Metal Production- Secondary Steelmaking Continued Flashcards
Principle for vacuum degassing and how this occurs
Principle is that when a liquid metal is exposed to a vacuum any dissolved will be liberated. Occurs by two different processes.
Formation of bubbles if the quantity of dissolved gas is large (fast).
The migration of gas atoms to the surface and their movement across the metal/vacuum interface (surface desorption - slow).
Effect of vacuum on oxygen% vs carbon% graph
1/x shape curve. Gets closer to axes and origin for increasing vacuum (decreasing pressure)
Why are Al and Si deoxidisers still needed after vacuum processing?
Despite the theory that a vacuum should give better removal of oxygen than conventional deoxidisers, in practice the full removal of oxygen is never achieved due to the very slow speed of CO removal via surface desorption
What else does vacuum degassing improve?
Improves the cleanness of the steel and some dépassées are used primarily for this purpose
Ladle degasser
The ladle is placed in a large chamber that is evacuated down to 1/1000th of an atm. A CO boil occurs with unskilled steel (one that hasn’t been deoxidised) and some H and O are removed in the initial stages. Fresh metal must be continually brought into contact with the vacuum so some sort of stirring process must be employed. Argon bubbling or EM stirring
Stream degasser
Different principle to ladle degasser. Metal transferred into ladle which is held within a vacuum chamber. When metal contacts the vacuum, gas bubbles form and the metals stream turns into droplets. This aids the removal of H, N, O because of the turbulence and enormous surface are of the droplets. Probably most cost effective method of removing H.
DH degassing process
Dortmunder-Heraeus process. Small amounts of the melt are sucked up via a snorkel into a vacuum chamber positioned above the ladle. The treated metal is returned to the ladle after exposure to the vacuum and the lower end of the snorkel remains submerged at all times. Good mixing occurs and the whole ladle can be treated in around 25mins.
RH degassing process
Ruhrstahl-Heraeus process. Uses two snorkels held beneath the surface of the melt. Liquid steel is drawn up into the vacuum chamber in the same way as the DH process.
Benefits of degassing
Removal of oxygen and hydrogen.
Mixing beneficial to temperature and chemical analysis.
When alloying additions made to the melt they are distributed evenly because of the stirring actions in the ladle.
Also no problems with losses due to oxidation when these additions are made under vacuum.
Problem with degassing procedures
They don’t have any reheating capacity
Describe ladle steel making units and what they are for
All have heating facilities usually comprising electrodes which arc into surface of melt. Means steel can be held at a certain temperature and if heat losses have occurred then can be reheated to correct temperature. Units provide a buffer between steelmaking and casting and allow for delays in steelmaking shop. Divided into vacuum steelmaking units and units with reheating capabilities only.
What further refining processes can improve the cleanness of the steel?
Consumable electrode processes such as vacuum arc remelting (VAR) or electro-slag refining (ESR).
Similarities of the VAR and ESR processes
The steel to be refined is cast or forged into a cylindrical shape and will act as the electrode in the refining process. Heat applied to the steel which begins to melt. Droplets of molten liquid fall and accumulate in a water cooled crucible. Molten pool solidifies and forms the refined ingot. Amount of liquid metal in the molten pool is always very small and solidification occurs almost unidirectionally (no segregation effects or significant porosity.
Specifics of VAR
Remelting is carried out in a vacuum chamber, an arc is struck between the steel electrode and the crucible. As the droplets fall into the pool they are exposed to a vacuum and virtually all the dissolved hydrogen, nitrogen and oxygen (as CO) is removed.
Specifics of ESR
Heat supplied in a slightly different fashion to in VAR. A current is passed through a molten fluorspar slag between the crucible and the electrode. The droplets fall through the slag and inclusions are absorbed. The slag also forms a skin between the crucible and the ingot so a very good surface finish is obtained.
Benefits of consumable electrode refining
Only VAR is effective at removing dissolved gases. Only ESR is effective in removing sulfur. Both together produce ultra clean steel where inclusions are reduced in number and size and distributed more evenly throughout the ingot. Suitable for the production of high grade steel for the power generation and aerospace industries.
Disadvantages of consumable electrode refining
Very slow processes and high costs involved
Effects of hydrogen in steel
Generally not good. Can cause steel to be more brittle and fail in service. Can be cause of stress corrosion cracking and corrosion fatigue. Harmful in welding of high strength steel and can cause cracking in thick sections.
Sources of hydrogen in steel
Mainly water (wet scrap, ferro alloys, refractories, CaO in slag, etc).
Removing hydrogen
Vacuum processes most effective. Tank degasser and RH degasser can be used. RH degasser very effective at removing H because it recirculated the liquid steel. Removal of H even more effective when use inert gas like argon to stir the steel. Bubble the argon in through the bottom of the ladle or inject it through the top and it stirs the steel
How does hydrogen content depend on volume of argon stirring gas and pressure?
H content vs volume Ar stirring gases graph. An exponential decrease from a starting H content (at 0 volume Ar) with increasing volume Ar. The graph always starts from same point but gets lower for lower pressures (and high pressure makes it more linear).
Effects of nitrogen in steel
Can be good and bad for steel properties. Cheap additions to lots of steel grades can increase strength and corrosion resistance. But can also decrease formability, fracture strength and how the steels behave in service.
Does steel from EAF or BOS route usually have higher N content?
EAF route usually produces steels with a higher N content than BOS
Removing nitrogen and extra complications
Can use vacuum degassing techniques. But this is only effective if the steel has low sulfur content, low oxygen content and if high flow rates of Ar gas are used for stirring during degassing. Also need to ensure slag layer is intact to avoid nitrogen pick up.
