The Choice and Application of AOD Furnace Refractories

AOD steel refining method has been rapidly promoted since 1968 when it was first used by Jocelyn corporate in the USA. It has been the main method for stainless steel refining. But, the conditions of AOD furnaces are harsh and the linings are seriously eroded, so it is necessary to select high quality refractories to expand the service life of AOD furnaces. 

The principle of AOD method is to reduce the partial pressure of CO by the gas dilution method. Blow the mixture of argon and oxygen to increase the contact between the molten steel and bubble and slag, which is helpful to remove carbon, sulfur, and other non-metallic inclusions. In the smelting process, the composition and properties of slags in the AOD furnace change too much. The alkalinity varies from 0.6 to 4.0-4.5. So, refractory materials are exposed to acid slags and alkali slags. The main reason for the corrosion of refractory materials is the vortex caused by the gas stirring, which causes the rapid flow and scouring of molten steel and slags. The high refining temperature (1700-1750℃) and its fluctuation can cause the thermal and structural spalling. At high temperature, the slags with a greater change of alkalinity can the penetration and ablation of the linings. So, AOD furnace refractories have good high temperature strength, good thermal shock resistance, good slag resistance, good erosion resistance and good wear resistance.

 

In the USA and UK, directly bonded or rebounded magnesia chrome bricks are used; in the western European countries, magnesia dolomite brick and dolomite brick are used; in China, magnesia chrome brick and magnesia dolomite brick are used. 

 

(1) Magnesia chrome refractories

The lining of AOD furnaces can be built with MgO-Cr2O3 bricks, rebounded and semi rebounded MgO—Cr2O3 bricks. Directly bonded magnesia bricks can be made of high quality magnesia sand and chrome ore. Its micro-structure is characterized with the direct bonding of main crystal phases and isolated existence of low-melting silicate phases. Rebounded magnesia chrome bricks are made by crushing magnesia chrome bricks and then firing it at high temperature. It has better slag resistance than direct bonded magnesia chrome bricks. Semi rebounded magnesia chrome bricks combine the advantages of the former two. 

The main reason for magnesia chrome bricks in AOD furnaces is the structural spalling caused by the penetration of molten slags. In order to improve their performance, the chemical composition of the bricks and the particle size of the raw materials should be controlled well. The 18%-20%of Cr2O3 is ideal. The content of SiO2 has great impact on the corrosion resistance of the bricks. Its content should be controlled less than 1%. The content of CaO should be controlled too. 

(2) Magnesia calcium refractories

The magnesia calcium refractories have rich source of raw materials and low price, but it is easy to be hydrated. The main magnesia calcium refractories used in AOD furnaces include burned magnesia dolomite brick and fused non burnt magnesia dolomite brick. Burned magnesia dolomite brick is made of high quality sintered magnesium dolomite sand as the main raw material by high-pressure molding and high temperature firing above 1600℃. Fused non burnt magnesia dolomite brick is made of fused magnesia dolomite sand as raw material by high-pressure molding and low-temperature drying. Since fused magnesia dolomite sand is melted fully, it has complete crystalline structure and lower activity, so that fused magnesia dolomite brick has high corrosion resistance and water resistance. 

 

Both magnesia chrome refractories and magnesia calcium refractories have pros and cons, but given all factors, the latter has more advantages and has been a trend in AOD furnaces. 

 

Magnesia calcium refractories have the following advantages:

(1) The main components MgO and CaO have high melting point (2800℃, 2570℃), low vapor pressure and stable thermodynamic properties.

(3) The dihedral angle of MgO-CaO is bigger than that of MgO-MgO, which can improve the slag resistance and penetration. Magnesia calcium refractories have better thermal shock resistance than magnesia chrome refractories.

(2) The presence of CaO can purify the molten steel, so magnesia calcium refractories are especially suitable for clean steel smelting.

(3) Abundant resources of raw materials, low cost, and no chromium pollution.

 

Stainless steel is mainly made by the AOD method. Magnesia chrome and magnesia calcium refractories are the main refractories used in AOD furnaces and the latter has an advantage over the former. But magnesia calcium refractories have a single product variety, so better varieties need to be developed to meet the development of AOD furnaces.