JPH11147759A - Zirconia-graphite refractory - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the thermal impact resistance of a zirconia-graphite refractory without deteriorating the corrosion resistance of the zirconia-graphite refractory and maintain the balance of both the characteristics. SOLUTION: This refractory contains zirconia granules having granule diameters of >=0.2 mm, zirconia granules having granule diameters of <0.2 mm, and graphite as main components. Therein, the zirconia granules having granule diameters of >=0.2 mm are coated with coating layers comprising a mixture consisting mainly of zirconia granules having granule diameters of <0. 2 mm and graphite, and the rate of the graphite at parts brought into contact with the zirconia of the coating layers is higher than that at parts except the coarse zirconia granules.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an immersion nozzle used for injecting molten metal from a tundish into a mold in continuous casting of molten metal such as molten steel, and an injection of molten metal from a ladle to a tundish. The present invention relates to a zirconia-graphite refractory that can be suitably applied to a portion of a continuous casting nozzle, such as a long nozzle, used for contact with slag, or a stopper for controlling flow rate.

[0002]

2. Description of the Related Art Conventionally, continuous casting nozzles have been designed to be suitable for long-term use by combining graphite and various oxides such as zirconia, alumina and silica. In particular, where the nozzle is immersed in molten steel and the surface of the nozzle comes into contact with slag or mold powder, the erosion due to chemical erosion is large,
It often affects the life of the nozzle.

[0003] Among these oxides, zirconia has low reactivity with oxide slag as compared with alumina and silica, and has excellent corrosion resistance, especially to slag having low basicity and strong erosion.

The zirconia-graphitic refractory obtained by combining zirconia and graphite has excellent thermal shock resistance by being combined with graphite having characteristics of excellent thermal shock resistance due to low elastic modulus and high thermal conductivity. At the same time, it is superior in corrosion resistance to alumina-graphite refractories and alumina-silica-graphite refractories.

For example, in the case of an immersion nozzle, a zirconia-graphite refractory is applied to a portion in contact with mold powder to obtain excellent corrosion resistance. In addition, a zirconia-graphite refractory is applied to a portion of the long nozzle in contact with the slag, thereby obtaining excellent corrosion resistance to slag having a low basicity and a high erodibility.

[0006] In order to eliminate the disadvantage that graphite is easily dissolved in molten metal such as molten steel and easily oxidized while taking advantage of the characteristics of zirconia-graphitic refractory having excellent corrosion resistance, graphite at high temperatures is used. Metals such as aluminum and silicon which react with carbon to form a strong bond are also added, and as a result, the abrasion resistance of the zirconia-graphite refractory was improved.

Further, various attempts have been made to further improve the excellent corrosion resistance of zirconia-graphitic refractories. For this purpose, it is most effective to reduce the amount of graphite and increase the amount of zirconia. . However, there is a problem that the thermal shock resistance is reduced due to the reduced amount of graphite.

As means for solving this problem, Japanese Patent Laid-Open No. 7-2
The 14260 publication, cubic and the ZrO ₂ partially stabilized ZrO ₂ as a main component 75 to 90 wt%, zirconia purity contains 10 to 25 wt% of 94 wt% or more of graphite in the fixed carbon - Graphitic refractories are disclosed. As described above, attempts have been made to specify a raw material to be used or a particle size composition, but a sufficient improvement in thermal shock resistance has not been obtained.

[0009]

An object of the present invention is to improve the thermal shock resistance of the zirconia-graphite refractory without deteriorating its corrosion resistance and to improve the balance between the two properties.

[0010]

SUMMARY OF THE INVENTION The present invention provides a zirconia
The present invention was completed based on the finding that the balance between corrosion resistance and thermal shock resistance can be improved by optimizing the arrangement of zirconia and graphite in a graphite refractory.

A conventional zirconia-graphitic refractory is prepared by adding a binder mainly composed of phenolic resin to zirconia and graphite to obtain a uniform mixture, followed by molding and firing. Therefore, no particular consideration is given to the arrangement of graphite with respect to zirconia, and graphite and zirconia are generally uniformly dispersed.

Coarse-grained zirconia having a large particle diameter is effective in improving corrosion resistance, and has the effect of bypassing cracks when cracks occur, which is effective in improving durability. However, when the particle diameter is large, there is a portion where the elastic modulus is extremely large, and the elastic modulus is increased as a whole, so that there is a problem that the thermal shock resistance is reduced.
However, when the amount of the graphite covering the coarse zirconia is increased, the graphite plays a role of a buffer, and the increase in the elastic modulus is suppressed.

The effect of the present invention is remarkable for coarse zirconia having a zirconia particle diameter of 0.2 mm or more. Intermediate grains of less than 0.2 mm and finely divided zirconia hardly affect the increase in the elastic modulus, so that the thermal shock resistance hardly decreases.

The weight ratio of coarse zirconia having a particle diameter of 0.2 mm or more to the total amount of zirconia is 5 to 50.
% Is preferred. If the content is less than 5%, the influence of the increase in the elastic modulus due to the presence of the coarse particles is small, so that the necessity of applying the present invention is small. If it is 50% or more, a large amount of graphite must be coated in order to suppress the increase in the elastic modulus, so that the graphite in the fine powder portion is relatively significantly reduced, and the fine powder is agglomerated, resulting in substantially the same form as coarse particles. Therefore, there is a problem that the elastic modulus is increased.

Hereinafter, coarse zirconia having a particle diameter of 0.2 mm or more is simply referred to as coarse zirconia.

In the present invention, it is most preferable that a coating layer having a high proportion of graphite is formed for all of the added coarse zirconia. The effect is improved. In addition, the effect of reducing the elastic modulus is obtained as the proportion of the graphite covering the coarse zirconia increases, but when the graphite gathers only around the coarse zirconia, the graphite in the fine zirconia portion is relatively reduced, and the fine zirconia portion is reduced. Since agglomeration occurs and the form becomes substantially the same as that of the coarse particles, there is a disadvantage that the elastic modulus is increased. To give an example of a preferred range, the amount of graphite covering the coarse zirconia is suitably about 1.2 to 2.6 times the proportion of graphite in the portion of the refractory other than the coarse zirconia.

The range of the thickness of the coating layer, which covers the coarse zirconia and optimizes the amount of graphite, cannot be uniquely defined because it is affected by the amount of fine powder and the amount of graphite. It is preferable to form a coating layer in a range of 0.05 mm or more and 0.3 mm or less. If the thickness of the coating layer is too small, the effect of reducing the elastic modulus is reduced, and if it is too large, the strength may be reduced. Therefore, the thickness is preferably adjusted to an appropriate range. It should be noted that there is no problem if a small amount of coarse particles is appropriate even if the coating thickness is out of the range.

The coarse zirconia is coated with a raw material mixture mainly composed of zirconia having a particle diameter of less than 0.2 mm and graphite, and the portion of the coating layer in contact with the coarse zirconia, that is, the proportion of graphite in the first layer is determined by the fire resistance. 0.2 mm particle diameter
By setting the graphite ratio higher than the graphite ratio in the portion excluding the zirconia, the elastic modulus is reduced and the thermal shock resistance is improved as compared with the case where the same amount of graphite is uniformly dispersed in the structure. You.

That is, the zirconia-graphitic refractory of the present invention comprises coarse zirconia particles, zirconia particles having a particle diameter of less than 0.2 mm and graphite as main components, and the coarse zirconia particles have a particle diameter of 0%. .2 mm and covered with a coating layer composed of a mixture mainly composed of graphite and zirconia particles.

The portion of the coating layer in contact with the coarse zirconia, that is, the graphite ratio of the first layer is higher than the graphite ratio of the portion excluding the coarse zirconia in the refractory. And

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to increase the amount of graphite covering coarse zirconia, for example, a binder is first added to zirconia excluding coarse zirconia and coarse particles containing a larger amount of graphite than the average of the whole. After kneading and kneading to form a coating layer, a method of adding the remaining raw materials and binder, kneading again, and performing heat treatment after molding by a usual method can be adopted.

The coating layer of coarse zirconia may be formed as a single layer or a plurality of layers depending on the initial kneading method or the kneading method with the remaining raw materials, but the first layer in contact with the coarse zirconia may be formed. , The particle size of the refractory is 0.2
A sufficient effect can be obtained by having a graphite ratio higher than the graphite ratio in a portion excluding zirconia of mm or more.

It should be noted that the present invention is not limited to simple zirconia, but zirconia may contain Si, Si for preventing the oxidation of graphite.
The same applies to the case where C, B ₄ C and the like are added in small amounts up to about 3% by weight, and the case where ZrB ₂ is used instead of zirconia.

Example 1 This example confirms the effect of the proportion of the graphite in the first layer in contact with the coarse zirconia in the coating layer on the coarse zirconia.

Table 1 shows an example of the applied composition. No. 1 shows a conventional product for comparison. Two
No. 4 shows an embodiment of the present invention.

In the table, No. The kneading of No. 1 was carried out by mixing the entire amount of graphite and the entire amount of zirconia, and adding phenolic resin thereto. In addition, in Example No. Two
No. No. 4 first kneaded the raw materials for pre-kneading in advance to increase the amount of graphite coated on coarse zirconia having a particle size of 0.2 mm or more. Particles having a greater graphite coverage than the sample No. 1 were prepared, and a raw material for post-kneading was added thereto, followed by further kneading to obtain a blend.

The obtained composition was CIP-molded into a nozzle at a pressure of 1000 kg / cm ² , embedded in coke, and reduced and fired at a maximum temperature of 1000 ° C. By observing the cross section of the sample after firing with a microscope, one or two coating layers can be confirmed around the coarse zirconia, and the thickness of the first coating layer in contact with the coarse zirconia is calculated according to Table 1. The value almost coincided with the value.

Further, the bending strength, the elastic modulus, and the coefficient of thermal expansion were measured from the fired nozzle. Table 1 shows the measurement results and the results of the thermal shock resistance coefficient representing the spall resistance. The bending strength was measured by a three-point bending method, the elastic modulus was measured by an ultrasonic method, and the thermal expansion coefficient was measured by a commercially available thermal dilatometer, and showed an average linear expansion coefficient up to 1500 ° C. The thermal shock resistance coefficient was calculated by the following equation since the Poisson's ratio was almost constant.

(Bending strength) / [(elastic modulus) × (thermal expansion coefficient)] The larger the number, the better the spall resistance. As is clear from the measurement results, the No. 1 No. 1, which is an embodiment of the present invention, as compared with No. 1. It is clear that Nos. 2 to 4 have improved thermal shock resistance coefficients due to the effect of lowering the elastic modulus. However, when the content of graphite was no. No. 1 which is about 2.6 times that of No. 1 No. 4 is No. As compared with No. 3, the strength and the thermal shock resistance coefficient were reduced, and an increase in the amount of graphite beyond this value was inappropriate because the improvement effect was small. Accordingly, it was found that the more preferable ratio of graphite in the coating layer is about 1.2 to 2.6 times the ratio of graphite in the portion of the refractory other than coarse zirconia.

[0030]

[Table 1] Example 2 In this example, the effect of the thickness of the coating layer was confirmed. 5 to No. 5 10 shows six types of formulation examples and the characteristics of the obtained refractories. No. 5 shows a comparative example. Kneaded uniformly as in Example 1. No. 6-N
o. Reference numeral 10 denotes an embodiment of the present invention. 2-No. As in the case of No. 4, the amount of graphite coating on the coarse particles was increased. The molding, firing, quality evaluation, and the like after the preparation of the blend were performed in exactly the same manner as in Example 1. As a result of the quality evaluation, No. 1 of the embodiment of the present invention was obtained. 6 ~
No. No. 10 of the conventional product. It is apparent that the modulus of elasticity is smaller than that of No. 5 and the thermal shock resistance is excellent. In Examples, the thickness of the coating layer was 0.05 mm or less.
No. 6 and 0.3 mm or more. 10 is inferior in thermal shock resistance, and the preferable coating thickness is 0.05 mm to 0.3 m.
It is clear that m.

[0031]

[Table 2] Example 3 Table 3 shows the results of studies on the amount of coarse zirconia added. In the table, No. 11 to No. 14 is an example of the present invention, which has a low elastic modulus and is good. No. of the comparative example.
No. 15 is outside the scope of claim 4 of the present invention, and has a high elastic modulus.

[0032]

[Table 3] Example 4 This shows the result of being subjected to an actual furnace test. l and No. 1 which is an embodiment of the present invention. The material of No. 3 was applied to a powder line to prepare an immersion nozzle, which was subjected to an actual furnace test using a continuous slab casting machine. Ladle capacity is 300
ton, casting time per ch is about 40 minutes, same T
No. D 1 and No. The immersion nozzle to which No. 3 was applied was set. The number of test pieces was five each, and each piece was used for about 300 minutes on average. The used nozzles were collected and the erosion rate in the powder line was examined. The results were almost the same. No. No. 3 is No. It was found that the product of the present invention had the same corrosion resistance and contributed to stable casting because it was superior in thermal shock resistance as compared with Comparative Example 1. Further, superior thermal shock resistance means that there is room for further increasing the content of zirconia, and it is clear that the present invention also contributes to improvement in durability.

[0033]

(1) The spalling resistance is remarkably improved as compared with the conventional zirconia-graphitic refractory using the same amount of graphite.

(2) Since the increase in elastic modulus due to the addition of coarse particles is suppressed, the content of coarse zirconia can be increased as compared with the conventional one, so that the corrosion resistance is further improved.

Claims (4)

[Claims] 1. A zirconia having a particle diameter of at least 0.2 mm, zirconia having a particle diameter of less than 0.2 mm, and graphite as a main component, wherein the coarse zirconia has a particle diameter of less than 0.2 mm. In the zirconia-graphitic refractory covered with the coating layer composed of a mixture containing as the main component, the proportion of graphite in a portion of the coating layer in contact with the coarse zirconia is a portion excluding the coarse zirconia in the refractory. A zirconia-graphitic refractory, characterized by having a higher graphite proportion than 2. The graphite ratio of the portion of the coating layer in contact with the coarse zirconia is 1.2 to 2.6 times higher than the weight ratio of graphite in the portion of the refractory other than the coarse zirconia particles. The zirconia-graphitic refractory according to claim 1. 3. The zirconia-graphite refractory according to claim 1, wherein the proportion of the coarse zirconia to the total amount of zirconia is 5 to 50% by weight. 4. The zirconia-graphitic refractory according to claim 1, wherein the thickness of the coating layer in contact with the coarse zirconia is 0.05 mm or more and 0.3 mm or less. .