MERCURY MODEL ON FLOW IN CONTINUOUS CASTING OF SLAB WITH EMBR. Kang DENG, Zhenqiang ZHANG, Zuosheng LEI, Zhongming REN & Weizhong DING

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1 May 21 st 23 rd 214, Brno, Czech Republic, EU MERCURY MODEL ON FLOW IN CONTINUOUS CASTING OF SLAB WITH EMBR Kang DENG, Zhenqiang ZHANG, Zuosheng LEI, Zhongming REN & Weizhong DING Shanghai University, Shanghai, China, Abstract A mercury model has been developed to investigate the influences of Electromagnetic Mold Brake Ruler (EMBr-Ruler) and Flow Control Mold (FC-Mold) on metal flow in continuous casting of slab with the practical casting velocity 1., 1.3 and 2. m/min respectively. The experiment shows that the FC-Mold can efficiently repress the surface flow and its fluctuation. The expanding space of the jets is compressed by EMBr-Ruler and FC-Mold respectively, then the flow passage where the vertical velocity of flow increases sharply is developed near the narrow wall with EMBr-Ruler and FC-Mold. It is bad for the development of the plug like flow. Only the nozzle ports are placed in the braking magnetic field region and the casting speed is suitable, EMBr can be beneficial to the formation of the plug like flow. The flow regime is improved by FC-Mold when the casting velocity is high (such as 2 m/min), and EMBr can improve the flow field when the casting velocity to be medium (such as 1.3 m/min), but for slow casting velocity (such as 1 m/min), the effects of above two types of EMBr are unsatisfactory at all. Keywords: continuous casting of slab; electromagnetic braking (EMBr); metal flow; mercury model. 1. INTRODUCTION Electromagnetic Braking (EMBr) is widely applied as an useful method to control metal flow in the mold of continuous casting of slab [1-2]. Several years ago, the second generation (Electromagnetic Mold Brake Ruler, EMBr-Ruler) and the third generation (Flow Control Mold, FC-Mold) of EMBr have replaced the first generation of local electromagnetic brake (EMBr) gradually and then resolve the defects of the local EMBr in which the braking region is not enough and the molten steel tends to flow to without magnetic field region under the nozzle. At the same time, Yu et al. [3] built a three-dimensional finite-volume mathematical model of region electromagnetic brake to study the influences of the magnetic field intensity on the effect of EMBr. Cukierski et al. [4] applied a computational fluid flow model to investigate the effects of varying submergence depth of submerged entry nozzle (SEN) and magnetic field intensity of EMBr on metal flow in the mold. Harada et al. [5] built a mercury model to study the brake effect of a local magnetic field and a level magnetic field. Now the research about the comparison of the influences of different types of EMBr (EMBr-Rule and FC-Mold) on metal flow is few, especially applying the physical simulation. In this paper, the velocity distribution of mercury (which is instead of motel steel in the experiment) in the physical model of slab continuous casting mold is measured by ultrasound Doppler velocimetry (UDV), from which the influences of EMBr-Ruler and FC-Mold on metal flow in mold with different casting velocity can be investigated. 2. EXPERIMENTAL APPARATUS AND METHOD In the experiment the mold model is based on a scale of 1/6 of the size of a slab continuous casting mold and the technique processes of BAOSTEEL. The physical simulation apparatus and the parameters of molten metal using in the experiment can be found in [5]. It has two pairs of magnetic poles, one of the centre lines of the fields imposed is at the meniscus and the other is at 78 mm below the meniscus. Both of the fields cover the entire width of the mold. The magnetic field intensities of the magnetic fields can be adjusted respectively in order to simulate EMBr-Ruler and FC-Mold. The geometric and technical details of the industrial prototype and the model are presented on the following Table.

2 May 21 st 23 rd 214, Brno, Czech Republic, EU Table 1 Basic parameters relating to model experiment and production Parameter Prototype value Model value Model height, mm Model thickness, mm Model width, mm Outer diameter of SEN, mm 12 2 Inner diameter of SEN, mm 7 12 Port height, mm 8 13 Port width, mm 6 1 Port angle, deg -15 (downward) -15 (downward) SEN submergence depth, mm Magnets width, mm 24 4 Magnets length, mm Casting velocity, m/min 1, 1.3, 2.41,.52,.82 In the experiment, the similarities between the industrial prototype and the model are maintained by the Froude number (Fr) and the Stuart number (St). There Fr makes sure the flow field similarity, and St makes sure the magnetic field similarity. According to production practice, the magnetic field intensity of the down pair of magnetic poles should be strong in the EMBr-Ruler processes, which is in order to shorten the impact depth, promote the formation of the plug like flow under the nozzle and stabilize the flow field as soon as possible. In the experiment, the magnetic field intensity of the down pair of magnetic poles remains the maximum value.5 T (responding to the industrial value.28 T practically). In the experiment, the regression curves of magnetic flux density in the centre section between two poles of magnets can be shown in Fig.1. The maximum magnetic field intensity of the down pair of magnetic poles (B 2) is.5 T, the maximum magnetic field intensity of the upper pair of magnetic poles (B 1) is increased from to.5 T gradually. There the case of B 1= T & B 2=.5 T is about equal to EMBr-Ruler and the another one of B 1=.5 T & B 2=.5 T is about equal to FC-Mold. Both the positive and negative sign show respectively the opposite directions. y, mm B 1 =T, B 2 =.5T B 1 =.5T, B 2 =.5T B, T Fig. 1 Magnetic flux density distribution in the mold model Also in the experiment, the mercury velocity in mold model is measured by ultrasound Doppler velocimetry (UDV). The principle and the test method can be found in [6]. A line array of eighteen 2-MHz transducers is assembled at the outer wall of the mold aligned with the midsection of the narrow face with a distance of 15 mm between two adjacent transducers. The hightest sensor is assembled at 1mm under the liquid surface.

3 May 21 st 23 rd 214, Brno, Czech Republic, EU At the same time, a line array of eighteen 2-MHz transducers is assembled at the liquid surface aligned with the midsection of the narrow face with a distance of 5 mm between two adjacent transducers. The first sensor is assembled at a distance of 7mm from the narrow wall. Using this configuration, the velocity profiles are recorded along the wide face of the mold from the sidewall to the submerged entry nozzle. When the flow field in the mold is stable, the continuous testing lasts 12 s. The time-average of the velocity is as the velocity of the metal flow. The direction of flow is defined positive when it is identical with the coordinate direction. The root mean square of the mean velocity and the instant velocity (turbulivity) is utilized as the indicator to estimate the stability of the metal flow. 3. Experimental Results and Discussion 3.1 Flow Field in Mold Model With Casting velocity.41 m/min The flow pattern in the mold model is shown in Fig.2 for the cases of (a) no magnetic field, (b) EMBr-Ruler and (c) FC-Mold. It can be obtained that with EMBr-Ruler, the lower magnetic poles repress the dispersion of the jet from the SEN outport, delay the metal flow upward, then make the jet flow have relatively big horizontal momentum when it impacts the narrow wall. It enhances the horizontal impact strength to the narrow wall. With FC-Mold, the upper magnetic poles also limit the space of the jet flow and enlarge the angle of the jet. It enlarges the distance that jet flow spreads to the narrow wall. Then with the coupled function of the two pairs of magnetic poles, the maximum velocity impinging the narrow wall and the impinging position are both smaller than the values with EMBr-Ruler. In addition, from the flow regime perspective, there are vortex centers in the upper recirculating flow region in the model when no magnetic field or with EMBr-Ruler, and the velocity of the upper recirculating flow is relatively big. But with FC-Mold, the velocity of the upper recirculating flow is small, and the vortex center is not obvious. This indicates that FC-Mold gets the result in terms of stabilizing the surface flow. From the downward flow near the narrow wall perspective, the flow passage is developed near the narrow wall of the mold with EMBr making the velocity of the downward flow enlarge, especially with FC-Mold, the channel effect is more obvious than with EMBr- Ruler. This indicates that with EMBr, the inhibitory action of the magnetic field on the flow makes the liquid flow redistribute. When the space distribution of the jet from the SEN outport is restrained, the major flow of the jet is released near the narrow wall, then the channel effect is developed. It is not benefit for the formation of the plug like flow under the nozzle..4-t-t(28) t-t(28) t-.5t(28) (a) without EMBr (b) EMBr-Ruler (c) FC-Mold Fig. 2 mercury velocity distribution in the model with casting velocity.41 m/min Moreover, the experiment verifies that the horizontal velocity near the liquid surface decreases on the whole with the magnetic field intensity of the magnetic poles increasing. The horizontal velocity near the surface is higher than that near the narrow wall, and it decreases linearly with the reduction of the distance to the

4 May 21 st 23 rd 214, Brno, Czech Republic, EU nozzle. The experiment shows that in the case of without EMBr, the maximum horizontal velocity near the surface is.36 m/s, which is higher than that one (.35 m/s) with EMBr-Ruler. In addition, from the stability of the surface flow perspective, EMBr makes the stability of the surface flow increase obviously, in which the maximum turbulent intensity near the surface, compared with that without EMBr, is decreased 25% in EMBr- Ruler and 8% in FC-Mold. On the whole, EMBr-Ruler fails to solve the problem of the surface flow, but it increases the stability of the surface flow; and yet, FC-Mold can control the surface flow effectively, but it makes the horizontal velocity near the surface be so slow that it is easy to cause the meniscus freezing. 3.2 Flow Field in Mold Model With Casting velocity.52 m/min The magnetic field is the same as the condition with casting velocity.41 m/min. Fig.3 shows the flow regime and velocity distribution of the mercury in the mold model. The results are similar as that on Fig. 2, but the channel effect of the downward flow near the narrow wall of the mold decreases somewhat in EMBr-Ruler..52-T-T(28).52-.5T-T(28).52-.5T-.5T(28) (a) without EMBr (b) EMBr-Ruler (c) FC-Mold Fig. 3 mercury velocity distribution in the model with casting velocity.52 m/min Also from the horizontal velocity near the liquid surface perspective, the maximum horizontal velocities near the surface are.114,.16 and.26 m/s corresponding respectively to the cases of without EMBr, EMBr- Ruler and FC-Mold. It shows a decreasing trend. In the cases of without EMBr and with EMBr-Ruler, both of the horizontal velocity distributions on the surface can be shown approximately parabola type, and the positions of the maximum horizontal velocities are all near 1/4 of the mold wide wall. But in FC-Mold the horizontal velocity distribution on the surface shows approximately linearity type, and the horizontal velocity and the turbulent intensity near the surface are low on the whole. Their maximum values are respectively 1/5 and 1/6 in comparison with that without EMBr. It shows the braking effect of FC-Mold on the surface flow and its fluctuate behavior. This is because the inhibition effect of the upper magnetic poles on the upper recirculating flow makes the upward flow turn to the horizontal direction under the upper magnetic poles. 3.3 Flow Field in Mold Model With Casting velocity.82 m/min Fig.4 shows the flow regime and velocity distribution of the mercury in the mold model. The results show that with high model casting speed (.82 m/min), corresponding to the cases of without EMBr, with EMBr-Ruler and with FC-Mold, the maximum velocities of the jet from the outport are respectively.37,.32 and.29 m/s, and the maximum horizontal velocities impinging the narrow wall of the model are.71,.46 and.49 m/s respectively. On the whole, the maximum velocities decrease gradually with the magnetic field intensity increasing, but the impinging positions on the narrow wall are respectively 55, 4 and 55 mm under the liquid surface, which is oscillating with the magnetic field intensity increasing. Furthermore, the result of the horizontal velocity near the liquid surface shows that the maximum horizontal velocities of the surface flow are respectively.136,.85 and.28 m/s corresponding to the cases of

5 May 21 st 23 rd 214, Brno, Czech Republic, EU without EMBr, with EMBr-Ruler and with FC-Mold. Clearly, EMBr-Ruler has certain inhibition effect, but the braking effect of FC-Mold is better, which shows that the whole horizontal velocities of the surface flow in the mold is decreasing and stable to above three cases..82-t-t(28) t-t(28) t-.5t(28) (a) without EMBr (b) EMBr-Ruler (c) FC-Mold Fig. 4 mercury velocity distribution in the model with casting velocity.82 m/min From the horizontal velocity near the liquid surface with different casting speeds perspective, no matter with or without EMBr and EMBr-Ruler, both of the velocity and the fluctuant range of the surface flow increase with the casting velocity increasing, but FC-Mold can effectively decrease the velocity and fluctuation of the surface flow only. 4. CONCLUSIONS (1) As for the slow casting velocity (such as.41 m/min in the experiment, which is corresponding to the practical value 1 m/min), EMBr-Ruler enhances the vertical velocity near the narrow wall, with the result that the erosion to initial solidification shell can be intensified; FC-Mold is throretically beneficial to inhibit the upper recirculating flow and to decrease the velocity of surface flow, but its effect is very limited practically in this case. (2) As for the medium casting velocity (such as.52 m/min in the experiment, which is corresponding to the pactical value 1.3 m/min), from the inhibition on the velocity and fluctuation of the surface flow perspective, the braking effect of FC-Mold is better than EMBr-Ruler. It shows that the velocity of the surface flow is stable and the fluctuation of the liquid surface decreases. (3) As for the fast casting velocity (such as.82 m/min in the experiment, which is corresponding to the practical value 2 m/min), EMBr-Ruler could reduce the horizontal velocity near the liquid surface in a certain extent, but enhance the channel effect near the narrow wall. Theoretically, It can enhance the erosion to initial solidification shell. FC-Mold could effectively decrease the velocity and fluctuation of the surface flow and steady the upper recirculating flow field, and it does not enhance the erosion effect on the narrow wall obviously. FC-Mold has the better effect to control the flow field. REFERENCES [1] Klaus T, Sven E, Gunter G. Experimental Investigation of the Flow in a Continuous-Casting Mold under the Influence of a Transverse, Direct Current Magnetic Field. Metall Mater Trans B, 211, year 42, nr. 1, pages [2] Timmel K, Eckert S, Gerbeth G, et al. Experimental Modeling of the Continuous Casting Process of Steel Using Low Melting Point Metal Alloys the LIMMCAST Program. ISIJ Int., 21, year 5, nr. 8, pages [3] Yu H Q, Wang B F, Li H Q, et al. Influence of electromagnetic brake on flow field of liquid steel in the slab continuous casting mold. J Mater Process Techn., 28, nr. 22, pages

6 May 21 st 23 rd 214, Brno, Czech Republic, EU [4] Cukierski K, Thomas B G. Flow Control with Local Electromagnetic Braking in Continuous Casting of Steel Slabs. Metall Mater Trans B, 28, year 39B, pages [5] Harada H, Toh T, Ishii T, et al. Effect of Magnetic Field Conditions on the Electromagnetic Braking Efficiency. ISIJ Int., 21, year 41, nr. 1, pages [6] Jia H, Wang M, Zhang Z Q, et al. Effects of Nozzle Submerged Depth and Casting Velocity on Liquid Metal Flow with EMBr in Continuous Casting of Slab. Shanghai Met., 212, year 34, nr. 1, pages (in Chinese)