磁场对金属材料凝固的影响

控制并得到合适的凝固组织一直是材料工作者研究的方向。近年来人们对金属在磁场中的凝固做了一系列研究,取得了一定成果。主要介绍了磁场对金属凝固过程的影响,包括磁场驱动流体流动、控制宏观偏析、改善凝固组织等,并对未来的研究工作进行了展望。     

钢锭内部的金属流动与宏观偏析

钢锭内部的宏观偏析取决于凝固过程中凝固界面处的溶质分配及固－液两相区中的溶质流动。溶质分配和流体流动受钢锭合金成分、冷却条件及凝固形式等影响。根据钢锭中宏观偏析形成的最新理论，从枝晶间流动、凝固组织、成分、冷却条件等方面，详细论述了钢锭中宏观偏析的控制。     

连铸坯的半宏观偏析

1.序言现已查明,在连铸坯轴心部位存在的偏析中,历来认为不成问题的小型偏析(半宏观偏析),也是妨碍钢板质量均质性的重要因素。因此,人们开始重视半宏观偏析问题,并对连铸条件、凝固组织的影响和偏析程度等进行了调查研究。半宏观偏析主要是伴随凝固收缩而形成的高浓度液相的流动、积聚凝固而产生的。它的大小介于宏观偏析和微观偏析之间。     

连铸坯凝固过程中传输现象基础知识系列讲座(待续)──第三讲 连铸坯凝固过程中传输现象的研究概况(上)

应用数学模型和物理模型对连铸坯凝固过程中的传输现象进行研究，近些年来得到飞速发展，利用数学模型预测连铸坯的液相流动、传热和偏析状况，并在研究传热的基础上开发了应力计算模型，用于分析裂纹的形成原因，并介绍了国内外有关专家在这一领域的研究概况。     

连铸坯凝固过程中传输现象基础知识系列讲座（待续）：第三讲 连铸坯凝固过…

应用数学模型和物理模型对连铸凝固过程中的传输现象进行研究，近些年来得到飞速发展，利用数学模型预测连铸坯的液相流动，传热和偏析状况，并在研究传热的基础上开发了应力计算模型，用于分析裂纹的形成原因，并介绍了国内外有关专家在这一领域的研究概况。     

采用电磁搅拌技术改善铸坯质量的探讨

一、前言电磁搅拌技术早在1922年Mcneill就论述了熔融金属的流动对凝固组织、中心偏析和夹杂物分布的影响。1934年Bruchanov利用电转磁场研究了钢水的流动对凝固组织形态的影响,以及对减少钢中气体含量、改善     

连铸方坯拉坯方向凝固行为与偏析控制研究进展

连铸方坯是棒线类钢铁产品的主要母材,其宏观/半宏观偏析等缺陷,一定程度反映了铸坯成分分布和组织性能的不均匀,是影响高端钢材质量提升的重要因素。从二维横断面入手对铸坯内部低倍组织进行表征、分析与优化是目前铸坯内部偏析缺陷调控的主要手段;然而,铸坯内部凝固枝晶和中心偏析均具有三维空间形貌和分布特征,导致仅通过二维横断面进行研究可能会存在一定误差;同时,连铸拉坯方向（垂直于横断面）上凝固枝晶与偏析的波动特征能够更直观地反映连铸坯内部组织性能和质量沿长度方向的波动,这也是影响高品质钢连铸坯及后续产品质量稳定性的关键之一。从凝固行为特点、组织波动变化规律、中心偏析控制等3个方面总结讨论近年来连铸方坯拉坯方向的凝固行为与偏析控制研究进展。CET转变是铸坯凝固组织调控的重点,其判定依据及其与偏析相关性的研究表明,拉坯方向上CET位置存在波动性且与中心偏析形貌特征相关。液相穴内非对称流场及温度变化会造成拉坯方向凝固枝晶的波动特征以及凝固终点的周期性波动;同时,实际铸坯以及相关定性定量模型也表明了拉坯方向偏析元素的分布具有波动特征,并可以利用时间序列技术等方法展开解析。中心偏析控制技术的发展明显提升了连...     

连铸坯微观及宏观偏析数学模型的研究进展

对连铸坯微观和宏观偏析模型及树枝晶间液相流动的研究进展进行了评述，采用近平衡凝固过程溶质再分配理论并结合连铸传热数学模型对连铸坯微观及宏观偏析的定量解析方法进行了分析。     

钢水流动对凝固组织影响的模型

在大方坯、小方坯铸造中，为了防止中心偏析和中心疏松，在结晶器内和凝固末端，采用电磁搅拌使凝固组织等轴晶化，在凝固末期采用轻压下技术来防止凝固收缩时的钢水流动。但是对偏析晶粒影响很大的等轴晶率和等轴晶粒大小，因钢种、铸坯尺寸、铸造速度、钢水过热度及流动条件等会发生怎样的变化，目前还未建立定量的推定方法。     

钢的凝固组织的成因、显示、识别和控制

一、前言钢的凝固以及在冷却过程中产生的偏析、三、四次夹杂物以及其他各种低、高倍缺陷对钢质造成的不良影响,在以后的热处理和压力加工过程中也难以完全消除。因此,应该重视对钢凝固组织和凝固过程的控制。各种不同的凝固条件(例如不同的温度梯度,冷却速度、凝固速度、溶质分布、钢液流动等),得到的凝固组织也不同。因此,我们可以从已经凝固了的钢的     

高碳钢矩形坯轻压下中心负偏析形成机理

高碳钢82A断面180 mm × 240 mm矩形坯轻压下试验中,铸坯心部碳元素产生了负偏析现象.将轻压下工艺与负偏析形成条件相结合,对中心负偏析形成机理进行了探讨.结果表明,轻压下消除了枝晶间富集溶质钢液向铸坯中心流动通道,铸坯中心受枝晶间富集溶质液体流动影响较小;凝固过程中心负偏析产生条件为vr/R＜-0.042 2,即液相流动速度vr和凝固速度R方向相反为形成中心负偏析的必要而非充分条件;轻压下易满足该条件并在此基础上提出了2种形成机理,即轻压下挤压作用导致心部钢液流动速度方向的改变或者脱落枝晶在铸坯心部先形核凝固导致凝固速度方向的改变都可能导致中心负偏析的形成.     

Modeling of solute redistribution in the mushy zone during solidification of aluminum-copper alloys

A mathematical model has been established to predict the formation of macrosegregation for a unidirectional solidification of aluminum-copper alloys cooled from the bottom. The model, based on the continuum formulation, allows the calculation of transient distributions of temperature, velocity, and species in the solidifying alloy caused by thermosolutal convection and shrinkage-induced fluid flow. Positive segregation in the casting near the bottom (inverse segregation) is found, which is accompanied by a moving negative-segregated mushy zone. The effects of shrinkage-induced fluid flow and solute diffusion on the formation of macrosegregation are examined. It is found that the redistribution of solute in the solidifying alloy is caused by the flow of solute-rich liquid in the mushy zone due to solidification shrinkage. A higher heat-extraction rate at the bottom increases the solidification rate, decreasing the size of the mushy zone, reducing the flow of solute-rich liquid in the mushy zone and, as a result, lessening the severity of inverse segregation. Comparisons between the theoretical predictions from the present study and previous modeling results and available experimental data are made, and good agreements are obtained.     

Influence of Forced/Natural Convection on Segregation During the Directional Solidification of Al-Based Binary Alloys

We analyzed the columnar solidification of a binary alloy under the influence of an electromagnetic forced convection of various types and investigated the influence of a rotating magnetic field on segregation during directional solidification of Al-Si alloy as well as the influence of a travelling magnetic field on segregation during solidification of Al-Ni alloy through directional solidification experiments and numerical modeling of macrosegregation. The numerical model is capable of predicting fluid flow, heat transfer, solute concentration field, and columnar solidification and takes into account the existence of a mushy zone. Fluid flows are created by both natural convection as well as electromagnetic body forces. Both the experiments and the numerical modeling, which were achieved in axisymmetric geometry, show that the forced-flow configuration changes the segregation pattern. The change is a result of the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. In a forced flow, the pressure difference along the front drives a mush flow that transports the solute within the mushy region. The channel forms at the junction of two meridional vortices in the liquid zone where the fluid leaves the front. The latter phenomenon is observed for both the rotating magnetic field (RMF) and traveling magnetic field (TMF) cases. The liquid enrichment in the segregated channel is strong enough that the local solute concentration may reach the eutectic composition.     

Macrosegregation in Rotated Remelted Ingots

A computer model is presented for predicting macrosegregation in rotated electroslag or vacuum arc remelted ingots. Sample calculations of segregation are carried out for ingots of the model alloy Sn-12 pet Pb in which the liquid density increases during solidification and for two hypothetical alloys; in one, the liquid density decreases during solidification, and in the other, liquid density first increases and then decreases during solidification. In alloys such as Sn-Pb in which liquid density increases during solidification, segregation is positive at the ingot centerline and if solidification is sufficiently slow, “freckles” form near the centerline. Positive segregation and freckles are found at the outer periphery of the ingot when liquid density decreases during solidification. Positive segregation and freckles are found at midradius when liquid density first increases and then decreases during solidification, and when the solidus isotherm changes shape abruptly at midradius (with density increasing during solidification). Ingot rotation, by introducing a radial component to the force field, alters interdendritic flow behavior and therefore macrosegregation. Modest rotation speeds eliminate “freckles” and reduce macrosegregation in all modeling studies conducted. Greater rotational speeds can accentuate the segregation. Experiments were conducted on simulated remelted ingots of Sn-Pb alloy. The ingots were 8 cm diam, rotated at speeds up to 119 rpm and solidified at rates from 5.3 × 10^?3 to 1.36 × 10^?2 cm/s. Segregation behavior obtained agrees qualitatively and quantitatively with theory.     

高强度油井套管钢偏析与含Nb析出相研究

摘要：点状偏析是连铸坯中的常见半宏观缺陷之一，严重影响后续产品的加工性能和服役性能。点状偏析的形成与枝晶形貌以及凝固末期钢液流动有关，经热轧会遗留至管材中变成带状偏析，两者在成分和位置上存在一致性。热力学计算分析表明,该钢种圆坯点状偏析内存在富Nb碳化物，该析出相是凝固过程中分散型剩余液相溶质偏析的产物。通过SEM观察到，在常规加热过程中该富Nb碳化物并不能完全回溶，在管材带状偏析处仍然可以发现未溶碳化物的存在。     

A model for macrosegregation and its application to Al-Cu castings

A macrosegregation model has been developed to evaluate solute redistribution during solidification of casting alloys. The continuum formulations were used to describe the macroscopic transports of mass, energy, and momentum, associated with the microscopic transport phenomena, for two-phase systems. It was assumed that liquid flow is driven by thermal and solutal buoyancy, as well as by solidification contraction. The movement of free surface was also considered to ensure volume con-servation. In numerical calculations, the solidification event was divided into two stages. In the first stage, the liquid containing freely moving equiaxed grains was described through the relative vis-cosity concept. In the second stage, when a fixed dendritic network formed after dendrite coherency, the mushy zone was treated as a porous medium. After validation of the proposed model for the case of segregation in a bottom-chilled unidirectionally solidified casting of Al-Cu alloys, the nu-merical model was applied to the study of three different castings with simple geometry. It was found that solutal convection tends to decrease the macrosegregation generated by thermal convec-tion. When shrinkage-driven convection was also considered, segregation was again increased, with highly segregated areas forming away from the riser and next to the mold wall. It was demonstrated that solidification contraction has a stronger effect on the liquid flow in the mushy region than buoyancy. The model also was applied to assess the probability of pore formation based on the pressure drop concept. While in the absence of experimental data for the critical pressure drop it was not possible to uniquely predict the formation of porosity, it was possible to indicate the regions where porosity may form preferentially.     

Channel segregation during solidification and the effects of an alternating traveling magnetic field

The model proposed by Felicelli, Heinrich, and Poirier is used to simulate the solidification of a small two-dimensional domain of Pb-10 wt pct Sn alloy in the presence of electromagnetic stirring by different traveling fields, with or without gravity. Results show (a) enrichment of the bulk liquid by mush solute draining; (b) spontaneous formation of vertical channels, acting as ducts, significantly modified by the electromagnetic flow; and possibly (c) a finer periodic structure of subchannels. Only the last feature is sensitive to the mesh size and permeability value. Scaling analysis is used to balance Darcy, buoyancy, and electromagnetic phenomena. Attention is focused on the gradient zone at the solidification front. Electromagnetic forces can change the flow structure in the bulk liquid. In this way, they modify the pressure differences at the solidification front, changing the channel segregation pattern. Although they cannot eliminate the channels, they can control their positions and partly prevent the unsteadiness of buoyancy effects.     

铝液凝固过程中显微组织对溶质元素偏析的影响

以溶质元素分配系数小于1的元素Fe、Si、Ga、Zn为研究对象,通过分析工业试验条件下铝液凝固过程中杂质元素含量变化,以及对应的显微组织(包括晶粒形貌、尺寸和金属间化合物),并与热力学计算软件Factsage计算的Scheil-Gulliver冷却条件下杂质元素含量和析出相进行了比较。研究发现,铝液凝固初期固相中杂质元素含量最低,且均大于理论计算值,随着凝固的进行,杂质元素含量逐渐增高且与理论值偏差越来越大,出现以上现象的原因包括：1)铝液实际凝固过程中存在边界层效应,即从固相中排出的杂质元素没有完全扩散到液相中;2)铝液实际凝固过程中存在微观偏析现象,即沿着晶界处有Al-Fe-Si中间相析出。另外,发现本研究试验条件下铝液凝固组织有粗大柱状晶、细小柱状晶、等轴晶,粗大的柱状晶更有利于提高部分杂质元素的偏析提纯效率,通过控制提高强制冷却可以促进粗大柱状晶的形成。     

Modeling of the formation of under-riser macrosegregation during solidification of binary alloys

The formation of macrosegregation in a rectangular ingot with reduced cross section from the riser to the casting, chilled from the bottom, has been studied numerically. In addition to positive inverse segregation occurring near the chilled surface, very severe negative segregation around the under-riser region and moderate positive segregation near the top corners of the casting were found. Although large circulating vortexes are created by natural convection in the under-riser region during the early stage of solidification, the fluid flow in the mushy zone is dominated by solidification shrinkage. As a result, the final solute distribution in the casting is determined by the flow of solute-rich liquid in the mushy zone owing to the combined effects of solidification shrinkage and change of cross section from casting to riser. Detailed explanations regarding the effect of different flow phenomena on the formation of the segregations are provided. The effects of riser size and cooling condition at the bottom of the ingot on the formation of macrosegregation also were studied. The predicted negative and positive macrosegregations in the casting compared very well with the available experimental data.     

Chimney Formation in Solidifying Ga-25wt pct In Alloys Under the Influence of Thermosolutal Melt Convection

The directional solidification of Ga-25wt pct In alloys within a Hele-Shaw cell under the influence of thermosolutal convection was observed by means of X-ray radioscopy. The unstable density stratification at the solidification front causes the formation of rising plumes containing solute-rich liquids. The development of the chimneys and the probability of their surviving depend sensitively on the spatial and temporal properties of the flow field. Variations of the vertical temperature gradient along the solidification cell lead to the observation of different mechanisms for chimney formation. Perturbations of the dendritic structure are the origin of development of segregation freckles in case of low temperature gradients. The long-term stabilities of these segregation channels are strongly influenced by the transient nature of the melt convection. The situation at higher temperature gradients is characterized by two dominating convection rolls in the liquid phase which are driven by a lateral temperature gradient and the convex shape of the solidification front. The penetration of this flow pattern into the mushy zone results in continuous accumulation of solute in the central part of the mushy zone followed by a remelting of the solid fraction and the occurrence of a stable chimney.