Important aspects of induction furnace operation
As liquid steel is excited by current opposite to current flowing in induction coil, it is agitated to raise its surface in the center. Surface of liquid steel is risen higher as frequency becomes lower, i.e. agitation of the liquid steel occurs stronger in low-frequency furnace than in high-frequency furnace. This effect of agitation makes it possible to ensure uniform temperature of the liquid steel and its uniform quality as well as to promote entrapment of material charged and fusion of chemical composition adjusting agents, specially carbon addition. On the other hand, excessive agitation may cause such troubles as oxidative wearing of liquid steel and fusing out of refractories or danger of spattering of liquid steel.
Once the melting is complete, the slag is skimmed off. Slag generated during melting has tendency to stick on the furnace wall. This reduces volume of furnace hence reduces metal output per heat. Superheating of metal is done at higher temperature and held for few minutes. This inhibits slag to deposit on the furnace lining keeping furnace clean with full volume.
The composition of the slag varies depending on the specific process being used and the type of steel being produced. The compositions of furnace and ladle slags are often very complex. The slag which is formed is the result of complex reactions between silica, iron oxide from steel scrap, other oxidation by products from melting, and reactions with refractory linings. The slag consists of a complex liquid phase of oxides of iron, manganese, magnesium and silicon, silicates and sulfides plus a host of other compounds, which may include alumina, calcium oxides and sulfides, rare earth oxides and sulfides etc.
While producing the steel, the chemistry of end product is controlled. The chemical analysis of all the input materials is done to have a decision on the charge mix. After completing 50 % charging of the input materials, a bath sample is analyzed for chemical composition. Based on the chemical analysis of the bath sample at this stage calculations are made for further additions of the metallics. If the bath sample at this stage shows high percentage of carbon, sulphur and phosphorus then the direct reduced iron content of the charge is increased. Final bath sample is taken when 80 % melting is completed. Based on the analysis of this sample, another adjustment is made in the charge. The lower content of carbon in the sample is corrected by increasing the quantity of pig iron/cast iron in the charge. Silicon and manganese in the metal is oxidized by the iron oxide of the direct reduced iron. Sulphur is also diluted by the direct reduced iron. Because of use of direct reduced iron the trace elements in the steel made in the induction furnace remains under control.
The liquid steel is the desired output of the induction furnace. The quantity depends upon the capacity of the furnace, and the quality depends upon the raw materials and the steel composition. The tapping temperature depends upon the type of steel and the super heat needed in the liquid steel for its end use. Tapping of steel at high temperatures increases refractory erosion and power consumption.
Unnecessary superheating of liquid steel to high temperature costs to energy significantly. Minimizing the overheating of molten bath saves energy. Depending on steel specification and temperature loss during transfer of liquid steel to continuous casting machine, superheat temperature is to be decided. In every heat, temperature of the liquid steel bath is to be measured and monitored to get optimum energy saving. Proper power control systems with potentiometer adjustment need to be provided for minimizing energy losses due to overheating.
Tilting of the furnace is to effect pouring of the melt is a last operational activity before casting. The furnace is usually tilted to achieve an angle of 90 degree or greater for complete pouring of the liquid steel.