Application of magnesia-carbon bricks in iron and steel metallurgy

Magnesia-carbon bricks are a new type of refractory material that emerged in the 1970s. They are unburned refractories made from high-temperature sintered magnesia or fused magnesia and carbonaceous materials, using various carbonaceous binders. Magnesia-carbon bricks retain the advantages of carbonaceous refractories while completely overcoming the inherent disadvantages of traditional alkaline refractories, such as poor spalling resistance and easy slag absorption.

Currently, magnesia-carbon bricks are still widely used in the iron and steel metallurgical industry. Their application has improved various technical and economic indicators of converters and reduced refractory material consumption. Furthermore, as an unburned product, compared to traditional fired magnesia-dolomite bricks, magnesia-carbon brick production saves at least 80% in fuel consumption.

Because magnesia-carbon bricks are composed of magnesia oxide and carbon, both of which have high melting points and are immiscible, they possess high resistance to melting.

Magnesia-carbon bricks are a composite structure, primarily composed of magnesium oxide clinker, which exhibits strong resistance to slag erosion by alkaline slags, and carbon, which has poor wettability with molten slag. Therefore, they possess excellent slag erosion resistance. In particular, they have a strong resistance to molten slag penetration; compared to older fired alkaline bricks, the penetration layer of magnesia-carbon bricks is much shallower.

Because dry graphite possesses excellent thermal shock resistance, magnesia-carbon bricks, inheriting these excellent properties, exhibit high thermal conductivity, a low coefficient of linear expansion and elastic modulus, and high high-temperature strength, effectively preventing structural damage and spalling caused by cracking during use.

In addition to the aforementioned excellent properties, magnesia-carbon bricks also possess good hot creep resistance, showing particularly good resistance to deformation compared to other ceramic-bonded bricks.

Since the advent of oxygen converters, furnace lining materials have undergone three stages of evolution: tar-dolomite bricks – fired alkaline oil-impregnated bricks – magnesia carbon bricks. Since the 1980s, steelmaking converters and their refractory linings have seen significant development. Steelmaking converters have completed the process of development towards large-scale and automated production. Currently, they are developing towards combined blowing and high-temperature production.

During the smelting process, the operating conditions and damage conditions of different parts of the converter vary, and the refractory materials used for lining different parts of the converter under different operating conditions also differ.

(1) Furnace Mouth: Due to drastic temperature changes at the furnace mouth, the erosion caused by molten slag and high-temperature exhaust gases is quite severe. Furthermore, the furnace door is impacted during scrap removal and charging. Therefore, the refractory material used at the furnace mouth must possess high thermal shock resistance and slag resistance, be resistant to slag and high-temperature exhaust gas erosion, and be easy to clean and prevent steel from being easily attached.

(2) Furnace Cap: The furnace cap is the part most severely affected by slag corrosion, as well as temperature changes, carbon oxidation, and the erosion caused by dust-laden exhaust gases. Therefore, magnesia-carbon bricks with strong slag resistance and thermal shock resistance are required.

(3) Charging Side: During blowing, the splashing action of slag and molten steel easily causes chemical erosion, wear, and scouring to the charging side. The charging side is also subjected to direct impact and erosion from the added scrap steel and molten iron, resulting in severe mechanical damage. Therefore, magnesia-carbon bricks must possess not only high slag resistance and considerable high-temperature strength, but also excellent thermal shock resistance. High-strength magnesia-carbon bricks with added anti-oxidants are typically used. (4) Tapping Side: The tapping side is largely unaffected by mechanical damage during charging and has minimal thermal shock. However, it is subjected to thermal shock and scouring from the molten steel during tapping, resulting in a much slower rate of damage compared to the charging side. When using the same material as the charging side, a thinner lining is used to maintain a balanced lifespan for the converter lining.

(5) Slag Line Area: The slag line is the area where the furnace lining is in long-term contact with the molten slag and suffers severe slag corrosion. On the tapping side, the slag position changes over time and is often not noticeable. On the slag discharge side, severe slag corrosion and other effects experienced by the furnace belly during blowing cause significant damage. Therefore, magnesia-carbon bricks with excellent slag resistance are required.

(6) Trunnion Sides: Besides being damaged during blowing, the trunnion sides lack a protective layer, making repair difficult. Consequently, the carbon in the lining material is easily oxidized, leading to severe damage. High-grade magnesia-carbon bricks with excellent slag resistance and oxidation resistance should be used. (7) Hearth and Bottom: These areas are subjected to intense erosion by molten steel during blowing, but the damage is generally less severe compared to other areas. Low-carbon magnesia-carbon bricks or tar-dolomite bricks can be used. When high-speed blowing is used and the molten pool is shallow, the central part of the hearth may suffer severe damage. Furthermore, when bottom blowing is used, the damage to these areas may be exacerbated, and the same material as the charging side of the hearth should be used.

Currently, to improve the technical and economic indicators of converters, integrated lining is commonly used.

Magnesia Carbon Bricks