Numerical Investigation and Experimental Validation of Motion and Distribution of Nonmetallic Inclusions in Argon Protection Electroslag Remelting Process

Effect of Electromagnetic Frequency on the Flow Behavior in Mold during Bloom Casting

Considering solidification, a large eddy simulation (LES) model of two-phase flow was established to simulate the thermal–magnetic flow coupled fields inside a jumbo bloom. The magnetic field was calculated based on Maxwell’s equations, constitutive equations, and Ohm’s law. An enthalpy–porosity technique was used to model the solidification of the steel. The movement of the free surface was described by the volume of fluid (VOF) approach. With the effect of electromagnetic stirring (MEMS), the vortices in the bloom tended to be strip-like; large vortices mostly appeared in the injection zone, while small ones were found near the surface of the bloom. It is newly found that even though the submerged entry nozzle (SEN) is asymmetrical about the bloom, a biased flow can also be found under the effect of MEMS. The reason for this phenomenon is because the magnetic force is asymmetrical and transient. A high frequency will reduce the period of biased flow; however, the frequency should not be too high because it could also intensify meniscus fluctuations and thus entrap slag droplets in the mold. The velocity near the solidification front can also be increased with a higher frequency.

Numerical Simulation on Motion Behavior of Inclusions in the Lab-Scale Electroslag Remelting Process with a Vibrating Electrode

In order to meet the requirement of high-quality ingots, the vibrating electrode technique in the electroslag remelting (ESR) process has been proposed. Non-metallic inclusions in ingots may cause serious defects and deteriorate mechanical properties of final products. Moreover, the dimension, number and distribution of non-metallic inclusions should be strictly controlled during the ESR process in order to produce high-quality ingots. A transient 2-D coupled model is established to analyze the motion behavior of inclusions in the lab-scale ESR process with a vibrating electrode, especially under the influence of the vibration frequency, current, slag layer thickness, and filling ratio, as well as type and diameter of inclusions. Simulation model of inclusions motion behavior is established based on the Euler-Lagrange approach. The continuous phase including metal and slag, is calculated based on the volume of fluid (VOF) method, and the trajectory of inclusions is tracked with the discrete phase model (DPM). The vibrating electrode is simulated by the user-defined function and dynamic mesh. The results show that when the electrode vibration frequency is 0.25 Hz or 1 Hz, the inclusions will gather on one side of the slag layer. When it increases from 0.25 Hz to 1 Hz, the removal ratio of 10 μm and 50 μm inclusions increases by 5% and 4.1%, respectively. When the current increases from 1200 A to 1800 A, the flow following property of inclusions in the slag layer becomes worse. The removal ratio of inclusions reaches the maximum value of 92% with the current of 1500 A. The thickness of slag layer mainly affects the position of inclusions entering the liquid-metal pool. As the slag layer thickens, the inclusions removal ratio increases gradually from 82.73% to 85.91%. As the filling ratio increases, the flow following property of inclusions in the slag layer is enhanced. The removal ratio of 10 μm inclusions increases from 94.82% to 97%. However, for inclusions with a diameter of 50 μm, the maximum removal ratio is 96.04% with a filling ratio of 0.46. The distribution of 50 μm inclusions is significantly different, while the distribution of 10 μm inclusions is almost similar. Because of the influence of a vibrating electrode, 10 μm Al2O3 and MnO have a similar removal ratios of 81.33% and 82.81%, respectively.

Role of Electrode Rotation on Improvement of Metal Pool Profile in Electroslag Remelting Process

A comprehensive transient model is developed to study the effect of electrode rotation on the evolution of metal pool profiles and the solidification quality of ESR ingots. Magnetohydrodynamic flow, heat transfer, solidification, and electrode melting are considered simultaneously in the model. The growth of the ESR ingot is predicted using the dynamic layering method. The numerical results show that the productivity reaches a maximum of 15.97% at the rotating speed of 40 rpm without increasing power. With the increasing rotating speed, the maximum temperature of the melt decreases, and the temperature distribution becomes more uniform. Compared with the static one, the pool profiles are flattened by −1.19%, −8.52%, and 12.44% at the rotating speeds of 20, 40, and 60 rpm, respectively. The metal pool profile was improved remarkably, but only at the higher speed (i.e., 60 rpm). The effect of rotating speed on the metal pool profile depends on the competition between the melting rate and slag temperature. Meanwhile, the local solidification time and the secondary dendrite arm spacing are slightly improved at lower rotating speeds but are significantly worse at higher rotating speeds.

Large Eddy Simulation of Multi-Phase Flow and Slag Entrapment in Molds with Different Widths

Slag entrapment is a critical problem that affects the quality of steel. In this work, a three-dimensional model is established to simulate the slag entrapment phenomenon, mainly focusing on the slag entrapment phenomenon at the interface between slag and steel in molds with different widths. The large eddy simulation (LES) model and discrete particle model (DPM) are used to simulate the movements of bubbles. The interactions between phases involve two-way coupling. The accuracy of our mathematical model is validated by comparing slag–metal interface fluctuations with practical measurements. The results reveal that the average interface velocity and transverse velocity decrease as the mold width increases, however, they cannot represent the severity of slag entrapment at the interface between slag and steel. Due to the influence of bubble motion behavior, the maximum interface velocity increases with mold width and causes slag entrapment readily, which can reflect the severity of slag entrapment. On this basis, by monitoring the change of impact depths in different molds, a new dimensionless number “C” is found to reveal the severity of slag entrapment at the interface between slag and steel. The results show that the criterion number C increases with mold width, which is consistent with the results of flaw detection. Therefore, criterion number C can be used to reflect the severity of slag entrapment in different molds.

Numerical Simulation on Saffman Force Controlled Inclusions Removal during the ESR Process

Electroslag remelting (ESR) is an effective method for removing nonmetallic inclusions from steels or alloys. The main stage of inclusion removal during ESR is the aggregation of liquid metal film (LMF) to form droplets at the consumable electrode tip. In this study, a lab-level ESR experiment was carried out. The number and size of inclusions at the characteristic position of the electrode were quantitatively counted. The number of inclusions in the center position of LMF were larger than that in other regions. To elucidate these phenomena, a two-dimensional mathematical model was established to study the migration of inclusions in LMF. The results indicate that due to the large velocity gradient in LMF, the Saffman force is strong enough to offset the buoyant force and drag the inclusions toward the slag/LMF interface (SFI), where the inclusions will be dissolved in the SFI region by the molten slag. This study demonstrates that the Saffman force plays a key role in the removal of nonmetallic inclusions in LMF during the ESR process.

Titanium production by magnesium thermal reduction in the electroslag process

Titanium is widely used in specific applications due to its high strength, low density and good chemical stability. Despite it is one of the most abundant elements in the earth's crust, it is very expensive, because production of pure metallic titanium is very complex. Kroll process is the way how most of the titanium is produced nowadays. Shortages of this process are that it is batch process and it is very energy exhaustive, because titanium sponge material after reduction reaction needs complex post processing to isolate pure titanium. In this work we describe and experimentally investigate technology for Ti production from titanium tetrachloride using combined Kroll and electroslag process. Such process allows to achieve better reaction product separation by molten slag and process can potentially be continuous, thus technological process to produce metallic titanium can be significantly shortened.