JPH0818118B2 - Control method of fluctuation of molten metal level in continuous casting - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for controlling a molten metal level fluctuation in which a molten metal level fluctuation in a mold is estimated by a casting condition factor in continuous casting of steel and the like.

[Prior Art] In continuous casting of steel, molten steel in a tundish is poured into a mold through an immersion nozzle immersed in the molten steel in the mold. 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, a powder that keeps the molten steel in the mold warm floats on the surface of the molten metal in the mold, and when the powder melts, it intervenes between the mold and the solidified shell and has a function of lubricating the two.

[Problems to be Solved by the Invention] However, when the molten steel flow from the tundish causes a so-called molten metal rampage phenomenon in which the molten metal surface in the mold fluctuates, powder on the molten metal surface mixes into the molten steel in the mold and It is trapped at the solidification interface inside and causes inclusion defects in the slab. On the other hand, even if the level of the molten metal in the mold is too quiet, there arises a problem that, for example, lubrication by molten slag between the mold and the slab is not smoothly performed.

The present invention has been made in view of the above circumstances, and it is possible to manufacture a high-quality slab by controlling the fluctuation of the molten metal surface in the mold within a predetermined range, and the fluctuation of the molten metal surface in continuous casting. It aims at providing the control method of.

[Means for Solving the Problems] A method for controlling the fluctuation of the molten metal surface in continuous casting according to the present invention is a method for controlling the fluctuation of molten metal surface in continuous casting for thick slabs in which molten metal is injected into a mold from a molten metal container through an immersion nozzle. In the control method, the discharge flow rate Q of the molten metal from the immersion nozzle, the collision velocity v and the collision velocity θ when the molten metal flow collides with the inner wall of the mold, and the molten metal flow from the position where the molten metal collides with the inner wall of the mold. The cross-sectional size of the slab to be cast such that the variation index R (Q, v, θ, D) obtained by using the following mathematical expression (1) as a function of the collision depth D falls within the range of 1 to 10. After determining, the shape of the immersion nozzle is selected, the discharge flow rate Q of the molten metal from the immersion nozzle is controlled, and the amount of gas blown into the immersion nozzle is controlled. .

R = ρQv (1-sinθ) / 4D (1) where ρ: molten metal density (kg / m ³ ), Q: molten metal flow rate (m ³ / sec), v: molten metal collision speed, θ: molten metal collision Angle, D: Collision depth of molten metal (m).

When the jet velocity immediately before the jet from the immersion nozzle collides with the inner wall of the short side of the mold is v, the mass of one of the two jets is m ₀ (= ρQ / 2), and the collision angle of the jet is θ.
The momentum of one jet is m ₀ v (= ρQv / 2), and this vertical component is m ₀ vsinθ. Further, immediately after the jet collides with the inside wall of the mold short side is jet was 2 Tsuniwaka mass m _d toward the mass m _u and downward upward along the inner wall, each of the speed collision just before the velocity v Therefore,
The upward momentum of the molten metal flow is m _u v, and the downward momentum of the molten metal flow is m _d v. The following equation holds from the law of conservation of momentum in the vertical direction.

m ₀ vsin θ = m _d v − m _u v The following formula is derived by eliminating v from both sides of the above formula.

m ₀ sin θ = m _{d −} m _{u Further} , the following equation holds from the law of conservation of mass before and after the impingement of the jet on the inner wall of the short side of the mold.

m ₀ = m _u + m _d Therefore, when the upper mass m _u and the lower mass m _d are obtained from these relationships, respectively, the following relationships are obtained.

m _u = m ₀ · (1-sin θ) / 2 = ρQ (1-sin θ) / 4 m _d = m ₀ · (1 + sin θ) / 2 = ρQ (1 + sin θ) / 4 The mass m _{u of the} molten metal stream that is distributed upward that affects

Next, the momentum (m _u v) of the upper mass m _u is attenuated before reaching the vicinity of the molten metal surface and shaking the molten metal surface, so the damping coefficient is obtained. The collision velocity v of the molten metal flow at the collision point of the inner wall of the mold is attenuated by the frictional force received from the inner wall of the mold until reaching the molten metal surface, and becomes the velocity v _s of the molten metal flow on the molten metal surface.
When estimating the degree of damping, the width of the short side cannot be ignored compared to the width of the long side in a continuous casting machine for thick-walled slabs, so the "velocity damping model of radial wall jet" is applied. In this model, there is a relationship of v _s ∝1 / D, and n = 1
Becomes Multiplying the upper mass m _u by 1 / D as the damping coefficient,
The variation index R (Q, v, θ, D) is obtained as the above-mentioned mathematical expression (1) as a concept introduced to estimate the degree of level variation.

Furthermore, the actual melt level fluctuation is detected so that this fluctuation index R falls within the range of 1 to 10, and based on the detection result, the immersion depth of the immersion nozzle, the amount of gas blown into the immersion nozzle,
At least one of the electromagnetic stirring strength to the molten metal in the mold is changed. This reduces the momentum of the vertically upper component of the molten metal flow and suppresses fluctuations in the molten metal surface.

[Examples] The inventors of the present application conducted extensive research into factors governing the fluctuation of the molten metal surface in the mold, and as a result, when the molten metal flow discharged from the immersion nozzle collided with the inner wall of the mold and branched upward and downward. In addition, it was thought that the momentum of the rising flow of molten steel toward the molten metal surface had a great influence on the fluctuation of the molten metal surface. The variation index R (Q, v, θ, corresponding to the momentum of such molten steel flow
Examples of D) include those represented by the following formula (1).

R = ρQv (1-sinθ) / (4D) (1) where ρ: molten steel density (kg / m ³ ), Q: molten steel flow rate (m ³ / sec), v: collision velocity of molten steel (m / sec) ), Θ: collision angle of molten steel, D: collision depth of molten steel (m).

The factors showing the molten steel flow conditions are shown in FIG. The immersion nozzle 1 is immersed in the molten steel 3 in the mold 2, and the powder 4 floats on the molten steel surface of the molten steel. In this case, the locus of the center of the molten steel flow is shown by the arrow 5, and the molten steel flows from the discharge port of the immersion nozzle 1 toward the inner wall of the mold along a quadratic curve. The collision angle θ is expressed as an angle formed by the flow direction of the molten steel when the molten steel collides with the inner wall of the mold and the direction orthogonal to the inner wall of the mold 2. The collision depth D is the distance between the position where this molten steel collides with the inner wall of the mold and the molten steel surface.

The dipping nozzle 1 normally has two discharge ports, but in this case, the injection flow rate of the molten steel discharged from each discharge port is Q / 2. When the velocity before collision (collision velocity) is v, the momentum of the molten steel flow at the time of collision is ρQv / 2. The molten steel flow after the collision is (1-sin θ) / 2 upwards and (1 + sin θ) / 2 downwards.
Is distributed. Therefore, the momentum of the upward molten steel flow after the collision is expressed as (ρQ / 2) (1/2) v (1-sin θ). It is considered that the momentum held at the time of the collision is attenuated by the time when the molten steel flow rises and reaches the molten metal surface. Therefore, it is considered that the momentum retained when the molten steel flow reaches the molten metal surface is 1 / D ⁿ (usually n is about 1) of the momentum retained at the time of collision. Therefore, the ascending flow of the molten steel in the mold has the momentum shown by the following equation (1) on the molten metal 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 rate Q is determined by the size of the mold slab and the casting speed. On the other hand, the collision angle θ and the collision depth D can be obtained from the trajectory of molten steel flow from the discharge port of the immersion nozzle 1. This locus can be approximately expressed by the regression equation shown in the following equation (2), where the x-axis is the direction from the center of the immersion nozzle 1 to the inner wall of the mold and the y-axis is the direction from the molten steel outlet. You can

y = (b ₁ + a ₁ α) G ₁ x ² − (b ₂ + a ₂ α) G ₂ x (2) where α is the inclination angle of the discharge port of the immersion nozzle (the downward direction is positive) , A ₁ , a ₂ , b ₁ , b ₂ are constants determined by the shape of the immersion nozzle. In order to prevent the inclusions in the molten steel from adhering to the inner wall of the immersion nozzle, Ar is placed inside the immersion nozzle.
Although gas such as gas may be blown in, the gas blown into this immersion nozzle also affects the flow trajectory of the molten steel.
The amount of this blown gas is G ₁ and G ₂ in (2) above.
G ₁ and G ₂ are represented by the following equation (3).

However, θ ₁ is the direction (substantial discharge angle) of the molten steel flow immediately after being discharged from the immersion nozzle, and Q ₁ is the amount of gas blown into the immersion nozzle. Further, c ₁ , c ₂ , m ₁ , and m ₂ are constants determined by the immersion nozzle. θ ₁ can be expressed by the following equation (4), for example.

θ ₁ = −tan ⁻ (dy / dx), (x = 0.1) (4) The regression equation (2) simulates molten steel flow in continuous casting for various α, Q, θ ₁ and Q _1. The flow trajectory can be obtained by a water model experiment, and a regression calculation can be performed based on the obtained data. This regression equation differs depending on the shape of the immersion nozzle. That is, as shown in FIG. 2, the immersion nozzle has an inverted Y-shape in which molten steel is discharged as it is downwardly inclined with respect to the axial direction of the nozzle, and as shown in FIG. There is a pool type, etc., which is discharged diagonally downward after it has fallen to. Further, there are discharge ports having a circular shape or a rectangular shape, and the inclination angles thereof are different. Since the flow path of the discharged molten steel flow differs depending on the type of the immersion nozzle, it is necessary to obtain the regression equation of the flow path for each nozzle shape. The constants of the expressions (2) and (3) are as follows for the immersion nozzle having a pool-shaped circular hole.

Q = 0.005 to 0.012 m ³ / sec Q ₁ = 0 to 3.3 × 10 ^-4 m ³ / sec a ₁ = 0.003 a ₂ ＝ 0.01466 b ₁ ＝ 0.1779 b ₂ ＝ 0.2684 c ₁ = 0 c ₂ ＝ 0.3551 m ₂ ＝ 1.2739 The flow of the molten metal observed by the water model experiment was recorded by a video camera, and a plot of the flow locus obtained based on this recording result and an example of its regression equation are shown in Fig. 4.
In FIG. 4, the horizontal axis represents the horizontal distance x from the center of the immersion nozzle, and the vertical axis represents the depth from the molten steel discharge start point. FIG. 4 shows the case where the flow rate of the molten metal is 4.98 tons / min. In FIGS. 4 (a), (b) and (c), the inclination angle α of the discharge port of the immersion nozzle is −15 ° and −35, respectively. Data for ° and −45 °. As is clear from these graphs, the regression equation and the experimental data correspond well, and the variation of the experimental data from the regression equation is small. Therefore, by obtaining such a regression equation (regression curve) for each nozzle, the flow trajectory of the molten steel discharged from the immersion nozzle can be estimated. In other words, if two tundishes are installed in each tundish, the distance from the center of each immersion nozzle to the mold wall is 1 / the width w of the casting cross section of the mold.
Therefore, by substituting x into (1/4) w into equation (2), the collision depth D can be obtained as the value of y at that time,
The collision angle θ can be calculated from the following equation (5).

^{θ = -tan - (dy / dx} ), (x = w / 4) ... (5) FIG. 5 (a) and (b), when the melt flow rate is 3.65 ton / min, the gas to the immersion nozzle It is a graph which shows the influence which a blowing amount gives to the flow path of a molten metal, (a) is the inclination angle of the discharge port of an immersion nozzle being -15 degrees, (b) is-.
This is the case of 35 °. As is clear from this figure, as the gas injection amount increases, the collision angle θ decreases and the collision depth D decreases. Therefore, as is clear from the above equation (1), it is estimated that when the gas injection amount increases, the fluctuation index R representing the fluctuation of the molten metal surface becomes large and the fluctuation of the molten metal surface becomes severe. In other words, the level fluctuation can be adjusted to some extent by adjusting the gas blowing amount.

Next, the regression equation of the flow velocity v will be described. The flow velocity v of the discharge flow from the immersion nozzle can be expressed by the following equation (6).

v = {d + fQ / (60S)} L ^-k (6) where S: inner cross sectional area of the immersion nozzle (m ² ) L; distance from the discharge port along the flow path (m) k; in the shape of the immersion nozzle Constant (0.4 to 0.7) d, f; Constant determined by immersion nozzle However, in the case of a pool type immersion nozzle having a circular discharge port, d is 0.01703 and f is 0.09152.

^{θ = -tan - {dy / dx} }, x = w / 2 ... (7) regression equation of the speed can be determined by observation with water model experiment. The curve of the regression equation thus obtained is shown in FIG. 6 together with the data obtained by the water model experiment. As is clear from FIG. 6, the collision speed v can be obtained by the above equation (6).

As described above, the collision angle θ, the collision depth D, and the collision velocity v can be obtained under each casting condition factor. Then, by substituting this data into the equation (1), the fluctuation index R can be calculated, and the fluctuation of the molten steel level in the mold can be estimated by the magnitude of the fluctuation index R. FIG. 7 is a graph showing the relationship between the fluctuation index R on the horizontal axis and the fluctuation of the molten metal 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 fluctuation amount of the molten metal surface, and the fluctuation amount R determined by the casting condition factor is lower and the fluctuation amount of the molten metal surface is highly accurate. Can be estimated. Fluctuation level 1 to 7m
Since it is necessary for the quality of the slab to be in the range of m, the variation index R must be in the range of 1 to 10, preferably 2 to 7. Therefore, after the cross-sectional size of the cast piece to be cast is determined, the dipping nozzle is selected or a casting factor such as the molten steel flow rate of casting is appropriately selected so that the fluctuation index R falls within this predetermined range.

[Effects of the Invention] According to the present invention, in continuous casting, it is possible to highly accurately estimate the fluctuation of the molten metal surface caused by the flow of the molten metal in the mold, using the fluctuation index R including the momentum of the molten metal flow. Therefore, the variation index R can be made to fall within a predetermined range by modifying an appropriate casting condition factor, and as a result, the melt level variation can always be kept within the optimum range. Can be produced.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the fluctuation index R, FIGS. 2 and 3 are cross-sectional views showing an 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 (b) are graphs showing the effect of gas injection into the immersion nozzle, Fig. 6 is a graph showing the validity of the regression curve of velocity, and Fig. 7 is a fluctuation index R. It is a graph which shows the relationship between the molten metal level change. 1; Immersion nozzle, 2; Mold, 3; Molten steel, 4; Slag

Front Page Continuation (72) Inventor Issei Issei Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Tube Co., Ltd. (72) Inventor Toru Kitagawa 1-2-1 Marunouchi Chiyoda-ku, Tokyo In-house (72) Inventor Toshio Teshima 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (1)

[Claims] 1. A method for controlling the level fluctuation of a molten metal in continuous casting for a thick slab in which the molten metal is injected from a molten metal container through a dipping nozzle into a mold, wherein the molten metal discharge flow rate Q from the dipping nozzle and the molten metal flow are Variations obtained using the following mathematical formula (1) as a function of the collision velocity v and the collision velocity θ when colliding with the inner wall, and the collision depth D from the molten metal surface where the molten metal collides with the mold inner wall. The index R (Q, v, θ, D) is 1
After the cross-sectional size of the cast piece to be cast is determined so that it falls within the range of 10 to 10, the shape of the immersion nozzle is selected, the discharge flow rate Q of the molten metal from the immersion nozzle is controlled, and the amount of gas blown into the immersion nozzle is controlled. A method for controlling fluctuations in molten metal surface in continuous casting, which is characterized by controlling the temperature. R = ρQv (1-sinθ) / 4D (1) where ρ: molten metal density (kg / m ³ ), Q: molten metal flow rate (m ³ / sec), v: molten metal collision speed, θ: molten metal collision Angle, D: Collision depth of molten metal (m).