Ladle Metallurgy: Basics, Objectives and Processes
- 2. Outline Importance of Steel Secondary steel making – What, When, Why and How? Objectives of Ladle metallurgy Reheating / Stirring Ladle Furnace Deoxidation Desulphurization Degassing AOD CAS – OB process
- 3. Worldwide steel production and India's position ■ In CY 2019, the world crude steel production reached 1869 million tones (MT) and showed a growth of 3% over CY 2018. ■ China - World’s largest crude steel producer in 2019 (996 MT) ■ India (111 MT) is World’s second largest crude steel, followed by Japan (99 MT) and the USA (88 MT) - based on rankings released by the World Steel Association ■ India is 2nd largest finished steel consumer in the world after China & USA in 2019,
- 4. What is Ladle Metallurgy? • Metallurgical reactions happening in a Ladle - Ladle Metallurgy • After taping from converter or electric furnace, molten steel for high quality or special applications is subjected to further refining in a number of alternative processes collectively called as Ladle metallurgy or Secondary steel making • Cost-efficient to operate the primary furnace as a high-speed melter and to adjust the final chemical composition and temperature of the steel after tapping. • Equipment design and operation of ladle helps certain metallurgical reactions to be more efficiently performed • Ladle – An open-topped cylindrical container made of heavy steel plates and lined with refractory - Used for holding and transporting liquid steel.
- 5. Process flow – Iron and Steel making
- 6. Purpose of ladle metallurgy
- 7. Objectives of ladle metallurgy 1. Homogenization 2. Deoxidation 3. Desulphurization 4. Decarburization 5. Degassing 6. Inclusion modification & Cleanliness 7. Alloying
- 8. VD – Vacuum Degassing VOD – Vacuum Oxygen Decarburization IGP – Inert gas purging IM – Injection Metallurgy VAD – Vacuum Argon Decarburization LF – Ladle Furnace Objectives and Processes
- 9. Reheating / Stirring Ladle stirring is an essential operation during secondary steelmaking in order to: ■ homogenize bath composition; ■ homogenize bath temperature; ■ Fasten deoxidation ■ facilitate slag-metal interactions essential for processes such as desulfurization; ■ accelerate the removal of inclusions in the steel - Argon is purged through the liquid steel, either via a submerged lance, or by porous plugs in the bottom of the ladle (Inert Gas purging)
- 10. Ladle Furnace • The ladle with liquid steel is brought to the LF station, where a top cover is placed on the ladle and graphite electrodes are introduced. • In the LF, deoxidation and composition adjustments are carried out by additions, and the temperature of the melt is adjusted by arc heating (graphite elecrodes) • Bath stirring is achieved by means of argon purging from the bottom •Formation of slag layer that protects refractory from arc damage, concentrates and transfer heat to the liquid steel, trap inclusions and metal oxides, and provides means for desulphurization. • Additions of ferro alloys to provide for bulk or trim chemical control. • Cored wire addition for trimming and morphology control. • Provide a means for deep desulphurization & dephosphorization • Act as a buffer for downstream equipment and process. Ref: Iron and steel making Book, Ahindra Ghosh & Amit chatterjee
- 11. Deoxidation Lowering of dissolved oxygen levels in molten steel by the addition of strong oxide formers such as Mn, Si, Al, Ca (as ferromanganese, ferrosilicon, silico-manganese, aluminum, calcium silicide) in the ladle is known as deoxidation. Solubility of oxygen in solid steel is negligibly small. Therefore, during solidification of liquid steel, the excess oxygen is rejected by the solidifying metal. This excess oxygen causes defects by reacting with C, Mn, Si, etc. resulting in the formation of blowholes (primarily CO) and oxide inclusions (FeO–MnO, SiO2, Al2O3, etc.). Strong to weak deoxidizers Ca > Al > Si > Mn
- 12. Desulphurization ■ Reduction of sulfur concentrations as low as 0.002% ■ Injection of desulphurization reagents - Lime, calcium carbide, soda ash, magnesium or calcium- silicon alloy containing 30 percent calcium (effective desulfurizer). ■ Metallic calcium desulfurizes by forming the very stable compound calcium sulfide (CaS), and it is alloyed with silicon because pure calcium reacts instantaneously with water and is therefore difficult to handle. ■ Inclusion shape control – CaS inclusions are formed instead of MnS.
- 13. ■ Removal of gases like N and H from the molten steel is referred as Degassing ■ When liquid steel solidifies, excess nitrogen forms stable nitrides of Al, Si, Cr, etc. Dissolved nitrogen affects: - Toughness and ageing characteristics of steel - Tendency towards stress corrosion cracking - Strain hardening effect Dissolved hydrogen causes a loss of ductility of steel Degassing is effective under Vacuum. Types: ■ Ladle degassing processes (VD, VOD, VAD) ■ Stream degassing processes ■ Circulation degassing processes (DH and RH). Degassing
- 14. RH Degassing (Ruhrstahl Heraus) • Molten steel is contained in the ladle. The two legs of the vacuum chamber are immersed into the melt. • Argon is injected into the upleg. Rising and expanding argon bubbles provide pumping action and lift the liquid into the vacuum chamber, where it disintegrates into fine droplets, gets degassed and comes down through the downleg snorkel, causing melt circulation. • Provisions are available for argon injection from the bottom, heating, alloy additions, sampling Ref: Iron and steel making Book, Ahindra Ghosh & Amit chatterjee
- 15. Vacuum Treatment VOD – Vacuum Oxygen Decarburization • VOD is often used to lower the carbon content of high-alloy steels without also over oxidizing such oxidizable alloying elements as chromium • A modification of the tank degassers is the vacuum oxygen decarburizer (VOD), which has an oxygen lance in the centre of the tank lid to enhance carbon removal under vacuum. • Tank degassers that have electrodes installed like a ladle furnace, thus permitting arc heating under vacuum - Vacuum arc degassing, or VAD .
- 16. • Argon oxygen decarburization (AOD) is a process primarily used in stainless steel making and other high-grade alloys with oxidizable elements such as chromium and aluminum. • After initial melting, the metal is then transferred to an AOD vessel where it will be subjected to three steps of refining namely (i) decarburization, (ii) reduction, and (iii) desulphurization. • The key feature in the AOD vessel is that oxygen for decarburization is mixed with argon or nitrogen inert gases and injected through submerged tuyeres. This argon dilution minimizes unwanted oxidation of precious elements contained in specialty steels, such as chromium. AOD Process
- 17. CAS - OB ■ CAS-OB stands for Composition Adjustment by Sealed Argon Bubbling with Oxygen Blowing)process ■ Developed in by Nippon Steel Corporation in 1980’s ■ The process allows alloy additions to be made under an inert argon environment. It allows simultaneous addition of Al and O2 gas blown through a top lance. These react to form Al2O3 and generate a considerable amount of heat due to exothermic nature of the reaction. The CAS-OB process, therefore results into chemical heating of the liquid steel.
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