JP2001011525A - Device for reducing inclusion in molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inclusion reducing device having simple structure, low equipment cost and high efficiency. SOLUTION: AC current is supplied into an annular induction coil 6 parallel disposed with a ladle 1 bottom surface in a refractory at the ladle bottom part, and the fine inclusion in molten steel is fluidized on the ladle bottom part and caught to the refractory at the bottom part with Lorentz force caused by the mutual action between magnetic field induced with the AC current and eddy current in the molten steel 2 induced with the induced magnetic field. Further, the remaining fine inclusion is coagulated and grown with the fluidity of the molten steel to promote the floatation and the separation thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an apparatus for reducing inclusions in molten steel in a process of refining molten steel.

[0002]

2. Description of the Related Art Conventionally, the following technology has been disclosed as an inclusion reducing device using electromagnetic force.

(A) A method using a rotating magnetic field to increase the stirring intensity in a ladle to promote a refining reaction, and to increase the agglomeration efficiency of inclusions due to turbulence to separate them by flotation (for example, Japanese Patent Publication No. 62-11049) Gazette, JP-A-01-279
706, JP-B-59-29083).

(B) A method in which a magnetic field is applied to a channel having a cylindrical or rectangular cross section to capture inclusions on a refractory wall of the channel by a pinch force acting on molten steel (for example, see Japanese Patent Application Laid-Open No.
-90342, JP-A-7-90343, JP-A-8-60263).

(C) In addition to these electromagnetic methods, a method of blowing an inert gas into molten steel to improve the ability to reduce inclusions (see, for example, JP-A-9-122846,
295109).

[0006] The problem with these conventional inclusion reducing devices is that the magnetic shielding effect of the steel shell occurs because a magnetic field is applied from the outer surface side of the steel shell of a refining vessel such as a ladle.
Due to this shield effect, most of the applied magnetic field is attenuated, magnetic penetration into the molten steel is reduced, and the power use efficiency is reduced, resulting in a problem that the stirring efficiency is reduced and the inclusion reduction effect is reduced. .

In order to solve such problems, a method using a non-magnetic material for the steel shell has been proposed. However, the equipment cost is high, and since the material is mostly made of metal, it is induced here. The eddy current causes power loss and lowers power efficiency.

Further, in each of the above methods (a), there is a limit to the promotion of coagulation and hypertrophy only by increasing the stirring intensity, and fine inclusions remain. There was a harmful effect that the material was caught in the molten steel again.

In each of the above methods (b), the pinch force acting on the molten steel tries to capture not only the small inclusions but also the large inclusions, so that the inclusions are significantly deposited on the surface of the refractory, and the channels are closed. There was a fatal problem.

In each of the above methods (c), since the effect of gas injection is limited to the vicinity of the injection hole, the number of injection holes must be increased in order to exert an effect on all the molten steel in the vessel, and the apparatus needs to be used. This leads to complications and, consequently, increased remodeling costs.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to improve the efficiency of reducing inclusions, reduce eddy current loss induced in a steel shell, and improve the structure in refining molten steel using electromagnetic force. In order to reduce the cost of the apparatus.

[0012]

Means for Solving the Problems The present inventors focused on a ladle refining process as a basic process for removing inclusions in molten steel, and obtained the following knowledge.

(A) To improve the power efficiency of molten steel stirring,
It is preferable to arrange an induction coil in a refractory of a ladle and apply a magnetic field directly to molten steel without passing through a steel shell.

(B) A ring-shaped induction coil is arranged at the bottom of the ladle in parallel with the bottom surface, and when an alternating current is applied, an induction magnetic field penetrating the molten steel in a vertical direction is generated. An eddy current is induced around the central axis of the pot. The Lorentz force is generated by the interaction between the eddy current and the induction magnetic field.

(C) If the size of the induction coil is made slightly smaller than the diameter of the bottom surface of the ladle, the distribution of Lorentz force near the bottom surface is maximum just above the induction coil and minimum at the center, and In the nearby molten steel, a flow from the center to the ladle wall occurs. This molten steel flow becomes the stirring power.

(D) The Lorentz force acts on the molten steel as a conductor in the upward direction or in the direction of the center of the ladle, so that a volume force acts on the molten steel toward the center or upward of the ladle. On the other hand, since the Lorentz force does not act on the inclusion that is a non-conductive material, a relatively opposite force (pinch force) acts in the opposite direction (the bottom direction or the outer peripheral direction of the ladle). as a result,
The inclusions move toward the ladle bottom side wall relatively to the molten steel flow.

(E) Due to the stirring of the molten steel, the fine inclusions are coagulated, enlarged, and easily floated, and float relatively to the molten steel flow generated in the entire ladle. In addition, the fine inclusions move outward along the ladle bottom in accordance with the flow of molten steel, but also move downward due to the pinch force, so that they are trapped on the bottom after colliding with the bottom refractory.

(F) If the refractory on the bottom surface is made porous, the minute refractory that has collided with or contacts the refractory is more easily captured.

(G) Since the induction coil is located below the bottom of the ladle,
The flow velocity of molten steel is higher at the bottom of the ladle and lower at the top of the steel bath. Therefore, the inclusions floating on the upper part of the steel bath easily float even if the particle size is not so large. Further, the inclusions floating on the steel bath surface do not get caught in the steel bath again.

The present invention has been completed based on the above findings, and the gist thereof is as described in (1) and (2) below.

(1) A device for reducing inclusions in a molten steel while applying a magnetic field to agitate molten steel in a ladle together with slag to promote a refining reaction, wherein a refractory-lined removal device is provided. A ladle, a ring-shaped induction coil, and a power supply, wherein the induction coil is disposed in the ladle bottom refractory and parallel to the ladle bottom, and the power supply supplies an alternating current to the induction coil. The Lorentz force caused by the interaction between the induced magnetic field induced by the alternating current and the eddy current in the molten steel induced by the induced magnetic field causes the minute inclusions in the molten steel to flow to the bottom of the ladle, thereby causing the ladle bottom to be refractory. As well as
An apparatus for reducing inclusions in molten steel, which is a power supply device for flowing molten steel, coagulating and thickening inclusions, and promoting flotation.

(2) The apparatus for reducing inclusions in molten steel according to the above (1), wherein a part or all of the surface of the refractory at the bottom of the ladle is covered with a porous refractory.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram showing the configuration of an inclusion reducing device according to the present invention. In the figure, reference numeral 1 denotes a ladle, 2 denotes molten steel, 3 denotes an iron skin, 4 denotes a refractory, 5 denotes a heat insulating lid,
6 is an induction coil, 7 is an AC power supply, 8 is an inverter, 9 is a porous refractory, 10 is a slag, and 11 is a frequency control device. The molten steel 2 is, for example, after being decarburized and dephosphorized in a converter, before being subjected to continuous casting in the next step, is refined by the inclusion reducing device of the present invention, and is produced before or during refining. Inclusions are removed. In the figure, the inner surface of the ladle 1 is lined with a refractory 4, and an induction coil 6 is arranged on the bottom of the refractory 4 in parallel with the bottom surface of the ladle. An AC current having a controlled frequency, voltage, and current is applied to the induction coil 6 from a commercial frequency AC power supply 7 via an inverter 8. This control is performed by the frequency control device 11 to control the AC magnetic field induced in the molten steel. AC power supply 7, inverter 8, and frequency control device 1
1 is collectively called a power supply device. During smelting, slag 10 for smelting is added to ladle 1, and lid 5 for heat insulation for heat retention.
Is placed.

The induction coil 6 is formed so as to be located concentrically with the ladle center axis so that the diameter of the induction coil 6 is slightly smaller than the inner diameter of the ladle bottom at the maximum.

The refractory surface at the bottom of the ladle 1 is partially or entirely covered with a porous refractory 9. In order to reduce the cost of the porous refractory, it is preferable to mainly coat the outer periphery of the ladle bottom concentrically when partially covering the refractory. That is, as will be described later, the stirring flow of the molten steel by electromagnetic induction becomes a flow from the center to the outer peripheral side along the bottom surface, the flow velocity is smaller toward the outer peripheral side, and inclusions in the molten steel are trapped in the porous refractory 9 toward the outer peripheral side. This is because it is easy to be done. When the outer peripheral side of the bottom surface is covered with the porous refractory, the diameter of the uncoated portion is preferably set to 1/2 to 1/3 of the inner diameter of the bottom portion of the ladle.

It is desirable that the induction coil 6 be as close as possible to the molten steel, but it is necessary to ensure the thickness of the refractory in consideration of the wear of the refractory. Since the induction coil is exposed to the heat generated by Joule heat and the heat load of the molten steel via the refractory 4, it is desirable to provide a cooling structure. As a known cooling method, a method in which a hollow copper wire is used as a conductor of the coil and a cooling medium is circulated inside the copper wire may be used. However, in order to prevent the conductor of the induction coil 6 from being melted and damaged to cause a steam explosion and to generate a flammable gas and cause a fire, the cooling medium is water, flammable mineral oil,
It is not preferable to use fats and oils. Use a non-flammable or flame-retardant liquid such as CFC, carbon tetrachloride, or liquid nitrogen, or blow an inert gas such as nitrogen or argon at high pressure to adiabatic expansion in a conductive coil and use the heat absorption effect at that time to make use of the coil. It is better to cool.

The frequency of the current flowing through the induction coil 6 is preferably selected in such a range that the power conversion efficiency of the inverter 8 is high, the conversion efficiency to the Lorentz force generated by the induction magnetic field is high, and the equipment cost does not increase so much. . This frequency is preferably 1 to 15 Hz, and more preferably 8 to 10 Hz, in the range of commercially available devices currently available.

The amount of electric power supplied to the coil is preferably 2.2 to 3.6 kW per ton of molten steel. At less than 2.2 kW / t, almost no molten steel agitation effective for agglomeration of inclusions is obtained. Even if the agitation exceeds 3.6 kW / t, the agitation power required for refining is saturated. This is because the refractory of the slag line becomes severely worn.

Although the power is supplied as described above, the induced magnetic field induced is about 10 ³ to 10 ⁵ AT / t (ampere turn / ton), although it depends on the shape and specific resistance of the induction coil.

FIG. 2 is a schematic longitudinal sectional view showing the relationship among a magnetic field, an induced eddy current, and an acting electromagnetic force when an alternating current is applied to the induction coil 6. FIG. In the case of the first half period of the phase, FIG. 12B shows the case of the second half period. In FIG. 5A, the symbol on the left side of the cross section of the induction coil 6 (circled by .circle-solid.) Indicates that the applied current 21 is in the direction from the back of the paper to the front, and the symbol on the right side (× surrounded by the circle). ) Similarly indicates that the applied current 21 goes from the front to the back of the page. In FIG. 2A, the applied current 21 is the induction magnetic field 2
The eddy current 23 is induced in the molten steel 2 by the induction magnetic field 22. The eddy current 23 is on the left side in the direction from the front to the back of the page, and is opposite to the direction of the applied current 21.

In the molten steel region of the induction magnetic field 22 and the eddy current 23, a body force 24 called Lorentz force acts by an interaction according to Fleming's left-hand rule. FIG.
As shown in (b), the induced magnetic field 22 and the eddy current 2
Although the direction of 3 is reversed, the body force 24 acts in the same direction as in FIG.

FIG. 3 is a longitudinal sectional view schematically showing the magnitude and direction of the body force in the section of the molten steel. In the figure, the body force 24 has an upward direction with respect to the molten steel in the ladle bottom layer portion, and has a maximum value immediately above the coil 6. In a situation where such a body force 24 acts, the molten steel 2 containing inclusions has a function of eliminating inclusions by circulating flow and pinch force.

First, with regard to the circulating flow, the volume force 24 acts as a force for raising the molten steel 2 over the entire cross section of the ladle, but since the direction and the size are different in the radial direction, the volume force 24 is applied to the molten steel. Flow force is generated.

FIG. 4 is a longitudinal sectional view schematically showing the flow of molten steel in the ladle. As shown in the figure, a circulating flow is generated in the molten steel 2 such that the flow is ascending near the side wall of the ladle and descending at the center. This circulating flow has a direction opposite to the thermal convection generated by cooling the ladle side wall, but the eddy current generated at the bottom heats the molten steel at the bottom, so it is possible to maintain sufficient stirring power It is.

On the other hand, as shown in FIG. 2, near the center axis of the ladle and away from the bottom surface, the body force 24 is directed toward the center of the ladle and acts as a pinch force in the direction of reducing the volume of the molten steel. However, the inclusions are non-conductive and no such body forces act. Therefore, the inclusions are removed from the molten steel in this portion, and a force toward the bottom of the ladle acts. Although upward bulk force acts on the molten steel in this part, the ladder is circulated downward at the center of the entire ladle as described above. Reach.

At this time, the minute inclusions are adsorbed on the surface of the refractory 4 at the bottom of the ladle shown in FIG. Further, in a preferred embodiment of the present invention, the porous refractory 9 is arranged on the bottom of the ladle, and the fine inclusions enter the porous gap and are trapped. However, large inclusions are unlikely to be adsorbed on the refractory surface or the porous refractory 9 at the bottom of the ladle, are separated from the bottom of the ladle by the circulating flow of the molten steel, and float along the ladle side wall to form a surface layer of the molten steel. It will be captured by the slag 10.

In the present invention, it is considered that different behaviors are taken depending on the size of the inclusion. Large inclusions (having a particle size of more than about 50 μm) have a sufficient levitation force, so that they float and reach the surface of the molten steel in the ladle regardless of the electromagnetic stirring of the present invention.

Micro inclusions (particle size less than about 10 μm)
Has little buoyancy, and flows on the flow of molten steel according to the present invention (downward flow at the center of the ladle, upward flow near the side wall). Things. The non-agglomerated microinclusions are adsorbed by the refractory (porous refractory in a preferred embodiment) on the side of the ladle in the downward flow at the center of the ladle or the flow from the center of the ladle bottom to the side wall by the action according to the present invention. You.

Medium particle size inclusions (particle size approximately 10 to 50 μm)
m) has some levitation force but is weak. In addition, since the specific surface area is not as large as that of the minute inclusions, the possibility of being adsorbed by the refractory on the bottom of the ladle is small. Therefore, it flows along with the molten steel flow, but in the process, a part thereof is coagulated and enlarged, the particle diameter becomes large, a sufficient levitation force is obtained, and the steel floats regardless of the molten steel flow. Since the higher the flow velocity, the more easily the coagulation occurs, in this process, the enlargement progresses in the downward flow at the center of the ladle and the flow from the center to the side wall at the bottom of the ladle. Inclusions that could not be sufficiently enlarged rise along the ladle side wall and ride the flow from the outer periphery toward the center. As can be seen from the distribution of the body force 24 in FIG. 3, since the flow velocity in this part (upper half of the steel bath) is low, the medium particle size inclusions have a sufficient floating time even if the floating speed is low, and the steel bath Reach the surface.

In the present invention, since the flow of molten steel near the surface of the steel bath is small, inclusions that have once reached the surface of the steel bath do not flow out into the molten steel again.

[0041]

EXAMPLE In a ladle having an upper diameter of 2.4 m, a lower diameter of 2.2 m, a height of 2.5 m, and a molten steel capacity of about 80 t, an induction coil having a circular shape with four turns and a radius of 0.8 m was used. A study according to the invention was performed. The current supplied to the induction coil was 10 Hz, 10 ⁶ AT (ampere turn). In addition, a porous refractory was placed on the entire bottom of the ladle. Average pore size of porous refractory is 0.5m
m and a thickness of 0.05 m.

As a prior art for comparison, A was measured from the bottom of the ladle.
An apparatus for blowing r gas was used. In this comparative example, Ar gas at 50 Nl / min per ton of molten steel was blown from the center of the bottom of the ladle of the same size as the ladle of the apparatus of the present invention.

In Examples of the present invention and Comparative Examples, the same steel type was used.
The performances of the two types of inclusions of various sizes that were initially present in the same treatment of molten steel after a 10-minute treatment were compared based on the reduction ratio after the treatment. Table 1 shows the results.

[0044]

[Table 1]

As shown in Table 1, with respect to large inclusions among the initially existing inclusions, the same reduction effect as that of the apparatus of the present invention example was observed in the Ar gas blowing apparatus.

With respect to minute inclusions, the device of the present invention showed a remarkable improvement effect as compared with the device of the comparative example. In addition, even when the ladle using the apparatus of the present invention was used many times, no molten steel contamination or the like due to the fine inclusions accumulated at the bottom was observed, and a stable removal effect of the fine inclusions was observed.

[0047]

According to the inclusion reducing apparatus of the present invention, equipment cost can be reduced and molten steel refining with high inclusion reducing efficiency can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration of an inclusion reducing device of the present invention.

FIG. 2 shows a magnetic field when an alternating current is applied to an induction coil;
It is a vertical cross-sectional schematic diagram showing a relationship between an induced eddy current and an acting electromagnetic force, wherein FIG. 12A shows a case of a first half cycle of an alternating current phase, and FIG. .

FIG. 3 is a longitudinal sectional view schematically showing the magnitude and direction of body force in a molten steel section.

FIG. 4 is a longitudinal sectional view schematically showing a flow of molten steel in a ladle.

[Explanation of symbols]

 1. Ladle 2. Molten steel 3. Iron skin 4. Refractory 5. Heat retention lid 6. Induction coil 7. AC power supply 8. Inverter 9. Porous refractories 10. Slug 11. Frequency control device 21. Applied current 22. Induction magnetic field 23, eddy current 24. Body force

Claims (2)

[Claims] An apparatus for reducing inclusions in a molten steel while applying a magnetic field to agitate molten steel in a ladle together with slag to promote a refining reaction, wherein the ladle has a refractory lining. And a ring-shaped induction coil, and a power supply device, wherein the induction coil is disposed parallel to the ladle bottom within the ladle bottom refractory, the power supply supplies an alternating current to the induction coil, The Lorentz force caused by the interaction between the induced magnetic field induced by the alternating current and the eddy current in the molten steel induced by the induced magnetic field causes the small inclusions in the molten steel to flow to the bottom of the ladle, thereby forming a refractory at the bottom of the ladle. As well as capture
An apparatus for reducing inclusions in molten steel, which is a power supply device for flowing molten steel, coagulating and thickening inclusions, and promoting flotation. 2. The apparatus for reducing inclusions in molten steel according to claim 1, wherein part or all of the surface of the refractory at the bottom of the ladle is covered with a porous refractory.