Effect of electromagnetic braking on metallurgical behavior in thin slab mold

This article describes the effect of electromagnetic braking on thin slab molds.

Keywords: high drawing speed; thin slab; magnetic field; electromagnetic braking; numerical simulation

In order to further improve the application effect of the multi-mode 5-hole block electromagnetic braking system in the high-efficiency continuous casting process, the FTSC thin slab mold and the new multi-module continuous electromagnetic braking (MM-EMB) system of a steel plant were used as prototypes. The characteristics of the flow field in the mold and the fluctuation degree of the steel slag interface were used as the evaluation criteria. Numerical simulation methods were used to perform multi-physics coupling calculations on the flow field of the mold under different electromagnetic conditions, focusing on revealing that under the condition of high pulling speed of 6 m/min , when different current intensities are applied to the electromagnetic braking system, the influence of different coil combinations in the multi-module continuous electromagnetic braking system on the flow field characteristics in the thin slab continuous casting mold. The research results show that based on the structural characteristics of the 5-hole nozzle, the electromagnetic brake (MM-EMB) system can divide the 5 sets of coils into 2 sets of control units according to their scope of action. The upper control unit contains 2 sets of coils, and the main scope of action is In the reflow area of the molten steel flow field, when the applied current of the two sets of coils increases from 400 A to 1000 A, it can effectively reduce the speed of the molten steel flow hitting the steel slag interface and reduce the fluctuation of the steel slag interface; the lower control unit contains three sets of coils , mainly plays a stabilizing role in the lower reflow in the crystallizer. When the current of the three sets of coils increases to 800 A, the vortex center position and the narrow surface impact point decrease significantly, and the intensity and range of the lower reflow are effectively controlled. When two sets of control units work together on the molten steel flow field in the mold, and the coil current of the lower control unit is 800 A, the best braking effect can be achieved by applying a current of 800 A to the two sets of coils of the upper control unit. The numerical simulation results were applied to industrial tests. Through on-site data tracking, the slag inclusion rate and crack rate of the slab were further reduced. The application of numerical simulation methods provided theoretical basis and technology for the multi-module continuous electromagnetic braking system to further optimize the mold flow field. support.

The core of efficient continuous casting is high casting speed. Thin slab continuous casting and rolling technology is subject to the influence of high-speed conditions, which may affect the surface quality of the cast slab, or may lead to steel breakout accidents. Electromagnetic braking technology is an effective means to improve the behavior of the molten steel flow field in the mold. . In recent years, due to the continuous advancement of the application of magnetohydrodynamics in metallurgy and the continuous development of electromagnetic braking technology, domestic and foreign scholars have conducted a large number of studies on it. The continuous casting production process is a complex and huge system, and numerical simulation is an effective research method. The advantage of simulation is that the physical phenomena in the continuous casting and rolling process can be studied more intuitively through the combination of physical simulation and numerical simulation. Domestic and foreign scholars have done a lot of research using numerical simulation methods: CUKIERSKI K et al. used the FLUENT software of the ANSYS platform to study the impact of magnetic induction intensity on the flow of molten steel in the mold. LEI S W et al. further coupled the influence of the electromagnetic braking device on the flow of molten steel in the thin slab immersed nozzle mold under multi-physical fields through programming methods. By comparing with physical experiments, they found that the numerical simulation results were similar to the physical simulation results. Li Baokuan et al. applied numerical simulation technology to study the impact of full-width one-section electromagnetic braking on the flow of molten steel in the thin slab continuous casting mold. The results showed that after the second generation electromagnetic braking was applied, the eddy current shape in the mold changed, which played a role. Significantly suppresses the effect of eddy currents, but does not completely eliminate them. GARCIA-HERNANDEZ S et al. analyzed the influence of electromagnetic force on the flow of molten steel from the perspective of stress. When the magnetic induction intensity reaches 0.1 T near the nozzle, the turbulent flow in the mold can be effectively controlled. The application of electromagnetic braking technology can reduce the size of the mold. The flow velocity at the molten steel surface reduces the peak flow velocity of the molten steel at the molten steel surface. However, with the further development of thin slab equipment and processes, after the thin slab drawing speed reaches 6 m/min, the impact of the structural optimization, installation position, and electromagnetic intensity of the electromagnetic braking system on the metallurgical behavior of the molten steel in the mold is unclear. There are reports. This study uses the FTSC thin slab mold and the new multi-module continuous electromagnetic braking (MM-EMB) system as prototypes. The flow field characteristics in the mold and the fluctuation degree of the steel slag interface are used as the evaluation criteria. Numerical simulation methods are used for modeling. The coupling calculation of multi-physics focuses on revealing the effect of different coil combinations in the multi-module continuous electromagnetic braking system on the crystallization of thin slab continuous casting when different current intensities are applied to the electromagnetic braking system at a high pulling speed of 6 m/min. The influence of the flow field characteristics in the device provides theoretical basis and technical support for further improving the application effect of the multi-module electromagnetic braking system in the high-efficiency continuous casting process.

1) The application of the newly designed 5-hole immersed nozzle can, to a certain extent, reduce the flow rate of the molten steel stream from the side hole of the nozzle impacting the narrow side of the mold, and reduce the fluctuation of the steel liquid level in the mold. However, when the pulling speed is increased to 6 m/min, the fluctuation of the mold steel liquid level intensified, and the fluctuation height near the narrow side reached a maximum of 15 mm. Therefore, it is necessary to cooperate with electromagnetic braking equipment to better suppress the fluctuation of the crystallizer liquid level, reduce the probability of slag entrainment, and improve the quality of the cast slab.

2) There are 5 sets of electromagnetic coils in the multi-module electromagnetic braking system, which can be divided into 2 control units according to the distribution position and scope of the coils. Under high pulling speed conditions, when different current values are applied to the coils A and B of the upper control unit, the shape of the molten steel flow field does not change significantly, but when the applied current increases from 400 A to 1000 A, the upper surface of the mold The scope of the vortex in the recirculation area is enhanced, the position of the vortex center continues to decrease, the intensity of the eddy current near the steel-slag interface continues to weaken, and the speed of the molten steel stream impacting the steel-slag interface in the recirculation area of the flow field decreases.

3) When the current intensity of the three sets of coils of the lower control unit C, D, and E continues to increase, the intensity and range of the backflow under the molten steel flow field are significantly suppressed, and the vortex center position and narrow surface impact point are also significantly reduced. However, when the current reaches 800 A and continues to increase, the molten steel flowing through this area will generate excessive Lorentz force, and the kinetic energy of the molten steel will become smaller, making it difficult to impact the narrow surface of the crystallizer and causing crystallization The area of the reflow area on the device becomes larger, which is not conducive to the floating of bubbles and inclusions.

4) Under the condition that the pulling speed is 6 m/min, when the current of the C, D, and E coil control units is set to 800 A unchanged, the steel slag can be minimized by applying different currents to the A and B groups of coils. Interface surface flow velocity and fluctuation intensity, but when the current increases to 800 A, as the current flowing into the A and B coils continues to increase, the downward movement rate of the peak point of the surface flow velocity curve decreases. When the mold steel liquid level fluctuations and the current are 800 A There is no obvious change, so the current applied to the coils of groups A and B should not exceed 800 A.

5) In the actual application process of the multi-module electromagnetic braking system on site, combined with the numerical simulation results, the currents of the C, D, and E coil control units were set to 800 A. By adjusting the A and B coil currents, the slag inclusion rate of the slab was equal to The crack rate has been reduced, especially the control of slag inclusion defects is more significant.

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