With the rapid growth of production capacity of high-quality steel such as stainless steel and clean steel in my country, and the successful application of domestic magnesia-calcium refractories in refining equipment such as AOD furnaces, my country’s magnesia-calcium refractories have entered a new development period. The number of manufacturers continues to increase, the scale of the original manufacturers continues to expand, the variety continues to enrich, and new magnesium-calcium refractory materials are continuously developed, especially magnesium-calcium unshaped refractory materials, such as ladle gunning materials, converter gunning materials, RH furnace Gunning material, self-flowing hot material, tundish coating material, dry ramming material, etc., and will soon be put into large-scale production. Its characteristics mainly include the following six aspects:
01 “High refractoriness, good stability”
The melting point of calcium oxide is 2600 °C, while the melting point of magnesium oxide is higher, reaching 2800 °C, which is higher than the melting point of many commonly used oxide refractory materials.
CaO is one of the most stable common oxides, and it hardly reacts with various molten metals. According to the research of Chuantong and other scholars, at 1500 ° C, the absolute value of the formation free energy (negative value) of some oxides is in the order of It is: CaO≥HfO2>ZrO2>Al2O3>MgO>TiO2>SiO2>Cr2O3, at the melting temperature of metals and alloys, the amount of CaO reduced by active alloying elements is much less than other industrial materials.
02 “Purification of Molten Steel”
The principle of purifying molten steel is mainly: free CaO in magnesia-calcium bricks has a very low oxygen partial pressure, and its solubility in molten steel is extremely small. It is an oxide that can effectively avoid re-oxidation of molten steel; free CaO can absorb molten steel [S], [P], [O] and non-metallic inclusions such as Al2O3, SiO2, etc., to purify the molten steel. At the same time, the slag reacts with MgO-CaO to form C2S with high refractoriness, which can increase the wetting angle of slag and viscosity to further improve the desulfurization effect of molten steel. At the same time, the low melting material formed by the reaction will float up into the slag, which can prevent the nozzle from being blocked. Japanese scholars pointed out after research that it is better to use non-fired magnesia-calcium bricks when smelting stainless steel.
03 “High Alkalinity”
Since the free CaO in the MgO-CaO-based refractories will preferentially react with SiO2 in the slag to generate high refractoriness (melting point 2130°C) and high viscosity C2S when the basicity is low at the initial stage of refining, the slag wetting angle increases, Viscosity increases, which prevents further intrusion of slag into the interior, and improves the unfavorable situation of thickened metamorphic layer. In addition, the ability of magnesia-calcium bricks to resist high-alkalinity slag (R>1.5) is better than that of low-alkalinity slag. CaO, 2CaO SiO2, 3CaO SiO2, etc.), it is difficult to penetrate into the brick. Low-alkalinity slag has low viscosity, low melting point, and contains some low-melting silicate phases (such as CaO·MgO·SiO2, etc.), which are easy to invade into bricks. In the steelmaking process, the smelting operation under the condition of high alkalinity slag can be carried out to improve the service life of the furnace lining.
Magnesia-calcium refractory materials also have strong resistance to high-iron slag, because during use, iron ions in the slag will diffuse into the brick, which will not reduce its slag resistance, but can make it close to the side of the slag belt , forming a direct combination of a high-temperature solid phase, which plays a shielding role and hinders the further diffusion of iron ions.
In terms of the influence of its own composition on slag resistance, the research of Huang Bo et al. shows that with the increase of CaO content in magnesium-calcium materials, the ability to resist CaO-SiO2 slag increases, while the ability to resist CaO-Al2O3 slag decreases. . The main reason is that CaO can form a large amount of calcium silicate (C3S) with high melting point on the working surface with SiO2. These C3S surround magnesium oxide and form a bridge structure. This high-temperature phase can prevent the erosion of slag; while CaO-Al2O3 slag It will cause more calcium magnesium olivine (CMS) and calcium aluminum feldspar (C2AS) with low melting point to be generated in the working face. At the same time, high-calcium MgO-CaO materials with w(CaO) ≥ 50% have strong slag resistance to acidic and alkaline slag, while low-calcium magnesium-calcium bricks have strong corrosion resistance to alkaline slag, but they are The penetration resistance of acid slag is weak.