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Flow Control in Six-Strand Billet Continuous Casting Tundish With Different Configurations
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-9ScienceDir e d JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(7): 07-12
Flow Control in Six-Strand Billet Continuous Casting Tundish With Different Configurations ZHONG Liang-cai'
,
WANG Ming-an'
,
CHEN Bo-yuZ,
WANG Can-rong'
,
Z H U Ying-xiong'
(1. Institute of Ferrous Metallurgy, Northeastern University, Shenyang 110004, Liaoning, China i 2. Steelmaking Plant, Sanming Iron and Steel Co, Sanming 365000, Fujian, China)
Abstract: A 1 2 2. 5 scale tundish model was set up in laboratory for a six-strand billet continuous casting tundish with different configurations to investigate fluid flow characteristics under different operational conditions by measuring residence time distribution curves. It was found that minimum residence time, maximum concentration time and average residence time of the three strands on the same side of the tundish with the former configuration under normal operation, that is, six strands were open, were small and non-uniform and the tundish had large dead volume fraction. Vortexes easily formed on the liquid surface in the pouring zone of the tundish. The fluid flow characteristics in the tundish with the optimal turbulence inhibitor and baffles were improved and became less non-uniform among the strands. Vortexes were not found on the pouring zone surface in the optimal tundish. For non-normal operation, that is, one strand was close, it was important to choose which strand to be closed for.maintaining flow characteristics of the rest two strands. It was found from this investigation that fluid flow characteristics in the optimal configuration tundish with closing strand 2 were better than those with closing strand 3 on the same side. Key words: billet continuous casting; six-strand tundish; flow control; physical modeling; operational mode
Tundishes are important metallurgical vessels in molten steel continuous casting. For multi-strand tundishes more than 3 strands, there are great differences in residence time distribution ( R T D ) curves, temperature and concentration distributions among strands due to complicated geometric shape of the tundishes and different distances between molten steel impacting point from ladle shroud and tundish outlets, if the tundish configuration and its flow control devices designed are not optimized. Such differences will influence quality and castability of molten steel. Tundishes not only make fluid flow from their outlets stable and distribute molten steel into different moulds, but also have an important role with regard to cleanliness and thermal homogeneity of molten steel and in providing stable operation. These are closely related with molten steel flow behavior in tundishes. Reasonable flowing of molten steel in tundishes is a prerequisite for a smooth operation. For uniform strand performance
in multi-strand tundishes, there should be uniformity in temperature and residence time among different outlets. A very short residence time may induce a breakout because of hotter metal, slag entrainment and turbulence at the outlet in close proximity of the inlet stream. On the other hand, a very long residence time at an outlet may give rise to clogged phenomena because of large heat loss. Therefore, homogeneity in flow characteristics for different outlets is expected besides the optimal flow characteristics in multi-strand tundish operation. Many researcher^^'-^' investigated the fluid flow in multi-strand tundishes with different flow control devices, such as different turbulence inhibitors and baffles. In the present work, fluid flow in a six-strand billet continuous casting tundish with different flow control devices was investigated under two operation modes; six strand open, named normal operation mode, and five strand open with one of strands close, un-normal operation mode. A
Foaad.tioa Item: Item Sponsored by National High Technology Research and Development Plan of China (2007AA042194) BiogRphy:ZHONG Liang-cai( 1958-), Male, Doctor., Professor; E-mail: zhonglc@l26. com; Received Date: July 5, 2009
tundish model was set up in laboratory t o conduct physical modeling experiments and determine reasonable flow control devices and tundish configuration for improvement and homogeneity in the tundish fluid flow performance at the same time. In addition, which one of outlets should be closed in un-normal casting is still a problem in production t o be solved. Therefore, flow characteristics in the tundish with one of outlets closed were measured in the present work.
1
Vol. 17
Journal of Iron and Steel Research, International
08.
Experimental
In order to ensure that the fluid flowing between the model tundish and prototype tundish for isothermal and non-reactive systems was similar, geometrical and dynamic similarities must be satisfied between the two vessels. In the present work, the ratio of geometrical similarity of model tundish to the prototype was chosen to be 1 : 2. 5. Dynamic similarity required simultaneous equality of both turbulent Reynolds and Froude numbersr" , but it was impossible t o keep the condition satisfied in reduced scale modeling studies. T h e experimental work of Singh and KoriaC5' showed that the magnitude of turbulent Reynolds number under turbulent flow range in different tundishes was very similar. Therefore, Froude number between the model tundish and the prototype was maintained t o be equivalent in this work. With this condition, the water flow rate, Q, was calculated from the liquid steel flow rate, Q, by the following equation: Q, =)\"."Q, (1) T h e experimental apparatus was shown in Fig. 1. 0. 2 g / m L NaCl solution of 200 mL was used a s the tracer in the water model experiments. After
fluid flow in the model tundish reached stable, the tracer was injected into the tundish though the shroud of ladle. Probes were located under the outlets of the tundish t o measure the variation of water conductivity with time, that is, residence time distribution (RTD) curves. The probes were connected t o conductivity meters and the signals were recorded with a data acquisition system and a computer. From the RTD curves measured, fluid flow characteristics in a certain tundish configuration were calculated. In order to compare the flow characteristics among strands for multi-strand tundish, the calculation methodC6' of flow characteristics for single strand tundish was introduced in the present work by using a sub-tundish concept, that is, the n-strand tundish is consist of n sub-tundish. T h e liquid volume, V,, of a sub-tundish for strand i can be written a s
v =4'v
(2)
iq,
'
,=I
where, qr is the volumetric flow rate of strand i ; V is the total liquid volume of the tundish. If the flow rates of strands in n-strand tundish are identical, one can obtain V=nV, from Eqn. ( 2 ) . T h e theoretic mean residence times for the i sub-tundish and the n-strand tundish, t c , zand t , , can be calculated by tc,t=----
V,L q1
v
-
,=1
T h e actual mean residence time, i is given by
2 c, ( t,
t8.l
=
,=I
t a . t , for
outlet
t , At,
m
Cc,(t,)At,
,=1
Tundish
Fig. 1
(3)
tc
iq,
Schematic of experimental apparatus for physical modeling
(4)
Flow Control in Six-Strand Billet Continuous Casting Tundish With Different Configurations
Issue 7
As suggested in Ref. [S] , the dead volume fraction for the sub-tundish i is determined by Vd.z ---
V,
1- -eC., Qa.t-
(5)
Q,
The plug flow volume fraction for the sub-tundish i is calculated with the following equation in this work (6)
Last, the well-mixed volume fraction for the outlet i is given by
where, V, is the liquid volume in the sub-tundish i and Vd., , V,,, and V,,, are the volumes of dead zone, plug flow and well-mixed flow in the sub-tundish, respectively. Therefore, the flow parameters of and for these volume fractions, vd/v, the whole tundish can not be simply obtained by adding vd,,/v,,V,.,/V, and V,,,/V, from all strands, respectively. Otherwise, some addition values among these three volume fractions, vd/v, v,/v and v,/v, for the n-strand tundish would be larger than 1 or 100%.
v,/v,
v,/v
2 2.1
Results and Discussion Normal operation mode
T h e baffle used in the former tundish configuration is shown in Fig. 2. It has three faces with 5 holes in total, one being in the middle face and two in each side face. Fig. 3 shows the R T D curves measured in this tundish configuration and its flow characteristics are presented in Table 1. It is known from Fig. 3 and Table 1 that the minimum residence time of the first strand is the longest, while that of the third strand is the shortest in this case. T h e maximum concentration time of the second strand is the lowest. Because the third strand outlet is the nearest to the impact zone of ladle stream
i-
. _
-4
(b) Former baffle. Former tundish configuration and baffle
( a ) Former tundish configuration:
Fig. 2
+
2.0
1.0
I
(7)
3.0
Dimensionlesstime
Fig. 3 Table 1
- 9 .
1 4.0 4.0
RTD curves of former tundish configuration
Flow characteristics for former tundish configuration
Strand tmIn.,/st m a x , t / s
t..,/s
V,.,/V,
Vd.,/V,
Vm.,/V,
2
45.30 25. 65
111.30 327.88 5 7 . 3 0 298. 72
20.29% 10. 75%
25.18% 30. 51%
54.54% 58.75%
3
16.95
88.20
13.62%
33.06%
53.32%
1
288.07
and there is a hole in the middle face of the baffle, the tracer flowing into the tundish runs very quickly to the third strand outlet, which makes the strand have the shortest minimum residence time. T h e distance between the first strand nozzle and impact zone is the longest. Therefore the tracer needs much time to travel to the outlet and its minimum residence time is longer than those of the other strands by about 20-30 s. Much amount of tracer flows directly to the second and third strand outlets through the holes on the faces of the baffle, which induces the shortest maximum concentration time for these two strands. As shown in Table 1, minimum residence and maximum concentration times of the second and third strands in the tundish configuration are short and their dead zones are large. There is a large difference in flow characteristics among the three strands at the same side of the tundish with the former configuration. Such tundish arrangement is not favorable to removal of inclusions from liquid steel and operation stability. It was found from the experiment by direct observation that some of large eddies appeared on the surface of the impact zone in the former tundish configuration due to the smaller impact zone inside the baffle and the formation of conflux flow on the surface from the six side faces in the impact zone. Such eddies would cause slag entrainment during casting and deteriorate billet quality. In order to improve the flow characteristics in
Vol. 17
Journal of Iron and Steel Research, International
10
the tundish, it was decided to optimize the tundish configuration with new flow control devices. A square turbulence inhibitor ( T I ) was used under the long shroud of ladle in the impact zone. T h e former holistic baffle was divided into two baffles with rectangular holes for bigger impact zone. T h e improved baffles 1 and corresponding tundish arrangement are shown in Fig. 4 ( a > and Fig. 4 ( b ) , respectively. The RTD curves and flow characteristics in such tundish configuration are shown in Fig. 5 and Table 2 , respectively. It can be known from Fig. 5 that the RTD curves in tundish with baffle 1 are nearer each other in the vicinity of maximum concentration time, which means that the difference in the flow characteristics among the three strands becomes less. Much amount of tracer flows to strand 1 and its maximum concentration time becomes short. It is evident in Table 2 that mini-
+
(a) Tundish configuration 1 or 21
( b ) Baffle 1 I
t
(c) Baffle 2.
Fig. 4 Tundish configuration 1 or 2 with baffle 1 or 2 , respectively
8
i8$
1.0
8
0.6
E
1 .r!
.s 0.2
1.0
2.0 Dimensionlesstime
0
Fig. 5
3.0
4.0
RTD curves of tundish configuration with baffle I
mum residence time of each strand is increased and fraction of dead volume is decreased. T h e application of the T I restricts the influence of high speed stream from the long shroud on tundish fluid flow and prolongs flowing route of fluid, which increases the minimum residence time. In the other way, the T I makes the surface flow in impact zone stable and does not form eddies due to disappearance of the conflux flow on liquid surface. T h e two baffles 1 enlarge the volume of impact zone, which not only decreases turbulent energy of fluid flow in the impact zone, but also avoids quick outflow of the fluid from the impact zone to the two side zones of the tundish. In addition, the change of section and angle of the rectangle hole in baffle 1, different from the two holes in the former baffle, decreases flowing speed and varies the flowing direction, which is favorable to the improvement of flow characteristics, as shown in Table 2. In experiments, it was found that a waterfall formed from the rectangular hole in the baffle 1 during tundish filling. In order to prevent long time appearance of the waterfall when liquid steel flows from the holes in baffle 1 during casting start to reduce the second oxidation and avoid the impingement of the liquid steel stream to powder added into the tundish, a circle hole was opened under the rectangle hole in the baffle 1, as shown in Fig. 4 ( c ) , named baffle 2. T h e flow characteristics in the tundish with baffle 2 are presented in Table 3. It can be seen from Table 3 that the flow characteristics in tundish with baffle 2 are similar to those in tundish with baffle 1, that is, the circle hole with suitable size and angle under the rectangle hole in baffle 1 does not influence flow characteristics in the tundish much. It was known by visible observation in the modeling experiments that fluid first flows through the circle holes in baffle 2 into the two sides of the tundish during casting start and then through the rectangle holes when the liquid surface reaches the lower margin of the rectangle hole. Table 3 Flow characteristics for tundish confirmration 2 with baffle 2
Table 2 Flow characteristics for tundish configuration 1 with baffle 1 Strand tmBn.,/stmx.,/S
t..,/s
V,.,/V,
Vd.s/Vs
V,.,/V,
321.04
17.82%
23.11%
59.08%
1
58.05
2
42.75
89.70
310.94
17.16%
24.57%
58.28%
3
55.20
152.55
329.77
26.91%
20.38%
52.71%
79.50
1
66.90
97.50
334.83
21.30%
23.08%
55.63%
2
48.00
87.45
310.78
17. 55%
26.45%
56.00%
3
51.00
172.65
319.74
28.97%
24.98%
46.06%
Issue 7
Flow Control in Six-Strand Billet Continuous Casting Tundish With Different Configurations
In this way, the second oxidation and impingement of liquid steel stream to powder during casting start would be avoided. Experiments for investigating flow characteristics in tundish configuration 3 with baffle 3, as shown in Fig. 6 , was carried out. Baffle 3 was built from two baffles 2 which were shorten and connected each other so as to connect the two sides of the tundish. T h e experimental results are presented in Table 4. It is clear in the table that baffle 3 has little effect on the flow characteristics of strand 1 and 2 , but on those of strand 3. Minimum residence and maximum concentration time for strand 3 become low and dead volume turns large. T h e reason for such results is that baffle 3 decreases the volume of impact zone of the tundish, which induces short residence time for tracer in this zone. Therefore, the flow characteristics of the third strand are affected.
2.2
Non-normal operation mode In production, sometimes one has to cast molten steel in multi-strand tundish with one strand close due
Fig. 6 Tundish configuration 3 (a) and baffle 3 (b) Table 5
Table 4 Flow characteristics for tundish configuration 3 1 2 3
57.45 49.95 33.30
88.65 333.82 76.35 309.51 110.85 304.60
18.93% 16.36% 18.68%
23.93% 26.96% 29.05%
Strand
thn,i/s
t,..i/s
3
1
2
2 1 3
51.60 30.90 68.10 19.20
138.30 91.20 200.40 91.50
t..i/s
387.42 360.30 394.30 338.02
vp.i/vi
vd,i/vi
vm.i/v,
20.51% 13.19% 29.00% 11.96%
23.94% 28.92% 23.03% 33.08%
55.56% 57.90% 47.98% 54.97%
Table 6 Flow characteristics for tundish configuration 1 with one strand close Strand closed
Strand
3
1 2 1 3
2
tm,n.z/s
tm..,,/s
t..,/s
71.10 46.05 88.80 62.70
135.30
437.76 407.30 430.99 415.70
76.20 112.50 123.90
57.15% 56.69% 52.28%
to some reasons. This is a kind of non-normal casting operation. Flow characteristics of tundish for such case were investigated for the above tundish configurations with strand 2 or 3 close. Investigation results for the former tundish configuration are shown in Table 5. It is known from the table that flow characteristics of strand 2 with strand 3 close is better than those of strand 3 with strand 2 close due to the fact that the outlet of strand 3 is the nearest to the impact zone. For baffle 1, flow characteristics with one strand close are listed in Table 6. It can be seen from the table that flow characteristics of tundish with baffle 1 are in general better than those with the former baffle. For baffle 1 with one big rectangle open, fluid first flows to the outlet of strand 2 when this outlet is close, then to outlets of strand 1 and 3, respectively. Such flowing can prolong the minimum residence, maximum concentration and average residence times of strand 1 and 3 and decrease their dead volumes. Compared Table 1 to Table 4 with Table 5 and Table 6 , it is known that the minimum and average residence times of the rest two strands with one strand close become longer. T h e reason is that the total flow rate at the same side with one strand close decreases, and therefore the residence times of these two strands at this side increases.
Flow characteristics for former tundish configuration with one strand close
Strand closed
11
Vp.,/V,
Vd.a/Va
Vm.,/V,
22.29%
19.07% 23.56%
58.65% 63.24%
18.84% 21.20%
59.43% 58.66%
13.21% 21.74% 20.16%
12
3
Journal of Iron and Steel Research, International
Conclusions
1 ) T h e minimum and average residence times and maximum concentration time .of the three strands on the same side of the tundish with the former configuration are high and the non-uniformity of flow characteristics among the strands and their dead volumes are large. It is easy to form vortex on liquid surface of impact zone in such tundish configuration. 2 ) After the tundish configuration optimization with optimal turbulence inhibitor and baffles, the flow characteristics of each strand are improved and at the same time their non-uniformity of flow characteristics becomes less. The liquid surface of impact zone is stable and no vortex appears. 3 ) Among the improved baffles studied in this research, baffle 1 is the best to improve the tundish flow cha.racteristics for hormal and non-normal casting operation. Its residence times are long and its dead zone volume is small. It is found that the fluid flow characteristics for the optimal configuration tundish with closing strand 2 were better than those with closing strand 3 on the same side in non-normal operation mode.
Vol. 17
4) In order to prevent long time spring flowing out from the rectangle open in baffle 1 at casting start, reduce secondary oxidation of liquid steel and avoid the steel spring impacting tundish powder added into the tundish at casting start, baffle 2 can be used as a flow control device for the tundish. References : r11
CZl C31
c41
c51
161
Morales R D. Barreto J J , Lopez-Ramirez S. Melt Flow Control in a Multistrand Tundish Using a Turbulence Inhibitor [J]. Metallurgical and Materials Transactions B, 2000, 31 ( 11 ) : 1505. Jha P K , Dash S K. Kumar S. Fluid Flow and Mixing in a Six Strand Billet Caster Tundish: A Parametric Study [J]. ISIJ International, 2001, 41(12): 1437. Sahay S K , De T K , Basy D S. Strand Performance Improvement Through Use of Asymmetric Baffles in Tundish of Six Strand Billet Caster at DSP [J]. Iron and Steelmaker, 2001. 71 ( 6 ) : 71. Mazumdar D, Guthrie R. Physical and Mathematical Modeling of Continuous Casting Tundish Systems [J]. ISIJ International, 1999, 3 9 ( 6 ) : 524. Singh S . Koria S. Physical Modeling of Steel Flow in Continuous Casting Tundish [J]. Ironmaking and Steelmaking, 1993, 20(3): 221. Sahai Y , Emi T. Melt Flow Characterization in Continuous Casting Tundish [J]. ISIJ International, 1996, 36(6): 667.
-9ScienceDir e d JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(7): 07-12
Flow Control in Six-Strand Billet Continuous Casting Tundish With Different Configurations ZHONG Liang-cai'
,
WANG Ming-an'
,
CHEN Bo-yuZ,
WANG Can-rong'
,
Z H U Ying-xiong'
(1. Institute of Ferrous Metallurgy, Northeastern University, Shenyang 110004, Liaoning, China i 2. Steelmaking Plant, Sanming Iron and Steel Co, Sanming 365000, Fujian, China)
Abstract: A 1 2 2. 5 scale tundish model was set up in laboratory for a six-strand billet continuous casting tundish with different configurations to investigate fluid flow characteristics under different operational conditions by measuring residence time distribution curves. It was found that minimum residence time, maximum concentration time and average residence time of the three strands on the same side of the tundish with the former configuration under normal operation, that is, six strands were open, were small and non-uniform and the tundish had large dead volume fraction. Vortexes easily formed on the liquid surface in the pouring zone of the tundish. The fluid flow characteristics in the tundish with the optimal turbulence inhibitor and baffles were improved and became less non-uniform among the strands. Vortexes were not found on the pouring zone surface in the optimal tundish. For non-normal operation, that is, one strand was close, it was important to choose which strand to be closed for.maintaining flow characteristics of the rest two strands. It was found from this investigation that fluid flow characteristics in the optimal configuration tundish with closing strand 2 were better than those with closing strand 3 on the same side. Key words: billet continuous casting; six-strand tundish; flow control; physical modeling; operational mode
Tundishes are important metallurgical vessels in molten steel continuous casting. For multi-strand tundishes more than 3 strands, there are great differences in residence time distribution ( R T D ) curves, temperature and concentration distributions among strands due to complicated geometric shape of the tundishes and different distances between molten steel impacting point from ladle shroud and tundish outlets, if the tundish configuration and its flow control devices designed are not optimized. Such differences will influence quality and castability of molten steel. Tundishes not only make fluid flow from their outlets stable and distribute molten steel into different moulds, but also have an important role with regard to cleanliness and thermal homogeneity of molten steel and in providing stable operation. These are closely related with molten steel flow behavior in tundishes. Reasonable flowing of molten steel in tundishes is a prerequisite for a smooth operation. For uniform strand performance
in multi-strand tundishes, there should be uniformity in temperature and residence time among different outlets. A very short residence time may induce a breakout because of hotter metal, slag entrainment and turbulence at the outlet in close proximity of the inlet stream. On the other hand, a very long residence time at an outlet may give rise to clogged phenomena because of large heat loss. Therefore, homogeneity in flow characteristics for different outlets is expected besides the optimal flow characteristics in multi-strand tundish operation. Many researcher^^'-^' investigated the fluid flow in multi-strand tundishes with different flow control devices, such as different turbulence inhibitors and baffles. In the present work, fluid flow in a six-strand billet continuous casting tundish with different flow control devices was investigated under two operation modes; six strand open, named normal operation mode, and five strand open with one of strands close, un-normal operation mode. A
Foaad.tioa Item: Item Sponsored by National High Technology Research and Development Plan of China (2007AA042194) BiogRphy:ZHONG Liang-cai( 1958-), Male, Doctor., Professor; E-mail: zhonglc@l26. com; Received Date: July 5, 2009
tundish model was set up in laboratory t o conduct physical modeling experiments and determine reasonable flow control devices and tundish configuration for improvement and homogeneity in the tundish fluid flow performance at the same time. In addition, which one of outlets should be closed in un-normal casting is still a problem in production t o be solved. Therefore, flow characteristics in the tundish with one of outlets closed were measured in the present work.
1
Vol. 17
Journal of Iron and Steel Research, International
08.
Experimental
In order to ensure that the fluid flowing between the model tundish and prototype tundish for isothermal and non-reactive systems was similar, geometrical and dynamic similarities must be satisfied between the two vessels. In the present work, the ratio of geometrical similarity of model tundish to the prototype was chosen to be 1 : 2. 5. Dynamic similarity required simultaneous equality of both turbulent Reynolds and Froude numbersr" , but it was impossible t o keep the condition satisfied in reduced scale modeling studies. T h e experimental work of Singh and KoriaC5' showed that the magnitude of turbulent Reynolds number under turbulent flow range in different tundishes was very similar. Therefore, Froude number between the model tundish and the prototype was maintained t o be equivalent in this work. With this condition, the water flow rate, Q, was calculated from the liquid steel flow rate, Q, by the following equation: Q, =)\"."Q, (1) T h e experimental apparatus was shown in Fig. 1. 0. 2 g / m L NaCl solution of 200 mL was used a s the tracer in the water model experiments. After
fluid flow in the model tundish reached stable, the tracer was injected into the tundish though the shroud of ladle. Probes were located under the outlets of the tundish t o measure the variation of water conductivity with time, that is, residence time distribution (RTD) curves. The probes were connected t o conductivity meters and the signals were recorded with a data acquisition system and a computer. From the RTD curves measured, fluid flow characteristics in a certain tundish configuration were calculated. In order to compare the flow characteristics among strands for multi-strand tundish, the calculation methodC6' of flow characteristics for single strand tundish was introduced in the present work by using a sub-tundish concept, that is, the n-strand tundish is consist of n sub-tundish. T h e liquid volume, V,, of a sub-tundish for strand i can be written a s
v =4'v
(2)
iq,
'
,=I
where, qr is the volumetric flow rate of strand i ; V is the total liquid volume of the tundish. If the flow rates of strands in n-strand tundish are identical, one can obtain V=nV, from Eqn. ( 2 ) . T h e theoretic mean residence times for the i sub-tundish and the n-strand tundish, t c , zand t , , can be calculated by tc,t=----
V,L q1
v
-
,=1
T h e actual mean residence time, i is given by
2 c, ( t,
t8.l
=
,=I
t a . t , for
outlet
t , At,
m
Cc,(t,)At,
,=1
Tundish
Fig. 1
(3)
tc
iq,
Schematic of experimental apparatus for physical modeling
(4)
Flow Control in Six-Strand Billet Continuous Casting Tundish With Different Configurations
Issue 7
As suggested in Ref. [S] , the dead volume fraction for the sub-tundish i is determined by Vd.z ---
V,
1- -eC., Qa.t-
(5)
Q,
The plug flow volume fraction for the sub-tundish i is calculated with the following equation in this work (6)
Last, the well-mixed volume fraction for the outlet i is given by
where, V, is the liquid volume in the sub-tundish i and Vd., , V,,, and V,,, are the volumes of dead zone, plug flow and well-mixed flow in the sub-tundish, respectively. Therefore, the flow parameters of and for these volume fractions, vd/v, the whole tundish can not be simply obtained by adding vd,,/v,,V,.,/V, and V,,,/V, from all strands, respectively. Otherwise, some addition values among these three volume fractions, vd/v, v,/v and v,/v, for the n-strand tundish would be larger than 1 or 100%.
v,/v,
v,/v
2 2.1
Results and Discussion Normal operation mode
T h e baffle used in the former tundish configuration is shown in Fig. 2. It has three faces with 5 holes in total, one being in the middle face and two in each side face. Fig. 3 shows the R T D curves measured in this tundish configuration and its flow characteristics are presented in Table 1. It is known from Fig. 3 and Table 1 that the minimum residence time of the first strand is the longest, while that of the third strand is the shortest in this case. T h e maximum concentration time of the second strand is the lowest. Because the third strand outlet is the nearest to the impact zone of ladle stream
i-
. _
-4
(b) Former baffle. Former tundish configuration and baffle
( a ) Former tundish configuration:
Fig. 2
+
2.0
1.0
I
(7)
3.0
Dimensionlesstime
Fig. 3 Table 1
- 9 .
1 4.0 4.0
RTD curves of former tundish configuration
Flow characteristics for former tundish configuration
Strand tmIn.,/st m a x , t / s
t..,/s
V,.,/V,
Vd.,/V,
Vm.,/V,
2
45.30 25. 65
111.30 327.88 5 7 . 3 0 298. 72
20.29% 10. 75%
25.18% 30. 51%
54.54% 58.75%
3
16.95
88.20
13.62%
33.06%
53.32%
1
288.07
and there is a hole in the middle face of the baffle, the tracer flowing into the tundish runs very quickly to the third strand outlet, which makes the strand have the shortest minimum residence time. T h e distance between the first strand nozzle and impact zone is the longest. Therefore the tracer needs much time to travel to the outlet and its minimum residence time is longer than those of the other strands by about 20-30 s. Much amount of tracer flows directly to the second and third strand outlets through the holes on the faces of the baffle, which induces the shortest maximum concentration time for these two strands. As shown in Table 1, minimum residence and maximum concentration times of the second and third strands in the tundish configuration are short and their dead zones are large. There is a large difference in flow characteristics among the three strands at the same side of the tundish with the former configuration. Such tundish arrangement is not favorable to removal of inclusions from liquid steel and operation stability. It was found from the experiment by direct observation that some of large eddies appeared on the surface of the impact zone in the former tundish configuration due to the smaller impact zone inside the baffle and the formation of conflux flow on the surface from the six side faces in the impact zone. Such eddies would cause slag entrainment during casting and deteriorate billet quality. In order to improve the flow characteristics in
Vol. 17
Journal of Iron and Steel Research, International
10
the tundish, it was decided to optimize the tundish configuration with new flow control devices. A square turbulence inhibitor ( T I ) was used under the long shroud of ladle in the impact zone. T h e former holistic baffle was divided into two baffles with rectangular holes for bigger impact zone. T h e improved baffles 1 and corresponding tundish arrangement are shown in Fig. 4 ( a > and Fig. 4 ( b ) , respectively. The RTD curves and flow characteristics in such tundish configuration are shown in Fig. 5 and Table 2 , respectively. It can be known from Fig. 5 that the RTD curves in tundish with baffle 1 are nearer each other in the vicinity of maximum concentration time, which means that the difference in the flow characteristics among the three strands becomes less. Much amount of tracer flows to strand 1 and its maximum concentration time becomes short. It is evident in Table 2 that mini-
+
(a) Tundish configuration 1 or 21
( b ) Baffle 1 I
t
(c) Baffle 2.
Fig. 4 Tundish configuration 1 or 2 with baffle 1 or 2 , respectively
8
i8$
1.0
8
0.6
E
1 .r!
.s 0.2
1.0
2.0 Dimensionlesstime
0
Fig. 5
3.0
4.0
RTD curves of tundish configuration with baffle I
mum residence time of each strand is increased and fraction of dead volume is decreased. T h e application of the T I restricts the influence of high speed stream from the long shroud on tundish fluid flow and prolongs flowing route of fluid, which increases the minimum residence time. In the other way, the T I makes the surface flow in impact zone stable and does not form eddies due to disappearance of the conflux flow on liquid surface. T h e two baffles 1 enlarge the volume of impact zone, which not only decreases turbulent energy of fluid flow in the impact zone, but also avoids quick outflow of the fluid from the impact zone to the two side zones of the tundish. In addition, the change of section and angle of the rectangle hole in baffle 1, different from the two holes in the former baffle, decreases flowing speed and varies the flowing direction, which is favorable to the improvement of flow characteristics, as shown in Table 2. In experiments, it was found that a waterfall formed from the rectangular hole in the baffle 1 during tundish filling. In order to prevent long time appearance of the waterfall when liquid steel flows from the holes in baffle 1 during casting start to reduce the second oxidation and avoid the impingement of the liquid steel stream to powder added into the tundish, a circle hole was opened under the rectangle hole in the baffle 1, as shown in Fig. 4 ( c ) , named baffle 2. T h e flow characteristics in the tundish with baffle 2 are presented in Table 3. It can be seen from Table 3 that the flow characteristics in tundish with baffle 2 are similar to those in tundish with baffle 1, that is, the circle hole with suitable size and angle under the rectangle hole in baffle 1 does not influence flow characteristics in the tundish much. It was known by visible observation in the modeling experiments that fluid first flows through the circle holes in baffle 2 into the two sides of the tundish during casting start and then through the rectangle holes when the liquid surface reaches the lower margin of the rectangle hole. Table 3 Flow characteristics for tundish confirmration 2 with baffle 2
Table 2 Flow characteristics for tundish configuration 1 with baffle 1 Strand tmBn.,/stmx.,/S
t..,/s
V,.,/V,
Vd.s/Vs
V,.,/V,
321.04
17.82%
23.11%
59.08%
1
58.05
2
42.75
89.70
310.94
17.16%
24.57%
58.28%
3
55.20
152.55
329.77
26.91%
20.38%
52.71%
79.50
1
66.90
97.50
334.83
21.30%
23.08%
55.63%
2
48.00
87.45
310.78
17. 55%
26.45%
56.00%
3
51.00
172.65
319.74
28.97%
24.98%
46.06%
Issue 7
Flow Control in Six-Strand Billet Continuous Casting Tundish With Different Configurations
In this way, the second oxidation and impingement of liquid steel stream to powder during casting start would be avoided. Experiments for investigating flow characteristics in tundish configuration 3 with baffle 3, as shown in Fig. 6 , was carried out. Baffle 3 was built from two baffles 2 which were shorten and connected each other so as to connect the two sides of the tundish. T h e experimental results are presented in Table 4. It is clear in the table that baffle 3 has little effect on the flow characteristics of strand 1 and 2 , but on those of strand 3. Minimum residence and maximum concentration time for strand 3 become low and dead volume turns large. T h e reason for such results is that baffle 3 decreases the volume of impact zone of the tundish, which induces short residence time for tracer in this zone. Therefore, the flow characteristics of the third strand are affected.
2.2
Non-normal operation mode In production, sometimes one has to cast molten steel in multi-strand tundish with one strand close due
Fig. 6 Tundish configuration 3 (a) and baffle 3 (b) Table 5
Table 4 Flow characteristics for tundish configuration 3 1 2 3
57.45 49.95 33.30
88.65 333.82 76.35 309.51 110.85 304.60
18.93% 16.36% 18.68%
23.93% 26.96% 29.05%
Strand
thn,i/s
t,..i/s
3
1
2
2 1 3
51.60 30.90 68.10 19.20
138.30 91.20 200.40 91.50
t..i/s
387.42 360.30 394.30 338.02
vp.i/vi
vd,i/vi
vm.i/v,
20.51% 13.19% 29.00% 11.96%
23.94% 28.92% 23.03% 33.08%
55.56% 57.90% 47.98% 54.97%
Table 6 Flow characteristics for tundish configuration 1 with one strand close Strand closed
Strand
3
1 2 1 3
2
tm,n.z/s
tm..,,/s
t..,/s
71.10 46.05 88.80 62.70
135.30
437.76 407.30 430.99 415.70
76.20 112.50 123.90
57.15% 56.69% 52.28%
to some reasons. This is a kind of non-normal casting operation. Flow characteristics of tundish for such case were investigated for the above tundish configurations with strand 2 or 3 close. Investigation results for the former tundish configuration are shown in Table 5. It is known from the table that flow characteristics of strand 2 with strand 3 close is better than those of strand 3 with strand 2 close due to the fact that the outlet of strand 3 is the nearest to the impact zone. For baffle 1, flow characteristics with one strand close are listed in Table 6. It can be seen from the table that flow characteristics of tundish with baffle 1 are in general better than those with the former baffle. For baffle 1 with one big rectangle open, fluid first flows to the outlet of strand 2 when this outlet is close, then to outlets of strand 1 and 3, respectively. Such flowing can prolong the minimum residence, maximum concentration and average residence times of strand 1 and 3 and decrease their dead volumes. Compared Table 1 to Table 4 with Table 5 and Table 6 , it is known that the minimum and average residence times of the rest two strands with one strand close become longer. T h e reason is that the total flow rate at the same side with one strand close decreases, and therefore the residence times of these two strands at this side increases.
Flow characteristics for former tundish configuration with one strand close
Strand closed
11
Vp.,/V,
Vd.a/Va
Vm.,/V,
22.29%
19.07% 23.56%
58.65% 63.24%
18.84% 21.20%
59.43% 58.66%
13.21% 21.74% 20.16%
12
3
Journal of Iron and Steel Research, International
Conclusions
1 ) T h e minimum and average residence times and maximum concentration time .of the three strands on the same side of the tundish with the former configuration are high and the non-uniformity of flow characteristics among the strands and their dead volumes are large. It is easy to form vortex on liquid surface of impact zone in such tundish configuration. 2 ) After the tundish configuration optimization with optimal turbulence inhibitor and baffles, the flow characteristics of each strand are improved and at the same time their non-uniformity of flow characteristics becomes less. The liquid surface of impact zone is stable and no vortex appears. 3 ) Among the improved baffles studied in this research, baffle 1 is the best to improve the tundish flow cha.racteristics for hormal and non-normal casting operation. Its residence times are long and its dead zone volume is small. It is found that the fluid flow characteristics for the optimal configuration tundish with closing strand 2 were better than those with closing strand 3 on the same side in non-normal operation mode.
Vol. 17
4) In order to prevent long time spring flowing out from the rectangle open in baffle 1 at casting start, reduce secondary oxidation of liquid steel and avoid the steel spring impacting tundish powder added into the tundish at casting start, baffle 2 can be used as a flow control device for the tundish. References : r11
CZl C31
c41
c51
161
Morales R D. Barreto J J , Lopez-Ramirez S. Melt Flow Control in a Multistrand Tundish Using a Turbulence Inhibitor [J]. Metallurgical and Materials Transactions B, 2000, 31 ( 11 ) : 1505. Jha P K , Dash S K. Kumar S. Fluid Flow and Mixing in a Six Strand Billet Caster Tundish: A Parametric Study [J]. ISIJ International, 2001, 41(12): 1437. Sahay S K , De T K , Basy D S. Strand Performance Improvement Through Use of Asymmetric Baffles in Tundish of Six Strand Billet Caster at DSP [J]. Iron and Steelmaker, 2001. 71 ( 6 ) : 71. Mazumdar D, Guthrie R. Physical and Mathematical Modeling of Continuous Casting Tundish Systems [J]. ISIJ International, 1999, 3 9 ( 6 ) : 524. Singh S . Koria S. Physical Modeling of Steel Flow in Continuous Casting Tundish [J]. Ironmaking and Steelmaking, 1993, 20(3): 221. Sahai Y , Emi T. Melt Flow Characterization in Continuous Casting Tundish [J]. ISIJ International, 1996, 36(6): 667.