USS122G钢锭真空电弧重熔工艺的数值模拟

 超高强度不锈钢以其超高强度和良好的韧性以及优异的耐腐蚀性能而广泛地应用在航空、航天等领域。真空自耗重熔(VAR)作为生产超高强度不锈钢铸锭的主要生产技术,具有去除钢中有害杂质、改善钢中元素偏析的功能。为了研究新型Cr-Co-Ni-Mo合金体系超高强度不锈钢USS122G的真空电弧重熔过程,通过工艺仿真优化软件(Melf-Flow-VAR),对VAR过程的宏观传热、传质和流动现象进行模拟,建立USS122G合金VAR过程的二维轴对称数学模型,预测了不同冷却速度下的温度场和熔池形貌,并着重分析了特定熔速下的温度场、流场的演变,有无氦气冷却的元素宏观偏析情况,最后以模拟工艺制备了USS122G合金660 mm铸锭进行验证。结果表明,熔速增加,熔池深度加深,熔池形貌由低熔速扁平状圆弧状高熔速深“U”变化,熔炼速率为4.5 kg/min的熔池形貌具有较窄的糊状区,在此熔速下,熔池形貌呈现圆弧状,且真空自耗炉的输入功率较低,流场模拟结果显示流体的流动方向沿边部向下,中部向上,在铸锭右侧呈现顺时针运动规律;模拟熔池在达到稳态后深度为132 mm,此时模拟熔池深度与实测结果基本一致;在熔炼过程中Cr和C元素均发生正向偏析,采用氦气冷却的铸锭中元素偏析程度较小,在距钢锭1/2R处到边部Cr和C元素分布规律与模拟结果吻合较好。本项研究成果为钢的工业化稳定生产提供了可靠的数据支撑。     

On the Formation of Macrosegregations in Steel Ingot Castings

A multiphase approach is used to study macrosegregation phenomena that occur during solidification of steel ingot castings. The goal is to enhance the understanding of different mechanisms of macrosegregation formation. 4 different cases are presented consecutively with increasing complexity of the model assumptions and increasing dimensions: (1) feeding‐induced macrosegregations in 1‐dimentional unidirectional solidification situation, (2) macrosegregations caused by thermosolutal buoyancy driven flow in a 2‐dimensional axially symmetric benchmark ingot, (3) macrosegregations caused by grain sedimentation in the same 2‐dimensional ingot, and (4) macrosegregations which form during mixed equiaxed‐columnar solidification in a full 3‐dimensional benchmark ingot.     

Ti2AlNb合金锭真空自耗过程宏观偏析的数值模拟

Ti₂AlNb合金锭的真空电弧重熔(VAR)是一种超高温且不透明冶金过程,很难对这一过程中的熔体流动行为和宏观偏析的形成过程进行试验研究。发展了基于欧拉多相流的电磁场、温度场、流场、溶质场的多场强耦合数学模型,研究了真空自耗过程中的多物理场相互作用机制,对Ti₂AlNb合金锭中成分偏析形成过程及分布规律进行了预测。模拟结果表明,电磁力主要分布于熔池表面,自感电磁力推动金属液由中心向下流动而加深熔池;搅拌电磁力的离心效应则大幅提升熔池的温度场均匀度,促使熔池内金属液中的溶质混合均匀。尽管铸锭外围和中心分别形成了大范围的正、负偏析区,但区域内的成分较为均匀。在搅拌和沉降的作用下,金属熔池中的等轴晶极大地缩短了铸锭中的柱状晶区。该模型的模拟结果在熔池深度与宏观偏析分布方面与试验结果吻合良好,可进一步应用于预测和研究工业级大型铸锭中的成分偏析。     

On the Modeling of Thermal Radiation at the Top Surface of a Vacuum Arc Remelting Ingot

Two models have been implemented for calculating the thermal radiation emitted at the ingot top in the VAR process, namely, a crude model that considers only radiative heat transfer between the free surface and electrode tip and a more detailed model that describes all radiative exchanges between the ingot, electrode, and crucible wall using a radiosity method. From the results of the second model, it is found that the radiative heat flux at the ingot top may depend heavily on the arc gap length and the electrode radius, but remains almost unaffected by variations of the electrode height. Both radiation models have been integrated into a CFD numerical code that simulates the growth and solidification of a VAR ingot. The simulation of a Ti-6-4 alloy melt shows that use of the detailed radiation model leads to some significant modification of the simulation results compared with the simple model. This is especially true during the hot-topping phase, where the top radiation plays an increasingly important role compared with the arc energy input. Thus, while the crude model has the advantage of its simplicity, use of the detailed model should be preferred.     

镍基高温合金真空自耗数值模拟

摘要：基于某特钢厂生产过程,对镍基高温合金的真空自耗过程进行数值模拟,研究了不同电流强度、熔速和通入氦气对铸锭Nb元素宏观偏析、黑斑形成的影响规律。铸锭在顶部和1/4处产生较为严重的宏观偏析。随着熔炼电流强度增加,铸锭顶部的磁场增加,顶部宏观偏析和1/4处黑斑逐渐加重。随着熔速的增加,熔池深度增加,铸锭顶部的Nb元素偏析加重,铸锭1/4处黑斑增加。熔炼时通入氦气,铸锭冷却速率大幅度提高,铸锭的元素偏析程度和黑斑明显减少,且最大偏析部位向铸锭顶部移动。     

Numerical simulation of vacuum arc remelting nickel based superalloy

Based on the process in a special steel plant, the vacuum arc remelting process of nickel based superalloys was simulated to investigate the effect of current intensity, melting rate, and helium gas cooling on macrosegregation of Nb and formation of freckles. The ingot has severe macrosegregation in the upper and 1/4 part of the ingot. With the current intensity increasing, the magnetic field in the upper part of the ingot increases, and macrosegregation and freckles increase gradually both in the upper and 1/4 part of the ingot. With the melting rate increasing, the depth of the molten pool increases, the macrosegregation of Nb increases in the upper part of the ingot, and the freckles increase significantly in the 1/4 part of the ingot. When helium is introduced into the ingot during smelting, the cooling rate increases greatly, the macrosegregation and freckles reduce significantly in the ingot, and the maximum segregation position moves to the top of the ingot.     

过热度对GCr15SiMn轴承钢钢锭宏观偏析的影响

针对GCr15SiMn钢锭易出现宏观偏析凝固缺陷的问题,研究了过热度对GCr15SiMn钢锭宏观偏析的影响规律,使用真空感应炉冶炼1 kg的GCr15SiMn钢锭,通过酸侵试验与OPA技术分别测定了钢锭的凝固组织与宏观偏析,并结合ProCAST软件分析了钢液流动的规律。结果表明,高过热度(70 ℃)时,中心下部出现一定程度的负偏析,中心上部形成了较严重的正偏析同时伴随疏松;中过热度(50 ℃)时,疏松范围较小,碳元素分布较均匀;低过热度(20 ℃)与极低过热度(-20 ℃)时,疏松范围扩大,凝固初期是严重负偏析,凝固末期是严重正偏析。过热度影响偏析的机理为,高过热度时,凝固过程热对流较强,溶质上浮,钢锭上部的正偏析严重;当过热度过低时,初期凝固大量形核并保留在钢锭底部,在底部形成严重的负偏析。     

真空自耗熔炼过程宏观偏析的数值模拟

基于国内某特钢厂真空自耗生产过程,利用ProCAST软件建立三维熔炼模型,研究了不同电流强度和熔速对铸锭熔池形状和宏观偏析的影响规律.铸锭侧面表层基本不发生宏观偏析,铸锭1/4处偏析度约为1.03的正偏析,铸锭中心处由于枝晶沉降形成偏析度为0.96的负偏析.电流强度从4kA增加至8kA,熔滴滴落温度增加,熔池深度加深,...     

Modeling of Species Transport and Macrosegregation in Heavy Steel Ingots

In the current study, two significant phenomena involved in heavy steel ingot casting, i.e., species transport and macrosegregation, were numerically simulated. First, a ladle–tundish–mold species transport model describing the entire multiple pouring process of heavy steel ingots was proposed. Carbon distribution and variation in both the tundish and the mold of a 292-ton steel ingot were predicted. Results indicate high carbon concentration in the bottom of the mold while low concentration carbon at the top of mold after the pouring process. Such concentration distribution helps in reducing both negative segregation in the bottom of the solidified ingot and positive segregation at the top. Second, a two-phase multiscale macrosegregation model was used to simulate the solidification process of industrial steel ingots. This model takes into account heat transfer, fluid flow, solute transport, and equiaxed grain motion on a system scale, as well as grain nucleation and growth on a microscopic scale. The model was first used to analyze a three-dimensional industry-scale steel ingot as a benchmark. Then, it was applied to study macrosegregation formation in a 53-ton steel ingot. Macrosegregation predicted by the numerical model was presented and compared with experimental measurements. Typical macrosegregation patterns in heavy steel ingots are found to be well reproduced with the two-phase model.     

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.     

The Experimental Characterization and Numerical Simulation of A-Segregates in 27SiMn Steel

A 500-kg 27SiMn steel ingot has been produced via low-oxygen purifying technology and then fully dissected in the longitudinal center plane to investigate the typical features and formation mechanism of A-segregates. The macrostructure, microstructure, inclusions, cavity and solute segregation surrounding the A-segregates were characterized and analyzed in detail. It shows that the main solute components such as C, Si, and Mn, and even some large cavities accumulate obviously within the A-type channels. Owing to the low content of oxygen and other impurities, there is no apparent enrichment of the inclusions. Based on the classical theory of macrosegregation, driven by thermo-solutal buoyancy, A-segregates in the body of a 27SiMn steel ingot have been simulated successfully by using the multi-component continuum model. They disappear in a referred 1045 steel ingot. By comparing the evolution processes during solidification of both steels, it is demonstrated that the strong thermo-solutal convection originated from the high content of Si element in 27SiMn steel destabilizes the mushy zone and therefore induces the A-segregates. In terms of the occurrence zones of A-segregates and corresponding distributions of Rayleigh number, its critical value to predict the A-segregates in industrial steel ingots was proposed and then applied successfully.     

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.     

Arc Distribution During the Vacuum Arc Remelting of Ti-6Al-4V

Currently, the temporal distribution of electric arcs across the ingot during vacuum arc remelting (VAR) is not a known or monitored process parameter. Previous studies indicate that the distribution of arcs can be neither diffuse nor axisymmetric about the center of the furnace. Correct accounting for the heat flux, electric current flux, and mass flux into the ingot is critical to achieving realistic solidification models of the VAR process. The National Energy Technology Laboratory has developed an arc position measurement system capable of locating arcs and determining the arc distribution within an industrial VAR furnace. The system is based on noninvasive magnetic field measurements and a VAR specific form of the Biot–Savart law. The system was installed on a coaxial industrial VAR furnace at ATI Albany Operations in Albany, OR. This article reports on the different arc distributions observed during production of Ti-6Al-4V. It is shown that several characteristic arc distribution modes can develop. This behavior is not apparent in the existing signals used to control the furnace, indicating the measurement system is providing new information. It is also shown that the different arc distribution modes observed may impact local solidification times, particularly at the side wall.     

Steady state segregation and heat flow in ESR

A combined theoretical and experimental study of steady-state heat flow and segregation in ESR is presented. The segregation model permits prediction of pressure gradients, hence, interdendritic flow velocities responsible for macrosegregation in the “mushy? zone of axisymmetric ESR ingots. The heat flow model considers the solidus isotherm as a moving boundary. The relationships between power and slag temperature as well as power and heat transfer coefficient are experimentally measured and included in the heat balance equation for the slag. Experiments on both a low-temperature simulated ESR apparatus and on a 200 mm diam ESR ingot mold verify both models.     

Numerical Modeling of Macrosegregation in Binary Alloys Solidifying in the Presence of Electromagnetic Stirring

A model for predicting solidification and solute segregation of binary alloys undergoing electromagnetic stirring has been developed. A dual-zone formulation was employed to describe the velocity fields in the mushy region. The key feature of this model lies in its accounting for flow damping in the suspended particle region via turbulent interactions the crystallite surfaces. The damping force is given in terms of the turbulent kinetic energy, fraction solid, and the crystallite sphericity. The computed macrosegregation results for Al-4.5 pctCu alloy were validated against, and were found to agree with, experimental measurements. The effect of final grain size and frequency on segregation was also determined. This validated model represents a rigorous mathematical framework for describing the flow behavior and solute segregation in electromagnetically stirred melts.     

Formation of macrosegregation by multicomponent thermosolutal convection during the solidification of steel

The formation of macrosegregation by multicomponent thermosolutal convection during the solidification of steel is simulated by simultaneously solving macroscopic mass, momentum, energy, and species conservation equations with full coupling of the temperature and concentrations through thermodynamic equilibrium at the solid/liquid interface. The flow field, solid fraction evolution, and macrosegregation patterns for four cases are presented. The results show both the formation of channel segregates and the formation of islands of mush surrounded by bulk melt. In examining the solidification of a ten-element steel, the global extent of macrosegregation of an element is found to be linearly dependent on its partition coefficient (more severe segregation for small partition coefficient), although such scaling is not possible locally. Results for the solidification of a binary Fe-C alloy (with the same carbon content as the ten-element alloy) are similar to those for the ten-element alloy due solely to the large contribution of carbon to buoyancy driven flow in the ten-element steel chosen for study. While including only those elements that make significant contributions to buoyancy driven flow reproduces the global extent of macrosegregation seen in the ten-element alloy, local differences in the predictions are visible. Finally, comparison of results for the solidification of the same ten-element steel using two different sets of data to describe the partition coefficients and change in liquidus temperature with concentration of the elements shows completely opposite behavior,i.e., upward flow through the mushy zone for one case and downward flow for the other. Thus, the need to have accurate phase-equilibrium data when modeling multicomponent macrosegregation is illustrated. Together, the results give an indication of what areas require more careful examination if accurate modeling of multicomponent solidification is to be accomplished.     

Liquid Metal Pool Behavior during the Vacuum Arc Remelting of INCONEL 718

Non-steady-state ensemble arc behavior has been observed during the Vacuum Arc Remelting (VAR) of 508-mm-diameter ingots of INCONEL 718. The liquid metal flow in the melt pool of a 508-mm ingot during VAR has been simulated under two alternative sets of conditions: (1) a steady-state axisymmetrical arc distribution, as has been typically used in modeling work previously; and (b) a transient asymmetrical arc distribution. Due to the computational requirements, neither mass flux nor solidification were modeled; instead, the pool shape was fixed from measurements from a 508-mm-diameter ingot, and a constant pool wall temperature of 1609 K was used. The transient simulation assumed a localized Gaussian arc whose effective center was located at a distance of 0.1 m from the ingot centerline; this simulation rotated clockwise around the centerline with a period of 36 seconds. The steady-state model was simulated with axisymmetrical distributions of current and power input to the pool top surface calculated by time averaging the transient current and power inputs. The standard k-ε solver of ANSYS CFX 5.6 software was used for both simulations. The transient model results suggest that 5 seconds of asymmetrical arc behavior is enough to change the pool from steady state to transient and that, after 30 seconds, the flow is almost fully developed (at least to the accuracy of the model) and dominated by the Lorentz force. Aspects of the model results agree with key features of the melt pool observed during VAR.     

Solidification of a Vacuum Arc-Remelted Zirconium Ingot

As the quality of vacuum arc-remelted (VAR) zirconium ingots is directly linked to their chemical homogeneity and their metallurgical structure after solidification, it is important to predictively relate these factors to the operating conditions. Therefore, a detailed modeling study of the solidification process during VAR has been undertaken. To this purpose, the numerical macromodel SOLAR has been used. Assuming axisymmetrical geometry, this model is based on the solution of the coupled transient heat, momentum, and solute transport equations, under turbulent flow conditions during the remelting, hot-topping, and cooling of a cylindrical ingot. The actual operating parameters are defined as inputs for the model. Each of them, mainly the melting current sequence, melting rate sequence, and stirring parameters (current and period), is allowed to vary with time. Solidification mechanisms recently implemented in the model include a full coupling between energy and solute transport in the mushy zone. This modeling can be applied to actual multicomponent alloys. In this article, the macrosegregation induced by solidification in a zirconium alloy ingot is investigated. In order to validate the model results, a full-scale homogeneous Zy4 electrode has been remelted, and the resulted ingot has been analyzed. The model results show a general good agreement with the chemistry analyses, as soon as thermosolutal convection is accounted for to simulate accurately the interdendritic fluid flow in the central part of the ingot.