GB2131791A

GB2131791A - Carbon-containing refractory

Info

Publication number: GB2131791A
Application number: GB08333238A
Authority: GB
Inventors: Hiroshi Kyoden; Hideaki Nishio; Shohei Hara; Yoichiro Kawabe; Toshihiro Matsumoto
Original assignee: Shinagawa Refractories Co Ltd
Current assignee: Shinagawa Refractories Co Ltd
Priority date: 1982-12-13
Filing date: 1983-12-13
Publication date: 1984-06-27
Also published as: GB2131791B; FR2537566B1; DE3344852A1; FR2537566A1; DE3344852C2; GB8333238D0

Abstract

A carbon-containing refractory comprises approximately 1 to 10 parts by weight of at least one metal powder selected from Al-Mg alloy powder, Al-Mg-Si alloy powder, and Al-Mg-Cr alloy powder per 100 parts by weight of graphite (3-50 pts. by wt.) and refractory aggregate (50-97 pts. by wt.). However, because the melting points of the metal alloy powders are up to 250 DEG C lower than the melting points of the conventionally used unalloyed metal powders, the oxidation-preventing effects of the metal alloy powders are greater in the low temperature range (from about 400 DEG C) than the effects of unalloyed metal powders. The resulting refractory has increased hot strength and decreased weight loss after oxidizing burning. The resistance to corrosion of the present refractory may be further increased by the admixture of approximately 0.3 to approximately 5 parts by weight of boron carbide per 100 parts by weight of graphite and refractory aggregate.

Description

SPECIFICATION A carbon-containing refractory The present invention relates to carbon-containing refractories and more specifically to burned and unburned Al203-C, MgO-C, and MgO-Al203-C refractories having improved resistance to oxidation, spalling, and corrosion, in addition to improved hot strength.

Refractories containing carbon in the form of graphite are widely used in metallurgy. When in contact with molten steel, or slag, these refractories exhibit excellent resistance to chemical corrosion.

Since graphite itself is resistant to wetting by molten iron or slag, its presence in refractories prevents the penetration of slag into the refractories. Further, because of graphite, the refractories can not be over-sintered by high temperatures during burning or actual use, and therefore thermal spalling does not readily occur. This, too, contributes to the high durability of graphite-containing refractories.

However, graphite is very easily oxidized by oxygen in the surroundings, and oxidation causes a graphite-containing refractory to lose its excellent durability. In order to obtain a graphite-containing refractory with good durability, it is extremely important to suppress the oxidation of graphite as much as possible. Various methods have been proposed for increasing the resistance to oxidation of this type of refractory, but at present no satisfactory method has been found.

Japanese Patent Laid Open No. 50-69106 discloses covering the surface of a carbon-containing moulded refractory material with a nitride or carbide of silicon and further coating it with a borosilicate glass comprising boron carbide and silicon dioxide in order to prevent oxidation. However, these covering layers are not resistant to attack by molten iron, molten steel, or slag, and if worn through by chemical attack will lose their anti-oxidizing effect. Accordingly, this method is not desirable.

Another method of preventing oxidation in carbon-containing refractories is to uniformly disperse metal powder in the refractory raw materials. Japanese Patent Laid Open No. 55-107749 discloses adding magnesium, aluminium, and silicon powder to carbon-containing refractory bricks, and Japanese Patent Laid Open No. 54-39422 discloses adding a metal powder having a greater affinity for oxygen than does carbon. In the latter case, at least one type of metal powder selected from the group consisting of Al, Si, Cr, Ti, and Mg is added. However, the resistance to oxidation and the hot strength of the resulting carbon-containing refractory, while improved, are not fully satisfactory.

The addition of metal powders to carbon-containing refractories has a number of beneficial effects. (1) From the temperature range of 200-3000C in which oxidation of the metal powders begins, carbon is protected from oxidation by the preferential oxidation of the metal powders. (2) When the metal powders oxidize, they expand in volume.As a result of this volume expansion, the refractory becomes more compact, and penetration of oxidizing materials into the refractory is decreased, with the resulting decrease in oxidation of the graphite. (3) When the metal powders oxidize, they form bonds with the refractory raw materials which increase the heat strength of the refractory. (4) From about 1000C, the volatile portions of the refractory binder such as water, tar, pitch, phenolic resins, and the like employed in moulding refractories begin to volatilize, leaving pores and passageways in the refractory into which oxygen can penetrate. Once the refactory reaches a sufficient temperature and the metal powders melt, the liquid metal expands in volume and flows into and fills the pores and passageways, preventing the penetration of oxygen.

However, the melting point of the metals conventionally admixed in carbon-containing refractories (e.g. 6600 C for aluminium and 649 OC for magnesium) are considerably higher than the temperature (around 4000C) at which oxidation of carbon begins. Accordingly, there is a temperature gap of approximately 2500C in which the ability of conventionally-used metal powders to suppress oxidation by melting and filling in pores is extremely low.

The present invention provides carbon-containing refractory comprising approximately 3 to approximately 50 parts by weight of graphite and approximately 50 to approximately 97 parts by weight of refractory aggregate. The refractory further comprises approximately 1 to approximately 10 parts by weight of at least one metal alloy powder selected from the group consisting of Al-Mg alloy powder, Al-Mg-Si alloy powder, and Al-Mg-Cr alloy powder per 100 parts by weight of graphite and refractory aggregate. This carbon-containing refractory may further comprise approximately 0.3 to approximately 5 parts by weight of boron carbide per 100 parts by weight of graphite and refractory aggregate.

As is well known, the melting point of a metal alloy is lower than the melting points of the metals which consitute the alloy. For example, Al-Mg alloys has a eutectic point of 451 OC, while unalloyed aluminium and magnesium having melting points of 6600C and 6490C, respectively, approximately 2000C higher than the eutectic point of the alloy.

In the present invention, metal alloy powders having a greater affinity for oxygen than does carbon are admixed instead of the unalloyed metal powders used in conventional carbon-containing refractories. Because of their low melting points, these alloy powders greatly increase the resistance to oxidation and the hot strength of the resulting refractory in the low temperature range (from about 400 C) in which oxidation of carbon begins. Because of this increased resistance to oxidation, the resistance to corrosion and hot strength of the refractory are increased. The resistance to corrosion may be further increased by admixture of boron carbide, as will be described below.

A carbon-containing refractory according to the present invention will be described below by way of example. It significantly differs from conventional carbon-containing refractories in that it contains one or more metal alloy powders selected from Al-Mg alloy powder, Al-Mg-Si alloy powder, and Al-Mg-Cr alloy powder.

The mechanism whereby metal alloy powders increase the resistance to oxidation of a carbon containing refractory in which they are incorporated is basically the same as the mechanism whereby conventionally-used non-alloyed metal powders do so. Namely, (1) the alloy powders have a greater affinity for oxygen than carbon and are preferentially oxidized; (2) when oxidized, the metal alloy powders undergo volume expansion which increases the compactness of the refractory; (3) the oxidized metal alloy powders form new bonds with the refractory aggregate, increasing the hot strength of the refractory; and (4) upon melting, the non-oxidized portions of the metal alloy powders flow into and fill pores left by volatilization of the binder used in moulding.

The big difference between the use of the metal alloy powder and unalloyed metal powder is the much lower melting point of metal alloy powders. Accordingly, the range in which metal alloy powders can suppress oxidation is greater than for unalloyed metal powders.

When the metal alloy powders used in the present invention are oxidized, the resulting Awl 203 and MgO coexist in an extremely active state and very readily form spinel (MgO awl203) when they reach 1 0000 C. This results in a greater expansion in volume of the refractory.

This expansion is particularly significant; the formation of spinel prevents the carbon-containing brick from falling out a wall formed of such bricks. Due to their smooth surfaces, carbon-containing bricks have a tendency to fall out of walls during use, but the volume expansion produced by the formation of spinel causes the tightening of the bricks in a wall, preventing them from falling out.

The metal alloy powder used in the present invention comprises one or more powders suitably selected from Al-Mg alloy powder, Al-Mg-Cr alloy powder, and Al-Mg-Si powder. Each of the powders used should preferably comprise from approximately 30% by weight to approximately 70% by weight of Al, with the weight ratio of Al to Mg preferably ranged from approximately 0.5 to approximately 1.5. If the weight ratio of Al to Mg falls outside this range, the previously described benefits of the metal alloy powder may not be fully exhibited.

From the standpoint of reactivity and uniform dispersibility it is desirable that the grain size of the metal alloy powder be not greater than approximately 0.125 mm. The amount of metal alloy powder used per 100 parts by weight of graphite and refractory aggregate should be approximately 1 to approximately 10 parts by weight. If less than approximately 1 part by weight is used, the effectiveness of the metal alloy powder is small, and if more than approximately 1 0 parts by weight are used, a moulded body having a compact texture can not be obtained and expansion upon heating it too large.

The refractory aggregate employed in the present invention may comprise oxides such as magnesia, spinel, alumina, silica, zircon, and zirconia, and non-oxides such as silicon carbide, silicon nitride, and boron nitride. There are no particular limits on the components, but it is desirable that the main components be magnesia, spinel, and alumina.

The graphite portion of the refractory aggregate may be natural graphite such as amorphous graphite or crystalline graphite, or it may be an artificial graphite such as that derived from electrode scraps, petroleum coke, or carbon black. However, it is preferable to use crystalline graphite with few impurities. The relative proportion of graphite used depends upon the type of refractory aggregate used and the intended use for the refractory. However, it is generally preferably to employ 3 to 50 parts by weight of graphite in 100 parts by weight of graphite and refractory aggregate. If the amount of graphite is less than 3 parts by weight, the graphite will not exhibit good resistance to wetting by molten iron or slag, in which case the entire refractory will have poor resistance to molten iron or slag.

Furthermore, if the graphite exceeds 50 parts by weight, the desired strength can not be obtained.

The resistance to corrosion or a carbon-containing refractory according to the present invention may be further increased by the admixture of boron carbide to the graphite and refractory aggregate.

When the surface of a carbon-containing refractory containing boron carbide is exposed to molten metal, boron carbide is oxidized and forms boron oxide. Boron oxide together with the refractory aggregate and the oxides of the metal alloy powder form a melt of high viscosity which covers the surface of the refractory and prevents oxidation of the graphite in the refractory.

However, in the present invention, if boron carbide is used, it is mandatory that it be admixed not alone but in combination with a metal alloy powder. When boron carbide is admixed to the graphite and refractory aggregate either by itself or with unalloyed metal powder, the hot strength and the strength after heating are low, and thus the beneficial effects are produced by the present invention can not be achieved.

Commercial boron carbide abrasive material is satisfactory for use as the boron carbide in the preferred embodiment of a carbon-containing refractory according to the present invention. In order to achieve good reactivity and uniform dispersion of the boron carbide, it is desirable that the grain size be at most 0.125 mm. Per 100 parts by weight of graphite and refractory aggregate, approximately 0.3 to approximately 5 parts by weight of boron carbide should be used. If less than approximately 0.3 parts by weight of boron carbide are used, its addition has no effect. If it exceeds approximately 5 parts by weight, the refractory exhibits excellent resistance to oxidation, but its not strength and durability decrease.

An unburned carbon-containing refractory according to the present invention may be produced by first blending the graphite, the refractory aggregate, and the grain-size-regulated metal alloy powder in the ratios mentioned above. At this time, boron carbide may also be admixed. A binder such as tar, pitch, phenolic resin, or furan resin is added. Using conventional methods, this mixture is moulded. After being dried at around 2000C, an unburned refractory is obtained. If it is burned at 900-1 5000 C, a burned refractory is obtained.

The following examples of a refractory according to the present invention illustrate the effects produced by various combinations of the components.

EXAMPLE 1 80 parts by weight of magnesia, 20 parts by weight of graphite, 3 parts by weight of aluminium magnesium alloy powder, and 5 parts by weight of resol-type phenolic resin as a binder were blended together and then moulded under a pressure of 1 500 kg/cm2 into standard bricks (230 x 114 x 65 mm) which were then dried at 2000C for 5 hours. At 1 4000C, the completed unburned bricks had a high hot modulus of rupture of 210 kg/cm2. After oxidizing burning at 10000C for 3 hours, the bricks had a decrease in weight of only 3.5%.

EXAMPLE 2-5 Using the same method as was used for Example 1, three additional examples of a carboncontaining refractory having various compositions were mixed and formed into unburned standard bricks. The components and physical properties of these refractories are shown in Table 1.

COMPARATIVE EXAMPLES 1'-3' For the purpose of comparison, three refractories having the compositions shown on the right side of Table 1 were blended and moulded into standard bricks using the same method as was used in preparing Example 1. Comparative Example 1' was identical to Example 1 except that the weight ratio of Al to Mg was 4, outside the preferred limits of approximately 0.5 to approximately 1.5. This refractory accordingly had a low hot strength and high weight loss after oxidizing burning.

Comparative Examples 2' and 3' contained metal powders in unalloyed form. These refractories also had much lower hot stength and greater weight loss after oxidizing burning than did Examples 1-4 containing metal alloy powders.

EXAMPLES 5-9 Using the same method as was used for Example 1 except for the addition of boron carbide during the blending stage, the five examples of carbon-containing refractories shown in the left half of Table 2 were prepared.

COMPARATIVE EXAMPLES 4'-6' Using the same method as was used for Examples 5-9, the refractories shown in the right half of Table 2 were prepared for the purpose of comparison. These refractories contained boron carbide, but did not contain any of the metal alloy powders used in the present invention. The hot strength of these refractories was much lower and the weight loss after oxidizing burning was much greater than for Examples 5-9, which contain both boron carbide and metal alloy powders.

TABLE 1

Example Number The present invention Comparative Examples 1 2 3 4 1' 2' 3' magnesia 80 60 80 80 80 a s spinel 40 95 0 < F alumina 35 LLO w rr silicon carbide 15 LLB z m crystalline graphite 20 40 10 5 20 20 20 gv, Al-Mg alloy powder* 3 2 1 Al-Mg alloy powder*2 3 O Al-Mg-Si alloy powder*3 4 Al-Mg-Cr alloy powder*3 5 2 Al 3 2 Mg 1 % weight loss after*4 v, oxidation burning at 3.5 4.5 2.8 2.0 5.0 6.0 5.5 s 1 0000C for 3 hours .. . D w hot modulus of hot modulus rupture w hot (kg/cm2) at 1400 C *1 Al/Mg = 1 *2 Al/Mg = 4 *3 Al = 45%, Mg = 45% *4 Total weight loss after 3 hours of burning minus weight loss corresponding to volatilization of binder.

TABLE 2

Example Number The present invention Comparative Examples 5 6 7 8 9 4' 5' 6' magnesia 80 60 80 80 80 80 spinel 30 20 B5 Q alumina 60 85 G isu W silicon carbide 10 ZF F crystalline graphite 20 10 20 5 20 20 20 20 ~ < Al-Mg alloy powder* 2 2 2 Q Al-Mg-Si alloy powder*2 4 0 Al-Mg-Cr alloy powder 5 Boron carbide 1 2 5 3 0. 1 0.5 0.5 Al 2 2 Mg 1 1 % weight loss after*3 Co oxidizing burning at 2.8 1.5 1.3 1.9 3.1 3.2 7.1 4.8 F D 1OOO0Cfor3 hours Fen hot modulus of rupture (kg/cm2)at14000C 210 168 170 175 230 175 146 185 *1 Al/Mg = 1 *2 Al = 45%, Mg = 45% *3 Total weight loss after 3 hours of burning minus weight loss corresponding to volatilization of binder.

Claims

1. A carbon-containing refractory comprising: approximately 3 to approximately 50 parts by weight of graphite; approximately 50 to approximately 97 parts by weight of refractory aggregate; and approximately 1 to approximately 10 parts by weight of at least one metal alloy powder selected from Al-Mg alloy powder, Al-Mg-Si alloy powder, and Al-Mg-Cr alloy powder per 100 parts by weight of the graphite and refractory aggregate.

2. A carbon-containing refractory as claimed in claim 1 , further comprising approximately 0.3 to approximately 5 parts by weight of boron carbide per 100 parts by weight of the graphite and refractory aggregate.

3. A carbon-containing refractory substantially as described in any of Examples 1 to 9.