Key Points of EAF/LF Steelmaking

This article is applicable for three phase AC EAF with top charging by scrap basket and spout taping, and may also be taken as reference for modern EAF like Consteel, Quantum, Ecoarc, CISDI-Green, SHARC.?

1.?Raw materials preparation

1.1 Scrap is major raw materials for EAF steel making, and account for 55~80% of total cost, so its quality is directly related with finished steel quality, production cost and efficiency.

1.2 Scrap shall be clean and free from heavy rust, grease and oil contamination;

1.3 Scrap shall not contain non-ferrous elements like lead/tin/zinc/cooper, slag, non-conducting materials, refractory and others.

1.4 For safety of production, scrap shall not contain sealed container, flammable and explosive and toxic materials.

1.5 Chemical composition of scrap shall be clear, no excess content of sulfur and phosphorus.

1.6 Size of scrap shall be suitable, say no more than 400x400mm in cross section, and no more than 1500mm in length of each piece.

1.7 Pig iron is usually charged to increase carbon content, no more than 30% of total burden.

2. Fettling

2.1 Usually after each heat, before materials charge, fettling is needed to repair furnace bottom, erosion part of slag line and other damaged part.

2.2 Quick fettling in high temperature to enable self-sintering of fettling material, firstly furnace door and two sides of tap hole, then heavily eroded bottom part under graphite electrode, finally other parts of furnace.

2.3 Fettling layer shall be thin ( 20~30mm thick) for quick self-sintering.

3. Raw materials charging and distribution

3.1 Before charging, limestone（grain size: <60mm）of about 1.5~2% of total burden weight shall lay down in furnace bottom, which is for slag forming in advance, early dephosphorization and speedy heating.

3.2 Carbon content shall be determined by burning loss of carbon in melting period, amount of decarbonization in oxidizing period, and recarburization amount in reducing period, basically, it shall be 0.3~0.6% higher than min. Level of finished steel in melt down period.

3.3 Principle: bottom dense, top loose; center uplift, around recess, no big and heavy materials in furnace door, for quick penetration and no bridge collapse.

3.4 Burden shall be dense enough and need reasonable collocation of heavy, medium and light scrap (usually, heavy scrap thickness≧15mm, 15mm>medium scrap thickness≧6mm, light scrap thickness≦4mm), say 15~20% of light scrap, 40~50% of medium scrap, 40% of heavy scrap.

During charging, 1/2 of light scrap shall lay down in bottom, above it ( in central part of EAF) heavy scrap, low carbon scrap and infusible materials shall be placed, then medium scrap shall be place above it and circumferential, finally, light scrap shall be place on the top.

3.5 If graphite electrode breaks are used, it shall be in size of 50~100mm and placed in bottom part of each burden.

3.6 If the 2nd or 3th charging is needed, it shall be dry and done when the 1st burden in EAF are melt 70~90%;

4. Melting

4.1 Definition: it’s period from power on phase till that all burden are melt down, usually, it take 50~70% of total tap to tap time and 70~80% of total power consumption, so the key task of melting is to heat and melt burden with the min. power, given furnace life is guaranteed.

4.2 Melting can be classified into 4 steps, say: a) arc striking, b) penetration, c) lift of electrode or main meting period, d) melt down period.

4.2.1 Arc striking: medium voltage and 2/3 input of rated power of transformer is suggested for protection of furnace roof.

4.2.2 Penetration: it takes about 1/4 of total melting time, because arc is completely surrounded by scrap and little damage to lining, max. Power and current to be used.

4.2.3 lift of electrode or main meting period：it takes 50~70% of total melting time, molten pool is formed, still apply max. Power and current.

4.2.4 Melt down period：3/4 of charged materials has been melt, to avoid?damage of roof and shell cased by strong radiation, medium voltage input is suggested.

4.3 Typical electricity supply:

4.4 Oxygen-fuel burning can start when power is on.

5. Oxidizing period

5.1 Task: dephosphorization, degassing, removal of impurities

5.2 Principle: for oxidizing, firstly phosphorus, then carbon; in temperature, firstly low then high; in slag forming, firstly more slag for dephosphorization, then little slag for decarbonization; in oxygen supply, firstly iron ore or comprehensive oxidizing, lastly oxygen blowing.

5.3 Oxygen blowing

5.3.1 Oxygen blowing shall start after 20 min. on power time, when charged materials are melt ≧80%; graphite electrode must be lifted during oxygen blowing;

5.3.2 Oxygen blowing shall be increased gradually, 0.6Mpa≦oxygen pressure≦0.8Mpa

5.3.3 Oxygen blowing under low temperature, solid material still in furnace and partial aeration is prohibited, to avoid materials collapse, violent boiling, bulge of furnace roof, liquid steel leakage, personal or equipment accident.

6. Reducing period

Definition: it’s time after skimming and before tapping, main task is to remove oxygen and sulfur, adjust temperature and chemical composition in liquid steel, prepare for casting.

* In modern EAF/LF route, whole reducing has been conducted in Lable Furnace refining process, so reducing atmosphere must be guaranteed during ladle furnace refining process, more operation key points can refer to below ladle furnace part.?.

7. Tapping

7.1 Temperature of liquid steel: ≥ 1640℃

7.2 Tap with reserve of liquid steel and slag, usually, reserve of liquid steel shall be 10~15% of total tap weight.

7.3 Argon stirring in ladle furnace shall be ready when ladle furnace are back to station after receiving liquid steel, pressure of Argon is 0.4~0.6Mpa and shall be adjusted accordingly to stirring situation.

7.4 During tapping, when 1/3 of total liquid steel is poured, formulated ferroalloy, deoxidizer?and slag forming materials shall be added into liquid steel, while, amount of ferroalloy shall be based on min. content of certain steel grade, and slag forming materials in 10kg/mt.

8. Ladle furnace refining

Key points:

8.1 reducing atmosphere inside ladle furnace;

8.2 Good argon stirring from the bottom of furnace

8.3 Sub-merged arc heating

8.4 White slag refining

8.5 refining time no less than 35 mins

Thanks for your time and attention, because I am not metallurgical professional and just compile/edit this paper based on many known or unknown sources, there must be errors or misunderstanding in this article, welcome correction and further discussion.