- Email: [email protected]
Numerical Simulation of Influence of Casting Speed Variation on Surface Fluctuation of Molten Steel in Mold
Available online at www.sciencedirect.com 1 1 1 1 0
ScienceDirect
JOURNAL OF IRON AND STEEL RESEARCH, INTERNpiTIONAL. 2010, 17(8): 15-19
Numerical Simulation of Influence of Casting Speed Variation on Surface Fluctuation of Molten Steel in Mold ZHANG Qiao-ying’
,
WANG Xin-hua’
(1. Ma’anshan Iron and Steel Co Ltd, Ma’anshan 243000, Anhui, China; Technology Beijing, Beijing 100081, China)
2. University of Science and
Abstract: The influences of casting speed variation on surface fluctuation of the molten steel in mold during continuous casting were investigated with numerical simulation method. It was found that when the casting speed was evenly
decreased from 1 . 4 m min-’ to 0. 6 m min-’ , the increase of the surface fluctuation of the molten steel in mold was observed only on time that was at the start of casting speed change. While, in experiment of increasing casting speed evenly from 0. 6 m min-’ to 1. 4 m min-’ , the increase of the surface fluctuation of the molten steel in mold was observed only a t the time when the casting speed was stopped to increase after it had been increased to 1.4 m min-I. For surface fluctuation of the molten steel in mold which was produced during the casting speed evenly increasing or decreasing period and at the time when increasing or decreasing the casting speed at low casting speed level (0. 6 m min-’ ) , the influence of casting speed change is very small. In addition, it is found that, at high casting speed level (1.4 m * min-’ ), even a little change of casting speed could result in remarkable increase of the surface fluctuation. Thus, a t high casting speed, changing casting speed should be avoided or much slower speed changing rate should be used. Key words: unsteady continuous casting; moid; casting speed; surface fluctuation; numerical simulation
Mold powder entrapment mainly takes place in so-called “unsteady casting” periods, e. g. the time of casting start, casting end, ladle exchange, mold width change, SEN change, etc. During the unsteady casting periods, usually, casting speed v, is largely varied which disturbs the flow of the liquid steel in the mold and results in severe level fluctuation. Hence, mold powders are easy to be carried into the liquid steel and some are entrapped by the solidified shell and exist as non-metallic inclusions in slabs. In recent years, surface defects of I F steel sheets caused due t o problems of steelmaking, secondary refining, etc. have been significantly. decreased owing t o the progress and improvement of the steelmaking and refining technologies and equipments. However, the ratio of the defects owing to the mold powder entrapment during continuous casting has increased“-”. Casting speed variation should be avoided during casting. However, in practical casting production, casting speed change is still Biography:ZHANG Qiao-ying(l977-),
inevitable. Investigating the influences of casting speed variation on surface fluctuation of the molten steel of mold was important for obtaining slabs good quality and improving production efficiency. Usually surface fluctuation of the molten steel of mold was investigated by water model and numerical simulation method^[^-^' because it is relatively difficult to study the molten steel of mold directly. T h e influences of casting speed variation on surface fluctuation of the molten steel of mold during continuous casting were investigated in this paper with numerical simulation method.
1 Experimental Method T h e operational parameters used in the present study are shown in Table 1. Increasing or decreasing the casting speed was intentionally arranged in the experiments to investigate the influence of the v, change on surface fluctuation of the molten steel of mold. In the experiments
Female, Doctor, Senior Engineers
E-mail: dyn19781223B163. comi
Received Date: July 4 , 2009
Journal of Iron and Steel Research, International
16
Table 1 Operational parameters Machine type Vertical length/m Type of SEN Port angle (downward)/(") Size of mold/mm SEN submerged depth/mm
Vertical bend 2.75 ' Bifurcated 15 1 150 X 230 X 900 170
of decreasing casting speed, after v, was maintained at 1 . 4 m min-' for more than 4 min, the casting speed was decreased evenly with the speed decreasing rate of 0.15 m min-'. After casting, fluctuation height, surface velocity, impact depth and impact velocity were taken from the flow field which was just in the mold during casting when the casting speed was decreased to 1 . 2 m min-' , 1 . 0 m min-' , 0.8 m min-' and 0. 6 m min-' , respectively. In the experiments of increasing casting speed, after v, was maintained at 0. 6 m min-' for more than 4 mint the casting speed was evenly increased with the rate of 0 . 1 5 m rnin-'. After casting, fluctuation height, surface velocity, impact depth and impact velocity were taken from the flow field which was just in the mold when the casting speed was increased to 0.8 m min-' , 1. O m min-' , 1 . 2 m min-' and 1. 4 m min-' , respectively. In addition, for comparison, fluctuation height, surface velocity, impact depth and impact velocity were also taken on the flow field which was normally cast at 0. 6 m min-' and 1 . 4 m min-'.
2
Mathematical Model Formulation
2.1
Assumptions in modeling 1) Neglecting influences of solidified shell on the flow field of the molten steel of mold. 2 ) Slag-steel interface is recognized to be momentum transfer. 3 ) The liquid steel inside the mold is incompressible Newtonian fluid and physical parameter is recognized constant. 4 ) Liquid steel flow is recognized to be turbulent flow. 2.2
Governing equation Based on above assumptions, the following governing equations were solved in the mathematical model : 1) The continuity equation; 2) The momentum equations; 3) The turbulence equation.
Vol. 17
The parameters that were recommended by Launder and Spalding were used in the mathematical model. Their values are C1= 1 . 4 4 , C2 = 1 . 9 2 , U , = 0 . 0 9 , G k = l . 0, u e = l . 3. Solution procedure Three-dimensional liquid transfer inside the mold has been investigated in this model. The distance from inlet to the outlet is long enough in order to attain a fully developed turbulent flow. The velocity at the inlet depends on experimental method. Estimation for k and E at the inlet can be arrived at by solving expressions which were recommended by Guthrie R I L. A no-slip velocity boundary condition is applied. At the exit, a pressure boundary condition is applied. Slag-steel interface was simulated by SOLA-VOF model. Volume function F ( 5 , y , z, t ) of slag-steel interface is defined as follows: 1 grids filled by liquid 0- 1 surface grids (non-entirely filled (1) F=[ by liquid) 0 empty grids Volume function equation 2.3
(2) Calculation is carried out in this model by use of CFD software package, CFX. The structured finite difference grid is adopted. A typical calculation for practical process of 2 min required 23 h of CPU time. The residual for the convergence criterion is
3
Results and Discussion
The parameters of fluid in the model include the following: density of mold powder, 2 700 kg m-' 5 specific heat of mold powder, 700 J kg-' K-'; viscosity of mold powder, 34.5 X lop6 m2 s-l; thermal conductivity of mold powder, 68 W m-' K-' ; density of molten steel, 7 020 kg m-' ; specific heat of molten steel, 755 J kg-' K-'; viscosity of molten steel, 5. 5 X lo-' kg m-' s-' ; thermal conductivity of molten steel, 41 W m-' K-' ; surface tension, 1 . 2 2 N m-'. Fig. 1 shows the volume fraction of the molten steel. The slag-steel interface was represented by the volume fraction, which is 0. 5. Variation of the slagsteel interface is the fluctuation. The velocity of slag-steel interfaces is the surface velocity. Fig. 1 shows fluctuation height and surface velocity of slag-steel interfaces at the time when the casting speed decreased to each v, in the experiment of decrea-
Issue 8
Numerical Simulation of Influence of Casting Speed Variation on Surface Fluctuation of Molten Steel
,.
rng d
11.6 , -
2
4
6
8
Timm
10
12
14
Fig. 1 Fluctuations and surface velocity of slagsteel interfaces in casting speed decreasing experiment
sing casting speed. Also, the casting speed change is illustrated in the figure. Before decreasing casting speed, the casting speed was stabilized at 1. 4 m min-' and the fluctuation height at the stable v, of 1.4 m min-' was about 8. 7 mm. It is seen in Fig. 1 that when decreasing casting speed was started and the v, was decreased from 1. 4 m min-' to 1 . 2 m min-' , the fluctuation height increased remarkably to 10 mm. Howeve r , in later casting speed decreasing period when v, was evenly decreased further to 1.0 m min-' and 0. 8 m min-' , fluctuation height were just between 7-8 mm less than the fluctuation height at 1. 4 m min-'. When casting speed changing was stopped after the v, was decreased to 0. 6 m min-' , a very slight increase of the fluctuation height was found. It is shown in Fig. 1 that surface velocity change is the same as fluctuation height during the decreasing casting speed experiments. T h e result of the mathematical simulation indicates that the flow of the liquid steel in the mold is much more largely influenced at the time when the casting speed is started t o decrease, which corresponds well to the result of the investigation on the effect of the casting speed change on the non-metallic inclusions in subsurface layers of the slabsL5'. Ref. [5] indicates that if the casting speed is decreased from high speed level (e. g. 1.4 m min-' , the mold powder entrapment owing to enhanced fluctuation height and surface velocity of slag-steel interfaces takes place only at the time which was at the initial casting speed changing period. This is because when casting speed decrease is started at higher casting speed, both the quantity and the velocity of the steel from the SEN are abruptly reduced. This sudden interfere on the flow of the liquid steel in the mold enhances the level fluctuation of the
17
steel and increases mold powder entrapment. However, in the subsequent v, decreasing period, the interfere of the casting speed change is gradually getting weaker and the flow of the liquid steel is getting relatively stable because the casting speed was evenly reduced in the experiment. So, in the later casting speed evenly decreasing period, fluctuation height and surface velocity of slag-steel interfaces was not increased. Moreover, when casting speed decrease is stopped at low v, level (e. g. 0. 6 m min-') , even though the flow of the steel in the mold is disturbed by the sudden change, its influence on fluctuation height and surface velocity of slag-steel interfaces is largely limited because the casting speed is low. This result corresponds weil to the result of the investigation on the effect of the casting speed change on the non-metallic inclusions in subsurface layers of the slabsCs1. It should be pointed out that a big increase of the fluctuations height and surface velocity of slagsteel interfaces was observed when the casting speed was only decreased from 1. 4 m min-' to 1. 2 m min-' in the experiment. This means that at relatively higher casting speed, a slight change of the casting speed can cause remarkable increase of the fluctuations height and surface velocity of slag-steel interfaces. Fig. 2 shows impact depth and impact velocity of the liquid steel in the mold at the time when the casting speed decreased to each v, of 1 . 2 m min-' , 1.0 m min-' , 0. 8 m min- '' and 0. 6 m min-' in the experiment of decreasing casting speed. Also, the casting speed change is illustrated in the figure. Before decreasing casting speed, the casting speed was stabilized at 1 . 4 m min-' and the impact depth at the stable v, of 1. 4 m min-' was about 0. 594 m. It is seen in Fig. 2 ( a > that when decreasing casting speed was started and the v, was evenly decreased from 1.4 m min-' to 1. 2 m min-' , 1. 0 m min-' , 0.8 m min-' and 0.6 m min-' , the impact depth was gradually increased to 0. 626 m. When casting speed changing was stopped after the v, was decreased to 0. 6 m min-' , a very slight decrease of the impact depth of 0. 612 m was found. It is seen in Fig. 2 ( b ) that with the change of casting speed, impact velocity was gradually decreased during the decreasing casting speed experiments. Fig. 3 shows fluctuation height and surface velocity of slag-steel interfaces at the time when the casting speed increased to each v, in the experiment of increa-
18
Vol. 17
Journal of Iron and Steel Research, International
1.4
$ o-620' 8 0.610 10.600. I o . 0.690
6
9
0
l
o
.
z
6 g
Fig.2 Impact depth (a) and impact velocity (b) of the liquid steel In casting speed decreasing experiment
Fig. 3 Fluctuations and surface velocity of slagsteel interfaces in casting speed increasing experiment
sing casting speed. Also, the casting speed change is illustrated in the figure. It is seen that, when casting speed increase was started at low casting level (0. 6 m min-'1 , even though the flow of the liquid steel in the mold was. disturbed, the fluctuation height of slag-steel interfaces were only slightly increased. In the subsequent casting speed increasing period, the flow in the mold became relatively stable because the v, was evenly increased and heat supply to the meniscus by the up flow of the steel was increased with increasing v,. As shown in Fig. 3, fluctuation height of slag-steel interfaces when the v, was increased to 1.0 m min-' and 1 . 2 m min-' was decreased compared with those at lower v,. During the casting speed evenly increasing period, the contents of the fluctuation height of slagsteel interfaces were between 6 - 9 mm. While,
when the casting speed increasing was stopped at 1.4 m min-' , the fluctuation height of slag-steel interfaces was remarkably increased to 11 mm. Later, when the v, was again stabilized at 1.4 m min-' , the fluctuation height of slag-steel interfaces was decreased to 9.02 mm. Similar to start decreasing v, at higher casting speed, when the casting speed increase is stopped at high casting speed level (1.4 m min-' ) , th; flow of the liquid steel in the mold is largely disturbed. The result of the mathematical simulation indicates that the flow of the liquid steel in the mold is much more largely influenced at the time when the casting speed is stopped at higher casting speed level, which corresponds well to the result of the investigation on the effect of the casting speed change on the non-metallic inclusions in subsurface layers of the slabsC5'. This sudden interfere enhances the level fluctuation of the liquid steel in the mold and increases the entrapment of the mold powders by the solidified shell. So, at high casting speed level, v, changing should be avoided or be made with slower speed changing rate in continuous casting. Fig.4 shows impact depth and impact velocity of the liquid steel in the mold at the time when the casting speed increased to each v, of 0.8 m min-' , 1.0 m min-' , 1 . 2 m min-' and 1.4 m min-' in the experiment of increasing casting speed. Also, the casting speed change is illustrated in the figure. Before increasing casting speed, the casting speed was stabilized at 0.6 m min-' and the impact 1.4
*
0.200 I 2.0
4.0
6.0
8.0
10.0
Timehin
12.0
0.6
10.4 14.0
Fig. 4 Impact depth (a> and impact velocity (b) of the liquid steel in casting speed increasing experiment
Issue 8
Numerical Simulation of Influence of Casting Speed Variation on Surface Fluctuation of Molten Steel
depth at the stable v, of 0.6 m min-' was about 0. 612 m. It is seen in Fig. 4 (a) that when increasing casting speed was started and the v, was evenly increased from 0. 6 m min-' to 0.8 m min-' and 1.0 m min-' , the impact depth was slightly increased to 0. 625 m In later casting speed increasing period when v, was evenly increased further to 1 . 2 m min-' and 1 . 4 m min-' , impact depth were gradually decreased to 0. 598 m. It is seen in Fig. 4 (b) that with the change of casting speed, impact velocity was gradually increased during the increasing casting speed experiments.
4
Conclusions
1 ) When casting speed was decreased from 1 . 4 m min-' to 0. 6 m min-' , the increases of the enhanced fluctuation height and surface velocity of slag-steel interfaces were observed only at the time which was at the initial casting speed changing period. The subsequent casting speed evenly decreasing period and stopping decreasing v, at 0. 6 m min-' had almost no bad influence on non-metallic inclusion contents of slabs. 2 ) When casting speed was evenly increased from 0. 6 m min-' to 1 . 4 m min-' , the increases of the enhanced fluctuation height and surface velocity were observed only at the time as the increasing
19
v, was stopped at 1. 4 m min-'. T o start increasing v, at low casting speed level ( 0. 6 m min-' ) and evenly increasing v, to 1 . 4 m min-' had almost no bad influence on fluctuation height and surface velocity. 3) At high casting speed level (1. 4 m min-' ) , even a little change of casting speed could result in the increase of fluctuation height and surface velocity. Thus, at high casting speed, casting speed changing should be avoided or slower speed changing rate in continuous casting should be used. References : c11
c21
c31
c41
C51
Iguchi M, Yoshida J , Shimizu T. Model Study on the Entrapment of Mold Powder Into Molten Steel [J]. ISIJ International, 2000, 40(7): 691. Tanizawa K. Influence of the Steelmaking Conditions on Nonmetallic Inclusions and Product Defects CJ]. Metallurgia Italiana, 1992, 84: 17. Gupta D, Lahiri A K. Cold Model Study of the Surface Profile in a Continuous Slab Casting Model: Effect of Second Phase [J]. Metallurgical and Material Transactions, 1996. 27B(8) : 695. Jonsson L, Jonsson P. . Modeling of Fluid Flow Conditions Around the Slag/Metal Interface in a Gas-Stirred Ladle [J]. ISU International, 1996, 36(9): 1127. ZHANG Qiac-ying, WANG Xin-hua. Influence of Casting Speed Variation During Unsteady Continuous Casting on NonMetallic Inclusions in IF Steel Slabs [J]. ISIJ International, 2006, 46(10): 1421.
ScienceDirect
JOURNAL OF IRON AND STEEL RESEARCH, INTERNpiTIONAL. 2010, 17(8): 15-19
Numerical Simulation of Influence of Casting Speed Variation on Surface Fluctuation of Molten Steel in Mold ZHANG Qiao-ying’
,
WANG Xin-hua’
(1. Ma’anshan Iron and Steel Co Ltd, Ma’anshan 243000, Anhui, China; Technology Beijing, Beijing 100081, China)
2. University of Science and
Abstract: The influences of casting speed variation on surface fluctuation of the molten steel in mold during continuous casting were investigated with numerical simulation method. It was found that when the casting speed was evenly
decreased from 1 . 4 m min-’ to 0. 6 m min-’ , the increase of the surface fluctuation of the molten steel in mold was observed only on time that was at the start of casting speed change. While, in experiment of increasing casting speed evenly from 0. 6 m min-’ to 1. 4 m min-’ , the increase of the surface fluctuation of the molten steel in mold was observed only a t the time when the casting speed was stopped to increase after it had been increased to 1.4 m min-I. For surface fluctuation of the molten steel in mold which was produced during the casting speed evenly increasing or decreasing period and at the time when increasing or decreasing the casting speed at low casting speed level (0. 6 m min-’ ) , the influence of casting speed change is very small. In addition, it is found that, at high casting speed level (1.4 m * min-’ ), even a little change of casting speed could result in remarkable increase of the surface fluctuation. Thus, a t high casting speed, changing casting speed should be avoided or much slower speed changing rate should be used. Key words: unsteady continuous casting; moid; casting speed; surface fluctuation; numerical simulation
Mold powder entrapment mainly takes place in so-called “unsteady casting” periods, e. g. the time of casting start, casting end, ladle exchange, mold width change, SEN change, etc. During the unsteady casting periods, usually, casting speed v, is largely varied which disturbs the flow of the liquid steel in the mold and results in severe level fluctuation. Hence, mold powders are easy to be carried into the liquid steel and some are entrapped by the solidified shell and exist as non-metallic inclusions in slabs. In recent years, surface defects of I F steel sheets caused due t o problems of steelmaking, secondary refining, etc. have been significantly. decreased owing t o the progress and improvement of the steelmaking and refining technologies and equipments. However, the ratio of the defects owing to the mold powder entrapment during continuous casting has increased“-”. Casting speed variation should be avoided during casting. However, in practical casting production, casting speed change is still Biography:ZHANG Qiao-ying(l977-),
inevitable. Investigating the influences of casting speed variation on surface fluctuation of the molten steel of mold was important for obtaining slabs good quality and improving production efficiency. Usually surface fluctuation of the molten steel of mold was investigated by water model and numerical simulation method^[^-^' because it is relatively difficult to study the molten steel of mold directly. T h e influences of casting speed variation on surface fluctuation of the molten steel of mold during continuous casting were investigated in this paper with numerical simulation method.
1 Experimental Method T h e operational parameters used in the present study are shown in Table 1. Increasing or decreasing the casting speed was intentionally arranged in the experiments to investigate the influence of the v, change on surface fluctuation of the molten steel of mold. In the experiments
Female, Doctor, Senior Engineers
E-mail: dyn19781223B163. comi
Received Date: July 4 , 2009
Journal of Iron and Steel Research, International
16
Table 1 Operational parameters Machine type Vertical length/m Type of SEN Port angle (downward)/(") Size of mold/mm SEN submerged depth/mm
Vertical bend 2.75 ' Bifurcated 15 1 150 X 230 X 900 170
of decreasing casting speed, after v, was maintained at 1 . 4 m min-' for more than 4 min, the casting speed was decreased evenly with the speed decreasing rate of 0.15 m min-'. After casting, fluctuation height, surface velocity, impact depth and impact velocity were taken from the flow field which was just in the mold during casting when the casting speed was decreased to 1 . 2 m min-' , 1 . 0 m min-' , 0.8 m min-' and 0. 6 m min-' , respectively. In the experiments of increasing casting speed, after v, was maintained at 0. 6 m min-' for more than 4 mint the casting speed was evenly increased with the rate of 0 . 1 5 m rnin-'. After casting, fluctuation height, surface velocity, impact depth and impact velocity were taken from the flow field which was just in the mold when the casting speed was increased to 0.8 m min-' , 1. O m min-' , 1 . 2 m min-' and 1. 4 m min-' , respectively. In addition, for comparison, fluctuation height, surface velocity, impact depth and impact velocity were also taken on the flow field which was normally cast at 0. 6 m min-' and 1 . 4 m min-'.
2
Mathematical Model Formulation
2.1
Assumptions in modeling 1) Neglecting influences of solidified shell on the flow field of the molten steel of mold. 2 ) Slag-steel interface is recognized to be momentum transfer. 3 ) The liquid steel inside the mold is incompressible Newtonian fluid and physical parameter is recognized constant. 4 ) Liquid steel flow is recognized to be turbulent flow. 2.2
Governing equation Based on above assumptions, the following governing equations were solved in the mathematical model : 1) The continuity equation; 2) The momentum equations; 3) The turbulence equation.
Vol. 17
The parameters that were recommended by Launder and Spalding were used in the mathematical model. Their values are C1= 1 . 4 4 , C2 = 1 . 9 2 , U , = 0 . 0 9 , G k = l . 0, u e = l . 3. Solution procedure Three-dimensional liquid transfer inside the mold has been investigated in this model. The distance from inlet to the outlet is long enough in order to attain a fully developed turbulent flow. The velocity at the inlet depends on experimental method. Estimation for k and E at the inlet can be arrived at by solving expressions which were recommended by Guthrie R I L. A no-slip velocity boundary condition is applied. At the exit, a pressure boundary condition is applied. Slag-steel interface was simulated by SOLA-VOF model. Volume function F ( 5 , y , z, t ) of slag-steel interface is defined as follows: 1 grids filled by liquid 0- 1 surface grids (non-entirely filled (1) F=[ by liquid) 0 empty grids Volume function equation 2.3
(2) Calculation is carried out in this model by use of CFD software package, CFX. The structured finite difference grid is adopted. A typical calculation for practical process of 2 min required 23 h of CPU time. The residual for the convergence criterion is
3
Results and Discussion
The parameters of fluid in the model include the following: density of mold powder, 2 700 kg m-' 5 specific heat of mold powder, 700 J kg-' K-'; viscosity of mold powder, 34.5 X lop6 m2 s-l; thermal conductivity of mold powder, 68 W m-' K-' ; density of molten steel, 7 020 kg m-' ; specific heat of molten steel, 755 J kg-' K-'; viscosity of molten steel, 5. 5 X lo-' kg m-' s-' ; thermal conductivity of molten steel, 41 W m-' K-' ; surface tension, 1 . 2 2 N m-'. Fig. 1 shows the volume fraction of the molten steel. The slag-steel interface was represented by the volume fraction, which is 0. 5. Variation of the slagsteel interface is the fluctuation. The velocity of slag-steel interfaces is the surface velocity. Fig. 1 shows fluctuation height and surface velocity of slag-steel interfaces at the time when the casting speed decreased to each v, in the experiment of decrea-
Issue 8
Numerical Simulation of Influence of Casting Speed Variation on Surface Fluctuation of Molten Steel
,.
rng d
11.6 , -
2
4
6
8
Timm
10
12
14
Fig. 1 Fluctuations and surface velocity of slagsteel interfaces in casting speed decreasing experiment
sing casting speed. Also, the casting speed change is illustrated in the figure. Before decreasing casting speed, the casting speed was stabilized at 1. 4 m min-' and the fluctuation height at the stable v, of 1.4 m min-' was about 8. 7 mm. It is seen in Fig. 1 that when decreasing casting speed was started and the v, was decreased from 1. 4 m min-' to 1 . 2 m min-' , the fluctuation height increased remarkably to 10 mm. Howeve r , in later casting speed decreasing period when v, was evenly decreased further to 1.0 m min-' and 0. 8 m min-' , fluctuation height were just between 7-8 mm less than the fluctuation height at 1. 4 m min-'. When casting speed changing was stopped after the v, was decreased to 0. 6 m min-' , a very slight increase of the fluctuation height was found. It is shown in Fig. 1 that surface velocity change is the same as fluctuation height during the decreasing casting speed experiments. T h e result of the mathematical simulation indicates that the flow of the liquid steel in the mold is much more largely influenced at the time when the casting speed is started t o decrease, which corresponds well to the result of the investigation on the effect of the casting speed change on the non-metallic inclusions in subsurface layers of the slabsL5'. Ref. [5] indicates that if the casting speed is decreased from high speed level (e. g. 1.4 m min-' , the mold powder entrapment owing to enhanced fluctuation height and surface velocity of slag-steel interfaces takes place only at the time which was at the initial casting speed changing period. This is because when casting speed decrease is started at higher casting speed, both the quantity and the velocity of the steel from the SEN are abruptly reduced. This sudden interfere on the flow of the liquid steel in the mold enhances the level fluctuation of the
17
steel and increases mold powder entrapment. However, in the subsequent v, decreasing period, the interfere of the casting speed change is gradually getting weaker and the flow of the liquid steel is getting relatively stable because the casting speed was evenly reduced in the experiment. So, in the later casting speed evenly decreasing period, fluctuation height and surface velocity of slag-steel interfaces was not increased. Moreover, when casting speed decrease is stopped at low v, level (e. g. 0. 6 m min-') , even though the flow of the steel in the mold is disturbed by the sudden change, its influence on fluctuation height and surface velocity of slag-steel interfaces is largely limited because the casting speed is low. This result corresponds weil to the result of the investigation on the effect of the casting speed change on the non-metallic inclusions in subsurface layers of the slabsCs1. It should be pointed out that a big increase of the fluctuations height and surface velocity of slagsteel interfaces was observed when the casting speed was only decreased from 1. 4 m min-' to 1. 2 m min-' in the experiment. This means that at relatively higher casting speed, a slight change of the casting speed can cause remarkable increase of the fluctuations height and surface velocity of slag-steel interfaces. Fig. 2 shows impact depth and impact velocity of the liquid steel in the mold at the time when the casting speed decreased to each v, of 1 . 2 m min-' , 1.0 m min-' , 0. 8 m min- '' and 0. 6 m min-' in the experiment of decreasing casting speed. Also, the casting speed change is illustrated in the figure. Before decreasing casting speed, the casting speed was stabilized at 1 . 4 m min-' and the impact depth at the stable v, of 1. 4 m min-' was about 0. 594 m. It is seen in Fig. 2 ( a > that when decreasing casting speed was started and the v, was evenly decreased from 1.4 m min-' to 1. 2 m min-' , 1. 0 m min-' , 0.8 m min-' and 0.6 m min-' , the impact depth was gradually increased to 0. 626 m. When casting speed changing was stopped after the v, was decreased to 0. 6 m min-' , a very slight decrease of the impact depth of 0. 612 m was found. It is seen in Fig. 2 ( b ) that with the change of casting speed, impact velocity was gradually decreased during the decreasing casting speed experiments. Fig. 3 shows fluctuation height and surface velocity of slag-steel interfaces at the time when the casting speed increased to each v, in the experiment of increa-
18
Vol. 17
Journal of Iron and Steel Research, International
1.4
$ o-620' 8 0.610 10.600. I o . 0.690
6
9
0
l
o
.
z
6 g
Fig.2 Impact depth (a) and impact velocity (b) of the liquid steel In casting speed decreasing experiment
Fig. 3 Fluctuations and surface velocity of slagsteel interfaces in casting speed increasing experiment
sing casting speed. Also, the casting speed change is illustrated in the figure. It is seen that, when casting speed increase was started at low casting level (0. 6 m min-'1 , even though the flow of the liquid steel in the mold was. disturbed, the fluctuation height of slag-steel interfaces were only slightly increased. In the subsequent casting speed increasing period, the flow in the mold became relatively stable because the v, was evenly increased and heat supply to the meniscus by the up flow of the steel was increased with increasing v,. As shown in Fig. 3, fluctuation height of slag-steel interfaces when the v, was increased to 1.0 m min-' and 1 . 2 m min-' was decreased compared with those at lower v,. During the casting speed evenly increasing period, the contents of the fluctuation height of slagsteel interfaces were between 6 - 9 mm. While,
when the casting speed increasing was stopped at 1.4 m min-' , the fluctuation height of slag-steel interfaces was remarkably increased to 11 mm. Later, when the v, was again stabilized at 1.4 m min-' , the fluctuation height of slag-steel interfaces was decreased to 9.02 mm. Similar to start decreasing v, at higher casting speed, when the casting speed increase is stopped at high casting speed level (1.4 m min-' ) , th; flow of the liquid steel in the mold is largely disturbed. The result of the mathematical simulation indicates that the flow of the liquid steel in the mold is much more largely influenced at the time when the casting speed is stopped at higher casting speed level, which corresponds well to the result of the investigation on the effect of the casting speed change on the non-metallic inclusions in subsurface layers of the slabsC5'. This sudden interfere enhances the level fluctuation of the liquid steel in the mold and increases the entrapment of the mold powders by the solidified shell. So, at high casting speed level, v, changing should be avoided or be made with slower speed changing rate in continuous casting. Fig.4 shows impact depth and impact velocity of the liquid steel in the mold at the time when the casting speed increased to each v, of 0.8 m min-' , 1.0 m min-' , 1 . 2 m min-' and 1.4 m min-' in the experiment of increasing casting speed. Also, the casting speed change is illustrated in the figure. Before increasing casting speed, the casting speed was stabilized at 0.6 m min-' and the impact 1.4
*
0.200 I 2.0
4.0
6.0
8.0
10.0
Timehin
12.0
0.6
10.4 14.0
Fig. 4 Impact depth (a> and impact velocity (b) of the liquid steel in casting speed increasing experiment
Issue 8
Numerical Simulation of Influence of Casting Speed Variation on Surface Fluctuation of Molten Steel
depth at the stable v, of 0.6 m min-' was about 0. 612 m. It is seen in Fig. 4 (a) that when increasing casting speed was started and the v, was evenly increased from 0. 6 m min-' to 0.8 m min-' and 1.0 m min-' , the impact depth was slightly increased to 0. 625 m In later casting speed increasing period when v, was evenly increased further to 1 . 2 m min-' and 1 . 4 m min-' , impact depth were gradually decreased to 0. 598 m. It is seen in Fig. 4 (b) that with the change of casting speed, impact velocity was gradually increased during the increasing casting speed experiments.
4
Conclusions
1 ) When casting speed was decreased from 1 . 4 m min-' to 0. 6 m min-' , the increases of the enhanced fluctuation height and surface velocity of slag-steel interfaces were observed only at the time which was at the initial casting speed changing period. The subsequent casting speed evenly decreasing period and stopping decreasing v, at 0. 6 m min-' had almost no bad influence on non-metallic inclusion contents of slabs. 2 ) When casting speed was evenly increased from 0. 6 m min-' to 1 . 4 m min-' , the increases of the enhanced fluctuation height and surface velocity were observed only at the time as the increasing
19
v, was stopped at 1. 4 m min-'. T o start increasing v, at low casting speed level ( 0. 6 m min-' ) and evenly increasing v, to 1 . 4 m min-' had almost no bad influence on fluctuation height and surface velocity. 3) At high casting speed level (1. 4 m min-' ) , even a little change of casting speed could result in the increase of fluctuation height and surface velocity. Thus, at high casting speed, casting speed changing should be avoided or slower speed changing rate in continuous casting should be used. References : c11
c21
c31
c41
C51
Iguchi M, Yoshida J , Shimizu T. Model Study on the Entrapment of Mold Powder Into Molten Steel [J]. ISIJ International, 2000, 40(7): 691. Tanizawa K. Influence of the Steelmaking Conditions on Nonmetallic Inclusions and Product Defects CJ]. Metallurgia Italiana, 1992, 84: 17. Gupta D, Lahiri A K. Cold Model Study of the Surface Profile in a Continuous Slab Casting Model: Effect of Second Phase [J]. Metallurgical and Material Transactions, 1996. 27B(8) : 695. Jonsson L, Jonsson P. . Modeling of Fluid Flow Conditions Around the Slag/Metal Interface in a Gas-Stirred Ladle [J]. ISU International, 1996, 36(9): 1127. ZHANG Qiac-ying, WANG Xin-hua. Influence of Casting Speed Variation During Unsteady Continuous Casting on NonMetallic Inclusions in IF Steel Slabs [J]. ISIJ International, 2006, 46(10): 1421.