CARBON CONTAINING REFRACTORY

Abstract: The refractory comprises, by weight: 5 to 60% of AZT clinker mainly constituted by ZrO₂, TiO₂, and SiO₂; 3 to 40% of a carbon source; and the balance comprising one or more refractories selected from alumina, magnesia, zirconia, mullite, spinel, calcia, and fused quartz.

Description: This invention relates to a carbon containing refractory for use in a molten metal refining furnace, superior in resistance to thermal shock and corrosion, and durable during use under severe conditions. Recently, carbon containing refractories are increasingly used as a refractory for a molten metal refining furnace in order to prevent permeation of molten metal and/or slag into the refractory body or to make the refractory body high-heat-conductive and low-expansible to improve the resistance to thermal shock. Generally, carbon containing refractories are formed as various combinations of carbon and refractory oxide aggregate, such as alumina-carbon, magnesia-carbon, zirconia-carbon, and so on, according to the conditions under which the refractory is used, including the type of furnace and the slag composition. In more detail, if the proportion of carbon is increased in the refractory oxide aggregate-carbon composition, the heat conductivity becomes higher and the thermal expansion coefficient becomes smaller, thereby improving the resistance to thermal shock as well as the anti-slag-permeation properties. In this case, however, the resistance to corrosion becomes lower since the content of the high-corrosion-resistant oxide aggregate is relatively reduced. If, conversely, the proportion of oxide aggregate is increased, the resistance to corrosion is improved but the heat conductivity becomes lower and the thermal expansion coefficient becomes higher, since the proportion of carbon becomes relatively small, resulting in a reduction in the resistance to thermal shock as well as a deterioration in the anti-slag-permeation properties. Under the present conditions, in the carbon containing refractory based on combinations of refractory oxide aggregate and carbon, the contents of respective components are restricted within suitable ranges according to the conditions under which the refractory is used, in order to balance the resistance to thermal shock, the anti-slag-permeation properties and the anti-corrosion properties with each other. US-A-4 646 950 discloses a sliding nozzle plate produced by firing a composition comprising non-alkaline refractory material, carbon, metal powder, and 2 to 5 wt.% of an alumina-zirconia-titania mixture having a silica content of at most 0.20 wt.%. This disclosure is discussed further below. It is an object of the present invention to provide a carbon containing refractory having improved and optimized anti-corrosion and wear resistance properties without reducing the resistance to thermal shock and the anti-slag-permeation properties. To this end, the present invention provides a carbon containing refractory substantially comprising, by weight: a) 5 to 60% of AZT clinker having 10 to 60% of ZrO₂, at least 20% of Al₂O₃, 3 to 40% of TiO₂, and 0.5 to 30% of SiO₂, the sum of the contents of TiO₂ and SiO₂ being smaller than 40%; b) 3 to 40% of a carbon source; and c) the balance comprising one or more refractories selected from alumina, magnesia, zirconia, mullite, spinel, calcia, and fused quartz. AZT clinkers which pertain to the present invention are clinkers containing alumina, zirconia, titania, and silica as main components and which are ordinarily formed in such a manner that raw-material powders are mixed to form respective compositions and are fused in an electric furnace so as to form clinker or that a composition mixture is formed as pellets and is, after being dried, burned at a temperature equal to or higher than 1500°C to form clinker. In the material composition of AZT clinker, besides corundum, baddeleyite, and mullite as main mineral phases, aluminum titanate which is low-expansible is formed as part of the corundum, and tetragonal or cubic stabilized zirconia phase is formed as part of non-stabilized monoclinic zirconia (baddeleyite), since TiO₂ is added. The AZT clinker comprises at least three of these types of mineral phases and exhibits a composition in which constituents coexist while being uniformly distributed. As is well known, mullite has a small thermal expansion coefficient, and aluminum titanate has a thermal expansion coefficient smaller than that of mullite. Baddeleyite exhibits a change in the volume (contraction) in response to crystal phase transformation from the monoclinic system to the tetragonal system, which is also a well-known fact. Therefore the AZT clinker which contains aluminum titanate and stabilized zirconia and which is constituted by at least three types of crystal phases of the above substances has a thermal expansion coefficient much smaller than those of corundum and baddeleyite. It is therefore possible to effectively reduce the thermal expansion and improve the resistance to thermal shock without increasing the carbon content, by adding this clinker to the materials to form a carbon containing refractory. In addition, in the carbon containing refractory containing AZT clinkers, fine microcracks are formed around particles and matrices of the AZT clinkers during heating because of difference between extents of thermal expansion of crystal phases coexisting in the AZT clinker particles (mismatch). This is effective in terms of reduction in the elastic modulus of the carbon containing refractory and, hence, reduction in thermal stress caused at the time of quick heating. In this respect also, the addition of AZT clinkers also contributes to improvement in the resistance to thermal shock. The basis on which the chemical composition of the AZT clinker is limited will be described below. If the content of ZrO₂ is less than 10 % by weight, the effect of the volume change in response to the phase transformation of baddeleyite is inadequate. If the content of ZrO₂ exceeds 60 % by weight, the volume change in response to the phase transformation becomes excessively large, which causes collapse of particles and makes the refractory unusable in fact. If the content of Aℓ₂O₃ is less than 20 % by weight, specific high anti-corrosion properties of alumina are not exhibited. If this content exceeds 90 % by weight, the majority of the main mineral phases are changed into corundum, and the effects of the other mineral phases become inadequate. If TiO₂ is less than 3 % by weight, stabilized zirconia and aluminum titanate having thermal expansion coefficients smaller than those of the other component minerals are not sufficiently produced. If, conversely, the content of TiO₂ exceeds 40 % by weight, the melting point of the resulting clinker becomes markedly reduced, which is not preferable in terms of resistance to corrosion. With respect to the TiO₂ content, an example of the composition of alumina-zirconia raw-material containing 2 % of TiO₂ has been reported [Refractory, 39 No. 9, pp 513 to 514 (1987)]. In the case where the TiO₂ content is less than 3 %, an increase in the tenacity is recognized but the effect of improvement in the resistance to thermal shock is inadequate. In the Japanese Patent Laid-Open No. 60-96567 and US-A-4 646 950 a sliding nozzle plate comprising Aℓ₂O₃-ZrO·TiO clinker and other materials has been disclosed. However, since this clinker is different from the clinker of the present invention and containing almost no SiO₂ in it, the growth of mullite is little and the effects to lower the thermal expansion is also small, so the sharp improvement in thermal spalling can not be expected. If the SiO₂ content is less than 0.5 %, mullite phase having small thermal expansion coefficient is not formed sufficiently, which is not preferable. If the SiO₂ content is increased, the melting point of the clinker becomes lower, and mineral phases of aluminum titanate and zirconia become reduced, which is not preferable. The effects of the present invention, however, are not lost so long as the sum of the contents of SiO₂ and TiO₂ is less than 40 %. If the proportion of AZT clinkers in the composition is less than 5 % by weight, the above-described advantages of AZT clinkers are not sufficiently utilized. In terms of resistance to corrosion, it is not preferable to set the proportion of AZT clinkers above 60 % by weight. A carbon raw-material has a heat conductivity markedly higher than that of the refractory oxide aggregate and, specifically, it is not easily wetted with molten metal and slag. In accordance with the present invention, therefore, carbon sources are added to an extent such that the anti-corrosion properties are sufficiently maintained, thereby preventing molten metal and/or slag permeating into pores of the carbon containing refractory as well as improving the resistance to thermal shock. Natural flaky graphite is preferable as a carbon source. It is possible to use other carbon sources as desired, including amorphous graphite, man-made graphite, coke, carbon black, electrode refuse, and so on. Other refractory aggregates, at least one or two types, may be selected from alumina, magnesia, zirconia, mullite, spinel, calcia (CaO), fused quartz, according to the conditions under which the refractory is used. It is possible to add other materials such as silicon carbide, boron carbide, metallic aluminum, metallic silicon, ferrosilicon, and so on, in order to improve the strength and anti-oxidation properties of the refractory. Embodiments of the present invention will be described below. Electro-fused AZT clinkers (AZT-1 to 8) having chemical compositions and mineral compositions shown in Table 1 were used to prepare compounds Nos.1 to 10 (weight percent) shown in Table 2. Of these compounds, the compounds Nos. 1 to 4 are alumina-graphite compounds, the compounds Nos. 5 to 7 are magnesia-graphite compounds, and the compounds Nos. 8 to 10 are zirconia-graphite compounds. A predetermined amount of phenol resin was added to each compound, and the compounds underwent mixing and kneading. Thereafter, the compounds Nos. 1 to 7 were press-formed at 98 MPa (1 t/cm²) with a uniaxial compression press while the compounds Nos. 8 to 10 were press-formed at 147 MPa (1.5 t/cm²) with a rubber press. The compacts underwent coking by being burned at 1000 °C in a non-oxidizing atmosphere of coke breeze. Table 2 shows comparison between general characteristics, anti-corrosion properties, and thermal shock resistance properties of burnt bodies thereby obtained. As is apparent from Table 2, the articles of the present invention, that is, all of alumina-graphite, magnesia-graphite,and zirconia-graphite refractories, have smaller thermal expansion coefficients and smaller elastic moduli, and are improved in the resistance to thermal shock, while they have smaller carbon contents and have improved anti-corrosion properties when compared with reference examples. An example of application of the carbon containing refractories in accordance with the present invention to a molten steel continuous casting nozzle will be described below. Compounds shown in Table 3 (weight percent) were prepared by using the AZT raw-materials shown in Table 1. A predetermined amount of phenol resin was added to each compound, and the compounds underwent mixing, kneading, and pelletization. Thereafter, the compounds were formed by a hydrostatic press into a long nozzle shape. The compacts underwent coking by being burned at 1000 °C in a non-oxidizing atmosphere of coke breeze. Table 3 shows comparison between general qualities, anti-corrosion properties, and thermal shock resistance properties of burnt bodies thereby obtained. As is apparent from table 3, the articles of the present invention are low-expandable and improved in the resistance to thermal shock as well as the resistance to corrosion when compared with the reference example 1. In a steel mill A where the same member as the reference example 1 was used as a molten steel continuous casting long nozzle, long nozzles in accordance with examples 2 and 3 of the present invention were used under the non-preheating condition. As a result, in the case of the conventional long nozzle in accordance with the reference example 1 used under the non-preheating condition, cracks take place due to thermal shock at a rate of 2 to 3 %, and the number of average times of use was 5 to 6 charges. In contrast, in the case of the long nozzles in accordance with the examples 2 and 3 of the present invention, there were no crack defects due to thermal shock,while the numbers of average times of use were 7 to 8 charges and 9 to 10 charges, respectively, thus, greatly improving the durability. Another example of application of the carbon containing refractories of the present invention to a molten steel casting sliding nozzle (hereinafter referred as SN) refractory will be described below. Compounds shown in Table 4 were prepared by using the AZT raw-materials shown in Table 1. A predetermined amount of phenol resin was added to each compound, and the compounds underwent mixing and kneading. Thereafter, the compounds were formed by a uniaxial compression press into SN plate bricks and upper and lower SN bricks. The plate bricks thereby obtained were burned at 1400 °C in a non-oxidizing atmosphere of coke breeze,while the upper and lower nozzle bricks underwent baking at 300 °C. Table 4 shows comparison between general qualities, anti-corrosion properties,and thermal shock resistance properties of the obtained bricks. As is apparent from Table 4, the articles of the present invention, namely both the plate bricks and upper and lower nozzle bricks are superior in the resistance to thermal shock compared with the reference examples and have anti-corrosion properties equal to or superior than those of the reference examples. In a steel mill B where bricks in accordance with the reference examples 1 and 5 were used as a ladle, SN plate bricks, and an upper nozzle, bricks in accordance with the examples 3 and 6 of the present invention were used as plate bricks and upper nozzles. As a result, in the case of the plate bricks in accordance with the reference example 1, the number of average times of use was limited to about 4 charges because of considerable edge breakages due to thermal shock. In contrast, in the case of the plate bricks in accordance with the example 3 of the present invention, there were reduced edge breakages, and the number of average times of use was 6 charges. With respect to the upper nozzle, while the number of average times of use of the conventional bricks in accordance with the reference example 5 was limited to 8 charges because of enlargement of the nozzle bore, the upper nozzles in accordance with the example 6 were used 12 times on the average. Also, cracks were remarkably reduced compared with the reference example. In a steel mill C where bricks in accordance with the reference examples 2 and 5 were used as continuous casting tundish SN plate bricks and a lower nozzle, bricks in accordance with the examples 4 and 7 of the present invention were used as plate bricks and lower nozzles. As a result, in the plate bricks in accordance with the reference example 2, there were many and deep cracks due to thermal shock, and the number of time of use was only one (1 tundish). In contrast, in the plate bricks in accordance with the example 4 of the present invention, cracks due to thermal shock were small, and it was possible to use these bricks two times (two tundish). With respect to the lower nozzle, the material in accordance with the reference example 6 sometimes caused air infiltration trouble by cracks due to thermal shock. The lower nozzles 7 in accordance with the example 7 of the present invention were free from this kind of trouble and were greatly improved in terms of prevention of enlargement of the nozzle bore compared with the conventional nozzle. As described above, the carbon containing refractory formed by adding AZT clinkers to the raw-material is apparently advantageous. The carbon containing refractory containing AZT clinkers in accordance with the present invention is effectively improved in the thermal shock resistance properties without impairing anti-corrosion properties.

Claims: A carbon containing refractory substantially comprising, by weight: a) 5 to 60% of AZT clinker having 10 to 60% of ZrO₂, at least 20% of Al₂O₃, 3 to 40% of TiO₂, and 0.5 to 30% of SiO₂, the sum of the contents of TiO₂ and SiO₂ being smaller than 40%; b) 3 to 40% of a carbon source; and c) the balance comprising one or more refractories selected from alumina, magnesia, zirconia, mullite, spinel, calcia, and fused quartz. A carbon containing refractory according to claim 1, wherein the carbon source comprises one or more substances selected from flaky graphite, amorphous graphite, man-made graphite, coke, carbon black, and electrode refuse. A carbon containing refractory according to claim 1 or 2, wherein, in the AZT clinker, the content of Al₂O₃ is 40 to 70% by weight, the content of ZrO₂ is 25 to 40% by weight, the content of TiO₂ is 3 to 15% by weight, and the content of SiO₂ is 1 to 20% by weight. A carbon containing refractory according to any of claims 1 to 3, further comprising at least one material selected from silicon carbide, boron carbide, aluminum, silicon, and ferrosilicon.

Inventors: ICHIKAWA, KENJI, IIDA, EISHI, IWADO, HITOSHI, TSUKAMOTO, NOBORU
Applicants: SHINAGAWA REFRACTORIES CO., LTD.
Application year: 1988
Main classification: C04B35
IPC codes: C04B35