**2. Model Description**

#### *2.1. Governing Equations and Boundary Conditions*

In order to simplify the numerical model and reduce its calculation time, the following assumptions were made:


The flow of gas in tundish during the process of protective casting can be described by the following governing Equations:

$$\frac{\partial \rho}{\partial t} + \nabla \cdot (\rho u) = 0 \tag{1}$$

$$\frac{\partial}{\partial t}(\rho \mathbf{u}) + \nabla \cdot (\rho \mathbf{u} \mathbf{u}) = -\nabla p + \nabla \cdot \left[ (\mu + \mu\_l) \left( \nabla \mathbf{u} + \nabla \mathbf{u}^T \right) \right] + \rho \mathbf{g} \tag{2}$$

where *ρ* is the mixture gas density, kg/m<sup>3</sup> ; *t* is time, s; *u* is the velocity vector of mixture gas, m/s; *<sup>p</sup>* is the local pressure, Pa; *<sup>µ</sup>* is the mixture viscosity, kg·m−<sup>1</sup> ·s −1 ; *µ*<sup>t</sup> is the turbulent viscosity, kg·m−<sup>1</sup> ·s −1 ; and **g** is the gravitational acceleration, m·s −2 .

The standard two-equation *k-ε* model with scalable wall function is applied to describe the turbulent behavior of flow field, where the equation of turbulent kinetic energy transport and its dissipation rate transport can be solved to obtain the turbulent viscosity:

$$
\mu\_t = \rho \mathbb{C}\_{\mu} \frac{k^2}{\varepsilon} \tag{3}
$$

Turbulent kinetic energy, *k*, m<sup>2</sup> ·s −2 :

$$\frac{\partial(\rho k)}{\partial t} + \nabla \cdot (\rho u k) = \nabla \cdot \left[ \left( \mu + \frac{\mu}{\sigma\_k} \right) \nabla k \right] + \mathcal{G}\_{\mathbf{k}} - \rho \varepsilon \tag{4}$$

where *G* is the generation of turbulent kinetic energy due to the mean velocity gradients and can be written as:

$$\mathbf{G}\_{\mathbf{k}} = \mu\_l \frac{\partial u\_j}{\partial \mathbf{x}\_l} \left( \frac{\partial u\_i}{\partial \mathbf{x}\_j} + \frac{\partial u\_j}{\partial \mathbf{x}\_i} \right) \tag{5}$$

The dissipation rate of turbulent kinetic energy, *ε*, m<sup>2</sup> ·s −3 :

$$\frac{\partial(\rho\varepsilon)}{\partial t} + \nabla \cdot (\rho u \varepsilon) = \nabla \cdot \left[ \left( \mu + \frac{\mu\_{\text{l}}}{\sigma\_{\varepsilon}} \right) \nabla \varepsilon \right] + \frac{\varepsilon}{k} (\mathsf{C}\_{1}\mathsf{G} - \mathsf{C}\_{2}\rho \varepsilon) \tag{6}$$

where *C*1, *C*2, *C*µ, *σ*k*, σ*<sup>ε</sup> are the empirical constants, whose values, as recommended by Launder and Spalding [23], are 1.38, 1.92, 0.09, 1.0, and 1.3, respectively.

The species transport equation was solved to obtain the volume fraction of air and argon, and then the volume fraction of oxygen can be obtained by the product of air volume fraction and 0.21 (the volume fraction of oxygen in air):

$$\frac{\partial}{\partial t}(\rho Y\_l) + \nabla \cdot (\rho u Y\_l) = -\nabla \cdot \left(-\left(\rho D\_{i,m} + \frac{\mu\_l}{Sc\_l}\right) \nabla Y\_l\right) + S\_l \tag{7}$$

where *Y<sup>i</sup>* is mass fraction for species *i*, non-dimensional; *Di*, m is the mass diffusion coefficient for species *i*, m<sup>2</sup> ·s −1 ; *Sc*t is the turbulent Schmidt number, 0.7; *S<sup>i</sup>* is the source term for species *<sup>i</sup>*, kg·(m−<sup>3</sup> ·s −1 ); and in the current study, *i* represents argon or air in tundish.

#### *2.2. Experimental Facility and Numerical Model*

Figure 1 shows the two-strand slab tundish schematic diagram. A tundish with a size of 7.77 × 1.68 × 1.32 m and a taper of 1.1 between the top face and bottom face has an injecting chamber and two symmetrical casting chambers. The injecting chamber has two baking holes, and each casting chamber has a baking hole and a stopper hole. The tundish cover is composed of an outer shell made of steel plates and filler made of refractory material. The argon pipe is buried inside the refractory material. Figure 2 shows the arrangement of the argon blowing pipe in the tundish cover. Fourteen argon blowing pipes with diameters of 30 mm were installed on the tundish cover, including six for the injecting chamber and four for each casting chamber. Before a tundish is used, the blast furnace gas is first adopted to bake the tundish through those baking holes for more than 30 min to completely eliminate water vapor. After baking the tundish, its temperature was greater than 1000 K. The process of ABTC will be conducted after baking tundish. Because the covering flux on the molten steel surface cannot be fully melted immediately, argon blown into tundish through those pipes may remove air and reduce molten steel secondary oxidation. During the period of empty tundish and normal casting, it is necessary to maintain the volume fraction of oxygen in tundish at a low level (<1%) to avoid the secondary oxidation of molten steel and improve its cleanliness.

During the period of empty tundish, the gas can be freely exchanged between the injecting chamber and casting chamber. However, during the period of normal casting, the injecting chamber and casting chamber are isolated by molten steel, which forms a separate injecting chamber and two separate casting chambers, in which the gas cannot exchange. Therefore, the mesh for the empty tundish and normal casting must be built separately. The mesh for empty tundish and normal casting is shown in Figure 3. Octahedron grids are adopted to a mesh in the calculating domain, and the numbers of meshes are 508,484 for empty tundish, 41,018 for isolated casting chamber, and 52,493 for isolated injecting chamber.

**Figure 1.** Structure diagram of a two-strand slab tundish. **Figure 1.** Structure diagram of a two-strand slab tundish. **Figure 1.** Structure diagram of a two-strand slab tundish.

*Metals* **2022**, *12*, x FOR PEER REVIEW 4 of 13

*Metals* **2022**, *12*, x FOR PEER REVIEW 4 of 13

**Figure 2.** Arrangement of the argon blowing pipe on the two-strand slab tundish cover (mm). **Figure 2.** Arrangement of the argon blowing pipe on the two-strand slab tundish cover (mm). **Figure 2.** Arrangement of the argon blowing pipe on the two-strand slab tundish cover (mm). injecting chamber.

**Figure 2.** Arrangement of the argon blowing pipe on the two-strand slab tundish cover (mm).

**Figure 3.** Mesh for empty tundish (**a**), isolated casting chamber (**b**), and isolated injecting chamber (**c**) (mm). **Figure 3.** Mesh for empty tundish (**a**), isolated casting chamber (**b**), and isolated injecting chamber (**c**) (mm).

**Figure 3.** Mesh for empty tundish (**a**), isolated casting chamber (**b**), and isolated injecting chamber (**c**) (mm). During the calculation, the velocity inlet boundary condition was used for those argon pipes, where the argon flow rate of each pipe is 1/14 of the total flow rate. The pressure outlet with a relative static pressure of zero was adopted for the opening hole, which is decided by the sealing scheme in the stage of empty tundish and the injecting hole and stopper hole in the normal casting according to the practical process. The non-slip wall boundary was used for all inner faces of tundish, which assumes that the gas velocity on the wall is zero. The initial condition of air volume fraction and temperature is assumed as 1 and 1000 K, respectively. The SIMPLEC algorithm was adopted to solve the transient problem. The timestep was set to 0.05 s for the current study. Calculation convergence was achieved when all residuals were lower than 10−<sup>4</sup> . The average volume fraction of oxygen

**Figure 3.** Mesh for empty tundish (**a**), isolated casting chamber (**b**), and isolated injecting chamber

(**c**) (mm).

is monitored throughout the calculation process. The tundish parameters and other related continuous casting process parameters are summarized in Table 1.

**Table 1.** Parameters of tundish and the casting process.


#### *2.3. Model Validation*

In order to verify the reliability of the numerical model, an industrial trial with an argon flow rate of 220 Nm3/h was conducted, and the injecting hole, baking holes, and stopper holes were kept open after tundish baking. The VF of oxygen at different times was measured using an Optima 7 gas analyzer. The measurement in the industrial trial was carried out 3 min after tundish baking due to the rapid pace of industrial production. A numerical simulation based on the process was also conducted. The data from the industrial test and numerical calculation are shown in Figure 4, demonstrating a better agreement between the experimental values and the simulation results. *Metals* **2022**, *12*, x FOR PEER REVIEW 6 of 13

**Figure 4.** Snapshot of the industrial test (**a**) and comparison of the industrial test data and numerical calculation (**b**). **Figure 4.** Snapshot of the industrial test (**a**) and comparison of the industrial test data and numerical calculation (**b**).

A verification of grid independence was also performed in the current study. The operating parameter of the argon flow rate of 220 Nm<sup>3</sup> /h, with sealing the injecting hole, baking holes and an open stopper hole, was chosen. The average VFtheoxygen in the tundish at 300 s was discussed and used as the verification criterion. Table 2 shows the statistical results for different meshes. As shown in Table 2, the errors decrease with an in-A verification of grid independence was also performed in the current study. The operating parameter of the argon flow rate of 220 Nm3/h, with sealing the injecting hole, baking holes and an open stopper hole, was chosen. The average VF of the oxygen in the tundish at 300 s was discussed and used as the verification criterion. Table 2 shows the statistical results for different meshes. As shown in Table 2, the errors decrease with an increased

creased number of cells. For the case of M3, the error relative to the finest mesh M4 is less than 5%, which is within the allowable error range. Therefore, considering the computa-

Average volume faction of oxygen 0.04846 0.04765 0.04591 0.04511 *δ*oxygen = |*VF*oxygen-i − *VF*oxygen-4|/*VF*oxygen-4 0.0743 0.0563 0.0179 –

3.1.1. Calculation of the Sealing Scheme of Tundish Cover Holes during the Period of

During practical production, there are about 5~10 min for blowing argon into tundish from the end of tundish baking to the start of casting. In order to improve the efficiency of protective casting and fully discharge oxygen, asbestos is usually used to seal some holes, such as the baking hole, injecting hole, or stopper hole. To determine the best sealing scheme during the period of empty tundish, three schemes were designed and described, as shown in Table 3. The average volume fractions of oxygen with argon flow

erage argon volume fraction with respect to time for different schemes are shown in Figure 5. As shown in Figure 5, the difference in average oxygen volume fraction at 10 min between the schemes 1, 3 and 4 is small, which is 0.0174, 0.0167, and 0.0170 respectively. Scheme 2, sealing the injecting hole and all baking holes, is the best, and its average oxy-

/h for different schemes were calculated in 10 min. The variations of av-

**Mesh M1 M2 M3 M4** Total cell number 192,236 325,618 508,483 612,348 Total node number 998,050 1,878,166 2,932,932 3,532,026

**Table 2.** Error statistics of different meshes.

gen volume fraction is 0.0132 at 10 min.

**3. Results and Discussion** *3.1. Numerical Simulation*

Empty Tundish

rates of 200 Nm<sup>3</sup>

number of cells. For the case of M3, the error relative to the finest mesh M4 is less than 5%, which is within the allowable error range. Therefore, considering the computational cost and efficiency, the mesh of M3 was adopted for the numerical calculation.

**Table 2.** Error statistics of different meshes.


#### **3. Results and Discussion**

*3.1. Numerical Simulation*

3.1.1. Calculation of the Sealing Scheme of Tundish Cover Holes during the Period of Empty Tundish

During practical production, there are about 5~10 min for blowing argon into tundish from the end of tundish baking to the start of casting. In order to improve the efficiency of protective casting and fully discharge oxygen, asbestos is usually used to seal some holes, such as the baking hole, injecting hole, or stopper hole. To determine the best sealing scheme during the period of empty tundish, three schemes were designed and described, as shown in Table 3. The average volume fractions of oxygen with argon flow rates of 200 Nm3/h for different schemes were calculated in 10 min. The variations of average argon volume fraction with respect to time for different schemes are shown in Figure 5. As shown in Figure 5, the difference in average oxygen volume fraction at 10 min between the Schemes 1, 3 and 4 is small, which is 0.0174, 0.0167, and 0.0170 respectively. Scheme 2, sealing the injecting hole and all baking holes, is the best, and its average oxygen volume fraction is 0.0132 at 10 min. *Metals* **2022**, *12*, x FOR PEER REVIEW 7 of 13

> **Table 3.** Sealing schemes of tundish cover holes. **Table 3.** Sealing schemes of tundish cover holes.


**Figure 5.** Variation of oxygen volume fraction in tundish for different schemes. **Figure 5.** Variation of oxygen volume fraction in tundish for different schemes.

Figure 6 shows the contour of oxygen volume fraction at 10 min. For Scheme 1, sealing all stopper holes and all baking holes, the oxygen volume fraction in the casting cham-

casting chamber and injecting chamber, which results in the oxygen in the casting chambers not being smoothly discharged and forming a dead zone. For Scheme 2, sealing the injecting hole and all baking holes, the oxygen volume fraction in the injecting chamber and casting chamber is relatively uniform, which indicates a better effect of discharging oxygen. For Scheme 3—sealing the injecting hole, stopper hole 2, and all baking holes the oxygen volume fraction in casting chamber 1 is lower than that in chamber 2, which indicates the negative effects of discharging oxygen due to the long distance from casting chamber 2 to the outlet (stopper hole 1). As for Scheme 4, opening all holes, the oxygen content in the injecting chamber is higher than that in the casting chamber due to the stronger backflow of air. Overall, Scheme 2 is the best and used for the analysis of argon

flow rate.

Figure 6 shows the contour of oxygen volume fraction at 10 min. For Scheme 1, sealing all stopper holes and all baking holes, the oxygen volume fraction in the casting chamber is higher than that in the injecting chamber. The dam and wall of tundish isolate the casting chamber and injecting chamber, which results in the oxygen in the casting chambers not being smoothly discharged and forming a dead zone. For Scheme 2, sealing the injecting hole and all baking holes, the oxygen volume fraction in the injecting chamber and casting chamber is relatively uniform, which indicates a better effect of discharging oxygen. For Scheme 3—sealing the injecting hole, stopper hole 2, and all baking holes—the oxygen volume fraction in casting chamber 1 is lower than that in chamber 2, which indicates the negative effects of discharging oxygen due to the long distance from casting chamber 2 to the outlet (stopper hole 1). As for Scheme 4, opening all holes, the oxygen content in the injecting chamber is higher than that in the casting chamber due to the stronger backflow of air. Overall, Scheme 2 is the best and used for the analysis of argon flow rate. *Metals* **2022**, *12*, x FOR PEER REVIEW 8 of 13

**Figure 6.** Contour of oxygen volume fraction at *z* = 0 m section, (**a**) for Scheme 1, (**b**) Scheme 2, (**c**) Scheme 3, (**d**) and Scheme 4. **Figure 6.** Contour of oxygen volume fraction at *z* = 0 m section, (**a**) for Scheme 1, (**b**) Scheme 2, (**c**) Scheme 3, (**d**) and Scheme 4.

#### 3.1.2. Calculation of Argon Flow Rate during a Period of Empty Tundish

3.1.2. Calculation of Argon Flow Rate during a Period of Empty Tundish Figure 7a shows the variation of average oxygen volume fraction with time under different argon flow rates. The volume fraction of oxygen shows similar tendencies for different argon flow rates, but the increase in argon flow rate accelerates the reduction of oxygen volume fraction, which shows that increasing the argon flow rate is conducive to discharging oxygen in tundish. Figure 7b shows the effect of argon flow rate on average volume fraction of oxygen at different times. The average volume fraction of oxygen linearly decreases with the increase in argon flow rate. It can be reduced to 1% after 10 min Figure 7a shows the variation of average oxygen volume fraction with time under different argon flow rates. The volume fraction of oxygen shows similar tendencies for different argon flow rates, but the increase in argon flow rate accelerates the reduction of oxygen volume fraction, which shows that increasing the argon flow rate is conducive to discharging oxygen in tundish. Figure 7b shows the effect of argon flow rate on average volume fraction of oxygen at different times. The average volume fraction of oxygen linearly decreases with the increase in argon flow rate. It can be reduced to 1% after 10 min when the argon flow rate is 220 Nm3/h, which indicates that the argon flow rate should be greater than 220 Nm3/h to ensure that the remaining volume fraction of oxygen is less than 1% and achieves a better protective casting effect during a period of empty tundish.

1% and achieves a better protective casting effect during a period of empty tundish.

**Figure 7.** Variation of volume fraction of oxygen with time for different argon flow rates (**a**), and

/h, which indicates that the argon flow rate should be

/h to ensure that the remaining volume fraction of oxygen is less than

/h

/h

/h

(**a**) (**b**)

effect of argon flow rate on the volume fraction of oxygen at different times (**b**).

when the argon flow rate is 220 Nm<sup>3</sup>

200 Nm<sup>3</sup>

220 Nm<sup>3</sup>

240 Nm<sup>3</sup>

 260 Nm<sup>3</sup> /h

greater than 220 Nm<sup>3</sup>

0 100 200 300 400 500 600

Time of argon blowing, s

0.00

0.05

0.10

Volume fraction of oxygen

0.15

0.20

0.25

Scheme 3, (**d**) and Scheme 4.

when the argon flow rate is 220 Nm<sup>3</sup>

greater than 220 Nm<sup>3</sup>

**Figure 7.** Variation of volume fraction of oxygen with time for different argon flow rates (**a**), and effect of argon flow rate on the volume fraction of oxygen at different times (**b**). **Figure 7.** Variation of volume fraction of oxygen with time for different argon flow rates (**a**), and effect of argon flow rate on the volume fraction of oxygen at different times (**b**).

**Figure 6.** Contour of oxygen volume fraction at *z* = 0 m section, (**a**) for Scheme 1, (**b**) Scheme 2, (**c**)

1% and achieves a better protective casting effect during a period of empty tundish.

Figure 7a shows the variation of average oxygen volume fraction with time under different argon flow rates. The volume fraction of oxygen shows similar tendencies for different argon flow rates, but the increase in argon flow rate accelerates the reduction of oxygen volume fraction, which shows that increasing the argon flow rate is conducive to discharging oxygen in tundish. Figure 7b shows the effect of argon flow rate on average volume fraction of oxygen at different times. The average volume fraction of oxygen linearly decreases with the increase in argon flow rate. It can be reduced to 1% after 10 min

/h, which indicates that the argon flow rate should be

/h to ensure that the remaining volume fraction of oxygen is less than

3.1.2. Calculation of Argon Flow Rate during a Period of Empty Tundish

#### 3.1.3. Calculation of Argon Flow Rate during a Period of Normal Casting

During a period of normal casting, the injecting chamber and casting chambers are isolated into independent chambers by molten steel. There is no gas exchange between those independent chambers. Compared with a period of empty tundish, the holding capacity of gas in the tundish largely decreases, and the argon flow rate should be adjusted during a period of normal casting. In order to determine the optimal argon flow rate, the variation of average oxygen volume fraction in an isolated injecting chamber and casting chamber was investigated, respectively. During the calculation, the oxygen volume fraction of Scheme 2 with an argon flow rate of 260 Nm3/h at 5 min was taken as the initial condition. During a period of normal casting, the baking holes are usually sealed except for the sampling operation, and the stopper holes and injecting hole remain open.

Figure 8a shows the variation of oxygen volume fraction with time in the isolated injecting chamber. The volume fraction of oxygen decreases with increasing argon blowing time, but the downtrend shows a tendency of increasing first and then decreasing with the increase in argon flow rate. The oxygen volume fractions after argon blowing for 5 min are 0.03, 0.0171, and 0.0259 when the argon flow rates are 60 Nm3/h, 80 Nm3/h, and 120 Nm3/h, respectively. Figure 8b shows the oxygen volume fraction at a given time under different argon flow rates. At a fixed time, the oxygen volume fraction first decreases and then increases with increase in argon flow rate, showing a minimum value at an argon flow rate of 80 Nm3/h.

In order to explain this phenomenon, the contours of oxygen volume fraction and velocity vector at *z* = 0 m section are abstracted and shown in Figures 9 and 10. As shown in Figure 9, when the argon flow rate is 80 Nm3/h, the oxygen volume fraction in the injecting chamber is uniform, except the region near the injecting holes, where it is slightly higher than that in the other region. With the argon flow rate increasing to 140 Nm3/h, the region of oxygen volume fraction of more than 0.027 in injecting chamber significantly increases, and even extends into the vicinity of baking holes. This phenomenon can be explained by the velocity vector, as shown in Figure 10. Argon jets into the injecting chamber by the argon pipe installed on both sides of the injecting hole. After the argon jet impacts the molten steel, two horizontal streams occur and collide at the center of the injecting hole, and then discharge from the injecting hole. With the discharge in argon, a large amount of air is drawn into the injecting chamber and results in an increase in oxygen volume fraction. The phenomenon of air entrainment increases with the increase in argon flow rate. Therefore, the best argon flow rate in the injecting chamber is 80 Nm3/h.

120 Nm<sup>3</sup>

flow rate of 80 Nm<sup>3</sup>

/h.

3.1.3. Calculation of Argon Flow Rate during a Period of Normal Casting

tion of Scheme 2 with an argon flow rate of 260 Nm<sup>3</sup>

During a period of normal casting, the injecting chamber and casting chambers are isolated into independent chambers by molten steel. There is no gas exchange between those independent chambers. Compared with a period of empty tundish, the holding capacity of gas in the tundish largely decreases, and the argon flow rate should be adjusted during a period of normal casting. In order to determine the optimal argon flow rate, the variation of average oxygen volume fraction in an isolated injecting chamber and casting chamber was investigated, respectively. During the calculation, the oxygen volume frac-

condition. During a period of normal casting, the baking holes are usually sealed except

Figure 8a shows the variation of oxygen volume fraction with time in the isolated injecting chamber. The volume fraction of oxygen decreases with increasing argon blowing time, but the downtrend shows a tendency of increasing first and then decreasing with the increase in argon flow rate. The oxygen volume fractions after argon blowing for 5

/h, respectively. Figure 8b shows the oxygen volume fraction at a given time un-

der different argon flow rates. At a fixed time, the oxygen volume fraction first decreases and then increases with increase in argon flow rate, showing a minimum value at an argon

for the sampling operation, and the stopper holes and injecting hole remain open.

min are 0.03, 0.0171, and 0.0259 when the argon flow rates are 60 Nm<sup>3</sup>

/h at 5 min was taken as the initial

/h, 80 Nm<sup>3</sup>

/h, and

**Figure 8.** Variation of oxygen volume fraction with time for different argon flow rates (**a**) and effect of argon flow rate on volume fraction of oxygen for different times (**b**) in an isolated injecting chamber. **Figure 8.** Variation of oxygen volume fraction with time for different argon flow rates (**a**) and effect of argon flow rate on volume fraction of oxygen for different times (**b**) in an isolated injecting chamber. volume fraction. The phenomenon of air entrainment increases with the increase in argon flow rate. Therefore, the best argon flow rate in the injecting chamber is 80 Nm<sup>3</sup> /h. volume fraction. The phenomenon of air entrainment increases with the increase in argon flow rate. Therefore, the best argon flow rate in the injecting chamber is 80 Nm<sup>3</sup> /h.

ing hole, and then discharge from the injecting hole. With the discharge in argon, a large amount of air is drawn into the injecting chamber and results in an increase in oxygen **Figure 9.** Contour of argon volume fraction at the z = 0 m section for different argon flow rates in the injecting chamber: (**a**) 80 Nm<sup>3</sup> /h, and (**b**) 140 Nm<sup>3</sup> /h. **Figure 9.** Contour of argon volume fraction at the *z* = 0 m section for different argon flow rates in the injecting chamber: (**a**) 80 Nm3/h, and (**b**) 140 Nm3/h. **Figure 9.** Contour of argon volume fraction at the z = 0 m section for different argon flow rates in the injecting chamber: (**a**) 80 Nm<sup>3</sup> /h, and (**b**) 140 Nm<sup>3</sup> /h.

The variation behavior of oxygen volume fraction in the isolated casting chamber **Figure 10.** Vector of argon velocity at *z* = 0 m section in the injecting chamber. **Figure 10.** Vector of argon velocity at *z* = 0 m section in the injecting chamber.

was also investigated and is shown in Figure 11. The volume fraction of oxygen in the isolated casting chamber continuously decreases with the increase in argon blowing time, and the downtrend shows a positive correlation with argon flow rate. Figure 11b shows

The variation behavior of oxygen volume fraction in the isolated casting chamber was also investigated and is shown in Figure 11. The volume fraction of oxygen in the

variation behavior of oxygen volume fraction in an isolated injecting chamber, the argon

the oxygen volume fraction at a given time under different argon flow rates. The oxygen volume fraction decreases with the increase in argon flow rate at a given time and achieves

variation behavior of oxygen volume fraction in an isolated injecting chamber, the argon

/h for the period of normal casting.

/h for the period of normal casting.

/h, which indicates the argon flow rate in

/h. However, combined with the

/h, which indicates the argon flow rate in

/h. However, combined with the

the isolated casting room should be greater than 80 Nm<sup>3</sup>

the isolated casting room should be greater than 80 Nm<sup>3</sup>

1% at 5 min when the argon flow rate is 80 Nm<sup>3</sup>

1% at 5 min when the argon flow rate is 80 Nm<sup>3</sup>

flow rate should be kept as 80 Nm<sup>3</sup>

flow rate should be kept as 80 Nm<sup>3</sup>

The variation behavior of oxygen volume fraction in the isolated casting chamber was also investigated and is shown in Figure 11. The volume fraction of oxygen in the isolated casting chamber continuously decreases with the increase in argon blowing time, and the downtrend shows a positive correlation with argon flow rate. Figure 11b shows the oxygen volume fraction at a given time under different argon flow rates. The oxygen volume fraction decreases with the increase in argon flow rate at a given time and achieves 1% at 5 min when the argon flow rate is 80 Nm3/h, which indicates the argon flow rate in the isolated casting room should be greater than 80 Nm3/h. However, combined with the variation behavior of oxygen volume fraction in an isolated injecting chamber, the argon flow rate should be kept as 80 Nm3/h for the period of normal casting. *Metals* **2022**, *12*, x FOR PEER REVIEW 11 of 13

**Figure 11.** Variation of oxygen volume fraction with time for different argon flow rate (**a**), and effect of argon flow rate on volume fraction of oxygen at different times (**b**) in an isolated casting chamber. **Figure 11.** Variation of oxygen volume fraction with time for different argon flow rate (**a**), and effect of argon flow rate on volume fraction of oxygen at different times (**b**) in an isolated casting chamber.

#### *3.2. Industrial Application 3.2. Industrial Application*

1 × 10−<sup>6</sup>

In order to evaluate the process effect of ABTC, plant trials of two consecutive tundishes for continuous casting were conducted, and each tundish was used for six ladles. The first tundish with ABTC, under Scheme 2, was taken as the trial group, and the second tundish without ABTC was taken as the control group. The process parameters of ABTC and continuous casting are listed in Table 4. In order to evaluate the process effect of ABTC, plant trials of two consecutive tundishes for continuous casting were conducted, and each tundish was used for six ladles. The first tundish with ABTC, under Scheme 2, was taken as the trial group, and the second tundish without ABTC was taken as the control group. The process parameters of ABTC and continuous casting are listed in Table 4.

**Table 4.** Process parameters of argon blowing tundish cover and continuous casting. **Table 4.** Process parameters of argon blowing tundish cover and continuous casting.


Casting speed, m/min 1.12 Section size of slab, mm × mm 1300 × 230 During trials, steel samples at the end of RH and in the tundish for each ladle were taken, and the contents of nitrogen, titanium, and aluminum in the steel samples were measured and are listed in Table 5. By applying ABTC, the average nitrogen content of steel samples in tundish decreases from 22 × 10 −6 to 16 × 10 −6 , and the average increased During trials, steel samples at the end of RH and in the tundish for each ladle were taken, and the contents of nitrogen, titanium, and aluminum in the steel samples were measured and are listed in Table 5. By applying ABTC, the average nitrogen content of steel samples in tundish decreases from 22 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 16 <sup>×</sup> <sup>10</sup>−<sup>6</sup> , and the average increased nitrogen content (4*w*[N]) from the end of RH to tundish decreases by 90% from <sup>10</sup> <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 1 <sup>×</sup> <sup>10</sup>−<sup>6</sup> . The average loss of titanium and aluminum decreases by 12.7%

nitrogen content (△*w*[N]) from the end of RH to tundish decreases by 90% from 10 × 10−<sup>6</sup>

loss of titanium and aluminum. Therefore, the protective casting effect could be signifi-

to 55 × 10−<sup>6</sup>

. The average loss of titanium and aluminum decreases by 12.7% from 63 × 10−<sup>6</sup>

, respectively. In general, the ABTC process is

to

to

cantly improved by applying ABTC.

from 63 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 55 <sup>×</sup> <sup>10</sup>−<sup>6</sup> and 7.1% from 70 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 55 <sup>×</sup> <sup>10</sup>−<sup>6</sup> , respectively. In general, the ABTC process is conducive to decreasing the increased nitrogen content of molten steel in tundish and the loss of titanium and aluminum. Therefore, the protective casting effect could be significantly improved by applying ABTC.

**Table 5.** Mass fractions of nitrogen, titanium, and aluminum of steel samples at the end of RH and in tundish.


#### **4. Conclusions**

In this study, the feasibility and application effect of ABTC during a continuous casting period were investigated and evaluated through simulations and plant trials. The following conclusions can be drawn:


**Author Contributions:** Conceptualization, Y.L. and X.Y.; Data curation, C.W.; Investigation, Y.L.; Methodology, X.X.; Software, Y.L.; Supervision, X.M.; Validation, J.C.; Writing—original draft, Y.L.; Writing—review and editing, C.W., X.X. and L.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is supported by the Pangang Group Research Institute Co., Ltd.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest that are relevant to the content of this article.

#### **References**


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