JP3093862B2 - Refractories for continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to refractories for continuous casting, and more particularly to materials for immersion nozzles, long nozzles and the like used in continuous casting.

[0002]

2. Description of the Related Art Among refractories for continuous casting, nozzles for continuous casting such as immersion nozzles and long nozzles connect a ladle, a tundish, and a mold. It is formed to regulate the flow of the molten steel and to prevent reoxidation of the molten steel. Since the molten steel flows in the holes, a large temperature difference occurs between the inner and outer surfaces, and cracks due to thermal stress are likely to occur. For this reason, as the material of these continuous casting nozzles, fused silica based on low thermal expansion and graphite based materials with high thermal conductivity have been used. Alumina-graphite graphite materials showing stable erosion resistance to steel types have become mainstream.

[0003] However, even this graphite-based material does not always provide sufficient durability.

For example, an immersion nozzle is particularly important for preventing vertical cracks due to thermal shock and for the corrosion resistance of the slag line to the mold powder, while an alumina-graphite immersion nozzle is problematic for the corrosion resistance of the powder line in contact with the mold powder. There is a possibility that the immersion nozzle may be broken due to erosion of the powder line portion, so that it cannot be used for a long time. Therefore, as a countermeasure against erosion of the powder line portion, a zirconia-graphite system having relatively good corrosion resistance is applied to the mold powder, and the powder line portion is thickened in a form corresponding to the amount of erosion. Because the corrosion resistance of the product itself is insufficient, sufficient durability has not been obtained.

In the long nozzle, as in the immersion nozzle, there is a problem of preventing vertical cracks due to thermal shock and improving corrosion resistance.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a submerged nozzle having improved longitudinal crack prevention and corrosion resistance due to thermal shock.
An object of the present invention is to provide a refractory material for continuous casting used for long nozzles and the like.

[0007]

SUMMARY OF THE INVENTION The present invention provides an alumina
Romia ceramic matrix
Phase) and the second phase aggregates dispersed therein.
The matrix contains fine cracks and the second
Is the phase a homogeneous mixture of matrix and unstable zirconia?
Alumina-chromia sinter containing cohesive grains
It is a refractory for continuous casting characterized by being a body .

The matrix is made of Al ₂ O ₃ 100-40.
% By weight, Cr ₂ O ₃ 0-60% by weight, TiO ₂
1.5% by weight or less.

The dispersed second phase is agglomerated particles composed of a homogeneous mixture of a matrix and unstable zirconia, and has a size of 10 to 200 μm.
Uniform agglomeration in which the proportion of the phase is 10 to 35% by volume, the particle size of the unstable zirconia in the second phase is 0.3 to 20 μm and the proportion of the unstable zirconia in the second phase is 5 to 100% by volume It is a grain.

In this sintered body, the size of the second phase is 10 to 2
The aggregated particles having a size of 00 μm are
Agglomerated particles and a matrix powder are mixed and prepared so as to be 5% by volume, and the mixture is formed into a desired shape.
It is obtained by sintering at a temperature of 0 ° C. or higher. At this time, instead of uniformly dispersing unstable zirconia that undergoes transformation expansion during firing, by adding in the form of aggregated particles, the amount of transformation expansion of aggregated particles is substantially proportional to the amount of unstable zirconia added. It has the feature that the amount of expansion of the aggregated particles can be controlled.

[0011]

The excellent thermal shock resistance of the alumina-chromia material according to the present invention is mainly due to the crack branching effect due to the controlled and appropriate size of microcracks, and secondly the zirconia in the second phase which is rich in unstable zirconia. This is achieved by stress-induced transformation due to transformation, and thirdly, by crack deflection at the boundaries of agglomerated grains.

The first controlled microcracks having an appropriate size have a crack width of about 3 to 20 μm. When the cracks are appropriately distributed, crack branching occurs when the cracks develop. In addition, it absorbs and disperses the breaking energy of the cracks, thereby preventing the cracks from developing.

[0013] The stress-induced transformation due to zirconia transformation in the second phase, which is rich in unstable zirconia, occurs when cracks enter the second phase in which unstable zirconia is present. The absorption of the breaking energy due to the transformation expansion and the compressive stress generated inside the second phase cause a compressive force to act on the tip of the crack, thereby inhibiting the progress of the crack.

In the third crack deflection at the boundary of the agglomerated grains, a tensile stress acts on the boundary between the second phase and the matrix. When the crack reaches this boundary, the crack is deflected in the tangential direction of the boundary. Crack progress is hindered.

The amount of transformation expansion of the agglomerated particles is substantially proportional to the amount of unstable zirconia in the agglomerated particles, and by controlling the agglomerated particle size and the amount of the agglomerated particles added in the matrix. The expansion amount of agglomerated particles can be controlled,
The amount, size and distribution of cracks generated inside the matrix can be arbitrarily controlled.

The present invention is characterized in that unstable zirconia that undergoes transformation expansion during firing is not uniformly dispersed, but is added in the form of aggregated particles.

[0017]

【Example】

Example 1 A material was prepared in which the amount of the matrix and the amount of the second phase added were changed, the thermal shock resistance was investigated, and a comparison was made with a conventional product.

Average particle size 0.4 μm as matrix material
m, 50% by weight of aluminum oxide, 50% by weight of chromium oxide having an average particle size of 0.3 μm, titanium oxide or talc powder, an organic binder, and purified water as sintering aids. The resulting slurry was mixed and dispersed by a lighter for 3 hours, and the obtained slurry was granulated by a spray drier to obtain a matrix granular powder. Average particle size is 50μ
m.

Next, powder obtained by adding 50% by volume of unstable zirconia having an average particle diameter of 2 μm to 100% by volume of the same material and the same composition as the matrix material as the second phase aggregated particles is used. Are weighed and mixed, a predetermined amount of an organic binder and purified water are added, and the mixture is preliminarily mixed with a ball mill for 24 hours, and then mixed and dispersed with an attritor for 3 hours,
The obtained slurry was mixed with a spray dryer to obtain aggregated granules of the second-phase granular powder. This particle size was 50 μm on average.

Further, the matrix granules and the second phase granules were mixed at a mixing ratio (volume ratio) shown in Table 1 with a V-type mixer for a certain period of time to obtain a mixed powder. This mixed powder was mixed with a uniaxial molding machine at a pressure of 1.4 ton / cm ² for 12 hours.
It was formed into a shape of 0 square x 12 mmt. For comparison, a matrix having only a matrix alone without the addition of the zirconia-rich second phase was also formed.

The obtained substrate was placed in an electric furnace under atmospheric air.
It was baked while being kept at 50 ° C. for 2 hours. The bulk density and apparent porosity of the sintered body were measured by the Archimedes method. Further, the room temperature bending strength was measured according to JIS-R1601. The thermal shock resistance is such that a bending sample conforming to JIS-R1601 is held at a predetermined temperature for 1 hour, rapidly dropped into water, and then the bending strength of a dried sample is measured and compared with the bending strength at room temperature. The difference between the holding temperature and the water temperature at which the strength changed abruptly was defined as ΔT (° C.), and the higher the ΔT, the better the thermal shock resistance.

Table 1 shows the results of firing the above-mentioned base material and the characteristics of the fired body in comparison with the alumina-graphite system which is a conventional refractory for continuous casting.

[0023]

[Table 1] From the results in Table 1, it can be seen that the reference numerals 4 to 7 according to the present invention are significantly improved as compared with ΔT of Comparative Example 1 in which the second phase is not added. In Example 9, cracks occurred and a satisfactory sintered body was not obtained. It is considered that the reason for this is that in Example 9, the amount of zirconia added was large, and the generated cracks were connected to each other, causing large cracks in the sintered body.

Further, an improvement in ΔT was recognized even in comparison with the conventional AG brick.

Example 2 The refractories according to the present invention were compared with known zirconia dispersion strengthened ceramics.

Based on the description in JP-B-59-25148. A powder using alumina-chromia as a matrix was prepared for comparison. 50% by weight of aluminum oxide having an average particle diameter of 0.4 μm as a matrix and an average particle diameter of 0.3 μm
Table 2 shows the unstable zirconia (average particle size: 2 μm) used in Example 1 for a powder composed of 50% by weight of chromium oxide of m, titanium oxide as a sintering aid, and talc + 1.0% by weight (outer surface). The mixture was weighed at a ratio (volume%), a predetermined amount of an organic binder and purified water were added, and the mixture was premixed for 24 hours with a ball mill, mixed and dispersed with an attritor for 3 hours, and the obtained slurry was dried with a spray dryer. Granulation and matrix granule powder were obtained.

The molding and firing were performed in the same manner as in Example 1. Table 2 shows the characteristics of the sintered body obtained by this method in comparison with those of the present invention shown in Example 1.

[Table 2] Observation of the microstructure with a scanning electron microscope revealed that in Examples 11 and 12, the matrix was very fine, and the unstable zirconia was uniformly dispersed.

On the other hand, the matrix portions 5 and 7 of the present invention have almost the same large particle size as Comparative Example 10, and the zirconia-rich second phase having a size of about 35 to 40 μm is uniformly contained in the matrix. The second phase was dispersed and comprised a fine matrix of about 5 microns and unstable zirconia.

Here, the volume percentage of the unstable zirconia in the sintered bodies of Examples Nos. 5 and 7 in Example 1 was 5, 10%, respectively.
% By volume.

Therefore, Comparative Example 11 and Invention 5 and Comparative Examples 12 and 7 have completely different dispersion states of zirconia, but have the same unstable zirconia volume% in the sintered body.

From the results shown in Table 2, it is clear that in Nos. 11 and 12 based on JP-B-59-25748, the thermal shock resistance is improved by the uniform dispersion of the unstable zirconia, but the effect is not so remarkable as in the present invention. It is.

Example 3 A test was performed on the amount of zirconia added in the second phase.

The same unstable zirconia as used in Example 1 was used, and the addition ratio (volume%) of the matrix and the unstable zirconia in the aggregated particles of the second phase was as shown in Table 3. Second-phase aggregated particles were produced in the same manner as in the granule production process shown in Example 1. The matrix composition is the same as in Example 1.

The average particle size of the obtained granules was about 50 μm. The obtained various second-phase aggregated particles having different amounts of unstable zirconia and matrix granules were mixed at a volume percentage shown in Tables 4 to 9 and evaluated by the same method as in Example 1. It was described in.

[0035]

[Table 3]

[Table 4]

[Table 5]

[Table 6]

[Table 7]

[Table 8]

[Table 9] From the results in Table 4, it can be seen that when the amount of the unstable zirconia in the aggregated particles of the second phase is 100% by volume, the optimum addition amount is 3 to 20% by volume.

From the results shown in Tables 5, 6 and 7, when the amount of unstable zirconia in the aggregated particles of the second phase is 67% by volume, the optimum addition amount is 10 to 30% by volume. When the amount of unstable zirconia is 50% by volume, the optimum addition amount is 10 to 40% by volume. When the amount of unstable zirconia in the aggregated particles of the second phase is 33% by volume, the optimum addition amount is 10 to 50% by volume. It can be seen that it is.

Further, from the results in Table 8, it can be seen that when the amount of the unstable zirconia in the aggregated particles of the second phase is 5% by volume, the optimum addition amount is 30 to 70% by volume.

However, from the results shown in Table 9, when the amount of unstable zirconia in the second-phase aggregated particles is 3% by volume, the thermal shock resistance can be obtained no matter how the mixing ratio of the matrix granules and the second-phase aggregated particles is changed. No improvement in properties is observed. That is, based on the above results, 5% by volume of unstable zirconia in the aggregated particles of the second phase.
If less than the above, the effect of the present invention is not recognized. Therefore, in the present invention, the proportion of the unstable zirconia in the aggregated particles of the second phase is 5 to 5.
It is defined as 100% by volume.

The amount of the second-phase aggregated particles varies with the amount of the unstable zirconia in the second-phase aggregated particles and the optimum addition ratio of the second-phase aggregated particles. When the amount of unstable zirconia added is 5 to 100% by volume,
It can be seen that the optimum amount of the second phase aggregated particles is 3 to 70% by volume.

Example 4 Two kinds of alumina-graphite bricks of the present invention 4 and 6 of Example 1 and a comparative example were prepared and subjected to an erosion test in a high-frequency induction furnace. Table 10 shows the results.

[0041]

[Table 10] The test conditions were a sample of 10 × 10 × 80 mm, which was connected to an alumina pipe with mortar, and a previously dried sample was held in an iron holder, and immersed in molten steel for 1 hour without preheating (holding on a furnace for 1 minute or less).

The depth of the sample immersed in the molten steel without rotation was about 40 mm. The steel type was a low carbon aluminum killed steel, the molten steel free oxygen was 8 to 9 ppm, and the molten steel temperature was 1550 ° C. To evaluate the corrosion resistance, the sample after the immersion test was cut at the center of the sample with a diamond cutter, the dimensions of the eroded portion of the molten steel were measured with a micrometer and compared with the dimensions before erosion, and the erosion rate (mm / min) was measured. Calculated. As a result, the product of the present invention showed excellent durability.

That is, as is clear from Table 10, in the conventional AG (alumina-graphite) brick used in the comparative example, the molten steel portion was completely eroded by immersion for one hour, and the erosion rate was 0.08 mm / min. As described above, all the products of the present invention were 0.001 mm / min or less, and no erosion was observed within the measurement accuracy. In addition, the conventional zirconia dispersion-strengthened ceramics 11 and 12 are inferior in thermal shock resistance, so that an erosion test cannot be performed.
I can't compare.

As described above, the product of the present invention was found to have very good thermal shock resistance and corrosion resistance.

[0045]

According to the present invention, a refractory for continuous casting having improved thermal shock resistance and corrosion resistance as compared with the prior art can be obtained.
It can greatly contribute to stability and life extension, quality control and labor saving.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hatsuo Taira 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Yasuhiro Yamada 20-1 Shintomi, Futtsu-shi, Chiba New Japan (56) References JP-A-60-51659 (JP, A) JP-A-56-165549 (JP, A) JP-A-2-255248 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) B22D 11/10 330 B22D 41/54 C04B 35/101 C04B 35/12 C04B 35/48

Claims (1)

(57) [Claims] 1. A alumina - and chromia electrolyte matrix continuous phase consists of a second phase aggregate particle dispersed therein, Ma
Tricks contain fine cracks and the second phase
A refractory for continuous casting, characterized in that it is an alumina-chromia sintered body containing agglomerated particles consisting of a homogeneous mixture of powder and unstable zirconia .