Application of Refractory Materials in Steelmaking Equipment Electric Arc Furnace

(1) Operating Conditions for Refractory Materials

Electric arc furnaces, which use the arc between the electrode tip and the charge material as the heat source for steelmaking, are characterized by unique conditions for refractory material usage. The development of direct current (DC) arc furnaces, high-power operation, bottom gas stirring, and bottom tapping have been introduced in recent years. The furnace roof is built with magnesia-chrome or magnesia-spinel refractory materials, which exhibit excellent stability against basic slags, metals, and silicate melts. However, the operating conditions for refractories in this environment are quite severe, dictated by the specifics of arc melting.

The arc melting process is approximately twice as fast as the open-hearth process, subjecting refractories to frequent temperature changes and prolonged high temperatures. During the charging phase, the furnace roof is removed, exposing it to uneven heating, with the temperature at the center significantly higher than at the periphery. This uneven heating is exacerbated by the irregular nature of the working arc. Consequently, the center of the roof deteriorates quickly. For instance, in a 100-ton furnace, the central roof section erodes at a rate of 4 to 4.4 mm per heat, compared to 2 to 2.6 mm per heat at the edges. This uneven wear leads to uneven bulging of the roof and, occasionally, brick spalling.

In DC arc furnaces, the single-electrode design eliminates hot spots, and the water-cooled roof area is expanded, slightly improving refractory conditions. However, as furnace capacity increases and specific power rises, the working conditions for the roof become even harsher. Furnace roofs are circular and often built using fan-shaped arches or ring bricklaying techniques. Bricks are laid without binders or cement, secured by metal spacers with sharp ends. Openings for electrodes, gas extraction, and oxygen injection reduce the roof's weight. In some cases, the areas around these openings are cast using high-alumina cement or phosphate-bonded castables. Measures are also taken to eliminate electrical short circuits.

The service life of furnace roofs for furnaces with capacities below 100 tons is typically 60–120 heats, while larger furnaces with capacities above 100 tons achieve 60–80 heats. Total refractory consumption for an electric furnace per ton of steel is approximately 10–12 kg, with the roof accounting for 6–7 kg.

(2) Selection of Roof Refractories

The choice of refractory materials for electric arc furnace roofs is still evolving. Although MgO-Cr₂O₃-based refractories offer high resistance to slag and metals, their load-softening temperature is relatively low. Another drawback of basic refractories is their significant thermal expansion at high temperatures, which can cause brick joints to crack and the roof to deform. To prevent joint cracking, a mix of fired and unfired bricks is used, allowing shrinkage to offset the expansion of fired bricks. Some literature suggests combining fired and metal-coated unfired magnesia-chrome products.

Innovative refractory materials for furnace roofs are being tested, including corundum-chromite, mullite-corundum, and combinations of basic and high-alumina materials. Special attention is paid to chromium-containing refractories, as spalling of chrome bricks may introduce chromium into the steel, which is not permissible for certain grades of steel.

With advancements in metallurgical technologies such as atomic energy furnaces, continuous casting machines, and large electric arc furnaces, the role of electric furnaces in steelmaking is expected to grow significantly. Electric furnaces offer advantages over open-hearth and converter furnaces, including flexibility in adjusting steel composition and producing various steel grades. This expansion is also economically favorable due to the continuous increase in scrap metal availability.

In China, high-alumina bricks are commonly used for furnace roofs, with ramming mixes applied around the center and electrode holes of smaller furnace covers. However, with the development of large ultra-high-power electric furnaces, the service life of high-alumina bricks has declined, prompting further use of basic bricks.

(3) Application of Water-Cooling Technology

The development of modern electric steelmaking is closely linked to the adoption of high-power transformers, ensuring unit power levels of 600–800 kV·A/t during the melting phase. Preheating of charge materials, oxygen injection into the molten bath, and heating the furnace with gas-oxygen burners are among the advanced features. Artificially cooled components are increasingly used to replace portions of refractory linings.

During intensive oxygen supply to the molten bath, dust formation (15–40 g/m³) increases the optical density of the gaseous medium, turning its blackness close to 1. This nearly completely obscures the molten bath and furnace roof, reducing the roof temperature. Slag-forming roofs and walls incorporate various water-cooled structures, with movable cooling shields protecting wall hot spots.

The introduction of artificially cooled components has led to new electric arc furnace designs, reducing the ratio of refractory lining surface area to metallic surface area. Improved heat exchange conditions lower the heat load on the molten bath, which increases as the refractory lining is reduced or eliminated. Structures with adjustable lining development allow for horizontal water-cooled roofs that can move downward along water-cooled walls as the melting process progresses.

The use of artificial cooling in steelmaking furnaces removes limitations imposed by thermal loads and lining temperatures, creating conditions for intensified smelting. Although water-cooled structures slightly increase electricity consumption, the reduction in downtime and increased productivity enhance overall economic efficiency. Refractory material consumption is significantly reduced (almost to the minimum), electrode consumption per unit is lowered by 15%, and the heavy manual labor associated with refractory bricklaying is reduced.

Experimental development of furnaces with water-cooled lining components has shown that the energy and cost savings in related industries (refractory production, transportation, electrode manufacturing, etc.) exceed the additional energy costs associated with water-cooled equipment.