JPH08176647A - Production of highly cleaned steel with less inclusion - Google Patents

Abstract

PURPOSE: To provide a manufacturing method of a highly cleaned steel by fine bubbles. CONSTITUTION: A fine bubble generating device is provided on the bottom part of a ladle 4 corresponding to the intermediate position between a rising tube 1 and a descending tube 11 of an RH device to blow an inert gas in a circulating molten steel 3. It is preferable to use the fine bubble generating device in which one ends of a plurality of ceramics nozzles 8 whose outer diameters are each <=2mm are embedded in a porous brick 5 and the other end is projected in the molten steel, and refractory coating is applied to the surface of the porous brick 5 in contact with the molten steel. The impurities in the molten steel are efficiently and rapidly removed to manufacture the highly cleaned steel at a low cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing highly clean steel with few inclusions.

[0002]

2. Description of the Related Art In recent years, due to stricter quality requirement levels and intensifying competition with other materials, the high cleanliness required for steel, particularly the reduction of non-metallic inclusions in steel, has become stricter year by year. Conventionally, in order to remove inclusions in steel, agglomeration between inclusions,
Macro bubbling method focusing on promoting levitation,
Methods such as electromagnetic stirring and vacuum degassing are used. However, it is difficult to sufficiently remove fine inclusions having a size of several tens of μm or less existing in the molten steel by these methods.

Recently, attention has been focused on a method of removing inclusions which separates the inclusions by the trapping effect of the fine bubbles.
Japanese Patent Publication No. 2-197515 discloses "a step of blowing fine bubbles of inert gas into molten steel through a porous plug provided at a lower portion outside a decompression container such as a ladle, and decompressing the molten steel. The method for smelting a high cleanliness ultra-low carbon steel, which comprises a step of circulating the steel into a container. In this method, in order to effectively contain the fine bubbles 9 in the molten steel 3, it is better to provide the porous brick 5 directly below the rising pipe 1 and / or the descending pipe 11 in which the molten steel flow is violent as shown in FIG. It is said that it is effective in dispersing the fine bubbles 9 in the molten steel 3.

However, when a large number of fine bubbles are generated from a wide area at the bottom of the ladle as shown in FIG. 9, there is a possibility that the fine bubbles generated in the circulating molten steel collide with each other and agglomerate into large bubbles. high.

Further, the inclusions in the ascending flow near the inlet of the ascending pipe move quickly upward along with the ascending flow, so it is difficult to be trapped by the bubbles. Further, since the bubbles in the downflow near the outlet of the downcomer are affected by the flow of the downflow and the floating speed becomes slow, this phenomenon also makes it difficult to capture inclusions.

As will be described later, since the wettability between the porous brick and the molten steel is extremely poor, it is impossible to stably generate fine bubbles in the molten steel with the porous brick.

Therefore, in the above method, the effect of removing inclusions is not good, the treatment time is long, and a large amount of inert gas is required.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for solving the above problems of the prior art and effectively removing inclusions by fine bubbles to produce a highly clean steel containing few inclusions. To do.

[0009]

The gist of the present invention resides in the following methods (1) and (2) for producing highly clean steel.

(1) A device for generating fine bubbles is provided at the bottom of the ladle corresponding to the intermediate position between the ascending pipe and the descending pipe of the vacuum degassing device (RH), and this device is inert to molten steel in circulation. A method for producing highly clean steel with few inclusions, characterized by blowing gas.

(2) As the above-mentioned fine bubble generator,
A device in which one end of a plurality of ceramics nozzles having an outer diameter of 2 mm or less is embedded in a porous brick, the other end is projected into molten steel, and a refractory paint is applied to the surface of the porous brick in contact with the molten steel. A method for producing highly clean steel with few inclusions, which comprises blowing an inert gas into molten steel during circulation by using the method.

The fine bubbles referred to in the present invention have a diameter of 5
Indicates a size of mm or less.

[0013]

1 and 2 show an example of an apparatus to which the method of the present invention is applied.
This will be described below.

FIG. 1 is a vertical sectional view showing an RH vacuum degassing apparatus (hereinafter referred to as RH). This equipment uses Ar for reflux
A vacuum chamber 12 having an ascending pipe 1 having a tuyere for blowing gas 2 and a descending pipe 11, a ladle 4, and a refractory material 13 at the bottom thereof.
It is provided with a device for generating fine bubbles 9 provided in the above. In FIG. 1, reference numeral 3 is molten steel, and 10 and 10 'are slags.

FIG. 2 is a vertical cross-sectional view showing a structural example of a device for generating fine bubbles 9. This generator comprises a porous brick 5 embedded in a refractory 13 at the bottom of a ladle 4, one end of which is embedded in the porous brick 5 as shown in the drawing, and the other end of the molten steel 3
A plurality of nozzles 8 and porous bricks 5 that protrude inside
The refractory paint 7 is applied to the surface of the molten steel 3 in contact with the molten steel 3.

As shown in FIG. 1, the ascending pipe 1 and the descending pipe 11 are immersed in the molten steel 3 in the ladle 4, the vacuum chamber 12 is evacuated, and the reflux Ar gas 2 is blown from the ascending pipe 1 side. The molten steel 3 is circulated as shown by an arrow to perform refining such as degassing. At this time, the Ar gas 6 for generating fine bubbles is further supplied from the porous brick 5 to generate fine bubbles 9 from the nozzle 8, and the fine inclusions in the molten steel 3 are separated by the trapping effect of the bubbles 9 to obtain a high-clean steel. To manufacture.

In the method of the present invention, the fine bubbles 9 are generated in the rising pipe 1.
It is generated from a fine bubble generator provided at a position corresponding to the refractory material 13 at the bottom of the ladle 4 at an intermediate position of the downcomer pipe 11. That is, the fine bubble generator is installed in the refractory 13 at the bottom of the ladle 4 so that the center of the fine bubble generator shown in FIG. 2 is located at the intermediate position between the ascending pipe 1 and the descending pipe 11.

In practice, this installation position is almost at the center of the bottom of the ladle, and some deviations that occur during actual operation are allowed.

A thin tube made of ceramics (specifically, alumina, magnesia, silica, etc.) is used for the nozzle 8. The desirable size range of the nozzle is 0.6 to 2 mm in outer diameter and 0.4 to 1 mm in inner diameter. The desirable range of the protruding length into the molten steel is 5 to 10 mm, and it is desirable to mount the nozzles so that the level at the gas outlet end is as uniform as possible. The desirable distribution density range of the nozzles is 0.5 to 1 nozzle / cm ² . The nozzle-to-nozzle spacing (gap) is made larger than the diameter of the fine bubbles generated from the nozzle outlet, and its desirable range is about 1.5 to 3 times the diameter of the fine bubbles. A desirable range of the embedded depth of the nozzle in the porous brick is 5 to 10 mm. The embedding of the nozzle into the porous brick may be carried out by a usual method by perforating the porous brick, a fire resistant adhesive, or the like.

Alumina, alumina graphite and the like are used as the material of the porous brick, and the porosity range is 20 to 60.
It is desirable to set it as%. The refractory paint material is SiO ₂ -Al ₂ O
₃ or the like can be used, and a desirable coating thickness range is 2 to 5 mm.

A desirable range of the total flow rate of Ar gas for generating fine bubbles is 2 to 5 Nm ³ / min, and a flow rate of Ar gas for reflux is also 1 to 1.5 Nm ³ / min. In addition to Ar, an inert gas such as N ₂ can be used as these gases.

The generation mechanism of fine bubbles in the method of the present invention will be described with reference to FIG. When Ar gas 6 for generating fine bubbles is blown from the porous brick 5, since the refractory paint 7 is applied to the surface of the porous brick 5 which is in contact with the molten steel 3, this gas enters the molten steel 3 only from the outlet of the nozzle 8. Come out.
Since the nozzle 8 is made of ceramics, it has good wettability with molten steel. Therefore, the fine bubbles 9 can be stably generated in the molten steel 3 by the nozzle 8.
However, when the outer diameter of the nozzle 8 exceeds 2 mm, the diameter becomes 5
It is difficult to obtain fine bubbles of mm or less. That is,
Since the wettability between the molten steel and the nozzle is poor, the bubble diameter generated increases as the nozzle outer diameter increases. Further, when the gap between the nozzles becomes equal to or smaller than the diameter of the fine bubbles generated from the nozzle outlet, the bubbles generated adjacent to each other coalesce, and it becomes impossible to obtain the fine bubbles of a desired size.

Since the position of this generator is provided at the bottom of the ladle in consideration of the positions of the ascending pipe and the descending pipe as described above, the fine bubbles do not diffuse into a wide range in the molten steel,
Since it is less likely to be disturbed by the circulation flow, it is less likely that the generated bubbles will collide with each other and aggregate to form a large bubble. As a result, the effect of capturing and separating fine inclusions of fine bubbles can be maintained high.

In order to clarify the influence of the installation position of the fine bubble generator on the removal effect of inclusions in molten steel in RH, the inventor of the present invention has a fine bubble generator including only a water model device of RH and a porous brick. Experiments were performed using and.

FIGS. 3, 4 and 5 (a) are longitudinal sectional views showing the structure of this water model device, and the basic structure is the same as that of the device shown in FIG. Figure 5 (b) is line A in Figure 5 (a).
It is a horizontal sectional view and a top view in A. 3 to 5, reference numeral 14 is a container for water 15, 16 is a model vacuum tank, 17 is an ascending tube, 18 is a descending tube, and 19 is Ar for reflux.
Gas is 20, polystyrene particles, 21 is paraffin liquid, 22 and 22 'are porous bricks, 23 is fine bubbles and 24 is Ar gas for generating fine bubbles.

The water container 14 has an inner diameter of 300 mm and a height of 400 mm. For generating fine bubbles, Ar for bubble generation is generated through porous bricks 22 and 22 'having a porosity of 30%.
Gas 24 is blown in and the average diameter is 10μ as an alternative inclusion.
m polystyrene particles 20 were used.

The model vacuum tank 16 is evacuated and water 15
After sucking a part of the above from the container 14 into the tank, the circulating Ar gas 19 was blown from the side of the rising pipe 17 to circulate the water 15. At this time, the paraffin liquid 21 was added to the water surface in the model vacuum tank 16 to float the polystyrene particles 2
0 was absorbed in paraffin liquid 21 as an alternative slag.

The condition (case 1) shown in FIG. 3 is the condition (case 2) shown in FIG. 4 when the fine bubble generator is not provided.
Is provided with two circular porous bricks 22 at the bottom of the container 14 corresponding directly below the ascending pipe 17 and the descending pipe 18 of the model vacuum tank 16, and the diameter of the porous brick 22 is 10-8.
When changed in the range of 0 mm, the condition (case 3) shown in FIG. 5 is that the container 1 corresponding to the intermediate position between the ascending pipe 17 and the descending pipe 18 as shown in FIG.
This is the case where it is provided at the bottom of No. 4. The length of the rectangular porous brick 22 'shown in FIG. 5 is 2/3 of the inner diameter of the container 14, and the width is such that the area of the porous brick 22' is equal to the total gas generation area of the two circular porous bricks 22 of the case 2. Changed to.

The flow rate of Ar gas for generating fine bubbles is 0.01
The gas generation area of the porous brick was changed in the range of ˜0.1 Nm ³ / min in the range of 78 to 5026 mm ² .

A sample is taken from the container 14 at regular time intervals, the number concentration is calculated by counting the number of polystyrene particles per unit time, and the number concentration of the particles at the sampling time and the initial number concentration are calculated. The ratio was used to evaluate the effect of removing polystyrene particles. An example of this result is shown in FIG.

FIG. 6 is a diagram showing an example of the relationship between the number concentration ratio and the treatment time in the water model experiment. this is,
This is the case where the gas generation area of the porous brick is 1000 m ² and the flow rate of the Ar gas for generating fine bubbles is 0.05 Nm ³ / min. As shown in the figure, the effect of removing particles is best in case 3, then worse in order of case 2 and case 1, and this tendency was observed under any experimental condition not shown. . This is because in case 3, there is less agglomeration between the generated fine bubbles and the efficiency of trapping inclusions by the fine bubbles is high.

From the above results, even in the case of molten steel, it is best to provide the device for generating fine bubbles at the bottom of the ladle corresponding to the intermediate position between the ascending pipe and the descending pipe.

However, when the present inventor uses a porous brick as a device for generating fine bubbles when targeting molten steel,
It was found that bubbles generated from the pores on the surface of porous bricks tend to coalesce with each other to form large bubbles, and it is impossible to maintain fine bubbles in molten steel. FIG. 7 is a diagram schematically illustrating this phenomenon.

Since the wettability between the porous brick and the molten steel is very poor, the bubbles 24 generated from the pores 23 on the surface of the porous brick 5 first grow along the surface as shown in FIG. This is because the large bubbles 25 tend to coalesce with each other.

On the other hand, FIGS. 1 and 2 used in the method of the present invention.
In the device for generating fine bubbles shown in Fig. 3, gas is generated only from the nozzle outlet of the ceramic thin tube protruding into the molten steel, and because the wettability between the ceramic nozzle and molten steel is good, the bubbles do not spread in the lateral direction. Leave the molten steel from the outlet. Furthermore, even if a plurality of nozzles are installed, the distance between the nozzles is larger than the diameter of the fine bubbles generated from the nozzle outlets, so that the coalescence of bubbles as in the case of using the porous brick shown in FIG. It does not occur, and the generated bubbles are stable and fine.

Although only the nozzle may be used only for the purpose of generating fine bubbles, in order to prevent molten steel from leaking from the nozzle by combining with a porous brick like the fine bubble generator used in the method of the present invention. It is not necessary to provide complicated devices required for controlling the gas pressure and flow rate of the gas.

[0037]

EXAMPLES (Invention Example) Using 100 ton RH having the apparatus configuration shown in FIGS. 1 and 2, a test for producing highly clean steel was conducted under the following conditions.

Steel type: Al killed steel Molten steel temperature: 1650 to 1700 ° C. Porous brick: length 2000 mm, width 50 mm Porous brick material: high alumina (porosity: 40%) Nozzle material: alumina (outer diameter 1.5 mm, inner diameter 1 mm) ) Nozzle distribution density: 1 nozzle / cm ² Projection height of molten steel into nozzle: 20 mm Nozzle embedded in porous brick: 8 mm Immersion tank pressure: 100 to 1 Torr Ar gas flow rate for fine bubble generation: 1 Nm ³ / min Refluxing Ar gas flow rate: 1.2 Nm ³ / min Refractory paint material: 30% SiO ₂ -70% Al ₂ O ₃ refractory paint coating thickness: 3 mm (Comparative Example 1) Nozzle and refractory paint Was used, and the other conditions were the same as those of Example 1 of the present invention.

Comparative Example 2 The conventional apparatus shown in FIG. 9 was used. The diameter of each of the two circular porous plugs was 360 mm, and the total flow rate of the Ar gas for generating fine bubbles was the same as that of Example 1 of the present invention and Comparative Example 1.

The removal rates of inclusions in the above three types of methods were compared by the same evaluation method as in the model experiment described above.
The results are shown in Fig. 8.

As shown in FIG. 8, it is understood that the removal rate of inclusions is most excellent in the case of the present invention.

[0042]

According to the method of the present invention, inclusions in molten steel can be efficiently and quickly removed, and highly clean steel can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing an apparatus example (RH) for carrying out the method of the present invention.

FIG. 2 is a vertical cross-sectional view of a fine bubble generator for carrying out the method of the present invention.

FIG. 3 is a vertical cross-sectional view of RH used in a water model experiment (case 1).

FIG. 4 is a vertical cross-sectional view of RH used in a water model experiment (case 2).

FIG. 5 is a diagram showing RH used in a water model experiment (case 3). (a) is a vertical sectional view, (b) is a horizontal sectional view and a plan view taken along line AA of (a).

FIG. 6 is a diagram showing the results of a water model experiment.

FIG. 7 is a schematic diagram illustrating a mechanism of generating large bubbles in molten steel when the fine bubble generator is a porous brick.

FIG. 8 is a diagram showing a comparison of inclusion removal rates in Examples.

FIG. 9 is a vertical cross-sectional view of RH showing a conventional method.

[Explanation of symbols]

1,17: Ascending pipe, 2,19: Ar gas for reflux, 3: Molten steel, 4: Ladle, 5, 22, 22 ': Porous brick,
6,24: Ar gas for generating fine bubbles, 7: Refractory paint,
8: Nozzle, 9,23: Micro bubbles, 10,10 ': Slag, 11,18: Downcomer, 12: Vacuum tank, 13: Ladle bottom refractory, 14: Container, 15: Water, 16: Model vacuum Tank, 20: polystyrene particles, 21: paraffin liquid, 23: pores, 24: bubbles, 25: large bubbles

Claims (2)

[Claims] 1. A vacuum bubble degasser (RH) is provided with a device for generating fine bubbles at the bottom of a ladle corresponding to an intermediate position between an ascending pipe and a descending pipe, and an inert gas is supplied from this device to molten steel in circulation. A method for producing high-cleanliness steel with few inclusions, which comprises blowing air. 2. A device for generating fine bubbles according to claim 1, wherein one end of a plurality of ceramics nozzles having an outer diameter of 2 mm or less is embedded in a porous brick, the other end is projected into molten steel and is in contact with molten steel. A method for producing highly clean steel with few inclusions, characterized in that an inert gas is blown into the circulating molten steel using an apparatus in which a refractory paint is applied to the surface of a porous brick.