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Water modeling of mold powder entrapment in slab continuous casting mold
Journal of University of Science and Technology Beijing Volume 14, Number 5, October 2007, Page 399
Metallurgy
Water modeling of mold powder entrapment in slab continuous casting mold Qiaotong Lu, Rongguang Yang, Xinhua Wang, Jiongming Zhang, and Wanjun Wang Metallurgical and Ecological Engineering School, University of Science and Technology Beijing, Beijing 100083, China
(Received 2006-08-25)
Abstract: The optimal parameters were determined by the water modeling of slab casting. It was found that there are mainly three types of mold powder entrapment in slab continuous casting, i.e., the entrapment caused by the shearing flow near the narrow face of mold, the entrapment caused by vortexes around the submerged entry nozzle (SEN), and the entrapment caused by the Ar bubbling. Both the velocity of the surface flow and the level fluctuation of the liquids are enlarged with increasing the casting speed, reducing the submersion depth of SEN, decreasing the downward angles of the nozzle outlets, and increasing the Ar flowrate, all of which increase the tendency of mold powder entrapment. Among the four above-mentioned factors, casting speed has the largest effect.
Key words: continuous casting; slab; mold; mold powder; water model
1. Introduction With the development of clean steel production, the content of impurity elements in deep and superdeep drawing steel used for automobile and home appliances has been reduced to a low level. Now, the main reason of the surface defects of cold rolled strips is mold powder entrapment [l]. It is very important to study the flow of the molten steel in the mold and the mold powder entrapment by water modeling and numerical simulation. Until now, most water modeling studies were based on a model without slag on the surface of the molten steel [2-71. There were, however, some studies with some slag on the surface of the molten steel [ l , 8-10]. Some studies focused on high-speed slab casting [8] while others focused on thin slab casting [9]. The present experiments were carried out with a 1:l scale water model to discover the flow pattern of the molten steel in mold and the characteristics of the mold powder entrapment.
2. Water modeling Based on the similarity principle, for the isothermal steady flow of incompressible viscous fluid, the Re number and Fr number of water modeling must be equal to that of the real slab casting. A water model of 1:l was used. The width and thickness of the mold were 1500 and 250 mm, respectively. The water, oil mixture, and the polyethylene partiCorresponding author: Qiaotong Lu, E-mail: [email protected]
cle were used to simulate the molten steel, the liquid slag (molten mold powder) layer, and the solid powder slag layer, respectively. The effect of the viscosity and the density of the molten mold powder were studied, and by way of dimensionless analysis, the viscosity must meet the requirement of the following equation: (1)
Vslag/Vsteel=Voil/Vwater
where v is the kinematical viscosity, m2/s. The ratio of the molten mold powder kinematical viscosity to that of liquid steel is 54-97. According to Eq. (l), the kinematical viscosity of the oil in the experiment must be about (5.4-9.7)~10-~m2/s. The kinematical viscosity of the oil mixture used in the experiment was about 8 . 8 6 ~ 1 0 -m2/s. ~ The thickness of the oil layer was more than 10 mm. Model DJ800's Multi-functional Water Engineering Observation System made in China was used to measure the height of the wave on the surface without oil, and the measured data was collected using a computer. The flow velocity of the oiUwater interface was measured using a Model LS1206B Propeller Current Meter. This device can be used to measure the offset velocity that is parallel to the axis of the propeller. The minimal measurement velocity of flow was 0.05 m/s with a precision of 21.0%.
3. Results and discussion 3.1. Mechanism of mold powder entrapment Fig. 1 depicts the photographs taken at a casing Also available online at www.sciencedirect.com
J. Univ. Sci. Technol. Beijing, Vo1.14,No.5, Oct 2007
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speed of 1.4 d m i n . As seen in Fig.1, there are three types of mold powder entrapment in slab casting
mold.
Fig. 1. Photographs showing different types of mold slag entrapment in the water model: (a) entrapment caused by shearing flow; (b) entrapment caused by vortexes (high symmetrical flow field); (c) entrapment caused by vortexes (unsymmetrical flow field); (d) entrapment caused by Ar bubbles.
(1) Entrapment caused by the shearing flow. The entrapment caused by the shearing flow occurs at 400 mm apart from the narrow face of the mold. In this region, the stream from the outlet of the submerged entry nozzle (SEN) is divided into two branches reaching the narrow face of the mold; one is the up branch and the other is the down branch. After the up branch reaches the surface, the stream changes its direction and flows toward the SEN. This kind of surface current gives rise to a shearing force on the slag layer (simulated by oil) making it thin and even exposition of molten steel. The shearing current gathers the slag in the region apart from the narrow face of the mold at about 400 mm, and forms a downward protuberance. Under the joint action of the free surface fluctuation and the turbulent flow, the drippings at the tip of the protuberance are very easy to break away from the slag layer and to be entrapped. If the slag drippings cannot return to the slag layer to be taken by the liquid steel or to be trapped by the solid shell, the mold powder entrapment will occur (as shown in Fig. l(a)). The higher the casting speed, the shallower the SEN submersion depth, and the shallower the downward angles of the SEN outlets, the easier is this kind of mold powder entrapment. (2) Entrapment caused by vortexes. As seen in Fig. 2, the surface currents toward the SEN meet together near the nozzle. When the flow
field in the mold is high symmetrical, the two surface currents at both sides of the SEN meet in the region between the SEN and the mold walls. Owing to the difference in the surface velocities of currents, it is possible to form a vortex at points A or B as shown in Fig. 2 and Fig. 1 (b).
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Fig. 2. Illustration of vortexes in the area around the SEN.
The unsymmetrical streams from two outlets will bring an unequal surface flow velocity at both sides of the SEN. When the difference in velocity of the surface flow reaches some extent, vortexes will be formed at the joint points (as shown in Fig. 2, C , D, E, and 0. If the energy of such vortexes is large enough, the slag can be entrapped (as shown in Fig.1 (c)). Increasing the casting speed or reducing the submersion depth of SEN or decreasing the downward angles of the SEN outlets will make this kind of mold powder entrapment easy to occur.
( 3 ) Entrapment caused by the Ar bubbles at the interface between the liquid steel and the mold powder. To prevent the nozzle from clogging, Ar gas is always used to inject into the nozzle. During casting,
Q.T. Lu et al., Water modeling of mold powder entrapment in slab continuous casting mold
most of the Ar gas carried by the pouring stream will break away from the main stream and directly float to the surface. When the Ar bubbles reach the interface between the liquid steel and the mold powder, the bursting of the bubbles and the release of the Ar gas cause a violent agitation to the interface. Under the joint action of the wave fluctuation and the turbulent flow, some slag drippings break away form the slag layer and are entrapped into the steel (as shown in Fig. l(d)). This kind of entrapment will easily occur when the Ar flowrate is large or the nozzle submersion depth is shallow or the downward angles of the SEN outlets are shallow.
meniscus near the narrow faces, i.e., the easier is the entrapment caused by the shearing flow. In addition, with the increase of the casting speed, the kinetic energy of the turbulent flow and the unsymmetry of the flow field in the mold will be increased, and the probability of forming vortex will also be increased. Fig. 3 shows the effect of casting speed on level fluctuation and velocity of the surface flow. From Fig. 3, it can be seen that with the increase of the casting speed, the velocity of the surface flow and the level fluctuation measured along the width of the mold are increased. As seen in Figs. l(a)-l(c), at the casting speed of 1.4 d m i n , except for some few solid powder particles, the slag layer can hardly be seen at the meniscus near the narrow faces. At the position about 400 mm apart from the narrow face of the mold, mold powder entrapment caused by the shearing flow occurs, and vortexes near the SEN are formed.
3.2. Effect of technical parameters ( 1 ) Effect of the casting speed. The water model experiment showed that with the increase of the casting speed, the slag layer near the narrow faces of the mold becomes unstable. The higher the casing speed, the easier it is to expose the 14
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Fig. 3. Effects of casting speed on level fluctuation and velocity of the surface flow: (a) effect on the level fluctuation; (b) effect on the velocity of the surface flow. Angle of SEN outlet: -15'; submersion depth: 120 mm; Ar flowrate: 0 L/min.
(2) Effect of the submersion depth of SEN. As seen in Fig. 4,when the immersion depth of the nozzle becomes shallow, the velocity of the surface flow and the level fluctuation of the liquid increase, and therefore, the mold powder entrapment will easily occur. With the decrease of the submersion depth, the slag on the surface layer becomes unstable. At a depth of 120 mm, the slag layer near the narrow face of the mold is thin, and molten steel is easy to be exposed as shown in Fig. 1 because of a stronger impact of the stream on the surface. Therefore, the velocity of the
surface flow and the level fluctuation are increased. By the action of the shearing force, the molten slag is pushed from the narrow faces of the mold to the SEN. When the submersion depth of the SEN is 120 mm, the slag on the surface is gathered at 400 mm apart from the narrow faces of the mold, and when the submersion depth of SEN is 200 and 250 mm, the slag is easy to gather in the region from the narrow faces of the mold to 400 mm apart from the narrow face of the mold. The gathered slag forms a downward protuberance, where the mold powder entrapment easily occurs.
J. Univ. Sci. Technol. Beijing, Vo1.14,NOS, Oct 2007
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In addition, the decrease of the mold submersion depth will increase the probability of forming vortexes
near the nozzle, and therefore the probability of mold powder entrapment will increase. 0.30
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Fig. 4. Effects of the submersion depth of SEN on the level fluctuation and velocity of the surface flow: (a) effect on the level fluctuation; (b) effect on the velocity of the surface flow. Angle of SEN outlet: -15"; casting speed: 1.4 ∈ Ar flowrate: 0 L/min.
(3) Effect of the downward angle of the SEN outlet. Fig. 5 shows the influence of the downward angle of the SEN outlet on the level fluctuation and the velocity of the surface flow. With the increase of the downward angle of the SEN outlet, the velocity of the surface flow and the level fluctuation are decreased. If
the angle increases over -25", the influence on the velocity of the surface flow and the level fluctuation is decreased. Thus, the smaller the downward angles of the SEN outlets, the stronger is the influence on the mold powder entrapment. 0.30
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Fig. 5. Effects of the outlet downward angle of SEN on the level fluctuation and velocity of the surface flow: (a) effect on the level fluctuation; (b) effect on the velocity of the surface flow. Submersion depth: 120 mm; casting speed: 1.4 d m i n ; Ar flowrate: 0 L/min.
(4) Effect of the Ar flowrate. Fig. 6 shows the change in velocity of the surface flow and the level fluctuation with Ar flowrate. With the increase of the Ar flowrate, the velocity of the sur-
face flow at various measuring points somewhat increases, and increases obviously near the SEN, because a good deal of Ar bubbles break away from the main stream and float to the surface near the nozzle causing considerably stronger collision and agitation
Q.T. Lu et al., Water modeling of mold powder entrapment in slab continuous casting mold
to the surface (as shown in Fig. 1). Also, with the increase of the Ar flowrate, the level fluctuation near the nozzle will increase (as shown in Fig. 6). Agitation brought by Ar injection can entrap the mold powder into the steel, as shown in Fig. l(d), and
403
with the change of the Ar flowrate, the agitation by Ar will give rise to the mold powder entrapment at different positions along the mold width. This phenomenon has a close relationship with the bubble density in the foaming and the position where the bubbles float up.
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Fig. 6. Effects of Ar flowrate on the level fluctuation and velocity of the surface flow: (a) effect on the level fluctuation; (b) effect on the velocity of the surface flow. Submersion depth: 120 mm; angle of SEN outlet: -15"; casting speed 1.4 ndmin.
(5) Interaction of technical parameters on the mold powder entrapment. From above mentioned results, it is clear that casting speed has the largest effect on the mold powder entrapment, followed by the submersion depth. In case of high casting speed, to reduce or to avoid mold powder entrapment, it is necessary to increase the nozzle submersion depth and the downward angle of the SEN outlet. In case of shallow nozzle submersion depth, decreasing the casting speed and increasing the downward angle of the SEN outlet appropriately must be taken into account. In case of higher Ar flowrate, decreasing the casting speed and increasing the nozzle submersion depth and downward angle of the SEN outlet appropriately must be taken into account. It is found that when 1.4 d m i n of casting speed is used for casting slabs of 1900 mm in width, the velocity of surface flow and the level fluctuation of the fluid in the mold can be controlled to prevent and reduce the mold powder entrapment with the downward angle of the SEN outlet of 25", submersion depth of SEN of 250 mm, and Ar flowrate of 10 NL/min.
(b) the mold powder entrapment caused by vortexes around the submerged entry nozzle (SEN); and (c) the mold powder entrapment caused by Ar bubbling. (2) Both the velocity of the surface flow and the fluctuation level of the bath in the mold are enlarged with increasing the casting speed, reducing the submersion depth of SEN and decreasing the downward angle of the nozzle outlet, and increasing the tendency of the mold powder entrapment.
(3) With the increase of the Ar flowrate, both the velocity of the surface flow and the level fluctuation are increased. For different positions of Ar bubbling, the tendency of powder entrapment is different.
(4)The parameters such as casting speed, submersion depth of SEN, downward angle of the SEN outlet, and Ar flowrate have influence on the mold powder entrapment. Among the four above-mentioned factors, casting speed has the largest effect. The negative influence caused by one parameter can be reduced or eliminated by proper selection of another parameter or some other parameters.
References
4. Conclusions (1) There are mainly three types of mold powder entrapment that occur in slab continuous casting: (a) the mold powder entrapment caused by the shearing flow 400 mm apart from the narrow face of the mold;
[ I ] M. Iguchi, J. Yoshida, and T. Shimizu, Model study on the entrapment of mold powder into molten steel, ISZJ, 40(2000), No.7, p.685. [2] D. Gupta and A.K. Lahiri, A water model study of the flow asymmetry inside a continuous slab casting mold, Metall. Mater: Trans. B, 27B(1996), p.757.
404
[3] P. Audrzejewski, K.U. Kbhler, and W. Pluschkell, Model investigations on the fluid flow in continuous casting moulds of wide dimensions, Steel Res., 1992, No.6, p.242. [4] F.M. Najjar, B.G Thomas, and D.E. Hershey, Numerical study of steady turbulent flow through bifurcated nozzles in continuous casting, Metall. Matel: Trans. B, 26B(1995), p.749. [5] D. Gupta and A.K. Lahiri, Water-modeling study of the surface disturbances in continuous slab caster, Metall. Matel: Trans. B, 25B(1994), p.227. [6] Toshio Teshima, Jun Kubota, Mikio Suzuki, et al., Influence of casting conditions on molten steel flow in continuous casting mold at high speed casting of slabs, Iron Steel (in Japan), 79( 1993), No.5, p.576.
J. Univ. Sci. Technol. Beijing, Vol.14, NOS, Oct 2007 [7] X.G Wan, C.J. Han,K.K. Cai, et al., Experimental research of submerged entry nozzle of slab continuous casting, Iron Steel (in Chinese), 35(2000), No.9, p.20. 183 H. Lei, H.H. Xu, M.Y.Zhu, et al., Mechanism and control of slag entrapment in high casting mould, Iron Steel (in Chinese), 34(1999), N0.8, p.20. [9] Y.P. Bao, J.Q. Zhu, W. Jiang, et al., Water modeling research on slag-entrapment in the mould of thin slab continuous caster, J. Univ. Sci. Technol. Beijing (in Chinese), 21(1999), N0.6, p.530. [lo] J. Yoshida, T. Ohmi, and M. Iguchi, Cold model study of the effects of density difference and blockage factor on mold powder entrainment, ISIJ, 45(2005), No.8, p.1160.
Metallurgy
Water modeling of mold powder entrapment in slab continuous casting mold Qiaotong Lu, Rongguang Yang, Xinhua Wang, Jiongming Zhang, and Wanjun Wang Metallurgical and Ecological Engineering School, University of Science and Technology Beijing, Beijing 100083, China
(Received 2006-08-25)
Abstract: The optimal parameters were determined by the water modeling of slab casting. It was found that there are mainly three types of mold powder entrapment in slab continuous casting, i.e., the entrapment caused by the shearing flow near the narrow face of mold, the entrapment caused by vortexes around the submerged entry nozzle (SEN), and the entrapment caused by the Ar bubbling. Both the velocity of the surface flow and the level fluctuation of the liquids are enlarged with increasing the casting speed, reducing the submersion depth of SEN, decreasing the downward angles of the nozzle outlets, and increasing the Ar flowrate, all of which increase the tendency of mold powder entrapment. Among the four above-mentioned factors, casting speed has the largest effect.
Key words: continuous casting; slab; mold; mold powder; water model
1. Introduction With the development of clean steel production, the content of impurity elements in deep and superdeep drawing steel used for automobile and home appliances has been reduced to a low level. Now, the main reason of the surface defects of cold rolled strips is mold powder entrapment [l]. It is very important to study the flow of the molten steel in the mold and the mold powder entrapment by water modeling and numerical simulation. Until now, most water modeling studies were based on a model without slag on the surface of the molten steel [2-71. There were, however, some studies with some slag on the surface of the molten steel [ l , 8-10]. Some studies focused on high-speed slab casting [8] while others focused on thin slab casting [9]. The present experiments were carried out with a 1:l scale water model to discover the flow pattern of the molten steel in mold and the characteristics of the mold powder entrapment.
2. Water modeling Based on the similarity principle, for the isothermal steady flow of incompressible viscous fluid, the Re number and Fr number of water modeling must be equal to that of the real slab casting. A water model of 1:l was used. The width and thickness of the mold were 1500 and 250 mm, respectively. The water, oil mixture, and the polyethylene partiCorresponding author: Qiaotong Lu, E-mail: [email protected]
cle were used to simulate the molten steel, the liquid slag (molten mold powder) layer, and the solid powder slag layer, respectively. The effect of the viscosity and the density of the molten mold powder were studied, and by way of dimensionless analysis, the viscosity must meet the requirement of the following equation: (1)
Vslag/Vsteel=Voil/Vwater
where v is the kinematical viscosity, m2/s. The ratio of the molten mold powder kinematical viscosity to that of liquid steel is 54-97. According to Eq. (l), the kinematical viscosity of the oil in the experiment must be about (5.4-9.7)~10-~m2/s. The kinematical viscosity of the oil mixture used in the experiment was about 8 . 8 6 ~ 1 0 -m2/s. ~ The thickness of the oil layer was more than 10 mm. Model DJ800's Multi-functional Water Engineering Observation System made in China was used to measure the height of the wave on the surface without oil, and the measured data was collected using a computer. The flow velocity of the oiUwater interface was measured using a Model LS1206B Propeller Current Meter. This device can be used to measure the offset velocity that is parallel to the axis of the propeller. The minimal measurement velocity of flow was 0.05 m/s with a precision of 21.0%.
3. Results and discussion 3.1. Mechanism of mold powder entrapment Fig. 1 depicts the photographs taken at a casing Also available online at www.sciencedirect.com
J. Univ. Sci. Technol. Beijing, Vo1.14,No.5, Oct 2007
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speed of 1.4 d m i n . As seen in Fig.1, there are three types of mold powder entrapment in slab casting
mold.
Fig. 1. Photographs showing different types of mold slag entrapment in the water model: (a) entrapment caused by shearing flow; (b) entrapment caused by vortexes (high symmetrical flow field); (c) entrapment caused by vortexes (unsymmetrical flow field); (d) entrapment caused by Ar bubbles.
(1) Entrapment caused by the shearing flow. The entrapment caused by the shearing flow occurs at 400 mm apart from the narrow face of the mold. In this region, the stream from the outlet of the submerged entry nozzle (SEN) is divided into two branches reaching the narrow face of the mold; one is the up branch and the other is the down branch. After the up branch reaches the surface, the stream changes its direction and flows toward the SEN. This kind of surface current gives rise to a shearing force on the slag layer (simulated by oil) making it thin and even exposition of molten steel. The shearing current gathers the slag in the region apart from the narrow face of the mold at about 400 mm, and forms a downward protuberance. Under the joint action of the free surface fluctuation and the turbulent flow, the drippings at the tip of the protuberance are very easy to break away from the slag layer and to be entrapped. If the slag drippings cannot return to the slag layer to be taken by the liquid steel or to be trapped by the solid shell, the mold powder entrapment will occur (as shown in Fig. l(a)). The higher the casting speed, the shallower the SEN submersion depth, and the shallower the downward angles of the SEN outlets, the easier is this kind of mold powder entrapment. (2) Entrapment caused by vortexes. As seen in Fig. 2, the surface currents toward the SEN meet together near the nozzle. When the flow
field in the mold is high symmetrical, the two surface currents at both sides of the SEN meet in the region between the SEN and the mold walls. Owing to the difference in the surface velocities of currents, it is possible to form a vortex at points A or B as shown in Fig. 2 and Fig. 1 (b).
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Fig. 2. Illustration of vortexes in the area around the SEN.
The unsymmetrical streams from two outlets will bring an unequal surface flow velocity at both sides of the SEN. When the difference in velocity of the surface flow reaches some extent, vortexes will be formed at the joint points (as shown in Fig. 2, C , D, E, and 0. If the energy of such vortexes is large enough, the slag can be entrapped (as shown in Fig.1 (c)). Increasing the casting speed or reducing the submersion depth of SEN or decreasing the downward angles of the SEN outlets will make this kind of mold powder entrapment easy to occur.
( 3 ) Entrapment caused by the Ar bubbles at the interface between the liquid steel and the mold powder. To prevent the nozzle from clogging, Ar gas is always used to inject into the nozzle. During casting,
Q.T. Lu et al., Water modeling of mold powder entrapment in slab continuous casting mold
most of the Ar gas carried by the pouring stream will break away from the main stream and directly float to the surface. When the Ar bubbles reach the interface between the liquid steel and the mold powder, the bursting of the bubbles and the release of the Ar gas cause a violent agitation to the interface. Under the joint action of the wave fluctuation and the turbulent flow, some slag drippings break away form the slag layer and are entrapped into the steel (as shown in Fig. l(d)). This kind of entrapment will easily occur when the Ar flowrate is large or the nozzle submersion depth is shallow or the downward angles of the SEN outlets are shallow.
meniscus near the narrow faces, i.e., the easier is the entrapment caused by the shearing flow. In addition, with the increase of the casting speed, the kinetic energy of the turbulent flow and the unsymmetry of the flow field in the mold will be increased, and the probability of forming vortex will also be increased. Fig. 3 shows the effect of casting speed on level fluctuation and velocity of the surface flow. From Fig. 3, it can be seen that with the increase of the casting speed, the velocity of the surface flow and the level fluctuation measured along the width of the mold are increased. As seen in Figs. l(a)-l(c), at the casting speed of 1.4 d m i n , except for some few solid powder particles, the slag layer can hardly be seen at the meniscus near the narrow faces. At the position about 400 mm apart from the narrow face of the mold, mold powder entrapment caused by the shearing flow occurs, and vortexes near the SEN are formed.
3.2. Effect of technical parameters ( 1 ) Effect of the casting speed. The water model experiment showed that with the increase of the casting speed, the slag layer near the narrow faces of the mold becomes unstable. The higher the casing speed, the easier it is to expose the 14
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Fig. 3. Effects of casting speed on level fluctuation and velocity of the surface flow: (a) effect on the level fluctuation; (b) effect on the velocity of the surface flow. Angle of SEN outlet: -15'; submersion depth: 120 mm; Ar flowrate: 0 L/min.
(2) Effect of the submersion depth of SEN. As seen in Fig. 4,when the immersion depth of the nozzle becomes shallow, the velocity of the surface flow and the level fluctuation of the liquid increase, and therefore, the mold powder entrapment will easily occur. With the decrease of the submersion depth, the slag on the surface layer becomes unstable. At a depth of 120 mm, the slag layer near the narrow face of the mold is thin, and molten steel is easy to be exposed as shown in Fig. 1 because of a stronger impact of the stream on the surface. Therefore, the velocity of the
surface flow and the level fluctuation are increased. By the action of the shearing force, the molten slag is pushed from the narrow faces of the mold to the SEN. When the submersion depth of the SEN is 120 mm, the slag on the surface is gathered at 400 mm apart from the narrow faces of the mold, and when the submersion depth of SEN is 200 and 250 mm, the slag is easy to gather in the region from the narrow faces of the mold to 400 mm apart from the narrow face of the mold. The gathered slag forms a downward protuberance, where the mold powder entrapment easily occurs.
J. Univ. Sci. Technol. Beijing, Vo1.14,NOS, Oct 2007
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In addition, the decrease of the mold submersion depth will increase the probability of forming vortexes
near the nozzle, and therefore the probability of mold powder entrapment will increase. 0.30
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Distance from narrow side of mold / mm
Fig. 4. Effects of the submersion depth of SEN on the level fluctuation and velocity of the surface flow: (a) effect on the level fluctuation; (b) effect on the velocity of the surface flow. Angle of SEN outlet: -15"; casting speed: 1.4 ∈ Ar flowrate: 0 L/min.
(3) Effect of the downward angle of the SEN outlet. Fig. 5 shows the influence of the downward angle of the SEN outlet on the level fluctuation and the velocity of the surface flow. With the increase of the downward angle of the SEN outlet, the velocity of the surface flow and the level fluctuation are decreased. If
the angle increases over -25", the influence on the velocity of the surface flow and the level fluctuation is decreased. Thus, the smaller the downward angles of the SEN outlets, the stronger is the influence on the mold powder entrapment. 0.30
-
(b)
Downward angle of SEN outlet:
h
0.25
-*--15" -W--25"
E
v 1
Downward angle of SEN outlet:
5
0.20
6: 0 a3
-0-1 5"
0.15
-O--25" -A--34"
vl
b)
E 0
0.10
.-b 0 2
>" 100
200
300
400
500
6.
Distance from narrow side of mold / mm
0.05 0.00
0
100
I
I
200
300
#
400
500
Distance from narrow side of mold / mm
Fig. 5. Effects of the outlet downward angle of SEN on the level fluctuation and velocity of the surface flow: (a) effect on the level fluctuation; (b) effect on the velocity of the surface flow. Submersion depth: 120 mm; casting speed: 1.4 d m i n ; Ar flowrate: 0 L/min.
(4) Effect of the Ar flowrate. Fig. 6 shows the change in velocity of the surface flow and the level fluctuation with Ar flowrate. With the increase of the Ar flowrate, the velocity of the sur-
face flow at various measuring points somewhat increases, and increases obviously near the SEN, because a good deal of Ar bubbles break away from the main stream and float to the surface near the nozzle causing considerably stronger collision and agitation
Q.T. Lu et al., Water modeling of mold powder entrapment in slab continuous casting mold
to the surface (as shown in Fig. 1). Also, with the increase of the Ar flowrate, the level fluctuation near the nozzle will increase (as shown in Fig. 6). Agitation brought by Ar injection can entrap the mold powder into the steel, as shown in Fig. l(d), and
403
with the change of the Ar flowrate, the agitation by Ar will give rise to the mold powder entrapment at different positions along the mold width. This phenomenon has a close relationship with the bubble density in the foaming and the position where the bubbles float up.
P 0.24
-
0.22 -
0.20 0.18 -
Ar flowrate: - 0 - 0 L/min
-0-4 Limin -A-10 L/min I
1
I
I
I
I
100
200
300
400
500
600
Distance from narrow side of mold / mm
-.-
0.16 c
4 L/min -A- 10 Limin
0.14 n
171
"",O
100
200
300
400
500
Distance from narrow side of mold / mm
Fig. 6. Effects of Ar flowrate on the level fluctuation and velocity of the surface flow: (a) effect on the level fluctuation; (b) effect on the velocity of the surface flow. Submersion depth: 120 mm; angle of SEN outlet: -15"; casting speed 1.4 ndmin.
(5) Interaction of technical parameters on the mold powder entrapment. From above mentioned results, it is clear that casting speed has the largest effect on the mold powder entrapment, followed by the submersion depth. In case of high casting speed, to reduce or to avoid mold powder entrapment, it is necessary to increase the nozzle submersion depth and the downward angle of the SEN outlet. In case of shallow nozzle submersion depth, decreasing the casting speed and increasing the downward angle of the SEN outlet appropriately must be taken into account. In case of higher Ar flowrate, decreasing the casting speed and increasing the nozzle submersion depth and downward angle of the SEN outlet appropriately must be taken into account. It is found that when 1.4 d m i n of casting speed is used for casting slabs of 1900 mm in width, the velocity of surface flow and the level fluctuation of the fluid in the mold can be controlled to prevent and reduce the mold powder entrapment with the downward angle of the SEN outlet of 25", submersion depth of SEN of 250 mm, and Ar flowrate of 10 NL/min.
(b) the mold powder entrapment caused by vortexes around the submerged entry nozzle (SEN); and (c) the mold powder entrapment caused by Ar bubbling. (2) Both the velocity of the surface flow and the fluctuation level of the bath in the mold are enlarged with increasing the casting speed, reducing the submersion depth of SEN and decreasing the downward angle of the nozzle outlet, and increasing the tendency of the mold powder entrapment.
(3) With the increase of the Ar flowrate, both the velocity of the surface flow and the level fluctuation are increased. For different positions of Ar bubbling, the tendency of powder entrapment is different.
(4)The parameters such as casting speed, submersion depth of SEN, downward angle of the SEN outlet, and Ar flowrate have influence on the mold powder entrapment. Among the four above-mentioned factors, casting speed has the largest effect. The negative influence caused by one parameter can be reduced or eliminated by proper selection of another parameter or some other parameters.
References
4. Conclusions (1) There are mainly three types of mold powder entrapment that occur in slab continuous casting: (a) the mold powder entrapment caused by the shearing flow 400 mm apart from the narrow face of the mold;
[ I ] M. Iguchi, J. Yoshida, and T. Shimizu, Model study on the entrapment of mold powder into molten steel, ISZJ, 40(2000), No.7, p.685. [2] D. Gupta and A.K. Lahiri, A water model study of the flow asymmetry inside a continuous slab casting mold, Metall. Mater: Trans. B, 27B(1996), p.757.
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[3] P. Audrzejewski, K.U. Kbhler, and W. Pluschkell, Model investigations on the fluid flow in continuous casting moulds of wide dimensions, Steel Res., 1992, No.6, p.242. [4] F.M. Najjar, B.G Thomas, and D.E. Hershey, Numerical study of steady turbulent flow through bifurcated nozzles in continuous casting, Metall. Matel: Trans. B, 26B(1995), p.749. [5] D. Gupta and A.K. Lahiri, Water-modeling study of the surface disturbances in continuous slab caster, Metall. Matel: Trans. B, 25B(1994), p.227. [6] Toshio Teshima, Jun Kubota, Mikio Suzuki, et al., Influence of casting conditions on molten steel flow in continuous casting mold at high speed casting of slabs, Iron Steel (in Japan), 79( 1993), No.5, p.576.
J. Univ. Sci. Technol. Beijing, Vol.14, NOS, Oct 2007 [7] X.G Wan, C.J. Han,K.K. Cai, et al., Experimental research of submerged entry nozzle of slab continuous casting, Iron Steel (in Chinese), 35(2000), No.9, p.20. 183 H. Lei, H.H. Xu, M.Y.Zhu, et al., Mechanism and control of slag entrapment in high casting mould, Iron Steel (in Chinese), 34(1999), N0.8, p.20. [9] Y.P. Bao, J.Q. Zhu, W. Jiang, et al., Water modeling research on slag-entrapment in the mould of thin slab continuous caster, J. Univ. Sci. Technol. Beijing (in Chinese), 21(1999), N0.6, p.530. [lo] J. Yoshida, T. Ohmi, and M. Iguchi, Cold model study of the effects of density difference and blockage factor on mold powder entrainment, ISIJ, 45(2005), No.8, p.1160.