JP2020158821A - Porous plug - Google Patents

Abstract

To provide a porous plug capable of improving air permeability and reducing infiltration of molten steel.SOLUTION: A porous plug is composed of 40-90 mass% of medium particles consisting of irregularly-shaped alumina having a particle diameter of 1-3 mm, 0-20 mass% of coarse particles consisting of irregularly-shaped alumina having a particle diameter of 3-5 mm, and the remainder being fine powder composed mainly of alumina having a particle diameter of less than 1 mm.SELECTED DRAWING: None

Description

The present invention relates to a porous plug used to blow gas into molten steel.

The porous plug is a porous refractory that is attached to the bottom of the molten metal container and has air permeability for blowing gas into the molten metal, and is used for purposes such as stirring molten steel and floating molten steel inclusions.

As a form of wear of the porous plug, peeling due to steel infiltrated into the porous plug and melting loss due to oxygen cleaning can be mentioned.

After receiving the molten steel and before starting gas injection, the molten steel infiltrates the porous plug and solidifies to form an infiltration layer. Since sufficient gas is not blown by this permeation layer, the permeation layer on the surface of the porous plug is blown off (peeled) by gas pressure.

A similar osmotic layer is formed even after the gas injection is completed. This osmotic layer is subjected to oxygen cleaning because it significantly reduces the air permeability of the gas. Oxygen cleaning is performed by blowing oxygen from the container side and nitrogen or Ar from the opposite side to the porous plug after discharging the molten steel, and removing the solidified steel by permeating the permeated steel while oxidizing and burning it.

Due to the above-mentioned treatment, the porous plug is worn out and the service life is shortened, or the porous plug does not function sufficiently. Therefore, the porous plug is required to reduce the infiltration of molten steel.

Therefore, in order to reduce the infiltration of molten steel, particles having a small particle size are used to reduce the pore diameter, and spherical particles are applied as an aggregate to facilitate the formation of continuous ventilation holes and improve the air permeability. As a result, a method has been adopted in which the porosity can be reduced while maintaining air permeability (for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 9-52168

Porous plugs are often required to have a large flow rate in order to improve the stirring efficiency of molten steel. Further, the porous plug may be used for the purpose of promoting the metallurgical reaction other than the purpose of stirring. For example, it may be used for the purpose of nitrogen blowing, absorbing, and reacting with molten steel, but since nitrogen takes time, a large flow rate is required.

However, the conventional porous plug using spherical fine particles as an aggregate cannot blow a large flow rate of gas because the pore diameter is small. In order to blow a large flow rate of gas, it is conceivable to increase the pore diameter, but since continuous ventilation holes are formed when spherical particles are used as aggregate, a short time from after receiving steel to blowing gas. There was a risk that the molten steel would be greatly infiltrated and gas could not be blown.

Therefore, an object of the present invention is to provide a porous plug capable of improving air permeability and reducing infiltration of molten steel.

In the invention according to claim 1, the porous plug has 40-90% by mass of medium particles made of irregularly shaped alumina having a particle size of 1-3 mm and coarse particles made of irregularly shaped alumina having a particle size of 3-5 mm. A technique in which the grains are composed of 0 to 20% by mass and the balance is fine powder containing alumina as a main component having a particle size of less than 1 mm, and the total of the medium grains and the coarse grains is 75 to 95% by mass. Use the means.

In the invention according to claim 2, in the porous plug according to claim 1, the medium grain is composed of 65-85% by mass, the coarse grain is composed of 5-15% by mass, and the balance is composed of the fine powder. , The technical means that the total of the medium grain and the coarse grain is 75-95% by mass is used.

In the invention according to claim 3, in the porous plug according to claim 1 or 2, the technical means that the medium grain, the coarse grain and the fine powder are crushed grains is used.

The invention according to claim 4 uses the technical means that the porosity is 20-35% in the porous plug according to any one of claims 1 to 3.

In the invention according to claim 5, in the porous plug according to any one of claims 1 to 4, the fine powder contains at least one of chromia, magnesia, spinel, zircon and silica. The technical means is used.

According to the porous plug of the present invention, by using an aggregate having a large particle size, the air permeability can be improved and a large amount of gas can be introduced into the molten steel. Further, as compared with the case of using a spherical raw material, continuous pores are less likely to be formed, and infiltration of molten steel can be reduced.

It is explanatory drawing explaining the calculation of the cross-sectional shape and the pore diameter of a porous plug. FIG. 1 (A) is a cross-sectional photograph, FIG. 1 (B) is a binarized image of FIG. 1 (A), and FIG. 1 (C) is a diagram showing the pores divided in order to calculate the pore diameter. .. It is explanatory drawing which shows the relationship between the pore diameter and the permeation amount.

The present inventors have conducted diligent research to solve the above-mentioned problems, and have found a porous plug that can achieve both improvement of air permeability and reduction of infiltration of molten steel. The technical concept of material design is shown below.

Since it is necessary to increase the pore diameter in order to improve the air permeability of the porous plug, particles having a large particle size are used. On the other hand, since the infiltration of molten steel increases when the pore diameter is increased, a particle shape is adopted in which the length of communication of the pores is short and a structure having a complicated pore path can be formed to prevent the infiltration of molten steel. It was configured to reduce.

Hereinafter, the configuration of the porous plug of the present invention (hereinafter referred to as a porous plug) will be described.

The porous plug has 40-90% by mass of medium particles made of irregularly shaped alumina having a particle size of 1-3 mm and 0-20% by mass of coarse particles made of irregularly shaped alumina having a particle size of 3-5 mm. The balance is composed of fine powder containing alumina as a main component having a particle size of less than 1 mm, and the total of medium and coarse particles is 75-95% by mass. Further, it is preferably composed of 65-85% by mass of medium grains, 5-15% by mass of coarse grains, and fine powder as the balance, and the total of medium grains and coarse grains is 75-95% by mass. ..

In the present invention, the particle size is defined as "coarse grain" when the particle size is 3 mm or more and less than 5 mm, "medium grain" when the particle size is 1 mm or more and less than 3 mm, and "fine powder" when the particle size is less than 1 mm. Here, the particle size is obtained by sieving using a test sieve specified in Japanese Industrial Standards JIS Z 8801-1: 2006.

The "irregular shape" mainly indicates a shape other than a spherical shape, and for example, crushed particles produced by crushing a sintered body are particles having an irregular shape. In this embodiment, crushed grains were used as the medium grains, coarse grains, and fine powders. Since the crushed grains have high irregularity in shape, they can be preferably used.

The porous plug is mainly composed of medium particles and coarse particles having a larger particle size than the particles constituting the conventional porous plug in order to increase the pore diameter. Here, if the total of the medium grains and the coarse grains is small, the amount of fine powder increases and the porosity decreases, and if it is large, the binding force becomes insufficient and the strength decreases, so the total of the medium grains and the coarse grains decreases. Was 75-95% by mass.

If the amount of medium grains is small, the amount of fine powder increases and the area constituting the matrix increases, so that the pore diameter becomes small and the air flow rate decreases, and if the amount is large, the amount of fine powder decreases relatively to form the matrix. Since the region is reduced and the strength is lowered, it is set to 40-90% by mass, more preferably 65-85% by mass.

The structure having coarse grains improves the air permeability, but if the amount is too large, the pore diameter becomes too large and the infiltration of molten steel increases. Therefore, the amount of coarse grains was 0-20% by mass when the amount of medium grains was 40-90% by mass, and 5-15% by mass when the amount of medium grains was 65-85% by mass.

The fine powder constitutes the balance, and the medium and coarse grains are combined to form a matrix. The fine powder may contain at least one or more of chromia, magnesia, spinel, zircon and silica in addition to alumina. Chromia has the effect of improving slag erosion resistance. Magnesia and spinel are binding aids and have an effect of improving corrosion resistance and also have an effect of improving peelability (breathing recovery property) in order to reduce heat-resistant spalling property. Zircon is a binding aid and has an effect of improving corrosion resistance. Silica is a binding aid.

Irregularly shaped particles were used as the raw material for the following reasons.

In order to improve the air permeability of the porous plug, it is necessary to increase the pore diameter, but increasing the pore diameter shows the contradictory property that the infiltration of molten steel increases.

Conventionally, when non-spherical particles are used as the main aggregate, the filling of the aggregate becomes uneven, the pore shape becomes irregular, and the air permeability is insufficient (for example, Patent Document 1 No. 3). Paragraph).

However, the inventor has found that by using irregularly shaped particles having a large particle size, air permeability is ensured, and infiltration of molten steel can be reduced due to uneven packing of particles and irregular pore shape. By optimizing the particle size, compounding ratio, etc., it was possible to achieve both contradictory properties.

If the porosity of the porous plug is too high, the infiltration of molten steel will increase and the strength will also decrease. On the other hand, if it is too low, the air permeability will decrease, so 20-35% is preferable. Depending on the blending ratio of the medium grain, coarse grain and fine powder of the present invention, the porosity can be a porous plug in a suitable range.

The method for manufacturing the porous plug of the present invention is as follows. First, each raw material and a sintering aid (several%) are weighed and mixed, and after kneading, they are molded into a predetermined shape. A porous plug can be obtained by firing this molded product in the air at a firing temperature of 1500 ° C. or higher.

The porous plug of the present invention can be suitably used for applications that require a large flow rate of gas to be blown into molten steel. For example, in a mass refining process in which the amount of molten steel processed is large, it is necessary to increase the amount of gas blown, so that it can be preferably used. Further, even when a plurality of porous plugs are required, the number of porous plugs can be reduced in order to obtain the same gas injection.

Further, when a metallurgical reaction such as nitrogenation is caused by gas introduction, it is required to blow a large flow rate of gas, so that the porous plug of the present invention can be preferably used.

(Effect of embodiment)
According to the porous plug of the present invention, by using an aggregate having a large particle size, the air permeability can be improved and a large amount of gas can be introduced into the molten steel. Further, as compared with the case of using a spherical raw material, continuous pores are less likely to be formed, and infiltration of molten steel can be reduced.

The present invention will be described below by way of examples. However, the present invention is not limited to these examples. Table 1 shows the blending ratio, porosity, bulk specific gravity, pore diameter, air volume, and permeation layer thickness of the raw materials of Examples 1-3 and Comparative Example 1-4, which are the porous plugs of the present invention.

Here, Comparative Examples 1 and 2 mainly use spherical particles. Although crushed grains are used in Comparative Examples 3 and 4, the blending ratio is outside the scope of the claims of the present invention.

All the samples were kneaded with the various raw materials shown in Table 1 and pressure-molded with a 300t friction press to prepare a truncated cone-shaped molded product having a diameter of 80 x φ140 x h330 mm, and the obtained molded product was dried at 150 ° C. for 24 hours. After that, it was produced by firing at a temperature of 1500 ° C. or higher for 15 hours.

The porosity was calculated by the following method. The dimensional bulk specific gravity was calculated by cutting out a rectangular parallelepiped sample and measuring the dimensions and weight after polishing. The apparent specific gravity was measured by the Archimedes method (vacuum), and the true density was measured by the gas method (Japanese Industrial Standard JIS Z 8807: 2012).

(Equation 1)
-Total porosity: (1-dimension bulk specific gravity / true density) x 100
-Sealed porosity: (1-apparent specific gravity / true density) x 100
・ Apparent porosity: total porosity-sealed porosity

The pore size was evaluated as follows. FIG. 1 shows a method for evaluating the pore diameter of Example 1. First, a micrograph was taken by magnifying the cross section of the sample 70 times (FIG. 1 (A)). Next, the photographed micrograph was subjected to black-and-white binarization processing using image processing software ImageJ (Wayne Rasband) (FIG. 1 (B)). Here, the occupancy rate of the black portion corresponds to the area ratio occupied by the pores. Then, the pores were divided in order to calculate the pore diameter (FIG. 1 (C)), and the equivalent diameter was calculated from the area to obtain the pore diameter.

As shown in FIG. 1A, it was confirmed that in the porous plug of the present invention, since the shape of each raw material is irregular, the pore shape is complicated and continuous pores are difficult to be formed.

Raw material filling rate, air volume, and permeation layer thickness were adopted as evaluation items.

The raw material filling rate was evaluated as follows. First, a graduated cylinder having a capacity of 500 ml is prepared, and the raw material particles are filled up to a scale of 500 ml. Next, the weight of the filled particles was measured, and the raw material filling rate was calculated by the following formula. Here, since the spherical raw material corresponding to the particle size of the medium grain and the coarse grain could not be obtained, the filling rates of the spherical raw material having a particle size of 0.5-1 mm and the irregularly shaped raw material were compared.

(Number 2)
Filling rate (%) = Filling weight (g) / (Raw material bulk density x 500 (ml)) x 100

The filling rate of the irregularly shaped raw material having a particle size of 0.5-1 mm was 49%, the filling rate of the spherical raw material was 58%, and the filling rate of the irregularly shaped raw material was smaller than the filling rate of the spherical raw material. It is considered that the filling rate of the irregularly shaped raw material is smaller than the filling rate of the spherical raw material even in the medium grain and the coarse grain.

The air volume was evaluated as follows. A sample formed into a cylinder having a diameter of 50 × 50 mm was attached to an airtight holder, and the flow rate of air at 20 ° C. when the holder was blown at a pressure of 0.01 MPa was measured.

The thickness of the permeation layer was evaluated as follows. A rotary erosion test was carried out on a sample cut out in a block shape of 50x85x55x115 mm (upper side x lower side x height x trapezoidal length), the cross section of the sample after the test was observed, and the maximum permeation layer thickness was measured. In the rotary erosion test, a sample was arranged in a cylindrical shape, molten steel and synthetic slag components were put therein, and a cycle of rotating and holding the sample at 1650 ° C. for 2 hours was carried out for 10 cycles.

In the examples, the amount of coarse particles added decreases in the order of Examples 1, 2, and 3. As the amount of coarse particles added decreased, the amount of aeration decreased and the amount of permeation tended to decrease.

In Comparative Examples 1 and 2 in which spherical particles were used as a raw material, the air permeability was low, and in Comparative Example 1 having a large pore diameter, the permeation amount was larger. Further, in Comparative Example 3 in which the number of coarse particles was larger than the range of the present invention, the permeation amount was large. In Comparative Example 4 in which crushed particles having a small particle size were used, the aeration amount was small.

The air flow rate of the examples was larger than that of the comparative examples, and a large flow rate exceeding about 70 NL / min could be obtained. With this air volume, it was confirmed that the porous plug of the present invention can be applied to applications requiring a large flow rate.

FIG. 2 shows the relationship between the pore diameter and the permeation amount. In Comparative Examples 1 and 2 using the spherical particles, when the pore diameter is increased, the permeation amount rapidly increases, whereas in the examples, the increase rate of the permeation amount is small with respect to the increase rate of the pore diameter. If the pore diameter is increased in order to increase the air permeability, the permeation amount rapidly increases when spherical particles are used, which is unsuitable for use in an actual machine. On the other hand, in the examples, it was confirmed that the permeation amount did not increase sharply even if the pore diameter was increased.

From the above, it was confirmed that the porous plug of the present invention can improve the air permeability and reduce the infiltration of molten steel.

Claims (5)

Medium grains made of irregularly shaped alumina with a particle size of 1-3 mm were 40-90% by mass.
Coarse particles made of irregularly shaped alumina with a particle size of 3-5 mm were added to 0-20% by mass.
The balance is composed of fine powder containing alumina as a main component with a particle size of less than 1 mm.
A porous plug characterized in that the total of the medium grains and the coarse grains is 75-95% by mass. The medium grain was 65-85% by mass,
The coarse grains were added to 5-15% by mass.
The rest is composed of the fine powder.
A porous plug characterized in that the total of the medium grains and the coarse grains is 75-95% by mass. The porous plug according to claim 1 or 2, wherein the medium grain, the coarse grain and the fine powder are crushed grains. The porous plug according to any one of claims 1 to 3, wherein the porosity is 20-35%. The porous plug according to any one of claims 1 to 4, wherein the fine powder contains at least one of chromia, magnesia, spinel, zircon and silica.