JPH11123506A - Method for removing inclusion in tundish for continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformize the fluidity of molten steel in a tundish without dividing the tundish with a weir, etc., and to efficiently and inexpensively remove the inclusion in the molten steel. SOLUTION: In the tundish 1 for continuous casting, the molten steel 9 in the tundish is heated with a plasma torch 6 and also, a static magnetic field is impressed to the molten steel plasma-heated near the plasma torch. Then, Lorentz's force developed with the plasma current and the static magnetic field is used act the molten steel toward the upstream side of the tundish to remove the inclusion. At this time, the removing efficiency of the inclusion is further improved by changing magnetic flux density in the static magnetic field which depends on the plasma current and the average molten steel flow rate in the tundish.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing inclusions in a tundish for continuous casting, which can efficiently remove nonmetallic inclusions in molten steel injected into a tundish.

[0002]

2. Description of the Related Art FIG. 6 is a view schematically showing the flow of molten steel in a conventional tundish for continuous casting. The molten steel 9 injected into the tundish 1 is different from the molten steel 9 in the tundish 1. Since the temperature is high, the molten steel 9 injected into the tundish 1 from the ladle 2 through the long nozzle 4 has a characteristic of floating upward, as shown in FIG. Since it passes through the vicinity of the molten steel surface in the tundish 1 at a high speed and then descends near the immersion nozzle 5 and flows into the mold 3 through the immersion nozzle 5, non-metallic inclusions suspended in the molten steel 9 (hereinafter, referred to as , "Inclusion") is short, and therefore, it is generally difficult for the tundish 1 to remove the inclusion.

[0003] Since these inclusions cause defects such as surface flaws in the final product, they need to be separated and removed as much as possible.
Various measures to reduce inclusions have been implemented.

For example, Yamada et al. Describe materials and processes "Vol.
9 (1996), p233 ”(hereinafter referred to as“ prior art 1 ”), the steel receiving part of the tundish is divided into a cylindrical rotating tank, and a rotating electromagnetic stirrer is arranged around the tank. It has been reported that when the molten steel is swirled horizontally in a rotating tank with a stirrer, inclusions in the molten steel are aggregated and coalesced, flotation and separation are promoted, and the molten steel is cleaned.

In addition, Taguchi et al. Described iron and steel, Vol. 71 (1985), No.
12, S-993 "(hereinafter referred to as" prior art 2 ").
In the tundish, a refractory weir having a number of holes of the collision hole type is provided to partition the tundish, and molten steel passes through the holes of the weir, so that Al ₂ O ₃ suspended in the molten steel forms the weir. It reports that it can adhere to the inside of the hole and clean the molten steel.

[0006]

The prior art 1 is a very excellent method for increasing the size of inclusions and causing them to float, but it is necessary to provide a rotary tank separately in a tundish. Dish refractory costs are very high. In addition, simply increasing the size of the inclusions cannot sufficiently improve the cleanliness of the molten steel.To improve cleanliness, the size of the inclusions must be increased and the molten steel flow in the tundish must be uniform. There is a need to.

In the prior art 2, the flow of the molten steel in the tundish is made uniform by the weir and a sufficient floating time is secured. However, the weir is a consumable and must be newly installed for each use. However, like the prior art 1, the cost of the tundish refractory increases. Further, when a tundish in recent years is re-used hot, a weir cannot be installed manually in a high-temperature tundish, and a dedicated machine for installing the weir is required, resulting in an increase in manufacturing costs.

[0008] The present invention has been made in view of the above circumstances,
An object of the present invention is to provide a method for making the flow of molten steel in a tundish uniform without dividing the tundish with a weir or the like, and efficiently and efficiently removing inclusions suspended in the molten steel at low cost.

[0009]

According to a first aspect of the present invention, there is provided a method for removing inclusions in a tundish for continuous casting, wherein the molten steel in the tundish is heated by a plasma torch in the tundish for continuous casting. The present invention is characterized in that a static magnetic field is applied to molten steel that has been plasma-heated, and Lorentz force generated by the plasma current and the static magnetic field acts on the molten steel toward the upstream of the tundish.

The method for removing inclusions in a tundish for continuous casting according to the second invention is a method for removing inclusions according to the first invention, wherein the inclusion is removed depending on the plasma current and the average flow rate of molten steel in the tundish. It is characterized in that the magnetic flux density of the magnetic field is changed.

When a counter electrode is provided in the molten steel in the tundish and a static magnetic field is applied to the molten steel near the surface of the molten steel being plasma-heated, the plasma current and the static magnetic field are applied to the molten steel in the tundish based on Fleming's left-hand rule. The Lorentz force acts on the molten steel near the molten steel surface. In the present invention, since this Lorentz force acts on the upstream side of the tundish, that is, toward the steel receiving portion from the ladle of the tundish, the molten steel flow near the molten steel surface is decelerated. As a result, the fast flow near the molten steel surface caused by the temperature difference with the molten steel in the tundish is reduced, and the molten steel flow velocity distribution in the vertical section from the molten steel surface in the tundish to the bottom of the tundish becomes uniform, Since the floating time of the inclusions can be sufficiently secured, the cleanliness of the molten steel is improved.

At this time, if the magnetic flux density of the static magnetic field is determined depending on the plasma current, that is, the output of the plasma heating, and the average flow velocity of the molten steel in the tundish, the flow of the molten steel in the vicinity of the molten steel surface can be properly controlled. Therefore, the cleanliness of the molten steel can be ensured at any casting speed.
Incidentally, the average molten steel flow velocity, the molten steel casting volume per unit time determined by the sectional area of the cast slab to be cast and the slab withdrawal speed, the sectional area of the tundish, that is, the inner surface width of the tundish, and stored It is a calculated value obtained by dividing by the product with the molten steel height.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to the drawings.
FIG. 1 is a schematic view of a tundish for continuous casting showing one example of an embodiment of the present invention, in which (a) is a front sectional view,
(B) is a side sectional view.

A ladle 2 is disposed above a cuboid tundish 1 having an inner surface made of a refractory, and molten steel 9 in the ladle 2 is a long nozzle 4 installed at the bottom of the ladle 2. The tundish 1 is injected into the tundish 1 with one short side wall 11 side as a steel receiving portion. or,
An immersion nozzle 5 is provided at the bottom on the other short side wall 12 side opposite to the steel receiving portion from the ladle 2, and molten steel 9 injected into the tundish 1 is passed through the immersion nozzle 5 for 1 It is cast in the mold 3 with the molten steel casting amount per minute being Q (t / min). Then, it is cooled and solidified in the mold 3 to form a slab 10.

A plasma torch 6 is installed between the steel receiving portion from the ladle 2 and the mounting position of the immersion nozzle 5 to heat the molten steel 9 in the tundish 1. The location of the plasma torch 6 is determined by examining the molten steel flow in the tundish 1 in advance, and if the molten steel flow exists in the vicinity of the molten steel surface, the flow control of the molten steel 9 can be performed accurately. The position may be substantially intermediate between the steel receiving portion from the pan 2 and the mounting position of the immersion nozzle 5. Then, the counter electrode 7 for the plasma generation power supply is connected to the tundish 1 vertically below the plasma torch 6.
Install at the bottom of With this arrangement, the direction p of the plasma current is vertically downward. Although the counter electrode 7 is provided to penetrate the bottom of the tundish 1 in the drawing, it is not necessary to penetrate the bottom, and the wiring may be laid on the lower surface of the refractory at the bottom of the tundish 1. Also, the installation position of the counter electrode 7 may not be strictly vertically below the plasma torch 6,
The present invention can be implemented as long as the direction p of the plasma current is in the downward direction.

A static magnetic field generator 8 whose magnetic flux density is controlled by a DC power supply (not shown) is opposed to a side surface of the tundish 1 near the place where the plasma torch 6 is installed. And a static magnetic field is applied to the molten steel 9 near the molten steel surface. With this arrangement, the direction s of the static magnetic field is horizontal. At this time, if the outer shell of the tundish 1 is constructed of non-magnetic stainless steel or the like, the attenuation of the magnetic flux density can be suppressed. The strength of the static magnetic field may be about 0.4 Tesla at maximum, which is commonly used in industry.

Then, the Lorentz force F acts on the molten steel 9 toward the direction of the steel receiving portion from the ladle 2 in accordance with the Fleming's left-hand rule by the direction p of the plasma current and the direction s of the static magnetic field, The flow velocity of the molten steel near the molten steel surface is reduced. At that time, since the static magnetic field is not applied to the bottom side of the tundish 1, the Lorentz force F does not act on the molten steel 9 on the bottom side of the tundish 1. Therefore, the flow speed of the molten steel on the bottom side of the tundish 1 is increased by an amount corresponding to the deceleration on the molten steel surface, and the flow speed of the molten steel in the vertical section of the tundish 1 is made uniform.

It is preferable that the Lorentz force F acting on the molten steel 9 is suitable for the molten steel flow velocity near the molten metal surface in the tundish 1. That is, even if excessively slow down,
Even if the degree of deceleration is excessively weak, the molten steel flow cannot be made uniform. Here, the Lorentz force F is determined by the product of the plasma current and the magnetic flux density of the static magnetic field, and the flow velocity of the molten steel near the molten metal surface in the tundish 1 increases and decreases in proportion to the average flow velocity of the molten steel. Therefore, first calculate the average molten steel flow velocity, and then, taking into account the plasma current at that time, determine the magnetic flux density of the static magnetic field so that the Lorentz force is commensurate with the average molten steel flow velocity. Can work. Incidentally, the average molten steel flow rate may be obtained by dividing the molten steel casting amount Q per unit time by the product of the inner surface width W of the tundish 1, the molten steel storage height H, and the molten steel specific gravity.

The temperature of the molten steel 9 in the tundish 1 rises due to the plasma heating. Therefore, considering the temperature rise due to the plasma heating in advance, the molten steel 9 in the ladle 2 before casting is removed.
Is preferably adjusted. If the temperature of the molten steel in the tundish 1 is too high, the same effect can be obtained by weakening the plasma current, that is, suppressing the plasma heating and increasing the magnetic flux density of the static magnetic field.

FIG. 1 is an illustration of a tundish 1 of single-strand casting. However, the present invention is not limited to single-strand casting, but a steel receiving portion is provided at the center and molds 3 are arranged on the left and right. Also applicable to multi-strand casting. In this case, it is necessary to install the plasma torch 6 and the static magnetic field generator 8 on the left and right sides of the tundish 1, but the plasma heating method and the static magnetic field applying method may be performed as described above.

[0021]

An embodiment using a tundish for continuous casting shown in FIG. 1 will be described below. The tundish capacity is 30 tons, the inner surface width W of the tundish is 800 mm, the molten steel storage height H is 1000 mm, and the molten steel storage length is 5300 mm. The cast slab was a rectangular type having a thickness of 250 mm and a width of 2100 mm, and was cast from a low-carbon aluminum killed steel having a carbon concentration of 0.03 wt% at a slab drawing speed of 3.0 m / min. Under these conditions, the molten steel casting per minute is 12.3.
6 tons / min, and the average molten steel flow rate is 33 mm / sec.
c.

The plasma torch and the static magnetic field generating device are arranged almost in the middle between the steel receiving portion from the ladle and the mounting position of the immersion nozzle.
m, the maximum coil current is 1000 A, and the maximum magnetic flux density is
At 3 Tesla, the maximum plasma output was 1 MW, the maximum plasma current was 7000 A, and Ar gas was used as the plasma gas.

Casting was performed with the static magnetic flux density at the center of the tundish and at the center line of the opposing magnetic poles set to 0.25 Tesla and the plasma current set to 7000A.
For comparison, a casting in which only a static magnetic field having the same magnetic flux density was applied (Comparative Example) and a casting in which neither plasma heating nor application of a static magnetic field was applied (Conventional Example) were performed.

FIG. 2 shows the result of measuring the flow velocity of molten steel in the tundish in the tundish longitudinal section at that time. The flow velocity was determined from the stress acting on the refractory rod due to the flow of the molten steel, with the refractory block attached to the tip of the refractory rod and immersed in the molten steel. As shown in the figure, the flow rate immediately below the surface of the molten steel in the tundish is 100 mm / sec at maximum in the conventional example, whereas it is reduced to 60 mm / sec in the comparative example, and further reduced in the example of the present invention. The maximum flow rate becomes 35 mm / sec,
The speed was reduced to a flow rate almost equal to the average molten steel flow rate. From these, it can be seen that in the comparative example, the motor was decelerated by the so-called electromagnetic braking effect due to the static magnetic field, but in the embodiment of the present invention, the motor was further decelerated by adding the plasma current.

FIG. 3 shows a sample for inclusion investigation from the slabs of the above embodiment, comparative example, and conventional example, and the number of inclusions having a diameter of 50 μm or more was examined by microscopic observation. FIG. 3 is a diagram showing an index assuming that the number of inclusions is 1; According to the embodiment of the present invention, it is reduced to 1/5 as compared with the conventional example and 1/3 as compared with the comparative example.

Next, the optimum value of the Lorentz force acting on the molten steel was investigated by changing the slab drawing speed from 0.5 m to 3.0 m / min with the above slab dimensions. Since the Lorentz force is determined by the product of the plasma current and the magnetic flux density of the static magnetic field, the Lorentz force was adjusted by changing the plasma current and the magnetic flux density of the static magnetic field. The optimum value of the Lorentz force was determined when the flow rate of the molten steel in the vertical section in the tundish was measured and the difference between the maximum flow rate and the minimum flow rate was 20 mm / sec. In addition, the flow velocity distribution of molten steel in the tundish vertical section is
The measurement was performed according to the method using the refractory thin rod described above.

FIG. 4 shows the result. FIG. 4 shows the condition where the above slab drawing speed is 3.0 m / min, that is, the magnetic flux density of the static magnetic field is 0.25 Tesla and the plasma current is 7000.
FIG. 3 is a diagram showing, assuming that the product of the magnetic flux density and the plasma current at the time of A is 1, an index for each slab drawing speed. As shown in the figure, in order to apply the optimum Lorentz force, it is necessary to increase or decrease the product of the magnetic flux density and the plasma current almost in proportion to the slab drawing speed, that is, the average molten steel flow velocity in the tundish. I understand.

FIG. 5 shows that the slab drawing speed is 1.0, 2..
Under the conditions of 0 and 3.0 m / min, casting was performed by increasing or decreasing the product of the magnetic flux density and the plasma current in the range shown in FIG. With the above slab drawing speed of 3.0 m / min,
That is, it is a figure showing the result of investigating the number of slab inclusions when casting was performed under constant conditions of a magnetic flux density of a static magnetic field of 0.25 Tesla and a plasma current of 7000 A. In the figure, ●
Is the case where the product of the magnetic flux density and the plasma current is changed, and when the product of the magnetic flux density and the plasma current when the casting speed is 3.0 m / min is indexed as 1.0, the casting speed is extracted. Is 0.33, 2.0 at 1.0 m / min
It was cast at 0.67 at m / min.
In addition, ○ indicates a case where the product of the magnetic flux density and the plasma current is constant. Inclusion investigation was carried out according to the above-mentioned method, and the index was shown as an index when the slab drawing speed was 3.0 m / min as 1. Things. As shown in the figure, it can be seen that inclusions can be further reduced by applying an appropriate Lorentz force corresponding to the slab drawing speed.

[0029]

According to the present invention, the flow of molten steel in a tundish can be made uniform without providing a weir or the like in the tundish, and as a result, inclusions in the molten steel can be efficiently removed at low cost. The industrial benefits are enormous.

[Brief description of the drawings]

FIG. 1 is a schematic view of a tundish for continuous casting showing an example of an embodiment of the present invention, in which (a) is a front sectional view and (b) is a side sectional view.

FIG. 2 is a diagram showing the flow rate of molten steel in a vertical section of a tundish in an example, a comparative example, and a conventional example.

FIG. 3 is a diagram showing the number of inclusions in a cast slab in an example, a comparative example, and a conventional example.

FIG. 4 is a diagram showing conditions under which Lorentz force acting on molten steel is optimized.

FIG. 5 is a diagram showing a comparison of the number of inclusions in a slab when the Lorentz force acting on molten steel is changed and when it is not changed.

FIG. 6 is a view schematically showing a flow of molten steel in a conventional tundish.

[Explanation of symbols]

 Reference Signs List 1 tundish 2 ladle 3 mold 4 long nozzle 5 immersion nozzle 6 plasma torch 7 counter electrode 8 static magnetic field generator 9 molten steel 10 cast slab

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Noriko Kubo, 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Inside Nihon Kokan Co., Ltd. (72) Hiroshi Shimizu 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan (72) Inventor Katsuhiko Murakami 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (2)

[Claims] In a tundish for continuous casting, a molten steel in a tundish is heated by a plasma torch, and a static magnetic field is applied to the molten steel heated by plasma in the vicinity of the plasma torch. A method for removing inclusions in a tundish for continuous casting, characterized in that the applied Lorentz force acts on molten steel toward the upstream side of the tundish. 2. The method for removing inclusions in a tundish for continuous casting according to claim 1, wherein the magnetic flux density of the static magnetic field is changed depending on the plasma current and the average molten steel flow velocity in the tundish. .