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Reduction in skull loss in billet caster tundish through water modelling studies
Abstract
During continuous casting, some amount of metal is left in the tundish at the end of the casting sequence to avoid slag entrapment into continuous casting mould. This tundish loss often called as skull loss. It reduces the caster yield and is directly related to the size of the tundish. JSW Steel operates with 44 T, 8 strand billet caster tundish which had ~7T tundish loss under previous operational practice. Water modelling studies were carried out using a 0.25 scale model to design a new tundish bottom to reduce tundish skull loss. Three different tundish bottom configurations were studied and the corresponding effect on tundish flow profile and vortex formation was also investigated. Based on water modelling results, the optimized tundish design was used for plant trials. A 47% reduction in skull loss was obtained with no adverse effect on steel cleanliness from different strands.
... In view of the high temperature characteristics of continuous casting tundish, the optimization of its structure is mainly carried out by water modelling [12][13][14] and numerical simulation [15][16][17]. For example, Tang et al. [18] optimized a 4-strand tundish in a steel mill through the above method. ...
Cracking con-rod is an advanced high-precision connecting structure based on brittle expansion, breaking and reconnection of steel, to solve the problem of assembly circle missing. High carbon micro-alloyed steel C70S6, as a dominant material for the production of cracking con-rod, has extremely strict requirements on non-metallic inclusions in steel and microstructure stability. Continuous casting tundish plays an important role in removing large-sized inclusions and stabilizing casting quality. Aiming at the inconsistent casting quality of C70S6 steel produced by a three-strand asymmetric tundish and the frequent occurrence of slag entrapment problems in Xining Special Steel, the tundish structure was optimized by means of physical modelling combined with numerical simulation, and the quality of the bloom castings and subsequent hot-rolled products before and after optimization were compared based on volume production. The results show that a new flow control design to the tundish can effectively improve the consistency of its metallurgical effect for each of the three strands and the following overall product quality, in which the flow field and temperature field in the tundish are more uniform. This is due to the adoption of a vortex inhibitor and an optimized wall structure according to the measured RTD curve, ink trajectory and numerical simulation on the 3-D streamline contours and temperature distribution in the tundish. The peak concentration of outlet 1 is decreased from 6.5 before optimization to less than 2.0 after optimization, which means the elimination or alleviation of the local short-circuit flow. The maximum temperature difference of C70S6 molten steel measured at the outlets of the tundish three strands is decreased from 2–5 °C to 1–3 °C, which is in good agreement with the numerical simulation results. The difference in columnar crystal ratio of the corresponding bloom castings is decreased from 2.27–3.17% to 1.26–1.85%, and the consistency of central carbon segregation index is also significantly improved. In addition, the difference in oxygen content among the three strand blooms is decreased from 1.7–3.5 ppm to 0.8–1.9 ppm. As a result, the overall mechanical properties and microstructure stability of the hot-rolled products are improved statistically, in which the hardness fluctuation is decreased from 84 HBW to 60 HBW, the inclusion grade of types B + C + D + Ds is reduced to 1.105, and the occurrence rate of Ds dropped to 0.118%. Accordingly, the failure rate of the cracking con-rod is controlled stably within 4‰, and the fracture is generally smoother than that before tundish optimization. In summary, the flow field optimization to a multi-strand asymmetric tundish has a clear effect on improving the overall quality of its bloom castings and rolled products, which should be paid more attention industrially. Meanwhile, the present study provides a reliable theoretical and experimental reference for the improvement of metallurgical effects of an asymmetric-typed tundish commonly used in special steel production.
The free-surface vortex at the end of tundish casting sequence induces slag carryover into the mold, which can induce reduction in metal yield, deterioration in billet quality, and disruption to production. In this paper, the results of multiphase flow behavior in both tundish and mold during the formation process of free-surface vortex calculated by large eddy simulation (LES) model the RNG k-ɛ model were firstly compared, then the effect of rotating speed of stopper-rod on vortex and mold was numerically investigated based on the LES model. The results showed that the LES model offers a more precise simulation of multiphase flow, providing a better representation of tundish swirl evolution, fluctuations at the steel/slag interface, and slag phase motion. When the rotating speed of stopper-rod is 60 r/min, the fluctuation of mold flux is minimized, with a maximum surface velocity of 0.12 m/s, a slag carryover velocity of 0.122 kg/s at the moment of vortex penetration, and the amount of residual steel is reduced by about 600 kg, the critical height of the 1st and 2nd strands is reduced by 13.5 mm and 15.5 mm, respectively, which performs clear advantage over the schemes of 0, 30, and 90 r/min. The present study can offer theoretical guidance on suppressing free-surface vortex in tundish on the premise of improving the multiphase flow in mold.
In this paper, the flow characteristics and behavior of liquid steel in a six-strand tundish under unsteady casting state are studied by means of the combination of numerical and physical simulation. The results show that there is no obvious change in the consistency of the flow response of each flow medium in the unsteady casting state, but the flow characteristics and behavior of the liquid steel in the tundish will be greatly affected. At the same time, the fluctuation of the liquid level is enhanced, and the wave height difference is inconsistent at different positions affected by the change of flow state, among which the fluctuation is the strongest at position 5. Compared with the normal casting state, the blocking flow can change the original flow state and path. The dead zone volume fraction is increased by 1–1.5 times, the average residence time of liquid steel is increased, and the removal rate of inclusions is increased by 2.22%, 2.65% and 3.40%, respectively. The flow behavior of liquid steel in the tundish changes when the casting speed is changed. When the casting speed of outlet1 is increased, the removal rate of inclusions decreased by 1.14% on average, and the dead zone volume fraction decreased by 4.3% under the physical simulation results. When the outlet3 casting speed is reduced or the casting speed of each strand is reduced, the removal rate of fine inclusions is improved, the removal rate of inclusions increases by 2.39% and 11.42% respectively, but the dead zone volume fraction increases and the overall fluidity of liquid steel becomes worse. In addition, variable casting speed increases the instability of steel surface, and the liquid surface flow rate is basically proportional to the casting speed. When the liquid surface velocity is greater than 0.1 m/s, the protection slag has the risk of being blown open and exposed. It often occurs near the inlet of ladle nozzle. At the same time, the fluid dead zone critical velocity of 0.008 m/s defined in this study can be used for velocity calibration of fluid dead zone, and its accuracy is mutually verified by numerical and physical simulations.
The flow state of the melt inside the continuous casting tundish plays a decisive role in the realisation of metallurgical function. With the application of tundish induction heating, much attention has been paid to the influence of the internal non-isothermal state caused by supplementary heating on the flow field. In this study, hydraulic and numerical simulations combined with particle image velocimetry are used to quantitatively study the internal flow field under channel-type tundish with induction heating. Simultaneously, the effect of induction heating on the flow state of the flow field is investigated. The results show that the internal heating of the tundish channel not only enhances the range and intensity of turbulence distribution, but also significantly improves the flow state of the flow field. By comparing the velocity distribution of the tundish liquid surface measured by hydraulic simulation, it is found that the average velocity under non-isothermal conditions is 0.066 m/s, which is 47% higher than that under isothermal conditions. Finally, the standard deviation of peak time and actual residence time for the residence time distribution curve of tundish changes from 278 and 217 s to 202 and 148 s, respectively, before and after supplementing channel heating, which shows that it can indeed improve the consistency of each flow.
In recent years, there has been significant development in the large-scale utilisation of round bloom continuous casting, which has led to an increase in the strand spacing of the tundish. However, this increase often accompanies issues such as poor consistency among each strand. This study focuses on the three-strand asymmetric tundish used in super-large round bloom continuous casting and uses a combination of numerical simulation and physical experiment to investigate the impact of different dam and retaining wall structures on the flow, temperature and removal of inclusions in the super-large round bloom tundish. The reliability of the model is verified through isothermal Water model experiments. The results indicate that by employing an optimised flow control structure with a U-shaped retaining wall featuring small diversion holes and a dam near each of strands No.1 and No.3, several improvements are achieved compared to the prototype tundish. The dead zone ratio of the tundish is reduced by 16.33%, the standard deviation of average residence time decreases by 167.07 s, the volume of the tundish's low temperature zone decreases by 7.79% and the total inclusion removal ratio increases by 14.44%. By appropriately incorporating dams, reducing the area of diversion holes and modifying the retaining wall structure in the tundish, the flow, temperature and inclusion removal consistency of each strand can be effectively improved. This enhances the overall metallurgical efficiency of the tundish. Even when encountering strand blocking operation, the dead zone ratio can be further reduced to 22.44% using the optimised structure proposed in this study. This demonstrates that the optimised structure not only improves the steady-state metallurgical behaviour of the asymmetric three-strand tundish with large strand spacing for super-large round bloom but is also suitable for addressing unsteady-state metallurgical behaviour caused by on-site working conditions, such as production scheduling or high casting speed. By implementing the findings of this study, it is expected that the efficiency and effectiveness of the super-large round bloom continuous casting process will be significantly enhanced.
Physical and numerical modelling methods were followed to identify mechanism(s) for tundish slag entrainment in a bare tundish and one with a flow control device (FCD). The physical and numerical models made use of water and paraffin to model steel and slag respectively. Observations from the physical model showed that the steel-slag interface remains immobile in both cases. Entrained paraffin formed droplets approximately 1 mm in diameter. Results from both models (numerical and physical) showed that in both cases (bare and FCD case), areas of high entrained slag concentration exist near the inlet region. The entrained slag concentration decreases towards the tundish endwalls. Flow patterns and velocities tangential to the steel-slag interface from the numerical model showed that slag entrainment in both the bare tundish and tundish with a FCD possibly takes place via two mechanisms. First, the slag moves across the steel-slag interface via mass transfer; secondly small velocities tangential to the interface at depths greater than 10 mm below the interface carry the already 'entrained' slag into the bulk steel phase. These tangential flow patterns are dominant in the inlet region, hence the high concentration of entrained slag in this region.
The present study focuses on the fluid dynamics of vortex formation in tundish operations using a physical water analogue model and Particle Image Velocimetry (PIV) measurements. The aim is to get fundamental knowledge of the nature of vortex formation that clearly represents this flow phenomenon. The results indicate that vortex formation is directly related to the highly turbulent flows present in the tundish. Great changes of velocity fields during short times and large distances between the upper edge of the dam and the free surface of the bath enhance the formation of vortex flows. Therefore, increasing tundish capacity does not guarantee steel cleanliness through long residence times if the dam height is not adjusted to the new level of liquid in the tundish.
A mathematical model was developed using ANSYS 12.0 in order to simulate non-isothermal melt flows in a delta shaped four strand billet caster tundish. The fluid inside the tundish considered was water, so that the CFD model could later be validated against water model experiments. The buoyancy term was included in the momentum equation using Boussinesq's approximation. Experiments were performed for both the bare tundish, and the tundish fitted with an impact pad. For the bare tundish, step inputs of 5-15 degrees C hotter fluid resulted in significantly stronger natural convection currents towards the extremities of the tundish. On the other hand, for cases of a tundish fitted with an impact pad, the effect of buoyancy driven flows due to step inputs of hot water, was much less pronounced since the pad itself had a big effect on regulating the fluid flow patterns. Step-down conditions were also simulated, where 10-15 degrees cooler fluid was introduced into a hotter liquid within the tundish. Detailed calculations were performed using the DPM, in order to evaluate the number of inclusions passing through the SENs under such transient conditions. While the step-up conditions facilitated the flotation of inclusions because of upward buoyancy driven flows, the step-down condition generated catastrophic results in terms of liquid metal quality. One third and full scale water model experiments were done to validate the numerical model and it was found that the mathematical predictions were in good agreement with the experimental results.
To develop an effective technology to prevent vortex forming in ladles, the mechanism underlying vortex formation during steel teeming must be studied. For simulations using the numerical simulation software Fluent, a geometric model of the ladle was generated, and initially water teeming in the ladle was simulated for different initial tangential velocities. The results obtained help to verify the validity of the numerical computations. Similar simulation conducted for steel showed that the tangential velocity increases from the liquid level to the bottom of the ladle, and the flow at the bottom is related to vortex formation. The inference is that vortex formation begins from the ladle bottom. Causes for vortex formation during teeming are discussed to help lay a theoretical basis for ladle design in preventing vortex formation during steel teeming.
Slag entrainment during steel teeming-drain operations from a steel ladle impacts negatively steel cleanliness and quality. In the present work water modeling and mathematical simulations using a multiphase model for momentum and heat transfer were employed to understand the mechanisms of vortex funnel drain and sink drain flows. The critical bath height for vortex development increases with steel throughput and valve gate opening. Six stages during vortex development are identified, passing from a dimple formation on the bath surface until sink drain which begins when the bath level has the same magnitude as the nozzle diameter. This later drain pattern is independent from any other teeming-draining variable being only a function of the bath height and nozzle diameter; when they are about the same the metal-interface collapses. The temperature gradients originated by the heat losses of the ladle to the surroundings provide buoyancy forces that are large enough to influence liquid motion in the ladle. At large bath levels steel observes long-vertical recirculating flows and at low bath levels these flows change to horizontal-circular recirculating flows that become a seed for later vortex development. These buoyancy forces increase the critical height for vortex development and slag entrainment.
A mathematical model was developed to study effects of the vortex and the short circuit flow phenomena during tundish operation on the inclusion removal for a wide range of particle diameters. The model was solved using FLUENT1commercial software; for the particle tracking the Lagrangian discrete phase model was employed. Even when the vortexes drag about 50% of the inclusions, the most detrimental phenomenon for inclusion removal is the short circuit flow. It is important to stress that for the global inclusion behavior the removal rate decreases as the tundish level increases. It can be concluded that when increasing the tundish capacity without any flow control device adjustment, the steel cleanliness is deteriorated significantly.
Despite significant work done till date on single/dual strand tundish (those applied to slab casting) not much information on an equivalent multi-strand tundish (which are of significance to billet casting) is available regarding residence time distribution (RTD) measurements and its analysis. The proportions of various flow volumes estimation (e.g., dead, plug, well mixed, etc.) on the basis of a single/dual strand tundish cannot be directly extrapolated to a multi-strand tundish from its individual strand data only. In the present study, therefore, the principle and the methodology underlying the derivation for concentration vs. time curves (C curves) of a multi-strand tundish is addressed to provide a basis for the evaluation of RTD parameters. To this end, several experiments in a four-strand water model tundish were carried out for RTD measurements and estimation of associated flow volumes. The tracer dispersion experiments were performed by injecting potassium chloride (20 mL solution) as a pulse into the submerged inlet stream and the variation of conductivity of water caused by the differential mixing of the injected tracer into the bath were recorded continuously at the exits of the water model tundish with a 75 L volume. These conductivity values of individual strands were converted into corresponding concentrations, and the dimensionless concentrations were plotted against dimensionless time to derive the characteristic C curves. The individual C curves effect the strand to strand variations whereas the overall C curve (average of individual C curves) is used to determine the flow volumes in a multi-strand tundish system.
The Reynolds-averaged Navier-Stokes equations have been numerically solved to obtain a steady, three-dimensional velocity field inside the six-strand billet caster tundish using the standard k-epsilon model of turbulence. These steady flow fields so obtained are then used to predict the removal of inclusions from molten steel by solving the inclusion transport equation with the help of a commercial CFD software Fluent 6.1.22. The effects of height and position of dams in the tundish and size of the inclusion particles on percentage inclusion removal were studied. To simulate the chaotic effect of the turbulent eddies on the particle paths; a discrete random walk model was applied during inclusion trajectory calculations. The computational model was first validated with the results reported in the literature a good match was found between the two. It was found that with the increase in the height of the dams, the inclusion removal tendency increased whereas the shifting of the dams towards the outlets decreases the inclusion removal. An increase in the inclusion removal was found with an increase in the inclusion particle size.
The introduction of the appropriate size and precise location of flow control devices such as dam, turbulence inhibitor, etc., helps to modify the flow pattern and minimizing short circuiting and dead zone. Beside this, these also create the surface directed flow and maximize the residence time available for the flotation of inclusions and assimilation of the reaction products from the molten steel into the slag phase. These can be products of deoxidation, reoxidation, precipitation, emulsification and/or entrainment of refractory components into the melt and thus encompass both indigenous and exogenous inclusions. To this end, both the numerical and physical simulations were carried out mainly for three cases: a) in absence of flow control devices (i.e., bare tundish), b) in the presence of a dam, and c) with the application of turbulence inhibiting device (TID) and dam combination in the existing tundish configuration. The commercial CFD (computational fluid dynamics) package FLUENT (R) was used to predict the flow field prevalent in the water model tundish at steady state, whereas in the experimental program, both Particle Image Velocimetry (PIV) techniques for flow measurements and tracer dispersion experiments for concentration measurements were applied in the present study. Among all types of configurations applied in the present study, a combination of TID with holes+a dam work reasonably found to be an optimum configuration of the four-strand tundish regarding inclusion floatation. A superior strand similarity is also achieved in this configuration. Also the predicted time averaged horizontal and vertical components agreed within +/- 10% with the experimentally derived ones.
In the present work a numerical study was carried out to investigate the inclusion behav-iour in a four strand asymmetric billet caster tundish. A parameter called separation effi-ciency was used to compare inclusion behaviour quantitatively. An open source CFD code called OpenFOAM was used to model the inclusions through Lagrangian Particle Tracking approach. First, the Lagrangian particle class was defined in the existing simpleFoam solver of the OpenFOAM and the same customized solver was used to track particles in the already obtained velocity field. Inclusions were modelled as spheres and various forces act-ing were also considered. Present numerical results were validated using available exper-imental results and found to be in good agreement. Further, such inclusion behaviour was also extended for a real industrial case tundish. Inclusion flotation characteristics in ther-mally induced flow was analyzed in detail. This is, perhaps, for the first time such an exhaustive study on inclusion analysis being reported considering the forces acting on the inclusions particles and with OpenFOAM.
Physical modelling using water in a one-third scale model was carried out to ascertain the influence of various types of baffles with inclined holes on the liquid flow in a six strand round bloom continuous casting tundish. To characterise the flow in the tundish, residence time distribution (RTD) curves were measured for different types of baffles with inclined holes. Because there is no well known analysis model to characterise the melt flow in multistrand tundishes, a new model was presented to analyse RTD curves and its reasonability was discussed. Furthermore, a new approach for quantifying the similarity among the strands was proposed and the baffle was optimised to improve the inclusion floatation and strand similarity in the tundish.
Mixing phenomena in a six strand billet caster tundish has been studied by numerically solving the Navier Stokes equations along with the species concentration equation in a boundary fitted coordinate system comprising the geometry of the tundish. The solution of the species concentration equation has been utilized to compute the mix, dead and plug volume of the tundish under different flow conditions. The numerical procedure and solution algorithm has been first verified by comparing the numerically obtained residence time distribution curve, which agree well with that of the experiments done for a single strand bare tundish by Singh and Koria.44) It has been observed that the ratio of the mix to dead volume for the six strand tundish has a maximum value for a particular position of the outlets. At that particular position of the outlets (where mixing is best), an APB is placed on the bottom of the tundish surrounding the incoming inlet jet and the height of the APB has been varied to see the effect on mixing in the tundish. It has been observed that the ratio of mix to dead volume further increases with the use of APB and attains a peak value after which it decreases with the increase of the height of the APB signifying the existence of an optimum APB height. At this optimum height of the APB, the shroud immersion depth was made to change from 0 to 400 mm. It was also observed that there exists an optimum immersion depth of the shroud where the ratio of mix to dead volume still attains another peak signifying still better mixing. However, increasing the immersion depth to higher values spoils mixing significantly.
At the late stage of continuous casting (CC) ladle teeming, sink vortex can suck the liquid slag into tundish, and cause negative influences on the cleanliness of molten steel. To address this issue, a two-phase fluid mechanical modeling method for ladle teeming was proposed. Firstly, a dynamic model for vortex suction process was built, and the profiles of vortex flow field were acquired. Then, based on the level set method (LSM), a two-phase 3D interface coupling model for slag entrapment was built. Finally, in combination with high-order essentially non-oscillatory (ENO) and total variation diminishing (TVD) methods, a LSM-based numerical solution method was proposed to obtain the 3D coupling evolution regularities in vortex suction process. Numerical results show that the vortex with higher kinetic energy can form an expanded sandglass-shape region with larger slag fraction and lower rotating velocity; there is a pressure oscillation phenomenon at the vortex penetration state, which is caused by the energy shock of two-phase vortex penetration coupling.
During tapping of steel from converters and ladles, vortices may develop which take down in their core undesired slag. Formation of vortices has been studied with models involving water and mercury. Countermeasures, e. g. floating objects, flow obstacles, modified outlets, and electromagnetically induced flow were tested. Gas injection was investigated on a paraffin-water model and in plant operation on a BOF unit.
The effect of a stopper rod on the formation of a vortex and an air core in the nozzle has been investigated with cold models using water and mercury as liquids. It was found that in the presence of a stopper rod the air core develops from below, whereas it forms from the surface in the absence of a stopper rod. Another effect of the stopper rod is surface oscillations. In this case the vortex may become detached from the stopper rod and spin around it. The frequency of the wave can be predicted from wave theory. The results also draw attention to other surface wave phenomena in metallurgy.
To assess and quantify the relative importance of Reynolds and Froude numbers in reduced scale model studies (these cannot be simultaneously respected when the scale factor is less than unity), aqueous model investigations were carried out on three different laboratory scale tundish models. The experimental tundish systems included two strand, five strand skewed delta shaped and six strand rectangular shaped vessels. Experimental observations show that the depth of liquid in the model would only correspond to that in the full scale system, provided the model flow rate is scaled in accordance with the relationship: Qm = λ5/2 Qf.s, in which, λ is the geometrical scaling factor. Furthermore, on the basis of residence time distribution measurements in two different configurations of the five strand tundish, it was demonstrated explicitly that flow phenomena in tundish systems are largely dominated by inertial forces and are therefore, essentially Froude dominated.
To suppress slag entrapment by vortexes during steel teeming, and to improve steel cleanliness, key factors affecting free surface vortex formation have been analyzed in this study. It was found that Coriolis forces have little effect on vortex formation. The initial tangential disturbance is the main factor for vortex formation. And when the nozzle position is central or eccentric, the effects of initial tangential velocity, nozzle diameter on the critical height are different. Physical properties of liquid steel have a small effect on the critical height. A formula for calculating the height was discussed.
Mechanism of slag carry-over during drainage of metallurgical vessel is studied in a physical model. Vortex and drain sink formation are found to be the main mechanism of carry-over of slag to underlying vessel. Empirical equations are obtained to determine vortex and drain sink height under a wide range of draining conditions. Macroscopic mass and energy balance are used to determine vortex time. The vortex time is discussed in relation to the synchronization of the ladle change-over time during sequence casting.
Analysis of a single strand 36 t thin slab caster tundish has been conducted with different sets of furniture using a three-dimensional computation fluid dynamics model taking heat losses into account. Five distinct and optimised cases and a base case were used for simulation. The cases were built considering tundish furniture that is readily and economically available and provides ease of maintenance, thus targeting an optimal set of furniture. The performance of different sets of furniture was assessed based on residence time parameters like plug volume fraction, mixed volume fraction, dead volume fraction, etc. Other performance indicators used in the analysis were temperature distribution, observing cold spot, surface velocity and nature of flow in the tundish. Insight from the base case reveals the desired flow characteristics that help to achieve the target performance. Inferred results suggested the use of a turbulence inhibitor in combination with a dam as the optimal set of furniture. Use of a non-isothermal model is important, as it was found that even a small change in temperature (2.3 degrees C) plays a vital role in the fluid flow inside the tundish.
The tundish plays a major role in the continuous casting process. The flow in a tundish has a very substantial effect on the quality of the final product and on efficient casting conditions. Efforts are being made worldwide to obtain the most favourable shape of tundish interior by using dams, weirs and gas curtains. The aim of these flow control devices is to reduce the dead zone areas and improve the conditions for the separation of non-metallic inclusions. Numerous model studies are being carried out to explain the effect of the tundish working space shape and steel flow conditions on the inclusions floating processes. The presented article shows the results of investigations performed to obtain the mass exchange characteristics in the investigated tundish. The measurements were done directly at the steel plant during normal working conditions. By controlling the changing content of manganese in steel, the residence time distribution (RTD) characteristics were acquired. The RTD characteristics are also obtained with a water model of the tundish with dimensional scale of 1:3. Parallel to the water model, numerical simulation based on mathematical modelling of fluid flow, relying on the system of differential equations, is employed in the research work. Numerical simulations were carried out with the finite-volume commercial code FLUENT using the standard k-ε turbulence model. The primary purpose of the investigations carried out is to present the characteristics describing the transitory zone in a six-strand tundish. It is shown that the F-curve, describing the transitory zone, can be obtained by using different measurement techniques. Tracer concentration characteristics for the model of tundish obtained from both modelling techniques - physical as well as numerical - are very similar.
Tundish in the steelmaking circuitry, like BOF/EAF and ladles, plays a pivotal role in determining plant performance and steel quality. Thus, beyond its traditional role as a buffer vessel, a steelmaking tundish is currently designed and operated to ensure maximum yield, superior cleanliness, negligible energy and materials losses and longer life. Naturally therefore, modern day steelmakers are emphasizing tundish management and practice. Adequate understanding of the physical, chemical and thermal interactions among steel, slag, gas and refractory phases is a pre-requisite to fully exploit the potential of any given tundish system. In the present work, a brief account of the theory of tundish processing is first presented. Following such, phenomena such as, inclusion removal, temperature drop, skull losses, grade intermixing as well as slag emulsification and entrainment in different tundish systems are discussed. In such context, decade long association with half a dozen domestic steel industries namely, JSW-Ispat, JSPL, JSW, RINL, Hospet steel and MUSCO is highlighted and our approach and efforts towards tundish process performance enhancement demonstrated.
New types of furniture (also termed as flow modifiers or baffles) were incorporated in industrial scale, slab and bloom casting tundish systems with an aim to reduce residual metal loss (i.e., tundish skull) at the end of sequence casting. To this end, water model experiments were carried out in which, slag vortexing phenomena during emptying of tundish was studied embodying different types of furniture into existing tundish designs. These in general indicate that a wedge shaped bottom together with an embedded pouring box applied in conjunction have the potential to reduce tundish skull and improve yield losses significantly. In addition, limited residence time distribution measurement experiments were made to investigate metallurgical performance of modified design tundish systems. These indicate that deployment of new furniture with minor design modifications, despite contributing to a reduction in tundish capacity (10–12%), do not influence metallurgical performance of steelmaking tundish systems to any significant extent. Accordingly, designs of currently employed slab (32 and 37 tonnes capacity respectively) and bloom casting tundish (10 and a 17 tonnes capacity respectively) systems were modified in four different steel mills and plant trials conducted to assess the extent of yield improvement. Significant improvements in yield losses, to the extent of 50–60%, have been confirmed by the industry during sequence casting.
Mechanism of vortex formation in continuous casting moulds was investigated using a full scale water model. It was found that vortices occurring near the SEN was due to interaction between the two surface streams flowing inwards. Biased flow as such does not generate vortices in the mould, but increases its depth and frequency. Vortex depth increases with increasing casting speed. Vortex formation in the mould can be reduced by diminishing biased flow and optimising the SEN design. When casting wide slab at a shallow SEN immersion depth, vortex formation takes place near the narrow faces.
Considerable efforts have been made in academia and industry over the last two decades to fully exploit and enhance the metallurgical performance of continuous casting tundish systems. Towards these goals, numerous physical and mathematical modelling studies embodying both industrial and water model tundishes have been carried out and reported in the literature. Based on an extensive literature search, we now present a summary, discussion and analysis of these. For the sake of convenience and clarity of presentation, the studies have been categorised into three major groups: (1) physical modelling (2) mathematical modelling and (3) combined physical and mathematical modelling. In each of these categories, a great number of publications on various aspects of tundish metallurgy, such as, modelling criteria, turbulent fluid flow, residence time distributions (RTD), inclusion transport and separation, heat lass and temperature drop, grade transition and intermixing, etc. have been reported. These works have lead to considerable improvements in our understanding of the various transport processes (viz, RTD, inclusion float out, thermal energy transport, etc.) associated with tundish operations. Comprehensive and sufficiently reliable mathematical models are also currently available and these also allow one to carry out full scale predictions and useful engineering design and process calculations. None the less, certain obscurities and uncertainties remain. These are reviewed together with suggestions of areas where further research is needed.
A model of a tundish has been developed that takes into account the steel, slag, and refractory phases. Predicted temperatures and velocities in the steel and refractory from the model were earlier found to agree well with measured velocities and temperatures. The model was also used to determine the optimal location of flow devices, making the temperature distribution in the steel more even and enhancing the removal of inclusions to the slag. In this study, the focus was on using the model to study the slag/steel interface in the tundish. Predictions showed that slag is dispersed into the steel close to the interface as well as close to the ladle shroud. In order to confirm these predictions, the momentary interfacial solidification sampling (MISS) method was developed. Using this method, a sample of the steel/slag interface could be taken that represented almost an instantaneous picture of the interface. The MISS sampler was used for sampling low-carbon steel in the tundish. Samples were analyzed using ultrasonic testing, optical microscopy, and scanning electron microscopy (SEM). Analysis results confirmed the presence of nonmetallic particles close to the slag/steel interface and close to the ladle shroud, as suggested by the modeling results. The analyses also showed that the slag/steel interface is very irregular, despite the low velocities.
- Z Liangcai
- W Mingan
- C Boyu
Liangcai Z, Mingan W, Boyu C, et al. J Iron Steel Res Int. 2010;17:7-12.
- G S Diaz
- A R Banderas
- J D Barreto
Diaz GS, Banderas AR, Barreto JD, et al. AISTech. 2006;1:977-984.
- P K Tripathi
- D S Kumar
- T Rajendra
Tripathi PK, Kumar DS, Rajendra T, et al. Proceedings of NMD-ATM
2016; Nov 2016, Kanpur, India.