JPS6316841A - Control method for molten surface fluctuation in continuous casting - Google Patents

Abstract

PURPOSE:To optimize the fluctuation of molten surface and to obtain high quality of a cast slab by correcting a fluctuation index operated from flowing data of molten metal into a mold from a submerged nozzle by the fluctuation of molten surface caused by change of the nozzle and adjusting as giving an electromagnetic force in the prescribed range. CONSTITUTION:The fluctuation index R is operated 12 by the equation, which uses Q for a discharging flow quantity of molten metal 5 flowed in the mold 2 from the submerged nozzle 1, V for a colliding velocity against the inner wall of the mold 2, theta for a colliding angle, D for a depth of colliding position from the molten surface 3 and delta for the molten metal density. Further, the fluctuation of molten surface caused by a change with the passage of time of the nozzle 1 with time is detected by a vortex range finder 20, 21, to correct the operation 12 by a changing device. The molten surface is so adjusted 11 by the electromagnetic giving means 10 that the compensated fluctuation index R exists within the range of the prescribed values 1-10. The molten surface in the mold 2 is always fluctuated under the optimum condition in spite of the change of nozzle 1, to obtain the cast slab having high quality.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention provides a method for continuously casting steel, etc., in which the molten steel in the tundish is immersed in the molten steel in the mold. It is injected into the mold through a submerged nozzle. The discharge port of the immersion nozzle is inclined with respect to the axial direction of the nozzle body, and molten steel is discharged from this discharge port. On the other hand, powder that keeps the molten steel in the mold warm is floating on the surface of the molten metal in the mold, and when this powder melts, it is interposed between the mold and the solidified shell and has the effect of creating m* between the two.

[Problems to be Solved by the Invention] However, when the molten steel flow from the tundish causes a so-called "win" phenomenon in which the molten metal level in the mold fluctuates, the powder on the molten metal surface mixes into the molten steel in the mold and is trapped at the solidification interface of the slab and causes inclusion defects in the slab. On the other hand, if the molten metal level in the mold is too quiet, problems arise, such as the molten slag between the mold and the slab not being able to move smoothly.

This invention has been made in view of the above circumstances, and aims to manufacture high-quality slabs by controlling fluctuations in the level of the molten metal in the mold within a predetermined range with high precision. Means for controlling] A method for controlling scene fluctuations in continuous casting according to the present invention is a method for controlling scene fluctuations in continuous casting in which molten metal is injected into a mold from a molten metal container through an immersed nozzle. Flow! As a function of IQ, the collision speed V and collision angle θ when the molten metal flow collides with the inner wall of the mold, and the collision depth D from the molten metal surface at the position where the molten metal flow collides with the mold inner wall, the following equation (1) is calculated. The fluctuation index R (
Q.

■, θ, D), a detection step to detect scene variations, and based on the scene variation detection results, the hot water level variation conforms to the predetermined relationship between the variation index and scene variation. a changing step of changing at least one casting condition so that the casting condition is changed, and an adjusting step of adjusting the scene fluctuation to a predetermined value by applying an electromagnetic force in the flow direction of the molten metal flow from the immersion nozzle or in the opposite direction. It is characterized by

R=, OQv (1-sinθ)/(4D) ・(
1) However, ρ: molten metal degree (direction/13), Q: molten metal flow I (■3/sec), V: molten metal collision velocity (-7 sec), θ: molten metal collision angle, D: molten metal Impact depth -).

[Function] The fluctuation index R, which includes the molten metal flow rate Q corresponding to the weight of the molten metal and the collision speed V, is a variable corresponding to the momentum of the molten metal, and the fluctuation of the molten metal level can be estimated with high accuracy by this fluctuation index. be able to. Therefore, there is a range of the fluctuation index R such that the amount of fluctuation in the hot water level falls within a predetermined range. For this reason, so that the fluctuation index R falls within this predetermined range (for example, 1 to 10),
By applying electromagnetic force to the molten metal, scene fluctuations can be controlled within a predetermined range. However, as casting progresses, the shape of the immersion nozzle changes due to, for example, melting damage. Then, the relationship between the fluctuation index and the hot water level fluctuation deviates from a predetermined state. Therefore, the scene variation is actually measured using an eddy current distance meter, etc., and the immersion depth of the immersion nozzle or, if applicable, the immersion nozzle is adjusted so that the scene variation corresponds to the variation index R. Change the casting conditions such as the amount of Ar gas blown or the strength of electromagnetic stirring in the mold. Therefore, in this invention,
Despite changes over time in casting conditions such as erosion of the immersion nozzle, the relationship between the scene fluctuation and the fluctuation index can always be maintained at a predetermined relationship, and based on this relationship, it is possible to maintain the relationship between the scene fluctuation and the fluctuation index. By adjusting the electromagnetic force, scene fluctuations can be controlled within a predetermined range.

[Example] The inventors of the present application have conducted extensive research and experiments on the factors that control the fluctuations in the mold surface. As a result, the present inventors have found that when the molten metal flow discharged from the immersion nozzle collides with the mold inner wall and branches upward and downward. In addition, we came to the conclusion that the momentum of the upward flow of molten steel toward the molten metal surface has a large effect on scene fluctuations. The fluctuation index R(Q, v,
θ, D) can be expressed by the following equation (1).

R-ρQV(1-sinθ)/(4D)-(
1) However, ρ: Molten steel density (direction/1), Q: Molten steel flow (-3/sec), ■ = Collision speed of molten steel (m/sec), □θ: Collision angle of molten steel, D = Collision angle of molten steel Impact depth ai).

Figure 1 shows factors indicating each molten steel flow condition.

The immersion nozzle 1 is immersed in the molten steel 3 in the mold 2,
Powder 4 is floating on this molten steel scene. In this case, the trajectory of the center of the molten steel flow is indicated by an arrow 5, and the molten steel flows from the discharge port of the immersion nozzle 1 toward the inner wall of the mold almost along a quadratic curve.

The collision angle θ is expressed as the angle between the flow direction of the molten steel when the molten steel collides with the inner wall of the mold and the direction perpendicular to the inner wall of the mold 2. The collision depth D is the distance between the point at which the molten steel collides with the inner wall of the mold and the molten steel surface.

The immersion nozzle 1 usually has two discharge ports, and in this case, the injection flow rate of molten steel discharged from each discharge port is Q/2. Further, if the velocity before the collision (collision velocity) is V, the momentum of the molten steel flow at the time of collision is expressed as ρQV/2. The flow of molten steel after the collision is upward (1-sinθ)/2 and downward (1
+sin θ)/2. Therefore, the momentum of the upward molten steel flow after the II thrust is (ρQ/2)(1
/2)v(1-sinθ). It is thought that the momentum possessed at the time of this collision is attenuated by the time the molten steel flow rises and reaches the molten metal surface. Therefore, the momentum retained by the molten steel flow when it reaches the molten metal surface is considered to be 1/D (usually n is approximately 1) of the momentum retained at the time of collision. Therefore, the upward flow of molten steel in the mold has a motion expressed by the following equation (1) on its wetted surface.

R-ρQV (1-sinθ)/(4D) ・(1
) In this equation (1), the density ρ of the molten steel may be input as a constant, and the flow IQ is determined by the size of the cast slab and the casting speed. On the other hand, the collision angle θ and the collision depth D can be determined from the locus of the flow of molten steel from the discharge port of the submerged nozzle 1. If the direction from the center of the immersion nozzle 1 toward the inner wall of the mold is taken as the X-axis, and the direction from the molten steel discharge port downward is taken as the y-axis, this trajectory can be approximately expressed by the regression equation shown in equation (2) below. Can be done.

V= (bx +at α)GI X2- (b2+8
2 cN02X - (2) However, α is the inclination angle of the discharge port of the immersion nozzle (downward is positive),
al, G2, bl, and b2 are constants determined by the shape of the immersion nozzle. In order to prevent inclusions in the molten steel from adhering to the inner wall of the immersion nozzle, gas such as Ar gas is sometimes blown into the inside of the immersion nozzle. Affect trajectory. The amount of this blown gas is G in the equation (2) above.
GI and G2, which are expressed by the following equation (3).

Gl =exp [-Ct (Qsinθt)-=
QtlG213XIl [-02(Qslnθt)
-"Qt]...(G3 However, G1 is the direction (actual discharge angle) of the molten steel flow immediately after being discharged from the immersion nozzle, and Ql is the amount of gas blown into the immersion nozzle. Also, C1, C2, ml, m2
is a constant determined by the immersion nozzle. This G1 can be expressed by the following equation (4), for example.

θs −−tan −(dy/dx), (x−o,
i)...(4) Regression equation (2) is calculated by calculating the flow trajectory using a drain water model experiment that simulates the flow of molten steel in continuous casting for various α, Q1θ1, and Ql, and then using the data as a regression model. 1
It can be obtained by performing calculations. This regression equation differs depending on the shape of the immersion nozzle. That is, as shown in FIG. 2, the immersion nozzle includes a pool type in which molten steel is discharged downwardly at an angle with respect to the axial direction of the nozzle. Also,
The shape of the discharge port may be circular or square, and the angle of inclination thereof also differs. Since the flow locus of the discharged molten steel flow differs due to the different types of immersion nozzles as described above, it is necessary to obtain a regression equation for the flow locus for each nozzle shape. The constants in equations (2) and (3) above are as follows for a submerged nozzle having a pool-shaped circular hole.

Q-0,005~0.012 Theory 3/sec Q1=0~
3.3X10-4 m3/sec at-0,003 a2=0.01466 bl =0. 1 779 b2-0.2684 C2-0,3551 m2-1. 2739 The flow of dissolution and depletion observed in the water model experiment was recorded with a video camera, and a plot of the flow trajectory determined based on the recorded results and an example of its regression equation are shown in FIG. In Figure 4, the horizontal axis is the horizontal distance X from the center of the immersion nozzle.
, and the vertical axis is the depth from the molten steel discharge starting point. Fourth
The figure shows the case where the amount of molten Si is 4.98t-27min. In this figure, (a), (b) and (C) are respectively,
The inclination angle α of the discharge port of the immersion nozzle is -15°, -35°
and -45°. As is clear from these graphs, the regression equation and the experimental data correspond well, and the variation in the experimental data from the regression equation is small. Therefore, a regression equation (regression curve) like this is created for each nozzle.
By determining , it is possible to estimate the flow trajectory of the molten steel discharged from the immersion nozzle. In other words, since the distance from the center of each immersion nozzle to the mold wall is 1/2 of the width W of the casting cross section of the mold, if X is set to (1/2) W and substituted into equation (2) above, collision The depth D can be determined as the value of y at that time, and the collision angle θ can be determined from the following equation (5).

θ=-jan” (dy/dx), (x=w
/2)...(5) Figures 5 (a) and (b) show that the molten S** is 3.65 tons/
FIG. 3 is a graph showing the influence of the amount of gas blown into the immersion nozzle on the flow trajectory of the molten metal when the inclination angle of the discharge port of the immersion nozzle is 115°;
is the case of −35″.As is clear from this figure,
As the amount of gas blown increases, the collision angle θ becomes smaller and the collision depth D becomes shallower. For this reason, the above (1)
As is clear from the equation, as the amount of gas blown increases,
It is estimated that the fluctuation index R, which indicates the fluctuation of the hot water level, becomes larger and the fluctuation of the hot water level becomes more severe.

In other words, by adjusting the amount of gas blown, the fluctuations in the hot water level can be adjusted to some extent.

Next, a regression equation for the flow velocity V will be explained. The flow velocity V of the discharge flow from the immersion nozzle can be expressed by the following equation (6).

v= (dfQ/ (608)), ・(6)
However, S: Internal cross-sectional area of the immersed nozzle (-2) L: Distance from the discharge port along the flow trajectory all (1) k: Constant determined by the shape of the immersed nozzle (0.4 to 0.7) d, f: Constants determined by the immersion nozzle However, in the case of a pool-type immersion nozzle with a circular discharge port, d is 0.01703 and f is 0.0915.
It is 2.

The regression equation for this velocity can also be determined from the observation results of water model experiments. FIG. 6 shows the curve of the regression equation thus obtained together with the data obtained from the water model experiment. As is clear from FIG. 6, the collision velocity V can be determined by the equation (6).

As described above, with each casting condition factor below, the collision angle θ
, the collision depth D and the collision velocity V are determined.

Then, by substituting this data into the above equation (1), a fluctuation index R can be calculated, and a fluctuation in the molten steel level in the mold can be estimated based on the magnitude of this fluctuation index R. FIG. 7 is a graph showing the relationship between the fluctuation index R on the horizontal axis and the fluctuation in the hot water level determined by the water model experiment on the vertical axis. As is clear from this figure, there is an extremely strong correlation between the fluctuation index R and the amount of mold level fluctuation.
The amount of fluctuation in the hot water level can be estimated using the high II jet. Since it is necessary for the quality of the slab to have the amount of fluctuation in the melt level in the range of 1 to 7+m, the fluctuation index R needs to be in the range of 1 to 10, preferably in the range of 2PJ to 7.

FIGS. 8 and 9 are diagrams showing the state of implementation of the invention, with FIG. 8 being a longitudinal sectional view of the mold, and FIG. 9 being a plan view of the mold. A slab continuous casting mold 2 having a rectangular casting cross section is provided with an immersion nozzle 1 at its center.
A discharge port formed at the bottom of the mold is immersed in the mold self-melting steel 3. Powder 4 is floating on the molten steel 3, and the molten steel 3
It is designed to keep warm. The molten steel 3 is cooled by the mold 2 to form a solidified shell 6, and the slab containing unsolidified molten steel surrounded by the solidified shell 6 is continuously drawn from the mold.

Two pairs of electromagnetic force applying devices 10 are installed on the longer sides of the mold 2 so as to sandwich the mold 2 therebetween. This electromagnetic force adjusting device 10 applies a force F or F' indicated by an internal arrow to the molten steel flow (indicated by an arrow 5) in the mold 2. The force F acts in a direction opposing the flow direction of the molten steel, and acts as a braking force on the flow of the molten steel. On the other hand, the force F' acts in the direction of flow of the molten steel and acts as an accelerating force on the flow of the molten steel. The direction and magnitude of the force (F or F') in this electromagnetic force applying device 10 are adjusted by an adjusting device 11. The calculation device 12 calculates the variation index R as described above, and applies electromagnetic force to the molten steel via the electromagnetic force application device 10 so that the variation index R falls within the range of 1 to 10. Above the mold self-melting steel surface, there is a 14m distance meter 20.
is installed, and this eddy current distance meter 20 measures bloodletting fluctuations and outputs the measurement results to the change device 13.

In the embodiment of the present invention, the variation index R changes when the handling speed of the slab is changed or the width of the casting cross section of the mold is changed during the continuous casting operation. For example, when changing the width, the discharge angle of the immersion nozzle does not change, so assuming that the slab drawing speed is constant,
When the width of the mold is narrowed, the molten steel flow IQ decreases and the collision position becomes shallower. On the other hand, since the distance between the immersion nozzle and the mold wall decreases, the impact velocity (2) increases. As a result, the variation index R changes compared to before the width change is performed.

In this case, the fluctuation index R is 10. Preferably, when it exceeds 7, the fluctuation in the scene becomes large, so a braking force F is applied to the molten steel flow in order to reduce the collision speed V of the molten steel and the fluctuation index R.

That is, the calculation device 12 calculates the fluctuation index R, and the adjustment device 11 calculates the braking force F necessary for the fluctuation index R to become 10 or 7 or less. Then, the electromagnetic force applying device 10 applies an electromagnetic force F in a direction opposite to the flow direction of the molten steel, thereby applying a braking force to the molten steel and suppressing scene fluctuations.

On the other hand, when the fluctuation index R becomes less than 1 or 3, the electromagnetic force applying device @10 applies an electromagnetic force to the molten steel flow in the direction of the flow, thereby applying an accelerating force to the molten steel. This results in
The stirring force at the corners of the slab is strengthened, reducing slab defects caused by powder.

By the way, as the casting progresses, the immersion nozzle gets melted and damaged.
The size and shape of the outlet vary. In this case, the discharge flow rate, collision speed, collision angle, collision depth, etc. of molten steel will change from the initial state of casting, and the fluctuation index R
changes, and the hot water level changes as shown in Fig. 7. Therefore, in order to control the scene fluctuations within a predetermined range, electromagnetic force is applied based on the relationship shown in Figure 7. Even if the variation index R is changed by adjusting the electromagnetic force of 10, the scene variation will actually deviate from its target range.

Therefore, in this embodiment, the scene change is actually measured by the eddy current distance meter 20, and the detection result is output to the change device 13. The calculation device 12 calculates the fluctuation index R of the casting conditions at that time.
Change the scene changes in! The changing device 113 compares the actual value of the scene variation detected by the eddy current distance meter 20 with the scene variation in the variation index R, and outputs a signal to change the casting conditions so that they match. ,
For example, it is output to a process computer controlling the casting operation. Such casting conditions to be changed include the immersion depth of the immersion nozzle, the amount of Ar gas or the like blown into the immersion nozzle, and the intensity of electromagnetic stirring within the mold.

By changing these casting conditions, the melt level fluctuation corresponding to the fluctuation index R at that time matches the scene fluctuation actually measured by the eddy current distance meter 20. Therefore, after the casting conditions are changed, the fluctuation index R and the melt level fluctuation correspond to each other in the correlation shown in FIG. 7, and the adjustment of the scene fluctuation by the adjustment device 11 can be carried out in degrees am. Therefore, according to this embodiment, the situation of self-melting steel in the mold can always be controlled within a predetermined range with high precision.

If a sliding nozzle is used to control the amount of molten steel injected into the mold from the tundish, the flow rate of the molten steel discharged from the two discharge ports of the submerged nozzle may be mutually controlled within the mold. There may be a difference in the When such a drift occurs, the fluctuation of the melt level differs between the left and right sides of the immersion nozzle, and it is difficult to uniquely estimate it only by the fluctuation index R. However, if eddy current distance meters 20 and 21 are installed on the left and right sides of the immersion nozzle 1 as shown in Fig. 8, and the fluctuations in the hot water level are measured by the pair of eddy current distance meters, the fluctuations in the hot water level will be out of the appropriate range. It is possible to estimate the hot water level fluctuation from the fluctuation index R by using the actual measured value of the hot water level fluctuation as a correction term, such as adjusting the fluctuation index R according to the change.

[Effects of the Invention] According to the present invention, in continuous casting, it is possible to estimate with high accuracy the fluctuation in the melt level caused by the flow of the molten metal in the mold using the fluctuation index R that includes the momentum of the molten metal flow. and,
Electromagnetic force is applied to the molten steel so that this fluctuation index R falls within a predetermined range. That is, when the fluctuation index R is large, a braking force is applied to the flow of molten steel to reduce the fluctuation index R. On the other hand, when the fluctuation index R is small, an accelerating force is applied to the molten steel flow to increase the fluctuation index R. In addition, since we are actually measuring the fluctuations in the melt level, we are measuring 8 as the casting progresses! Even if the submerged nozzle is damaged by erosion, the relationship between the fluctuation index R and the fluctuation in the hot water level can be corrected from the actual measurement value, and the fluctuation in the hot water level can always be controlled within a predetermined range with high precision. High quality slabs can be manufactured.

[Brief explanation of drawings]

Fig. 1 is a diagram explaining the variation index R, Figs. 2 and 3 are cross-sectional views showing the immersion nozzle, and Figs. 4(a) to 4(C).
The figure is a graph showing the validity of the regression curve of the flow trajectory.
Figures (a) and 5(b) are graphs showing the influence of gas injection into the submerged nozzle, Figure 6 is a graph showing the validity of the velocity regression curve, and Figure 7 is the variation index R. Graphs illustrating the relationship between molten metal and hot water level fluctuations, and FIGS. 8 and 9 are diagrams illustrating the state of implementation of the present invention. 1: Immersion nozzle, 2; Mold, 3; Molten steel, 4; Slag, 6
: Solidified shell, 10: Electromagnetic force application device, 11: Adjustment device, 1
2; Arithmetic device, 13: Modification device, 20.21: Eddy current distance meter Applicant's agent, Patent attorney Takehiko Suzue Figure 2 Figure 3 Insect)... 03♀17')O U) '') Ant 5A Kenpeki C) Issumi...- δ89!LoO Moejuku-5"cr rhyme r/ε ηN 2 no 8 Figure 10 15 Dullity

Claims (1)

[Claims] In a method for controlling molten metal level fluctuation in continuous casting in which molten metal is injected into a mold from a molten metal container through an immersion nozzle, the flow rate Q of molten metal discharged from the immersion nozzle causes the molten metal flow to collide with the inner wall of the mold. As a function of the collision speed V and collision angle θ at the time of collision, and the collision depth D from the molten metal surface at the position where the molten metal flow collides with the inner wall of the mold, the variation index R (Q,
V, θ, D), a detection step to detect the hot water level fluctuation, and a hot water level fluctuation that is calculated in advance between the fluctuation index and the hot water level fluctuation based on the detection result of the hot water level fluctuation. a change step of changing at least one casting condition to match the relationship; and an adjustment step of adjusting the molten metal level fluctuation to a predetermined value by applying electromagnetic force in the flow direction of the molten metal flow from the immersion nozzle or in the opposite direction. A method for controlling melt level fluctuation in continuous casting, comprising: R-ρQV(1-sinθ)/(4D)...(1) However, ρ: Molten metal density (kg/m^3), Q: Molten metal flow rate (m^3/sec), V: Molten metal collision speed (m/sec), θ: Collision angle of molten metal, D: Collision depth of molten metal (m).