The characteristic of induction furnaces for melting iron and steel is that the melt does not add carbon, like arc furnaces. According to the structure of induction furnaces, there are crucible furnaces without iron core rods (high frequency and low frequency) and tube furnaces with iron core rods (low frequency).
The typical induction furnace metal pool has strong electric stirring, and the furnace wall thickness is 10~15cm. High temperature (tube furnace), large amount of metal in the molten pool, considerable erosion of the lining and intense heat exchange (crucible furnace). Compared with other steelmaking equipment, the lining of the induction furnace may be the most demanding use condition. It is damaged by the soft melting of the lining, chemical erosion, wear under the load of the furnace charge and strong stirring of the liquid metal due to long-term exposure to high temperature.
Therefore, the refractory bricks material should have high stability and high strength to chemical action, high thermal shock resistance, and fixed volume. To meet these requirements, it is better to use amorphous refractory clay as a whole to ram the lining rather than small pieces. The lining of the steelmaking induction furnace is made of corundum-added periclase mud. This mud forms spinel during use, and the volume increases to compensate for the shrinkage of periclase. The overall lining volume is stable, and even the ramming hot zone is slightly enlarged and a pre-reaction state is created. The variable porosity of the ramming material increases from the hot zone to the cold zone, which increases the life of the lining. The ramming of baddeleyite mud in the lining of the vacuum induction furnace has achieved good results when smelting precision alloys. The use of ramming baddeleyite mud instead of periclase and corundum reduces the number of non-metallic inclusions by 1.5 times, improves the casting quality, its plasticity and deformability, and a series of special properties (magnetic sensitivity, magnetic permeability, etc.). When smelting cast iron, ordinary pig iron and ordinary steel, the lining of the induction furnace uses quartz or high-alumina mud.
In the application of refractory bricks materials, in order to increase the service life of the furnace lining, the following matters should be noted:
1. Balanced furnace masonry should achieve equal life of each part of the lining, increase the thickness and use higher quality products in places with greater damage. For example, high-quality tar magnesium dolomite refractory materials or high-quality magnesia carbon bricks that are heat-treated for trunnion masonry.
2. Appropriate gunning of the converter lining should have systematic gunning measures to obtain a long service life. The application of a systematic gunning system can ensure that the converter obtains any specified technical and economic indicators according to the service life of the lining. The gunning material generally uses a dolomite-magnesia sand mixture, and the CaO content is less than 10% of pure fine powder. The better gunning method is flame gunning. In order to improve the efficiency of gunning, the degree of lining damage must be known as accurately as possible. The relationship between the shell temperature and the degree of lining damage of the oxygen converter is measured using a dark box infrared radiation device. The increase in lining damage causes the shell temperature to increase and the infrared radiation intensity to change. The famous laser lining thickness gauge, which can check the change in lining thickness, has an accuracy of less than 1mm according to the operating period of the converter. The thickness gauge can monitor the lining status, objectively evaluate the influence of process factors on the lining damage rate, and determine the distribution of the gunning layer.
3. Improve the oxygen blowing method. The oxygen blowing method of oxygen converter steelmaking production has been continuously improved, and the top blowing from above and the bottom blowing from bottom to top, while blowing solid powdered alloy, using inert gas to stir in the molten pool, etc. Under such conditions, to improve the life of refractory brick materials, it is necessary to improve the purity of raw materials. Since the slag and metal wet and protect the refractory lining, the lining should not be built mechanically, the damage of the lining should be accurately monitored, and the spraying method should be improved.
4. Use slag splashing to protect the furnace. According to the principle that the MgO concentration in the slag is oversaturated and then splashed on the furnace lining to form a "molten tumor" (described above), someone in the United States invented the converter slag splashing furnace protection technology (making the slag splash evenly on the entire furnace lining, balancing its melting rate requires sophisticated equipment and accurate operation) to make the converter lining life reach 10,000~15,000 furnaces. The life of China Baosteel's 300t converter reached 14,000 furnaces, solving the converter lining problem.
