JPS623072A - Silicon carbide base sintered body and manufacture - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a silicon carbide sintered body, particularly a silicon carbide sintered body containing aluminum (Al) and a boride, and a method for producing the same.

[Prior art and its problems] Silicon carbide has high hardness and rigidity, low coefficient of thermal expansion,
It is also a covalent compound with a high decomposition temperature, and its high-density sintered body has high strength at high temperatures and high thermal shock resistance.
Applications are opening up as lightweight, high-temperature components with excellent wear resistance.

Well, hot press method, normal pressure sintering method, reaction sintering method, CVD method, etc. are known to obtain a high density sintered body of silicon carbide, but the pressureless sintering method is practically useful. Of course, even in the hot press method, high density cannot be obtained without a sintering aid. Therefore, research on sintering aids has been actively conducted.

In the past, research to obtain sintered bodies with particularly high strength has been active, and examples of using boron or boron compounds and carbon as sintering aids are disclosed in JP-A-50-78809 and JP-A-51-148.
712, etc. Although these auxiliaries have the advantage of promoting densification in small amounts and do not deteriorate in strength even at high temperatures, they have drawbacks such as low fracture toughness and high firing temperatures.

On the other hand, an example of using AI203 as a sintering aid has been proposed in JP-A-57-42577, etc., and a sintered body manufactured under favorable conditions has high strength and high fracture toughness. Excellent as a normal high temperature structural member. However, when used in applications that come into contact with steel materials at high temperatures, Fe-5i-C-based low-melting compounds (cementite, ferrosilicon, etc.) may be produced due to the reaction between silicon carbide and iron, resulting in wear. Of course, even if a small amount of the aforementioned boron or boron compound is used as a sintering aid, this drawback cannot be overcome.

On the other hand, BN, TiB2. Borides such as ZrB2 and CrB2 are known as materials with high corrosion resistance against iron, but it is difficult to obtain high-density sintered bodies of these borides, and even if they are obtained, they have a high fracture toughness value. It was difficult to obtain or the oxidation resistance was insufficient. Further, when manufacturing a composite sintered body of these borides and silicon carbide, if a large amount of these borides is added to silicon carbide, sintering becomes difficult.

Furthermore, JP-A-57-27975 proposes a silicon carbide sintered body containing TiB2 by an atmospheric pressure sintering method.
Because boron and carbon are used as sintering aids, high fracture toughness cannot be obtained.
According to JP-A-59-101702, in order to impart electrical conductivity to black silicon carbide powder with a particle size of approximately
Silicon carbide sintered bodies obtained by hot pressing with the addition of rBz and VB2 are also known, but A [20
Even if No. 3 is used as a sintering aid, the shape of the silicon carbide crystal particles remains uniformly glazed, and therefore the fracture toughness value is considered to be low.

[Object of the Invention] The present invention seeks to solve the above-mentioned problems of the prior art.

That is, the present invention has high density, maintains excellent mechanical properties such as high bending strength and high fracture toughness,
The present invention provides a silicon carbide sintered body that has high corrosion resistance and can be manufactured into various shapes, and a method for manufacturing the same.

[Structure of the Invention] The present invention comprises silicon carbide, 0.5 to 35% by weight of Al and/or a refractory Al compound in terms of Al based on silicon carbide, and 2 to 89% by weight based on silicon carbide. 4A in the 4th, 5th or 6th period of %.

5A, one or more selected from borides of fiA group elements as a main component, and a carbonization characterized in that half of the silicon carbide crystal particles have a structure consisting of columnar and/or plate-shaped silicon carbide crystal particles. Silicon sintered body, silicon carbide, and 0.2 in terms of Al with respect to silicon carbide
A type selected from 5 to 40% by weight of Al and/or an Al compound, and 2 to 120% by weight of a boron compound of a group 4A, 5^ or 6A element in the 4th, 5th or 6th period based on silicon carbide. The molded product containing the above is heated to 1800 to 23
This is a method for producing a silicon carbide sintered body characterized by sintering at 00°C.

A body of silicon carbide! The silicon carbide sintered body of the present invention basically consists of three components. The first component is silicon carbide. The second component is Al and/or a refractory Al compound.

The third component is a 4A, 5A or BA group element of the 4th, 5th or 6th period, namely titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum ( Ta), chromium (Or),
It is one or more selected from borides of molybdenum (No) and tungsten (IN).

The silicon carbide sintered body of the present invention preferably consists of only these three components, but may contain small amounts of other components as long as the objectives and effects of the present invention are not impaired. In addition, even if the product consists essentially only of these three components, for example, trace impurities such as Fe in the raw material silicon carbide powder, or oxidation of the surface of the raw material silicon carbide powder (such as 5i02 minute origin), etc. It may contain unavoidable impurities originating from the raw materials, and further unavoidable impurities mixed in during the grinding/mixing process of the raw material powder.

The second component, Al and/or the refractory Al compound, acts as an aid to promote sintering, and particularly controls the structure of silicon carbide crystals in the sintered body. Al
There are many compounds, such as aluminum hydroxide and aluminum isopropoxide, that release moisture and organic matter through thermal decomposition and change into Ah03 (alumina), etc., which are stable at high temperatures9. The compound refers to an Al compound that is stable at high temperatures, such as Al2O3. Another fire-resistant Al compound is AlN.

Examples include AlaC3, aluminum oxynitride (e.g. Al23027NS), AlB2, etc.1
The second component of the present invention is Al and/or Al2O.
3, particularly Al2O3 is preferred because the raw materials are easy to obtain and handle.

The second component is 0.5 to 35% by weight (hereinafter simply referred to as % unless otherwise specified) in terms of Al based on silicon carbide, which is the first component. If it exceeds 35%, it causes a decrease in high temperature strength and thermal shock resistance, and the content is preferably 25% or less. If it is less than 0.5%, sintering becomes difficult, and it is preferably 1.0% or more. In order to obtain a sintered body with better physical properties, the content is particularly set at 3.5 to 15%. In addition,
"In terms of rAl" means Al2O for silicon carbide.
If 3 is 10%, it means 5.3% in terms of Al.

The third component, the boride, is thought to work to improve the corrosion resistance of the entire sintered body against iron, etc. In a non-oxidizing atmosphere, the boride itself has the property of not being easily wetted by iron, etc. It is thought that this prevents silicon carbide from reacting.

Examples of such borides are TiB and TiB2.

ZrB, ZrB2,)lfB2. V3 Ba,
Nb5B4, TaB2. CrB, CrB2. M
OB2.

Examples include NaB4, WB2, WB4, among others, borides of group 4A or 6A elements, especially T.
iB2. ZrB2°CrB2, MOB2, and WB2 are preferred because the oxide layer formed on the surface protects the interior even when used in an oxidizing atmosphere.

Although the detailed mechanism is unknown, in the case of TiB2, T
FeTih is generated by reacting with i02 and iron, and prevents internal silicon carbide from directly reacting with iron.

In the case of ZrB2, Z rS ion reacted with Z r02 and Si02 by oxidation of silicon carbide shows high corrosion resistance to iron. In the case of CrB2, Gr203 shows high corrosion resistance to iron. MOB2 and WB2 In this case, No. 03 and No. 103 are considered to act as a protective layer. Note that since Hf has almost the same ionic radius as Zr,
Although ZrB2 can form a solid solution with HfB2, such a solid solution may be sufficient.

The third component is 2 to 99% relative to the first component, silicon carbide.
%. If it exceeds 99%, sintering becomes extremely difficult and oxidation resistance and fracture toughness decrease. More preferably, it is 49% or less.

If it is less than 2%, good corrosion resistance cannot be obtained. Preferably it is 5% or more. In order to obtain a sintered body with better properties, the content is preferably 10 to 29%.

Silicon carbide, which is the first component, has an α type and a β type, and the sintered body may contain either one or a mixed crystal of both.

Microstructure of the silicon carbide sintered body In the silicon carbide sintered body of the present invention, more than half of the silicon carbide crystal grains are composed of columnar or plate-shaped silicon carbide crystal grains.

The structure of the sintered body can be confirmed by observation using various optical microscopes or electron microscopes. In the microstructure of a general silicon carbide sintered body, silicon carbide crystal grains include equiaxed grains with approximately equal length on each side, columnar grains elongated in the axial direction, and plates short in the axial direction. shaped particles are known.

In the silicon carbide sintered body of the present invention, such columnar and/or plate-like particles account for half or more of all silicon carbide crystal particles, preferably 70% or more, particularly 90% or more. Such a structure inhibits crack propagation and provides high fracture toughness.

It is preferable for the aspect ratio of such columnar and/or plate-like particles to be 2.0 or more, more preferably 3.0 or more in order to improve fracture toughness. In addition, the major axis of such columnar and/or plate-like particles is 0.5μ or more, particularly 2.0μ.
When it is more than μ, grain growth from equiaxed grains is exhibited, and the aspect ratio is also high, which is preferable. Also, if the major axis is too large, it will cause defects, and this major axis should be less than 100 l, especially 3
If it is 0p or less, high strength may be exhibited.

In the silicon carbide sintered body of the present invention, the columnar and/or plate-like particles are preferably randomly oriented,
It is also desirable that they intersect with each other. This improves the fracture toughness and creep resistance of the sintered body. Furthermore, in the silicon carbide sintered body of the present invention, it is desirable that the number of silicon carbide equiaxed particles is as small as possible, but they may coexist with columnar and/or plate-shaped particles. is preferably 30μ, particularly 20pL or less.

In most of the silicon carbide sintered bodies of the present invention, a second component phase and a third component phase are further observed in terms of structure. The second component phase, for example, the Al2O3 phase, often exists as an intergranular phase at triple points, and in some cases exists as crystal grains with the same size as silicon carbide crystals, or is observed as a mixture of both. Ru. The third component phase is often observed as crystal grains, especially equiaxed grains. The average representative diameter of the second component phase is preferably 30 squares or less from the viewpoint of high temperature strength and creep resistance, and the average representative diameter of the third component phase is preferably 15 squares or less from the viewpoint of strength and corrosion resistance. .

In the silicon carbide sintered body of the present invention, the silicon carbide crystal particles, the second component phase, and the third component phase are each randomly
In addition, it is desirable that the particles be uniformly dispersed from a macroscopic perspective because the strength and corrosion resistance will be uniform within the sintered body.

′! Raw materials for silicon monocide bodies In the uninvented method for producing silicon carbide sintered bodies, the raw materials basically consist of three components. The first component is silicon carbide. The second component is Al and/or an Al compound. The third component is 4A, 5 of the 4th, 5th or 6th period.
A or BA group elements, i.e. Ti, Zr, ) If, V
, Nb, Ta, Cr, No, and W boron compounds.

Silicon carbide as a raw material may be predominant in either the α-type or the β-type, or it may be a mixture of the two, but if it is the β-type, it changes to the 4H type of the α-type. However, it is desirable because it facilitates the formation of columnar or plate-like particles.

It is desirable that the raw material silicon carbide has a high purity, and in order not to deteriorate the mechanical properties especially at high temperatures, the content of Na, K, and Ca should be 0.2% or less in terms of metal, and further, It is preferable that it is 0.05% or less, and as mentioned above,
It is desirable that the amount of 5i02 derived from surface oxidation is also small.

As the second component, Al, Al2O3, Al(OH)3
, AlN.

(i-C3H70)3Al (aluminum triisopropoxide), Al4C3, AlC+oH+30aN2(
Aluminum diaminetetraacetate), among others, Al, Al2O3, Al(OH)3,
AlN is preferable because active fine powder with good sinterability can be easily obtained. Generally, one type of these is employed, but two or more types may be used in combination.

The second component is A with respect to silicon carbide, which is the first component.
It is 0.25 to 40% in terms of l. The numerical value is slightly different compared to the component ratio in the sintered body mentioned above, but this is because the Al content is generally easy to volatilize during the sintering process, and in addition,
This is because it can also be supplied from the atmosphere. Preferably 25%, particularly 15% or less, preferably 2.0%
%, especially 3.5% or more. The reason for the limitation is the same as that described for the sintered body.

As the third component, the various borides mentioned above as the third component in the sintered body are employed for the same reason. Borate such as TiEO3, CrBO3, Zr(B
Hn)a, I5[B(W30+o)4]a5
A boron compound such as H20 can also be used alone or in combination with the above-mentioned borides.

The third component has a ratio of 2 to silicon carbide, which is the first component.
~120%. The numerical value is slightly higher than the component ratio in the sintered body described above, but this is because boron compounds other than borides can be used, and in the sintering process, the third component itself or the constituent elements of the third component This is because a part of it may evaporate. Preferably 49%, especially 29%
% or less, preferably 5%, particularly 10% or more. The reason for the limitation is the same as that described for the sintered body.

Regarding the raw material for the silicon carbide sintered body of the present invention, it is preferable that the ingredients that are actively left in the sintered body consist of only these three types.

It may contain small amounts of other components within the range that does not impair the desired effect of the present invention.

Manufacturing process of silicon carbide sintered body: A predetermined amount of such raw materials is weighed out and pulverized and mixed in a dry or wet process, preferably to reduce the average particle size of the first component raw material to 0.
8 hemorrhoids, especially 0.4μ or less, the average particle size of the raw materials for the second and third components is 10μ, especially 1g or less, and if necessary, a molding binder etc. is added and mixed, and cast. A molded article is formed by an appropriate molding method such as molding, press molding, injection molding, or extrusion molding.

The molded body thus obtained is stripped of the molding binder and the like, if necessary, and then sintered by heating to 1800 to 2300° C. in a vacuum or in a non-oxidizing atmosphere of 10 atm or less.

Al and/or Al compound added as a second component becomes Al and/or refractory Al compound during the sintering process, and some reacts with silicon carbide and 5iOz present on the surface of silicon carbide to form a liquid. give rise to phases. It is thought that by repeating dissolution and precipitation of fine silicon carbide particles in this liquid phase, grain growth occurs in a columnar or plate shape, resulting in a microstructure with high strength and fracture toughness. Eventually, the liquid phase will be mainly refractory Al compounds.

The third component, the boron compound, undergoes thermal decomposition during the sintering process.
Alternatively, it reacts with Al and/or an Al compound or silicon carbide to form a stable boride, but at this sintering temperature, significant grain growth is unlikely to occur.

If the raw material powder is pulverized to 0.8 μm or less, it will have high reactivity when sintered at 10 atmospheres or less, and will easily generate a liquid phase.
Sintering at 10 atm or less makes it easier to achieve densification; This is because products with complex φ large shapes can be fired in a furnace with a simple structure, and it is particularly preferable to perform the firing at around atmospheric pressure or under reduced pressure.

When the sintering temperature is lower than 1800°C, the amount of the liquid phase produced is small and sintering does not proceed, and when it is 1900°C or higher, the density of the sintered body becomes high, which is preferable.

On the other hand, if the temperature exceeds 2300°C, the reaction between silicon carbide and the liquid phase will be intense and decomposition will proceed.It is more preferable if the temperature is 2200°C or lower.

The non-oxidizing atmosphere is preferably N2. Examples include Ar, N2. C: The atmosphere may be composed of O, Nh, etc., or may contain these.

In addition, it is preferable that the non-oxidizing atmosphere contains vapor of Al and/or Al compounds. In other words, some of the Al and/or Al compounds in the compact may volatilize during sintering. This is because this can be prevented or adjusted.

In the method for producing a silicon carbide sintered body of the present invention, the sintered body thus obtained is preferably further heat-treated at 1800 to 2300°C in a non-oxidizing atmosphere of 20 atmospheres or more. By such heat treatment, for example, even if the density is about 90% (ratio to the theoretical density, the same applies hereinafter) in the previous sintering stage, it can be densified to a density of 85% or more, resulting in improved strength and corrosion resistance.

The reason why the pressure is set at 20 atmospheres or higher is to promote densification. In this case, if the pressure exceeds 3,000 atm, the container will become too large and is not practical.In particular, it is better to set the pressure to 50 to 2,000 atm.
The temperature is set at 0°C or higher in order to generate a liquid phase again and promote densification, and the temperature is set at 2300°C or lower, preferably at 22°C.
The reason why the temperature is 00°C or lower is to prevent decomposition of the sample. The non-oxidizing atmosphere is preferably N2. An example is Ar, but an atmosphere consisting of or containing N2°CO, NH3, etc. may also be used. In addition, the non-oxidizing atmosphere in such heat treatment is Al and/or A
1 may contain vapors of the compound.

The sintered body thus produced can be used as is or after being processed into a predetermined shape. Since it can be manufactured by , any shape can be easily obtained in actual purchase, including not only simple shapes but also complex shapes. Also, regarding size, since there are no restrictions on manufacturing equipment as in the hot press method, large products can be easily obtained.

Process of silicon carbide sintered body The silicon carbide sintered body of the present invention is a fine powder with high purity! −
4, it has a specific microstructure characterized by being obtained by so-called pressureless sintering using Al and/or an Al compound as a sintering aid, and has high strength, high toughness value, high thermal shock resistance, Since it has relatively excellent oxidation resistance, it can be used as a variety of high-temperature structural members. Furthermore, the silicon carbide sintered body of the present invention can be used as a member that comes into contact with high-temperature molten metal and high-temperature iron (or steel) materials by taking advantage of its excellent corrosion resistance, especially against iron. The silicon carbide sintered bodies containing a large amount of Al2O3 and AlM have an electrical resistivity of 10ΩC1 at 15°C.
Since the temperature coefficient is negative in the above, it exhibits semiconductor characteristics,
When the surface is oxidized, it becomes a good insulator, so it can be used as a semiconductor or insulator used at high temperatures.

Preferred applications include continuous casting rollers, conveyor rollers, skid buttons (or skid rails) for steel heating furnaces,
Examples include skid buttons (or skid rails) for steel heat treatment furnaces, members for blast furnaces, converters, open hearths, and electric furnaces, hot metal (or steel) contact members, 5 electrocouple protection tubes, stirring propellers, filters, etc.3. The present invention will be explained below.

[Example 1 As a raw material, ■Pale green β type 5iC5 with a purity of 88% or more and an average particle size of 0.3 or a purity of 98% or more and an average particle size of 0.
α-type SN with a purity of 98% or more and an average particle size of 0.5 jL, or Al. ■Purity 98% or more, average particle size Q, 5 JL) Ti82. ZrB2, I(fB2
, V3Ba, NbaBn.

Using TaB, Cr8;

It was molded to a size of IIIm.

Si is placed in a graphite container with a volume slightly larger than the dimensions of the compact.
A mixed powder of G powder and Al2O3 powder was added, a molded body was buried in this mixed powder, and sintered under the firing conditions shown in Table 1. The mixed powder is a mixture of α1-type SiC having an average particle size of 1p and α-type A 1203 described in (2) above so that the weight ratio of SiC to Al content is the same as that in the compact. Test pieces were prepared from the obtained sintered body and subjected to various tests. The test results are also shown in Table 1.

In Example 3, after sintering under the firing conditions shown in Table 1, sintering was performed at 1850°C in an Al atmosphere at 80 atm.
In Example 0 15 and 1 llt, which show sintered bodies obtained by heat treatment in the same manner, in Example 9 17, in which no boron compound was added, a graphite die with a diameter of 80 mm was used. 350kg/c in Ar at atmospheric pressure
The figure shows a sintered body with a thickness of 10 mm obtained by hot pressing at a pressure of m2.

The composition of the sintered body obtained in Example 1 is Si 53.4LC2
3.8%, Al 2.75%, Ti 13.5%, 85
．． 86%, 00.89$, A+ amount is 3.6 compared to Korekara SiC! , the amount of TiB2 was found to be 24.7 $.

The composition of the sintered body obtained in Example 9 was Si 52.8X, G
22.8$, Al 3-07$, Ta 18.0%, 8
1.01%, O2, 72$, and from this it was found that A[fi was 4.1% and TaBffi was 24.0 for SiC. In addition, all of these microstructures were structures in which columnar and/or plate-like crystals of silicon carbide with a major axis of 5 to 20 JL and an aspect ratio of 3 to 6 were interlaced. The composition of the obtained sintered body is not much different from that of the molded body as in Examples 11 and 9, and its microstructure is similar to that of the sintered bodies obtained in Examples 1 and 9. The microstructure of the sintered body obtained in Example 17 consisted of glaze particles of 1 to 5 w.

In Table 1, the bending strength is the sample cross section of 3×3 m1. Distance between lower supports 20cm, crosshead speed 0.5mm/w
It was measured by three-point bending. Fracture toughness value (K+c)
was measured by the chevron notch method.

In the iron piece contact test, the amount of erosion was measured when an iron piece was left in close contact with the surface of a sample measuring 20 x 20 mm for 48 hours at 1200° C. in the atmosphere.

[Effects of the Invention] As described in detail above, according to the present invention, it combines the advantages of silicon carbide such as high strength, high hardness, and high heat resistance with the high corrosion resistance of boride, and uses an Al-based sintering aid. A highly tough silicon carbide sintered body that has been effectively used is provided, which solves the problems that have arisen when contacting high-temperature steel materials, and provides a useful new material that can be used in an extremely wide range of industries.

In addition, this sintered body has a fine structure that is difficult to obtain with the conventional hot press method, and is produced through a so-called pressureless sintering process using fine powder raw materials and an Al-based sintering aid. There is a specificity. Furthermore, the present invention provides a technology that contributes to the development of industry by taking advantage of the so-called pressureless sintering method, which allows large-sized products and complex-shaped products to be manufactured cheaply and stably.

Claims (20)

[Claims] (1) Silicon carbide and 0.5 to 35% by weight of Al and/or refractory Al in terms of Al based on silicon carbide
a fourth compound in an amount of 2 to 99% by weight based on silicon carbide;
A structure in which at least half of the silicon carbide crystal grains are columnar and/or plate-shaped silicon carbide crystal grains, the main component being at least one type selected from borides of group 4A, 5A, or 8A elements in the 5th or 6th period. A silicon carbide sintered body characterized by having: (2) 1.0 to 25 in terms of Al to silicon carbide
The silicon carbide sintered body according to claim 1, which contains % by weight of Al and/or a refractory Al compound. (3) The silicon carbide sintered body according to claim 1 or 2, which contains 5 to 49% by weight of one or more selected from the borides based on silicon carbide. (4) The boride is a 4th, 5th or 6th period
The silicon carbide sintered body according to any one of claims 1 to 3, which is a boride of a group A or 6A element. (5) The silicon carbide sintered body according to claim 4, wherein the boride is TiB_2. (6) The silicon carbide sintered body according to claim 4, wherein the boride is ZrB_2. (7) The silicon carbide sintered body according to claim 4, wherein the boride is CrB_2. (8) The silicon carbide sintered body according to any one of claims 1 to 7, which is used for a member that comes into contact with a high-temperature molten metal or a nearly high-temperature iron or steel material. (9) The silicon carbide sintered body according to claim 7, which is used for skid buttons or skid rails in steel heating furnaces or steel heat treatment furnaces. (10) silicon carbide, 0.25 to 40% by weight of Al and/or an Al compound in terms of Al based on silicon carbide, and a fourth content of 2 to 120% by weight based on silicon carbide;
A molded article containing one or more boron compounds of group 4A, 5A, or 6A elements in the fifth or sixth period is heated to 18 ml in vacuum or in a non-oxidizing atmosphere of 10 atm or less.
A method for producing a silicon carbide sintered body, characterized by sintering at a temperature of 00 to 2300°C. (11) The method according to claim 10, wherein the molded body contains 2.0 to 25% by weight of Al and/or an Al compound in terms of Al based on silicon carbide. (12) The method according to claim 10 or 11, wherein the molded body is a molded body containing one or more types selected from the boron compounds in an amount of 5 to 49% by weight based on silicon carbide. (13) The method according to any one of claims 10 to 12, wherein the boron compound is a boride. (14) The method according to any one of claims 10 to 13, wherein the boride is TiB_2. (15) The method according to any one of claims 10 to 13, wherein the boride is ZrB_2. (16) The method according to any one of claims 10 to 13, wherein the boride is CrB_2. (17) The method according to any one of claims 10 to 16, wherein the silicon carbide has an average particle size of 0.8 μm or less. (18) The production method according to any one of claims 10 to 17, wherein the average particle size of the Al and/or Al compound and the boron compound is 10 μm or less. (19) The non-oxidizing atmosphere is Al and/or Al
Claims 10 to 1 containing vapor of the compound
The manufacturing method according to any of Item 8. (20) The molded body is sintered at 1,800 to 2,300°C in a vacuum or in a non-oxidizing atmosphere of 10 atm or less, and then further sintered at 1,800 to 2,300°C in a non-oxidizing atmosphere of 20 atm or more.
The manufacturing method according to any one of claims 10 to 19, characterized in that heat treatment is performed at 2300°C.