PREPARATION OF REFRACTORY FOR MAKING STEEL INGOTS

FIELD OF THE INVENTION

The present invention relates to a method for preparing a refractory having a double-layer structure for making alumina impurity-free steel ingots.

BACKGROUND OF THE INVENTION

A general process for making steel ingots, schematically illustrated in

Fig.l involved the steps of passing a molten steel through refractory conduit which consists of inflow tube 1, sleeve 2, center brick 3, runner brick 4, outflow tube 5 and an ingot mold 6, and cooling cast steel obtained in step (a) to obtain steel ingots. The refractory for making steel ingots is commonly made of alumina- based materials. However, when a stream of molten steel is introduced through the alumina-based refractory and brought into contact with the surface of the refractory, alumina may be incorporated into the molten steel as impurities. In order to prevent such a problem, there have been attempts to use refractory materials containing no alumina, e.g., MgO, SiC, zircon, zirconia and silica. However, such methods still suffer from the problem of incorporation of alumina impurities during the press-molding procedure due to the use of a binder containing alumina.

Korean Application Patent Publication No. 2005-4827 discloses a continuous-casting nozzle obtained by laminating a CaO-containing refractory sleeve on an alumina substrate. However, this method has proven to be not viable due to the fact that the CaO-containing refractory

- i -

sleeve tends to get detach from the nozzle substrate.

STTMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an improved method for preparing a refractory for making steel ingots, which is free from the above-mentioned problems.

In accordance with one aspect of the present invention, there is provided a method for preparing a refractory having a double-layer structure for making steel ingots comprising:

(a) preparing a first refractory composition and a second refractory composition separately, wherein the first refractory composition contains alumina in an amount of 5 % by weight or less based on the total weight of the first refractory composition; (b) introducing to a press mold, which comprises a cylindrical outer mold, a cylindrical inner mold disposed inside the outer mold and a cylindrical separating mold which is disposed between the outer mold and the inner mold, the first refractory composition and the second refractory composition obtained in step (a) such that the first refractory composition is introduced into the space between the inner mold and the separating mold and the second refractory composition is introduced into the space between the outer mold and the separating mold of the press mold, and pressing to obtain a press-molded refractory having a double-layer structure consisting of a first refractory layer and a second refractory layer surrounding the first refractory layer; and (c) drying and optionally sintering the press-molded refractory obtained in step (b).

In accordance with another aspect of the present invention, there is

provided a method for preparing a refractory having a double-layer structure for making steel ingots comprising:

(a) preparing a first refractory composition and a second refractory composition separately, wherein the first refractory composition contains alumina in an amount of 5 % by weight or less based on the total weight of the first refractory composition;

(b) molding the second refractory composition obtained in step (a) into a cylindrical form, followed by drying and sintering to form a cylindrical second refractory layer; and introducing the first refractory composition obtained in step (a) into the cylindrical second refractory layer by casting or spray coating to obtain a press- molded refractory having a double-layer structure consisting of the first refractory layer and the second refractory layer surrounding the first refractory layer; and (c) drying and optionally sintering the press-molded refractory obtained in step (b).

Brief Description of the Drawings

The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings:

Fig. 1 is a schematic view of an ingot making process; Fig. 2 is a schematic view showing a mold divided by inserting a separating mold, according to one embodiment of the present invention; (a) and (b) of Fig. 2 are a top view and a longitudinal cross-sectional view of the mold, respectively; and

Figs. 3 and 4 are schematic views of slurry casting and spray coating processes, respectively, according to one embodiment of the present invention.

<Brief description of the reference numerals in drawings>

1 : inflow tube 2: sleeve

3 : center brick 4: runner brick

5: outflow tube 6: ingot mold

10 : inner mold 20 : outer mold 30: separating mold 40: molten steel flow channel

100: first refractory composition 200: second refractory layer 300: slurry flow control cover

400: spray nozzle 500: mold

DETAILED DESCRIPTION OF THE INVENTION

The present invention is characterized that the refractory used for making steel ingots is fabricated in the form of a double-layer structure, wherein the inner first refractory layer which is to contact molten steel directly, is free of alumina or contains of alumina in an amount of less than 5 % by weight.

The method of preparing the refractory having the double-layer structure for making steel ingots according to the present invention is described in detail as follows:

<Step (a)>

A first refractory composition and a second refractory composition are prepared, separately.

i) First refractory composition

The first refractory composition comprises 75 to 95 % by weight of an alumina-free refractory matrix component based on the total weight of the first refractory composition.

The alumina-free refractory matrix component may be any conventional component used for preparing a refractory, and representative examples thereof include SiC, zirconia, magnesia(MgO), olivine, calcia(CaO), dolomite, silica and a mixture thereof.

The first refractory composition may further comprise an additional refractory matrix component such as chamotte, chromite, pyrophyllite and clay, the amount of which ranges from 0 to 20 % by weight based on the total weight of the first refractory composition.

In order to enhance the plasticity, the first refractory composition may still further comprise a binder. Representative examples of the binder used in the present invention include silica sol, lignin sulfonic acid, magnesium sulfate, magnesium chloride or molasses, which can be used in an amount ranging from 0.5 to 25 % by weight based on the total weight of the first refractory composition.

For the purpose of strengthening after drying, the first refractory composition may further comprise phosphoric acid, sodium phosphate or water glass, the amount of which ranges from 1 to 10 % by weight based on the total weight of the first refractory composition.

The total amount of alumina in the first refractory composition is

maintained in the range of 0 to 5 % by weight based on the total weight of the first refractory composition.

In order to match the sintering temperatures of the first refractory composition, the first refractory composition used in the present invention may further comprise an alkali component, e.g., Na ₂ O, K ₂ O and TiO ₂ as a sintering aid. In the present invention, the alkali component may be used in an amount of less than 1 % by weight based on the total weight of the first refractory composition.

ii) Second refractory composition

In the present invention, the second refractory composition may comprise any of the conventional matrix components used in the fabrication of refractory materials, preferably an alumina-silica-based material.

<Step (b)>

The first refractory composition and the second refractory composition obtained in step (a) are introduced into a press mold, followed by (i) press- molding and (ii) slurry casting or spray coating to obtain a refractory having a double-layer structure.

(i) Press-molding method

According to the present invention, various mold types for manufacturing a double-layered refractory can be used, and the mold used in the present invention preferably comprises a cylindrical outer mold, a cylindrical inner mold disposed inside the outer mold and a cylindrical separating mold which is disposed between the outer mold and the inner mold.

For example, when a sleeve having a separating mold which is

disposed between the outer mold and inner mold to create two separate space portions shown in Fig. 2 is used, the first refractory composition and the second refractory composition obtained in step (a) are introduced into the space between the inner mold 10 and the separating mold 30 and the space between the outer mold 20 and the separating mold 30 of the press mold, respectively. Then, the separating mold 30 is removed from the press mold, followed by pressing, to obtain a press-molded refractory having a double layer structure consisting of a first refractory layer and a second refractory layer surrounding the first refractory layer. Alternatively, for example, the first refractory composition is introduced into the space between the inner mold 10 and the separating mold 30 to press, and the separating mold 30 is removed from the mold to form a mold having a first refractory layer. Then, the second refractory composition is introduced into the space between the outer mold 20 and the first refractory layer containing the first refractory composition, followed by pressing to obtain a press-molded refractory having a double layer structure consisting of a first refractory layer and a second refractory layer surrounding the first refractory layer.

In case of using a press-molding method according to the present invention, the first or second refractory composition may be previously mixed with water in a mixer for the introduction to the press mold. At this time, water is preferably used in an amount ranging from 1 to 10 parts by weight, based on 100 parts by weight of each refractory composition.

In the present invention, the inner first refractory layer, which is to be brought directly into contact with molten steel, preferably has a thickness ranging from 3 to 25 mm. When the thickness of the first refractory layer is too thin, alumina from the alumina-containing outer refractory layer may be

incorporated into the molten steel due to the corrosion of the first refractory layer.

The second refractory layer may have a various thicknesses depending on the thickness of the first refractory layer, and the thickness of the second refractory layer preferably ranges from 10 to 40 mm.

(ii) Slurry casting or spray coating method

According to one embodiment of the present invention, the refractory having a double-layer structure consisting of a first refractory layer and a second refractory layer may be obtained by molding the second refractory composition obtained in step (a) into a cylindrical form, followed by drying and sintering to form a cylindrical second refractory layer; and introducing the first refractory composition obtained in step (a) into the cylindrical second refractory layer by casting or spray coating to form a first refractory layer coated on the surface of the cylindrical second refractory layer.

For example, in case of using the slurry casting method shown in Fig. 3, the outlet of the mold 500 having the second refractory layer 200 is equipped with the slurry flow control cover 300, and the first refractory layer is introduced into the inlet of the mold by casting to fill the mold and is kept for about 20 seconds to 3 minutes. The slurry flow control cover 300 is removed from the mold resulting in the drainage of the casting solution while the slurry coats the surface of the second refractory layer 200.

Alternatively, for example, when the refractory having a double-layer structure is manufactured using the spray coating method shown in Fig. 4, the first refractory composition 100 is spray-coated on the surface of the second refractory layer 200 using a spray nozzle 400 introduced into the mold 500 having the second refractory layer 200.

In case of using the casting or spray coating method in accordance with the present invention, the first refractory composition is preferably used in the form of slurry obtained by mixing with water, alcohol, cinna or a mixture thereof in a conventional mixer before its introduction to the mold. At this time, water, alcohol, cinna or a mixture thereof is preferably used in an amount ranging from 10 to 40 parts by weight, based on 100 parts by weight of the first refractory composition, and the slurry may comprise 10 to 70 % by weight, preferably 50 to 70 % by weight of the first refractory composition based on the total weight of the slurry. In the present invention, the first refractory layer preferably has a thickness ranging from 3 to 5 mm, and the second refractory layer preferably has a thickness ranging from 15 to 40 mm.

<Step (c)> Subsequently, the press-molded refractory with a double-layer structure obtained in step (b) may be dried and optionally sintered to obtain the inventive refractory having a double-layer structure.

The drying process may be carried out for 12 to 24 hours in air and for 8 to 24 hours at 150 to 250 ^° C. The sintering process is preferably carried out at 1,200 to 1,500 ⁰ C for 3 to 6 hours.

As described above, the refractory having a double-layer structure prepared in accordance with the present invention has a good corrosion resistance, and the incorporation of alumina impurity into the steel ingot product can be prevented or reduced. The following Examples and Comparative Examples are given for the purpose of illustration only and are not intended to limit the scope of the invention.

<Assay of refractory properties>

The physical properties of the refractory obtained in the following Example and Comparative Example was evaluated in accordance with the following methods.

(1) Cold crushing strength (kg/cm ² )

The cold crushing strength of a refractory specimen was measured with a crush tester according to a KSL 3115 method.

(2) Thermal shock resistance

A refractory specimen was kept at 1500 ^° C for 30 minutes and cooled in water for 10 minutes, followed by cooling in air for 20 minutes (1 cycle). The above procedure was repeated until the refractory specimen was detached and the number of cycle was counted.

(3) Corrosion resistance

A refractory specimen was fabricated in the form of a square column and heated at 1650 ⁰ C using a gas burner. A batch of slag and steel was introduced to the square column and replaced with a fresh batch, which was repeated 24 times every 30 minutes. The degree of the column damaged was expressed by corrosion index based on a standard specimen.

(4) Heat resistance

The inner portion of a press-molded double-layered refractory was heated at 1650 ^° C for 3 hours, and the thickness of cracks appeared on the outer potion was measured.

Example 1

The first refractory composition and the second refractory composition prepared using the components listed in Table 1 (the amounts of the components are on the basis of parts by weight) were respectively introduced into the space between the inner mold (10) and the separating mold

(30) and the space between the outer mold (20) and the separating mold (30) of the press mold, as shown in Fig. 2, followed by press-molding. The press- molded refractory with a double-layer structure consisting of a first refractory layer with thickness of about 15 mm and a second refractory layer with thickness of about 20 mm was separated from the mold. The press-molded refractory thus obtained was dried for 24 hours in air, and for 12 hours at

200 ^° C, followed by sintering at 1420 ^° C to obtain a refractory having a double-layer structure.

Comparative Example 1

Al ₂ θ ₃ /Siθ ₂ -containing mono-layer refractory (SK-38, Wonjin Worldwide Co., Ltd.) having the same size as that fabricated in Example was prepared according to a conventional method.

The physical properties of the refractories thus obtained in Example 1 and Comparative Example 1 were evaluated as described before. The results are shown in Table 1.

Table 1

Example 2

The first refractory composition and the second refractory composition prepared using the components listed in Table 2 (the amounts of the components are on the basis of parts by weight) were prepared, respectively. The second refractory composition was dried at 200 ^° C, followed by sintering at 1420 ^° C to obtain the cylindrical second refractory layer (200) with thickness of 30 mm. The outlet of the mold (500) having the cylindrical second refractory layer (200) was equipped with the slurry flow control cover (300), and the first refractory layer was introduced into the inlet of the mold by casting to fill the mold and was kept for about 20 to 30 seconds. Then, the slurry flow control cover (300) was removed from the mold resulting in the drainage of the casting solution, while the slurry containing the first refractory composition coated the surface of the second refractory layer (200) in the thickness of 3 to 5 mm. The press-molded refractory thus obtained was dried for 24 hours in air, and for 12 hours at 200 ^° C, followed by sintering at 1420 ^° C to obtain a refractory with a double-layer structure.

Example 3

The first refractory composition and the second refractory composition prepared using the components listed in Table 2 (the amounts of the components are on the basis of parts by weight) were prepared, respectively. The second refractory layer (200) with thickness of 30 mm inside the press mold (500) was formed according to the same method as in Example 2. Then, the first refractory composition was spray-coated on the surface of the second refractory layer (200) using a spray nozzle introduced

into the mold (500) having the second refractory layer (200) to form the first refractory layer with the thickness of 3 to 5 mm on the surface of the second refractory layer (200). The press-molded refractory thus obtained was dried for 24 hours in air, and for 12 hours at 200 ⁰ C, followed by sintering at 1420 °C to obtain a refractory with a double-layer structure.

The physical properties of the refractories thus obtained in Examples 2 and 3 were evaluated as described before. The results are shown in Table

2.

Table 2

As shown in Tables 1 and 2, the refractory having a double-layer

structure for making steel ingots prepared in accordance with the present invention exhibit excellent corrosion resistance and heat resistance.

While the invention has been described with respect to the above specific embodiments, it should be recognized that various modifications and changes may be made to the invention by those skilled in the art which also fall within the scope of the invention as defined by the appended claims.