Steel Slag Interface Research Articles

Multiphysics Numerical Simulation Model and Hydraulic Model Experiments in the Argon-Stirred Ladle

The argon-stirred ladle is a standard piece of steelmaking refining equipment. The molten steel quality will improve when a good argon-stirred process is applied. In this paper, a Multiphysics model that contained fluid flow, bubble transport, alloy transport, bubble heat flux, alloy heat flux, alloy melting, and an alloy concentration species transport model was established. The fluid model and bubble transport model that were used to calculate the fluid velocity were verified by the hydraulic model of the ladle that was combined with particle image velocimetry measurement results. The numerical simulation results of the temperature fields and steel–slag interface shape were verified by a ladle that contained 25 t of molten steel in a steel plant. The velocity difference between the hydraulic model and numerical model decreased when the CL (integral time-scale constant) increased from 0 to 0.3; then, the difference increased when the CL increased from 0.3 to 0.45. The results showed that a CL of 0.3 approached the experiment results more. The bubble heat flux model was examined by the industrial practice, and the temperature decrease rate was 0.0144 K/s. The simulation results of the temperature decrease rate increased when the initial bubble temperature decreased. When the initial bubble temperature was 800 °C, the numerical simulation results showed that the temperature decrease rate was 0.0147 K/s, and the initial bubble temperature set at 800 °C was more appropriate. The average melting time of the alloy was 12.49 s and 12.71 s, and the mixture time was approximately the same when the alloy was added to two slag eyes individually. The alloy concentration had fewer changes after the alloy was added in the ladle after 100 s.

Solid reactions between CaO–Al2O3 and Si–Ti-containing steel at 1273 K

Solid reactions between Si–Ti-containing steel and calcium aluminates with various CaO/Al2O3 ratios at 1273 K were studied. Particle precipitation zones (PPZs) were observed and analyzed for the composition and thickness. After heating at 1273 K for 10 h, 30 h, and 50 h, the composition of precipitated particles was Ca–Ti–O and TiOx in diffusion couples with a slag phase of CaO·Al2O3 (CA). The precipitates changed into mainly Ca–Si–O when the slag phase was 3CaO·Al2O3 (C3A) or 12CaO·7Al2O3 (C12A7). Elements of calcium and oxygen diffused from the slag phase to the steel phase were proved not to be from the decomposition of calcium aluminates, but ion diffusion under concentration gradients. The thickness of the PPZ increased from 1.5 μm to 14.8 μm with the increase of CaO/Al2O3 ratio of the slag phase and the heating time. The dissolved oxygen at the slag-steel interface calculated by the dynamic model demonstrated that the oxygen delivery capacity of calcium aluminates increased with their CaO/Al2O3 ratios. The evolution path of Ca–Ti–O to Ca–Si–O calculated by FactSage primely explained the precipitation variance of diffusion couples with various slag phases.

Reaction between molten steel and CaO–SiO2–MgO–Al2O3-FetO slag under varying amounts of converter carryover slag

Converter carryover slag during tapping has been a trouble for improvement of steel quality. In order to give an insight into the action mechanism of carryover slag on steel during tapping, a serious of molten steel-slag reaction experiments was conducted to elucidate the reaction mechanism between 3% Al steel and CaO–SiO2–MgO–Al2O3-FetO type slag at 1600 °C as well as its resulting effect on cleanliness under varying amount of converter carryover slag was investigated in laboratory. Composition evolutions in molten steel and slag during the reaction and microstructure on the steel-slag interface were investigated by employing different experimental techniques. 2[Al] + (Fe2O3) = 2[Fe] + (Al2O3) and 4[Al] + 3(SiO2) = 3[Si] + 2(Al2O3) were confirmed as a major reaction in the system. The initial (Fe2O3) content in experiments increased from 3% to 15%. The results indicated the metal-slag reaction tended to stable after about 10 min, the mass transfer of [Al] in molten steel was the rate-controlling step [Als] was more prone to react with (SiO2) for slags with initial (Fe2O3) less than 4.07%, but it reacted with both (Fe2O3) and (SiO2) when (Fe2O3) content was more. It was therefore suitable addition of Al alloy should take (SiO2) into consideration in addition to (Fe2O3) and dissolved [O]. Increase of carryover slag amount represented by increase of initial (Fe2O3) decreased the sulfur distribution but promoted the formation of MgO·Al2O3 inclusion. The sulfur distribution ratio decreased from roughly 5000 to 2000 but the number density of inclusion increased from 22.125/mm2 to 45.875/mm2 with increase of initial (Fe2O3) from 3% to 15%.

Numerical investigation of particle motion at the steel—slag interface in continuous casting using VOF method and dynamic overset grids

The capillary interactions are prominent for a micro-sized particle at the steel—slag interface. In this study, the dynamics of a spherical particle interacting with the steel—slag interface is numerically investigated using the volume of fluid method in combination with the overset grid technique to account for particle motion. The simulations have shown the particle’s separation process at the interface and successfully captured the formation and continuous evolution of a meniscus in the course of particle motion. A sensitivity analysis on the effect of different physical parameters in the steel—slag—particle system is also conducted. The result indicates that the wettability of particle with the slag phase is the main factor affecting particle separation behavior (trapped at the interface or fully separated into slag). Higher interfacial tension of fluid interface and smaller particle size can speed up the particle motion but have less effect on the equilibrium position for particle staying at the interface. In comparison, particle density shows a minor influence when the motion is dominated by the capillary effect. By taking account of the effect of meniscus and capillary forces on a particle, this study provides a more accurate simulation of particle motion in the vicinity of the steel—slag interface and enables further investigation of more complex situations.

Water modeling on slag entrainment in the slab continuous casting mold

A water model is built to investigate the transport phenomena in the slab continuous casting mold. The gas flow rate, casting speed, and slab width on the slag entrainment are studied. The sliver defect in the IF steel plate is analyzed by the scanning electron microscope. The slag entrainment is recorded by the high-speed camera and the velocity is measured by the PIV. The results show that sliver defects contain Al, Ca, Si, Mg, Na, and O, which are mainly derived from the mold slag entrainment. With the casting speed increase, the fluid velocity from the submerged entry nozzle rises clearly and the slag droplets detach from slag layer. As the gas flow rate increases, the liquid moves upward with bubble floating and attacks the slag-steel interface. It is suggested the gas flow rate should be less than 3.3 NL/min for the slab width of 1300 mm and the casting speed of 1.2 m/min. With the slab width increasing from 1300 mm to 1700 mm, no slag entrainment zone is reduced. The gas flow rate should be below 2.2 NL/min with the casting speed increasing to 1.4 m/min, both for the slab width of 1300 mm and 1700 mm.

Comparison of the Flow Field in a Slab Continuous Casting Mold between the Thicknesses of 180 mm and 250 mm by High Temperature Quantitative Measurement and Numerical Simulation

In the present work, the flow field in a slab continuous casting mold with thicknesses of 180 and 250 mm are compared using high temperature quantitative measurement and numerical simulation. The results of the numerical simulation are in agreement with those of the high temperature quantitative measurement, which verifies the accuracy and reliability of the numerical simulation. Under the same working conditions, the velocities near the mold surface with the thickness of 180 mm were slightly higher than those of the mold with the thickness of 250 mm. The flow pattern in the 180 mm thick mold maintains DRF more easily than that in 250 mm thick mold. The kinetic energy of the jet dissipates faster in the 250 mm thick mold than in the 180 mm mold. For double-roll flow (DRF), as the argon gas bubbles can be flushed into the deeper region under the influence of strong jets on both sides, the argon bubbles distribute widely in the mold. For single-roll flow (SRF), as the argon bubbles float up quickly after leaving the side holes, the bubble distribution is more concentrated in the width direction, which may cause violent interface fluctuation and slag entrainment. The fluctuation at the steel-slag interface in the mold with 180 mm thickness is greater than that in the mold with 250 mm thickness but less than 5 mm. The increase of mold thickness may lead to a decrease of the symmetry of the flow field in the thickness direction and uniformity of mold powder layer thickness. In summary, the steel throughput should be increased in the 250 mm thick mold compared with that in the 180 mm thick mold.

Effect of CaO/Al2O3 ratio on desulphurization and non-metallic inclusions in low-density steel

ABSTRACT To determine the effect of the CaO/Al2O3 ratio on desulphurization and inclusion evolution in low-density steel, a series of laboratory-scale experiments are performed. The results show that violent reaction occurs at the interface between refining slag and molten steel. As the slag-steel reaction progresses, (CaO) and (MgO) are reduced to [Ca] and [Mg] by dissolved [Al]. The deoxidation product Al2O3 dissolves into the slag and accumulates at the slag-steel interface. The evolution process of the sulphide inclusions is CaS → MgS → MnS. The sulphide outer layer wraps AlN and spinel inclusions, floats up and is absorbed by the top slag. During refining process, the accumulated Al2O3 at the slag-steel interface easily forms MgO·Al2O3. Finally, a dense spinel layer can be formed at the slag-steel interface, which inhibits the desulphurization reaction. Reducing the C/A ratio can reduce the formation of MgO·Al2O3 and has a positive effect on the desulphurization in low-density steel. The influence of the C/A ratio on desulphurization kinetics should be considered in the design of slag composition.

Cone Clogging of Submerged Entry Nozzle in Rare Earth Treated Ultra-Low Carbon Al-Killed Steel and Its Effect on the Flow Field and Vortex in the Mold

Two submerged entry nozzles (SENs) used for casting 1300 tons and 260 tons of Al-killed steel were dissected. Several parameters including block rate, nozzle clog angle, port width, and port height of the clogged nozzle were introduced to describe the geometry of clogs in the SENs based on the dissection; furthermore, a geometry model was established to describe the characteristics of the nozzle clogging of the SENs. A large-eddy simulation (LES) coupled with the volume of fraction (VOF) method was adopted to simulate the steel–slag interface’s interaction behavior. The vortex visualization and rotation magnitude were characterized by the Liutex method. Quantitatively, the influence of nozzle clogging resulted in block rates of 0% to 45.9% on the flow and vortex distribution in the mold, and the characteristics of the steel–slag interface fluctuation were well verified in the industrial experiment.

Effect of rare earth oxide on the crystallization behavior of CaO-Al2O3-based mold flux for rare earth heat-resistant steel continuous casting

New CaO-Al2O3-based mold flux for rare earth steel continuous casting was devised to restrain the slag-steel interface reaction. The effect of Ce2O3 on the crystallization behavior were investigated. The continuous cooling transformation tests show that the addition of Ce2O3 not only reduces the critical cooling rate of the mold flux but also reduces the initial crystallization temperature. Besides, the distribution of the initial crystallization temperature became more stable. The temperature time transformation tests show that the crystallization incubation time is slightly increased, the addition of Ce2O3 inhibit the isothermal crystallization process to a certain extent. The crystalline phases changed from LiAlO2+CaO to LiAlO2+CaCeAlO4. The precipitation of CaO was restrained by adding Ce2O3. With the combination of Ca2+, Ce3+, and [AlO69−]-octahedron units, CaCeAlO4 formed and precipitated in the crystallization process. Besides inhibiting the crystallization of CaO-Al2O3-based mold flux, adding rare earth oxide also can reduce the viscosity and the breaking temperature.

Effect of Exit Shape of Submerged Entry Nozzle on Flow Field and Slag Entrainment in Continuous Casting Mold

In steel continuous casting with submerged entry nozzles at a domestic steel plant, nozzle clogging was largely alleviated and defect rates of hot-rolled and cold-rolled sheets relating to slag entrainment were reduced for a nozzle with an approximately oval exit relative to a nozzle with a rectangular exit. Numerical and physical models were built to investigate the above phenomena. A turbulent model was applied in large eddy simulation to obtain the evolution of turbulent features within the nozzle and mold, while particle image velocimetry was adopted to monitor fluid flow beneath the slag–steel interface. Main results were that the flow speed of the main stream from the approximately oval exit was higher than that from the rectangular exit, while the zone of a velocity magnitude below 0.2 m/s within the approximately oval exit was smaller than that within the rectangular exit, indicating that inclusions tended to reside for longer in the latter. At the same casting speed, that slag entrainment occurred more frequently for the nozzle with a rectangular exit was found to have no direct relationship with the velocity magnitude and vorticity at 1/4 mold width on the top surface. Meanwhile, slag entrainment always occurred at the moment that vorticity changed most rapidly. The current study thus provided new insight into the structural parameter optimization of the submerged entry nozzle.

Kinetic Modeling of Nonmetallic Inclusions Behavior in Molten Steel: A Review

The kinetic modeling for the nucleation, size growth, and compositional evolution of nonmetallic inclusions in steel was extensively reviewed in the present article. The nucleation and initial growth of inclusion in molten steel during deoxidation as well as the collision growth, motion, removal, and entrapment of inclusions in the molten steel in continuous casting (CC) tundish and strand were discussed. Moreover, the recent studies on the prediction of inclusion composition in CC semiproducts were introduced. Since the 1990s, the development of thermodynamic model and relevant databases for inclusion engineering has been initiated by the steel industry. Later, the commercial software FACTSAGE employing the FACT database was widely used to predict the gas (atmosphere/bubble)–liquid (steel/slag/inclusion)–solid (refractory/slag/steel/inclusion) multiphase equilibria. With the help of the comprehensive thermodynamic database and solution models in conjunction with the development of user-friendly computing packages, the kinetics of inclusion evolution in molten steel can be successfully predicted based on several kinetic models such as the coupled reaction (CR) model, reaction zone model, and tank series recirculation (TSR) model. However, some parameters are needed to represent the real processes according to the model employed at different operational or experimental conditions. The effect of reoxidation on the evolution of inclusions in the ladle and tundish, which was experimentally confirmed, can be simulated by the effective equilibrium reaction zone (EERZ) model. The complex slag–steel interfacial reaction phenomena have been successfully explained by the interfacial kinetic model based on the dynamic interfacial tension and oxygen adsorption/desorption characteristics at the slag-steel interface.

Numerical Simulation on Flow Characteristic of Molten Steel in the Mold with Freestanding Adjustable Combination Electromagnetic Brake

The general flow characteristics of the molten steel and the behavior of molten steel and liquid slag interface (level fluctuation) in the mold with a new type of electromagnetic brake, freestanding adjustable combination electromagnetic brake (FAC-EMBr), was investigated by numerical simulation. The influences of the magnetic induction intensity, the submerged entry nozzle (SEN) immersion depth, the SEN port angle, and casting speed on the effect of FAC-EMBr are investigated. The results show that electromagnetic field formed by FAC-EMBr can cover the jet flow impact region, the upward backflow region, and the meniscus region simultaneously. With the increase of magnetic flux density, the jet flow impact intensity and the molten steel velocity in the upward and downward backflow regions are effectively suppressed, the meniscus wave height also decreases gradually, and the level fluctuation tends to be stable. For the influence of SEN port angle and SEN depth on the effect of FAC-EMBr mold, the results show that the molten steel velocity in the three main influence regions are all effectively depressed with increase of the value of SEN port angle and SEN depth. Even though under the condition of high casting speed (VC = 2.2 m/min), the application of FAC-EMBr can achieve effective braking effect.

Effect of Turbulence Inhibitors on Molten Steel Flow in 66-Ton T-Type Tundish with Large Impact Area

A multiphase numerical simulation of the steel-slag flow was established by using the volume of fluid (VOF) model to study the effect of different turbulence inhibitors on the improvement of the steel-slag flow in the tundish. The steel-slag interface fluctuation was studied by vorticity magnitude and transient fluctuation change. A prediction model of residence time distribution (RTD) curve was established based on mathematical simulation and the error of prediction model can be controlled below 6% by comparing with the hydraulic results. The results show that jet flow into the tundish generated very different flow patterns. Case 1 produced a double-roll flow pattern and case 2 produced a four-roll flow pattern in the impact area. The ratio of vorticity magnitude above 1.00 s−1 near the ladle shroud was 2.60% in case 1 and the ratio of vorticity magnitude above 1.00 s−1 near the ladle shroud was 13.15% in case 2, which indicates case 2 increased the possibility of slag entrainment via the upward flow mechanism and shear layer instability. Surface velocity fluctuations in case 2 were much more severe near the ladle shroud. The thickness of the slag layer was 60 mm, the interface fluctuation towards surface in case 2 was close to 20 mm. Meanwhile, case 1 involved very small volume-fraction contours near interface. The turbulence inhibitor with internal ripples (case 1) showed a better optimization effect and the results could provide a theoretical basis for the selection of a suitable turbulence inhibitor for the 66-ton T-type tundish.

Investigation of rheological behavior for commercial mold slags

Rheological behaviors of several commercial mold slags, which were designed for continuous casting of high-carbon steel, peritectic steel, and high-Al steel were studied using rotational viscometer with various rotating speeds. The crystallization rates, which were correlated with rheological behaviors, were detected by in-situ observation. The effect of steel-slag interface reaction between high-Al steel and CaO–Al2O3-based slag on the non-Newtonian fluid behaviors was also discussed. The main results demonstrated that these mold slags all belonged to pseudoplastic fluid and exhibited shear thinning characteristics. The index of flow characteristics in high temperature zone was less than 1 and fluctuated around 0.9, while in low temperature zone, n rapidly decreased from 0.9 to 0.3. The results show that the low temperature zone is more obvious than the high temperature zone for the shear thinning characteristics of the mold fluxes below the break temperature. The shear thinning was closely related to the crystallization rate and grain size of crystals precipitated within melts. Steel-slag interface reaction decreased the content of glass formers, such as SiO2 and B2O3. The crystallization capacity was enhanced, followed by a reduction of lubrication ability. It is found non-Newtonian behaviors of commercial mold slag are closely associated with their crystallization capacity, and the discussion range of viscosity temperature is more in line with the actual temperature drop range of molten flux during continuous casting, which is of great significance to understand the mechanism of shear thinning and optimize the comprehensive performances of commercial mold slags.

A Simulation and Optimization Study of the Swirling Nozzle for Eccentric Flow Fields of Round Molds

In continuous casting, the nozzle position may deviate from the center under actual operating conditions, which may cause periodic fluctuation of the steel-slag interface and easily lead to slag entrapment and gas absorption. Swirling nozzles can reduce these negative effects. A mathematical simulation method based on a round mold of steel components with a 600 mm diameter is applied to study the flow field of molten steel in a mold. The swirling nozzle is optimized through the establishment of a fluid dynamics model. Meanwhile, a 1:2 hydraulic model is established for validation experiments. The results show that, when the submerged entry nozzle (SEN) is eccentric in the mold, it results in serious bias flow, increasing the drift index in the mold up to 0.46 at the eccentric distance of 50 mm. The impact depth of liquid steel and turbulent kinetic energy can be decreased by increasing the rotation angle of the nozzle. The nozzle with one bottom hole, which significantly decreases the bottom pressure and turbulent kinetic energy, greatly weakens the scour on nozzle and surface fluctuation. In the eccentric casting condition, using the optimized swirling nozzle that employs a 5-fractional structure, in which the rotation angle of 4 side holes is 30° and there is one bottom outlet, can effectively restrain bias flow and reduce the drift index to 0.28, a decline of more than 39%.

Effect of Interfacial Reaction between CaO–BaO–Al2O3‐Based Mold Fluxes and High‐Mn–High‐Al Steels on Fundamental Properties and Lubrication of Mold Flux

Contact experiments between low‐reactivity CaO–BaO–Al2O3‐based mold fluxes, F1 and F2, and 1.4–2.2 mass% of Al‐containing steels are conducted to investigate the effect of interfacial reaction on mold flux properties and the ultimate limit of reaction duration capacity of the developed mold flux. F1 contains 30.76% CaO, 17.03% Al2O3, 13.40% BaO, and 10.70% SiO2, whereas F2 consists of 27.90% CaO, 23.60% Al2O3, 12.90% BaO, and 9.57% SiO2 of the mass. The pre‐ and post‐experimental wettability of slag samples on steel substrates and the lubrication behavior of slags are discussed. The main results reveal that SiO2, Na2O, and B2O3 in the mold flux react with [Al] in the molten steel at steel–slag interface. The variations in slag properties indicate that both reaction durations between F1 reacting with twinning‐induced plasticity (TWIP) and F2 reacting with 20Mn23AlV steel can last for 40 min, whereas F2 has wider potential applicability for the continuous casting of Al‐containing steels. Interfacial reaction deteriorates the wettability of slag F2 on 20Mn23AlV steel substrate and reduces slag consumption, as equilibrium contact angle increases from approximately 3° to 37°, but slag consumption prediction is 0.22 kg m−2, which is likely to meet the requirement for lubrication.

Numerical Investigations on Bubble Behavior at a Steel–Slag Interface

Herein, the volume of fluid (VOF) model is applied to simulate the phenomenon caused by the bubble motion from the molten metal into a slag. First, the mathematical model is validated by the experimental data from the literature. Then, the model is used to quantitatively predict the process of a single argon bubble passage from the liquid steel into the liquid slag. Finally, a parametric study is conducted to determine how the bubble diameter and the interfacial tension influence the bubble movement. The simulation results show that the bubble passing patterns at the steel–slag interface are oscillation‐pass, oscillations‐breakup, oscillations‐pass, pass, and pass‐breakup for the 3, 5, 7, 10, and 15 (20) mm bubbles, respectively. For a 5 mm bubble, it is found that an increase in the interfacial tension from 0.04 to 0.8 N m−1 results in a delayed bubble passage time. The results also show that the bubble experiences an oscillations‐breakup process if the interfacial tension value is up to 1.15 N m−1. However, a higher interfacial tension value (1.8 N m−1) can make the bubble pass again but with a longer passage time.

Simulation research on oxygen mass transfer between steel and slag in IF steel refining process

The effects of oxidizing slag on oxygen mass transfer and inclusions in different stages of IF (interstitial-free) steel refining were investigated by several heat simulation experiments. The results of the experiments showed that the oxidizability of slag changed considerably during different refining stages. Keeping the content of FeO in the slag within 1 wt.% would narrow the difference of slag oxidizability, stabilize the content of [Al]s in the steel, avoid secondary oxidation of molten steel by the slag, and reduce the inclusions. When the mass transfer of FeO in the slag phase was a limiting step, the secondary oxidation reaction occurred at the steel–slag interface; when the diffusion of oxygen in the molten steel was a limiting step, the secondary oxidation reaction took place inside the molten steel. The oxygen transfer rate was affected by the mass transfer coefficient of oxygen. For every 0.0001 m/s increase in mass transfer coefficient, the oxygen transfer rate increased by about 2.2 × 10−6 min−1. By changing the mass transfer coefficient, the oxygen transfer rate of the slag to the molten steel can be controlled.

Calculation of Static Suspension Depth and Meniscus Shape of a Solid Spherical Inclusion at the Steel–Slag Interface

A method to calculate the meniscus shape around an inclusion on the steel–slag interface is presented. The meniscus shape is related to the properties of steel and slag and the size of the inclusion. The large-size inclusions have a large suspension depth and interaction range. At the small-size range of inclusions, the suspension depth increases slowly with the size, while the interaction range is sensitive to the size.

Simulation for Mass Transfer Kinetics at Slag-Steel Interface during High Al Steel Continuous Casting

Simulation for Mass Transfer Kinetics at Slag-Steel Interface during High Al Steel Continuous Casting