Study on Electro-Capillary Oscillation in ESR System

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Electrode reactions in dc electroslag remelting of steel rod

Abstract

The non-metallic inclusion content increased significantly when a steel rod of Fe-Ni was remelted by dc electroslag remelting. The silicon content increased slightly. The manganese and sulphur contents did not change. The total aluminium content in the ingot was max. 0·7%, while that in the electrode was only 10 ppm. The aluminium cations Al³+ in the slag are reduced to metallic aluminium at the slag/electrode interface, while O² - anions are oxidised to dissolved O in the metal pool. This Al and O subsequently recombine to form alumina inclusions in the metal pool. The inclusion content was dependent on the alumina content in the slag. When a rod of plain carbon steel was remelted, however, the increase in nonmetallic inclusion content was as little as one-tenth of that for the remelted Fe-Ni rod. The non-metallic inclusion content was independent of the polarity of the electrode.     

板坯凝固过程夹杂物运动行为

 本文使用离散相模型,利用数值模拟的方法对结晶器中的钢液流动、传热、凝固以及夹杂物的运动进行了耦合计算。通过追踪夹杂物的运动轨迹,并在钢渣界面处对夹杂物进行采样分析,最终计算出夹杂物在结晶器中的上浮率。研究表明,夹杂物在结晶器中的上浮率与其尺寸及拉速的大小均有关系,但受夹杂物密度的影响很小。夹杂物越大、拉速越小,越有利于夹杂物上浮至自由液面。小颗粒夹杂在结晶器中并不能被有效去除。对于粒径为50μm的夹杂物,当拉速为1m/min时其上浮率仅为46%,有37%的夹杂物被凝固坯壳捕捉,主要分布在铸坯表皮下10~25mm处。夹杂物被宽面坯壳捕捉的位置多集中在宽面靠近窄面处,在水口下方被捕捉的夹杂物较少。以往的研究认为只要夹杂物上浮至钢渣界面就能够被保护渣吸收,J.Strandh等的研究表明,夹杂物能否被吸收还取决于保护渣的粘度和润湿性等因素。因此,对于粒径较小的夹杂物,必须在精炼后的软吹氩过程中适当增大钢液的静置时间,尽量减少钢液中小颗粒夹杂的数量。另外,结晶器保护渣的选用对钢液中夹杂物的去除也很重要,不仅要满足其对钢液的保温润滑作用,还要考虑其对夹杂物吸附的影响。     

钢中夹杂物去除技术的进展

钢中夹杂物去除的主要环节为夹杂物的长大、上浮和分离。钢中夹杂物去除技术的主要进展有:气体搅拌-钢包吹氩、中间包气幕挡墙和RH-NK-RERM法;电磁净化-钢包电磁搅拌、中间包离心分离和结晶器电磁制动;渣洗技术;过滤器技术。文中分析了各种夹杂物去除技术的冶金功能,将不同夹杂物去除技术进行合理组合,实现多功能精炼,可取得最佳冶金效果。     

电渣重熔过程电源频率对电渣锭洁净度的影响

电渣重熔采用低频供电可以提高功率因数、降低电耗,并实现电力系统的三相平衡。然而,其对电渣锭冶金质量特别是洁净度的影响还缺乏足够的数据支撑。为了研究电源频率特别是低频操作对电渣重熔锭洁净度的影响,采用实验室小型低频电渣重熔炉,以304奥氏体不锈钢、GCr15轴承钢为研究对象,详细分析了不同的电源频率对电渣锭化学成分、气体含量、夹杂物分布的影响规律。研究结果发现,与工频电渣重熔相比,不论是不锈钢还是轴承钢,当采用低频电源(2、1、0.4、0.1 Hz)电渣重熔后(在其他工艺参数如渣系、渣量、电流、电压、气氛等完全相同的情况下),电渣锭中的氧质量分数(0.010%～0.013%)大幅增加,对氮含量影响很小。电渣锭中的铝含量明显增加,而其他化学成分变化很小。与此相对应,低频电渣重熔锭的夹杂物数量也明显增加,且增加的夹杂物主要以氧化铝为主,但是夹杂物主要以小于10μm的细小夹杂为主,大颗粒夹杂物略有增加,但是数量较少。氧含量增加的主要原因是低频电源的直流倾向增大,使重熔渣池中的氧化铝发生了电解(30%Al₂O₃+70%CaF₂渣系...     

Numerical modelling of the transport and removal of inclusions in an industrial gas-stirred ladle

A full-scale, three-dimensional, transient CFD modelling approach capable of predicting the three-phase fluid flow characteristics and the inclusion removal in a gas-stirred ladle was developed. The comparison with experimental data indicates that this model can accurately predict the multiphase fluid flow and slag eye behaviour. The transport and removal of the inclusions in the gas-stirred ladle were predicted by tracing the movement of individual inclusions through computing their particle trajectories and considering a fluctuant top slag layer. The effects of inclusion size, gas flow rates, and injected bubble diameters as well as various removal mechanisms including slag capture, bubble attachment, and ladle wall adhesion on the removal of inclusions were investigated. It is shown that the slag capture is the prevailing mechanism for inclusion removal and the gas flow rate is the most important parameter for enhancing the inclusion removal efficiency.     

Effect of initial large-sized inclusion content on inclusion removal during electroslag remelting of H13 die steel

The current paper focuses on the influence of initial large-sized inclusion content in the consumable electrode on inclusion removal during electroslag remelting (ESR) of H13 die steel. Considering the relationship between the inclusion size and the interfacial energy change of the slag/inclusion/steel system during the inclusion transfer across the steel/slag interface, the thermodynamic conditions for inclusion removal from steel to the slag were put forward. The results showed that the content of large-sized inclusions in final ESR ingot was decreased by approximately 13.66% with the increase in large-sized inclusion content in the consumable electrode from 11.36?mg/10?kg to 16.50?mg/10?kg. The interfacial energy change of the slag/inclusion/steel system decreases with the increase in inclusion radius during the absorption process of inclusions by slag, which is beneficial for inclusion removal.     

导电结晶器电渣重熔中非金属夹杂物的去除

导电结晶器电渣重熔渣池局部发热密度最高值在电极角部和结晶器壁附近,电极端部下方仍存在"高温区",对非金属夹杂物去除提供热力学与动力学条件。电渣重熔过程中非金属夹杂物的去除主要发生在自耗电极端头熔滴形成阶段以及熔滴穿过熔渣层阶段。综合自耗电极端部熔化至熔滴形成过程阶段、熔滴滴落并穿过渣池至金属熔池阶段和金属熔池至铸锭凝固非金属夹杂物的上浮阶段的去除率,导电结晶器电渣重熔去除非金属夹杂物的能力不低于传统电渣重熔。     

电渣重熔过程中夹杂物的控制

电渣重熔过程中非金属夹杂物的去除主要发生在自耗电极端头熔滴形成期以及熔滴滴落穿过熔渣层阶段。电渣重熔过程中原生夹杂物去除的同时将产生新的夹杂物,为了有效地控制电渣锭中的夹杂物,使用复合脱氧剂对自耗电极进行终脱氧,采用合理的重熔速度、熔渣的化学组成和供电制度,以及严格控制电渣炉内氧位。     

Effect of the Slag Composition on the Process Behavior,Energy Consumption,and Nonmetallic Inclusions during Electroslag Remelting

Electroslag remelting (ESR) is an important process to produce high-quality tool steels. The slag composition has a strong effect on the remelting behavior, particularly on energy consumption and the removal of nonmetallic inclusions (NMI). The latter aspect is strongly related to chemical reactions between the slag and the metal and determines the necessary composition of the slag. Also, the electrical conductivity of the slag is determined by the slag composition, and a high resistivity is desirable. The effect of different slag compositions with 0%–60% CaF₂ and a corresponding wide range of electrical conductivities is investigated regarding slag movement, slag surface temperature, and slag skin thickness, as well as their impact on chemical reactions and the removal of NMI. Therefore, a laboratory-scale ESR unit and the plastic mold steel X40Cr14 are used for the experimental trials. The results show a strong impact on the remelting behavior as well as on the specific energy consumption ranging from ≈900 to over 1700 kWh h⁻¹. The findings from the chemical analysis and detection of NMI indicate that a similar metallurgical behavior is feasible, leading to comparable amounts of dominantly Al₂O₃–MgO-type inclusions with some variation due to different activities in the slag.     

Modeling of Electromagnetic Field and Liquid Metal Pool Shape in an Electroslag Remelting Process with Two Series-Connected Electrodes

A three-dimensional finite-element model has been developed to understand the electromagnetic field and liquid metal pool shape in an electroslag remelting (ESR) process with two series-connected electrodes. The magnetic vector potential is introduced into the Maxwell’s equations, and the nodal-based method is used to solve a three-dimensional harmonic electromagnetic field. The heat transfer of the solidifying processes of ingot is modeled by a source-based enthalpy method, and the Joule heating is included in an inner source. The results show the main part of the current flows through the slag cap and a little enters into ingot in a two-series-connected electrode ESR system. As the interaction of self-induced and mutual-induced of two electrodes occurs, the skin effect is significantly suppressed by the neighbor effect. A symmetrical pattern of magnetic flux density in a two-series-connected electrode ESR system is displayed. The magnetic flux density between two electrodes is reinforced and reduced at the outside of two electrodes. The maximum Joule heat power density is located at the interface of slag and electrodes, and it decreases with an increase of the electrode immersion depth. The averaged Joule heat power density increases when slag cap thickness is reduced. With the increase of ingot height, the liquid metal pool shape changes from arc shaped to “V” shaped. When the ingot height is more than the diameter in the ESR processes, the liquid metal pool shape is constant.     

电渣重熔体系电毛细振荡的动态模拟

通过理论分析和交、直流电动态模拟试验证实了交流电渣重熔体系存在电毛细振荡;交流电频率越高,渣－金界面的振荡频率越大,振幅随之减小;在直流电渣重熔过程中,渣－金界面不存在电毛细振荡。     

Numerical Modeling of Inclusion Removal in Electromagnetic Stirred Steel Billets

To investigate the inclusion removal in billets cast under electromagnetic stirring (EMS) influence, a numerical model has been developed to compute the magnetic field, the Lorentz force, the steel flow velocities, and the particle transport within the liquid pool. The electromagnetic field was described by the Maxwell equations and the finite element method was applied using a commercial package. The turbulent fluid flow was described by the Navier‐Stokes equation and by the Reynolds Stress model and the finite volume method was applied using another numerical package. The time average of the Lorentz force was calculated in each element center and this value was applied as a body force in the Navier‐Stokes equation. The magnetic flux density profile was compared with the data obtained in the stirrer of the steel plant. The particle transport model includes the drag force, the buoyancy force and the random walk model, to include the turbulence effects on the particle trajectory. The inclusion removal was calculated and analysed in function of casting speed and stirring current for one size section of mold. The inclusions considered in the calculations have a fixed density and four values of diameter. The numerical results of the electromagnetic model are in agreement with the experimental measurements. A good relationship between the electromagnetic model and the fluid flow model could be shown. An interesting effect is the break of the rotation motion due to the EMS by the jet from the nozzle. The fraction of inclusions removed by the top surface of the mold was improved due to the EMS.     

电渣重熔体系电毛细振荡的研究

分析了电毛细振荡产生的机理和电渣重熔过程中去硫、去氧和去夹杂行为。通过交、直流两用电渣炉试验证实了交流电渣重熔过程存在电毛细振荡现象，并且交流电渣炉去硫和去夹杂效果优于直流正接和直流反接电渣炉。     

Numerical Study on the Effect of Low-Frequency Power Supply on Desulfurization in the Electroslag Remelting Process

In the electroslag remelting (ESR) process, low-frequency power supply can significantly reduce power consumption and achieve three-phase balance of power supply. Therefore, a transient coupling model of fluid flow, heat transfer, and component transport in the ESR process, which is coupled to the electromagnetic field calculated using Maxwell 3D software, is established to study the influence of low-frequency power supply on desulfurization. When a 50 Hz power supply is used, a skin effect is observed in the metal, and the direction of the Lorentz force at the slag/metal interface changes. However, this effect becomes less pronounced with decreasing current frequency. Sulfur is mainly transferred at the electrode tip, and the desulfurization rate is approximately 50%. Electrochemical reactions mainly occur at the electrode tip/slag interface and the metal pool/slag interface. The removal rate of sulfur using direct current (DC) power supply is less than that using an alternating current power supply. The DC reverse polarity power supply leads to higher desulfurization rate than DC straight polarity, which is 74% and 31%, respectively. The sulfur removal rate increases from 81.37% to 84.59% as the frequency decreases from 50 to 2 Hz because of the longer electrochemical reaction time at this lower frequency.     

电渣重熔渣系对GCr15轴承钢洁净度的影响

针对目前在电渣重熔(ESR)GCr15轴承钢D类夹杂物超标问题,设计不同渣系,并分别借助Factsage软件和经验公式计算了渣系的熔化特性、黏度、电导率等物性参数,采用实验室渣金平衡实验及现场2.5 t电渣重熔实验分析得出：最优ESR渣系为55CaF₂-25Al₂O₃-15CaO-5MgO。使用新渣系平均全氧含量较传统渣系降低41.98%,并且能够降低电渣锭不同位置的全氧含量;对于1～5μm夹杂物个数较原始渣系下降了31.25%。通过渣系对夹杂物调控,改善了GCr15轴承钢中D类夹杂物,评级级别可达到0.5级,能够更好地“净化”电渣锭。     

高速钢电渣锭夹渣缺陷的控制

通过对高速钢电渣锭夹渣缺陷机理分析,采取合理设定重熔工艺参数和重熔前进行捅渣操作,减少了结晶器挂渣现象,大幅度减少了电渣锭夹渣缺陷。     

The contribution of electrochemical reactions to sulfur transfer from electrode to slag in electroslag remelting

In order to assess the contribution of electrochemical reactions to refining reactions, such as sulfur removal, in electroslag remelting, electrochemical experiments which simulate conditions at the electrode/slag, and liquid metal pool/slag interfaces have been carried out. Liquid copper electrodes and a CaO-Al₂O₃-CaF₂ electrolyte were used. The experimental technique involved chronopotentiometry at constant current. Sulfur reversion from slag to metal is shown to be a diffusion controlled, reversible electrochemical process involving two electrons. On the other hand sulfur transfer from metal to slag occurs both by the diffusion controlled electrochemical reaction and the direct exchange reaction. A hanging drop electrode system was used to simulate conditions at the electrode/slag interface. Further experiments on the hanging drop electrode with and without applied current showed that direct reaction was a minor contribution to sulfur removal. The results are compared to findings from industrial practice.     

不同工艺参数下电渣重熔过程的数值模拟

为研究工艺参数对电渣重熔过程的影响,建立了能够考虑电磁现象并耦合动量和热量传输的三维瞬态数学模型,运用商业软件Fluent进行了模拟研究,计算值与测量值吻合较好。模拟结果表明:在极间距相同的2种工况下,监测点的电流密度值基本相同,说明当渣池深度变化时,可以通过调节电极浸入深度,以符合设定的操作制度。电势降主要发生在渣池区域,渣金界面处的电压仅为0.1V;焦耳热最大值出现在电极端角与渣池接触区域,最小值出现在渣金界面;电磁力最大值出现在电极边角附近,随着纵向深度增加,轴向分量越来越小。极间距为20mm时,电极端角与渣池接触区域的速度和温度均出现最大值,金属熔池深度也最大,其大小分别为0.045m/s,2 250K,41.5mm。