Effect of Nozzle Structure on Slab Continuous Casting Mold Flow Field

Abstract: The flow field of molten steel in the mold is the basis for studying the transmission behavior in the continuous casting machine. The numerical simulation method was used to analyze the flow pattern of molten steel in the mold under the nozzle structure (concave bottom, flat bottom and pointed bottom), mold width and casting speed. Numerical results show that the molten steel flow field in the mold is similar under the three immersed nozzle structures of concave bottom, flat bottom and pointed bottom. However, the liquid surface velocity of the molten steel under the nozzle of the pointed bottom structure is the largest, followed by the flat bottom, and the concave bottom is the smallest. As the width of the copper mould tube bottom increases, the range of the upper and lower reflow zones of the copper mould tube increases. As the casting speed increases, the mold surface velocity increases, but the impact position of the molten steel stream on the narrow surface of the mold remains unchanged.

Keywords: continuous casting; copper mould tube; slab; immersed nozzle; numerical simulatio

Continuous casting is a key link in modern steelmaking production, and the copper mould tube is the core equipment of the continuous casting machine. Field practice and existing research have shown that an unreasonable mold flow field may cause the molten steel surface flow rate to be too high, causing slag entrainment, or the impact solidification billet shell speed to be too high, resulting in the solidification billet shell to be too thin or even a steel breakout accident. Therefore, the optimization of the flow field in the mold has always been the research focus of the majority of continuous casting work. With the development of computer hardware and software, numerical simulation has become a necessary means for metallurgists to study the transmission process in the continuous casting machine. In view of the actual production of Nanjing Steel Steel Plant, this study uses numerical simulation methods to study the effects of the immersed nozzle structure, the width of the mold and the casting speed on the flow behavior of molten steel in the mold. Provide theoretical guidance for slab continuous casting production control.

1 Mathematical model of flow field

1.1 Basic assumptions

(1) The flow of molten steel in the copper mould tube is a steady-state flow of single-phase incompressible Newtonian fluid;

(2) The physical parameters of molten steel are all constant;

(3) Because the inclusions in the molten steel are very low, their impact on the flow of the molten steel is negligible;

(4) Ignoring the influence of the vibration of the mold and the protective slag on the surface of the molten steel on the flow of the molten steel, it is considered that the liquid level of the mold is flat;

(5) Ignore the tilting effect of the solidified shell and the inner wall of the copper mould tube.

1.2 Governing equations

The flow of molten steel in the mold can be determined by the continuity equation, momentum conservation equation and standard k-ε double equation.

continuity equation

In the formula: ρ is the density of molten steel, kg/m3; v is the velocity of molten steel, m/s; x is the coordinate;

Momentum conservation equation:

In the formula: p is the pressure of molten steel, Pa; g is the acceleration of gravity, m/s2; μeff is the effective viscosity of molten steel. In the basis, the effective viscosity is determined by k-ε double equation turbulence simulation.

2 Calculation area and boundary conditions

Considering the symmetry of the continuous casting mold, the calculation area can only take 1/4 of the slab, thereby greatly reducing the number of meshes and effectively reducing the calculation time.

2.1 copper mould tube wall and nozzle wall

The normal velocity component perpendicular to the wall is set to zero, the pressure, velocity, turbulent kinetic energy and turbulent kinetic energy dissipation rate parallel to the wall adopt the no-slip boundary, and the wall function is adopted at the near-wall position.

2.2 Water entrance

According to the conservation of inlet and outlet flow, the copper mould tube inlet speed can be determined by the following formula:

In the formula: vin is the entrance speed of the immersed nozzle, m/s; vpull is the casting speed, m/s; smold and ssen are the cross-sectional area of the crystallizer outlet and the cross-sectional area of the immersed nozzle entrance, respectively, m2.

2.3 Symmetry plane

The velocity component perpendicular to the symmetry plane is set to zero, and the gradients of all other physical quantities along the normal direction of the symmetry plane are zero.

2.4 Free surface

The slip boundary condition is adopted on the free liquid surface, that is, the velocity component perpendicular to the free liquid surface is set to zero, and the gradients of all other physical quantities along the normal direction of the weighing surface are zero.

2.5 Export

The gradients of all physical quantities along the normal direction of the outlet are zero.

3 Calculation methods

Figure 1 shows that the calculation domain is 1/4 of the slab volume, and the relevant calculation parameters are shown in Table 1.

The calculation domain area is divided using hexahedral mesh, and the mesh quality is greater than 0.9. The finite volume method is used to discretize the partial differential equations into difference equations. The coupling calculation of velocity and pressure is performed using the SIMPLE algorithm. Each calculation scheme requires about 5 000 to 6 000 iteration steps to obtain a converged solution. The convergence criteria for simulation calculations are that the residuals of each variable are less than 10-7 and the inlet and outlet flows are less than 1%.

Figure 1 copper mould tube calculation area

Table 1 Physical parameters of the model

parameter numerical value
Cast slab section (width×thickness)/mm×mm 2 070×220
Cast slab height/mm 5 000
Submersible nozzle section (length×width)/mm×mm 98×54
Casting speed/m·min-1 1.2
Nozzle immersion depth/mm 100
Nozzle outlet angle/(°) 15

4 Results and discussion

4.1 Immersed nozzle structure

Figure 2 is a schematic diagram of the bottom structure of the immersed nozzle. The bottom of the immersed nozzle can adopt three structures: concave bottom, flat bottom and pointed bottom. At the entrances of these three types of nozzles, the velocity of the molten steel is all 1.72 m/s, the cross-sectional areas of the entrances are exactly the same, and the cross-sections of the exits are also the same. The only difference is the shape of the bottom.

Figure 3 is a schematic diagram of the flow field of the copper mould tube under different nozzles. Figure 3 shows that although the bottom structure of the immersed nozzle is different, the molten steel flow field in the copper mould tube is very similar. After the molten steel vertically enters the immersed nozzle, it hits the bottom of the nozzle in a straight line, and then divides into left and right streams and flows out of the immersed nozzle; the molten steel flowing out from the immersed nozzle impacts the narrow surface of the copper mould tube in the form of a jet and then divides into two upper and lower streams. . One of the streams reaches the liquid level along the narrow surface and then flows along the liquid surface to the water outlet, finally forming an upper recirculation area. Another stream flows downward along the narrow surface. After reaching a certain depth, part of the fluid changes its flow direction to form a lower return zone, and part of the fluid continues downward.

4.2 copper mould tube width

Figure 4 is a schematic diagram of the copper mould tube flow field under different copper mould tube widths. Figure 4(a) shows that when the copper mould tube width is 1200 mm, the areas where the upper reflux and the lower reflux flow are smaller. The vortex center of the upper recirculation zone is 450 mm from the free liquid surface and 300 mm from the central symmetry plane of the narrow surface; the vortex center of the lower recirculation zone is 1500 mm from the free surface and 300 mm from the central symmetry plane of the narrow surface. Figure 4(b) shows that the shape of the upper reflux area of the flow field when the copper mould tube width is 2070 mm is similar to Figure 4(a). The difference is that the upper reflux and lower reflux areas are larger. The vortex center of the upper recirculation zone is 900 mm from the free liquid surface and 520 mm from the central symmetry plane of the narrow surface; the vortex center of the lower recirculation zone is 3000 mm from the free surface and 520 mm from the central symmetry plane of the narrow surface. Figure 4(b) also shows that partial fluid backflow occurs at the outlet. This is because under the same flow rate, the larger the width of the copper mould tube, the smaller the kinetic energy of impact on the narrow surface, and the larger the lower swirl zone.

Figure 2 The bottom structure of the immersed nozzle

Figure 3 copper mould tube flow field under different nozzles

Figure 4 Crystallizer flow field under different copper mould tube widths

4.3 Influence of immersed nozzle structure

Figure 5 is a diagram showing the influence of different immersed nozzle structures on liquid surface velocity distribution. Figure 5 shows the horizontal velocity of the nozzle and liquid surface. The nozzle and the copper mould tube liquid level are separated by the immersed nozzle refractory material, so the speed in Figure 5 is divided into left and right sections. In the immersed nozzle, the velocity of the molten steel is along the axis of the nozzle, so the horizontal velocity of the molten steel is close to zero. The velocity distribution of molten steel on the liquid surface is uneven, and the velocity distribution presents a “parabolic” shape. Different immersed nozzle structures have basically the same liquid surface velocity distribution. The difference is that the maximum value of the molten steel velocity and its occurrence position are slightly different. When a sharp bottom nozzle is used, the liquid surface velocity reaches a maximum value of 0.36 m/s at a distance of 0.47m from the nozzle. When a concave bottom nozzle is used, the liquid surface velocity reaches a maximum value of 0.34 m/s at a distance of 0.45 m from the nozzle. This is because different nozzle bottom structures will cause different kinetic energy losses of the molten steel. When the bottom structure of the nozzle adopts a concave surface, the liquid steel loses the most kinetic energy, the liquid surface velocity is small, the liquid level is stable, and slag entrainment is less likely to occur; when an inclined surface nozzle is used, the liquid surface velocity is larger, which is conducive to slag removal.

Figure 6 is a diagram showing the influence of different immersed nozzles on the narrow surface impact point. Figure 6 shows that under immersed nozzles with different bottom structures, the position where the molten steel impacts the narrow surface is basically the same and the velocity distribution of the fluid on the narrow surface is also similar. The molten steel flowing out from the immersed nozzle rushes towards the narrow surface located 0.47 m below the liquid surface at high speed. The velocity of the fluid rising along the narrow surface reaches the maximum value at 0.28 m from the free liquid surface, and then slows down to flow to the free liquid surface; The fluid velocity flowing downward along the narrow surface reaches the maximum value at 0.79 m from the surface of the molten steel, and then slows down and flows toward the outlet.

Figure 5 Effects of different immersed nozzle structures on liquid surface velocity distribution

Figure 6 The influence of different immersed nozzles on the narrow surface impact point

4.4 Influence of billet drawing speed

Figure 7 is a diagram of the mold liquid surface velocity at different casting speeds when using a pointed bottom structure nozzle. Figure 7 shows that under different pulling speeds, the velocity distribution of the molten steel at the free liquid surface is similar, but the velocity values of the molten steel show large differences. The greater the casting speed, the greater the peak value of molten steel surface velocity, and the peak value appears at a distance of 0.45 m from the nozzle. When the pulling speed is 1.4 m/min, the maximum speed is 0.42 m/s; when the pulling speed is 1.2 m/min, the maximum speed is 0.29 m/min, a difference of 0. 13 m/min. Therefore, the greater the pulling speed, the more severe the free liquid surface velocity fluctuation, and the easier it is for slag entrapment to occur.

Figure 7 Effect of billet drawing speed on liquid surface velocity distribution

Figure 8 Effect of billet drawing speed on narrow surface impact point

Figure 8 shows the velocity of molten steel along the narrow surface of the mold at different casting speeds when using a pointed bottom structure nozzle. Under different pulling speeds, the position where the molten steel stream impacts the narrow surface is basically the same, which is 0.47 m; however, the maximum value of the fluid velocity when the molten steel stream flows upward and downward along the narrow surface is different. When the pulling speed is 1.4 m/min, the maximum upward flow velocity of molten steel is 0.21 m/s, and the maximum downward flow velocity is 0.28 m/s. When the pulling speed is 1.2 m/min, the maximum upward flow velocity of molten steel is 0. 18 m/s, and the maximum downward flow velocity is 0.24 m/s. Therefore, the greater the pulling speed, the greater the impact depth of molten steel, and the easier it is to cause a steel breakout accident.

5 Conclusion

(1) Under different immersed nozzles (concave bottom, flat bottom and pointed bottom), the flow field of molten steel in the mold is similar. After the fluid flows out from the immersed nozzle, it impacts the narrow surface of the calculation domain and forms two streams. The ascending stream reaches the free liquid level and forms an upper reflux area in the upper part of the copper mould tube. The downflow stream forms a lower reflux area in the lower part of the copper mould tube.

(2) When the copper mould tube width is 1200 mm, the upper and lower reflow areas are smaller; when the copper mould tube width is 2070 mm, the upper and lower reflow areas are larger.

(3) Although the bottom structure of the immersed nozzle is different (concave bottom, flat bottom and pointed bottom), the velocity distribution of the liquid surface is basically similar, both showing a “parabolic” shape, but the concave bottom nozzle leads to the smallest flow rate of the liquid surface, followed by the flat bottom , the largest tip. The impact point position of the molten steel stream on the narrow surface under different immersed nozzles is the same.

(4) Under different casting speeds, the distribution of mold liquid surface velocity is similar. As the casting speed gradually increases, the mold liquid surface velocity gradually increases, but the position where the molten steel stream impacts the narrow surface remains unchanged.

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