Refractory for casting and process for producing same

Abstract

A refractory for steel casting comprising, based on the total weight of the refractory in each case, 5-80% by weight of ZRM, 10-40% by weight of carbon powder, 0-20% by weight of one or more of SiC, Si.sub.3 N.sub.4, metallic silicon and ferrosilicon, 0-30% by weight of fused silica and 0-60% by weight of alumina powder containing at least 70% by weight of Al.sub.2 O.sub.3 based on the total weight of the alumina powder. The ZRM includes the main mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of 25-85% by weight of Al.sub.2 O.sub.3, 10-70% by weight of ZrO.sub.2 and 5-25% by weight of SiO.sub.2. A process for producing the refractory for steel casting.

Description

BACKGROUND OF THE INVENTION

This invention relates to a highly durable carbon powder-containing refractory for steel casting and more particularly, to a highly durable carbon-containing refractory nozzle system of a casting machine and a process for producing such a refractory nozzle system.

Of late, with the aim to increase the yield rate and reduce the cost in the production of steel castings, the so-called continuous charge casting system has been increasingly employed in place of the so-called batch charge system and particularly, in these last years the steel casting operation has been in most cases consecutively performed from several charges up to several ten charges.

In a steel casting machine, the refractory nozzle is employed between the ladle tundish and between the tundish and mould, respectively and serves to prevent oxidation of molten steel and regulate the flow direction of the molten steel. Thus, the refractory nozzle is a quite important refractory component in the steel casting machine.

However, when the refractory nozzle is employed in the continuous casting operation referred to hereinabove, since the nozzle tends to be submerged in the molten steel pool for a fairly long time period, the portion of the nozzle which is designed to come to contact with a highly erosive powder layer such as silica or alumina which is spread over the top of the molten steel pool is attacked by the erosive powder.

In order to protect the particular portion of the refractory nozzle from the erosive material, various attempts have been made up to date. The conventional attempts are (1) the formation of the nozzle with Al.sub.2 O.sub.3 and graphite, (2) the mounting of a protective ring on the powder contacting portion of the nozzle brick and (3) the simultaneous formation of the powder contacting portion and the rest of the refractory nozzle by the use of a high erosion resistance material (for example, zirconia or the like) and steel, respectively, to thereby reinforce the refractory nozzle powder contacting portion. However, none of these conventional attempts have been found perfectly satisfactory. Although the attempt (3) has been extensively studied and widely practiced because the attempt can use the expensive material such as zirconia with a high efficiency, since the nozzle is formed of a combination of different materials, this nozzle encounters difficulty in the formation of the same. In addition, the powder contacting portion of the nozzle formed of the different material combination can not be always maintained in contact with the powder layer over the molten steel pool because the refractory nozzle moves upwardly and downwardly in the casting operation to thereby render some portion of the refractory nozzle other than the powder contacting portion thereof to be exposed to the erosive powder layer which may result in giving damage to the area adjacent to the powder contacting portion of the nozzle and thus, the attempt has been found not perfectly satisfactory.

When a long nozzle the tip of which is adapted to be always submerged in the molten steel pool during the casting operation is employed, the submerged tip of the nozzle is attached by the heat of the molten steel flowing down through the refractory nozzle and wears off prematurely to thereby reduce the designed service life of the nozzle. Furthermore, when a submerged nozzle is used, alumina deposit from the molten steel tends to adhere to the orifices in the nozzle to clog up them which may also reduce the service life of the nozzle.

In order to prevent the nozzle orifices from being clogged up by the alumina deposit from the molten steel, it has been proposed to blow gas such as argon gas into the orifices. However, in the conventional refractory nozzle formed of a combination of Al.sub.2 O.sub.3 and graphite, the alumina deposit cannot be perfectly removed from the orifices and such gas blowing has been found not fully effective. And the gas blowing tends to disturb the molten steel pool to thereby accelerate the wearing-off of the refractory nozzle. The wearing-off progresses from the walls of the refractory nozzle orifices into the body of the nozzle.

Thus, there has been a demand for a nozzle for use in the continuous steel casting operation which can effectively solve the problem of orifice clogging-up by alumina deposit without the blowing of gas or by full realization of the effects of gas blowing and that of wearing-off of the nozzle.

A patent has been granted to the formation of a highly durable nozzle for the continuous steel casting operation with a combination of Al.sub.2 O.sub.3 and graphite with zircon or zirconia added thereto. However, since zircon or zirconia is employed in its original form, such additive cannot be perfectly and uniformly distributed within the Al.sub.2 O.sub.3 and graphite combination. In addition, since zircon and zirconia have a higher fire resistance than those of the other components of the refractory nozzle, such additive materials take a rather long time period until they form a highly viscous glass film through their reaction with the other refractory nozzle components and the other components tend to be adversely affected by the heat of molten steel before the glass film is formed.

And in order to prevent the refractory nozzle orifice clogging-up problem, although it has been proposed to add silica in an increased amount to the Al.sub.2 O.sub.3 and graphite combination, the addition of such increased amount of silica is still not able to appreciably reduce the possibility of refractory nozzle orifice clogging-off. And it is difficult to increase the amount of the additive in proportion to the amount of alumina deposit to prevent wearing-off and orifice clogging-up whereby the additive cannot be added in an amount to cope with variation in the type of steel material to be cast.

SUMMARY OF THE INVENTION

Therefore, the purpose of the present invention is to provide a novel and improved highly durable refractory for nozzles for use in continuous steel casting which can effectively eliminate the disadvantages inherent in the conventional refractory nozzle and be continuously employed for a long time period and a process for producing the refractory.

The inventors have previously developed a highly durable refractory nozzle for use in the continuous steel casting based on the discovery that a refractory nozzle formed of an electrically fused or burnt refractory material clinker comprising a refractory component including the principal mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of Al.sub.2 O.sub.3, ZrO.sub.2 and SiO.sub.2 within a certain blending ratio (the refractory material clinker will be referred to as "ZRM" hereinafter) and the refractory nozzle is the subject of Japanese patent application No. 87459/1979. As disclosed in this Japanese patent application, the highly durable refractory nozzle for use in the continuous steel casting is prepared by admixing, based on the total weight of the refractory in each case, refractory component including the principal mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of 30-60% by weight of Al.sub.2 O.sub.3, 20-60% by weight of ZrO.sub.2 and 5-25% by weight of SiO.sub.2 based on the weight of the chemical composition, respectively, 20-40% by weight of graphite powder, 0-10% by weight of SiC, 0-25% by weight of fused silica and 0-45% by weight of alumina powder containing at least 70% by weight of Al.sub.2 O.sub.3 based on the weight of the alumina powder at cold temperature or under heating by the use of resin or pitch binder, moulding the mixture and burning the moulded product in non-oxidation atmosphere.

However, through subsequent researches, the inventors have found that the ZRM nozzle positively forms highly viscous glass films thereon with the heat from molten steel to thereby improve the heat resistance of the inner and outer surfaces of the nozzle, that the presence of the thus formed glass films on the inner and outer surfaces of the refractory nozzle improve wear resistance and anti-orifice clogging-off of the refractory nozzle and that the nozzle is imparted thereto a high durability even when the blending ratio of the components in the refractory component and the blending amount of the refractory component in the refractory are varied within a certain range and particularly, even when the refractory component is employed in the mixture in amounts ranging from 5-30% by weight based on the total weight of the mixture, the highly durable nozzle for casting to which the present invention is directed can be employed.

PREFERRED EMBODIMENTS OF THE INVENTION

According to the present invention, a highly durable refractory nozzle for use in the continuous steel casting can be provided by admixing, based on the total weight of the mixture in each case, 5-80% by weight of refractory component including the principal mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of 25-85% by weight of Al.sub.2 O.sub.3, 10-70% by weight of ZrO.sub.2 and 5-25% by weight of SiO.sub.2, respectively, based on the total weight of the chemical composition, 10-40% by weight of graphite powder, 0-20% by weight of one or more members selected from SiC, Si.sub.3 N.sub.4, metallic silicon and ferrosilicon, 0-30% by weight of fused silica and 0-60% by weight of alumina powder containing at least 70% by weight of Al.sub.2 O.sub.3 based on the total weight of the alumina powder at cold temperature or under heating by the use of resin or pitch binder, moulding the mixture and burning the moulded mixture in non-oxidation atmosphere.

As to the refractory component including the principal mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of 25-85% by weight of Al.sub.2 O.sub.3, 10-70% by weight of ZrO.sub.2 and 5-25% by weight of SiO.sub.2 based on the total weight of the chemical composition, clinker prepared by fusing zircon and alumina as principal materials or clinker prepared by fusing zircon and alumina with the addition of baddeleyite thereto can be employed. The present invention is not limited to the use of ZRMs prepared by fusing zircon and alumina with or without the addition of baddeleyite thereto, but clinkers prepared by sintering zircon and alumina with or without the addition of baddeleyite thereto and grinding the sinter to adjust the grain size thereof can be also employed.

Zirconia has an excellent erosion resistance and as mentioned hereinabove, although zirconia is suitably employed to reinforce the powder contacting portion of the refractory nozzle, the material is prohivitively expensive. In addition, since zirconia has a relatively high specific gravity and as a result, the erosion resistance material adds weight to the resulting refractory nozzle to thereby make it inconvenient to handle the refractory nozzle. On the other hand, stabilized zirconia has a higher coefficient of thermal expansion than those of oxides such as corundum and zircon which are employed as the principal components of a carbon-containing submerged nozzle. Unstabilized zirconia tends to expand excessively to the extent that the material cracks.

With the aim to provide a nozzle for use in steel casting which has spalling, wear and orifice clogging-up resistance properties and which can form a glass film easily while retaining excellent erosion resistance against the erosive powder layer, the inventors have conducted extensive researches on various materials and have found that in ZRM zirconia crystals deposit within and about mullite and corundum crystals to protect the mullite and corundum crystals and the zirconia crystals exhibit excellent erosion resistance effect. And since ZRM has a relatively lower resistance as compared with alumina, zirconia and zircon, ZRM can easily form a glass film in which zirconia is dispersed and has a high viscosity. Furthermore, ZRM has a relatively low coefficient of thermal expansion such as 7.times.10.sup.-6 and is characterized by its low coefficient of thermal expansion such as 2-4.times.10.sup.-6 within the high temperature range of 1000.degree.-1600.degree. C. The inventors have come to the conclusion that these properties of ZRM make ZRM suitable as the principal refractory component for submerged nozzles for use in continuous steel casting which is required to have high erosion, wear, orifice clogging-up and spalling resistance properties.

When ZRM contains ZrO.sub.2 in amounts less than 10% by weight based on the total weight of ZRM, the amount of zirconia which deposits within and about mullite and corundum crystals is insufficiently small and ZrO.sub.2 in such small amounts fails to improve the erosion resistance of the nozzle. On the other hand, when the amount of ZrO.sub.2 exceeds 70% by weight, since the ZrO.sub.2 is present in the form of monoclinic crystals, any abnormal expansion occurs in ZrO.sub.2 to thereby undesirably reduce the spalling resistance of the nozzle. Thus, the amount of ZrO.sub.2 to be contained in ZRM should be within the range of 10-70% by weight based on the total weight of ZRM.

And in ZRM the amounts of Al.sub.2 O.sub.3 and SiO.sub.2 should be within the ranges of 25-89% by weight and 5-25% by weight, respectively, based on the total weight of ZRM.

ZRM having the above-mentioned chemical composition occupies 5-80% by weight based on the total weight of the mixture for the refractory nozzle. When ZRM is employed in amounts less than 5% by weight in the mixture, the above-mentioned properties of ZRM cannot exhibit their full effects and on the other hand, when the amount of ZRM exceeds 80% by weight, the spalling resistance property of the resulting submerged nozzle is insufficient.

The amount of graphite in the mixture should be within the range of 10-40% by weight based on the total weight of the mixture. When graphite is employed in amounts less than 10% by weight, the resulting nozzle has insufficient erosion and spalling resistances. On the other hand, when the amount of graphite exceeds 40% by weight, graphite melts into the molten steel pool and presents as impurity in the molten steel. In place of graphite, amorphous graphite, acheson graphite, kish graphite, thermally decomposed graphite, petroleum pitch coke, coal coke, anthracite, charcoal, carbon black, thermally decomposed carbon of carbonhydride, thermally decomposed carbon of carbon hydrate, thermally decomposed carbon of synthetic resin or glassy carbon can be equally employed within the scope of the invention.

The amount of one or more members selected from the group consisting of SiC, Si.sub.3 N.sub.4, Metallic silicon and ferrosilicon should be within the range of 0-20% by weight and the one or members of the group form a SiO.sub.2 glass film when they oxidize. The glass film contributes to the prevention of oxidation. When the amount of the one or more members of the group exceeds 20% by weight, the material or materials melt into the molten steel pool and present as impurity or impurities in the molten metal. The amount of fused silica should be within the range of 0-30% by weight based on the total weight of the mixture and the fused silica in amounts within the range serve to impart spalling and orifice clogging-up resistance properties to the resulting nozzle. When silica is employed in amounts in excess of 30% by weight, the nozzle tends to be damaged by the molten metal.

The amount of alumina containing up to 70% by weight of Al.sub.2 O.sub.3 based on the total weight of the alumina should be within the range of 0-60% by weight based on the total weight of the refractory mixture. However, since Al.sub.2 O.sub.3 is also contained in ZRM, alumina is added to the refractory mixture as the case may be.

According to the present invention, the above-mentioned components of the refractory mixture are mixed together by the use of resin or pitch binder at cold temperature or under heating, the resulting refractory mixture is moulded and the moulded product is burnt in non-oxidation environment.

Examples of the present invention will be given hereinbelow by way of illustration together with controls for comparison purpose, but not for limiting the present invention thereto.

Table 1 shows properties of various ZRMs employed as the principal components in the production of refractory mixtures for nozzles for use in continuous steel casting and Tables 2 and 3 show various blending ratios of ZRMs and other components together with properties of refractory mixtures obtained. In each case, the mixing was carried out by the use of an airich mixer, the moulding was carried out by the use of a rubber press under the pressure of 1200 kg/cm.sup.2 and the burning was carried out by the conventional practice with the moulded product embedded in coke. It has been found that nozzles formed of the refractory mixtures containing ZRMs exhibited excellent orifice clogging-up, wear and spalling resistance properties and also erosion resistance property against erosive powder layer. No cracks occurred in the powder contacting portions of the nozzles and the obtained nozzles are superior to the conventional comparative nozzles.

TABLE 1__________________________________________________________________________Properties of ZRMChemical Composition Physical Property Coefficient of ThermalBy weight (%) Bulk Specific Apparent Expansion (%)No. ZrO.sub.2 Al.sub.2 O.sub.3 SiO.sub.2 Gravity Porosity (%) At 1000.degree. C. Abnormality__________________________________________________________________________ZRM 1 10.0 67.0 18.0 3.05 4.0 0.65 noneZRM 2 35.0 63.0 16.0 3.15 4.1 0.65 "ZRM 3 35.0 48.0 16.0 3.65 2.5 0.70 slightZRM 4 45.0 40.0 14.0 3.80 2.9 0.80 "ZRM 5 55.0 33.0 10.0 3.95 2.7 0.70 "ZRM 6 70.0 20.0 10.0 4.20 3.1 0.70 mediumZRM 7 80.0 13.0 5.0 4.35 3.8 0.75 great__________________________________________________________________________ TABLE 2-A__________________________________________________________________________ (By weight %) Control 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example__________________________________________________________________________ 6Crystalline flaky graphite 30 30 30 30 30 30 30ZRM 5 15 30 30 30 60 (ZRM 3) (ZRM 4) (ZRM 4) (ZRM 4) (ZRM 4) (ZRM 4)Electrically fused 60 50 40 30 15 10aluminaFused silica 5 5 15 20Silicon carbide 5 5 5 5 5 5 5Metallic silicon 5 5 5 5 5 5 5Phenol resin binder +10 +10 +10 +10 +10 +10 +9.5Bulk specific gravity 2.48 2.43 2.45 2.65 2.51 2.40 2.59Apparent porosity (%) 18.0 18.5 18.8 18.7 18.3 19.3 20.1Crushing strength(kg/cm.sup.2) 350 280 290 315 300 280 310Spalling resistance (1500.degree. cracksC. .times. 15 minute water cool- occurred none none none none none no cracking,repeated 4 times) at 3rd water coolingCorrosion resistance index 100 93 90 77 95 100 55Number of charges with 3 5 5 5 5 5 6tundish for 200 ladle charges charges charges charges charges charges chargesNumber of charges with 1 1 1 -- -- -- --tundish for 50 chargee charge charge__________________________________________________________________________ TABLE 2-B__________________________________________________________________________ (By weight %) Example 7 Example 8 Example 9 Example 10 Example 11 Example 12__________________________________________________________________________Crystalline flaky graphite 30 30 20 30 25 20ZRM 60 60 60 60 70 80 (ZRM 5) (ZRM 3) (ZRM 4) (ZRM 6) (ZRM 3) (ZRM 3)Electrically fusedaluminaFused silica 10Silicon carbide 5 5 5 5Metallic silicon 5 5 5 5 5Phenol resin binder +9.5 +9.5 +9 +9.5 +9 +9Bulk specific gravity 2.69 2.57 2.72 2.71 2.74 2.76Apparent porosity (%) 19.7 18.8 18.4 19.1 19.6 20.0Crushing strength(kg/cm.sup.2) 320 280 305 275 280 280Spalling resistance fine cracks fine cracks(1500.degree. C. .times. 15 minute water no crack no crack no crack occurred at occurred at no crackcooling, repeated 4 times) 4th water 4th water cooling coolingCorrosion resistance index 45 70 55 45 40 35Number of charges with 6 6 -- -- -- --tundish for 200 ladle charges chargesNumber of charges with -- -- 2 2 2 2tundish for 50 ladle charges charges charges charges__________________________________________________________________________ TABLE 2-C__________________________________________________________________________ (By weight %) Control 2 Control 3 Control 4 Control 5 Control 6 Control 7__________________________________________________________________________Crystalline flaky graphite 30 20 30 30 30 30ZRM 60 70 40 30 60 20 (zirconia) (zirconia) (zirconia) (zircon) (zircon) (zirconia) (zircon) 20Electrically fusedalumina 20 30 20Fused silicaSilicon carbide 5 5 5 5 5 5Metallic silicon 5 5 5 5 5 5Phenol resin binder +8.5 +8 +9.5 +10 +10 +9.5Bulk specific gravity 2.70 3.15 2.64 2.53 2.62 2.66Apparent porosity (%) 20.3 19.7 20.1 18.3 18.5 17.9Crushing strength(kg/cm.sup.2) 250 330 260 280 330 290Spalling resistance cracks cracks cracks cracks(1500.degree. C. .times. 15 minute water occurred occurred occurred occurredcooling, repeated 4 at 2nd at 1st at 2nd no crack no crack at 3rdtimes) water water water water cooling cooling cooling coolingCorrosion resistance index 40 30 70 105 90 85Number of charges with 4 charges, -- 4 3 -- 3tundish for 200 ladle cracks charges charges charges occurredNumber of charges with 1 -- -- -- 1 --tundish for 50 chargee charge__________________________________________________________________________ TABLE 3__________________________________________________________________________ (by weight %) Example 14 Example 15 Example 16 Control 8 Control 9 Control 10 long submerged submerged long submerged submerged Example 13 nozzle nozzle nozzle nozzle nozzle nozzle__________________________________________________________________________Crystalline flaky graphite 30 30 30 30 30 30 30ZRM 5 10 25 20 -- -- -- (ZRM 3) (ZRM 3) (ZRM 3) (ZRM 3)Electrically fusedalumina 40 35 22 35 45 47 55Fused silica 20 20 15 5 20 15 5Silicon carbide 5 5 5 5 5 5 5Metallic silicon -- -- 3 5 -- 3 5Phenol resin binder +10 +10 +10 +10 +10 +10 +10Bulk specific gravity 2.25 2.25 2.30 2.40 2.25 2.30 2.40Apparent porosity (%) 14.5 14.5 16.5 17.0 14.5 17.0 17.5Crushing strength(kg/cm.sup.2) 270 290 240 270 255 245 280Spalling resistance(water cooling for 15 min. no crack no crack no crack no crack no crack no crack no crackat 1500.degree. C., repeated4 times)Number of charges with 11 13 -- -- 6 -- --200 ladle charges charges chargesNumber of charges with -- -- 4 charges*.sup.1 6 charges*.sup.2 -- 2 charges*.sup.1 5 charges*.sup.2tundish for 200 ladle no trouble orifice substantial clogging- damage by off heat__________________________________________________________________________ Note: *.sup.1 = high aluminum steel, gas bubbling and laminated layers of zirconia and graphite *.sup.2 = medium aluminum steel, gas bubbling and laminated layers of zirconia and graphite

Claims

1. A highly durable refractory for casting comprising, based on the total weight of said refractory in each case, 5-80% by weight of refractory component including the principal mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of 25-85% by weight of Al.sub.2 O.sub.3, 10-70% by weight of ZrO.sub.2 and 5-25% by weight of SiO.sub.2, respectively, based on the total weight of said chemical composition and 10-40% by weight of carbon powder.

2. The highly durable refractory for casting as set forth in claim 1, further comprising, based on the total weight of said refractory in each case, up to 20% by weight of one or more members selected from the group consisting of SiC, Si.sub.3 N.sub.4, metallic silicon and ferrosilicon, up to 30% by weight of fused silica and up to 60% by weight of alumina powder containing at least 70% by weight of Al.sub.2 O.sub.3 based on the total weight of said alumina powder.

3. A process for producing a highly durable refractory for casting comprising the steps of admixing, based on the total weight of said refractory in each case, 5 -80% by weight of refractory component including the principal mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of 25-85% by weight of Al.sub.2 O.sub.3, 10-70% by weight of ZrO.sub.2 and 5-25% by weight of SiO.sub.2, respectively, based on the total weight of said chemical composition and 10 -40% by weight of carbon powder by the use of a binder, moulding said mixture and burning the resulting moulded product in non-oxidation environment.

4. A process for producing a highly durable refractory for casting comprising the steps of admixing, based on the total weight of said refractory in each case, 5-80% by weight of refractory component including the principal mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of 25-85% by weight of Al.sub.2 O.sub.3, 10-70% by weight of ZrO.sub.2 and 5-25% by weight of SiO.sub.2, respectively, based on the total weight of said chemical composition, 10-40% by weight of carbon powder, up to 20% by weight of one or more members selected from the group consisting of SiC, Si.sub.3 N.sub.4, metallic silicon and ferrosilicon, up to 30% by weight of fused silica and up to 60% by weight of alumina powder containing at least 70% by weight of Al.sub.2 O.sub.3 based on the total weight of said alumina powder by the use of a binder, moulding said mixture and burning the resulting moulded product in non-oxidation environment.

5. The process as set forth in claim 3 or 4, in which said binder is resin.

6. The process as set forth in claim 3 or 4, in which said binder is pitch.

7. The process as set forth in claim 3 or 4, in which said admixing step is performed at cold temperature.

8. The process as set forth in claim 3 or 4, in which said admixing step is performed under heating.

9. A highly durable refractory nozzle for casting comprising, based on the total weight of said refractory in each case, 5-80% by weight of refractory component including the principal mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of 25-85% by weight of Al.sub.2 O.sub.3, 10-70% by weight of ZrO.sub.2 and 5-25% by weight of SiO.sub.2, respectively, based on the total weight of said chemical composition and 10-40% by weight of carbon powder.

10. A process for producing a highly durable refractory nozzle for casting comprising the steps of admixing, based on the total weight of said refractory in each case, 5-80% by weight of refractory component including the principal mineral phase consisting of mullite, baddeleyite and corundum and having the chemical composition consisting of 25-85% by weight of Al.sub.2 O.sub.3, 10 -70% by weight of ZrO.sub.2 and 5-25% by weight of SiO.sub.2, respectively, based on the total weight of said chemical composition, 10-40% by weight of carbon powder, up to 20% by weight of one or more members selected from the group consisting of SiC, Si.sub.3 N.sub.4, metallic silicon and ferrosilicon, up to 30% by weight of fused silica and up to 60% by weight of alumina powder containing at least 70% by weight of Al.sub.2 O.sub.3 based on the total weight of said alumina powder by the use of a binder, moulding said mixture and burning the resulting moulded product in non-oxidation environment.