FR2727340A1 - COWL WITH AN EXTERNAL LAYER CAPABLE OF FORMING A GAS WATERPROOF LAYER - Google Patents

Abstract

A steel casting stopper rod comprising a body (2) with a tip (5) made of a carbon-containing refractory material. The tip (5) is at least partially coated with an outer layer (4) capable of forming a gas-tight, oxidised, densified sintered layer at temperatures higher than 1000 DEG C. The layer (4) preferably consists of a refractory material comprising sintering precursors such as calcined alumina, reactive calcined alumina, silica fumes or clays.

Description

COWL WITH AN EXTERNAL LAYER CAPABLE OF FORMING A
WATERPROOF GAS LAYER
In the continuous casting of steel, pieces of refractory material are used to channel and regulate the flow of liquid steel. In particular, a stopper rod is used to regulate and / or stop the flow of liquid steel from a ladle to a distributor and from the distributor to a continuous casting mold. The refractory material is subject to severe conditions of use. It undergoes thermal stresses, erosion by steel, oxidation. and generally all reactions which result from interactions between the constituents of the refractory material and the steel.

The refractory materials used generally contain carbon. Frequently they use a carbon bond and are composed of refractory materials such as alumina, zirconia. Clay, magnesia, silica. silicon carbide or other dense grains. These refractories also generally contain significant amounts of carbon in the form of graphite. amorphous graphite, carbon black and an additional amount of carbon from the binder used.

The present invention relates to a stopper rod comprising a body terminated by a nose made of a refractory material comprising carbon.

We already know (GB-A-2 095 612) a distaff of this type. It has a body and a reinforced nose ending the body and made of a material different from that of the body. The material of the body and that of the nose are compressed in a single operation. In other words two powders of different composition. for example graphite alumina for the body and zirconia or magnesia for the nose. are introduced simultaneously into the same mold then pressed and cooked simultaneously.

However, in a distaff of this type. the cohesion of the alumina, zirconia and / or magnesia grains is obtained by a carbon type bond, namely a bond in which the carbon contained in the mixture constitutes by hot polymerization a network which encloses the various grains.

Aggressive steels with high oxygen content which are currently being poured and which are not always calmed, for example with aluminum or silicon or which are insufficiently so. erode the nose of such a distaff. This results in a short lifespan of the stopper rod and requires frequent replacement.

On the other hand the reactions between chemical compounds, particularly gaseous, which can form at high temperature in the refractory material constituting the nose of the stopper rod and in the liquid steel react. For example, carbon monoxide reduces certain elements present in the liquid steel on the surface of the nose and causes on this surface the precipitation of oxides, in particular of alumina oxide. Oxide deposits gradually prevent complete closure of the sprue.

The present invention specifically relates to a stopper rod which overcomes these drawbacks. It proposes a distaff in which the reactions between chemical compounds, particularly gaseous. which can form at high temperature in the refractory material constituting the nozzle nose and in the liquid steel are prevented.

This result is obtained, in accordance with the invention. by the fact that the stopper's nose has an outer layer which covers it partially or entirely. this layer being capable of forming a sintered, dense, oxidized and gas-impermeable layer when it is brought to a temperature above 1000 C.

Thanks to the presence of this layer, the resistance of the nose to steels which are not calm, or which are insufficiently so. is increased very significantly. The lifespan of the stopper rod is extended and this results in significant savings for the user.

Oxide deposits are also avoided on the surface of the nose so that the steel regulation is not disturbed. The watertight closure of the drainage hole is always possible, even after a long casting sequence.

Preferably the outer layer of the nose is made of a refractory material comprising sintering precursors. These precursors are intended to promote the sintering phenomenon. that is, the grain-to-grain bond. They allow sintering to occur at a lower temperature and to run in a shorter time.

These sintering precursors are chosen in particular from the group comprising calcined alumina, reactive calcined alumina, silica smoke, clays.

Preferably the outer layer of the nose is made of a material comprising at most 9% by weight of carbon, including the carbon contained in the binder used. Ideally the total carbon, expressed by weight, does not exceed 5%.

The outer layer may consist of a cap made separately from the body of the stopper rod and then assembled with this body. It can also be co-pressed at the same time as the cattail body.

Preferably, the same binder is used to bond the material constituting the body of the stopper rod and the material constituting the outer layer. The use of the same binder provides greater ease of manufacture in the case where the stopper rod is compressed.

Indeed, in the latter case, it would be very difficult, if not impossible, to compress a stopper rod using two different binders.

According to a preferred embodiment, the material of the external layer of the nose comprises agents for reducing the permeability. These agents are preferably chosen from the group consisting of: metal additions and in particular silicon, borax, silicon carbide. boron carbide. The purpose of these permeability reduction agents is to create a layer with reduced permeability which is added to the dense oxidized layer impermeable to gases formed by sintering the outer layer of the nose.

In a preferred embodiment, the outer layer consists of at least 80% of refractory oxides. It has a thickness of less than 10 mm and the thickness of the dense sintered layer impermeable to gases is less than 5 mm.

The invention further relates to a method of implementing a stopper rod according to the invention. According to this process, a dense sintered and gas impermeable layer is formed on the surface of the nose during a heat treatment step. Preferably, the heat treatment step is carried out by bringing the nose to a temperature of 1000 ° C. in less than 20 minutes. The heat treatment can be carried out during the preheating of the stopper rod or prior to this preheating.

Other characteristics and advantages of the present invention will become apparent on reading the following description of an exemplary embodiment given by way of illustration with reference to the appended figures. In these figures
Figure 1 is a longitudinal sectional view of a stopper rod according to the
present invention
Figures 2 and 3 are partial views on an enlarged scale of part of the nose
of the stopper rod shown in figure 1
Figure 4 is a diagram illustrating the preferred preheating mode used
for the creation of a dense sintered layer impermeable to gases in a
stopper rod according to the present invention.

In Figure 1, the stopper rod has a body 2 of elongated shape. An axial channel 3 is left in this body by the pressing mandrel. The axial channel 2 extends from the upper end of the stopper rod to a small distance from its lower end. The upper part of the body can be connected. by means not shown, to a lifting mechanism which allows it to be moved vertically in order to regulate the flow of liquid steel.

At its lower end the stopper has a rounded nose 5. The body 2 of the stopper is made of a traditional refractory material, for example a material comprising from 20 to 30% of carbon and one or more refractory oxides such as alumina, zirconia, silica, magnesia etc.

The outer layer 4 of the nose 5 is made of a refractory material having a low graphite content. The total loss on ignition of this material is less than 9 6XG.

This means that when this material is oxidized during the preheating stage of the stopper, the graphite it contains and the carbon contained in the binder represent 9% or less of the weight of refractories. In addition, the outer layer 4 includes a large amount of a refractory oxide such as alumina. Finally, the material constituting the outer layer 4 of the nose comprises sintering precursors, in particular calcined alumina, reactive calcined alumina, silica smoke or clays. The total amount of refractory oxide. is at least 80 7c. Sintering precursors are generally small grains. that is to say grains with a large specific surface. As a result, the contact surface between the grains is increased. The calcined alumina has a large specific surface and the calcined alumina reactivates an even greater specific surface. Silica smoke produces an alumina-silica reaction to create mullite. The densification of layer 4 is then carried out by mullitization. Clay type systems also create ceramic bonds at a relatively low temperature of the order of 1000 C to 1100 C.

Thanks to the presence of one or more of these sintering precursors, it is possible to create, at a relatively low temperature, for example 10000C, a grain-to-grain bond between the alumina grains (ceramic bond). This layer is dense, hard and has pores of small diameter. It is therefore impermeable to gases. This layer is preferably formed during the preheating of the stopper rod, but it can also be carried out previously. The preheating operation oxidizes the carbon contained in the outer layer 4 and thus eliminates it. This gives a carbon-free layer on the outer surface of the nose 5. It should be noted that this carbon-free layer has a small thickness. For example, if the thickness of layer 4 is 10 mm. The thickness of the decarburized layer will typically be 3 mm and a maximum of 5 mm. It can thus be seen that a large part of the thickness of layer 4 is not decarburized during preheating. In fact, during this operation, two simultaneous phenomena are observed. On the one hand, the oxidation of carbon which increases the permeability of the jacket material in a proportion all the greater the higher the carbon content. This is the reason why in general. the carbon content of the jacket material must not be removed and in any case it must not be greater than 9%. On the other hand, in parallel with the oxidation of carbon, the sintering phenomenon occurs which tends to contrary to creating an impermeable layer which opposes the further decarburization inwards of the refractory material. In order for the stopper rod to function satisfactorily, the sintering of the upper layer must quickly prevail over its oxidation. This is the reason why the sintering precursors which have been mentioned previously and which are intended to facilitate and accelerate it are provided.

The stopper rod shown in Figure I was produced by the process known as isostatic copressing. Two mixtures, one corresponding to the composition of the body 2 of the stopper rod, the other to that of the external layer 4 were placed simultaneously in a deformable mold comprising an axial mandrel intended to form a recess corresponding to the channel 3. L he assembly was subjected to isostatic pressing. The same binder was used for the body 2 and for the outer layer 4. The use of the same binder is a great advantage because it makes the part more cohesive and ensures better connection between the body 2 and the outer layer 4.

There is shown in Figures 2 and 3 a part of the stopper rod of Figure I before the preheating operation (Figure 2) and after preheating (Figure 3). In FIG. 2, a distinction is made between layer 2 corresponding to the body of the stopper rod and layer 4 corresponding to the thickness of the outer layer before preheating. In FIG. 3, the layer 2 forming the body has remained identical. On the other hand, layer 4 is now broken down into a layer 4a which constitutes the dense sintered and gas impermeable layer described above and a layer 4b which has not been oxidized because it has been protected from oxidation by layer 4a. . Its composition therefore remained identical to the initial composition that it had before preheating. We therefore note that the nozzle which at the start consisted of two distinct layers now consists of three different layers. Preferably, agents for reducing the permeability are also included in the layer 4. These waterproofing agents are, for example, silicon metal. borax. boron carbide (B4C), boron nitride (BN). The purpose of these agents is to reduce the permeability of the layer 4b so as to form an additional barrier to oppose the circulation of gases between the liquid steel contained in the pocket or in the distributor and the body of the refractory material 2.

FIG. 4 shows a graph which illustrates the correct way of preheating a stopper rod of the invention. According to curve N., the temperature of the nozzle was rapidly raised to a temperature at least equal to 1000; C. This temperature was measured in the refractory material inside the channel 3. This was done in a period of less than 20 min. Indeed. as explained previously, two phenomena occur simultaneously during preheating, on the one hand the oxidation of the carbonaceous layer and on the other hand the creation of a dense sintered layer.

If the dense impermeable sintered layer 4a shown in FIG. 3 does not form quickly, the oxidation will continue throughout the thickness of the outer layer 4 and may also reach the body 2. To prevent this from happening, it is necessary to quickly reach the sintering temperature, that is to say a temperature at least equal to 1000 C. as shown diagrammatically in FIG. 4. It is therefore necessary that the power of the burners used for preheating is sufficient to allow this temperature to be reached quickly. Curve B illustrates a too slow rise in temperature. The temperature of 1000oC necessary for the sintering to be carried out under good conditions is reached only after too long a duration, clearly greater than 20 min. Under these conditions the decarburization of the outer layer 4 has occurred excessively and it will not be possible to obtain a sufficiently waterproof layer. On the curve C. the temperature rise is rapid but the maximum temperature reached remains below 1000 C. Consequently, in this case also, the sintering of the layer 4a will not occur under good conditions.

EXAMPLE
The composition of an example of a mixture is given below for the constitution of a sintered layer according to the invention and the physical properties of this layer before sintering / oxidation.

Composition% by weight
Tabular alumina (Al203) 66
Calcined alumina (A12O3> 21
Graphite (C)
Binder 6
Silicon metal 3
Clay i
Silica smoke
100
Physical properties
Rupture module at room temperature 10.40 MPa
Density 2.913
Porosity (%) 16,190
Gravity (g / cm3) 3.475
Modulus of elasticity 23.02 GPa
Hot break module 4.34 MPa

Claims (1)

CLAIMS 1. Cattail comprising a body (2) terminated by a nose (5) made of a refractory material comprising carbon, characterized in that it comprises an external layer (4) partially or entirely covering the nose (5), this layer (4) being able to form a dense fnttée layer oxidized and impermeable to gases when brought to a temperature higher than  1000 C. 2. Stopper rod according to claim I characterized in that the outer layer (4)  is made of a refractory material comprising sintering precursors. 3. distaff according to claim 2 characterized in that the precursors of  sintering are chosen from the group comprising calcined alumina. Alumina  calcined reactive. silica smoke. clays. 4. distaff according to any one of claims í to 3 characterized in  that the outer layer (4) is made of a material comprising at most 9% in  carbon weight.  Stopper rod according to any one of Claims 1 to 4, characterized in that  that the outer layer (4) consists of an insert manufactured separately from the  body (2) then assembled to this body. 6. Stopper rod according to any one of claims 1 to 4 characterized in  that the outer layer (4) is co-pressed with the body (2) of the stopper rod. 7. distaff according to any one of claims I to 6 characterized in that  that the same binder is used for the material constituting the body (2) of the  stopper rod and the material constituting the outer layer (4). 8. Stopper rod according to any one of claims I to 7, characterized in that  that the material of the outer layer (4) comprises agents for reducing the  permeability. 9. distaff according to claim 8 characterized in that the reducing agents  of the permeability are chosen from the group comprising the additions  metallic and particularly silicon, borax, silicon carbide,  boron carbide, boron nitride. 10. Stopper rod according to any one of claims 1 to 9 characterized in  that the outer layer (4) consists of at least 80% of refractory oxides. 11. Stopper rod according to any one of claims I to 10 characterized in  that the outer layer (4) has a thickness of less than 10 mm. 12. Stopper rod according to any one of claims I to 11, characterized in that  that the thickness of the dense, gas-tight sintered outer layer (4a) is  less than 5 mm. 13. Method for implementing a stopper rod according to any one of  Claims I to 12, characterized in that a dense sintered layer is formed  and impermeable to gases (4a) at the surface of the nose (5) during a step of  heat treatment. 14. Method according to claim 13 characterized in that the treatment step  thermal is carried out by bringing the distaff to a temperature of 1000 C in  less than 20 min. 15. The method of claim 13 or 14 characterized in that the stamp  heat treatment is carried out during a preheating step of the  distaff.