Evolution and Influence of Flow Field Affected by Coupled Swirling Flows in Round Billet Mold

Abstract

The quality of round billet is affected by the flow characteristics of molten steel during continuous casting. The application of nozzle swirling flow combined with electromagnetic stirrer (M-EMS) can increase the number of equiaxed crystal and alleviate macrosegregation, which has been demonstrated in our previous industrial test. However, flow field cannot be visually observed in industrial tests. In this paper, for the first time, the evolution of flow field and its influence on the macrosegregation and liquid level fluctuation under the action of coupled swirling flows were studied. Compared with that by using only mold stirring, the flow field was distributed more symmetrically in the upper part of mold and its impact depth decreased from 350 mm to 246 mm when the rotating speed of stirring propeller was 70 r/min. The centerline segregation level was reduced owing to the inward flow weakened by the outward nozzle swirling flow. Additionally, the tangential velocity at 1/2 radius near the surface was decreased from 0.589 m/s to 0.469 m/s, leading to the reduction of liquid level fluctuation. Consequently, both the internal quality and the surface quality can be improved by coupled swirling flows, provided that the rotating intensities of nozzle swirling flow and M-EMS are selected properly in the actual continuous casting production.

1. Introduction

Due to the radial flow in the mold during continuous casting of round billet, solute elements precipitated in the solidification process of molten steel are transported into the billet center, causing macrosegregation defect.^1,2,3,4,5,6) Electromagnetic field plays a significant role in the process of steel production.^7,8,9,10) To regulate flow field in mold and thereby improve the billet quality, mold electromagnetic stirrer (M-EMS) is applied to accelerate overheat dissipation and refine grain.^11,12) Besides, the forced flow induced by M-EMS breaks the dendrite side arms and forms more equiaxed crystals during the solidification.^{13,14,15,16,17,18)} The increase in number and decrease in size of equiaxed crystals can reduce macrosegregation to a certain level.^19,20) However, when the stirring of M-EMS is vigorous, the secondary flow in the radial direction is strengthened,^21,22) which could aggravate negative segregation and positive segregation formed near the billet surface and in the billet center, respectively.^23,24,25,26) Additionally, increasing the magnitude of secondary flow can cause serious liquid level fluctuation, which does not benefit the surface quality of round billet.

The flow characteristics in the submerged entry nozzle (SEN) affects the flow field in the mold.^27,28,29) To tackle the negative influence by M-EMS on the flow field in mold, some researchers proposed various solutions to generate a swirling flow out of the SEN.^{30,31,32,33,34,35,36,37,38)} In our previous industrial test, it was found that electromagnetic swirling flow in the nozzle (EMSFN) approach combined with M-EMS could take good effect on improving the quality of round billet.^39,40,41) When the rotating direction of nozzle swirling flow was opposite to that induced by M-EMS, the number of equiaxed crystal increased, compared with that by using only M-EMS. Besides, both the centerline segregation level and liquid level fluctuation were reduced.

However, the industrial test results cannot visually depict the flow field in mold, and the evolution of flow field affected by coupled swirling flows is still unknown. Therefore, a water model experiment is imperatively carried out in this paper. As electromagnetic force is unable to directly act on water, a flow field in the mold was artificially created by mechanical stirring. Based on it, the fluid flow trajectory in the mold of round billet under the action of coupled swirling flows was investigated. Afterwards, to further illustrate the influence of coupled swirling flows on the billet quality, the research on macrosegregation in the round billet and liquid level fluctuation near the free surface of mold were also provided. The results were compared with that by using either a mold stirring or a single nozzle swirling flow for a better understanding.

2. Experimental Methods

The water model experiment, numerical simulation and industrial test were carried out at different stages of this research.

2.1. Water Model Experiment

The 1/1.625 scaled water model experiment system for casting round billet was established according to the similarity principle.^42,43) The Reynolds numbers of prototype and water model are both in the same self-modeling domain, so that the criterion of dynamic similarity is independent of Reynolds number. Only the Froude number, Fr, needs to be kept equal in these two systems, which is described as follows:

  

$$F{r}_1=F{r}_2$$(1)

  

$$\frac{u_1}{\sqrt{g{l}_1}}=\frac{u_2}{\sqrt{g{l}_2}}$$(2)

  

$$\lambda =\frac{l_1}{l_2}$$(3)

Where Fr₁ and Fr₂ are the Froude numbers of prototype and water model respectively. u₁ and u₂ are the flow velocities out of the SEN in prototype and water model respectively, m/s. l₁ and l₂ are the characteristic lengths in prototype and water model respectively, m. g is the acceleration of gravity, m/s². λ is 1.625 in this experiment.

Table 1 shows the parameters of water model experiment, and the schematic diagram of the experiment system is depicted in Fig. 1. Water in the tundish flows into the mold through the SEN. A removable swirling rotor was installed inside the SEN to generate nozzle swirling flow. When using it, the structure of SEN outlet was in divergent shape to enhance the magnitude of nozzle swirling flow.³⁷⁾ Otherwise, it was in straight shape. In the lower part of mold, a stirring propeller was applied to make water rotate. Its rotating speeds in this experiment were 60 r/min, 70 r/min, and 80 r/min. Its rotating direction was always opposite to that of the nozzle swirling flow.

Table 1. Parameters of water model experiment.

Item	Prototype	Water model
Geometric dimension ratio	1.625	1
Mold diameter (mm)	650	400
Nozzle outer diameter (mm)	110	68
Nozzle inner diameter (mm)	40	25
Nozzle height (mm)	940	578
Nozzle submerged depth (mm)	130	80
Casting speed (m/min)	0.23	0.18

Fig. 1. Schematic diagram of water model experiment system. (Online version in color.)

It is noted here when using stirring propeller, the flow field in the whole mold is not completely consistent with that generated by M-EMS in the actual continuous casting process. When the nozzle swirling flow is applied with the M-EMS, the main influence area of nozzle swirling flow is limited in the upper part of mold.⁴⁴⁾ Therefore, the purpose of using stirring propeller is to create a flow field in the upper part of mold, where the variant trend of flow velocity is similar to that by using M-EMS. Besides, liquid level fluctuation at free surface can be artificially produced.

The fluid flow trajectory in the mold was observed by adding black ink as the tracer. After multiple tests, the tracer showed the best following performance in water when the mixing ratio of water and ink was 700:1.

As shown in Fig. 1, two capacitive wave sensors were used to measure liquid level fluctuation along the radial direction, which were located at 40 mm and 146 mm from the inner wall of mold, respectively. When the water surface fluctuates, the depth at which the sensor is submerged changes, resulting in a change in capacitance. After the change in capacitance is converted and amplified, the output current reflects the fluctuation of the water surface. The measure range and precision of the wave sensors are 0–100 mm and 0.3 mm respectively. 100 fluctuation data were collected in every second during the period of 1 minute in the experiment.

A flow velocity sensor (LS300A) was used to measure the tangential velocity at three locations along the radial direction. The flow velocity sensor is driven by hydraulic pressure and its built-in signal device generates a speed signal. Therefore, the flow velocity can be calculated based on the total number of signals generated by the sensor during a certain period. The tangential velocity can be calculated as follows:

  

$$v=a+b\frac{R}{T}$$(4)

Where v is the average flow velocity in the measurement period, m/s. a is the constant of sensor, m/s. b is the hydraulic pitch of propeller in the sensor, m. T is the measurement period, s. R is the total number of revolutions of sensor rotor during the time. a and b are fixed values, so the flow velocity can be calculated based on T and R. The average velocity was obtained within one minute. The final velocity was calculated according to the average value of multiple measurement results.

2.2. Numerical Simulation

2.2.1. Governing Equations

To determine whether the flow field produced by stirring propeller is similar to that by M-EMS in the upper part of mold, the numerical simulation software ANSYS FLUENT was used to calculate the velocity variation of fluid flow in this water model experimental system. The numerical simulation was verified by measurement result.

It is assumed that the flow is steady-state and incompressible Newtonian. The flow is governed by continuity equation and momentum equation as follows:

  

$$\frac{\partial \rho }{\partial t}+\nabla \cdot \left(\rho u\right)=0$$(5)

  

$$\frac{\partial \left(\rho u\right)}{\partial t}+\nabla \cdot \left(\rho u\times u\right)=u_{\text{eff}}{\nabla }^{2}u-\nabla p+\rho g$$(6)

Where ρ, t, u, p and u_eff represent water density, time, velocity, static pressure and effective viscosity, respectively.

In this paper, RNG k-ε model is used to calculate the flow field in the mold and the equations are described as follows:

  

$$\begin{array}{l}\frac{\partial \left(\rho k\right)}{\partial t}+\frac{\partial \left(\rho k{u}_i\right)}{\partial x_i}=\frac{\partial }{\partial x_j}\left({\alpha }_k{u}_{\text{eff}}\frac{\partial k}{\partial x_j}\right)\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.5em}+G_k+G_b-\rho \epsilon -Y_m+S_k\end{array}$$(7)

  

$$\begin{array}{l}\frac{\partial \left(\rho \epsilon \right)}{\partial t}+\frac{\partial \left(\rho \epsilon u_i\right)}{\partial x_i}=\frac{\partial }{\partial x_j}\left({\alpha }_{\epsilon }{u}_{\text{eff}}\frac{\partial \epsilon }{\partial x_j}\right)+C_{1\epsilon }\frac{\epsilon }{k}\left(G_k+C_{3\epsilon }{G}_b\right)\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}-C_{2\epsilon }\rho \frac{{\epsilon }^2}{k}-R_{\epsilon }+S_{\epsilon }\end{array}$$(8)

Where k, ε, G_k is turbulent kinetic energy, turbulent dissipation rate and turbulent kinetic energy generated by mean velocity gradient, respectively. G_b is turbulent kinetic energy induced by buoyancy. Y_m is the influence of wave expansion of compressible turbulence on total dissipation rate. C_1ε, C_2ε, C_3ε, S_k and S_ε are model constants. α_k and α_k are the reciprocals of the effective Prandtl number for k and ε, respectively, which are 1.393 in high Reynolds number flow. C_1ε=1.42, C_2ε=1.68.

The effective viscosity u_eff is calculated as follows:

  

$$\text{d}\left(\frac{{\rho }^{2}k}{\sqrt{\epsilon \mu }}\right)=1.72\frac{\hat{v}}{\sqrt{{\hat{v}}^3-1+C_{\text{V}}}}d\hat{v}$$(9)

Where $\hat{v}=u_{\text{eff}}/\mu$; C_V ≈ 100.

For effective viscosity at high Reynolds number, the equations are described as follows:

  

$$u_{\text{eff}}=\mu +{\mu }_t$$(10)

  

$${\mu }_{\text{t}}=\rho C_{\mu }\frac{k^2}{\epsilon }$$(11)

Where C_μ = 0.0845. μ is kinetic viscosity and μ_t is turbulent viscosity.

R_ε in Eq. (4) can be represented as follows:

  

$$R_{\epsilon }=\frac{C_{\mu }\rho {\eta }^3\left(1-\frac{\eta }{{\eta }_0}\right)}{1+\beta {\eta }^3}\frac{{\epsilon }^3}{k}$$(12)

Where η = Sk/ε; η₀ = 4.38; β = 0.012.

2.2.2. Boundary Conditions, Geometry and Mesh

Table 2 lists the boundary conditions. The required parameters are shown in Table 1 and Fig. 1. The mesh of the computational domain is Poly-Hexcore structured, and the number of meshes is 9138190, as shown in Fig. 2.

Table 2. Boundary conditions.

Location	Description
Inlet	inlet velocity is 0.77 m/s
Outlet	pressure boundary condition is imposed at the computational domain outlet
Solid wall	no slip boundary condition
Free surface	no shear boundary condition

Fig. 2. Mesh in the mold: (a) overall fluid mesh in mold and (b) mesh on the longitudinal section of mold. (Online version in color.)

2.3. Industrial Test

The industrial test was carried out in a Φ650 mm round billet caster. The chemical composition of molten steel and the process parameters of continuous casting are listed in Tables 3 and 4, respectively. The installation positions of EMSFN and M-EMS devices are depicted in Fig. 3. To study macrosegregation, four holes on the cross section of the round billet were drilled along the radial direction, which were located at 15 mm, 40 mm, 80 mm and 165 mm from the billet surface. The carbon contents were measure by LECO infrared carbon and sulfur analyzer, and carbon segregation indexes were calculated. The segregation index is expressed by r=C/C_A, where C is the element concentration at a specific location, and C_A is the average element concentration collected in one billet sample. If r is greater than 1, it indicates the positive segregation. Otherwise, if r is less than 1, it is negative segregation.

Table 3. Chemical composition of molten steel (mass fraction: %).

C	S	P	Mn	Si	Cr	Mo
0.415	≤0.015	≤0.018	0.660	0.200	1.010	0.170

Table 4. Process parameters of continuous casting.

Item	Value
Steel grade	42CrMo
Diameter (mm)	650
Casting speed (m/min)	0.23
Superheat (°C)	25
M-EMS current frequency (Hz)	1.2
M-EMS current intensity (A)	256
EMSFN current frequency (Hz)	50
EMSFN current intensity (A)	200–600

Fig. 3. Installation positions of EMSFN and M-EMS on the continuous caster.

3. Results and Discussion

3.1. Feasibility Analysis when Using Stirring Propeller

Figure 4 gives the tangential velocity along the radial direction at a distance of 280 mm above the stirring propeller when using only stirring propeller at 80 r/min. The numerical simulation results showed the maximum velocity was located near the inner wall of mold. As the radius decreased, the velocity gradually decreased. The trend of measurement result was well consistent with that of numerical simulation. Figure 5 gives the tangential velocity in the longitudinal section in the mold when using only stirring propeller at 80 r/min. The velocities decreased from the stirring propeller to the free surface. This was consistent with other researcher’s result about the velocity variation in continuous casting round billet mold.⁴⁵⁾ Therefore, the application of stirring propeller to artificially produce the swirling flow in the upper part of mold is feasible.

Fig. 4. Tangential velocity along the radial direction of mold with stirring propeller at 80 r/min. (Online version in color.)

Fig. 5. Velocity distribution in the longitudinal section of mold with stirring propeller at 80 r/min. (Online version in color.)

3.2. Fluid Flow Trajectory in the Mold

3.2.1. Without Any Swirling Flow

Before investigating the flow field affected by the swirling flow, the fluid flow trajectory without any stirring was observed first, as shown in Fig. 6. Without any stirring, the impact depth reached 427 mm when the tracer left the SEN at the 2nd second. Excessive impact depth cannot enhance the overheat dissipation in molten steel, which is not conducive to the formation of equiaxed crystal. Thus, to obtain more equiaxed crystals and reduce the centerline segregation level, it is necessary to stir the molten steel in mold.

Fig. 6. Fluid flow trajectory in the mold without any stirring. (Online version in color.)

3.2.2. When Using a Single Swirling Flow

Figure 7 shows the fluid flow trajectory in mold under the single action of stirring propeller with different rotating speeds. When the stirring propeller applied at 60 r/min, the impact depth didn’t change significantly at the 2nd second. After it reached 70 r/min, the impact depth decreased to 350 mm at the 2nd second. In continuous casting, the reduction of impact depth cannot only accelerate the overheat dissipation, which is beneficial to grain refinement in the billet, but also promote the upward movement of bubbles and inclusions in the molten steel. However, even though the mold stirring can bring some positive effort on the billet quality, negative influence on the flow field in mold is still obvious. Under the action of stirring, the bias flow occurred, so that the flow pattern distributed asymmetrically along the axial direction of mold. It results from that the turbulence in mold is enhanced by the stirring.⁴⁵⁾ As the stirring intensity increased, the bias flow was gradually strengthened. Influenced by this bias flow, the molten steel could impact solid-liquid interface at the early stage of solidification, leading to the uneven thickness of solidification shell and the macrosegregation of solute element in final product. The recirculation appeared after the 4th second. The radial flow from the mold inner wall to the stirring center can transport the solute elements precipitated into the center, aggravating centerline segregation in the billet.

Fig. 7. Fluid flow trajectory in the mold with only stirring propeller at (a) 60 r/min, (b) 70 r/min and (c) 80 r/min. (Online version in color.)

Unlike the mold stirring, the flow direction of nozzle swirling flow was from the outlet of SEN to the inner wall of mold, and its magnitude gradually decreased. After the flow reached the mold wall, the recirculation began to occur. Figure 8 shows the fluid flow trajectory in mold under the single action of swirling rotor installed in the SEN. The flow out of the SEN outlet dispersed and its impact depth reduced significantly, compared to that with no stirring or using only stirring propeller. From the 1st to the 4th second, the impact depths measured were 205 mm, 275 mm, 295 mm, and 320 mm, respectively. Moreover, after the flow left the SEN, it distributed symmetrically, indicating that the bias flow was almost eliminated. However, the nozzle swirling flow can only influence the initial stage of solidification, and can hardly break more dendrite side arms during solidification. Thus, the advantages of nozzle swirling flow and the forced flow induced by M-EMS should be both considered in continuous casting.

Fig. 8. Fluid flow trajectory in the mold with only swirling rotor. (Online version in color.)

3.2.3. When Using Coupled Swirling Flows

Figure 9 displays the fluid flow trajectory in mold under the action of coupled swirling flows. When the rotating speed of stirring propeller was 60 r/min, the flow pattern was of no significant difference from that using only stirring propeller. When the speed reached 70 r/min, both the deflection angle of bias flow and the impact depth decreased. After the speed increased to 80 r/min, the impact depth further decreased and almost no bias flow occurred. With the stirring propeller at 70 r/min and 80 r/min, the impact depths could reach 246 mm and 268 mm, respectively. However, due to the intensive stirring, the flow pattern formed in a “tornado” shape, indicating a large amount of solute element transported to the billet center under the action of inward radial flow, and thus the severity of macrosegregation aggravated. Therefore, when the magnitude of nozzle swirling flow cannot be changed, the stirring intensity of M-EMS should be decreased to a certain extent in order to obtain excellent internal quality in the billet.

Fig. 9. Fluid flow trajectory in the mold affected by nozzle swirling flow and stirring propeller at (a) 60 r/min, (b) 70 r/min and (c) 80 r/min. (Online version in color.)

According to the results shown above, Fig. 10 simply explains the variation of fluid flow trajectory affected by using these coupled swirling flows. H_n and H_m represent the influence heights affected by nozzle swirling flow and the forced flow induced by mold stirring, respectively. When the magnitude of nozzle swirling flow is unchanged, H_n decreases gradually with increasing the intensity of mold stirring. As the time goes on, H_m increases, because the forced flow induced by mold stirring gradually dominates from the center to the upper part of the mold in the axial direction. It is noted here that with increasing the stirring intensity of mold stirring, the increase of H_m does not appear at the same time and it may be delayed as shown in Fig. 9. The increase of H_m indicates the more solute elements could be transported into the stirring center under the action of inward flow. The radial flow in the mold plays a significant role on this variation. Figure 11 shows the flow field in the upper part of mold by using single and coupled swirling flows. The direction of nozzle swirling flow is from the mold center to the mold wall when leaving the SEN outlet, and its magnitude decreases with increasing the radius. The secondary flow induced by the M-EMS provides the inward flow, which tangential velocity decreases gradually from the mold wall to the stirring center. When the magnitude of nozzle swirling flow is increased to a certain level, it can weaken the inward flow induced by M-EMS and decrease H_m, leading to the reduction of macrosegregation level.

Fig. 10. Schematic diagram of flow trajectory variation in the mold when using coupled swirling flows in the water model experiment. (Online version in color.)

Fig. 11. Schematic diagram of flow field in the upper apart of mold, (a) by the M-EMS and nozzle swirling flow in the longitudinal direction, (b) by the M-EMS only, (c) by the nozzle swirling flow with different stirring direction and (d) by the coupled swirling flows in transverse direction at the outlet of SEN. (Online version in color.)

3.3. Macrosegregation in Round Billet

To verify the influence of flow field on the round billet quality when using coupled swirling flows, the macrosegregation was studied. Figure 12 shows the carbon segregation indexes with different distances from the billet surface at different current intensities of EMSFN device. It is noted that unlike the results in the water model experiment, the results obtained in the industrial test were based on unchangeable stirring intensity of M-EMS, while gradually increasing the magnitude of nozzle swirling flow. When the current intensity of EMSFN device was 0 A that means using only M-EMS, the segregation indexes at the locations with 15 mm–80 mm from the billet surface were all less than 1, indicating negative segregation. While at 1/2 radius, 165 mm from the billet surface, the segregation index was 1.145, which was serious positive segregation. This is the reason why using only M-EMS is not favorable to the internal quality of billet. With increasing the current intensity of EMSFN device, the magnitude of nozzle swirling flow increased. The outward nozzle swirling flow weakened the inward secondary flow induced by M-EMS, so that the transportation of solute elements was slowed down into the billet center, leading to both degrees of negative segregation and positive segregation reduced. It can be understood based on the conservation of matter. If solute elements accumulate in the early stage of solidification, the centerline segregation is alleviated in the later stage.

Fig. 12. Carbon segregation indexes with different distances from the surface of 42CrMo steel round billet at different current intensities of EMSFN device. (Online version in color.)

However, with further increasing the nozzle swirling flow magnitude with the current intensity of EMSFN device more than 400 A, the lower recirculation induced by nozzle swirling flow was strengthened, leading to the reoccurrence of negative segregation. However, the segregation index in the middle of billet did not increase again, owing to the accelerated overheat dissipation and thereby more equiaxed crystals obtained.^37,39,40) When the stirring intensity of M-EMS remains constant, the secondary flow velocity in the upper part of mold is related to the magnitude of nozzle swirling flow, which determines the difficulty of solute element transportation.⁴⁴⁾ Therefore, the appropriate combination of the intensities of nozzle swirling flow and M-EMS is the key factor to improve the billet quality in the industrial application.

3.4. Liquid Level Fluctuation at the Free Surface

The surface quality of round billet is related to the liquid level fluctuation, which is affected by the flow velocity near the free surface in mold. Figure 13 gives the tangential velocity at 20 mm under the free surface along the radial direction when using only stirring propeller or coupled swirling flows. With only stirring propeller, the velocity decreased from the mold wall to the mold center at a certain stirring intensity. With increasing the stirring intensity, the velocities increased gradually.

Fig. 13. Tangential velocity near free surface when using stirring propeller at (a) 60 r/min, (b) 70 r/min and (c) 80 r/min. (Online version in color.)

By using the coupled swirling flows, the tangential velocity still showed a decreasing trend along the radial direction. However, the secondary flow generated by stirring propeller was weakened by the nozzle swirling flow in opposite rotating direction, so that the tangential velocity was reduced, especially near the 1/2 radius of the mold. When the rotating speeds were 70 r/min and 80 r/min, the velocities at 80 mm from the mold wall decreased from 0.589 m/s and 0.718 m/s to 0.469 m/s and 0.583 m/s, respectively. Therefore, based on the above results of the flow field and the velocity near the free surface, the flow characteristics was much favorable when using swirling rotor combined with stirring propeller at 70 r/min in this water model experiment.

To verify that the decrease of velocity was beneficial for stabilizing the free surface, the liquid level fluctuation was measured when using stirring propeller at 70 r/min with or without swirling rotor. As shown in Fig. 14, compared with using only stirring propeller, the liquid level fluctuation affected by the coupled swirling flows was alleviated within the measurement period at both the mold inner wall and nozzle outer wall, which is consistent with the result obtained in previous industrial test.⁴¹⁾ The fluctuation ranges of liquid level were calculated and shown in Fig. 15. With using only stirring propeller, the fluctuation ranges near the inner wall of mold and outer wall of SEN were 4.99 mm and 7.00 mm, respectively. Under the action of coupled swirling flows, those values decreased to 3.99 mm and 4.61 mm, respectively. The stability of liquid level fluctuation is beneficial for the surface quality of continuous casting round billet.

Fig. 14. Liquid level fluctuation when using stirring propeller at 70 r/min, measured at (a) 40 mm and (b) 146 mm from the inner wall of mold. (Online version in color.)

Fig. 15. Fluctuation ranges when using stirring propeller at 70 r/min, measured at (a) 40 mm and (b) 146 mm from the inner wall of mold. (Online version in color.)

4. Conclusions

In this study, the fluid flow trajectory, macrosgregation defect and liquid level fluctuation in the mold of round billet under the action of coupled swirling flows were investigated by using water model experiment and industrial test methods, the conclusions are as follows:

(1) With coupled swirling flows, the impact depth in the round billet mold was reduced. At a certain rotating speed of stirring propeller, the flow out of the SEN could maintain uniform distribution in the mold.

(2) When the magnitude of nozzle swirling flow was unchanged, the influence height of nozzle swirling flow decreased with increasing the stirring intensity of mold stirring, leading to more solute elements transported into the stirring center by the inward flow.

(3) Compared with using only M-EMS, the macrosegregation level could be alleviated, owing to the inward flow weakened by the nozzle swirling flow. However, the combination of the magnitude of nozzle swirling flow and the stirring intensity of M-EMS must be carefully selected.

(4) Under the experimental condition in this paper, when the rotating speed was 70 r/min, the tangential velocity at the 1/2 radius near free surface could be decreased from 0.589 m/s to 0.469 m/s. Meanwhile, the liquid level fluctuation was alleviated.

Thus, the flow field induced by M-EMS can be optimized by using nozzle swirling flow in the opposite rotating direction, which benefits the further improvement of both internal quality and surface quality in round billet.

Author Declaration

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgements

The authors are grateful to the Basic Scientific Research Fund Project of the Educational Department of Liaoning Province (No. LJ222411430033), Liaoning Institute of Science and Technology Doctoral Research Initiation Fund Project (No. 2307B20 and No. 2307B04), the Liaoning Nature Science Foundation (No. 2022-MS-365), the National Natural Science Foundation of China (No. U1560207) and the Hebei Natural Science Foundation (No. E2019501008).

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