D e s c r i p t i o n

The invention concerns a composition and a method for the manufacture of a refractory lining on a surface. In particular, the invention concerns a refractory composition, in particular in the form of a so-called dry setting mix (also referred to as “dry vibratable mix” in the art), which is used to produce a refractory lining in the form of a wear lining on a tundish. In that regard, the method in accordance with the invention also concerns, in particular, the manufacture of such a wear lining on a tundish.

Continuous casting plants for the casting of steel have an intermediate metallurgical vessel, referred to as a “tundish” in the art, from which the steel melt can be poured into the mould via an immersion nozzle. A tundish comprises a steel trough for holding the molten steel. On the inside, the steel trough is lined with a refractory material, the so-called permanent lining. In order to protect the permanent lining from the molten steel, a refractory material is applied to the permanent lining which is in direct contact with the molten steel in the tundish. This refractory material, which is attacked by the steel melt, is also referred to as wear lining or wear compound. In order to apply the wear lining to the permanent lining, a mould, the so- called template, is introduced into the tundish in such a way that a gap remains between the permanent lining and the template, which is filled with a composition in the form of a refractory composition.

This composition is then heated, which hardens the composition and gives it strength or green strength, respectively. The template can then be removed from the tundish and the tundish can be used for casting steel. When casting steel, the composition comes into contact with the molten steel, causing the composition to sinter.

Corresponding compositions or refractory masses for the production of such a wear lining are particularly known on the basis of magnesia. It is known, however, that magnesia reacts with water to form magnesium hydroxide. In this respect, the magnesia present in a composition can react with water, for example with water from components of the composition or with air humidity, to form magnesium hydroxide. Such a reaction of magnesia to magnesium hydroxide can occur more rapidly and to a greater extent, especially at high air humidity.

However, this reaction of magnesia to magnesium hydroxide is disadvantageous for the use of the composition, since the composition, when heated, hardens all the less the more magnesia has already been converted to magnesium hydroxide. Accordingly, the green strength of a wear lining formed from the composition decreases to the extent that magnesia has been converted to magnesium hydroxide. If, when using such a refractory composition as a wear lining, a too high proportion of the magnesia has already reacted to magnesium hydroxide, this may result in a strength, in particular a green strength, of a wear lining which can be produced from this composition, being so low that it can no longer be used as a wear lining.

A further disadvantage of the reaction of magnesia to magnesium hydroxide lies in the fact that the composition might agglomerate and harden, making the further use of the composition more difficult or even impossible.

The object to be solved by the invention is providing a composition comprising magnesia, whereby a reaction of the magnesia with water can be suppressed. In particular, the object to be solved by the invention is providing such a composition in which a reaction of magnesia with water can be suppressed or even completely prevented before the composition has been heated after application to the surface of the tundish. In particular, the object to be solved by the invention is providing such a composition which can be used in the form of a dry setting mix, in particular in the form of a dry setting mix for producing a wear lining in a tundish. In particular, the object to be solved by the invention is providing such a dry setting mix in which a reaction of magnesia with water can be suppressed or even completely prevented before the composition has been heated after application to the surface of the tundish.

A further object to be solved by the invention is providing a method for the manufacture of a refractory lining on a surface using this composition. In particular, the object to be solved by the invention is providing such a method to produce a wear lining on the surface of a tundish. A further object to be solved by the invention is providing the use of such a composition as a dry setting mix.

In its most general form, a composition comprising the following components is provided: magnesia, wherein at least a portion of said magnesia is at least partially covered with an oil; and at least one hygroscopic additive.

The invention is based in particular on the following surprising findings. Insofar, it was surprisingly found that the presence of a hygroscopic additive in a composition comprising magnesia allows water to be bound to the hygroscopic additive to such an extent that a reaction of water with magnesia can be suppressed. However, in so far as water components should remain in the composition which are not bound by the hygroscopic additive, it was surprisingly found that magnesia can be effectively protected against a reaction with these water components by covering the magnesia with oil. This oil, which covers the magnesia, preferably lies as a thin film over the magnesia. In this respect, it was surprisingly found, according to the invention, that this synergistic interaction of the hygroscopic additive in the composition and the covering of the magnesia with an oil can considerably suppress or even completely prevent a reaction of the magnesia with water in the composition.

The oil by which at least part of the magnesia is at least partially covered may be in the form of an oil or in the form of a mixture of several oils. In principle, any oil can be used that covers magnesia or forms a thin film on the magnesia. In this respect, all oils which have such a viscosity at room temperature that they can wet the magnesia grains in the composition and form a permanent coating on the magnesia grains can be considered as oils. Furthermore, the oils should be stable at room temperature, i.e. they should not volatilize at room temperature, and should have a flash point below 300°C, or even more preferably below 250°C. This is because when the composition is used to produce a wear lining on a tundish, it is preferably heated at temperatures in the range from about 200 to 300°C in order to harden the binders present in the composition, so that the composition forms a green strength.

In this respect, the oil should have a flash point below these temperatures so that it can volatilize when the composition is heated to temperatures in the range from about 200 to 300°C and does not subsequently impede the hardening of the composition.

Preferably, the oil has a flash point below 250°C, and even more preferably below 200°C.

Furthermore, the oil preferably has a viscosity in a range from 10 to 25 cP (centi Poise). In accordance with the invention, it was found that an oil with a viscosity in this range can particularly advantageously form a coating on the magnesia - especially in the form of a thin film - which completely envelops or covers the magnesia grains. In accordance with the invention, it was found that mineral oil in particular had the aforementioned properties, so that the magnesia of the composition according to the invention is preferably covered with an oil in the form of mineral oil.

The composition comprises the oil preferably in such a proportion that the magnesia is covered as completely as possible by the oil. According to the invention it was found that a complete covering of the magnesia is possible from a proportion of 0.5 mass % oil in the composition. Furthermore, it was found that the proportion of oil in the composition should not be too high, as this can hinder the hardening of the composition, especially when the composition is used to produce a wear lining. In accordance with the invention, it was found that the proportion of oil in the composition should therefore not exceed 2.5 % by mass. In addition, it was found according to the invention that the optimum proportion of oil in the composition is 1.0 % by mass. Preferably, it is therefore provided that the oil is present in the composition in a range from 0.5 to 2.5 % by mass, even more preferably in a proportion from 0.5 to 2.0 % by mass and even more preferably in a proportion from 0.8 to 1.8 % by mass.

All of the information given herein in % by mass is based on the total mass of the composition, unless otherwise stated in the individual case.

In order to suppress a reaction of magnesia with water as effectively as possible in the composition, it is preferably provided that at least 90 % by mass of the magnesia, particularly 100 % by mass of the magnesia, in relation to the total mass of the magnesia in the composition, is at least partially covered with the oil. Furthermore, in order again to be able to suppress a reaction of magnesia with water as effectively as possible, it is preferably provided that at least 90 % by mass of the magnesia, in particular 100 % by mass of the magnesia, in relation to the total mass of magnesia in the composition, is completely covered with the oil.

The composition may preferably contain magnesia in such proportions as are contained in dry setting mixes for the preparation of a state of the art wear lining. In accordance with the invention, it was found that the composition according to the invention is particularly suitable for the production of a wear lining if it has a magnesia content of 91 % by mass. Preferably, it may therefore be provided for the composition to have a magnesia content of at least 60 % by mass. According to one embodiment it may be provided that the composition comprises a proportion of magnesia in the range from 60 to 95% by mass, more preferably in a proportion in the range from 70 to 94% by mass, more preferably in a proportion in the range from 80 to 94% by mass and even more preferably in a proportion in the range from 88 to 94% by mass.

Magnesia is present in the composition in the form of grains, i.e. as a quantity of grains or in granular form. In accordance with the invention, it was found that the particle size distribution of magnesia can be important in the composition. In this respect, it was found that with increasing fineness of the magnesia grains, an increasing proportion of oil is required in the composition in order to cover the magnesia grains as completely as possible with oil. On the other hand, the grain size of the magnesia grains must not be too high, as this may impede the hardening and sintering of the composition. According to the invention, it was found that at least 40 % by mass of magnesia, relative to the total mass of magnesia in the composition, should have a grain size in the range from 0.2 mm to 3.0 mm. According to a particularly preferred embodiment, magnesia has the following particle size distribution, whereby the mass fractions are in relation to the total mass of magnesia in the composition:

0.6 to 3.0 mm: 15 to 35 % by mass,

0.2 to < 0.6 mm: 25 to 55 % by mass,

< 0.2 mm: 11 to 41 % by mass.

Magnesia with a grain size of more than 3.0 mm is preferably provided in small or no proportions, preferably less than 5 % by mass, especially with 0 % by mass, based on the total mass of magnesia in the composition.

The grain size is determined according to the standard DIN EN ISO 1927- 3 :2013-03.

In principle, magnesia can be present in the composition in the form of any magnesia known from the state of the art, for example in the form of at least one of the following types of magnesia: Fused magnesia or sintered magnesia. Due to its good sintering properties, magnesia is particularly preferred in the composition in the form of sintered magnesia.

The at least one hygroscopic additive may be one or more additives with hygroscopic properties. The at least one hygroscopic additive is different from magnesia and is therefore not present in the form of magnesia. Furthermore, the at least one hygroscopic additives is different from binders, i.e. does not exist in the form of a binder, especially not in the form of a binder known from the state of the art for dry setting mixes for the production of wear linings.

In principle, the at least one hygroscopic additive may be present in the form of one or more hygroscopic additives which exhibit hygroscopic properties, are stable at room temperature and preferably do not react with the other components of the composition. Preferably, the at least one hygroscopic additive is one or more inorganic additives. The at least one hygroscopic additive may preferably be present in the form of at least one of the following hygroscopic additives: silica gel, alkali metal hydroxide or boron oxide.

Particularly preferably, the at least one hygroscopic additive is present in the form of boron oxide.

Boron oxide exhibits hygroscopic properties to the extent that it reacts with water to form boric acid according to the following reaction equation:

B2O3 + 3H2O ® 2H3BO3.

The use of boron oxide as a hygroscopic additive also has the particular advantage that it acts not only as a hygroscopic additive but also as a sintering aid.

In this respect, it was found that boron oxide, even if it reacts to boric acid (H ₃ BO ₃ ), promotes the sintering of magnesia in the composition when the composition sinters in the tundish by contact with a steel melt. In that regard, boron oxide in the composition initially acts as a hygroscopic additive to suppress a reaction of magnesia with water in the composition. Later, the boron oxide acts as a sintering aid for the magnesia during sintering of the composition.

Furthermore, it was surprisingly found that the use of boron oxide as a hygroscopic additive in the composition can also prevent an unwanted premature reaction of binders in the composition by the boron oxide binding water to itself and thus preventing a reaction of the water with the binders.

Accordingly, according to a preferred embodiment, the at least one hygroscopic additive is present in the form of at least one mineral raw material. As set forth above, the advantage of a hygroscopic additive which is present in the form of a mineral raw material lies in the fact that it withstands the temperatures when the composition is heated up and, hence, can fulfil even further functions when the composition is heated up to a temperature where the composition is sintered. Especially preferred, as set forth above, the at least one hygroscopic additive is present in the form of at least one mineral raw material which acts as a sintering aid for the magnesia during sintering of the composition. Accordingly, such a hygroscopic additive, exhibiting hygroscopic properties, being present in the form of at least one mineral raw material and acting as a sintering aid for magnesia, initially acts as a hygroscopic additive to suppress a reaction of magnesia with water in the composition and later as a sintering aid for the magnesia during sintering of the composition. In accordance with the invention, it was found that from a proportion of hygroscopic additive in the composition, in particular a hygroscopic additive in the form of boron oxide, of 0.5 % by mass, a reaction of water with magnesia can be reduced. Furthermore, it was found that the properties of the composition, in particular its hardening and sintering, can also be impaired if the proportion of hygroscopic additive exceeds 3.0 % by mass, in particular if the hygroscopic additive is in the form of boron oxide. The optimum proportion of hygroscopic additive, especially in the form of boron oxide, is 1.0 % by mass. In this respect, it may preferably be provided that the at least one hygroscopic additive is present in the composition in a proportion in the range from 0.5 to 2.5 % by mass and even more preferably in a proportion in the range from 0.5 to 2.0 % by mass.

According to the invention, the effectiveness of the at least one hygroscopic additive for suppressing a reaction of water with magnesia has been found to be particularly effective when the hygroscopic additive is not covered with oil. In this respect, the at least one hygroscopic additive in the composition according to the invention is preferably not covered with the oil with which the magnesia is covered. This can be achieved in particular by the method described below for preparing the composition according to the invention.

Preferably, the at least one hygroscopic additive is present solid, preferably fine-grained, especially preferred as powder. In this respect, a hygroscopic additive in the form of a boron oxide powder is particularly preferred.

Preferably, the composition according to the invention comprises at least one binder, in particular at least one binder as known from the state of the art for dry setting mixes for the production of a wear lining. The at least one binder is particularly preferred in the form of one or more binders which harden under temperature. In this respect, the binders are preferably in the form of so-called thermosetting binders. The advantage of thermosetting binders is in particular that the influence of temperature on the composition not only enables the binders to harden, but also (as explained above) the oil covering the magnesia can volatilize. The binders are preferably in the form of thermosetting binders, which harden under the influence of temperature in the range from 200 to 300°C. The binders can be used in the form of thermosetting binders. Preferably, the at least one binder is present in the form of inorganic binders. Particularly preferably, the at least one binder is present in the form of at least one of the following inorganic, thermosetting binders: Silicate binders, phosphate binders or sulphate binders. Particularly preferably, the at least one binder is present in the form of at least one of the following binders: Magnesium sulphate, sodium silicate or sodium phosphate. Preferably, the at least one binder is present in a proportion in the range from 3 to 13 % by mass, even more preferably in a proportion in the range from 4 to 10 % by mass and even more preferably in a proportion in the range from 5 to 8 % by mass.

According to one embodiment of the invention it may be provided that the composition according to the invention (beside the magnesia) comprises a refractory base material. This refractory base material may consist of one or several refractory components. The refractory components may consist of refractory raw materials, especially such refractory raw materials which compositions of the generic kind, that is to say especially dry setting mixes, in accordance with the state of the art may comprise. Particularly, each of such components may consist of a non-metallic, inorganic raw material, especially a granular, non-metallic, inorganic raw material, such as olivine. The composition may comprise such a refractory base material in a proportion in the range from 0 to 30 % by mass. According to one embodiment, the composition may comprise such a refractory base material as a remainder in a proportion, completing the further components of the composition to 100 % by mass.

Preferably, the composition according to the invention consists largely or completely of magnesia, the oil, the at least one hygroscopic additive, the at least one binder and the refractory base material. According to a preferred embodiment, the total mass of the magnesia, the oil, the at least one hygroscopic additive, the at least one binder and the refractory base material is in the range from 97 to 100 % by mass, more preferably in the range from 98 to 100 % by mass, more preferably in the range from 99 to 100 % by mass and more preferably at 100 % by mass.

Accordingly, it may be provided that, in addition to the above-mentioned components, i.e. magnesia, the oil, the at least one hygroscopic additive, the at least one binder and the refractory base material, further components are present in the composition in a proportion below 3 % by mass, in particular in a proportion in the range from 0 to 3 % by mass, even more preferably in a proportion in the range from 0 to 2 % by mass, even more preferably in a proportion in the range from 0 to 1 % by mass and even more preferably at 0 % by mass.

Surprisingly, it turned out according to the invention that the composition according to the invention is particularly suitable for the production of a wear lining for a tundish for casting ultra low carbon steels, i.e. so-called ULC steels, as long as the composition contains only inorganic binders. After the oil has evaporated after the composition has been heated, it no longer contains any carbon.

In this respect, it is intended according to a preferred embodiment that the composition in accordance with the invention is used to produce a wear lining for a tundish for casting ultra low carbon steels, in particular according to the method disclosed herein.

In order to produce the composition in accordance with the invention, it is preferably provided that the magnesia is mixed with the oil in order to coat the magnesia at least partially with the oil. Preferably, magnesia and oil are mixed such, in particular for such a duration and intensity, that the magnesia or magnesia grains are completely covered with the oil. The magnesia and the oil can preferably be mixed together in a mixer, in particular a compulsory mixer. In order to prevent the at least one hygroscopic additive of the composition according to the invention from being covered by the oil, it may be preferably provided, according to the invention, that the magnesia and the oil are mixed together without the presence of the at least one hygroscopic additive. Rather, according to the invention, it can be preferably provided that the at least one hygroscopic additive is combined with the magnesia after the magnesia has already been mixed with or covered by the oil. It may also be preferred that the other components of the composition according to the invention, in particular the at least one binder, are only combined with the magnesia after the magnesia has already been covered with the oil. The at least one hygroscopic additive and the binders can be mixed with the magnesia already covered with oil, preferably also in a mixer, in order to obtain the composition according to the invention. The composition according to the invention is preferably used in the form of a dry mass, i.e. a so-called dry setting mix, for the lining of a tundish, i.e. in the form of a wear lining. As explained above, according to a particularly preferred embodiment, the composition can be used as a wear lining for the lining of a tundish for the casting of ultra low carbon steels. The use of the composition according to the invention as wear lining for the lining of a tundish can take place as known from the state of the art. In this respect, the composition provided by the invention can first be applied to the surface of the tundish. In particular, the composition can be applied to the surface of a permanent lining of the tundish. As is known from the state of the art, this application can be carried out with the aid of a template which runs at a distance from the surface to which the composition is to be applied. This surface can, as explained above, be formed in particular by the permanent lining of the tundish. A gap is formed between the template and the surface and the composition is filled into this gap. The composition is then heated. Heating can be done by state of the art technologies such as hot air, burners or electrical resistance. According to the invention, the composition is heated to a temperature above the flash point of the oil, preferably to a temperature in the range from 200 to 300°C. Furthermore, the composition is preferably heated for such a duration that the oil completely evaporates. In this respect, the composition can, for example, be heated for a period in the range from 1 to 2 hours. Furthermore, the composition is simultaneously heated to a temperature such that the binder hardens as it is heated, so that it gives the composition such a green strength that the template can be removed after heating. Then a refractory lining is obtained on the surface of the tundish, i.e. on the permanent lining of the tundish, in the form of a wear lining. The tundish can then be used as a tundish for casting steel. During the casting of steel, the wear lining formed from the composition according to the invention comes into contact with the molten steel, causing the composition or the wear lining formed from it to sinter. If a hygroscopic additive in the form of boron oxide has been used, it can act as a sintering aid.

Further features of the invention result from the claims and the example of the invention described below.

All features of the invention can be combined individually or in combination.

An example of the invention is described in more detail below.

According to the example, an exemplary embodiment of a composition according to the invention was first produced. First, a sintered magnesia was provided, which had the following particle size distribution, whereby the mass fractions are related to the total mass of the magnesia:

0.6 to 2.36 mm: 20 % by mass,

0.2 to < 0.6 mm: 50 % by mass,

< 0.2 mm: 30 % by mass.

The grain size is determined according to the standard DIN EN ISO 1927- 3 :2013-03.

This magnesia was then mixed in a mixer with 1.65 % by mass mineral oil, based on the total mass of magnesia without the mineral oil, for about 5 minutes until all grains of magnesia were completely covered with the mineral oil. The mineral oil had a flash point of 190°C and a viscosity of 18 cP.

The grains of magnesia covered with oil were then mixed with boron oxide and binders so that an embodiment of a composition according to the invention was obtained.

The composition contained magnesia with 91 % by mass, oil with 1.5 % by mass, boron oxide with 1.5 % by mass and the binders with 6 % by mass.

Boron oxide was present in the form of a powder.

The binders were also provided in dry powder form. The binders were exclusively in the form of inorganic, thermosetting binders, which are able to cure when heated to a temperature in the range of about 200 to 300°C. Specifically, the following three binders were provided in the following proportions in the composition:

Magnesium sulphate: 4.5 % by mass,

Sodium silicate: 1.0 % by mass,

Sodium phosphate: 0.5 % by mass.

In order to test how the environmental conditions during storage of this composition affect its curing properties during heating, three samples of the composition were exposed to different environmental conditions: A first sample (Sample 1) was stored for 24 hours at room temperature and low humidity. A second sample (Sample 2) was stored for 24 hours at 50°C and low humidity. A third sample (Sample 3) was stored for 24 hours at 50°C and high humidity (50 %). Subsequently, the curing properties of Samples 1 - 3 were tested. At the same time, the properties of Samples 1 - 3 were to be tested for the use as a dry setting mix to produce a wear lining on a tundish. For this purpose, the corresponding use was simulated in the laboratory as indicated below.

First, the Samples 1 - 3 were each fed into a box open at the top. This simulated the filling of the composition into the gap between a template and a permanent lining in a tundish.

The Samples 1 - 3 fed into the boxes were then placed in an oven and subjected to a temperature of 200°C for 2 hours. This simulated the heating of the composition behind the template in a tundish to harden it.

Finally, the boxes with the Samples 1 - 3 inside were taken out of the oven and the Samples 1 - 3 were taken out of the boxes. Samples 1 - 3 remained dimensionally stable, which showed a high green strength of Samples 1 - 3. In order to quantify the green strength of Samples 1 - 3, the cold compressive strength of Samples 1 - 3 was measured according to ABNT NBR 11222:2010- 06-17. Afterwards the Samples 1 - 3 showed the following cold compressive strength:

Sample 1 : 1.42 MPa,

Sample 2: 1.32 MPa,

Sample 3 : 1.38 MPa.

Samples 1 - 3 thus showed a similar level of cold compressive strength. In this respect, it was found that the environmental conditions to which Samples 1 - 3 were exposed did not lead to any deterioration in curing and strength. Samples 1 - 3 would therefore have been well suited for the production of a wear lining.

For comparison purposes, a state of the art composition was tested. For this purpose, a composition was first produced which contained 94 % by mass of the magnesia and 6 % by mass of the binders specified above. In this respect, this composition was identical to the example of the above-mentioned composition according to the invention, the only difference being that the example of the composition according to the invention also contained boron oxide and the magnesia grains were coated with oil.

Three samples (Samples I S - 3S) were taken from this comparative composition and Sample I S was treated according to Sample 1 , Sample 2S according to Sample 2 and Sample 3S according to Sample 3.

Already when taking the Samples 2S and 3S from the boxes after heating in the oven, it was evident that they did not exhibit any green strength, as they could not be taken from the boxes in a dimensionally stable manner, but already partially disintegrated when they were taken out.

The measurement of the cold compressive strength of Samples I S - 3S according to ABNT NBR 1 1222:2010-06-17 produced the following results:

Sample I S: 1.06 MPa,

Sample 2S: 0.00 MPa,

Sample 3S: 0.00 MPa. From this it can be concluded that the environmental conditions to which the Samples I S - 3S were exposed during storage had led to hydration of the magnesia in the composition. As a result, the composition could not cure as good during heating in the furnace as the composition according to Samples 1 - 3. Samples 2S and 3S did not cure at all and could not have been used to produce a wear lining.