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.     

薄板坯连铸结晶器内钢水凝固传热数值模拟

为研究薄板坯连铸高拉速下结晶器内坯壳生长过程,利用ANSYS有限元模拟软件,建立了CSP薄板坯连铸结晶器内钢水凝固传热数学模型,并通过实际测量得到的凝固坯壳厚度验证了模型的合理性。对结构用钢Q235B钢种,1 500 mm×60 mm规格在结晶器内凝固过程进行了模拟,研究了不同拉速、不同结晶器冷却水量对凝固的影响。研究结果表明,随着拉速降低,坯壳厚度增加;随着结晶器水量增大,坯壳厚度增加,模拟结果与实测结果相符合,为实际生产优化工艺参数提供了理论参考。     

Effect of superheat,mold, and casting materials on the metal/mold interfacial heat transfer during solidification in graphite-lined permanent molds

Heat transfer during the solidification of an Al-Cu-Si alloy (LM4) and commercial pure tin in single steel, graphite, and graphite-lined metallic (composite) molds was investigated. Experiments were carried out at three different superheats. In the case of composite molds, the effect of the thickness of the graphite lining and the outer wall on heat transfer was studied. Temperatures at known locations inside the mold and casting were used to solve the Fourier heat conduction equation inversely to yield the casting/mold interfacial heat flux transients. Increased melt superheats and higher thermal conductivity of the mold material led to an increase in the peak heat flux at the metal/mold interface. Factorial experiments indicated that the mold material had a significant effect on the peak heat flux at the 5% level of significance. The ratio of graphite lining to outer steel wall and superheat had a significant effect on the peak heat flux in significance range varying between 5 and 25%. A heat flux model was proposed to estimate the maximum heat flux transients at different superheat levels of 25 to 75 °C for any metal/mold combinations having a thermal diffusivity ratio (α _R) varying between 0.25 and 6.96. The heat flow models could be used to estimate interfacial heat flux transients from the thermophysical properties of the mold and cast materials and the melt superheat. Metallographic analysis indicated finer microstructures for castings poured at increased melt superheats and cast in high-thermal diffusivity molds.     

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。     

Spatial interfacial heat transfer and surface characteristics during gravity casting of A356 alloy

As one of the key boundary conditions during casting solidification process, the interfacial heat transfer coefficient (IHTC) affects the temperature variation and distribution. Based on the improved nonlinear estimation method (NEM), thermal measurements near both bottom and lateral metal-mold interfaces throughout A356 gravity casting process were carried out and applied to solving the inverse heat conduction problem (IHCP). Finite element method (FEM) is employed for modeling transient thermal fields implementing a developed NEM interface program to quantify transient IHTCs. It is found that IHTCs at the lateral interface become stable after the volumetric shrinkage of casting while those of the bottom interface reach the steady period once a surface layer has solidified. The stable value of bottom IHTCs is 750 W/(m²·°C), which is approximately 3 times that at the lateral interface. Further analysis of the interplay between spatial IHTCs and observed surface morphology reveals that spatial heat transfer across casting-mold interfaces is the direct result of different interface evolution during solidification process.     

Microstructure and solidification thermal parameters in thin strip continuous casting of a stainless steel

The present work focuses on the relationships between solidification thermal parameters and the dendritic microstructure of an AISI 304 stainless steel solidified both in a strip casting pilot equipment (twin-roll) and in a directional solidification simulator. Experimental studies were conducted with a stainless steel strip casting obtained in a twin-roll continuous caster pilot equipment and in samples solidified in a directional solidification simulator with two different melt superheats. In both cases, the surface of the substrates was similar, with mean surface roughness of about 0.3 μm. After solidification, the specimens were cut at different positions from the metal/mold interface and etched for metallographic examination. An empirical equation from the literature relating secondary dendrite arm spacing and cooling rates was used to demonstrate the similarity of the cooling efficiency. The results have shown that the simulator can be used in the determination of transient metal/mold interface coefficients (h_i) and in the preprogramming of the strip casting operational conditions as a function of roll materials and surface roughness.     

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.     

Interfacial heat transfer behavior at metal/die in finger-plated casting during high pressure die casting process

Heat transfer at the metal-die interface has a great influence on the solidification process and casting structure. As thin-wall components are extensively produced by high pressure die casting process(HPDC), the B390 alloy finger-plate casting was cast against an H13 steel die on a cold-chamber HPDC machine. The interfacial heat transfer behavior at different positions of the die was carefully studied using an inverse approach based on the temperature measurements inside the die. Furthermore, the filling process and the solidification rate in different finger-plates were also given to explain the distribution of interfacial heat flux(q) and interfacial heat transfer coefficient(h). Measurement results at the side of sprue indicates that qmax and hmax could reach 9.2 MW·m~(-2) and 64.3 kW ·m~(-2)·K~(-1), respectively. The simulation of melt flow in the die reveals that the thinnest(T_1) finger plate could accelerate the melt flow from 50 m·s~(-1) to 110 m·s~(-1). Due to this high velocity, the interfacial heat flux at the end of T_1 could firstly reach a highest value 7.92 MW·m~(-2) among the ends of T_n(n=2,3,4,5). In addition, the q_(max) and h_(max) values of T_2, T_4 and T_5 finger-plates increase with the increasing thickness of the finger plate. Finally, at the rapid decreasing stage of interfacial heat transfer coefficient(h), the decreasing rate of h has an exponential relationship with the increasing rate of solid fraction(f).     

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.     

Heat transfer between WC-Co coating and aluminum alloy substrate during high-velocity oxygen-fuel (HVOF) spraying

Mathematical simulation of the heat transfer between a WC-Co coating and an aluminum alloy (Al-4%Cu) substrate during HVOF spraying is provided. This simulation includes the investigation of tem-perature evolution, coating solidification, fusion and solidification in the substrate interfacial region, and particular features of the substrate-coating thermal interaction. Optimal thermal conditions for forming the coating structure are estimated. The results obtained are used in another paper (Ref 15), “Formation of Structure of WC-Co Coating on Aluminum Alloy Substrate During High-Velocity Oxygen-Fuel (HVOF) Spraying,” to predict the structural parameters, which agree well with the experimental data.     

热成形淬火过程模具/板料界面接触热阻的测量计算方法概述

分析了固体界面接触热阻的形成机理和影响接触热阻数值的诸多因素,并根据热成形淬火过程模具/板料界面的传热模型,基于傅里叶定律对其界面传热情况进行分析,利用热阻的思想推导出界面接触热阻的表示方法。对已有的测量、计算热成形模具/板料界面接触热阻的方法进行概述:逆热传导法基于导热微分方程和傅里叶定律,通过测得的热流密度计算得到接触热阻;间接法依据接触电阻和接触热阻有相同的产生原因,通过特定装置测定接触电阻再根据接触电阻和接触热阻的关系计算得到接触热阻。最后从测量装置、测量计算原理和测量计算的过程等方面对两种方法进行了比较,并分析了界面接触热阻对热成形模具以及新型材料研发的重要性。     

Metal/quenchant interfacial heat flux transients during quenching in conventional quench media and vegetable oils

The determination of quench severity and the quantification of the boundary conditions at the metal/quenchant interface would be of considerable utility to the heat treating community. In the present work, an attempt has been made to determine the quench severity of various quench media, including three vegetable oils, by the Grossmann Hardenability Factor method and by estimation of heat flux transients by inverse modeling of heat conduction in 304 stainless steel quench probes. The heat flux transient technique was found to be more accurate than the Grossmann technique in assessing the severity of quenching. This finding was supported by the hardness data and microstructure obtained with the quenched steel specimens. New heat flux parameters are proposed to assess the severity of quenching. The boundary heat flux transients during end quenching of AISI 1040 steel specimens were also estimated. The estimated heat flux transients could be used for modeling of heat transfer during quenching. An attempt has also been made in the present work to assess the feasibility of three vegetable oils, namely coconut, sunflower, and groundnut oils, as quenching media. Further investigation is required in this direction to explore the suitability of these oils for industrial heat treating applications. This application would have immense environmental and economical benefits.     

板坯电磁连铸结晶器内钢/渣界面波动及流动行为的数值模拟

建立了板坯电磁连铸结晶器内钢/渣界面波动行为的三维数学模型,利用数值模拟方法研究了磁场与流场耦合作用下不同工艺参数和电磁参数对结晶器内钢/渣界面波动行为及流场的影响,通过VOF方法对不同条件下的钢/渣界面进行捕捉,讨论不同磁极位置、水口倾角、拉速及线圈电流强度对结晶器内钢/渣界面波动行为和流动的影响。模拟结果表明：电磁制动的施加可以显著降低钢/渣界面波高,减小射流对结晶器窄面的冲击。拉速和水口浸入深度恒定时,磁极位置和水口角度直接影响结晶器内流场形式：当[P＝]40 mm时,增加线圈电流可以降低结晶器内钢/渣界面波高和表面流速,从而减小由液面波动引发卷渣的概率;当磁极距离水口较远时[（P＝]80 mm）,随着线圈电流强度的增大,水口射流的冲击方向向上偏转,引起上回流的流动强度增强,导致钢/渣界面波高增加,增大卷渣发生的概率。     

热型连铸固液界面位置和形状的控制分析

在对横引式金属圆杆、圆管热型连铸试验基础上,分析了固液界面位置和形状的影响因素,提出通过提高铸型的温度梯度来加强对固液界面位置和形状的控制,以扩大热型连铸工艺参数配合的范围。     

铸件热应力模拟中铸件/型(芯)相互作用的研究进展

铸造过程热应力分析是当前铸件数值模拟领域的研究重点和热点之一.其中铸件/型(芯)之间的相互作用的处理直接影响铸件铸造过程热应力分析的准确性.介绍了国内外处理铸件/型(芯)热力相互作用的研究进展,接触单元法是三维复杂铸件应力模拟发展的趋势.文章还介绍了砂型材料高温力学性能的研究情况.