Application of magnesia carbon bricks in the steel and metallurgical industry

Magnesia carbon brick is a new type of refractory material that emerged in the 1970s. It is an unburned refractory material made of high-temperature sintered magnesia or fused magnesia and carbon materials and various carbonaceous binders. Magnesia carbon bricks not only maintain the advantages of carbon refractory materials, but also completely change the inherent disadvantages of poor spalling resistance and easy absorption of slag in the past alkaline refractory materials.
So far, magnesia carbon bricks are still widely used in the steel and metallurgical industry. The application of magnesia carbon bricks has improved various technical and economic indicators of converters and reduced the consumption of refractory materials. In addition, as an unburned product, compared with traditional fired magnesia dolomite bricks, the fuel consumption of magnesia carbon brick production is saved by at least 80%.

1. Properties of magnesia carbon bricks

Magnesium carbon bricks have high melting resistance because their composition materials are magnesium oxide and carbon, both of which have high melting points, and the two components do not melt into each other.

Magnesium carbon bricks are a composite structure, the main part of which is magnesium oxide clinker with strong slag resistance to alkaline slag and carbon with poor wettability with slag, so they have excellent slag resistance. In particular, they have strong resistance to slag penetration. Compared with the old fired alkaline bricks, the penetration layer of magnesium carbon bricks is much shallower.

Since dry graphite has excellent heat shock resistance, magnesium carbon bricks that inherit the excellent characteristics of graphite have high thermal conductivity, relatively small linear expansion coefficient and elastic modulus, and relatively large high temperature strength, which basically avoids the organizational damage and peeling caused by cracking during use.

In addition to the above-mentioned excellent characteristics, magnesium carbon bricks also have good thermal creep resistance. Compared with other ceramic bonded bricks, magnesium carbon bricks show particularly good creep resistance.

2. Application of magnesia carbon bricks in converter steelmaking

Since the advent of oxygen converter, the lining materials have undergone three stages of evolution: tar dolomite bricks-burned alkaline oil-soaked bricks-magnesia carbon bricks. Since the 1980s, steelmaking converters and their lining refractory materials have made great progress. As for steelmaking converters, they have completed the process of development towards large-scale and automation. Currently, they are developing towards re-blowing and high temperature.

During the smelting process, the use conditions and damage conditions of various parts of the converter are different. For various parts of the converter under different use conditions, the refractory materials used are also different.

(1) Furnace mouth: Due to the drastic temperature change at the furnace mouth, the scouring of slag and high-temperature exhaust gas is more harmful, and the furnace door is hit when removing scrap steel and adding materials. Therefore, the refractory materials used for the furnace mouth must have high thermal shock resistance and slag resistance, be resistant to the scouring of slag and high-temperature exhaust gas, not easy to hang steel and easy to clean.

(2) Furnace cap: The furnace cap is the part that is most seriously affected by slag erosion. It is also affected by temperature changes, carbon oxidation and scouring of dusty exhaust gas. Therefore, magnesia carbon bricks with strong slag resistance and thermal shock resistance are required.

(3) Charging side: The splashing of slag and molten steel during blowing can easily cause chemical erosion, wear and scouring on the charging side.

The charging side is also directly impacted and eroded by the scrap steel and molten iron, causing serious mechanical damage. Therefore, it is required that

magnesia carbon bricks must have good thermal shock resistance in addition to high slag resistance and high high temperature strength. High-strength magnesium carbon bricks with antioxidants are usually used.

(4) Steel tapping side: The steel tapping side is basically not subject to mechanical damage during charging, and the thermal shock effect is also small, but it is subject to the thermal shock and erosion of molten steel during steel tapping, and the damage rate is much lower than that of the charging side. When the same material as the charging side is used, in order to maintain the balanced life of the converter lining, a thinner structure than the charging side is used for masonry.

(5) Slag line: The slag line is the part of the furnace lining that is severely corroded by slag due to long-term contact with the slag. On the steel tapping side, the position of the slag changes with the steel tapping time and is not obvious. On the slag discharge side, due to the strong slag corrosion and the combined influence of other effects on the belly of the furnace during the blowing process, it is severely damaged. Therefore, it is necessary to build magnesia carbon bricks with excellent slag resistance.

(6) Both sides of the ear shaft: In addition to the damage caused by blowing, the two sides of the ear shaft are not covered with a protective layer and are not easy to repair. Therefore, the carbon in the furnace lining material is easily oxidized, resulting in serious damage. High-grade magnesia carbon bricks with excellent slag resistance and strong oxidation resistance should be built.

(7) Furnace hearth and furnace bottom: These parts are violently eroded by molten steel during blowing, but compared with other parts, the damage is generally lighter. Magnesia carbon bricks with low carbon content can be selected, or tar dolomite bricks can be used. When high-speed blowing is used and the molten pool is shallow, the central part of the furnace bottom may be severely damaged. In addition, when bottom blowing is used, the damage to these parts may be aggravated, and the same material as the charging side of the furnace belly should be used.

At present, in order to improve the technical and economic indicators of the converter, comprehensive masonry is generally used.

3. Use of magnesia carbon bricks in EAF

At present, almost all the walls of electric furnaces are built with magnesia carbon bricks. Therefore, the life of magnesia carbon bricks determines the service life of electric furnaces. The main factors that determine the quality of magnesia carbon bricks for electric furnaces include the purity of magnesia sand, impurity types, bonding state of periclase grains and grain size of MgO source; the purity, crystallization degree and flake size of flake graphite as the source of carbon introduction; thermosetting phenolic resin is usually used as a binder, and the main influencing factors are the addition amount and residual carbon content. It has been proven that adding antioxidants to magnesia carbon bricks can change and improve their matrix structure, but when used under normal operating conditions of electric furnaces, antioxidants are not necessary components of magnesia carbon bricks. Only when used in electric arc furnaces with high FeOn slag, such as using direct reduced iron or irregularly oxidized parts and hot spots of electric furnaces, can various metal antioxidants be added to become an important part of magnesia carbon bricks.

The corrosion behavior of magnesia carbon bricks used in slag lines is manifested as the formation of obvious reaction dense layers and decarburized loose layers. The reaction dense zone is also called the slag invasion zone, which is the erosion area where the high-temperature liquid slag penetrates into the brick body after the magnesia carbon brick is decarburized to form a large number of pores. In this area, FeOn in the slag is reduced to metallic iron, and even the desolvated phase and intergranular Fe2O3 dissolved in MgO are also reduced to metallic iron. The depth of slag penetration into the brick is mainly determined by the thickness of the decarburized loose layer, and usually ends at the place where the residual graphite remains. Under normal circumstances, the decarburization layer in the magnesia carbon brick is relatively thin due to the presence of graphite.

There are two ways to tap the steel at the tapping port of the electric furnace: tapping with the tapping trough tilting and tapping from the furnace bottom. When the tapping trough is tilted to tap the steel, magnesia carbon bricks are basically not used, but Al2O3 or ZrO2 are selected, and non-oxygen substances such as C, SiC and Si3N4 are added. When tapping from the furnace bottom is used, the tapping port is composed of an outer sleeve brick and an inner tube brick. The furnace bottom tapping port uses magnesia carbon brick tube bricks. The hole size of the tube brick is determined by factors such as furnace capacity and tapping time. The general inner diameter is 140~260mm.

A steel plant electric furnace uses medium-range and low-range magnesia carbon bricks at the tapping port, and the two sides of the copper tapping port replace the sintered magnesia bricks used originally. Initially, good results have been achieved, and the furnace life has increased from about 60 furnaces to more than 1 times. After use, the magnesia carbon bricks at the slag line remain relatively intact and do not stick to slag. The slag line does not need to be supplemented, which reduces labor intensity and improves the purity and productivity of molten steel.

4. Use of Aluminum Magnesium Carbon Bricks in Ladle

When MgO-C bricks are used in refining ladle furnaces and ladles, they are mainly used in the clearance and slag line. According to the operating conditions, the refractory materials used in these parts must be resistant to high temperature, thermal shock, and mechanical corrosion caused by slag erosion. Therefore, in the past, magnesia-chromium refractory materials were used in these parts, but considering the pollution of chromium to the environment, its use has been reduced, and now magnesia-carbon bricks are used.

Due to the serious damage of magnesia-carbon bricks in the new ladle during the preheating process, the loose decarburization layer can reach 30~60mm thick. This layer is washed away during the injection of molten steel and the magnesia sand particles are brought into the slag. Obviously, preventing the carbon in the magnesia-carbon bricks from being burned during preheating has become one of the important steps to improve the service life of magnesia-carbon bricks in the clearance and slag line of the ladle. Its technical measures, in addition to adding composite antioxidants to the magnesia-carbon bricks, the key is to cover the surface of the magnesia-carbon bricks with alkali-containing low-melting glass phase liquid after lining to protect the carbon in the magnesia-carbon bricks from being burned during the preheating process of the ladle.

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