CA1337205C

CA1337205C - Immersion nozzles for metal melts

Info

Publication number: CA1337205C
Application number: CA000605931A
Authority: CA
Inventors: Ernst Luhrsen; Albert Ott
Original assignee: Didier Werke AG
Current assignee: Didier Werke AG
Priority date: 1988-08-12
Filing date: 1989-07-18
Publication date: 1995-10-03
Anticipated expiration: 2012-10-03
Also published as: EP0354304A1; EP0354304B1; ES2048230T3; DE3827424C2; DE58906144D1; IL90588A0; PL156669B1; JPH02258150A; ZA894622B; BR8903992A; CN1040336A; KR900002868A; DE3827424A1

Abstract

Disclosed is a method of manufacturing an immersion nozzle for metal melts, particularly steel melts, from a starting mixture including aluminium oxide and boron nitride with the use of at least one fluxing agent, a bonding agent and optionally further conventional additives by preparing the starting mixture with liquid, shaping, drying and optionally firing the shaped and to an immersion nozzle manufactured in accordance with the method.
The method is characterised in that a) a portion of the aluminium oxide used is replaced by one or more other refractory materials which have a lesser tendency of the deposits formation than aluminium oxide, and b) aluminium oxide has a maximum grain size of 250 m. The immersion nozzle manufactured by the method has an advantage that it has little or no tendency to adhere to oxide components in the melt and nevertheless has a sufficiently good resistance to changes in temperature.

Description

~337205 The invention relates to a method for manufacturing an immersion nozzle for metal melts, particularly steel melts, from a starting mixture containing aluminium oxide and boron nitride with the use of a fluxing agent, a bonding agent and optionally further conventional additives by preparing the starting mixture with liquid, shaping, drying and optionally firing the shaped body.
Immersion nozzles are used in the processing of metal melts, particularly in the continuous casting of steel. DE-C-3003046 discloses a ceramic composition based on aluminium oxide or zirconium oxide or silicon oxide, carbon and bonding agents for manufacturing such immersion nozzles, which contains 5 to 15 wt.%
calcium silicon and/or ferrosilicon and/or boron nitride. Due to the boron, nitride a reduction of the melting point of the solid oxide suspensions present in the steel, e.g. alumina particles of 1 to 30mm is supposed to be achieved, so that these become liquid and do not adhere to the refractory wall. A mixture of different refractory materials is however not proposed in this DE-C-3003046 and further nothing is said about the grain size of the corundum (aluminium oxide) used. Additionally, DE-A-3439954 discloses a refractory wear part for pouring liquid melts, which is manufactured from a mixture of A1203, graphite, a fluxing agent combination and a carbon-containing bonding agent and a metallic powder and in which the fired wear part is wholly or partially rough glazed and optionally subsequently fired in an oxidising atmosphere. However, in the manufacture of this refractory wear part, only aluminium oxide is used as the refractory material and no boron nitride is used. Immersion nozzles, in which a layer of -2- 13~7~2 05 boron nitride is applied to the surfaces which come into contact with the steel, are disclosed in DE-C-3401999.
The problem referred to above of the adhesion of oxide components contained in the steel melt is particularly present with steel melts when using immersion nozzles, whereby a clogging of the immersion nozzle may result in an interruption in the pouring of the metal melt. It is absolutely important to make the pouring time as long as possible for cost reasons. A further problem with immersion nozzles is their resistance to changes in temperature which should be sufficiently high in order to avoid the formation of cracks and a premature replacement of the immersion nozzles.
It has now been found that in immersion nozzles as a result of the use of fine grained aluminium oxide in conjunction with fluxing agents and boron nitride such a growth of oxide slag constituents of the metal melt can be avoided, but that when using fine aluminium oxide the resistance to changes in temperature is reduced. If aluminium oxide is used for the manufacture of immersion nozzles it was previously used with grain sizes of up to 0.5mm. When using such coarse grained aluminium oxide there was however always a deposition of oxide components, particularly of aluminium oxide from the steel.
It is thus the objection of the present invention to provide an immersion nozzle which not only exhibits a reduced tendency to deposits or clogging but which also has a sufficiently high resistance to changes in temperature.
According to an aspect of the present invention, there -is provided a method of manufacturing an immersion nozzle of the type described above, which comprises:
[A] preparing a starting mixture using a liquid, the said starting mixture containing:
(a) finely grained aluminium oxide having a maximum grain size of 250~m, (b) a refractory material which (i) is other than aluminium oxide and (ii) has a lesser tendency of formation of deposits than aluminium oxide when used in the metal melt, (c) boron nitride, (d) a fluxing agent, and (e) a bonding agent, [B] shaping the starting mixture;
[C] drying the shaped mixture; and [D] where required, firing the dried mixture.
Two major characteristic features of the method according to the present invention are:
a) the combination of aluminium oxide and the other refractory material having a lesser tendency to form deposits than aluminium oxide, and b) the use of finely grained aluminium oxide.
Preferably the aluminium oxide has a maximum grain size of 150~m, more preferably 90~m and particularly preferably 44~m.
The smaller is the maximum, grain size of the aluminium oxide used, the smaller is the tendency to adhesion of oxide components from the metal melt but on the other hand if the structure is too finely grained, the resistance to changes in temperature tends to become worse.
The other refractory material which has a lesser tendency to form deposits than aluminium oxide, may thus have a coarser grain size than the aluminium oxide used, i.e. preferably with a minimum grain size of 44~m, more preferably with a minimum grain size of 90~m and particularly preferably with a minimum grain size of 150~m, whereby the maximum grain size of these other refractory materials is determined only by technical requirements, i.e. the maximum grain size extends up to the usual values of l.Omm.
The effect of the combination of finer grained aluminium oxide and coarser grained other refractory materials is that the resistance to changes in temperature of the finished immersion nozzle can be improved so that only the problem of the clogging of the immersion nozzle but also the problem of the good resistance to changes in temperature is achieved which is at least as good as in an immersion nozzle manufactured with a coarse grained alumina.
The aluminium oxide used in the method in accordance with the invention can be a conventional material used for refractory purposes, e.g. appropriately fine grained fused corundum or tabular alumina.
Examples of the other refractory or oxide materials which have a lesser tendency of deposits formation than aluminium oxide, are mullite, silicon carbide, silicon nitride, vitreous silica, broken porcelain and fused lime. These other refractory materials can in each case be used individually or as a mixture together with the aluminium oxide.
The total amount of the fine grain aluminium oxide and the other refractory material is not particularly critical and is in a range of common use. Usually it is from about 30 to about 80 wt.%, preferably from about 40 to about 60 wt.% with respect to the stating material.
The weight ratio of aluminium oxide to the other refractory materials is not very critical, and in a preferred embodiment, it is 30:70 to 70:30. This results on the one hand in the good refractory characteristics of aluminium oxide and on the other hand an adequate resistance to changes in temperature is achieved by use of the other refractory material. The grain size of the other additives normally ranges up to a maximum grain size of l.Omm, preferably 0.5mm, i.e. their grain size corresponds to the usual grain sizes used in the manufacture of such immersion nozzles.
In a further preferred embodiment graphite and/or elemental silicon can be added as a further additive to the starting mixture. Flakey graphite is advantageously used as the graphite and the elemental silicon is commonly used in finely divided form, i.e. with a maximum grain size of 0.2mm. Due to the use of flakey graphite expensive boron nitride can be saved.
Elemental silicon serves to strengthen and to protect the carbon compound against oxidation.
When manufacturing the immersion nozzles in accordance with the invention at least one fluxing agent is used. This can be selected from fluxing agents commonly used in the manufacture of immersion nozzles. This can be boron-free or boron-containing fluxing agents, examples of which are glass frits, feldspars, boric acid, borax etc.
In a preferred embodiment a fluxing agent combination comprising a glass frit and an alkaline feldspar are used as the flowing agents. The glass frit used can be either a boron-containing glass frit, e.g. a glass frit with an approximate oxide composition of 30% SiO2, 6% A1203, 4% Fe203, 8% CaO, 1% MgO, 0.4%
Mn304, 25% Na20, 2% P205, 3.2% BaO and 20% B203. Furthermore, it is also possible to use so-called glass frits, e.g. with an approximate oxide composition of 66% SiO2, 23% A1203, 5% CaO, 4%
ZnO and 2% LiO2. When using such boron-free glass frits a boron-containing fluxing agent, as were referred to above, is advantageously also used.
In the manufacture of the immersion nozzles in accordance with the invention a conventional temporary bonding agent is also used. In an advantageous embodiment a resin, particularly a synthetic resin in the form of a novolak or a resol resin or a conventional pitch is used as the temporary bonding agent, whereby synthetic resin and pitch can also be used as a mixture. When using a curable synthetic resin a suitable hardener, e.g. hexamethylenetetramine, can be added in the amount necessary to harden the synthetic resin added, for instance when using novolaks. When using resol resins a hardener additive is normally not necessary. The temporary bonding agent is decomposed during the firing step.
The boron nitride used in the manufacture of the immersion nozzles in accordance with the invention may be hexagonal boron nitride commonly used in fine grain form, i.e.
with a grain size not more than lOO~m.
The amount of the boron nitride used, with respect to the dry starting mixture, is usually in the range of 5 to 30 and advantageously in the range of 8 to 25 wt.%. The amounts of optionally added elemental silicon are in the usual ranges of 3 to 8 wt.% with respect to the dry starting mixture and the amounts of graphite in the range of 0 to 20 wt.%.
The fluxing agents are used in amounts between 2 and 12 wt.%, with respect to the dry starting mixture.
The temporary bonding agent is generally used in amounts of 5 to 20 wt.%, with the respect to the dry starting mixture.
The invention will be described in more detail by way of the following examples.
Example 1 A starting mixture of 12 parts by weight fused corundum with a maximum grain size of 120~m, 14.5 parts by weight tabular alumina with a maximum grain size of 44~m, 21 parts by weight comminuted vitreous silica with a grain size of 0.09 to 0.5mm, 4 parts by weight feldspar, 1.5 parts by weight borax, 2.5 parts by weight of a boron-containing glass frit with a B203 content of 20%, 15 parts by weight flakey graphite, 10 parts by weight boron nitride with a maximum grain size of lOO~m and 5 parts by weight finely divided silicon with a maximum grain size of 75~m was thoroughly premixed in a mixer. Subsequently 14.5 parts by weight of a resol resin solution were added. The resol resin solution was sufficient as the mixing liquid.
After thorough mixing an immersion nozzle was moulded from the mixture, dried for 4 hours at 120C and subsequently hardened for 6 hours at 180C.
This nozzle was subsequently slowly heated in a reducing atmosphere and fired for 4h at 1000C.
In use, the immersion nozzle exhibited only a very small tendency to adhesion of oxide components to the surfaces coming into contact with liquid steel and had an adequately good resistance to changes in temperature.
Examples 2 to 5 The procedure of Example 1 was repeated with the starting components given in the following table, whereby immersion nozzles were produced which also had only a small tendency to the accumulation of slag components from a steel melt and had a sufficiently high resistance to changes in temperature.

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Claims

1. A method of manufacturing a refractory immersion nozzle for a metal melt, which comprises:
[A] preparing a starting mixture using a liquid, the said starting mixture containing:
(a) finely grained aluminium oxide having a maximum grain size of 250µm, (b) a refractory material which (i) is other than aluminium oxide and (ii) has a lesser tendency of formation of deposits than aluminium oxide when used in the metal melt, (c) boron nitride, (d) a fluxing agent, and (e) a bonding agent, [B] shaping the starting mixture;
[C] drying the shaped mixture; and [D] where required, firing the dried mixture.

2. The method as claimed in claim 1, wherein the refractory material other than aluminium oxide has a grain size coarser than the aluminium oxide but up to 1.0 mm.

3. The method as claimed in claim 2, wherein the refractory material other than aluminium oxide is one or more members selected from the group consisting of mullite, silicon carbide, silicon nitride, vitreous silica, broken porcelain and fused lime.

4. The method as claimed in claim 1, 2 or 3, wherein the aluminium oxide has a maximum grain size of 150µm.

5. The method as claimed in claim 1, 2 or 3, wherein the aluminium oxide has a maximum grain size of 90µm.

6. The method as claimed in claim 1, 2 or 3, wherein the aluminium oxide has a maximum grain size of 44µm.

7. The method as claimed in claim 1, 2 or 3, wherein the other refractory material has a minimum grain size of 44µm.

8. The method as claimed in claim 1, 2 or 3, wherein the other refractory material has a minimum grain size of 90µm.

9. The method as claimed in claim 1, 2 or 3, wherein the other refractory material has a minimum grain size of 150µm.

10. The method as claimed in claim 1, 2 or 3, wherein the weight ratio of the aluminium oxide to the other refractory material is in the range of 30:70 to 70:30.

11. The method as claimed in claim 1, 2 or 3, wherein the starting material also contains at least one member selected from the group consisting of graphite and elemental silicon.

12. The method as claimed in claim 1, 2 or 3, wherein the fluxing agent contains a glass frit and a feldspar.

13. The method as claimed in claim 12, wherein the fluxing agent further contains a boron-containing fluxing agent.

14. The method as claimed in claim 1, 2 or 3, wherein a resin or pitch is used as a temporary bonding agent.

15. A method of manufacturing a refractory immersion nozzle for molten steel, which comprises:
[A] preparing a starting mixture using water, the said starting mixture containing:
(a) finely grained aluminium oxide having a maximum grain size of 250µm, (b) a refractory material which has a lesser tendency of deposit formation than aluminium oxide when used in the molten steel and is one or more members selected from the group consisting of mullite, silicon carbide, silicon nitride, vitreous silica, broken porcelain and fused lime, the said refractory material having a grain size coarser than the aluminium oxide up to 1.0 mm, (c) boron nitride having a maximum grain size of not more than 100µm, (d) a fluxing agent, and (e) a temporary bonding, [B] shaping the starting mixture;
[C] drying the shaped mixture; and [D] firing the dried mixture.

16. The method as claimed in claim 15, wherein the starting mixture further contains:

(f) at least one member selected from the group consisting of graphite and elemental silicon.

17. The method as claimed in claim 16, wherein the starting mixture contains:
from about 30 to about 80 wt.% of the total of the aluminium oxide (a) and the refractory material (b);
from about 5 to about 30 wt.% of the boron nitride (c);
from about 2 to about 12 wt.% of the fluxing agent (d);
from about 5 to 20 wt.% of the temporary bonding (e);
from about 3 to about 8 wt.% of elemental silicon; and from 0 to about 20 wt.% of graphite.

18. The method as claimed in claim 15, 16 or 17, wherein the aluminium oxide has a maximum grain size of 150µm.

19. The method as claimed in claim 15, 16 or 17, wherein the aluminium oxide (a) is a mixture of tabular aluminium oxide having a maximum grain size of 44µm and fused corundum having a maximum size of up to 150µm.

20. The method as claimed in claim 15, 16 or 17, wherein the weight ratio of the aluminium oxide to the other refractory material is in the range of 30:70 to 70:30.

21. The method as claimed in claim 19, wherein the weight ratio of the aluminium oxide to the other refractory material is in the range of 30:70 to 70:30.

22. An immersion nozzle for metal melts manufactured in accordance with the method of any one of claims 1, 2, 3, 15, 16 and 17.