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Simulation study on liquid steel flow and inclusion removal in bottom-blown gas tundish

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Through the 1:3 water model experiment of the 40t tundish, the flow field of the molten steel in the 6-flow tundish of billet continuous casting was measured. The effects of five different bottom blowing schemes on the flow characteristics and inclusion removal of molten steel in the tundish were studied.

Keywords: tundish; bottom blow; water simulation; inclusions

Blowing air from the bottom of the tundish can effectively improve the flow characteristics of the molten steel and facilitate the removal of non-metallic inclusions, improving the cleanliness of the molten steel. This paper uses a water model to study the impact of bottom blowing at different positions on the flow field of the tundish, and simulates inclusion removal using polystyrene plastic particles.

Water simulation experiment

Experimental principle

According to the principle of similarity, Froude’s quasi-number equality is adopted to ensure the geometric similarity and dynamic similarity between the model and the prototype. In actual production, the steel ladle capacity is 90t and the intermediate ladle capacity is 40t. This experiment is based on the 6-stream tundish of billet continuous casting, with a similarity ratio of 1:3. A organic glass model is used to simulate the tundish, tap water simulates molten steel, and compressed air simulates argon (Figure 1). Dispersed breathable brick 3 used in the experiment. The tundish structure and blowing position are shown in Figure 2.

Figure 1 Schematic diagram of water model experimental device

Figure 2 Schematic diagram of the prototype structure of the tundish and the air blowing position in the water simulation experiment​

Experimental methods

According to the similarity principle, the relationship between the model and prototype volume flow rates is Qm=λ⁵²Qp. During the experiment, after the flow field in the tundish is stabilized, a certain amount of saturated KCl solution is pulsed at the long nozzle of the ladle. The conductivity probe measures the change pattern of conductivity with time at each outlet. After reaching the specified time, the RTD curve of each outlet is drawn for analysis based on the data recorded by the conductivity meter. Considering the symmetry of the tundish, only three water outlets on one side need to be measured, specifically the 1, 2, and 3 water outlets in Figure 2. The experimental results are based on the model proposed by Ahuja et al., and the volume fractions V₄, V, and Vm of the tundish dead zone, plug flow and total mixed flow are calculated and analyzed. At the same time, the standard deviation (S) is used to examine the dispersion of flow characteristics between each outlet. Discussed (1) the tundish bag, that is, the bottom of the bag does not blow (scheme Y). (2) Blow air at position ① in the flow injection area (Plan I). (3) Blow air at the ② position between the proximal flow and the middle flow outlet (Plan Ⅱ). (4) Blow air at position ③ between the middle flow and the far flow outlet (Plan Ⅲ). (5) On the basis of Scheme II, a square turbulator is added to the flow injection area, and the position is shown in ④ in Figure 2 (Option IV). (6) On the basis of Plan III, a square turbulator is added in the flow injection area (Plan V).

Polystyrene plastic particles are used to simulate inclusions, with a density of ρ=640kg/m³. A stepwise addition method was used to measure inclusion removal, and alcohol was used as a dispersant for particles. A certain amount of particles is added each time with the injection flow of the ladle, and is collected by sieves at the exits of the six flows of the tundish. The collection is stopped after 1200 s (twice the theoretical residence time). After the collected particles were dried, the removal of inclusions was statistically analyzed.

Experimental results and analysis

For the tundish non-blowing experiment, it can be seen from the results of the mixed model in Table 1 and RTD curve Figure 3(a):

(1) The peak time is short, the average value is only 64 s, and the peak conductivity value is high, that is, most of the flows flow out of the outlet quickly, and the flow path is single to form a short-circuit flow;

(2) The dead zone ratio reaches 60%, the old and new molten steel are not fully mixed, and the temperatures of the components of each stream are uneven;

(3) The RTD curve is “peak-shaped”. The sharply changing curve causes the molten steel to stay in the tundish for a short time, which is not conducive to the floating of inclusions.

Table 1 The residence time and volume fraction of the flow mode of molten steel in the tundish measured experimentally for each scheme

plan nozzle Dead time (tmin)/sPeak time (tmax)/s Actual average dwell time/sFlow pattern/%
piston areadead zonefull mixed zone
No blowingaverage value41642578.3959.0632.55
Ystandard deviation2926184.433.001.43
planaverage value539335611.5943.4444.97
standard deviation5319344.095.471.57
planaverage value3716738816.2438.1045.66
standard deviation2750745.9411.705.91
planaverage value389634010.7445.8443.42
standard deviation2192478.947.482.89
planaverage value3816239916.0535.8148.14
standard deviation2379548.168.670.58
planaverage value468332510.3548.0541.60
standard deviation2560686.8510.894.28

It can be seen from Table 1 that when the breathable brick is installed at position ① in the flow injection area, the stagnation time and peak time are improved compared to the experiment without air blowing. The average values of each flow are 53 and 93 s respectively, the standard deviation value is small, and the flow characteristics of each flow are relatively consistent. The dead zone ratio decreased from 59.06% in the no-air blowing experiment to 43.44%. Installing a breathable brick at position ① and blowing air will generate fine bubbles that move upward, which reduces the turbulence intensity of the molten steel flowing out of the long nozzle of the ladle and disperses the flow. After the molten steel passes through the diversion hole, the tendency of the molten steel to move toward the liquid surface weakens and the flow rate decreases. This not only increases the residence time of the molten steel in the ladle, is conducive to the floating of inclusions, but also reduces the impact of the molten steel on the tundish liquid level and refractory materials. of scouring. Figure 3(b) shows that when air is blown at position ①, the peak conductivity value of the RTD curve decreases and the trend tends to be flat.

When installing breathable bricks at position ② between the near-flow and mid-flow nozzles, it can be seen from Table 1 and Figure 3 (c, e) that the RTD curves of each flow of molten steel in the tundish are flatter and closer to a normal distribution. The peak time and actual residence time are significantly prolonged, and the dead zone ratio is reduced to 38.10%. Scheme II has a piston area ratio that is doubled compared to scheme Y, and the standard deviation of each indicator is small. After the molten steel passes through the diversion hole, it is driven and stirred evenly by the upward moving bubbles generated by the breathable brick at position ②. It effectively improves the flow field distribution, fully activates the molten steel in the tundish, and also extends the actual residence time of the molten steel in the pouring area, so that inclusions have sufficient time to float up and be eliminated. At the same time, the floating bubbles can also capture inclusions, increase the probability of collision and improve the removal rate. Compared with Scheme II, the peak conductivity of adding a square turbulator (Plan IN) in the flow injection area is reduced, the synchronization of each water outlet is improved, the dead zone ratio is 35.81%, and the flow effect in the tundish is the best.

When the breathable brick is installed at position ③ between the middle flow and the far flow outlet (Figure 3d, f), it only improves the flow characteristics of the far flow and does not have much impact on the near flow and middle flow. As far as the entire tundish is concerned, the effect is not ideal.

It can be seen from Figure 4 that the removal of inclusions at the far flow outlet is better, while the removal of inclusions at the near flow and middle flow outlet is worse than that at the far flow. When blowing air at position ①, the bubbles generated by the breathable bricks in the flow injection area alleviate the turbulence intensity, disperse the flow streams, and extend the residence time. At the same time, the large turbulence intensity in the injection zone can break the bubbles into dispersed small bubbles and carry inclusions to the surface. When blowing at position ②, due to the blocking effect of the air curtain wall produced by the breathable bricks between the near flow and the middle flow, the inclusion removal effect near the outlet is worse than when blowing at position ①. When blowing at position ③, the inclusion removal effect is not significantly improved compared to the non-blowing plan Y. When blowing air at positions ② and ③, whether a turbulence device is added to the injection area (i.e., II and V schemes compared, III and V schemes) has little impact on the inclusion removal effect.

Figure 3 Residence time distribution (RTD) curve of molten steel in tundish: Plan Y, no air blowing (a); Air blowing plan-1(b), l(c), III(d), N(e), V(f)

Figure 4 Experimental results of inclusion removal using different solutions

Conclusion

(1) When the prototype tundish is not blown, there is an obvious short-circuit flow. The flow rate of the molten steel in the tundish is relatively large, which causes greater erosion of the refractory material and covering agent. The stagnation time and actual residence time are short, which is not conducive to the floating of inclusions; the dead zone ratio is large, which is not conducive to the uniformity of composition and temperature.

(2) Blowing air at position ① in the flow injection area prevents the molten steel from flowing directly to the nozzle, avoids short-circuit flow, weakens the turbulence intensity, and prolongs the residence time of the molten steel in the tundish. At the same time, the large turbulence intensity in the injection zone can break the bubbles into dispersed small bubbles, which promotes the floating of inclusions and achieves the best inclusion removal effect.

(3) When blowing air at position ②, the removal effect of inclusions near the water outlet is slightly worse than when blowing air at position ①. Blowing air at position ③ only improves the flow field characteristics at the far outlet. When blowing at position ② or ③ and adding a turbulator, the inclusion removal effect is not as good as blowing in the injection zone.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

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