CA1256679A - Method of manufacturing nozzles for continuous casting - Google Patents

Abstract

METHOD OF MANUFACTURING NOZZLES
FOR CONTINUOUS CASTING

Abstract of the Disclosure A method of manufacturing long or immersion type nozzles to be used for the continuous casting is provided, in which the refractory material containing flake graphite is kneaded by controlling its kneading force to prevent serious breakage of the graphite flakes. Thus the lowering of plastic deform-ability of the material may be prevented and the products of improved thermal shock resistance and highly stable quality can be obtained. As a result, the service life of the nozzles can be improved remarkably.

Description

~:5~ E;79 Back~round of the Invention This invention relates to a method of manufacturing long or immersion type nozzles to be used for continuou~ casting -hereinafter will be called as "casting no7zles" - having highly stable quality, and high thermal shock- and corrosion-resistance.
Casting nozzles are very important and effective members in the continuous casting, for improving the quality of cast metal products. In response to the recent increasing demand for high-quality steel material, continuous casting operations using casting nozzles have been remarkably increased. As the continuous casting process is locating at an early s~age of the steel production line, any trouble occuring in this process may affect the subsequent processes remarkably. Accordingly, the casting nozzles for use in this process have always been required to have a long service life and a highly stable quali-ty. For instance, they are imposed such severe requirements that the occurrence of troubles in the casting nozzles must be lessthan once per one or two thousands pieces of nozzles.
Besides that, the use of the continuous casting is ex-tended to the wide variety of steels, including the kinds of highly corrosive to the refractories. Also there is an in-creased use of multl-ladle casting which requires long time continuous operations. Therefore, there is an increased demand for the development of casting nozzles having highly stable

2~ quality and high reliability, as well as high thermal shock-and corrosion-resistance.
According to our investigation of various troubles occurred in actual operations which seem to have a relation-~s, 1 ~5~

ship to the quality of casting nozzle, such troubles may be classified into two major types, i.e., 1). Occurrence of cracking or breakage which seems to be often caused at the beginning of casting operations.
2). Occurrence of pitting or boring on the inner surface of the nozzles due to local erosion which may be caused at the inter-mediate stage of casting operations.
Detailed study of these defective nozzles and other products of the same lot revealed that in the microscopic tests, there is a tendency of lack of the uniform distribution of the refractory constituent in one body of the casting noz-zle on one hand, while on the other hand in the physical and chemical test results, such as porosity, specific gravity, bending strength and modulus of elasticity, considerably wide fluctuation was observed in the results of measurements. This may indicate that the cracking or abnormal erosion takes place at the weak portion caused by the lack of uniform quality in the body of the casting nozzles.
Generally, two counterplans are considered to avoid such troubles:
1). Improving physical and chemical properties such as anit-spalling or anti-corrosion characteristics of the refractories used for the casting nozzles.
2). Minimizing the fluctuation of the quality in one body of the casting nozzle.

6~3 ~ oth counterplan~ are intending, after all, to minimize the occurrence of the weak point described above.
As for the counterplan 1), efforts are being made to raise the refractoriness, corrosion-resistivity and anti-spalling characteristics of the ma-terial by controlling its composition an~ treatments. However, for the counterplan 2), there has been found no effective procedures to minimize the fluctuation of the quality, and this may be also fundamentally important procedure to promo-te counterplan.l).
Next, we have studied the cause of the fluc-tuation of the quality in connection with the manufacturing process of the casting nozzles.
In the production of casting nozzles, natural Elake graphite, alumina and fused silica are generally used as main refractory materials. As the na-tural flake graphite has low wetting properties against molten metals and slags as well as high thermal conductivity, casting nozz:Les containing this material have low modulus of elasticity and high tharmal shock-and corrosion-resistivity. As a result, they have a long ser-vice life.
However, particles of fla]ce graphite are flaky and fle~i-ble, -these properties are different from those of other kinds of refractory materials - ,so that it is difficult to obtain uniform dispersion of particles when the refractory materials containing such flake graphite are kneaded, and when the mate-rial is strongly kneaded, par-ticles of flake graphite tend to be broken to small size by the s-trong shearing force and there-by cause the increase of modulus of elasticity and increase the brittleness of the products~
Further, at the time of molding, the flakes of graphite have the tendency of being arranged in a same direction perpendicular to the direction of the compression, and also the flakes are apt to be dis-torted.
These and other peculiar characteristics of the flake graphite have led us to the conclusion that the special con-sideration must be paid on the manufacturing method of the casting nozzles, and particularly on the knead.ing process of raw material.

Summary of the Invention __ In the method of manufacturing casting nozzles of the present invention, the refractory material containing flake graphite is kneaded by controlling its kneading force to prevent serious breakage of the flakes of the graphite.
The method comprises the steps of;~a) kneading refractory material containing flake graphite by a granulation kneading and/or a stirring kneading, b) aging said kneaded material and molding to the nozzle shape; and c) sintering by heating said molded material to form nozzle body.
Thus the lowering of plastic deformability of the material may be prevanted by this method, and the products of high ther-mal shock resistance and highly stable quality can be obtained.
As a result, the service life of the nozzles can be improved remarkably.
The refractory material used for the present invention has the following composition, i.e., 20 - 60% by weight of natural flake graphite, ~0 - 70% by weight of alumina, 5 - 20 by weight of fused s~ilica, and 2 ~ 20~ by weight of bond re-inforcing material and anti-oxidant with the addition of phenol resin as a binder. Refractory material ls granulated to about lQ0 - 1,5Q0 ~m size granules in said kneading step.
The method of the present invention will be ~escribed in more details with reference to the accompanying drawings.

B ef Description of the Drawings Fig. 1 illustrates a series of graphs of stress-strain characteristics of the specimens prepared by three different methods showing the degree of the fluctuation of test results;
and Fig. 2 illustrates a graph showing AE total count numbers as a ~unction of the thermal shock application time on the specimens prepared by three different methods.

Detai'led Description of the Invention . , . . __ . . . . . . . .
As the natural flake graphite has low wetting properties against molten metals and slags as well as high thermal con-2G ducti~ity, casting nozzles containing t'his material have lowmodulus of elasticity, and high thermal shock~ and corrosion-resistance as described above.
However, the graphite is easily oxidized at high tempera-ture, so that the amount of -the flake graphite may be limited to some extent. A preferrably range may be 20 - 60g by weight.
As the particles of this graphite are flaky, coarse grains thereof may lower the dispersion properties and also may be easily broken to pieces/ while fine grains thereof may have less ability for lowering modulus of elasticity of the mate-rial and more easily oxidized. Therefore, grain size of the flake graphite withi~ the range of 1,000 - ~0 ~um, more preferr-ably within the range of 700 - lOO~um may be used.
Fused alumina and/or sintered alumina may be used for alumina. As the alumina has high refractoriness and corrosion-resistivity against the molten metals and slags, casting noz-zles containing the same have a high thermal shock- a~d corrosion-resistance. ~0 - 70% by weight of alumina powder of less than 120~um grain size may be used preferrably.
The use of such fine grain size may improve the dispersion properties of alumina in the material. The use of less than 40% by weight may cause the lowering of the corrosion resist-ance and the use of more than 70% by weight may cause the lowering of the thermal shock resistance because of the decrease of graphite and fused silica contents.
E'used silica, having a low coefficient of thermal expan-sion, gives high thermal shock resistance to the casting noz-zles containing the same. 5 ~ 25% by weight of fused silica of 500 - lOO~um grai~ size may be used preferrably for the purpose. Fused silica of more than 500~um grain size may cause lowering of dispersion properties, and fused silica of less than 100 ~m grain size may lower the thermal shock resistance due to the devitrification of silica during the use in a high temperature. The use of less than 5~ by weight of fused silica may no~ sufficient for the purpose and the use of more than 25% by weight may lower the corrosion resistance.
2 - 20% by weight of bond reinforcing material and anti~

oxidant may be added to the main raw material. Mullite, feldspar, sillimanite, aluminosilicates such as clay minerals, fine silicon carbide powder, fine alumina powder, fine silica powder, ~rit and the like materlal may be used as the bond reinforcing material and anti-oxidant. To the mixture of these powders, phenol resin may be added as a binder.
Preferrably, powdery and liquid form phenol resins may be used concurrently.
The above mixture is then kneaded by a granulation kneading to make granules of about 100 - 1,500~um in size by the use of, for instance, a controlled roller kneading machine and/or by a stirring kneading by the use of such as a screw beater.
Kneading by the ordinary type roller kneading machine may break the flake graphite severely, but in the granulation kneading, although broken or deformed parts of flake graphite may increase, a mosaic packing pattern of granules surrounded by the deformed graphite flakes may be formed.
Stirring type kneading has the advantages that the breakage ~0 of graphite flakes hardly occurs and they are uniformly mixed, and dispersed with other components of the material.
The kneaded mixture is -then aged by preserving for several days, after that the mixture is molded to the nozzle shape by the use of rubber press. The molded refractory material is then sintered by heating the same under the reducing atmosphere to form nozzle body. In another way of aging, the kneaded mixture is fed into the rotary drum, and the hot air is blown into the drum, while the same is rotated, then a part of the volatite components, free water and condensation water contained in the mixture may be removed and the mixture may be brought into a condition suitable for the subsequent molding process in a short period of time.
Advantages of the present invention will be explained more clearly by the following examples.
Table 1 illustrates the physical test results of the products prepared by the method in accordance with the present invention (methods II and III), with those of the products made by the conventional method (method I) for comparison.
First, the methods I, II and III were compared using the material composition (a), then the material composi~ions (a), (b), (c), (d) were compared, in which the same kneading method III being used.
In the Table, modulus of quasi-elasticit~ E' is defined by the following equation, E~ = (P60 P30)L _____--- - (1) 48(~60 -~30)I
20 where P60 and P30 are 60 percent and 30 percent load respect~ve-ly of maximum load value Pmax in the stress-strain curve obtain-ed by a three-point precision bending strength test made on a square pillar specimen having a rectangular section; and ~60 and ~30 are strain values at the load P60 and P30 respectively, and I is the moment of inertia of area of the specimen, and L
is the distance between the supporting points.
The reason of the use of the modulus of quasi-elasticity for the evaluation of material quality is that the firebrick 6~7~
materi.als containing conside:rable amount of flake graphite shows relatively large pl.astic deformation and that a large error would appear in the measurement of elasticity within a very narrow elastic range. Actually, it was seen that the evaluation based on the modulus of quasi-elasticity, including the range of plastic deformation, can explain the quality of the material more really.

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Ei7~

From the results listed in Table 1, it will be seen that the products of this invention have physical values nearly equal to those of conventional products, but they have lower modulus of ~uasi-elasticity than that of conventional products, and the thermal shock resistance is improved considerahly.
In comparing the results obtained with the compositions (a), (b), (c), (d) in the method III, the following has become clear that composition (b) having a greater amount of graphite than composition (a) shows slightly lower strength than (a) but has lower modulus of quasi~elasticity E', so that it shows an increased plastic deformability. Composition (C) having a greater amount of silica than composition (a) also shows a similar trend. Composition (d) having a greater amount of bond reinforcing material than composition (a) shows a higher strength than (a), but shows considerabl~ high modulus of ~uasi-elasticity E', so that it shows an increased of brittle-ness.
Fig. 1 shows stress ~B - strain ~ curves obtained by the bending test on several specimens prepared by the three diE-ferent kneading methods Ia, IIa and IIIa. It will be seenthat the methods IIa and IIIa of the present invention cause less fluctuation of test results than conventional method Ia and are indicating the stabilization of quality.
Fig. 2 shows the results of a thermal shock resistance test conducted by an AE (Acoustic Emission) method. When a piece of brick is heated from one sid~ surface thereof, remarkably large stress occurs within said brick. In case this stress exceeds the strength of the brick, cracks may ii$6~

occur and are transmitted through the brickO A part of the energy liberated at this time can be detected by the AE
measuring apparatus as elastic waves. AE data are given in the form of the AE count number, and as the AE count number corresponds to a number of occurrence of cracks, the larger AE count number shows much cracks occurred and transmitted, and indicates lower thermal shock resistance.
It will be seen from Fig. 2, that according to the methods IIa and IIIa of ~he present invention, products of higher thermal shock resistance may be obtained than those made by the conventional method Ia.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of manufacturing nozzles for continuous cast-ing comprising steps of;
a) kneading refractory material containing flake graphite by a granulation kneading and/or a stirring kneading;
b) aging said kneaded material and molding to the nozzle shape;
c) sintering by heating said molded material to form nozzle body.

2. The method of claim 1, wherein said refractory material is granulated to about 100 - 1,500 µm size granules in said kneading step.

3. A method of manufacturing nozzles for continuous cast-ing comprising steps of;
a) preparing refractory material comprising 20 - 60%
by weight of flake graphite, 40 - 70% by weight of alumina, 5 - 20% by weight of fused silica, and 2 - 20% by weight of bond reinforcing material and anti-oxidant with the addition of phenol resin as a binder;
b) kneading said refractory material by a granulation kneading and/or a stirring kneading;
c) aging said kneaded material;

d) molding said aged material to the nozzle shape;
e) sintering by heating said molded material to form nozzle body.