JPH05154632A - Method for controlling level in casting nozzle - Google Patents

Abstract

PURPOSE:To prevent the entrapment of powder into a mold by changing Ar gas flow rate to control molten steel level in a nozzle. CONSTITUTION:At the time of pouring the molten steel 14 from a tundish 13 into the continuous casting mold 15 through the casting nozzle 3, the molten steel level (h1) in the nozzle 3 is estimated from the density and the pouring rate of the molten, steel 14 and the pressure in the casting nozzle 3. The pressure in the nozzle is controlled so that the value becomes the prescribed value obtd. from discharging flow speed V0 of the molten steel assumed with molten steel surface level in the tundish 13 as the supplying side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a level control method in a casting nozzle for supplying molten steel from a tundish to a mold in a continuous casting machine.

[0002]

2. Description of the Related Art For controlling the flow rate of molten steel in a continuous casting machine, it is common to use a sliding nozzle (hereinafter abbreviated as SN) as an operating end in order to realize highly accurate position control.

The SN is used to control the molten steel level in the mold to a predetermined position necessary for operation by sliding the refractory plate and changing the opening area to adjust the amount of molten steel supplied.

However, a space is formed in the dipping nozzle (hereinafter abbreviated as IN) used to prevent the molten steel from being oxidized and to stabilize the supply in order to slide refractories having a thickness, and Disturbance of molten steel flow such as one-sided flow (uneven flow) occurs.

For this reason, the outflow amount of IN appears as an imbalance in the outflow amount, which causes a malfunction of the powder entrainment level control in the mold.

In order to prevent clogging of the nozzle, S
Ar gas is blown from the upper part of N and the IN part, and the Ar gas is blown from here, but not all the gas blown to the upper part of SN is in the molten steel in the tundish, the molten steel level rises and the molten steel flows out. As the speed increases, the amount of gas mixed in the IN increases.

For this reason, the pressure inside the IN rises and the meniscus level inside the nozzle falls, and finally it comes out as boil from the IN discharge port. Therefore, the powder that floats on the molten steel in the mold is caught up (Fig. 1).

However, the molten steel level in the IN is important for quality control, but since the IN is made of refractory material, the inside state cannot be seen from the outside.

There are examples in which a temperature measuring element such as a thermocouple is attached to the outer wall of IN in order to estimate the internal level, or the molten steel level inside is investigated by using X-rays.

[0010]

If a temperature measuring element such as a thermocouple is used to estimate the molten steel level inside the IN, a plurality of temperature measuring elements must be installed for position estimation. It becomes impossible to measure when it becomes lower than the molten steel level in the mold.

Further, since there is an operator, it is not preferable to use radiation such as X-rays, and similarly to the method using the temperature measuring element, the level below the molten steel level in the mold cannot be measured.

[0012]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention, when injecting molten steel from a tundish with gas into a continuous casting mold through a casting nozzle, the density, the outflow amount and the casting of the molten steel. The molten steel level in the nozzle is estimated from the pressure in the nozzle, and the pressure in the nozzle is manipulated so that this value becomes a predetermined value obtained from the pouring flow velocity of the liquid estimated by the liquid surface position of the tundish on the supply side. It is a level control method in a casting nozzle characterized by the above.

[0013]

The present invention provides a method for solving these problems.

If the inside of the IN is a vacuum, the molten steel rises by about 1.3 m under atmospheric pressure, so the inside of the IN should be filled with the molten steel.

The reason why this is not the case is that clogging is prevented.
This is because the Ar gas blown into the upper portion of SN or IN for the floating of inclusions has a great influence.

That is, it is Ar gas that has a great influence on the molten steel level in the IN, and it is most rational to use this as an operating end for detecting the molten steel level in the nozzle and controlling it at a predetermined position. Target.

A method of estimating the molten steel level inside the nozzle based on the operation signal and the internal pressure of IN will be described below with reference to FIG.

For simplification, as shown in FIG. 2, a model is approximated by a model in which liquid is poured from the upper part into a glass container sealed with IN from the upper part and discharged from the lower part.

The atmospheric pressure is P ₀ , the pressure in the container is P ₁ , the outlet position of the liquid is h ₀ , the liquid level in the container is h ₁ , and the outflow rate of the liquid is v.
₀ , the velocity of liquid drop is v ₁ , and the density of the liquid is ρ,
The following equation holds from Bernoulli's theorem.

[0020]

[Equation 1] If h ₀ = 0,

[0021]

[Equation 2]

That is, it can be seen that the liquid surface position inside the nozzle has a term relating to the velocity of the liquid and a term relating to the pressure inside and outside.

In an actual plant, if the width of the mold is W, the thickness is D, and the drawing speed is Vc, the outflow amount Q _out is Q _out = Vc · D · W

Considering that IN has two discharge ports, if the discharge port area is s, the flow velocity v ₀ from the discharge port can be expressed as follows (when the number of discharge ports is two or more, it is expressed by 2 is only the number of outlets). v ₀ = Q _out / 2s = (Vc · D · W) / 2s

If the drawing speed is constant, v₀,
v₁Assuming that the nozzle cross-sectional area is S, the ratio of₀= 2s · v₁Than v₁/ V₀= S / 2s Therefore h₁= 1/2 [{(Vc · D · W) / 2s}² -{(Vc
・ D ・ W) / 2s}²・ (S / 2s)²] / G + (P ₀−
P₁) / Ρg = Vc²・ W²・ D²・ (4s²-S²) / 32
s^Four/ G + (P ₀-P₁) / Ρg

Since all variables except P ₁ are known,
The molten steel level h ₁ in the nozzle can be estimated by measuring the pressure in IN.

Next, a method for controlling the molten steel level in the nozzle using this value will be described with reference to FIG.

Assuming that the molten steel level in the tundish (distance from the molten steel surface to the SN outlet) is H, the distance from the SN outlet to the upper end of the mold is l ₁ and the molten steel level in the mold is l ₀ , Bernoulli's theorem is used as described above. Using molten steel flow velocity v ₀ at SN
Can be asked.

However, since the cross-sectional area of the tundish is sufficiently larger than the SN opening area, the flow velocity v is approximately

[0030]

[Equation 3] k: Outflow resistance correction coefficient (approx. 0.8).

If the internal pressure of the nozzle is atmospheric pressure, the molten steel level in the nozzle balances at the same position as the molten steel level in the mold, so the reference value of the molten steel level in the mold should be taken at the molten steel level position in the mold. Therefore, the falling distance of the molten steel is calculated from the above-mentioned formula h ₁ by the molten steel drop distance from SN = l ₀ + l ₁ −h ₁

[0032]

[Equation 4] The velocity v when reaching the molten steel surface in IN is

[0033]

[Equation 5] Becomes

The smaller the value, the weaker the flow velocity at the IN discharge port, which is desirable. However, if the Ar flow rate is reduced indefinitely, the effect of flowing Ar is lost and nozzle clogging progresses.

That is, it is desirable that the Ar flow rate is set to a level at which the nozzle clogging does not occur and the molten steel velocity when the molten steel level inside the IN is reached can be kept to a minimum.

Since the clogging can be evaluated from the imbalance between the inflow amount and the outflow amount of the molten steel into the mold, for example, when the SN opening area (calculated from the SN opening) is A, the inflow amount into the mold is Q _in : _in = A · v ₀ . If the index x of clogging is defined from the ratio of this and the above-mentioned Q, x = Q _out / Q _in · 100 (%) is obtained.

As a result of various experiments, an appropriate range of the index x is about 80 to 100%, and it is known that if the index x is 70% or less, clogging is likely to occur and quality is badly affected.

Therefore, the lower limit value is the flow rate at which x is 70% or less, and the upper limit value of the flow rate is the point at which the molten steel level in IN is the same as the molten steel level in the mold.

The molten steel level in the IN, which is necessary to alleviate the flow rate imbalance at the IN discharge port, varies with the discharge angle of the nozzle, the viscosity of the molten steel, the nozzle size, etc., so it is difficult to determine by a mathematical expression. Coefficient f (about 1.0 to 1.2) that determines the target value of the flow velocity of molten steel inside
Is set (the appropriate value is obtained by experiment), and v is f · v ₀
The Ar flow rate is changed so as to coincide with.

[0040]

EXAMPLES The results of tests conducted in an actual plant are shown in FIG.

Pulling speed Vc = 1.48 (m / mi
n), slab width W = 1800 (mm), thickness D = 280
(Mm), IN cross section S = 4²・ Π (cm²), IN discharge
Mouth cross section s = 7²(Cm²), Molten steel density ρ = 8 (g / cm
³), H ₁= (P ₀-P₁) /Ρg-1.99 (cm) Nozzle external pressure difference ΔP = P₀-P₁Is measured to be 300
mmH₂It was O. From now on, the level h inside the nozzle is
It is estimated to be 1.76 cm.

At this time, the molten steel level in the tundish is H
= 1000 mm, the distance from the SN exit to the top of the mold is l
₁= 300 mm, the level of the molten metal is l₀= 100 mm
Therefore, the molten steel flow velocity v on the SN exit side₁Is 3.5 m / sec
The flow velocity when reaching the molten steel level in the nozzle is
4.4 m / sec is obtained. Select the coefficient f to 1.1
If this speed is v_aim= 3.896 m / sec
It will be. h₁= V_aim/ 2g-k²H-1 ₀-L₁ More h should be 26.55 cm.

It was estimated that the target flow velocity was reached at ΔP = 2312 mmH ₂ O as the Ar gas blown into the interior was reduced.

The nozzle clogging amount x at this time was 95%, and it can be said that the operation was stable.

[0045]

INDUSTRIAL APPLICABILITY The present invention controls the internal pressure by changing the flow rate of Ar gas to the internal molten steel level so as to keep the molten steel flow velocity in the IN nozzle constant. It is effective in preventing powder entrapment in the mold.

By setting the upper limit of the flow rate to the molten steel level in the mold and the lower limit of the flow rate to the upper limit of the nozzle clogging amount, while maintaining the effect of flowing Ar gas,
The surface of the bath can be stabilized.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a relationship between an Ar flow rate, a molten steel level in a nozzle, a drift and Ar boil.

FIG. 2 is an explanatory diagram of a relationship between a liquid level in a closed container and an internal pressure.

FIG. 3 is an explanatory diagram of a molten steel level calculation method in a nozzle.

FIG. 4 is a diagram showing an online test result.

[Explanation of symbols]

 1 Ar bubbles 2 Porous 3 IN 4 Molten steel level in IN when internal pressure is low 5 Molten steel level in IN when internal pressure is high 6 Bowder layer 7 Powder particles entrained 8 Flow direction of molten steel 9 Closed container 10 Liquid level in container 11 Outlet faucet 12 Inlet side faucet 13 Tandishyu 14 Molten steel 15 Molten steel level in mold 16 Mold

Claims (1)

[Claims] 1. When injecting molten steel from a tundish with gas into a continuous casting mold through a casting nozzle, from the density of molten steel, the outflow amount and the pressure in the casting nozzle,
Casting characterized in that the molten steel level in the nozzle is estimated and the pressure in the nozzle is manipulated so that this value becomes a predetermined value obtained from the pouring flow velocity of the liquid estimated by the liquid surface position of the tundish on the supply side. Nozzle level control method.