WO1999047472A1

WO1999047472A1 - Low silica refractory

Info

Publication number: WO1999047472A1
Application number: PCT/US1999/005214
Authority: WO
Inventors: Theodore Daggett; Daniel Leech; Martin Morris; Gilbert C. Rancoule
Original assignee: Vesuvius Crucible Company
Priority date: 1998-03-18
Filing date: 1999-03-10
Publication date: 1999-09-23
Also published as: AU2902899A

Abstract

A refractory article such as a slide gate plate, nozzle, or similar refractory component for use in the continuous casting of steel is disclosed which comprises about 50-90 wt.% particulate alumina, about 5-28 wt.% alumina-zirconia fused grain, about 3-10 wt.% zirconia, and less than about 2.5 wt.% silica. Firing at temperatures in excess of 1370 °C produces an oxygen-bonded article with excellent resistance to thermal shock. The fired article also resists corrosion and erosion by molten steel, including high manganese and calcium grades of steel.

Description

LOW SILICA REFRACTORY

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to materials and articles for use in handling molten metals, and, more particularly, to refractory compositions and articles, which are resistant to the thermal shock and erosive effects of liquid steel.

Description of the Prior Art

In the continuous casting of molten metal, slide gate valves are commonly used to control flow from metallurgical vessels, such as ladles or tundishes. Slide gate valves employ refractory plates having apertures, the alignment of which may be varied so as to regulate the amount of molten metal flowing from the vessel. These refractory plates and associated nozzles are used in extremely hostile environments where resistance to thermal shock and chemically erosive attack from molten metal and slag are critical factors and correlate with refractory service life.

The steel industry has continued to introduce increasingly chemically aggressive grades of steel and the demands on steel refractories have increased commensurably. The primary factors affecting refractory life are thermal shock-resistance and chemical erosion resistance. Unfortunately, refractory compositions with greater thermal shock-resistance have poorer erosion resistance. Conversely, refractory compositions with greater erosion resistance have lower thermal shock. Carbon-bonded alumina-graphite refractory compositions have been used for slide gate plates. While exhibiting excellent thermal shock, the carbon portion of this material is subject to oxidation and subsequent erosion. These materials must also be fired in reducing atmospheres to prevent oxidation of the graphite component .

Alumina-mullite refractory compositions have also been used in slide gate plates and nozzles. Mullite is an alumina silicate having excellent thermal shock- resistance, which is caused, at least in part, by the silica. With conventional grades of steel, alumina- mullite refractories are sufficiently resistant to chemical erosion. These materials are also available at a reasonable cost.

U.S. Patent Nos . 5,055,433 and 5,214,010 disclose slide gate refractories comprising carbon-bonded alumina-zirconia compositions. These materials contain a substantial amount of carbon, which is known to improve thermal shock-resistance and decrease chemical erosion resistance. Conversely, zirconia has very good erosion resistance and poor thermal shock-resistance. Commonly, these refractories are fired above 1000°C in reducing atmospheres to prevent oxidation of the carbon. Alternatively, they may be thermally cured at lower temperatures, impregnated with tar, pitch or similar hydrocarbon, and subsequently baked. Besides alumina- zirconia, practitioners also impregnate other refractory compositions, such as magnesia and alumina. U.S. Pat. No. 5,403,794 teaches an improved alumina-zirconia composition for slide gate plates. This alumina-zirconia composition is oxide-bonded, not carbon-bonded, and consists essentially of alumina, zirconia and silica. It is believed that silica reacts with zirconia at intermediate firing temperatures to form zircon. At higher temperatures, the zircon dissociates and the freed silica reacts with alumina to form a fine crystalline, well-distributed mullite phase. Mullite provides excellent thermal shock resistance and compensates for the absence of carbon-bonding. The patent describes the silica/zircon/mullite transformation as essential to producing a finished article. The alumina-zirconia-silica composition is described as having good thermal shock-resistance combined with good erosion resistance. A further advantage to this composition is the absence of carbon, which lowers material costs and permits firing absent a reducing atmosphere.

Despite these advances in slide plate refractories, further improvements are needed as commercial grades of steel continue to be more chemically erosive. In particular, a need exists for greater thermal shock-resistance and, especially, improved chemical erosion resistance. SUMMARY OF THE INVENTION

The present invention relates to an improved alumina-zirconia refractory composition, which may be manufactured into various shapes, including slide gate plates, collector nozzles, and other shapes for use in the handling of molten metals. Slide gate plates made from the subject composition are described as exhibiting improved chemical erosion resistance and thermal shock- resistance, very good hot strength, and cost savings over carbon-bonded and zirconia refractory compositions. The composition of the present invention is considered particularly resistant to the erosive effects of calcium-silicon treated grades of steel.

In a broad aspect, the invention explains how reduced silica content and elevated firing temperature can combine to produce an alumina-zirconia refractory composition without the expected decreases in resistance to chemical erosion and thermal shock^'.

One aspect of the invention describes an improved refractory composition consisting essentially of, on a dry mix basis, 60-90 wt.% particulate alumina, about 5- 28 wt.% alumina-zirconia fused grain, about 3-10 wt.% monoclinic zirconia, and less than 2.5 wt.% microsilica. Such compositions may be fired at temperatures greater than about 1370°C without the need for a reducing atmosphere .

A present preferred embodiment is described as a mix comprising about 80 wt.% alumina, 13 wt.% fused alumina-zirconia, 5 wt.% monoclinic zirconia, and 2 wt.% microsilica. The alumina comprises 65 wt.% tabular alumina and 35 wt.% calcined alumina. The fused alumina-zirconia is about 75 wt.% alumina.

In another aspect of the invention, the refractory composition may be blended with binder, dispersant and water. The blended composition may be formed into a desired shape, such as a slide gate plate or nozzle, and fired at a temperature in excess of 1370°C and preferably above about 1500°C. The fired composition consists essentially of about 65-96 wt.% alumina, about 2-33 wt.% zirconia, and less than 2.5 wt.% silica. After firing, the preferred embodiment is described as consisting essentially of about 90 wt.% alumina, about 8 wt.% zirconia, and about 2 wt.% silica.

The fired composition exhibits a typical modulus of rupture of 3500-5000 psi at room temperature, an apparent specific gravity of about 3.5 to 4.0, an apparent porosity of 10.0%-15.0%, and a bulk density of about 3.20 to about 3.50 g/cm³. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side elevation view of a slide gate valve including refractory shapes according to the present invention particularly suitable for use in controlling molten metal flow from a steelmaking ladle in a continuous casting operation,-

FIG. 2 is a slightly enlarged, plain view of the bottom plate of the ladle slide gate valve of FIG. 1 ; and

FIG. 3 is a cross-sectional side elevation view of the bottom plate and integral collector nozzle taken along line III-III of Fig. 2. DESCRIPTION OF THE PREFERRED EMBODIMENT

The refractory composition of the present invention is particularly suitable for manufacturing the refractory components of a slide gate valve, including plates and nozzles, and may be used anywhere good erosion resistance and thermal shock-resistance are required.

Slide gate valves may contain either two or three refractory plates. The slide gate valve 2 shown in FIG. 1 is a two-plate type normally used between a ladle and a tundish. The valve 2 is mounted on the bottom of the ladle (not shown) to control the flow of molten metal from the ladle to the tundish. The slide gate valve 2 normally includes a rigid frame 4 which carries a stationary upper plate 6, which is connected to an inner nozzle or well block nozzle 8. The well block nozzle 8 and upper plate 6 have a bore 10, through which molten metal may flow. The well block nozzle 8 is adapted to fit into a well block orifice (not shown) at the bottom of the ladle in a manner well- known in the art. A slidable bottom plate 12 and collector nozzle 14 having a bore 16 are mounted in a moveable carrier 18. The movement of the moveable carrier 18 may be controlled by any conventional means, for example, by a hydraulic cylinder.

As seen in FIGS. 2 and 3, the collector nozzle 14 may be secured to the bottom plate 12 with refractory cement 13. The top and bottom plate assemblies may also be encased in a protective metal shell 15.

In the closed position depicted in FIG. 1, the bores 10 and 16 are offset from one another and the flow of molten metal is prevented. When the bottom plate 12 slides to the right, the bores 10 and 16 align to open the valve 2 and permit molten metal to flow. The top and bottom plates 6 and 12 and their respective nozzles 8 and 14 are thereby exposed to the erosive, corrosive and thermal shock effects of molten metal . The refractory composition of the present invention is particularly suited for the manufacture of slide gate valve plates and nozzles. The composition is able to tolerate the harsh operating conditions existing during pouring of a molten metal, particularly when the molten metal is an aggressive grade of steel, such as, for example, low carbon, high manganese or high calcium steels . Prior art alumina-zirconia compositions for slide gate refractories require greater than 3 wt.% silica and preferably about 5 wt.% silica. Silica is believed to impart thermal shock-resistance, and, in combination with alumina as mullite, is thought to improve hot strength. Failure to have at least the minimum amount of silica was considered deleterious to physical and chemical properties. In contrast, the present invention requires 2.5 wt.% silica or less. High levels of silica in prior art mixes limit the permissible range of firing temperatures. Under-firing impedes mullite formation and leads to decreased thermal shock-resistance and hot strength. Firing temperatures above 1500°C cause excessive sintering, which is known to cause larger pore sizes and disadvantageously affect physical properties, including erosion resistance, thermal shock-resistance and hot strength. The firing temperature of high silica, alumina-zirconia mixes should remain below about 1500°C. Prior art teaches, therefore, both a lower limit on the amount of silica required and an upper limit on the firing temperature. In the present invention, silica is necessarily kept below about 2.5 wt.% and firing is above about 1500°C. The low-silica, alumina-zirconia matrix has excellent corrosion and erosion resistance to molten steel. Suprisingly, the present invention also possesses good thermal shock-resistance. By comparison, when fired above about 1500°C, compositions with greater than about 2.5 wt.% silica yield a product with large pore sizes and correspondingly poor erosive qualities. A representative composition of the present invention consists essentially of 52 wt.% tabular alumina, 28 wt.% calcined alumina, 13 wt.% fused alumina-zirconia, 5 wt.% monoclinic zirconia, 1.5 wt.% microsilica, and 0.5 wt.% bentonite. Another 3-4 wt.% of binder and water, on a dry mix basis, are added to facilitate processing.

The tabular alumina constituent is a so-called "dead burned" alumina, which has been fired at a high temperature to produce a dense, low porosity grain. Tabular alumina provides maximum strength and good erosion resistance. The calcined alumina constituent is in a fine, particulate form and is made from an alumina material, which has been fired at a lower temperature than the tabular alumina. As a result, calcined alumina is not fully densified and, thus, possesses a higher surface area compared to that of tabular alumina. This higher surface area causes calcined alumina to be more reactive and function as a bonding phase during firing of the refractory. The present invention uses substantially higher levels of calcined alumina than prior art alumina-zirconia refractories. Preferably, tabular alumina is present in about a 2:1 weight ratio with calcined alumina, whereas prior art limited calcined alumina to about 20 wt.% of the total amount of alumina. A size-graded mixture of alumina is employed to maximize packing density and may include a fused alumina grain, wherein at least about 15 wt.% of the mixture is calcined alumina of a fine -325 mesh size.

The alumina-zirconia grain mix constituent is made from a pre- fusion (melt) of alumina and zirconia materials. A presently preferred alumina-zirconia composition consists of a fusion product comprising about 75 wt.% alumina and about 25 wt.% zirconia. The fused alumina-zirconia material is believed to be beneficial m providing increased erosion resistance compared to fused zirconia-mullite grains, and is more economical than zirconia grain. The zirconia constituent m the above-mentioned composition is a monoclinic variety; although, it is anticipated that stabilized forms of zirconia may also be used. The silica constituent is preferably microsilica, also known in the art as fumed silica or volatized silica. Microsilica is extremely finely divided powder having a typical surface area of 10-200 mVgram. Silica may also be introduced as a constituent in a chemical compound including, for example, silicates, mullite, zircon, clay, and mineral silicates. The bentonite constituent contains kaolin as its manor constituent. The binder system is conventional and is employed to provide the necessary green strength to the composition after forming. The composition may be formed by pressing, extrusion or injection molding. Typical, well-known binder systems include starch, resin or ligno-sulfmate .

The raw materials of the representative composition may be mixed and subsequently pressed into the form of a slide gate plate. The reduction of silica in an alumina-zirconia composition may improve pressability. Improved pressability implies increased grain density and decreased likelihood of pressure- related rejects caused by, for example, delammation. The composition is particularly suitable for use as a plate in a ladle slide gate valve where higher resistance to aggressive grades of steel, such as calcium-silicon treated grades is needed. The composition is also useful for making plates for tundish gate valves, furnace valves, and inserts and nozzles for these various valve components .

The pressed shapes may be fired at a temperature of about 1565°C. The preferred firing temperature is between about 1500°C and about 1650°C. Unlike carbon- bonded refractory materials, firing may take place without the need for a special or reducing atmosphere. After firing a representative composition may consist essentially of about 90 wt.% alumina, about 8 wt.% zirconia, and about 2 wt.% silica. Typical physical properties may be:

Modulus of Rupture (MOR) 4900 psi @ room temperature Apparent porosity 11.5%

Bulk density 3.40 g/cm³

Specific gravity 3.86

The fired plates may be impregnated with a carbonaceous material such as tar or resin in a conventional manner prior to service. EXAMPLE I

A number of slide gate plates were made from a composition according to the present invention. The plates were fired at 1565°C (2850°F) . The fired composition consisted essentially of about 90 wt.% alumina, about 8 wt . % zirconia, and about 2 wt.% microsilica. Sets of these slide gate plates were installed in a commercial steel mill slide gate valve. A trial was conducted using relatively low corrosive steel having less than 0.9 wt.% manganese and less than 20 ppm calcium. Plates made from the composition of the present invention were considered serviceable for an

10 average of 4.4 heats compared to only 4.0 heats for prior art alumina-zirconia plates containing greater than 3 wt.% silica. Evaluation of the plates after removal from the slide gate valve revealed that bore erosion in the low silica plates of the present invention was only 2.52 mm/hr compared to 3.28 mm/hr for the prior art high silica plates. EXAMPLE II

Slide plates made from a composition of the present invention were trialed using steel with less than 0.02 wt.% carbon and with 0.14 wt.% to 0.72 wt.% manganese. The plates had a fired composition consisting essentially of about 90 wt.% alumina, about 8 wt.% zirconia, and about 2 wt.% microsilica. Sets of these slide gate plates were installed in a large bore, slide gate valve at a commercial steel mill. Commercial plates typically survive three heats under these conditions. Test plates made from the composition of the present invention were run for three heats and compared to commercial, alumina-mullite and high silica alumina-zirconia plates that had been subjected to the same conditions. The test plates were judged as serviceable as the commercial plates, and showed minimal cracking caused by thermal shock. It was believed that the test plates would last, on average, 0.5 heat longer than the commercial plates.

While we have shown and described a present preferred embodiment of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

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Claims

We claim :

1. A refractory article resistant to thermal shock and erosion by molten metals, and exhibiting excellent hot strength, the article made from an unfired composition consisting essentially of, on a dry mix basis, about 60-90 wt.% particulate alumina, about 5-28 wt.% alumina-zirconia fused grain, about 3-10 wt.% zirconia and an amount of silica characterized in that the amount of silica is less than 2.5 wt.%.

2. The refractory article of claim 1 wherein the particulate alumina comprises a mixture of calcined alumina and at least one member selected from the group consisting of tabular alumina and fused alumina.

3. The refractory article of claim 2 wherein the particulate alumina comprises about 50 wt.% tabular alumina and about 28 wt.% calcined alumina based on a total dry mix weight.

4. The refractory article of claim 1 wherein the alumina-zirconia fused grain comprises about 25 wt.% zirconia.

5. The refractory article of any one of the previous claims having a fired composition consisting essentially of about 65-96 wt.% alumina, about 2-33 wt.% zirconia, and less than 2.5 wt.% silica.

6. The refractory article of any one of the previous claims having a fired composition consisting essentially of about 90 wt.% alumina, about 8 wt.% zirconia, and about 2 wt.% microsilica.

7. The refractory article of any one of the previous claims in the form of a slide gate plate.

8. The refractory article of claim 7 wherein the article is impregnated with a carbonaceous material.

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9. The refractory article of claim 8 wherein the carbonaceous material is tar or resin.

10. The refractory article of any one of claims 1- 6 in the form of a collector nozzle.

11. The refractory article of any one of claims 1- 6 in the form of a well block nozzle.

12. A method of making a refractory article comprised of:

(a) mixing a composition consisting essentially of 60-90 wt.% particulate alumina, about 5-28 wt.% alumina- zirconia fused grain, about 3-10 wt.% zirconia, and less than 2.5 wt.% silica;

(b) forming the composition into a shape; and

(b) firing the shape at a temperature greater than about 1370┬░C.

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