JP2011143449A - Method for removing inclusion in tundish for continuous casting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for removing inclusions in a tundish which can satisfactorily remove inclusions even under a broad range of casting speed conditions. <P>SOLUTION: In a continuous casting tundish into which a molten metal is poured, a weir is provided between a pouring point of the molten metal and an outflow hole through which the molten metal is led out to a casting mold, and a molten metal stream is led from the pouring point side to the outflow hole side through a flow part provided in the weir. In this case, an inert gas is blown into the molten metal through the bottom of the tundish on the outflow hole side under predetermined conditions. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a tundish for relaying and supplying molten metal injected from a ladle in a continuous casting process to a mold, and includes oxide-based non-metallic inclusions in the molten metal injected into the tundish. The present invention relates to an efficient removal method.

In continuous casting of molten metal, for example, molten steel, a predetermined amount of molten steel in the ladle is supplied to the tundish through an injection nozzle (also referred to as a “long nozzle”) installed at the bottom of the ladle. While the molten steel stays, the molten steel in the tundish is distributed and injected into each mold through molten steel outflow holes installed at the bottom of the tundish to produce a slab.
In the molten steel, oxide-based non-metallic inclusions such as alumina originating from the deoxidation product (hereinafter referred to as “inclusions”) are suspended, and the inclusions are contained in the solidified layer when the molten steel solidifies. If it is taken in, it causes defects due to inclusions in the final product such as a thin steel plate. Therefore, the tundish is also required to have a function of floating and separating inclusions.

  Conventionally, in order to float and separate inclusions in the tundish, weirs have been installed in the tundish and the molten steel flow in the tundish has been controlled by this weir. Can now be manufactured. However, inclusions with a small particle size, such as 50 μm or less, are difficult to float and separate because the floating speed in molten steel is extremely slow. In recent years, high-quality materials such as bearing steel are required to have very high cleanliness, and advanced inclusion separation techniques that can remove minute inclusions are also required.

As a method for promoting the floating and separation of inclusions in the tundish, a method of blowing an inert gas such as argon into the tundish has been proposed.
That is, in Patent Document 1, it is said that the removal of inclusions can be promoted by installing a fine nozzle so that the tip protrudes at a plurality of locations in the bottom of the tundish and blowing fine inert gas.

  Further, in Patent Document 2, a mixed gas of argon and a non-oxidizing gas that can be dissolved in molten steel is blown into the molten steel in the tundish or into the molten steel injected into the tundish, and the molten steel in the tundish. Is poured into a mold. Furthermore, it is described that the mixed gas is blown from porous bricks arranged at the bottom of the tundish or the lower part of the side wall.

Japanese Patent Laid-Open No. 9-10900 Japanese Patent Laid-Open No. 2004-066335

  By the way, as for the inert gas or mixed gas, the rising flow of the molten steel is promoted as the flow rate of blowing is increased, and the fine inclusions can be lifted and removed. On the other hand, if the gas flow rate is increased too much, the molten steel bath surface in the tundish Swaying and entraining the tundish slag floating on the bath surface, causing an increase in inclusions.

  In this regard, Patent Document 1 discloses that the effect of removing inclusions and the re-entrainment of the bath surface slag are prevented by refining bubbles by blowing gas from a fine nozzle. However, if the molten steel flow velocity at the position where the inert gas is blown is small, the bubbles of inert gas are not dispersed by the molten steel flow, and the bubbles are concentrated at the upper portion of the blowing position. A very strong upward flow is generated at the upper position. Since the molten steel flow velocity varies depending on the shape of the tundish and the casting speed, bath surface slag can be involved depending on the conditions even under the same gas blowing conditions. In particular, Patent Document 1 is insufficient because it does not describe the value of the gas blowing amount with respect to the tundish shape and casting conditions. That is, in recent high-clean steel, since the casting speed is often kept low, the method using the same inert gas blowing conditions regardless of casting conditions as in Patent Document 1 sufficiently removes inclusions. Difficult to do.

  On the other hand, although the technique described in Patent Document 2 uses a non-oxidizing gas that can be dissolved in molten steel, there is a case where it is desired to avoid using a non-oxidizing gas that can be dissolved in molten steel from the viewpoint of cost. In such a case, it is extremely meaningful if inclusions can be removed only with an inert gas.

  The present invention has been made in view of the above circumstances, and the object of the present invention is to suppress the fluctuation of the bath surface when the amount of inert gas blown is increased, and the inert gas blow is sufficient. An object of the present invention is to provide a method for removing inclusions in a tundish, which can remove inclusions.

The gist configuration of the present invention is as follows.
1. In a tundish for continuous casting in which molten metal is injected, a weir is disposed between the injection point of the molten metal and an outflow hole through which the molten metal is led out to the mold, and the injection is performed through a flow section provided in the weir. When the molten metal flow is guided from the point side to the outflow hole side, an inert gas is blown into the molten metal from the bottom of the tundish on the outflow hole side of the weir under the condition satisfying the following formula (1). Inclusion removal method in tundish for continuous casting.
Record
0.00014 ≦ Ql · A / (Q2 · W) ≦ 0.00025 (1)
However,
Ql: Inert gas blowing flow rate per strand (standard state conversion Nm ³ / s)
Q2: Molten steel flow rate per strand (m ³ / s)
A: Total cross-sectional area (m ² ) of the circulation part of the weir
W: Average width of tundish at the weir position (m)

2. 2. The inclusion removal method in a continuous casting tundish according to the above 1, wherein the amount of the inert gas blown is adjusted according to a casting speed in continuous casting.

  According to the present invention, in order to optimize the amount of inert gas blown in accordance with the shape of the tundish and the flow rate of the molten steel, it is possible to intervene with the inert gas without causing entrainment of the bath surface slag under a wide casting condition. An object removal effect can be obtained. In addition, by controlling the amount of inert gas blown to an appropriate value in response to fluctuations in the casting speed, an excellent inclusion removal effect can be maintained even when the casting speed is changed during casting.

It is a figure which shows the outline of the tundish for continuous casting. It is a schematic diagram which shows the floating behavior of the molten steel in a gas blowing area | region. It is a figure which shows the shape of the other weir in the tundish for continuous casting. It is a figure which shows the definition of the average width W of a tundish. It is a figure which shows the structure of the other weir in the tundish for continuous casting. It is a figure which shows the outline of the other tundish for continuous casting from which a weir arrangement differs. It is a graph which shows the relationship between gas blowing conditions and the number of inclusion outflows to a casting_mold | template.

Hereinafter, the present invention will be described in detail with reference to the drawings.
In FIG. 1, the outline of the tundish 1 for continuous casting used for implementation of the method of this invention is shown. The continuous casting tundish 1 is, for example, a rectangular parallelepiped container having an open top surface. The molten steel 3 is supplied from a ladle (not shown) through the injection nozzle 2 and the stored molten steel 3 is provided at the bottom. In the illustrated example, the two outflow holes 4a and 4b are respectively supplied to molds (not shown).
That is, in the illustrated example, the injection nozzle 2 is inserted and disposed in the center of the tundish 1 in the width direction (longitudinal direction). If the opening position of the tip of the injection nozzle 2 at this time is the molten steel injection point P, this molten steel injection Outflow holes 4a and 4b for distributing and supplying the molten steel 3 to the mold are formed at the bottoms of the sides facing each other across the point P across the point P. Further, the molten steel injection point P and the molten steel outflow holes 4a and 4b are formed. In between, the weirs 5a and 5b which make a pair on both sides of the molten steel injection | pouring point P are installed.

  These weirs 5a and 5b are provided with a rectangular flow portion 6 in the illustrated example for guiding the molten steel 3 injected from the injection nozzle 2 to the outflow holes 4a and 4b (see FIG. 1B). Further, at the bottom between the weirs 5a and 5b and the outflow holes 4a and 4b, there are blow-in ports 8a and 8b made of porous brick for blowing the inert gas 7.

  When the molten steel 3 is continuously injected through the injection nozzle 2 into the tundish 1 having the above-described configuration, the molten steel 3 that has flowed flows into the blowing region of the inert gas 7 through the flow portions 6 of the weirs 5a and 5b. In the region where the inert gas 7 is blown, the buoyancy works as the bubbles 7 a are dispersed and raised in the molten steel 3, and an upward flow is given to the molten steel 3. At that time, inclusions in the molten steel 3, particularly minute inclusions of 50 μm or less, float on the bath surface along with the upward flow of the molten steel 3. Since the inclusion removal efficiency can be increased by promoting the floating of the inclusions, the inclusion removal efficiency becomes higher as the floating flow rate of the molten steel increases in the range where the entrainment of the bath surface slag does not occur. Since the ascending flow rate of the molten steel increases as the inert gas blowing amount increases, it is desirable to blow in the inert gas having the maximum flow rate within the range of the ascending flow rate at which fluctuation does not occur on the bath surface. Therefore, the conditions for blowing the inert gas at the maximum flow rate were studied earnestly as follows.

First, the floating behavior of molten steel in the gas blowing region is schematically shown in FIG. The levitation component Vy of the molten steel flow velocity after the molten steel has passed through the inert gas blowing region is expressed by using the buoyancy F by the inert gas and the buoyancy application time Δt,
Vy∝F · Δt (2)
It can be expressed.

Next, the buoyancy F due to the inert gas is proportional to the volume fraction of the bubbles in the molten steel. Here, since the volume fraction of bubbles can be considered constant in the gas blowing region, the buoyancy F is
F = γQ1 · ρ · g / (Vg · W · X) (3)
Where γ: expansion ratio of inert gas in molten steel
Q1: Amount of inert gas blown (standard state conversion) (Nm ³ / s)
ρ: Density of molten steel (kg / m ³ )
g: Gravitational acceleration (= 9.8 m / s ² )
Vg: Average rising speed of bubbles (m / s)
W: Average width of tundish (m)
X: Length of the inert gas blowing area in the tundish width direction (m)
It can be expressed. In addition, said (gamma), (rho), and Vg are values determined by the operating conditions of continuous casting, and the specification of a tundish installation.

On the other hand, as the buoyancy imparting time Δt, when using the horizontal velocity Vx of the molten steel near the weir and the tundish width direction length X (m) of the inert gas blowing region,
Δt = X / Vx = X · A / Q2
And the horizontal speed Vx is
Vx = Q2 / A
Here, Q2: Molten steel flow rate per strand (m ³ / s)
A: Total cross-sectional area (m ² ) of the circulation part of each weir
Because
Δt = X / Vx = X · A / Q2 (4)
It becomes.
Therefore, from the above equations (2), (3), and (4), the ascending flow velocity Vy of the molten steel is Vy∝Ql · A / (Q2 · W) (5)
It becomes.

As described above, the removal efficiency of inclusions in molten steel and the presence or absence of slag entrainment due to fluctuations in the surface of the tundish bath surface are determined by the rising flow velocity Vy, so that the inert gas flow rate Ql (m ³ / s) By adjusting the total cross-sectional area A (m ² ) of the flow part, the molten steel flow rate Q2 (m ³ / s) per strand and the average width W (m) of the tundish, the inside of the tundish when the inert gas is blown Inclusion removal capability can be maximized.

  Therefore, the value of Ql · A / (Q2 · W) was changed in various ways, and the relationship between the removal efficiency of inclusions in the molten steel and the occurrence of hot water surface fluctuation on the bath surface was investigated. The result is shown in FIG. 7 in an example described later, and when the inert gas is blown under the condition that Ql · A / (Q2 · W) exceeds 0.00025, the tundish bath surface greatly fluctuates. It will cause entrainment of bath slag and splash of molten steel. On the other hand, if the value of Ql · A / (Q2 · W) is less than 0.00014 depending on the conditions when the inert gas is blown, the floating of the molten steel is too weak and the inclusions cannot be removed sufficiently.

  In addition, let the total cross-sectional area A of the above-mentioned distribution part of molten steel be the sum total of the cross-sectional area of the part which can distribute molten steel in a weir. For example, in the case of the weir 5 having the shape shown in FIG. 3, the sum of the area of the plurality of circular flow portions 6a and the area of the rectangular flow portion 6b due to the lower end of the weir being lower than the bath surface is the cross-sectional area of the flow portion. A.

In addition, the average width W of the tundish is, as shown in FIG. 4 taking the case of the weir shown in FIG. The displayed part) is defined as a value (W = S / H) obtained by dividing the part by the molten steel depth H.
Incidentally, the side wall of the tundish does not necessarily have to be vertical. When the side wall of the tundish is inclined or the side wall is curved, the average width may be obtained from W = S / H using the total cross-sectional area S and the depth H in the same manner as described above.

  Furthermore, in order to remove inclusions stably, it is better to place the inert gas blow position as close to the weir as possible unless there is a problem of interference in terms of equipment. The distance to the gas blowing position is preferably within 1000 mm. As the inert gas, argon, nitrogen, or the like may be used alone or in combination.

  The height and width of the weir, the opening shape of the circulation part, etc. may be any shape as long as the above formula (1) is satisfied with respect to the area A of the circulation part, but if the circulation part of the weir is too large Since the flow of molten steel in the inert gas blowing region is uneven and stable floating characteristics of the molten steel may not be obtained, the area A of the flow section satisfies A / S <0.5 with respect to the total cross-sectional area S of the weir cross section. It is desirable to regulate the range.

  Furthermore, as shown in FIG. 5, when there is one outflow hole 4 of the tundish molten steel 3, the weir 5 is provided alone on the outflow hole 4 side of the injection point P, and on the outflow hole 4 side of the weir 5. What is necessary is just to inject the inert gas 7 from the blowing port 8 of the tundish bottom.

In adjusting the amount of inert gas blown in accordance with the above, the casting speed during continuous casting is monitored, the molten steel flow rate is calculated according to the casting speed, and the inert gas blown so as to satisfy the above-described equation (1). It is preferable to adjust the amount. This is because in an actual continuous casting operation, the casting speed may change due to ladle exchange or slab width change, and the flow rate of molten steel flowing into the tundish may change from moment to moment. Therefore, by changing the inert gas flow rate to the optimum value in accordance with the molten steel flow rate condition, it is possible to maintain the optimum inclusion removal condition according to the operation condition at that time.
In addition, although the method of calculating | requiring a molten steel flow volume according to a casting speed should just use a well-known method, the following methods can be utilized, for example. That is, when the casting speed of the continuous casting machine is Vc (m / s), the slab width is Wc (m), and the slab thickness is Dc (m), the molten steel flow rate Q2 (m ³ / s) per strand is Q2. = Vc x Wc x Dc
Can be calculated as

  Using two tundishes with molten steel capacities of 75t and 48t, continuous casting of the molten steel was carried out, and the presence or absence of bath surface fluctuations in the tundish and the amount of inclusions flowing into the mold were investigated. The presence or absence of bath surface fluctuation in the tundish is an observation result in this example. The amount of inclusions flowing into one mold was calculated by a numerical analysis method described later. This is because, when the amount of inclusions is obtained from the obtained slab, factors other than the inert gas are included, and the effect of the present invention becomes unclear in such measurement results.

Here, the conditions regarding the tundish were large dams 5a and 5b covering the bath surface on the side close to the injection point P as shown in FIG. Blowing ports 8a and 8b made of porous brick were provided at the bottom of the tundish on the outflow hole side in the tundish width direction 600mm as viewed from the large weir, and argon was blown from there.
Incidentally, the molten steel flow per strand, 75t tundish 0.0119m ³ / s and 48t tundish was 0.00660m ³ / s. The width of the tundish is 75t tundish 0.107m and 48t tundish 0.106M, also circulation portion the total area of the large weir 75t tundish is 0.0728M ² and 48t tundish is 0.0776m ^2.

  In each tundish, continuous casting was performed by changing the blowing amount of argon. At that time, the presence or absence of fluctuation of the bath surface in the tundish was visually observed.

On the other hand, the amount of inclusion spillage is calculated by calculating the number of inclusions spilled into the mold by molten steel flow analysis and inclusion particle trajectory analysis under the above tundish conditions. This was calculated and compared as the amount of inclusion outflow.
That is, in the molten steel flow analysis, an inflow / outflow boundary condition was given to the injection point and the outflow hole in accordance with the molten steel flow rate shown in Table 1 using the k-ε turbulent model, and the molten steel flow was calculated. In the inclusion particle trajectory analysis, 50 μm inclusion particles were injected from the injection point, and the motion trajectory of the inclusion particles was calculated according to the molten steel flow analysis results. At this time, under the test conditions in which the molten metal surface fluctuation is caused by the above-mentioned visual observation (Test Nos. 1, 2, 9 and 10 in Table 1 are applicable), the inclusion adsorption on the molten metal surface above the Ar gas blowing position does not occur. No inclusion particle trajectory analysis is performed. In addition, under the test conditions in which no molten metal surface fluctuation occurred (Test Nos. 3, 4, 5, 6, 7, 8, 11, 12, and 13 in Table 1 apply), all the inclusions that reached the molten metal surface were bathed. Inclusion particle trajectory analysis is performed as adsorbed on the surface slag. From the inclusion particle activation analysis result, the number of inclusions flowing into the mold was calculated under each test condition.

Furthermore, the method for obtaining the inclusion index is as follows. The condition of No. 8 in Table 1 below, that is, the condition in which inert gas was not blown in the 75 t tundish was used as the reference condition. The number of 50 μm inclusions flowing out into the mold under this reference condition was defined as a reference value (index 1). The other conditions were expressed as a ratio of the number of inclusion outflows to the reference value, and this was used as the inclusion index.
Table 1 shows each test condition, Ql · A / (Q2 · W), presence / absence of bath surface fluctuation in the tundish, and inclusion index from which inclusions flowed out from the tundish to the mold. Further, FIG. 7 is a graph showing the relationship between the inclusion index at which inclusions flow out from the tundish to the mold and the value of Ql · A / (Q2 · W) when casting is performed under each test condition in Table 1. .

  The larger the inclusion index in Table 1 and FIG. 7, the more inclusions flowed from the tundish to the mold. In the scope of the present invention, the inclusion index is kept low. In addition, where the value of Ql · A / (Q2 · W) is smaller than the range of the present invention, the outflow of inclusion occurs due to insufficient floating of the inclusion, and the value of Ql · A / (Q2 · W) is the present invention. Above the range, inclusion spillage occurs due to bath surface fluctuations in the tundish.

  From the above, it was confirmed that the best inclusion removal performance could be achieved without causing the fluctuation of the bath surface if the inert gas was blown in the conditions satisfying the scope of the present invention.

  The inclusion removal method of the present invention is not limited to molten steel, but can also be applied to non-ferrous metals such as aluminum and titanium other than steel.

DESCRIPTION OF SYMBOLS 1 Tundish 2 Injection nozzle 3 Molten steel 4a, 4b Outflow hole 5, 5a, 5b Weir 6 Distribution part 7 Inert gas 8a, 8b Blowing inlet

Claims (2)

In a tundish for continuous casting in which molten metal is injected, a weir is disposed between the injection point of the molten metal and an outflow hole through which the molten metal is led out to the mold, and the injection is performed through a flow section provided in the weir. When the molten metal flow is guided from the point side to the outflow hole side, an inert gas is blown into the molten metal from the bottom of the tundish on the outflow hole side of the weir under the condition satisfying the following formula (1). Inclusion removal method in tundish for continuous casting.
Record
0.00014 ≦ Ql · A / (Q2 · W) ≦ 0.00025 (1)
However,
Ql: Inert gas blowing flow rate per strand (standard state conversion Nm ³ / s)
Q2: Molten steel flow rate per strand (m ³ / s)
A: Total cross-sectional area (m ² ) of the circulation part of the weir
W: Average width of tundish at the weir position (m)   The method for removing inclusions in a tundish for continuous casting according to claim 1, wherein the blowing amount of the inert gas is adjusted according to a casting speed in continuous casting.

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