JPS5910864B2 - Porous refractories and their manufacturing method - Google Patents

Description

Detailed Description of the Invention The present invention relates to a bolus refractory used when blowing gas into molten steel from the bottom or side wall of a molten steel container such as a tundish in continuous casting equipment to promote the floating of inclusions, and the like. Regarding the manufacturing method.

In continuous casting, molten steel is poured into a tundish from a ladle, flows through the tundish in its longitudinal direction, and is supplied into the mold from a tundish nozzle attached to the bottom.

The molten steel poured from the ladle contains various non-metallic inclusions, and when these enter the mold and are captured by the solidified shell, inclusion defects occur in the slab.

For this reason, a plurality of bolus plugs made of highly breathable bolus refractories are arranged in parallel in the width direction of the tundish between the position where the molten steel is poured from the ladle in the tundish and the position where the tundish nozzle is installed. By blowing an inert gas such as Ar or N2 into the molten steel through the bolus plug and forming an upward flow of bubbles by the inert gas to promote floating separation of non-metallic inclusions in the molten steel, Attempts have been made to remove nonmetallic inclusions.

However, in this case, as shown in FIG. 9, which is an enlarged schematic view of the area surrounding the bolus plug 7 fitted into the bottom brick of the tundish 10, the diameter of the ventilation hole of the porous plug 7 itself is 5.
Although the diameter of the bolus plug 7 is extremely small (approximately 0 μm), since the distance between the vent holes is not sufficient compared to the diameter of the vent holes, the bubbles generated by injecting inert gas into the molten steel coalesce with each other, and the diameter of the bolus plug 7 ( The problem is that the bubbles become as large as a few centimeters (usually several centimeters), and that the molten steel is stirred in vain and a sufficient effect cannot be obtained in removing non-metallic inclusions.

In other words, air bubbles have a surface area per unit volume of air bubbles (specific surface area).
Since the molten steel is small, it is disadvantageous for capturing inclusions, and there is also the disadvantage that new inclusions are generated due to slag entrainment due to excessive stirring of molten steel. This is because when manufacturing refractories for plugs, only the porosity, which controls the quality of air permeability, is taken into consideration, but no consideration is given to the pore diameter and mutual spacing of the bolus refractories. be.

That is, conventional methods for manufacturing porous refractories include a method of forming the refractory using coarse aggregate and a method of obtaining air permeability using fibers.

The former involves forming relatively coarse aggregate (refractory material) into a bolus refractory, but there are gaps between aggregate particles;
Through this gap, pores are formed that communicate and connect from one end of the porous refractory to the other, producing a bolus refractory having a random but somewhat constant porosity.

However, in this case, the porosity of the porous refractory can be adjusted by adjusting the particle size of the aggregate particles.
The diameter of the pores and the distance between the pores cannot be adjusted at all, resulting in the disadvantage that air bubbles are generated as described above.

In addition, the latter is a bolus refractory in which small diameter fibers are arranged in the same direction or in a base shape and embedded in the aggregate, and when this is fired, the fibers burn and disappear, and the areas where the fibers were present become pores. Manufactured.

In this case, the direction of ventilation can be set in the desired direction by adjusting the direction in which the fibers are arranged, but because the strength of the fibers is low, it is impossible to control the distance between the pores to the desired value. Nearby, even this porous refractory cannot suppress the formation of air bubbles.

The present invention was made in view of such circumstances, and by carefully studying the pore diameter and pore spacing in order to form a bubble upward flow in which fine bubbles float orderly in molten steel,
An object of the present invention is to provide a porous refractory that can effectively float and separate nonmetallic inclusions in molten steel, and a method for manufacturing the same.

The present invention will be specifically explained below.

First, the results of a water model experiment regarding the relationship between the pore diameter of the bolus refractory and the bubble diameter of the bubbles generated from the bolus refractory will be explained.

A bolus plug made of bolus refractory material having a certain pore size is fitted into the bottom of a gutter-shaped container made of transparent acrylic resin and manufactured to simulate a reduced tundish, and the filtrate flows through the container. Using experimental equipment configured to blow gas through the porous plug to form an upward flow of bubbles, bubbles were formed by the porous plug using various bolus plugs with different pore sizes. The relationship between the pore diameter and the pore diameter was investigated.

As a result, when the flow rate of the filtrate flowing through the container is low, less than 500 in Raynozzle number, the following equation (1) holds true between the bubble diameter db immediately after generation and the pore diameter d.

However, db: bubble diameter (cm) d: pore diameter (crn) σ: surface tension (about 1 sooayn/cm) g: gravitational acceleration (about 980 dyn/g) ρ: molten steel specific gravity (about 7.0g fake) ρ: Gas specific gravity (g/ffl) Here, ρ》ρ2, so ρ−ρ2F! When approximated to ρ, equation (1) becomes as shown in equation (2) below.

Therefore, the bubble diameter db is expressed by the following equation (3).

In order to prevent the bubbles generated from the pores of the porous plug at the tundish bottom from combining with the bubbles generated from adjacent pores and growing into large bubbles, the distance P between the pores must be at least as large as possible. It needs to be larger than the bubble diameter immediately after.

Therefore, the distance P between the pores of the porous refractory is expressed by the following equation (4) from equation (3).

However, P: distance between pores (am) Next, the results of an investigation using the water model experimental equipment described above regarding the bubble diameter in an upward bubble flow that can effectively remove nonmetallic inclusions in molten steel will be explained.

Acrylic fine particles were added to simulate inclusions in the water in the container, and the relationship between the removal rate of the fine particles and the bubble diameter was investigated.

FIG. 1 is a graph showing the relationship between the bubble diameter on the horizontal axis and the particle removal rate on the vertical axis.

As described above, when the bubble diameter becomes small, such as 0.5 cm or less, the removal rate of fine particles increases significantly.

Regarding the bubble shape, the bubble diameter is 0. When the bubble diameter is 5 an or less, the bubble maintains its spherical shape and floats up in an orderly manner according to Stokes' theorem, just like solid floating objects, but when the bubble diameter is large (0.5 to 1.0 an), it becomes spherical during the floating process. It deforms into an elliptical shape, and its levitation trajectory also takes on a spiral shape.
Furthermore, the bubbles collide and coalesce with each other, and the upward flow of the bubbles becomes extremely random.

For this reason, and because the specific surface area is small, air bubbles are thought to have a low effect on promoting the floating of fine particles, and in order to effectively float and remove nonmetallic inclusions in molten steel, it is necessary to have zero. It is necessary to form an upward flow with bubbles having a diameter of 5 ann or less.

The following equation (5) is established from this db≦0.5 and equation (3).

d≦0. 0 8 (am) = (5) That is, the pore diameter d needs to be 0.08 cm or less.

As mentioned above, in order to effectively remove inclusions in molten steel, it is necessary to form an upward flow of minute bubbles with a diameter of 0.5 cm or less in the molten steel, and for this purpose, the pore diameter must be 0.08 cm.
In addition, in order to prevent the generated bubbles from coalescing and growing into large bubbles, the distance P between the pores should be smaller than that according to equation (4).

Next, a method for manufacturing a porous refractory that can adjust the pore diameter and pore spacing in this manner will be described.

The method for producing a bolus refractory according to the present invention comprises placing a plurality of mask plates made of a thermofusible or combustible material and having a diameter of d or more on the surface of an appropriately shaped porous refractory body, mutually disposing a plurality of mask plates having a diameter of d or more. A fluid refractory is applied onto the porous refractory element body to fill the gap between the mask plates, and then the mask plates and the fluid refractory are heated. It is characterized by sintering the fluid refractory while removing it by melting or burning.

This will be explained in detail below.

FIG. 2 is a schematic perspective view showing the implementation state of this bolus refractory manufacturing method.

The bolus refractory element body 3 is a bolus refractory manufactured by a conventional method.

That is, the bolus refractory body 3 is manufactured by molding using coarse aggregate as described above, or by embedding fibers, molding, and firing, and the pore diameter and pore spacing are not adjusted in any way. However, it has a substantially constant porosity.

Reference numeral 1 is a thin copper plate in the shape of a disc with an appropriate thickness.

As the plan view of the copper thin plate 1 is shown in Fig. 3, its diameter is equal to or slightly larger than the pore diameter d, which is determined to satisfy equation (5) when designing a bolus refractory. It is something.

Then, a large number of thin copper plates 1 are connected to thin wires 2 made of copper or the like so that the spacing between the copper thin plates 1 is equal to the pore spacing P determined to satisfy equation (4) in the design of bolus refractories. Connect in a mesh pattern.

As shown in the schematic perspective views of FIG. 2 and FIG.
Place the wire so that the joint between the wire and the thin wire 2 faces downward.

Next, in order to fill the gap between the thin copper plates 1 placed on the bolus refractory body 3 with the refractory 4, fluid refractory 4 is sprayed onto the bolus refractory body 3, and the thin copper plates 1 and A layer of refractory material 4 of the same thickness is formed.

That is, the refractory material 4 sprayed onto the bolus refractory material body 3
The thin copper plates 1 are scattered at predetermined intervals in the layer to form a layer in which both surfaces are flush with each other and the surfaces of the thin copper plates 1 are exposed.

Next, when this copper thin plate 1 and refractory material 4 are heated to a high temperature,
The thin copper plate 1 is melted and removed, and the refractory 4 is sintered.

As a result, the bolus refractory material body 3 having a constant porosity
A thin layer of refractory material 4 is formed on the surface of the refractory material 4, and this refractory material 4
In the refractory material 4, there is a hole with a diameter d in the part where the copper thin plate 1 has eluted.
It is formed to penetrate through the layer in the thickness direction, and a bolus refractory is manufactured in which the discharge surface of the pores has a pore diameter d and a pore interval P in an orderly manner.

Therefore, when a porous plug is manufactured using this bolus refractory and gas is blown into molten steel from the porous plug, an upward flow of bubbles in which minute bubbles with a bubble diameter of db {see formula (3)} float up in an orderly manner is generated into the molten steel. can be formed inside.

As is clear from the above description, since the thin copper plate 1 functions as a mask plate when spraying the fluid refractory 4, the required spacing can be maintained during spraying and firing of the refractory 4. Any material can be used as the mask plate instead of the thin copper plate 1 as long as it has strength and a melting point or combustion temperature that allows it to elute or burn when heated during firing.

From this point of view, instead of the copper thin plate 1, a plastic thin plate or a chain hydrocarbon resin thin plate having similar dimensions may be used.

Next, the effects of the present invention will be explained.

First, the results of a water model experiment comparing the upward flow of bubbles formed by a bolus nozzle made from the bolus refractory manufactured according to the present invention and the upward flow of bubbles formed by a conventional bolus nozzle will be explained in Fig. 5. 6, a bolus plug 6 according to the present invention (FIG. 5) and a conventional bolus plug 7 (FIG. 6) are fitted and installed at the bottom of a transparent acrylic bucket-shaped container 5, and the bolus plug 6, Various flow rates of air were blown into the water via 7.

The inner diameter of the container 5 is 30 cm, and the water depth is 20 cm.
It is m.

In addition, both bolus plugs 6 and 7 have a diameter of 2 mm.
It is in the shape of a cylindrical table with a height of 3 ann.

Table 1 shows the pore diameter d and pore spacing P of the bolus plugs 6 and 7.

In this way, in the bolus plug 6, d and P satisfy equations (5) and (4), respectively, and in the porous plug 7, d is (
5) formula is satisfied, but P is small and varies.

Therefore, the air flow rate is 1. In the case of OJJ/min, as shown in Fig. 5, the bubbles formed by the air blown into the water from the bolus plug 6 float to the surface in a laminar flow state, and become slightly turbulent within a range of about 2 cm below the water surface. On the other hand, as shown in FIG. 6, the air bubbles formed by the air blown into the water from the porous plug 7 are air bubbles and are in a turbulent state over almost the entire floating area.

Table 2 shows the relationship between the air flow rate and the state of bubble upward flow.

The unit of air flow rate is l/min, layer means laminar flow, and turbulence means turbulent flow.As described above, in the case of the bolus plug 6 according to the present invention, even if the air flow rate is increased, the bubbles float to the surface in a laminar flow state. In the case of the bolus plug 7, when the air flow rate exceeds 0.3 J; 3/min, turbulent flow occurs.

Next, the results of investigating the effect of removing inclusions by installing the bolus plugs 6 and 7 in an actual tundish will be explained.

FIG. 7 is a schematic vertical sectional view of the mustland type continuous casting equipment, and FIG. 8 is a schematic plan view of the tundish 10.

Molten steel (S i
-A7 killed steel) is passed through the tundish 10 toward both ends thereof, and from the nozzles 10a, 10a to the mold 1111.
It is cast into slabs 12, 12.

In order to prevent the molten steel injected from the ladle 8 into the tundish 10 from being oxidized by air, the upper opening of the tundish 10 and around the nozzle 8a are covered with a hood 9, and Ar gas is introduced into the hood 9. It is sealed with Ar.

Between the ladle injection flow position at the bottom of the tundish 10 and the nozzles 10a and 10a, bolus plugs 6 and 7 are inserted in the width direction of the tundish 10, respectively.
It has been installed.

The diameter of this porous plug 6 or 7 is <2 as described above.
an, and the distance between the edges is 3an.

Ar gas was blown into the molten steel from each of the porous plugs 6 and 7 at a flow rate of 0.517 minutes.

In this case, samples taken from the molten steel in the ladle 8, samples taken from slabs cast on the strand on the side where the porous nozzle 6 was installed, and samples taken from the slab cast on the strand on the side where the porous nozzle 7 was installed. After polishing the sample, the number of inclusions was visually observed using a microscope.

The results are shown in Table 3. Bolus nozzle 7 like this
The number of inclusions in the slab using the method is 3.9/1, which is approximately the same as the number of inclusions in the molten steel in the ladle, 4.2/1, and the inclusion removal rate by the porous nozzle 7 is On the other hand, when the bolus nozzle 6 is used, the number of inclusions in the slab is 1.7 pieces/d, which shows an extremely high inclusion removal rate, and the bolus nozzle 6 has an excellent inclusion removal effect. That is clear.

As detailed above, the porous refractory according to the present invention is a bolus refractory installed in a molten steel container to form air bubbles in molten steel, and the pore diameter d and the distance P between each pore are determined by the formula (4) above. The bolus refractory according to the present invention is characterized in that the pore diameter d is set to satisfy the relationship of formula (5) above. For example, when gas is blown into molten steel flowing through a tundish using a device, an upward flow of minute bubbles with a diameter of 0.5 ann or less can be formed in the molten steel, and the bubbles can coalesce with each other. Since it is possible to form a bubble flow that floats in an orderly laminar flow state without forming air bubbles, the effect of promoting the floating of inclusions in molten steel is high, and inclusions can be effectively removed. , the present invention has extremely excellent effects.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between bubble diameter and particulate removal rate.
2 and 4 are schematic perspective views showing the implementation state of the bolus refractory manufacturing method according to the present invention, FIG. 3 is a schematic view illustrating the diameter and spacing of the thin copper plates 1, and FIG. Fig. 6 is an explanatory diagram of a water model experiment showing the effects of the present invention, Fig. 7 is a schematic vertical cross-sectional view of continuous casting equipment, and Fig. 8 is a tundish 10.
FIG. 9 is a schematic plan view of a conventional porous plug. 1... Copper thin plate, 2... Thin wire, 3...
... Porous refractory body, 4 ... Refractory material, 6
,7...Porous plug.

Claims (1)

[Claims] 1. In a porous refractory installed in a molten steel container to form air bubbles in molten steel, the pore diameter d and the distance P between each pore are defined as follows: σ: surface tension g: gravitational acceleration ρ: A porous refractory characterized in that both the specific gravity of molten steel satisfy the relationship according to the CGS unit system. 2. On the surface of an appropriately shaped porous refractory body, a plurality of mask plates made of a thermofusible or combustible material and having a diameter of d or more are placed at a distance of at least P from each other, and the space between the mask plates is A fluid refractory is applied onto the bolus refractory element to fill the gap, and then the mask plate and the fluid refractory are heated, thereby melting or burning off the mask plate and removing the fluid refractory. A method for producing a bolus refractory, characterized by sintering. However, P: Air-drying L interval d: Pore diameter σ: Surface tension g: Gravitational acceleration ρ: Molten steel specific gravity All are based on the CGS unit system.