Fused zirconia refractory materials having high-temperature heat resistance and corrosion resistance and a method for producing the same

Abstract: Fused zirconia refractory materials having high-temperature resistance and corrosion resistance which includes 1 - 30 wt% of calcia and 0.05 - 2 wt% of yttria or rare earth minerals including yttria as a stabilizer of zirconia, which are produced by the process comprising adding and mixing calcia and yttria or rare earth minerals including yttria to zirconia materials as a stabilizer, fusing the mixed materials in an electric arc furnace, cooling slowly the fused materials to obtain an ingot, crushing the ingot to particles and oxidizing and annealing the particles.

Description: This invention relates to a fused zirconia refractory material having high-temperature heat resistance and corrosion resistance and a method for producing the same, and more particularly to a fused zirconia refractory material of zirconia-calcia-yttria system and a method for producing the same. Prior Arts: Having the properties such as a high melting point of about 2700°C, high corresion resistance and low thermal conductivity, zirconia(ZrO₂) has been generally used for refractory materials. On the ether hand, zirconia is a multi-modification mineral which is monoclinic from a room temperature to about 650°C, tetragonal up to about 1100°C and cubic up to about 2700°C. It shows an extreme thermal expansion and shrinkage of about 5% at the phase transition between monoclinic and tetragonal. Accordingly, when subjected to a heat cycle, i.e. repetition of heating and cooling, zirconia is cracked and finally fractured. In view of the above, zirconia for refractory materials has been stabilized to restrain the extreme thermal expansion and shrinkage by adding CaO or MgO as a stabilizer and applying a high-temperature heat treatment and thereby substituting a portion of Zr⁴⁺ with Ca²⁺ or Mg²⁺ in the form of a solid solution. Said stabilized zirconia has different properties in accordance with a stabilizer and its quantity to be used. For example, four variations have been known when CaO is used as a stabilizer. 1) Quantity = 2.5wt% Stabilisation rate is 60%. Grain strength is hiqh. Thermal expansion rate is low. Hysteresis, that is, the difference between expansion rate and shrinkage factor during a heat cycle, is high. 2) Quantity = 4wt% Stabilization rate is about 80%. Grain strength is rather smaller than 2.5wt% CaO. Thermal expansion rate is higher. Hysteresis is low. 3) Quantity = 7.5 - 12wt% Stabilization rate is 100%, that is a fully stabilized zirconia. Grain strength is comparatively lower and thermal expansion rate is higher in comparison with 2.5 and 4 wt% CaO. No hysteresis is observed. 4) Quantity = 25 - 30wt% Consisting of a fully stabilized zirconia and CaZrO₃. Grain strength is lower than 100% stabilized zirconia. No hysteresis is observed. The development of the stabilized zirconia has contributed the spread of continuous casting method which can omit a reheating process of iron, and thereby the yield of products has been increased. Further air contact can be minimized by the continuous casting method, and the quality of iron has been improved. The stabilized zirconia refractory materials, however, are not free from a destabilization. As a result, it has not been satisfactory yet in a recent clean steel technology for producing a high tention steel. Then the development of novel refractory materials have been strongly waited for. CaO stabilization zirconia has been produced by adding CaO to zirconia and fusing and solidifying said mixture. In that process, there were need to exclude carbide and dioxide produced in a fusing process and diminish a strain in a crystal caused in cooling or solidifying process. Accordingly, annealing operation was added later for oxidization and decarburization as welt as removal of strain in order to restrain the occurrence of destabilization which causes the phase transition. It is true that CaO stabilization zirconia is prevented from occurrence of destabilization in a high-temperature region of above about 1400°C. However, when it is used for a tundish nozzle brick, long nozzle brick or submerged nozzle brick in a continuous casting method, CaO elutes at the contact with a molten steel, whereby destabilization is accelerated. Further grain strength deteriorates remarkably at a high temperature so that destruction and dissolution of nozzle are accelerated. On the other hand, MgO stabilization zirconia has a high strength at a room temperature. But in a wide region of below 1100°C, periclase(MgO) and monoclinic zirconia coexist in a crystal structure, so that destabilization easily occurs at a heat cycle. In order to prevent the occurrence of destabilization, annealing operation shall be made for a long period of time, so that it is not suitable for industrial production. Furthermore, MgO mixed into a molten steel is hardy removed in a later process compared to CaO, and it forms a bar against the achievement of clean steel technology. There have been developed various refractory materials and refractory brick in order to solve the above problems. Japanese patent publication No. 50-30035 discloses a method for preventing the occurrence of destabilization of stabilized zirconia, which consists of processes comprising crushing a CaO or MgO stabilization zirconia into a lump, heating the crushed zirconia to higher than 1200°C, cooling it to below 900°C and repeating said processes more than three times. Japanese patent application laid open under No. 62-138327 discloses a method for obtaining a fused stabilized zirconia in an efficient manner without using annealing operation for oxidation, by oxidizing fusing zirconia by O₂ blown into an electric furnace and modifying a stabilization rate by controlling a cooling rate for solidification. The above two methods, however, are not practical in industrial production because the processes take much time and are very complicated. Besides, as far as either CaO or MgO is used independently, the above mentioned problems can not be solved. An improvement of components has been also attempted. Document EP-A-49366 mentions stabilization of ZrO₂ with CaO, MgO, Y₂O₃, rare earth elements or their mixed oxides wherein, preferably, stabilization is effected by 3.7 to 5 wt% CaO and/or MgO. The zirconium oxide stabilized in the cubic phase is melted in an arc furnace, cooled, and crushed. Japanese patent publication No. 63-1274 discloses a heat-resistant stuructural matierial of Y₂O₃ - ZrO₂ system consisting of single crystal and multi crystal, which is obtained by solidifying fused materials including zirconia of 89 to 99 mol% and yttria of 1 to 11 mol%. Japanese patent application laid open under No. 61-68372 discloses zirconia partially stabilized by yttria having high hardness and high toughness, which is obtained by cooling a uniform cubic crystal of high temperature which includes zirconia and yttria of 1 - 10 mol% and thereby forming a structure Of rhombohedral crystal and/or tetragonal crystal. These materials are heat-resistant structural materials which are obtained by solidifying fused materials. Although it can be conjectured that they are superior in grain strength, heat resistance and corrosion resistance, it is not practical from an economical point of view to use yttria of over 1 mol% (1.82 wt%, approximately 2 wt%) for producing refractory materials, because yttria is expensive. Generally the addition of over 3 mol% (5.3 wt%) is required when yttria is used as a stabilizer. A large quantity of yttria which amounts to about 15 wt% is required for obtaining a fully stabilized zirconia. In case yttria content is less than 2 wt%, stabilization effect is poor. Another refractory material has been disclosed in the Japanese patent application laid open under No. 60-51663. Said application discloses fused cast refractories of thermal shock resistant zirconia including MgO of 1 to 5 wt% and CeO₂ of 0.2 to 6 wt%. But the employment of MgO badly affects on the stability and the destabilization as mentioned above is apt to be induced. As mentioned above, conventional refractory materials are not well restrained from destabilization. There are also such problems that the corrosion resistance to a molten steel is low and the processes are time consuming, complicated and uneconomical. As a result they are not suitable for the clean steel technology. It is therefore an object of the present invention to solve all of the above problems and to provide fused zirconia refractory materials having high properties in high-temperature heat resistance and in corrosion resistance and to provide a method for producing the same. We have found that we can restrain the occurrence of destabilization of zirconia during a heat cycle between high temperature region and room temperature region by adding 0.05 to 2 wt% of yttria or rare earth oxide including yttria to refractory materials of zirconia-calcia system and fusing and solidifying the mixture and thereby we can obtain refractory materials having high corrosion resistance to a molten steel. Namely, the present invention provides fused zirconia refractory materials having high properties in high-temperature heat resistance and in corrosion resistance, which include 1 to 30 wt% of calcia and 0.05 to 2 wt% of yttria or rare earth minerals including yttria as a stabilizer. The above refractory materials of zirconia-calcia-yttria system can be produced according to an electrofusing processe using a general electric arc furnace. Namely a method for producing fused zirconia refractory materials in accordance with the present invention comprises adding and mixing calcia and yttria or rare earth minerals including yttria as a stabilizer to zirconia materials, fusing the mixed materials in an electric arc furnace, cooling slowly and solidifying the fused materials to get an ingot, crushing the ingot and oxidizing and annealing the crushed materials. The fluid materials may, for example, be allowed to cool naturally to room temperature. The additive amount is preferred to be 1 to 30 wt% of calcia and 0.05 to 2 wt% of yttria or rare earth minerals including yttria. Although the conventional refractory materials of zirconia-calcia system can be employed in the clean steel technology by the reason that calcia eluted into a molten steel can be removed by an additional treatment, destabilization is accelerated by the elution of calcia so that grain strengh deteriorates remarkably at a high-temperature region and grain fracture occurs. On the other hand, yttrial stabilization zirconia is superior in grain strengh and toughness and also stable in a high-temperature region. But the additive amount is required to be more than 5 wt%. Such a large quantity of yttria is too expensive to be employed for refractory materials. Yttria stabilization zirconia is hardly eluted in a molten steel, but yttria once eluted can not be easily removed. In accordance with the above ellucidation, we have found that we can solve the problem of refractory materials of zirconia-calcia system and provide novel refractory materials which in restrained from the occurrence of destabilization during a heat cycle between high temperature region and room temperature region and which is superior in grain strength by adding a small amount of yttria to the zirconia-calcia system. We have confirned that the addition of yttria is effective in every additive amount of calcia described before and that 1 to 2.5 wt% of calcia are also affective owing to the synergistic effect with yttria, so that 1 to 30 wt% of calcia may be used as a stabilizer. The additive amount of yttria or rare earth oxide including yttria is within the range of 0.05 to 2 wt% in order to limit the used amount of expensive yttria and to minimize the contamination of a molten steel when the refractory materials including yttria are dissolved into a molten steel. A combination of yttria with another stabilizer such as MgO or TiO₂ instead of CaO is not preferred from the view point of destabilization. A small amount of other stabiliser(s) such as, for example, MgO and/or TiO₂ may, howewer, be used in addition to Y₂O₃ and CaO, and may partially replace the CaO in the refractory material. In such circumstances the mixture of CaO with MgO and/or TiO₂ is preferably 1 to 30 wt% of the refractory material. The ratio of MgO, if present, to CaO is preferably ≦ 0.3:1, and the ratio of TiO₂, if present, to CaO is preferably ≦ 0.2:1. When more than one other stabiliser (e.g. MgO and TiO₂) are used, the ratio of their combined weights to the weight of CaO is preferably ≦ 0.3:1. Thus the stabiliser is mainly composed of yttria and calcia. Another rare earth oxides such as CeO₂, Yb₂O₃, Gd₂O₃, Sm₂O₃, La₂O₃, Nd₂O₃ or Er₂O₃ are not preferred because these rare earth oxides shall be added in large quantities in order to accomplish the object. As a result the refractory materials obtained becomes very expensive and besides such rare earth oxides can not be easily removed when they elutes in a molten steel. The fused zirconia refractory materials in accordance with the present invention can be produced using an electrofusing process, so that refractory materials of high grain strength can be easily obtained compared with a conventional binding process or annealing process. In accordance with the present invention, desired fused zirconia refractory materials can be obtained by the synergistic effect of calcia and yttria or rare earth oxide including yttria used as a stabilizer of zirconia. The addition of yttria or rare earth oxide including yttria restrains the elution of calcia, so that destabilization of zirconia is effectively prevented. Grain strength does not deteriorates at a high temperature region. Corrosion resistance to a molten steel is improved so that the dissolving destructure of the refractory materials do not occur. Then the refractory materials obtained is available in the clean steel technology. By virtue of the combination with calcia, the additive amount of yttrla or rare earth oxide including yttria can be decreased. As a result, the refractory materials obtained are also satisfactory from the economical point of view. EXAMPLES There were prepared Baddeleyite (96% ZrO₂) as zirconia source and quick lime (98% CaO) ns calcia source. Yttria source and rare earth oxide source including yttria comprised following components shown in Table 1. Yttria(wt%) Rare earth oxide including Yttria (wt%) Y₂O₃ 99.9 60 Fe₂O₃ 0.0015 0.013 Na₂O 0.0015 - K₂O 0.0015 - SiO₂ 0.001 0.05 La₂O₃ - <10 CeO₂ - <10 Nd₂O₃ - <10 Yb₂O₃ - 6 Er₂O₃ - 6.5 Sm₂O₃ - 2 Materials mixed as shown in Table 2 were fused in an electric arc furnace with the secondary voltage of 95 V, average loading electric power of 300 kW, total operation time of two hours and total amount of electric power of 600 kWh. After the operation, fused materials were cooled slowly in an atmosphere to get an ingot (that is, the materials were allowed to cool naturally to room temperature in the air). The ingot obtained was then crushed into particles or below 3 mm using a crusher such as jaw crusher or roll crusher. Oxidizing and annealing treatment were then conducted using a gas furnace to remove a carbide or dioxide induced in the fusing process and also to remove a strain caused in a crystal by quenching in the cooling process. Said treatment was effected in such a manner that the temperature was raised up to 1400°C at the rate of 5°C/min. and kept unchagned for three hours and thereafter was cooled to a room temperature at the rate of 5°C/min. Samples thus obtained were classified by using a sieve to otatin the particles of 2.83 - 2.38 mm for measurement of stabilization rate and grain strength. Stabilization rate is the ratio of the sum of cubic phase and tetragonal phase to total zirconia phase. It can be determined using X-ray powder diffraction method. The following arithematic equation can be used for measuring the stabilization rate I (C+T) on the basis of diffraction peak intensity of cubic I C (111), tetragonal I T (111), monoclinic I M (111) and I M (11 1 ). In order to test the probability of destabilization and grain strength variation due to a heat cycle, the samples were subjected to a thermal shock test which repeated the cycle 10 times comprising heating the samples at 1450°C for 10 minutes in a muffle furnace (ERREMA furnace) and thereafter cooling by water. The variations of stabilization rate and grain strength are as shown in Table 2. Measurement of grain strength was made using a testing machine, AUTOGRAPH of Shimazu Corporation. 50 grains were measured is each sample and average value was determined as the grain strength (strength per grain). Reduction rate of the stabilization rate and deterioration rate of strength per grain shown in Table 2 are represented by the percentage of the difference between an original value and a value after thermal shock to the original value. Inorder to test the corrosion resistance, test pieces of zirconia-carbon system of 20 x 20 x 100 mm were prepared by mixing 80 parts by weight of the above samples below 3 mm, 13 parts by weight of F.C and 7 parts by weight of SiC. Said test pieces were subjected to a corrosion test using a cryptol furnace. In addition to said test pieces, iron and continuous casting powder were loaded. After one hour operation at 1600°C, the test pieces were cooled and dissolving destruction was determined at a cut surface. The results are shown in Table 2 in the form of index by setting the amount of dissolving destruction of Sample No. 12 (4 wt% calcia - 96 wt% zirconia) to be 100. In Table 2, samples No. 11 to 15 in comparative examples are conventional calcia stabilization zirconia which includes no yttria or which includes a very small amount of yttria. The stabilization rate thereof is originally 60 - 100 %, but 40 - 86 % after the thermal shock test, which shows 14 - 33 % reduction. The deterioration rate of strength per grain is as high as 23 - 39 %. Samples No. 17 and 18 are another comparative examples i.e. yttria stabilization zirconia which includes yttria only. Both the stabilization rate and the deterioration rate after the thermal shock test is not preferred. Samples No. 16, 19 and 20 in comparative examples show preferableresults in the stabilisation rate, grain strength and dissolving destruction. But they are not practical in the industrial production because yttria is expensive and the removal of yttria eluted in a molten steel is difficult. On the contrary, samples No. 1 - 10 which are the embodiments of the present invention show preferable results. Namely the reduction rate of stabilization rate is less than 12% and the deterioration rate of grain strength is less than 15%. Besides the dissolving restructure of samples No. 1 - 10 is less than 60 while conventional calcia stabilization zirconia shows the dissolving dextructure of 95 - 200. As mentioned above, the refractory materials of zirconia-calcium-yttria system in accordance with the present invention have such superior properties that the grain strength is high at a room temperature, the destabilization of zirconia and the reduction of grain strength caused by a heat cycle are restrained and the amount of dissolving destruction is small, that is, the corrosion resistance is high. Accordingly, the refractory materials in accordance with the present invention is preferably used in the clean steel technology.

Claims: Fused zirconia refractory materials having high-temperature resistance and corrosion resistance including 1 - 30 wt% of calcia and 0.05 - 2 wt% of yttria or rare earth minerals including yttria as a stabilizer. Method for producing fused zirconia refractory materials comprising adding and mixing calcia and yttria or rare earth minerals including yttria to zirconia materials as a stabilizer, fusing the mixed materials in an electric arc furnace, cooling slowly the fused materials to obtain an ingot, crushing the ingot and oxidizing and annealing the crunhed materials. Method for producing fused zirconia refractory materials claimed in Claim 2, wherein the additive amount of calcia is 1 - 30 wt% and the additive amount of yttria or rare earth minerals including yttria is 0.05 - 2 wt%. Method for producing fused zirconia refractory materials as claimed in claim 2, wherein, as well as the calcia and yttria or rare earth mineral including yttria, one or more other stabilisers is also mixed with the zirconia material. Method as claimed in claim 4, wherein the ratio of other stabiliser or total of other stabilisers to calcia is ≦ 0.3:1. Method as claimed in claim 5, wherein the other stabiliser(s) comprise MgO and/or TiO₂, the TiO₂ if present being in a ratio to calcia of ≦ 0.2:1. Method as claimed in any one of claims 2, 4, 5 and 6, wherein the additive amounts are 0.05 to 2 wt % of yttria or rare earth mineral including yttria, and 1 to 30 wt % of other stabiliser component comprising calcia. A fused zirconia refractory material including 0.05 to 2 wt % of yttria or rare earth mineral including yttria as stabiliser, and 1 to 30 wt % of other stabiliser component comprising calcia wherein the ratio of other stabiliser or total of other stabiliser, if present, to calcia is ≦ 0.3:1. A fused zirconia refractory material as claimed in claim 8, wherein the calcia-comprising stabiliser component includes MgO and/or TiO₂ upto a maximum ratio of TiO₂:calcia of 0.2:1.

Inventors: ONODA YOSHIHIRO, TAMAMAKI MASAHIRO, TSUDA KOJI, YAMASHITA HIROYUKI, ONODA, YOSHIHIRO, TAMAMAKI, MASAHIRO, TSUDA, KOJI, YAMASHITA, HIROYUKI
Applicants: JAPAN ABRASIVE, JAPAN ABRASIVE CO., LTD.
Application year: 1991
Main classification: C04B35
IPC codes: C04B35, C01G25, B22D11