Steel Manufacturing Process from Scrap
STEEL MAKING FROM SCRAP BY ELECTRIC ARC FURNANCE (EAF):
The basic principle of electric steelmaking today is that the furnace is a “melting machine” that produces liquid steel with required chemistry, temperature, and mass in time to feed steel to successive ladle treatments and continuous caster, which finally determines the production rhythm.
Typical tap-to-tap times are in the range of 40–60 min, which is also the total furnace time per heat.
Melting Practice and Foaming Slag:
Although Ultra High Power (UHP) furnaces are used, fast melting by using only electric power is difficult and not the most economic practice either. Importing extra energy and assisting melting technique can greatly accelerate scrap melting and bring economic benefits.
Accordingly, the current state of the art in EAF steelmaking is to use as much as possible chemical energy, besides electric energy, to accommodate tap-to-tap times to the pace of the downstream continuous caster.
Regarding the application of electric energy, at the start of melting, after basket charge, not all the available power can be applied, as the electrodes may be still in a high position, too close to the roof. Then, when the melting operation has advanced further, changing to the tap maximum power may be applied. This is not the case for 100% flat bath operations like in Consteel EAFs or 100% DRI/HBI charging through the fifth hole.
Chemical energy is introduced by oxygen, carbonaceous materials, and natural gas, more and more through injectors rather than lances. The energy-generating reactions are:
The refining step usually does not require full power, which with already flat bath could be dangerous for the lining. At that time, the foaming of the slag is a must. For the slag to foam, the production of CO gas is necessary, by means of the injection of carbon and oxygen through lances or burners. For foaming purposes, several carbonaceous materials are useful, depending on local cost and availability: anthracite, petroleum coke, coke breeze.
Another requirement for good slag foaming has to do with slag chemistry. Basicity should be more than 1.5 and FeO content should be neither very low nor very high.
The main slag formers in the EAF are lime, dolomitic lime, oxides in scrap, gangue in DRI, oxidation products from metallic charge, ash of carbonaceous additions, MgO picked-up from refractories as well as the slag remaining from the previous heat.
Refining in EAF:
- For most steelmaking operations, refining in the EAF is limited to dephosphorization, decarburization, and temperature adjustment.
- Phosphorus in the charge could be at higher than usual levels for DRI, HBI, pig iron, and hot metal, depending on the iron ore source.
- Some steels require particularly low P levels to avoid too high ductile/brittle transition temperature or tempering brittleness.
- From the equilibrium point of view, lower temperatures, high slag basicity, and high oxidation of the bath favor dephosphorization.
- Slag/metal interaction is important from a kinetic point of view but is not always attainable in the EAF.
- Reversion of phosphorus from slag to steel may take place when heating to the aimed temperature, close to the end of the process.
- Then, if there is some slag carry over to the ladle, and steel and slag have a low oxidation level, P reversion is again possible.
- Therefore, in such phosphorus critical cases slag carry over to the ladle should be carefully prevented.
- That can be done by slag detection–slag stopping system or by EBT, which is stopped before the end of the steel.
- In such a “hot heel” practice, a significant fraction of liquid steel is left in the furnace for the next heat.
- Hot heel practice not only prevents steel from rephosphorization, but also speeds up the melting process; however, it has a smaller charge weight in the ladle as a drawbacks.