metals

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

Peng Lin, Yan Jin * , Fu Yang, Ziyu Liu, Rundong Jing, Yang Cao, Yuyang Xiang, Changgui Cheng and Yang Li

Key Lab for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China; [email protected] (P.L.); [email protected] (F.Y.); [email protected] (Z.L.); [email protected] (R.J.); [email protected] (Y.C.); [email protected] (Y.X.); [email protected] (C.C.); [email protected] (Y.L.) * Correspondence: [email protected]; Tel.: +86-156-9718-0966



Received: 18 April 2020; Accepted: 19 May 2020; Published: 25 May 2020


Abstract: 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%.

Keywords: round mold; swirling nozzle; flow field; eccentric casting; mathematical simulation

1. Introduction The mold is the key factor in quality control of casting molding. The molten steel flow in a mold directly effects thickness and uniformity of the solidified shell [1,2], which is a complex turbulent flow process. Its main characteristics are irregularity, rotation, three-dimensionality, diffusivity and dissipativity [3,4]. As the inlet of the mold, the outlet of the nozzle influences the flow pattern of liquid steel directly in the mold and affects the flow field and liquid surface behavior of steel slag [5–8]. Therefore, it is very important for the shape and control of the flow field in the mold. According to the actual operation of a steel works with high productivity and low cost, a tundish pours the round strands with diameters of 500 mm and 600 mm at the same time. This causes the submerged entry nozzle of a 600 mm mold to deviate from the center by 50 mm during the casting process. The operation results in uneven distribution of the flow field, a large surface wave and uneven thickness of the solidified shell [9,10]. It was found that the swirling flow field driven by swirling nozzle could inhibit the asymmetrical flow in mould and could suppress the negative effects on the surface and/or internal quality of strand caused by asymmetrical flow [6,7]. Based on previous work [9–12] and through the study of numerical simulation and water modeling on the eccentric flow field of the mold, in this paper,

Metals 2020, 10, 691; doi:10.3390/met10050691 www.mdpi.com/journal/metals Metals 2020, 10, x FOR PEER REVIEW 2 of 23 inhibit the asymmetrical flow in mould and could suppress the negative effects on the surface and/or internal quality of strand caused by asymmetrical flow [6,7]. Based on previous work [9–12] and through the study of numerical simulation and water modeling on the eccentric flow field of the mold, Metals 2020, 10, 691 2 of 23 in this paper, a reasonable swirling-type submerged entry nozzle is designed, and the swirling nozzle is adopted to optimize the flow field in the mold to improve the casting environment and enhance thea reasonablequality of the swirling-type round bloom. submerged entry nozzle is designed, and the swirling nozzle is adopted to optimize the flow field in the mold to improve the casting environment and enhance the quality of the 2. roundMathematical bloom. and Physical Models

2.1.2. The Mathematical Research Subject and Physical Models

2.1.The The subject Research investigated Subject in this study is a round mold in a caster, which is used in conjunction with a 46 t four-flow tundish. To research the flow state and distribution of molten steel in the mold of differentThe subject nozzle investigated structures under in this the study conditions is a round of molddifferent in a caster,eccentric which degrees is used on incasting, conjunction the originalwith a five-fractional 46 t four-flow tundish.radial nozzle To research (the original the flow SEN) state and and swirling distribution five-fractional of molten tangential steel in the nozzle mold of (thedi ffswirlingerent nozzle SEN) structures are adopted. under The the SEN conditions structure of and diff parameterserent eccentric are degreesshown in on Figure casting, 1 and the originalTable 1. five-fractional radial nozzle (the original SEN) and swirling five-fractional tangential nozzle (the swirling SEN) are adopted. The SEN structure and parameters are shown in Figure1 and Table1.

(a) (b)

FigureFigure 1. 1.StructuralStructural schematic schematic of ofthe the mold mold and and submerged submerged entry entry nozzle. nozzle. (a) ( aFive-) Five- fractional fractional SEN SEN with with outletsoutlets in inradial radial direction direction (b ()b Five-) Five- fractional fractional SEN SEN with with outl outletsets in intangential tangential direction. direction.

Table 1. Geometric parameters of various SENs. Table 1. Geometric parameters of various SENs. Inside Diameter Outside Diameter Tangential Side Hole Side Hole Nozzle Type Inside Outside Diameter Tangential Side Hole Side Hole Nozzle Type (mm) (mm) Angle (◦) Height (mm) Width (mm) Diameter (mm) (mm) Angle (°) Height (mm) Width (mm) Five-fractional radial 50 100 0 48 32 Five-fractionalnozzle radial (a) Five-fractional 50 100 0 48 32 nozzle (a) 50 100 15, 30 48 32 tangential nozzle (b) Five-fractional 50 100 15, 30 48 32 tangential nozzle (b) 2.2. The Mathematical Model 2.2. The Mathematical Model 2.2.1. Model Assumptions 2.2.1. ModelAccording Assumptions to the physical characteristics of liquid steel and its flow features, the following assumptions have been created: According to the physical characteristics of liquid steel and its flow features, the following assumptions(1) Neglecting have been the influence created: of a surface covering agent on the flow, the top surface of the molten steel is a free-slip surface that is satisfied with k = ε = 0 and ∂u/∂z = ∂v/∂z = ∂k/∂z = ∂ε/∂z = w = 0; (2) The fluid in the mold is a viscous and incompressible fluid, which has unsteady flow and the

initial temperature is a uniform distribution; (3) The effects of the shrinkage of the round bloom and the vibration of the mold on the flow of molten steel are not considered when the steel solidifies; Metals 2020, 10, 691 3 of 23

(4) The latent heat of δ-γ phase transformation is far less than that of solidification, neglecting the effect of the γ metal solid phase transformation and the influence of solidification of the billet shell on the molten steel flow in the mold; (5) Argon bubbles and inclusions are spherical, without considering collision and growth; (6) The inclusions do not influence the flow field, but the flow field does influence the movement of the inclusions.

2.2.2. The Governing Equations and Boundary Conditions As a complex process, the flow behavior and temperature change of molten steel have been simulated by the continuity equation, the momentum equation (the Navier-Stokes equation), the k-ε two equation describing the turbulence model and energy equation (coupled with momentum and continuity equations). The Discrete Phase Model (DPM) is established to simulate the trajectories and distributions of the inclusions and argon bubbles in the mold:

(1) The continuity equation: ∂ρ/∂t + ∂ρuj/∂xj = 0 (1)

In the formula, uj represents the velocity component (m/s); xj represents coordinate (m); ρ and t are fluid density (kg/m3) and time (s), respectively; (2) The Navier-Stokes equation:

∂(ρu )/∂t + ∂(ρu u )/∂x = ∂p/∂x + ∂[µ (∂u /∂x + ∂u /∂x )]/∂x + ρg (2) i i j j − i eff i j j i i i

where ui and uj denote the velocity of i and j direction (m/s); xi and xj stand for coordinates of the i and j directions (m); p is pressure (Pa); µeff indicates the coefficient of effective viscosity (Pa s) 2· determined by the available turbulence model; and gi stands for the gravity component, (m/s ); (3) The k-ε two equation model. The k equation of turbulent kinetic energy is given by the expression:

∂(ρk)/∂t ∂(ρku )/∂x = ∂[(µt/σ µ)(∂k/∂x )]/∂x + µt (∂u /∂x )(∂u /∂x + ∂u /∂x ) ρε (3) − j j k − i j j i i j j i −

In this formula, k represents turbulent kinetic energy (m2/s2), and ε denotes the dissipation rate of turbulent flow energy (m2/s3); The ε equation of dissipation rate of turbulent energy:

2 ∂(ρε)/∂t + ∂(ρεu )/∂x = ∂[(µt/σε µ)(∂ε/∂x )]/∂x + C (ε/k)µt(∂u /∂x )(∂u /∂x + ∂u /∂x ) C ρ(ε /k) (4) j j − i i 1 j i i j j i − 2 2 where µt can be calculated by Equations µt = ρCµk /ε and µeff = µ + µt; in the formula, µt represents the turbulent viscosity (Pa s); µ is the laminar viscosity (Pa s); C , C , Cµ, σε and σ are · · 1 2 k empirical constants, C1 = 1.43, C2 = 1.93, Cµ = 0.09, σk = 1.0, σε = 1.43. (4) The energy transfer equation. The energy transfer equation was coupled with the flow field equations. Without considering the effect of temperature on the density of molten steel, the flow field and temperature field of the mold are calculated by single-phase coupling. The energy transfer equation is shown below:

∂(ρujH)/∂xi = ∂[keff(∂T/∂xi)]/∂xi (5)

where keff can be calculated by equation keff = µ/σT + µt/σt,T; in the formula, H represents the specific enthalpy of molten steel (J/kg); T stands for molten steel temperature (K); and keff is the effective thermal conductivity (W/m/K); σ = 1.00 (K s2/m2); σ = 0.9 (K s2/m2). T · t,T · Metals 2020, 10, 691 4 of 23

(5) The DPM Model. In the integral Lagrangian coordinate system, the trajectory of a particle (an inclusion or an argon bubble) is described by the particle force differential equation:

∂up/∂t = F (u up) + g (ρp ρ)/ρp + F (6) D − i − i  2  FD = 18µi/ρpdp (CDRe/24) (7)

In the formula, u and up are the velocity of the fluid phase and the particle phase (m/s); µi is the dynamic viscosity of the fluid (Pa s); ρ and ρp are the density of the fluid phase and the particle 3 · phase (kg/m ); dp is the particle diameter (m); Re is the Reynolds number of the particle; FD is the momentum exchange coefficient of drag force (1/s); Fi is the liquid inertia force per particle mass 2 acting on particle as particle accelerating (i = {x, y, z}) (m/s ); CD is the drag force coefficient (-). In the model, the range of particle size for simulated inclusions is 1~50 µm. The density of inclusions is 3000 kg/m3, and the diameter distribution of particles is discontinuous and is divided into 11 sizes, including 1, 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50 µm. For the simulation of argon bubbles, the drag force, buoyancy force and virtual mass force on bubbles are considered in calculation. The density of argon is 0.275 kg/m3, and the particle size range of argon bubbles is 0.6–3.0 mm and its mean diameter is 2 mm according to the previous experiments. The particle size distribution was set as Rosin-Rammler distribution, in which the different bubble size ranges are divided into discrete size groups, as shown in Equation (8): n Y = exp [ (dp/da,avg) ] (8) d − where Yd is the mass fraction of particles; da,avg is the mean bubble diameter (m); n is the distribution index. Boundary conditions of the model have been summarized as follows: (1) The entrance is defined at the inlet of liquid steel in the upper part of the submerged nozzle, entrance velocity (Vin) is determined by casting speed in steady pouring according to the principle of mass conservation of molten steel; the temperature of molten steel at the entrance is 1803 K; the turbulent kinetic energy (kinlet) and turbulent energy dissipation rate (εinlet) at the entrance 2 3/4 3/2 are calculated by kinlet = (3/2) (VinTi) and εinlet = Cµ k /l respectively, in which Ti of the turbulence intensity is 3.7%, and l of the turbulence length scale is 0.07d, among which d is nozzle diameter; and Cµ is taken as 0.09; (2) The exit is set up at the bottom of the calculation domain as free exit “outflow”; (3) The liquid surface of the mold is set as a free surface, and the upper surface temperature of the liquid slag layer is a constant temperature of 1500 K, with the velocity component perpendicular to the liquid surface of a constant zero; (4) The wall of the mold with a no-slip surface is treated by the standard wall function of Fluent, and the height distance from the meniscus level dependant heat flux distributionon with the average heat flux (745 kW/m2) is employed as the heat transfer condition. When using a discrete phase model to simulate the process of argon blowing or inclusion movement, argon bubbles blow into the mold from the submerged nozzle with the flow rate of 2 L/min. For argon bubbles, the boundary condition of the outlet of mold is set to “escape” condition, the liquid surface is set to “trap” condition, and the wall of the mold is set to “reflect” boundary condition. The inclusions are injected into the inlet at the same velocity as that of the molten steel, and the diameter distribution of inclusions is set to “uniform”, which means that the number of inclusion particles for each diameter is the same. The inclusions can be trapped at the liquid surface and reflected by the wall of mold, and the model outlet is the escape outlet. According to the assumptions, Equations and boundary conditions of the model, a geometric model of the mold is established using ICEM software (15.0, Ansys Inc., Canonsburg, PA, USA) to carry on the grid division. The meshes of the mold computational domains included non-uniform Metals 2020, 10, 691 5 of 23 grids with about 1.55 million cells, and the maximum mesh size is set to 16mm. In order to improve the calculation accuracy of complex structures of nozzle, the nozzle area is encrypted with the maximum grid size of only 8mm. Finally, the mathematical model is set up in FLUENT software (ver. 15.0, Ansys Inc.) to complete the calculation. The unsteady state calculation is employed to simulate molten steel flow with the time step of 0.005 s. To ensure the development of the flow field in the mold, the calculation domain of the model has also been extended to twice the actual mold length. The process parameters and thermal physical properties are shown in Tables2 and3[ 12]. Among them, the immersionMetals 2020, 10 depth, x FOR ofPEER the REVIEW nozzle is defined as the distance from the upper end surface of the SEN5 sideof 23 hole to the surface of molten steel. The numerical model widely applies the SIMPLE algorithm by utilizingcarry on commercialthe grid division. software The to meshes simulate of thethe three-dimensionalmold computational flow domains field and included temperature non-uniform field in thegrids mold. with Theabout movement 1.55 million track cells, of inclusion and the maximum particles or mesh bubbles size in is theset moldto 16mm. is simulated In order byto Coupledimprove 4 algorithm.the calculation The accuracy residual valueof complex equation structures is less than of nozzle, 10− of the the nozzle convergence area is standard encrypted solving with the continuitymaximum equation,grid size of momentum only 8mm. equation Finally, andthe mathematical turbulence. The model process is set of up building in FLUENT the FLUENT software model (ver. is15.0, shown Ansys in FigureInc.) to2 .complete the calculation. The unsteady state calculation is employed to simulate molten steel flow with the time step of 0.005 s. To ensure the development of the flow field in the mold, the calculation domain of theTable model 2. Parameters has also been for the extended mold. to twice the actual mold length.

TheSection process Diameter parametersLiquid and Surface thermal of Liquid physical Surface prop of ertiesImmersion are Depthshown inCasting Table Speed 2 and TableCasting 3 [12]. Among/ (mm)them, the immersionStable Casting depth (mm) of theOverflow nozzle (mm) is definedof Nozzle as the (mm) distance from(m/min) the upperTemperature end surface (K) of the SEN600 side hole to the 900 surface of molten 950 steel. The numerical 160 model widely 0.30 applies the 1803 SIMPLE algorithm by utilizing commercial software to simulate the three-dimensional flow field and temperature field in the mold.Table The 3. Thermal movement physical track properties of inclusion of molten particles steel. or bubbles in the mold is −4 simulated by Coupled algorithm. The residualMolar Mass value equationSpecific is Heat less than 10Effective of the Thermal convergence Density (kg/m3) Viscosity (Pa s) · (g/mol) Capacity (J/(kg K)) Conductivity (W/(m2 K)) standard solving the continuity equation, momentum equation and· turbulence. The process· of building the7000 FLUENT model 0.0065 is shown in Figure 55.85 2. 750 41

Figure 2. The process of building the FLUENT model. Figure 2. The process of building the FLUENT model. 2.3. The Physical Model According to the principle of similarity,Table 2. Parameters in order to for ensure the mold. the similarity of fluid motion behavior betweenSection the modelLiquid and Surface prototype, Liquid and to Surface consider theImmersion effects of inertia force,Casting gravity, surfaceCasting tension andDiameter viscous force ofof theStable melt, it is necessaryof Overflow that theDepth Reynolds, of Nozzle Froude, andSpeed Webber numbersTemperature for the prototype(mm) and modelCasting are (mm) equal, and the(mm) relevant calculation(mm) formulas are detailed(m/min) in Equations(K) (9), (10) and (11).600 According to900 the Equation (9) 950used for the calculation 160 of the Reynolds 0.30 number, the1803 Reynolds numbers of the model and the prototype are by far more than 105, thus belonging to the second self-modelling zone. BecauseTable the 3.flow Thermal is in physical the same proper self-modelingties of molten zone, steel. the velocity distribution of theDensity fluid is not affectedViscosity by ReynoldsMolar number, Mass andSpecific the viscous Heat Capacity force can be ignoredEffective in the Thermal establishment of the water model. Therefore, the similarity of the model and prototype can be ensured when the (kg/m3) (Pa·s) (g/mol) (J/(kg·K)) Conductivity (W/(m2·K)) Froude and Weber numbers are equal, which does not require that the Reynolds number be equal. Considering7000 the accuracy0.0065 of the experiment 55.85 and the experimental 750 conditions, the similarity 41 ratio of 1:2 between the model and the prototype is adopted to build physical model by applying plexiglass 2.3. The Physical Model According to the principle of similarity, in order to ensure the similarity of fluid motion behavior between the model and prototype, and to consider the effects of inertia force, gravity, surface tension and viscous force of the melt, it is necessary that the Reynolds, Froude, and Webber numbers for the prototype and model are equal, and the relevant calculation formulas are detailed in Equations (9), (10) and (11). According to the Equation (9) used for the calculation of the Reynolds number, the Reynolds numbers of the model and the prototype are by far more than 105, thus belonging to the second self-modelling zone. Because the flow is in the same self-modeling zone, the velocity distribution of the fluid is not affected by Reynolds number, and the viscous force can be ignored in the establishment of the water model. Therefore, the similarity of the model and prototype can be

Metals 2020, 10, x FOR PEER REVIEW 6 of 23 ensured when the Froude and Weber numbers are equal, which does not require that the Reynolds numberMetals 2020 be, 10 equal., 691 Considering the accuracy of the experiment and the experimental conditions,6 ofthe 23 similarity ratio of 1:2 between the model and the prototype is adopted to build physical model by applyingmaterials, plexiglass as shown inmaterials, Figure3. as The shown specific in parametersFigure 3. The of thespecific model parameters and the prototype of the model are detailed and the in prototypeTable4. are detailed in Table 4.

Figure 3. Water modeling experiment schematic. (1) Casting Tundish; (2), (5) slide gate; (3) Submerged Figurenozzle; 3. (4) Water Mold; (6)modeling High speed experiment camera; (7)schematic. DJ800 hydraulic (1) Casting acquisition Tundish; treatment (2), (5) system. slide gate; (3) Submerged nozzle; (4) Mold; (6) High speed camera; (7) DJ800 hydraulic acquisition treatment system. Table 4. Water model experiment parameters.

ParametersTable 4. Water model experiment parameters. Prototype Model Section diameterParameters (mm) Prototype 600 Model 300 CastingSection speed diameter(mm) (m/min) 600 0.3 300 0.212 3 Water flowCasting speed speed(m/min) (m /h) 5.09 0.3 0.212 0.900 Mold length (mm) 900 1000 Water flow speed(m3/h) 5.09 0.900 Immersion depth (mm) 160 80 Mold length(mm) 900 1000 Eccentric distance of SEN (mm) 50 25 Immersion depth(mm) 160 80 Diameter of SEN (mm) 50 25 Steel- slag interfacialEccentric tension distance coe offfi cientSEN(mm) (N/m) 501.4 25 - Water- oil interfacialDiameter tension of coe SEN(mm)fficient (N /m) 50 - 25 0.05 Steel- slagReynolds interfacial number tension coefficient(N/m) 2.65 1.4 108 - 1.27 107 × × Water- oilFroude interfacial number tension coefficient(N/m) 1.06 - 0.05 1.06 WeberReynolds number number 2.651.30 × 101082 1.27 × 101.307 102 × × Froude number 1.06 1.06 Weber number 1.30 × 102 1.30 × 102 Molten steel and the specific viscosity of the slag are simulated by water and the proper proportion

of the mixed oil. The inflow and outflow of water are adjusted by the rotameter to simulate the different Molten steel and the specific viscosity of the slag are simulated by water and the proper speeds. The height of liquid surface is collected by a DJ800 (China Institute of Water Resources and proportion of the mixed oil. The inflow and outflow of water are adjusted by the rotameter to Hydropower Research, Beijing, China) acquisition and processing system, and the measuring range simulate the different speeds. The height of liquid surface is collected by a DJ800 (China Institute of and accuracy of the wave-height sensor are 15 cm and 0.5% full scale (F.S.). Methylene blue is employed Water Resources and Hydropower Research, Beijing, China) acquisition and processing system, and as a tracer dye on the flow field in the mold through camera equipment to monitor and record the the measuring range and accuracy of the wave-height sensor are 15 cm and 0.5% full scale (F.S.). distribution and development of the tracer in various technological conditions: Methylene blue is employed as a tracer dye on the flow field in the mold through camera equipment to monitor and record the distribution and development of the tracer in various technological Re = uodo/ν (9) conditions: 2 Fr = uo /gdo (10)

2 We = ρuo do/σ (11) Metals 2020, 10, 691 7 of 23

In these formulas, Re is the Reynolds number; Fr is the Froude number; We is the Weber number; uo is the liquid flow velocity of the SEN outlet (m/s); do is the inside diameter of the SEN (m); ν is the kinematic viscosity of liquid (m2/s); σ is the interfacial tension coefficient (N/m) [13]. After determining the proportion of water model parameters, it is necessary to determine the similar conditions of steel slag interface. The dimension analysis shows that in order to simulate the flow of steel slag interface, the medium used to simulate the mold powder should satisfy the following condition [11]: νslag/νsteel = ν1/ν2 (12) In the formula, ν is the kinematic viscosity, (m2/s); slag, steel, l, 2 represent mold powder, molten steel, simulated mold powder and simulated molten steel, respectively. According to the various parameters in Table5, the kinematic viscosity of water at room temperature is not much di fferent from that of liquid steel. Using the mixture model of aviation kerosene and vacuum pump oil can ensure that the kinematic viscosity is similar to that of mold powder. The viscosity of the mixed oil selected in this experiment is 75 mm2/s, the mixture ratio between 3# aviation kerosene and N100 vacuum pump oil is 0.11 L:2.46 L.

Table 5. Physical parameters of different substances [11].

Dynamic Viscosity Kinematic Viscosity Liquid Phase Density (kg/m3) (Pa s) (m2/s) · Water 1000 1.0 10 3 1.0 10 6 × − × − Molten steel 7020 6.5 10 3 0.95 10 6 × − × − 3# aviation kerosene 800 2.0 10 3 2.5 10 6 × − × − N100 vacuum pump oil 880 88.0 10 3 100 10 6 × − × − Mold powder 2500~2900 20~800 10 3 8~296.3 10 6 × − × −

The experiment with the water model to physically verify the numerical simulation results employs two kinds of SEN in different eccentric distance conditions to observe the fluctuation of the liquid surface and the change of the flow field in the mold.

3. Experimental Results and Analysis

3.1. Effect of SEN Eccentric Distance on the Flow Field Much research indicates that the drift phenomenon in industrial fields and production procedures is mainly reflected in the level fluctuation of the mold and the melting distribution of mold powder on the steel-slag interface. Mold level fluctuations can be measured by technical means, but the distribution status of mold fluxes cannot be quantitatively described. To describe and analyze the drift phenomenon of the mold quantitatively, the drift index (B) has been adopted in this experiment to measure eccentricity of the flow field [14], calculated by the method shown in Equation (13):

B = (F F )/[(F + F )/2] (13) R − L R L where FR and FL are the values of liquid surface fluctuation close to the wall area on two sides of the mold along the eccentric direction of SEN, which are defined by the data of wave height variation at the same position and recorded by the wave height sensor arranged near the wall on both sides of the SEN. The data in Equation (13) are the average wave heights over a period of time. The eccentric distance of the SEN is the most significant factor affecting the drift index in a mold. The numerical simulation is applied to study and analyze the drift degree in the mold under the conditions of different eccentric distances of the original SEN. According to the preliminary simulation test, the casting speed of 0.3 m/min and the immersion depth of 160 mm are selected as the experimental conditions. Metals 2020, 10, x FOR PEER REVIEW 8 of 23

conditions of different eccentric distances of the original SEN. According to the preliminary Metals 2020, 10, 691 8 of 23 simulation test, the casting speed of 0.3 m/min and the immersion depth of 160 mm are selected as the experimental conditions. The distribution of the flowflow fieldfield of molten steel in the mold in which the original SEN is located in thethe middlemiddle position position of of the the mold, mold, which which is inis in a non-eccentric a non-eccentric state state with with an eccentrican eccentric distance distance of 0 mm,of 0 ismm, shown is shown in Figure in Figure4. 4.

(a) (b) (c)

Figure 4. The distribution of flow field of molten steel in mold. (a) Contour; (b) Vector graph; Figure 4. The distribution of flow field of molten steel in mold. (a) Contour; (b) Vector graph; (c) (c) Streamline graph. Streamline graph. As shown in Figure4, the molten steel through the bottom hole of SEN forms a vertical downward flowAs that shown is called in Figure a central 4, the stream. molten Its steel velocity through gradually the bottom decreases hole of withSEN forms the increase a vertical of downward the impact depth,flow that the is shortestcalled a central distance stream. from Its the velocity liquid grad surfaceually of decreases the mold with to a the certain increase position, of the impact in which depth, the averagethe shortest downward distance velocity from the of theliquid fluid surface on the of cross the sectionmold to of a the certain mold position, is equal toin the which casting the speed,average is defineddownward as the velocity impact of depth. the fluid When on the the cross center section stream of reaches the mold a certain is equal depth, to the it casting forms anspeed, upward is defined reflux stream.as the impact Similarly, depth. the When molten the steel center through stream the reaches 4 side a holescertain of depth, the SEN it forms forms an 4 jets.upward The reflux jets gradually stream. expandSimilarly, and the diverge molten insteel the through flow process the 4 side and holes their velocitiesof the SEN are forms gradually 4 jets. The reduced. jets gradually When reaching expand and impactingdiverge in thethe wallflow of process mold, theand jets their can veloci formties upward are gradually and downward reduced. flows. When The reaching downward and flowsimpacting of molten the wall steel of flow mold, to the jets center can of form the moldupwa afterrd and the downward flows impact flows. to a The certain downward depth, forming flows of a largermolten range steel flow of lower to the reflux center area. of the Additionally, mold after th thee flows downward impact flows to a certain are related depth, to forming the distribution a larger ofrange flow of field lower at reflux the lower area. region Additionally, of mold the and downwa the crystalrd flows structure are related after the to the round distribution bloom enter of flow the secondaryfield at the coolinglower region zone. of The mold upward and the flows crystal can disturbstructure the after steel-slag the round interface, bloom but enter they the have secondary a great influencecooling zone. on the The floating upward of flows the inclusions can disturb and the the steel-s fluctuationlag interface, of the but liquid they surface have a andgreat the influence heating on of the moldfloating flux, of the and inclusions they determine and the the fluctuation heat transfer of the of liquid liquid surface surface and and the the heating fluctuating of the behavior mold flux, of theand steel-slagthey determine interface. the heat transfer of liquid surface and the fluctuating behavior of the steel-slag interface.Figure 5 shows the velocity maps on the surface of YOZ (longitudinal section on the direction of castingFigure speed) 5 shows when the the velocity eccentric maps distances on the of surface the original of YOZ SEN (longitudinal are 0, 12.5, section 25, 37.5 on and the 50 direction mm in the of casting condition.speed) when Among the eccentric them, the distances cloud images of the or representiginal SEN the are size 0, of 12.5, the molten25, 37.5 steeland 50 flow mm field. in the castingAs condition. shown in FigureAmong5, them, when the the cloud original images SEN repr is inesent the eccentric the size conditionof the molten and steel when flow the field. eccentric distanceAs shown of the in SEN Figure increases, 5, when the the center original stream SEN is appears in the eccentric obviously condition asymmetrical, and when shifting the eccentric to the edgedistance of theof the round SEN bloom, increases, and the it washescenter stream into the appears region obviously of initial as solidifiedymmetrical, shell. shifting From to the the vector edge diagramof the round of flow bloom, fields and in it Figure washes5, it into can the be foundregion thatof initial with solidified the increase shell. of the From eccentric the vector distance diagram of the of nozzle,flow fields the in flow Figure pattern 5, it can of double be found reflux thatformed with the by increase the center of the stream eccentric near distance the outlet of the of the nozzle, mold the is graduallyflow pattern replaced of double by one reflux large formed and one by weak the returncenter flow,stream and near eventually the outlet become of the only mold one is large gradually reflux stream,replaced which by one is large because and the one reflux weak streamreturn inflow, the an eccentricd eventually direction become of the only nozzle one large is inhibited reflux stream, greatly. Thewhich continuous is because development the reflux stream of this in one-sided the eccentri largec refluxdirection stream of the leads nozzle to the is deviationinhibited ofgreatly. the center The stream.continuous Meanwhile, development the velocity of this one- of thesided free large surface reflux at stream the eccentric leads to side the is deviation also largely of the imbalanced, center stream. up toMeanwhile, 0.113 m/s whenthe velocity the eccentric of the free distance surface of at the the SEN ecce isntric 50 mm. side is also largely imbalanced, up to 0.113 m/s when the eccentric distance of the SEN is 50 mm.

Metals 2020, 10, 691 9 of 23 Metals 2020, 10, x FOR PEER REVIEW 9 of 23

(a)

(b)

FigureFigure 5. 5. FlowFlow fields fields of ofmolten molten steel steel of ofthe the SEN SEN with with different different eccentric eccentric distances. distances. (a) Contour; (a) Contour; (b) Vector(b) Vector graph. graph.

TheThe asymmetric asymmetric flow flow field field results results in in a a significant significant difference difference in in the the impact impact degree degree on on the the shell shell from from thethe side-hole side-hole jets jets around around the the nozzle. nozzle. The The stronger stronger impact impact brings brings a ahigher-velocity higher-velocity upward upward stream stream and and aa greater greater disturbance disturbance to to the the meniscus. meniscus. The The difference difference of ofthe the impact impact degree degree leads leads to a to great a great difference difference in thein thefluctuation fluctuation of surface of surface level level on onboth both sides sides of ofthe the nozzle. nozzle. The The large large magnitude magnitude of of surface surface level level fluctuationfluctuation at at one one side side of of mold mold can can be be propagated propagated to to the the other other side side of of mold mold with with small small magnitude magnitude of of surfacesurface level level fluctuation fluctuation in in the the form form of of surface surface wa wave.ve. The The wave wave transfer transfer process process may may cause cause a a periodic periodic fluctuationfluctuation of of liquid liquid surface surface in in the the mold, mold, which which results results in in mold mold fluxes fluxes infiltration infiltration when when the the velocity velocity of of thethe free free surface surface is is too too high. high. At At the the same same time, time, the the strong strong drift drift flow flow can can greatly greatly destroy destroy the the stability stability of of thethe flow flow field field and and cause cause excessive excessive fl fluctuationsuctuations of of the the meniscus meniscus and and an an increase increase of of the the velocity, velocity, resulting resulting inin slag slag entrapment. entrapment. This This is iscalled called the the phenom phenomenonenon of ofdrift drift flow flow or orthe the drift drift phenomenon. phenomenon. TheThe velocity velocity distribution distribution of ofthe the steel-slag steel-slag interfac interfacee when when the eccentric the eccentric distance distance of the of original the original SEN changesSEN changes is shown is shownin Figure in Figure6, where6, whereit can be it canobserved be observed that when that the when SEN the is SENlocated is locatedin the middle in the positionmiddle of position the mold, of the the mold, flow velo the flowcities velocitiesof both sides of both of the sides nozzle of the are nozzle basically are the basically same in the the same steel- in slag interface. When the nozzle deviates from the mold center of 25 mm, there are obvious differences

Metals 2020, 10, 691 10 of 23

Metals 2020, 10, x FOR PEER REVIEW 10 of 23 the steel-slag interface. When the nozzle deviates from the mold center of 25 mm, there are obvious diin ffvelocitieserencesin between velocities the betweentwo sides the of the two nozzle sides ofin the steel-slag nozzle in interface, the steel-slag increasing interface, significantly increasing on significantlythe deviated on side the of deviated the SEN, side which of the SEN,is due which to th ise dueSEN to deviation the SEN deviationfrom the fromcenter the of center the ofmold. the mold.Additionally, Additionally, the vortex the center vortex location center locationof the back offlow the backflow changes, changes,causing upward causing reflux upward to be reflux obviously to be obviouslystrengthened, strengthened, while the erosion while the for erosion the initial for thesolid initialified solidifiedshell from shell molten from steel molten flow steelon the flow deviated on the deviatedside of SEN side also of SENincreases. also increases.

Figure 6. The effect of eccentric distance on the velocity of the steel-slag interface. Figure 6. The effect of eccentric distance on the velocity of the steel-slag interface. In order to research further the effect of drift phenomenon on bubble distribution and inclusion removalIn order efficiency to research in the mold,further a seriousthe effect eccentric of drift flow phenomenon field with on SEN bubble eccentric distribution distance ofand 50 inclusion mm was selectedremoval asefficiency an example in the to mold, compare a serious with the eccentric simulation flow resultsfield with of theSEN mold eccentric with SENdistance eccentric of 50 mm distance was ofselected 0 mm. as an example to compare with the simulation results of the mold with SEN eccentric distance of 0 mm.The distribution of argon bubbles in the mold is shown in Figure7. As seen in Figure7, when the nozzleThe is locateddistribution in the of center argon of bubbles the mold, in thethe distribution mold is shown of argon in Figure bubbles 7. As on bothseen sidesin Figure of the 7, SEN when is uniform,the nozzle and is located the bubbles in the with center diameter of the overmold, 2 the mm distribution are mainly distributedof argon bubbl aroundes on the both nozzle. sides This of the is mainlySEN is uniform, due to large-size and the bubbles bubbles with with diameter larger buoyancy, over 2 mm which are mainly are less distributed affected by around the drag theforce nozzle. of moltenThis is mainly steel and due can to float large-size quickly bubbles near the with nozzle, larger but buoyancy, it is easy which to cause are the less increase affected of by fluctuation the drag offorce liquid of molten level near steel the and nozzle. can float The bubblesquickly withnear diameterthe nozzle, over but 1 it mm is easy are widely to cause distributed the increase in the of flowfluctuation field, which of liquid is mainly level becausenear the the nozzle. buoyancy The ofbubbles small-size with bubbles diameter is small, over and1 mm the bubblesare widely are greatlydistributed influenced in the byflow the field, drag which force ofis moltenmainly steel,because so that the theybuoyancy can impact of smal furtherl-size withbubbles molten is small, steel. Whenand the the bubbles eccentric are distance greatly ofinfluenced SEN is 50 by mm, the thedrag symmetry force of ofmolten argon steel, bubbles so that distribution they canon impact both sidesfurther of with the SEN molten is less steel. affected, When and the theeccentric main influencedistance isof thatSEN the is distribution50 mm, the ofsymmetry argon bubbles of argon in thebubbles whole distribution mold is not on uniform. both sides On of the the side SEN far is from less theaffected, eccentric and direction the main of influence the nozzle, is that there the is somedistribution areas thatof argon the argon bubbles bubbles in the cannot whole reach, mold whichis not uniform. weakens On the the effect side of far argon from blowing the eccentric in the mold.direction The of movement the nozzle, behavior there is ofsome argon areas bubbles that isthe similar argonto bubbles that of cannot the non-eccentric reach, which SEN, weakens but due the to theeffect eff ofect argon of the blowing deviation in the of the mold. center The stream, movement the disturbancebehavior of argon of the bubbles flow field is similar in the lower to that part of the of thenon-eccentric mold increases, SEN, andbut due the impactto the effect depth of of the most devia bubblestion of blown the center out from stream, the bottomthe disturbance hole decreases. of the However,flow field therein the are lower very part few of small the mold bubbles increases, separated and from the theimpact main depth argon of blowing most bubbles range toblown obtain out a largerfrom the impact bottom depth. hole Considering decreases. However, the force ofthere small-size are very bubbles, few small these bubbles separated separated bubbles from are the not main easy toargon float blowing with other range bubbles, to obtain and cana larger eventually impact stay depth. in the Considering solidified shell.the force of small-size bubbles, these separated bubbles are not easy to float with other bubbles, and can eventually stay in the solidified shell.

Metals 2020,, 10,, 691x FOR PEER REVIEW 1111 of of 23 Metals 2020, 10, x FOR PEER REVIEW 11 of 23

(a) (b) (a) (b) Figure 7. Distribution of argon bubbles of different SEN eccentric distances. (a) SEN eccentric Figure 7. Distribution of argon bubbles of different SEN eccentric distances. (a) SEN eccentric Figuredistances 7. Distributionis 0 mm; (b) ofSEN argon eccentric bubbles distances of different is 50 SEN mm. eccentric distances. (a) SEN eccentric distances isdistances 0 mm; ( bis) 0 SEN mm; eccentric (b) SEN distances eccentric isdistances 50 mm. is 50 mm. The movement of inclusions in mold is also simulated in mathematical model. As shown in Figure The movement of inclusions in mold is also simulated in mathematical model. As shown in 8, withThe the movement increase of of eccentric inclusions distance in mold of theis also SEN, simu thelated removal in mathematical rate of inclusions model. in theAs shownmold decreases. in Figure Figure8, with the increase of eccentric distance of the SEN, the removal rate of inclusions in the mold The8, with lowest the increaseremoval of rate eccentric is 40.10%, distance when of the the SEN SEN, ecce thentric removal distance rate isof 50 inclusions mm, which in the decreases mold decreases. by more decreases. The lowest removal rate is 40.10%, when the SEN eccentric distance is 50 mm, which thanThe lowest15.8% removalcompared rate to isthe 40.10%, removal when rate the (47.64%) SEN ecce of thentric mold distance with is the 50 nozzlmm, whiche in the decreases middle position. by more decreases by more than 15.8% compared to the removal rate (47.64%) of the mold with the nozzle in Becausethan 15.8% the comparedsmall inclusions to the inremoval the mold rate are (47.64%) mainly of dr theiven mold by the with drag the force nozzl ofe the in themolten middle steel, position. and in the middle position. Because the small inclusions in the mold are mainly driven by the drag force of theBecause eccentric the small flow field,inclusions the large in the drift mold flow are of mainly molten dr steeliven inby the the mold drag causesforce of small the molten inclusions steel, to and flow in the molten steel, and in the eccentric flow field, the large drift flow of molten steel in the mold causes downthe eccentric with it andflow stay field, at thethe largedeeper drift location flow of themolten liquid steel core, in butthe moldthey do causes not flo smallat upward inclusions so that to theyflow small inclusions to flow down with it and stay at the deeper location of the liquid core, but they do not aredown gathered with it insideand stay the at round the deeper bloom. location of the liquid core, but they do not float upward so that they floatare gathered upward inside so that the they round are gatheredbloom. inside the round bloom.

Figure 8. Inclusion removal rates of different SEN eccentric distances. Figure 8. Inclusion removal rates of different SEN eccentric distances. In order to studyFigure the 8. e ffInclusionect of eccentric removal rates flow of field different on the SEN temperature eccentric distances. field of the mold and the solidificationIn order shellto study in the the round effect bloom, of eccentric the temperature flow field on distribution the temperature in different field positions of the mold of the and mold the wassolidification studiedIn order byshellto takingstudy in the the the round effect SEN bloo of eccentric eccentricm, the temperature distance flow fie ofld 50ondistribution mmthe temperature with in larger different driftfield positions of phenomenon the mold of the and asmold the an example,wassolidification studied and by shell compared taking in thethe withroundSEN eccentric the bloo temperaturem, distancethe temperature fieldof 50 ofmm thedistribution with mold larger with in drift thedifferent phenomenon SEN in positions the center as ofan the position.example, mold Someandwas studiedcompared representative by takingwith positionsthe the temperatureSEN wereeccentric selected field distance of to analyzethe of 50mold mm the with with temperature thelarger SEN drift distribution,in phenomenon the center including position. as an example, the Some the and compared with the temperature field of the mold with the SEN in the center position. Some radialrepresentative temperature positions distribution were selected of liquid to analyze level, radial the temperature temperature distribution, distribution including of mold the outlet the radial plane representative positions were selected to analyze the temperature distribution, including the the radial (temperatureZ = 900 mm ),distribution the center axisof liquid temperature level, radial distribution temperature of the distribution mold and theof mold center outlet axis ofplane the SEN.(Z = temperature distribution of liquid level, radial temperature distribution of mold outlet plane (Z = Because900mm), thethe nozzlecenter deviatesaxis temperature from the distribution center axis ofof thethe mold,mold theandtemperature the center axis diff oference the SEN. between Because the 900mm), the center axis temperature distribution of the mold and the center axis of the SEN. Because centerthe nozzle axis deviates of the mold from and the the center center axis axis of the of themold SEN, the is temperature selected to judge difference the temperature between the distribution center axis the nozzle deviates from the center axis of the mold, the temperature difference between the center axis insideof the mold the mold. and the center axis of the SEN is selected to judge the temperature distribution inside the mold.of the mold and the center axis of the SEN is selected to judge the temperature distribution inside the mold.

Metals 2020, 10, 691 12 of 23 Metals 2020, 10, x FOR PEER REVIEW 12 of 23

Under the same conditions, the effects effects of the nozzl nozzlee eccentric distances (0, (0, 12.5, 12.5, 25, 25, 37.5 37.5 and and 50 50 mm) mm) forfor the the drift drift index index of of the the mold mold are shown in Figure 99.. AccordingAccording toto thethe datadata inin FigureFigure9 9,, the the theoretical theoretical relationship between the eccentric distance data and the drift index (B) (B) of of mold mold is is derived, derived, as as shown shown in in Equation (14): (14): B =B a= a ×[1 [1 −exp exp ( (−bb × DD)])] (14) (14) × − − × where D represents the values of the eccentric distance of SEN, mm; and a and b are the empirical constants, a = −0.021,0.021, b b == −0.063.0.063. − −

(a) (b)

(c) (d)

Figure 9. The effect of eccentric distance on the temperature field of mold when: (a) The temperature Figure 9. The effect of eccentric distance on the temperature field of mold when: (a) The temperature distribution along radial direction as the SEN eccentric distance is 0 mm; (b) The temperature distribution distribution along radial direction as the SEN eccentric distance is 0 mm; (b) The temperature along central line as the SEN eccentric distance is 0 mm; (c) The temperature distribution along radial distribution along central line as the SEN eccentric distance is 0 mm; (c) The temperature distribution direction as the SEN eccentric distance is 50 mm; (d) The temperature distribution along. along radial direction as the SEN eccentric distance is 50 mm; (d) The temperature distribution along. From Figure 10 and Equation (14), it is concluded that there exists an exponential relationship betweenFrom the Figure drift index10 and (B Equation) and the eccentric(14), it is distanceconcluded (D) that of SEN, there and exists the driftan exponential index of mold relationship increases betweenwith the increasethe drift ofindex SEN (B eccentric) and the distance. eccentric Whendistance the (D eccentric) of SEN, distance and the is drift less index than 5%of mold (30 mm) increases of the withdiameter the increase of the mold, of SEN the drifteccentric index distance. increases When slightly, the but eccentric both are distance less than is 0.1.less Atthan this 5% time, (30 themm) peak of thevalue diameter of the stream of the mold, at the eccentricthe drift index side of increases the nozzle slig ishtly, higher but than both the are other less than side, 0.1. yet At the this basic time, type the of peakflow fieldvalue also of the remains stream unchanged. at the eccentric When side the of eccentric the nozzle distance is higher of nozzle than the is more other than side, 5% yet (30 the mm) basic of typethe mold of flow diameter; field also however, remains the unchanged. drift index When increases the eccentric dramatically distance and of the nozzle maximum is more is overthan 0.45,5% (30 in mm)which of the the velocity mold diameter; distribution however, of flow the field drift in index the mold increases completely dramatically changes and and thethe maximum disturbance is over of 0.45,the liquid in which surface the from velocity the stream distribution of the eccentricof flow sidefield increases. in the mold It is easycompletely to form changes a vortex and near the disturbance of the liquid surface from the stream of the eccentric side increases. It is easy to form a

Metals 2020, 10, x FOR PEER REVIEW 13 of 23 vortex near the nozzle of the mold, which can cause adverse effects such as slag entrapment. Therefore, when the eccentric distance of SEN is 50 mm, the phenomenon of eccentric flow is most serious. Based on the above analysis and the actual production situation, the eccentric distance of 50 Metals 2020, 10, 691 13 of 23 Metalsmm is 2020 selected, 10, x FOR to studyPEER REVIEW eccentric flow field and optimize the SEN. 13 of 23 nozzlevortex ofnear the the mold, nozzle which of canthe causemold, adverse which ecanffects cause such adverse as slag entrapment.effects such Therefore,as slag entrapment. when the eccentricTherefore, distance when the of SENeccentric is 50 distance mm, the of phenomenon SEN is 50 mm, of eccentric the phenomenon flow is most of eccentric serious. Basedflow is on most the aboveserious. analysis Based andon the the above actual analysis production and situation, the actual the production eccentric distancesituation, of the 50 mmeccentric is selected distance to study of 50 eccentricmm is selected flow field to study and optimizeeccentric theflow SEN. field and optimize the SEN.

Figure 10. The effect of eccentric distance on the drift index.

3.2. Effect of Selection of The Bottom Structure of The SEN on Eccentric Flow Field

In the selection and optimization of the SEN, the existence of the bottom outlet has a great influence on the flow fieldFigure in 10. theThe mold. effect The of eccentric original distance SEN is on the the nozzle drift index. with a bottom hole, so the influence of the bottomFigure shapes 10. of The the effect SEN of oneccentric the fl owdistance field on of the mold drift is index. directly investigated under 3.2. Effect of Selection of the Bottom Structure of the SEN on Eccentric Flow Field eccentric casting conditions. When the casting speed is 0.3 m/min with an immersion depth of 160 3.2.mm Effect andIn the eccentric of selection Selection distance and of The optimization Bottomis 50 mm, Stru ofthecture the distribution SEN, of The the SEN existence of on turbulent Eccentric of the ki Flow bottomnetic Field energy outlet hasin the a great liquid influence surface onof the theIn mold, flow the fieldselection with in and the and without mold. optimization The a bottom original ofhole, SEN the is isSEN, shown the nozzlethe in existence Figure with a 11. bottom of the hole,bottom so theoutlet influence has a ofgreat the bottominfluenceFrom shapes on Figure the of flow 11, the it SENfield is seen on in thethatthe flow mold.when field theThe ofSEN original mold has is a SEN directlybottom is the outlet, investigated nozzle the withturbulent under a bottom eccentrickinetic hole, energy casting so the in conditions.influencethe mold decreasesofWhen the bottom the significantly. casting shapes speed of Th theis is 0.3SEN because m on/min the withthe fl owexistence an field immersion of of mold the depthbottom is directly of outlet 160 investigated mm canand play eccentric the under role distanceeccentricof distribution. iscasting 50 mm, The conditions. the dip distribution angle When of the ofthe side turbulent casting hole in kineticsp theeed SENis energy 0.3 is m/min upward-sloping in the with liquid an surface immersion to effectively of the depth mold, improve of with 160 andmmthe temperature withoutand eccentric a bottom of distance the hole, steel-slag is is 50 shown mm, interface, inthe Figure distribution and 11it .is also of turbulentbeneficial kitonetic the removal energy in of the inclusions. liquid surface of the mold, with and without a bottom hole, is shown in Figure 11. From Figure 11, it is seen that when the SEN has a bottom outlet, the turbulent kinetic energy in the mold decreases significantly. This is because the existence of the bottom outlet can play the role of distribution. The dip angle of the side hole in the SEN is upward-sloping to effectively improve the temperature of the steel-slag interface, and it is also beneficial to the removal of inclusions.

Figure 11. Turbulent kinetic energy distribution of liquid surface with different nozzle bottoms. Figure 11. Turbulent kinetic energy distribution of liquid surface with different nozzle bottoms. From Figure 11, it is seen that when the SEN has a bottom outlet, the turbulent kinetic energy in the mold decreases significantly. This is because the existence of the bottom outlet can play the role of distribution. The dip angle of the side hole in the SEN is upward-sloping to effectively improve the temperature of the steel-slag interface, and it is also beneficial to the removal of inclusions. However,Figure 11. when Turbulent the mold kinetic is at energy state of distribution high casting of liq speed,uid surface the upward-sloping with different nozzle nozzle bottoms. can aggravate level fluctuation. The bottom outlet can split the molten steel flows of four side holes so that the

Metals 2020, 10, x FOR PEER REVIEW 14 of 23

However, when the mold is at state of high casting speed, the upward-sloping nozzle can aggravate level fluctuation. The bottom outlet can split the molten steel flows of four side holes so that the velocity of liquid steel inside the hole is not too large to cause slag entrapment or the liquid surface to be exposed. MetalsUnder2020, 10 ,the 691 same conditions and at an eccentric distance of 50 mm, pressure distribution 14at ofthe 23 different SEN bottoms is shown in Figure 12. As the SEN bottom has an outlet, the pressure of the SEN bottom from molten steel is greatly reduced, which is beneficial to decrease the impact of molten steelvelocity on the of liquid SEN and steel therefore inside the is holeof positive is not too signi largeficance to cause in improving slag entrapment the service or the life liquid of the surface SEN. to be exposed.The above analysis shows that, under the eccentric casting condition, the SEN with a bottom outletUnder can significantly the same conditions reduce the and bottom at an pressure eccentric and distance turbulent of 50 ki mm,netic pressureenergy, weaken distribution the erosion at the ofdi fftheerent nozzle SEN and bottoms decrease is shown the fluctuation in Figure 12 of. Asthe theliqu SENid surface. bottom Therefore, has an outlet, the thebottom pressure outlet of plays the SEN an importantbottom from role molten in thesteel eccentric is greatly flow reduced,field improvement. which is beneficial to decrease the impact of molten steel on the SEN and therefore is of positive significance in improving the service life of the SEN.

Figure 12. Variation of pressure at different bottoms of a submerged nozzle. Figure 12. Variation of pressure at different bottoms of a submerged nozzle. The above analysis shows that, under the eccentric casting condition, the SEN with a bottom 3.3.outlet Effect can of significantly Selection of reduceThe Rotation the bottom Angle pressureof the Side and Hole turbulent on Eccentric kinetic Flow energy, Field weaken the erosion of the nozzle and decrease the fluctuation of the liquid surface. Therefore, the bottom outlet plays an 3.3.1.important Effect role of the in theRotation eccentric Angl flowe of field the Side improvement. Hole on Flow Field

3.3. ETheffect choice of Selection of different of the Rotation rotation Angle angles of the on Side the Hole side on hole Eccentric of the Flow SEN Field can affect the flow field distribution of the mold in different degrees. According to the analysis of section 3.2, based on reservation3.3.1. Effect of of the the bottom Rotation outlet, Angle the of effects the Side of Holerotation on Flowangles Field (0°, 15° and 30°) on liquid wave height, turbulent kinetic energy and impact depth of the mold are analyzed in the following section. The choice of different rotation angles on the side hole of the SEN can affect the flow field Figure 13 shows the velocity vector diagram of the steel-slag interface. Combined with Figure 4, distribution of the mold in different degrees. According to the analysis of Section 3.2, based on it can be found that under the condition of non-eccentric flow field, the four upward-sloping side reservation of the bottom outlet, the effects of rotation angles (0 , 15 and 30 ) on liquid wave height, holes of the SEN bring four upward streams to the liquid surface◦ of◦ mold. The◦ streams then hit the turbulent kinetic energy and impact depth of the mold are analyzed in the following section. wall of the mold, and a part of the molten steel continues to rise, flowing through the steel-slag Figure 13 shows the velocity vector diagram of the steel-slag interface. Combined with Figure4, it interface and reflowing down near the nozzle, resulting in four high-speed regions and the vortex can be found that under the condition of non-eccentric flow field, the four upward-sloping side holes core regions in a direction perpendicular to the liquid surface, which can bring enough heat fluxes of the SEN bring four upward streams to the liquid surface of mold. The streams then hit the wall of and flows. However, excessively high speed can also cause adverse effects such as slag entrapment. the mold, and a part of the molten steel continues to rise, flowing through the steel-slag interface and In Figure 13, when the rotation angle of the side hole is not 0°, four upward flows impact the reflowing down near the nozzle, resulting in four high-speed regions and the vortex core regions in a wall of mold along the vertical direction, rise to the steel slag interface, and then reflow evenly to the direction perpendicular to the liquid surface, which can bring enough heat fluxes and flows. However, nozzle, and the liquid surface is stable. However, the molten steel that directly reflows from the excessively high speed can also cause adverse effects such as slag entrapment. nozzle has a large downward velocity, increasing the impact depth of the center stream. When the

Metals 2020, 10, x FOR PEER REVIEW 15 of 23 rotation angle of side hole is 15°, four rising streams impact the wall of mold according to a certain angle of rotation. Thus, the molten steel that reaches the steel-slag interface also forms four high speed streams with a certain angle of rotation. Additionally, the horizontal swirling flow around the nozzle is formed to weaken the downward velocity of the reflux, but the process that the molten steel reflows evenly to the nozzle is still partially present. When the rotation angle of the side hole is 30°, the process of direct even reflux in the steel-slag interface is completely replaced by the swirling process of four high speed streams, forming a plurality of horizontal swirling centers near the nozzle, a phenomenonMetals 2020, 10, 691of swirling flow that is more obvious. 15 of 23

(a) (b) (c)

Figure 13. Velocity vector of the steel-slag interface in different rotation angles of the side hole. (a) The Figure 13. Velocity vector of the steel-slag interface in different rotation angles of the side hole. (a) rotation angles on the side hole of the SEN is 0◦;(b) The rotation angles on the side hole of the SEN is The rotation angles on the side hole of the SEN is 0°; (b) The rotation angles on the side hole of the 15◦;(c) The rotation angles on the side hole of the SEN is 30◦. SEN is 15°; (c) The rotation angles on the side hole of the SEN is 30°.

In Figure 13, when the rotation angle of the side hole is not 0◦, four upward flows impact the wall of moldWhen along the the SEN vertical eccentric direction, distance rise tois the0 mm steel with slag interface,a casting andspeed then of reflow 0.3 m/min evenly and to the an nozzle, SEN immersionand the liquid depth surface of 160 is stable.mm, different However, rotation the molten angles steel of the that side directly holes reflows cause fromeffects the on nozzle the liquid has a wavelarge downwardof the mold velocity,and the increasingturbulence the kinetic impact ener depthgy of of liquid the center surface, stream. as shown When in the Figure rotation 14. angle of In order to make the measured wave height data of numerical simulation and modeling side hole is 15◦, four rising streams impact the wall of mold according to a certain angle of rotation. experimentThus, the molten comparable, steel that all reacheswave height the steel-slag data have interface become also dimensionless: forms four high speed streams with a certain angle of rotation. Additionally, the horizontalFD = 100FS/( swirlingλD) flow around the nozzle is formed(15) to weaken the downward velocity of the reflux, but the process that the molten steel reflows evenly to the In this formula, FD represents dimensionless wave height; FS is the measured wave height data nozzle is still partially present. When the rotation angle of the side hole is 30◦, the process of direct of liquid surface, mm; λ and D represent the value of similarity ratio and the horizontal section even reflux in the steel-slag interface is completely replaced by the swirling process of four high speed diameter of different mold models, mm. streams, forming a plurality of horizontal swirling centers near the nozzle, a phenomenon of swirling Figure 14 shows that when the outlets of side holes have rotation angles at tangential directions, flow that is more obvious. the average wave height of the liquid surface tends to decrease with increasing rotation angle. The When the SEN eccentric distance is 0 mm with a casting speed of 0.3 m/min and an SEN immersion side holes with rotation angle can obviously reduce the turbulent kinetic energy of the mold surface depth of 160 mm, different rotation angles of the side holes cause effects on the liquid wave of the mold and eliminate the peak region of turbulent kinetic energy. This is because the rotation angle of the Metalsand the2020 turbulence, 10, x FOR PEER kinetic REVIEW energy of liquid surface, as shown in Figure 14. 16 of 23 side holes increases the horizontal velocity of streams and brings the horizontal swirls to the mold when the molten steel flows out through SEN. The existence of horizontal swirling flows can weaken the velocity of the molten steel flow in the Z direction (axis of mold) so that the upward reflux velocity decreases, thereby reducing the vortex center position of upward reflux and disturbance to the free surface from it. In this condition, the liquid wave height of the mold also decreases. At the same time, the elimination of the peak value in Figure 14b can effectively prevent the excessive fluctuation of the area that can cause many problems, such as liquid steel exposure.

(a) (b)

FigureFigure 14. 14. TheThe effect effect of of rotation rotation angle angle of of side side holes holes to to liquid liquid surface surface of of the the mold. ( (aa)) Liquid Liquid wave wave height;height; ( (bb)) Turbulence Turbulence kinetic kinetic energy energy of of liquid liquid surface. surface.

The side-hole jets and central stream have comprehensively effect on impact depth. As shown in Figure 15, with the increase of the rotation angle of the side hole, the impact depth of the stream in the mold gradually decreases. Like in Figure 14, this is because of the rotation angle so that swirling flows in the horizontal direction are formed in the mold. The presence of these swirling flows can disperse the initial kinetic energy of molten steel and reduce the velocity component in the Z direction; the horizontal swirls also improve the impact so that the streams are gentler. Therefore, the increase of the rotation angle can decrease the impact depth. The greater the rotation angle and the higher the intensity of horizontal swirling flows are, the more obvious the effect will be. From the above analysis, it is concluded that the horizontal swirling flows occur in the mold with the increase of the rotation angle of the side holes with other conditions unchanged. The impact depth of stream and the fluctuation of the liquid surface gradually reduce, and the turbulent kinetic energy of liquid surface decreases significantly. To fully utilize the metallurgical capability of horizontal swirling flows, it is recommended that the rotation angle of the side hole be 30°.

Figure 15. The effect of rotation angle of side holes for the impact depth of liquid steel.

3.3.2. Effect of the Swirling SEN Under Eccentric Casting Under the condition of eccentric casting, the flow field behaviors of the mold employing the optimized swirling SEN, which is the swirling SEN with the rotation angle of side holes of 30°, are analyzed to verify the metallurgical effect of the optimized parameters in the following section.

Metals 2020, 10, 691 16 of 23

MetalsIn 2020 order, 10, x to FOR make PEER the REVIEW measured wave height data of numerical simulation and modeling experiment16 of 23 comparable, all wave height data have become dimensionless:

FD = 100FS/(λD) (15)

In this formula, FD represents dimensionless wave height; FS is the measured wave height data of liquid surface, mm; λ and D represent the value of similarity ratio and the horizontal section diameter of different mold models, mm. Figure 14 shows that when the outlets of side holes have rotation angles at tangential directions, the average wave height of the liquid surface tends to decrease with increasing rotation angle. The side holes with rotation angle can obviously reduce the turbulent kinetic energy of the mold surface and eliminate the peak region of turbulent kinetic energy. This is because the rotation angle of the side holes increases the horizontal velocity of streams and brings the horizontal swirls to the mold when the molten steel flows out through SEN. The existence of horizontal swirling flows can weaken the velocity of the molten steel flow in the Z direction (axis of mold) so that the upward reflux velocity decreases, thereby reducing(a) the vortex center position of upward reflux( andb) disturbance to the free surfaceFigure from 14. it. The In thiseffect condition, of rotation the angle liquid of side wave holes height to liquid of the surface mold alsoof the decreases. mold. (a) AtLiquid the samewave time, the eliminationheight; (b) Turbulence of the peak kinetic value energy in Figure of liquid 14b cansurface. effectively prevent the excessive fluctuation of the area that can cause many problems, such as liquid steel exposure. The side-hole jets andand centralcentral streamstream have have comprehensively comprehensively e ffeffectect on on impact impact depth. depth. As As shown shown in inFigure Figure 15 15,, with with the the increase increase of of the the rotation rotation angle angle of of the the side side hole, hole, thethe impactimpact depthdepth of the stream in thethe mold mold gradually gradually decreases. decreases. Like Like in in Figure Figure 14,14, this this is is because because of of the the rotation rotation angle angle so so that that swirling swirling flowsflows in the horizontal direction are formed in the mold. The The presence presence of of these swirling flows flows can disperse the initial kinetic energy of molten steel an andd reduce the velocity component in the Z direction; thethe horizontal horizontal swirls swirls also also improve improve the the impact impact so so th thatat the the streams streams are gentler. Therefore, Therefore, the increase of the the rotation rotation angle angle can can decrease decrease the the impact impact depth depth.. The The greater greater the the rotation rotation angle angle and and the thehigher higher the intensitythe intensity of horizontal of horizontal swirling swirling flows flowsare, the are, more the obvious more obvious the effect the will effect be. willFrom be. the From above the analysis, above itanalysis, is concluded it is concluded that the horizontal that the horizontal swirling flows swirling occur flows in the occur mold in thewith mold the withincrease the of increase the rotation of the anglerotation of anglethe side of the holes side with holes other with otherconditions conditions unchanged. unchanged. The Theimpact impact depth depth of ofstream stream and and the the fluctuationfluctuation of the liquid surface gradually reduce, and the turbulent kinetic energy of liquid surface decreases significantly. significantly. To To fully utilize the metallurgical capability of horizontal swirling flows, flows, it it is recommendedrecommended that the rotation angle of the side hole be 3030°.◦.

Figure 15. The effect of rotation angle of side holes for the impact depth of liquid steel. Figure 15. The effect of rotation angle of side holes for the impact depth of liquid steel.

3.3.2. Effect of the Swirling SEN Under Eccentric Casting Under the condition of eccentric casting, the flow field behaviors of the mold employing the optimized swirling SEN, which is the swirling SEN with the rotation angle of side holes of 30°, are analyzed to verify the metallurgical effect of the optimized parameters in the following section.

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3.3.2. Effect of the Swirling SEN Under Eccentric Casting Under the condition of eccentric casting, the flow field behaviors of the mold employing the optimized swirling SEN, which is the swirling SEN with the rotation angle of side holes of 30◦, are Metalsanalyzed 2020, to10, verifyx FOR PEER the metallurgical REVIEW effect of the optimized parameters in the following section.17 of 23 Figure 16 shows the distribution of flow fields of the optimized swirling SEN in mold when the castingFigure speed 16 is shows 0.3 m/ minthe distribution with an immersion of flow depthfields of 160the mmoptimized and an swirling eccentric SEN distance in mold of 0 when or 50 mm.the castingComparing speed the is flow0.3 m/min fields withwith thosean immersion of the original depth nozzle of 160 (Figuremm and5), an Figure eccentric16 indicates distance that, of 0 when or 50 mm.using Comparing the swirling the SEN, flow the fields side with holes those with of a the tangential original rotation nozzle (Figure angle can 5), formFigure horizontal 16 indicates swirling that, whenflows inusing the mold,the swirling weakening SEN, thethe impactside holes of high with speed a tangential jets on therotation wall ofangle the can mold. form The horizontal ability of swirling flows SEN to in form the mold, a wide weakening range of horizontal the impact swirling of high flowspeed inside jets on the the mold wall is of similar the mold. to that The of abilitythe M-EMS of swirling (Danieli SEN Metallurgical to form a wide Equipment range of &horizontal Service (China)swirling Co flow Ltd, inside Changshu, the mold China) is similar device, to thatwhich of accelerates the M-EMS the horizontaldevice, which movement accelerates of molten the steel horizontal by electromagnetic movement force of andmolten forms steel swirling by electromagneticflow field inside theforce mold. and Theforms maximum swirling horizontal flow field swirling inside velocitythe mold. of moltenThe maximum steel near horizontal the outlet swirlingof the side velocity hole of of the molten swirling steelSEN near canthe reachoutlet 0.6of the m/ s,side and hole the of maximum the swirling swirling SEN can velocity reach near0.6 m/s, the andwall the of mold maximum is over swirling 0.075 m /velocitys, which near is close the to wall the maximumof mold is horizontalover 0.075 swirlingm/s, which velocity is close (generally to the maximum0.05~0.3 m /horizontals) near the swirling wall of mold veloci whenty (generally using the 0.05~0.3 M-EMS m/s) device near [15 the–18 wall], so of it canmold be when considered using thatthe M-EMSthe swirling device SEN [15–18], has a so similar it can metallurgicalbe considered ethatffect the with swirling M-EMS SEN to has a certain a similar extent. metallurgical The horizontal effect withswirling M-EMS flows to formeda certain by extent. swirling The nozzle horizontal is beneficial swirling to flows more formed molten by steel swirling moving nozzle towards is beneficial the shell. toAnd more to a molten certain extent,steel moving its tangential towards velocity the shell. promotes And to the a breakagecertain extent, of the its front tangential end of columnar velocity promotescrystals by the molten breakage steel of to the form front a large end of number columnar of dendrite crystals fragments,by molten steel which to canform be a used large as number crystal ofnuclei, dendrite and increasefragments, the which probability can be of used equiaxed as crystal crystal nuclei, formation. and increase Moreover, the probability the horizontal of equiaxed swirls at crystalthe meniscus formation. can stabilize Moreover, the meniscusthe horizontal fluctuation, swirls reduceat the themeniscus probability can ofstabilize slag entrapment, the meniscus and fluctuation,provide a heat reduce source the forprobability the meniscus of slag to entrapment, improve molten and provide slag. The a heat molten source steel for flowing the meniscus out from to improvethe side holemolten also slag. forms The double molten streams steel flowing of the risingout from and the falling side flows.hole also Compared forms double with the streams original of theSEN, rising however, and falling the existence flows. ofCompared horizontal with swirling the or flowsiginal weakens SEN, however, the velocity the ofexistence double streamsof horizontal in the swirlingZ direction flows and weakens the reflux the center velocity has of been double greatly streams improved. in the BasedZ direction on utilization and the reflux of the center optimized has beenswirling greatly nozzle improved. in an eccentric Based on distance utilization of 50 of mm,the optimized the bias phenomenon swirling nozzle on in center an eccentric stream hasdistance been ofeff ectively50 mm, the restrained bias phenomenon without noticeable on center eccentric stream stream.has been effectively restrained without noticeable eccentric stream.

(a) (b)

Figure 16. Flow fields of molten steel in the mold with different conditions. (a) Five-furcated SEN Figure 16. Flow fields of molten steel in the mold with different conditions. (a) Five-furcated SEN with outlets in tangential direction with the same axis; (b) Five-furcated SEN with outlets in tangential with outlets in tangential direction with the same axis; (b) Five-furcated SEN with outlets in tangential direction with deviation from the center of 50 mm. direction with deviation from the center of 50 mm. Figure 17 shows the velocity vector diagram of the steel-slag interface with the original nozzle and theFigure optimized 17 shows swirling the velocity nozzle vector under diagram the eccentric of the condition.steel-slag interface As seen with in Figure the original 17, compared nozzle and the optimized swirling nozzle under the eccentric condition. As seen in Figure 17, compared with the non-eccentric condition in Figure 13, when the nozzle eccentric distance is 50 mm, using the original nozzle, the molten steel close to the eccentric sidewall of the mold can still return uniformly to the nozzle. However, at the same time, the liquid steel of other areas forms an auxiliary reflux center near the center area of mold, which is the single high-speed reflux vortex and can accelerate the reflux process for molten steel far away from the region of the nozzle to compensate for the structural unbalance of the flow field caused by the nozzle that deviates from the center. The single

Metals 2020, 10, 691 18 of 23 with the non-eccentric condition in Figure 13, when the nozzle eccentric distance is 50 mm, using the original nozzle, the molten steel close to the eccentric sidewall of the mold can still return uniformly to the nozzle. However, at the same time, the liquid steel of other areas forms an auxiliary reflux center near the center area of mold, which is the single high-speed reflux vortex and can accelerate the Metalsreflux 2020 process, 10, x FOR for PEER molten REVIEW steel far away from the region of the nozzle to compensate for the structural18 of 23 unbalance of the flow field caused by the nozzle that deviates from the center. The single high-speed high-speedreflux vortex reflux can, vortex however, can, easily however, cause easily slag cause entrapment slag entrapment of molten steel.of molten After steel. using After the optimizedusing the optimizedswirling nozzle, swirling compared nozzle, compared with the singlewith the reflux single vortex reflux in thevortex flow in field the offlow the field original of the nozzle, original the nozzle,swirling the nozzle swirling produces nozzle fourproduces high-speed four high-spe upwarded streams upward with streams the same with rotationthe same angle rotation so that angle the sowidespread that the widespread horizontal swirlinghorizontal flows swirling are formed flows are in the formed flow fieldin the of flow mold. field Therefore, of mold. there Therefore, are also theremultiple are also centers multiple of horizontal centers of swirling horizontal flows swirling formed flows close formed to the close wall to of the the wall mold of the in the mold steel-slag in the steel-slaginterface. interface. The maximum The maximum horizontal horizontal swirling velocityswirling isvelocity 0.045 m is/s, 0.045 and them/s, downward and the downward velocity in velocitythe center in the of swirlingcenter of flowsswirling is lessflows than is less 0.015 than m /0.015s, which m/s, is which much is lower much than lower the than minimum the minimum critical criticalvelocity velocity for slag for entrainment slag entrainment of 0.264 of m 0.264/s [19 m/s]. Furthermore, [19]. Furthermore, the density the density of the vectorof the invector the velocity in the velocityvector diagramvector diagram of the swirling of the swirling SEN decreases SEN decr greatlyeases comparedgreatly compared with the with diagram the diagram of the ordinary of the ordinarySEN under SEN the under same the conditions, same conditions, this indicates this indica thattes the that high-speed the high-speed flows reduce flows reduce significantly significantly and the andsteel-slag the steel-slag interface interface is more is stable. more Accordingly, stable. Accordin thesegly, not these only greatlynot only weaken greatly the weaken size and the velocity size and of velocitythe reflux of vortexthe reflux near vortex the center near of the the center interface of tothe suppress interface the to phenomenonsuppress the ofphenomenon slag entrapment of slag but entrapmentalso stabilize but the also flow stabilize field of the molten flow steel,field of eff moltectivelyen steel, reducing effectively the inconsistency reducing the of inconsistency reflux process of in refluxdifferent process regions. in different At the same regions. time, At because the same the time, optimized because swirling the optimized SEN can swirling form similar SEN horizontalcan form similarswirling horizontal flows around swirling the centerflows streamaround at the the center lower stream region ofat mold,the lower it can region also greatly of mold, inhibit it can the also bias greatlyphenomenon inhibit ofthe center bias phenomenon stream in the of mold. center stream in the mold.

(a) (b)

Figure 17. Velocity vector of steel-slag interface under the eccentric condition. (a) The original SEN; Figure 17. Velocity vector of steel-slag interface under the eccentric condition. (a) The original SEN; (b) The optimized swirling SEN. (b) The optimized swirling SEN. Compared with Figures 18a and8, the optimized swirling SEN can improve the overall removal rateCompared of 1~50 µm with inclusions, Figure 18a and and the Figure maximum 8, the removal optimized rate swirling is 51.70%. SEN However, can improve under the theoverall SEN removaleccentric rate conditions, of 1~50 μm the inclusions, overall removal and the e ffimaximumciency of removal inclusions rate in isthe 51.70%. mold However, still decreased under fromthe SEN51.7% eccentric to 50.36%, conditions, and the the diff erenceoverall isremoval only 2.6%. efficiency But compared of inclusions with in the the removal mold still rate decreased under diff fromerent 51.7%conditions to 50.36%, of the and original the difference nozzle, the is only removal 2.6%. effi Butciency compared is still with improved. the removal rate under different conditions of the original nozzle, the removal efficiency is still improved.

Metals 2020, 10, 691 19 of 23 Metals 2020, 10, x FOR PEER REVIEW 19 of 23 Metals 2020, 10, x FOR PEER REVIEW 19 of 23

((aa)) (b(b) ) Figure 18. Inclusion removal rates and distribution of bubbles for the optimized swirling SEN. Figure 18. Inclusion removal rates and distribution of bubbles for the optimized swirling SEN. (a) (aFigure) Inclusion 18. Inclusion removal rates;removal (b) rates Distribution and distribution of argon bubbles.of bubbles for the optimized swirling SEN. (a) InclusionInclusion removal removal rates; rates; ((bb)) DistributionDistribution of argon bubbles.bubbles. As shown in Figure 18b, under the SEN eccentric conditions, compared with the distribution of bubblesAsAs inshown shown the original in in Figure Figure SEN 18b,18b, of Figure underunder7 the, when SEN argon eccentric is blown conditions,conditions, by optimized compared compared swirling with with SEN,the the distribution distribution the horizontal of of swirlingbubblesbubbles flowin in thethe can originaloriginal effectively SENSEN promote ofof FigureFigure the 7, horizontal when argon movement isis blownblown of bubbles, byby optimizedoptimized inhibit swirling theswirling non-uniformity SEN, SEN, the the ofhorizontalhorizontal bubbles swirling distribution swirling flow flow in can can the effectively effectively mold caused promote by bias thee flow, horizontalhorizontal reduce movement movement the impact of of bubbles, depth bubbles, of inhibit bubbles,inhibit the the andnon- non- at theuniformityuniformity same time, of of bubbles makebubbles the distdist large-sizeributionribution bubbles inin the mold with acaca diameterusedused byby bias ofbias more flow,flow, than reducereduce 2 mm the the move impact impact farther depth depth in of the of horizontalbubbles,bubbles, and and direction at at the the same same so that time, time, the makemake liquid the level large-size fluctuation bubblesbubbles near withwith the SENa a diameter diameter is weakened. of of more more than than 2 2mm mm move move farther in the horizontal direction so that the liquid level fluctuation near the SEN is weakened. fartherBy in comparing the horizontal the temperature direction so distributions that the liquid of dilevelfferent fluctuation positions near in the the mold SEN shown is weakened. in Figures 9 By comparing the temperature distributions of different positions in the mold shown in Figure and 19By, whencomparing using the the temperature optimized swirling distributions SEN, theof different temperature positions difference in the between mold shown the center in Figure axis 19 and Figure 9, when using the optimized swirling SEN, the temperature difference between the of19 theand mold Figure and 9, when the center using axis the ofoptimized the SEN swir hasling been SEN, eliminated the temperature below 400 difference mm. Although between there the center axis of the mold and the center axis of the SEN has been eliminated below 400 mm. Although iscenter still axis a certain of the temperature mold and the di centerfference axis on of the the liquidSEN has surface been eliminated of the mold, below it is 400 controlled mm. Although within there is still a certain temperature difference on the liquid surface of the mold, it is controlled within 5there K. The is still temperature a certain temperature distribution difference at the outlet on planethe liquid (Z = surface900 mm) of the of the mold, mold it is is controlled uniform and within the 5 K. The temperature distribution at the outlet plane (Z = 900 mm) of the mold is uniform and the temperature5 K. The temperature in most areas distribution of the center at the is controlledoutlet plane around (Z = 900 1750 mm) K. Compared of the mold with is theuniform original and SEN, the temperature in most areas of the center is controlled around 1750 K. Compared with the original SEN, thetemperature temperature in most at the areas outlet of planethe center (over is 1760contro K)lled decreases around by 1750 more K. Compared than 10 K which with the can original promote SEN, the the temperature at the outlet plane (over 1760 K) decreases by more than 10 K which can promote the uniformthe temperature distribution at the of outlet the shell plane thickness (over 1760 around K) decreases the cross sectionby more of than the mold.10 K which All these can indicatepromote that the uniformuniform distribution distribution ofof thethe shellshell thicknessthickness around thethe crosscross sectionsection of of the the mold. mold. All All these these indicate indicate thethat eccentric the eccentric phenomenon phenomenon of the of temperature the temperature field fie distributionld distribution caused caused by the by driftthe drift flow flow of molten of molten steel hasthatbeen the eccentric greatlyimproved. phenomenon of the temperature field distribution caused by the drift flow of molten steelsteel has has been been greatly greatly improved. improved.

(a) (b) (a) (b) Figure 19. Temperature field of the mold with the optimized swirling SEN. (a) The temperature Figure 19. Temperature field of the mold with the optimized swirling SEN. (a) The temperature distribution along radial direction; (b) The temperature distribution along the center line. Figuredistribution 19. Temperature along radial fielddirection; of the (b )mold The temperature with the optimized distribution swirling along theSEN. center (a) Theline. temperature distribution along radial direction; (b) The temperature distribution along the center line.

Metals 20202020,, 10,, x 691 FOR PEER REVIEW 2020 of of 23 23

As shown in Figure 20, in the eccentric flow field, the optimized swirling nozzle has a shallower impactAs depth shown than in Figure the original 20, in nozzle, the eccentric which flow is bene field,ficial the to optimized floating and swirling removing nozzle inclusions. has a shallower When theimpact eccentric depth distance than the originalreaches nozzle,50 mm, which the drift is beneficial index of tothe floating original and SEN removing reaches inclusions. 0.46, which When causes the theeccentric flow field distance to be reachescompletely 50 mm, unbalanced the drift with index turbulent of the original kinetic SENenergy reaches and the 0.46, surface which disturbance causes the increases.flow field However, to be completely in the eccentric unbalanced casting with condition, turbulent the kinetic optimized energy swirling and the nozzle surface can disturbance obviously reduceincreases. the However,drift index in to the 0.28, eccentric a decline casting of more condition, than 39%. the In optimized such conditions swirling the nozzle drift canphenomenon obviously ofreduce the mold the drift is suppressed index to 0.28, in the a decline lower oflevel more so thanthat it 39%. does In not such affect conditions the normal the driftdistribution phenomenon of flow of field.the mold is suppressed in the lower level so that it does not affect the normal distribution of flow field.

Figure 20. Impact depth and drift index of molten steel for SEN before and after optimization. Figure 20. Impact depth and drift index of molten steel for SEN before and after optimization. 3.4. Comparison with Water Modeling Experiment 3.4. ComparisonAccording With to the Water simulation Modeling results Experiment of different SEN structures and process parameters obtained in theAccording previous experiments,to the simulation the structural results of parametersdifferent SEN of SENstructures have beenand process optimized parameters and the optimized obtained inswirling the previous nozzle is experiments, as follows: five-furcated the structural nozzle parameters with a bottom of holeSEN andhave four been side optimized holes, thetangential and the optimizedrotation angle swirling of side nozzle holes is 30as ◦follows:. Under five-furcated the same parameters nozzle with of mold, a bottom the water hole modeling and four experimentside holes, thehas tangential been applied rotation as a contrast angle testof side to verify holes the is practical30°. Under effect the of same the optimized parameters swirling of mold, nozzle the and water the modelingoriginal nozzle experiment in the eccentrichas been applied flow field. as a contrast test to verify the practical effect of the optimized swirlingAs shownnozzle inand Figure the original 21, according nozzle to in the the results eccentric of experimentflow field. in the eccentric distance of 50 mm, it is foundAs shown that in the Figure flow 21, field according in the mold to the employing results of experiment the original in nozzle the eccentric is obviously distance biased of 50 to mm, the itside is found on which thatthe the nozzle flow field is close in the to themold wall. employing Two seconds the original after tracer nozzle injection, is obviously there isbiased an obvious to the sideimbalance on which in the the flow nozzle field is of close mold. to Inthe contrast, wall. Two with seconds the use after of optimized tracer injection, swirling there nozzle is an casting, obvious the imbalancecentral stream in the of moltenflow field steel of doesmold. not In give contrast, rise to with bias the flows use and of optimized the nonuniform swirling flow nozzle field hascasting, been thesignificantly central stream improved. of molten The steel results does of waternot give modeling rise to bias experiment flows and and the numerical nonuniform simulation flow field in has the beenprevious significantly sections areimproved. consistent. The results of water modeling experiment and numerical simulation in the previousFigure 22 sections shows are that consistent. with the use of the original nozzle in the process of eccentric casting, the phenomenon of slag entrapment is obviously observed (Figure 22a). In addition, when the casting speed increases to 0.4 m/min, the mold powder at the stream impact point is washed away by high-speed liquid flow so that it leads to surface exposure of liquid steel. However, after using the optimized swirling nozzle in Figure 22b, there is no slag entrapment or molten steel exposed in the mold under the same conditions of casting speed. This illustrates that the optimized swirling nozzle not only can effectively suppress the drift flow but also can significantly reduce the quality defects of the round bloom.

Metals 2020, 10, x FOR PEER REVIEW 21 of 23

Metals 2020, 10, 691 21 of 23 Metals 2020, 10, x FOR PEER REVIEW 21 of 23

(a) (b) (c) (d)

Figure 21. The flow field of the mold at different times while using different SENs in water model. (a) Original nozzle at t = 0.5 s; (b) Original nozzle at t = 2 s; (c) Optimized swirling nozzle at t = 0.5 s; (d) Optimized swirling nozzle at t = 2 s.

Figure 22 shows that with the use of the original nozzle in the process of eccentric casting, the phenomenon of slag entrapment is obviously observed (Figure 22a). In addition, when the casting speed increases to 0.4 m/min, the mold powder at the stream impact point is washed away by high- speed liquid flow so that it leads to surface exposure of liquid steel. However, after using the optimized swirling(a) nozzle in Figure( b22b,) there is no slag entrapment(c) or molten steel exposed(d) in the mold under the same conditions of casting speed. This illustrates that the optimized swirling nozzle Figure 21. The flow field of the mold at different times while using different SENs in water model. not onlyFigure can 21. effectively The flow field suppress of the moldthe drift at different flow but times also while can significantlyusing different reduce SENs in the water quality model. defects (a) of (a) Original nozzle at t = 0.5 s; (b) Original nozzle at t = 2 s; (c) Optimized swirling nozzle at t = 0.5 s; the roundOriginal bloom. nozzle at t = 0.5 s; (b) Original nozzle at t = 2 s; (c) Optimized swirling nozzle at t = 0.5 s; (d) (d) Optimized swirling nozzle at t = 2 s. Optimized swirling nozzle at t = 2 s.

Figure 22 shows that with the use of the original nozzle in the process of eccentric casting, the phenomenon of slag entrapment is obviously observed (Figure 22a). In addition, when the casting speed increases to 0.4 m/min, the mold powder at the stream impact point is washed away by high- speed liquid flow so that it leads to surface exposure of liquid steel. However, after using the optimized swirling nozzle in Figure 22b, there is no slag entrapment or molten steel exposed in the (a) (b) mold under the same conditions of casting speed. This illustrates that the optimized swirling nozzle not onlyFigure can 22. effectivelyMorphology suppress of steel-slag the drift interface flow in but mold also for SENcan significantly before and after reduce optimization. the quality (a) Original defects of Figure 22. Morphology of steel-slag interface in mold for SEN before and after optimization. (a) the roundnozzle; bloom. (b) Optimized swirling nozzle. Original nozzle; (b) Optimized swirling nozzle. 4. Conclusions 4. Conclusions In this study, the flow fields of a mold equipped with different SEN have been simulated through the methodIn this ofstudy, physical the andflow mathematical fields of a models.mold equipped The effects with of SENdifferent structure SEN parameters have been for simulated turbulent throughkinetic energy, the method the impact of physical depth andanddrift mathematical index are analyzedmodels. The and optimizedeffects of SEN in the structure mold when parameters the SEN foris in turbulent an eccentric kinetic state, energy, namely, the a impact mold indepth an eccentric and drift flow index field. are Theanalyzed following and optimized aspects are in concluded the mold whenin this the paper: SEN is in an eccentric state, namely, a mold in an eccentric flow field. The following aspects (a) (b) are concluded in this paper: (1) The eccentric SEN can result in serious bias flow in the mold, causing asymmetrical distribution 1) FigureofThe flow eccentric 22. field Morphology for SEN molten can of result steel. steel-slag in The serious driftinterface index bias in flow reachesmold in forthe up SEN mold, to 0.46 before causing in theand eccentric asymmetricalafter optimization. distance distribution of (a 50) mm, Originalwhichof flow leadsnozzle; field tofor (b problems) moltenOptimized steel. of slagswirling The entrapment drift nozzle. index and reaches liquid up steel to exposed0.46 in the in eccentric production. distance of 50 (2) Withmm, which the increase leads to of problems the rotation of angleslag entrapme on the SENnt and side liquid holes, steel the horizontalexposed in swirlingproduction. flows are 4. Conclusionsformed in the flow field of mold so that the impact depth of the stream and turbulent kinetic

Inenergy this study, of liquid the surface flow fields decrease of a accordingly. mold equipped with different SEN have been simulated through(3) The the SEN method with a of bottom physical outlet and can mathematical significantly models. reduce theThe bottom effects pressureof SEN structure and turbulent parameters kinetic for turbulentenergy andkinetic weaken energy, the the scour impact of level depth fluctuation and drift and index molten are analyzed steel for the and SEN, optimized thereby in prolonging the mold when thethe serviceSEN is in life an of eccentric nozzle. state, namely, a mold in an eccentric flow field. The following aspects are(4) concludedIn the eccentric in this paper: casting condition with an eccentric distance of 50 mm, the use of the optimized swirling SEN, which employed a five-furcated nozzle with a bottom hole and four side holes and 1) The eccentric SEN can result in serious bias flow in the mold, causing asymmetrical distribution a tangential rotation angle of the side holes of 30 , can effectively inhibit the phenomenon of bias of flow field for molten steel. The drift index re◦aches up to 0.46 in the eccentric distance of 50 flow in the mold and reduce the mold drift index to 0.28, a decline of more than 39%. Compared mm, which leads to problems of slag entrapment and liquid steel exposed in production.

Metals 2020, 10, 691 22 of 23

to a water modeling experiment, it is found that the optimized swirling nozzle can greatly reduce eccentric flow field and the level fluctuation significantly so that slag entrapment and exposed molten steel disappear. (5) The optimized swirling SEN can improve the inclusion removal efficiency in model although it is used in the eccentric casting condition, so it is suggested that the optimized swirling SEN should be used in actual continuous casting operation with eccentric casting condition. (6) The conclusions of the mathematical and physical models in this paper are related to similarity in non-perfect conditions and must be further compared to industrial experience which can be made in future.

Author Contributions: All authors contributed significantly. Y.J., C.C. and Y.L. designed the project. P.L., Y.J., F.Y. and Z.L. performed the numerical simulations and data collection. P.L., Y.J., F.Y. and Z.L. analyzed the data. All authors contributed to the discussion of the results. P.L. and Y.J. wrote and revised the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Nature Science Foundation of China (NSFC) under Grant No. 51974213 and 51874215. Conflicts of Interest: The authors declare no conflict of interest.

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