The composition and properties of slag refined outside the furnace such as argon oxygen furnace and vacuum argon oxygen furnace vary greatly during the smelting process, and the basicity changes from about 0.6 in the initial slag to 4.0~4.5 or higher.
The refractory lining is corroded by highly corrosive acidic slag and alkaline slag at high temperature. Therefore, it is meaningful to clarify the relationship between the basicity of slag and the slag resistance of refractory materials, whether for the selection of suitable refractory materials or for trying to improve the life of lining by improving steelmaking operations.
For magnesia-chrome bricks, the slag erosion is the smallest when the basicity is about 1.4; in the low basicity range, the slag erosion decreases with the increase of basicity, but in the high basicity range, on the contrary, the slag erosion increases rapidly with the increase of basicity, and greatly exceeds the erosion at low basicity.
In other words, magnesia-chrome bricks have high slag resistance to acidic slag, but poor resistance to high basicity slag.
The main reasons for the damage of MgO-Cr2O3 refractory lining used in AOD furnace
(1) High smelting temperature and long acidic slag action time lead to melting and penetration of the furnace lining;
(2) Thermal spalling and structural spalling caused by temperature fluctuations;
(3) Rapid gas → slag → molten steel eddy erosion, especially the erosion caused by the violent stirring of slag and molten steel. Therefore, the refractory materials for A0D furnaces should have good thermal shock stability (TWB), high slag resistance, strong resistance to high temperature molten steel and slag erosion and mechanical damage, and at the same time, the refractory materials should have high refractoriness, low porosity, and sufficient strength.
When the AOD furnace is built with magnesia-chrome bricks, it can be roughly divided into three parts: furnace cap, furnace lining and tuyere bricks.
Stove cap
Usually, the damage to this part is relatively minor. Therefore, high-aluminum (80% A1.O) plastic combined with phosphoric acid is used, and metal or ceramic is used to anchor the furnace cap shell. Or 50% MgO-Cr2O3 waste brick particles/50% castable (90% Al2O3) is used for casting. Ordinary magnesia-chrome bricks can be used for masonry.
Furnace lining
Due to the high smelting temperature of the AOD process, the long-term erosion of the acidic slag and the rapid gas-slag-molten steel eddy current, the refractory material is seriously corroded.
When MgO-Cr2O3 bricks are used as lining, direct-bonded magnesia-chrome bricks are usually selected, and magnesia-chrome bricks and rebonded magnesia-chrome bricks are sintered together for masonry. The United States proposes that the MgO of various magnesia-chrome bricks is 40%~60%, Cr2O3>18%, and the content of easily reducible oxide Fe2O3 should be as low as possible Cr2O3/Fe2O3≥2.5, and Cr2O3/Fe2O3>3 for the main load parts.
The AOD furnace lining was originally made of directly bonded MgO-Cr2O3 bricks, which are made from sintered magnesia with low impurity content and chromium concentrate sand after mineral processing, and are fired at temperatures above 1700°C or even higher.
After high-temperature firing, the main crystal phases form direct bonds, while the low-melting-point silicate phase exists in the form of islands. The high-temperature strength, slag resistance and high-temperature volume stability of this kind of brick are better than those of traditional Mg-Cr2O3 bricks or Cr2O3-MgO bricks.
Among them, the direct bonded magnesia-chromium brick with high secondary spinel content has the best performance. Among brick-making methods that produce high secondary spinel content, Cr2O3 spinel is the most effective. The most effective way is to add Cr2O3 powder to the ingredients.
In order to adapt to the operating conditions of the AOD furnace, a co-sintered magnesia-chrome brick was developed. It is a pre-synthesized low-impurity magnesia sand and chrome ore at 1800℃, which is fully reacted to obtain synthetic MgO-Cr2O3 sand, and then crushed and granulated, formed, and sintered at high temperature to obtain a uniform distribution of spinel and a low silicate content. Its resistance to slag erosion and penetration are better than that of directly bonded magnesia-chrome bricks.
Another type of magnesia-chrome brick used in AOD furnace is a fused-cast MgO-Cr2O3 brick made by melting a mixture of MgO and chrome ore in an electric furnace. It not only has a relatively uniform distribution of spinel, but also a low porosity. Although it has high erosion resistance, its thermal shock resistance is not good.
The fused magnesia-chrome sand is crushed and granulated, formed, and then fired at high temperature. This product is called rebonded magnesia-chrome brick. It has good resistance to slag erosion and higher thermal shock resistance than fused cast magnesia-chrome brick.
Japan Xingqi Steel Plant has successfully developed a composite blowing method for AOD furnace, blowing mixed gas 02+Ar (N2) from the side of the furnace and oxygen from the top. The 20tAOD furnace uses rebonded magnesia-chrome bricks to build the lining, and its life span reaches 235 furnaces, and the unit consumption of the furnace lining is only 6.7kg/t steel.
Semi-rebonded magnesia-chrome brick is made of fused magnesia-chrome sand and magnesia sand (or co-sintered magnesia-chrome sand) as raw materials. Therefore, it has some excellent properties of co-sintered magnesia-chrome bricks and rebonded magnesia-chrome bricks, but overcomes the shortcomings of both.
According to the different service life of different types of MgO-Cr2O3 bricks in AOD furnace and the inconsistent damage of furnace lining, comprehensive lining measures can be adopted to achieve the purpose of balanced erosion and increased furnace life. In addition, the upper lining of the tuyere with serious damage is generally built with combined MgO-Cr2O3 bricks or special MgO-Cr2O3 bricks.
Air outlet brick
If the AOD furnace lining is divided into three areas: the wind side, the trunnion side and the steel tapping side, their corrosion severity decreases in this order, and the tuyere bricks are particularly critical.
Usually, high-purity direct-bonded magnesia-chrome bricks and rebonded magnesia-chrome bricks are selected as tuyere bricks for AOD furnaces.
In terms of service life, there is almost no difference between these two bricks when used as tuyere bricks for AOD furnaces;
The results of use show that when semi-rebonded magnesia-chrome bricks and special magnesia-chrome bricks are selected as tuyere bricks for A0D furnaces, a longer service life can be obtained.
However, when MgO-Cr2O3 bricks are used as linings for the tuyere of A0D furnaces, there are also some disadvantages. The main disadvantages are:
(1) Discontinuous corrosion and less than ideal spalling resistance;
(2) Magnesia-chrome bricks are very sensitive to the use temperature. When the smelting temperature is above 1700℃, the corrosion of MgO-Cr2O3 bricks will increase by 1~2 times for every 40K increase in temperature;
(3) The chromium in MgO-Cr2O3 bricks will be absorbed by molten steel, making it difficult to control the Cr content in chromium-containing molten steel;
(4) Magnesia-chrome bricks are compatible with alkaline desulfurization slag (CaO/SiO2>2) because high CaO slag will dissolve chromium-containing spinel;
(5) Used MgO-Cr2O3 waste bricks will cause pollution to the environment;
(6) In addition, compared with MgO-CaO bricks, their prices are higher.
