晶器冷却强度以及结晶器与铸坏间的接触压力越小，因结晶器振动而产生的弯月面温度波动亦越小，据此认为，连铸弯月面温度波动是导致铸坯表面缺陷产生的一个重要原因，基于此“温度波动”现象，分析了包括高频小振幅振动，软接触结晶器电磁连铸，热顶结晶器等诸多技术改善坯表面质量的机理。     

Correlative thermal methodology for castability simulation of ductile iron in ADI production

The present paper investigates the simulation analysis of simultaneous mold filling and solidification of ductile iron casting in a permanent mold by virtue of its thermal characteristics. Thermal analysis was performed to determine the solidification behavior and nature of alloy of the melt during its solidification. It revealed the variation in the nature of alloy due to the variations in eutectic freezing and carbon equivalent of the melt. The obtained thermal parameters from the thermal analysis were further used for the casting simulation of the melt. The simulation results show a progressive solidification behavior of the casting. There is a significant decrease in the overall heat transfer coefficient with time during the solidification process. The simulation results were further verified experimentally. The experimental results show porosity defects at the top section of the casting. Two distinct zones (center and outer) were observed on the produced samples based on the average graphite nodule counts and average graphite nodule size.     

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.     

采用正交设计优化单晶合金定向凝固数值模拟参数

采用正交设计方法研究单晶高温合金定向凝固过程数值模拟所需参数对温度场和冷却曲线的影响.结果表明:单晶高温合金凝固曲线对模拟参数十分敏感,特别敏感于铸件与铸型之间的界面换热系数.通过正交设计获得边界条件参数使模拟结果与试验结果吻合良好.确定符合DD6单晶高温合金凝固过程的数值模拟边界条件.     

Heat transfer characteristics of lost foam casting process of magnesium alloy

1 Introduction The growing demand for mass reduction in aerospace and automotive industries has greatly increased the magnesium application. Currently, casting is the main industrial forming method for magnesium alloys, but the lag of research and develop…     

Application of inverse method to estimation of boundary conditions during investment casting simulation

Inverse method was used in single crystal superalloy DD6 processing simulation during solidification. Numerical modeling coupled with experiments has been used to estimate the interface heat transfer coefficient (IHTC) between the surface of slab casting and inner mold. Calculated temperature dependent values of IHTC were obtained from a numerical solution. The calculated temperatures agreed well with the measurement of cooling profile.