Product Development and Flow Optimization in the Tundish by Modelling and Simulation

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1 Product Development and Flow Optimization in the Tundish by Modelling and Simulation Gernot Hackl 1, David Wappel 1, Daniel Meurer 1, Marcos Tomas 1, Rick Komanecky 2 1 RHI AG Technology Center Magnesitstrasse 2 A Leoben, Austria Phone (0) Fax (0) gernot.hackl@rhi-ag.com; david.wappel@rhi-ag.com, daniel.meurer@rhi-ag.com; marcos.tomas@rhi-ag.com 2 RHI US Ltd Kister Court Ashtabula, OH rick.komanecky@rhi-ag.com Keywords: CFD, water modelling, thermodynamic modelling, impact pot, flow modifiers, wear lining INTRODUCTION The tundish is an intermediate vessel in the continuous casting machine, used to transfer liquid steel from the ladle to the mould. Its primary functions are to guarantee continuous operation and an even distribution of steel to the mould(s). The tundish is lined with different types of refractory materials that should provide adequate thermal insulation and be thermochemically resistant to steel and slag, thereby avoiding the formation of detrimental exogenous inclusions. In addition, appropriate refractory selection has an impact on the maximum possible sequence length of a specific tundish setup. Depending on the tundish flux used, which is dictated by the steel grade being cast, optimized raw material concepts need to be used to maximize the lining lifetime, since an inappropriate choice may lead to premature wear and reduce caster productivity. Thermodynamic modelling can provide important information regarding the potential reaction processes and complex dissolution mechanisms that can occur. Besides the aforementioned primary functions, the tundish has become increasingly important as a refining reactor since it provides several metallurgical operations such as efficient removal of nonmetallic inclusions as well as thermal homogenization. In this regard, fluid flow in the tundish significantly influences metallurgical performance and as a consequence the product quality. Therefore, detailed understanding of flow phenomena is crucial for optimizing the process. However, due to the harsh operating environment, direct measurements cannot be conducted in an efficient manner and appropriate modelling techniques are required. With respect to the operating conditions, one can distinguish between two types of event: Steady-state casting and transient periods. For the performance under steadystate casting conditions, the approach of measuring the residence time distribution (RTD) has been commonly applied by many researchers. The residence time of a fluid in a reactor, such as the tundish, is defined as the time a single fluid element will remain in the reactor. Usually, flow in any tundish is accompanied by different residence times for different fluid elements, resulting in a distribution function of residence times. From this curve important parameters to characterize the performance can be derived, such as the minimum residence time (t min ), mean residence time (t mean ) as well as the dead volume (V d ), well mixed volume (V m ), and the plug flow volume (V p ). [1] In order to maximize the flotation behaviour and avoid reoxidation in a given tundish under steady-state casting conditions, it is necessary to ensure the following points: [2] - Minimum spread of the residence time - Minimum dead volume - Large ratio of plug to dead volume and plug to mixed volume - Surface-directed flow - Quiescent slag layer - Contained regions of mixing AISTech 2014 Proceedings by AIST. 1911

2 Transient casting conditions such as the start of casting and a ladle or grade change are well established as being detrimental to the steel quality. These events frequently result in the formation of additional inclusions by reoxidation. Splashing at the beginning of the cast, slag emulsification during a ladle change, or vortex formation at the end of a sequence, which can all be the source of additional contamination, need to be avoided as much as possible. Modelling techniques such as water modelling, computational fluid dynamics (CFD), and thermodynamic modelling play an important role in deepening process and material understanding and provide information to improve existing setups in a systematic and efficient manner in order to derive the maximum benefit tailored to the customer s requirements. Thermodynamic Modelling MODELING APPROACHES The chemical wear of refractory materials occurs via reaction and dissolution mechanisms. Dissolution, which is diffusion controlled, can be expressed by Fick s first law: c j D S c 0 (1) Where j is the mass flux of a certain ion species, D is the effective diffusion coefficient, δ is the boundary layer thickness, whilst c s and c 0 are the solubility of the refractory component and the oxide concentration in the melt, respectively. According to the equation, c s -c 0 strongly influences the corrosion rate and is a crucial parameter to evaluate the corrosion resistance of refractory components against any melt composition. [3] For many decades phase diagrams were used to evaluate solubilities and phase equilibria in refractory corrosion. Nowadays, self-consistent thermochemical databases have proved to be so advanced that computational thermodynamics can be successfully applied to multicomponent phase equilibria. For a given set of constraints, such as temperature, pressure, and mass of each element, the software calculates the equilibrium conditions by minimizing the total Gibbs energy of the system. This is mathematically equivalent to solving all the equilibrium constant equations simultaneously. This approach may be used to examine the wear lining material in contact with the tundish flux or carryover slag from the ladle. Based on the process slag chemistry, appropriate refractory material selection can be performed in order to minimize corrosion and increase the tundish sequence length. Water Modelling Physical modelling, by means of water modelling, is an efficient way to understand steel flow inside a tundish. The fundamentals of water modelling require the system to approximate, as closely as possible, conditions in the actual setup. To achieve this, certain similarities between the real application and the model must be fulfilled, which include geometric, dynamic, kinematic, and thermal considerations, as previously described. [1,4] In general not all of these criteria can be fulfilled simultaneously; however, either full or scaled-down models are able to provide useful information about the tundish flow characteristics and can help optimize tundish performance. In the past, several studies were performed that compared the results of full and scaled-down models. From the observations it can be concluded that a model tundish scaled down on the basis of geometric similarity and fulfilling the Froude similarity criteria is likely to simulate flow phenomena of the corresponding full-scale system quite accurately. [4] The Froude number (Fr), which is the ratio between inertial and gravitational forces, is defined as: 2 u Fr gl (2) Where u is the velocity, g is the acceleration due to gravity, and l is the characteristic length of the system. During transient operations, such as ladle change, temperature gradients in the melt can be high and variations in density may occur. Under this situation, temperature-induced buoyancy forces needs to be considered. This is expressed by the Richardson number (Ri), which is the ratio of buoyancy to inertial force. Tgl Ri 2 u (3) 1912 AISTech 2014 Proceedings by AIST.

3 Where β is the thermal expansion coefficient and ΔT is a characteristic temperature difference in the tundish, for example between the inlet and outlet. Computational Fluid Dynamics (CFD) Modelling Computational fluid dynamics provides a framework for simulating complex three-dimensional fluid flow as well as related phenomena in the tundish. It consists of three major steps: Pre-processing (creation of a geometric model and meshing), solving, and post-processing (data analysis, visualization, and validation of results). In contrast to water models, it can provide a much more detailed image of velocity, turbulence, or temperature distribution, based on real operating conditions and melt properties. In order to simulate turbulent, transient, nonisothermal fluid flow, a set of conservation equations, such as continuity, momentum, and energy need to be solved. The effect of turbulence is typically modelled using an eddy viscosity approach, such as the k-ε model. A detailed model description can be found elsewhere. [5] Subsequently, the flow model can be extended to investigate the flotation behaviour of nonmetallic inclusions, for example as a dispersed phase in the melt, in relation to specific furniture configurations or the influence of a gas curtain injected through a purging beam. TUNFLOW and Flow Modifiers FLOW MODIFICATION AND OPTIMIZATION In order to guarantee a safe casting start without splashing and optimize steel flow to maximize the flotation rate of nonmetallic inclusions, an effective flow control arrangement needs to be implemented. The development of flow modifiers is usually performed with the aid of CFD and water modelling. Depending on the requirements, the product or combinations thereof, such as an impact pot and dam/weir combination need to be investigated under the specific casting conditions. With the TUNFLOW impact pot series, RHI is able to deliver a design portfolio that provides optimum flow conditions for any tundish configuration. The first example below depicts typical CFD simulation results, in this case where a competitor product is compared to the RHI TUNFLOW using a vector plot in the central plane of a two strand slab caster tundish (Figure 1). The corresponding RTD data provided evidence of the superior TUNFLOW performance, since t min, t mean, and V p were increased whereas V d was reduced in the comparison (Figure 2 and Table I). The RTD parameters were calculated according to the model of Sahai and Emi, considering the dispersed plug flow volume. [1] The second example demonstrates the positive impact of TUNFLOW in a three-strand bloom tundish when compared to a bare installation comprising an impact plate (Figure 3). One problem with the original configuration was that strand 3 (an external strand close to the tundish corner) suffered from parasitic solidification in the casting channel, which caused several casting stops. After installing TUNFLOW, the heat transfer to this external strand improved, minimizing the temperature differences between the strands. On-site temperature measurements confirmed the CFD simulation findings. (a) Figure 1. Vector plot in the centre plan of the tundish: (a) competitor impact pot and TUNFLOW AISTech 2014 Proceedings by AIST. 1913

4 Figure 2. RTD curve for both designs Competitor TUNFLOW design t min [s] t mean [s] V p [%] V d [%] V m [%] Table I. Corresponding RTD parameters (a) (c) Figure 3. Temperature distribution in a tundish: (a) bare, TUNFLOW, and (c) simulated temperature drop (T inlet T outlet ) for all strands comparing both configurations With the aid of a water model study, a new baffle configuration was designed to optimize the residence time distribution in a three-strand delta tundish. The original configuration was equipped with a single rectangular hole baffle, where the hole was located in the centre quite close to the bottom. The relatively small cross section of the hole induced a strong focused jet that hit the front face of the tundish. The observed flow pattern was quite imbalanced, leading to a short circuited flow towards the central strand and very low flow velocity towards the external strands. A snapshot of both configurations during a dye injection test is shown in Figure 4. The measured RTD curves for each individual strand provided evidence of the flow imbalance (Figure 5a). After several optimization steps, a baffle with four rectangular holes located close to the tundish side wall was designed. The result was a significant flow improvement. The minimum residence time as well as the RTD curves for each strand were equalized (Figure 5b). (a) Figure 4. Snapshot taken during a dye injection test for the baffle design (a) before optimization and after optimization 1914 AISTech 2014 Proceedings by AIST.

5 (a) Figure 5. Measured RTD curves for each individual strand. Baffle design (a) before optimization and after optimization Transient Operations Transient tundish operations, such as a ladle change, can critically influence product quality. For example, it is well known that the steel contamination risk is high during this period, as a result of reoxidation, slag emulsification, or possible slag entrainment caused by vortices if the tundish is drained too much. The water model represents a very powerful tool to investigate these types of process conditions. The operation of nonsubmerged ladle shroud opening during a ladle change is common practice. This can have a farreaching, negative impact on quality. During the nonsubmerged period, severe emulsification of the tundish flux can take place, leading to a significant increase in the inclusion number. However, appropriate furniture installation will inhibit potential slag entrainment and reduce the number of inclusions during this transient period. The impact on emulsified slag transport is shown for tundish arrangements without and with a TUNFLOW in Figure 6. (a) Figure 6. Emulsified slag transport during a ladle change. Tundish equipped (a) with TUNFLOW and without One important aspect to consider during a ladle or grade change is the melt temperature. Even minimal differences between the temperature of the incoming stream and the residual steel in the tundish can have considerable impact on the flow situation and may change it completely. A study was carried out to investigate the effect of hot into cold and cold into hot melt scenarios, based on a six-strand billet caster. The study was performed fulfilling Froude and Richardson similarity under the assumption of a 15 C melt temperature difference. For reference, isothermal conditions were also modelled. Figure 7 clearly shows the difference in the mixing behaviour and the effect of temperature-induced buoyancy forces. Since a certain temperature difference during a ladle change will always occur, accurate process control of the water model (e.g. fluid temperature) is indispensable for a realistic representation of the system, since inaccurate control may lead to poor and questionable results. AISTech 2014 Proceedings by AIST. 1915

6 (a) (c) Figure 7. Ladle/grade change simulation under different thermal conditions at two discrete time points: (a) isothermal, hot into cold, and (c) cold into hot New Impact Pot Development Most modelling studies are performed under the assumption of a perfectly vertically aligned and centred ladle shroud. However, in steel plant operations this is frequently not the case and quite a large inclination angle may be observed (Figure 8). Figure 8. Misaligned ladle shroud Under such conditions, the functionality of conventional impact pots can be severely limited. Not only the RTD parameters and kinetic energy dissipation under steady state conditions deteriorate but also the risk of splashing at the start of casting may be increased. The recent development of a new TUNFLOW family aims to tackle this problem. The new product design features special shaped protrusions on the inner sidewalls that are able to dissipate the kinetic energy more effectively when compared to a standard design under both aligned and misaligned ladle shroud conditions. Consequently the steel stream is distributed more evenly, which increases the plug volume fraction and decreases the dead volume fraction. The new design is able to maintain the desired flow pattern under suboptimal operating conditions where other designs may reach their limit. The CFD results in Figure 9 demonstrate the improved performance of the newly developed TUNFLOW compared to a conventional lipped impact pot of the same gross dimensions. Both the regions of higher surface velocity and turbulence in the impact zone are lower, which minimizes the risk of an open eye and steel reoxidation. The influence on inclined ladle shroud positions was investigated using an oil layer on top of the water to simulate the tundish flux. The flow rate of the system was adjusted such that under the boundary conditions of a vertical ladle shroud the oil layer, using the newly design, started to break up. Comparative tests with this specific flow rate were performed to investigate both a standard lipped design and the new TUNFLOW. Figure 10 shows a representative snapshot of the different scenarios (i.e., vertical and 3.5 inclined ladle shroud) for both designs. (a) Figure 9. Surface velocity (left) and turbulence (right) for (a) a conventional lipped impact pot design and the newly developed TUNFLOW 1916 AISTech 2014 Proceedings by AIST.

7 (a) Figure 10. Snapshot of a water model study to investigate the surface turbulence for a vertical (left) and misaligned (right) ladle shroud installation: (a) conventional lipped impact pot design and newly developed TUNFLOW LINING OPTIMIZATION Thermochemical modelling is a proven method to calculate the interactions between the tundish cover powder or tundish slag with the basic wear lining. Since these calculations are based on theoretical equilibrium calculations, several different practical tests were performed with different basic wear linings and cover powder to evaluate the degree of conformity between the predicted and actual refractory material wear. To generate service-relevant results, calcium aluminate cover powder and a range of tundish mixes based on the different chemical compositions detailed in Table II were examined. Thermochemical modelling was performed in parallel using the commercial FactSage software. The results of these calculations are depicted in Figure 11. The diagram shows the amount of liquid refractory material phases formed at different percentages of cover powder addition. Table II. Chemical composition of wear lining mixes and cover powder Figure 11. Relationship between the amount of liquid phase formation and the percentage of cover powder added The calculations clearly showed that the mix with a high SiO 2 content strongly reacted with the tundish cover powder and high levels of liquid phases were formed, which would result in significant wear and very low refractoriness. The calculations indicated that even minimal amounts of cover powder would react with SiO 2 in the olivine ((Mg,Fe) 2 SiO 4 ) to form liquid phases. This reaction occurred until all the SiO 2 from the refractory material was consumed. Subsequently, the curve flattened because MgO was mainly present in the mix. The other mixes did not show such significant liquid phase formation when in contact with the tundish cover powder, but rather the formation of stable solid phases was indicated by the curves decreasing compared to the start values. Various laboratory trials were performed to practically verify the FactSage thermochemical modelling results. In the first experiment, test cylinders comprising one of the four tundish lining mixes and increasing amounts of tundish cover powder were prepared and fired at 1560 C. The height reduction of the fired cylinders was measured and plotted against the amount of cover powder added (Figure 12b). The high interaction between the CaO-rich cover powder and the SiO 2 -rich tundish wear lining mix was clearly visible (Figure 12a) and even the addition of a small amount of cover powder significantly enhanced the formation of liquid phases and consequently lowered the refractoriness. The interactions of the other tundish mixes with the cover powder were also comparable to the results obtained from the modelling. AISTech 2014 Proceedings by AIST. 1917

8 (a) Figure 12. (a) cylinders comprising the SiO 2 -rich tundish wear lining mix and increasing amounts of CaO-rich cover powder after firing at 1560 C and relationship between the measured height reduction of the test cylinders and the amount of cover powder addition In the second test series, crucibles prepared from the various tundish mixes were filled with tundish cover powder and fired at 1560 C. After cooling, the crucibles were cut and a mineralogical investigation was performed. The high SiO 2 -containing mix demonstrated the most significant interaction with the cover powder (Figure 13a). At the interface the crucible showed a broad area of high wear with a clear indication of liquid phases. All the other mixes did not have such low melting phases. A mineralogical investigation (Figure 13b) revealed complete corrosion at the reaction zone between the SiO 2 -rich tunidsh mix and the cover powder. The original mix matrix had completely disintegrated, liquid phases were formed, and the refractoriness was significantly decreased. High lime-containing mixes based on alpine sinter magnesia showed only minimal infiltration, low sintering, and marginal interaction with the cover powder. High SiO 2 mix High lime alpine sinter mix High SiO 2 mix High lime alpine sinter mix (a) Figure 13. (a) Crucible test with two different tundish mixes and the calcium aluminate cover powder and microstructure at the reaction zone Based on these results, the use of olivine-containing mixes in combination with calcium aluminate cover powders should be avoided and mixes based on alpine sinter magnesia with a high MgO or CaO content should be favoured. CONCLUSIONS As a global partner for the steel industry, RHI is always striving to offer the best customer solutions based on individual conditions and requirements. Technology in the steel industry is constantly developing and topics such as clean steel are becoming ever more important. This is affecting all metallurgical vessels including the tundish. Simulation techniques, such as CFD and water modelling can help deepen process understanding, since direct measurements during casting are difficult to perform because of the harsh environment. The impact of flow modifiers, such as impact pots, dams, weirs, and baffle plates, can be efficiently studied under specific conditions. The results provide information about improvement potentials for existing configurations and help to optimize them. Another important aspect is support regarding the development of innovative products. In addition, thermochemical modelling provides an efficient approach to evaluate lining materials under specific operating conditions. Based on the cover powder used and resulting slag chemistry, the most appropriate lining concepts can be selected. RHI offers a large range of materials in order to guarantee maximum lining lifetime and minimize 1918 AISTech 2014 Proceedings by AIST.

9 potential steel contamination by the mix. All the simulation methods are used to enhance the understanding of a specific setup and thereby deliver products and systems tailored to the customers needs. ACKNOWLEDGEMENTS The authors would like to thank Stollberg for providing the tundish cover powder and their considerable technical support. REFERENCES 1. Y. Sahai and T. Emi, Melt Flow Characterization in Continuous Casting Tundishes, ISIJ International, Vol. 36, No. 6, 1996, pp R. Ahuja and Y. Sahai, Fluid Flow and Mixing of Melt in Steelmaking Tundishes, Ironmaking and Steelmaking, Vol. 13, 1986, pp V. Reiter, F. Melcher, H. Harmuth and W. Netzer, Thermochemical Modeling of EAF Slags and Refractory/Slag equilibria, Proc. UNITECR D. Mazumdar and R. I. L. Guthrie, The Physical and Mathematical Modelling of Continuous Casting Tundish Systems, ISIJ International, Vol. 39, No. 6, 1999, pp B.A. Launder and D.B. Spalding The Numerical Computation of Turbulent Flows, Computer Methods in Applied Mechanics and Engineering, Vol. 3, No. 2, 1974, pp AISTech 2014 Proceedings by AIST. 1919

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