WO2015020752A1

WO2015020752A1 - Composite side dam

Info

Publication number: WO2015020752A1
Application number: PCT/US2014/045891
Authority: WO
Inventors: Sunilkumar C. Pillai; Duane L. Debastiani
Original assignee: Vesuvius Crucible Company
Priority date: 2013-08-09
Filing date: 2014-07-09
Publication date: 2015-02-12

Abstract

A composite that may be used in a side dam in a continuous twin roll caster system contains a working layer attached to a backing layer. The working layer contains hexagonal boron nitride and a second material containing least one metal nitride selected from the group consisting of silicon, aluminum and titanium nitride, and at least one stable metal oxide. The backing layer contains hexagonal boron nitride and a second material selected from the group consisting of SiAlON, silicates, nitrides, carbides, borides, oxides, mixed oxides, and combinations of more than one of these materials.

Description

TITLE OF THE INVENTION

Composite Side Dam BACKGROUND OF THE INVENTION

(a) Field of the Invention

[0001] The invention relates to a side dam for use in a continuous twin roll caster system.

(b) Description of Related Art

[0002] In the continuous casting method of manufacturing steel, molten (liquid) steel is cast directly into thin strip by a casting machine. The shape of the strip is determined by the mold of the casting machine, which receives the molten metal from a tundish and casts the metal into thin strip. The strip may be further subjected to cooling and processing upon exit from the casting rolls.

[0003] In a twin roll caster, molten metal is introduced between a pair of counter- rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving casting roll surfaces, and are brought together at the nip between the casting rolls to produce a thin cast strip product, delivered downwardly from the nip between the casting rolls. The term "nip" is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the casting rolls so as to restrain the two ends of the casting pool.

[0004] The length of a casting campaign of a twin roll caster has been generally determined in the past by the wear cycle on the core nozzle, tundish and side dams. Multi-ladle sequences can be continued so long as the source of hot metal supplies ladles of molten steel, by use of a turret on which multiple ladles of molten metal can be transferred to operating position. Therefore, the focus of attention in the casting has been extending the life cycle of the core nozzle, tundish and side dams, and thereby reducing the cost per ton of casting thin strip. When a nozzle, tundish or side dam would wear to the point that one of them had to be replaced, the casting campaign would have to be stopped, and the worn out component replaced. This would generally require removing other unworn components as well since otherwise the length of the next campaign would be limited by the remaining useful life of the worn but not replaced refractory components, with attendant waste of useful life of refractories and increased cost of casting steel. Further, all of the refractory components, both replaced and continued components, would have to be preheated the same as starting the original casting campaign before the next casting could be done. Graphitized alumina, boron nitride and boron nitride-zirconia composites are examples of suitable refractory materials for the side dams, tundish and core nozzle components. Also, since the core nozzle, tundish and side dams all have to be preheated to very high temperatures approaching that of the molten steel to withstand contact with the molten steel over long periods, considerable waste of casting time between campaigns resulted.

[0005] Also, the side dams wear independently of the core nozzles and tundish, and independently of each other. The side dams must initially be urged against the ends of the casting rolls under applied forces, and "bedded in" by wear so as to ensure adequate seating against outflow of molten steel from the casting pool. The forces applied to the side dams may be reduced after an initial bedding-in period, but will always be such that there is significant wear of the side dams throughout the casting operation. For this reason, the core nozzle and tundish components in the metal delivery system could have a longer life than the side dams, and could normally continue to be operated through several more ladles of molten steel supplied in a campaign if the useful life of the side dams could be extended. The tundish and core nozzle components, which still have useful life, are often changed when the side dams are changed to increase casting capacity of the caster. Further, the core nozzle must be put in place before the tundish, and conversely the tundish must be removed before core nozzle can be replaced, and both of these refractory components wear independently of each other.

[0006] In addition, no matter which refractory component wears out first, a casting run will need to be terminated to replace the worn out component. Since the cost of thin cast strip production is directly related to the length of the casting time, unworn components in the metal delivery system are generally replaced before the end of their useful life as a precaution to avoid further disruption of the next casting campaign. This results in attendant waste of useful life of refractory components.

[0007] Each side dam is generally held in place during casting by a side dam holder. The side dam typically includes a V-shaped beveled bottom portion and the side dam holder typically includes a V-shaped receptacle into which the V-shaped beveled bottom portion of the side dam is seated. The V-shape configuration serves to position and hold the side dam in place during casting. However, such side dam assemblies limit the useful life of the side dams before adversely impacting the edges of the cast strip and risking serious damage to the casting equipment. Specifically, the worn side dams and side dam holders may allow bleeding molten metal if the side dams are allowed to wear past a certain point, and result in damage to the casting equipment. Therefore, the side dams are usually replaced before such damage to the edges of the cast strip and to the casting equipment can occur limiting the duration of the casting campaign. As explained above, when the side dams are changed, the removable tundish and nozzle core will generally also be changed and a new casting campaign started. The casting costs per ton of thin strip cast thus could be considerably reduced if the useful life of the side dams could be extended.

[0008] It has been further observed that greater pressure is exerted between the side dam and casting rolls adjacent the nip and has resulted in increased localized wear of the side dam adjacent the nip. This additional wear adjacent the nip had led to a groove or channel forming in the side dam in that area. Further, the increased wear in this location reduces the useful life of the overall side dam, which further reduces

productivity of a continuous caster system because of the need to change side dams more often. [0009] In addition, during longer heating cycles, the overall wear of the side dams will be so high that their strength goes down. This will lead to cracking of the side dams which can be disastrous as the molten metal may leak out.

BRIEF SUMMARY OF THE INVENTION

[0010] Accordingly, a side dam has been developed in which a working layer is laminated to a backing layer. In this side dam, the backing layer has higher strength and higher fracture toughness than currently-used side dam materials, and the materials of the working and the backing layer are selected so that their respective coefficients of thermal expansion do not lead to cracking. In certain embodiments of the invention, the backing layer has at least 25% greater strength and at least 25% greater fracture toughness that does the working layer. For a side dam in which the average strength of the working layer is 100 MPa and the fracture toughness is 3 MPa · m^1/2, the average strength of the backing layer would be at least 125 MPa and the fracture toughness would be at least 3.75 3 MPa · m^1/2. In certain strip casting applications, the backing layer is machinable, and can contain holes that may be threaded. These holes may accommodate fastening devices so that the backing layer can be connected to a side dam holder.

[0011] The working layer is composed of a composite pressure-sintered material comprising a continuous phase of hexagonal boron nitride and, dispersed therein, a second material comprising: (a) at least one metal nitride selected from the group consisting of silicon, aluminium or titanium nitride, and (b) at least one stable metal oxide; the amount of metal oxide being such that the second material does not contain more than 35% by weight of oxygen.

[0012] The material used in the working layer possesses a relatively low thermal expansion coefficient and therefore exhibits good thermal shock resistance. Another characteristic of this material is its low wettability by molten steel which is thus responsible for improved resistance to liquid metal and reduces the occurrence of steel solidification thereon. Finally, it has been observed that this material exhibits exceptional mechanical wear resistance.

[0013] The major constituent of the backing layer may be an silicate (such as mullite), a nitride (such as silicon nitride), SiAION, a carbide (such as silicon carbide), a boride, or a combination of more than one of these materials. The backing layer also contains hexagonal boron nitride in a weight percentage in the range from and including 5% to and including 40%. Optionally, the backing layer contains at least one metal oxide, such as magnesium oxide or yttrium oxide.

[0014] In terms of physical properties, the material of the backing layer is selected to have a thermal expansion coefficient comparable to that of the working layer in order to avoid cracks associated with a thermal expansion mismatch. In certain embodiments, the thermal expansion coefficients of the working layer and the backing layer may differ by less than 0.5 χ Ιθ τ¹, by less than 0.8 χ 10^-6κΛ by less than 1.0 x 10^~6K^~1, by less than 1.2 x 10 K , or by less than 1.5 x 10 K . The presence of boron nitride in the backing layer produces machinability. For example, boron nitride may be present in the material of the backing layer in a range from and including 5 wt% to and including 50 wt%, from and including 5 wt% to and including 40 wt%, from and including 10 wt% to and including 40 wt%, from and including 10 wt% to and including 30 wt%, or from and including 20 wt% to and including 40 wt%.

[0015] In some embodiments of the invention, the material of the backing layer is selected to have a chemistry comparable to that of the working layer so that the working and backing layers will be bonded together during the consolidation process. For example SiAION, which has a chemical formula represented by given by Si6-z Al_z O_z

N8-z, wherein z is has a value between 0 and about 4.5, between 1 and 4.5 or between

2 and 3, can be used in the invention so that the "z" value of the backing layer is lower than the "z" value of the working layer. If z is equal to zero for a sample of SiAION, the resulting compound is Si₃N₄ or silicon nitride, which may be used in some embodiments of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] The composite side dam of the invention contains a working layer and a backing layer that have been attached together. The working layer may be referred to herein as a first layer, and the backing layer may be referred to herein as a second layer. Attachment may be accomplished by lamination, co-pressing, or by mechanically attaching the layers. In use in a metallurgical process, the side dam of the invention is installed so that the working layer faces the interior of the mold of a casting machine, and the backing layer is disposed between the working layer and the side dam holder.

[0017] The backing layer can be hot-pressed along with the working layer in a co-hot pressing process. A side dam with these two layers resists crack propagation.

[0018] The crystalline structure of hexagonal boron nitride is essentially constituted of planes which are supposed to play a role in the prevention of cracks propagation.

Therefore, the material of the working layer must contain a continuous phase of hexagonal boron nitride. It has been determined that an amount of at least 45% by weight of hexagonal boron nitride, or at least 55% by weight of hexagonal boron nitride, produces a continuous phase of boron nitride.

[0019] However, uncombined hexagonal boron nitride is too soft and has low mechanical strength with the result that the material has a high tendency to chipping and wearing. Therefore, the material of the working layer should contain less than 80% by weight of hexagonal boron nitride, or less than 70% by weight.

[0020] Therefore, a composition that may be used in the working layer of the invention is a composite pressure-sintered material comprising from 45 to 80% by weight of hexagonal boron nitride and from 55 to 20% by weight of a second material comprising: (a) at least one metal nitride selected from the group consisting of silicon, aluminium and titanium nitride, and (b) at least one stable metal oxide in an amount such that the second material does not contain more than to 35% by weight of oxygen. Suitable results have been obtained with working layer materials comprising 57.5% by weight of hexagonal boron nitride.

[0021] According to the invention, the composite material of the working layer comprises at least one metal nitride selected from the group consisting of silicon, aluminium and titanium nitrides. For example, silicon nitride is used.

[0022] In a particular embodiment of the invention, the second material in the working layer may contain at least one stable metal oxide in an amount such that the second material does not contain more than 35% by weight of oxygen. For example, the second material may contain at least 2.5% by weight of oxygen.

[0023] It is necessary that the selected stable metal oxides, if any, are able to form a solid solution into said metal nitride. This is generally the case when the atomic number of the heavier metal atom of said stable metal oxides is not higher than the atomic number of the heavier metal atom of said metal nitride. Suitable stable metal oxides comprise, but are not limited to, oxides of aluminum, titanium, silicon and magnesium or mixtures thereof. For example, aluminum oxide may be used.

[0024] In a particular embodiment of the invention, use is made of Sialon as a second material containing oxygen. Sialon is a well-known material and, as the name implies, it is a material composed of Si-AI-O-N and may be described as a solid solution of alumina in silicon nitride. The normal chemical formula of Sialon is given by Si6_-Z Al_z O_z

Νβ-ζ, wherein z is has a value between 0 and about 4.5, between 1 and 4.5 or between 2 and 3.

[0025] It is to be understood that the composite material of the working layer may also comprise conventional additives such as yttrium, magnesium, calcium and/or cerium oxides which present melting phases at high temperature and can be used in addition to, or instead of, boron oxide. These additives may be added in minor amounts not exceeding 5% by weight of the mixture hexagonal boron nitride/second material. [0026] As starting materials for the production of the composite material of the working layer of the invention, use is advantageously made of hexagonal boron nitride powder having an oxygen content of from about 2 to about 8% by weight and a specific surface

2 2

of from about 5 m /g to about 30 m /g (measured by the BET method) and of the metal nitride and oxide powder, respectively, with a purity of at least about 95% in each case. These powders may be homogeneously mixed in a manner known, per se, in a standard mixing apparatus optionally with a temporary binder being used at the same time and then pressure-sintered until the density of at least about 94% of the theoretical density is achieved. In this process, the mixtures may be hot-pressed in graphite moulds, with axial pressure application at temperatures of from about 1500 degrees C. to about 1800 degrees C, or from about 1650 degrees C. to about 1750 degrees C, with a die pressure of from about 10 to about 40 MPa, or from about 15 to about 35 MPa. Alternatively, the mixtures may also be isostatically hot-pressed in a vacuum tight closed case at a temperature of from about 1400 degrees C. to about 1700 degrees C, or from about 1500 degrees C. to about 1600 degrees C. under a pressure of from about 100 to about 300 MPa, or from about 100 to about 200 MPa in a high-pressure autoclave using an inert gas as a pressure transfer medium. Suitable shaped parts with the required dimensions are machined out of the ingots thus obtained.

[0027] The material for the backing layer can be prepared in a manner similar to that of the preparation of the working layer. In this case, the selected material (such as SiAION, SiAION with a "z" value lower than the "z" value of SiAION in the working layer, a silicate (such as rriullite), a nitride (such as silicon nitride), a carbide (such as silicon carbide), a boride, an oxide, a mixed oxide, or a combination of more than one of these materials is homogeneously mixed with hexagonal boron nitride powder in a manner known, per se, in a standard mixing apparatus optionally with a temporary binder used at the same time. A suitable sintering aid can be used to enhance the densification of the backing layer. One or more metal oxides may be added to the backing layer material for this purpose. For example, when SiAION is selected, yttrium oxide and magnesium oxide can be used as sintering aids; when silicon nitride is selected, either a rare-earth oxide such as yttrium oxide or a combination of yttrium oxide and aluminum oxide or a combination of oxides can be used as sintering aid. Sintering aids may be present in the formulation in amounts from and including 0.1 wt.% to and including 10 wt.%, 0.1 wt.% to and including 5 wt.%, 0.5 wt.% to and including 3 wt.%, from and including 1 wt.% to and including 7 wt.%, from and including 1 wt.% to and including 5 wt.%, from and including 1 wt.% to and including 4 wt.%, and from and including 1 wt.% to and including 3 wt.%.

[0028] Consolidation of the backing layer with the working layer can be accomplished by a co-hot pressing process (1400°C - 1800°C), or an isostatic hot pressing process (1400°C - 1800°C). In some applications, the working layer can be mechanically bonded to the backing layer by means of joints or by bonding them together by chemical means. In this case, the backing layer can be consolidated separately and then bonded to the working layer through mechanical or chemical bonding.

[0029] The composite material according to the invention finds its main application as side dam plates for strip casting process, but also in other applications where its exceptional resistance to mechanical and thermal stresses and its excellent resistance against chemical or mechanical wear are of importance; for example as sliding plate, in a sliding gate for a metallurgical vessel such as a tundish or a ladle.

[0030] Therefore, the invention also relates to an improvement of the strip casting process wherein the lateral containment of the liquid metal in the casting space defined by the rolls is provided by side dam plates which are applied against the plane extremities of the rolls, and the side dam plates are constituted as described above.

[0031] The subject of the invention is explained in more details in the following examples:

EXAMPLES

[0032] The following powder mixtures have been prepared to produce working layers according to the invention: [0033] In the examples 1 to 5 according to the invention, the second material is silicon nitride containing oxygen, which has been introduced under the form of alumina and of magnesia. In these examples, use has been made of a Sialon with z=2 obtained by known methods such as solid reaction of silicon nitride with alumina or by carbo- reduction under ammoniac atmosphere of a mixture of silicon and aluminium oxides. Three percent (by weight of Sialon) of MgO is added to the Sialon. This leads to oxygen content of 1 1.45% for the second material.

[0034] In the examples 3 and 4, 1% (by weight of Sialon) of yttrium oxide (Y₂O₃) was also added.

TABLE I

[0035] By way of comparison, a powder mixture comprising 50% by weight of hexagonal boron nitride, 40% by weight of zirconia (ZrO2) and 10% by weight of silicon carbide was also prepared (example C1).

[0036] The powder mixtures prepared at examples 1 to 5 and C1 were hot pressed at a temperature of 1650 degrees C. with a die pressure of 20 MPa.

[0037] Table 2 shows the results achieved using the materials of example 1 to 5 and C1 .

TABLE 2

[0038] It is shown by comparison between examples 1 to 5 and example C1 that the material according to the invention exhibits a very low coefficient of thermal expansion and has then an excellent resistance to thermal stresses. This appears also from the values measured for the R factor (thermal shock resistance calculated by the formula:

εα

wherein sigma (σ) represents the flexural modulus (MOR), nu (v) represents the Poisson's ratio, epsilon (ε) represents the Young's modulus and alpha (a) represents the thermal expansion coefficient) showing that the material according to the invention may resist to 2 to 3 times the ΔΤ which is sufficient to cause crack in the material of the art. The values observed for the wettability show that the material according to the invention is poorly wettable by molten steel, this may also be observed in placing a drop of liquid stainless steel at 1550 degrees C. under an atmosphere of argon on a sample of the material. After removal of the steel, an interaction zone of 250 pm (micrometer) depth is observed with the material of example C1 , while in the case of the material according to the invention (example 5), an interaction zone of only 50 pm may be observed.

[0039] Backing layer compositions according to the invention may contain

a) a substance selected from mullite, silicon carbide, SiAION, and silicon nitride in amounts in the range from and including 60 wt% to and including 90 wt% of the formulation, SiAION used in the backing layer composition, having a chemical formula of Si6-z Al_z O_z N8-z, may have a z value from and including 0 to and including 4.5, from and including 0 to and including 3, from and including 1 to and including 4.5, from and including 2 to and including 3, or may have a z value of 1 , 1.5 or 2.

b) boron nitride in an amount in a range from and including 5 wt% to and including 50 wt%, from and including 5 wt% to and including 40 wt%, from and including 10 wt% to and including 40 wt%, from and including 0 wt% to and including 30 wt%, or from and including 20 t% to and including 40 wt%,

c) optionally, magnesium oxide in an amount in a range from and including 0.01 wt% to and including 5 wt%, and

d) optionally, yttrium oxide in an amount from and including 0.5 wt% to and including 3 wt%.

[0040] Examples 6 - 10 are backing layer compositions according to the present invention:

[0041] Example 6: Sialon (z=2) = 77 wt%, magnesium oxide = 3 wt%, Yttrium oxide = 1 wt%; h-boron nitride = 20 wt%

[0042] Example 7: Sialon (z=1 ) = 77 wt%, magnesium oxide = 3 wt%, Yttrium oxide = 1 wt%; h-boron nitride = 20 wt%

[0043] Example 8: Silicon nitride = 76 wt%, yttrium oxide = 4 wt%, h-boron nitride = 20 wt%

[0044] Example 9: Silicon carbide = 65 wt%, boron carbide = 5 wt%, h-boron nitride = 30 wt%

[0045] Example 10: Mullite = 80 wt%, h-boron nitride = 20 wt%

[0046] According to the invention, a composite article for use in refractory applications may comprise (a) a first layer, comprising a first material being a continuous phase of hexagonal boron nitride; and a second material dispersed in the boron nitride comprising at least one metal nitride selected from the group consisting of silicon nitride, aluminum nitride, titanium nitride, zirconium nitride and mixtures thereof, and up to 35 wt. % oxygen as at least one stable metal oxide; and b) a second layer in contact with the first layer, comprising a first material being hexagonal boron nitride; and a second material selected from the group consisting of silicates, nitrides, Si₆._z Al_z O_z Ne-z, carbides, borides, oxides, mixed oxides, and combinations of these materials. The first layer may further comprise Si6-z Al_z O_z Ns-z, the second layer may comprise Sie-z Alz

O_z Νδ-ζ, and the value of z in the backing layer may be smaller than the value of z in the working layer. The first layer may further comprise a metal oxide, which may be selected form the group consisting of boron oxide, yttrium oxide, magnesium oxide, calcium oxide, cerium oxide and combinations thereof. The second layer may comprise a metal oxide, which may be selected form the group consisting of yttrium oxide, aluminum oxide, magnesium oxide and combinations thereof. The coefficient of thermal expansion of the first layer and the coefficient of expansion of the second layer may differ by less than 0.5 x 10^"6K^"1 , 0.8 x 10^{" 1} , 1.0 x 10^"V¹ , 1.2 x 10^"V¹ , or 1.5 χ

-G -1

10 K . The second layer may comprise boron nitride in an amount in the range from and including 5 wt% to and including 40 wt%, from and including 10 wt% to and including 40 wt%, or from and including 20 wt% to and including 40 wt°/¾>. The second layer may comprise Sie-z Alz Oz Νβ-ζ, with a value of z in the range from and including 0 to and including 3. The second material in the second layer may be present in an amount in the range from and including 60 wt% to and including 90 wt%. The second layer may comprise magnesium oxide in an amount in the range from and including 0.1 wt% to and including 5 wt%. The second layer may comprise yttrium oxide in an amount in the range from and including 0.5 wt% to and including 3 wt%. The second layer may comprise a material selected from the group consisting of an oxide such as aluminum oxide or magnesium oxide, a mixed oxide such as mullite or spinel, and combinations of these materials.

[0047] Examples 1 1 and 12 are working layer compositions according to the present invention. [0048] Example 1 1 is a sample composed of 44% Z-3 SiAION and 56% BN. Additionally, it contains 3 wt% MgO and 1 wt% Y2O3 as measured with respect to the total weight of SiAION and BN. It has a porosity of 0.25%, apparent specific gravity of 2.42, bulk density of 2.42 g/cm³, modulus of rupture - press direction 109 ± 6 Mpa, thermal expansion - transverse direction (20°C -1400°C) 2.89 x 0^~6/°C, thermal conductivity - press direction 8.44 W/m-K at 1000°C, and fracture toughness 2.97 +Λ- 0.05 Mpa-m^1/2.

[0049] Example 12 is a sample composed of 42% Z-3 SiAION and 58% BN.

Additionally, it contains 3 wt% MgO and 1 wt% Y2O3 as measured with respect to the total weight of SiAION and BN. It has a porosity of 0.27%, apparent specific gravity of 2.46, bulk density of 2.40 g/cm³, modulus of rupture - press direction 114 + 6 Mpa, thermal expansion - transverse direction (20°C -1400°C) 2.43 x 10^'6/°C, and thermal conductivity - press direction 8.48 W/m-K at 1000°C.

[0050] Examples 13-17 are backing layer compositions according to the present invention

[0051] Example 13 is a sample composed of 70% Z-2 SiAION and 30% BN.

Additionally, it contains 3 wt% MgO and 1 wt% Y₂O₃ as measured with respect to the total weight of SiAION and BN. It has a porosity of 0.86%, apparent specific gravity of 2.71 , bulk density of 2.70 g/cm³, modulus of rupture - press direction 146 ± 9 Mpa, thermal expansion - transverse direction (20°C -1400°C) 3.95 x 10^"6/°C, and thermal conductivity - press direction 7.46 W/m-K at 1000^oC.

[0052] Example 14 is a sample composed of 70% Z-1 SiAION and 30% BN.

Additionally, it contains 3 wt% MgO and 1 wt% Y2O3 as measured with respect to the total weight of SiAION and BN. It has a porosity of 0.99%, apparent specific gravity of 2.81 , bulk density of 2.78 g/cm³, modulus of rupture - press direction 162 ± 8 Mpa, thermal expansion - transverse direction (20°C -1400°C) 4.34 x 10^"6/°C, thermal conductivity - press direction 8.58 W/m-K at 1000°C, and fracture toughness 3.70 +/- 0.45 Mpa-m ² [0053] Example 15 is a sample composed of 80% Z-1 SiAION and 20% BN. Additionally, it contains 3 wt% MgO and 1 wt% Y₂O₃ as measured with respect to the total weight of SiAION and BN. It has a porosity of 1.15%, apparent specific gravity of 2.92, bulk density of 2.89 g/cm³, modulus of rupture - press direction 193 ± 21 Mpa, thermal expansion - transverse direction (20°C -1400°C) 4.79 x 10^"6/°C, thermal conductivity - press direction 10.16 W/m-K at 1000°C, and fracture toughness 3.60 +1- 0.44 Mpa-m^{1 2}.

[0054] Example 16 is a sample composed of 90% Z-1 SiAION and 10% BN.

Additionally, it contains 3 wt% MgO and 1 wt% Y₂O₃ as measured with respect to the total weight of SiAION and BN. It has a fracture toughness of 5.60 +/-0.40 Mpa-m '².

[0055] Example 17 is a sample composed of 80% Z-0 SiAION and 20% BN.

Additionally, it contains 3 wt% MgO and 1 wt% Y₂O₃ as measured with respect to the total weight of SiAION and BN. It has a porosity of 1.31 %, apparent specific gravity of 2.97, bulk density of 2.93 g/cm³, modulus of rupture - press direction 189 ± 9 Mpa, thermal expansion - transverse direction {20°C -1400X) 4.66 x 10^"6/°C, thermal conductivity - press direction 1 1.76 W/m-K at 1000°C, and fracture toughness 4.10 +/- 0.40 Mpa-m¹'²

[0056] Examples 18-21 are experimental results of exposure of samples to casting conditions. The sample of Example 18 was a monolayer sample having the

composition of Example 12. The samples of Examples 19-21 are bilayer plates with a working layer according to Example 12 and a backing layer according to Example 13. The samples of Examples 19-21 were hot isostatically pressed.

[0057] The sample of Example 18 was exposed to two heats with a tundish temperature in the range of 2950°C - 2975X having a total heating time of 180 minutes. Three cracks were observed in the sample. [0058] The sample of Example 19 was exposed to two heats with a tundish temperature in the range of 2950°C - 2975°C having a total heating time of 166 minutes. Two cracks were observed in the sample.

[0059] The sample of Example 20 was exposed to two heats with a tundish temperature in the range of 2950°C - 2975°C having a total heating time of 63 minutes. Two cracks were observed in the sample.

[0060] The sample of Example 21 was exposed to two heats with a tundish temperature in the range of 2950°C - 2975°C having a total heating time of 170 minutes. Two cracks were observed in the sample.

[0061] The bilayer samples of Examples 19, 20 and 21 , when exposed to the same number of heats having comparable total heating times, exhibited fewer cracks than did the monolayer sample of Example 18.

[0062] Numerous modifications and variations of the present invention are possible. It is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described.

Claims

CLAIMS We claim:

1. (Originally presented) A composite article for use in refractory

applications, comprising

a) a first layer, comprising

a continuous phase of hexagonal boron nitride; and

a second material dispersed in the boron nitride comprising at least one metal nitride selected from the group consisting of silicon nitride , aluminum nitride, titanium nitride, zirconium nitride and mixtures thereof, and up to 35 wt. % oxygen as at least one stable metal oxide; and b) a second layer in contact with the first layer, comprising

hexagonal boron nitride; and

a second material selected from the group consisting of silicates, nitrides, Si6-z Al_z O_z Ne-z, carbides, borides, oxides, mixed oxides and

combinations of these materials.

2. (Originally presented) The composite article of claim 1 ,

wherein the first layer further comprises Si6-z Al_z O_z Ns-_Z, wherein the second layer comprises Si6-_Z Al_z O_z Νβ-_Ζ, and

wherein the value of z in the backing layer is smaller than the value of z in the working layer.

3. (Originally presented) The composite article of claim 1 , wherein the first layer further comprises a metal oxide,

4. (Originally presented) The composite article of claim 3, wherein the metal oxide is selected form the group consisting of boron oxide, yttrium oxide, magnesium oxide, calcium oxide, cerium oxide and combinations thereof.

5. (Originally presented) The composite article of claim 1 , wherein the second layer comprises a metal oxide.

6. (Originally presented) The composite article of claim 5, wherein the metal oxide is selected form the group consisting of yttrium oxide, aluminum oxide, magnesium oxide and combinations thereof.

7. (Originally presented) The composite article of claim 1 , wherein the

coefficient of thermal expansion of the first layer and the coefficient of

-Θ -1

expansion of the second layer differ by less than 1 10 K .

8. (Originally presented) The composite article of claim 1 , wherein the

second layer comprises boron nitride in an amount in the range from and including 5 wt% to and including 40 wt%.

9. (Originally presented) The composite article of claim 1 , wherein the

second layer comprises boron nitride in an amount in the range from and including 0 wt% to and including 40 wt%.

10. (Originally presented) The composite article of claim 1 , wherein the

second layer comprises boron nitride in an amount in the range from and including 20 wt% to and including 40 wt%.

11. (Originally presented) The composite article of claim 1 , wherein the

second layer comprises Si6-_Z Al_z O_z Ns-z, and

wherein the value of z in the second layer is in the range from and including 0 to and including 3.

12. (Originally presented) The composite article of claim 1 , wherein the second material in the second layer is present in an amount in the range from and including 60 wt% to and including 90 wt%.

13. (Originally presented) The composite article of claim 1 , wherein the

second layer comprises magnesium oxide in an amount in the range from and including 0.1 wt% to and including 5 wt%.

14. (Originally presented) The composite article of claim 1 , wherein the

second layer comprises yttrium oxide in an amount in the range from and including 0.5 wt% to and including 3 wt%.

15. (Originally presented) The composite article of claim 1 , wherein the

second layer comprises a material selected from the group consisting of aluminum oxide, magnesium oxide, mullite, spinel and combinations of these materials.