JPH1025168A - Ramming material for induction furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a ramming material for an induction furnace, capable of safely operating by solving peeling off of a sintered layer on the furnace wall in a high power rapid melting and large-sized high frequency induction furnace in recent years and forming a long life furnace wall. SOLUTION: This ramming material for induction furnace contains 3-20wt.% magnesia, 3-20wt.% spinel and 0.2-1.0wt.% boric acid, and alumina as the balance. It is preferable that the base powder grain size of the magnesia of the ramming material for induction furnace is <=0.3mm and that that of the spinel is <=1.0mm. Further, it is preferable that the boric acid is ortho-boric acid and has <=0.3mm grain size powder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ramming material for an induction furnace, and more particularly to a ramming material for an induction furnace applicable mainly to a lining material of a large induction furnace operated at a high temperature for melting cast steel.

[0002]

2. Description of the Related Art Conventionally, as a lining material of an induction furnace in which cast steel or the like is melted at a high temperature, a basic ramming material mainly composed of magnesia, such as magnesia-alumina or magnesia-spinel, and an alumina-magnesia material are used. A neutral ramming material based on alumina is used. Magnesia-alumina material and magnesia-spinel material of the basic ramming material have a high melting point of 2800 ° C. as a main component of magnesia, and have properties chemically stable to a basic molten metal or slag. ing. However, magnesia has a large coefficient of thermal expansion, poor resistance to thermal shock resistance, and cannot be applied to the furnace wall of an induction furnace in which melting and tapping are performed in a relatively short time and rapid thermal quenching is continuously repeated. . For example, when the induction furnace wall in which the above-described heat cycle occurs is formed of a basic ramming material, cracks are easily generated on the inner surface of the furnace wall due to a rapid temperature change, and troubles such as hot water are caused by the generated cracks. It is easy to happen. It has been well known that when an induction furnace wall as described above is formed of a basic ramming material, the trouble caused by cracks generated in the furnace wall during operation is proportional to the size of the furnace. Therefore, the basic ramming material is usually used only in small furnaces of 1 ton class or less in consideration of operational safety.

[0003] On the other hand, the alumina-magnesia neutral ramming material is also called an alumina-spinel ramming material, the main component of which is alumina having a high melting point of 2000 ° C. or more and high corrosion resistance, to which magnesia is added. Things. This alumina / spinel material has a smaller coefficient of thermal expansion compared to magnesia as a main component of the above-described basic ramming material, has a volume stability during hot, has excellent thermal shock resistance, and is in operation. Then, alumina and magnesia react with each other to generate spinel (secondary spinel), and the generation of cracks is reduced due to residual expansion at the time of generation, and the expansion of cracks is prevented. When a neutral ramming material is used for the induction furnace wall as described above, even if cracks occur on the operating surface, it is minor and has higher safety than the basic ramming material, so it is particularly suitable for melting cast steel. Neutral ramming material is suitably used as a 5-ton class large high-frequency induction furnace wall lining material operated at a high temperature.

[0004]

When the induction furnace wall is formed of the above-described ramming material, a method is generally employed in which a dry powder of a predetermined blended raw material called a dry ramming material is filled and installed. The dry ramming material is sintered using a predetermined molten metal to increase the inner surface strength and subjected to the same treatment as before in operation to form a stable sintered layer. In this case, in order to increase the electric efficiency in the high-frequency induction furnace,
Normally, the wall of the neutral ramming material furnace is designed to be as thin as about 60 to 90 mm, but the inner wall surface in contact with the high-temperature molten metal has a high temperature, while the outer surface in contact with the water-cooled induction coil has a low temperature.
For this reason, the inside of the furnace wall has an extremely large temperature gradient, and the sintering layer is formed on the working surface side of the ramming material furnace wall, but the unsintered layer (powder layer) is formed on the back side thereof. Will remain. As the raw material powder constituting the alumina-magnesia neutral ramming material forming the furnace wall of a large-scale 5-ton induction furnace, high-purity fused alumina and fused magnesia with poor sinterability have been used. It was normal to do. Therefore, the phenomenon that only the surface sinters is more remarkable especially in the furnace wall of a large high-frequency induction furnace, and only the very surface of the operating surface sinters, and most of the sintered layer immediately on the rear side is not yet formed. It is a powder layer for sintering. As a result, structural spalling due to infiltration of slag components, thermal spalling due to rapid heat quenching, physical impact at the time of charging molten material, etc.
Troubles such as the peeling of the sintered thin layer formed only on the operating surface may occur, and the durability and stability of a large high-frequency induction furnace are problematic.

The present invention has a problem when a neutral ramming material is used as a refractory for a wall of a large high-frequency induction furnace for a cast steel in which the above-described melting and full tapping are performed and a rapid temperature change occurs. In consideration of the spalling phenomenon of the sintered layer, it has excellent resistance to thermal shock due to thermal cycle and spalling due to infiltration of slag components, and there is no peeling of the working surface sintered layer, and the melting and tapping of cast steel is stable. It is an object to provide a ramming material for a furnace wall that can be repeated. The inventors reexamined various conventional ramming materials to achieve the above object. As a result, they have found that the conventional problems described above can be solved by positively adding spinel without waiting for the formation of secondary spinel by the reaction between alumina and magnesia, and completed the present invention.

[0006]

According to the present invention, the composition contains 3 to 20% by weight of magnesia, 3 to 20% by weight of spinel and 0.2 to 1.0% by weight of boric acid, and the remainder contains alumina. A ramming material for an induction furnace is provided. In the induction furnace ramming material of the present invention,
The raw material powder particle size of magnesia is preferably 0.3 mm or less, the raw material powder particle size of spinel is preferably 1.0 mm or less, and the boric acid is orthoboric acid, and the powder having a particle size of 0.3 mm or less is preferable. preferable.

The present invention is configured as described above, and since a predetermined amount of spinel is added, the generated sintered layer can suppress the infiltration of the slag component from the beginning. In addition, since boric acid as a sintering aid is in a predetermined amount, the strength of the sintered layer on the working surface is secured, and the entire ramming material is not cured, and even if cracks occur, the cracks are prevented from expanding, Further, it is possible to prevent the molten metal from reaching the back surface and leaking.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The ramming material for an induction furnace according to the present invention contains alumina, magnesia, and spinel as main components, and is formed by adding boric acid thereto. In the ramming material of the present invention, magnesia reacts with alumina during the melting operation of the molten metal for cast steel to generate secondary spinel, and the expansion of cracks can be prevented and reduced from the residual expansion that rapidly expands simultaneously with the generation. The magnesia content is 3-20% by weight, preferably 5%
１５15% by weight. If magnesia is less than 3% by weight, the effect of reducing cracks cannot be expected. On the other hand, if the content exceeds 20% by weight, the above-mentioned expansion becomes excessively large, and the refractory structure is relaxed and the corrosion resistance is lowered, which is not preferable. The ramming material of the present invention, by blending magnesia with the other components in the above-mentioned range, causes little cracking even in a large-scale high-frequency induction furnace of, for example, 5 ton class, and causes troubles such as molten metal leakage. Etc. are prevented. The raw material magnesia powder usually uses electrofused magnesia and has a particle size of 0.3.
mm or less, preferably 0.1 mm or less. This is for accelerating the reaction with alumina and facilitating the formation of secondary spinel.

In the ramming material of the present invention, spinel is a refractory of a composite oxide of alumina and magnesia.
It has a high melting point of 000 ° C or higher and has excellent corrosion resistance at high temperatures.
The slag component in the molten cast steel has the effect of suppressing infiltration into the refractory structure, can prevent the sintering layer from peeling off due to structural spalling, and improve the corrosion resistance to prevent the occurrence and expansion of cracks. be able to. The content of spinel is 3 to 20% by weight, preferably 5 to 15% by weight. If the spinel content is less than 3% by weight, the above effects cannot be expected, while if it exceeds 20% by weight, the thermal expansion coefficient becomes large and cracks are more likely to occur. This is because spinel has a smaller coefficient of thermal expansion than magnesia, but is larger than alumina. As the raw material spinel powder, an electrofused spinel is usually used, and its particle size is 1 mm or less, preferably 0.1 mm or less.
It is better to be 5 mm or less. Since the reaction between the molten metal component and the refractory and the infiltration of the slag component into the refractory structure are performed mainly in the matrix portion that joins the skeleton portion of the refractory, the spinel should be evenly dispersed in the matrix portion. The spinel is preferably as fine as possible, and preferably has a particle size of 1 mm or less.

In the ramming material of the present invention, as the alumina constituting the main component together with the magnesia and spinel, high-purity fused alumina or sintered alumina conventionally used as a refractory lining of an induction furnace may be used. it can. Alumina has a melting point of 2,000 ° C. or higher like the spinel described above, has high corrosion resistance, has a low coefficient of thermal expansion as described above, and has excellent volume stability of the operating surface in contact with the high-temperature molten metal.
In the present invention, a high-strength sintered layer is formed together with other components on the operating surface, and a secondary spinel is formed with the magnesia component to prevent crack propagation. The particles of the raw material alumina powder are preferably used in combination of several kinds of large and small. This is because a dense layer can be formed. Usually, particle size 1
Particles of 5 mm, 1 mm or less, and 0.3 mm or less are used in combination.

In the ramming material for an induction furnace of the present invention, the boric acid added to the above three main components has the effect of forming a dense sintered layer on the operating surface and of forming a hardened layer for backing up the sintered layer. In addition, the sinterability of the ramming material on the inner wall of the induction furnace is improved, the resistance to thermal spalling and physical impact is improved, and the peeling of the sintered layer can be prevented. The amount of boric acid added is 0.2-1.0% by weight, preferably 0.3-0.1%.
8% by weight. If the amount is less than 0.2% by weight, improvement in sinterability cannot be expected. On the other hand, if the content exceeds 1.0% by weight, the melting point of the refractory decreases, the corrosion resistance decreases, and the hardening proceeds from the working surface to the back side of the ramming material, and the same as the conventional neutral ramming material such as molten metal leakage. It is not preferable because a problem may occur in terms of safety. As the boric acid used in the present invention, it is usually preferable to use orthoboric acid (H ₃ BO ₃ ) from the viewpoint of storability. The particle size is preferably 0.3 mm or less, and more preferably 0.2 mm or less. This is because the sintering strength of the ramming material is quickly developed.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to embodiments. However, the present invention is not limited by the following examples. Examples 1 to 3 and Comparative Examples 1 to 3 FIG. 1 is an explanatory cross-sectional view of a main part of a method for building a high-frequency induction furnace used in the present example. The furnace shown in FIG. 1 has a coil protection refractory 2 arranged inside an outermost induction coil 1 and an insulating sheet 3 set on the inner surface of the coil protection refractory 2. A predetermined amount of each of the ramming material blends in the ratio was charged into the hearth 4 and was constructed with an air rammer. After the completion of the construction of the hearth 4, the construction surface of the hearth 4 is finished to be smooth, and the furnace cylinder 5 is provided with the heat insulating sheet 3 and the space 6 having a predetermined width. A plurality of partition plates (not shown) having a predetermined height are vertically set in the annular space 6 so as to be in contact with the heat insulating sheet 3 and the furnace cylinder 5 so as to divide the annular space 6 into equal parts. In the same manner as in the hearth, a predetermined amount of each ramming material compound was charged up to the height of the partition plate. Usually, it is charged so as to have a height of about 60 to 70 mm at a time. After charging, the surface of each compound was smoothed, the partition plate was pulled out to the upper part of the furnace, and the product was constructed using an air rammer. After that, roughing the construction surface to prevent lamination at the joint surface,
A partition plate was set again on the ramming material thus constructed, and the same procedure was repeated, and furnace wall construction was performed to the upper part of the furnace using the ramming material. As described above, the furnace construction of the high-frequency induction furnace with a capacity of 300 kg was completed. The furnace cylinder 5 was left as it was, and was melted together with pig iron charged thereafter.

The high-frequency induction furnace of 300 kg capacity constructed as described above has a furnace cylinder 5 having a diameter of 3 mm.
10 mmφ, and the height of the constructed hearth 4 is 200 m
m. The annular space 6 had a width of 70 mm, and the height of the furnace wall ramming material constructed on the hearth 4 was 600 mm. Next, pig iron was charged into the built high-frequency induction furnace, and a melting test was performed. In the dissolution test, pig iron was 200k
g, dissolving and holding at a high temperature of 1660-1680 ° C. for 5 hours, each of which is performed twice, each thickness of the sintered layer, the solidified layer and the powder layer, the depth of erosion and infiltration, the maximum crack The width was measured, and the results were averaged and shown in Table 1.

[0014]

[Table 1]

As is apparent from Table 1, (1) In Examples 1 to 3 of the ramming material of the present invention in which magnesia and spinel are blended in an amount of 5 to 15% by weight, neither erosion nor infiltration is observed at all. . In addition, it is observed that a solidified layer is formed on the back side of the sintered layer with a thickness of 25 to 35 mm, and the crack width is as small as 1 mm at the maximum, which indicates that it is very good. On the other hand, in (2) Comparative Example 1 in which no spinel or hydrated boric acid was used, erosion and infiltration were observed. In addition, the sintered layer was as thin as 15 mm, and only 5 mm of the solidified layer on the back side was formed, indicating that the sinterability was poor. Furthermore, (3) Comparative Example 2 using 25% by weight of magnesia and Comparative Example 3 using 25% by weight of spinel all show erosion of 1 to 2 mm and 0.3 to
It can be seen that a large crack of 0.4 mm is generated, and there is a possibility of molten metal leakage, which is not preferable.

[0016]

The ramming material for an induction furnace according to the present invention is superior in corrosion resistance, slag resistance and infiltration resistance to alumina by mixing spinel from the beginning, and has a predetermined amount of boric acid. The back side of the layer has a hardened layer as a backup layer with an appropriate thickness, and the powder layer remains on the back side. The sintering layer, which is a problem in induction furnaces, can be prevented from peeling off, and large-scale high-frequency induction furnaces can be safely and stably operated, and can be used for a long time and are industrially useful. .

[Brief description of the drawings]

FIG. 1 is an explanatory sectional view of a main part of a high-frequency induction furnace used in an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Induction coil 2 Coil protection refractory 3 Heat insulation sheet 4 Hearth 5 Furnace cylinder 6 Space (furnace wall)

Claims (3)

[Claims] 1. Magnesia 3-20% by weight, spinel 3
A ramming material for an induction furnace, characterized by containing -20% by weight and 0.2-1.0% by weight of boric acid, and the balance of alumina. 2. The raw material powder of magnesia has a particle size of 0.3.
mm or less, and the raw material powder particle size of the spinel is 1.0
2. The ramming material for an induction furnace according to claim 1, which has a thickness of not more than mm. 3. The method according to claim 1, wherein the boric acid is orthoboric acid,
3. The ramming material for an induction furnace according to claim 1, which is a powder having a particle size of not more than m.