What is the main objective of in secondary metallurgy?
Steel deoxidation
Achieve aim composition (target alloy)
Reduce S, N2, H2 and C content
preparation for casting
Why is chemical and thermal homogenization important in steelmaking?
It ensures uniform composition and temperature throughout the steel batch.
Name the possible operations in secondary metallurgy.
Additions
Bath stirring
Formation of a new slag
Ladle desulfurization
Degassing
Ca treatment
other specific operation eg. Argon Oxygen Decarburisation for stainless steels
What is the purpose of ladle slag formation?
capture impurities and assist in their removal from the steel
thermal and chemical insulation
How does ladle stirring benefit the steelmaking process?
It promotes chemical homogeneity and helps remove inclusions.
What is the role of additives in secondary metallurgy?
Additives adjust the final steel composition and aid in deoxidation.
Why is degassing an essential operation in secondary metallurgy?
Degassing removes gases like hydrogen and nitrogen, preventing defects and improving steel quality.
What are the typical contents of nitrogen and hydrogen in steel during secondary metallurgy?
Nitrogen is usually around 40 ppm, while hydrogen is around 5 ppm.
What are the minimum achievable element compositions in the ladle?
Which molecule is commonly used for deoxidation?
Al2O3
What is the purpose of ladle stirring?
Thermal and Chemical homogenization
favour inclusion flotation and removal
Increase reaction kinetics
What is the stokes law of setting and why is it important for secondary metallurgy?
The law describes the velocity of the particle towards the surface of the melt.
d describes the melt
rho m is the particle density and rho s the steel density
What are the stages of two-stage deoxidation?
Initially, deoxidation with manganese/silicon, followed by completion with aluminum.
Why is argon stirring used in ladle metallurgy?
To facilitate homogenization and inclusion removal, and to increase reaction kinetics.
What is the definition of the Recovery rate in Secondary Metallurgy?
The weight percentage of additive that is transferred into the steel (the rest goes into the slag)
What are the steps of ladle desulfurization?
Adding a synthetic CaO based Slag
Al deoxidizing to very low oxygen activity
Vigourous stirring under vacuum
Name the reasons for a temperature drop in the ladle furnace.
radiation losses at tapping
heat exchange with the refractories of the ladle
radiation losses in the ladle
enthalphy reduction because of additives
losses during stirring
losses during powder injection
losses during degassing
losses from transfering to tundish (feed for ingot casting)
How does the ladle furnace differ from the electric arc furnace?
less power
shorter arcs
Explain the DH (Dortmund-Hörde) method for degassing.
The steel is sucked into a low pressure enviroment in the tank through a snorkel.
The process is not continous
Explain the RH (Ruhrstahl-Hereaus) method for degassing.
The steel is sucked into a low pressure enviroment in the tank through a snorkel and released through the other, which leads to degassing
The process is continous
How can hydrogen inclusions be prevented?
Propper degassing
Dehydrogenation cycles
Plastic working with large deformations
Why should hydrogen inclusions be avoided?
They cause very high local embrittlement and can lead to local cracks.
What is the problem with denitrogenization?
It needs a very low content of sulfur (around 30ppm)
Normally only 20% can be removed
Name the production steps of the Vacuum Oxygen Decaburization (VOD) for stainless steel.
Addition of HC FeCr
Blowing of O2 (1/10 atm)
Vacuum (1/100 atm)
strong de-C
Addition of CaO,FeSi
What are the effects of the calcium treatment and how is it conducted?
uses:
elimination of Aluminum
helps reduce inclusions
makes steel castable
Insertion of Calciumsilicon or Calciumcarbide wire
for good melting wire has to be inserted at high velocity.
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