Another type of refractory material used in AOD furnaces is MgO-CaO refractory materials. The technical advantages of MgO-CaO refractory materials used in AOD furnace lining over MgO-Cr2O3 refractory materials are:
(1) This material does not contain chromium, so it will not increase chromium inclusions in molten steel. It is very important to control the chromium content of not only chromium-free molten steel but also chromium-containing molten steel;
(2) MgO-CaO refractory materials are chemically compatible with high-quality desulfurized lime slag;
(3) MgO-CaO refractory materials have strong resistance to redox reactions, even in AOD furnaces and at high temperatures;
(4) MgO-CaO refractory materials show obvious creep properties at temperatures as low as 1260<. This indicates that this type of material has a strong resistance to spalling on the hot surface, so it can become a dense brick with the greatest corrosion resistance when used;
(5) The raw materials used for MgO-CaO refractory materials are easy to source and can be obtained locally;
(6) MgO-CaO refractory materials contain highly reactive f-CaO, which can repair the cracks of this type of material and form a very dense and impermeable working surface, so it has a high corrosion resistance.
Through the analysis of the laboratory slag resistance test results and the research results of the microstructure of magnesia-calcium bricks used in AOD furnaces, it is concluded that the erosion resistance of magnesia-calcium refractory materials against AOD slag increases with the decrease of CaO/MgO ratio in the material, while the spalling resistance increases with the increase of CaO/MgO ratio in the material; however, the material with end member CaO/MgO=0 (magnesia brick) is not suitable for the operating conditions of AOD furnaces, while the material with end member CaO/MgO→∞ (CaO brick) has higher durability when used in AOD furnaces. In recent years, in order to extend the life of the furnace lining and reduce the consumption of refractory materials, many tests have been carried out with dolomite bricks, and good results have been achieved, with the life reaching or exceeding that of magnesia chrome bricks.
Performance advantages of magnesia-calcium refractories
1. Slag resistance
Magnesia calcium bricks have large dissolution loss in the initial slag and small erosion in the later slag. At the same time, the slag that invades the brick reacts with the CaO of the brick to form highly refractory dicalcium silicate, which increases the viscosity of the slag and the wetting angle, thereby limiting the penetration of the slag into the deep part of the brick and preventing the formation of a thick metamorphic layer. In addition, during the use of magnesia-calcium refractories, due to the recrystallization of the calcium oxide phase, a very dense unmetamorphosed dolomite layer is formed on the hot surface of the brick, which can completely prevent the penetration of slag.
2. High temperature strength
Forced stirring is usually adopted for refining outside the furnace, and the scouring of refractory materials by steel slag is very serious, so the refractory materials are required to have good high temperature wear resistance. The wear resistance of refractory materials can be compared according to their high temperature strength. It is generally believed that the high-temperature strength of magnesia calcium bricks is not as good as that of magnesia chrome refractory materials. However, the United States has produced a high-density magnesia-calcium brick, whose high-temperature strength is roughly the same as that of directly bonded magnesia chrome bricks.
3. Thermal shock resistance
Out-of-furnace refining furnaces are usually operated intermittently, with drastic temperature changes, and refractory materials are often damaged by thermal shock. Since magnesia calcium refractory materials contain a lot of free CaO, they have greater plasticity at high temperatures, which can buffer the thermal stress caused by temperature fluctuations. In addition, they will not produce a thick metamorphic layer when used, and their thermal shock resistance is better than that of magnesia chrome bricks.
4. Stability under high-temperature vacuum
Out-of-furnace refining is mostly carried out under high-temperature vacuum, so the stability of refractory materials must be considered. The results show that high-purity magnesia calcium bricks are very stable under high-temperature vacuum, and the weight loss rate is very small; magnesia-chrome refractory materials have a large weight loss rate, significantly increased porosity, and are unstable under high-temperature vacuum.
